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

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

In this work, amorphous TiO2 and SiO2-coated Ni composite microspheres were successfully prepared by a two-step method. The phase purity, morphology, and structure of composite microspheres are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Due to the presence of the insulator SiO2 shell, the core–shell Ni–SiO2 composite microspheres exhibit better antioxidation capability than that of pure Ni microspheres. The core–shell Ni–SiO2 composite microspheres show the best microwave absorption properties than those of pure Ni microspheres and Ni–TiO2 composites. For Ni–SiO2 composite microspheres, an optimal reflection loss (RL) as low as −40.0 dB (99.99% absorption) was observed at 12.6 GHz with an absorber thickness of only 1.5 mm. The effective absorption (below −10 dB, 90% microwave absorption) bandwidth can be adjusted between 3.1 GHz and 14.4 GHz by tuning the absorber thickness in the range of 1.5–4.5 mm. The excellent microwave absorption abilities of Ni–SiO2 composite microspheres are attributed to a higher attenuation constant, Debye relaxation, interface polarization of the core–shell structure and synergistic effects between high dielectric loss and high magnetic loss.

Investigation of the electromagnetic absorption properties of Ni@TiO2 and Ni@SiO2 composite microspheres with core-shell structure.

Random composites with nickel networks hosted randomly in porous alumina are proposed to realize double negative materials. The random composite for DNMs (RC-DNMs) can be prepared by typical processing of material, which makes it possible to explore new DNMs and potential applications, and to feasibly tune their electromagnetic parameters by controlling their composition and microstructure. Hopefully, various new RC-DNMs with improved performance will be proposed in the future.

Random Composites of Nickel Networks Supported by Porous Alumina Toward Double Negative Materials

Electromagnetic (EM) wave absorption materials possess exceptionally high EM energy loss efficiency. With vigorous developments in nanotechnology, such materials have exhibited numerous advanced EM functions, including radiation prevention and antiradar stealth. To achieve improved EM performance and multifunctionality, the elaborate control of microstructures has become an attractive research direction. By designing them as core-shell structures with different dimensions, the combined effects, such as interfacial polarization, conduction networks, magnetic coupling, and magnetic-dielectric synergy, can significantly enhance the EM wave absorption performance. Herein, the advances in low-dimensional core-shell EM wave absorption materials are outlined and a selection of the most remarkable examples is discussed. The derived key information regarding dimensional design, structural engineering, performance, and structure-function relationship are comprehensively summarized. Moreover, the investigation of the cutting-edge mechanisms is given particular attention. Additional applications, such as oxidation resistance and self-cleaning functions, are also introduced. Finally, insight into what may be expected from this rapidly expanding field and future challenges are presented. 

Dimensional Design and Core–Shell Engineering of Nanomaterials for Electromagnetic Wave Absorption

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.

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

Electromagnetic properties of hybrid composite materials containing copper and permalloy (Fe53Ni47 alloy) particles have been investigated in the RF to microwave frequency range up to 20 GHz. Double negative permittivity and permeability spectra have been observed in the percolated state of the hybrid composite material. The negative permittivityspectra in this composite can be attributed to the low frequency plasmonic state produced by the percolatedCu and permalloy cluster chains as well as the dielectric resonance of the isolated metal clusters. The refractive indexspectra which were calculated from the measured permittivity and permeability data indicated the negative refraction from 200 MHz to 1.8 GHz. The near zero or zero refractive index state can be obtained near the two zero crossing frequencies in the refractive indexspectra.

Double negative electromagnetic properties of percolated Fe53Ni47/Cu granular composites

The relative complex permittivity and permeability spectra of the coagulated copper and yttrium iron garnet (Cu/YIG) hybrid granular composite materials have been studied in the microwave range. The insulator to metal transition was observed at the percolation threshold of Cu particle content (φ Cu  = 16.0 vol. %) in the electrical conductivity. In the percolation threshold, the low frequency plasmonic state caused by the metallic Cu particle networks was observed. The percolated Cu/YIG granular composites show simultaneous negative permittivity and permeability spectra under external magnetic fields.

Teruhiro Kasagi

Papers

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

Double negative electromagnetic properties of percolated Fe53Ni47/Cu granular composites

Negative permittivity and permeability spectra of Cu/yttrium iron garnet hybrid granular composite materials in the microwave frequency range