Molten Salt Electrodeposition of Silicon in Cu-Si
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Molten Salt Electrodeposition of Silicon in Cu-Si by Samira Sokhanvaran A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Departments of Materials Science and Engineering University of Toronto © Copyright by Samira Sokhanvaran 2014 Molten Salt Electrodeposition of Silicon in Cu-Si Samira Sokhanvaran Doctor of philosophy Departments of Materials Science and Engineering University of Toronto 2014 Abstract Widespread use of solar energy has not been realized to date because its cost is not competitive with conventional energy sources. The high price of solar grade silicon has been one of the barriers against photovoltaic industry achieving its much anticipated growth. Therefore, developing a method, which is energy efficient and will deliver inexpensive silicon feedstock material is essential. The electrodeposition of Si from a cryolite-based melt was investigated in the present work as a possible solution. This study proposed electrowinning of Si in molten Cu-Si alloy, to decrease the working temperature and increase the efficiency. Solvent refining can be used to recover Si from Cu-Si and also as a second purification method. The physicochemical properties of the potential electrolyte, cryolite–SiO2 melts, were studied in the first step of this work. The deposition potential of Si on a graphite cathode was measured to determine the working potential and the effect of SiO2 concentration on it. In the next step, the deposition potential of Si from cryolite– SiO2 melt on Cu and Cu-Si cathodes was determined using cyclic voltammetry. Next, the cathodic and the anodic current efficiencies of the process were measured. Continuous analysis of the evolved gas enabled the instantaneous measurement of the current efficiency and the kinetics of the deposition. Finally, the effectiveness of the process in delivering high purity Si ii was investigated. Si dendrites were precipitated out of the Cu-Si cathode and recovered to determine the purity of the final product as the final step of this study. The produced Si was separated from the alloy matrix by crushing and acid leaching and the purity was reported. The findings of this research show that the proposed method has the potential to produce high purity silicon with low B content. Further development is required to remove some metallic impurities that are remained in Si. iii Acknowledgments This research bears the imprint of many people who shared with me their knowledge and experience. First and foremost, I wish to express my sincere gratitude to my supervisor, Dr. Mansoor Barati, for his invaluable assistance, guidance and support through completion of this research. I would like to thank him for teaching me how to solve the challenging and applied problems and for being there whenever I needed him at all stages of this research. Deep gratitude is also due to the members of the supervisory committee, Dr. R. Ravindran, Dr. C. Jia and Dr. K. Lian whose assistance was elemental in successful completion of this study. Also, I would like to take this opportunity to thank the Department of Materials Science and Engineering for providing me the necessary facilities and warm environment to carry out the research work. A special appreciation goes to Late Prof. T. Utigard, for the amenities and support he provided on the characterization of electrolyte. I would also like to thank Sal Boccia and Dan Grozea from the Department of Materials Science and Engineering and Mr. S. Salavati the Department of Mechanical and Industrial Engineering for their advice and assistance in sample preparation and SEM analysis. I also would like to thank RioTinto Alcan, NSERC and MSE department for providing the financial support for this research. I really appreciate the support of all members of the Sustainable Materials Processing Research Group, specially S. Thomas, M. Li. I also thank a tireless man, K. Danaei, for his effort both in the lab and in the office. Without his wise advice, invaluable help this research would not have been possible. I owe my deepest gratitude to my parents for their dedication and endless support and love through the years. I gained so much drive under their watchful eyes. I also appreciate my two sisters who always made me smile even on tough times when nothing worked well in the lab. Lastly and most importantly, I would like to thank my life partner Mojtaba for standing beside me through thick and thin. He was always there cheering me up on the dull days. His unwavering love was undeniably the bedrock upon which the past twelve years of my life have been built and I dedicate this dissertation to him. iv Table of Contents Acknowledgments.......................................................................................................................... iv Table of Contents .............................................................................................................................v List of Symbols ............................................................................................................................. xii List of Abbreviations ................................................................................................................... xiv List of Tables .................................................................................................................................xv List of Figures .............................................................................................................................. xvi Chapter 1 Introduction .....................................................................................................................1 1.1 Motivation for the thesis ............................................................................................................1 1.2 Objectives of the study...............................................................................................................3 1.3 Organization of the thesis ..........................................................................................................4 Chapter 2 Literature Review ............................................................................................................5 2.1 Silicon ........................................................................................................................................5 2.1.1 Metallurgical grade silicon .....................................................................................................6 2.1.2 Semiconductor grade silicon ...................................................................................................7 2.1.3 Solar grade silicon...................................................................................................................8 2.2 SoG-Si production methods .......................................................................................................9 2.2.1 Refining of MG-Si ................................................................................................................10 2.2.1.1 Acid leaching .....................................................................................................................10 2.2.1.2 Reactive gas blowing .........................................................................................................11 2.2.1.3 Slagging .............................................................................................................................12 2.2.1.4 Solvent refining ..................................................................................................................12 2.2.1.5 Electrorefining ...................................................................................................................13 2.2.2 Silica reduction .....................................................................................................................16 v 2.2.2.1 Carbothermal reduction of silica ........................................................................................16 2.2.2.2 Reduction by metals and compounds ................................................................................17 2.2.2.3 Electrodeposition ...............................................................................................................17 2.3 Electrowinning of silicon .........................................................................................................17 2.3.1 Electrowinning of solid Si ....................................................................................................18 2.3.1.1 Organic solvents.................................................................................................................18 2.3.1.2 Molten salts ........................................................................................................................19 2.3.1.2.1 Deposition from halide melts ......................................................................................... 19 2.3.1.2.2 Deposition from mixture of oxide without halides ........................................................ 20 2.3.1.2.3 Deposition from mixture of halides and silica ............................................................... 20 2.3.2 Electrowinning of molten Si .................................................................................................22 2.3.2.1 Above the melting temperature of Si .................................................................................22 2.3.2.2 Below the melting temperature of Si .................................................................................23