Effects of Bi₂O₃ Addition in Micro- and Nanoscale on the Structural and Electrical Properties of Zn_1-xBi_xO varistors

Abstract

Two similar sets of Zn_1-xBi_xO ceramic varistors with various x values (0.00 ≤x≤ 0.20) have been prepared by using Bi₂O₃ additions with two different sizes. In the first set, Bi₂O₃ nanoparticles (≈200 nm) were used, while Bi₂O₃ microparticles (≈5 μm) were used in the second set. It was found that addition of Bi up to 5 % for both sets did not affect the wurtzite-type hexagonal structure of ZnO, but with increasing Bi above 5 %, some unknown lines were clearly observed in XRD spectra. The grain sizes are increased in both sets with increasing Bi content up to 2.5 %, followed by a decrease with further increase of Bi up to 20 %, and their values for microparticle additions were larger than that of the sets containing nanoparticle additions. Two nonlinear regions were formed in the I–V curves of ZnO due to Bi₂O₃ nanoparticle additions above 5 %. However, this behavior was completely absent in the samples containing Bi₂O₃ microparticles. Moreover, the breakdown field and nonlinear coefficient decreased with Bi₂O₃ addition up to 5 % for both sets, followed by an increase with further increase of Bi up to 20 %, and their values were higher for nanoparticle additions than that of microparticles. A reverse behavior was recorded for the electrical conductivity. The results have been discussed in terms of Bi₂O₃ nanosize grains which may be localized at the grain boundaries of ZnO ceramics.