Thermodynamic study of the effect of calcium on removal of phosphorus from silicon by acid leaching treatment
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3/3/04
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Thermodynamic Study of the Effect of Calcium on Removal of Phosphorus from Silicon by Acid Leaching Treatment TOMOHITO SHIMPO, TAKESHI YOSHIKAWA, and KAZUKI MORITA The interaction between calcium and phosphorus in molten silicon was investigated for predicting the removal of phosphorus from silicon by an acid leaching treatment with calcium addition. In the present study, two immiscible liquids of silicon and lead were equilibrated, and the interaction parameter between calcium and phosphorus and the self-interaction parameter of phosphorus in molten silicon at 1723 K were determined. PCa in Si 14.6(1.7)
PP in Si 13.8(3.2)
In the derivation process, the following activity coefficients and the interaction parameters of metallic impurities in molten silicon and lead at 1723 K were also obtained: lng0Pb in Si 3.73(0.010)
lng0Si in Pb 3.39(0.040)
PPb in Si 4.08(0.80)
PSi in Pb 23.8(18)
Ca Pb in Si 8.36(1.6)
Ca Si in Pb 14.3(7.0)
The effect of the calcium addition to silicon improves the subsequent removal of phosphorus in the acid leaching treatment, and the increase in removal fraction of phosphorus was observed remarkably with the addition of calcium in silicon.
I. INTRODUCTION
SOLAR energy will shortly be in great demand, since it is inexhaustible and cleaner than any conventional energy resource. Silicon is the most widely used material to convert solar energy into electricity. Expensive off-grade silicon for semiconductors has been used for solar cells, but it is anticipated that solargrade silicon (SOG-Si) will be short of supply with increasing demand for solar cells. A new metallurgical refining process of SOG-Si using metallurgical-grade silicon (MG-Si) as a starting material has been developed in Japan to produce SOG-Si economically.[1,2] In the process, MG-Si is purified to SOG-Si through a two-step treatment: (1) electron-beam melting for phosphorus removal, followed by the directional solidification for removal of most metallic impurities; and (2) plasma melting with Ar/H2O gas blowing, for boron and carbon removal with another directional solidification. Phosphorus is one of the major impurities in silicon and should be reduced to lower than 0.1 wt ppm for solar cells.[3] The segregation coefficient of phosphorus in silicon reported (0.35[4]) is larger than that of most metallic impurities in silicon, indicating that it is difficult to remove phosphorus efficiently by solidification. Since phosphorus, however, has a high vapor pressure in molten Si,[5] phosphorus is preferentially removed, especially by the vacuum-melting treatment, as in the newly developed metallurgical refining TOMOHITO SHIMPO, formerly Graduate Student, Department of Materials Engineering, University of Tokyo, is Engineer with Showa Shell Sekiyu Corporation, Yokkaichi Mie 510-0851, Japan. TAKESHI YOSHIKAWA, Graduate Student, and KAZUKI MORITA, Associate Professor, are with the Department of Materials Engineering, University of Tokyo, Tokyo 113-8656, Japan. Contact
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