The diffusion of Na ions in type-I silica glass was studied by using 22Na as a radioactive tracer at 400 to 700C. It was found that the Na tracer diffusivity in the as-received glass was described well by an Arrhenius equation. The residual radioactivity profiles which were observed after pre-annealing in normal and wet air at 900 to 1100C, and diffusion annealing at 650C, could not be described by a unique diffusion coefficient. A marked residual radioactivity decrease occurred near to the surface, and was followed by a much smaller decrease in the underlying bulk. This type of profile was attributed to the production of a near-surface region, having a reduced Na mobility, during the pre-annealing. A mathematical model was developed for analysing the observed profiles, and was used to determine Na tracer diffusion coefficients in the near-surface region and in the bulk. It was found that the Na tracer diffusion coefficient in the near-surface region was some 2 orders of magnitude smaller than that in the bulk This immobilization of the Na was suggested to be due to the occurrence of a water-induced structural relaxation of the near-surface region during pre-annealing.

Effect of Water Incorporation on the Diffusion of Sodium in Type-I Silica Glass. L.Tian, R.Dieckmann, C.Y.Hui, Y.Y.Lin, J.G.Couillard: Journal of Non-Crystalline Solids, 2001, 286[3], 146-61