Electromagnetic-wave-absorption properties of wire-like structures self-assembled by FeCo nanocapsules

Abstract

Al2O3-coated FeCo nanocapsules were synthesized by the arc-discharge method and three-dimensional wire-like macro-aggregates were self-assembled simultaneously by nanocapsules. The electromagnetic (EM) parameters (i.e. the relative permeability and permittivity) were measured at 2–18 GHz. As a consequence of the surface- and shape-anisotropy energy of the nanocapsules, a natural resonance at 6 GHz is dominant in the microwave absorption properties. A reflection loss (RL) exceeding −20 dB was obtained in the frequency range 4.2–18 GHz for absorber thicknesses of 1.3–6 mm. An optimal RL of −51 dB was found at 10.2 GHz for an absorber thickness of 2 mm. As a result, the Al2O3-coated FeCo nanocapsules show good prospects of being applied in EM wave absorptive devices.

Large Scale Synthesis of Superparamagnetic Face-centered Cubic Co/C Nanocapsules by a Facile Hydrothermal Method and their Microwave Absorbing Properties

“…This frequency range covers the absorption frequency range of the traditional sintered ferrites 26,27 . The thickness range is thinner than that of many earlier reported nanocomposites 5,[8][9][10][11] . The good microwave absorbing properties of FCC-Co/C nanocapsules is considered to result from the excellent synergetic effect of the multi-dielectric relaxation loss and the multi-magnetic resonance loss.…”

“…The FCC-Co/C nanocapsules-paraffin composite was prepared by uniformly mixing FCC-Co/C nanocapsules with paraffin, and the detailed descriptions can be found elsewhere [7][8][9][10][11] . The mixture was compressed into a cylinder-shaped compact, and then cut into a toroidal shape with 7.00 mm outer diameter and 3.04 mm inner diameter.…”

“…In our previous papers, the resonance frequency at 6.4 GHz is due to the large anisotropy energy. The anisotropy energy of particles of small size, especially in nanometer scale, may be remarkably increased due to the shape/surface anisotropy affected by the size effects 5,[8][9][10][11] . The large resonance band may be interpreted as a consequence of size and morphology of the FCC-Co/C nanocapsules.…”

“…Therefore, the magnetic loss in the present system may be caused mainly by the natural resonance and exchange resonance. According to the transmission line theory, when a wave is normally incident to a absorber layer with a backed metal plate, the reflection loss (RL) curves at a given absorber thickness can be calculated from the complex permeability and permittivity by means of the following expressions [8][9][10][11] :…”

“…The core-shell structure improves the impedance matching between the nanostructures and the incident microwaves due to the coupling of the electromagnetic field with both the magnetic core and the dielectric shell. Some nanocapsules of this type, including FeNiMo/C, Fe/ZnO, FeCo/Al 2 O 3 , Ni/Cu oxides, etc., have been studied [7][8][9][10] . The ferromagnetic resonance and dielectric resonance of nanocomposite, essential for enhanced microwave absorption, is determined by a few physical parameters of the nanocomposites, namely, anisotropy coefficient K, damping parameter α, saturation magnetization M S , and particle shape 1 .…”