Preparation And Characterization Of Fine Structural Hydroxide Aluminum And Alumina Powders

In this dissertation powders of alumina with peculiar morphology and different properties have been prepared with aluminum sulfate as precursor, ethanol-water as solvent by hydrothermal treatment methods. During the preparing process NaOH and urea were respectively introduced as precipitating agent. The effects of water -alcohol ratio, precipitant concentration, hydrothermal temperature and hydrothermal time on the properties of alumina powders were investigated systemically by SEM, XRD and Size analysis.The results indicate that aluminum hydroxide sulfate with cubic structure was prepared without precipitating agent, and the particle size is about 10μm. The grain size can be obviously refined by adding NaOH. As-synthesized powders show a nearly spherical structure with loose surface and their phase still keep in aluminum hydroxide sulfate.When urea introduced as precipitating agent, the ethanol concentration become the major factor on the microstructure and structure of hydrothermal product. The hydrothermal product is flower-like structure boehmite in the water -alcohol ratio 2:4.There are three kinds of morphology of the hydrothermal products in the water -alcohol ratio 4:2. With the hydrothermal synthesis time extended, the crystal grain by the smooth surface of solid ball into the core-shell structure which surface is composed of nano-flakes, and finally into a perforated hollow spherical structure. The conversion time is shorten with increasing hydrothermal temperature, but solid ball disappear when hydrothermal temperature higher than 160℃.The XRD analysis shows that the solid ball corresponded to the aluminum hydroxide sulfate phase, the core-shell structure corresponded to the mixed-phase structure which was composed of aluminum hydroxide sulfate and AlOOH,while the hollow spheres corresponded to boehmite phase structure. In addition, theγphase transformation temperature for aluminum hydroxide sulfate toγ-Al2O3 is 900℃, while theγphase transformation temperature for boehmite is 600℃.The mechanism of crystal growth on three kinds of alumina powders was analyzed and discussed in details. The results indicate that the crystal growth was in the way of anionic coordination polyhedron growth units and precipitation from supersaturated solution.