Sodium tracer diffusion in sodium boroaluminosilicate glasses

Sodium tracer diffusion coefficients, D* _Na, have been measured using the radioactive isotope Na-22 in sodium boroaluminosilicate (NBAS) glasses containing either a small amount of As ₂O ₃ or Fe ₂O ₃. The chemical compositions of the first type of glasses are given by the formula [(Na ₂O) _0.71(Fe ₂O ₃) _0.05(B ₂O ₃) _0.24] _0.2[(SiO ₂) _x(Al ₂O ₃) _{1 - x}] _0.8 and those of the second type of glasses correspond to the formula [(Na ₂O) _0.73(B ₂O ₃) _0.24(As ₂O ₃) _0.03] _0.18[(SiO ₂) _x(Al ₂O ₃) _{1 - x}] _0.82. Tracer diffusion measurements were performed at different temperatures between 198 and 350 °C. Pre-annealing of the glass samples at their glass transition temperatures in common air was found to lead to changes in the values of sodium tracer diffusion coefficients. For the NBAS glasses containing Fe ₂O ₃, after pre-annealing for 5 h, the activation enthalpy derived for the sodium tracer diffusion increases almost linearly from 57.5 to 71.3 kJ/mol with a decrease in the alumina content while the pre-exponential factor of the sodium tracer diffusion coefficient increases from 2.1·10 ^{- 4} to 5.3·10 ^{- 4} cm ²/s. For the iron-free NBAS glasses pre-annealed for 5 h, the activation enthalpy varies between 63.9 and 71.4 kJ/mol while the pre-exponential factor varies between 1.5·10 ^{- 4} and 1.2·10 ^{- 3} cm ²/s. In addition, it was observed that the considered glasses take up water when annealed at 300°C in wet air with P H ₂O = 474 mbar.