Effective modulation of electromagnetic characteristics by composition and size in expanded graphite/Fe3O4 nanoring composites with high Snoek's limit

Fe3O4 nanoring (NR) composites anchored on the nanolayers of expanded graphite (EG) were prepared through solvothermalesurface modificationesintering approach to break Snoek's limit. The samples were characterized by XRD, EDX, XPS, FESEM, TEM, STEM, and Raman spectroscopy analyses. Fe3O4 NR content, NR size, and EG size were well adjusted by changing precursor mass and size and ball milling EG for various times. The composites exhibited controllable saturation magnetization and remarkably enhanced permittivity and permeability. A proper Fe3O4 content (6.38 wt.%-22.12 wt.%), large EG size, and large NR size favor the enhancement of permeability and permittivity due to the synergistic effect of easy-plane anisotropy, high magnetization (Ms), microcurrent induced plasmonic resonance, and microcurrent induced electromagnetic coupling. Specifically, the composites exhibit light weight, strong absorption, and broad-absorbing band compared with those of pure EG and Fe3O4 NRs. The optimal microwave absorption property was found in composites containing 29.82 wt.% Fe3O4 with a minimum RL value of -45.8 dB at 8.55 GHz, frequency range (RL <= -20 dB, 99% absorption) of 12.97 GHz, and mass fraction of 10 wt.%. This work provides a significant guide for designing and synthesizing microwave absorbers with high Snoek's limit, broad bandwidth, and light weight. (C) 2017 Elsevier B.V. All rights reserved.