Peitao Xie

An overview of metamaterials and their achievements in wireless power transfer

Metamaterials have been deployed for a wide range of fields including invisible cloak, superlens, electromagnetic wave absorption and magnetic resonance imaging, owing to their peculiar electromagnetic properties. However, few investigations on metamaterials were focused on wireless power transfer (WPT). WPT is the transmission of electrical energy from a power source to an electrical load without conductors like wires or cables. Metamaterials can enhance the transfer efficiency and enlarge the transfer distance due to their ability of focusing magnetic flux, which opens up a novel approach to promoting the development and application of WPT. This review paper aims to provide an overview of the fabrications, exotic properties, and their applications especially in the WPT field. Meanwhile, the perspective and future challenges of metamaterials and WPT are proposed.

The tunable negative permittivity and negative permeability of percolative Fe/Al2O3 composites in radio frequency range

The electromagnetic properties including ac conductivity, permittivity, and permeability of percolative Fe/Al2O3 composites, which were prepared by in-situ synthesis process, are investigated in the radio frequency range. There is an obvious percolation transition with the increase of iron contents. When iron content is beyond but near the percolation threshold, the negative permittivity and permeability are simultaneously obtained from 631 MHz to 1 GHz in sample FA30. Further increasing iron content, the Fano-like resonances are observed, and the resonance frequency where the permittivity changes from negative to positive shifts to lower frequency. In addition, the frequency region of negative permittivity and negative permeability are not overlapped any more. Hopefully, the percolative Fe/Al2O3 composites with tunable negative permittivity and negative permeability can be used as electromagnetic wave absorber.

Silica microsphere templated self-assembly of a three-dimensional carbon network with stable radio-frequency negative permittivity and low dielectric loss

Percolative composites always suffer from their unstable and filler-loading dependent microstructures and negative electromagnetic parameters. Here, stable negative permittivity is achieved by in situ constructing a three-dimensional carbon network in the silica spherical matrix after a self-assembly and pyrolysis process. An electrical percolation phenomenon appears with the formation of a carbon network. Once the carbon network is formed, further increasing carbon loadings will only influence the porosity rather than the connectivity due to the nature of the porous carbon itself. Hence, the microstructure and plasma-like negative permittivity are not sensitive to carbon loading, leading to a negligible carbon loading dependent permittivity behavior. Moreover, negative permittivity with small values (−100 < e′ < 0), beneficial for matching with permeability, was effectively adjusted by changing the carbonization temperature. The carbon composites with negative permittivity showed an extremely low dielectric loss (tan δ = 1–7) compared with metal composites (usually tan δ = 10–100). This work provides a convenient means to obtain stable negative permittivity properties. The carbon composites can be regarded as a promising candidate for metamaterials and will facilitate the applications of materials with negative electromagnetic parameters.

Regulation mechanism of negative permittivity in percolating composites via building blocks

Percolating composites with negative permittivity can be promising candidates for metamaterials; however, building blocks of negative permittivity have not yet been put forward in percolating composites. Here, the dielectric properties of a ternary composite with Fe and SiO2-coated Fe particles dispersed in a polymer matrix were investigated in the range of 10 MHz–1 GHz. By gradually controlling the Fe/coated-Fe ratio (x), a three-dimensional conductive network could be constructed when x exceeds 0.75. The Drude-type negative permittivity was achieved by the conductive network, and its Lorentz-type dispersion was mainly attributed to dielectric resonance of coated-Fe particles. Equivalent circuit analysis demonstrated that the inductive conductive network was the decisive building block to achieve negative permittivity. Moreover, the dielectric resonance caused by coated-Fe particles was LC resonance, and this indicated that the capacitive isolated metallic particles acted as another building block to con...

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

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.

Generation mechanism of negative permittivity and Kramers–Kronig relations in BaTiO3/Y3Fe5O12 multiferroic composites

Recently, negative parameters such as negative permittivity and negative permeability have been attracting extensive attention for their unique electromagnetic properties. Usually, negative permittivity is well achieved by plasma oscillation of free electrons in conductor-insulator composites or metamaterials, while some attention has been paid to attaining negative permittivity in ceramics to reduce dielectric loss. In this paper, negative permittivity in barium titanate and yttrium iron garnet composites are reported which was well fitted by the Lorentz model. Further, negative permittivity behavior was verified via Kramers-Kronig relations, and it revealed that the causal principle still valid for negative permittivity resulted from dielectric resonance. The interrelationships among negative permittivity, capacitive-inductive transition and ac conductivity are discussed.

Strategy of adjusting negative permittivity with invariant permeability property in metallic granular percolating composites

The electromagnetic properties including ac conductivity, reactance, permittivity, and permeability of percolating Fe/Epoxy composites are investigated at radio-frequency range. Percolating behavior is observed in the composites. Below percolation threshold, ac conductivity spectra follows the Jonscher’s power law indicating the weakened trend of hopping conductive behavior, while the skin effect is dominant above percolation threshold. Plasma-type negative permittivity is attributed to the low frequency plasmonic state explained by Drude model. The frequency region and value of negative permittivity are effectively adjusted by SiO2-coated iron particles’ controlling percolating network, while permeability property could be almost kept invariant. Invariant permeability property is attributed to suppressing current loops by SiO2 layers. This strategy with tunable permittivity and invariant permeability provides a method of suppressing the strong electromagnetic coupling effect in intrinsic metamaterials, and can facilitate applications of negative permittivity materials.

Metacomposites: functional design via titanium nitride/nickel(II) oxide composites towards tailorable negative dielectric properties at radio-frequency range

Functional metacomposites towards negative dielectric properties via percolating behavior have triggered tremendous fundamental and practical interest. In this paper, titanium nitride was selected to construct percolating metacomposites. Hence, adjusting the frequency region and the value of negative permittivity was effectively realized by uniformly building different ratio x of nickel(II) oxide/titanium nitride composites. Occurrence of percolation phenomenon and change of conductive mechanism were observed when alternating the ratio x. Two different types of negative permittivity (i.e., dipole-type and plasma-type) were observed in the composites. The dipole-type negative permittivity behavior in the composite with low titanium nitride content (i.e., x = 0.5) was ascribed to the resonance-induced electric dipole generated from the isolated titanium nitride particles, which could be explained by Lorentz model. While the plasma-type negative permittivity with titanium nitride content exceeding the percolation threshold could be well explained by the low frequency plasmonic state generated from conductive titanium nitride networks using Drude model. Besides, the electrical properties influenced by percolating phenomenon including ac conductivity, dielectric loss, and impedance were investigated. This work presents a systematic and novel investigation on negative dielectric properties of percolating metacomposites and will greatly facilitate the practical applications of metacomposites.

Tunable negative permittivity and permeability of yttrium iron garnet/polyaniline composites in radio frequency region

The present work demonstrated the tunable negative permittivity and permeability properties of a two-phase composite material consisting of conductive matrix [polyaniline (PANI)] and magnetic filler [yttrium iron garnet (YIG)]. The negative permittivity observed in the YIG/PANI composites is accompanied by the formation of conductive PANI network, which induced the plasma oscillation of carriers in PANI obeying the Drude model. Through theoretical calculation, negative permeability can be realized and further adjusted by external magnetic field via the domain wall and spin resonances of YIG.

Large-Area, Low-Cost Infrared Metamaterial Fabrication Via Pulsed Laser Deposition with Metallic Mesh as a Shadow Mask

Metamaterials are artificial periodic structures with negative permittivity and permeability. Several interesting properties can be obtained in metamaterials, such as negative index behavior, which can be used for building perfect lenses, cloaking, antennas, etc. As the metamaterial’s properties are determined by its structure, the key challenge is to reduce the fabrication cost of the periodic structure on the micrometer or nanometer scale for realistic applications. In this paper, we experimentally demonstrate a new one-step method for the fabrication of a large-area infrared metamaterial at extremely low cost. A metallic mesh is used as a shadow mask during the pulsed laser deposition (PLD) process to fabricate a FeNi/SiC/FeNi multilayer sandwich structure on Si substrate (cm2 level). The sample shows a strong absorption peak in the infrared frequency range, and the absorption intensity changes with the sample’s geometry.