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

Tunable and weakly negative permittivity in carbon/silicon nitride composites with different carbonizing temperatures

The flexible metacomposites with tunable negative permittivity have great potential in wearable cloaks, stretchable sensors, and thin-film capacitors, etc. In this paper, the flexible graphene/poly...

Tunable Negative Permittivity in Flexible Graphene/PDMS Metacomposites

Random composites of iron particles hosted in porous alumina were prepared from a facile impregnation-reduction process. Interestingly, when the iron content exceeds the percolation threshold, the interconnection of iron particles results in the formation of iron networks. The composites then change from capacitive to inductive and the conductive mechanism changes from hopping conduction to metal-like conduction. The negative permittivity was attributed to the plasma oscillation of delocalized electrons in iron networks, while the negative permeability could be ascribed to the strong diamagnetic response of current loops in iron networks. The negative permittivity behavior of the iron/alumina composite was analyzed using Drude model. Additionally, the fitting results indicated that the effective plasma frequency of the iron/alumina composite is much lower than bulk iron. Further investigations show that, the iron content and reduction temperature can easily tune the amplitude and frequency ranges of the negative permittivity and permeability. Moreover, the negative permittivity region and the negative permeability region can be pushed to the same frequency region by adjusting the iron content and reduction temperature. The impregnation-reduction process opens a new way for the realization of tunable negative permittivity and permeability in random composites, and has great potential for the preparation of new types of double negative materials.

Preparation of Iron Networks Hosted in Porous Alumina with Tunable Negative Permittivity and Permeability

While metal is the most common conductive constituent element in the preparation of metamaterials, one-dimensional conductive carbon nanotubes (CNTs) provide alternative building blocks. Here alumina (Al2O3) nanocomposites with multi-walled carbon nanotubes (MWCNTs) uniformly dispersed in the alumina matrix were prepared by hot-pressing sintering. As the MWCNT content increased, the formed conductive MWCNT networks led to the occurrence of the percolation phenomenon and a change of the conductive mechanism. Two different types of negative permittivity (i.e., resonance-induced and plasma-like) were observed in the composites. The resonance-induced negative permittivity behavior in the composite with a low nanotube content was ascribed to the induced electric dipole generated from the isolated MWCNTs. The frequency dispersions of such negative permittivity can be fitted well by the Lorentz model, while the observed plasma-like negative permittivity behavior in the composites with MWCNT content exceeding the percolation threshold could be well explained by the low frequency plasmonic state generated from conductive nanotube networks using the Drude model. This work is favorable to revealing the generation mechanism of negative permittivity behavior and will greatly facilitate the practical applications of metamaterials.

Radio frequency negative permittivity in random carbon nanotubes/alumina nanocomposites

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.

L. W. Yin

Papers

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