Influence of praseodymium oxide/cobalt oxide ratio on microstructure and electrical properties of zinc oxide varistor ceramics

The microstructure and electrical properties of varistor ceramics, which are composed of (99.5–x–y)ZnO+xPr6O11+yCoO+0.5Dy2O3 system, were investigated with Pr6O11/CoO mole ratio (x/y=0.5/0.5, 0.5/1.0, 1.0/0.5, 1.0/1.0) and sintering temperature. The density of varistor ceramics with Pr6O11=1.0 was almost constant with sintering temperature, whereas it was increased noticeably in Pr6O11=0.5. Increasing Pr6O11 content enhanced the densification for any CoO content and the density was greatly affected not by CoO content but by Pr6O11 content. The varistor ceramics with Pr6O11/CoO=0.5/1.0 exhibited a higher nonlinearity than any other composition ratios. In particular, the varistor ceramics sintered at 1350 °C. exhibited the best electrical properties, with nonlinear exponent of 37.8, leakage current of 7.6 μA, and dissipation factor of 0.059. It was found that Pr6O11/CoO ratio greatly affects various characteristics of varistor ceramics.