Porous boron nitride Fiber-Based 3D skeleton reinforced phenolic aerogel for lightweight thermal protection systems

Advanced thermal protection systems require ultralight, robust, thermally insulating, and ablation‑resistant composites. While promising, the three-dimensional (3D) skeleton/matrix design is challenged by fabricating an ideal skeleton with structural continuity, high‑temperature stability, interfacial compatibility, and scalability. Herein, we fabricate composite aerogels using an internally porous boron nitride (BN) fiber‑based 3D aerogel as the reinforcing skeleton and a phenolic (PF) aerogel as the matrix. The resulting PF/BN composite aerogels exhibit ultralow densities of 0.141–0.167 g/cm 3 . Excellent interfacial compatibility via π‑π interactions and hydrogen bonding ensures efficient stress transfer to the robust BN skeleton, preventing structural collapse during ambient pressure drying (APD) and endowing enhanced mechanical properties: up to 70% compressive strain and compressive strengths of 3.4–7.8 MPa. The aerogels also show outstanding thermal insulation, with the PF/BN2.0 sample exhibiting a low thermal conductivity of 45.3 mW·m −1 ·K −1 . Furthermore, under butane flame ablation (∼1300°C), a 20  mm‑thick PF/BN2.0 maintains a backside temperature of only 370°C after 15  min, and simply attaching it to an aluminum plate enables effective high‑temperature infrared stealth. This performance is attributed to a hierarchical structure formed in situ during ablation, consisting of a dense outer layer and a porous inner layer. This work not only demonstrates the effectiveness of the 3D BN fiber‑based skeleton as a high‑performance reinforcement, but also provides a new strategy for developing lightweight, multifunctional composites for extreme‑environment applications.