Tracer diffusion of 22Na and 45Ca, ionic conduction and viscosity of two standard soda-lime glasses and their undercooled melts

The tracer diffusivities of 45Ca in two different high purity standard soda-lime silica glasses have been measured by the radiotracer method below and above their calorimetric glass transition temperatures. Calorimetric glass transition temperatures (Tg) of 845 K and 867 K have been obtained for standard glasses I and II, respectively, using differential scanning calorimetry (DSC) at a heating rate of 20 K/min. In this paper, we focus on the results of 45Ca diffusion and conductivity of the two standard soda-lime glasses and compare them with 22Na diffusivities also obtained in our laboratory [E.M. Tanguep Njiokep, H. Mehrer, Solid State Ionics 177 (2006) 2839, E.M. Tanguep Njiokep, H. Mehrer, Defect Diffus Forum 237–240 (2005) 282]. The 45Ca diffusion coefficients obtained are found to follow the Arrhenius law, both below (Tanguep Njiokep and Mehrer, 2006, 2005) and above Tg. In the Arrhenius diagram a change of slope of the 45Ca diffusivities appears at 835 K for standard glass I and at 790 K for standard glass II. At the same time, the ionic conductivities display a change in slope at 790 K and 778 K for standard glasses I and II, respectively. These temperatures are somewhat smaller than the calorimetric glass transition temperatures obtained at a heating rate of 20 K/min. Rather, they appear to be close to values of Tg obtained by extrapolation to a vanishing heating rate (Tanguep Njiokep and Mehrer, 2006). The viscosity diffusion of standard glass I is considerably smaller than the conductivity diffusion coefficient and both tracer diffusivities. In both glasses the ionic conductivity is essentially due to the motion of Na ions. The contribution of Ca ions to the conductivity is negligible.