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

Abstract

This work reports the formation of α-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 °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 °C for 1-h. The eta (η) and theta (θ) transitional aluminas were detected until complete transformation to highly crystalline α-alumina occurred at ~1200 °C. Moreover, starting at 1100 °C the sample derived from the sulfate salt showed intense crystalline peaks corresponding to α-alumina. The morphological study for the samples of α-alumina calcined at 1200 °C was carried out by field emission scanning electron microscopy where a vermicular structure was observed. These α-alumina samples were also analyzed by high resolution-transmission electron microscopy. In addition, fast Fourier transform and selected area electron diffraction patterns of α-alumina samples calcined at the highest temperature were obtained to calculate the experimental interplanar distances whose values were compared to reported theoretical values.