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 (AlCl 3 ·6H 2 O) 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/cm 3 and 0.028 W/(m·K), whereas the maximum SSA and Young's modulus are 364.74 m 2 /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‐Al 2 O 3 caused severe skeleton coarsening and degraded thermal insulation. This study provides a potentially cost‐effective and efficient fabrication route for monolithic alumina aerogels.