Removal of Boron from Silicon by Solvent Refining Using Ferrosilicon Alloys
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SILICON is the most common base material for semiconductor devices. It accounts for over 90 pct of today’s PV materials.[1–3] Ultra pure silicon (9 N) for solid state electronic devices is industrially prepared by the distillation and subsequent thermal decomposition of volatile silicon compounds.[4–6] Considering the cost and energy required for the above process and the difference in silicon specification requirements for application in microelectronics and in PV industry (6-7 N), an interest has grown to develop technologies for mass production of low cost solar grade silicon. Metallurgical techniques have been particularly focused on because of their ability to deliver large production rates at low cost.[3,4,7–12] Solvent refining is one of the techniques that have been considered as a cost efficient, less energy intensive purification step for producing solar grade silicon. Basically the process employs an alloying element as a solvent for crude Si, from which pure Si crystals are later precipitated upon controlled cooling and solidification. The purification that takes place during crystal growth is mainly due to impurity rejection by the solidification front. Previously, Al,[7,8,13–17] Cu,[18–20] Ni,[21] Sb[22], and Sn[23] have been used as the alloying elements, or impurity getter, in solvent refining of silicon. In a new approach to metallurgical refining of silicon by a combination of solvent refining and heavy media LEILI TAFAGHODI KHAJAVI, Ph.D. Candidate, and MANSOOR BARATI, Associate Professor, are with the Department of Materials Science and Engineering, University of Toronto, 184 College Street, Suite 140, Toronto, ON M5S 3E4, Canada. Contact e-mail: [email protected] KAZUKI MORITA, Professor, and TAKESHI YOSHIKAWA, Associate Professor, are with the Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan. Manuscript submitted July 15, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
separation, iron has been successfully utilized as the getter.[10,11,24] Unlike most metallic impurities, phosphorus and boron have relatively large segregation coefficients[4] that render them unresponsive to solidification refining processes. Consequently, it is crucial to study the feasibility and efficiency of phosphorus and boron removal through solvent refining with iron. In a recent study,[25] thermodynamics of phosphorus distribution between purified silicon crystals and the ironsilicon melt has been investigated. Aiming to establish the thermodynamic fundamentals of the process, the present study was performed to investigate the distribution coefficient of boron between silicon and the alloy melt. Furthermore, the interaction parameter between iron and boron was estimated to determine the affinity of iron for boron.
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MATERIALS AND METHODS
Sample preparation involves melting silicon, iron, and boron together to form an alloy with the desired composition. According to iron-silicon phase diagram (Figure 1), the binary alloy should contain more than 58.2 wt pct sili
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