Effect of water incorporation on the diffusion of sodium in Type I silica glass

The diffusion of sodium ions in Type I silica glass was studied experimentally by using Na-22 as a radioactive tracer at temperatures between 400°C and 700°C. It was found that the sodium tracer diffusion coefficient in as-received glass was well described by an Arrhenius equation. All residual radioactivity profiles observed after pre-annealing glass samples in common and wet air at temperatures between 900°C and 1100°C and performing diffusion anneals at 650°C cannot be described by a single diffusion coefficient. A strong residual radioactivity decrease occurred near the surface, which was followed by a much smaller decrease in the bulk below. This type of profile was attributed to the generation of a near-surface region with a reduced sodium mobility during the pre-annealing. A mathematical model for the analysis of the observed profiles was developed and used to determine sodium tracer diffusion coefficients in the near-surface region and in the bulk. It was found that the sodium tracer diffusion coefficient in the near-surface region was by about two orders of magnitude smaller than that in the bulk. This immobilization of Na is believed to be due to a water-induced structural relaxation in the near-surface region during pre-annealing.