Microstructural analysis of varistors prepared from nanosize ZnO

ZnO nanoparticles were prepared by a solid state pyrolysis reaction of zinc acetate dihydrate and oxalic acid dihydrate at 500°C. The course of reaction at various temperatures was followed by XRD. Subsequently varistors were fabricated from this nano-ZnO material by solid state mixing with various oxide additives and sintering to 1050°C. The microstructure of the sintered material was studied using XRD, field emission SEM (FESEM), and EDX, and ZnO grains, bismuth rich regions and spinel phases were identified. Discs made from oxide doped nano-ZnO show considerably higher breakdown voltage (656±30 V mm^-) compared to those prepared from micrometre sized ZnO (410±30 V mm^-1) and commercial varistors (454±30 V mm^-1). However, varistors made from the nano-ZnO show very low densification and high leakage current, making them unsuitable for device fabrication.