Preparation Of High Potential Gradient ZnO-based Varistor Ceramics

ZnO varistors with highly nonlinear characteristic are widely used to protect electricalcircuits in the fields of electrical and electronics. With the development of power system andemhancement of power transmission line voltage, it is of practrical significance to developZnO varistor with high potential gradient. In this dissertation, ZnO varistor ceramics wasprepared by conventional solid state reaction process and the effects of multi-doping on thephase composition, microstructure and electrical properties was investigated. The obtainedoptimal samples sintered at1150℃have a potential gradient of above460V/mm,nonlinearity coefficient of89, leakage current of less than0.2μA, and the surge current（8/20μs wave） discharge capacity is above2.74KA/cm².First of all, the effects of Sb₂O₃doping on the properties of ZnO-based varistor ceramicwas studied. We found that spinel phase (Zn₇Sb₂O₁₂) formed during the sintering processhindered the ZnO grain growth when the Sb₂O₃doping amounts was between0mol%and2.8mol%. The reduce of ZnO grain size caused the increase of potential gradient and thedecrease of relative dielectric constant. Appropriate amount of Sb₂O₃doping （<1.6mol%）decreased the donor concentration and increased the barrier height, which will result in theincrease of nonlinearity coefficient and decrease of leakage current. In addition, Sb₂O₃dopingpromoted the sintering process, improved the density and the uniformity of the grain size.Therefore, the high current performance of the samples were improved. The sinteringtemperature was crucial to electrical properties. The weting effect of rich-Bi liquid becomedmore notable with the higher sintering temperature.On the basis of Sb₂O₃doping, we added SiO₂in order to obtain higher potential gradient.Zn₂SiO₄formed in the corners of the ZnO grains. Glass phase formed from SiO₂during thesintering process increased the viscosity of the liquid phase. This two factors resulted in thedecrease of grain size and the increase of potential gradient. The nonlinear characteristic canbe improved with appropriate amount of SiO₂doping （<0.3mol%） because SiO₂dopingenhanced the impurity segregation concentration of extrinsic interface states, density of interface states and barrier height. Being as a non-conductor, Zn₂SiO₄lowered the dischargecapacity of the samples.On the basis of Sb₂O₃, SiO₂doping, we added MgO in order to further improve thenonlinearity coefficient. We found that Mg₂SiO₄phase formed from MgO and SiO₂hinderedthe grain growth and increased the potential gradient. Owing to the increase of homogeneityof barrier height and width, the nonlinearity coefficient increased greatly with the MgOaddition in a certain range. Potential gradient and nonlinearity coefficient had no significantchange when MgO doping amount was more than0.2mol%. Leakage current changed slightlyafter doping because Mg²⁺was a neutral defect and it had a limited impact on barrier height.The structure of crystallization phase becomed homogeneous in a certain range of MgOamounts, resulting in slight increase of discharge capacity and decrease of residual voltageratio.