International audienceVaristor ceramics were prepared by spark plasma sintering (SPS) using two doped zinc oxide based powders. Optimized sintering cycle for thecommercial powder (mixture of oxides) yielded dense parts (>99%) containing mainly ZnO grains (1 mm) and additional Bi2O3 and Zn7Sb2O12oxides. Sintering at the low oxygen partial pressure inherent to SPS leads to the reduction of Bi2O3 into metallic Bi. To reduce the grain size,improve the distribution of dopants and the grain size distribution, we used co-precipitation to synthesize a powder with the same formulation as thecommercial powder. Due to the high reactivity of this powder, fully dense parts were obtained at temperature as low as 400 ◦C under air atmosphereusing carbide tools. Such conditions led to limited ZnO grain growth and bismuth reduction during the thermal cycle. The fine microstructureobtained led to varistances with to high electrical properties (threshold field higher than 2300 V/mm)

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International audienceThe leakage current and the non-linearity coefficient (α) are crucial parameters in varistors. This work, deals with the optimization of the electrical characteristics of medium and high voltage varistors based on the ZnO–Bi2O3–Sb2O3 system. First, the aim was to allow the formation and the stabilization of the spinel phase by reducing the heating rate. To do so, the right sintering temperature and dwell time have to be chosen to obtain a breakdown field suitable for industrial requirements. During cooling, the spinel phase can return to a pyrochlore phase, which contributes to the increase of leakage current. In a second part, this reaction was prevented by faster cooling in an appropriate temperature range. The fine-tuning of both the heating and cooling phases leads to a significant decrease of the leakage current. Moreover, the value of the non-linearity coefficient was increased by 80%, due to better and more homogeneous wetting of the ZnO grains by the Bi-rich phase

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