Low-temperature synthesis of α-alumina based on sol-gel processes

Typically, synthesis of α-alumina (α-Al2O3) has been performed by annealing the aluminium hydroxide (AH) at temperatures higher than 1000°C, which leads to a high energy consumption. Accordingly, α-Al2O3 is desired to be synthesised by low-temperature processes for reducing energy consumption. This review describes our studies on the low-temperature synthesis of α-Al2O3 based on a sol-gel reaction. AHs were prepared by the hydrolysis of aluminium alkoxide and homogeneous precipitation from aluminium inorganic salt (alkoxide AH and inorganic AH, respectively). Alumina sols (ASs) were prepared by peptising the alkoxide AH and the inorganic AH with acids (alkoxide AS and inorganic AS, respectively). The ASs were converted to alumina gels (AGs) by dehydration in air at room temperature (alkoxide AG and inorganic AG, respectively). The ASs were seeded with α-Al2O3 nanoparticles and/or treated hydrothermally. Inorganic AS and AG were suitable to produce α-Al2O3 compared to alkoxide AS and AG. The use of formic acid as peptiser promoted crystallisation of AG to α-Al2O3. The seeding and the hydrothermal treatment also promoted the crystallisation. The alumina started to be crystallised to α-Al2O3 at a temperature as low as 400°C with the above three conditions.