Water diffusion in thin silica sheets : Effect of swelling stress on diffusivity

Water diffuses into silica glass surfaces reacting with the SiO₂ structure under hydroxyl generation. Swelling of water-containing silica at high temperatures was reported in literature. As a consequence of volume swelling and restriction of free expansion by the bulk material, swelling stresses are caused. The effect of these stresses on diffusivity and equilibrium constant of the water/silica reaction was studied in several papers for thin diffusion layers negligible compared to the thickness of the bulk material. In very thin glass specimens the diffusion problem becomes more complicated when the diffusion zone size competes with the thickness of the glass specimen. In this case the swelling stresses must decrease with time due to the mechanical equilibrium condition. In the present investigation we concentrate predominantly on the low-temperature reaction mechanism (<500°C) and apply an analytical solution for transparent first-order considerations. In addition, some first numerical results are reported.