Effects Of Pre-synthetic Additives On The Structure And Varistor Properties Of TiO₂ Varistors

TiO₂ varistor ceramic is a kind of sensitive ceramic material with typical grain boundary properties.It has the advantages of low voltage,high dielectric constant,simple production process,low production cost and good absorption of noise and surge.It is widely used in electronic circuits nowadays.However,because of the nonuniformity of composition and distribution of grain boundary phase,it is difficult to realize low pressure because of the unevenness of grain boundary properties of TiO₂.If the grain boundary components in the TiO₂ varistor ceramic formula are pre-synthesized as amorphous glass and then added to the ceramic batch,the sintering temperature can be reduced while improving the uniformity of the composition and distribution of the ceramic grain boundary phases.,Reduce the grain size,improve the performance of varistor ceramics.For this reason,TiO₂ is used as the base material,and it is added to the synthetic boundary additive,TiO₂ varistor ceramics are prepared by two sintering methods:resistance and microwave heating.The structure and composition of the samples were characterized by means of XRD,SEM and other methods.The electrical properties of the samples were evaluated by means of varistors DC parameters and automatic component analyzers.Based on the phase diagram of Bi2O3-SiO₂-B2O3 ternary system,four additives with different compositions were designed.They were prepared by two methods of melting and sol-gel methods,respectively,using XRD,SEM,DSC-TG differential thermal analysis and infrared spectroscopy and other means to study the composition of additives,softening temperature,crystallization conditions.The softening temperature of the additive when the composition is 35%Bi2O3-15%SiO7-50%B2O3 is up to 580?.During the sintering of the sample,the amount of volatilization is small,and almost no crystals precipitate and the degree of vitrification is the highest.Four kinds of TiO₂ based varistors were prepared by adding 2wt%,4wt%,6wt%and 8wt%of pre-synthesized additives in TiO₂.At the same time,TiO₂ varistors were prepared with the ratio of 99.7mol%TiO₂+0.3mol%Bi2O3.After the above ingredients are subjected to ball milling,granulation,pressure proofing and degumming,resistance sintering is performed at three temperatures of 1200?,1300? and 1400?.And microwave sintering is performed at temperatures of 920?,970? and 1020? respectively.After that,the silver electrode was coated on the sintered sample,and parameters such as pressure sensitive parameters,dielectric properties,and impedance were measured.The results show that the sintering temperature of TiO₂ varistor decreases from 1400? to 1300? after the introduction of pre-synthetic additives.At the same time,the grain size of TiO₂ decreases from 40-50?m to 20-30?m,the varistor decreases,and the nonlinear coefficient persists.Increasing,wherein,when the additive amount is 6wt%,the obtained TiO₂ varistor has a bulk density of 4.16g/cm3,a linear shrinkage of 16.73%,a nonlinear coefficient a of 25.62,and the varistor voltage is lowering to 20.1 V/mm,the dielectric constant??r?is 2.04×107.When the blending amount is 2%,the varistor voltage of the TiO₂ varistor reaches a minimum of 9.56V/mm.Compared to conventional sintering,the temperature required for microwave sintering is reduced to 1000?.,and the sintering time is shortened to half that of conventional sintering.The bulk density p of the TiO₂ varistor is 4.12 g/cm3,and the linear shrinkage is 16.67%.The varistor voltage is as low as 15.62V/mm,the nonlinear coefficient is 17.99,and the dielectric constant??r?is 2.96×108.