Spectral properties and temperature-dependence of semiconductivity of various doped bismuth trioxide polycrystals

Abstract

Several samples of Nb20~-doped Bi203 and Y203-doped Bi203 were carefully prepared and sintered at 700 ~ for 3 hours. Extensive measurements were carried out on these samples, including X-ray diffraction spectra, infrared absorption spectra and the temperaturedependence of the DC-electrical conductivity in the solid state. The results obtained were discussed, correlated and interpreted. Finally, the optimum compositions were established and recommended for doped-Bi203 in the electronics industry.The interest in the spectral and semiconducting properties of various doped polycrystals is due to the use of Bi20 3 as a sintering aid for the preparation of ZnO high-field varistors. A varistor is a variable resistor in that its resistance decreases rapidly with increasing voltage above a threshold value [1,2]. Since this breakdown voltage is stropgly dependent on grain size, it is important to limit the size of the ZnO grains for a high-field device. Varistors are usually prepared from a mixture of metal oxides such as ZnO, CoO, Bi203 and PbO.Although several theories have been proposed to explain the effects of certain oxides on the properties of varistors, their roles are not completely understood or characterized. While it is thought that Bi203 does not affect the electrical properties of the ZnO grains themselves, it has been postulated that Bi203 affects the electrical properties of the grain boundaries of the varistors [1][2][3][4].Bismuth sesquioxide (Bi203) has four polymorphic forms: a monoclinic a-form stable below 730 ~ a cubic 7-form stable above 730 ~ to the melting point (825~ a metastable tetragonal r-form, and a bcc 7-form which appears during cooling of the v-form. It has been reported that the v-form has a deficient fluorite structure, with