Lightweight, High-Strength, and Anisotropic Structure Composite Aerogel Based on Hydroxyapatite Nanocrystal and Chitosan with Thermal Insulation and Flame Retardant Properties

Advanced thermal management materials with low thermal conductivity and robustness have been a research hotspot for energy conservation and sustainable development. However, the brittleness of inorganic materials and the high flammability of polymers remain a challenge for industrial applications. Herein, we demonstrated a novel strategy to fabricate an organic/inorganic composite aerogel based on a combination of hydroxyapatite (HAP) and chitosan (CS). The combination of chemical cross-linking and unidirectional freeze-drying methods can significantly improve the mechanical properties and thermal stability, and the obtained anisotropic microstructure has a significant effect on thermal conductivity. Compared with the uncross-linked HAP-Si/CS composite aerogel, the cross-linked HAP-Si/CSG composite aerogel has high mechanical strength (0.82–2.37 MPa) and high specific modulus (41.22–129.20 kN m kg–1). In addition, the as-prepared HAP-Si/CSG composite aerogel exhibits a lower radial-direction thermal conductivity (28.16–37.43 mW m–1 K–1) than that of the axial direction. Meanwhile, the composite aerogel constructed by the HAP nanostructure embedded in the CS sheets better limits the heat transfer and blocks the combustion of organic compounds, showing excellent fire resistance. Thus, the biomass-based composite aerogel will be a sustainable and renewable functional material with high mechanical properties and thermal insulation, which is further expected to promote the high-value utilization of biopolymers.