Water In Silica Glass Research Articles

Fast diffusion path for water in silica glass

Fast diffusion path for water in silica glass

Diffusion of water in silica glass in the absence of stresses

AbstractIn this paper, we consider the diffusion of water vapor 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 liquid water, the concentration of water at the surface quickly achieves saturation value. In gas, the concentration of water at the surface does not achieve a constant value, but builds up with time eventually reaching a steady state value. Here, 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 consistent with published data. We suggest that barrier formation is a consequence of the structure of the silica glass at the free surface and the way that water reacts at the surface in contrast to the way it reacts in the bulk glass. This description gives a quantitative explanation of the development of a diffusion barrier and provides a contrast between the way liquid water and water vapor behave at a free surface.

Diffusion of water in silica glass

The diffusion of water into silica glass is modeled to result from the diffusion of molecular water into the glass and its reaction with the silicon-oxygen network to form SiOH groups. Equations for this diffusion-reaction mechanism are presented and compared with experimental diffusion profiles. At temperatures above about 500 °C the reaction goes to equilibrium, but at lower temperatures it does not, leading to a time dependence of the concentration of surface-reacted OH groups and of their apparent diffusion coefficient. At higher temperatures, the OH groups are nearly immobile, but diffuse far enough to sample neighboring OH groups, leading to a bimolecular reverse reaction. At lower temperatures only OH pairs react, giving a first-order reaction. When water tagged with O18 diffuses into silica, the O18 exchanges with O16 in the silicon-oxygen network of the glass. This process is also controlled by the rate of diffusion of molecular water into the glass, and the rate of O18-O16 exchange. This diffusion-reaction mechanism gives a unified description of the diffusion of water in silica glass from 160 °C to 1200 °C at least.

The Irreversible and Reversible Changes in Silica Glasses Observed by Electrical Properties (Part 2)

The effect of heat treatment on electrical properties of a quartz glass prepared by flame fusion was investigated and the following results were obtained. (1) When the glass was heated at 1000°C, the electrical conductivity increased irreversibly with time at first and then decreased. (2) The increase in conductivity went on uniformly in the interior of the glass. On the other hand, the decrease in the conductivity started at the surface layer of the glass and spread to the interior. (3) The glass approached finally a state in which the electrical conductivity changed reversibly between high and low conductive states depending on the secondary heat treatment conditions. Such irreversible and reversible changes can be explained by considering the chemical equilibrium between OH group and molecular water in silica glass, outward diffusion of molecular water and protonic conduction.

Water Diffusion in Glasses: Influence of Pressure and Fugacity on Diffusion Coefficient of Hydroxyl and Molecular Groups

Diffusion of water in silica glass was studied under high pressure and water fugacities. To clarify the role of each parameter, experiments were conducted by varying the pressure at constant fugacity and by varying the fugacity at constant pressure. Concentrations of diffusing substances were obtained using FTIR spectroscopy. The solubility of molecular water species was observed to decrease with pressure while it increased linearly with fugacity. The diffusivities of hydroxyl and molecular water species were obtained at different pressures and fugacities.

Effect of residual water in silica glass on static fatigue

Static fatigue of water-soaked silica glasses was investigated in formamide after various heat treatments. It was found that the fatigue resistance increased with increasing heat treatment temperature. Water concentration profile near the surface of the similarly treated silica glass was obtained from infrared spectra of successively etched sample. The increasing fatigue resistance of specimens heat treated at higher temperature was attributed to the reduced amount of residual water in the glass surface and the consequent increase in crack initiation stress.

ChemInform Abstract: EFFECT OF STRESS ON WATER DIFFUSION IN SILICA GLASS

The diffusion of water in silica glass was measured as a function of applied uniaxial stress and hydrostatic pressure at selected temperatures. It was found that the diffusion coefficient of water increased exponentially with increasing tensile stress and decreased with increasing compressive stress and increasing hydrostatic pressure. The activation volume for water diffusion in silica glass was found to be ∼170 cm3/mol at 192°C and ∼72 cm3/mol at 350°C. The solubility of water in glass showed a trend opposite to the diffusion coefficient, namely, it decreased exponentially with increasing tensile stress and increased with increasing compressive stress and hydrostatic pressure. These stress (or pressure) dependences were attributed to the shift of the glass-water reaction equilibrium.

Effect of Stress on Water Diffusion in Silica Glass

The diffusion of water in silica glass was measured as a function of applied uniaxial stress and hydrostatic pressure at selected temperatures. It was found that the diffusion coefficient of water increased exponentially with increasing tensile stress and decreased with increasing compressive stress and increasing hydrostatic pressure. The activation volume for water diffusion in silica glass was found to be ∼170 cm3/mol at 192°C and ∼72 cm3/mol at 350°C. The solubility of water in glass showed a trend opposite to the diffusion coefficient, namely, it decreased exponentially with increasing tensile stress and increased with increasing compressive stress and hydrostatic pressure. These stress (or pressure) dependences were attributed to the shift of the glass‐water reaction equilibrium.

Water in silica glass

Water in silica glass