Influence of Chromium on Microstructure and Electrical Properties of ZnO-Based Varistor Materials

The addition of Cr2O3 into a ZnO-based varistor composition was systematically investigated. All samples had the general chemical formula was Zn0.96Bi0.02Co0.02-xCrxOβ (x = 0, 0.005, 0.01 and 0.02) and prepared by solid-state processing. Addition of chromium had a great influence on several microstructural characteristics which, in turn, resulted in different varistor behaviors. All samples could be sintered up to 90% at 900 °C which could be attributed to liquid-phase sintering; though densification rate of Cr-added samples was greatly reduced especially below 900 °C. Such retardation in sintering might be possibly related to differences in phase formation among different compositions. X-ray diffraction (XRD) of the samples sintered at 1000 °C revealed a higher degree of multi-phasic nature upon increasing amount of chromium. Secondary phases include ZnCr2O4, Bi2O3 and Zn-Bi-O compounds. Scanning electron microscopy (SEM) showed that the first phase was distributed all over the microstructure. The latter two phases, however, were found mostly in the grain boundary region and are likely associated with the mechanism of liquid-phase sintering. Surprisingly, these two liquid phases could not be detected in the non-Cr sample by XRD in spite of the similar initial amounts of Bi2O3 in all samples. This finding indicates possible alterations in both crystallization and phase formability in this varistor system. Besides the phasic unconformity, grain growth was also affected. Increasing amount of chromium significantly reduced the averaged grain size of the ZnO main phase (from 6.3 μm at x = 0.005 to 4.5 μm at x = 0.02). The smaller grains among these samples could be related to the presence of chromium itself which somehow possibly plays a role in atomic diffusion during sintering. In addition, the induced crystallization of the liquid phases, as confirmed by XRD, might also be responsible for grain boundary pinning, inhibiting grain growth during heat treatment. Samples with smaller grain sizes yielded a better varistor property; the breakdown voltages were improved (up to 740 V/cm) when the amount of chromium was increased from 0 to 0.02. This improvement was likely related to a higher volume of grain boundary phases which were widely proposed to facilitate the non-linear behavior in ZnO-based varistor materials.