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

Abstract

Sodium tracer diffusion coefficients, DNa*, have been measured in sodium borosilicate glasses of the type (Na2O)0.2[(BO1.5)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, BO1.5, 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 80kJ/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.