Compact shape as a relevant parameter for sintering ZnO–Bi2O3 based varistors

Ceramic varistors based on the ZnO–Bi2O3–Sb2O3 system, are usually sintered at 1150–1200 °C in the presence of a Bi-rich liquid phase that forms at 700–800 °C. Bismuth is critical for the formation of electrically active grain boundaries, and the Bi-rich liquid phase plays a major role in the grain growth kinetics. Therefore, uncontrolled bismuth oxide vaporization during sintering can heavily deteriorate the electrical response of the varistor, and the area–volume ratio of the green compact becomes a relevant parameter, along with the sintering temperature and soaking time. In the present work, we have studied the microstructure and the current–voltage characteristics of ceramic varistors with different area–volume ratios and sintering cycles. We find that, depending on the sintering cycle, there exists a value for the area–volume ratio that leads to a strong deterioration of the varistor electrical properties.