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ZnO varistor samples generally have poor preparation repeatability. In this work, ZnO varistor samples A and B with the same preparation technique showed significant differences in properties, and the reasons were explored in detail. Nonlinear coefficient (α) values of 42.59 and 72.68, and leakage current density of 0.013 mA/cm2 and 0.002 mA/cm2 were found for samples A and B, respectively, which are markedly different between the samples. However, samples A and B showed the same phases. In addition, samples A and B exhibited mean grain size (d) of 2.21 μm and 2.47 μm, double Schottky barrier height (φB) of 0.45 eV and 0.48 eV, and grain boundary resistivity (ρgb) of 5.32 × 106 Ω m and 5.40 × 106 Ω m, respectively, which were all similar between the two samples. According to the low-temperature dielectric spectral analyses, the values of k1 of samples A and B, reflecting the concentration of interstitial zinc, were 14.06 and 11.09, and the values of k2 reflecting the concentration of oxygen vacancy were 7.48 and 3.35, respectively. Therefore, the obvious difference in the electrical properties between samples A and B may be caused mainly by the obvious difference in the concentration of oxygen vacancy. Thus, this work provides evidence of the effects of oxygen vacancy on the electrical properties of ZnO varistors. [ABSTRACT FROM AUTHOR]

Cobalt (Co) as an additive has a remarkable impact on the microstructure and electrical properties of zinc oxide (ZnO) varistor ceramics. In this paper, ZnO varistor ceramics were prepared by the conventional solid-phase method, based on the base formula of ZnO-Bi2O3-Sb2O3-MnO2-Cr2O3 with Co2O3 doped and Co3O4 doped respectively, and their microstructure and electrical properties were compared. It's found that doped Co mainly affects the average grain size of ZnO varistor ceramics, and the average grain size of samples doped Co2O3 s greater than that of samples doped Co3O4. In addition, the Co3O4 doped samples has a higher nonlinear of 51. 40, a higher breakdown field of 338. 29 V/mm, a lower leakage current density of 1.33 µA/cmµ, and a higher barrier height of 0.69 eV, which has excellent comprehensive performance. Therefore, Co3O4 s better than Co2O3 to improve microstructure and electrical properties of ZnO varistor ceramic. [ABSTRACT FROM AUTHOR]

The effects of sintering temperatures on the microstructure, electrical and dielectric properties of (VO-BiO-MnO-YO-CrO-CoO)-doped ZnO varistors had been researched systematically. The results showed that, when the sintering temperature increased from 870 to 930 °C, the following changes occurred: the average grain size enlarged from 0.83 to 1.67 μm and the density increased from 4.97 to 5.3 g/cm. The conclusion can be drawn that the optimal microstructure and uniformity could be obtained at 910 °C; Meanwhile, it was observed that the breakdown electrical field E decreased from 5127.3 to 2687.5 V/cm and the nonlinear coefficient α was likely to increase as the sintering temperature increased, however, the latter decreased to 15.6 sharply at 930 °C; Simultaneously, it was noted that the relative dielectric constant ε and the dielectric loss tan δ increased first and then decreased as the sintering temperature increasing when the frequency was 10 kHz. Another conclusion can be drawn that the best electrical and dielectric properties were obtained at 910 °C, where α was 27, E was 3456.5 V/cm; when the frequency was 10 kHz the values of ε and tan δ were 194 and 0.29 at this sintering temperature, respectively. Meanwhile, the temperature coefficient of resistance α was only −0.67 % and the impedance was also fairly stable at this sintering temperature. [ABSTRACT FROM AUTHOR]