A novel lightweight ceramizable Si-B hybrid phenolic aerogel composite with excellent thermal insulation and ablation...

Lightweight phenolic aerogel composites represent a crucial component of future thermal protection systems in aerospace applications. Multifunctional hybrid modification is considered an effective method to enhance the thermal stability and ablation resistance of phenolic aerogel composites. In this study, silicone and boric acid were crosslinked with phenolic resin (SiBRx) to introduce high-bond-energy Si-O and B-O bonds. Subsequently, the SiBRx can be used to prepare a lightweight aerogel by the sol-gel method. The specific surface area of the SiBRx aerogel reached 180.25 m² g^-1, representing a 59% increase compared to the unmodified phenolic resin (PR) aerogel. Furthermore, the SiBRx aerogel exhibited superior thermal stability, with a maximum decomposition temperature of 603.2 °C, 12% higher than that of the PR aerogel. Quartz fiber-reinforced phenolic aerogel composites (QF/SiBR_x) were fabricated by incorporating quartz fiber mats and varying the content of the curing agent hexamethylenetetramine (HMTA). At a HMTA content of 15%, the QF/SiBR_15% aerogel composite achieved a bending strength of 14.17 MPa, with compressive strength increasing from 9.99 MPa to 12.26 MPa. Moreover, the QF/SiBR_x composites demonstrated outstanding ablation resistance, with a remarkably low linear ablation rate of 0.077 mm s^-1. The incorporation of Si and B forms a dense composite ceramic layer (C, SiO₂, and B₂O₃) on the ablated surface, preventing pyrolytic degradation of the material structure and enhancing ablation resistance.