Robust and thermally insulating SiBCN/SiOC ceramic aerogel with superior electromagnetic wave absorption performance and high-temperature stability up to 1600 °C

SiBCN ceramic aerogels have emerged as a new generation of integrated thermal insulation and microwave absorption materials but face great challenges in terms of mechanical properties, high-temperature stability, and absorption bandwidth in practical applications. Herein, SiBCN/SiOC composite ceramic aerogels were prepared by solvent thermal crosslinking, freeze-drying, and pyrolysis of precursors. Polyhydromethylsiloxane (PHMS) was introduced in situ by the hydrosilane addition reaction during the solvothermal process, which endowed the precursor aerogel with a complex and robust three-dimensional network structure and further resulted in a 260% improvement in the compressive strength of the SiBCN/SiOC composite aerogel compared with that of the pure SiBCN aerogel. Additional investigations revealed that the SiBCN/SiOC composite aerogel enjoyed a low thermal conductivity (0.044–0.051 W·m^−1·K−1) and a light weight (0.13–0.16 g·cm⁻³), which was favorable for thermal barrier material. Notably, the SiBCN/SiOC composite aerogel exhibited excellent microwave absorption performance with an effective absorption bandwidth of 6.7 GHz and a reflection loss of −43.89 dB at a thickness of 2.5 mm due to improved impedance matching, multiple reflections, and enhanced interfacial polarization. Furthermore, the introduction of SiOC significantly inhibited the crystallization of SiBCN at high temperatures. After heat treatment at 1600 °С, the composite aerogel retained its amorphous nanoparticle pearl-chain-like structure, with thermal conductivity remaining as low as 0.052 W·m^−1·K−1. The in situ introduction of PHMS provided novel insight and a promising strategy for enhancing the overall performance of SiBCN ceramic aerogels, expanding their application in high-temperature environments.