Preparation, characterization and dielectric response of a high-breakdown-field ZnO-based varistor

In this study, ZnO-based varistors with a high breakdown field were obtained through BaCO₃ doping and sintering at a low temperature of 950 °C. The grain size of the ZnO samples decreased to 1.28 μm, and the breakdown field was enhanced to 3845 V/mm, which is approximately one order of magnitude higher than that of ordinary ZnO-based varistors. The dielectric responses of the ZnO-based varistors were measured in a wide frequency and temperature range. At 193 K, two dielectric relaxation peaks with activation energies of 0.22 and 0.35 eV were observed and considered intrinsic defects; these peaks did not vary with the preparation process. Another dielectric relaxation peak observed in a wide temperature range (283–463 K) was characterized in an impedance plot. This relaxation peak was ascribed to extrinsic defects related to the grain boundary. The resistance values of the grain boundary increased by two orders of magnitude through BaCO₃ doping and lowering of the sintering temperature. The corresponding activation energy also increased from 0.53 to 0.74 eV. A parallel resistance–capacitance circuit model was proposed to interpret the variation in the electrical properties of ZnO-based varistors.