Zidong Zhang

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

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.

Experimental realization of simultaneous negative permittivity and permeability in Ag/Y3Fe5O12 random composites

Ag/Y3Fe5O12 random composites of silver particles randomly hosted in porous Y3Fe5O12 were prepared using a facile impregnation–calcination process. With the increase of silver content, the silver particles interconnected with each other near the percolation threshold, leading to the formation of electrical conductive silver networks. The plasma oscillation of delocalized electrons in interconnected silver particles leads to the negative permittivity. The negative permittivity behaviour was analyzed using the Drude model and equivalent circuit models. Meanwhile, the diamagnetic response of silver networks combined with the magnetic resonance of Y3Fe5O12 results in the negative permeability. That is to say, simultaneous negative permittivity and negative permeability were realized in the Ag/Y3Fe5O12 random composites. Hopefully, tunable double negative property could be realized by adjusting the microstructure and composition of the composites. External DC magnetic field could also be applied to control the electromagnetic properties of the composites.

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.

Random copper/yttrium iron garnet composites with tunable negative electromagnetic parameters prepared by in situ synthesis

Negative parameter materials (NPMs) with negative permittivity and/or negative permeability have attracted increasing attention in recent years. In this work, the tunable negative electromagnetic parameters of copper/yttrium iron garnet (Cu/YIG) composites, which were prepared by an in situ synthesis process, were investigated in a radio frequency regime. When they reached the percolated state, Fano-like resonances are observed and the permittivity changes from negative to positive. In addition, the combined contributions of the magnetic resonance of ferrimagnetic YIG particles and the diamagnetic response of the current loop bring about negative permeability in high frequency. Furthermore, the negative permittivity and permeability could be controllable by an external magnetic field. Hopefully, it is indicated that the in situ synthesis process offers a facile and versatile approach to fabricate NPMs.

Experimental realization of tunable negative permittivity in percolative Fe78Si9B13/epoxy composites

Negative permittivity is one of the most important properties in the realization of double negative medium or negative index materials. In this paper, tunable negative permittivity in the radio frequency range has been obtained in composites with Fe78Si9B13 amorphous alloy dispersed in an epoxy matrix. The microstructure and dielectric properties of Fe78Si9B13/epoxy composites are investigated in detail. The results indicate that when the Fe78Si9B13 content is beyond the percolation threshold, the plasma oscillation of delocalized electrons in interconnected Fe78Si9B13 leads to negative permittivity. By controlling the effective concentration of free electrons, the negative permittivity of the Fe78Si9B13/epoxy composites could be easily adjusted. Additionally, the frequency dispersion behaviors of the conductivity conform to the Jonschers power law below percolation threshold, demonstrating that the conductive mechanism is hopping conduction. The realization of tunable negative permittivity in Fe78Si9B13/epoxy composites gives a new and high-efficient way toward double negative materials.

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.

Ultra low percolation threshold and significantly enhanced permittivity in porous metal–ceramic composites

Iron–alumina composites consisting of different loadings of iron particles dispersed in an alumina matrix were prepared via a facile impregnation–calcination process. The frequency dispersions of conductivity and permittivity were investigated in detail. An ultra low percolation threshold of 2.3 vol%, which is much lower than that of dense metal–ceramic composites, was obtained. Meanwhile, a significant enhancement of permittivity e′ (from ∼7.5 to ∼800) was achieved when the iron content increases from 0 to 4.2 vol% at 10 MHz. The ultra low percolation threshold can be explained by the fact that the porous microstructure of the composites will facilitate the formation of a layer of two dimensional conductive networks on the pore wall of porous alumina. And the significant enhancement of permittivity should be attributed to the interfacial polarization phenomenon that takes place at the iron–alumina interfaces. This paper demonstrates that the loading of a conductive component into a porous matrix is an effective way to fabricate composites with simultaneously high permittivity and ultra-low percolation threshold. Hopefully, various porous metal–ceramic composites with tailored dielectric properties could be fabricated using the impregnation–calcination process.

Microstructure and dielectric properties of ion-doped La0.7Sr0.3MnO3 lossy ceramics at radio frequencies

In this paper, the microstructure and dielectric properties of ion-doped La0.7Sr0.3MnO3 are investigated in detail. The polycrystalline ceramics of La0.7Sr0.3Mn1−yMyO3 (M = Fe, Ni, Cu; y = 0.3 or 0.5) are prepared by the sol–gel/sintering method. The symmetry of the crystal structure is improved and the dielectric properties obtained in the frequency range from 80 MHz to 1 GHz can be tuned by the doping of Fe, Ni, and Cu ions. With regard to the dielectric properties in lossy ceramics, the decrease of real permittivity e′r for the doping of Fe and Ni will be effective for impedance-matching; the enhanced dielectric losses in the Ni and Cu doping samples will promote stronger absorption. Thus, the improved dielectric properties make ion-doped La0.7Sr0.3MnO3 promising candidates for lossy ceramics in microwave electronics. In addition, the experimental results of permittivity were checked by K–K relations, and the frequency dispersion behaviors of the conductivity within a certain frequency range accord with the Jonschers power law in the form of σ′ac(f) ∝ (2πf)n, demonstrating that the conductive mechanism is hopping conduction.

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.

Percolative cobalt/silicon nitride composites with tunable negative electromagnetic parameters

In this paper, the relationship between the microstructures and electromagnetic properties of cobalt/silicon nitride (Co/Si3N4) composites synthesized by an impregnation–calcination process are discussed. The enhanced interconnectivity of cobalt particles led to the appearance of percolation phenomenon and the change of conductive mechanism from hopping conduction to metal-like conduction. The composites above percolation threshold exhibited the negative permittivity and negative permeability behavior, which were mainly ascribed to the plasma oscillation of free electrons and the diamagnetic responses of conductive cobalt network, respectively. The frequency region and magnitude of such negative electromagnetic parameters closely associated with the cobalt content. When the cobalt content reached 35 wt%, simultaneous negative permittivity and negative permeability were realized in the frequency range from 550 MHz to 1 GHz. The experimental exploration of Co/Si3N4 composites by tailoring compositions and microstructures has great significance on the development of tunable negative electromagnetic parameter materials.