Study On High-performace Rare-earth Doped ZnO-based Varistor Materials

One of the main target for studying ZnO varistors is to prepare high varistor-voltage ZnO varistor for high voltage application. In this paper, via a conventional ceramic process, several rare-earth doped ZnO-based varistor materials were prepared. The effects of doping elements and doping level on microstructure and electrical properties were investigated. It was found that:(1) In ZnO-Bi₂O₃-Y₂O₃-based varistor materials, Y₂O₃ acted as an inhibitor to the growth of ZnO grains. When the doping level of Y₂O₃ was small, the nonlinear coefficientsαand the breakdown voltages E would increase with the amounts of Y₂O₃ incrasing, and the leakage current IL would increase slightly; however, when the doping level of Y₂O₃ was high,αand E would decrease sharply with more Y₂O₃ doped, but IL would increase dramatically. During sintering, Bi₂O₃ formed liquid phase, promoting the sintering process, and the liquid phase was helpful for the growth of ZnO grains. Bi₂O₃ of itself had volatility, resulted in decrease in relative density. With the doping levels of Bi₂O₃ increasing,αand E would increase at first then decrease, but IL would decrease at first then increase.The average grain size of material with Sb₂O₃ and Y₂O₃ co-doped samples was smaller in comparison to the samples with only Y₂O₃ added. With the atom ratios of Y and Sb increasing, the weight loss of the sintered bodies decreased, and resulted in increase in material densities. The electrical properties of material with Sb₂O₃ and Y₂O₃ co-doped samples were better in comparison to the samples with only Y₂O₃ or Sb₂O₃ added, when the atom ratio was 5, E,αand IL had a best value of 777 V/mm and 23, and 0.17 mA/cm2 respectively.(2) In ZnO-Pr₆O₁₁-Fe₂O₃-based varistor materials, the average grain size of samples would decrease with Fe₂O₃ doping, and the densities of sintered bodies would increase. When the doping level of Fe₂O₃ was small, Fe atoms would dissolve into ZnO grains, providing extra carriers and oxygen, therefore,αand E would increase, IL would decrease; But with more Fe₂O₃ doped, more Fe atoms would segregate at the grain boudaries, decreasing the resistance of the grain boundaries, destroying the electrical properties of the material. Pr6O11 would act with Fe atoms producting PrFeO3 during sintering, inhibitoring the growth of the grains. With the doping amounts of Pr6O11 increasing, more intergranular phases formed,αand E would increase at first then decrease, but IL would decrease at first then increase. The optimum ZnO-based varistor material had electrical property of breakdown voltage of 801 V/mm, nonlinear coefficient of 32.4, and leakage current of 0.04 mA/cm2.(3) In ZnO-Pr6O11-WO3-based varistor materials, the average grain size of samples would decrease with WO3 doping, but because of the high melting point of WO3, the densities of the samples would drop down dramatically with more WO3 doped. The optimum ZnO-based varistor material had electrical property of breakdown voltage of 745 V/mm,nonlinear coefficient of 20,and leakage current of 0.02 mA/cm2.