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

SiBCN ceramic aerogel has emerged as a new generation of thermal insulation and microwave absorption integrated material, but are facing great challenges in terms of mechanical property, 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. The polyhydromethylsiloxane (PHMS) was in situ introduced by the hydrosilane addition reaction during the solvothermal process, which endowed the precursor aerogel to form a complex and robust three-dimensional network structure, and further resulted in a 260% imprvement in the compressive strength of the SiBCN/SiOC composite aerogel compared to that of 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 an excellent microwave absorption performance with the effective absorption bandwidth of 6.7 GHz and the reflection loss of -43.89 dB at a thickness of 2.5 mm, due to the improved impedance matching, multiple reflections and enhanced interface polarization. Furthermore, the introduction of SiOC significantly inhibited the crystallization of SiBCN at high temperatures. After heat treatment at 1600 ºС, 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 the PHMS provided novel insight and a promising strategy for enhancing the overall performance of SiBCN ceramic aerogel, expanding their application in high-temperature environments.