Effect Of PrMnO₃Precuresor On ZnO-Pr₆O₁₁ Based Variistors

The ZnO based varistor using ZnO-Bi2O3（ZnBiO） based varistor ceramics as a majorraw material,which was the most widely used on the market to protect the circuit in case ofovervoltage.ZnO-Pr₆O₁₁（ZnPrO） based varistor ceramics have been discussed extensivelynowadays because of its good characteristic compared with ZnBiO,such as simplemicrostructure，excellent chemical stability, aging resistance and outstanding electricalnonlinearity.To obtain those distinctive properties,ZnPrO based varistor must be dopantCr,Co and Mn metallic oxide in ceramics as well as ZnBiO based varistor.Also,to clarifythe mechanism of doping has become the basis of research and application of ZnPrO basedvaristor ceramic.The perovskite-type PrMnO₃particle formed easily on the grainboundaries when MnO₂was added,but the role of Mn doping with this compound in theZnPrO based varistor is not clear.Therefore,in this paper,the pure PrMnO₃precuresor wasformed by solid-phase synthesis, and the effect of PrMnO₃precuresor doped from0.25to1mol%on ZnO-Pr₆O₁₁-Co₂O₃-Cr₂O₃-PrMnO₃varistor ceramics was studied,which wasprepared by conventional mixed oxide process, the content of Pr is keep at3mol%. Usingthe simplified phenomenological grain growth kinetics equation to investigated the effectof different content of PrMnO₃precursor on grain growth.Finally, analysis comparativelythe relative density,phase composition,microstructure and electrical properties of ZnPrO based varistor,Which doping withdifferent forms of MnO₂,MnCO₃,PrMnO₃.The main conclusions as following:High density,well-distributed microstructure,large average grain size and excellentelectrical characteristics ZnPrO based varistor is obtained by doping0.25mol%PrMnO₃stintered at the atmosphere of air for1h at1300℃,which the pellets were sintered atdifferent temperature（1250¹350℃） with soaking time（1⁶h） and doping content （0.25¹mol%）.With the content of PrMnO₃precursor increasing,the grain growth exponent value（n）and the apparent activation energy for this system（Q） were obtained from4.29to 5.13,858.7to1212.5KJ/mol respectively.The effect of hinder the ZnO grain growth inZnPrO based varistor is mainly attribute to mass transfer in liquid phase and the particlephase pinning.ZnPrO based varistor doped with different forms of Mn in MnO₂,MnCO3andPrMnO₃have the uniform phase composition.The samples have highest relative density,well-distributed microstructure by doping PrMnO₃precursor at whichever sinteringtemperatures,but its electrical properties is not well improved compare with the other twokinds of doped,one explanation for this failure is attribute to the majority of Mn is “fixed”in precursor,which can not well-distributed at grain boundaries in the form of ionic topromote grain-boundary defect barrier. Furthermore,an effective way to adjust themicrostructure and properties artificially of ZnPrO based varistor ceramics by doping fromthe research results.