Hydrogen diffusion in fused silica at high temperatures

Hydrogen diffusion in a silica glass at high temperature (750–1500 °C) was investigated. The starting material was a silica glass that was melted in hydrogen atmosphere and contained a high concentration of hydroxyl and hydride. After heat treatment at various temperatures in air, the effective hydrogen diffusion coefficients were evaluated from the removal rates of hydroxyl and hydride. The obtained diffusion coefficients exhibited an unusual temperature dependence, with the lower diffusion coefficient being observed at higher temperature in the temperature range of 1000–1500 °C. This was attributed to the higher reactivity of hydrogen with the glass network at higher temperatures, effectively slowing down the hydrogen mobility. Below 1000 °C, the effective hydrogen diffusion coefficient exhibited a normal temperature dependence. This reversal of the temperature dependence of the effective hydrogen diffusion coefficient appears to be related to the reversal of the temperature dependence of the hydroxyl (or hydride) solubility in silica glass.