Coral-Inspired Thermodynamically Stable-Phase Core-Shell Strategy Enables α-Al2O3@SiO2 Aerogels with Thermal Stability up to 1500 °C and Superhydrophobicity

Alumina aerogels have attracted great interest for extreme thermal insulation. However, traditional alumina aerogels inevitably undergo severe structural degradation above 1000 °C due to the metastable-to-α phase transformation, which fundamentally limits their ultrahigh-temperature applications. In nature, red coral constructs its robust porous skeleton directly from calcite, the thermodynamically stable polymorph of CaCO3, thereby achieving exceptional long-term structural durability. Inspired by this "thermodynamically stable-phase construction" wisdom, we herein propose a thermodynamically stable-phase core-shell (TSC) strategy that employs thermodynamically stable α-Al2O3 nanoparticles as building blocks. The in situ sol-gel growth of a methyl-containing SiO2 shell on the nanoparticle surfaces directly yields three-dimensional continuous α-Al2O3@SiO2 core-shell aerogels (SAAs). The resulting SAAs exhibit outstanding ultrahigh-temperature thermal stability, retaining a non-collapsed porous skeleton with a high specific surface area retention of 23.6% after calcination at 1500 °C in air for 2 h. They also display excellent superhydrophobicity, with a water contact angle as high as 163.0° and a rolling angle as low as 2.9°. Furthermore, the SAAs deliver remarkable thermal insulation performance, exhibiting a room-temperature thermal conductivity of only 0.0394 W·m-1·K-1. When exposed to a 1300 °C butane torch flame for 120 s, a merely 4-mm-thick SAAs plate registers a cold-side temperature rise of only 141 °C. Molecular dynamics simulations reveal that the SiO2 shell acts as an efficient physical and diffusion barrier, effectively suppressing sintering neck formation and atomic diffusion between particles, thereby preserving the nanostructure at extreme temperatures. This work not only provides a coral-inspired stable-phase construction pathway toward lightweight ceramic aerogels for extreme environments but also establishes a general approach for constructing sintering-resistant nanostructures based on thermodynamically stable-phase nanoparticles via core-shell engineering.