Studies Of The Improvement Of The Electrical Properties Of The ZnO-Bi₂O₃-Based Varistor Ceramics

The ZnO-Nb2O5, Bi2O3-MoO3, Bi2O3-B2O3conductive ceramics and the ZnO-Nb2O5-Bi2O3-PbO, ZnO-Bi2O3-TiO2-Co2O3-MnO2-Nb2O5-Sb2O3varistor ceramics were prepared by solid state reaction method. The microstructure of the samples were charactered by means of XRD and SEM; the resistivity, carrier concentration and defects of samples were tested by the Hall effect and the positron lifetime; the threshold voltage(V1mA), the leakage current(IL) and the nonlinear coefficient(α) of the ZnO-Nb2O5-Bi2O3-PbO and ZnO-Bi2O3-TiO2-Co2O3-MnO2-Nb2O5-Sb2O3were measured by a Ⅴ-Ⅰ source/measure unit. The effect of composition, microstructure, and carrier concentration on the electrical properties of the samples were investigated. The experiental results are as follows:(1) The electron density of ZnO-Nb2O5conductive ceramic varies with the content of Nb2O5. With the increase of the content of Nb2O5, the mean positron lifetime of the ZnO decreases firstly and then increases, or the electron density of the ZnO increases firstly and then decreases. As the content of Nb2O5is1.5mol%, the mean positron lifetime of the sample reaches a minimum value, that is, the electron density of the smaple reaches a maximum value.(2) ZnO-Nb2O5is an N-type semiconductor ceramic. With the increase of the Nb2O5content in the sample, the Zn3Nb2O8phase present in the sample, the grain size of ZnO increases and its distribution becomes more homogenius, while the resistivity of the sample decreases. As the Nb2O5content is2.0mol%, on increasing the sintering temperature, the grain size of the ZnO increases, meanwhile, the resistivity of the sample gradually decrease.(3) For the Bi2O3-B2O3semiconducting ceramics, with the increase of the content of B2O3, the mean positron lifetime increases, while the electron density decreases, results in the decrease of the resistivity of the sample.(4) For the Bi2O3-MoO3, Bi2O3-PbO semiconducting ceramics, with the increase of dopants (MoO3or PbO) content, the grain size of the sample increases and its distribution becomes more homogenius, at the same time the resistivity of the sample decrease.(5) For the ZnO-Nb2O5-Bi2O3-PbO varistor that made of semi-conducting ZnO-Nb2O5and semi-conducting Bi2O3-PbO, as the the sintering temperature increasing, the varistor voltage and leakage current of the varistor decrease firstly and then increase, while the nonlinear coefficient of the varistor increases firstly then decreases. When the sintering temperature is900℃, the threshold voltage(V1mA), the leakage current(IL) and the nonlinear coefficient(a) of the varister are147V,349μA and9.5, respectively.(6) The ZnO-Bi2O3-TiO2-Co2O3-MnO2-Nb2O5-Sb2O3varistor, which is prepared based on the considering the influence of various factors on the electrical properties of ZnO-based varistors ceramics, shows quite good electrical properties, the threshold voltage(V1mA), the leakage current(IL) and the nonlinear coefficient(a) of this sample are238V,3μA and36, respectively.