The effect of processing conditions on varistors prepared from nanocrystalline ZnOElectronic supplementary information (ESI) available: 1) EDX associated with HRTEM of the nano-ZnO sample before and after coating with Sb, Bi and Co; 2) HRTEM and EDX associated with HRTEM of the varistor powder calcined at 300 �C after the addition of all the dopants and 3) EDX associated with FESEM of the core shell varistor sample sintered at 1050 �C. See http://www.rsc.org/suppdata/jm/b3/b306280e/

Nanoparticles of ZnO were prepared by the reaction of ethanolic solutions of zinc acetate and oxalic acid followed by drying (80 uC) and calcination (500 uC). 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 uC) 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 (Vc ~ 850 ¡ 30 V mm21) as compared to a sample prepared by mixing with metal oxides (Vc ~ 683 ¡ 30 V mm21) or commercial varistor discs (Vc ~ 507 ¡ 30 V mm21). The high breakdown voltage obtained is attributed to the formation of more varistor-active grain boundaries per unit area