The electromagnetic ͑EM͒ characteristics of ZnO-coated Fe nanocapsules synthesized by arc discharging were studied at 2-18 GHz. A reflection loss ͑RL͒ exceeding −20 dB was obtained in the frequency range of 6.1-15.7 GHz for an absorber thickness of 1.5-5 mm. An optimal RL of −57.1 dB was found at 7.8 GHz for an absorber thickness of 3.00 mm. The excellent microwave-absorption properties are a consequence of a proper EM match in the nano-microstructure, a strong natural resonance, as well as multipolarization mechanisms. ZnO-coated Fe nanocapsules may be attractive candidates for EM-wave-absorption materials.

No abstract

The electromagnetic characteristics of Ni/ZnO nanocapsules were studied at 2-18 GHz. The dual nonlinear dielectric resonance and strong natural resonance at 16.6 GHz contribute to excellent electromagnetic absorption. A reflection loss ͑RL͒ exceeding Ϫ20 dB was calculated in 14-18 GHz for an absorber thickness of 2.05 mm, and RL exceeds Ϫ10 dB in the whole X-band ͑10-12.4 GHz͒ and the whole Ku-band ͑12.4-18 GHz͒ for a thickness of 2.50 mm. The equivalent circuit model was used to explain the dual nonlinear dielectric resonance, which is ascribed to a cooperative consequence of the core/shell interfaces and the dielectric ZnO shells.

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.

Microwave absorption properties of core double-shell FeCo/C/(x)BaTiO₃ nanocomposites were investigated in the 1-18 GHz frequency range. High resolution transmission electron microscopy studies reveal the core double-shell type nanocomposite with FeCo nanoparticles as the center, while carbon and BaTiO₃ are the inside and the outside shells, respectively. Enhanced relative permittivity made the core double-shell FeCo/C/(x)BaTiO₃ nanocomposites with better electromagnetic impedance matching than that of a FeCo/C and BaTiO₃ mixture. Reflection loss (RL) values of FeCo/C/(20 wt%)BaTiO₃-paraffin composite are almost double those of the FeCo/C-paraffin composite at the absorbent thickness from 2 to 7.5 mm due to enhanced interfacial effects. The RL value of the FeCo/C/(20 wt%)BaTiO₃-paraffin composite is -41.7 dB at 11.3 GHz at the absorbent thickness of 2 mm and a broad absorption bandwidth of 5.1 GHz (RL values exceeding -10 dB) covers the 9.4-14.5 GHz frequency range.

Electromagnetic-wave absorption by FeCo/C nanocapsules has been investigated. In contrast to earlier reported materials, including other nanocapsules, the absorption amplitude of FeCo/C nanocapsules is found not to decrease with increasing absorption-layer thickness. A reflection loss ͑RL͒ exceeding Ϫ20 dB can be obtained for all frequencies within the 2-18 GHz range by choosing an appropriate layer thickness between 1.6 and 8.5 mm. The broadest bandwidth ͑RL values exceeding Ϫ10 dB͒ from 10 to 18 GHz, covering half of the X-band and the whole K u-band, is obtained for a 2 mm layer.

No abstract

FeCo microspheres, self-assembled by Al 2 O 3 -coated FeCo nanocapsules, were prepared by means of the arc-discharge technique to investigate application in electromagnetic-wave absorption. For the FeCo microspheres, a reflection loss ͑RL͒ exceeding −20 dB was obtained in the frequency range of 10.9-15.6 GHz for an absorber thickness of 1.2-2.0 mm. An optimal RL of −30.8 dB was found at 11.4 GHz for an absorber thickness of 2.0 mm. The excellent microwave-absorption properties are a consequence of a proper electromagnetic match in the microstructure of a strong natural resonance and of the shape anisotropy.