Enhancement of electrical conductivity by Al-doped ZnO ceramic varistors

Zn_1−xAl_xO ceramic samples with various x values (0.00≤x≤0.20) are sintered in air at temperatures of 850 °C for 10 h and then quenched to room temperature. Structural, surface morphology and I-V characteristics of the samples are investigated using X-ray diffractometer (XRD), scanning electron microscope (SEM) and dc electrical measurements. It is found that addition of Al up to 0.05 does not influence the well-known peaks related to wurtzite structure of ZnO ceramics, and other unknown peaks could be formed above 0.05 of Al. The cell parameters of Al-doped samples are a little shorter than the undoped ZnO, and also the shape and size of grains are clearly affected. Average crystalline diameters, deduced from XRD analysis, are between 39.90 and 47.18 nm, which are 25 times lower than those obtained from SEM micrographs. Although breakdown field, nonlinear coefficient and barrier height are generally decreased by Al addition, the electrical conductivity is improved. These results are discussed in terms of the interaction mechanism between atoms of Al and Zn in both under and over-doped regions.