Distinctly enhanced permeability and excellent microwave absorption of expanded graphite/Fe3O4 nanoring composites

Abstract

To break Snoek's limit and obtain high permeability, expanded graphite/Fe₃O₄ nanoring composites have been synthesized via a solvothermal-surface modification-sintering approach. A series of characterizations have confirmed the formation of the composites. Studies of the influence of compound mode, Fe₃O₄ shape, and filling mass fraction on the EM parameters reveal that the recombination of Fe₃O₄ NRs and EG can distinctly enhance permeability and permittivity. The ε′ and ε′′ values of the composites are 1.5–70.0 and 4.0–858.0 times as many as EG′ and 22.2–26.0 and 214.0–611.0 times as many as Fe₃O₄ NRs', respectively. Their μ′ and μ′′ values are around 2.8–3.0 and 2.2–100 times the Fe₃O₄ NRs', respectively. This significant enhancement is caused by the synergistic effect of the planar anisotropy, plasmon resonance, electromagnetic coupling, and interfacial polarization. The EG/Fe₃O₄ NR composites with a mass fraction of 10 wt% achieved the maximum R_L value of −24.8 dB at 6.8 GHz and the corresponding frequency range (R_L ≤ −20 dB, 99% absorption) is 8.0 GHz. Our findings confirm that the above composites are not only excellent microwave absorbers of broad bandwidth but are also light weight and can break Snoek's limit.