Water diffusion into silica glass at a low temperature under high water vapor pressure

Abstract

Water in glass has disproportionately large effects on various properties of oxide glasses, such as chemical durability and mechanical strength. Silica glass is the basis of many oxide glasses. Thus, there have been numerous studies on water diffusion into silica glasses. But most past research on water diffusion into silica glasses has been conducted at high temperatures, where water diffusion is accompanied by structural changes of the glass. In the present study, water diffusion into silica glass at low temperature, 250°C, was measured using IR spectroscopy as the main research tool. Saturated high water vapor pressure was used to increase the amount of water diffusing into the glass so that the water can be detected by the IR method after a reasonably short heat-treatment time. It was found that both water solubility and diffusion coefficient were time dependent. Furthermore, both hydroxyl and molecular water were detected in the glass. From the concentrations of hydroxyl and molecular water, an apparent equilibrium constant, K=[SiOH]n/[H2O] was evaluated. At high temperatures, water reacts with silica glass by Si–O–Si+H2O↔2SiOH, and the local reaction equilibrium has been established at the value of n being 2. In the present study of low temperature water diffusion in silica glass, unlike the local reaction equilibrium in high temperature studies, the value of n was found to be close to 1. This indicates that the two hydroxyls formed in the glass during the reaction are not independent of each other.