Effects of SiO2 on the microstructure and electrical properties of ZnO-based varistors (ZBSCCM) prepared by two-step sintering routework

Abstract

In this work, ZBSCCM varistors were fabricated by precalcining of raw materials, including nanosized ZnO, Bi2O3, Sb2O3, Cr2O3, Co2O3, MnO2 and different amount of SiO2 powders at 800 °C and then sintering at 950 °C, and the effects of the SiO2 content on the microstructure and electrical properties of the varistors were explored. SiO2 doping formed two additional phases that played opposite roles; that is, Zn2SiO4 spinel, which was pinned at ZnO grain boundaries and restrained ZnO grain growth, and a Bi12SiO20 glass phase could promote ZnO grain growth. Nevertheless, all the samples were highly densified above 99%. The SiO2 dopant reacted with ZnO and diffused faster in or reacted with liquid Bi2O3. SiO2 doping, conversely, slowed the diffusion of MnO2, Co2O3, and Cr2O3 in the liquid and facilitated their reaction with ZnO, which increased the interfacial state density and barrier height. Therefore, the nonlinear coefficient and leakage current density increased and decreased, respectively. At 1.0 mol% SiO2, the ZBSCCM varistor exhibited the optimal comprehensive electrical properties, including a breakdown field, nonlinear coefficient, and leakage current density of 982.7 V/mm, 74.0, and 0.4 μA/cm2, respectively.