Si Purification by Removal of Entrapped Al during Electromagnetic Solidification Refining of Si-Al Alloy
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EXTENSIVE exploration of solar energy has triggered a strong demand for high-purity solar-grade silicon (SoG-Si, 99.9999 pct). Currently, the Siemens process is the main technology for manufacturing SoG-Si,[1,2] but its cost of production is still high and its manufacturing process can still cause environmental problems. Hence, a more economical and efficient SoG-Si fabrication technology is required for the sustainable development of the photovoltaic (PV) industry. Solvent refining with an Al-Si alloy[3–5] is a very promising process for low-cost SoG-Si production and one of the few metallurgical Si (MG-Si) purification processes realized on the industrial scale.[6] As compared to traditional metallurgical routes, solvent refining with Al-Si melts can reduce costs because the refining temperatures of this process, which ranges from 850 K to 1473 K (577 C to 1200 C), are much lower than the Si melting point [1683 K (1410 C)]. On the other hand, the segregation coefficient of impurities between solid Si and Al-Si melts is significantly smaller than that between solid and liquid Si, especially for B and P.[7] Furthermore, the recyclability of the solvent metal is considered beneficial for the sustainability of this process.
WENZHOU YU, ZHONG ZHENG, WEIYAN JIANG and JIE LI are with the College of Materials Science and Engineering, Chongqing University, Chongqing, 400045 China. Contact e-mail: [email protected] WENHUI MA and YUN LEI are with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093 China. Manuscript submitted October 7, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
In addition to the Al-Si solvent, other Si-based solvent such as Si-Sn,[8] Si-Ni,[9] Si-Fe,[10] and Si-Cu[11] solvents have been investigated for the removal of impurities. Among these solvent-refining systems, the Al-Si solvent may be more attractive because of the relative low price of Al (as compared to those of Cu and Sn) and the low melting point (as compared to Fe-Si and Ni-Si). Also, the by-product of Al-Si solvent refining (Al-Si alloy) can be employed as the raw materials in aerospace and automotive industry favored for its properties of high wear resistance, low density, corrosion resistance, and high thermal stability.[12] In the last decade, there have been many outstanding achievements in Si purification through solvent refining with Al-Si melts. Lei et al.[13] showed that the removal efficiency of B impurities can be enhanced dramatically by adding small amounts of Ti, Hf, and Zr, and the amount of residual B in the refined Si could be reduced to 0.46 ppmw, which meets the requirements for manufacturing solar cells. Additionally, Yoshikawa and Morita[14] reported that P could be removed effectively because the segregation ratio of P between refined Si and Al-Si melt is only 8 9 102 [1273 K (1000 C)]. In their work, the removal efficiency of P was as high as 97.4 pct. Clearly, B and P impurities and other metal impurities can be removed effectively in the Al-Si solvent refining
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