| In the rapid development era of power electronics, ZnO varistor can ensure the system run normally and defend the system as an over voltage protector. Its advantages are not only stable and fast response in a system, but also environmental-friendly, low cost and long working life. At present, Zn O lightning arrester is mainly used in large power station, substation, huge equipment as well as a variety of huge constructions to protect them from the damage of over voltages. Simultaneously, Zn O is also the main research object in lightning protection project. In this thesis, the effects of sintering temperatures and ion doping on microstructure and electrical properties of ZnO varistor ceramics are studied, and the mathematical model of the maximum nonlinear coefficient of ZnO varistor ceramic was obtained from data simulation.First, the influence of sintering temperature on microstructure, electrical properties of low-temperature sintering Zn-Bi system derived 95.5ZnO-0.5V2O5-2.0Bi2O3-0.5Mn3O4-0.5Y2O3-0.5Cr2O3-0.5Co2O3(mol%) had been studied, and the loss performance of the prepared sample was discussed in this part. The experiment result showed that when the sintering temperature was 910?, the following characteristics were obtained: microstructure of ZnO varistor was quite regular with distinguishable grain boundaries; it was very clear that considerable spinel phases distributed regularly on ZnO varistor; the nonlinear coefficient ? and breakdown electrical field E1 mA were 27 and 3456.5V/cm, respectively. Simultaneously, the study of dielectric properties indicated that the dielectric loss tan? was in the range of 0.19 and 0.29, which was a relatively high value. In this thesis, the detailed explanations of the high value were discussed.Next research topic was the effect of Bi2O3 doping on microstructure, electrical properties, and loss performance of(96.0-x)ZnO-2.0V2O5-1.0Y2O3-1.0 Cr2O3-xBi2O3 varistors. The phase analysis demonstrated that all varistors had a same main crystalline phase of hexagonal wurtzite as well as crystal phases of YVO4, Bi2O3 and Zn0.982Cr0.018 O were also comprised. Meanwhile, XRD result provided the strongest diffraction peaks of main crystal phase shifted to low angle direction as the Bi2O3 doping concentration increased. Further microstructure study indicated that the mean grain size of Zn O varistor ceramics increased at first and then decreased with the increase of Bi2O3 doping concentration. Grain boundaries became clear and grain boundary particles were present here obviously. When the Bi2O3 doped concentration was 2.5mol%, the mean grain size of Zn O varistor ceramics reached its maximum value of 2.25?m and the electrical properties of ZnO varistor ceramics are optimum: the breakdown field strength E1 mA was 4598V/cm, the nonlinear coefficient was 58, simultaneously, the resistance loss tangent value was 0.08 and relative dielectric constant ?r was 231.5 if the order of experimental frequency was 105 Hz. Compare with the undoped sample, the ?r of ZnO varistor increased 85.8% and tan? decreased 72.4%. In addition, the resistivity of the varistors was in the range of 1200 M?·cm to 1300 M?·cm under same doping condition when the experiment temperature was below 150?.Third, the impact of Pr6O11 doping on the microstructure, electrical properties and loss performance of(92.5-x)ZnO-2.0NiO-1.0Co2O3-1.0Cr2O3-2.5Bi2O3-1.0Sb2O3-xPr6O11 varistors had been investigated under the condition that the sintering temperature was 910°C and the doping concentration of Bi2O3 was 2.5mol%. Hexagonal wurtzite structure was still the main crystalline phase for all varistors, and the mean grain size of Zn O varistor ceramics decreased with the increase of Pr6O11 doping concentration. Besides above mentioned phenomenon, the breakdown field strength E1 mA enhanced gradually and the nonlinear coefficient increased at beginning and then decreased as well as the leakage flow showed completely opposite result in comparison with that of the nonlinear coefficient. After comprehensive analysis, ZnO varistor ceramics showed the best preformance when the Pr6O11 doping concentration was 0.6mol%: mean grain size of the varistor ceramics was 5.3?m, and the microstructure of ceramics was perfectly regular; breakdown field strength E1 mA was 3550V/cm; nonlinear coefficient reached the maximum value of 66 and leakage current is the minimum value of 0.3?A.Finally, the effect of additive ionic radius Ri on the maximum nonlinear coefficient ?max of ZnO varistors was systematically investigated. The functional relation of ?max and Ri was obtained by date analysis and curve fitting, additionally, the influence of relative deviation ?Rrel of the additive ionic radius Ri and the zinc ionic radius R on the value of ?max was also discussed in detail. The results indicated that the relation between ?max and Ri could be described by the formula(6-1), and the values of ?max calculated by this formula coincided with the real nonlinear coefficient ?rea reported in some references very well; the value of ?max increased with an increase of the relative deviation ?Rrel, and ?max approached saturation when the values of ?Rrel were greater than 30%, beyond which ?max was over 64; for the additive ions with different Ri and same ?Rrel, the value of ?max was enhanced when Ri was greater than that of zinc ionic. The above results were explained by using formation mechanism of the solid solution and the principle of minimum energy.ZnO varistor was the core component of the lightning arrester, and the ZnO lightning arrester was most widely used as over-voltage protection device because of its stable performance and fast response. Meanwhile, ZnO arrester had the advantages of long service life, environmental protection, low cost and so on. The performance parameters of ZnO varistors in this paper and other researches had been compared, and the results showed that the performance of Zn O varistors in this paper had outstanding low loss and high nonlinear coefficient. The maximum value of nonlinear coefficient can reach 66, and the loss tangent can reduce to the value of 0.08, which could ensure the normal operation of power electronic systems, and had lower loss during use. |
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