The effect of processing conditions on varistors prepared from nanocrystalline ZnO

Nanoparticles of ZnO were prepared by the reaction of ethanolic solutions of zinc acetate and oxalic acid followed by drying (80°C) and calcination (500°C). Subsequently varistor materials were fabricated from this nanoparticular ZnO via two separate routes:- a) from a "core shell" material using metal salts as additives; b) by using a conventional solid state mixing of metal oxides. Sintering (1050°C) and subsequent electrical studies were carried out for each of these samples and they were compared with commercial varistor samples prepared under similar conditions. "Core shell" type varistor material showed considerably higher breakdown voltage (V_c = 850 ± 30 V mm^-1) as compared to a sample prepared by mixing with metal oxides (V_c = 683 ± 30 V mm ^-1) or commercial varistor discs (V_c = 507 ± 30 V mm^-1). The high breakdown voltage obtained is attributed to the formation of more varistor-active grain boundaries per unit area.