High-Temperature Stable and Metal-Free Electromagnetic Wave-Absorbing SiBCN Ceramics Derived from Carbon-Rich Hyperbranched Polyborosilazanes

High-temperature stable and metal-free siliconboron carbonitride ceramics with high electromagnetic (EM) wave-absorbing efficiency were achieved through the structural design and pyrolysis of carbon-rich hyperbranched polyborosilazane precursors with pendent phenyl groups. The introduction of benzene rings into the precursors dramatically changes the microstructure and the EM wave-absorbing property of ceramics. It reveals that the ceramics pyrolyzed from the benzene ring-containing preceramic precursors have a higher carbon content and a larger number of sp2 carbons and generate crystalline carbons (graphitic carbons and tubular carbons) in situ, which lead to excellent EM wave-absorbing properties. The EM wave absorption efficiency and effective absorption bandwidth (EAB, reflection coefficient (RC) below −10 dB) can be tuned via annealing of the ceramics. The ceramics stable at 1320 °C exhibit their optimized EM wave-absorbing performance with a minimum RC (RCmin) of −71.80 dB and an EAB of 3.65 GHz (8.2–11.85 GHz). We believe that the research extends the design strategy of advanced EM wave-absorbing functional materials, which have great potential as promising absorbers in commercial or military applications.