Research On Zinc Oxide Varistors Doped With Rare Earth Oxides

ZnO varistor (variable resistors) is a sintered ceramic semiconductor device based on ZnO combined with varistor-forming oxides, such as Bi₂O₃ and Pr₆O₁₁, and a number of other metal oxides, ZnO varistors are widely used in high-voltage transmission lines, urban metro lines and rail network systems.High-performance ZnO varistor as a core component of ZnO surge arresters, its properties and manufacturing process directly restricts the development of ZnO surge arresters. Improving the performance of ZnO varistors is an important research topic for the grid system to enhance lightning surge protection. In this thises, the effect of rare earth oxides doped in Bi₂O₃- ZnO varistors on the constituents prepareing high-voltage ZnO varistors were investigated. The grain growth kinetics under the different sintering conditions was discussed. The development of new Pr₆O₁₁- ZnO varistors is expected to overcome the high volatility and reactivity of Bi₂O₃.The effect of the rare earth oxide (Y₂O₃, Er₂O₃) doped with 0.2 0.8mol% on the microstructure and electrical properties of ZnO-Bi₂O₃ has been investigated. It is aim to obtain the most optimum rare earth oxides and doping. The results show that: When the amount of Y₂O₃ increased to 0.6mol%, the ZnO varistor achieved the optimum electrical characteristics: V1mA/d=505V/mm;α=29.6; IL=345μA.The grain growth kinetics of Y₂O₃-doped ZnO-Bi₂O₃ varistors has been studied. The results show that: with the increase of sintering temperature and sintering time, the average grain size of samples growed up. When doped with 0.6mol% Y₂O₃, sintered at 1030 oC for 2h, the average grain size of ZnO varistors gain the smallest, about 1.5μm; whereas largest about 4.2μm at 1070 oC sintered for 8h. According to grain growth kinetics equation, 0.6mol% Y₂O₃-doped ZnO-Bi₂O₃ varistors have the grain growth kinetics of index n = 4, grain growth activation energy Q = 1182kJ/mol.In order to discuss the feasibility of ZnO-Pr₆O₁₁ varistors used in the fields of high-voltage power transmission. The microstructure and electrical properties of ZnO-Pr₆O₁₁ varistors doped with various amount of CeO2 have been investigated in this article. The resualts of experiment indicate that adding CeO₂ would greatly increase the threshold voltage and the nonlinear coefficient of the ceramics, whereas the leakage current is almost unchangeable. When the amount of CeO₂ increased to 1.0mol%, the ZnO varistor achieved the optimum electrical characteristics, the threshold voltage reaches 548V/mm and the nonlinear coefficient is 42. The SEM and XRD analyses testify that the microstructure of CeO2-doped ZnO-Pr₆O₁₁ varistor ceramics was composed of ZnO grain and intergranular layer: Pr₆O₁₁,Pr₂O₃ and CeO₂ phases only. The grain size of ZnO is diminished by Zni·, which is controlled by the amount of CeO₂. It also improves the structure and component of grain boundary.The effect factors of milling, molding process and sintering in ZnO-Pr₆O₁₁ Varistors was discussed. The results indicate that the system ZnO varistor material milling process should be used for milling ethanol liquid medium, should not use distilled water. Twice successively with pressure, reduce the molding pressure and increase the appropriate holding time can be reduced to suppress the process of spall fracture occurs. The ideal heating rate is 50 oC, when the pore is lowest in the sintered samples.