Construction of master sintering curves to understand the effect of Na addition on ZnO-based varistors

This work describes the effect of a small sodium addition on the densification of Bi2O3-based ZnO varistors. The characterisation was performed using both the master sintering curve tool and the analyses of structural and microstructural characteristics of the materials. It was found that sodium-free varistor material exhibited very low activation energy for densification (149.2 kJ/mol). This was explained by the liquid phase promoting diffusion through the enhanced reaction between ZnO and Bi2O3. For sodium-doped material, the calculated activation energy was much higher (292.5 kJ/mol) and was very close to the one obtained for pure ZnO, which is often related to the grain boundary solid-state diffusion. It was shown that a small sodium addition prevented Bi2O3 from satisfactorily reacting with ZnO. Finally, a mechanism was proposed to explain how a slight sodium amount altered the reactivity between Bi2O3 and the ZnO matrix, resulting in some major changes in the final microstructures.