Preparation of bimodal‐scale whiskers reinforced alumina aerogel composites via a vacuum freeze‐drying process

The nanoporous skeleton structure of alumina aerogel gives it a series of excellent properties, but it also greatly reduces its mechanical properties and thermal stability, which severely restricts its practical application. To address this challenge, herein we propose to simultaneously improve the thermal and mechanical properties of alumina aerogel by reinforcing the matrix with bimodal‐scale whiskers, namely microscale SiC whisker (SiC MW ) and nanoscale mullite whisker (Mullite NW ), through an efficient vacuum freeze‐drying process. The prepared SiC MW /Mullite NW /Al 2 O 3 aerogel composites are crack‐free, with a uniform microstructure, a well‐developed mesoporous network featuring ink‐bottle‐type pores, low density, and high specific surface area. The whiskers are well‐bonded with the alumina matrix. At room temperature, the SiC MW /Mullite NW /Al 2 O 3 composite has low thermal conductivity, which increases with the SiC MW content. At high temperatures (1400°C), Mullite NW inhibits the transformation of the alumina matrix into dense α‐Al 2 O 3 , thus helping retain the nanoporous skeleton. Combined with the infrared shielding effect of SiC MW , the composites exhibit superior thermal insulation and stability. Moreover, due to the effective inhibition of SiC MW and Mullite NW on microcrack initiation and propagation, the mechanical properties are significantly enhanced. This study presents an effective strategy to prepare high‐performance alumina aerogel composites for potential practical applications and commercialization.