Effect of a Bi-Cr-O synthetic multi-phase on the microstructure and electrical properties of ZnO-Bi2O3 varistor ceramics

ZnO-Bi2O3-MnO2 (ZBM)-based varistors were fabricated via doping a novel synthetic multi-phase (SMP) additive produced by calcining the mixture of 18Bi2O3·Cr2O3 at a given temperature. The effects of the SMP on the microstructural and electrical properties of ZBM varistors were investigated. It was found that the SMP dopant was a compound crystalline phases including Bi-Cr-O phases (Bi7.38Cr0.62O12+x and CrBi18O30) and small amounts of Bi2O3 rather than a synthesized polycrystal. The Bi-Cr-O phases were not emerged for samples with x=1, indicating that the amount of it is tiny and the small Bi2O3 may accelerate ZnO grain growth. With more SMP doping (x>1) in the ZBM ceramics, it acted as a barrier inhibiting grain growth. For samples with x=5, excellent electrical properties were obtained: the nonlinear coefficient α increased up to 50.19 corresponding to the highly barrier height of 2.62 eV; the leakage current IL reduced to 0.3 μA. The dielectric constant εa is proportional to the ratio of the grain size d to the thickness of the depletion layer width t, which explained the εa increased at f=1 kHz for the samples with x=1 and 5. The improvement of the electrical properties can be explained by the oxygen absorption mechanism.