Surface and bulk structural relaxation kinetics of silica glass

The structural relaxation kinetics of a silica glass were measured by following the IR structural band positions, which are directly correlated with the average Si-O-Si bond angle as well as with the fictive temperature of the glass, as a function of heat-treatment time, temperature and the water vapor pressure. Both surface relaxation and bulk relaxation kinetics were determined by measuring the IR reflection and absorption band positions, respectively. The surface relaxation was much faster than the bulk relaxation and had a smaller activation energy. Also, both relaxation kinetics were faster in the presence of water vapor. The apparent bulk relaxation time determined from the IR absorption band shift was a composite relaxation rime consisting of both the relaxation time of the water-catalyzed near surface layer and the true bulk relaxation time of the glass interior which is unaffected by water vapor. The true bulk relaxation time was evaluated and found to have an activation energy consistent with that of the viscous flow.