Preparation and characterisation of boehmite precursor and sinterable alumina powder from aqueous aluminium chloride-urea reaction

Abstract

The role of the process conditions on the formation of submicrometre size boehmite powders by aqueous aluminium chloride-urea reaction at 373 K has been studied. In all the reactions, initially a gelatinous precipitate was formed at pH 6.5. Further refluxing of the reaction solution containing this precipitate led to the formation of a granular powder. The kinetics of gelation and powder formation, and the final pH of the medium, were influenced by the aluminium to urea ratio (the lesser the ratio the faster the kinetics and the higher the final pH). The granular powder formation was accompanied by transformation of the amorphous gel into crystalline boehmite, as revealed by XRD. The particle size distribution of the precursor powders obtained by the sedimentation technique was in the range 0.3 to 0.8 μm. Increasing the aluminium concentration and aluminium to urea ratio increased the particle size. Morphological studies by SEM exhibited the powders as elongated fibrillar bundles of boehmite. The specific surface area data obtained by the BET method showed that the particles are porous agglomerates. Upon calcination at 1673 K, the alpha alumina powders formed retained nearly the same particle size distribution and morphology of the precursor. However, the specific surface area decreased due to the sintering of the crystallites within the individual particles. The compacts obtained from these powders could be sintered to densities above 95% of the theoretical at 1823 K and above.