| ZnO varistors have been widely used in the protection of electrical equipment and electronic system due to their excellent U-I characteristics,fast operation,long service life and excellent surge current absorption capacity.In recent years,with the rapid development of power,electronics and automation technology in China,metal arresters are developing towards high energy,miniaturization and ultra-high voltage.Therefore,it is necessary to develop ZnO varistors with better electrical performance.Due to its high activity and high degree of freedom,the ceramic grain boundary zone and the grain form a shell core structure.Its volume proportion,uniformity and segregation have a significant impact on the performance of the component.The grain boundary zone with uniform distribution of structural elements is the key to improve the performance of the component,and the uniformity of grain boundary elements can also be used as the basis for component selection.In order to improve the uneven distribution of elements in the grain boundary zone in the solid phase method,this thesis adopts the preparation process of sol-coated ZnO to ensure the uniformity of the grain growth environment during sintering,so that the Bi2O3 can fully wet the grain boundary when it is melted.In this thesis,the pH value of the sol,the preparation process were explored,and the ratio of composite additives,calcination temperature,and the preparation of four element and five element varistors by sol coating method were studied,and the properties of varistors prepared by solid-state method and composite additive method were compared.The results are as follows:In order to prepare stable,uniform and transparent sol,the sol with pH=3.00,pH=4.00,pH=5.00 and pH=6.00 was prepared,and the aging experiment was carried out.The results show that only the sol with pH=6.00 can be stable,and the sol with small pH value is easy to precipitate.When pH is neutral or alkaline,the hydrolysis reaction of bismuth nitrate to form basic salt is positive,and precipitation will destroy the stability of the sol.Through the thermo gravimetric experiment of xerogels,the gel has been decomposed completely at580℃and the calcination temperature has been set to 580℃.On the basis of solid-pHase formulation,the varistor ceramics were prepared by orthogonal test with the mass fraction of cobalt nitrate,manganese nitrate and nickel nitrate of±10%respectively.Taking the nonlinear coefficient as the index,the index factors were analyzed.It was proved that the formula with the highest nonlinear coefficient was consistent with the formula optimized by solid-pHase method.The process of preparing composite additives by sol-gel was adjusted,and the ZnO powder was evenly coated by sol coated ZnO powder and calcined.The four component system（Co,Mn,Ni,Bi）and five component system（Co,Mn,Ni,Bi,Sn）varistors were prepared by solid state method,composite additive method and sol coating method,respectively,and the micro and electrical properties were tested.The results show that:by comparing the standard difference of EDS point scanning of grain boundary elements,coating method<composite additive method<solid state method,it is proved that coating method can achieve uniform distribution of grain boundary additives.SEM analysis shows that compared with quaternary system,In the five element system,spinel pHase is formed in the triangle region of grain boundary,pinned at the grain boundary,which limits the grain growth and increases the number of grain boundaries per unit thickness.It is beneficial to improve the voltage gradient of the device.The electrical properties of the Quaternary varistors and the penta varistors prepared by the three methods were tested respectively.The test results show that the electrical properties of the varistor prepared by sol-gel coating method are better.The voltage gradient,nonlinear coefficient and leakage current are 395.2V/mm,38.90 and 2.695μa respectively.The voltage gradient is 415.2V/mm,the nonlinear coefficient is 39.21,and the leakage current is 5.143μA.The introduction of Sn as donor impurity increases the varistor gradient and nonlinear coefficient,as well as the carrier concentration and leakage current. |
