PREPARATION AND CHARACTERIZATION OF BOEHMITE PRECURSOR AND SINTERABLEALUMINA POWDER FROM AQUEOUS ALUMINUM CHLORIDE-UREA REACTION

The role of the process conditions on the formation of submicrometre s ize boehmite powders by aqueous aluminium chloride-urea reaction at 37 3 K has been studied. In all the reactions, initially a gelatinous pre cipitate was formed at pH 6.5. Further refluxing of the reaction solut ion containing this precipitate led to the formation of a granular pow der. The kinetics of gelation and powder formation, and the final pH o f the medium, were influenced by the aluminium to urea ratio (the less er the ratio the faster the kinetics and the higher the final pH). The granular powder formation was accompanied by transformation of the am orphous gel into crystalline boehmite, as revealed by XRD. The particl e size distribution of the precursor powders obtained by the sedimenta tion technique was in the range 0.3 to 0.8 mu m. Increasing the alumin ium concentration and aluminium to urea ratio increased the particle s ize. Morphological studies by SEM exhibited the powders as elongated f ibrillar bundles of boehmite. The specific surface area data obtained by the BET method showed that the particles are porous agglomerates. U pon calcination at 1673 K, the alpha alumina powders formed retained n early the same particle size distribution and morphology of the precur sor. However, the specific surface area decreased due to the sintering of the crystallites within the individual particles. The compacts obt ained from these powders could be sintered to densities above 95% of t he theoretical at 1823 K and above.