J.T. Jiang

Microwave absorption properties of FeNi3 submicrometre spheres and SiO2@FeNi3 core–shell structures

Nearly monodispersed FeNi3 submicrometre spheres with an average diameter of 220u2009nm were synthesized by a simple low temperature reduction method. SiO2@FeNi3 core–shell structured submicrometre spheres with 25u2009nm thick SiO2 shell were then fabricated by a sol–gel process. A significant enhancement of electromagnetic absorption (EMA) performance was achieved by the silica coating over the 2–18u2009GHz. The reflection loss (RL) exceeding −20u2009dB of the composite was obtained over 6.7–15.1u2009GHz by choosing an appropriate sample thickness between 2.1 and 3.3u2009mm, and an optimal RL of −61.3u2009dB was obtained at 8.7u2009GHz with a thin absorber thickness of 2.9u2009mm. The coating of the dielectric silica shell significantly enhanced the EMA performance due to the enhancement of interface polarization at the alloys and dielectric interfaces.

Electromagnetic properties of FeNi alloy nanoparticles prepared by hydrogen-thermal reduction method

FeNi alloy nanoparticles were prepared by hydrogen-thermal reduction in nickel ferrite nanoparticles at 400u2009°C for 1 h. FeNi alloy nanoparticles with average particle size of 150 nm in diameter are composed of Fe, Ni, and FeNi solid solution. The effective complex permittivity and complex permeability of FeNi alloy nanoparticles in the frequency range of 2–18 GHz were measured, and the microwave reflection loss of FeNi/wax with different assumed coating thicknesses was calculated according to the transmission line theory. The hydrogen-thermal reduced FeNi alloy nanoparticles were found to possess high permeability and superior microwave absorption performance in the microwave band.

Synthesis of CoFe/Al2O3 composite nanoparticles as the impedance matching layer of wideband multilayer absorber

CoFe/Al2O3 composite nanoparticles were successfully prepared by hydrogen-thermally reducing cobalt aluminum ferrite. Compared with CoFe alloy nanoparticles, the permeability of CoFe/Al2O3 composite nanoparticles was remarkably enhanced and an improved impedance characteristic was achieved due to the introduction of insulated Al2O3. A multilayer absorber with CoFe/Al2O3 composite nanoparticles as the impedance matching layer and CoFe nanoflake as the dissipation layer was designed by using genetic algorithm, in which an ultrawide operation frequency bandwidth over 2.5–18 GHz was obtained. The microwave absorption performance in both normal and oblique incident case was evaluated by using electromagnetic simulator. The backward radar cross-section (RCS) was decreased at least 10 dB over a wide frequency range by covering the multilayer absorber on the surface of perfect electrical conductive plate.

The influence of Fe content on the magnetic and electromagnetic characteristics for Fex(CoNi)1−x ternary alloy nanoparticles

Fex(CoNi)1−x (xu2009=u20090.14, 0.20, 0.25) ternary alloy nanoparticles were fabricated by a self-catalyzed reduction method at a low temperature. The investigation of static magnetic properties revealed both saturation magnetization and coercive force enhanced with increasing Fe content. By dispersing alloy nanoparticles into paraffin matrix homogeneously, the electromagnetic properties of them were investigated experimentally and the electromagnetic absorption performances were calculated according to transmission line theory. Significant dielectric relaxation was found in the low Fe content sample, which is dominant in dielectric loss. The magnetic loss was attributed to natural resonance and the resonance peaks’ shift to high frequency region with increasing Fecontent. The enhanced electromagnetic absorption performances were obtained by adjusting Fe content to balance electromagnetic parameters, because the electromagnetic parameters can be varied by structural and magnetic properties.

Effect of microstructure on the electromagnetic properties of Al18B4O33w/Co and Al18B4O33w/FeCo composite particles

The effect of microstructure on electromagnetic properties was investigated in Al18B4O33w/Co and Al18B4O33w/FeCo composite particles. Resistivity (ρ) and saturation magnetization (Ms) were introduced to determine the relationship between microstructure and electromagnetic properties indirectly. The ρ of Co coatings decreased by nine orders of magnitude when annealed in H2, which substantially affected permittivity and permeability. Intense dielectric relaxation occurred in 2u2009GHz to 18u2009GHz band, particularly when the ρ was lower than 1.16u2009×u200910−3 Ω·cm, and a strong eddy current effect began to emerge when the ρ decreased to 5u2009×u200910−4 Ω·cm. An unusual increase in the permeability of the Co coatings was identified, and enhanced exchange-coupling interaction was correspondingly revealed, which was assumed to be related to microstructure evolution. The influence of microstructure observed in Al18B4O33w/Co was further found in the Al18B4O33w/CoFe composite particles. This study suggested that the introduction of ...

Edge treatment for sidelobe reduction of parabolic reflector antenna with a two-layer absorber

In this paper, we present a method to reduce the sidelobe levels of a given parabolic reflector antenna by coating it with a 2-layer absorber. The absorber is designed by using Genetic Algorithm and an ultra-wideband performance (4.6–18 GHz) is obtained. The simulation results show that the sidelobe level can be reduced by coating the edge of the parabola with 2-layer absorber without compromising the gain.

The electromagnetic wave absorption properties of FeNi 3 nanospheres and FeNi 3 @C nanocapsules

FeNi3 nanospheres (NS) with an average diameter of 400 nm were synthesized by a simple low temperature reduction method. FeNi3@C core-shell structured nanocapsules (NC) were then fabricated with a hydrothermal process. The electromagnetic (EM) properties of paraffin matrix composites containing FeNi3 NS and FeNi3@C NC were studied at 2–18 GHz. The effect of dielectric carbon coating on the EM properties and electromagnetic wave absorbing (EMA) performance was directly observed.

Effect of microstructure on electromagnetic properties of ferromagnetic/dielectric composite particles

The Ferromagnetic/dielectric composite particles are attracting more and more interesting for their potential technology importance in electromagnetic industry [1–3]. To adjusting and control the electromagnetic parameters become a crucial problem to meet various application requirements. In view of materials science, to properly tailor the microstructure of Ferromagnetic/dielectric composite particles would be an effective way to optimal the EM performance. Although the effect of the microstructure on the EM properties has been investigated for some years [1, 4], the results were not intensive and systemic enough to guide the related design or further study. The main obstacle in the investigation is that to find out direct relationships between microstructures and EM properties is rather difficult. EM properties are more or less thought to stand off from microstructures and almost all expressions of permeability or permittivity dont include parameters on materials microstructure. Fortunately, the dependence of EM properties on static electrical or magnetic properties have been extensively investigated [5–7], and also the influence of microstructures on static magnetic properties and resistivity has been thoroughly studied. Thus to link microstructures and EM properties together by introducing static electrical or magnetic properties as a bridge may be a valid way to promote the investigation of the microstructure-EM properties relationship.