The Preparation And Properties Study Of ZnO Varistor By Means Of Chemical Co-precipitation Method

The thesis first introduced the development status, the application range and the requirements for performance of ZnO varistors, leading to the research on increasing the breakdown voltage gradient and improving the surge absorption capability. Judging from the research status at home and abroad, the properties of ZnO varistors could be improved by the improvement of preparation process of the additive powders. So the main research contents to solve the problem that the surge absorption capability of high-voltage ZnO varistors should be improved by improving the preparation process of additive powders. Based on the present formula of the high-voltage ZnO varistors, the experiment began with the preparation of nano compound additive by chemical co-precipitation method. Through analyzing the improviment effect of nano compound additive on varistors' microstructure, further analyses the effect on the electrical performances of ZnO varistors.In this thesis, chemical co-precipitation was used to prepare Co?Mn?Bi ternary system and Co?Mn?Bi?Ni?Sn quinary system composite additives, and optimization of the experimental process of titration environment and presintering temperature. By the XRF analysis on ternary system and quinary system composite additives prepared in different titration environment, the composition content of compound additive powder were the closest to standard experimental formula when the titration condition was pH = 10.00. By the XRD analysis on ternary system and quinary system composite additives?pH = 10.00?, the diffraction peak of Bi₂O₃?Co₃O₄?Mn₂O₃ were observed in ternary system, diffraction peak of the Bi₂O₃ were the most obvious; only the diffraction peak of Bi₂O₃ were observed in quinary system. With re-presintering of the 1g of ternary system and quinary system composite additive powders?pH = 10.00? under the conditions of the presintering temperature of 730?, 760?, 800?, the quality of the powders decreased with the temperature increasing. Through the separate precipitation experiment of Co, Mn, Bi, Ni elements, the precipitates presintering at 680?, 730?, 760?, 800?, the record of the powders quality change, analysis of the XRD diffraction on the additive powders, the presintering temperature was enhanced to 750?.The varistors' breakdown voltage gradient is 388 v/mm, nonlinear coefficient is 33, leakage current is 5 mA, flow value is 1850 A, change rate of the shock of before and after varistor voltage is 7%, which was prepared by chemical co-precipitation?ternary composite additive?. Nonlinear coefficient and flow value were better than varistors prepared by solid phase method. Through observation of SEM on the varistor, it could be known that grain size was uniform, average size was about 7 microns, distribution of additive on grain boundary were more uniform than solid-phase method, the stomatas were smaller than solid-phase method.Thermal aging and electric aging were used to process ZnO varistor. The experimental results showed that thermal aging process could reduce the breakdown voltage gradient, but had little influence on flow ability. Electric aging experiment, with 2.5 times and 3 times the breakdown voltage value, the waveform is 8/20?s lightning current components for 1 to 3 times, could enhance the surge absorption capability of the varistor, the minimum voltage change rate is 5.16%, varistor qualification rate could reach 100%.