NiFe2O4 nanosheets have been synthesized via a simple surfactant-assisted solution route, which are confirmed by X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. The NiFe2O4 sample exhibits the sheet-like structure, with width varying from 200 to 800 nm and thickness ranging from 20 to 60 nm. The electromagnetic properties of NiFe2O4 nanosheets-paraffin composites have been deeply investigated. The multiple dielectric relaxation loss in NiFe2O4 nanosheets is attributed to the size distribution and morphology of the NiFe2O4 nanosheets. The magnetic loss in the present system is caused mainly by the natural resonance. An absorber with a thickness of 4.3 mm exhibits an optimal reflection loss (RL) value of -47.1 dB at 7.67 GHz. RL values exceeding -20 dB in the 2.68-17.96 GHz range are obtained by choosing an appropriate absorption-layer thickness between 1.9 and 10 mm. Not only the RL peak frequency but also the number of the peaks can be well explained by the quarter-wavelength cancellation model.

AbstractThree-dimensional butterfly-like Ni architectures were fabricated by a surfactant-assisted hydrothermal method. The Ni architectures, with lengths of about 20 μm and widths of 4-6 μm, were assembled from tens of Ni nanorods with the averaged diameter of about 200 nm. The Ni nanorods were coated by oxide shells. The saturation magnetization of the Ni architectures was found to be 66.2 emu/g. The magnetic loss in the Ni architectures was mainly caused by the natural resonance, while the dielectric relaxation loss was originated from the interfacial relaxation between the oxide shells and the Ni nanorods in addition to the size distribution and morphology of the Ni architectures. Absorbers with a thickness of 2.1 mm exhibited an optimal reflection loss (RL) value of -38.9 dB at 12.8 GHz. RL values exceeding -20 dB in the 8.0-17.8 GHz range were obtained by choosing absorber thicknesses between 1.5 and 3.2 mm. A quarter-wavelength cancellation model was used to explain the observed thickness dependence of RL peak frequency.

A large scale direct hydrothermal method has been used to prepare face-centered cubic (FCC) Co/C nanocapsules with a core of FCC-Co nanoparticles and a shell of amorphous C. The size distribution is 5-20 nm. The TB of FCC-Co/C nanocapsules is determined as 85 K, indicating that FCC-Co/C nanocapsules are superparamagnetic at 300 K. For FCC-Co/C nanocapsules-paraffin composite, an optimal reflection losbs (RL) of -33.4 dB is observed at 9.6 GHz for the 2.6 mm thick layer. RL values exceeding –20 dB in the 7.4 –15 GHz range are obtained by choosing an appropriate absorption-layer thickness between 1.7 and 3.3 mm. 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. Quantitative calculation demonstrated that not only RL peak position but also the number of the peaks are determined by the quarter-wavelength cancellation model.