Influence Of Basic Ingredient On New Low-temperature Sintered ZnBiMnNbO-based Varistor Ceramics

ZnO-based varistor ceramic is a typical resistance material whose internal resistance changes nonlinearly with applied voltage.Due to its high non-linear coefficient and strong surge absorption capacity,ZnO-based varistor ceramics have been widely used as voltage surge arrestors in various electronic circuits and power protection systems.At present,both natural resources and energy are depleting at an alarming speed.Meanwhile,the demand for low-voltage ZnO based varistors are rising quickly.These trends require that ZnO varistor ceramics should have the characteristics of simplified composition,low sintering temperature,and low leakage current.At present,ZnBiMnNbO-based varistor ceramic is one of the few varistor ceramics that can simultaneously meet the above new requirements.However,the theoretical and application-oriented researches about this new ceramic are still in its infancy.The influence of the basic ingredients on its microstructure and properties is yet to be revealed.Based on the status quo and existing problems in this field,the traditional solid-state sintering technique was used to prepare ZnBiMnNbO-based varistor ceramics.Then the effect of Bi₂O₃,MnCO₃ and SnO₂ variations on the as prepared ceramics were studied by SEM,EDS,XRD and the conventional voltage field-current density（E-J）test.The main conclusions of this study are as follows:0.25mol%-1mol%Bi₂O₃ doped ZnBiMnNbO-based varistor ceramics can be densified in the range of 850-950℃.The sintered samples consist of ZnO equiaxed grains and some secondary particle phases,such as Bi₂O₃,Mn₃O₄,Bi₅Nb₃O₁₅.The Mn and Nb modified Bi-rich liquid phase contributes mainly to the abnormal low-temperature sintering of ZnBiMnNbO-based varistor ceramic samples.The0.5mol%Bi₂O₃ doped ZnBiMnNbO varistor ceramic sintered at 900℃ for 3 h has the best varistor characteristics:the nonlinear coefficient is 58.68,the breakdown voltage is 488.04（V/mm）,and the leakage current density is 3.15（μA/cm²）.Using the ZnBiMnNbO-based ceramic with the optimal Bi₂O₃ content,the influence of 0-1.5mol%MnCO₃ was studied.The change in MnCO₃ concentration content change barely influences the crystalline phase constituents and average grain diameter of ZnBiMnNbO-based varistor ceramics.However,adding MnCO₃ up to0.5 mol%can significantly improve the electrical nonlinearity of the ZnBiMnNbO-based varistor ceramics.Above this critical value,the further increase in MnCO₃ content gradually deteriorates the performance of the ZnBiMnNbO-based varistor ceramics.The ZnBiMnNbO-based varistor ceramic with 0.75 mol%MnCO₃shows the best overall properties under the condition of being sintered at 900℃ for 3h.The nonlinear coefficient is 52.47,the breakdown voltage is 453.90（V/mm）,and the leakage current density is 1.80（μA/cm²）.In the last part of this thesis,the influence of 0-0.75mol%SnO₂ was studied using the above-mentioned optimum model material.SnO₂ influences little the low-temperature sintering of the ZnBiMnNbO-based varistor ceramic.With the increase of SnO₂ content,increasing amount of Bi₂Sn₂O₇ forms as the new secondary phase.Therefore,the average grain diameter of the sample decreases,and the breakdown voltage increases accordingly.Compared with other SnO₂ doped samples,ZnBiMnNbSnO-based varistor ceramics sintered at 875℃ for 3 h shows its best electrical nonlinearity at 0.75 mol%SnO₂.The nonlinear coefficient is 52.64,the varistor voltage is 1024.37（V/mm）,and the leakage current density is 1.55（μA/cm²）.The results of this thesis provide good references for the research and development of a new generation of ZnO-based varistor ceramics with simple composition,low-temperature sintering and high-performance characteristics.