Sodium tracer diffusion in glasses of the type (Na2O)0.2[(BO1.5)x(SiO2)1 − x]0.8

Sodium tracer diffusion coefficients, D_Na*, have been measured in sodium borosilicate glasses of the type (Na₂O)_0.2(BO_1.5)_x(SiO₂)_1 − x]_0.8 as a function of temperature and the composition parameter x. In these glasses, which can alternatively also be described by using the formula Na₂O·(2B₂O₃)_x·(4SiO₂)_1 − x, one network former unit, SiO₂, is replaced by another one, BO_1.5, while keeping the sodium concentration constant. At constant temperature, the tracer diffusion coefficient of sodium as a function of x has a shallow minimum at about x = 0.7. At temperatures below about 310 °C the temperature dependences of the measured tracer diffusion coefficients are of Arrhenius-type; at higher temperatures one observes an increase in the temperature dependence with increasing temperature. The activation energy derived from sodium tracer diffusion data for temperatures up to about 310 °C increases about linearly with increasing x from about 70 to 80 kJ/mol. The pre-exponential factor as a function of x varies by about one order of magnitude and has a minimum at about x = 0.4. Values derived for the Haven-ratio are smaller than one and show a shallow minimum as a function of x at around x = 0.75. Furthermore, it was investigated whether there is a significant, directly measurable uptake of water during annealing in moist atmospheres and whether water taken up from moist atmospheres can influence the diffusion of sodium.