Huahui He

Optimization of two-layer electromagnetic wave absorbers composed of magnetic and dielectric materials in gigahertz frequency band

Both experimentally and theoretically, a two-layer electromagnetic (EM) wave absorber exhibits the possibility of meeting the demand for effective EM wave absorbers. The first layer, made up of magnetic micpowder (MMP), has a large permeability and magnetic loss, while the second layer, comprised of MMP and nanotitanium powder, has a frequency dispersion with the parameters of permittivity and permeability to match the incidence free space over a wide frequency range. The predicted results are based on the modulus of permittivity (permeability) which obeys a logarithmic law of mixtures, and the loss tangent is related through a linear law of mixtures. A linear regression analysis performed on the data points provides constants that can be used to predict the effective parameters at different frequencies. Finally, a program is presented that computes the optimum amount of MMP in the second layer and the required thickness for each layer. The predicted results agree quite well with the measured data.

Centimeter- and Millimeter-Wave Attenuation Properties of Carbonyl Iron Fiber-Filled Foam Composites

We have formulated carbonyl iron fiber-filled foam composites and measured their complex permittivity and permeability at x-band (8.2-12.4 GHz) by the transmission and reflection waveguide method. The results show that carbonyl iron fiber with smaller diameter and higher aspect ratio exhibits higher complex permittivity and permeability. The reflection loss measurement shows that lightweight carbonyl iron fiber-filled foam composites possess good absorbing properties in both centimeter- and millimeter-wave bands.

Absorbing properties of the magnetic composite electromagnetic wave absorber

Ba3Co2Ti1.2Fe22.4O 41 hexagonal ferrite was obtained by conventional ceramic technology. Fe-5wt%Ni alloy was also prepared by mechanical alloying method. Electromagnetic (EM) characteristics of the two materials were measured in a frequency range of 2-18 GHz. The absorption performances of metal-backed single-layered coating and double-layered coating were numerically simulated. Results show that preferable properties of EM absorption can be obtained by the double-layered design of ferrite and Fe-Ni alloy. The minimum reflection loss is -32.96 dB and the bandwidth of -10 dB is over 7 GHz at 3 mm. Thus, it is possible to make thinner broadband EM wave absorber by adjusting input impedance of the composite materials

Thin electromagnetic wave absorber containing Fe-Co alloy for quasi-microwave band

The need for effective thin-layer electromagnetic wave absorbers for operation in quasi-microwave band has fueled to much research on both the materials and design methods. Magnetic absorbing materials are one of the popular absorbers because they have lower work frequencies and are relatively thinner. Many attentions are paid to the improvements of the complex permeability and most of them are focus on the amorphous ferromagnetic materials. It was reported that the reflection loss maps are constructed as functions of d (thickness) and the material parameters such as the dielectric loss tangen tg /spl epsi/, the magnetic loss tangen tg /spl mu/ and the ratio of /spl epsi//sub r///spl mu//sub r/. Thus, performances of the absorbers should be adjusted from the panoramic view. In our work, Fe-Co alloy with suitable electromagnetic characteristics was fabricated by mechanical alloying (MA) and was design for absorbers in the quasi-microwave band. Compared with amorphous alloy-ferrite-epoxy composite, it is thinner (about 2.5 mm thick) and easier to be prepared.

Micromagnetic Simulation of a Magnetic Film With Surface Roughness

We have simulated micromagnetically the static magnetic structure and magnetic spectrum of a magnetic film with surface roughness, using a finite stripe film model. We found that the rough surface can lead to a ripple magnetic structure, which results in increasing the damping factor of the film, and that the damping factor increases with the increase of the root-mean-square roughness. For a particular film with a given rough surface, the damping factor first decreases because of a decrease of the magnetic dispersion, then increases because of the surface-roughness-induced demagnetizing effect with increasing the external field.

Microwave characterization of carbonyl Fe/sub 55/Ni/sub 45/ fiber-filled composites

Fiber-filled composites have the ability of tailoring the electric and magnetic properties and can be suitable for use as microwave lenses, high-strength low weight electromagnetic interference (EMI) shielding materials, radar absorbing materials (RAM), antennas and waveguides, etc. The subject of this paper is to examine the microwave characterization of carbonyl Fe/sub 55/Ni/sub 45/ fiber-filled composites. With the method of the transmission and reflection waveguide, the complex permittivity and permeability of carbonyl Fe/sub 55/Ni/sub 45/ fiber-filled composites in S-band (2-4 GHz) were measured by a vector network analyzer. The reflection loss of these composites was also calculated in this frequency range. The results show that the strong skin effect can decrease the conductivity of fiber and ultimately reduce the effective permittivity of fiber-filled composites. The lower aspect ratio of carbonyl Fe/sub 55/Ni/sub 45/ fiber gives rise to the decrease of the effective magnetic field and the permeability. It was also found that the reflection loss of the composites is influenced more greatly by the low permittivity than that by thickness.