Pre-synthetic Of Grain Boundary Phase Of ZnO Varistor And Microwave Sintering Studies

The ZnO-based varistor is the core component of the overvoltage protection device in the circuit.The electrical properties of ZnO varistor are mainly determined by the microstructure and properties of their grain boundaries.If the grain boundary component in the ZnO varistor were pre-synthesized and then introduced in the form of addition,it is expected to improve the composition and distribution uniformity in grain boundary phase,the sintering temperature and the grain size will be reduce,and improve the performance of the pressure-sensitive ceramic.Compared with traditional sintering,microwave sintering has the advantages of uniform heating,lower sintering temperature and shorter sintering time.It can improve the microstructure of the material.Microwave sintering can be used for the preparation of pre-synthesized grain boundary phase of ZnO varistor,which may further improve the comprehensive properties of the material.Therefore,in this study,the grain boundary additives in ZnO pressure-sensitive ceramic batching were separated and processed by high-temperature melting,water quenching and ball grinding to obtain pre-synthesized frit powder,which was used to replace the grain boundary additives in the ceramic ingredients.Using traditional sintering and microwave sintering were adopted to prepare ZnO varistor.The compositional changes of ZnO grains and grain boundaries were characterized by means of XRD,SEM and EDS.The performance change of the varistor was evaluated by means of the varistor voltage,nonlinear coefficient,leakage current,capacitance-voltage characteristics,impedance and relative dielectric constant of the sample.The effects of composition and content of pre-synthetic additives,microwave sintering temperature and time on varistor were discussed.Bi₂O₃,B₂O₃ and SiO₂ were compounded according to the formulation ratio of ZnO varistor ceramics.After mixing,melting,water quenching and pulverization,pre-synthetic with particle size close to the particle size of ZnO and uniform particle size distribution is obtained.The sdditives has a melting temperature of 650 ?.The experimental results show that the sintering temperature of the sample with pre-synthetic additives is 1000 ?,which is 50 ?lower than the sample with multi-component oxide additives.It is beneficial to the uniformity of the grain boundary of the varistor ceramic,compact sintering,increase effective interface state density.The sample with the addition amount of 6 wt%has relatively good microstructure and electrical properties,the grain boundary barrier is increased to 1.83 eV,and the nonlinear coefficient a is increased to 60.5.Meanwhile,the number of semi-conductive grains is increased.The grain resistance is lowered,the grain boundary insulating is increased to 6.31×106 ohm,the dielectric constant is increased to 640.5,the dielectric loss is decreased to 0.2.The microwave sintering temperature of the varistor of the same composition can be reduced by 200 ? compared to conventional sintering,and the sintering time can be shortened by 5/6.When the sintering time is 40 min,ZnO varistor exhibits the best comprehensive performance,the density is 5.49 g/cm3,the linear shrinkage is 13.7%,the varistor voltage is 234.50 V/mm,and the nonlinear coefficient is 51.1.The resistance value of the boundary is 5.99×105 ohm,the grain resistance is 17.31 ohm,the dielectric constant is 363.50,the dielectric loss is 0.61,and the barrier height is 1.77 eV.