ИСТИНА

Gas defects, gas content, gas release and gas diffusion processes in glassy and crystalline silicas have been studied by the method of kinetic thermodesorption mass spectrometry. The method has been also developed for polydispertional samples with different dispersion functions of particle sizes and with different shapes of particles. The comparative study of diffusion processes in silica glasses of standard and non-standard types and crystalline quartzes of variable origin has been performed for the first time. Diffusion coefficients and activation energies for water diffusion in dissimilar silicas have been determined. Gas transport processes in glassy and crystalline silicas are different within two temperature ranges. The numerous processes, such as surface diffusion, grain boundary diffusion, decrepitation, surface chemical reactions (dehydroxylation a.o.) can be the rate-determining ones at low temperatures. At high temperatures (higher than 800 C), the limiting stage is the volume diffusion in structure of the silicas. A new structural-geometric model of water diffusion transport in silicas has been proposed at high temperatures. This model takes into account the interaction of water and silica generating hydroxyl groups as well as the change of mean effective diameter of the diffusion channels formed by the rings from silicon-oxygen tetrahedra. The model provides reasonable interpretation of many peculiarities of water diffusion in vitreous and crystalline silicas, including concentration dependence of water diffusion coefficient.