Research On The Nonlinear Varistor Ceramics Based On Nano-Zno-Pr₆O₁₁

With the rapid progress in electronic information and household electricalappliance industry, it has been required that growing demand and highperformance for varistors, especially the higher request from modernizationand informatization of military equipment. Thus the ceramics must meet twobasic requirements: one is the low sintering temperature. The other is thebetter nonlinear properties. In recent years, Pr₆O₁₁-based ZnO varistors arebeing actively studied aiming to find a substitution for Bi2O₃-based ZnOvaristors, which have some drawbacks due to Bi2O₃having high volatility andreactivity. Additional, the advantage of Pr₆O₁₁-based ZnO varistors aresummarized by the less kind of additives, simple microstructure consisted ofonly two phases, high nonlinear and their stability against DC acceleratedaging stress. However, they also have some flaws to improve, namely, thehigh sintering temperature and production cost, which hinders its large-scaleapplications and commercial.The traditional ZnO varistors are prepared with micron-ZnO powders asraw material. Their performance will be limited by the physical, chemical andthermodynamic properties of micron-ZnO powders. Nano-materials andtechnology application in ceramics provide a new idea and a lot to someexperience for reference for ZnO varistors. Therefore， in this paper,Pr₆O₁₁-based ZnO varistors were investigated systematically based onnano-ZnO powders, including sintering kinetics, microstructure, electricalproperties, the effect of Y₂O₃and Al2O₃doping on Pr₆O₁₁-based ZnOvaristors with traditional process and Pr₆O₁₁-based ZnO varistors preparedwith sol-gel and nano-coating process. The main research results are asfollows:（1） ZnO-Pr₆O₁₁-MnCO₃-CoO varistors （ZPMC in short） were preparedby traditional methods. The relative densities of the sintered samplesincreased from72%to96%of TD with the increasing sintering temperatureand time. All the samples showed uniform equiaxed grain and were free ofabnormal grain growth. In general, the grain size increased from about2.3to8.3μm with the increasing temperature and holding time. An optimumsintering temperature of1300℃gave the best electrical properties, which was about50-100℃lower than that of samples prepared with micron-ZnOmaterial. From1050to1300℃, the grain growth kinetic exponent is about3.7and the apparent activation energy for grain growth is around306±61kJ/mol.The breakdown field （E21mA/cm） decreased with the increasing sinteringtemperature. The nonlinear exponent （α） increased firstly as sinteringtemperature increased. Further increase in sintering temperature caused adecrease in the α value. While the variation of Jleakvalue was opposite to thatof α value. The Jleakwas not ideal and needed to be further optimized.（2）The sintered density for ZnO-Pr₆O₁₁-MnCO₃-CoO-Y₂O₃（ZPMCY inshort） system increased as sintering temperature increased. However, thesintered densities of all ZPMCY samples are slightly lower than those ofZPMC samples at the same sintering temperature. A two-phase microstructurewas detected in ZPMCY systems comprising ZnO primary grains and Pr andY-rich secondary oxide phase. The addition of Y₂O₃led to the decrease of thedensification and the grain size, which is attributed to segregation of Y₂O₃,which is nearly insoluble in ZnO grains, to grain boundaries. The varistorvoltage （E21mA/cm） of ZPMCY samples decreased over a wide range from7100V/cm to2470V/cm with the increase of sintering temperature. TheZPMCY-based varistor ceramics sintered at1275℃exhibited the bestperformance in E–J characteristics. These varistors were characterized by55.3in the nonlinear exponent and1.53μA in the leakage current.（3） The microstructure of ZnO-Pr₆O₁₁-MnCO₃-CoO-Y₂O₃-Al2O₃（ZPMCYAl in short） system was composed of the ZnO phase, the ZnAl₂O₄spine phase and Pr-rich phases. The ZnAl₂O₄spine phase can be formed inthe ZnO-based varistor cermamics only sufficiently large additions of Al2O₃,in amounts exceeding the solubility of aluminum in ZnO phase. The ZnAl₂O₄spine phase does not change with the increasing Al2O₃content. Al2O₃dopingdid not significantly modify the densification process and grain growth. Theaverage grain size （d） and the sintered density （ρ） were observed to increaserespectively from approximately3.3μm to3.7μm and5.41to5.53g/cm³withthe increasing Al2O₃content. The varistor voltage （E21mA/cm） of ZPMCYAlsamples decreased over a wide range from5530V/cm to1844V/cm with theincreasing Al2O₃contents. The nonlinear exponent （α） increased firstly withthe increasing Al2O₃content whereas the further doping caused it to decrease. The samples with0.01mol%Al2O₃exhibited a highest α value of53.4. Inaddition, the JLvalue increased in a wide range of1.53to200μA/cm2withadditions of Al2O₃.（4） The ZPMC varistor ceramics were prepared by sol-gel process. Theproperties of powders and varistor ceramics were investigated. The resultsshow that the four-additive powders calcined at700℃for2h are sphericalwith the average size of200nm. The excellent electrical properties can beobtained for the Pr-doped ZnO varistor ceramics sintered at1220℃, which isabout80℃lower than those of samples prepared by conventional ceramicsprocessing. The microstructure of the varistor ceramics consists of only twophases ZnO grain and an Pr-rich phases as intergranular layer. These varistorsare characterized by3.2μm in the average grain size,910V/mm in varistorvoltage （E1mA）,40.2in the nonlinear exponent and0.3μA in the leakagecurrent.（5） The ZPMC varistor ceramics were prepared by liquid nano-coatingprocess. The ZnO composite powder calcined at750℃was spherical with theaverage size in the range of100-200nm. The diameter of the additive variedparticles from5to10nm. The optimal sintering temperature of varistors was1250℃, which was about50℃lower than that of samples prepared byconventional ceramics process. The microstructure of the varistor ceramicsconsisted of only two phases ZnO grain and Pr-rich phases as intergranularlayer. These varistors are characterized by4.4μm in the average grain size,790V/mm in varistor voltage （E1mA）,36.9in the nonlinear exponent and7μAin the leakage current.