Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp Purification of metallurgical-grade silicon using Si–Sn alloy in presence of Hf, Zr, or Ti Yun Lei a,b, ⁎ , Wenhui Ma a,b, ⁎ , Jijun Wu b , Kuixian Wei b , Guoqiang Lv a , Shaoyuan Li b , Kazuki Morita c a State Key Laboratory of Complex Non-ferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, PR China b National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, PR China c Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ARTICLE INFO Keywords: Si–Sn alloy Si purification Additive Impurity Intermetallic Electron probe microanalysis ABSTRACT Hf, Zr, or Ti was employed as additive to enhance removal of the main impurities from metallurgical-grade Si (MG–Si) during the Si–Sn alloy refining process. The microstructure of Si-Sn alloy without/with Zr, Hf, or Ti was observed and analyzed by electron probe microanalysis (EPMA) to investigate the distribution of impurities. Compositions of intermetallic phases precipitated in Sn or among the grain boundaries of Si were quantitatively analyzed using EPMA. The mechanisms of their formation and the removal of impurities are discussed. The results of Si refining showed that the addition of Hf or Zr slightly enhanced the removal of Al. The high refining temperature in the Si-Sn alloy refining process was responsible for the low extraction of B. Most of Zr, Hf, or Ti was removed simultaneously with other impurities during the Si–Sn alloy refining process. 1. Introduction Si is still the main material for manufacturing solar cells to convert solar energy to electrical energy. Si is thus an important material in the fields of sustainable and clean energy. As the impurities in Si decrease the photovoltaic conversation efficiency significantly, metallurgical–- grade Si (MG–Si, purity: > 99%) must be upgraded to solar–grade Si (SoG–Si, purity: > 99.9999%) to maintain high photovoltaic con- versation efficiency. SoG-Si is mainly manufactured by the Siemens process, which is a chemical manufacturing approach. This process is costly and may cause environmental problems because of its complex chemical processes. Therefore, a more economic and environmentally friendly technology is necessary and many approaches have been proposed and developed. Alloy refining (or solvent refining) is proposed because MG-Si can be refined below its melting temperature and no waste gases or slags will be discharged into the environment. The segregation coefficients of impurities in MG-Si decrease with decrease in refining temperature, indicating that low refining temperatures are preferred for Si purifica- tion. Many alloy refining approaches have been proposed such as Si–Sn [1–3], Al–Si [4–8], Si–Ga [9], Si–Fe [10,11], Si–Na [12,13], Si–Cu [14,15], Al–Si–Sn [16], and Al–Si–Zn [17]. Although alloy refining is efficient for the removal of impurities, the residual concentrations of impurities in the refined Si are still larger than that required for SoG-Si, particularly for B (boron), which is more difficult to remove than others. Small amounts of transition metals such as Zr, Hf, and Ti are em- ployed as additives to enhance B removal because of their strong affi- nity for B. The segregation coefficients of these elements between the solid/liquid phases are extremely small (4.9 × 10 -6 [18], 1.6 × 10 -8 [19], and 2.0 × 10 -6 [19] for Hf, Zr, and Ti, respectively, at the melting point of Si, 1687 K). This indicates that these elements can be removed with the impurities and that their addition will not con- taminate the refined Si. For example, extraction ratios of Zr, Hf, and Ti were reported as 99.998% (from 32000 ppmw to 0.6 ppmw; ppmw indicates per million by weight), 99.9994% (from 62000 ppmw to 0.4 ppmw), and 99.997% (from 17000 ppmw to 0.5 ppmw), respectively, in our previous study when Al–Si alloy was the refining solvent [20]. Si–Sn alloy is also one of the promising solvents for Si refining because the yield of refined Si was significantly larger than that using other solvents to refine Si (i.e., the loss of Si in the solvent after Si refining is significantly small) according to the Si–Sn binary phase diagram [21]. Some impurities were not removed efficiently using Si–Sn solvent and a more efficient approach is required to enhance impurity removal [1–3]. https://doi.org/10.1016/j.mssp.2018.07.039 Received 16 April 2018; Received in revised form 10 July 2018; Accepted 31 July 2018 ⁎ Corresponding authors at: State Key Laboratory of Complex Non-ferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, PR China. E-mail addresses: leiyn2008@163.com (Y. Lei), mwhsilicon@126.com (W. Ma). Materials Science in Semiconductor Processing 88 (2018) 97–102 1369-8001/ © 2018 Elsevier Ltd. All rights reserved. T