Knudsen effusion study of silica

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 ΔGT= 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 αv=(2.2 ± 0.8)× 10–2 in the experimental temperature range.