The Influence Of High Dispersion System And Doping On The Performance Of ZnO Varistor

ZnO varistor is a kind of semiconductor ceramic resistor with nonlinear volt-current characteristics.Optimizing the application of ZnO varistors in electronic equipment,high/low voltage power system and semiconductor industry is a hot research field.ZnO varistor is always used as surge protector（SPD）and lightning arrester and the parameters.The former pays more attention to improving the stability under the impact of large current pulse of the varistors,while the latter pays more attention to the flow capacity of the varistors.In this paper,both of the two kinds of ZnO varistors applied in different fields are researched.For the resistor sheet used in SPD,the relationships between the doping of Ti O₂ and the high dispersion system,microstructure,electrical properties and degradation behavior has been studied respectively.Besides,the stability of ZnO varistor under electrical impedance surge shock is improved and the optimal amount of Ti O₂ to reduce the potential gradient of ZnO varistor was obtained basing on the stability of the sample.As for the resistor used in lightning arrester,the influence of impurities which is associated with raw materials on its micromorphology and comprehensive electrical properties is studied,and the influence mechanism is explored and the results of the researches can provide guidance to enterprises on the controlling of the raw materials.（1）ZnO varistors with different types of adhesives and different content of adhesives were prepared,and the relationship between the type and amount of adhesives on the dispersion system,microstructure,comprehensive electrical properties,especially the degradation behavior of ZnO varistors were systematically explored.The results show that,in addition to binding effect and enhancing mechanical strength of powder and billet,adhesives also affect the dispersion effect of paste,the uniformity of porcelain body and the electrical properties of resistor sheet to a great extent.In the process of production,it is necessary to choose the type of adhesive suitable to the formula system and adjust its dosage.Little use of adhesive will cause disadvantageous factors such as poor dispersion of slurry and low mechanical strength of billet body.Excessive use of adhesive will make the paste easy to aggregate,high porosity of porcelain body,increase the cost,etc.Under this formulation system,the sample using0.9 wt%HR-3010 had the best comprehensive properties.After 20 groups of surge shock at 20 k A,the forward and reverse change rates were-1.99%and-2.15%,respectively,and the clamping voltage ratio at 20 k A was 1.92.（2）The refining method of additive oxides is optimized,which not only greatly improves the production efficiency,but also further improves the dispersibility and the uniformity of the final sample.What’s more,the stability of the additive oxides against the surge impact.The slurry obtained from the additive oxides which were ball-milled for 6 h and sanded for 15 min had the best dispersion and concentrated particle size distribution.After 20 groups of surge shock at 20 k A,the forward and reverse change rates are-1.47%and 0.08%,respectively,and the clamping voltage ratio at 20 k A is1.96,which further optimizes the performance of ZnO varistors.（3）In order to adapt to more using scenarios and produce ZnO varistors with lower gradients,Ti O₂ was selected as the doping on the basis of the formula system of the above experiment,and the influence mechanism of its content on the microstructure and electrical properties of the sample,especially the stability of anti-surge impact was studied.The experimental results show that the doping of Ti O₂ can promote the grain growth and significantly reduce the potential gradient of ZnO varistors.However,it will also intensify the degradation behavior of ZnO varistors for the direct doping of Ti O₂ will cause the anisotropy of grain growth.There is still a controllable range.While the doping amount is 0.4 mol%,the potential gradient is reduced by 32 V/mm.After 20groups of 20 k A surge,the forward and reverse change rates are-5.64%and-4.71%,respectively,and the clamping voltage ratio is 2.45.（4）Impurity elements are often associated with raw materials for preparing varistors.ZnO varistors are prepared by doping different content of Na⁺,and the mechanism of the introduction of Na⁺on the microstructure and comprehensive electrical properties of ZnO varistors was studied.Small amount of Na⁺can decrease the mobility of grain boundary and inhibit the growth of ZnO grains.However,excessive doping will increase the viscosity of the liquid phase and the inhibiting effect of grain growth will gradually disappear.In addition,the micro-amount of Na⁺can be used as amphoteric dopant to improve some properties of the resistors.Excessive doping of Na⁺is mainly used as acceptor element,which occupies the lattice position and causes lattice distortion,so that the properties of the resistor chip deteriorate rapidly.（5）Iron often exists in the raw material as an impurity element.Varistor was prepared by doping different content of Fe³⁺,and the mechanism of the introduction of Fe³⁺on the microstructure and comprehensive electrical properties of ZnO varistors was studied.With the doping of Fe³⁺,a large number of Zn Fe₂O₄ spinel phases would be generated,restraining the growth of ZnO grains and increasing the varistor voltage.The surface state density of ZnO varistors can be improved by doping small amount of iron ions,which is helpful to the preparation of high-gradient ZnO varistors for arrestors.However,excessive introduction of iron will reduce the grain conductivity,cause lattice distortion and slow deterioration of other properties of ZnO varistors.