Guohua Fan

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.

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., xu2009=u20090.5) 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.

Correction: An overview of metamaterials and their achievements in wireless power transfer

Correction for ‘An overview of metamaterials and their achievements in wireless power transfer’ by Kai Sun et al., J. Mater. Chem. C, 2018, DOI: 10.1039/c7tc03384b.

Negative permittivity behavior of titanium nitride/polyphenylene sulfide “metacomposites” under radio frequency

Random composites with negative permittivity or negative permeability have developed rapidly while few were accomplished using ceramic materials as the functional fillers. In this work, titanium nitride/polyphenylene sulfide (TiN/PPS) composites with different TiN content were prepared by hot-pressing method. The electrical percolation threshold phenomenon occurred and the conductive mechanism transferred from hopping conduction to metal-like conduction when TiN’s content increased. Negative permittivity in composite with high TiN content was derived from low frequency plasmonic state of free electrons and the plasm-like negative permittivity spectra was analyzed by Drude model. Besides, it’s indicated that the dielectric loss was dominated by conduction loss and polarization loss in composites with high TiN content. Equivalent circuit model was utilized to investigate the impedance properties and suggested that parallel inductances played critical role in achieving negative permittivity.

Three-dimensional graphene network supported by poly phenylene sulfide with negative permittivity at radio-frequency

Percolative composites with negative permittivity can be promising candidates for metamaterials, but there is still a need about efficient method to adjust the permittivity value. Here graphene/poly phenylene sulfide composites were prepared and the dielectric property was studied. By gradually increasing the graphene content, three-dimensional conductive network could be constructed, accompanied by a change in the conductive mechanism from hopping conduction to metal-like conduction. Negative permittivity was achieved, and the Drude model indicated that negative permittivity came from the plasma oscillation in graphene network. The equivalent circuit analysis demonstrated that interconnection of graphene was the most crucial factor to adjust the negative permittivity value. That is, negative permittivity value can be effectively adjusted by graphene content. These findings may pave a way to simply and efficiently control negative permittivity and imply promising applications in modern electrical and electronic industries.

Tunable and weakly negative permittivity at radio frequency range based on titanium nitride/polyethylene terephthalate composites

Metacomposites have been induced widespread concern in the realization of negative permittivity. In this paper, composites with titanium nitride (TiN) particles homogeneously dispersed in polyethylene terephthalate (PET) resin were prepared by high energy ball-milling and adhesive hot-pressing method. The influences of TiN networks in composites on the electric and dielectric properties were investigated in detail. With the formation of conductive TiN networks, the conductance mechanism changed from hopping conduction to metal-like conduction. The tunable and weakly negative permittivity in the radio frequency range was obtained in TiN/PET composites by adjusting the frequency range and volume fraction (v) of TiN in composites. Negative permittivity behavior raises from the low-frequency plasma oscillation of free electrons in TiN networks, which could be analyzed by the Drude model. The impedance of TiN/PET composites were investigated by the equivalent circuit analysis, demonstrating the capacitive or inductive of the composites. This paper shows an effective way toward the tunable and weakly negative permittivity, which will promote practical applications of metacomposites in electromagnetic shielding and impedance matching fields.

Meta-composites: NiO supported 3D carbon networks structured by 1D building blocks towards tailorable negative permittivity

Meta-composites have drawn significant attention due to their preferable applications in electronic devices and promising mass production-scale. Compared with metallic particles as common conductive units in metamaterials or meta-composites fabrication, one-dimensional (1D) carbonaceous building blocks [e.g. multi-walled carbon nanotubes (MWCNTs) or carbon fibers] could provide preferable alternations. In this paper, nickel-modified carbon fibers and MWCNTs were served as 1D building blocks to fabricate meta-composites. Negative permittivity behavior in meta-composites were investigated at radio-frequency region. Herein, two different types of negative permittivity (i.e. dipole-type and plasma-type) were observed and analyzed by Lorentz model and Drude model respectively. Different variation trends of alternative conductivity spectra were followed by Jonscher’s power law or Drude model, indicating conductive mechanism change from hopping conduction to metal-like conduction. Equivalent circuit analysis to impedance response of meta-composites manifested correspondence between inductive characteristic and negative permittivity. This work not only presents novel routes to meta-composites designations by 1D carbon building blocks, but also further clarifies negative permittivity generation mechanism, which will facilitate applications in impedance matching, electromagnetic shielding and multi-layer high-k capacitors etc.

Flexible acrylic-polyurethane/copper composites with a frequency and temperature-independent permittivity

Capacitors with a high dielectric constant and low dielectric loss are highly demanded for electronic applications, such as electric devices and energy storage. To enhance the performance of those electronic applications, it is of great significance to achieve capacitors with stable dielectric property both in a broad ranges of frequency and temperature. In this research, flexible Acrylic-polyurethane/copper (APU/Cu) composites with different filler fractions were successfully synthesized. Excellent flexibility of APU/Cu composites is maintained even when Cu particles reached to 19.11xa0vol%. The dielectric and thermal properties of APU/Cu composites were systematically investigated. Enhanced dielectric constant and lower dielectric loss were achieved. Meanwhile, it is found that the permittivity fluctuated slightly with the increase of frequency and temperature, suggesting that permittivity was independent on frequency and temperature over a broad range. This research shed lights on the application of APU/Cu composites in flexible thin-film capacitors and stretchable electronic devices.

Weakly Radio-Frequency Negative Permittivity of Poly(vinylidene fluoride)/Ti3SiC2 MAX Phase Metacomposites

While metal or carbon materials served as conductive phase in fabricating metamaterials or metacomposites have been widely investigated, MAX phases could provide alternative route. In this paper, Poly(vinylidene fluoride)/Ti3SiC2 MAX phase metacomposites with different Ti3SiC2 content were fabricated. Electrical and dielectric properties of metacomposites were analyzed. Percolating phenomenon was observed over the percolation threshold (fc). Below fc, ac conductivity spectra were explained by Jonscher’s power law, indicating hopping conduction behavior. Above fc, ac conductivity of composites follows Drude model, suggesting the metal-like conductive behavior. Weakly negative permittivity behavior was observed and explained by Lorentz and Drude model, suggesting the combinative contribution of induced electric dipole resonance and low-frequency plasmonic oscillation. The impedance performance of composites were also clarified by Nyquist plots and equivalent circuit analysis, manifesting the capacitive-inductive shift of composites. This work presented a novel route to metacomposites with weakly negative permittivity which greatly benefitted the practical applications of MAX phase in metacomposites.