Effects of Sb2O3 on the microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics fabricated by two-step solid-state reaction route

In this work, the ZnO–Bi₂O₃–Cr₂O₃–Co₂O₃–MnO₂ varistors doped with different content of Sb₂O₃ were prepared by two-step solid-state reaction route, including a pre-calcining of the mixtures of nanosized ZnO and the other additives at an optimized temperature, followed by a consequent sintering process at different temperatures. Meanwhile, the effects of Sb₂O₃ on the sintering temperature, microstructure and electrical properties of the objective varistors were investigated. It was found the densification temperature went up in a proper range and the content of pyrochlore phase, spinel phase and β-Bi₂O₃ phase increased with the increasing content of Sb₂O₃, while the grain size of ZnO–Bi₂O₃-based varistor reduced. The results demonstrated that at the same sintering temperature, the second particles increased with the increasing amount of Sb₂O₃, which was helpful to control the grain growth, leading to a higher breakdown voltage. However, the decrease of α-Bi₂O₃ phase (melting point of α-Bi₂O₃ phase is 825 °C), which is the main component of the liquid Bi₂O₃ phase in the sample during sintering process, leads to the increase of the sintering temperature of the green pallet. As a result, the ZnO varistor doped with 3.0 mol% Sb₂O₃ sintered at 1000 °C exhibited the highest breakdown voltage of 1863.3 V/mm. By contrast, the ZnO varistor without Sb₂O₃ doping sintered at 900 °C had the optimum nonlinear coefficient of 59.8.