Self-assembled arrays of ZnO nanoparticles and their application as varistor materials

Linear arrays of ZnO nanoparticles have been successfully prepared by a simple sol–gel condensation reaction involving chemical modifiers, followed by drying (80oC) and calcination (500 oC). The calcined material (nanoarray ZnO) is composed of approximately spherical nanoparticles of average diameter 21+/-3 nm, selfassembled to form arrays extending in length to 2–4 mm. The morphology of the ZnO is found to depend sensitively on the amounts of chemical modifiers present. In their absence the ZnO produced (nano-ZnO) is an unstructured agglomerate of nanoparticles. The mechanism for formation of these linear arrays has been investigated by examining the intermediates formed at 80 oC and 250 oC using XRD and TEM and by following the decomposition reactions using TGA and DSC. Varistors prepared from the nano-array ZnO by sintering (1050 oC) with ppropriate mixtures of metal oxides showed a breakdown voltage of 786 +/- 30 Vmm-1, which is substantially higher than that of samples prepared under similar conditions from either micronsized commercial ZnO (507 +/-30 V mm-1) or from nano-ZnO (683 +/-30 V mm-1)