Diffusion in single and mixed-alkali borate glasses

Diffusion of Na-22 was studied in x Na2O (.) (1-x) B2O3 glasses with x = 0.2 and x = 0.3 using the radiotracer serial sectioning method. A slight decrease of the diffusion coefficients with increasing diffusion time was observed and is attributed to a structural relaxation of the glass network. The temperature dependence of diffusion was measured below and also in a small temperature range above the glass transition temperature (T-G). Below T-G the diffusion coefficients obey Arrhenius relations. Diffusion in the glass with higher Na content is considerably faster. The results are compared with do conductivity data. The pressure dependence of Na-22 diffusion in 0.3 Na2O .0.7 B2O3 yields an activation volume of 0.52 molar volumes. This relatively high value indicates a significant relaxation of the glass network associated with a Na jump. In sodium-rubidium borate glasses with a total alkali content of 30 mol percent and Na/(Na+Rb) ratios between 0.4 and 1.0 diffusion of Na-22 and Rb-86 was investigated at 673 K. Diffusion of alkali ions in this mixed-alkali borate glasses shows a concentration dependence which is different from published mixed-alkali effects for diffusion in silicate glasses. Sodium diffusion is always faster than rubidium diffusion. Na diffusion shows a minimum and Rb diffusion a maximum for a relative concentration of about 60% sodium. However, the ionic conductivity deduced from the diffusivities displays a minimum as a function of the relative composition of sodium like in other mixed-alkali oxide glasses.