Masahiro Shinmei

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.

Synthesis of fluorine doped silica gel and its properties at high temperature

Fluorine doped silica gels were synthesized by using the sol-gel processes of (A) SiF4(g) and H2O(1) and (B) the mixed solution of Si(OC2H5)4, C2H5OH, H2O and H2SiF6. By the former process we obtained a gel of relatively high fluorine content (8–12 at.%F), while we could synthesize the gel of 0–12 at.% F by adjusting the F/Si ratio of the starting solution mixtures by the latter process. The defluorination behavior and the structural change of these gels at high temperature were studied by heating-mass spectrometry, IR and ESR measurements. The results revealed the following: (1) defluorination by liberation of SiF4(g) was admitted from temperatures at about 400°C and was controlled by the diffusion of fluorine in the gel bulk. (2) The peak separation analysis for the IR band of 1300-900 cm−1, where the stretching vibrations of SiO and SiF appear, showed that the change of the band shape resulted from the increase or the decrease of the SiF bonds and the change of the bond angle of SiOSi as well as the change of the force constant accompanied by fluorination or defluorination. (3) The defects of the Si E′ center were induced by X-ray irradiation depending on the degree of the defluorination, and were reduced by the heat treatment. However, with the heat treatment at temperatures higher than 1000°C, the E′ center increased again. The IR spectra suggest that this behavior might correspond to the gel-glass trasition.