Reaction Mechanism and Kinetics of Boron Removal from Molten Silicon via CaO-SiO 2 -CaCl 2 Slag Treatment and Ammonia In
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gy crises and environmental degradation have led to higher demands on the installed capacity of solar cells, which has increased from 7.7 GW in 2009 to 72.9 GW in 2017.[1] Silicon-based solar cells accounted for more than 94 pct of the global photovoltaic (PV) market in 2017 owing to their low cost, high-photoelectric conversion efficiency, and stability.[2] Because of the fast growth of the PV market,[3] the main source of
HUI CHEN is with the School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, P.R. China and also with the Department of Materials Engineering, The University of Tokyo, Tokyo 113-8654, Japan. XIZHI YUAN, YANJUN ZHONG, and YE WANG are with the School of Chemical Engineering, Sichuan University. Contact e-mail: [email protected] KAZUKI MORITA is with the Department of Materials Engineering, The University of Tokyo. XIAODONG MA is with the School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. ZHIYUAN CHEN is with the Department of Materials Science and Engineering, Delft University of Technology, 2628 CD, Delft, The Netherlands. Manuscript submitted May 5, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
solar-grade silicon (SOG-Si) today is the non-prime electronic-grade silicon (EG-Si), which is produced by conventional processes (e.g., modified Siemens process and fluidized bed process) with more economical parameters (e.g., higher production efficiency and lower purity grades).[4] However, an extra doping process is required to achieve high-photoelectric conversion efficiency of the solar modules; this reduces the purity and increases the manufacturing cost.[5] In the metallurgical route, which has immense potential for the large-scale production of SOG-Si from metallurgical-grade silicon (MG-Si) directly, boron (B) removal is most difficult.[6] In the CaO-MgO-Al2O3-SiO2 slag treatment process, the distribution coefficient of B does not change much when changing the slag composition,[7] while the addition of Al2O3 decreases the reaction rate because of the increased viscosity.[8] The CaCl2-CaO-SiO2 slag system has shown to be potentially beneficial for B removal due to both oxidization and chlorination.[9] However, the final B concentration in Si does not yet meet the industrial demand.[10] A method using the iron catalyst and ammonia (NH3) was recently reported, where NH3 displays a better selectivity for B.[11] In this work, a method for B removal from molten Si via CaO-SiO2CaCl2 slag treatment and NH3 injection was proposed to achieve a higher removal rate, and a kinetic model was presented to clarify the reaction mechanism of the volatile slag-NH3 gas-Si system. Similar to other models for their target impurities,[12,13] this model can predict the change in the B concentration in Si with time to some extent. The cross-sectional configuration of the shaft furnace system in our experiments is detailed in Figure 1. The samples were heated in a graphite crucible with MoSi2 heating elements. All reactions were performed with high-purity anhydrous chem
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