Comparative characteristics of yttrium oxide and yttrium nitric acid doping in ZnO varistor ceramics

The effect of different molar ratios of Y2O3 and Y(NO3)3 on the microstructure and electrical response of ZnO-Bi2O3-based varistor ceramics sintered at 1 000 °C was investigated, and the mechanism by which this doping improves the electrical characteristics of ZnO-Bi2O3-based varistor ceramics was discussed. With increasing amounts of Y(NO3)3 or Y2O3 in the starting composition, Y2O3, Sb2O4 and Y-containing Bi-rich phase form, and the average grain size significantly decreases. The average grain size significantly decreases as the contents of rare earth compounds of Y(NO3)3 or Y2O3 increase. The maximum value of the nonlinear coefficient is found at 0.16% Y(NO3)3 or 0.02% Y2O3 (molar fraction) doped varistor ceramics, and there is an increase of 122% or 35% compared with the varistor ceramics without Y(NO3)3 or Y2O3. The threshold voltage VT of Y(NO3)3 and Y2O3 reaches at 1 460 V/mm and 1 035 V/mm, respectively. The results also show that varistor sample doped with Y(NO3)3 has a remarkably more homogeneous and denser microstructure in comparison to the sample doped with Y2O3.