Ionic Conduction in Alkali and Thallium Silicate Glasses

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Abstract
Electrical conductivities of binary alkali and thallous silicate glasses have been measured as a function of composition, temperature, and frequency. The best approximation to the activation energy for dc conduction can probably be obtained by extrapolating its frequency dependence to zero frequency, although values obtained at frequencies below 2500 hz do not differ greatly from the dc values. The plot of activation energy for conduction against modifier content consists of two straight lines of different slope. For all the alkali silicate glasses the break in slope occurs near 25 mole % alkali oxide and at an activation energy near 14.7 kcal/mole. The slope of the line below 25 mole % alkali oxide is proportional to the alkali ion radius. The behavior of thallous silicate glass is similar but not identical to that of the alkali silicates. It is concluded that (1) the reported conduction behavior does not result primarily from phase separation, (2) the principal contribution to the activation energy is probably the work required for a mobile ion to pass through the glass network rather than to leave its initial position, and (3) a structural change independent of the nature of the modifier cation occurs near a modifier content of 25 mole %.

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