Water diffusion, oxygen vacancy annihilation and structural relaxation in silica glasses

Abstract

Water diffusion into silica glasses causes structural changes which can be readily detected by ultraviolet (UV) and infrared (IR) spectroscopy. One such change is the annihilation of the oxygen vacancies which exist in some silica glasses. This process can be monitored using the UV absorption at ∼ 240 nm (∼ 5.0 eV). Water can also accelerate local structural relaxation of silica glass. This relaxation can be monitored by measuring the IR absorption peak at 1873 cm −1. The diffusion coefficients for water penetration and local structural relaxation were measured for various silica glasses with low water concentrations in the high temperature range of 750–1200°C under a constant water vapor pressure of 355 mm Hg. The diffusion coefficient for oxygen vacancy annihilation was also measured for oxygen-deficient glasses under the same conditions. The water diffusion coefficient was nearly identical for all glasses investigated. The diffusion coefficients for oxygen vacancy annihilation in oxygen-deficient glasses and for local structural relaxation of stoichiometric silica glass were higher than the water diffusion coefficient even though both structural changes are accelerated by water. Water diffusion into silica at high temperatures has been explained in terms of the local equilibrium of the reaction between mobile molecular water and silica glass to form immobile hydroxyl water. The phenomena observed here can be explained by postulating the presence of a minute amount of molecular water species which are not restricted by the equilibrium condition for the reaction and can diffuse faster than the majority of the molecular water species.