Effect of Process Parameters on the Characteristics of Nanocrystalline Alumina Particles Synthesized by Solution Combustion Process

Nanocrystalline ceramic oxide particles with high purity can be synthesized efficiently by the solution combustion synthesis route, which is based on redox reactions between metal salts and reducing agents such as urea and glycine. In the present study, nanocrystalline alumina powders were synthesized using aluminium nitrate nonahydrate as the metal salt and urea as the fuel. The powders were characterized primarily for the phases present and crystallite size (from X-ray diffractometry) and morphology (by scanning electron microscopy). When stoichiometric amounts of the starting chemicals were taken, X-ray diffraction (XRD) analysis of the synthesized powder revealed it to be phase-pure α-Al₂O₃, with a crystallite size of 42 nm. Electron microscopy of the synthesized powder revealed a flaky morphology. Further, the pH value of the solution containing the stoichiometric amounts of the aluminium salt and the fuel was systematically varied by dissolving in liquid ammonia. It was observed (from XRD analysis) that an increase in the pH progressively stabilized the metastable γ-Al₂O₃ phase. An increase in the fuel (urea) content had no effect on the phase stability, but decreased the crystallite size of α-Al₂O₃. A crystallite size of 29 nm could be achieved with an excess fuel ratio of 1.5 over the stoichiometric value.