Synthesis and characterization of ZnO-based nano-powders: study of the effect of sintering temperature on the performance of ZnO–Bi2O3 varistors

In this paper, we investigate the improvement of varistor devices with a ZnO-based material. In this goal, pure and 1% Bi-doped ZnO nano-powders have been synthesized at different sintering temperatures by a soft chemistry method: the sol–gel route. The used temperatures were carefully chosen to obtain the desired phases: (α-Bi 2 O 3 , β-Bi 2 O 3 , and δ-Bi 2 O 3 ). Characterizations were made by XRD and TEM to determine the structural properties and particle sizes of Bi phases. XRD spectra confirmed the würtzite structure and the presence of many transition phases for each sintering temperatures with an average grain size varying from 42 nm to 76 nm. TEM and SEM images of the samples allowed us to study the location of different phases, their morphologies, and the size of particles, and they show a mixture of nano-objects of different sizes and shapes. The electrical characteristics J ( E ) were measured to correlate these results with the varistor effect which indicate a good nonohmic behavior for all samples; the threshold voltage V B ranges between 242 and 701 V/cm and the coefficient of nonlinearity between 10.26 and 14.5. The total absence of phase transitions between the grains of the triple point in the ZnO–Bi 2 O 3 -based varistor is presented. Moreover, we show that the majority of phase transitions are located at the boundary of grains and forms a Shottky barrier which is suitable to improve the protective effect of surge arresters for hugely suppressing over-voltages in power and network systems.