Fashen Li

Enhanced microwave absorption of Fe nanoflakes after coating with SiO2 nanoshell.

Fe nanoflakes were prepared by the ball-milling technique, and then were coated with 20 nm-thick SiO(2) to prepare Fe/SiO(2) core-shell nanoflakes. Compared with the uncoated Fe nanoflakes, the permittivity of Fe/SiO(2) nanoflakes decreases dramatically, while the permeability decreases slightly. Consequently, reflection losses exceeding - 20 dB of Fe/SiO(2) nanoflakes are obtained in the frequency range of 3.8-7.3 GHz for absorber thicknesses of 2.2-3.6 mm, while the reflection loss of uncoated Fe nanoflakes almost cannot reach - 10 dB in the same thickness range. The enhanced microwave absorption of Fe/SiO(2) nanoflakes can be attributed to the combination of the proper electromagnetic impedance match due to the decrease of permittivity and large magnetic loss due to strong and broadband natural resonance. The key to the combination is the coexistence of the nanoshell microstructure and the nanoflake morphology.

One-pot synthesis of CoFe2O4/graphene oxide hybrids and their conversion into FeCo/graphene hybrids for lightweight and highly efficient microwave absorber

CoFe2O4/graphene oxide hybrids have been successfully fabricated via a facile one-pot polyol route, followed by chemical conversion into FeCo/graphene hybrids under H2/NH3 atmosphere. These magnetic nanocrystals were uniformly decorated on the entire graphene nanosheets without aggregation. The morphology, chemical composition and crystal structure have been characterized in detail. In particular, FeCo/graphene hybrids show significant improvement in both permeability and permittivity due to the combination of the high magnetocrystalline anisotropy of metallic FeCo and high conductivity of light-weight graphene. This leads to remarkable enhancement in microwave absorption properties. The maximum reflection loss of FeCo/graphene hybrids reaches −40.2 dB at 8.9 GHz with a matching thickness of only 2.5 mm, and the absorption bandwidth with reflection loss exceeding −10 dB is in the 3.4–18 GHz range for the absorber thickness of only 1.5–5 mm. Moreover, the experimental relationship between matching thickness and frequency is found to obey the quarter-wavelength matching model, facilitating the design of FeCo/graphene hybrid film for practical application. The results suggest that the FeCo/graphene hybrids developed here can serve as an ideal candidate for the manufacture of light-weight and high-efficiency microwave-absorbing devices.

Erratum to: Synthesis and Magnetic Properties of Nearly Monodisperse CoFe2O4 Nanoparticles Through a Simple Hydrothermal Condition

Nearly monodisperse cobalt ferrite (CoFe2O4) nanoparticles without any size-selection process have been prepared through an alluring method in an oleylamine/ethanol/water system. Well-defined nanospheres with an average size of 5.5 nm have been synthesized using metal chloride as the law materials and oleic amine as the capping agent, through a general liquid–solid-solution (LSS) process. Magnetic measurement indicates that the particles exhibit a very high coercivity at 10 K and perform superparamagnetism at room temperature which is further illuminated by ZFC/FC curves. These superparamagnetic cobalt ferrite nanomaterials are considered to have potential application in the fields of biomedicine. The synthesis method is possible to be a general approach for the preparation of other pure binary and ternary compounds.

Analyses on double resonance behavior in microwave magnetic permeability of multiwalled carbon nanotube composites containing Ni catalyst

The double resonance behavior of microwave magnetic permeability has been observed for multiwalled carbon nanotube composites containing Ni catalyst. One of them is due to the natural resonance at 6.00GHz and another is due to the exchange resonance at 10.11GHz. The natural resonance is dependent on magnetocrystalline anisotropy and shape anisotropy of Ni nanostick catalyst and the calculated result of exchange resonance mode with a few modifications was close to the experiment. It is believed that the coexistence of natural resonance and exchange resonance is benefial to large bandwidth as a microwave absorber.

Reflection loss mechanism of single layer absorber for flake-shaped carbonyl-iron particle composite

The reflection loss (RL) properties including the frequency, intensity, and bandwidth of absorption peaks for the flake-shaped carbonyl-iron particle/paraffin composite were investigated in detail. By measuring the reflection coefficient of the composite without (S11-OPEN) and with (S11-SHORT) a backed metal plate, the origin of the RL peak was directly verified. The separated electromagnetic wave energy reflected at the air-absorber interface and the absorber-metal plate interface was achieved from S11-OPEN and S11-SHORT parameters, and they were used to explain the intensity variation of the RL peak. The bandwidth of the RL peak was also deduced from viewpoint of interface reflection waves. The calculated bandwidth from the derived formula agreed well with the experimental data.

Microwave permeability spectra of flake-shaped FeCuNbSiB particle composites

The effective permeability of flake-shaped FeCuNbSiB particles/nonmagnetic matrix composition in high frequency was measured and calculated. In contrast to the relatively larger size and irregular shape particles, the flake particles have higher permeability. The results are attributed to the different magnetic loss mechanisms. According to the skin-effect criterion, we find that the magnetic loss in flake particles is mainly caused by the natural resonance, compared with the eddy current effect in the larger size and irregular shape particles. Using the shape anisotropy, the flake soft magnetic particles overcome the difficulty of the relatively small intrinsic anisotropies and increase the natural resonant frequencies to the gigahertz range, leading to the higher real part and imaginary part of the permeability. The resonance peak of flake particles was simulated using the combination of the Landau–Lifshitz–Gilbert equation and Bruggeman’s effective medium theory considering a random spatial distributio...

1D Magnetic Materials of Fe3O4 and Fe with High Performance of Microwave Absorption Fabricated by Electrospinning Method

Fe3O4 and Fe nanowires are successfully fabricated by electrospinning method and reduction process. Wiry microstructures were achieved with the phase transformation from α-Fe2O3 to Fe3O4 and Fe by partial and full reduction, while still preserving the wire morphology. The diameters of the Fe3O4 and Fe nanowires are approximately 50–60 nm and 30–40 nm, respectively. The investigation of microwave absorption reveals that the Fe3O4 nanowires exhibit excellent microwave absorbing properties. For paraffin-based composite containing 50% weight concentration of Fe3O4 nanowires, the minimum reflection loss reaches −17.2 dB at 6.2 GHz with the matching thickness of 5.5 mm. Furthermore, the calculation shows that the modulus of the ratio between the complex permittivity and permeability |ε/μ| is far away from unity at the minimum reflection loss point, which is quite different from the traditional opinions.

Microwave reflection characteristics of surface-modified Fe50Ni50 fine particle composites

Surface-modified planar-anisotropy Fe50Ni50 particle-resin composites have been studied in the microwave frequency range. After surface modification, the eddy current effect is suppressed effectively while the permeability is almost unchanged. High permeability and appropriate permittivity are obtained at the meantime. Simulated microwave reflection result shows that the matching thickness, corresponding frequency, μ and e is coincident with the quarter wavelength matching condition. The surface-modified Fe50Ni50/resin composites can be attractive candidates for thinner microwave absorbers in L-band (1–2 GHz).

BaFe12O19 Single-Particle-Chain Nanofibers: Preparation, Characterization, Formation Principle, and Magnetization Reversal Mechanism

BaFe(12)O(19) single-particle-chain nanofibers have been successfully prepared by an electrospinning method and calcination process, and their morphology, chemistry, and crystal structure have been characterized at the nanoscale. It is found that individual BaFe(12)O(19) nanofibers consist of single nanoparticles which are found to stack along the nanofiber axis. The chemical analysis shows that the atomic ratio of Ba/Fe is 1:12, suggesting a BaFe(12)O(19) composition. The crystal structure of the BaFe(12)O(19) single-particle-chain nanofibers is proved to be M-type hexagonal. The single crystallites on each BaFe(12)O(19) single-particle-chain nanofibers have random orientations. A formation mechanism is proposed based on thermogravimetry/differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM) at six temperatures, 250, 400, 500, 600, 650, and 800 °C. The magnetic measurement of the BaFe(12)O(19) single-particle-chain nanofibers reveals that the coercivity reaches a maximum of 5943 Oe and the saturated magnetization is 71.5 emu/g at room temperature. Theoretical analysis at the micromagnetism level is adapted to describe the magnetic behavior of the BaFe(12)O(19) single-particle-chain nanofibers.

Microwave absorption properties of the hierarchically branched Ni nanowire composites

Microwave absorber using hierarchically branched Ni nanowires based composite was introduced in this work. The nanostructure constituents greatly suppressed the eddy current and improved the magnetic anisotropy. Their composite with wax matrix showed strong and broadband electromagnetic attenuation around 8.23 GHz where the magnetic mode was excited. The resonance absorption character was discussed by fitting the permeability spectrum using well-known theoretical equations (Landau-Lifshitz-Gilbert equation and Kittel formula).