Synthesis and properties of rod-like ZnO composite powders by the reflux method

Abstract

The rod-like (Bi, Co) co-doped ZnO powders were directly synthesized by refluxing method and the effect of reaction time on its structure and optical properties was explored. These powders were consolidated into dense varistors discs by compaction, sintering, and evaluated for their non-linear I–V characteristics. X-ray diffraction (XRD) results revealed that the doped ZnO nanopowders have a hexagonal wurtzite structure and the Bi₂O₃ phase is found. The half-height width of the (002) plane of the ZnO phase decreases as the reaction time increases, which indicates the improvement of the crystal quality. Scanning electron microscope (SEM) results show that the samples present a uniform rod-like structure. As the refluxing time increases from 2 to 4 h, the average length of ZnO composites increases from 900 to 2250 nm and decreases to 530 nm for 6 h. The presence of functional groups and the chemical bonding is confirmed by Fourier transformation infrared spectra (FTIR). X-ray photoelectron spectroscopy (XPS) demonstrates that Bi element is successfully doped into ZnO in the trivalent valence state while Co element in the form of divalent and trivalent states. The Z-2 varistor ceramics sintered in air at 1200 °C for 2 h showed the highest nonlinear coefficient of 30.8 and the strongest breakdown voltage of 282.8 V/mm. These studies demonstrate the feasibility of direct synthesis of doped ZnO powders for varistor ceramics by refluxing method.