The Study On Process Of ZnO Varistors With High Voltage-gradient And High Surge Current Withstanding Capactiy

ZnO varistors have been widely used in the field of power systems and electroniccircuits for its excellent nonlinear voltage-current properties and surge current adoptioncapacity. However, this field is still expecting to improve the surge current withstandingcapability as well as voltage-gradient. At present, the voltage-gradient of domestic factoryproduction is about200V/mm, and there is a big difference from foreign level, thus thisthesis focuses on the process and production of high voltage-gradient varistor with highsurge current withstanding capability while maintaining other electrical characteristicsincluding pre-synthesis and oxidation treatment.First of all, MgO and SiO₂compounds were pre-synthetized and doped to improve thedensities. The results indicate that when MgO and SiO₂compounds were pre-synthetized,the densities of zinc oxide varistors are improved significantly, but the breakdown electricalfield is reduced. The breakdown electrical field of the varistors is340V/mm whilenonlinear coefficient is89, and the leakage current density is0.07μA/cm².Then, different proportions of Bi₂O₃and Sb₂O₃compounds were pre-synthetizedthrough solid-state reaction method to produce high voltage-gradient varistor with highsurge current withstanding capability, and the effect of the compounds on the densities,grain structure and electrical properties of zinc oxide varistors were studied. The resultsindicate that when the compound proportion of Bi₂O₃and Sb₂O₃is0.7:1, electricalproperties of zinc oxide varistors are the best, and uniformly dense grain structures areobtained. After wave surge current impulse of5KA current with a time length of8/20μs,the residual voltage ratio and breakdown voltage variation are2.6and2.5%respectively.The breakdown electrical field is361V/mm while nonlinear coefficient is86, and theleakage current density is0.07μA/cm².At last, the varistors were treated with oxidation processing to reduce the residualvoltage ratio and improve its stability. The effect of different oxidation temperature on grainstructure and electrical properties of zinc oxide varistors were studied. The results indicatethat when the varistors are oxided under900°C, the comprehensive performance of varistors is improved. The breakdown electrical field is390V/mm while nonlinearcoefficient is95, and the leakage current density is0.14μA/cm², and after wave surgecurrent impulse of5KA current with a time length of8/20μs, the residual voltage ratio andbreakdown voltage variation are2.2and2.1%respectively.