SPHERICAL AND ROD-LIKE ZINC-OXIDE MICROCRYSTALS - MORPHOLOGICAL CHARACTERIZATION AND MICROSTRUCTURAL EVOLUTION WITH TEMPERATURE

Spherical ZnO microcrystals obtained by spray pyrolysis and thermal decomposition methods as well as rod-like ZnO particles (prismatic and needle shaped) prepared from precipitation in aqueous solutions, have been characterized by electron microscopy, X-ray diffraction and infrared spectroscopy. Very different sizes of ZnO particles were obtained from spray pyrolysis. However, only the larger particles (0.7-mu-m) were found to be slightly deformed by infrared spectroscopy. From thermal decomposition of zinc acetate, fine particles of average size 0.05-mu-m, rather spherical and agglomeration free were obtained. The role of initial size and morphology in the thermal evolution is fundamental: very fine spherical particles (0.01-0.02-mu-m), can be sintered to give particles of 0.1-0.3-mu-m at 875-degrees-C with unchanged morphology. When the temperature induces a change in spherical shape, the first microstructural changes appear to take place through the crystallographic c-axis. However, for rod-like particles, changes begin from the a, b axes, being faster for needle-shaped microcrystals.