Effect of the Additives on the Nonlinear Coefficient of ZnO Varistor

In order to improve the characteristics of multilayer chip varistors, we have investigated the effects of additives on the varistor voltage and the nonlinear coefficient of ZnO-based chip varistor, whose fundamental components were ZnO:CoO:Pr6O11:Al2O3=100:1.0:0.1:0.005 (molar ratio). Selected additives were Cr2O3, MoO3, WO3, and Nb2O5, the extent of addition being from 0.05 to 0.1mol%, respectively. Thermal shrinkage and grain-growth rate were observed. The Capacitance-Voltage (C-V) method was applied for evaluating the electrical properties when the sintering temperature was varied from 1120 to 1230°C All the additives remarkably decreased the shrinkage rate above 1000°C. While the grain growth was controlled by the addition of any of the selected additives. its rate depended on each species. Additives were not soluble but segregated within the grain and/or at grain boundaries, irrespective of the kind of additive. While the varistor voltage was generally lowered and the nonlinear coefficient became higher with increasing sintering temperature, their absolute values depended on the kind of additive. Addition of Cr2O3 and MoO3 made available a high nonlinear coefficient with a relatively high varistor voltage, thus much improved varistor characteristics. A factor which influenced the increase of the nonlinear coefficient was interpreted by considering a relative increment of the barrier height, based on the double Schottky barrier model.