How to Tune the Alumina Aerogels Structure by the Variation of a Supercritical Solvent. Evolution of the Structure During Heat Treatment

Alumina aerogels are obtained by supercritical drying (SCD) in four solvents of different chemical natures. The phase transformations, the morphological changes, and the thermal stability during heat treatment in the range 20–1200 °C are investigated. The as-prepared samples obtained by SCD in MeOH are identified as crystalline aluminum hydroxide methoxide, and those obtained by SCD in i-PrOH consist of alumina with small size of crystallite grains. In contrast to alcohol-dried samples, the initial ether-dried aerogels are amorphous (as determined by X-ray diffraction). Calcination of the samples causes their crystallization and recrystallization. After heat treatment at 1200 °C, all samples consisted only of the α-alumina phase. TEM study demonstrated that the nature of the solvent used for SCD significantly affects the morphology of the aerogels. Low-temperature nitrogen adsorption investigation of the aerogel samples confirms the influence of the nature of supercritical fluid (SCF) on the structure of the aerogels. Moreover, the nature of SCF affects both the specific surface area and its evolution during heat treatment.