Cation diffusion and ionic conductivity in soda-lime silicate glasses

We have studied the mobilities of calcium and sodium ions in silicate glasses of compositions xNa2O (3 - x)CaO x 4SiO2 with x = 0.0, 0.1, 0.3, 1.0 and 3.0 by means of radiotracer diffusion, electrical conductivity measurements, and dynamic mechanical thermal analyses. In glasses containing sodium oxide, the Na+ ions are much more mobile than the Ca2+ ions, and are, therefore, governing the electrical conductivity. In the pure calcium silicate glass, the activation energy of Ca2+ diffusion is higher than the activation energy of the electrical conductivity. This provides strong evidence that the electrical conductivity of this glass is not determined by the migration of Ca2+ ions, but by impurity charge carriers, which are most likely Na+ ions. We sketch the composition-dependent mobilities of Na+ and Ca2+ ions in soda-lime silicate glasses with variable Na2O and CaO content. Our results indicate that the coordination environment of Ca2+ ions remains unchanged when CaO is replaced by Na2O which is consistent with recent results of molecular dynamic simulations. Moreover, our results confirm the formation of dissimilar Na-Ca pairs which lead to a non-random mixing of the cations in the glass. The formation of such pairs was recently deduced from nuclear magnetic resonance spectra of soda-lime silicate glasses.