Influences of sintering process on (Bi₂O₃–V₂O₅–Mn₃O₄–Y₂O₃–Co₂O₃–Cr₂O₃)-doped ZnO varistors

The effects of sintering temperatures on the microstructure, electrical and dielectric properties of (V₂O₅–Bi₂O₃–Mn₃O₄–Y₂O₃–Cr₂O₃–Co₂O₃)-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/cm³. 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 _1mA 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 _r and the dielectric loss tanδ 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 _1mA was 3456.5 V/cm; when the frequency was 10 kHz the values of ε _r and tanδ were 194 and 0.29 at this sintering temperature, respectively. Meanwhile, the temperature coefficient of resistance α _T was only −0.67 % and the impedance was also fairly stable at this sintering temperature.