Effects of sintering temperature on microstructure and varistor performances of ZnO-SrCO3-Co2O3 ceramics

Sintering temperature is decisive for optimizing the grain boundary environment to obtain the high performance ZnO based varistor ceramic. In this work, the impacts of sintering temperature on microstructure and electrical properties of ternary Zn-Sr-Co varistor were investigated. It was found that the distribution of Sr, the critical factor to form grain boundary, was heavily sensitive to sintering temperature. The considerable Sr ions precipitated at grain boundaries and formed the clusters of SrZnO₂ while the sintering temperature increases from 1150 °C to 1190 °C. Besides, the precipitation of Sr led to the large segregation of Co at grain boundaries. The enrichment behavior of Sr and Co contributed to the optimization of grain boundaries, resulting in the enhanced barrier height. As a result, the excellent nonlinear current-voltage performances, i.e., the high nonlinear coefficient of 56.47 and the low leakage current density of 0.73 μA/cm² were obtained in the ternary ZnO-SrCO₃-Co₂O₃ varistor sintered at 1190 °C. However, the grain boundary environment would be destroyed by the excessive temperature of 1210 °C, resulting in the degradation of grain boundary barrier and especially a surge in the leakage current I_L from 0.73 to 92.19 μA/cm². In addition, varying sintering temperature has the important effects on impedance and dielectric properties of the ZnO-SrCO₃-Co₂O₃ varistors. The findings provide new perspectives for developing the high-performance ternary ZnO-SrCO₃-Co₂O₃ varistor ceramics by optimizing the initial grain boundary environment at different sintering temperatures.