Yuan-Xun Gong

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 400 °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 and microwave electromagnetic properties of CoFe alloy nanoflakes prepared with hydrogen-thermal reduction method

CoFe alloy nanoflakes (NFs) with diameter and thickness on nanoscale were prepared by hydrogen-thermal reduction in CoFe2O4 flakes at 400 °C for 60 min. The effective complex permittivity and permeability of CoFe alloy NFs/paraffin composites were measured and compared with that of CoFe alloy nanoparticles (NPs)/paraffin composites. Due to the two-dimensional shape character, the real part of permittivity and permeability of CoFe alloy NFs was rather higher than that of CoFe alloy NPs. Electromagnetic wave absorbing (EMA) performance of both CoFe alloy NFs and NPs was evaluated by using transmission line theory. The effective EMA band position of the coating with CoFe alloy NFs as fillers was found to locate in the range of 2–4 GHz, while the effective EMA band position of the coating containing CoFe alloy NPs as fillers was located in the range 8–18 GHz. A maximum reflection loss (RLmax) of −57.8 dB was achieved in a coating containing CoFe alloy NFs as fillers, which is much higher than the −16.6 dB of ...

Synthesis of Fe–ferrite composite nanotubes with excellent microwave absorption performance

Fe–ferrite composite nanotubes were successfully prepared by thermal hydrogen reduction of α-FeOOH nanowires. The nanotubes have diameters of about 100 nm and lengths of tens of micrometres. The formation mechanism of Fe–ferrite composite nanotubes is discussed, and the non-equilibrium diffusion between hydrogen and oxygen was found to be responsible for the formation of the hollow interior structure. Because of the high shape anisotropy of the 1-D shape, the coercivity of composite nanotubes was higher than that of reported granular Fe–ferrite composite nanoparticles. Since the eddy current is effectively suppressed by the thin wall characteristic of nanotubes, the composite nanotubes exhibit higher permeability than that of the reported ferromagnetic metal nanowires. Due to the better impedance matching and higher dissipation efficiency, a superior microwave absorption performance was obtained in Fe–ferrite composite nanotubes, in which the maximum reflection loss is −18 dB and the effective absorption band (<−10 dB) covers the entire frequency band of 12.5–17.5 GHz.

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.

Co7Fe3 and Co7Fe3@SiO2 Nanospheres with Tunable Diameters for High-Performance Electromagnetic Wave Absorption

Ferromagnetic metal/alloy nanoparticles have attracted extensive interest for electromagnetic wave-absorbing applications. However, ferromagnetic nanoparticles are prone to oxidization and producing eddy currents, leading to the deterioration of electromagnetic properties. In this work, a simple and scalable liquid-phase reduction method was employed to synthesize uniform Co7Fe3 nanospheres with diameters ranging from 350 to 650 nm for high-performance microwave absorption application. Co7Fe3@SiO2 core-shell nanospheres with SiO2 shell thicknesses of 30 nm were then fabricated via a modified Stöber method. When tested as microwave absorbers, bare Co7Fe3 nanospheres with a diameter of 350 nm have a maximum reflection loss (RL) of 78.4 dB and an effective absorption with RL > 10 dB from 10 to 16.7 GHz at a small thickness of 1.59 mm. Co7Fe3@SiO2 nanospheres showed a significantly enhanced microwave absorption capability for an effective absorption bandwidth and a shift toward a lower frequency, which is ascribed to the protection of the SiO2 shell from direct contact among Co7Fe3 nanospheres, as well as improved crystallinity and decreased defects upon annealing. This work illustrates a simple and effective method to fabricate Co7Fe3 and Co7Fe3@SiO2 nanospheres as promising microwave absorbers, and the design concept can also be extended to other ferromagnetic alloy particles.

Permeability calculation in composite media with low filler concentration: A new method of effective media theory application

A new method was established to calculate the intrinsic or effective permeability in composite media with low filler concentrations. The calculation instability of Bruggeman effective media theory was avoided through a media reconstruction, and thus the prediction accuracy and calculation consistence were greatly improved. The established method has been tested in Fe/SiO2 based composite media, and its validity been preliminarily proved. This study then proposed a new way to extend the applicability scope of the Bruggeman effective media theory.

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.

Design of a two-layer ultra-wideband microwave absorber

In this paper, we present the design of a two-layer microwave absorber with ultra-wideband performance, covering the range of 4.6 to18 GHz. An analysis of the impedance transformation characteristics of the two-layer system is included to help understand how the low impedance of a PEC plane is mapped into the matched-condition by the two layers.