Bottom-Up Synthesis of High-Entropy Alloy/Carbon Nanofiber with Magnetic–Dielectric Synergy for Low-Filler and Broadband Electromagnetic Wave Absorption

Rational design of the composition and microstructure has been recognized as an effective strategy for fabricating high-efficiency electromagnetic wave (EMW) absorption materials. Herein, hierarchical FeCoNiMnCu high-entropy alloys (HEAs)/carbon nanofiber (FeCoNiMnCu/CNF) composites were fabricated by using a bottom-up synthesis method involving electrospinning followed by calcination. The nanosized FeCoNiMnCu nanoparticles are uniformly embedded within the CNF, constructing partial core–shell structures and abundant interfaces. Adjusting the calcination temperature can effectively regulate the crystallinity, graphitization degree, and electromagnetic parameters. The FeCoNiMnCu/CNF composites exhibit excellent EMW absorption performance, with strong absorption of −64.4 dB and broadband absorption of 4.1 GHz at a low filling of 10 wt % and thin thickness at 1.9 mm. Conductive loss, interface polarization, eddy current loss, and magnetic–dielectric synergy of FeCoNiMnCu/CNF composites collectively contribute to the superior EMW absorption performance. This study underscores the great potential of bottom-up synthesis in combining HEAs with CNF to develop low-filling and broadband HEA-based EMW absorption materials.