Influence of Cr2O3 on highly nonlinear properties and low leakage current of ZnO–Bi2O3 varistor ceramics

ZnO–Bi2O3–Sb2O3–Co2O3–MnO2–xCr2O3 (ZBSCM–xCr2O3, 0≤x≤0.6 mol%) varistors were fabricated through the conventional solid state method, and the effects of Cr2O3 on the microstructures and electrical properties were investigated. Results showed that the secondary phases CrBi18O30 and Co2Cr0.5Sb0.5O4 emerged when x ranges from 0.2 to 0.4. In these compositions, Cr2O3 acted as a donor and decreased the electrical properties of ZBSCM. For samples with x=0.5, the secondary phases transformed to MnCr2O4 and the electrical properties increased significantly: the nonlinear coefficient α sharply increased up to 80.71 and the barrier height ϕb reached 3.88 eV. This indicates that the donor effect of Cr2O3 disappeared. In addition, with the increase of Cr2O3, the average grain size of ZnO decreased from 7.48 μm to 5.46 μm, which in turn resulted in an increase of breakdown voltage E1mA from 216.17 V/mm to 362.50 V/mm. Besides, all the samples showed the low value of leakage current of lower than 0.1 μA. This varistor might be a promising candidate for highly effective applications.