Grain Growth by Ordered Coalescence of Nanocrystals in Ceramics Jianfeng Hu Doctoral thesis 2013 Department of Materials and Environmental Chemistry Arrhenius Laboratory, Stockholm University SE-10691 Stockholm, Sweden Faculty Opponent Prof. Viveka Alfredsson Division of Physical Chemistry Lund University, Sweden Evaluation committee Dr.Magnus Eklund Sandvik Coromant, Sweden Dr. Peter Alberius SP Technical Research Institute of Sweden, Sweden Dr. Feifei Gao Department of Materials and Environmental Chemistry Stockholm University, Sweden Substitute Prof. Stefan Jonsson Division of Mechanical Metallography KTH, Sweden ©Jianfeng Hu, Stockholm 2013 ISBN 978-91-7447-677-4 Cover illustration: The electron microscope micrographs of SrTiO3 and Si3N4 ceramics Printed in Sweden by US-AB, Stockholm 2013 Distributor: Department of Materials and Environmental Chemistry, Stockholm University - ii - To Zhang and Yifang - iii - - iv - Abstract Sintering is the most widely used consolidation method of polycrystalline materials in powder metallurgy and ceramic industries. Grain growth and densification process play the two most crucial roles on the microstructure evolution and the achieved performances during sintering of ceramics. In this thesis, the grain growth of SrTiO3, BaTiO3-SrTiO3 solid solutions and Si3N4 ceramics during spark plasma sintering (SPS) were investigated by electron microscopy. SrTiO3 ceramics starting from nanopowders were fabricated by SPS. A novel grain growth mechanism was discovered and named ordered coales- cence of nanocrystals. This mechanism involved nanocrystals as building blocks and is distinguished from atomic epitaxial growth (AEG) in classical sintering theory. The results also revealed that the dominant grain growth mechanism can be changed by varying heating rates. Low rate (10°C/min) gives AEG, whereas high rates (≥ 50°C/min) yields three-dimensional coa- lescence of nanocrystals, i.e. ordered coalescence. BaTiO3-SrTiO3 sintered bodies were made by SPS of BaTiO3 and SrTiO3 nanopowders mixtures. A novel Sr1-xBaxTiO3 ―solid solution‖ with mosaic- like single crystal structure was manufactured by ordered coalescence of the precursor crystallites. This reveals a new path for preparation of solid solu- tion grains or composites with unique structure. Si3N4 ceramics were prepared from α- or β-Si3N4 nanopowders at the same SPS conditions. The anisotropic ordered coalescence of precipitated β- Si3N4 crystallites gives elongated β-Si3N4 grains at 1650°C using α-Si3N4 nanopowder. The metastable α- to β-Si3N4 phase transformation and ordered coalescence of crystallites accelerates anisotropic grain growth. In contrast, AEG leads to the equi-axed β-Si3N4 grains using β-Si3N4 nanopowder. Grain motions contribute to the densification process during pressureless sintered 3Y-ZrO2 (>87%TD) or SPS of SrTiO3 (>92%TD) ceramics. This extends the sintering range for active grain re-arrangement over that pre- dicted by classical theory. In this thesis a new grain growth mechanism, i.e. ordered coalescence of nanocrystals, is discovered and proved to occur in both solid-state-sintered and liquid-phase-sintered ceramics by using SPS and nanopowders. By or- dered coalescence the microstructural evolution can be manipulated to achieve unique microstructures. - v - - vi - List of papers This thesis is based on the following papers I. Jianfeng Hu and Zhijian Shen. ―Grain Growth by Multiple Ordered Coalescence of Nanocrystals during Spark Plasma Sintering of SrTiO3 Nanopowders.‖ Acta Materialia 60(18): (2012) 6405–6412. In this paper, I performed all the experiments, analysed and interpreted all the experimental results, and did the majority of the writing. II. Jianfeng Hu and Zhijian Shen. ―Ordered Coalescence of Nano Crystallites Contributing to the Rapid Anisotropic Grain Growth in Silicon Nitride Ceramics.‖ Scripta Materialia (2013) In press. Doi: 10.1016/j.scriptamat.2013.04.017. In this paper, I performed the TEM experiments, analysed and interpreted all the experimental results, and did the majority of the writing. III. Jianfeng Hu and Zhijian Shen. ―Grain Growth Kinetics Determined by the Heating Rate during Spark Plasma Sintering: 2D Nucleation verus Ordered Coalescence of Nanocrystals.‖ Submitted. In this paper, I performed all the experiments, analysed and interpreted all the experimental results, and did the majority of the writing. IV. Jianfeng Hu and Zhijian Shen. ―Mosaic-like Structure in Barium Strotium Titanate Solid Solution.‖ In manuscript. In this paper, I performed all the experiments, analysed and interpreted all the experimental results, and did the majority of the writing. V. Mathias Herrmann, Zhijian Shen, Ingrid Schulz, Jianfeng Hu and Bostjan Jancar. ―Silicon Nitride Nanoceramics Densified by Dynamic Grain Sliding.‖ Journal of Materials Research 25(12): (2010) 2354– 2361. - vii - In this paper, I performed the TEM experiments, analysed and interpreted the TEM experimental results. VI. Yan Xiong, Jianfeng Hu, Zhijian Shen, Vacclav Pouchly and Karel Maca. ―Preparation of Transparent Nanoceramics by Suppressing Pore Coalescence‖. Journal of the American Ceramic Society 94(12): (2011) 4269–4273. In this paper, I performed the SEM experiments, analysed and discussed the experimental results. VII. Yan Xiong, Jianfeng Hu and Zhijian Shen. ―Dynamic Pore Coales- cence in Nanoceramic Consolidated by Two-Step Sintering Procedure.‖ Journal of the European Ceramic Society 33 (2013) 2087- 2092. In this paper, I performed the SEM experiments, analysed and discussed the experimental results. Reprints were made with permission from the publishers Papers not included in the thesis VIII. Zhijian Shen, Haixue Yan, Daniel Grüner, Lyubov M. Belova, Yasuhiro Sakamoto, Jianfeng Hu, Ce-Wen Nan, Thomas Höche, and Michael J. Reece. ―Ferroelectric Ceramics with Enhanced Remnant Polarization by Ordered Coalescence of Nano-Crystals.‖ Journal of Materials Chemistry 22(44): (2012) 23547. - viii - Abbreviations AEG Atomic Epitaxial Growth BEI Backscattered Electron Image BF Bright Field DF Dark Field ECS Equilibrium Crystallographic Shape EDS Energy Dispersive Spectrum FFT Fast Fourier Transform HAADF High Angle Annular Dark Field HRTEM High Resolution Transmission Electron Microscopy IFFT Inverse Fast Fourier Transform LSW Lifshitz-Slyozov-Wagner SE Secondary Electron SEM Scanning Electron Microscope STEM Scanning Transmission Electron Microscopy SPS Spark Plasma Sintering TD Theoretical Density TEM Transmission Electron Microscope XRD X-ray diffraction 2D Two Dimension 3Y-ZrO2 3 mol% Y2O3 stabilised ZrO2 - ix - - x - Contents Abstract .................................................................................................................... v List of papers ...................................................................................................... vii Abbreviations ........................................................................................................ ix 1. Introduction ...................................................................................................... 1 1.1 Mass transport mechanisms in classical sintering theory ........................... 1 1.1.1 Densification ................................................................................................. 3 1.1.2 Grain growth ................................................................................................. 3 1.2 Premelting .............................................................................................................. 4 1.3 Grain growth during Spark Plasma Sintering (SPS) ..................................... 5 1.3.1 SPS ................................................................................................................. 5 1.3.2 Grain growth behaviors during SPS ........................................................ 8 1.3.2.1 The effects of the electric SPS parameters on grain growth .... 8 1.3.2.2 The effect of heating rate on grain growth ................................... 9 1.4 Case studies on grain growth mechanisms .................................................... 9 1.4.1 Strontium titanate ceramics...................................................................... 9 1.4.2 SrTiO3-BaTiO3 solid solution ceramics .................................................. 10 1.4.3 Silicon nitride ceramics ............................................................................ 10 1.5 Aim of thesis ........................................................................................................ 11 2. Experiments ................................................................................................... 13 2.1 Starting powders ................................................................................................ 13 2.1.1 Titanate nanopowders .............................................................................. 13 2.1.2 Silicon nitride nanopowders .................................................................... 14 2.1.3 Zirconia nanopowder ................................................................................ 14 2.2 Spark Plasma Sintering ..................................................................................... 15 3. Results and Discussion ............................................................................... 19 3.1 Grain growth mechanisms in solid-state-sintered SrTiO3 ceramics by SPS ...............................................................................................................................