| Zn O-Bi2O3 based varistors have been widely used as surge protection devices in electrical and electronic systems due to their highly nonlinear current-voltage(I-U)characteristics, which are also the most widely studied systems of varistors. With the development of the technology of electronics, multilayer chip varistors have been used more and more widely. In order to reduce the manufacturing cost of multilayer devices, it is important to research the Zn O varistor materials having low sintering temperatures and excellent electrical properties. In this paper, a low melting point sintering additive was doped to reduce the sintering temperatures and the effects of several elements on the Zn O varistors were investigated. The main contents are as follows:1. The effects of low melting point additive B2O3 on the sintering temperatures and nonlinear electrical properties of Zn O-Bi2O3 based varistors were investigated. The results show that B2O3 could reduce the sintering temperature of the varistor ceramics to 850 o C.B2O3 could react with Bi2O3 to form a new secondary phase, Bi4B2O9, resulting in an enhancement of varistor performance. The nonlinear coefficient of the varistors with 0.5mol% B2O3 reached 56.8, the leakage current was 0.3 μA and the breakdown voltage field was 486 V/mm.2. The effects of different Co valence on the Zn O varistor samples sintered were investigated separately. The results show that all these cobalt oxides could effectively enhance the varistor performance, while the addition of same amount of cobalt cations in various cobalt oxides had different effects on varistor samples. Among them, the effect of cobalt cations in Co3O4 was the best. Analysis of microstructures indicated that the cobalt cations were distributed both in grain regions and grain boundary regions. The addition of cobalt oxides could change the electrical conductivities of both grains and grain boundaries,improving the varistor performance. It should be noted that the breakdown voltage fields in varistor samples doping with cobalt oxides increased slightly. Best electrical properties were obtained for the varistor sample sintered at 880 o C for 4 hours containing Co3O4, in which the nonlinearity coefficient was 28.5, the leakage current was 3.4 μA and thebreakdown voltage field was as low as 260 V/mm.3. The effects of Sn O2 on the microstructure and the electrical characteristics of Zn O-Bi2O3-Co2O3-based varistors were investigated. The results indicate that small amounts of Sn could diffuse into Zn O phase and affect its lattice constant. Moreover, the addition of Sn O2 caused a decrease in the leakage currents. It was found that Bi2O3 would not react with Sn O2 to form pyrochlore phase untill the ratio of Sn O2/Bi2O3 reaching up to3/7.4. The microstructure and electrical properties of Zn O-Bi2O3-Mn O2-based varistor ceramics doped with different amounts of Cr2O3 were investigated. With the increase of Cr content, there was a series of transformation of minor secondary phases. As 0.2 mol%Cr2O3 added, the average grain size increased obviously, resulting in a decrease in the breakdown voltage. However, the nonlinear coefficient was also reduced.5. The effect of the synthesized multi-phase dopant containing Cr on the Zn O based varistors was investigated. The Bi-Cr-O synthesized dopant was obtained by sintering a certain molar ratio of Bi2O3 and Cr2O3, and then added to Zn O-Bi2O3 based varistor powders forming starting materials. It was found that with the doping level up to 5 wt%,this dopant could effectively increase the grain growth as well as the nonlinear coefficients.For the varistor samples sintered at 930 o C, the nonlinearity coefficient could reach up to 50,while the leakage current was 0.1 μA and the breakdown voltage field was 221 V/mm.
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