Chuanbing Cheng

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.

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...

Tunable negative permittivity behavior of random carbon/alumina composites in the radio frequency band

A random metamaterial, carbon/alumina (C/Al2O3) composite, was prepared using a precursor infiltration and pyrolysis method, which has potential applications in novel antennas, microwave absorbing and shielding. The microstructures, radio-frequency dielectric property and conductivity behavior of the composites with different carbon contents were investigated in detail. It was found that the carbon membrane spread out on the pore walls of the alumina matrix. As the carbon content increased, the composites underwent a percolation phenomenon, and the conductive mechanism changed from hopping conduction to metal-like conduction due to the formation of conductive carbon networks. A negative permittivity behavior was observed in the composites above the percolation threshold, and this was ascribed to the low frequency plasmonic state produced by the carbon networks. The frequency dispersion of such negative permittivity efficiently agreed with the Drude model. The negative magnitude of permittivity in the testing frequency was small, ranging from −370 to −28, which originated from the lower carrier concentration in the conducting carbon networks. This work will greatly facilitate the practical application of random metamaterials with tunable electrical properties, and has great significance for the development of metamaterials.

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.