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 (80°C) and calcination (500°C). The calcined material (nano-array ZnO) is composed of approximately spherical nanoparticles of average diameter 21 ± 3 nm, self-assembled to form arrays extending in length to 2-4 μrn. 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°C and 250°C using XRD and TEM and by following the decomposition reactions using TGA and DSC. Varistors prepared from the nano-array ZnO by sintering (1050°C) with appropriate mixtures of metal oxides showed a breakdown voltage of 786 ± 30 V mm^-1, which is substantially higher than that of samples prepared under similar conditions from either micron-sized commercial ZnO (507 ± 30 V mm^-1) or from nano-ZnO (683 ± 30 V mm^-1).