This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substra...

Recent progress in some composite materials and structures for specific electromagnetic applications

Flexible polydimethylsiloxane/multi-walled carbon nanotubes membranous metacomposites with negative permittivity

Complex permeability spectra μ*=μ′−iμ″ for two types of spinel ferrites (Ni–Zn ferrite and Mn–Zn ferrite) and their composite materials have been investigated. The contribution of domain-wall and natural resonance to the permeability spectra was estimated by the numerical fitting of actual measurement data to a simple formula. Frequency dispersion type of each component, relaxation or resonance, can be estimated from one of the fitting parameters, damping factor. In sintered Mn–Zn ferrite, domain-wall contribution is dominant and gyromagnetic spin resonance or relaxation-type magnetization rotation is large in Ni–Zn ferrite. However, relaxation character is dominant in both Mn–Zn and Ni–Zn ferrite composite materials. In composite materials, the permeability value can be scaled by the ferrite particle content using a simple model concerning demagnetizing field. This analysis is useful in designing the permeability spectra of ferrite composite materials.

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Frequency dispersion of complex permeability in Mn–Zn and Ni–Zn spinel ferrites and their composite materials

Low-temperature sintering of NiZnCu ferrite and its permeability spectra

A bio-gel derived strategy was used to construct Ni/C nanocomposites consisting of three-dimensional (3-D) carbon networks with embedded nickel nanoparticles. The amorphous carbon prevents agglomeration of the nickel nanoparticles and thus contributes to a good impedance match. The microwave absorption properties of the Ni/C nanocomposites were optimized according to percolation theory for good impedance matching. As a result, microwave absorbing coatings, which have the advantages of thin thickness (1.75 and 1.5 mm) and light weight (25 and 30 wt%), were achieved with excellent absorbing properties (90% microwave absorption) and a broad bandwidth (13.6–18 GHz and 13.2–18 GHz). The absorbing properties were mainly attributed to the dielectric relaxation processes at 2–18 GHz from the multilevel interface, porous carbon materials and nanoscale nickel nanoparticles in the Ni/C nanocomposites. It is believed that this work not only helps to elucidate the mechanism of absorption but also provides a new design paradigm for determining the optimal content of absorbers using percolation theory. The bio-gel derived strategy paves a possible way for the mass synthesis of microwave absorbers.

Bio-gel derived nickel/carbon nanocomposites with enhanced microwave absorption

Permeability spectra in Ni‐Zn ferrite composite materials were studied at the volume loading of ferrite above 30% and at temperatures from 100 to 400 K. The permeability decreased with decreases in the volume loading of ferrite. This decrease was much larger than that expected from the empirical mixing law. This was attributed to the demagnetizing field, generated by the magnetic poles on the surface of the ferrite particles. Simultaneously, the demagnetizing field increased spin resonance frequency. For the sintered ferrite, the primary peak of the permeability was just below the Curie temperature. The peak becomes obscure and disappeared as the volume loading decreased. The temperature dependence of the spin resonance frequency was lower in the ferrite composite material than that in the sintered ferrite. These features were also discussed from the view point of the demagnetizing field.

Frequency dispersion and temperature variation of complex permeability of Ni‐Zn ferrite composite materials

Frequency dispersion of permeability in ferrite composite materials

Complex permeability spectra of Permalloy granular composite materials have been studied in the microwave frequency range. The heat-treated Permalloy particles in the air at several hundreds of °C have a high surface electrical resistance; the eddy current effect in the high frequency permeability spectra can be suppressed in the composite structure containing the percolated particles. A negative permeability has been obtained above 5GHz due to the natural magnetic resonance in the 70vol% particle content composite material. In this content, electrical permittivity spectra show a nonmetallic characteristic. This permeability dispersion can be applied for the left-handed media.

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Negative permeability spectra in Permalloy granular composite materials

We have studied the relative complex permittivity (e r = e r′- ie r″) of copper granular composite materials containing coagulated Cu particles in the microwave range as well as the electrical conductivity. The insulator to metal transition was observed at the percolation threshold φ c = 16.0 vol. %. The enhancement of permittivity in the insulating state can be described by the Effective Cluster Model. Above the percolation threshold φ c, it was found that the Cu granular composites show negative permittivity spectra below a characteristic frequency f 0 indicating the low frequency plasmonic state. Characteristic frequency tends to increase with particle content.

Low frequency plasmonic state and negative permittivity spectra of coagulated Cu granular composite materials in the percolation threshold

Relative complex permeability spectra ( μ r = μ r ′ - i μ r ′ ′ ) and the dc magnetic field effect on them for a yttrium iron garnet (YIG) and its granular composite materials have been studied to evaluate the negative permeability characteristics. In the sintered YIG, two distinct peaks corresponding to the domain wall and the gyromagnetic spin resonance were observed in the imaginary part μ r ′ ′ under zero magnetic field; the real part of complex permeability μ r ′ shows a small negative value in a certain frequency range. The Lorentz type magnetic resonance with the negative permeability dispersion was observed under dc magnetic field. Permeability spectra were evaluated by the numerical fitting of actual measurement data to a resonance formula using six parameters (resonance frequencies, static susceptibilities, and damping factors of the domain wall motion and the gyromagnetic spin rotation). The dc magnetic field suppresses the domain wall contribution and the spin component becomes dominant. In the YIG granular composite material, the permeability dispersion frequency shifts to higher frequency region due to demagnetizing field; the spin component becomes dominant. Negative permeability spectra were also observed in the high content YIG composites under the dc field. The negative permeability spectra of YIG composite materials can also be applied to the left-handed material as well as the sintered YIG.

Kenichi Hatakeyama

Papers

Frequency dispersion and temperature variation of complex permeability of Ni‐Zn ferrite composite materials

Frequency dispersion of permeability in ferrite composite materials

Negative permeability spectra in Permalloy granular composite materials

Low frequency plasmonic state and negative permittivity spectra of coagulated Cu granular composite materials in the percolation threshold

Permeability spectra of yttrium iron garnet and its granular composite materials under dc magnetic field