Cation diffusion and ionic conductivity in soda-lime silicate glasses

Abstract

We have studied the mobilities of calcium and sodium ions in silicate glasses of compositions xNa₂O·(3 − x)CaO·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.