Sol-Gel Aluminum Hydroxides and Their Thermal Transformation Studies for the Production of α-Alumina

This work reports the formation of alpha-alumina from pseudoboehmite synthesized from three different hydrated inorganic aluminum salts such as aluminum nitrate, aluminum chloride and aluminum sulfate using the sol-gel method in aqueous medium at 80 A degrees C. All the aluminum hydroxides were thermally treated and all showed similar crystalline conversion behavior; and, their solid-phase transformations to aluminum oxides were investigated. Following the synthesis, all products were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry-thermogravimetric analysis and X-ray diffraction. The hydrolysis products were calcined at 200, 300, 600, 800, 1100 and 1200 A degrees C for 1-h. The eta (eta) and theta (theta) transitional aluminas were detected until complete transformation to highly crystalline alpha-alumina occurred at similar to 1200 A degrees C. Moreover, starting at 1100 A degrees C the sample derived from the sulfate salt showed intense crystalline peaks corresponding to alpha-alumina. The morphological study for the samples of alpha-alumina calcined at 1200 A degrees C was carried out by field emission scanning electron microscopy where a vermicular structure was observed. These alpha-alumina samples were also analyzed by high resolution-transmission electron microscopy. In addition, fast Fourier transform and selected area electron diffraction patterns of alpha-alumina samples calcined at the highest temperature were obtained to calculate the experimental interplanar distances whose values were compared to reported theoretical values.