| ZnO-Bi2O3 varistor(ZNR)is an important component in protection circuit because of its high response voltammetry.The high response of ZNR is related to the distribution and concentration changes of ions during sintering.Sintering system,raw material type and raw material ratio can significantly affect the electrical properties of ZNR,such as MnO2,Cr2O3 and other metal oxides with donor effect;La2O3,Er2O3 and other lanthanide oxides with donor effect;Metal oxides,such as Ag2O and Cu2O,play the role of acceptor.In addition,the practicability of ZNR determines that it must have high electrical stability,which is specifically manifested in the ability to withstand pulse stimulation,large current impact and longterm DC resistance.The degradation mechanism of samples under DC aging is more recognized as the migration of zinc ions.This subject has studied the influence of donor and acceptor co-doping on the physical properties of ZnO-Bi2O3 system varistor ceramics,and also studied several factors affecting the stability of the electrical properties of the samples.ZnO-Bi2O3-MnO2-Co2O3-Sb2O3(ZBMCSB)and ZnO-Bi2O3-MnO2Cr2O3-SiO2(ZBMCSI)varistors were studied.At the sintering temperature of 980℃,ZBMCSB varistors samples have the highest response,and the nonlinear coefficient,breakdown field strength,barrier height and leakage current density are 50.59,510.99 V/mm,1.56 eV and 19.34 μA/cm2,the grain boundary resistance determines the stability of the grain boundary to a certain extent.At 990℃ sintering temperature,ZBMCSI ceramic samples have the highest response,and the nonlinear coefficient,breakdown field strength,barrier height and leakage current density are 43.36,435.10 V/mm,1.98 eV and 1.89 μA/cm2,respectively,test frequency in the range of 103-106 Hz tanδ below 0.07.On the basis of the above selected formula,the effect of donoracceptor dual-effect dopant TiN on the electrical properties of ZBMCSB varistor ceramics was studied.EDS analysis of ZBMCSB varistor ceramics doped with 1.0 mol%TiN shows that N is mainly distributed in grains,and some active N can replace O ions at grain boundaries to form acceptor defects,the distribution of Ti in the grain and grain boundary of ZnO sample is similar,and part of Ti entering the ZnO grain displaces Zn,forming donor defects.Part of Ti participated in the formation of bismuthrich phase and spinel phase.According to the energy spectrum distribution of N and Ti elements and the influence of TiN doping on the interface state and donor concentration of ZBMCSB varistor ceramics,it is proved that TiN can be used as a donor and acceptor dual effect dopant.When 1.0 mol%TiN is doped,the response of the sample is the highest,and the nonlinearity,breakdown field strength,barrier height and leakage current density are 46.99,447.70 V/mm,0.79 eV and 5.03 μA/cm2 respectively.The effects of Si3N4 doping on the average grain size,phase composition,element distribution and grain boundary barrier height of ZBMCSB varistor ceramic samples were compared and studied.The capacitance-voltage(C-V)test of the samples made by mixing different raw materials shows that Si3N4 doping can significantly improve the density of interface states and donor concentration of the samples.The difference is that using the traditional raw material mixing process,N element is mainly distributed in the grain,and using the pre-synthesis raw material mixing process,N element is mainly distributed in the grain boundary layer.When 1.0 mol%Si3N4 is doped,the samples prepared by the mixing process of the two raw materials have the highest response.The nonlinear,breakdown field strength,barrier height and leakage current density of the samples prepared by the traditional mixing process of raw materials are 42.29,753.51 V/mm,3.62 eV and 2.26 μA/cm2 respectively,the nonlinearity,breakdown field strength,barrier height and leakage current density of the mixed process of presynthetic raw materials are 64.65,868.54 V/mm,2.36 eV and 1.96 μA/cm2 respectively,it can be seen that the response of the sample made by the mixing process of the presynthetic raw material is significantly improved compared with the traditional raw material mixing process.Finally,the influence of dual-effect dopant Na2CO3 on the grain boundary phase change,microstructure,voltammetric characteristics,C-V characteristics,impedance and DC aging of ZBCMSI varistor ceramics was discussed.The DC aging test shows that the samples are degraded to different degrees due to the stimulation of DC voltage for a long time.Because the stable grain boundary phase cannot be formed without adding Sb2O3,the doping of Na2CO3 does not significantly improve the DC aging resistance of the samples.It can be seen that the electrical stability of the samples is affected by many factors such as grain boundary structure,intergranular phase and interstitial zinc ion concentration.It was found that different electrode positions directly affect the path of current passage,so the point electrode design is carried out for the ZNR sample with a diameter of 12 mm,the relationship between the breakdown voltage and the nonlinear coefficient of the sample is theoretically deduced,and the point electrodes at different positions of the same sample are connected and measured respectively.The relationship between electrode distance and breakdown field strength is consistent with the theoretical analysis results. |
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