| The developments of devises for miniaturization and integration have resulted in an increasing demand for low voltage varistors. However, there are some problems exist in the field of low voltage ZnO varistors, such as poor performance in dealing with high-current shocks, easy to moisture absorption and a low rate of qualified products, etc. Based on the above mentioned situation, we have carried out exploring researches to solve these problems. In addition, in order to reduce the fabricating expense of varistors, low temperature sintering of ZnO varistors was also studied in this paper. ZnO-Bi2O3and ZnO-V2O5based varistors were prepared by conventional solid-state reaction method. The phase structure, microstructure and electrical properties of materials were investigated. The main contents are as follow:ZnO-Bi2O3based varistors were chosen as the research objective. In order to produce low-voltage of ZnO varistors, TiO2was added as an additive to enhance the grain growth. The influences of TiO2addition on the microstructure and electrical properties of ZnO varistor were studied. The results show that the voltage-gradient of the samples decreased to minimum value of30.6V/mm when the TiO2content is0.6mol%, but increased with further increasing TiO2content. The variation trend of average grain size is just the opposite of the voltage-gradient. Samples have a more unify microstructure when the doping amount of TiO2is0.6mol%.To study the effects of different doping form of Ti on the low-voltage ZnO varistors, Bi4Ti3O12were introduced and prepared by mixing TiO2and Bi2O3in proper molar ratio and pre-sintering. The results show that the varistors doped with Bi4Ti3O12can reduce the formation of Zn2TiO4spinel phase and promote the growth of ZnO grain, the average grain size increase from22.4μm to24.2μm at the same temperature. The varistors doped with Bi4Ti3O12sintered at1130℃exhibit the best performance of IL=2.9μA, E1mA=29.7V/mm, α=30.2, and excellent surge absorption capability, with%△E1mA=-3.5%for500A8/20μs impulse current. In addition, the varistors exhibited the most stable accelerated aging characteristics and with%△E1mA=-0.3%for DC accelerated aging stress of125℃/7h.Doping with Al can decrease significantly grain size ofnZnO. with an increase of Al (NO3)3·9H2O, the average grain sizes of ZnO decrease, and the voltage-gradient of the samples increase gradually. The nonlinear coefficient was increased by the addition of Al, while larger additions caused it to decrease, and the break down voltage and the leakage current increased sharply. The Al(NO3)3·9H2O concentration of the Zn-Bi based ceramics for low voltage application should be strictly restricted below0.02wt%. When doped with0.02wt%Al(NO3)3·9H2O sintered at1130℃, the ZnO varistors exhibited a stable accelerated aging characteristics with%△E1mA=-0.3%for DC accelerated aging stress of125℃/7h, a low nonlinear coefficient and leakage current. But the surge absorption capability is poor, with a variation of E1mA under8/20μs impulse current%△E1mA=-22.3%.In the low temperature sintering study of ZnO varistors, ZnO-V2O5based varistors was chosen as the research objective. The effects of SbVO4addition on the phase structure, microstructure, electrical properties and characteristics of grain and grain boundary of ZnO-V2O5based varistor ceramics were studied. The results show that the addition of SbVO4can lower the sintering temperature to940℃.The varistor doped with0.3mol%SbVO4sintered at940℃for4h exhibited the largest resistance of grain boundary and good nonlinear properties, with the nonlinear coefficient of51, leakage current of13.4μA and breakdown voltage of416V/mm. |
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