Effect of thermal history on conductivity and electrical relaxation in alkali silicate glasses

Abstract

Electrical conductivity σ 0 and electric field relaxation measurements have been carried out as a function of thermal history for two alkali silicate glasses, Na 2O3SiO 2 and K 2O3SiO 2. Specimens of each glass with three different thermal histories, two of the anneal-and-quench type and one of the rate-cool type, were studied. The average structural or fictive temperature T f of each of the specimens was characterized by measuring their indices of refraction. Effects of thermal history on σ 0 and its activation enthalpy H σ ∗ were in accord with results of previous investigators. That is, for a given type of thermal history σ 0 was lower and H σ ∗ higher the lower T f. In addition it was found that for two specimens with the same T f or index of refraction but different thermal histories the rate-cooled specimen exhibited a lower conductivity than the annealed-and-quenched specimen, in accord with the results of Ritland. The distribution of relaxation times τ σ for decay of the electric field due to ionic migration was found to be due primarily to a distribution in the pre-exponential term ln τ σ ∗ in the equation ln τ σ = ln τ σ ∗ + H ∗/RT ; the distribution in H ∗ was extremely narrow. Differences in thermal history caused small differences in the distribution of τ σ , but no difference in the average activation enthalpy 〈H ∗〉 for τ σ . From this result it appeared that the dependence of the conductivity activation enthalpy H σ ∗ on thermal history was due to the effect of thermal history on the temperature dependence of the distribution in τ σ .