Institute of Materials Science - Technische Universität Darmstadt

Annual Report 2013 Faculty of Materials and Geo Sciences

Contents Dean’s Office .................................................................................................................. 4 Institute of Materials Science ......................................................................................... 6 PHYSICAL METALLURGY ....................................................................................................... 12 CERAMICS GROUP .............................................................................................................. 21 ELECTRONIC MATERIAL PROPERTIES...................................................................................... 30 SURFACE SCIENCE .............................................................................................................. 38 ADVANCED THIN FILM TECHNOLOGY ..................................................................................... 50 DISPERSIVE SOLIDS ............................................................................................................. 53 STRUCTURE RESEARCH........................................................................................................ 70 MATERIALS ANALYSIS ......................................................................................................... 75 MATERIALS MODELLING DIVISION ......................................................................................... 87 MATERIALS FOR RENEWABLE ENERGIES ................................................................................. 98 PHYSICS OF SURFACES....................................................................................................... 111 JOINT RESEARCH LABORATORY NANOMATERIALS .................................................................. 116 MECHANICS OF FUNCTIONAL MATERIALS ............................................................................. 120 FUNCTIONAL MATERIALS ................................................................................................... 126 ION-BEAM MODIFIED MATERIALS........................................................................................ 135 MOLECULAR NANOSTRUCTURES.......................................................................................... 142 COLLABORATIVE RESEARCH CENTER (SFB) .......................................................................... 146 DIPLOMA THESES IN MATERIALS SCIENCE ....................................................................... 150 BACHELOR THESES IN MATERIALS SCIENCE ..................................................................... 152 MASTER THESES IN MATERIALS SCIENCE......................................................................... 154 PHD THESES IN MATERIALS SCIENCE ............................................................................. 155 MECHANICAL WORKSHOP ............................................................................................. 157 ELECTRICAL WORKSHOP ............................................................................................... 157 Institute for Applied Geosciences .............................................................................. 158 PREFACE .................................................................................................................... 158 PHYSICAL GEOLOGY AND GLOBAL CYCLES ............................................................................ 160 HYDROGEOLOGY .............................................................................................................. 172 ENGINEERING GEOLOGY .................................................................................................... 177 GEOTHERMAL SCIENCE AND TECHNOLOGY ........................................................................... 187 APPLIED SEDIMENTOLOGY .................................................................................................. 197 GEO-RESOURCES AND GEO-HAZARDS .................................................................................. 203 GEOMATERIAL SCIENCE ..................................................................................................... 212 ELECTRON CRYSTALLOGRAPHY ........................................................................................... 221 TECHNICAL PETROLOGY WITH EMPHASIS IN LOW TEMPERATURE PETROLOGY............................. 223 ENVIRONMENTAL MINERALOGY .......................................................................................... 226 DIPLOMA THESES IN APPLIED GEOSCIENCES .................................................................... 230 MASTER THESES IN APPLIED GEOSCIENCES...................................................................... 230 MASTER THESES TROPHEE IN APPLIED GEOSCIENCES ...................................................... 231 BACHELOR THESES IN APPLIED GEOSCIENCES .................................................................. 232 PHD THESES IN APPLIED GEOSCIENCES .......................................................................... 233

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Dean’s Office Staff Members Dean:

Prof. Dr. Ralf Riedel

Vice dean:

Prof. Dr. Christoph Schüth

Dean of studies Materials Science:

Prof. Dr. Lambert Alff

Dean of studies Applied Geosciences:

Prof. Dr. Matthias Hinderer

Scientific coordinator, department and Materials Science:

PD Dr. Boris Kastening

Scientific coordinator, Applied Geosciences:

Dr. Karl Ernst Roehl

Secretary of department:

Renate Ziegler-Krutz

Secretary of personnel and finances:

Christine Hempel

Competence center for materials characterization:

Dr. Joachim Brötz

IT group:

Dipl. –Ing. (BA) Andreas Hönl

Building services:

Dipl. –Ing. Heinz Mohren

Coordination of the KIVA project:

Dr. Silvia Faßbender

Public relations:

Marion Bracke

Media Design:

Thomas Keller

Publications of Permanent Members of the Dean's Office Boris Kastening Anisotropy and universality in finite-size scaling: Critical Binder cumulant of a twodimensional Ising model Phys. Rev. E 87, 044101/1-4 (2013), arXiv:1209.0105, DOI:10.1103/PhysRevE.87.044101 Siol, Sebastian; Straeter, Hendrik; Brueggemann, Rudolf; Broetz, Joachim; Bauer, Gottfried H.; Klein, Andreas; Jaegermann, Wolfram; PVD of copper sulfide (Cu2S) for PIN-structured solar cells; JOURNAL OF PHYSICS D-APPLIED PHYSICS Volume: 46 Issue: 49 Article Number: 495112 (2013)

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Pfeifer, Verena; Erhart, Paul; Li, Shunyi; Rachut, Karsten; Morasch, Jan; Broetz, Joachim; Reckers, Philip; Mayer, Thomas; Ruehle, Sven; Zaban, Arie; Mora Sero, Ivan; Bisquert, Juan; Jaegermann, Wolfram; Klein, Andreas; Energy Band Alignment between Anatase and Rutile TiO2; JOURNAL OF PHYSICAL CHEMISTRY LETTERS Volume: 4 Issue: 23 Pages: 41824187 (2013) Labrini, Mohamed; Saadoune, Ismael; Scheiba, Frieder; Almaggoussi, Abdelmajid; Elhaskouri, Jamal; Amoros, Pedro; Ehrenberg, Helmut; Broetz, Joachim; Magnetic and structural approach for understanding the electrochemical behavior of LiNi0.33Co0.33Mn0.33O2 positive electrode material; ELECTROCHIMICA ACTA Volume: 111 Pages: 567-574 (2013) Muench, Falk; Oezaslan, Mehtap; Rauber, Markus; Kaserer, Sebastian; Fuchs, Anne; Mankel, Eric; Broetz, Joachim; Strasser, Peter; Roth, Christina; Ensinger, Wolfgang; Electroless synthesis of nanostructured nickel and nickel-boron tubes and their performance as unsupported ethanol electrooxidation catalysts JOURNAL OF POWER SOURCES Volume: 222 Pages: 243-252 (2013)

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Institute of Materials Science Preface Dear colleagues and friends, The year 2013 was another successful period for the Department of Materials and Geo Sciences of TU Darmstadt. Details of the activities and achievements related to the individual departmental institutes, namely Materials Science and Applied Geosciences, are highlighted below. We would like to express our gratitude to all members of the Department – the mechanical workshop staff, technical and administrative staff, students working on their diploma and bachelor theses, Ph.D. students, and postdocs – for the outstanding effort and remarkable enthusiasm they put into their work. Without their contributions the performance and the results presented here would not have been possible. We aim to sustain and promote the motivating and fruitful atmosphere at our institute in order to continue our commitment and success in the time to come.

Materials Science The amount of acquired third party funding has reached a nearly constant value in the order of 10 million Euro. Presently, the total number of students (bachelor & master) in materials science amounts round about 500. The number of freshmen of the bachelor study course Materials Science in the winter semester 2013/14 reached 94 (see Figure 1). The new master course Energy Science and Engineering which is an interdisciplinary field of study and which is administratively organized by our Department successfully developed in its second year. The master course presently counts 61 students of which 36 were freshmen in the WS 2013/14. Last not least, the master course Energy Science and Engineering was accredited in spring 2013. The Materials Science and Geo Sciences Department’s Materialium Graduate School has been further strengthened and now accommodates 30 PhD students, while the total number of PhD students of Materials Science exceeds by now 150. The research-oriented doctorate program culminating in award of the degree of “Dr.-Ing.” or “Dr. rer. nat.” fosters an interdisciplinary integration of the various Ph.D. studies between research groups inside and outside of the Materials Science Department. During specific events, Ph.D. students present their current scientific problems and methods, providing a forum for close interdisciplinary problem solving that stimulates synergy between research groups. Research is always a collaborative enterprise! Professors of Materialium are particularly committed to supporting their Ph.D. students. For instance, they strongly encourage participation at international conferences and publication in refereed research journals, which is bolstered by the high number of coordinated research programs in Materials Science at TU Darmstadt. Moreover, Materialium is a member of Ingenium, the umbrella organisation of graduate schools at TU Darmstadt.

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Fig. 1: All students (except Ph.D. students) and freshmen (Diplom until WS 07/08, B.Sc. from WS 08/09) of Materials Science at TU Darmstadt.

Coordinated Research Proposals The institute was actively involved in a variety of coordinated research project applications. Among them one new proposal was successfully evaluated in 2013 in the frame of the LOEWE Priority Program supported by the Hessian State Government. The scientific topic of this research program is related to “The Reduction and Substitution of Rare Earth Elements in High Performance Permanent Magnets” (Response) and is coordinated by Prof. Gutfleisch. The official start of this coordinated research is January 01, 2014. This initiative marks the interdisciplinary approach the university is promoting and for which the Department of Materials and Geo Science is ideal since its subjects combine various sciences like chemistry, physics, electrical and mechanical engineering.

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Faculty Members In 2013 we had two important events related to the faculty staff members. First, in spring 2013, Prof. Dr. Karsten Durst started as the new head of the group Physical Metallurgy. He is the successor of Prof. Dr. Martin Heilmaier, who left the Department of Materials and Geosciences to take up a position as Director at the Karlsruhe Institute of Technology (KIT) in December 2011. Second, in late autumn 2013 Prof. Dr. Jürgen Rödel, the head of the group Nonmetallic Inorganic Materials, was elected as Vice President for research of TU Darmstadt. We wish both colleagues a successful start for their new and responsible positions.

Prof. Dr. Karsten Durst

Prof. Dr. Jürgen Rödel

Buildings and Lab/Office Space In the course of the completion of the new “Hörsaal- und Medienzentrum” the street names of campus Lichtwiese have been renamed since autumn 2013. Accordingly, the address of the Dean’s office as well as that of the Materials science building has been changed to Alarich-Weiss-Str. 2. The office building L1|08, where the groups of Prof. Albe, Prof. Hahn, Prof. Jaegermann, Prof. Krupke, Prof. Riedel, and Prof. Xu are hosted, have now the postal address Jovanka-Bontschits-Str. 2. In September 2013 we moved into our new lab and office building denoted as M3 which stands for “Molecules, Magnets and Materials”. In an official ceremonial act, the building was inaugurated in the presence of the TU President and Chancellor on October 29. The Functional Materials group of Prof. Gutfleisch as well as the Physics of Surfaces group of Prof. Stark have found their new homes in this state of the art and functional building.

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New M3 lab and office building.

Honours, Awards and Special Achievements In 2013, the following precious awards were granted to faculty members of the materials science department: Prof. Fueß received an honorary doctorate of the University of Vilnius in Lithuania for his outstanding research and for his continuing and fruitful scientific collaboration with the university. Prof. Hahn was awarded with the Franklin Mehl Award of The Minerals, Metals and Materials Society (TMS), USA, for his exceptional research in Materials Science. Dr. Robert Dittmer received a young investigator award, namely the “Nachwuchspreis” of the “Deutsche Gesellschaft für Materialkunde” (DGM).

Prof. Dr. Hartmut Fueß Preface

Prof. Dr. Horst Hahn

Dr. Robert Dittmer 9

At the suggestion of the Department of Materials and Geosciences, the TU Darmstadt has solemnly conferred Prof. Dr. Jean Etourneau, Professor Emeritus at the University of Bordeaux 1, the honorary doctorate. Thereby the TU Darmstadt recognizes his pioneering contributions to the field of materials chemistry and materials science as well as his great commitment to advance scientific communication and cooperation in Europe.

TU-President Hans Jürgen Prömel (right) with TU-Honorary Doctor Prof. Jean Etourneau. Photo: Felipe Fernandes

As usual, the annual awarding of the "MaWi Prize" formed part of the MaWi summer party. The 1st prize was awarded to Ruben Heid from the division PhM for his Diploma thesis on “Einfluss von gießtechnischen Prozessschwankungen auf das Eigenschaftsspektrum crashrelevanter Aluminium-Druckgusslegierungen;” 2nd prizes were awarded to Tim Niewelt from the division OF for his Diploma thesis about “Analyse von Defekten in kristallinem Silizium” and to Joachim Langner from the division EE for his Diploma thesis about “Ionische Flüssigkeiten als Elektrolyt, Co-Katalysator und Stabilisator in Brennstoffzellen“. The 3rd prize was awarded to Christian Lohaus for his Bachelor thesis about “Synthese verschiedener rußgeträgerter Pt-Ru-Au Katalysatoren und Untersuchung des Degradationsverhaltens.”

Social Events and Awards As every year, our annual summer party was scheduled shortly before the summer break, being one of the most important social events of the Materials Science Institute. It has become a tradition to use this festivity to award the MaWi prize to the three best students having accomplished their Diploma or Master in the past winter semester.

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The 1st prize was awarded to Christoph Rakousky from the division EE for his diploma thesis “Neue Kohlenstoffkomponenten für Gasdiffusionsschichten. The award comes with prize money of € 500. 2nd prizes for Diploma with honours were awarded to Andreas Liess from the division EM and Maybritt Kühn from the division OF. Laura Ahmels from the division PhM and Hans Justus Köbler from the division ST were awarded for best Bachelor degree. In December 2013 we celebrated the year-end ceremony for all research groups, staff members and students, including the formal graduate celebration, where Bachelor, Master and PhD students received their certificates. The celebration including the social programme was organized by the Deanery´s team, in particular by PD Dr. Boris Kastening, Heinz Mohren, Dr. Silvia Faßbender and our workshop team. For the first time this ceremonial act took place in the new lecture hall and media centre, the “Hörsaal- und Medienzentrum” on the Lichtwiese campus. Students passing their Bachelor, Master and Diploma with honours in the past summer semester were awarded with the MaWi prize which will be given away twice a year from now on. Namely, Andreas Taubel from the division PoS was awarded for his Bachelor, Cornelia Hintze from the division DF for her FAME-Master, Heide Humburg from the division NAW and Michel Kettner from the division OF for their Master, and Jens Wehner from the division MM and Verena Pfeifer from the division OF for their Diploma. In the past summer semester four graduate students passed their PhD with honours. Robert Dittmer from the division NAW, Erwin Hildebrandt from the division DS, Falk Münch from the division MA, and Mahdi Seifollahi Bazarjani from the division DF received their PhD certificate with honours during the ceremonial act.

Dean with Prize winners in the December awarding ceremony: Dr. E. Hildebrandt, J. Wehner, V. Pfeifer, A. Taubel, Prof. R. Riedel, C. Hintze, H. Humburg, M. Kettner, Dr. R. Dittmer, Dr. M. Seifollahi Bazarjani.

On the following pages, this annual report shall provide you with some information on the most prominent research activities of the individual groups conducted in 2013. Prof. Ralf Riedel Dean of the Department

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Physical Metallurgy The research group Physical Metallurgy at the TU Darmstadt in the department of materials science works on the structure-property relationship of structural metallic materials and thin hard coatings, focusing on the mechanical properties on both macroscopic as well as microscopic length scales. The group is headed by Prof. Dr. Karsten Durst, who joined in Mai 2013 TU Darmstadt from an affiliation at FAU Erlangen-Nürnberg. The group utilizes and develops new testing methods for enhancing our understanding of the deformation mechanism of structural materials on all length scales. Of main interest are the mechanical properties of materials under various loading conditions (uniaxial, fatigue, wear or creep), specifically relating the macroscopic material response to the micromechanical properties at small length scales. New insights in the materials response are achieved by in-situ mechanical testing approaches, where the material is being mechanical loaded and the deformation is monitored by microscopic or spectroscopic means. Coupling the information of the materials microstructure with the processing condition and the mechanical properties, the group supports the development or enhancement of new structural materials and coatings. Currently the research deals with steels, Al-alloys, Cu and Ni-based alloys as well as a-C:H coatings and nickel-base superalloys. One important class of materials are so called ultrafine-grained or nanocrystalline materials, which are being processed by severe plastic deformation processes. The materials microstructure is strongly refined by these processes, leading to both strong and ductile materials. During processing, residual stresses can arise in the microstructure. The research currently focuses on post treatment conditions for adjusting the mechanical properties together with the residual stress for different applications. Residual stresses are also important for the application of hard coatings on ductile substrate. Together with partners, new processing conditions are being developed, which allow for a design of the coating with respect to residual stress and mechanical properties also under contact loading conditions. The determination of residual stress for both ultrafine-grained metals and amorphous carbon coatings using local methods is also shown as this years research highlight. Staff Members Head

Prof. Dr. K. Durst

Research Associates

Dr. E. Bruder

Prof. Dr. C. Müller

Technical Personnel

Ulrike Kunz Claudia Wasmund

Petra Neuhäusel Sven Frank

Secretaries

Christine Hempel

PhD Students

Dipl.-Ing. Vanessa Kaune Dipl.-Ing. Thorsten Gröb Dipl.-Ing. Jan Scheil MSc. Farhan Javaid

Dipl.-Ing. Jennifer Bödeker Dipl.-Ing. Jörn Niehuesbernd Dipl.-Ing. Christoph Schmid

Diploma Students

Frederik Brohmann Thorsten Gröb Anke Scherf

Aletta Böcker Moritz Elsaß

Master Students

Aniruddh Das

Jitendra Singh Rathore Adb Alaziz

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Bachelor Students

Paul Braun Tobias Schmiedl Kim Bergner Oskar Kowalik

Theresa Schütz Silke Innertsberger Romana Schwing

Research Projects “Effect of Load Frequency on the Fatigue Life of Aluminum Wrought Alloys in the VHCFRegime”, joint project with SzM-Darmstadt within the DFG Priority Programm 1466, DFG, since 04/2010 “Microstructure and Mechanical Properties of Bifurcated Sheet Profiles”, in SFB 666 of the DFG “Integral Sheet Metal Design with Higher Order Bifurcations”, since 06/2005. “Technologies of Surface Modification of Bifurcated Profiles”, in SFB 666 of the DFG “Integral Sheet Metal Design with Higher Order Bifurcations”, since 06/2009. “Subsequent Formability of Bifurcated Profiles” in SFB 666 of the DFG “Integral Sheet Metal Design with Higher Order Bifurcations”, since 06/2013. “New Synthesis Methods top-down” in Priority Program LOEWE “Response Ressourcenschonende Permanentmagnete durch optimierte Nutzung seltener Erden“, since 01/2014 “Damage Mechanims in Carbon Layer Systems”, DFG since 12/2011 “Influence of Glass Topology and Medium Range Order on the Deformation Mechanism in Borosilcate Glasses – a Multiple Length Scale Approach”, DFG since 07/2012 DAAD scholarship Farhan Javaid since 07/2012 Publications [1] Depner-Miller, U., Ellermeier, J., Scheerer, H., Oechsner, M., Bobzin, K., Bagcivan, N., Brögelmann, T., Weiss, R., Durst, K., Schmid, C.: Influence of application technology on the erosion resistance of DLC coatings, Surface and Coatings Technology 237 (2013) pp. 284 [2] Ahmed, F., Krottenthaler, M., Schmid, C., Durst, K.: Assessment of stress relaxation experiments on diamond coatings analyzed by digital image correlation and micro-Raman spectroscopy, Surface and Coatings Technology 237 (2013) pp. 255 [3] Wheeler, J.M., Maier, V., Durst, K., Göken, M., Michler, J.: Activation parameters for deformation of ultrafine-grained aluminium as determined by indentation strain rate jumps at elevated temperature, Materials Science and Engineering A 585 (2013) pp. 108 [4] Ast, J., Durst, K: Nanoforming behaviour and microstructural evolution during nanoimprinting of ultrafine-grained and nanocrystalline metals, Materials Science and Engineering A 568 (2013) pp. 68 [5] Maier, V., Merle, B., Göken, M., Durst, K.: An improved long-term nanoindentation creep testing approach for studying the local deformation processes in nanocrystalline metals at room and elevated temperatures, Journal of Materials Research 28 (9) (2013) pp. 1177

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[6] Krottenthaler, M., Schmid, C., Schaufler, J., Durst, K., Göken, M.: A simple method for residual stress measurements in thin films by means of focused ion beam milling and digital image correlation, Surface and Coatings Technology 215 (2013) pp. 247 [7] Schmid, C., Maier, V., Schaufler, J., Butz, B., Spiecker, E., Meier, S., Göken, M., Durst, K.: Highly resolved analysis of the chemistry and mechanical properties of an a-C:H coating system by nanoindentation and auger electron spectroscopy, Thin Solid Films 528 (2013) pp. 263 [8] Hay, J., Maier, V., Durst, K., Göken, M Strain-rate sensitivity (SRS) of nickel by instrumented indentation, Conference Proceedings of the Society for Experimental Mechanics Series 6 (2013) pp. 47 [9] Kriegsmann, A., Müller, C.: Richtungsabhängigkeit der Rissausbreitung bei einem gefügebedingten Übergang von rauigkeits- zu plastizitätsinduzierter Rissschließung an der Legierung Ti-6Al-4V, Mat.-wiss. u.Werkstofftech. 2013, 44, No. 9, 749-752 [10] Schäfer, S.; Abedini, S.; Groche, P.; Bäcker, F.; Ludwig, C.; Abele, E.; Jalizi, B.; Müller, C.; Kaune, V.; Verbindungstechniken durch die Technologie des SFB 666, In: Bauingenieur, Springer VDI-Verlag, Düsseldorf, Vol. 1 (2013), 8-13 [11] Ludwig, C.; Hammen, V.; Groche, P.; Kaune, V.; Müller, C.; Fertigung qualitätsoptimierter Spaltprofile durch Variation schnell änderbarer Prozessgrößen und deren Einfluss auf die Materialeigenschaften, Materialwissenschaft und Werkstofftechnik, Vol. 44 (2013), 601-611 [12] Karin, I.; Niehuesbernd, J.; Bruder, E.; Lipp, K.; Hanselka, H.; Müller, C.: FiniteElement analysis of a rolling contact model with anisotropic elastic material properties, Materialwissenschaft und Werkstofftechnik, Vol. 44, 2013, 298-303 [13] Niehuesbernd, J.; Müller, C.; Pantleon, W.; Bruder, E.: Quantification of local and global elastic anisotropy in ultrafine grained gradient microstructures, produced by linear flow splitting, Materials Science and Engineering A, Vol. 560, 2013, 273-277 [14] Steitz, M.; Scheil, J.; Müller, C.; Groche, P.: Effect of Process Parameters on Surface Roughness in Hammer Peening an Deep Rolling, Key Engineering Materials, 554-557 (2013), 1887-1901 [15] Scheil, J.; Müller, C.; Steitz, M.; Groche, P.: Influence of Process Parameters on Surface Hardening in Hammer Peening and Deep Rolling. Key Engineering Materials, 554557 (2013), 1819-1827 [16] Groche, P.; Steitz, M.; Engels, M.; Scheil, J.; Müller, C.; Bräuer, G.; Weigel, K.: Effizienzsteigerung im Werkzeugund Formenbau durch maschinelle Oberflächeneinglättung, EFB: Europäische Forschungsgesellschaft für Blechverarbeitung e.V., (2013) 1-119. ISBN-13: 978-3867763981 [17] Steitz, M.; Weigel, K.; Weber, M.; Scheil, J.; Müller, C.: Coating of deep rolled and hammer peened deep drawing tools, Advanced Materials Research, 769 (2013), 245-252

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Advanced Methods for the Determination of Residual Stresses in complex Material Systems J. Niehuesbernd, C. Schmid, E. Bruder, C. Müller and K. Durst

Introduction Residual stresses are present in many engineering components such as complex shaped metallic profiles but also in thin protective coatings. These intrinsic stresses can originate from diverse processing steps during manufacturing of the component like e.g. high plastic deformation during forming processes of bulk materials or thermal mismatch between substrate and coating during deposition at elevated temperatures. Since residual stresses can strongly influence the lifetime and the overall performance of the final component in operation, the knowledge of their magnitude as well as proper measurement methods are crucial for reliable product design. Nowadays, residual stress measurement techniques such as the hole drilling method for bulk materials and thicker coatings or curvature measurement methods for thin amorphous films are frequently used and well established. However, these measurement techniques still suffer from certain disadvantages and limitations. The present article, which summarizes the main results of the publications by Niehuesbernd et al. and Schmid et al. [1,2], describes the measurement of residual stresses of two different material systems, a bulk material as well as a thin amorphous coating, using different advanced characterization methods. On the one hand, the hole drilling method in combination with Electron Back Scatter Diffraction (EBSD) measurements and Finite Element Modeling (FEM) is employed to assess the stress gradient within a graded and elastic anisotropic steel profile. The results are compared to the isotropic case to analyse the influence of texture and elastic anisotropy on the determination of the residual stress value. On the other hand, a method based on combined Focused Ion Beam milling (FIB) and Digital Image Correlation (DIC) is applied to determine the residual stress state of tungsten modified amorphous carbon coatings of tailored mechanical properties deposited on steel substrates. Moreover, the obtained properties of the coatings are correlated to the applied process parameters during deposition. Influence of gradients in the elastic anisotropy on the reliability of residual stresses determined by the hole drilling method Modern forming processes often introduce large strains and in most cases significant strain gradients in the material, which generally cause residual stresses. Since plastic deformation also leads to the development of crystallographic textures, the influence of these textures and texture gradients on Fig. 1: a) Function principle of linear flow splitting, determined residual stress distributions b) profile and defined coordinate system needs to be considered. Linear flow splitting (LFS) is a process which involves complex forming conditions with steep strain gradients and spacially varying deformation modes. By subjecting the edges of a sheet to

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Fig. 2: Orientation distribution functions of measurements in 50 µm, 200 µm and 1000 µm beneath the split surface. Only sections of the Euler space with a constant angle ϕ2 = 45° are shown.

severe plasticdeformation flanges are produced, leading to profiles with a double-Y shape (Fig. 1). The heterogeneous material flow in combination with the severe strains leads to steep microstructure [3] and yield strength gradients (Fig. 3), as well as strong crystallographic textures and texture gradients in flange thickness direction [4,5]. In the present investigation the ferritic stainless steel X6Cr16 (AISI 430/ 1.4016) was examined. The initial sheet thickness was 2 mm and the flange length and thickness after LFS was 10 mm and 1 mm respectively. Orientation distribution funcions (ODF) obtained by EBSD on cross sections of the flanges revealed typical rolling textures with partial α-fibers (red/vertical) and γ-fibers (blue/horizontal) in near-surface layers (Fig. 2). With increasing distance to the split surface the texture intensity decreases significantly and the rolling texture vanishes while shear components like the Goss orientation ({110}) appear. The orientation data obtained by EBSD was also used to calculate direction dependent elastic properties. By rotating the single Fig. 3: Approximated yield strength and Young’s crystal stiffness tensor of iron (C11 = 230.1 modulus in feed direction (TD) in dependence of the distance to the split surface. GPa, C12 = 134.6 GPa, C44 = 116.6 GPa, [6]) according to the measured grain orientations and averaging over all measurement points, the direction dependent Young’s Modulus can be determined. In the present investigation the geometric mean was utilized for averaging, which has proven to be a suitable approximation [5,7,8]. The Young´s modulus in feed direction (TD) steeply drops from 244 GPa in near-surface layers to approximately 212 GPa at the bottom surface (Fig. 3). Residual stress measurements were performed by the hole drilling method at the Fig. 4: Residual stress levels in feed direction (TD) flange top surface in combination with FEMfor the isotropic and the orthotropic case obtained by FE-modelling Simulations of the drilling process.

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The geometry of the flange and the hole was modeled using the commercial FE solver Abaqus. The flange was partitioned into 50 µm thick layers, which were assigned specific yield strengths and stiffness tensors, corresponding to the experimental data. The residual stress distribution was introduced by assigning each layer initial stress values in RD and TD. In order to obtain depth dependent strain data, 50 µm thick slices were removed in a series of steps and the resulting surface strains were acquired. These were compared to the measured ones and the initially assigned stresses were varied iteratively until the differences amounted to less than 0.5%. The same procedure was carried out for the elastic isotropic case to assess the impact of anisotropy on the determined residual stress values. The results of the FE-simulations of the drilling process reveal very high residual stress levels in the feed direction (TD) of the split profiles (Fig. 4). Nearly 800 MPa of tensile stress in a depth of 0.2 mm can be observed for the isotropic as well as for the anisotropic (orthotropic) case. Up to this depth the relaxations are dominated by plastic deformation in the vicinity of the hole, owing to the high residual stress levels. Therefore, the determined residual stresses in both case show the same behavior. In a depth between 0.3 mm and 0.4 mm the stress levels for the orthotropic model are about 20 % higher than the ones for the isotropic model. In this depth, the difference between the Young’s moduli of the two models is only 3 %. It is reasonable to assume, that the stiffer upper layers in the orthotropic model diminish the surface relaxation caused by the removal of a layer with a lower stiffness in the examined direction. This means that for the isotropic model the residual stresses in lower layers are highly underestimated. It is therefore concluded that for the hole drilling method anisotropic elastic properties do not only influence the determination of residual stresses in layers where anisotropy is present, but also in subjacent layers which might have isotropic properties. Residual stress measurement of thin amorphous coatings by means of FIB and DIC Residual stresses of thin amorphous coatings are commonly assessed by means of curvature measurement methods. These methods are based on the measurable change in curvature of the substrate due to deposition of a coating with intrinsic stresses. The residual stress state of the coating is then evaluated from the difference in curvature by use of Stoney’s equation [9]. However, in this approach the used substrate has to meet certain requirements i.e. it has to be a thin, elastically isotropic plate that is free to bend [10]. These substrates like e.g. thin flat silicon wafers are mostly not relevant for technical applications of hard coatings. A method capable to measure residual stresses of thin coatings, regardless of whether they are amorphous or crystalline, on substrates of technical relevance was proposed by Kang et al. [11]. This method is based on the relaxation of residual stresses by focused ion beam (FIB) milling and tracking of the resultant displacements by means of digital image correlation (DIC). The residual stress state in the coating is quantified by evaluation of the observed displacement fields by either analytical solutions or FE analysis using the elastic properties of the coating e.g. determined by Fig. 5: FIB cross-sections of the coating nanoindentation. In the literature different relaxation system CS2 revealing the basic structure geometries, like a single slot [11], annular trenches or [2]. Institute of Materials Science - Physical Metallurgy

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pillars [12,13] are proposed and successfully applied to determine residual stresses of different kind of coating/substrate systems. Here, a similar method was applied to assess residual stresses of three different tungsten modified hydrogenated amorphous carbon (a-C:H:W) coatings CS1-CS3 with predefined hardness values, ranging from 10 up to 16 GPa. The a-C:H:W coatings with a thickness of 1.6 µm were deposited on polished disks of cold work tool steel 1.2379 by reactive unbalanced magnetron sputtering of a binder-free WC target in argon-ethine atmosphere using an industrial coating equipment. Sufficient adhesion of the coating was achieved by depositing an adhesive layer, consisting of different Cr- and WC-based layers. Fig. 5 exemplarily shows a FIB cross-sections of the coating revealing the basic structure consisting of adhesive layer and a-C:H:W functional layer which exhibits a weak columnar microstructure. In order to obtain three coatings of predefined hardness, negative bias voltage Ubias was adapted during deposition of the a-C:H:W layer according to a previously created regression model. This model was obtained in previous work by variation of four main process parameters of the used deposition process according to a central composite design and measuring their influence on the mechanical properties of the a-C:H:W coating by nanoindentation similar to the approach used in [14]. Fig. 6 shows the relation between the hardness of the coating and the process parameters ethane flow rate ((C2H2)) and Ubias and the good accordance between the measured hardness of the three different coatings with the predicted values by the regression model. Fig. 6: a) Regression model describing the relation between the For the assessment of the hardness of the coating and the process parameters (C2H2) and residual stress state of the Ubias with given positions of the selected coating systems CS1-CS3. b) of the coatings measured by nanoindentation in coatings, a double slit geometry Hardness comparison with predicted hardness values given by the regression as described in [15] was model [2]. employed for relaxation of internal stresses. The used relaxation geometry leads to a linear and symmetric displacement gradient across the remaining bar, facilitating the evaluation of the displacements and the quantification of the corresponding residual stress state. Therefore, two high resolution SEM images of the area of interest, one before and one after the Fig. 7: Experimental approach used for the determination of residual stresses exemplarily demonstrated on a hydrogenated amorphous carbon coating with residual compressive stresses of ca. -3 GPa. Clearly a symmetric and linear displacement gradient across the remaining bar can be observed.

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FIB milling procedure are taken. The milling of the double-slit geometry was conducted by an automated procedure, which has been optimized to reduce FIB damage and to attain high milling accuracy. Fig. 7 summarizes the experimental approach, exemplarily demon- Fig. 8: Representative displacement gradients of coating CS1-CS3 obtained by strated on a hydro- DIC and corresponding displacement vs. position plots of all five measurements per coating system for the evaluation of relief strain (slope of genated amorphous linear regression) [2]. carbon coating with residual compressive stresses of ca Table 1: Comparison of mechanical properties, relief strain and residual ca. -3 GPa. To enable stresses of the coatings [2]. simple calculation of coating H in GPa E in GPa εrel. in % σres. in GPa residual stresses by use CS1 9.4 ± 1.4 117 ± 12 0.34 ± 0.06 0.40 ± 0.07 of Hooke’s law, the CS2 12.2 ± 1.0 145 ± 13 0.54 ± 0.07 0.79 ± 0.11 applied milling geometry, depth d and CS3 14.8 ± 2.2 172 ± 20 0.92 ± 0.10 1.57 ± 0.18 distance w between the two slits, was previously optimized with regard to the coating thickness t. Further information about proper relaxation geometry can be found elsewhere [16]. A total of 5 measurements per coating were conducted for residual stress evaluation. Fig. 8 exemplarily shows one FIB milled double-slit geometry of each coating superimposed with the resulting displacement gradient obtained by DIC. A symmetric gradient across the remaining bar with maximum displacements at the edges is found. Since the bars expand, coatings are subjected to compressive residual stresses. Comparing all three gradients, it becomes evident that the resultant displacements increase from CS1 to CS3, i.e. with increasing Ubias. Additional to the gradients, the corresponding displacement vs. position plots of all five measurements per coating system are shown. The slopes of linear regression of the displacement vs. position plots (du/dx) give the respective relief strain εrel.. As already indicated by the displacements, the relief strain also increases considerably with increasing Ubias. For the coatings CS1-CS3, relief strain increases from 0.34 % to 0.92 % with corresponding residual stress values between -0.40 GPa to -1.57 GPa. Table 1 summarizes the determined properties of the coatings.

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Conclusions In the present article, two different approaches for the determination of residual stresses of complex material systems both based on material removal are presented. The methods were applied to a graded and elastically anisotropic steel profile and a thin amorphous coating. For the assessment of the stress gradient within the steel profile, the hole drilling method in combination with EBSD and FEM was employed and the influence of elastic anisotropy on the determined residual stress values was shown. Further, the residual stress state of three a-C:H:W coatings with tailored mechanical properties deposited on steel substrates were assessed by means of focused ion beam milling of a double-slit geometry, which causes the internal stresses to relax, and tracking of the resultant relief strain by digital image correlation. Here a direct correlation between the coating properties and the applied process parameters was obtained. The described methods are suitable for determination of residual stresses of both amorphous and elastically anisotropic metallic materials, giving important insight for further optimization of the materials. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

J. Niehuesbernd, E. Bruder, C. Müller, submitted to Adv. Mater. Res. (2013) C. Schmid, H. Hetzner, S. Tremmel, F. Hilpert, K. Durst, submitted to Adv. Mater. Res. (2013) T. Bohn, E. Bruder, C. Müller, J. Mater. Sci. 43 (2008), 7307-7312 E. Bruder, J. Mater. Sci. 47 (2012), 7751-7758 J. Niehuesbernd, C. Müller, W. Pantleon, E. Bruder, Mat. Sci. Eng. A 560 (2013), 273-277 J. J. Adams, D. S. Agosta, R. G. Leisure, H. Ledbetter, J. of Appl. Phys. 100 (2006), 113530 1-7 S. Matthies, M. Humbert, Phys. Status Solidi B 177 (1993), K47-K50 S. Matthies, M. Humbert, J. Appl. Crystallogr. 28 (1995), 254-266 G.G. Stoney, Proc. R. Soc. Lond. A 82 (1909), 172-175 R.P. Vinci, J.J. Vlassak, Annu. Rev. Mater. Sci. 26 (1996), 431-62 K. J. Kang, N. Yao, M. Y. He, A.G. Evans, Thin Solid Films 443 (2003), 71-77 A.M. Korsunsky, M. Sebastiani, E. Bemporad, Surf. Coat. Technol. 205 (2010), 2393-2403. M. Sebastiani, C. Eberl, E. Bemporad, G. M. Pharr, Mat. Sci. Eng. A 528 (2011), 7901– 7908 H. Hetzner, R. Zhao, S. Tremmel, S. Wartzack, in: K. D. Bouzakis, K. Bobzin, B. Denkena, M. Merklein (Eds.), Proceedings of the 10th International Conference THE ''A'' Coatings 2013, Shaker, Aachen, 2013, 39-49 [15] M. Krottenthaler, C. Schmid, J. Schaufler, K. Durst, M. Göken, Surf. Coat. Technol. 215 (2013), 247252 [14] F. Ahmed, M. Krottenthaler, C. Schmid, K. Durst, Surf. Coat. Technol. 237 (2013), 255–260

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Ceramics Group The emphasis in the ceramics group is on the correlation between microstructure and mechanical as well as functional properties. A number of processing methods are available in order to accomplish different microstructure classes, to determine their specific properties in an experiment and to rationalize these with straightforward modelling efforts. Thus, a materials optimization is afforded which allows effective interplay between processing, testing and modelling. In particular, new lead free piezoceramics and lead-free high-temperature dielectrics can be obtained and extensively characterized electrically and mechanically. The scientific effort can be grouped as follows: I.

Conductivity of Oxides Dr. Till Frömling

Modulation of conductivity of oxide ceramics is usually achieved by doping and temperature treatment in a large oxygen partial pressure range. However, electric and ionic conductivity can also be changed by mechanical modifications. In this research group conductivity is of oxide ceramics is modified by the following approaches a) Induction of dislocations: Dislocations are mechanically introduced into strontium titanate which can be plastically deformed even at room temperature. Changes of the electric and ionic conductivity are, amongst other methods, investigated by complex impedance spectroscopy and dcmeasurements. The aim of this project is to identify the defect chemical properties of dislocation cores in strontium titanate and related materials. b) Altering potential barriers in piezoelectric semiconductor materials: In this project Schottky-barriers and varistor material based on ZnO are investigated as a function of applied pressure. II. Development of new piezoceramics Dr. Wook Jo In response to the recent demands for environmental friendly piezoelectric materials for electrical and electronic applications, the principal focus of this group is the development of non-toxic piezoceramics with electromechanical performance comparable to their leadcontaining counterparts. Among all the potentially promising candidates special attention has been given to bismuth-based materials whose properties can be effectively tailored using the so-called morphotropic phase boundary (MPB) concept. Extensive compositional research has been performed on various bismuth-based solid solution systems that contain a MPB between separating different crystal symmetries of the members. To better understand the mechanisms governing the enhancement of electromechanical properties of materials and to make our search for alternative materials more effective fundamental scientific research on model systems have been performed in parallel to the compositional investigations. We employ various characterization techniques such as macroscopic dielectric, ferroelectric and ferroelastic property measurements as well as crystallographic structural analyses based on synchrotron and neutron diffractions, Raman, nuclear magnetic resonance, electron paramagnetic resonance spectroscopic techniques, and Institute of Materials Science - Ceramics Group

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transmission electron microscopy. We are also simultaneously establishing thermodynamic and phenomenological models which are verified by the first principles calculations. Currently, we have extensive and active international collaborations with eminent ferroelectric groups throughout the world. In the last year, we also added work on KNNbased piezoceramics (collaborations with Prof. Ke Wang (Tsinghua University, China) and Dr. Ruiping Wang (AIST, Japan) and work on BT-based piezoceramics to our research scheme. III. Mechanical properties of ferroelectrics Dr. Kyle Webber The focus of this research group is understanding the mechanical properties of ferroelectric materials, particularly the influence of stress on the phase transformation behavior and ferroelasticity at high temperature. Research over the last year has centered around development of a high temperature fracture testing setup for characterizing crack growth resistance behavior of ferroelastic materials as well as utilizing the newly developed experimental arrangement for characterizing small signal dielectric, piezoelectric, and elastic properties under large mechanical, electrical and thermal fields as a function of frequency. Preliminary results have already given insight into the impact of stress on the depolarization temperature of ferroelectric Pb(Zr,Ti)O3, which is commonly used in actuation and sensing applications. In addition, the Emmy Noether research group, lead by Kyle Webber, began in June and has been working on relaxor/ferroelectric composites and mixed conducting cathode materials for solid oxide fuel cells. Both of these projects are focused on understanding the influence of stress on the functional properties. Currently, equipment is being developed to allow for the mechanical characterization of samples in a atmosphere with an adjustable oxygen particle pressure.

Staff Members Head

Prof. Dr. Jürgen Rödel

Research Associates

Dr. Till Frömling Dr. Wook Jo Dr. Jurij Koruza Dipl. Phys. Irene Mieskes

Dr. Nikola Novak Dr. Eric Patterson Dr. Kyle Webber Dr. Ludwig Weiler

Technical Personnel

Dipl.-Ing. Gundel Fliß Dipl.-Ing. Daniel Isaia

Michael Heyse

Secretaries

Roswita Geier

Gila Völzke

PhD Students

M. Sc. Matias Acosta M. Sc. Azatuhi Ayrikyan Dipl.-Ing. Raschid Baraki Dipl.-Ing. Martin Blömker Dipl.-Ing. Laetitia Carrara Dipl.-Ing. Robert Dittmer Dipl.-Phys. Daniel Franzbach M. Sc. Philipp Geiger

Dipl.-Ing. Claudia Groh Dipl.-Ing. Christine Jamin Dipl.-Ing. Markus Jung Dipl.-Ing. Eva Sapper Dipl.-Ing. Florian Schader Dipl.-Phys. Deborah Schneider Dipl.-Ing. Yohan Seo M. Sc. Jiadong Zang

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Diploma/ Bachelor/Master Students

David Brandt Johannes Dingeldein Shenshen He Heide Humburg

Manuel Kloos Malte Vögler

Research Fellow

Dr. Ke Wang (AvH)

Dr. Haibo Zhang (AvH)

Guest Scientists

Prof. Dr. Satoshi Wada Prof. Dr. Derek Sinclair Prof. Dr. Mario Maglione Dr. Philipp Veber Dr. Ruiping Wang Dr. Akira Ando Dr. Soon-Jong Jeong Prof. Dr. Chae Ill Cheon

Research Projects 

Processing of textured ceramic actuators with high strain (SFB 595, 2003–2014)

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Mesoscopic and macroscopic fatigue in doped ferroelectric ceramics (SFB 595, 2003–2014)

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Development of new lead –free piezoceramics (ADRIA, state funding, 2008-2014)

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Development of new high-temperature piezoceramics (ADRIA, state funding, 2008-2013)

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Stress and strain fields in ferroelectrics (Graduate school “computational engineering” 20092017)

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High-temperature dielectrics (DFG 2010-2013)

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Mechanical compliance at phase transition points in lead-free ferroelectrics (DFG 20112014)

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Lead-free piezoelectric single crystals with high strain: orientation dependence, polarization rotation and morphotropic phase boundaries (DFG 2011-2014)

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Energy absorption of ZnO varistors (DFG 2011-2014)

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Ag-based electrical switches (state of Hesse / Umicore)

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Emmy Noether Program: The Influence of Mechanical Loads on the Functional Properties of Perovskite Oxides (DFG 2013-2018)

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Publications [1] Jamin, Christine ; Rasp, Tobias ; Kraft, Torsten ; Guillon, Olivier : Constrained sintering of alumina stripe patterns on rigid substrates: Effect of stripe geometry. [Online-Edition: http://dx.doi.org/10.1016/j.jeurceramsoc.2013.06.016] In: Journal of the European Ceramic Society, 33 (15-16) pp. 3221-3230. ISSN 09552219 [Artikel], (2013) [2] Yao, Fang-Zhou ; Glaum, Julia ; Wang, Ke ; Jo, Wook ; Rödel, Jürgen ; Li, Jing-Feng : Fatigue-free unipolar strain behavior in CaZrO3 and MnO2 co-modified (K,Na)NbO3-based lead-free piezoceramics. [Online-Edition: http://dx.doi.org/10.1063/1.4829150] In: Applied Physics Letters, 103 (19) 192907(1-4). ISSN 00036951, [Artikel], (2013) [3] Zhukov, Sergey ; Genenko, Yuri A. ; Acosta, Matias ; Humburg, Heide ; Jo, Wook ; Rödel, Jürgen ; von Seggern, Heinz : Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3x(Ba0.7Ca0.3)TiO3 ferroelectrics. [Online-Edition: http://dx.doi.org/10.1063/1.4824730] In: Applied Physics Letters, 103 (15) 152904(1-5). ISSN 00036951, [Artikel], (2013) [4] Kling, Jens ; Jo, Wook ; Dittmer, Robert ; Schaab, Silke ; Kleebe, Hans-Joachim ; Zhang, S. : Temperature-Dependent Phase Transitions in the Lead-Free Piezoceramics (1 - x y)(Bi1/2Na1/2)TiO3-xBaTiO3-y(K0.5Na0.5)NbO3Observed byin situTransmission Electron Microscopy and Dielectric Measurements. [Online-Edition: http://dx.doi.org/10.1111/jace.12493] In: Journal of the American Ceramic Society, 96 (10) pp. 3312-3324. ISSN 00027820 [Artikel], (2013) [5] Seo, Yo-Han ; Vögler, Malte ; Isaia, Daniel ; Aulbach, Emil ; Rödel, Jürgen ; Webber, Kyle G. : Temperature-dependent R-curve behavior of Pb(Zr1−xTix)O3. [Online-Edition: http://dx.doi.org/10.1016/j.actamat.2013.07.020] In: Acta Materialia, 61 (17) pp. 6418-6427. ISSN 13596454, [Artikel], (2013) [6] Amaral, Luís ; Jamin, Christine ; Senos, Ana M. R. ; Vilarinho, Paula M. ; Guillon, Olivier : Constrained sintering of BaLa4Ti4O15 thick films: Pore and grain anisotropy. [Online-Edition: http://dx.doi.org/10.1016/j.jeurceramsoc.2013.01.031] In: Journal of the European Ceramic Society, 33 (10) pp. 1801-1808. ISSN 09552219 [Artikel], (2013) [7] Cumming, D. J. ; Sebastian, Tutu ; Sterianou, Iasmi ; Rödel, Jürgen ; Reaney, Ian M.: Bi(Me)O3-PbTiO3 high TC piezoelectric multilayers. [Online-Edition: http://dx.doi.org/10.1179/1753555713Y.0000000067] In: Materials Technology: Advanced Performance Materials, 28 (5) pp. 247-253. ISSN 10667857, [Artikel], (2013)

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[8] Glaum, Julia ; Simons, Hugh ; Acosta, Matias ; Hoffman, Mark ; Feteira, A. : Tailoring the Piezoelectric and Relaxor Properties of (Bi1/2Na1/2)TiO3-BaTiO3via Zirconium Doping. [Online-Edition: http://dx.doi.org/10.1111/jace.12405] In: Journal of the American Ceramic Society n/a-n/a. ISSN 00027820, [Artikel], (2013) [9] Schader, Florian H. ; Aulbach, Emil ; Webber, Kyle G. ; Rossetti, George A. : Influence of uniaxial stress on the ferroelectric-to-paraelectric phase change in barium titanate. [Online-Edition: http://dx.doi.org/10.1063/1.4799581] In: Journal of Applied Physics, 113 (17) 174103(1-9). ISSN 00218979, [Artikel], (2013) [10] Tran, Vu Diem Ngoc ; Dinh, Thi Hinh ; Han, Hyoung-Su ; Jo, Wook ; Lee, Jae-Shin : Lead-free Bi1/2(Na0.82K0.18)1/2TiO3 relaxor ferroelectrics with temperature insensitive electrostrictive coefficient. [Online-Edition: http://dx.doi.org/10.1016/j.ceramint.2012.10.046], In: Ceramics International, 39 (Supplement 1) S119-S124. ISSN 02728842, [Artikel], (2013) [11] Jo, Wook ; Daniels, John E. ; Damjanovic, Dragan ; Kleemann, Wolfgang ; Rödel, Jürgen : Two-stage processes of electrically induced-ferroelectric to relaxor transition in 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3. [Online-Edition: http://dx.doi.org/10.1063/1.4805360] In: Applied Physics Letters, 102 (19) 192903(1-4). ISSN 00036951, [Artikel], (2013) [12] Han, Hyoung-Su ; Jo, Wook ; Kang, Jin-Kyu ; Ahn, Chang-Won ; Won Kim, Ill ; Ahn, Kyoung-Kwan ; Lee, Jae-Shin : Incipient piezoelectrics and electrostriction behavior in Sn-doped Bi1/2(Na0.82K0.18)1/2TiO3 lead-free ceramics. [Online-Edition: http://dx.doi.org/10.1063/1.4801893] In: Journal of Applied Physics, 113 (15) 154102(1-6). ISSN 00218979, [Artikel], (2013) [13] Schwarz, Sebastian ; Guillon, Olivier : Two step sintering of cubic yttria stabilized zirconia using Field Assisted Sintering Technique/Spark Plasma Sintering. [Online-Edition: http://dx.doi.org/10.1016/j.jeurceramsoc.2012.10.002] In: Journal of the European Ceramic Society, 33 (4) pp. 637-641. ISSN 09552219 [Artikel], (2013) [14] Seo, Yo-Han ; Franzbach, Daniel J. ; Koruza, Jurij ; Benčan, Andreja ; Malič, Barbara ; Kosec, Marija ; Jones, Jacob L. ; Webber, Kyle G. : Nonlinear stress-strain behavior and stress-induced phase transitions in soft Pb(Zr_{1−x}Ti_{x})O_{3} at the morphotropic phase boundary. [Online-Edition: http://dx.doi.org/10.1103/PhysRevB.87.094116] In: Physical Review B, 87 (9) 094116(1-11). ISSN 1098-0121, [Artikel], (2013)

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[15] Rödel, Jürgen ; Seo, Yo-Han ; Benčan, Andreja ; Malič, Barbara ; Kosec, Marija ; Webber, Kyle G. : R-curves in transformation toughened lead zirconate titanate. [Online-Edition: http://dx.doi.org/10.1016/j.engfracmech.2012.06.023] In: Engineering Fracture Mechanics, 100 pp. 86-91. ISSN 00137944, [Artikel], (2013) [16] Dittmer, Robert ; Webber, Kyle G. ; Aulbach, Emil ; Jo, Wook ; Tan, Xiaoli ; Rödel, Jürgen : Electric-field-induced polarization and strain in 0.94(Bi1/2Na1/2)TiO3–0.06BaTiO3 under uniaxial stress. [Online-Edition: http://dx.doi.org/10.1016/j.actamat.2012.11.012] In: Acta Materialia, 61 (4) pp. 1350-1358. ISSN 13596454, [Artikel], (2013) [17] Levin, I. ; Reaney, I. M. ; Anton, Eva-Maria ; Jo, Wook ; Rödel, Jürgen ; Pokorny, J. ; Schmitt, L. A. ; Kleebe, H-J. ; Hinterstein, Manuel ; Jones, J. L. : Local structure, pseudosymmetry, and phase transitions in Na_{1/2}Bi_{1/2}TiO_{3}– K_{1/2}Bi_{1/2}TiO_{3} ceramics. [Online-Edition: http://dx.doi.org/10.1103/PhysRevB.87.024113] In: Physical Review B, 87 (2) 024113(1-11). ISSN 1098-0121, [Artikel], (2013) [18] Zang, Jiadong ; Jo, Wook ; Rödel, Jürgen : Quenching-induced circumvention of integrated aging effect of relaxor lead lanthanum zirconate titanate and (Bi1/2Na1/2)TiO3-BaTiO3. [Online-Edition: http://dx.doi.org/10.1063/1.4788932] In: Applied Physics Letters, 102 (3) 032901. ISSN 00036951, [Artikel], (2013) [19] Dittmer, Robert ; Webber, Kyle G. ; Aulbach, Emil ; Jo, Wook ; Tan, Xiaoli ; Rödel, Jürgen : Optimal working regime of lead–zirconate–titanate for actuation applications. [Online-Edition: http://dx.doi.org/10.1016/j.sna.2012.09.015] In: Sensors and Actuators A: Physical, 189 pp. 187-194. ISSN 09244247, [Artikel], (2013) [20] Salje, Ekhard K. H. ; Carpenter, Michael A. ; Nataf, Guillaume F. ; Picht, Gunnar ; Webber, Kyle G. ; Weerasinghe, Jeevaka ; Lisenkov, S. ; Bellaiche, L. : Elastic excitations in BaTiO_{3} single crystals and ceramics: Mobile domain boundaries and polar nanoregions observed by resonant ultrasonic spectroscopy. [Online-Edition: http://dx.doi.org/10.1103/PhysRevB.87.014106] In: Physical Review B, 87 (1) 014106(1-10). ISSN 1098-0121, [Artikel], (2013) [21] Uhlmann, Ina ; Hawelka, Dominik ; Hildebrandt, Erwin ; Pradella, Jens ; Rödel, Jürgen: Structure and mechanical properties of silica doped zirconia thin films. [Online-Edition: http://dx.doi.org/10.1016/j.tsf.2012.08.007] In: Thin Solid Films, 527 pp. 200-204. ISSN 00406090, [Artikel], (2013)

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Rocking Curve X-Ray Diffraction for Quantifying Dislocations in SrTiO3 Single Crystals Eric Patterson, Till Frömling, Kyle Webber, and Jürgen Rödel

Introduction: Strontium titanate (SrTiO3 or STO) is frequently described as a model perovskite system and its electrical properties have been studied over a wide range of conditions, including oxygen partial pressure, temperature and electric field [1]. Single crystal STO shows a rare ability among oxide materials to plastically deform when compressively stressed. It also undergoes a ductile to brittle to ductile transition as a function of changing temperature that has previously been shown [2-5]. Because it remains cubic over a wide range of temperatures, the change in mechanical properties cannot be tied to phase transitions. Current investigations on this system are directed towards understanding the relationship between this plastic strain behavior, the dislocation network developed, and changes that arise in the electrical conductivity of these perovskite single crystals. In previous work high conductivity paths along dislocations were shown in STO single crystals via conductive atomic force microscopy [6]. In order to analyze these properties, it is therefore essential to have an accurate method to reliably quantify the plastic strain during deformation and correlate this to a change in the dislocation density in a crystal after a given amount of deformation. By investigating the relationship between dislocations and conductivity in STO, we may be able to ascertain the mechanism of changes in electromechanical properties in these materials. Experimental Procedure: In this work, strontium titanate single crystals oriented along the (001) growth direction with dimensions of 4x4x8 mm3 (Alineason Materials Technology, GmbH) and optical quality polished sides were examined. Compressive loading was done with a load frame (Zwick/Roell Z030) with a loading rate of 25 N/s in a displacement controlled manner using 32 µm limited steps, which were repeated 3 times in order to achieve approximately 1% plastic strain. The stress-induced uniaxial displacement of the specimen was measured by a linear variable differential transformer (LVDT). The deformations were peformed in a range of temperature from 25°C - 450°C. The samples were examined optically by polarized light microscopy to observed the birefringence around the dislocation slip planes. X-ray diffraction (XRD) rocking curve measurments were made before and after deformation of the samples to observe changes in dislocation density following the methodolgy of Ayers, et al [7]. The XRD rocking curve technique for dislocation density determination utilizes a positionlocked source with a Bartel’s monochromator attachment to focus the emerging x-ray beam to a width of a few arcseconds. The theta is adjusted by tilting the sample and the detector is moved as normal, albeit with an upper 2 limit of approximately 152°. Samples were mounted to a goniometer head and aligned flat with respect to the diffraction plane using a polarized light setup, such that the X-ray beam crosses lengthwise across the center of one of the sample faces. The sample () and detector (2) were next rotated to the selected diffraction planes, i.e. (001), (002), (003), and (004). Since the faces of the STO single crystals are oriented and not polycrystalline, a non-symmetric arrangement of -2 angles must be selected for all other peaks, i.e. (014), (024), (233), and (224). Additionally, in order to reach the higher equivalent angles of  necessary for subsequent Institute of Materials Science - Ceramics Group

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density calculations, a motorized stage to control  was introduced and rotated to scan the (233) and (224) planes at 33.69° and 45°, respectively. The sample itself is finally rotated over a small range of angles around the peak location for the given plane selected ( = -0.2° to 0.2°). The resulting peak is fitted via Gaussian or Pseudo-Voigt fit, depending on symmetry and number of peaks and the full width at half maximum (FWHM or ) is determined. Ideally only one peak should be present for a single crystal, however, after deformation multiple peaks can sometimes be observed due to increase and arrangement of the dislocation network. In Figure 1, a clear broadening of the (001) after deformation of approximately 0.25% plastic strain is shown. These measurements were made for eight diffraction planes on each sample with a step size of 0.001° and a dwell time of 10 seconds per step. In the cases where  angle rotation was needed, two equivalent scans were taken at 90± ° in order to account for differences in edges and potential cracks intercepted by the beam at these rotated angles. a)

b)

Figure 1: a) Schematic of Bartel’s monochromator and b) an example rocking curve result for a single reflection before and after compression testing

A linear regression is fitted to a plot of FWHM2 as a function tan2, from the slope and intercept of these lines two independent dislocation density values can be calculated. The slope is associated with the strain around the dislocation and the intercept is related to the angular rotation around the dislocation core. Results and Discussion: The polarized light microscopy revealed an increase in the density and uniformity of the slip lines, as seen in Figure 2 for the (100) and (010) polished faces of the crystals deformed at 25° and 300°C. The dark lines in the image are cracks that developed during loading and unloading. a)

b)

c)

25° C

(100 )

(010 )

300° C

(100 )

(010 )

Figure 2: a) Deformation of single crystal STO at various temperatures with resulting PLM at b) 25°C and c) 300°C

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From the stress-strain plots, it is clear that with increasing temperature, the yield point of the samples is decreased dramatically. It was lowered nearly by half from ~140MPa at 25°C to ~75MPa at 300°C. Next XRD rocking curve measurements were performed on the samples and dislocation density was calculated based on the results of linear regression, as shown for the examples in Figure 3 below. a)

b)

Figure 3: Measured FWHM2 as a function of tan2 same 450°C crystal and c) PLM images for the 450°C crystal.

c)

450° C

(100 (010 ) b) two faces ) of the -deformed, 25°C, and 300°C

A clear difference was found between non-deformed samples and those deformed to 1% plastic strain. There was both an increase in the intercept value and in the slope compared to the average of non-deformed samples. From further calculations, dislocation content was shown to increase by 4x from 2.1x108 cm-2 in the initial state to 8.2x108 cm-2 after 1% strain at 25°C. At 300°C, the dislocation content was increased even further to approximately 9.7x108 cm-2. At 450°C, however, a different behavior was found depending on the side of the crystal examined. In this case the (010) side was subsequently discovered to bulge in the center, whereas the (100) face remained flat. The (100) side gives a dislocation density value approximately the same as the 300°C case. This makes sense given the higher degree of cracking that is apparent in the 450°C sample (Figure 3 c)). The XRD technique will next be confirmed with further samples and with an etch pit density evaluation, both at the surface and in the interior of the crystal. Now that this method of increasing dislocations has been shown, it must be connected to changes in the conductivity through impedance spectroscopy and oxygen diffusion using 18O annealing and ToF SIMS analysis. This will be performed on these undoped samples and on Fe-doped single crystals. References 1 2 3 4 5 6 7

De Souza et al., Z. Metallkd. 94 [3], 218-225 (2003) Brunner et al., J. Am. Ceram. Soc., 84 [5], 1161–63 (2001) Brunner, Acta Materialia 54, 4999-5011 (2006) Brunner, Mat. Sci. Eng. A, 483-484 521-524 (2008) Yang, et al., J. Am. Ceram. Soc., 94 [9] 3104–3111 (2011) Szot, et al., Nature Materials 5, 312 - 320 (2006) Ayers, et al., J. Crystal Growth, 135, 71-77 (1994)

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Electronic Material Properties The Electronic Materials division introduces the aspect of electronic functional materials and their properties into the Institute of Materials Science. The associated research concentrates on the characterization of various classes of materials suited for implementation in information storage and organic and inorganic electronics. Four major research topics are presently addressed:    

Electronic and optoelectronic properties of organic semiconductors. Charge transport in inorganic semiconductor devices. Charge transport and polarization in organic and inorganic dielectrics. Photo- and photostimulated luminescence in inorganic phosphors.

For novel areas of application a worldwide interest exists in the use of organic semiconductors in electronic and optoelectronic components, such as transistors and lightemitting diodes. So far, multicolour and full colour organic displays have been implemented in commercially available cameras, car-radios, PDAs, mp3-players and even television sets. Organic devices reaching further into the future will be simple logic circuits, constituting the core of communication electronics such as chip cards for radio-frequency identification (RFID) tags and maybe one day flexible electronic newspapers where the information is continuously renewed via mobile networks. In view of the inevitable technological development, the activities of the group are concerned with the characterization of organic material properties regarding the performance of organic electronic and optoelectronic devices. The major aspect deals with the charge carrier injection and transport taking place in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs). In particular, the performance of unipolar and ambipolar light-emitting OFETs and the stability of OFETs and OLEDs are subjects of recent investigations. To conduct these demanding tasks, various experimental techniques for device fabrication and characterization are installed. Besides basic electric measurement setups, a laser spectroscopy setup used for time-of-flight as well as for life-time measurements and a Kelvin-probe atomic force microscope to visualize the potential distribution of organic devices with nanometer resolution are available. Even though organic electronics is an emerging field especially for consumer electronics applications today's electronic devices still mainly rely on conventional silicon technology. While organic semiconductors have excellent optoelectronic properties they in general suffer from low charge carrier mobilities limiting the switching rates in organic transistors. Yet, metal oxides like ZnO, InZnO (IZO) or InGaZnO (IGZO) can bridge the gap between the high mobility semiconductors like silicon and the low mobility organic semiconductors. Using metal-organic precursors or nanoparticulate dispersions easy processing procedures like spin-coating or printing can be applied and yield rather high field-effect mobilities in the order of 1-10 cm²V-1s-1 for the produced thin film transistors (TFTs). Current research activities in the group concentrate on the optimization of the processing procedures especially the decrease of annealing temperatures is desired to make the processes compatible with organic substrates. Furthermore, the influence of the layer morphology and the role of the gas atmosphere for the device performance as well as stability issues are investigated.

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Institute of Materials Science – Electronic Material Properties

In the field of polymer electrets current research comprises the characterization of surface charge distribution, charge stability, and charge transport properties of fluoropolymers, as well as their applications in acoustical transducers. Present investigations of charge transport and polarization in organic dielectrics are directed towards the basic understanding of polarization buildup and stabilization in PVDF and in novel microporous dielectrics. Latter are scientifically interesting as model ferroelectric polymers. Moreover, the fatigue behaviour of electrically stressed inorganic PZT ceramics is investigated. The focus lies on preventing the operational fatigue of ferroelectric devices due to cyclic and static electrical stress. The available equipment includes poling devices, such as corona and high voltage setups, and a thermally stimulated current setup to investigate the energetic trap structure in dielectrics as well as the thermal charging and discharging under high electric fields. In addition, the laser induced pressure pulse (LIPP) method allows to investigate the spatial distribution of stored charges in organic as well as in inorganic ferroelectrics. The field of photoluminescent and photostimulated luminescent (PSL) materials (phosphors) is concerned with the synthesis and characterization of suited inorganic compounds used as wavelength converters in fluorescent lamps and in scintillating and information storing crystals. Present work is focused on x-ray detection materials, providing improved resolution and high PSL-efficiency needed in medical imaging. In particular the storage phosphors CsBr:Eu2+ and BaFBr:Eu2+ are under investigation. Research is concentrated on the influence of humidity on the sensitivity of CsBr:Eu2+. Before and after the treatment the materials are studied by means of spectroscopic methods as well as scanning electron microscopy. The exchange of water during the thermal treatment is measured in situ by thermal analysis methods. New synthesis methods for BaFBr:Eu 2+ used in commercial image plates are of interest and new synthesis routes will be tested for other storage phosphors and scintillators. On the one hand the mechanism of PSL-sensitization, which is found to be mainly due to the incorporation of oxygen and water, is investigated. On the other hand the implementation of BaFBr:Eu2+ powders into organic binders in order to form image plates is in the focus of the work. In the field of scintillators undoped and doped CsI is investigated concerning the afterglow. This afterglow is unfavourable in medical applications like CT where a series of images is made in a very short time. The task is to find the physical reason for this afterglow and a way to suppress it.

Staff Members Head

Prof. Dr.-Ing. Heinz von Seggern

Research Associates

Dr. Andrea Gassmann Dr. Joachim Hillenbrand Dr. Sergej Zhukov

Dr. Corinna Hein Dr. Emanuelle Reis Simas Dr. Jörg Zimmermann

Technical Personnel

Gabriele Andreß Sabine Hesse

Helga Janning Bernd Stoll

Secretary

Gabriele Kühnemundt

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PhD Students

Tobias Könyves-Toth Fabian Knoch Oili Pekkola Riitta Savikoski

Elmar Kersting Paul Mundt Florian Pfeil Henning Seim

Bachelor Students

Frank Löffler Florian Weyland

Stefan Schlißke

Master Students

Ralph Dachauer

Guest Scientists

Juliana Eccher Dr. Anatoli Popov

Prof. Dr. Sergei Fedosov Prof. Dr. Lucas F. Santos

Research Projects Fatigue of organic semiconductor components (SFB 595 (DFG), 2003-2014) Phenomenological modelling of bipolar carrier transport in organic semiconducting devices under special consideration of injection, transport and recombination phenomena (SFB 595 (DFG), 2003-2014) Polarization and charge in electrically fatigued ferroelectrics (SFB 595 (DFG), 2006-2014) Development and optimization of tuneable optical filters and VCSEL based on piezoelectric and electret actuators (TICMO Graduiertenkolleg 1037, 2007-2013) Development of organic piezo sensors (LOEWE AdRIA 26200026, 2008-2014) Thin film dielectrics for high performance transistors (DFG, 2012-2015) Development of gate insulators for organic field effect transistors exploiting self-assembly of block-copolymers (IDS-FunMat (EU), 2012-2015) Piezoelectric properties of ferroelectrics (DFG, 2012-2015) Preparation and characterization of metal-oxide field-effect transistors (MerckLab, 20092015) High resolution, transparent image plates based on the storage phosphor CsBr:Eu2+ (DFG, 2013-2015) Metal oxide based field-effect transistors with top gate geometry (Helmholtz Virtual Institute, 2012-2017)

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Publications [1] Order Induced Charge Carrier Mobility Enhancement in Columnar Liquid Crystal Diodes Eccher, Juliana; Faria, Gregorio C.; Bock, Harald; Seggern, Heinz; Bechtold, Ivan H. ACS APPLIED MATERIALS & INTERFACES Volume: 5 Issue: 22 Pages: 11935-11943 Published: NOV 27 2013 [2] Molecular Origin of Charge Traps in Polyfluorene-Based Semiconductors Faria, Gregorio C.; deAzevedo, Eduardo R.; von Seggern, Heinz MACROMOLECULES Volume: 46 Issue: 19 Pages: 7865-7873 Published: OCT 8 2013 [3] Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2Ti0.8)O-3x(Ba0.7Ca0.3)TiO3 ferroelectrics Zhukov, Sergey; Genenko, Yuri A.; Acosta, Matias; Humburg, Heide; Jo, Wook; Roedel, Juergen; von Seggern, Heinz APPLIED PHYSICS LETTERS Volume: 103 Issue: 15 Article Number: 152904 Published: OCT 7 2013 [4] Continuum modeling of charging process and piezoelectricity of ferroelectrets Xu, Bai-Xiang; von Seggern, Heinz; Zhukov, Sergey; Gross, Dietmar JOURNAL OF APPLIED PHYSICS Volume: 114 Issue: 9 Article Number: 094103 Published: SEP 7 2013 [5] Self-consistent model of polarization switching kinetics in disordered ferroelectrics Genenko, Yuri A.; Wehner, Jens; von Seggern, Heinz JOURNAL OF APPLIED PHYSICS Volume: 114 Issue: 8 Article Number: 084101 Published: AUG 28 2013 [6] Transit phenomena in organic field-effect transistors through Kelvin-probe force microscopy. Melzer, Christian; Siol, Christopher; von Seggern, Heinz Advanced materials (Deerfield Beach, Fla.) Volume: 25 Issue: 31 Pages: 4315-9 Published: 2013Aug-21 (Epub 2013 Apr 29) [7] A new method to invert top-gate organic field-effect transistors for Kelvin probe investigations Kehrer, Lorenz. A.; Feldmeier, Eva. J.; Siol, Christopher.; Walker, Daniel; Melzer, Christian; von Seggern, Heinz APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING Volume: 112 Issue: 2 Pages: 431-436 Published: AUG 2013 [8] High-performance n-channel thin-film transistors with acene-based semiconductors Fapei Zhang; Melzer, Christian; Gassmann, Andrea.; von Seggern, Heinz; Schwalm, Thorsten; Gawrisch, Christian; Rehahn, Matthias Organic Electronics. Materials, Physics, Chemistry and Applications Volume: 14 Issue: 3 Pages: 888-96 Published: March 2013

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[9] Comparative study of the luminescence properties of macro- and nanocrystalline MgO using synchrotron radiation Popov, Anatoli. I.; Shirmane, Liana; Pankratov, Vladimir.; Lushchik , A; Kotlov, Aleksei, Serga, V. E.; Kulikova L. D.; Chikvaidze; Zimmermann, Jörg; NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Volume: 310 Pages: 23-26 Published: SEP 1 2013 [10] Eu2+-doped csbr photostimulable x-ray storage Phosphors — analysis of defect structure by High-frequency epr Peter Jakes, Jörg Zimmermann, Heinz von Seggern, Andrew Ozarowski, Johan van Tol and Rüdiger-A. Eichel, FUNCTIONAL MATERIALS LETTERS, Vol. 7, No. 1 (2013) 1350073 Published: Dec 2013

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The influence of triplet excitons on the lifetime of polymer-based organic light emitting diodes Oili Pekkola, Andrea Gassmann, and Heinz von Seggern In an organic light emitting diode (OLED) the ratio of singlet to triplet excitons formed is 1:3. In fluorescent devices the radiative decay of triplet excitons is forbidden by the spin selection rules, and consequently only the singlet excitons decay radiatively. The high concentration of non-radiatively decaying triplet excitons, combined with their long lifetimes on the order of several µs to ms [1,2], arises the question about the influence of the triplet excitons on the lifetime of fluorescent OLEDs. In the present work, the concentration of triplet excitons in a Poly(p-phenylene vinylene) (PPV)-based polymer is deliberately increased in order to investigate the influence of a higher concentration of triplets on the degradation of the diodes. The increase in concentration is achieved by converting part of the polymer singlet excitons to triplets. This is done by blending the polymer with a triplet sensitizer additive, an organometallic compound containing a platinum atom. The first excited singlet state of the sensitizer molecule lies energetically lower than that of the polymer, which allows for a singlet exciton transfer from the polymer to the sensitizer. On the sensitizer, a fast intersystem crossing takes place, facilitated by the weakened spin selection rules due to the presence of the heavy metal atom. The first excited triplet state of the sensitizer lies higher than that of the polymer, and the triplet exciton can be subsequently transferred back to the polymer. The energetic scheme of the complete singlet-to-triplet conversion is illustrated in Fig. 1.

Figure 1 Energy scheme showing the singlet-to-triplet conversion processes. First, a singlet exciton energy transfer (ET) from OC3C8-PPV to PtOEPK takes place, followed by an intersystem crossing (ISC) to the PtOEPK first excited triplet state. The triplet exciton is subsequently transferred back to OC3C8-PPV, which has an energetically lower first excited triplet state.

All presented devices are polymer-based OLEDs with a Poly(2-propoxy-5-(2'ethylhexyloxy)-phenylene (OC3C8-PPV) as the active material. In the sensitized diodes, the polymer was blended with Platinum (II) octaethylporphyrine ketone (PtOEPK). All devices consisted of an ITO anode, a PEDOT:PSS hole injection layer (40 nm), the active polymer layer (130 nm) and a Ca cathode (20 nm) protected by an Al layer (100 nm). The devices were fabricated in an inert nitrogen atmosphere. OC3C8-PPV was spin-coated from a toluene solution with a concentration of 7.5 mg ml-1. The PtOEPK concentrations in sensitized films were 0.1 and 1 wt%, respectively. After preparation, the polymer films were annealed in the glovebox at 130 °C for 5 min. Finally, the top electrodes were vacuum deposited through shadow masks at a base pressure of 10-6 mbar. The singlet-to-triplet conversion can be verified with photoinduced absorption measurements, where the intensity of the excited triplet state absorption of the polymer Institute of Materials Science – Surface Science

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increases in the presence of the triplet sensitizer. The conversion is also observed to take place in electrically driven devices, which is manifested through a decrease in the electroluminescence, pointing to a decreased population of the first excited singlet state of the polymer. Additionally, no emission from the sensitizer is detected, indicating a complete transfer of triplet excitons from the sensitizer back to the polymer. Fig. 2 plots the lifetimes of the diodes with pristine and sensitized OC3C8-PPV layers. The devices were operated at a constant current density of 50 mA/cm2. The graph shows the normalized luminance values for a better comparison of the curves; the measured luminance intensities are shown in the inset. The t5o lifetime of the diodes with pristine OC3C8-PPV layers is 160 hours, whereas the devices with 0.1 wt% PtOEPK reach a t5o of only 15 hours. This corresponds to a decrease of 91 %. Increasing the PtOEPK concentration to 1 wt% leads to a t5o lifetime of only 2 hours, or a decrease of 99 % from the value of the devices with the pristine polymer layers [3].

Figure 2 Luminance versus time plots of PLEDs with pristine OC3C8-PPV (black squares), OC3C8PPV:PtOEPK (0.1 %) (orange circles) and OC3C8PPV:PtOEPK (1 %) (red triangles) illustrating the deterioration of the t50 lifetime with increasing sensitizer content. The inset shows the measured absolute luminance values.

It can be concluded that the triplet excitons do shorten the lifetime of the PPV-based OLEDs stongly. It is postulated that the heat generated by the non-radiative decay of the triplet excitons could be partly responsible for the observed accelerated decay. In thermography measurements it was observed that the temperature of a diode with pristine OC3C8-PPV rises 5 K above room temperature after turning on the diode. After turning on, the temperature stays constant for several hours of operation at a constant current density of 50 mA/cm2. The observed temperature rise of a diode with 1 wt% PtOEPK is 10 K and therefore higher than that of the device with a pristine polymer layer. It is, however, likely that the temperature increase of 5 K alone is not sufficient to explain the complete acceleration in the degradation of the devices. Another factor that could influence the fatigue of the devices with an increased concentration of triplet excitons is the interaction of the triplet excitons with oxygen. The triplet excitons of conjugated polymers are known to undergo energy transfer to oxygen in triplet state [4–7], leading to the formation of singlet oxygen. It has been observed to attac the vinylene bonds of PPV derivates, leading to chain scission [8]. The process has been shown to take place even in inert atmosphere with oxygen concentration in the low ppm range [6,7]. The higher concentration of triplet excitons in the diodes with the sensitized layers is expected to lead to increased interaction with oxygen. This process could partly contribute to the observed accelerated degradation. It is, however, presently not possible to assign the phenomena responsible for the fatigue to a single process. Nevertheless, due to the generally high concentration of triplet excitons in fluorescent OLEDs, the observed

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accelerated degradation bears a significant importance for improving the stability of the devices by engineering the triplet state population. References: [1] [2] [3] [4] [5] [6] [7] [8]

L. Lin, H. Meng, J. Shy, S. Horng, L. Yu, C. Chen, H. Liaw, C. Huang, K. Peng, and S. Chen, Phys. Rev. Lett. 90, 3 (2003). H. Liao, H. Meng, S. Horng, J. Shy, K. Chen, and C. Hsu, Phys. Rev. B 72, 113203 (2005). O. Pekkola, A. Gassmann, F. Etzold, F. Laquai, and H. von Seggern, Phys. Status Solidi DOI:10.1002/pssa.201330411 (2014). R. D. Scurlock, B. Wang, P. R. Ogilby, J. R. Sheats, and R. L. Clough, J. Am. Chem. Soc. 117, 10194 (1995). A. Sperlich, H. Kraus, C. Deibel, H. Blok, J. Schmidt, and V. Dyakonov, J. Phys. Chem. B 115, 13513 (2011). H. Y. Low, Thin Solid Films 413, 160 (2002). B. H. Cumpston, I. D. Parker, and K. F. Jensen, J. Appl. Phys. 81, 3716 (1997). T. Zyung and J.-J. Kim, Appl. Phys. Lett. 67, 3420 (1995).

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Surface Science The surface science division of the institute of materials science uses advanced surface science techniques to investigate surfaces and interfaces of materials and materials combinations of technological use. For this purpose integrated UHV-systems have been built up which combine different surface analytical tools (photoemission, inverse photoemission, electron diffraction, ion scattering, electron loss spectroscopy, scanning probe techniques) with the preparation of thin films (thermal evaporation, close-spaced sublimation, magnetron sputtering, MOCVD) and interfaces. The main research interest is directed to devices using polycrystalline compound semiconductors and interfaces between dissimilar materials. The perspectives of energy conversion (e.g. solar cells) or storage (intercalation batteries) devices are of special interest. In addition, the fundamental processes involved in chemical and electrochemical device engineering and oxide thin films for electronic applications are investigated. The main research areas are: Electrochemical Interfaces The aim of this research activity is the better understanding of electrochemical interfaces and their application for energy conversion. In addition, empirically derived (electro-) chemical processing steps for the controlled modification and structuring of materials is investigated and further optimized. In the center of our interest are semiconductor/electrolyte contacts. Solar fuels The direct solar light induced water splitting is investigated using photoelectrochemical (electrode/electrolyte) or photocatalytic (particle) arrangements. New materials, design structures, as well as interface engineering approached with advanced catalysts are investigated. The catalysts are also tested for their application in water electrolysis. Intercalation Batteries The aim of this research activity is the better understanding of electronic properties of Liintercalation batteries and of their degradation phenomena. Typically all solid state batteries are prepared and investigated using sputtering and CVD techniques for cathodes and solid electrolytes. In addition, the solid-electrolyte interface and synthetic surface layers are investigated as well as composite systems for increasing the capacity. Thin film solar cells The aim of this research activity is the testing and development of novel materials and materials combinations for photovoltaic applications. In addition, the interfaces in microcrystalline thin film solar cells are to be characterized on a microscopic level to understand and to further improve the empirically based optimisation of solar cells. Organic-inorganic interfaces and composites In this research area we are aiming at the development of composites marterials for (opto-) electronic applications. The decisive factors, which govern the electronic properties of interfaces between organic and inorganic materials are studied.

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Semiconducting Oxides The aim of this research area is to understand electronic surface and interfaces properties of oxides. We are mainly interested in transparent conducting oxide electrodes for solar cells and organic LEDs but also in dielectric and ferroelectric perovskites. Surface analysis The UHV-surface science equipment and techniques using different and versatile integrated preparation chambers is used for cooperative service investigations. For the experiments we use integrated UHV-preparation and analysis-systems (UPS, (M)XPS, LEISS, LEED), spectromicroscopy (PEEM) coupled with UHV-STM/AFM. We further apply synchrotron radiation (SXPS, spectromicroscopy), scanning probe methods (STM, AFM), and electrochemical measuring techniques. UHV-preparation chambers dedicated for MBE, CVD, PVD and (electro)chemical treatment are available. The members of the group are involved in basic courses of the department’s curriculum and offer special courses on the physics, chemistry and engineering of semiconductor devices and solar cells, on surface and interface science, and on thin film and surface technology and electrochemistry.

Staff Members Head

Prof. Dr. Wolfram Jaegermann

Research Associates

Dr.Gennady Cherkashinin Dr. Lucangelo Dimesso Dr. René Hausbrand Dr. Alexander Issanin PD Dr. Bernd Kaiser

Apl. Prof. Dr. Andreas Klein Dr. Shunyi Li Dr. Eric Mankel Dr. Thomas Mayer Dr. Hermann Schimper Dr.Florent Yang

Technical Personnel

Dipl.-Ing. Erich Golusda Kerstin Lakus-Wollny

Christina Spanheimer

Secretaries

Leslie Frotscher

Marga Lang

PhD Students

Dipl.-Ing. Thorsten Bayer Dipl.-Ing. Dirk Becker M.Sc. Mercedes Carillo Solano M.Sc. Mariel Grace Dimamay Dipl.-Ing. Dominic Fertig Dipl.-Ing. Anne Fuchs M. Sc. Stephan Hillmann Dipl.-Ing. Mareike Hohmann Dipl.-Ing. Jan Morasch Dipl.-Ing. Markus Motzko

Dipl.-Ing. ThiThanh Dung Nguyen Dipl.-Ing. Ruben Precht Dipl.-Ing. Karsten Rachut Dipl.-Ing. Philip Reckers Dipl.-Ing. Anja Schneikart Dipl.-Ing. André Schwöbel M.Sc. Sebastian Siol Dipl.-Ing. Johannes Türck Dipl.-Ing. Mirko Weidner Dipl.-Ing. Jürgen Ziegler

Master Students

Richard Günzler Lukas Hamm Michael Kettner

Christian Lohaus Tobias Rödlmeier Hans Wardenga

Guest Scientists

Dr. Lili Wu

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Research Projects Function and fatigue of conducting electrodes in organic LEDs, SFB 595-D3 (DFG 20032014) Polarization and charge in electrically fatigued ferroelectrics, SFB 595-B7 (DFG 2007-2014) Integriertes Graduiertenkolleg SFB 595 (DFG 2008-2014) Tunable Integrated Components for Microwaves and Optics, Graduiertenkolleg 1037 (DFG 2004-2013) P-I-N solar cells with alternative highly-absorbing semiconductors (BMBF 2010-2013) LOEWE Schwerpunkt AdRIA (LOEWE-Hessen: 2008-2013) Kosteneffiziente Produktionsverfahren für CdTe Solarzellen und kupferfreie Rückkontakte (CTF Solar 2012 – 2013) Boundary layers and thin films of ionic conductors: Electronic structure, electrochemical potentials, defect formation and degradation mechanism SFB595-A3 (DFG 2003-2014) Morphology and Electronic Structure of Organic/Organic and Organic/Metal-Oxid Hybrid Systems, Innovation Lab GmbH Heidelberg oft he BMBF leading edge cluster Forum Organic Electronics (BMBF 2009 – 2014) 9D-Sense Autonomous Nine Degrees of Freedom Sensor Module (BMBF/VDI 2011 – 2014) Solid State Lithium Batterien mit organischen Kathoden (Novaled 2011 – 2014) All Oxide PV (EU 2012 – 2014) Inverted organic solar cells: Charge carrier extraction and interface characterization (DFG 2012- 2014) Photoelectrochemical water splitting using adapted silicon based semiconductor tandem structures (DFG 2012 – 2015) Coordination SPP 1613 Solar H2 (DFG 2012 – 2015)

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Publications [1] Trost, S.; Zilberberg, K.; Behrendt, A.; Polywka, A.; Gorrn, P.; Reckers, P.; Maibach, J.; Mayer, T.; Riedl, T.; Overcoming the "Light-Soaking" Issue in Inverted Organic Solar Cells by the Use of Al:ZnO Electron Extraction Layers, ADVANCED ENERGY MATERIALS, 3 (2013) 1437-1444. [2] Dimesso, L.; Spanheimer, C.; Jaegermann, W.; Influence of isovalent ions (Ca and Mg) on the properties of LiCo0.9M0.1PO4 powders; JOURNAL OF POWER SOURCES, 243 (2013) 668-675 [3] Junfeng Han; Ganhua Fu; Krishnakumar, V.; Cheng Liao; Jaegermann, W.; Besland, M.P.; Preparation and characterization of ZnS/CdS bi-layer for CdTe solar cell application; JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 74 (2013), 1879-83 [4] Nguyen, T. T. D.; Dimesso, L.; Cherkashinin, G.; Jaud, J.C.; Lauterbach, S.; Hausbrand, R.; Jaegermann, W.; Synthesis and characterization of LiMn1-x Fe (x) PO4/carbon nanotubes composites as cathodes for Li-ion batteries; IONICS, 19 (2013), 1229-1240, [5] Lebedev, M.V.; Kunitsyna, E.V. ; Calvet, W.; Mayer, T. ; Jaegermann, W.; Sulfur Passivation of GaSb(100) Surfaces: Comparison of Aqueous and Alcoholic Sulfide Solutions Using Synchrotron Radiation Photoemission Spectroscopy; JOURNAL OF PHYSICAL CHEMISTRY C, 117 (2013), 15996-16004 [6] Bohne, L.; Pirk, T.; Jaegermann, W.; Investigations on the influence of the substrate on the crystal structure of sputtered LiCoO2; JOURNAL OF SOLID STATE ELECTROCHEMISTRY 17 (2013), 2095-2099 [7] Han J.; Fu G.; Krishnakumar, V.; Liao, C.; Jaegermann, W.; CdS annealing treatments in various atmospheres and effects on performances of CdTe/CdS solar cells; JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 24 (2013), 2695-2700 [8] Hofle, S.; Do, H.; Mankel, E.; Pfaff, M.; Zhang, Z.H.; Bahro, D.; Mayer, T.; Jaegermann, W.; Gerthsen, D.; Feldmann, C.; Lemmer, U.; Colsmann, A.; Molybdenum oxide anode buffer layers for solution processed, blue phosphorescent small molecule organic light emitting diodes; ORGANIC ELECTRONICS, 14 (2013), 1820-1824 [9] Maibach, J.; Mankel, E.; Mayer, T.; Jaegermann, W.; Synchrotron induced photoelectron spectroscopy on drop casted donor/acceptor bulk heterojunction: Orbital energy line up in DH6T/PCBM blends, SURFACE SCIENCE, 612 (2013), L9-L11 [10] Dimesso, L.; Spanheimer, C.; Jaegermann, W.; Zhang, Y.; Yarin, A. L.; LiCoPO4-3D carbon nanofiber composites as possible cathode materials for high voltage applications; ELECTROCHIMICA ACTA, 95 (2013), 38-42 [11] Cherkashinin, G.; Ensling, D.; Komissinskiy, P.; Hausbrand, R.; Jaegermann, W.; Temperature induced reduction of the trivalent Ni ions in LiMO2 (M = Ni, Co) thin films; SURFACE SCIENCE, 608 (2013), L1-L4

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[12] Dimesso, L.; Spanheimer, C.; Jaegermann, W.; Effect of the Mg-substitution on the graphitic carbon foams-LiNi1-yMgyPO4 composites as possible cathodes materials for 5 V applications, MATERIALS RESEARCH BULLETIN, 48 (2013), 559-565 [13] Becker, D.; Cherkashinin, G.; Hausbrand, R.; Jaegermann, W.; XPS study of diethyl carbonate adsorption on LiCoO2 thin films; SOLID STATE IONICS, 230 (2013), 83-85 [14] Maibach, J.; Mankel, E.; Mayer, T.; Jaegermann, W.; The band energy diagram of PCBM-DH6T bulk heterojunction solar cells: synchrotron-induced photoelectron spectroscopy on solution processed DH6T:PCBM blends and in situ prepared PCBM/DH6T interfaces; JOURNAL OF MATERIALS CHEMISTRY C, 1 (2013), 7635-7642 [15] Seifollahi B., M.; Hojamberdiev, M.; Morita, K.; Zhu, G.; Cherkashinin, G.; Fasel, C.; Herrmann, T.; Breitzke, H.; Gurlo, A.; Riedel, R.; Visible Light Photocatalysis with c-WO3– x/WO3×H2O Nanoheterostructures In Situ Formed in Mesoporous Polycarbosilane-Siloxane Polymer, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 135 (2013), 4467-4475 [16] S. Hoefle, H. Do, E. Mankel, M. Pfaff, Z. Zhang, D. Bahro, T. Mayer, W. Jaegermann, D. Gerthsen, C. Feldmann, U. Lemmer, A. Colsmann; Molybdenum oxide anode buffer layers for solution processed, blue phosphorescent small molecule organic light emitting diodes; ORGANIC ELECTRONICS, 14 (2013) 1820-1824 [17] Vafaee, M. ; Baghaie Y. M. ; Radetinac, A. ; Cherkashinin, G. ; Komissinskiy, P. ; Alff, L.; Strain engineering in epitaxial La[sub 1−x]Sr[sub 1+x]MnO[sub 4] thin films; JOURNAL OF APPLIED PHYSICS, 113 (2013), 053906 [18] Babu, D. J.; Lange, M.; Cherkashinin, G.; Issanin, A.; Staudt, R.; Schneider, J.; Gas adsorption studies of CO2 and N2 in spatially aligned double-walled carbon nanotube arrays; J. CARBON, 61 (2013), 616-623 [19] H. Sträter, R. Brüggemann, S. Siol, A. Klein, W. Jaegermann and G. H. Bauer; Detailed photoluminescence studies of thin film Cu2S for determination of quasi-Fermi level splitting and defect level; J. APPL. PHYS., 114 (2013), 233506 [20] B. Siepchen, H. Schimper, A. Klein, W, Jaegermann; SXPS studies of single crystalline CdTe/CdS interfaces; J. ELECTRON SPECTR. REL. PHENOM,. 190 (2013), 54-63 [21] H. Sträter, R. Brüggemann, S. Siol, A. Klein, W. Jaegermann and G. H. Bauer; Spectral Calibrated and Confocal Photoluminescence of Cu2S Thin-Film Absorber; MAT. RES. SOC. SYMP. PROC., 1538 (2013) [22] V. Pfeifer, P. Erhart, S. Li, K. Rachut, J. Morasch, J. Brötz, P. Reckers, T. Mayer, S. Rühle, A. Zaban, I. Mora Seró, J. Bisquert, W. Jaegermann, A. Klein; Energy Band Alignment Between Anatase and Rutile TiO2 ; J. PHYS. CHEM. LETT., 4 (2013), 4182-4187 [23] S. Siol, H. Sträter, R. Brüggemann, J. Brötz, G. H. Bauer, A. Klein, W. Jaegermann; PVD of Copper Sulfide Cu2S for PIN-structured solar cells; J. PHYS. D: APPL. PHYS. 46 (2013), 495112

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[24] M.T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, M. Müller, H.-J. Kleebe, K. Rachut, J. Ziegler, A. Klein, W. Jaegermann ; Preparation of RuO2/TiO2 Mesoporous Heterostructures and Rationalization of Their Enhanced Photocatalytic Properties by Band Alignment Investigations; J. PHYS. CHEM. C, 117 (2013), 22098–22110 [25] V. Krishnakumar, A. Klein, W. Jaegermann; Studies on CdTe solar cell front contact properties using X-ray Photoelectron Spectroscopy; THIN SOLID FILMS, 545 (2013), 548-557 [26] S. Li, J. Morasch, A. Klein, C. Chirila, L. Pintilie, L. Jia, K. Ellmer, M. Naderer, K. Reichmann, M. Gröting, K. Albe; Influence of orbital contributions to valence band alignment of Bi2O3, Fe2O3, BiFeO3, and Bi0.5Na0.5TiO3 ; PHYS. REV. B, 88 (2013), 045428 [27] K. Morita, G. Mera, K. Yoshida, Y. Ikuhara, A. Klein, H.-J. Kleebe, R. Riedel; Thermal Stability, Morphology and Electronic Band Gap of Zn(NCN); SOLID STATE SCIENCES, 23(2013), 50-57 [28] A. Schneikart, H.-J. Schimper, A. Klein, W. Jaegermann; Efficiency limitations of thermally evaporated thin film SnS solar cells; J. PHYS. D, 46 (2013), 305109 [29] V. Krishnakumar, A. Barati, H.-J. Schimper, A. Klein, W. Jaegermann; A possible way to reduce absorber layer thickness in thin film CdTe solar cells; THIN SOLID FILMS 535 (2013), 233-236 [30] A. Klein; Transparent Conducting Oxides: Electronic Structure – Property Relationship from Photoelectron Spectroscopy with in-situ Sample Preparation; J. AM. CERAM. SOC., 96 (2013), 331-345 [31] M.H. Rein, M. Hohmann, A. Thøgersen, J. Mayandi, A. Klein, and E.V. Monakhov; An in situ XPS study of the initial stages of rf magnetron sputter deposition of indium tin oxide on p-type Si substrate; APPL. PHYS. LETT., 102 (2013), 021606 [32] E. M. Hopper, Q. Zhu, J. Gassmann, A. Klein, T.O. Mason; Surface electronic properties of polycrystalline bulk and thin film In2O3(ZnO)k compounds; APPL. SURF. SCI., 264 (2013), 811-815 Patents [1] Photo-electrochemical cell for production of hydrogen and oxygen in water or electrolyte based aqueous solution, has ion exchange film that is arranged between front and back electrodes of electrochemical-layer structure Patent Number: DE102012205258-A1; WO2013143885-A1 Patent Assignee: EVONIK IND AG Inventor(s): HOCH S., MATTHIAS B., BUSSE J., CALVET W., KAISER B., JAEGERMANN W., HAHN H., ZANTHOFF H., BLUG M.

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Investigations of interface reactions between lithium and solid electrolyte André Schwöbel, René Hausbrand and Wolfram Jaegermann All-solid state Li-ion batteries are a hot topic of research due to possible applications in various areas of energy storage such as micro-electronics and electro-mobility. All-solid state devices feature high safety, low self discharge, high cycling stability and high energy density. These favorable characteristics reflect the properties of inorganic solid electrolytes, such as high thermal stability, low reactivity and low electronic conductivity. The use of solid electrolytes allows the save use of metallic lithium in all-solid state devices or, if applied as protective layer, also in conventional Li-ion batteries with liquid electrolyte. Nevertheless, reactivity between lithium and solid electrolyte resulting in detrimental interlayer formation (solid electrode solid electrolyte interface layer, SESEI-layer) still poses a problem. A solid state electrolyte which is commonly used in thin film batteries is LiPON, a nitrogensubstituted lithium phosphate glass which was first reported by Bates and coworkers [1]. LiPON is easy to synthezise by sputtering, owing to a broad composition range with a reasonable ionic conductivity (10-6 S/cm). We apply a surface science approach to investigate the reactivity between LiPON and lithium. In the experiments, lithium is deposited stepwise onto a LiPON thin film substrate, and x-ray photo-electron spectroscopy (XPS) is performed after each step (Fig. 1). Principally, such an approach allows also insights into the formation of the electrochemical interface, i.e. the formation of the electrochemical double layer and energy level alignment [2]. The experiments have been performed on LiPON with different composition. In this contribution we report results for a LiPON film with metaphosphate (LiPO3) character (Li1.4 PO2.2N0.7) [3], which has proven functional in our model thin film cells.

Li

XPS

XPS

Li

XPS

Li EL

EL

EL

Increasing Li deposition time Figure 1: Schematic illustration of the interface experiment (EL: electrolyte). The approach allows the investigation of reaction layers and energy level alignment.

The evolution of the core level and valence band spectra is shown in figure 2. At the bottom, the spectra of the LiPON film before lithium deposition (denoted as is) are shown, further up the spectra after different deposition times and at the top the spectra of the sample after lithium deposition and additional oxygen exposure (denoted oxidized). The O1s and N1s emissions of the pristine LiPON surface show shoulders to high binding energies, demonstrating the presence of doubly coordinated oxygen and triply coordinated nitrogen within the network. Upon lithium deposition, these features disappear and new emissions appear to lower binding energies in all core level spectra (see figure 3 for a

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detail of the N1s emission). Using literature, the new emissions are attributed to the presence of Li2O, Li3N and Li3P, next to metallic lithium from the top layer.

Figure 2: Evolution of core level and valence band XP-spectra with lithium deposition time.

Figure 3: N1s XP-specta with triply (NT) and doubly (ND) coordinated nitrogen (left) and illustration of nitrogen coordination in LiPON (right).

We conclude that contact of lithium with metaphosphate-type LiPON results in formation of reduced nitrogen and phosphorous coupounds such as Li3N and Li3P, accompanied by the the disruption of the network and the formation of Li2O and orthophosphate Li3PO4. Institute of Materials Science – Surface Science

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Experiments conducted on LiPON films with a more pronounced orthophosphate character indicate that these compounds are more stable. We presume that the reaction is restricted to the interface region due to low electronic conductivity of the reaction products, forming an efficient passivation layer. Nevertheless, such layers can have a pronounced effect on the Li-ion transfer resistance, and further evaluation is ongoing.

References: [1]

Bates, J.B., et al., Electrical-Properties of Amorphous Lithium Electrolyte Thin-Films. Solid State Ionics, 1992. 53-6: p. 647-654.

[2]

Hausbrand, R., D. Becker, and W. Jaegermann, A Surface Science Approach to Cathode/Electrolyte Interfaces in Li-ion Batteries: Contact Properties, Charge Transfer and Reactions accepted to Progress in Solid State Chemistry.

[3]

Schwöbel, A., R. Hausbrand, and W. Jaegermann, In preparation.

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Energy Band Alignment between Anatase and Rutile TiO2 Verena Pfeifer, Shunyi Li, Karsten Rachut, Jan Morasch, Philip Reckers, Thomas Mayer, Wolfram Jaegermann, Andreas Klein Titanium dioxide (TiO2) has been intensivley studied in the last two decades because of its promising photocatalytic properties for energy-related applications. The two most common modifications of TiO2 are anatase and rutile. It was observed that the mixed anatase/rutile systems show more favorable photocatalytic properties than pristine ones of either modification.[1-2] This synergistic effect has been attributed to a built-in driving force for separation of photogenerated charge carriers. One of the explainations for this effect is related to the energy band alignment that forms an energy barrier at the interface blocking charge transfer between anatase and rutile. However, the exact band alignment of these two modifications is still unclear. Early models which either align the valence band maxium of anatase and rutile at the same level or locate the band edges of rutile inbetween those of anatase cannot convincingly explain the observed synergistic phenomena. A more feasible model would be a staggered energy band alignment, in which both the valence band maximum and conduction band minimum of rutile are higher than those of anatase, as suggested by Deák [3] and Scanlon.[4] In this case the photogenerated electrons move preferentially to anatase and holes to rutile due to the energy band offsets. In order to determine the band alignment between anatase and rutile TiO2 and to obtain further understanding to the synergistic effect, the interface properties and electronic structures of these two materials are studied by X-ray photoelectron spectroscopy (XPS) measurements and density functional theory (DFT) calculations. For the XPS experiments rutile single crystals and polycrystalline TiO2 thin films with pure anatase phase deposited with reactive magnetron sputtering are used as substrates. Degnerately Sn-doped In2O3 (ITO) and metallic RuO2 are deposited stepwise onto the substrates as contact materials. After each incremental deposition step, photoelectron spectra are recorded in situ to trace fhifts in the binding energies of core-levels emission lines. The energy band alignment is finally derived independently for both contact materials by applying transitivity rule, ΔEVB(A/R) = ΔEVB(A/X) – ΔEVB(R/X), where A, R, and X represent anatase, rutile, and ITO or RuO2. Fig. 1: Energy band diagrams for anatase/RuO2 and rutile/RuO2 interfaces derived from the interface experiments. The band alignment at the rutile/anatase interface is obtained using transitivity from the figure by omitting the central RuO2 layer and the band bendings. The resulting valence and conduction band discontinuities at the rutile/anatase interface derived from the photoemission experiment are indicated by superscript E, and those from DFT calculations are indicated by superscript T.

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For both contact materials, an offset between the valence band edges (ΔEVB) of anatase and rutile of 0.7±0.1 eV has been determined, as illustrated in Fig. 1.By using the literature values for the band gaps of anatase and rutile, a conduction band offset (ΔECB) of 0.5 eV can be derived. This corresponds to a staggered energy band alignment similar to those reported by Deák [3] and Scanlon.[4] In order to obtain further insight regarding the origin of the band offsets at the rutile/anatase interface, the electronics structure of both materials are analysed on a DFT level. Comparison of the desity of states (DOS) (Fig. 2a) yields an offset of 0.63 eV for valence band and 0.39 eV for conduction band, both in good agreement with the experimental values (see Fig. 1). Further inspection of the valence band structure shows that the DOSs of rutile and anatase are mostly very similar except the appearance of “tails” at both top and bottom in case of rutile. The band structure of rutile in Fig. 2b shows that the tails originate from a pronounced splitting of the topmost and bottommost levels in the vicinity of the Γ point, which is entirely absend in anatase. The electronic origin of this feature can be understood with the help of a Wannier function analysis,[5] which yields on sp2- and pz-like orbital for each oxygen atom, as shown in Fig. 2c,d. The projection of the band structure on this set of states yields the relative admixture illustrated by the color coding in Fig. 2b. This analysis reveals that the topmost valence band of rutile near Γ point is virtually exclusively of pz character and can thus be interpreted as a lone-pair orbital. This explains the downward dispersion of the band away from the center of Γ.

Fig. 2: (a) Comparison of the DOSs of rutile and anatase. The energy scales have been aligned based on the electrostatic potential at the Ti cores. (b) Band structure of rutile where the color scale indicates the respective admixture of oxygencentered (c) sp2-and (d) pz-like orbitals. In (c), only one of the three individual Wannier functions that contribute to the sp2-like orbital is shown. The remaining lobs are oriented along the other two O-Ti bonds.

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In comparison, the pz-like orbital in anatase does not play a prominent role near the valence band edge and the band structure generated in the same fashion does not exhibit a splitting of states around the Γ point. These results suggest, that in rutile the pz-like orbital are much closer to each other and exhibit stronger interaction and overlap, which results in a larger splitting of the energy bands and consequently in a higher valence band maximum energy and the appearance of the tail at the top of the valence band.

References: [1] [2] [3] [4]

[5]

Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C., J. Phys. Chem. B 2003, 107, 45454549. Ohno, T.; Tokieda, K.; Higashida, S.; Matsumura, M., Appl. Catal., A 2003, 244, 383−391. Deá k, P.; Aradi, B.; Frauenheim, T., J. Phys. Chem. C 2011, 115, 3443−3446. Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P.; Watson, G. W.; Keal, T. W.; Sherwood, P.; Walsh, A.; Sokol, A. A., Nat. Mater. 2013, 12, 798−801. Marzari, N.; Mostofi, A. A.; Yates, J. R.; Souza, I.; Vanderbilt, D., Rev. Mod. Phys. 2012, 84, 1419−1475.

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Advanced Thin Film Technology The Advanced Thin Film Technology (ATFT) group works on advanced thin film deposition techniques of novel materials. The group is specialized on physical vapor deposition techniques such as pulsed laser deposition (PLD), advanced oxide molecular beam epitaxy (ADOMBE) and dc/rf-magnetron sputtering. The ADOMBE system is an in-house development and has been jointly financed by Max-Planck-Institute for Solid State Research in Stuttgart and TU Darmstadt. PLD and ADOMBE are part of a cluster system allowing for in-situ sample exchange between the different deposition methods and characterization tools. The ADOMBE apparatus is a worldwide unique thin film deposition system which is dedicated to the growth of complex oxides beyond thermodynamic equilibrium. It allows for the simultaneous deposition of six elements from electron beam sources and further elements evaporated from effusion cells. The molecular beams of each element can be individually controlled by a feed back loop using electron impact emission spectroscopy. The group is working mainly on oxide ceramics which show a stunning variety of new functional properties. Examples are high-temperature superconductors, magnetic oxides for spintronics, high-k dielectrics, ferroelectrics, and novel thermoelectric materials. As a vision for future, new solid state matter can be created by building hetero- and composite structures combining different oxide materials. While present day electronic devices heavily rely on conventional semiconducting materials, a future way to create novel functional devices could be based (completely) on oxide electronics. The group uses a Rigaku SmartLab X-ray thin film diffractometer with rotating anode ("synchrotron in house"). Other characterization tools located in the Advanced Thin Film Technology group include powder X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), high-resolution scanning electron microscopy (HREM) with light element sensitive EDX, and SQUID magnetometry. A 16 Tesla magnet cryostat allowing measurements down to liquid helium temperature has been installed. Another magnet cryostat (10 T) lowers the available temperature range to below 300 mK. This cryostat also contains high-frequency feed-throughs for electrical characterization (40 GHz). The group is also using external large scale facilities as synchrotron radiation (ESRF, Grenoble) and neutron reactors (ILL, Grenoble / HMI and DESY, Berlin) for advanced sample characterization. Close cooperation exists in particular with the Max-Planck-Institute for Solid State Research in Stuttgart, with the Japanese company NTT in Atsugi near Tokio, with the University of Tokio, and Chalmers University of Technology. Throughout 2013 Lambert Alff was working also as a Dean of Studies in the faculty of Materials Science and head of the Graduate School Materialium. Lambert Alff has also worked as an elected a member of the Senat of TU Darmstadt.

Staff Members Head

Prof. Dr. Lambert Alff

Research Associates

Dr. Ewrwin Hildebrandt Dr. Jose Kurian

Dr. Philipp Komissinskiy

Technical Personnel

Dipl.-Ing. Gabi Haindl

Jürgen Schreeck

Secretary

Marion Bracke

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PhD Students

Dipl.-Ing. Mani Arzhang Dipl.-Ing. Alexander Buckow M. Sc. Dominik Gölden Dipl.-Ing. Aldin Radetinac M.Sc. Sareh Sabet MTech. Sharath Ulhas

Dipl.-Ing. Mehrdad Baghaie M. Sc. Niklas van Elten Dipl.-Ing. Stefan Hirsch Dipl. Phys. Reiner Retzlaff BTech. Vikas Shabadi

Research Projects Novel arsenic free pnictide superconductors (SPP 1458) (DFG 2013 - 2015) Doped SrTiO3 for Microwave Applications and Multiferroics as novel materials for tunable components, within DFG Research Training Group 1037 “Tunable Integrated Components in Microwave Technology and Optics” (DFG 2008-2013) Resistives Schalten in HfO2-basierten Metall-Isolator-Metall Strukturen für Anwendungen im Bereich nicht-flüchtiger Speicher (DFG 2012-2013) Novel oxid electrodes for all oxide varactors (DFG 2012-2014) LOEWE-Centre AdRIA: Adaptronik – Research, Innovation, Application (HMWK 2011 2014) Publications [1] P. Lemmens, V. Gnezdilov, G. J. Shu, L. Alff, C. T. Lin, B. Keimer, and F. C. Chou. Enhanced low-energy fluctuations and increasing out-of-plane coherence in vacancyordered NaxCoO2. Phys. Rev. B 88, 195151 (2013). [2] Inventor(s): L. Alff, S. Hildebrandt, T. Kober, R. Teipen, A. T.Tham, R. Werthschützky Plate-like glass structure for manufacturing pressure sensor, has recessed contour portion that is surrounded by planar edge area, and specific glass surface whose surface roughness value and diameter are set to predetermined ranges Patent Number(s): DE102011084457A1 ; EP2581722-A2. [3] M. Baghaie Yazdi, K.-Y. Choi, D. Wulferding, P. Lemmens, and L. Alff. Raman study of the Verwey transition in magnetite thin films. New Journal of Physics 15, 103032 (2013). [4] R. Hord, G. Pascua, K. Hofmann, G. Cordier, J. Kurian, H. Luetkens, V. Pomjakushin, M. Reehuis, B. Albert and L. Alff. Oxygen stoichiometry of low-temperature synthesized metastable T'-La2CuO4. Supercond. Sci. Technol. 26, 105026 (2013). [5] Mingwei Zhu, Philipp Komissinskiy, Aldin Radetinac, Mehran Vafaee, Zhanjie Wang, and Lambert Alff. Effect of composition and strain on the electrical properties of LaNiO 3 thin films. Appl. Phys. Lett. 103, 141902 (2013). [6] Sandra Hildebrandt, Philipp Komissinskiy, Marton Major, Wolfgang Donner, Lambert Alff. Epitaxial growth and control of the sodium content in NaxCoO2 thin films. Thin Solid Films 545, 291 (2013).

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[7] Alff, L. Magnetic Ceramics. in Ceramics Science and Technology Volume 4: Applications (eds R. Riedel and I.-W. Chen), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2013. [8] K.I. Lilova, R. Hord, L. Alff, B. Albert, A. Navrotsky.Thermodynamic Study of Orthorhombic Tx and Tetragonal T′ Lanthanum Cuprate, La2CuO4. J. Solid State Chem. 204, 91 (2013). [9] K. Y. Constantinian, Yu. V. Kislinskii, G. A. Ovsyannikov, A. V. Shadrin, A. E. Sheyerman, A. L. Vasil’ev, M. Yu. Presnyakov, P. V. Komissinskiy. Interfaces in superconducting hybrid heterostructures with an antiferromagnetic interlayer. Physics of the Solid State 55, 461 (2013). [10] Mehran Vafaee, Mehrdad Baghaie Yazdi, Aldin Radetinac, Gennady Cherkashinin, Philipp Komissinskiy, and Lambert Alff. Strain engineering in epitaxial La1−xSr1+xMnO4 thin films. J. Appl. Phys. 113, 053906 (2013). [11] A Buckow, R Retzlaff, J Kurian and L Alff. Growth of superconducting epitaxial LaNixBi2 pnictide thin films with a Bi square net layer by reactive molecular beam epitaxy Supercond. Sci. Technol. 26, 015014 (2013). [12] Gennady Cherkashinin, David Ensling, Philipp Komissinskiy, René Hausbrand, Wolfram Jaegermann. Temperature induced reduction of the trivalent Ni ions in LiMO2 (M = Ni, Co) thin films. Surface Science 608, L1–L4 (2013). [13] Ina Uhlmann, Dominik Hawelka, Erwin Hildebrandt, Jens Pradella, Jürgen Rödel. Structure and mechanical properties of silica doped zirconia thin films. Thin Solid Films 527, 200 (2013). [14] J. Kurian, A. Buckow, R. Retzlaff, L. Alff. Search for superconductivity in LaNiP2 (P = Bi, Sb) thin films grown by reactive molecular beam epitaxy. Physica C 484, 171 (2013).

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Dispersive Solids The main research interests of the group Dispersive Solids are directed towards the development of novel strategies suitable for the synthesis of inorganic, oxidic and nonoxidic materials with properties beyond the state of the art. The materials of interest are advanced oxidic and non-oxidic ceramics with extraordinary properties in terms of thermal stability, hardness and electronic structure. Therefore, synthesis methods such as polymer-pyrolysis, non-oxidic and oxidic sol-gel methods, chemical vapour deposition and novel high pressure methods have been further developed. The following topical issues are presently under investigation: Polymer-Derived Ceramics The thermolytic decomposition of suitable organosilicon polymers provides materials which are denoted as polymer-derived ceramics (PDCs). The main emphasis is on the synthesis and characterization of new ceramic materials in the B-C-N, Si-C-N, Si-O-C, Si(B,C)-N and Ti-(B-C)-N systems. The structural peculiarities, thermochemical stability, mechanical and electrophysical properties of the PDCs have been investigated in a series of PhD theses and research projects. Due to their outstanding thermochemical stability as well as excellent oxidation and creep resistance at very high temperatures, the PDCs constitute promising materials for high temperature applications. Another advantage of the PDC route is that the materials can be easily shaped in form of fibres, layers or bulk composite materials. Finally the correlation of the materials properties with the molecular structure of the used preceramic polymer is elaborated Molecular Routes to Nanoscaled Materials The aim is to develop concepts for the production of novel multifunctional inorganic materials with a tailor-made nanoscaled structure. In accordance with the so-called “bottom-up” approach, specific inorganic molecules are to be assigned to higher molecular networks and solid-state structures in the form of molecular nanotools by means of condensation and polymerisation processes. High Pressure Chemistry Ultra-high pressure techniques like laser heated diamond anvil cell (LH-DAC) or multi anvil devices have been applied to synthesise novel solid state structures which cannot be produced by other methods, for example, inorganic nitrides. Moreover, the materials behaviour under pressure such as phase transformations and decomposition can be analysed. Functional Materials Further research topics are related to the development of materials suitable for applications in the fields of microelectromechanical systems (MEMS), optoelectronics (LEDs), pressure, temperature and gas sensors as well as thermoresistant ceramic membranes for high temperature gas separation. The integration of state-of-the-art in situ and in operando spectroscopic methods is applied to understand the mechanisms responsible for sensing and catalytic properties.

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Staff Members Head

Prof. Dr. rer. nat. habil. Prof. h. c. Dr. h. c. Ralf Riedel

Research Associates

Dr. Dmytro Dzivenko Dr. Magdalena GraczykZajac PD Dr. Aleksander Gurlo Dr. Emanuel Ionescu

Technical Personnel

Dipl.-Ing. Claudia Fasel

Secretaries

Su-Chen Chang Tania Fiedler-Valderrama (EU project)

Natallia Hurlo (substitute) Shobha Herur (substitute)

PhD Students

Dipl.-Ing. Miria Andrade M. Tech. Mahdi Seifollahi Bazarjani M. Sc. Shrikant Bhat M. Tech. Maged Bekheet M. Tech. Yan Gao M. Sc. Sarabjeet Kaur Dipl.-Ing. Jan Kaspar Dipl.-Ing. Amon Klausmann M. Sc. Wenjie Li

Dipl.-Ing. Christoph Linck M. Tech. Ravi Mohan Prasad Dipl.-Ing. Lukas Mirko Reinold Dipl.-Ing. Felix Roth M. Sc. Cristina Schitco Dipl.-Ing. Lukas Schlicker Dipl.-Ing. Alexander Uhl M. Sc. Qingbo Wen M. Sc. Jia Yuan M. Sc. Cong Zhou

Dr. Gabriela Mera Apl. Prof. Dr. Norbert Nicoloso Dr. Ravi Mohan Prasad M. Sc. Ahmad Choudhary

Diploma and Master Omar Ariobi Students Oliver Genschka Xueying Hai

Cornelia Hintze Chinomso Nwosu

Bachelor Students

Laszlo Horak Moritz Liesegang Mathias Storch

Maximilian Wimmer Kerstin Wissel Alexander Zimpel

Guest Scientists

Prof. Dilshat Tulyaganov, Turin Polytechnic University in Tashkent, Tashkent, Uzbekistan Prof. Zhaoju Yu, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen China Michal Vetrecin, Institute of Inorganic Chemistry,Slovak Academy of Sciences, Bratislava, Slovakia Ondrej Hanzel, Institute of Inorganic Chemistry,Slovak Academy of Sciences, Bratislava, Slovakia

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Institute of Materials Science - Dispersive Solids

Dr. Monika Wilamowska, Department of Chemical Technology, Chemical Faculty, Gdansk University of Technology, Poland Prof. Dr. Corneliu Balan, Politehnica, University of Bucharest, Faculty of Enegetics, Hydraulics Departement, Bucharest, Romania Dr. Xingang Luan, Associate Professor, Northwestern University Polytechnical, Schule für Materialien, Xian, Shaanxi, PR China Prof. Linan An, Associate Professor and Director, Materials Processing Laboratory, University of Central Florida, USA Prof. Kathy Lu, Virginia Tech, College of Engineering, Department of Materials Science and Engineering, Blacksburg, USA Yohei Shimokawa, Department of Frontier Materials Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Japan

Research Projects SiHfC(N) and SiHfN(C)-based Ultrahigh-Temperature Ceramic Nanocomposites (UHTCNCs) for EBC/TBC Applications (China Council Scholarship (CSC), Oct. 2012 - Oct. 2016) Nanocomposites as anode materials for lithium ion batteries: Synthesis, thermodynamic characterization and modeling of nanoparticular silicon dispersed in SiCN(O) and SiCObased matrices (DFG, Aug. 2010 - July 2016) High-Temperature Piezoresistivity in SiOC - Untersuchungen zur HochtemperaturPiezoresistivität in kohlenstoffhaltigen Siliciumoxycarbid-Nanokompositen(DFG, May 2013 - April 2016) Sensors Towards Terahertz (STT): Neuartige Technologien für Life Sciences, Prozess- und Umweltmonitoring (HMWK-LOEWE, Jan. 2013 - Dec. 2015) Mestabiles Indiumoxidhydroxid (InOOH) und Korund-Typ Indiumoxid (In2O3): Gezielte Synthese, Einkristallzüchtung und in-situ Charakterisierung der Umwandlungspfade und transienten Intermediaten (DFG, SPP 1415 "Kristalline Nichtgleichgewichtsstoffe", Jan. 2013 - Dec. 2015) Ternary M-Si-N Ceramics: Single-Source-Precoursor Synthesis and Microstructure Characterization (M = transition metal) (China Council Scholarship (CSC), Nov. 2012 Nov. 2015) Molecular Routes to SiMBCN Ceramic Nanocomposites (M = Zr, HF) (China Council Scholarship (CSC), Donghua University, Shanghai, China, Sep. 2011 - Aug. 2015) Institute of Materials Science - Dispersive Solids

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FUNEA - Functional Nitrides for Energy Applications (Coordination, EU - Marie Curie Initial Training Network, Feb. 2011 - Jan. 2015) Novel functional ceramics with substitution of anions in oxidic systems (DFG, SFB 595, project A4, Jan. 2003 - Dec. 2014) Adaptronik - Research, Innovation, Anwendung (HMWK-Loewe-AdRIA, Oct. 2008 - Sep. 2014) Polymer-Processing of Dense and Crack-Free SiC Monoliths (Doctor Thesis, Oct. 2011 Sep. 2014) FUNEA - Gas seperation membranes (EU - Marie Curie Initial Training Network, Oct. 2011 - Sep. 2014) FUNEA - Multifunctional perovskite nitrides (EU - Marie Curie Initial Training Network, Oct. 2011 - Sep. 2014) Keramische SiCN-basierte Hartstoffschichten Substratwerkstoffe (DFG, June 2011 - June 2014)

für

thermisch

hochbeanspruchte

Untersuchung der Einflussparameter für die Biege- und Zugfestigkeitsverhalten oxidischer Verbundkörper mit gefüllter Polysiloxanmatrix (Diploma Thesis, Dec. 2013 - June 2014) High-Pressure High Temperature Synthesis of Novel Binary and Ternary Superhard Phases in the B-C-N System (DFG, Feb. 2011 - Jan. 2014) PrintSens: Nanoskalige gedruckte Hybridmaterialien als aktive Funktionselemente in mikrostrukturierteen Sensorbauteilen (Schwerpunkt Mikrosystemtechnik im Förderprogramm "IKT 2020 - Forschung für Innovationen" (BMBF VDI/VDE/IT, Jan. 2011 December 2013) Ceramic Nanocomposites for Applications in Extreme Environments (DAAD, Projekt-ID 54440408, Jan. 2012 - Dec. 2013) Investigation of polymer-derived, carbon-rich SiOC ceramics as potential Na-ion storage material (Bachelor Thesis, Sep. 2013 - Dec. 2013) Synthesis of Dense SiOC Ceramics with tailored Carbon Content (March 2013 - Oct. 2013) Thermoelektrika auf Basis von MSix/SiOC-Kompositen (Bachelor Thesis, June 2013 - Sep. 2013) Synthesis of Vanadium-Carbide-Based Nanocomposites from Single-Source Precursors (Diploma Thesis, April 2013 - Oct. 2013) Sol-gel derived SiOC materials as anodes for Li-ion batteries (DFG, SFB 595, Aug. 2013 Sept. 2013)

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Institute of Materials Science - Dispersive Solids

Carbon-coated new Si-based composite anode materials for Li-ion batteries (FAME intership, INP Grenoble-Phelma, May 2013 - Aug. 2013) Material Anticipatio Studies for Heat disspation in Electric Switches (Master Thesis FAME in Cooperation with Élève Ingénieure des Matériaux, Grenoble INP - PHELMA, France, April 2013 - Sep. 2013) Optmization of Mechanical and Conductivity Properties of Ply, Modified Polyethylene Glycol and a Blend of Poe: NPEG Reinforced by Nanocrystalline Cellulose and Crosslinking (Master Thesis FAME in Cooperation with Université Grenoble, LEPMI, Grenoble, France March 2013 - Aug. 2013) Multifunctional Graphene Nanocomposites (Master Thesis, Feb. 2013 - Aug. 2013) Herstellung und Eigenschften von polymerabgeleiteten (Bachelor Thesis, April 2013 - Aug. 2013)

SiOC-Precursorkeramiken

Thermoresistant Ceramic Membrane with Integrated Gas Sensor for High Temperature Separation and Detection of Hydrogen and Carbon Monoxide (DFG, Aug. 2010 - July 2013) Schwerpunkt Mikrosystemtechnik im Förderprogramm "IKT 2020 - Forschung für Innovationen" (BMBF VDI/VDE/IT, Jan. 2011 - June 2013) Au/Graphene Metamaterialstrukturen (EUMINAfab (EU) in Cooperation with university of Frankfurt and TCD, Dublin, Jan. 2013 – June 2013) Ionic liquids as electrolyte for Li-ion batteries (Bachelor Thesis, March 2013 – June 2013) Non Aqueous Sol-Gel Synthesis of Boron Carbide Based Materials (US Army International Technolog, Aug. 2009 - May 2013) Synthesis and characterization of rare-earth cation-doped silicon carbonitride phosphors (International Training Program of JSPS, May 2012 - April 2013) Nanostructure and Calorimetry of Amorphous SiCN and SiBCN (DFG, April 2010 - March 2013) Porous Carbon Impregnated with Polymer-Derived Ceramics as Anode Material for Lithium-Ion Batteries (Bachelor Thesis, Jan. 2013 - April 2013) Elektrische und mechanische Kontaktierung eines Hochtemperatur-Ultraschallwandlers (Bachelor Thesis, Dec. 2012 – Feb. 2013) Indium oxide (In2O3) under high pressure: rational design of new polymorphs and characterisation of their physico-chemical properties (DFG, since June 2009)

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Publications [1] Wilamowska, M.; Graczyk-Zajac, M.; Riedel, R.; Composite materials based on polymerderived SiCN ceramic and disordered hard carbons as anodes for lithium-ion batteries; JOURNAL OF POWER SOURCES, 244 (2013) 80-86. [2] Kaspar, J.; Graczyk-Zajac, M.; Riedel, R.; Lithium insertion into carbon-rich SiOC ceramics: Influence of pyrolysis temperature on electrochemical properties; JOURNAL OF POWER SOURCES, 244 (2013) 450-455. [3] Mera, G.; Menapace, I.; Widgeon, S.; Sen, S.; Riedel, R.; Photoluminescence of assynthesized and heat-treated phenyl-containing polysilylcarbodiimides: role of crosslinking and free carbon formation in polymer-derived ceramics; APPLIED ORGANOMETALLIC CHEMISTRY, 27(11) (2013) 630-638. [4] Hojamberdiev, M.; Prasad, R.M.; Fasel, C.; Riedel, R.; Ionescu, E.; Single-sourceprec2ursor synthesis of soft magnetic Fe3Si- and Fe5Si3-containing SiOC ceramic nanocomposites; JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 33(13-14) (2013) 2465–2472. [5] Sen, S.; Widgeon, S.J.; Navrotsky, A.; Mera, G.; Tavakoli, A.; Ionescu, E.; Riedel, R.; Carbon substitution for oxygen in silicates in planetary interiors; PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 110(40) (2013) 15904-15907. [6] Balazsi, C.; Dusza, J.; Lojkowski, W.; Riedel, R.; E-MRS 2012 Fall Meeting, September 17-21, Warsaw University of Technology Preface; JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 33(12) (2013) SI 2215-2215. [7] Morita, K.; Mera, G.; Yoshida, K.; Ikuhara, Y.; Klein, A.; Kleebe, H.-J.; Riedel, R.; Thermal Stability, Morphology and Electronic Band Gap of Zn(NCN); SOLID STATE SCIENCES, 23 (2013) 50-57. [8] Li, W.; Ionescu, E.; Riedel, R.; Gurlo, A.; Can we predict the formability of perovskite oxynitrides from tolerance and octahedral factors?; JOURNAL OF MATERIALS CHEMISTRY A, 1 (2013) 12239-12245. [9] Liu, G.; Kaspar, J.; Reinold, L.M.; Graczyk-Zajac, M.; Riedel, R.; Electrochemical performance of DVB-modified SiOC and SiCN polymer-derived negative electrodes for lithiumion batteries; ELECTROCHIMICA ACTA, 106 (2013) 101-108. [10] Gao, Y; Widgeon, S.J.; Tran, T.B.; Tavakoli, A.H.; Mera, G.; Sen, S.; Riedel, R.; Navrotsky, A.; Effect of Demixing and Coarsening on the Energetics of Poly(boro)silazaneDerived Amorphous Si-(B-)C-N Ceramics; SCRIPTA MATERIALIA, 69(5) (2013) 347–350. [11] Bekheet, M.F.; Schwarz, M.; Lauterbach, S.; Kleebe, H.-J.; Kroll, P.; Stewart, A.; Kolb, U.; Riedel, R.; Gurlo, A.; In-situ high-pressure high-temperature experiments in multi-anvil assembly’s with bixbyite-type In2O3 and synthesis of corundum-type and orthorhombic In2O3 polymorphs ; HIGH PRESSURE RESEARCH, 33(3) (2013) 697-711.

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[12] Reinold, L.M.; Graczyk-Zajac, M.; Gao, Y.; Mera, G.; Riedel, R.; Carbon-Rich SiCN Ceramics as High Capacity/High Stability Anode Material for Lithium-Ion Batteries; JOURNAL OF POWER SOURCES, 236 (2013) 224-229. [13] Nonnenmacher, K.; Kleebe, H.-J.; Rohrer, J.; Ionescu, E.; Riedel, R.; Carbon Mobility in SiOC/HfO2 Ceramic Nanocomposites; JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 96(7) (2013) 2058–2060. [14] Ionescu, E.; Terzioglu, C.; Linck, C.; Kaspar, J.; Navrotsky, A.; Riedel, R.; Thermodynamic Control of Phase Composition and Crystallization of Metal-Modified Silicon Oxycarbides; JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 96(6) (2013) 1899-1903. [15] Bekheet, M.F.; Schwarz, M.R.; Lauterbach, S.; Kleebe, H.-J.; Kroll, P.; Riedel, R.; Gurlo, A.; Orthorhombic In2O3 : a metastable polymorph of indium sesquioxide; ANGEWANDTE CHEMIE-INTERNATIONAL EDITION IN ENGLISH, 52(25) (2013) 6531-6535. [16] Ionescu, E.; Mera, G.; Riedel, R.; Polymer-Derived Ceramics: Materials Design towards Applications at Ultrahigh-Temperatures and in Extreme Environments; in „MAX Phases and Ultra-High Temperature Ceramics for Extreme Environments“, Eds. Low, J.; Sakka, Y.; Hu, C., IGI GLOBAL, Publishing (2013). [17] Gurlo, A.; Ceramic Gas Sensors; in Advanced Ceramics Science and Technology Volume 4: Applications (Eds. R. Riedel and I.-W. Chen), WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (2013). [18] Colombo, P.; Mera, G.; Riedel, R.; Sorarù, G.D.; Polymer-Derived Ceramics: 40 Years of Research and Innovation; in Advanced Ceramics, in Ceramics Science and Technology Volume 4: Applications (Eds. R. Riedel and I.-W. Chen), WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (July 2013) [19] Mera, G.; Ionescu, E.; Silicon-Containing Preceramic Polymers; ENCYCLOPEDIA OF POLYMER SCIENCE AND TECHNOLOGY, available online since 24th of May 2013. [20] Widgeon, S.; Mera, G.; Gao, Y.; Sen, S.; Navrotsky, A.; Riedel, R.; Effect of Precursor on Speciation and Nanostructure of SiBCN Polymer-Derived Ceramics; JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 96(5) (2013) 1651–1659. [21] Sellappan, P.; Guin, J.-P.; Rocherulle, J.; Celarie, F.; Rouxel, T.; Riedel, R.; Influence of diamond particles content on the critical load for crack initiation and fracture toughness of SiOC glass-diamond composites; JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 33(4) (2013) 847-858. [22] Hojamberdiev, M.; Bozgeyik, M.S.; Abdullah, A.M.; Bekheet, M.F.; Zhu, G.; Yan, Y.; Xu, Y.; Okada, K.; Hydrothermal-induced growth of Ca10V6O25 crystals with various morphologies in a strong basic medium at different temperatures; MATERIALS RESEARCH BULLETIN, 48(4) (2013) 1388-1396. [23] Sänze, S.; Gurlo, A.; Hess, C.; Monitoring Gas Sensors at Work: Operando RamanFTIR Study of Ethanol Detection by Indium Oxide; ANGEWANDTE CHEMIE-INTERNATIONAL EDITION IN ENGLISH, 52(13) (2013) 3607-3610. Institute of Materials Science - Dispersive Solids

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[24] Drogowska, K.; Flege, S.; Rogalla, D.; Becker, H.-W.; Ionescu, E.; Kim-Ngan, N.-T.H.; Balogh, A.G.; Hydrogen content analysis in hydrogen-charged PZT ferroelectric ceramics; SOLID STATE IONICS, 235 (2013) 32-35. [25] Bazarjani, M.S.; Hojamberdiev, M.; Morita, K.; Zhu, G.; Cherkashinin, G.; Fasel, C.; Herrmann, T.; Breitzke, H.; Gurlo, A.; Riedel, R.; Visible Light Photocatalysis with c-WO3x/WO3×H2O Nanoheterostructures In situ Formed in Mesoporous Polycarbosilane-Siloxane Polymer; JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 135(11) (2013) 4467- 4475. [26] Pashchanka, M.; Prasad, R.M.; Hoffmann, R.C.; Gurlo, A.; Schneider, J.J.; InkjetPrinted Nanoscaled CuO for Miniaturized Gas-Sensing Devices; EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 9 (2013) 1481-1487. [27] Miehe, G.; Lauterbach, S.; Kleebe, H.-J.; Gurlo, A.; Indium hydroxide to oxide decomposition observed in one nanocrystal during in situ transmission electron microscopy studies; JOURNAL OF SOLID STATE CHEMISTRY, 198 (2013) 364-370. [28] Knappschneider, A.; Litterscheid, C.; Dzivenko, D.; Kurzman, J.A.; Seshadri, R.; Wagner, N.; Beck, J.; Riedel, R.; Albert, B.; Possible Superhardness of CrB4; INORGANIC CHEMISTRY, 52 (2) (2013) 540-542. [29] Bekheet, M.F.; Schwarz, M.R.; Müller, M.M.; Lauterbach, S.; Kleebe, H.-J.; Riedel, R.; Gurlo, A.; Phase segregation in Mn-doped In2O3: in situ high-pressure high-temperature synchrotron studies in multi-anvil assemblies; RSC ADVANCES, 3(16) (2013) 5357-5360. [30] Mera, G.; Navrotsky, A.; Sen, S.; Kleebe, H.-J.; Riedel, R.; Polymer-Derived SiCN and SiOC Ceramics – Structure and Energetics at the Nanoscale; JOURNAL OF MATERIALS CHEMISTRY A, 1 (2013) 3826-3836. [31] Papendorf, B.; Ionescu, E.; Kleebe, H.-J.; Linck, C.; Guillon, O.; Nonnenmacher, K.; Riedel, R.; High-Temperature Creep Behavior of Dense SiOC-Based Ceramic Nanocomposites: Microstructural and Phase Composition Effects; JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 96(1) (2013) 272–280. [32] Jüttke, Y.; Richter, H.; Voigt, I.; Prasad, R. M.; Bazarjani, M. S.; Gurlo, A.; Riedel, R. Polymer derived ceramic membranes for gas separation; Chemical Engineering Transactions 2013, 32, 1891-1896. DOI:10.3303/CET1332316 Books Riedel, R. / Chen, I.-W.; Ceramics Science and Technology, Volume 4: Applications, WILEY-VCH, July 2013, ISBN: 978-3-527-31158-3.

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Orthorhombic In2O3: A Metastable Polymorph of Indium Sesquioxide Maged F. Bekheeta, Marcus R. Schwarzb, Stefan Lauterbacha, Hans-Joachim Kleebea, Peter Krollc, Ralf Riedela, and Aleksander Gurloa a

Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, 64287 Darmstadt (Germany) b Technische Universität-Bergakademie Freiberg, Freiberg High Pressure Research Centre, Institut für Anorganische Chemie, 09599 Freiberg (Germany) c Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX 7600190065 (USA) Angew. Chem. (International Ed. in English), 52(25) (2013) 6531-6535.

As our results led to some discussion in the community,[1] we set out to explore alternative high-pressure routes towards a large amount of the o’-In2O3 polymorph. The main goals of this work are as follows: 1) to synthesize macroscopic quantities of o’-In2O3; 2) to recover it to ambient pressure; and 3) to determine the crystal structure of o’-In2O3 under ambient conditions. It is important to note that other corundum-type sesquioxides, including Cr2O3,[2] Fe2O3,[3] and Al2O3,[4] transform to Rh2O3(II)-type structure under high pressure, but none of them have been recovered to ambient conditions to date. Therefore, the availability of Rh2O3(II)-type o’-In2O3 under ambient conditions will also contribute to better understanding of the structural chemistry and properties of other binary oxides. Our work differs from the previous studies[5,6,7] in three major aspects. First, we are guided by theoretical calculations that suggest using the metastable corundum-type rhIn2O3 (for the specimen details we refer to references [8, 9]) as starting material for the high-pressure synthesis of the orthorhombic o’-In2O3 polymorph. Computations indicate that o’-In2O3 is lower in enthalpy than the rh-In2O3 for pressures above 6.4 GPa (arrow 1 in Figure 1 a) and thus below the c- to o’-In2O3 transition (arrow 2 in Figure 1 a).[8] Both structures, rh-In2O3 and o’-In2O3, are connected by a diffusionless pathway via a common monoclininc P2/c subgroup (in analogy to Al2O3).[1a] We computed the activation barrier for the collective transition rh-In2O3  o’-In2O3 to 0.08 eV per atom, which corresponds to a temperature of about 650 8C at the transition pressure (Figure 1 b). Consequently, we expect a fast transformation rh-In2O3  o’-In2O3 under high-pressure high-temperature conditions. Second, as we aim at high-yield synthesis, we choose multi-anvil and toroid cell apparatus that allowed us to obtain macroscopic quantities (ca. 10–100 mm3) of o’-In2O3 polymorphs and also to grow macroscopic single crystals.[10] The synthesis in multi-anvil cells is considered as a step towards an industrial scale synthesis, for example, in a belt apparatus that allows circa 7 cm3 of material to be obtained under given conditions; a similar pressure is applied in industrial synthesis of diamond and cubic boron nitride.[11] Finally, we perform time-resolved synchrotron studies in multi-anvil assemblies to follow hase transformations in situ under high-pressure high-temperature conditions. The phase development in rh-In2O3 was monitored in situ by energy-dispersive X-ray diffractometry at the two-stage 6–8 MAX200X multi-anvil high-pressure diffractometer of the GFZ Potsdam (beamline W2, HASYLAB/DESY, Hamburg, Germany). New high-pressure/hightemperature multi-anvil assemblies for synchrotron studies developed at the Freiberg High Pressure Research Centre are employed.[12] These assemblies have low X-ray absorption and do not show any additional reflections from the sample environment (see the Supporting Information).[13] Institute of Materials Science - Dispersive Solids

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The complete transformation from rh-In2O3 to o’-In2O3 takes less than 20 seconds at 600 °C and 9 GPa (arrow 1 in Figure 2), indicating fast kinetics as expected for a diffusionless transition. The XRD pattern of material rapidly quenched at 9 GPa from 600 °C to room temperature possesses only o’-In2O3 reflections. During decompression at room temperature, o’-In2O3 partially transforms to corundum-type rh-In2O3 at pressures below 1.0 GPa (arrow 2 in Figure 2). The structure refinement of the specimen recovered to ambient pressure confirms the coexistence of o’-In2O3 (fraction: 80.0 wt%), rh-In2O3 (15.9 wt%), and o-InOOH (4.1 wt%) as a side phase (Figure 3b, Table 1). In the next step, we explored whether the synthesis of o’-In2O3 could be reproduced ex situ in a toroid-type highpressure device that allows even larger macroscopic quantities to be obtained, as well as a fast compression/decompression rate and less experimental preparation times compared to multi-anvil devices.[14] In a typical experiment, rh-In2O3 was compressed to 8 GPa and heated at about 1000–1100 °C for 10 minutes. Figure 3c shows the X-ray powder diffraction pattern and Rietveld difference plot of the recovered specimen. The structure refinement (Figure 3c, Table 1) confirmed our finding from the in situ multi-anvil experiments and shows the coexistence of o’-In2O3 (fraction: 63.8 wt%), rh-In2O3 (31.5 wt%), and o-InOOH (4.7 wt%). The o-InOOH probably arises from the reaction between In2O3 and water under high-pressure and high-temperature (hydrothermal) conditions.[15] Possible water sources include the pressure standard or the sample itself. Interestingly, o-InOOH was also obtained as a side phase in recent synthesis of InMnO3 and In-Mn-Fe-O perovskites and corundum-type In2-2xZnxSnxO3 oxides performed at 6 GPa/1100–1500 °C and 7 GPa/1000 °C, respectively.[16] In three In2O3 polymorphs, which are available at ambient conditions, indium is octahedrally coordinated and oxygen tetrahedrally coordinated (Figure 4); the structural differences between them lie in the stacking of {InO6} octahedra. In c-In2O3, the {InO6} octahedra share corners and edges; in the other two it is the edges and faces. The o’-In2O3 is an orthorhombic distortion of the rh-In2O3 structure, in which each {InO6} octahedron shares only two edges with other octahedra rather than three in rh-In2O3. The interatomic distances are similar in all three structures; that is, the mean In_O distance is in the range 2.182–2.189 Å. o’-In2O3 is the densest polymorph, and the volume reduction from c-In2O3 and rh-In2O3 to o’-In2O3 is about 6 and 3%, respectively. In summary, we succeeded in synthesizing the orthorhombic o’-In2O3 polymorph from rhombohedral corundumtype rh-In2O3 under moderate high-pressure high-temperature conditions (8–9 GPa, 600–1100 °C) in multi-anvil and toroid apparatus. We were able to recover the polymorph to ambient pressure and temperature and to confirm its crystal structure by X-ray and electron diffraction at these conditions to be the Rh2O3(II)-type. Our experimental setup makes the orthorhombic o’-In2O3 polymorph available in large quantities for further physico-chemical characterization and provides an opportunity to grow o’-In2O3 as single crystals.

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Figure 2. In situ energy-dispersive XRD patterns in multi-anvil assemblies of a rh-In2O3 specimen compressed at 9.0 GPa and heated up to 600 °C. The tick marks refer to the calculated Bragg positions of o’-In2O3 (bottom) and rh-In2O3 (top). Arrows indicate the complete phase transition rhIn2O3  o’-In2O3 (1) and the partial o’-In2O3 transformation to rh-In2O3 (2). Figure 1. a) A section of the enthalpy–pressure (H–p) diagram for indium oxide polymorphs; cIn2O3 is a reference structure. Arrows indicate transitions (1) rh-In2O3  o’-In2O3 and (2) c-In2O3  o’-In2O3. b) The relative enthalpy (per formula unit of In2O3) between o’-In2O3 and rh-In2O3 polymorphs at 0, 2, 4, and 6.4 GPa.

Table 1: Phase composition of initial and recovered materials.[a] Specimen

rh-In2O3

o’-In2O3

o-InOOH

(R c, Z=6)

(Pbcn, Z=4)

(P21nm, Z=2)

starting material (Figure 3a)

100%, a=5.4814 (5) c=14.4998(3)

-

-

recovered from 9 GPa/ 600 °C (Figure 3b)

15.9% a=5.4795(4) c=14.4224

80% a=7.9295(1) b=5.4821(2) c=5.5898(6)

4.1% a=5.2587(9) b=4.5660(5) c=3.2669(6)

recovered from 8 GPa/ ca. 1100 °C (Figure 3c)

31.5% a=5.4803(5) c=14.4484(1)

63.8% a=7.9208(1) b=5.4881(6) c=5.5977(1)

4.7% a=5.2611(8) b=4.5673(3) c=3.2709(4)

[a] Fraction (wt%) and lattice parameters a, b, c [Å].

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Figure 3. Structure refinement of the starting material rh-In2O3 (a) and specimens recovered from the in situ multi-anvil cell (b) and toroid (c) experiments, showing observed and calculated intensities. Tick marks refer to Bragg reflections of o’-In2O3, rh-In2O3, and o-InOOH (bottom). Table 1 summarizes the results of the structure refinement.

Figure 4. Coordination, density, and interatomic distances (in Å) in In2O3 polymorphs at ambient pressure. In and O atoms are shown as small and large balls, respectively.

Notes and references [1]

[2] [3] [4] [5] [6]

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a) B. Xu, H. Stokes, J. J. Dong, J. Phys. Condens. Matter 2010, 22, 315403; b) A. Möller, P. Schmidt, M. Wilkening, Nachr. Chem. 2009, 57, 239-251; c) F. J. Manjón, D. Errandonea, Phys. Status Solidi B 2009, 246, 9-31. C. Wessel, R. Dronskowski, J. Solid State Chem. 2013, 199, 149-153. G. K. Rozenberg, L. S. Dubrovinsky, M. P. Pasternak, O. Naaman, T. Le Bihan, R. Ahuja, Phys. Rev. B 2002, 65, 064112. J. F. Lin, O. Degtyareva, C. T. Prewitt, P. Dera, N. Sata, E. Gregoryanz, H. K. Mao, R. J. Hemley, Nat. Mater. 2004, 3, 389-393. H. Yusa, T. Tsuchiya, J. Tsuchiya, N. Sata, Y. Ohishi, Phys. Rev. B2008, 78, 092107. A. Gurlo, D. Dzivenko, P. Kroll, R. Riedel, Phys. Status Solidi RRL 2008, 2, 269-271.

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[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

a) D. Liu,W. W. Lei, B. Zou, S. D. Yu, J. Hao, K.Wang, B. B. Liu, Q. L. Cui, G. T. Zou, J. Appl. Phys. 2008, 104, 083506; b) J. Qi, J. F. Liu, Y. He, W. Chen, C. Wang, J. Appl. Phys. 2011, 109, 063520. A. Gurlo, P. Kroll, R. Riedel, Chem. Eur. J. 2008, 14, 3306-3310. a) M. Epifani, P. Siciliano, A. Gurlo, N. Barsan, U. Weimar, J. Am. Chem. Soc. 2004, 126, 4078-4079; b) A. Gurlo, S. Lauterbach, G. Miehe, H.-J. Kleebe, R. Riedel, J. Phys. Chem. C 2008, 112, 9209-9213. T. Irifune, Mineral. Mag. 2002, 66, 769-790. E. Horvath-Bordon, R. Riedel, A. Zerr, P. F. McMillan, G. Auffermann, Y. Prots, W. Bronger, R. Kniep, P. Kroll, Chem. Soc. Rev. 2006, 35, 987-1014. M. Schwarz, T. Barsukova, C. Schimpf, D. Šimek, C. Lathe, D. Rafaja, E. Kroke in HASYLAB Users´Meeting, Hamburg, Germany, 2010. M. F. Bekheet, M. Schwarz, M. Mueller, S. Lauterbach, H. J. Kleebe, R. Riedel, A. Gurlo, RSC Adv., 2013, 3, 5357-5360. L. G. Khvostantsev, V. N. Slesarev, V. V. Brazhkin, High Pressure Res. 2004, 24, 371-383. A. N. Christensen, N. C. Broch, Acta Chem. Scand. 1967, 21, 1046-1056. a) D. A. Rusakov, A. A. Belik, S. Kamba, M. Savinov, D. Nuzhnyy, T. Kolodiazhnyi, K. Yamaura, E. Takayama-Muromachi, F. Borodavka, J. Kroupa, Inorg. Chem. 2011, 50, 3559-3566; b) A. A. Belik, T. Furubayashi, H. Yusa, E. Takayama-Muromachi, J. Am. Chem. Soc. 2011, 133, 9405-9412; c) C. A. Hoel, J. M. G. Amores, E. Moran, M. A. Alario-Franco, J. F. Gaillard, K. R. Poeppelmeier, J. Am. Chem. Soc. 2010, 132, 16479-16487.

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Thermodynamic Control of Phase Composition and Crystallization of Metal-Modified Silicon Oxycarbides E. Ionescu,a C. Terzioglu,a C. Linck,a J. Kaspar,a A. Navrotsky,b and R. Riedela a

Technische Universität Darmstadt, Institut für Materialwissenschaft, D-64287 Darmstadt, Germany Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis, Davis, California 95616

b

J. Am. Ceram. Soc., 96(6) (2013) 1899–1903.

Silicon oxycarbides modified with main group or transitionmetals (SiMOC) are usually synthesized via pyrolysis of sol-gel precursors from suitable metal-modified orthosilicates or polysiloxanes. In this study, the phase composition of different SiMOC systems (M = Sn, Fe, Mn, V, and Lu) was investigated. Depending on the metal, different ceramic phases formed. For M = Mn and Lu, MOx/SiOC ceramic nanocomposites were formed, whereas other compositions revealed the formation of M/SiOC (M = Sn), MSix/SiOC (M = Fe) or MCx/SiOC (M = V) upon pyrolysis. The different phase compositions of the SiMOC materials are rationalized by a simple thermodynamic approach which generally correctly predicts which type of ceramic nanocomposite is expected upon ceramization of the metal-modified precursors. Calculations show that the thermodynamic stability of the MOx phase with respect to that of the C–O system is the most important factor to predict phase formation in polymer-derived SiMOC ceramic systems. A secondary factor is the relative stability of metal oxides, silicates, carbides, and silicides. Experimental Procedures The synthesis of the precursors was performed as described elsewhere for SiZrOC and SiHfOC7,8 via chemical modification of a polysilsesquioxane (MK Belsil PMS; Wacker, Burghausen, Germany) with Fe(acac)3, Mn(acac)3, V(acac)3, VO(acac)2, Sn(ac)2, and Lu(ac)3 (ac = acetate; acac = acetylacetonate). Thus, each 5 g of polysilsesquioxane PMS was reacted with the corresponding amount of metal precursor at room temperature. For the reactions with the Fe, Mn, V, and Sn containing precursors, xylene was used as a solvent, whereas the reaction with Lu(ac)3 was performed in acetone. The amount of the metal precursor was chosen to obtain after pyrolysis a weight ratio between SiOC and a possible MOx phase (lowest oxide, which was assumed to precipitate) close to 70:30. To calculate the needed amounts of metal precursors, a ceramic yield of 81 wt% upon conversion of PMS into SiOC has been taken into account.7,8 In Table I, the amounts of the metal precursors used for the chemical modification of PMS is presented. Thus, the SiMOC-based ceramics were expected to exhibit similar MOx contents, between 30.9 and 36.7 wt% (see Table I). After mixing PMS with the metal precursor, the reaction solution was stirred for 2 h at room temperature. Subsequently, the solvent was removed under vacuum (10-2 mbar). The metal-modified precursors were cross-linked at 250°C and pyrolyzed in argon at 1100°C. The ceramic yield of the precursor-to-ceramic transformation processes showed values between 51.6 and 71.6 wt% (Table I).

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Table I. Amounts of PMS and Metal Precursors Used as well as Ceramic Yields of the Syntheses of SiMOC Samples

Sample

Metal

PMS (g)

Metal precursor (g)

SiOC SiFeOC SiSnOC SiMnOC SiLuOC SiVOC SiVOC

Fe(III) Sn(II) Mn(III) Lu(III) V(III) V(IV)

5 5 5 5 5 5 5

7.8 3.1 8.6 3.2 9.0 6.9

Expected content of MOx in SiMOC (wt%) 34.1 36.7 36.2 30.9 33.1 36.3

Ceramic yield (wt%)

Phase composition upon Pyrolysis at 1100°C

81.00 67.02 71.60 61.62 71.50 51.61 64.58

a-SiOC Fe3Si/a-SiOC Sn/a-SiOC MnSiO3/a-SiOC Lu2O3/a-SiOC V8C7/a-SiOC V8C7/a-SiOC

Results and Discussion Pyrolysis of the metal-containing polyorganosiloxanes in Ar atmosphere at 1100°C results in the formation of SiMOC ceramics, which were shown by XRD to exhibit different crystalline phase compositions [Fig. 1(a)]. In SiFeOC, Fe3Si was observed, while the tincontaining precursor gave a Sn/SiOC ceramic composite. In both cases, Fe(III) and Sn(II) were reduced to Fe(0) (as in Fe3Si alloy) and Sn(0). It is thought that the reducing conditions during the pyrolysis of the precursors are responsible for the formation of the metallic phases and are mainly due to the release of hydrogen and CO upon ceramization.7 The Sn/SiOC ceramic did not change phase composition when annealed at 1300°C; whereas in Fe3Si/SiOC the crystallization of Fe5Si3 and b-SiC was found under the same conditions [Fig. 1(b)]. Similar behavior was reported previously for Fe3Si/SiCNO.18 Pyrolysis of the Mn-containing precursor led to a poorly crystalline SiMnOC ceramic. The XRD pattern revealed the presence of MnSiO3 [Fig. 1(a)], which was also observed upon annealing at 1300°C [Fig. 1(b)]. It is assumed that the phase separation of MnO (at temperatures between 800°C and 1100°C) and its subsequent reaction with the phase-separated silica at higher temperature leads to the formation of the MnSiO3 phase. Such formation of binary and ternary oxides is analogous to the behavior observed in SiZrOC and SiHfOC.7,9 However, the formation of MnSiO3 occurs at lower temperatures than those for ZrSiO4 and HfSiO4, which crystallize at temperatures exceeding 1400°C.7,9 Similar results were obtained in the case of the lutetiummodified precursor. Thus, at 1100°C poorly crystallized Lu2O3 was identified by XRD, whereas at 1300°C crystalline Lu2Si2O7 was found (Fig. 1). Different behavior was found for the vanadium-modified precursor. At both temperatures a poorly crystalline V8C7 was detected, which can result from the reaction of vanadium oxide with excess carbon (Fig. 1, as for SiVOC prepared upon pyrolysis of the V(ac)3modified precursor). Interestingly, both precursors, i.e., the V(III)- and the V(IV)-modified polysilsesquioxanes led upon pyrolysis to the crystallization of V8C7 (i.e., formation of SiOC/V8C7 nanocomposites). The strong effect of the precursor composition on the phase evolution upon ceramization reflects the reducing conditions during pyrolysis and annealing. Thus, it is obvious that the thermodynamic stability of the metal oxides generated during pyrolysis plays a crucial role. To assess this effect in more detail, thermodynamic data for the oxides (MOx) were used, as depicted in the Ellingham diagrams in Fig. 2. Since all samples were synthesized under the same pyrolysis conditions, the partial pressures of the volatiles (i.e., CO, CO2,

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H2, and CH4) were not considered explicitly here. However, since carbon is present in all cases, it is appropriate to make a direct comparison between the CO–C and MOx – M equilibria. Since carbon is present in large amount in the investigated samples, the oxygen fugacity is determined by the equilibrium 2 C + O2  2CO.

Fig. 1. X-ray diffraction (XRD) patterns for SiMOC (M = Fe, Sn, Mn, Lu, V) pyrolyzed at 1100°C (a) and 1300°C (b).

Fig. 2. Ellingham diagrams showing the Gibbs free energy change of different oxides with respect to the system C–O (the gray areas correspond to the temperature range in which our samples were prepared, i.e., between 1100°C and 1300°C). Oxides with Gibbs free energies located in the area above the CO line will get reduced by carbon to their corresponding metals upon CO gas release; whereas those located in the area below the CO line will be stable against conversion into metals (data taken from Ref. [24]).

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Conclusion In this study, we show that the thermodynamic stability of MOx with respect to the system C–O plays a crucial role within the context of the ceramization process of metalmodified polymers. Based on thermodynamic data of the respective oxides, the phase composition of SiMOC/SiMCNO ceramics upon annealing at high temperatures can be predicted for different metals. The prediction agrees with the experimental results from this study and those reported in the literature for both SiMOC and SiMCNO ceramic composites. However, in addition to the stability of the oxides with respect to reduction, some other aspects must be taken into account for predicting the phase composition of SiMOC/SiMCNO composites, such as thermodynamic stabilization through conversion into silicates (for MOx being stable with respect to carbothermal conversion into M) or into silicides or carbides (for MOx not being stable against carbothermal reduction). These factors are summarized in Fig. 3. A more rigorous computation of the thermodynamics of crystallization could employ free energy minimization techniques. However, this would require some knowledge or assumptions about the free energies of the metals dissolved in the initially homogeneous ceramics. Such information is not currently available. The main point of this study is that even a very simple thermodynamic approach predicts the observed phases formed with remarkable accuracy.

Fig. 3. Predicted phase compositions of SiMOC and SiMCNO upon pyrolysis at 1100°C–1300°C. The oxides of the red marked metals are stable with respect to their reduction and thus SiOC/MO x nanocomposites are expected. Depending on the stability of the corresponding silicates (MSiO x), solid-state processes between MO and the phase-separated silica may occur, as observed for the case of Mn (crystallization of MnSiO3) and Lu (Lu2Si2O7) in this study. The oxides of the blue colored metals are not stable with respect to reduction by carbon. Consequently, SiOC/M nanocomposites are predicted to form here. Also in this case, the relative thermodynamic stability of the corresponding silicides or carbides will determine whether SiOC/MSi x or SiOC/MCx nanocomposites will be generated.

References [1] E. Ionescu, C. Linck, C. Fasel, M. Müller, H. J. Kleebe, and R. Riedel, “Polymer-Derived SiOC/ZrO2 Ceramic Nanocomposites With Excellent High-Temperature Stability,” J. Am. Ceram. Soc., 93 [1] 241–50 (2010). [2] E. Ionescu, B. Papendorf, H. J. Kleebe, F. Poli, K. Muller, and R. Riedel, “Polymer-Derived Silicon Oxycarbide/Hafnia Ceramic Nanocomposites. Part I: Phase and Microstructure Evolution During the Ceramization Process,” J. Am. Ceram. Soc., 93 [6] 1774–82 (2010). [3] A. Francis, E. Ionescu, C. Fasel, and R. Riedel, “Crystallization Behavior and Controlling Mechanism of Iron-Containing Si-C-N Ceramics,” Inorg. Chem., 48 [21] 10078–83 (2009). [4] E. Ionescu, B. Papendorf, H. J. Kleebe, and R. Riedel, “Polymer-Derived Silicon Oxycarbide/Hafnia Ceramic Nanocomposites. Part II: Stability Toward Decomposition and Microstructure Evolution at T»1000 Degrees C,” J. Am. Ceram. Soc., 93 [6] 1783–9 (2010). [5] T. B. Reed, Free Energy of Formation for Binary Compounds. MIT Press, Cambridge, MA, 1971.

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Structure Research In the year 2013, we completed the home-made MBE-setup for metallic films. Now we can grow thin metallic samples in Ultra High vacuum and transfer them, without braking the vacuum, into a small x-ray baby chamber (see separate report). The baby chamber is equipped with a hemisperical aluminum window and is not only compatible with our sixcircle diffractometer, but also with comparable instruments at various synchrotron sources. Several neutron scattering campaigns were carried out in an effort to quantify the structure and dynamics of defects in Ba-doped bismuth sodium titanate. In addition to measurements of the diffuse scattering, we used the extended x-ray absorption fine structure to characterize the local environment of the different species. Data evaluation is under way. Staff Members Head

Prof. Dr. Wolfgang Donner Prof. Dr. Dr. h.c. Hartmut Fueß

Research Associates

Dr. Joachim Brötz Dr. Marton Major

Dr. Ljubomira Schmitt Dr. Azza Amin

Technical Personnel

Dipl. Ing. Heinz Mohren Jean-Christophe Jaud Maria Bense

Ingrid Svoboda Sabine Foro

PhD Students

M. Sc. Qiran Li M. Sc. Marco Léal

Dipl.-Ing. Florian Pforr Dipl.-Ing. Dominik Stürmer

Master

Michael Brilz

Guest Scientists

Prof. Dr. Ismael Saadoune, Université Cadi Ayyad, Maroc

Secretary

Prof. Dr. Anouar Njeh, University of Sfax, Tunesia

Research Projects Structural investigations into the electric fatigue in piezo-ceramics (DFG-SFB, 2011-2014) Development of electrode materials for high capacitance devices (IDS-FunMat, 2013-2015) Phase transitions in thin potassium sodium niobate films (IDS-FunMat, 2012-2015) Influence of biaxial strain and texture on the elastic properties of Barium Strontium Titanate thin films (AvH Lab Partnership, 2013-2015) Publications [1] Siol, Sebastian; Straeter, Hendrik; Brueggemann, Rudolf; Broetz, Joachim; Bauer, Gottfried H.; Klein, Andreas; Jaegermann, Wolfram; PVD of copper sulfide (Cu2S) for PIN-structured solar cells; JOURNAL OF PHYSICS D-APPLIED PHYSICS Volume: 46 Issue: 49 Article Number: 495112 (2013)

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[2] Pfeifer, Verena; Erhart, Paul; Li, Shunyi; Rachut, Karsten; Morasch, Jan; Broetz, Joachim; Reckers, Philip; Mayer, Thomas; Ruehle, Sven; Zaban, Arie; Mora Sero, Ivan; Bisquert, Juan; Jaegermann, Wolfram; Klein, Andreas; Energy Band Alignment between Anatase and Rutile TiO2; JOURNAL OF PHYSICAL CHEMISTRY LETTERS Volume: 4 Issue: 23 Pages: 41824187 (2013) [3] Labrini, Mohamed; Saadoune, Ismael; Scheiba, Frieder; Almaggoussi, Abdelmajid; Elhaskouri, Jamal; Amoros, Pedro; Ehrenberg, Helmut; Broetz, Joachim; Magnetic and structural approach for understanding the electrochemical behavior of LiNi0.33Co0.33Mn0.33O2 positive electrode material; ELECTROCHIMICA ACTA Volume: 111 Pages: 567-574 (2013) [4] Muench, Falk; Oezaslan, Mehtap; Rauber, Markus; Kaserer, Sebastian; Fuchs, Anne; Mankel, Eric; Broetz, Joachim; Strasser, Peter; Roth, Christina; Ensinger, Wolfgang; Electroless synthesis of nanostructured nickel and nickel-boron tubes and their performance as unsupported ethanol electrooxidation catalysts JOURNAL OF POWER SOURCES Volume: 222 Pages: 243-252 (2013) [5] Z.K. Heiba, M.B. Mohamed, H. Fuess, Structural and magnetic properties of Sm2-xMnxO3 nanoparticles, Materials Research Bull. 48, 3750-3755, 2013 [6] J. P. Patel, A. Senyshyn, H. Fuess, D. Pandey Evidence for weak ferromagnetism, isostructural phase transition, and linear magnetoelectric coupling in the multiferroic Bi0.8Pb0.2Fe0.9Nb0.1O3 solid solution Phys. Rev. B 88 (10) 104108, 2013 [7] A. Senyshyn, O. Dolotko, M.J. Mühlbauer, K. Nikolowski, H. Fuess, H. Ehrenberg Lithium Intercalation into Graphite Carbons Revisited: Experimental Evidence for Twisted Bilayer Behavior J. Electrochem. Soc. 160 (5) A 3198-A 3205, 2013 [8] S. Bhattacharjee, A. Senyshyn, H. Fuess, D. Pandey Morin-Type spin-reorientation transition below the Neel transition in the monoclinic compositions of (1-x) BiFeO3-xPbTiO(3) (x= 0.25 and 0.27): A combined dc magnetization and x-ray and neutron powder diffraction study Phys. Rev. B 87 (5) 05417, 2013 [9] H. Ehrenberg, A. Senyshyn, M. Hinterstein, H. Fuess 16. In Situ Diffraction Measurements: Challenges, Instrumentation, and Examples. In E.J. Mittermeijer & U. Welzel (Eds)., Modern Diffraction Methods (528). Weinheim: Wiley-VCH [10] Epitaxial growth and control of the sodium content in NaxCoO2 thin films Hildebrandt, S; Komissinskiy, P; Major, M; Donner, W; Alff, L THIN SOLID FILMS Volume: 545 Pages: 291-295 DOI: 10.1016/j.tsf.2013.08.072 Published: OCT 31 2013

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[11] Synthesis, structure and magnetic properties of Ni(II)-Co(II) heterodinuclear complexes with ONNO type Schiff bases as ligands Oz, S; Titis, J; Nazir, H; Atakol, O; Boca, R; Svoboda, I; Fuess, H POLYHEDRON Volume: 59 Pages: 1-7 DOI: 10.1016/j.poly.2013.04.047 Published: AUG 1 2013 [12] Local structure, pseudosymmetry, and phase transitions in Na1/2Bi1/2TiO3K1/2Bi1/2TiO3 ceramics Levin, I; Reaney, IM; Anton, EM; Jo, W; Rodel, J; Pokorny, J; Schmitt, LA; Kleebe, HJ; Hinterstein, M; Jones, JL PHYSICAL REVIEW B Volume: 87 Issue: 2 Article Number: 024113 DOI: 10.1103/PhysRevB.87.024113 Published: JAN 31 2013

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A portable X-ray analysis chamber with in vacuo vertical transfer Azza Amin, Herry Wedel, Michael Weber, Jochen Rank, and Wolfgang Donner The x-ray diffraction analysis of reactive surfaces requires an Ultra High Vacuum (UHV) environment and, at the same time, an x-ray transparent window. Furthermore, because of space constraints on x-ray diffractometers, a UHV analysis chamber has to be lightweight, compact and transferable. We designed and built a portable x-ray analysis chamber with a hemispherical aluminum window. The sample holder can be moved vertically from the transfer position to the measurement position using a dedicated in vacuo mechanism.

Fig. 1: Left: cross section of the UHV baby chamber in transfer position. The sample holder (dark green) is transferred through the left CF-flange (yellow). Right: sample holder (dark green) in measurement position inside the hemisperical window.

Figure 1 shows a cross section through the chamber: the black boxes represent the ion getter pumps, which are battery-driven to facilitate transferabilty. The stainless steel body ends with a CF100 flange that carries the x-ray window, mashined out of a solid piece of high-strength Al alloy. Since the transfer of the sample holder from the growth chamber takes place through the (yellow) CF38 flange on the left, the sample holder has to be lifted into the x-ray window for measurements. In order to be in an eucentric position of the diffractometer, the total height of the sample surface must not exceed 170 mm. This prohibits the use of outside bellows or magnetic transfer rods for the vertical movement. Instead, the vertical transfer has to be in vacuo. This design makes the chamber unique. A stepper motor-driven worm drive takes the entire sample stage, including heater and thermocouple contacts, on a 50 mm travel. All parts of the transfer mechanism are made of UHV compatible materials and are therefore fully bakable. Figure 2 shows an example of x-ray diffraction measurements made possible by the chamber: a radial scan along the surface normal ([H0H]-direction) of a thin indium film on tungsten. The film has been grown at a temperature of 130 K, annealed at 350 K, and then transferred. Oscillations due to the finite thickness of the sample (Laue oscillations) are visible on either side of the Bragg peak. A preliminary fitting reveals the thickness (24 monolayers) and the reason for the pronounced asymmetry of the Laue oscillations: the coherent epitaxial growth leads to a strain gradient close to the film-substrate interface.

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Fig. 2: Radial scan along the [H 0 H] direction of a 24 monolayer

(6.7nm)

epitaxial

indium

W(001).

The

film

thin on

asymmetric

Laue oscillations in the data (circles) can be reproduced assuming a model with a vertical

gradient

in

the

lattice parameter (straight line).

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Materials Analysis The Materials Analysis group participates in two of the five Research Clusters of the Technische Universität Darmstadt: New Materials and Nuclear and Radiation Science. On the one hand the group is concerned with the characterization of self-synthesized modern materials, on the other hand with effects on materials caused by exposition to detrimental influences like ion irradiation. The research aims for clarification of the correlation of materials properties and synthesis or exposition parameters, respectively, by investigation of the elemental composition and the chemical binding. Current research topics are: Advanced 3-D Nanoobjects: Nanochannels, -wires, -tubes, and –networks: In collaboration with the GSI Helmholtz Centre for Heavy Ion Research, nanoporous membranes are formed by ion irradiation of polymer foils producing latent ion damage tracks which are chemically etched to nanochannels. These ion track (nano) filters can be used for filtering particles from liquids, collecting aerosols, for gas separation, and for analyzing small (bio)molecules. In the latter case, the nanochannel walls are chemically modified so that the nanochannel sensor becomes sensitive and selective to certain molecular species. Apart from polymer-based nanochannels, anodically oxidized aluminium (AAO) is used. Filling the polymer or AAO nanochannels galvanically with metals, such as copper, gold or platinum, and dissolving the templates, nanowires are formed. Here, different metal deposition conditions are used in order to obtain monometal but also multimetal (e.g. CuCo- and CuFe) nanowires. By redox-chemical reactions, the nanochannel walls can be coated with metal or metal oxide films, such as Ni, Cu, Ag, Au, Pt, Pd, and ZnO, SnO2, TiO2, In2O3, FexOy. Thus, nanotubes can be formed. Here, different morphologies are available, ranging from smooth compact nanotube walls to nanoporous walls to rough or peaked structures. When the nanochannels are crossed, the resulting nanowires are interconnected, forming nanowire networks. Dimensions, surface topography, microstructure, and crystallinity of these nanostructures are investigated. Macroscopic properties such as thermal stability, electrical conductivity and catalytic activity are analysed. Additionally, the obtained properties are evaluated with respect to applications as sensors, for gas flow or acceleration measurements, catalysts, for chemical reactions in microreactors, or electrodes in fuel cells. Thin film and coating deposition and analysis: In thin film and coating technology, the identification of chemical compounds, phases and binding conditions is of basic importance. Different methods are used for the formation of thin films (nanofilms), thick films and coatings. Surface modifications and layer deposition are performed via a plasma process. With plasma immersion ion implantation (PIII) it is possible to alter several surface properties by ion implantation. Different gaseous species are used such as oxygen, nitrogen and hydrocarbons, depending on the property to be modified, e.g. hardness, wear resistance, lifetime and biocompatibility. Using hydrocarbon gases films of diamond-like carbon (DLC) are deposited. Research topics are the adhesion of the DLC films to different substrates and the influence of the addition of different elements, especially metals, to the DLC films. The films are investigated for their chemical and phase composition, microstructure, adhesion, and in relation to biological applications, tribological properties, corrosion and wear Institute of Materials Science - Materials Analysis

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protection of metal substrates, wettability, and temperature stability. Since the PIII technique is also suitable for complex shaped substrates, the treated substrates also include samples such as tubes, where the focus in on the treatment of their inner surfaces. Oxide films, such as lead-free piezoelectrics like sodium potassium niobate (NKN), are prepared by the sol-gel technique combined with spin coating. The addition of NKN powder to the films is investigated as a means to increase the film thickness. Materials in radiation fields: Irradiation of materials with energetic particles (protons, heavy ions) and electromagnetic radiation (X-rays, gamma-rays) may lead to degradation of the materials’ properties. This happens to components in space vehicles, in nuclear facilities and in particle accelerators. Polymers with their covalent bonds are particularly sensitive towards ionizing radiation. Polyimide, vinyl polymers and fiber-reinforced polyepoxides, which are components of superconducting beam guiding magnets at the future FAIR synchrotron and storage rings, oxides such as alumina which are used as beamdiagnostic scintillator screens, and semiconductor components such as CCDs are irradiated and characterized for their properties, such as polymeric network degradation, mechanical strength, electrical resistance, dielectric strength, and optical properties. Apart from basic questions on material’s degradation mechanisms by energetic radiation, the investigations are used to estimate service life-times of the materials/components. Staff Members Head

Prof. Dr. Wolfgang Ensinger

Research Associates

Dr. Mubarak Ali Dr. Adam G. Balogh Dr. Stefan Flege Dr. Ruriko Hatada

Dr. Peter Hoffmann Dr. Falk Münch Dr. Quoc Hung Nguyen

Technical Personnel

Renate Benz

Brunhilde Thybusch

PhD Students

Anton Belousov Eva-Maria Felix Umme Habiba Hossain Martin Hottes Renuka Krishnakumar Stephan Lederer Alice Lieberwirth

Vincent Lima Saima Nasir Cornelia Neetzel Tim Seidl Christian Stegmann Sebastian Wiegand

Diploma and Master Alexandra Bobrich Students Nico Dams Rene Fischer Anja Habereder Ulla Hauf

Nicolas Jansohn Mario Klaric Pejman Khamegir Sandra Schäfer

Bachelor Students

Adjana Eils Carolin Fritsch Tim Hellmann

Christoph Kober Jona Schuch David Wieder

Guest Scientists

Prof. Dr. Takaomi Matsutani Takehiko Matsuya Evgenija Ermakova

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Research Projects Preparation of lead free piezo electric thin films (LOEWE centre AdRIA 2008–2014) Simulations on the influence of swift ion irradiation on materials of FAIR-components (GSI, 2010–2013) NanoC – Preparation, modification and characterization of nanochannels in polymer membranes (Beilstein-Institut, 2009–2013) NanoMag – Spin-dependent scattering in magnetic and Kondo nanowires (Beilstein-Institut, jointly with Goethe Universität Frankfurt am Main, 2009–2013) 3-Dimensional micro-nano-integration for gas flow sensor technology (BMBF, 2011–2013, jointly with Institut für Elektromechanische Konstruktionen, TU Darmstadt) Electromechanical sensors with one-dimensional nano objects (BMBF, 2011–2013, jointly with Institut für Elektromechanische Konstruktionen, TU Darmstadt) Beam diagnosis and radiation damage diagnosis – Scintillator materials for high current diagnosis (BMBF/GSI 2012–2015) Beam diagnosis and radiation damage diagnosis – radiation damage of accelerator components made out of plastics and countermeasures (BMBF/GSI 2012-2015) New technologies for efficient solar energy systems (DFG, 2012–2013) Investigation of technologically important nanostructured materials by high resolution ion beam analysis (DLR, 2012-2013) Publications [1] F. Muench, M. Oezaslan, M. Rauber, S. Kaserer, A. Fuchs, E. Mankel, J. Brötz, C. Roth, W. Ensinger; Electroless Synthesis of Nanostructured Nickel and Nickel-Boron Tubes and their Performance as Unsupported Ethanol Electrooxidation Catalysts, JOURNAL OF POWER SOURCES, 222 (2013) 243-252. [2] S. Wiegand, S. Flege, O. Baake, W. Ensinger; Effect of different calcination temperatures and post annealing on the properties of 1,3 propanediol based Sol-Gel (Na0.5K0.5)NbO3 (NKN) thin films; JOURNAL OF ALLOYS AND COMPOUNDS, 548 (2013) 38-45. [3] F. Muench, A. Fuchs, E. Mankel, M. Rauber, S. Lauterbach, H.-J. Kleebe, W. Ensinger; Synthesis of nanoparticle / ligand composite thin films by sequential ligand self assembly and surface complex reduction; JOURNAL OF COLLOID AND INTERFACE SCIENCE, 389 (2013) 23-30. [4] K. Baba, R. Hatada, S. Flege, W. Ensinger, Y. Shibata, J. Nakashima, T. Sawase, T. Morimura; Preparation and antibacterial properties of Ag-containing diamond-like carbon films prepared by a combination of magnetron sputtering and plasma source ion implantation; VACUUM, 89 (2013) 179-184.

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[5] W. Ensinger, E. Marin, L.Guzman; Ion beam based composition and texture control of titanium nitride; VACUUM 89 (2013) 229-232. [6] B. Pollakowski, P. Hoffmann, M. Kosinova, O. Baake, V. Trunova, R. Unterumsberger, W. Ensinger, B. Beckhoff; Non-destructive and non-preparative chemical nanometrology of internal material interfaces at tunable high information depths; ANALYTICAL CHEMISTRY, 85 (2013) 193-200. [7] S. Nasir, P. Ramirez, M. Ali, I. Ahmed, L. Fruk, S. Mafe, W. Ensinger; Nernst-Planck model of photo-triggered, pH–tunable ionic transport through nanopores functionalized with “caged” lysine chains; JOURNAL OF CHEMICAL PHYSICS, 138 (2013) 034709. [8] S. Wiegand, S. Flege, O. Baake, W. Ensinger; Effect of Different Calcination Temperatures and Post Annealing on the Properties of Acetic Acid Based Sol-Gel (Na0.5K0.5)NbO3 (NKN) Thin Films; JOURNAL OF MATERIALS SCIENCE & TECHNOLOGY, 29 (2013) 142-148. [9] K. Drogowska, S. Flege, D. Rogalla, H.-W. Becker, E. Ionescu, N.-T.H. Kim-Ngan, A.G. Balogh; Hydrogen content analysis in hydrogen-charged PZT ferroelectric ceramics; SOLID STATE IONICS, 235 (2013) 32-35. [10] M. N. Tahir, M. Ali, R. Andre, W. E. G. Müller, H. C. Schröder, W. Tremel, W. Ensinger; Silicatein conjugation inside nanoconfined geometries through immobilized NTA–Ni(II) chelates; CHEMICAL COMMUNICATIONS 49, (2013) 2210-2212. DOI: 10.1039/C3CC38605H [11] P. Ramirez, V. Gomez, M. Ali, W. Ensinger, S. Mafe; Net currents obtained from zero-average potentials in single amphoteric nanopores; ELECTROCHEMISTRY COMMUNICATIONS, 31 (2013) 137-140. [12] A. A. Younis, W. Ensinger, M. M. B. El-Sabbah, R. Holze; Corrosion protection of pure aluminium and aluminium alloy (AA7075) in salt solution with silane-based sol–gel coatings; MATERIALS AND CORROSION, 64 (2013) 276-283. [12] W. Ensinger, S. Flege, R. Hatada, S. Ayata, T. Matsutani, K. Baba; Hermetic Protection of Rings by Ion Beam Sputter Coating with a Broad Beam Ion Source and a W-Shaped Hollow Sputter Target; TRANSACTIONS OF THE MATERIALS RESEARCH SOCIETY OF JAPAN, 38 (2013) 97-100. [13] M. Pavlovič, M. Miglierini, E. Mustafin, W. Ensinger, A. Šagátová, T. Seidl, M. Šoka; Influence of xenon ion irradiation on magnetic susceptibility of soft-magnetic alloys; Proceedings of the 19th International conference on APPLIED PHYSICS OF CONDENSED MATTER (eds. J. Vajda, I. Jamnický), 2013, p. 78-81 [14] B. Lyson-Sypien, A. Czapla, M. Lubecka, E. Kusior, K. Zakrzewska, M. Radecka, A. Kusior, A.G. Balogh, S. Lauterbach, H.-J. Kleebe; Gas sensing properties of TiO2–SnO2 nanomaterials; SENSORS AND ACTUATORS B: CHEMICAL, 187 (2013) 445–454.

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[15] E. ElHaddad, W. Ensinger, C. Schüth; Untersuchungen zur Sorptionsreversibilität von organischen Schadstoffen in Aktivkohle, Holzkohle und Zeolith Y-200; GRUNDWASSER, 18 (2013) 197-202. [16] M. Ali, S. Nasir, I. Ahmed, L. Fruk, W. Ensinger; Tuning nanopore surface polarity and rectification properties through enzymatic hydrolysis inside nanoconfined geometries; CHEMICAL COMMUNICATIONS, 49 (2013) 8770-8772. [17] M. Ali, S. Nasir, P. Ramirez, J. Cervera, S. Mafe, W. Ensinger; Carbohydrate-Mediated Biomolecular Recognition and Gating of Synthetic Ion Channels; THE JOURNAL OF PHYSICAL CHEMISTRY C, 117 (2013) 18234-18242. [18] S. Wiegand, S. Flege, W. Ensinger; Comparison of the influence of titanium and chromium adhesion layers on the properties of sol-gel derived NKN thin films; JOURNAL OF SOL GEL SCIENCE AND TECHNOLOGY, 67 (2013) 654-659. [19] Z. Tarnawski, Nhu-T. H. Kim-Ngan, K. Zakrzewska, K. Drogowska, A. Brudnik, A. G. Balogh, R. Kužel, L. Havela, V. Sechovsky; Hydrogen storage in Ti–TiO2 multilayers; ADVANCES IN NATURAL SCIENCES: NANOSCIENCE AND NANOTECHNOLOGY, 4 (2013) 025004. [20] A.A. Younis, W. Ensinger, R. Holze; Impedance measurements at sol-gel based polysiloxane coatings on aluminium and its alloys; in: LECTURE NOTES ON IMPEDANCE SPECTROSCOPY: Volume 4, 99-106, Ed. O. Kanoun, CRC Press, 2013, ISBN 9781138001404. [21] S. Quednau, F. Dassinger, M. Hottes, C. Stegmann, W. Ensinger, H.F. Schlaak; Integration und Charakterisierung von Nanostrukturen in Mikrosysteme für sensorische Anwendungen; in: Proceedings: MIKROSYSTEMTECHNIK 2013, Eds. GMM, VDI/VDE-IT, ISBN 978-38007-3555-6. [22] K. Drogowska, S. Flege, H.-W. Becker, Z. Tarnawski, K. Zakrzewska, A.G. Balogh Physical properties of multilayer thin films of Ti-V and their hydrides studied by ion beam analysis methods in: Nanotechnology 2013: Advanced Materials, CNTs, Particles, Films and Composites (Volume 1), p. 124-127, Ed. Nano Science and Technology, Institute, Crc PressI Llc, ISBN: 978-1-4822-0581-7.

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Ag-containing Diamond-like carbon films deposited on the interior surface of a tube R. Hatada, S. Flege, A. Bobrich, T. Matsutani, W. Ensinger Adhesive Diamond-like carbon (DLC) films can be prepared by plasma immersion ion implantation (PIII) which is also suitable for the treatment of 3D objects because it is a nonline of sight technique. The incorporation of a metal into the DLC film provides a possibility to change the characteristics of the DLC film. For an improved biocompatibility Ag, Cu or TiO2 nanoparticles can be added. One commonly used combination for this purpose is simultaneous sputtering and hydrocarbon PIII. The properties of Ag-DLC films prepared by silver magnetron sputtering and acetylene PIII were reported in refs. [1, 2]. If the coating is to be done on the inner surface of a tube, however, there is usually the problem of an inhomogeneous distribution of the metal inside of the tube. There would be a strong gradient of the metal concentration with increasing distance from the metal source. Here, a different approach was developed. An auxiliary metal electrode along the central axis of the tube is normally implemented to achieve a more homogeneous thickness of the coating and to increase the energy of the plasma ions. This auxiliary electrode can be used to provide metal ions, as well. Selecting a silver (or silver covered) electrode and applying a negative DC voltage to it, sputtering of the electrode will occur which will then distribute Ag particles inside of the tube. So, a two step process was developed: in a first step a DLC film was deposited on the inside of the tube. A negative high voltage pulser was connected to the tube, whereas the auxiliary electrode was non-grounded. The latter was done to increase the plasma density within the tube by directing the electrons towards the outside of the tube. In the second step the tube was grounded and a negative DC voltage was applied to the auxiliary electrode. When the plasma ions hit the electrode, they remove Ag atoms from the electrode which are then incorporated into the DLC film. The composition of the plasma gas was also changed for the second step, from a hydrocarbon gas to a mixture of argon and hydrocarbons.

Fig. 1: The two steps of the coating process, left: DLC deposition, right: Ag sputtering. AE: auxiliary electrode.

The resulting coating consists of a DLC film with Ag nanoparticles in its outer surface. Earlier investigations [2] have shown that a few percent of Ag are already enough to achieve an antibacterial effect. The samples were characterized by secondary ion mass spectrometry depth profiling, X-ray photoelectron and Raman spectroscopy and atomic force microscopy. The antibacterial properties were checked by growth and survival tests of Staphylococcus aureus bacteria.

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The silver concentration of the DLC films ranged from 0.6 to 6.6 at.% depending on the chosen experimental conditions. Higher concentrations could be achieved using a higher pressure during the deposition process. The sputtering time was between 1 and 2 hours for the samples with the higher Ag concentrations. The silver is mainly located in the top surface area of the samples. In depth profiles the Ag intensity decreases slowly over the first 10 nm by about one order of magnitude. The slow decrease is due to the agglomeration of the Ag into nanoscale silver crystals as can be seen in TEM images [2] and due to the surface roughness of the samples. According to AFM images the average surface roughness of a pure DLC sample is 0.4 nm. The average surface roughness increases with Ag content up to 4.9 nm for the 6.6 at.% Ag sample. This is due to the mentioned silver agglomeration but also because of roughening caused by some sputtering of the DLC film during the second step of the process. The ID/IG value of the samples from the Raman measurement changed only slightly and was about 2.0. The survival test of the S. aureus bacteria shows an obvious difference between a pure DLC sample and one with 6.6 at.% Ag. While the sample on the left in Fig. 2 shows large areas with surviving bacteria, the sample on the right does not show any sign of survived bacteria. Here, the color is the indicator; the growing bacteria change the pH value of the medium and thus cause a phenol red indicator to change its color.

Fig. 2: Results of the bacterial survival test. Left: pure DLC coating, right: DLC coating with 6.6 at.% Ag. The brighter color on the left indicates bacterial growth.

It could be shown that it is possible to prepare DLC films containing silver on the inside of a tube by the combination of plasma immersion ion implantation and DC sputtering of an auxiliary electrode. Although the achievable Ag concentrations are only in the range of a few atomic percent, this is sufficient to cause an antibacterial effect as demonstrated with S. aureus bacteria. References: [1] [2]

K. Baba, R. Hatada, S. Flege, W. Ensinger, Advances in Materials Science and Engineering, 2012 (2012), p. 536853. K. Baba, R. Hatada, S. Flege, W. Ensinger, Y. Shibata, J. Nakashima, T. Sawase, T. Morimura, Vacuum, 89 (2013), pp. 179-184.

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New activation processes for the electroless synthesis of metal nanotubes Falk Münch, Wolfgang Ensinger One focus of the materials analysis group is the fabrication of one-dimensional metal nanomaterials such as nanotubes and nanowires by applying electroless plating to ion-track etched templates. This class of metallization reactions can be categorized as autocatalytical, surface-selective deposition from metastable solutions containing at least a metal complex and a reducing agent [1]. In order to initiate the plating reaction on the substrate, its surface usually has to be covered with metal nanoparticles which act as seeds for the metal film nucleation. These substrate pre-treatments are called activation processes. The synthesis of well-defined, complex metal nanomaterials is demanding concerning both the quality of the plating and activation reactions [1]. In recent studies [2,3], we introduced a highly flexible and easily scalable process for the activation of polymer substrates for consecutive electroless plating which is suitable for the synthesis of metal nanomaterials. It is based on the absorption of a reducing agent by a slightly swollen polymer substrate, followed by the precipitation of metal nanoparticles (Fig. 1).

Fig. 1: Synthetic scheme of the new activation technique [2]. 1) In the presence of a suitable solvent or solvent combination, the polymer substrate swells and absorbs a dissolved reducing agent (sensitization). 2) When the sensitized substrate is brought into contact with metal salt solutions, nanoparticles precipitate on its surface (activation). 3) Surface-conformal metal deposition is achieved by electroless plating. Copyright (2014) Springer Publishing.

The outlined technique can be applied to polymers with significantly differing chemical properties (e.g. ABS, PC, PET and PVA [3]) and allows to adjust the density, size and metal type of the seeds [2,3]. Therefore, the substrate activity can be tailored for obtaining optimum results in consecutive electroless depositions. Aside from a sufficient density of 82

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small seeds, a high catalytic activity in the corresponding plating reaction proved to be essential for the conformal metallization of challenging substrate morphologies with nanoscale homogeneity [2,3]. The presented method is not restricted to the fabrication of metal nanotubes (Fig. 2a,b), but can also be utilized for the preparation of two-dimensional films (Fig. 2c) and the metallization of macroscopic work pieces (Fig. 2d). Future studies aim to adopt the novel activation approach in the synthesis of well-defined, mono- and multimetallic nanotubes for application in heterogeneous catalysis and sensing.

Fig. 2: a) SEM image of a field of free-standing copper nanotubes (the template was removed with dichloromethane). b) Top-view of the nanotubes shown in a). c) SEM image of a silver film on ABS foil. d) LEGO®-block electrolessly covered with silver. Copyright (2014) Elsevier B.V.

References: [1] [2]

[3]

F. Muench, S. Lauterbach, H.-J. Kleebe, W. Ensinger, Deposition of Nanofilms inside a Polymer Template: Formation of Metal Nanotubes, E-J. SURF. SCI. NANOTECH. 10 (2012), 578-584. F. Muench, S. Bohn, M. Rauber, T. Seidl, A. Radetinac, U. Kunz, S. Lauterbach, H.-J. Kleebe, C. Trautmann, W. Ensinger, Polycarbonate activation for electroless plating by dimethylaminoborane absorption and subsequent nanoparticle deposition, APPL. PHYS. A: MAT. SCI. PROCESS. 2014 (in press), DOI: 10.1007/s00339-013-8119-z F. Muench, A. Eils, M. E. Toimil-Molares, U. H. Hossain, A. Radetinac, C. Stegmann, U. Kunz, S. Lauterbach, H.-J. Kleebe, W. Ensinger, Polymer activation by reducing agent absorption as a flexible tool for the creation of metal films and nanostructures by electroless plating, SURF. COAT. TECHNOL. 2014 (in press), DOI: 10.1016/j.surfcoat.2014.01.024

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Synthesis of oxidic copper and cobalt nanostructures of different morphologies T. Matsutani, C. Neetzel, F. Muench, W. Ensinger The investigation of bundled one-dimensional nanostructures by self-assembly methods can be easily carried out by anisotropic crystal growth which is directly related to the crystal structure. However, this process is limited to a few materials. In order to break the symmetry of the crystal growth, nanowire synthesis can be driven by screw dislocation where low precursor supersaturation and the presence of appropriate dislocation sources is essential. [1] In order to prepare morphology-controlled copper oxide/cobalt oxide heterostructures in micro- and nanometer scale, we investigated two different synthesis routes that are related to each other according to their mechanism but can be distinguished by their chemical reaction route. However, in both cases a ligand is required to build strong complexes resulting in low concentrations of free metal ions in the precursor solution. In the case of copper and cobalt ions, we compared ammonia as well as tartrate. The synthesis process via the ammonia route is accomplished with a three step route where firstly a copper ammonia complex is generated, followed by a ligand exchange process leading to the precipitation of copper hydroxide. By annealing, CuO is produced via dehydratation. Concentration ratios of the precursor solution as well as the resulting yields measured by EDX are listed in Table 1. According to the yield composition, the turnover for copper oxide is higher which can be explained by stronger copper ammonia complexes according to the related complex stability constants. Table 1: Composition and yield of CuO/Co3O4 heterostructures

Sample # precursor composition Cu2+z1Co2+z2 yield composition CuO(x)Co3O4(y)

(1)

(2)

(3)

(4)

(5)

z1:z2 = 1:0

z1:z2 = 7:1

z1:z2 = 5:1

z1:z2 = 3:1

z1:z2 = 1:10

x:y = 1:0

x:y = 8.6:1

x:y = 6.9:1

x:y = 4.1:1

x:y = 1.2:10

Figure 1 depicts the resulting morphologies of samples (1) to (5). Pure copper oxide deposition results in a network composed of needle-like structures with diameters of about 10 nm and lengths of 2 µm. With increasing Co3O4 and decreasing CuO content the structure morphology changes from needle-like architectures to plate-formed shapes. Obviously, the bundled network-like formation degenerates with decreasing copper content in the precursor-solution which indicates that its concentration is crucial for the onedimensional growth process in aqueous solution under these conditions. Sample (5) shows clearly identifiable thin plate structures of round shapes with diameters of approximately 5 µm. The reason for this observation can be explained by the crystal growth of the intermediate cobalt hydroxide complex. Co(OH)2 consists of a layered structure where neighboring layers are bound to each other by weak van der Waals forces. Thus, the (100) plane is stable. [1] 84

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For the fabrication of Cu2O nanostructures with low CoO contents in aqueous solution, we applied the well-known Fehling’s reagent with -D-glucose as reducing and tartrate as complexing agent. Tartrate generates a coordination complex with copper and cobalt metal ions which on one hand decreases the concentration of the related free metal ions in the solution and on the other hand prevents the undesirable precipitation of Co(OH) 2 and Cu(OH)2. Additionally to the earlier procedure, the concentration of the reducing agent as well as the concentration of copper and cobalt salts in the precursor solution is indispensable. As listed in Table 2, the specific concentration of the Fehling and reducing solution were varied in our experiments. First, we synthesized pure Cu2O structures and varied the concentration of copper metal ions in the precursor solution (samples (6) and (7)). As it is clearly recognizable (see Figure 3(a) and 3(b)) needle-like structures can be obtained by adding a low concentration (Table 2) of metal ions. An increasing amount of copper(II)ions already causes the formation of particles with diameters of about 250 nm. Therefore, we kept the concentration of the Cu2+ precursor constant and varied the concentration of cobalt ions in order test the morphology influence of this species in a heterostructural precipitation. By adding 1:1 = Cu2+:Co2+ (sample (8)) we observed cubic like structures with uniform lateral edges of 100 nm as shown in Fig. 3. EDX as well as XRD measurements (not shown here) confirm the precipitation of both cobalt and copper oxidic structures. Table 2: Composition of initial Cu2+ and Co2+ for Fehling's reaction

Sample # CuSO4.5H2O [mol/L] CoSO4.7H2O [mol/L] Na2C4H4O6 [mol/L]

(6)

(7)

(8)

(9)

0.004

0.006

0.004

0.004

-

-

0.004

0.0004

0.005

0.007

0.009

0.005

We expected to obtain structural analogies to sample (7) by keeping the concentration of copper precursor ions constant and adding a 1:0.1 quantity of cobalt ions (sample (9)). However, SEM investigations show that the morphology seems to be different (Fig. 3(d)). Particles with diameters of approximately 50 nm agglomerate together at certain positions resulting in a crossed one-dimensional growth in each direction. Thus, branched networks could be obtained. The reason for these occurrences could be that -D-glucose acts as surfactant and promotes the growth in a certain direction as it also described in the literature. [2] On the other hand, such morphologies were not obtained for sample (7); therefore, the addition of cobalt ions leads to a drastic change of the structural material architecture. However, this synthesis route seems to be more sensitive towards small changes of concentration in the precursor solution. Moreover, the influence of the reducing material should be considered accurately for further investigations. The results indicate that, dependent on the precursor concentration of copper and cobalt ions, it is possible to prepare morphology controlled heterostructures in a reproducible way. Further investigations will be carried out for the fabrication of needle-like structures with high Co3O4 contents.

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Fig. 1: CuO/Co3O4 heterostructures of different ratios: (a) sample (1), (b) sample (2), (c) sample (3), (d) sample (4)

Fig. 2: CuO/Co3O4 sample (5)

nanoplates:

Fig. 3: CuO nanoneedles obtained by precipitation of Cu(OH)2 and annealing

References: [1] [2]

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Y. Li, Y. Wu, Chem. Mater. 2010, 22, 5537-5542. G. Filipic, U Cvelbar, Nanotechnol., 2012, 23, 1-16

Institute of Materials Science- Materials Analysis

Materials Modelling Division The research of the Materials Modelling Division is focused on multi-physics modelling of defect structures in functional oxides, metallic nanoalloys and energy materials. We are combining electronic structure calculations with atomistic modelling methods and continuum descriptions depending on time and length scales involved. Quantum mechanical calculations based on density functional theory are used for electronic structure calculations. Large-scale molecular dynamics with analytical interatomic potentials are the method of choice for studying kinetic processes and plastic deformation. Kinetic lattice Monte-Carlo simulations are extensively used for simulations of diffusional and transport processes on extended time scales. The group is operating several HPCcomputers and has access to the Hessian High Performance Computers in Frankfurt and Darmstadt. The current research topics are:  Functional oxides o New lead-free ferroelectrics: Ordering effects and defects o Defects and diffusion in TCOs o Finite-size effects in oxide nanoparticles  Nanoalloys o Plasticity of nanocrystalline metals and bulk metallic glasses o Metallic nanoglasses o Nanophase diagrams o Metallic nanoparticles under nanoextrusion  Energy materials o Interfaces in Li-intercalation batteries o Defects in CIS/CIGS absorber materials o High-pressure phases of nitrogen o Interfaces in Superalloys (Mo-Si-B) Within the bachelor program the Materials Modelling Division is offering classes on thermodynamics and kinetics as well as defects in materials. Lectures and lab classes on simulation methods and programming techniques are offered as elective courses in both, the bachelor and master program.

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Staff Members Head

Prof. Dr. Karsten Albe

Emeritus Professor

Prof. Dr. Hermann Rauh, M.A., C.Phys., F.Inst.P., F.I.M.

Secretary

Renate Hernichel

Research Associates

PD Dr. Yuri Genenko Dr. Galina Yampolskya Dr. Sergey Yampolskii Dr. Alexander Stukowski Dr. Jochen Rohrer Dr. Omar Adjaoud Dr. Uma Maheswari Sankara Subbiah Dr. Marc Radu Dr. Sabrina Sicolo

Scientific Employees PhD Students

M.Sc. Heide Humburg

Master Students

Konstanze Kalcher, Markus Mock

Bachelor Student

Leonie Koch

Research Fellow

Dr. Guang-Tong Ma (AvH)

Dipl.-Phys. Johan Pohl Dipl.-Ing. Jonathan Schäfer Dipl.-Ing. Manuel Diehm Dipl.-Ing Melanie Gröting Dipl.-Ing. Jonathan Schäfer Dipl.-Ing. Arno Fey Dipl.-Ing. Kai Meyer Dipl.-Ing. Tobias Brink M.Sc. Olena Lenchuk M.Sc. Nam Ngo

Research Projects Mikrostruktur und Stabilität von Nanogläsern (DFG AL 578/6-2) Quantenmechanische Computersimulationen zur Elektronen- und Defektstruktur oxidischer Materialien (SFB 595, Teilprojekt C1, 2007-2014) Atomistische Computersimulationen von Defekten und deren Bewegung in Metalloxiden (SFB 585, Teilprojekt C2, 2003-2014)

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Phänomenologische Modellierung von Injektion, Transport und Rekombination in Bauelementen aus organischen Halbleitern sowie aus nichtorganischen Ferroelektrika (SFB C5, 2003-2014) Erforschung der Phasenstabilität und Niederdrucksynthese von festem Stickstoff mittels atomistischer Computersimulationen und Experimenten (DFG AL 578/3-2, 2010–2014)) Beyond Ni-Base Superalloys: Atomistische Modellierung des Einflusses von Legierungszusätzen auf die Korngrenzeigenschaften in Mo-Si-B und Co-Re Superlegierungen (DFG Forschergruppe 727, AL 578/9-1, 2010–2013) Nanosilicon dispersed in SiCN(O) and SiCO-based ceramic matrices derived from preceramic polymers: new composite anode materials for lithium ion batteries. (DFG SPP 1473 „Wendelib“, DFG AL 578/10-1, 2011–2013) Mechanische und kinetische Eigenschaften metallischer Sekundärphasen (DFG AL578/13-1, 2011–2013)

Gläser

mit

nanoskaligen

Bleifreie Piezokeramiken, LOEWE-Schwerpunkt ADRIA (HMWK, 2011-2014) PPP Finnland, Atomic level simulations of structure and growth of nanoalloys (DAAD 2011–2013) Topological Engineering of Ultra-Strong Glasses (DFG AL 578/15-1, 2012-2014) Modeling the electrocaloric effect in lead-free relaxor ferroelectrics: A combined atomisticcontinuum approach (DFG AL 578/16-1, 2012-2014) HZB-Helmholtz Zentrum Berlin, Virtuelles Institut (HZB VH-VI-520 2012-2017)

Publications K. Albe, Y. Ritter and D. Şopu, 'Enhancing the plasticity of metallic glasses: Shear band formation, nanocomposites and nanoglasses investigated by molecular dynamics simulations', Mechanics of Materials 67, 94 (2013) H. Rauh and G. T. Ma, 'Hysteretic ac loss of a superconductor strip subject to an oscillating transverse magnetic field: Geometrical and electromagnetic effects', J. Appl. Phys. 114, 193902 (2013) S. Zhukov, Y. A. Genenko, M. Acosta, H. Humburg, W. Jo, J. Rödel, H. von Seggern, Heinz, Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2 Ti0.8)O3x(Ba0.7Ca0.3)TiO3 ferroelectrics', Appl. Phys. Lett. 103, 152904 (2013) A. Kobler, J. Lohmiller, J. Schäfer, M. Kerber, A. Castrup, A. Kashiwar, P. A. Gruber, K. Albe, H. Hahn, C. Kübel, 'Deformation-induced grain growth and twinning in nanocrystalline palladium thin films', Beilstein J. Nanotechnol. 4, 554 (2013)

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J. Schäfer and K. Albe, 'Plasticity of nanocrystalline alloys with chemical order: on the strength and ductility of nanocrystalline Ni–Fe', Beilstein J. Nanotechnol. 4, 542 (2013) Y. A. Genenko, J. Wehner and H. von Seggern, 'Self-consistent model of polarization switching kinetics in disordered ferroelectrics', J. Appl. Phys. 114, 084101 (2013) G. T. Ma and H. Rauh, 'Thermo-electromagnetic properties of a magnetically shielded superconductor strip: theoretical foundations and numerical simulations', Supercond. Sci. Technol. 26,105001 (2013) J. Rohrer and K. Albe, 'Insights into Degradation of Si Anodes from First-Principle Calculations', J. Phys. Chem. C 117, 18796 (2013) S. Goel, A. Stukowski, X. Luo, A. Agrawal and R. L. Reuben, 'Anisotropy of single-crystal 3C–SiC during nanometric cutting', Modelling Simul. Mater. Sci. Eng. 21, 065004 (2013) P. Erhart, P. Träskelin and K. Albe, 'Formation and switching of defect dipoles in acceptordoped lead titanate: A kinetic model based on first-principles calculations', Phys. Rev. B 88, 024107 (2013) K. Nonnenmacher, H.-J. Kleebe, J. Rohrer, E. Ionescu, R. Riedel and G. Soraru, 'Carbon Mobility in SiOC/HfO2Ceramic Nanocomposites', J. Amer. Ceram. Soc. 96, 2058 (2013) J. Pohl and K. Albe, 'Intrinsic point defects in CuInSe2 and CuGaSe2 as seen via screenedexchange hybrid density functional theory', Phys. Rev. B 87, 245203 (2013) K. A. Avchaciov, Y. Ritter, F. Djurabekova, K. Nordlund and K. Albe, 'Controlled softening of Cu64Zr36 metallic glass by ion irradiation', Appl. Phys. Lett. 102, 181910 (2013) A. Tolvanen and K. Albe, 'Plasticity of Cu nanoparticles: Dislocation-dendrite-induced strain hardening and a limit for displacive plasticity', Beilstein J. Nanotechnol. 4, 173 (2013) H. S. Ruiz, A. Badia-Majos, Y. A. Genenko and S. V. Yampolskii, 'Strong Localization of the Density of Power Losses in Type-II Superconducting Wires', IEEE Trans. Appl. Superconduct. 23, 8000404 (2013) S. Li, J. Morasch, A. Klein, C. Chirila, L. Pintilie, L. Jia, K. Ellmer, M. Naderer, K. Reichmann, M. Gröting, and K. Albe, 'Influence of orbital contributions to the valence band alignment of Bi2O3, Fe2O3, BiFeO3, and Bi0.5Na0.5TiO3', Phys. Rev. B 88, 045428 (2013) Proceedings W. Witte, M. Powalla, D. Hariskos, A. Eicke, M. Botros, H.-W. Schock, A. Abou-Ras, R. Mainz, H. Rodríguez-Alvarez, T. Unold, G. H. Bauer, R. Brüggemann, S. J. Heise, O. Neumann, M. Meessen, J. Christen, F. Bertram, A. Klein, T. Adler, K. Albe, J. Pohl, M. Martin, R. A. De Souza, L. Nagarajan, T. Beckers, C. Boit, J. Dietrich, M. Hetterich, Z. Zhang, R. Scheer, H. Kempa and T. Orgis, 'Chemical Gradients in Cu(In,Ga)(S,Se)2 ThinFilm Solar Cells: Results of the GRACIS Project', In: 27th European Photovoltaic Solar Energy Conference and Exhibition, Frankfurt am Main. EU PVSEC Proceedings (2013). 90

Institute of Materials Science - Materials Modelling Division

Degradation of Si anodes studied with first-priniciple methods Jochen Rohrer and Karsten Albe Silicon is considered as promising anode material for Li-ion batteries [1]. However, despite its high mass-specific capacity, which is approximately ten times larger than that of commercially used graphitic anodes, Si undergoes rapid degradation. The details of this degradation are complex. However, roughly we can distinguish two failure modes. The first is related to the consumption of electrolyte and a build up of a thick solid-electrolyte interphase. The second is related to internal degradation of Si itself by means of cracking and pulverization. Here [2] we focus on internal degradation. We use density-function-theory calculations to study the structure and thermodynamics of amorphous LixSi (x representing the ratio of Li to Si) that forms during Li intercalation and deintercalation. Our calculations predict the existence of critical two-phase regions during initial lithiation and initial delithiation. Within two-phase regions, large local and inhomogeneous volume changes may lead to crack initiation. We also point out, how these two-phase regions can be avoided and degradation due to internal cracking is minimized.

Fig. 1: Iterative replica scheme to generate amorphous Li-Si model structures.

Figure 1 illustrates our protocol used to generate model structures of amorphous LixSi with varying Li content. Starting from a pure crystalline Si model consisting 64 atom, LixSi models with increasing Li content x are generated using an iterative replica scheme. In each step, five replica of the current LixSi model are created and in each of the replica, eight Li atoms are inserted to construct candidate structures of Lix’Si. Each candidate structure is then subjected to an equilibration run using ab initio molecular-dynamics simulations at 700 K. The equilibrated systems are finally fully optimized and the lowest-energy system is chosen as Lix’Si model for the current Li concentration. The procedure is iterated up to a concentration of x=4.75. Deintercalcation is modeled accordingly by removing Li.

In Figure 2 we show calculated formation energies of Li-Si alloys as a function of the Li content x. The left panel focuses on intercalation in crystalline Si. In agreement with hightemperature experiments [3], Li4.4Si is identified as global minimum. Initially, at low Li content, a phase separation into crystalline Si and amorphous Li2.0Si is thermodynamically favorable. Within this two-phase region, local inhomogeneous volume changes of +130% take place and crack initiation can be expected. For x>2, a homogeneous one-phase region is predicted and cracking is suppressed. At to x=3.625, we find a saddle point and further intercalation again leads to a two-phase region where amorphous Li3.625Si and Li4.4Si coexist. At the saddle point, the thermodynamic driving force for further intercalation vanishes. This might be interpreted as a potential reason for crystallization of Li15Si4, which is typically observed at room temperature [4]. Institute of Materials Science - Materials Modelling Division

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Fig. 2: Structure and formation energies of amorphous LixSi model systems.

The right panel of Figure 2 focuses on delithiation. For a maximum Li content of x>3.75, initial delithiation leads to two-phase regions (either Li4.4Si/Li3.625Si or Li15Si4/Li2.0Si). Within these two-phase regions, there are again large inhomogeneous volume changes for which crack formation can be expected; -15% in the former or -40% in the latter two-phase region. At lower Li contents, a homogeneous one-phase region is predicted and full delithiation leads to amorphous Si. Reinsertion into amorphous Si then proceeds homogeneously and essentially without crack formation. From x=3.625 on, amorphous Si behaves similar to crystalline Si. In summary, our calculations identify various two-phase regions occurring during Li intercalation or deintercalation in Si anodes. During corresponding phase conversion, large local and inhomogeneous volume changes may initiate cracks which subsequently lead to degradation due to particle fracturing. In amorphous Si, two-phase regions occur only during delithiation and only if the Li content is larger than x=3.75. Thus, limiting the Li content well below this critical value leads to homogeneous volume expansion and contraction and minimizes internal particle fracture. As a consequence, capacity limited cycling of amorphous Si anodes [5] leads to significantly enhanced cycling stability. References: [1] [2] [3] [4] [5]

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U. Kasavajjula, C. Wang and J. A. Appleby, J. Power Sources 163, 1003(2007). J. Rohrer and K. Albe, J. Phys. Chem. C 117, 18796 (2013). R. A Sharma and R. N. Seefurth, J. Electrochem Soc. 123, 1763 (1976). M. N. Obrovac and L. Christensen, Solid-State Lett. 7, A93 (2004). A. Magasinski et al., Nat. Mater. 9, 353 (2010).

Institute of Materials Science - Materials Modelling Division

Low Temperature Heat Capacity of a Severely Deformed Metallic Glass Jonas Bünz, Tobias Brink, Koichi Tsuchiya, Fanqiang Meng, Gerhard Wilde, Karsten Albe Metallic glasses show distinct mechanical, electrical, and magnetical properties from their crystalline counterparts. Like all glassy materials, they show a low-frequency peak in the vibrational spectrum in excess of the Debye law and the spectrum of crystals. This so-called boson peak is due to quasi-localized transverse vibrational modes associated with “defective” soft local structures in the glass.[1-4] The boson peak, situated in the terahertz region of the vibrational spectrum, also leads to an excess contribution to the lowtemperature heat capacity. This becomes visible by plotting the heat capacity c devided by T3, so that the Debye T3-law reduces to a constant. Several different models for the structural origin of the boson peak have been proposed but they all agree that its origins are related to decreased elastic constants in spatially distributed regions. Plastic deformation in metallic glasses is localized in structurally disturbed regions called shear bands. The shear bands are therefore expected to influence the boson peak. To analyze this contribution of the shear bands, we performed heat capacity measurements on Zr50Cu40Al10 metallic glass. The glass was deformed under hydrostatic pressure torsion, which produces a large volume of regions with structural changes that can be considered a “macroscopic shear band”.[5] Heat capacity measurements were performed using differential scanning calorimetry at low temperatures before and after deformation. In addition, the undeformed and deformed samples were annealed and heat capacity was measured again. The results are shown in Figure 1a. The boson peak of the as-cast glass does not significantly change upon annealing. The deformation-induced boson-peak is increased over the as-cast state and reduces again with annealing. If the annealing temperature is high enough, the boson peak will even reduce below the as-cast state. X-ray diffraction measurements show no detectable crystallization. To elucidate the exact origins of the change in heat capacity, we Fig. 1 Difference in heat capacity between glass and crystal. a) conducted molecular dynamics Experimental data, annealing below 393 K was done for 7 days, simulations on a deformed annealing at 743 K for 10 s. b) Simulation data, dashed lines are sample of Cu64Zr36 metallic glass. data from shear bands, solid lines from the matrix. Institute of Materials Science - Materials Modelling Division

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We identified atoms belonging to the shear band using the von Mises local shear invariant η as implemented in OVITO.[6] We marked atoms with η > 0.2 as belonging to the shear band and all others as the matrix. The phonon density of states (PDOS) was calculated[7] separately for shear band and matrix and is shown in Figure 2. Using the harmonic approximation of the free energy, we calculated the heat capacity from the PDOS as Fig. 2 Phonon density of states from computer simulation. shown in Figure 1b. Solid lines in Dashed lines are data from shear bands, solid lines from the these graphs represent data from matrix. the matrix. Our results show that the boson peak of the matrix stays unchanged during deformation and annealing. All changes result from the shear band, represented by dashed lines. Upon deformation the boson peak increases as in the experiment. Subsequent annealing relaxes the shear bands again to a state with reduced boson peak. The simulations do not show a reduction of the boson peak below the as-cast state. The reason is that the shear bands do not fully relax in the computationally accessible time (40 ns in our simulations). To confirm a possible transition to a state similar to the experiment, a much longer time scale would be needed. All in all our findings clearly show the shear bands to be the origin of the deformationinduced boson peak, while the matrix stays intact during all processing. The experiment supports this perspective as the change of the boson peak height for the undeformed sample is minimal during annealing. Several studies find healing of shear bands under annealing even at temperatures well below Tg.[8-10] In contrast, our measurements indicate that the structure of the annealed shear band differs from the initial as-cast state. Diffusion studies show accelerated diffusion constants in shear bands, which could promote structural relaxation in these regions. The boson peak is an excess over the ordered state, therefore the results of decreasing boson peak suggest short or medium range ordering. Even beginning crystallization on the nanoscale is conceivable, as this would not be detectable using X-ray diffraction.

References: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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S. N. Taraskin et al., Phys. Rev. Lett. 86, 1255 (2001) H. Shintani and H. Tanaka, Nat. Mater. 7, 870 (2008) H. R. Schober, J. Non-Cryst. Solids 357, 501 (2011) A. I. Chumakov et al., Phys. Rev. Lett. 106, 225501 (2011) F. Meng et al., Appl. Phys. Lett. 101, 121914 (2012) A. Stukowski, Modelling Simul. Mater. Sci. Eng. 18, 015012 (2010) J. M. Dickey and A. Paskin, Phys. Rev. 188, 1407 (1969) W. H. Jiang et al., Acta Mater. 53, 3469 (2005) S. Xie and E. P. George, Acta Mater. 56, 5202 (2008) Y. Ritter and K. Albe, Acta Mater. 59, 7082 (2011)

Institute of Materials Science - Materials Modelling Division

Hysteretic ac loss of a superconductor strip subject to an oscillating transverse magnetic field H. Rauh1 and G.T. Ma1,2 1 Institute of Materials Science, Darmstadt University of Technology, 64287 Darmstadt 2 Applied Superconductivity Laboratory, Southwest Jiaotong University, 610031 Chengdu Thin type-II superconductor strips, plates and tapes have lately become the focus of much attention in scientific and engineering research, since they are deemed promising elements for both large-scale power and microelectronic device applications. It is not surprising therefore that their response to imposed ac transport currents and applied ac magnetic fields, or both, has been studied intensively. One of the most important characteristics regarding their use is the hysteretic ac loss caused by excitations of the named sort. Applied ac magnetic fields – our concern here – have been addressed in analytical calculations as well as numerical analysis. Selected aspects for superconductor strips thereby include, e.g., the effects of demagnetization fields or those of external magnetic shields. However fundamental these investigations may be, a systematic examination of geometrical and electromagnetic effects owing to the superconductor strips themselves does not seem to have come forth yet. An obvious approximation at hand is the limit of infinitesimally thin superconductor strips, ignoring variations of the induced current and the electromagnetic field across the thickness of the strips. This ansatz has frequently been taken as a basis for theoretical explorations and, particularly, invoked for estimating the hysteretic ac loss; it has the distinct advantage of getting along with the concept of sheet current which allows representations in mathematically comprehensible forms. Such an approach needs justifying though by establishing its range of validity in tests against models that contain surplus traits. Assessments of the influence on the hysteretic ac loss of the width/thickness aspect ratio of a superconductor strip, with an ac transport current imposed, already exist. We aim at similar investigations for a superconductor strip, with an ac magnetic field applied, which crucially rely on an adequate characterization of the strip. Like on a different occasion before, we go back to a ‘smoothed’ Bean model of the critical state made up of a current-field relation derived from experiment. We confine ourselves to a purely electromagnetic account, without coupling to a thermal field; that is, the environment of the superconductor strip – in practice pervaded by a cooling liquid – is simply treated as a vacuum. Such a course has proven to yield very accurate results even at high amplitudes of the applied magnetic field. To appraise geometrical and electromagnetic effects on the distributions of the magnetic induction, the electric field, the current density, the power loss density inside the superconductor strip, and whence on the hysteretic ac loss suffered by the superconductor strip due to the presence of an oscillating transverse magnetic field, numerical simulations were carried out, understanding that the strip is made of an yttrium-barium cuprate and operated at the liquid nitrogen temperature of 77 K. We choose geometrical and materials data that second-generation superconductors typically display. Proceeding from a superconductor rod with quadratic cross section, i.e. a degenerate shape of the superconductor strip for which the width/thickness aspect ratio e  2w d equals unity, we studied the evolution of the above properties when the thickness of the strip d was Institute of Materials Science - Materials Modelling Division

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reduced, while the width 2w was kept fixed, such that the aspect ratio followed the geometric progression e  1, 10, 100, 1000. In this procedure, the critical current density at operating temperature/zero field was adjusted according to jc0  eI c0 4w2 with the total critical current I c0 ,emulating its dependence on the thickness of the superconductor strip as compared  to a superconductor bulk. Some of our numerical evaluations are shown graphically below.   Fig. 1 portrays the distribution of the magnetic induction B inside the superconductor strip for different values of the amplitude of the applied magnetic field H a in the electromagnetic steady state. Generally, this distribution reveals symmetry about the two mirror planes of the superconductor strip. For thelowest amplitude of the applied magnetic field, H a  1 A mm , the magnetic induction B is essentially confined  to the marginal parts of the strip, leaving most of its interior free of magnetic flux; the spatial localization and the strength of the magnetic induction B near the edges of the strip build up when the aspect ratio e augments. For the slightlyincreased amplitude of the applied magnetic field,  H a  5 A mm , the penetration of the magnetic field into the interior of the strip already becomes tangible, giving rise  to a pronounced inhomogeneous distribution of the magnetic induction B throughout the strip; the spatial localization and the strength of the magnetic  induction B near the edges of the strip again build up when the aspect ratio e augments. For the highest amplitude of the applied magnetic field, H a  500 A mm , penetration of the magnetic field into the interior of the strip is developed to the full, bringing about a  virtually homogeneous profile of the magnetic induction B across the thickness of the strip   the amplitude of the as the aspect ratio e augments. Concisely and overall, an increase of  applied magnetic field H a eventually gives rise to complete filling of the strip with magnetic flux, while the variations of the magnetic induction B in the direction of the  applied magnetic field die away as the thickness of the strip abates.

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Fig. 1: Distribution of the magnetic induction B (unit T ) inside a superconductor strip in the electromagnetic steady state, calculated at maximum strength of an applied transverse magnetic field with amplitude H a  1 A mm (top), H a  5 A mm (centre) and H a  500 A mm (bottom), assuming the values of the aspect ratio e  1 , 10, 100, 1000. The scale of of the strip is enlarged by the respective factor e in each the thickness  case. 

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Fig. 2 illustrates the dependence of the normalized hysteretic ac loss U ac H a2 on H a H c , the normalized amplitude of the magnetic field applied to the superconductor strip in the electromagnetic steady state, addressing a series of values of the aspect ratio e , together with the prediction for the limiting case of an infinitesimally thin strip in the Bean model of  field H  the critical state. This introduces the characteristic magnetic c , which may be expressed as H c  I c0 2w , remembering the definition of the critical sheet current of such  and materials data a strip, and thus takes on the value H c  8 A mm from the geometrical implied. A general trait revealed for whichever choice of the aspect ratio e is that, starting  fromthe smallest value of the normalized amplitude of the applied magnetic field H a H c , a monotonic rise of the normalized hysteretic ac loss U ac H a2 towards a maximum occurs,  followed by an asymptotically converging descent, as H a H c augments. Whereas the  geometrical effect controlled by the aspect ratio e is minute at large values  of H a H c , it becomes prominent at low values of H a H c where the normalized hysteretic ac loss  U ac H a2 abates rapidly, with a waning gradient,  as the aspect ratio e augments. The Bean model of the critical state adapted to an infinitesimally thin strip, often used  for convenient  mathematical analysis neglecting variations of electromagnetic observables (like the  magnetic induction or the current density) across the thickness of the strip, obviously underestimates the normalized hysteretic ac loss U ac H a2at low and moderate values of 2 H a H c , but overestimates, or at least conserves, the normalized hysteretic ac loss U ac H a at large values of H a H c .

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Fig. 2: Normalized hysteretic ac loss U ac H a2 suffered by a superconductor strip as a function of the normalized amplitude H a H c of the applied transverse magnetic field in the electromagnetic steady state, assuming the values of the aspect ratio e  1 , 10, 100, 1000. The analytical result of an infinitesimally thin strip in the Bean model of the criticalstate (dashed lines) is shown for comparison. 

In conclusion, a theoretical  approach with simultaneous regard of finite-geometrical aspects and electromagnetic traits is called for if the hysteretic ac loss suffered by the superconductor strip is to be reliably addressed. This is especially true in the range of low and moderate amplitudes of the applied magnetic field, where ascertainments based on Bean’s model of the critical state adapted to an infinitesimally thin strip yield clear underestimates of the said observable. On the other hand, this limit seems sufficient for determining the hysteretic ac loss at high amplitudes of the applied magnetic field – irrespective of the real value of the aspect ratio of the strip – if only the field dependence of the induced current is taken into account.

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Materials for Renewable Energies Research in the Renewable Energies group focuses on electrochemical energy technologies, such as fuel cells and batteries. Novel catalysts, electrodes and electrode processing techniques are being developed, but also sophisticated methods for their in-situ characterization. Systematic structural and electrochemical characterization of the new materials is carried out in order to unravel the structure-properties correlation. Techniques used for structure analysis include X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), and X-ray diffraction (XRD), whereas the electrocatalytic performance is tested in both model experiments and under realistic operation conditions. The group’s recent scientific activities can be divided into the following three areas:

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New catalyst concepts

Our main focus is on the design of alternative support materials for fuel cells, which do not suffer from corrosion in the severe operation conditions and may be promising candidates to replace the standard carbon support. Different morphologies, e.g. fibres or hollow spheres, contribute to an improved control in 3D electrode design and thus allow for an efficient mass transport. Furthermore, shape-selected nanoparticles exposing highly active crystal facets are being investigated and indicate improved electrocatalytic activities.

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Functional electrode design

Beyond the conventional preparation techniques, advanced layer-by-layer (LbL) techniques are used in the fabrication of fuel cell electrodes allowing for a well-defined 3D architecture. A likewise promising approach, which also offers high flexibility and a facile up-scaling, is the electrospinning technique. Thin fibres with solid, porous, but also coreshell structure can be spun and deposited as an arbitrary mesh or in an aligned fashion. These structures have been used as electrodes in both fuel cells and batteries. Electron microscopy is applied for the electrodes’ detailed characterization. For this specific purpose, new techniques have been developed and established in the group, as for instance the focused ion beam (FIB) technique in cooperation with the HZB, Berlin. FIB/SEM was applied to obtain 3D reconstructions of the porous fuel cell electrodes before and after operation as well as for comparison of the different electrode processing techniques.

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In situ studies

In situ and operando X-ray absorption studies play an important role in our activities with respect to the systematic investigation of reaction and degradation mechanisms. A versatile in situ sample environment has been designed and successfully implemented at various synchrotron facilities. It enables the spatial and time resolved study of different areas of the fuel cell electrodes in various operation conditions (direct methanol, direct ethanol operation, but also for intermediate temperature PBI fuel cell studies). In addition to the

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conventional EXAFS analysis the novel delta µ XANES technique is applied in cooperation with Prof. David Ramaker, George Washington University. This technique enables us to study adsorbates attached to the active catalyst surface, so that reaction mechanisms can be followed directly during operation. The results provide important insights, which will help to further catalyst optimization. In 2012, the delta µ XANES technique has been applied to intermediate temperature PBI fuel cells. At the cathode side, the adsorption of phosphoric acid species could be followed temperature and potential dependent. The effect of anode humidification on CO poisoning at different temperatures was also studied, and the importance of water being present at the anode side underlined. In July 2012, Prof. Christina Roth was appointed full professor at the FU Berlin and now heads the group Applied Physical Chemistry. Financial support is provided by DFG, BMWi, BMBF, and EU as well as by the respective synchrotron facilities and industrial partners.

Staff Members Head

Prof. Christina Roth

Secretary

Maria Bense (joint with Prof. Donner and Prof. Xu)

PHD students

Dipl.-Ing. (FH) Hanno Butsch Dipl.-Ing. Benedikt Peter Dipl.-Ing. André Wolz

Diploma students

Anja Habereder

Dipl.-Ing. Sebastian Kaserer Dipl.-Ing. Alexander Schökel

Research Projects German-Canadian fuel cell cooperation (BMWi project 2010-2013) New developments in intermediate temperature fuel cells (EU project 2010-2012) New concepts for a controlled 3D design of porous electrodes (DFG project 2010-2013)

Publications [1] F. Muench, M. Oezaslan, M. Rauber, S. Kaserer, A. Fuchs, E. Mankel, J. Brötz, P. Strasser, C. Roth, W. Ensinger, Electroless Synthesis of Nanostructured Nickel and Nickel-Boron Tubes and their Performance as Unsupported Ethanol Electrooxidation Catalysts, Journal of Power Sources 222 (2013), 243 – 252.

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[2] S. Kaserer, K. M. Caldwell, D. E. Ramaker, C. Roth, Analyzing the Influence of H3PO4 as Catalyst Poison in High Temperature PEM Fuel Cells Using in-operando X-ray Absorption Spectroscopy, J. Phys. Chem. C 117 (2013), 6210−6217. [3] S. Kaserer, C. Rakousky, J. Melke, C. Roth, Design of a reference electrode for high-temperature PEM fuel cells, J. Appl. Electrochem. (2013) DOI: 10.1007/s10800-013-0567

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Incorporation of Indium Tin Oxide Nanoparticles in PEMFC Electrodes André Wolz, Susanne Zils, David Ruch, Nicholas Kotov, Marc Michel, Christina Roth

Introduction Carbon materials suffer from corrosion at the cathode of polymer electrolyte membrane fuel cells (PEMFCs). In the presence of water, carbon support materials are oxidized to carbon dioxide even at low potentials. Hence, nowadays it is very fashionable to look for alternative support materials, like oxides or conductive polymers. The choice of a support material other than carbon black makes it mandatory to think about the preparation method of the electrode layer and its resulting electrode structure. The nano-sized oxide particles have to be assembled differently from the sub-micrometer sized carbon black particles to yield an equally promising structure. A schematic of such an electrode design is depicted in Fig. 1.

Fig. 1. Schematic of a 3D electrode design incorporating nano-sized oxide support particles (Pt/ITO) and Nafion-coated multi-walled carbon nanotubes (MWCNT/Nafion) into a fast sprayed multi-layer electrode.

The electrode structure is known to have a significant impact on the cell performance [1]. A homogeneous and porous structure favors mass transport of the reactants, and a good accessibility of the Pt nanoparticles results in a high Pt utilization. Recently, a novel electrode preparation technique has been introduced by which it was possible to assemble 1D support materials into 3D networks [2, 3]. The networks had a multilayered architecture of polyaniline and carbon nanotubes, both decorated with Pt, and the electrodes reached 3 times higher Pt utilizations at the cathode side than conventional electrodes. This technique is referred to as the ‘fast multilayer’ technique. The same technique was used by Zils et al. [4] to manufacture electrodes with carbon black material and Nafion layers, which were then compared to an airbrushed MEA with the same catalyst composition. Focused ion beam tomography (FIB) measurements revealed a Institute of Materials Science - Materials for Renewable Energies

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much more homogenous structure with a small average pore size for the multilayer electrode than for the airbrushed one. Single-cell tests furthermore demonstrated a two times higher Pt utilization showing the suitability of this technique as a fast and easy method for fuel cell electrode preparation. This study shows the results for the incorporation of nano-sized alternative support materials into advanced electrode architectures. It will give a first impression of how oxide nanoparticles can be assembled in a fuel cell electrode. The obtained results will bridge the gap between the previous results of electrochemical studies and the performance as catalyst material in a real fuel cell environment.

Experimental Pt decoration of ITO nanoparticles ITO nanoparticles (NP) were decorated with Pt NP after a reduction of PtCl4 precursor by sodium borohydride (NaBH4). 120 mg ITO NP and 51.8 mg PtCl4 (99.99+%) were dispersed in 20 ml ultrapure water (MilliQ - MQ), the amount of PtCl4 corresponding to a Pt loading of 20 wt%. After a homogenous dispersion was obtained, a solution of 51.8 mg NaBH4 in 10 ml MQ was added. The dispersion turned black immediately. Afterwards, the solution was diluted with deionized water, filtered through a 0.02 µm Anopore™ Inorganic Membrane (Whatman®) and dried at 30°C under vacuum. Functionalization of the multiwall carbon nanotubes (MWCNT) The MWCNT were treated in concentrated acids in order to functionalize their surface and to remove remaining amorphous carbon species. 20 mg MWCNT were dispersed in 6 ml HNO3 (p.a., 65%) and 6 ml H2SO4 (ACS reagent, 95-98%) and sonicated for 30 min. Afterwards, the nanotubes were diluted with copious amounts of MQ, filtered with a 0.45 µm polycarbonate track-etch membrane, rinsed with MQ water and dispersed in a solution of 5 ml ethanol and MQ (80:20 by volume). A mixture of 0.34 ml Nafion ® solution in 5 ml ethanol/MQ (80:20) solution was prepared and the MWCNT dispersion added dropwise to the ionomer dispersion. As the second ink,40 mg of Pt/ITO (20 wt% Pt loading) was dispersed in 10 ml ethanol/MQ solution (80:20). Electrode preparation Polymer electrolyte membranes of Nafion® 117 were purchased from Ion Power Inc., USA. The membrane was mounted in a home built spraying plate with vacuum feature. The plate was heated up to 80°C and a solution of 80 mg Pt on Vulcan-XC72 (HiSPEC™ 3000, Johnson Matthey), 0.4 ml Nafion® 117 solution (5%), 3.6 ml MQ, and 12 ml ethanol was coated onto the membrane by EcoSpray containers (Labo Chimie France). The cathode was assembled by alternating layers of Pt/ITO and MWCNT/Nafion® (referred to as multilayer electrode: ML-MEA). For comparison, a second electrode was sprayed, consisting of a standard Pt/CB anode with the same loading used before and a Pt/CB cathode, with a Pt loading and Nafion® content equal to the Pt/ITO cathode.

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Structural characterization The ITO supported Pt NP were characterized by X-ray diffraction (XRD). The XRD was carried out with a X’Pert-Pro diffractometer in reflection geometry operating with Cu Kα1 and Kα2 radiation (λ=1.54060 Å). Rietveld refinement was used to estimate the particle sizes of ITO and Pt. Scanning electron microscopy (SEM) was applied for the characterization of the Pt/ITO electrode using a FEI Quanta 200 FEG, equipped with a field emission gun operating at 15 kV. Electrochemical characterization Cyclic voltammograms (CVs) were measured with a Gamry Reference 600 potentiostat (USA) in a standard glass three-compartment electrochemical cell (Bio-Logic SAS), with a glassy carbon working electrode (Ø 3 mm, BASi Instruments, USA), a Pt wire serving as counter electrode, and an Ag/AgCl reference electrode (ASL, Japan). The potential between the working electrode (WE) and reference electrode was cycled 10 times between -0.21.2 V with a sweep rate of 50 mV s-1. The electrolyte was prepared with MQ water and HClO4 (Sigma-Aldrich, 70%) at a concentration of 0.1 (M). The electrolyte was purged for 5 min with Ar. Polarization curves of the electrodes were collected with a FuelCon Evaluator C50 test bench (FuelCon AG, Germany), in which the Pt/ITO-MWCNT/Nafion® electrode was used as cathode and the standard Pt/CB electrode as anode. Humidified hydrogen was fed to the anode with a flow rate of 200 ml min-1 and high-purity oxygen was provided to the cathode at a flow rate of 100 ml min-1. The anode/cathode gas humidifiers were set to 80°C and the cell temperature to 75°C. The polarization curves were recorded automatically with the software package FuelWork by increasing the current in 0.1 A steps after a steady state potential has been reached.

Results and discussion Pt nanoparticles on indium tin oxide nanoparticles have been considered as possible fuel cell catalyst materials. However, tests in a real fuel cell environment are still lacking. After the successful deposition of Pt on ITO the effect of electrode structure on the fuel cell performance is studied. The assembly of Pt/ITO with 20 wt% Nafion® ionomer in the electrode did not show any performance at all. This could be attributed to the very dense electrode structure formed by the nano-sized support and the ionomer preventing the desired gas transport. To enhance the porosity of the electrode network, the incorporation of Pt/ITO catalyst into a multi-walled carbon nanotubes (MWCNTs) network (coated with Nafion®) is proposed. The nanotube network has the advantage to be highly electron conductive and the ionomer coating ensures the proton conductive character of the electrode. The catalytic active species Pt on ITO is embedded into the structure during the preparation process. Single cell tests of the proposed electrode design have been performed, and the polarization and power density curves can be found in Fig. 2. The novel electrode design reached a maximum power density of 73 mW cm-2 at a current density of 0.12 A cm-2. The power density is comparable to the conventionally prepared Pt/CB electrode. The Pt utilization for the multilayer electrode is 1468 W g(Pt)-1 compared to 1723 W g(Pt)-1 for the standard cathode. Institute of Materials Science - Materials for Renewable Energies

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Fig. 2. Comparison of the polarization and power density curves of the multilayer electrode design (Pt/ITOMWCNT/Nafion) and a conventionally prepared electrode (Pt/CB-Nafion).

It is remarkable that both MEAs show almost the same performance while possessing two completely different morphologies. The spherical carbon black particles are in the micrometer range with micro- and macropores, whereas the ITO particles are in the nanometer range. The surface area of carbon black peaked at around 200 m2 g-1 and is therefore much higher compared to ITO (27 m2 g-1 according to the supplier). SEM micrographs have been recorded after the single cell measurements (Fig. 3). In the crosssectional view, the electrode thickness was measured to be 5.4 µm. The low and high magnification micrographs of the electrode surface indicate the mentioned network structure provided by the multi-walled carbon nanotubes, in which the Pt/ITO component is embedded. In Fig. 3, right single nanotube fibers are visible, and the structure seems to be highly porous as intended.

Fig. 3. SEM micrographs of the advanced multi-layered electrode structure incorporating oxide-supported Pt nanoparticles; in high magnification the carbon nanotubes can be seen.

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Conclusion This study showed a simple preparation technique for advanced electrode structures, which succeeded in incorporating a nano-sized oxide supported Pt component (Pt/ITO) into a 3D porous electrode network. Commercially available indium tin oxide (ITO) nanoparticles ( carbonaceous particles > metal oxides. Calcium-rich particles and soot play a minor role. Similar results are obtained by quasi-parallel measurements with an online single particle laser ablation mass spectrometer (ALABAMA). All the tested techniques for measuring ice nucleating particles perform similar from a chemical point of view within the range of their uncertainties and low counting statistics due to the low particle concentrations in free-tropospheric air. Thus, for the first time most of the existing ice nucleation measurement techniques could be compared side by side under real-world atmospheric conditions. Acknowledgment This project is founded by the DFG (INUIT, FOR 1525)

Institute of Applied Geosciences – Environmental Mineralogy

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Diploma Theses in Applied Geosciences Adam, Christian; Ermittlung von Chlorisotopen-Fraktionierungsfaktoren beim Abbau von chlorierten Schadstoffen in wässriger Phase, 1.10.13 Brauner, Sebastian; Bemessung einer Rigole zur Reinfiltration Grundwasser einer Altlastensanierung, 27.09.2013

von

gereinigtem

Helm, Johannes; Äolische Sedimente in Südhessen, 28.06.2013 Hesse, Jan; Untersuchung der thermophysikalischen Eigenschaften von Bettungsmaterialien für Nieder- und Mittelspannungskabel, 05.11.2013 Schütze, Katharina; Organic pollutant and particle characterization from primary atmospheric emission sources in the Arctic, 10.05.2013 Tunon Vettermann, Gabino; Outcrop analogue study of the Minjur Formation, Kingdom of Saudi Arabia, 09.08.2013 Winicker, Jannes; Massenbilanzierung von persistenten organischen Schadstoffen in einem urban belasteten Gewässer, 29.04.2013

Master Theses in Applied Geosciences Dönges, Florian; Einsatz von Nanotechnologie zur Qualitätssicherung bei der Errichtung von Erdwärmesonden, 20.12.2013 Faißt, Tobias; GIS-based landslide susceptibility modeling in the Lesser Himalaya of Central Nepal, 15.08.2013 Heldmann, Claus; Die hydrothermalen Vorkommen im Zillertal, 11.01.2013 Preiß, Indriani; Anwendbarkeit einer Screeningmethode zur Bestimmung des Nitratabbaupotentials mittels Redoxprofilmessungen in Grundwasserstellen im Hessischen Ried, 28.02.2013

230

Diploma- and Master Theses in Applied Geosciences

Master Theses TropHEE in Applied Geosciences Agyare, Eunice Brago; Influence of small scale mining operations on surface water quality and possible treatment options in the Tarkwa area, 27.09.2013 Androulakakis, Andreas; Characterization of chromium species in surface and groundwater samples for the Olatha area, Bangladesh, 10.12.2013 Fatema, Suraiya; Chlorine Isotope Effects During Sorption Of Organic Compounds On Carbonaceous Materials, 11.01.2013 Gebrehiwot, Haftay Hailu; Assessing the Groundwater potential in Tigray region, Ethiopia, using hydraulic and hydro-chemical methods, 07.10.2013 Gomez, Shirin; Analysis of quarternary landscape features in an Environmental and economic aspect in NE Estonia using UDAR data + ArcGIS, 02.12.2013 Gorle, Tanuja; Laboratory experiments to study sorption reversibility of organic compounds on carbonaceous materials, 29.07.2013 Kanyamuna, Beatrice; Hydrochemistry of Waters in the Laughing Waters Area of Lusaka, Zambia, 30.9.13 Koju, Bishnu; A study of a medium deep BHE heat storage system for clean renewable energy using numerical modeling, 02.01.2013 LaForce, Pamela; Comparison of Nitrate Values in Groundwater Samples to Nitrate Values in Hessen Reed Dictated by Expected Nitrate Budget Calculation, 26.11.13 Nayebare, Gumoteyo Jacintha; Waste soil Interaction on deeply weathered bedrocks on the mobility of nutrients, 30.09.2013 Ngole, Terence; Hydrochemical Investigation of Groundwater Quality in Viotic Kifissos Basin (Greece) with Special Focus on Nitrate pollution, 07.01.2013 Odipo, Victor Onyango; Geo-information application in assessing on-farm soil loss at a watershed context in Lower Nyando river basin, Western Kenya, 11.01.2013 Rathnayake, Armada; GIS-based landslide susceptibility and hazard modelling in the Lesser Himalaya of Central Nepal, 22.03.2013 Thilakerathne, Asanka; Geochemical and isotopic characterization of groundwater from shallow limestone aquifer system of Murunkan Basin, Sri Lanka, 02.09.2013 Tögl, Anja; Development of a method to determine groundwater recharge using stable isotopes of soils with low water saturation, 09.10.2013 Uddin, Sehab; Sorption of Lead and Arsenic from Water to Hydroxyapatite & Ca-deficient hydroxyapatite, 24.10.2013 Xu, Shaojuan; Using LIDAR data, GIS and remote sensing to evaluate landuse changes associated with Estonian energy production, 05.12.2013

Master Theses TropHEE in Applied Geosciences

231

Bachelor Theses in Applied Geosciences Anschütz, Sascha; Einfluss des Wassergehaltes auf Ultraschallmessungen bei Sandsteinen des Pfälzer Waldes, 21.10.2013 Glock, Thimo; Ökologische Bewertung der Pollen- und Algenflora der miozänen Seesedimente aus der Forschungsbohrung Nördlingen 1973, 02.09.2013 Haffke, Paul; Hydraulische und hydrochemische Untersuchungen der Vernässungsproblematik im Bereich der Weidsiedlung bei Weinheim, 19.07.2013 Krepp, Robin; Kompilation von boden- und felsmechanischen Kennwerten für das Quartär und Tertiär des nördlichen bis mittleren Oberrheingrabens, 21.05.2013 Kuhn, Georg; Messung von Radonkonzentration in Boden- und Raumluft im Stadtgebiet Darmstadt zur Beurteilung von Radonmigration im geodynamischem Kontext, 06.08.2013 Kurka, Sebastian; Standsicherheitsberechnungen von Böschungen auf der Grundlage von Triaxialversuchen an Großproben, 31.03.2013 Kusch, Ramona; Entwicklung von Methoden zur Probenvorbereitung für geothermische Laboruntersuchungen, 23.03.2013 Marshall, Patrick; Zusatzkonzentrationsanalyse bei industriell beeinflussten Messstellen, 08.07.2013 Mentges, Simon; Erste palynologische Bearbeitung der lakustrinen Sedimente aus dem mitteleozänen Maar-See von Offenthal (Sprendlinger Horst, Süd-Hessen), 27.04.2013 Schildt, Sven; TEM Characterization of Electron Beam Irradiated Cu-Nanotubes, 01.11.13 Schröder, Daniel; Lithofazies und Architekturelementanalyse spätpleistozäner bis rezenter Sedimente im NW Fuerteventuras (Kanarische Inseln), 10.02.2013 Schumacher, Christina; Sedimentological detailed modeling of an alluvial fan (Illgraben, Switzerland) with GOCAD® based on ground penetrating radar data, 13.11.2013 Werner, Melanie; Ableitung homogener Sedimentkörper mit Hilfe von Bohrverzeichnissen am Westrand des Oberrheingrabens, 05.07.2013 Wewior, Stefan; Messung von Radonkonzentrationen in der Bodenluft zur Beurteilung der Aktivität von tektonischen Störungen im Raum Darmstadt, 14.05.2013 Zahn, Florian; Stratigraphie und ökologische Bewertung der Forschungsbohrung Messel 2004 A (Sprendlinger Horst, Nordhessen) mit Hilfe palynologischer Methoden, 30.09.2013

232

Bachelor Theses in Applied Geosciences

PhD Theses in Applied Geosciences Hussain Al Ajmi: Sedimentology, stratigraphy and reservoir quality of the Paleozoic Wajid Sandstone in SW Saudi Arabia, 15.03.2013 – Betreuer: Prof. Hinderer Karsten Fischer: Geomechanical reservoir modeling – workflow and case study from the North German Basin, 18.10.2013 – Betreuer: Prof. Henk Monika Barbara Hofmann: GIS-based analysis of Geo-Potentials in the Northern Periphery of Belo Horizonte, MG, Brazil, 15.11.2013 – Betreuer: Prof. Hoppe Xiaoke Mu: TEM study of the structural evolution of ionic solids from amorphous to polycrystalline phases in the case of alkaline earth difluoride systems – Experimental exploration of energy landscape, 20.08.2013 – Betreuer: Prof. Kleebe Stefanie Schultheiß; Pseudomorphe Mineralumwandlung von Calcit, Dolomit, Magnesit und Witherit, 31.10.2013 – Betreuer: Prof. Kleebe

PhD Theses in Applied Geosciences

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Materials Science: Alarich-Weiss-Straße 2 L2/01 64287 Darmstadt

Applied Geosciences: Schnittspahnstraße 9 B2 01/02 64287 Darmstadt

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For further information contact: Dr. Joachim Brötz, Phone: +49(0)6151 / 16-4392; eMail: [email protected] Dipl.-Ing.(BA) Andreas Chr. Hönl, Phone: +49(0)6151 / 16-6325, eMail: [email protected]

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Bachelor Theses in Applied Geosciences