Silicon Preparation and Purity from the Reaction of Sodium with Silicon Tetrafluoride and Silicon Tetrachloride: A Thermochemical Study

摘要

Thermochemical equilibrium computations for the preparation of silicon (Si) by the reaction between sodium (Na), either liquid or vapor, with silicon tetrafluoride (SiF sub 4 ) and silicon tetrachloride (SiCl sub 4 ) are presented. Computations indicate that SiF sub 4 reacts with either liquid or gaseous Na to produce temperatures sufficiently high to form molten Si. Liquid Na reacts with SiF sub 4 to produce substantially higher Si yields than does the free combustion reaction with Na vapor; however, the Na vapor/SiF sub 4 reaction, if temperature-constrained at the Si melting point, produces an expected Si yield close to 100%. A stoichiometric mixture of liquid Na and SiCl sub 4 vapor reacts to produce liquid Si, gaseous sodium chloride (NaCl), and a small concentration of Si subhalides. Gaseous Na, however, reacts with SiCl sub 4 to form entirely gaseous reaction products and a high yield of Si (liquid) but subhalide concentrations are greater than when liquid Na is used. The reactions of a number of impurity elements in Na, during the course of the Na-Si halide reaction, have been described. Of those considered, only calcium (Ca), magnesium (Mg), phosphorus (P), and strontium (Sr) are expected to co-exist to any extent in Na vapor and none is expected to be in the Si products if excess Si halide is used in the Na-Si halide reactions. (ERA citation 05:020279)