Study On High Voltage Gradient Zinc Oxide Varistor

Zinc oxide, as a wide band-gap semiconductor material, is utilized in varistors. The zinc oxide based varistors are widely used in electric appliances especially in high voltage lines as voltage surge protection devices. Owing to the excellent nonlinear current-voltage characteristics, zinc oxide based varistors become the core elements of the arrester and directly affect the security and the reliability of the power lines.At present, the rapid development of super high-voltage power net-work makes the pressing requirement for the arrester with more security, reliability and miniaturization. Owing to the zinc oxide surge arrester possessing excellent properties of high voltage gradient and energy absorption capability, it becomes the most famous modern high-voltage surge arrester. To make a further improvement in electric properties of zinc oxide based varistor, a systematic research had been carried out by optimizing the sintering process as well as the compositions of the additives. Various influencing factors, such as the sintering condition, dopant species and doping content, on the microstructural and electrical properties were investigated. Otherwise, the growth mechanism of crystals was discussed.The sintering behaviors of the ZnO varistor were investigated, and the influence of heating rate and sintering temperature on the electric properties of the ZnO varistor were studied. With the decrease of heating rate from 200℃/ h to 50℃/ h, the varistor voltage increased and the leakage current decreased. It suggests that lower heating rate is necessary to form an optimal microstructure for obtaining the superior electric properties. The varistor sintered at 1030℃exhibits the higher varistor voltage and lower leakage current.The ZnO varistor doped with various rare earth oxides were studied. The Y₂O₃ doped ZnO varistors exhibit higher varistor voltage as well as the nonlinear coefficient. As the amount of the rare earth oxides increased, the water absorption and leakage current decreased whereas the varistor voltage and nonlinear coefficient increased. When the amount of Y₂O₃ increased to 0.6 mol%, the ZnO varistor achieved the optimum varistor voltage of 505V/mm.A ZnO-Y₂O₃-In₂O₃ glass based varistor was prepared by the method of direct co-sintering synthesis of oxides. This kind of varistor not only overcomes the drawback of large Bi₂O₃ volatilization in ZnO-Bi₂O₃ based varistor, but also avoids the high price of Pr₆O₁₁ in ZnO-Pr₆O₁₁ based varistor. Moreover, ZnO-Y₂O₃-In₂O₃ glass based varistor exhibits more excellent electric properties compared with other systems. It has an average varistor voltage up to 552V/mm, nonlinear coefficient up to 47.