Zhi-cheng Shi

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.

Negative permittivity behavior and magnetic performance of perovskite La1−xSrxMnO3 at high-frequency

For double negative materials (DNMs), these are usually observed in artificial metamaterials or some metal/ceramic composites. Here, an interesting negative permittivity behavior of single-phase perovskite La1−xSrxMnO3 (LSMO) is found. The LSMO bulks with an x value of 0.2 or 0.5 were prepared by a sol–gel/sintering process. From microstructure characterization, the bulks are nearly in single phase and the major phase belongs to the perovskite structure; meanwhile, the bulks are of high density. Furthermore, the experimental data of the plasma-like negative permittivity are fitted well by a lossy Drude model, suggesting a plasma frequency of 3.85 GHz (x = 0.5). The complex permeability of LSMO presents a negative susceptibility (μ′ < 1) at a frequency range of 0.2–1 GHz (x = 0.2) and 0.6–1 GHz (x = 0.5). These results have important implications for the realization of double negative properties in single-phase LSMO as a promising candidate for DNMs.

Experimental and theoretical investigation on the high frequency dielectric properties of Ag/Al2O3 composites

The impedance and dielectric properties of Ag/Al2O3 composites are investigated experimentally in the frequency range from 100 MHz to 1 GHz. Besides, equivalent circuit analysis and numerical simulations were carried out. For the composites with sufficiently high silver loading, current paths were formed and negative permittivity appeared. The negative permittivity can be well described by lossy Drude model. Moreover, the negative permittivity sample manifests inductive characteristic and shunt inductors are added to its equivalent circuit. Numerical simulations show that the interconnection of silver particles results in negative permittivity, hence the serious attenuation of electromagnetic waves.

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.

Tunable negative permittivity behavior and conductor–insulator transition in dual composites prepared by selective reduction reaction

We use a selective reduction reaction to fabricate a new kind of dual composite, which has a “composite-within-a-composite” structure. Based on the different reduction behavior of Fe2O3, an Fe-rich structure (Fe–Fe3O4–Al2O3) is formed in an Al-rich structure (FeAl2O4–Al2O3). The Fe-rich structure can bring down the concentration of free electrons to reduce the energy loss, without losing its metallic behaviour at the same time. Near the percolation threshold, a conductor–insulator transition appears and the dual composites show totally different electromagnetic responses, which can be well explained by effective medium theory. By controlling the parameters of the selective reduction process, we can get a tunable negative permittivity, which implies the ability to continuously change their electromagnetic properties.

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.