Lightweight and Thermally Insulating Alumina-Based Nanofiber Aerogels Assembled from SiO₂-Doped Nanofibers

Lightweight alumina-based aerogels with high thermal stability are promising candidates for high-temperature thermal insulation; however, conventional particle-based aerogels suffer from severe sintering and structural degradation. In this work, controlled amounts of SiO₂ were introduced into Al₂O₃-based nanofibers (NFs) as a secondary phase to regulate their thermal stability, and the obtained SiO₂-containing NFs were further assembled into alumina-based nanofiber aerogels (AGs). The SiO₂-containing NFs were assembled into three-dimensional porous aerogels via a direct foaming and freeze-drying process, followed by high-temperature calcination. The introduction of SiO₂ within the nanofibers effectively suppresses grain growth and delays the γ-Al₂O₃ to α-Al₂O₃ phase transformation through a Zener-pinning effect, resulting in a stabilized fibrous skeleton. Consequently, the AGs exhibit reduced density, enhanced porosity, improved compressive strength, and lower thermal conductivity. For aerogels calcined at 1000 °C, higher SiO₂ incorporation within the nanofibers markedly improves the compressive response at 40% deformation, increasing the strength from 2.9 to 16.9 kPa, while the thermal conductivity measured at 1000 °C declines to 0.03597 W/(m·K) from an initial value of 0.03903 W/(m·K). This work demonstrates an effective strategy for designing thermally stable and mechanically robust oxide nanofiber aerogels for high-temperature insulation applications by tailoring the SiO₂ content in alumina nanofibers.