INFLUENCE OF NETWORK-FORMING CATIONS ON IONIC-CONDUCTION IN SODIUM-SILICATE GLASSES

The substitution of Si by Al or B in Na2O 3SiO(2) glass has different effects on ionic conductivity. Whereas the activation energy of ionic conduction, E(sigma), decreases with the amount of Al in sodium aluminosilicate (SAS) glasses, it shows a shallow minimum with respect to B in sodium borosilicate (SBS) glasses. This difference is explained in terms of the strain energy, Delta E(s), and bond energy, Delta E(b), components of E(sigma), which are deduced from recent studies of chemical bonding and local structure of these glasses. It is found that Delta E(s) increases and Delta E(b) decreases with increasing Al and B in both SAS and SBS glasses. The theory of polar covalence has been applied to explain the variation of Delta E(b) in both glass systems. The dielectric constant is important for evaluating the ionic part of the bond energy, rather than to account for the partially covalent character of the bonds.