Low-temperature sintering and electrical properties of Co-doped ZnO varistors

ZnO–Bi₂O₃–B₂O₃-based varistors doped with each kind of cobalt oxides were prepared by conventional ceramic processing. The effects of CoO, Co₂O₃ and Co₃O₄ on the microstructure and the electrical characteristics of varistor samples sintered at 880 °C were investigated separately. Analysis of microstructure indicated the cobalt cations were distributed both in grain regions and grain boundary regions and no crystalline phases containing cobalt were detected in XRD patterns for the samples with various cobalt oxides. All these cobalt oxides could effectively enhance the varistor performance by effectively increasing the nonlinear coefficient and lowing the leakage current, while the breakdown voltage fields increased slightly. Capacitance–voltage characteristics showed the potential barriers of varistor samples increased with the addition of each cobalt oxide. It was found that the addition of same amount of cobalt cations in various cobalt oxides had a different effect on the varistor samples. Best electrical properties were obtained for the varistor sample containing Co₃O₄, in which the nonlinearity coefficient is 28.5, the leakage current density is 3.4 μA and the breakdown voltage field is as low as 260 V/mm.