Molten-salt assisted synthesis of MAX@straw composites for electromagnetic absorption

The advancement of electromagnetic absorption materials faces critical challenges in achieving tunable electromagnetic parameters, broadband efficiency, and sustainable manufacturing. Biomass-derived carbon, while offering abundant advantages (hierarchical structures, heteroatom doping, etc.), exhibits excessive sensitivity to carbonization conditions (temperature, duration, etc.), resulting in poor reproducibility that hinders practical deployment. To overcome these limitations, we developed an innovative molten-salt assisted synthesis strategy for creating hierarchical MAX@straw composites. This approach enables in-situ growth of MAX phase layers (specifically Ti₃AlC₂) on straw-derived carbon framework while simultaneously converting naturally present silica into functional silicon carbide at significantly reduced temperatures (~1100 °C versus traditional >1500 °C requirements). The resulting architecture features a sophisticated “carbon fibrous cluster–MAX phase–TiCₓ transition layer–carbon framework” hierarchical structure that preserves natural structural advantages of biomass. It is worth noting that by regulating the MAX phase content rather than the heat treatment parameters, we achieved dual control over the conversion efficiency from silica to silicon carbide and the degree of biomass carbonization, preventing impedance mismatch caused by excessive graphitization. With a 40 wt% filler loading, the obtained sMAX-n composite exhibited excellent electromagnetic wave absorption performance in the 2–18 GHz frequency range with a maximum reflection loss exceeding −60 dB and a wide effective absorption bandwidth of 5.2 GHz. This breakthrough is attributed to the electromagnetic synergistic effect of dielectric–magnetic coupling (eddy currents in conductive networks), polarization enhanced by multiple heterojunction interfaces, and impedance matching optimized through structural hierarchy. Our approach establishes a universal platform for designing tunable, sustainable biomass-based electromagnetic wave absorbing materials with robust process stability, providing direct application pathways for 5G communication infrastructure (millimeter-wave absorption), military stealth technology (broadband radar attenuation), and electromagnetic-tunable green buildings (environmentally friendly electromagnetic protection).