Effect Of Mn Dopants On ZnO-Pr₆O₁₁ Based Varistors

Because of the excellent non-ohm electric properties, large capacity for surge handling andstability of ZnO-Pr₆O₁₁（ZnPrO）based varistor ceramic, this ceramic material has been wellinvestigated nowadays. To obtain Non-ohm electric properties, there must be dopant elementsin ceramic. As a result, clarifying the effect of each dopant is the foundation for ZnPrO basedvaristor. Mn dopant has a widespread use in commercial ZnO varistor ceramics （ZnBiO）.However, the role of Mn dopant in ZnPrO based varistor ceramics is remained unclear. Thus,MnO₂and MnCO₃doped ZnPrO based varistor ceramics were fabricated by traditional ceramicprocess respectively.The effect of Mn dopant on ZnPrO based varistor ceramic was investigatedby density measurement, XRD phase analyzing, SEM, EDS and standard I-V tests. From theexperimental data, some conclusions have been obtained:In this research, increasing Mn dopants within the range of0-1mol%decrease the relativedensity of sintered ceramics among the sintering temperature range of1250℃-1400℃. And theMnCO₃dopant has a remarkable effect to the relative density. Take the samples sintered at1350℃for1hour for an example, by Mn dopant increased from0mol%to1mol%, the relativedensity of MnO₂doped specimen decreased from99%to87%;and that of MnCO₃changed to79%directly.The Mn dopants do not affect the main phase of ZnO. And by adding of Mn, the secondaryphase changed from Pr₆O₁₁to Pr₆O₁₁and PrMnO3.Generally, MnO₂and MnCO₃additions could reduce the average size of ZnO grain. AndMnCO₃has a great impact. To the samples sintered at1350℃for1hour, by Mn dopantincreased from0mol%to1mol%, the average grain size of MnO₂doped specimen decreasedfrom16.4μm to8μm; and that of MnCO₃changed to6.2μm. The reason for that phenomenon isattributed to a uniform distribution in MnCO₃doped specimens.From theI-V test to the specimen of sintered at1350℃for1hour. The0.5mol%Mndopants （MnO₂, MnCO₃） forms have given a risen to the nonlinear coefficient, remarkably. Andthe nonlinear coefficient and breakdown voltage increased from20,2569V/cm of0mol%MnCO₃doped specimen to100,2951V/cm; and that of MnO₂doped changed from undoped17.4,1241V/cm to54,1275V/cm.With phenomenological grain growth kinetics theory, grain growth in ZnO doped with0.5mol%MnO₂and MnCO₃were investigated. The grain growth exponent is reduced from3,for0.5mol%MnCO₃, to2.7, for ZnO doped with0.5mol%MnO₂, while the apparentactivation energy for grain growth is reduced from290kJ/mol, for0.5mol%MnCO₃, to 270kJ/mol, for0.5mol%MnO₂-doped. Grain growth of Mn （MnO₂, MnCO₃） doped ZnPrOsystem varistor ceramics was mainly dominated by liquid phase assisting mechanism and theParticles Pinning effect. Because of the difference of particle distribution, the resistance toMnO₂doped ZnPrO system varistor ceramic was lower than that of MnCO₃doped samples.