| Zinx oxide varistor is a polycrystalline semiconductor ceramic component working on the grain boundary effect. Because of its excellent nonlinear volt-ampere characteristics and large energy absorption capacity, it has been widely used in electronic and electrical systems. This thesis first introduces the development status of ZnO varistors, leading to the necessity to improve the surge current withstanding capability of high-voltage varistors.To improve the surge current withstanding capability, the thesis describes the principle, advantages and disadvantages of Sol-Gel method in detail, then nanoparticle additives powders of Co3O4, MnO, Bi2O3 and Ni2O3 with pH of 6.00, 8.00 and 10.00 were prepared by Sol-Gel method using Co?CH3COO?2·4H2O, Bi?NO3?3·5H2O, Ni?NO3?2·6H2O and Mn?CH3COO?2·4H2O as precursors; additives powders with pH of 6.00, 8.00, 8.50, 8.80 and 10.00 were prepared using nitrates of Co, Mn, Bi and Ni; additives powders of Co3O4, MnO, Bi2O3, Ni2O3, SnO2 with pH of 8.50 and 10.00 were prepared with SnCl4·5H2O, and nitrates of Co?Mn?Bi?Ni; SnCl2·2H2O, Co?NO3?2·6H2O, Mn?NO3?2, Bi?NO3?3·5H2O and Ni?NO3?2·6H2O were also used to synthesized composite powders with pH of 8.60, 8.70 and 8.80. The samples were produced following the conventional ceramic processing which began with mixing the synthesized powders with the other raw materials. The microstructure and electrical characteristics of these varistors were tested. Compared to the ZnO varistors prepared via the conventional ceramic processing, the microstructure, grain size distribution and distribution of dopants of the varistors prepared by Sol-Gel were more homogeneous and its surge absorption capability was also better. The Zn O varistors made from the Co3O4, MnO, Bi2O3, Ni2O3 composite powders with pH of 8.50 using nitrates of Co, Mn, Bi, Ni had the best performances, of which the nonlinear coefficient was 37.53, the leakage current was 3.05?A, the variation of breakdown voltage was 6.48% after application of a pulse-current of 1919 A. While the performances of the varistors made from the synthesized powders with pH of 8.60 using SnCl2·2H2O, Co?NO3?2·6H2O, Mn?NO3?2, Bi?NO3?3·5H2O and Ni?NO3?2·6H2O were much better among the Co3O4, MnO, Bi2O3, Ni2O3, SnO2 composite powders with the nonlinear coefficient of 28.97, the leakage current of 7.12?A and the variation of breakdown voltage of 12.18% after application of a pulse-current of 1909 A.The influence of sintering temperature, electric aging, thermal aging and electrode area on the electronic and electrical characteristics of ZnO varistors were also studied in this thesis. Experimental results showed that the varistors prepared by Sol-Gel had the best performance when sintered at 1050?; the varistors after 650? or 680? thermal aging had a poorer electrical properties and its surge absorption capability were also not improved; for the varistors under pulse-current of different sizes and different times, the nonlinear coefficient decreased, the leakage current increased, the surge absorption capability was significantly improved, but the microstructure of varistors would be damaged when the charging voltage was too high. In addition, the electrode area of the samples had no effect on the electrical properties, while the larger the electrode diameter is, the better is the surge absorption capability. |
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