DC conductivity of illitic clay after various firing

Abstract

The temperature dependencies of the direct current (DC) conductivity of illitic clay, which contained 86 % of illite of 1 M polytype, were measured in a temperature range of 20–450 °C, on green samples and on the samples after thermal treatments at 400–1200 °C. The release of the physically bound water from a green sample takes place in three stages—at 72, 186 and 298 °C. After heating at 400 °C, the movement of dominant H+ and OH− ions is characterized by the conduction activation energy (CAE) of 0.31 ± 0.02 eV, below 220 °C. At higher temperatures, K+ ions are dominant charge carriers with a CAE of 1.12 ± 0.02 eV. Upon heating at 450 °C, the DC conductivity slightly increases; a slightly higher CAE (1.22 eV) indicates that K+ ions are moving in a more disordered structure, probably due to starting dehydroxylation. Significant structural changes, which influence the DC conductivity, are finished at 550 °C. After preheating from 550 up to 1000 °C, the defect structure, type, concentration, and mobility of charge carriers are not significantly changed as the temperature dependence of the DC conductivity is independent of the temperature of firing. During preheating at 1100 °C, vitrification and sintering are almost completed. The high mobility of K+ ions in the glassy phase results in a high DC conductivity with the CAE equal to 0.76 eV.