Temperature dependence of the AC conductivity of illitic clay

Electrical conductivity measurement was used to characterize thermophysical processes occurring in illitic clay during firing. In this study, illitic clay was subjected to heating up to 1100 °C. The AC conductivity was measured at 10 different frequencies, ranging from 500 Hz up to 2 MHz. In the low-temperature interval (<250 °C) and during dehydroxylation (from 450 °C to 750 °C), H+ and OH- ions were the dominant charge carriers. Above 600 °C, the mobility of alkali ions was high enough to enable them to contribute to conductivity. Formation of glassy phase at high temperature rapidly increases conductivity. The conduction activation energies (EA) were calculated during a second firing. Above 350 °C, the values of EA lay in the interval between 0.79 eV (500 Hz) and 0.66 eV (2 MHz), which correspond to the conduction EA of alkali ions in an amorphous matrix. The responsible conduction mechanism was identified to be ion hopping.