Mixed alkali effect of electrical conductivity in glass-forming silicate melts

Abstract

Electrical conductances of nine melts of the mixed alkali glass system (1 - x) Na2O · x K2O · 0.7 CaO · 4.8 SiO2 with x = 0.0, 0.125, 0.25, to 1.0 were measured at 50 Κ intervals between 900 and 1550 °C by applying a recently published, precise conductivity cell for molten glasses and salts. The high reproducibility, the large number of measurements, and the dense sequence of experimental temperatures allowed confident extrapolation of the data up to 2000 °C. Conductance, ϰ, and activation energy, Ea, exhibit strong mixed alkali effects at an identical melt composition. While the maximum of the function Ea = f(x), however, exists up to above 2000 °C, the minimum of lg ϰ = f(x) disappears at approximately 1900 °C. This is indicated at as low a temperature as 1300 °C, above which the potassium content of the melt which is characterized by the extreme values increases with increasing temperature thus becoming unity between 1850 and 1950 °C. The existence of a mixed alkali effect also in the molten state suggests that structural entities which cause this effect in solid glasses are still present in the melts. Besides, the different upper temperature limits for ϰ and Ea raise the question whether extreme values, by which the mixed alkali effect is usually defined, are characteristic also of the underlying physico-chemical phenomenon. Deviations from "ideal" behavior, however, also exhibit different upper temperature limits.