ON THE MECHANISM OF DIFFUSION OF WATER IN SILICA GLASS

Diffusion of water into silica glass has been studied at low temperatures. Hydrothermal treatments were made with water of normal isotopic composition and water enriched in O-18. Hydrogen and O-18 concentration profiles were measured employing nuclear reaction analysis. After short-term treatments (5 to 20 h) at temperatures between 100 and 200-degrees-C, the concentration ratio [O-18]/[H] was found to be 0.5. This allows the conclusion that water molecules are the diffusing species. Diffusion coefficients deduced from the profiles are D almost-equal-to 3 . 10(-13) cm2/s at 200-degrees-C and D almost-equal-to 6 . 10(-15) cm2/s at 100-degrees-C; the corresponding activation energy is about 58 kJ/mol. The near-surface concentration of hydrogen and O-18 increased with treatment time and temperature, indicating a non-equilibrium state. After long-term treatments (6 to 20 d) at 200-degrees-C, the near-surface concentration of hydrogen was found to have a nearly constant value of 7 . 10(20) atoms/cm3, and [O-18]/[H] ratios up to 1.7 were observed. It was concluded that an equilibrium or near-equilibrium state with regard to the concentrations of H2O molecules and SiOH groups had been reached, the SiOH groups being formed by (reversible) reactions of H2O molecules with the SiO2 network. These findings are discussed in the framework of the diffusion model of Doremus.