Temperature dependent low frequency dielectric and conductivity measurements of Argonne Premium coals

A study has been made of the dielectric properties of a series of Argonne Premium coals as a function of frequency and temperature. Measurements were made in the low frequency range, 0.01-105 Hz, over the temperature range 300-450 K. Two distinct processes were observed over the frequency range. At low frequency, real and imaginary components of the permittivity showed an increase with decreasing frequency, particularly at low frequency and high temperature. This type of behaviour is consistent with a Maxwell-Wagner-Sillars process. At higher frequency, a dipolar relaxation was observed for four of the coals and this was shown to be an activated process. The real conductivity of each coal measured at 0.01 Hz was shown to increase with temperature with the majority of coals showing a distinct break at 370-380 K