Effect of CoO on nonlinear electrical properties of praseodymia-based ZnO varistors

The microstructure, nonlinear properties, and dielectric characteristics of the varistors, which are composed of ZnO–Pr6O11–CoO–Cr2O3–Dy2O3-based ceramics, were investigated at various CoO contents. The increase of CoO content improved the densification of ceramics as increasing density in the range of 5.25–5.55 g/cm3. The CoO served as a prompter of grain growth as increasing average grain size from 9.9 to 27.2 μm. The varistor voltage decreased from 235.3 to 86.0 V/mm with increasing CoO content. The highest nonlinearity was obtained from the varistors with 1.0 mol% CoO, with 66.6 in nonlinear exponent and 1.2 μA in leakage current. The further CoO addition decreased the nonlinearity.