Role and effect of Co2O3 additive on the upturn characteristics of ZnO varistors

The effect of Co2O3 additions on the characteristics of electric field-current density was investigated at high current densities. With the increase of mole % of Co2O3, the onset of voltage upturn descends to lower current density due to the decrease in the conductivity in ZnO grains. This is confirmed by impedance measurements at high frequencies. Analysis on the capacitance-voltage characteristics shows that the decreased grain conductivity is attributed to the lower carrier density caused by the reduced concentration of interstitial Zni or oxygen vacancy, VO. In the case of the highly doped ZnO varistors containing more than 0.5 mole % Co2O3, the lowered carrier mobility improves the decrease in grain conductivity.