Peculiar effects of microwave sintering on ZnO based varistors properties

Non-ohmic properties of doped zinc oxide are widely used in varistors applications. It is well established that final properties of the component are strongly correlated with reactivity of the added phases during sintering process and with final microstructure. In this paper, the specific effects of the hybrid single-mode microwave sintering process on the microstructure and electrical properties of a ZnO-based composition are investigated. Nano-sized ZnO-based powder with a proper amount of Bi2O3, Sb2O3, CoO and MnO is synthesized by a liquid route and is sintered within a short time (less than 10 min) in a conventional (CV) or by an hybrid single-mode microwave (MW) furnaces. Distinct differences can be seen in the density, reaction kinetics and dopant diffusivity: higher kinetics of MW leads to denser pellet, faster reaction among dopants and faster diffusion of cobalt and manganese into ZnO grains although grain sizes are almost identical between CV and MW. These differences in terms of chemistry and microstructure lead to sharp contrasts in electrical properties.

► Standard composition for ZnO-based varistors discovered 40 years ago was chosen.
► Liquid route synthesis was implemented to get nanoparticles.
► Samples were sintered during similar short times by microwaves and conventionally.
► Microwaves sintering increases sintered samples’ densities and improve varsitors properties.
► Peculiar effects of microwaves: enhancement of the reactions kinetics and dopants diffusion.