Effect Of Doped NBT On Microstructure And Electrical Of Barium Titanate-Based Lead-Free PTC Ceramic

Nowadays, BaTiO₃-based PTC ceramics with high Curie temperaturemostly contain lead. However, lead is volatile at high temperature, which doesharm to people during the process of producing and using ceramics. Taking allthese into account, it is urgert to development lead-free PTC （PositiveTemperature Coefficient） ceramcis with high Curie temperature, low resisitivityroom temperature and obvious resistance jump. In order to substitute thelead-contained PTC ceramics, three kinds of BaTiO₃-based lead-free PTCceramic system were investigated in the thesis.In this paper, BaTiO₃-Na_0.5Bi_0.5TiO₃, BaTiO₃-BiYO₃and BaTiO₃-Na0.5Bi0.5TiO3-BiYO₃system ceramics were fabricated by the conventionalmixed oxide method. The results showed that the ceramic samples doped Nb₂O₅could obtain good PTC effect （ρ max ρmin>103）, low room temperature resistivity（ρRT¹03cm） when the content of dopant （Na0.5Bi0.5TiO3） was less than2mol%, but it was difficult to fulfill the request of the Curie point in practicalapplications. When the content of dopant exceeded2mol%, the ρRTreached106cm, so it was hard to be semiconducting for the ceamic samples. Therefore,BaTiO₃-BiYO₃and BaTiO₃-Na_0.5Bi_0.5TiO₃-BiYO₃system ceramics werefabricated by doping donors such as Nb₂O₅. In addition, the samples werecharacterized by X-ray diffraction, scanning electron microscope, impedancespectroscopy and the temperature dependence of resistivity, and the effects ofcomposition and preparation process on the PTC characteristics andmicrostructure were investigated.In order to reduce the room temperature resistivity and enhance theresistance jump, Nb₂O₅as donor and MnCO3as acceptor were introduced intoBaTiO₃-Na_0.5Bi_0.5TiO₃ceramics，and the semiconducting ceramics could beobtained by doping Nb₂O₅, then the influence of doping Nb⁵⁺and Mn²⁺on thePTC effects and microstructure of BaTiO₃-Na_0.5Bi_0.5TiO₃ceramics were stuied. The research results showed that the room temperature resistivity could besignificantly reduced with the addition of Nb⁵⁺and the resistance jump can beenhanced with the addition of Mn²⁺, but the Curie temperature was shifted to alower temperature with increasing of the content of Nb⁵⁺. In addition, theaverage grain size of samples decreased with increasing of the contents of donorNb⁵⁺and acceptor Mn²⁺. However, the inhibitory extent of donor Nb⁵⁺on thegrain growth was much greater than that of acceptor Mn²⁺.In order to search for new system ceramics with higher Curie temperature,we studied BaTiO₃-BiYO₃system ceramics. The semiconducting ceramics couldbe obtained by doping Nb₂O₅, and the influence of doping BiYO₃on the PTCeffects and microstructure of BaTiO₃-based ceramics were stuied. The researchresults showed that the Curie temperature was shifted to a higher temperaturewith increasing of the content of BiYO₃. The0.97BaTiO₃-0.03BiYO₃-0.002Nb₂O₅ceramics could obtain lowρRT（4000cm）, high Curie temperature（150℃） and good PTC effect （ρ max ρ>103min）. In addition, the average grainsize decreased with increasing of BiYO₃.In order to improve the Curie point significantly, Na0.5Bi0.5TiO3and BiYO₃as another end member of BaTiO₃-based solid solutions were addedsimultaneously, then the influence of doping Na0.5Bi0.5TiO3and BiYO₃on thePTC effects and microstructure of BaTiO₃-based ceramics were stuied. Theresearch results showed that the obtained0.88BaTiO₃-0.10Na0.5Bi0.5TiO3-0.02BiYO₃ceramics prepared by a reduction-reoxidation exhibited a high Curietemperature of162℃. In addition, the impedance spectra of BaTiO₃-Na0.5Bi0.5TiO3-BiYO₃ceramic samples were measured from the roomtemperature to180℃. The results demonstrated that the resistivity of grainboundaries took the dominant position in PTC effect. The resistivity of grainboundary quickly increased with increasing of the contents of Na0.5Bi0.5TiO3andBiYO₃, but the change of the resistivity of grains was very little.