The role of CeO₂ in the high electric field ZnO varistors

The high electric field ZnO-Bi₂O₃ based varistor materials are developed using CeO₂ in the recipe. Achieving' such varistors is commercially desired as the resulting device utilizes reduced volume of materials and thereby enhancing cost effectiveness. Although there may be a subtle role of the electrolytic nature of the CeO₂ in the form of ion transport within the secondary phases in the microstructures causing possible degradation, it is observed that the addition of CeO₂ plays a pivotal role for achieving high electric field varistors via reducing the average grain size to about 25%. The overall enhancement in the voltage gradient for such average grain size distribution exceeds 75%. The plausible role of the chemistry at the grain boundary interfaces for this observation is not ruled out beside the average grain size reduction. The voltage gradient is presumed to be the function of the grain size reduction and overall charge trapping at the grain boundary interfaces where the applied electric field is primarily experienced. The terminal capacitance-voltage (C-V) measurement as a function of applied frequency of the ac small-signal amplitude indicates strong influence of the trapping effect at the grain boundary interfaces assuming identical carrier density in the ZnO grains for each varistor material. The nonlinear coefficient obtained in the current-voltage (I-V) plot is not affected for the CeO₂ added varistors. The C-V data are confirming the presence of the back-to-back Schottky barrier as observed in the varistor samples without CeO₂.