Synthesis of an alumina coating from chelated aluminium alkoxides

Chelating agent, such as acetylacetone (AcAcH) and ethylacetoacetate (EAA) are used to avoid the undersirable precipitation of gelatinuous aluminum hydroxides during hydrolysis of aluminum-sec-butoxides, Al(OBus). This procedure leads to alumina sols with particle size distributions in the range of 1 to 15 nm which depend on the type of chelating agent as well as on the molar Al(OBus)3/chelating agent ratio. Generally, EAA leads to particles smaller than AcAcH. Because of the small particle size, gel powders are X-ray amorphous and do not crystallize at temperature < 750°C. Crystallization of á-alumina starts between 950 and 1000°C. Complete transformation to á-alumina is achieved between 1050 and 1100°C. Coating experiments with different sols on silica glass lead to nearly crack-free dried gel films with AcAcH system. An ultrafine, dense microstructure is obtained