Impedance spectroscopy studies of nanostructured ZnO based varistor materials

Abstract

Varistor material with 96 mol%ZnO + 4 mol% dopants in oxide form of Bi, Co, Sb, B, Cu and Sn in nanostructured from has been synthesized using colloidal suspension and centrifugal separation method. The synthesized powder sample specimen was compacted into a pellet and sintered in air at 1073 K for 3 h to get a bulk density of 96%. In situ impedance spectroscopy (IS) studies have been carried out for the sintered nanostructured specimen at various temperatures in oxygen and nitrogen atmosphere. The impedance spectroscopy results below 423 K show that the specimen contains three activation energy regions which are attributed to (i) pure ZnO core grain (ii) aliovalent cation diffused layer around the pure ZnO core and (iii) grain boundary (GB). The specimen changes its conducting nature at and above 423 K. The a.c. conductivity was measured at fixed frequencies (106 Hz and 102 Hz) as a function of temperature. The conductivity of bulk grain and grain boundary regions increases with the increase of temperature. Eventhough the conductivity value of bulk grain and boundary regions. increases with temperature, the rate of change of conductivity in grain boundary region is larger than that in bulk grain. The GB conductivity becomes almost equal to that of bulk grains at high temperatures. The change of grain boundary conductivity with temperature is faster in N2 atmosphere compared to the change in O2 atmosphere. Due to this dominant grain boundary conductivity, the varistor action of the sample is lost at high temperatures in nitrogen atmosphere (air).