Synthesis of Alumina Aerogels by Cost-Effective Inorganic Aluminum Salt Precursors and Vacuum Freeze-Drying Process

Alumina aerogels are attractive for thermal insulation due to their ultralow thermal conductivity and excellent high-temperature resistance, but the high cost of organic aluminum alkoxide precursors limits large-scale use. Inorganic aluminum salts are low cost and stable but unsuitable for conventional supercritical or ambient pressure drying. Herein, aluminum chloride hexahydrate (AlCl₃·6H₂O) with tert-butanol/water cosolvent was employed to prepare alumina aerogels via vacuum freeze-drying. The freeze-dried aerogels were crack-free with tunable properties. Increasing precursor concentration enhanced specific surface area (SSA) and mechanical strength, though with higher density and thermal conductivity. The minimum density and thermal conductivity are 0.066 g/cm³ and 0.028 W/(m·K), whereas the maximum SSA and Young's modulus are 364.74 m²/g and 343.07 kPa. The aerogels maintained thermal conductivity below 0.190 W/(m·K) up to 1200°C, but at 1400°C phase transformation to α-Al₂O₃ caused severe skeleton coarsening and degraded thermal insulation. This study provides a potentially cost-effective and efficient fabrication route for monolithic alumina aerogels.