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STRUCTURE and PROPERTIES of POLYMER-DERIVED Sibcn CERAMICS by YAOHAN CHEN B.S. Sichuan University, China, 2005 M.S. Sichuan Univ

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STRUCTURE and PROPERTIES of POLYMER-DERIVED Sibcn CERAMICS by YAOHAN CHEN B.S. Sichuan University, China, 2005 M.S. Sichuan Univ STRUCTURE AND PROPERTIES OF POLYMER-DERIVED SiBCN CERAMICS by YAOHAN CHEN B.S. Sichuan University, China, 2005 M.S. Sichuan University, China, 2008 A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Mechanical, Materials and Aerospace Engineering in the College of Engineering and Computer Science at the University of Central Florida, Orlando, Florida Spring Term 2012 Major Professor: Linan An © 2012 Yaohan Chen ii ABSTRACT Polymer-derived ceramics (PDCs) are a unique class of multifunctional materials synthesized by thermal decomposition of polymeric precursors. Due to their unique and excellent properties and flexible manufacturing capability, PDC is a promising technology to prepare ceramic fibers, coatings, composites and micro-sensors for high-temperature applications. However, the structure-property relationships of PDCs have not been well understood. The lack of such understandings drastically limited the further developments and applications of the materials. In this dissertation, the structure and properties of amorphous polymer-derived silicon carbonitride (SiCN) and silicoboron carbonitride (SiBCN) have been studied. The SiCN was obtained using commercially available polysilazane as pre-ceramic precursor, and the SiBCN ceramics with varied Si-to-B ratio were obtained from polyborosilazanes, which were synthesized by the hydroboration and dehydrocoupling reaction of borane and polysilazane. The structural evolution of polymer-derived SiCN and SiBCN ceramics from polymer to ceramics was investigated by NMR, FTIR, Raman, EPR, TG/DTA, and XRD. The results show a phase- separation of amorphous matrix and a graphitization of “free” carbon phase, and suggest that the boron doping has a great influence on the structural evolution. The electric and dielectric properties of the SiCN and SiBCNs were studied by I-V curves, LCR Meter, and network analyzer. A new electronic conduction mechanism and structure model has been proposed to account for the relationships between the observed properties and microstructure of the materials. Furthermore, the SiBCN ceramics showed the improved dielectric properties at characterization iii temperature up to 1300 ºC, which allows the fabrication of ultrahigh-temperature wireless microsensors for extreme environments. iv ACKNOWLEDGMENTS First and foremost I would like to thank Prof. Linan An, my advisor, for his enormous support, encouragement and guidance over the years. The working experience with him has been one of the best memories in my life. I would always look up to him as a great scientist, mentor, and above all a cherished friend. I greatly appreciate Dr. Jiyu Fang for his role as co-chair in my committee and valuable advice on my research and composition. I also would like to express my deep and sincere gratitude to my committee members, Dr. Xun Gong, Dr. Chengying Xu, Dr. Lei Zhai, and Dr. Qun Huo for their constructive comments, experimental assistances, and financial support. I wish to thank Dr. Nina Orlovskaya for allowing me to use her lab facilities. I feel great pleasure to acknowledge my sincere appreciation to Dr. John Shen for his assistance and expertise in dielectric characterization. My warm thanks go to Dr. Yong-ho Sohn for his support on my characterization. I would also like to thank Dr. Yi Liao for his kind help in my electrical experiment. I am thankful to Ms. Clara Hofmeister, Mr. Kirk Scammon and Mr. Mikhail Klimov at MCF for their assistance with my experiments. I also thank Dr. Hans van Tol, Dr. Andrew Ozarowski and Dr. Zhehong Gan for EPR and NMR analysis at National High Field Magentic Lab at Florida State University. v It is my pleasure to thank all my former and present colleagues for creating a friendly and cooperative working atmosphere. My special gratitude goes to Dr. Yiguang Wang, Dr. Weixing Xu, Dr. Jianhua Zou, Dr. Jianhua Liu, Dr. Sourangsu Sarkar, Dr. Yong Chen, Dr. Ligong Zhang, Dr. Yuxi Yu, Dr. Tao Jiang, Dr. Siamak Ebadi, Jinling Liu, Gang Shao, and Xinhua Ren for their friendly and fruitful helps in my academic career. I also want to thank my friends, Dan Chen, He Shen, Ni Li, Yan Chen, Dr. Jeremiah Folsom-Kovarik, and Dr. Xuejun Zhang for their supports and encouragements these years. My wife, Xueping Yang, deserves a special gratitude for her patience and understanding throughout this time. Without all of your love, encouragement and support, I would have never made it. Thank you for always being there for me and for being such a wonderful soul mate. Finally, I dedicate this dissertation to my dear family, especially to my parents, Ping Chen and Daihong Xu for their continued love and support. I know that you are proud of me! vi TABLE OF CONTENTS LIST OF FIGURES ...................................................................................................................... xii LIST OF TABLES ........................................................................................................................ xx CHAPTER ONE: INTRODUCTION ............................................................................................. 1 1.1 Motivation ............................................................................................................................. 1 1.2 Outline of Dissertation .......................................................................................................... 3 CHAPTER TWO: LITERATURE REVIEW ................................................................................. 4 2.1 Overview of Polymer-Derived Ceramics ............................................................................. 4 2.2 Preceramic Polymer Precursor Synthesis ............................................................................. 6 2.2.1 Synthesis of Poly(organosilazanes) ............................................................................... 8 2.2.2 Synthesis of Poly(organoborosilazane) .......................................................................... 9 2.3 Processing of Polymer-Derived Ceramics .......................................................................... 16 2.3.1 Shaping and Crosslinking ............................................................................................ 16 2.3.2 Polymer-to-Ceramic Conversion ................................................................................. 17 2.4 Microstructure of Polymer-Derived Ceramics ................................................................... 19 vii 2.4.1 Raman Spectroscopy .................................................................................................... 22 2.4.2 Multinuclear MAS-NMR ............................................................................................. 27 2.4.3 Electron Paramagnetic Resonance (EPR) .................................................................... 28 2.5 Electrical and Dielectric Properties of Polymer-Derived Ceramics ................................... 33 2.5.1 D.C - Conductivity of Polymer-Derived Ceramics ...................................................... 33 2.5.2 A.C- Conductivity of Polymer-Derived Ceramics ....................................................... 37 2.5.3 Dielectric Properties of Polymer-Derived Ceramics ................................................... 39 CHAPTER THREE: SYNTHESIS OF POLYBOROSILAZANES ............................................ 43 3.1 Introduction ......................................................................................................................... 43 3.2 Experiments ........................................................................................................................ 43 3.2.1 Synthesis of Polyborosilazanes .................................................................................... 43 3.2.2 Characterization ........................................................................................................... 46 3.3 Results and Discussion ....................................................................................................... 48 3.3.1 NMR Spectra of SiBCN Precursors ............................................................................. 48 3.3.2 FTIR Spectra of SiBCN Precursors ............................................................................. 51 viii 3.3.3 Synthesis Scheme of SiBCN Precursor ....................................................................... 53 3.3.4 Cross-linking of SiBCN Precursors ............................................................................. 54 3.4 Summary ............................................................................................................................. 60 CHAPTER FOUR: STRUCTURE OF POLYMER-DERIVED SiBCN CERAMICS ................ 61 4.1 Introduction ......................................................................................................................... 61 4.2 Experiments ........................................................................................................................ 62 4.2.1 Shaping of Cross-linked Precursors ............................................................................. 62 4.2.2 Pyrolysis and Annealing of Polysilazane and Polyborosilazanes Disks ...................... 62 4.2.3 Characterization ........................................................................................................... 63 4.3 Results and Discussion ......................................................................................................
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