Effect of Si Addition on the Electrochemical Reduction Rate of SiO 2 Granules in Molten CaCl 2
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UCTION
AS the global greenhouse effect becomes more serious, countries are striving to cut their CO2 emissions. Solar energy is valued as a clean and inexhaustible renewable energy source. In the past decade, there have been tremendous developments in the photovoltaic (PV) industry. The annual global capacity of installed PV cells reached 50.6 gigawatts in 2015, which was a 100-fold increase compared to the capacity in 2000.[1] Meanwhile, crystalline Si is the dominant raw material for the production of solar cells, with ~ 90 pct of the market share.[2] Due to the rapid growth predicted in the PV industry, a high yield production method of crystalline Si is required to satisfy current and future demand. A purity of 6N (99.9999 pct) is required for solargrade Si (SOG-Si). Usually, the process of producing SOG-Si begins with metallurgical-grade Si (MG-Si), which is obtained by the reduction of SiO2 via a
MING ZHONG, XIAO YANG, and TOSHIYUKI NOHIRA are with the Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan. Contact e-mail: [email protected] KOUJI YASUDA is with the Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto, 606-8501, Japan and also with the Agency for Health, Safety and Environment, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan. TAKAYUKI HOMMA is with the Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan. Manuscript submitted August 6, 2017.
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high-temperature carbothermic reaction. The MG-Si is then converted into trichlorosilane (SiHCl3) gas, which is purified by distillation. SOG-Si is obtained by the chemical vapor deposition (CVD) of the purified SiHCl3 gas. This is the dominant technology at present, despite its low productivity and large energy consumption.[3–5] However, several methods involving the pyrolysis of monosilane (SiH4) with a fluidized bed reactor (FBR) and the upgrading of MG-Si have attempted to reduce the cost and improve the productivity.[5–10] Our group has investigated the direct electrochemical reduction of solid SiO2 plates and granules in molten CaCl2 using a contacting electrode as a new method of producing Si.[11–25] When studying SiO2 plates,[11,13] it was noted that the reaction started from the point of contact between SiO2 and the Mo line, expanding across the whole plate by creating a three phase boundary between SiO2/CaCl2/conductor. This was secured by the high conductivity of Si at high temperature.[11] As part of a series of studies, we have also investigated the electrochemical reduction of SiO2 granules in molten CaCl2.[20,22] Related research on the electrochemical reduction of SiO2 in molten chlorides has been also reported by other groups.[26–33] The reaction behavior of SiO2 granules stratified on graphite plates was studied in our previous work.[20,22,23] The reduction started at the SiO2 granules in contact with the graphite plate, and then the molten CaCl2 penet
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