Oxidative Removal of Boron from Molten Silicon by CaO-based Flux Treatment with Oxygen Gas Injection
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PHOTOVOLTAIC power generation using solar cells has been receiving significant attention as an indispensable renewable energy source to build a smart energy society. Currently in Japan, dissemination of solar cells is one of the urgent issues to solve the serious problem on an optimum energy mix. From the point of view, the demand for solar cells, particularly polycrystalline silicon solar cells, is increasing throughout the world. The main feed-stock for silicon of solar cells depends on the scrap of high purity silicon used for forming semiconductors (semiconductor-grade silicon: SEG-Si). However, if the demand exceeds the available scrap, its shortage will impede dissemination of silicon solar cells and also affect the market and price of SEG-Si.[1] One of the most effective methods for solving this problem is to provide an alternative feed-stock. It is imperative to develop an innovative silicon refining process to produce solar-grade silicon (SOG-Si) in a price range lower than that of scrap of SEG-Si. Recently, some research groups in Japan[2–4] have proposed a new metallurgical refining process of silicon for SOG-Si using metallurgical-grade silicon (MG-Si) as a starting material, which consists of several
MITSURU TANAHASHI, Associate Professor (Lecturer), is with the Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Contact e-mail: [email protected]. ac.jp TOSHIHARU FUJISAWA, Professor, and CHIKABUMI YAMAUCHI, Emeritus Professor, are with the Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Manuscript submitted June 18, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
unit purification steps, such as acid leaching, oxidative refining with plasma melting, vacuum refining with electron beam melting, and directional solidification. In the proposed process, boron removal is the most difficult step in terms of practical application. Because of low vapor pressure and the large segregation coefficient of boron in silicon,[5] it is considered that the oxidative refining with plasma melting is the most promising metallurgical process that can achieve the boron removal to the required concentration for the SOG-Si (0.1 to 0.3 mass ppm). However, it takes around 18 ks (5 hours) to refine 300 kg of silicon.[2,3,6] Further decrease in energy cost for the boron removal is imperative to put the production of SOG-Si into practice. From the above point of view, Morita and co-workers have recently investigated boron removal from silicon by acid leaching,[7] solidification refining with a silicon-based melt at low temperature[8–13] and flux treatment.[14–16] In the present study, from the viewpoint of practical application to a new SOG-Si production process, basic flux treatment with oxygen gas injection was proposed as a simpler method for boron removal than the abovementioned processes. As a primary step for examining the refinability of silicon by th
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