Materials, Methods & Technologies, Volume 8, 2014

Abstract: Thermal treatment (>1200°C) of unstable intermediate phases of aluminum hydroxides and aluminum oxides inspires formation of thermodynamically stable α-Al₂O₃, which is widely applied in the production of ceramics with different functional purposes. Low-temperature phase transformation and obtaining of α-Al₂O₃ nanopowders is possible, if in the reaction mixture is added a small amount of up to 5% α-Al₂O₃ (during decomposition, hydrolysis, drying-heating and annealing of precursors). Modified polymer-precursor and sol-gel combustion techniques were used for obtaining α-alumina. Aluminum nitrates, aluminum alkoxides, oxidants and bifunctional compounds for producing organic-inorganic gels were used as initial precursors. Organic-inorganic gels were obtained by homogenization of aluminum and organic compounds within the range of 80-200°C and by carbonization of the obtained mass within the temperature range of 300-800°C. The addition of 3-5% α-Al₂O₃ seeds in the initial reactive mixture decreases γ-phase transformation temperature (1200°C) to 1020-1080°C. It was established, that during the interaction between aluminum nitrates and urea, aging of aluminum urea solution caused obtaining of amorphous aluminum oxide, annealing of which for 2h at 800°C in the open air gave α-Al₂O₃. There were studied possibilities of obtaining nano-phase powders by low temperature (10-90°C) corrosion of aluminum activated with mercury. Subsequent annealing of the obtained Al₂O₃·nH₂O amorphous powder at 1000-1100°C resulted in the formation of α-Al₂O₃ fibers. XRD analysis showed that Al₂O₃ fibers contained particles with primary crystallites of 10-30 nm sizes.