Low-Cost Process for Silicon Purification with Bubble Adsorption in Al-Si Melt
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AS the photovoltaic (PV) industry is affected by the global economical crisis, the cost of fabrication for the solar cells must be reduced as much as possible. Conventionally, the PV industry is dependent on the expensive, off-spec semiconductor-grade silicon (SEGSi) that leads to the high cost of solar cells. To reduce the expense of raw materials, some low-cost fabrication routes of solar-grade silicon (SOG-Si) for the PV industry should be proposed. In the last few decades, metallurgical routes for SOG-Si preparation that usually contain acid leaching,[1] vacuum melting,[2] oxidation treatment,[3] slag refining,[4] directional solidification,[5,6] electron beam melting,[7] etc. have been suggested to deal with the metallurgical-grade silicon (MG-Si) by removing the impurities from it for the demand of SOGSi, and it is considered a potential method for the cost reduction. Evidently, acid leaching can easily dissolve the impurities segregated in the grain boundaries of Si, and directional solidification is a well-known process for
WENZHOU YU, Ph.D. Candidate, WENHUI MA, Professor, YONGSHENG REN, Master Graduate Student, and YONGNIAN DAI, Professor, are with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, P.R. China, and also with the National Engineering Laboratory for Vacuum Metallurgy, State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology. Contact e-mail: [email protected] GUOQIANG LV, Associate Professor, is with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology. KAZUKI MORITA, Professor, is with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Manuscript submitted August 21, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
removing metallic impurities due to their strong segregation tendencies.[8,9] Additionally, for the removing of nonmetallic impurities especially P and B, directional solidification may be ineffective due to relatively large segregation coefficients (KP = 0.35, KB = 0.8)[10]; therefore, vacuum melting and slag refining is always applied to remove P and B, respectively. Among the purification methods mentioned above, Si must be repeatedly molten in each step to ensure the impurities can easily transfer to outside, which results in a lot of energy consumption. On the other hand, a new method called ‘‘solvent refining,’’ which can significantly cut down the temperature of melting, has been proposed for further cost reduction of PV industry. Yoshikawa and coworkers[11–13] and Gu et al.[14] proposed a Si-Al solvent to refine Si by using the enhanced segregation tendency of impurities between solid silicon and the Si-Al melt. Zhao et al.[15] and Ma et al.[16] suggested a metal-liquating method using the Sn-Si system that would have high efficiency for the removal of B. Additionally, Cu-Si[17] and Zn-Si[15] solvent refining methods
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