EFFECT OF ANNEALING ON THE AC LEAKAGE COMPONENTS OF THE ZNO VARISTOR .2. CAPACITIVE CURRENT

Following an earlier study on the temperature dependence of the resistive component of the leakage current of a ZnO varistor, a detailed study has been undertaken in this paper to compare the temperature dependence of the resistive (IR), capacitive (IC), and total (IT) leakage currents between unannealed and 600 °C annealed samples. It is shown that, as before, an Arrhenius plot of IC and IR versus temperature can best be represented by a break in the plot comprising a high-temperature regime (125-165°C) with a high activation energy and a low-temperature regime (30-100 °C with a low activation energy. The activation energies of IR-T curves are generally higher than those of IC-T curves in both temperatures regimes. Upon annealing at the critical temperature of 600 °C, the activation energy changes for both current components, accompanied by increased stability of the varistor. For IR, the activation energy becomes higher in the low-temperature regime in agreement with previous observation, and for IC, it becomes lower in the higher-temperature regime. The relative contributions of these two current components on total current are also different in the two temperature regimes. In the low-temperature regime, the total current is mostly capacitive, and in the high-temperature regime, it is mostly resistive. The varistor is thus prone to more Joule heating in the high-temperature regime than in the low-temperature regime.