High Temp. Mater. Proc., Vol. 30 (2011), pp. 379–385 Copyright © 2011 De Gruyter. DOI 10.1515/HTMP.2011.060 Reduction Behavior of SiCl 4 During its Aluminothermic Reduction Tadatoshi Kawabe, 1 Takeshi Yoshikawa, 1 Kunio Saegusa 2 and Kazuki Morita 1; 1 Institute of Industrial Science, The University of Tokyo, Tokyo, Japan 2 Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd., Tokyo, Japan Abstract. With a long term aim of utilization of SiCl 4 as a resource for solar grade silicon, we have investigated the aluminothermic reduction of SiCl 4 . The rate-controlling step for reduction of SiCl 4 with liquid aluminum with the gas flow at the melt surface was estimated to be mass trans- fer in the liquid aluminum. When the Ar-SiCl 4 gas was either blown onto or injected into the liquid aluminum, the maximum silicon contents obtained were below the satu- rated silicon content, and the formation of solid silicon pre- cipitates was not observed. This was believed to be because solid silicon film forms at the gas/liquid interface and acts as a protective layer, stopping further reaction. To overcome this, the injection of Ar-SiCl 4 gas into liquid aluminum kept under a temperature gradient was conducted, and the pre- cipitation of solid silicon during the reaction was achieved. Keywords. Solar grade silicon, SiCl 4 , aluminothermic reduction. PACS ® (2010). 81.05.Bx, 81.10.Dn, 81.15.Gh, 82.60.-s. 1 Introduction The production of crystalline silicon solar cells has in- creased significantly this decade. However, the supply of silicon feedstock has been unstable, and sudden increases in feedstock price have occurred, particularly over the last five years. This feedstock problem arises because the industry is reliant on expensive, off-spec semiconductor-grade sil- icon (SEG-Si). Although construction of silicon produc- tion plants has accelerated worldwide, the development of a low-cost silicon production process is essential for the sus- tainable development of silicon solar cells. Corresponding author: Kazuki Morita, Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan; E-mail: kzmorita@iis.u-tokyo.ac.jp. Received: January 31, 2011. Accepted: April 15, 2011. Purification of silicon through metallurgical treatments is a promising way for mass production of solar grade sil- icon (SOG-Si) from metallurgical grade silicon (MG-Si). Solidification of silicon is a well known process for re- moving metallic impurities such as iron and titanium which have strong segregation tendencies from silicon. However, removal of phosphorus and boron, which are important dopants in silicon, is practically impossible during solidi- fication because their segregation coefficients are not suffi- ciently small (k P D 0:35, k B D 0:8 [1]). Vacuum melt- ing and oxidation treatment are often employed to remove phosphorus [2] and boron [3]. However, these processes re- quire high temperatures and long times, leading to a high price for the silicon produced. If silicon raw material with phosphorus and boron levels below 1 ppma can be supplied at a low price, the situation for silicon solar cells will change dramatically. SiCl 4 gas mixture is generated as a by-product during the production of high purity quartz or semiconductor sil- icon. Although it is generally recycled through a special treatment which consumes a vast amount of energy, it may be possible to be used as a source of SOG-Si. Recently, metallothermic reduction of SiCl 4 for producing SOG-Si has been investigated using sodium [4], zinc [5, 6] and alu- minum [7] as reductants. Sodium vapor can be supplied for the reduction of SiCl 4 , which is favorable for the reaction rate. However, there is strong disquiet for the handling of sodium vapor in mass production because of its high chem- ical reactivity. Zinc can also reduce SiCl 4 in the gas phase due to its high vapor pressure, and fibrous silicon or sili- con deposition can be obtained [5,6]. Yoshizawa [8] con- ducted aluminothermic reduction of SiCl 4 in the 1960s, and showed the formation of a mixture of solid silicon along with liquid Al-Si alloy. Although the mixed product is gen- erally a disadvantage for separation of silicon product, we can expect further precipitation of silicon to occur during cooling of the alloy. More importantly, silicon can be highly purified during the partial solidification from the alloy, as reported by the authors [9–15]. Hence, the reduction of SiCl 4 with pure aluminum, followed by the partial solidifi- cation of silicon, is considered to be a promising process to produce SOG-Si. The phenomena during the aluminother- mic reduction of SiCl 4 , however, were not well understood. In the present work, we have investigated the alu- minothermic reduction of SiCl 4 by blowing and injecting Ar-SiCl 4 gas into the liquid aluminum. The influence of adding copper to the melt, which is regarded as not harmful