Silicon preparation and purity from the reaction of sodium with silicon tetrafluoride and silicon tetrachloride: a thermochemical study

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.