Study On Low-Temperature Sintering And Electrical Property Of ZnO Varistor With High Voltage-gradient

ZnO varistors have been widely used to protect electric and electronic circuits from transient over-voltage,owing to its excellent nonlinear voltage-current characteristics and high energy absorption capability.With the miniaturized and integrated development of electronics,multilayer varistor has received considerable attention due to its general purpose for surface mount applications.Some researches are studying the varistor ceramics sintered at low temperature so they can be used for fabricating multilayer varistors with silver electrode and of lower cost and high reliability.B₂O₃ are generally used as sintering aids to achieve low temperature sintering of the varistor ceramics.However,B₂O₃,H₃BO₃ or Zn₃（BO₃）₂ will react with the organic binder so they could only be slightly added,which is inadequate to lower the sintering temperature.In this work,B₄C is used as the sintering aid for the following reasons,B₄C will not react with the organic binder at room temperature but will be decomposed and oxidized at 500～700℃to form B₂O₃ and then reduce the sintering temperature of ZnO varistor ceramics to 900℃.It was found that the B₄C additives can successfully reduce the sintering temperature to 840℃and effectively improve the voltage gradient and nonlinear electrical properties.The influences of B₄C on the density,crystalline form,microstructure and electrical properties of ZnO varistor samples under the sintering temperature of 840℃were studied.The results suggest that the change in the content of B₄C could have potentially influenced the microstructural homogeneity and directly influenced the ZnO grain size of the samples.The sample with 0.75 wt%B₄C exhibited the optimum electrical properties:a theoretical density approached 97%,α=55.57,E_1mA=1120.43 V/mm,IL=0.91μA。Effects of sintering temperature and sintering holding time on the densification and electrical properties of samples were investigated.Results show that there is an improvement in electrical performance of sample with the sintering temperature of 840℃and holding time of 90 min:a theoretical density higher than 97%,α=56.03,E_1mA=995.38 V/mm,I_L=0.87μA.The effects of SiO₂ on the electrical properties and microstructures of ZnO varistor samples were further studied.The E_1mA increased sharply from 995.38 to 1359.58 V/mm with the doping of 0～0.5 wt%SiO2.However,the electrical properties of samples were deteriorated.When0.5～1 wt%SiO₂ was added,the grain size distribution and electrical properties were improved gradually.The sample with 1 wt%SiO2 showed a moderate recovery of electrical properties:α=32.8,E_1mA=1270.22 V/mm,I_L=4.16μA.