Removal of Phosphorus, Aluminum and Calcium by Evaporation in Molten Silicon

Abstract

Vacuum refining under the atmospheres of 8.0×10−3∼3.6×10−2 Pa was applied for removing phosphorus, aluminium and calcium in metallurgical grade silicon (MG-Si) at 1722∼1915 K, and the low-pressure refining under the atmospheres of 4.0∼190 Pa was applied for removing phosphorus at 1733∼1823 K.The phosphorus and calcium contents decreased under 0.1 mass ppm by vacuum refining at 1915 K. The removal rates of these elements from molten silicon were roughly expressed by the equations of the first order. The apparent rate constants were expressed as follows;(This article is not displayable. Please see full text pdf.) The activation energy of removing phosphorus, aluminium and calcium from molten silicon were estimated to be 130 kJ/mol for phosphorus, 186 kJ/mol for aluminium and 147 kJ/mol for calcium, respectively. It has been demonstrated that the overall removing rates of these elements are controlled by the diffusion in molten silicon and the evaporation from the silicon surface.The phosphorus content was also decreased by the low-pressure refining. The removal rate of phosphorus was roughly expressed by the equation of the first degree. It increased with decreasing atmospheric pressure, but did not change with increasing melting temperature.The evaporation rate of silicon observed by vacuum refining was expressed as follows:(This article is not displayable. Please see full text pdf.)