A Thermodynamic Analysis of the High-temperature Vaporization Properties of Silica.

A review and analysis were made of the thermochemistry of the vaporization of silica at high temperatures. Various possible reactions that can occur when silica vaporizes were considered. The free-energy change for each reaction was obtained as a function of temperature. The reaction SiO/sub 2/(1) yields SiO(g) + 1/2 O/sub 2/ (g) was considered The equilibrium constant of the reaction and the equilibrium vapor pressure of liquid silica were calculated from the heat of reaction and entropy change of the reaction. Heats of formation for SiO/sub 2/(1) and SiO(g) were calculated. The thermochemistry of the condensation and dissociation reactions of SiO(g) was thoroughly studied. Because it could lead to a higher effective pressure in an equilibrium mixture of silica. the dissociation of oxygen was studied in the temperature range from 2900 to 3100 deg K. In a reducing atmosphere, decomposition of silica to produce SiO(g) occurs at a much lower temperature. Therefore, reactions with three typical reducing agents, carbon, silicon, and hydrogen, were studied. Calculated vaporization data for SiO/sub 2/(1) were compared with published values. (M.C.G.)