Distinct crystal growth on the surface and in the interior of Na2O·2CaO·3SiO2 glass

Abstract

The growth rates of Na2O⋅2CaO⋅3SiO2 crystals on a polished surface, us, and in the volume, uvuv, of the soda-lime-silica glass of the same composition were measured in a temperature range spanning from 873 to 1023 K (T12 = 845 K corresponds to a viscosity η = 1012 Pa·s). We found that us is significantly higher than uv and this difference progressively increases with decreasing temperature reaching in our experiment more than two orders of magnitude. This fact indicates that at low temperatures the effective diffusion coefficient of the structural units determining the growth kinetics along the glass surface is significantly higher than that in the bulk of glass, and gradually approaches the latter with increasing temperature. We also showed that the temperature at which the decoupling between the surface and volume diffusion coefficients occurs is higher than the decoupling temperature between diffusion coefficients estimated from viscosity via Stokes–Einstein−Eyring equation and by crystal rates in the bulk of glass. Our main conclusion about relation between surface and volume diffusion based on the analysis of surface and volume crystal growth for silicate glass combines the measurements performed for some molecular glasses, which proved that surface diffusion is much faster than volume diffusion.