Broadband microwave absorption of CoNi@C nanocapsules enhanced by dual dielectric relaxation and multiple magnetic resonances

Abstract

Dual dielectric relaxation of the permittivity and multiple magnetic resonances of the permeability (including one natural resonance and two exchange resonance modes) are observed in CoNi@C nanocapsules in the same 5–17 GHz frequency range which leads to a better electromagnetic-wave absorption than earlier reported for nanocomposites. A reflection loss (RL) exceeding −25 dB is obtained in a wide frequency range of 5–17 GHz when an appropriate absorber thickness between 2 and 4.8 mm is chosen. For a 2 mm absorber layer, a RL value exceeding −10 dB is achieved in the broad frequency range 12–18 GHz, which covers the whole Ku-band.

Synthesis, permeability resonance and microwave absorption of flake-assembled cobalt superstructure

“…Based on the parameters, the natural resonance frequency is 7.8 GHz. The difference between the calculated and measured value of the natural resonance frequency may be ascribed to the interaction among the cobalt superstructure and the shape of the particles [12]. The two resonance peaks at 10.98 and 15.32 GHz is ascribed to exchange resonance.…”

“…According to the reported literatures, the magnetic core/shell structures have been designed and synthesized to achieve desired impedance matching [9], such as C/Ni nanocapsules [10], C/FeCo nanocapsules [11], grapheme/Fe 3 O 4 /SiO 2 /NiO nanosheets [2], C/ CoNi nanocapsules [12], etc.. The above materials are all coated by the materials with relatively low conductivity, which could make magnetic particles be isolated, which could effectively avoid the formation of conductive network, and then improve the impedance matching, and ultimately obtain the superior microwave absorption materials.…”

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

“…The value of ' µ decreases from 1.19 to 0.98 with increasing frequency, which shows excellent frequency dispersion. The '' µ has a resonance peak at 6.4 GHz, and the large resonance band is observed in the range of 4-14 GHz, which is corresponding to the multi-magnetic resonance, like in CoNi/C nanocapsules, HCP-Co nanoparticles and Co nanoflakes 20,23,24 . In our previous papers, the resonance frequency at 6.4 GHz is due to the large anisotropy energy.…”

“…FCC-Co nanoparticles embedded in an amorphous carbon shells can act as charge centers and can contribute to the enhancement of dielectric permittivity because of interfacial polarization 22 . According to the Debye relaxation theory 23 , for most dynamic processes of dielectric relaxation loss, ' ε and '' ε follow the equation of the Cole-Cole semicircle. Clear segments of the Cole-Cole semicircle between ' ε and '' ε for the present FCC-Co/C nanocapsules are displayed in Figure 4c, in which the multi-semicircles indicate the multi-dielectric relaxation loss.…”

Hydrothermal Synthesis of Three-dimensional Butterfly-like Ni Architectures as Microwave Absorbers

“…By increasing the thickness, the RL maximum shifts to a lower frequency. When the thickness is thicker This frequency range covers the absorption frequency range of the traditional sintered ferrites 22 . The good microwave absorbing properties of Ni architectures can be ascribed to the excellent synergetic effect of the dielectric relaxation loss and the magnetic resonance loss.…”