Temperature dependence of electrophysical characteristics of zinc oxide based varistors

ZnO-based varistors were fabricated by sintering zinc oxide micro crystals with several additives of metal oxides. To avoid the shortage of oxygen, the samples were annealed in a continuously oxygen injected oven at 400 °C for 3–6 h. Temperature dependence of current-voltage characteristics (CVC) and the electrical conductivity of these samples were investigated in the temperature range 25–190 °C. They exhibited high nonlinearity in their I-V characteristics. The nonlinear coefficient varied between 40 and 60 but the highest nonlinearity coefficient for an individual sample was β = 100. At high temperatures, nonlinearity in the I-V characteristics of samples disappeared and the conductivity became ohmic in nature. The observed nonlinearity, an exponential dependence of electric conductivity, and also reduction of samples opening voltage are related with zinc oxide grain boundaries. In all temperature intervals, the hysteresis on curves I = f (U) was observed.