Thermal Evolution of Silicon and Fluorine Species in Fluorosilane-Modified Carbon Aerogel Composites

Carbon aerogels are widely used for thermal protection in extreme high-temperature environments under inert atmospheres because of their excellent thermal insulation. However, their high specific surface area and open-pore structure promote moisture uptake, necessitating hydrophobic or low-surface-energy functionalization for practical use. Existing studies have primarily focused on room-temperature wettability after modification, while the high-temperature decomposition of functional layers, elemental evolution, and volatile release remain insufficiently understood. Herein, vapor-phase fluorosilane (FAS)-modified carbon aerogel composites were investigated by combining solid-state chemical characterization with evolved-gas analysis to clarify the thermal evolution of the functional layer under inert high-temperature conditions. FAS modification produced a superhydrophobic surface with an initial water contact angle of approximately 163°. During heating, the FAS-derived functional layer entered a rapid-decomposition stage at approximately 372 °C, followed by the principal fluorinated-volatilization stage near 443 °C. Perfluoroalkyl-chain cleavage progressively removed fluorine from the material through the release of C_xF_y-containing volatile species, accompanied by the disappearance of C-F, CF₂, and CF₃ structures. In contrast, part of the silicon-containing fraction was retained and evolved into a Si-C-related residual structure. Although high-temperature exposure eliminated FAS-derived hydrophobicity, the nanoparticle-assembled nanoporous carbon framework remained intact without obvious structural collapse. These findings establish a selective thermal-failure pathway in which fluorinated chains preferentially decompose and volatilize, whereas silicon-containing species are partially retained and reconstructed on the carbon framework, clarifying the failure mechanism of surface-modified carbon aerogel nanomaterials in extreme thermal environments.