Role of Structure in Ionic Conduction in Oxide Glasses
Abstract
The importance of structure for the ionic conductivity of oxide glasses has been established using: (1) binary Rb and K germanate glasses; (2) mixed (Rb,Ag) and (Rb,K) germanate glasses; (3) neutron irradiated high purity quartz and (4) sodium triborate glasses with different melt conditions. The electrical conductivity is measured from 10 Hz to 100 kHz and from room temperature to 620 ^circC. The interatomic distances, coordination number, degree of disorder and the fractions of BOs and NBOs for the germanate glasses are obtained using EXAFS and XPS techniques. In the alkali germanates, both GeO_6 and NBO units are produced for all alkali concentration <20 mol%. The molar volume shows a minimum with respect to alkali concentration without affecting appreciably the various interatomic distances, indicating a non-uniform spatial distribution of atoms in the glass structure. The structural disorder around alkali ions in germanate glasses is much higher than that in corresponding silicate glasses, suggesting that the modified random network model is less appropriate for the former. The composition dependence of d.c. conductivity is explained in terms of the 'unoccupied volume' as a pathway for diffusion. The cooperative ion movement model for alkali silicates is inappropriate for describing conductivity in germanate glasses. The local structure for the mixed mobile ion glasses does not seem to be the key for the development of the mixed mobile ion effect. The activation energy, density, and IR absorptivity of sodium borate glasses are affected by the melt conditions, indicating extremely slow relaxations and anomalous expansion in this system. In contrast to the literature, the present glasses do not show simple correlations between the non-ideality of electrical relaxation (Kohlrausch exponent beta) and the d.c. conductivity, or the nominal cation-cation distance. Instead, beta is strongly determined by the ion distribution and the Coulombic interactions between a mobile cation and its charge compensating center. A dose of 1times10 ^{19} (or greater) fast neutron/cm ^2, which is known to create appreciable disorder in quartz, enhances the non-ideality of the electrical relaxation. It introduces a second plateau in a.c. conductivity at high frequencies and a broadening of the electric modulus peak.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1996
- Bibcode:
- 1996PhDT........42H
- Keywords:
-
- IRRADIATION;
- INHOMOGENEITY;
- Engineering: Materials Science; Engineering: Electronics and Electrical; Physics: Electricity and Magnetism