Diffusion of water in silica in the absence of stresses
Sheldon M. Wiederhorn
In this paper we consider the diffusion of water vapour into silica glass at relatively low temperatures, 25°C to ≈500°C. Extensive studies of such diffusion by others have shown that water diffusion from inert gases behaves differently than diffusion from liquid water. In water the concentration of molecular water, or hydroxyl water (silanol groups) at the surface quickly achieves a saturation value. In gas, the concentration of molecular water, and hydroxyl water at the surface of the glass does not achieve a constant value, but builds up with time eventually reaching a steady state value. In this paper we show that published data can be explained by assuming the existence of a surface barrier to the diffusion of water into the silica glass. We develop equations that quantify the diffusion rate through the barrier; the diffusion results are found to be consistent with published data. We suggest that the barrier is a consequence of the particular structure of silica glass at a free surface and the way that water reacts at a free surface in contrast to the way it reacts in the bulk glass. This description gives a good quantitative explanation of the development of a diffusion barrier at a free surface of silica glass and provides a contrast between the way liquid water behaves at a free surface compared to water vapour.