Water diffusion and surface structural relaxation of silica glasses

Surface structural relaxation of silica glass takes place more rapidly than bulk structural relaxation when the glasses are heat-treated in the presence of water vapor at temperatures below the glass transition temperature. It appears that water enters into silica glass and accelerates the structural change. When silica glass with high hydroxyl content is heated in a relative dry atmosphere, the hydroxyl concentration in the surface decreases. Yet, surface structural relaxation still occurs more rapidly than bulk structural relaxation during such dehydration experiments. For commercially available silica glasses, water exists in the glass predominantly as hydroxyl, but a small fraction can exist as molecular water. The observed surface structural relaxation phenomena are consistent with the postulation that surface structural relaxation is promoted by the presence of this immeasurably small quantity of molecular water rather than the hydroxyl water. (C) 2001 Elsevier Science B.V. All rights reserved.