Highly Compressible Si₃N₄ Nanowire Aerogels with Thermal Stability up to 1400 °C

Compressible ceramic aerogels are highly desirable for thermal insulation in extreme environments, yet maintaining their structural and functional stability at elevated temperatures remains a major challenge. Here, we report a template-assisted pyrolysis strategy for the direct fabrication of three-dimensional Si₃N₄ nanowire aerogels by pyrolyzing a porous carbon fiber/polysiloxane skeleton in nitrogen, followed by thermal oxidation to remove the carbon template. The resulting aerogels consist of interwoven single-crystalline α-Si₃N₄ nanowires, with both aerogel density and nanowire diameter being simultaneously tailored from 5 to 97 mg cm^–3 and from 0.31 to 1.23 μm, respectively, by controlling the vacuum filtration time and pyrolysis temperature. The aerogels show polymer-foam-like compressive behavior, and the deformation mode evolves from a bending-dominated regime toward a more stretching-dominated regime with increasing nanowire diameter and density. Correspondingly, the oxidation resistance and structural stability are significantly improved. Notably, the Si₃N₄ nanowire aerogel with a density of 43 mg cm^–3 and an average nanowire diameter of 0.8 μm retains its macroscopic structure, microscopic morphology, and good compressibility after isothermal oxidation in air at 1400 °C for 30 min. These results demonstrate that single-crystalline Si₃N₄ nanowire aerogels can combine high compressibility, effective thermal insulation and exceptional high-temperature stability, suggesting their potential for thermal protection under extreme environments.