Highly thermally stable alumina-based aerogels modified by partially hydrolyzed aluminum tri-sec-butoxide

Highly thermally stable alumina-based aerogels are synthesized by the acetone–aniline in situ water formation method and modified by partially hydrolyzed aluminum tri-sec-butoxide at different temperatures (25, 45, and 60 °C). The effects of modification, especially modification temperature, on microstructure and thermal stability of alumina-based aerogels are investigated. After the modification, the morphologies of alumina-based aerogels change from the network structures with interconnected needle-like particles to those with stacked sheet-like particles, resulting in a better heat resistance. The thermal stability of alumina-based aerogels enhances with the increasing modification temperature, whereas the high temperature (more than 60 °C) would lead to the dissolution of wet gels during the modification process due to the high solubility. After annealing at 1200 °C for 2 h, the 45 °C-modified alumina-based aerogel exhibits the best thermal stability with the lowest linear shrinkage of ~7% and the highest specific surface area of 154 m²/g. In addition, the modified aerogels remain in the θ-Al₂O₃ phase while the unmodified one transforms into α-Al₂O₃ phase after 1300 °C annealing. The alumina-based aerogels are further reinforced by incorporating with mullite fiber felt and TiO₂. The obtained composites show ultralow thermal conductivities of 0.065, 0.086, and 0.118 W/mK at 800, 1000, and 1200 °C, respectively.