Investigation On Doping Modification And Electrical Properties Of SnO₂-Co₂O₃-Nb₂O₅Based Varistor Ceramics

The effects of various additions on DC property, surge performance, electrical stability,dielectric property and grain boundary barrier of SnO₂-based varistor were investigated.Pb₃O₄and CuO, which can easily form a liquid phase at high temperature, were doped toimprove the sintering properties of the SnO₂-Co₂O₃-Nb₂O₅-Cr₂O₃varistor system. It is foundthat the samples doped with0.132wt%CuO shows the optimum density （6.92g/cm3）, but thepresence of CuO is deleterious to the DC nolinear property. Doping Pb₃O₄can’t improve thedensity of material, but moderate doping of Pb₃O₄is efficient to improve withstanding surgecurrent performance of materials. The sample with0.5wt%Pb₃O₄can withstand a peakcurrent of4472A of8/20μs surge current wave.Cr is an important dopant to improve the nonlinearity of SnO₂varistor ceramic. Cr₂O₃isreplaced by Cr（NO₃）₃9H₂O, and the effect of Cr（NO₃）₃9H₂O compared with Cr₂O₃onSnO₂-Co₂O₃-Nb₂O₅-Pb₃O₄-based varistor system was studied. The results indicate dopingappropriate amount of Cr（NO₃）₃9H₂O is efficient to improve withstanding surge currentperformance of materials. The sample with8wt%Cr（NO₃）₃9H₂O possesses the optimalwithstanding peak current of5356A of8/20μs wave. Doping Cr in the form of Cr（NO3）39H2O leads to decrease of DC nonliear property, but the sample doped with10wt%Cr（NO₃）₃9H₂O can reach the nonlinear coefficient of36.In the SnO₂-Co₂O₃-Nb₂O₅-Cr（NO₃）₃9H₂O-SiO2varistor system, ZnO doping reducesthe grain resistivity and enhances significantly the surge performance of SnO₂varistorceramics. The sample with0.5wt%ZnO can withstand surge peak current of4732A of8/20μswave, and the grain resistivity is about1.04Ω cm.To investigate the electrical stability of materials, the change of current with time underfixed voltage was studied. The results indicates that electrical stability is affected by grainboundary barrier and Joule heating. Cr is more evenly distributed in the ceramic materialwhen Cr（NO₃）₃9H₂O is used then when Cr₂O₃is used, and more uniform distribution of Cr isbenificial to the electrical stability. Doping CuO causes the number of non-active barriers toincrease, which results in deterioration of electrical stability. The C-V and D-V characteristics at different frequences were tested. It is found that thereare a peak and a valley-point on the C-V curve, and there is a loss peak in D-V curve. Thepeaks and the valley-point shift to high voltage with the increase of frequency. The effect ofCr（NO₃）₃9H₂O on dielectric property of SnO₂varistor was investigated. At room temperature,the relaxation loss peak of oxygen vacancies shifts to low frequency with the increase of Crcontent in the frequency rang from40Hz to1MHz.Grain boundary barrier height of the sample doped with Cr（NO₃）₃9H₂O was tested bythree methods. For the same sample, the activation energy calculated by activation energymethod is about0.74eV, calculated by J-E-T method is about0.72eV, and measuredby C-V method under different frequencies, the minimum of which is1.85eV The effect ofdopants on the grain boundary barrier was investigated, and the grain boundary barrier heightof SnO₂varistor ceramics doped with Pb and Cu tends to decline overall. Compared with thesample doped with Cr₂O₃, that of doping Cr（NO₃）₃9H₂O has lower Chromium ionconcentration at grain boundaries, which causes the grain boundary barrier height to decrease.