Effect of microwave sintering on the microstructure and electrical properties of low-voltage ZnO varistors

Abstract

ABSTRACTCoarse-grained ZnO varistors for low-voltage applications were prepared by microwave sintering technique under different soaking times of 5–150 min. For comparison, a low-voltage ZnO varistor was also prepared through a conventional sintering process. Microwave sintering remarkably enhanced the grain growth rate of ZnO varistors. Average grain size of the sample prepared by microwave sintering in 15 min was about 20 µm, which is similar to the grain size of sample prepared conventionally in 150 min time. In addition to grain growth, an increase in microwave sintering time led to precipitation of zinc titanate (Zn2TiO4) on the top surface of samples which sintered for long dwell times. X-ray diffraction and scanning electron microscopy results from different points of the samples declared that precipitation of Zn2TiO4 phase is due to the high rate of bismuth evaporation of Bi-rich liquid from top surface and the reaction between remaining titanium ions on the surface with ZnO. The results showed that increasing sintering time from 5 to 150 min increased the grain size from 14 to 33 µm, consequently, the breakdown field decreased from 90 to 27 V/mm, respectively. These changes led to a switch in the varistor application, from low to very low voltage.