Study On Electric Field Assisted Sintering And Doping Modification Of Tianium Oxide Based Varistor Ceramics

TiO2 varistor ceramics have many outstanding advantages such as low varistor voltage,large nonlinear coefficient,and high dielectric constant.They are often used as overvoltage protection and surge absorber-in the low-voltage field.The sintering method usually adopts the traditional solid-phase method.However,because the traditional sintering temperature is too high,which will not only cause abnormal growth of crystal grains,but also cause a large amount of volatilization of some low melting point additives,which will deteriorate the performance of the material.The electric field assisted sintering(flash sintering)is a sintering technology developed in recent years,Its sintering temperature is low,the sintering time is short,and the ceramic grains are uniform and refined.It is a new process for preparing ceramics with high efficiency and energy saving.For this reason,this thesis uses electric field assisted sintering technology to prepare TiO2 varistor ceramics,studies its sintering performance,varistor performance,dielectric properties and impedance at different temperatures and applied electric field,and discusses the electric field assist of TiO2 varistor ceramics sintering mechanism.When using electric field assisted sintering technology to prepare TiO2-Bi2O3-Nb2O5 varistor ceramics,the changes of flash parameters(current,power density,initial flash temperature,etc.)are studied by adjusting the intensity of the electric field.And two flash sintering modes of constant voltage and constant temperature are used to discuss the change of sample density and phase composition,the micro morphology of the sample is observed,and its electrical properties are determined.The experimental results show that the onset flash temperature decreases with the increase of the applied electric fields,When the electric field is 40 V/mm,the maximum flash temperature is only 643℃,which is about 500℃ lower than the traditional sintering temperature.At the same time,the sintering time is greatly shortened to only 120 s,and the preparation efficiency is greatly improved.In the constant voltage mode,when the electric field is 80 V/mm,the varistor voltage is doubled compared with traditional sintering,reaching 688.8 V/mm,the nonlinear coefficient is increased from about 5 to 15.1,and the leakage current is significantly reduced by only 0.001 mA.In the constant temperature mode,when the electric field is 80 V/mm,the nonlinear coefficient is greatly increased compared with the constant voltage mode,reaching 146,and the leakage current is increased to 1.341 mA.Comparing the two modes,the effect of constant temperature flash sintering on dielectric electrical performance is more favorable.Through the introduction of conductive components such as ZrO2 and SiC to improve the performance of the sample,while studying the effect of each component on the micro structure and electrical properties of TiO2 varistor ceramics.The experimental results show that when the ZrO2 doping amount is 0.5 wt%,the maximum flash temperature is increased to 726℃,and the power density is increased from 17.43 mW/mm3 to 29.402 mW/mm3.In addition,the addition of ZrO2 improves the dielectric properties of the sample,and the constant temperature mode is more advantageous than the constant pressure flash sintering mode.The dielectric constant under 40 V/mm voltage and 1000 Hz increases to 4.44×103,and the dielectric loss decreases to 0.17.When the SiC content is 5 wt%,the highest flash temperature rises to 786℃,and the relative density of the sample rises to 97%.At the same time,the addition of SiC increased the grain boundary resistance of the sample.When the electric field is 100 V/mm,the grain boundary resistance increases to 5.24×107 ohm under constant temperature flash sintering mode.