Knudsen effusion study of silica

Abstract

Knudsen effusion experiments with SiO₂(β-cristobalite) on a thermomicrobalance have been carried out over the temperature range 1823–1983 K. Interpretation of the effusion data is based on the following reactions: (a) SiO₂(s)→ SiO(g)+½ O₂(g), and (b) O₂(g)→ 2O(g). Third law calculations give the standard Gibbs free energy of the reaction (a) as ΔG_T= 182 050–58.29 T± 2500 cal mol^–1 in the measured temperature range, the heat of formation of SiO(g) as ΔH^°₂₉₈=–27.9 ± 3.5 kcal mol^–1, and the dissociation energy of SiO(g) as D^°₀= 8.45 ± 0.20 eV. Determination of the vaporization coefficient of SiO₂(s) gives α_v=(2.2 ± 0.8)× 10^–2 in the experimental temperature range.