Yunpeng Qu

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., x = 0.5) 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.

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.

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.

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.

Adsorption mechanisms of metal ions on the potassium dihydrogen phosphate (1 0 0) surface: A density functional theory-based investigation

The adsorption of metal ions (K+, Na+, Ca2+, Cu2+, Al3+, Cr3+) on the (1 0 0) surface of potassium dihydrogen phosphate (KDP) has been studied using density functional theory (DFT). Calculation results show that all the investigated metal ions can be spontaneously adsorbed on the surface with negative adsorption energies. The adsorption stability increases in the order of Na+ < K+ < Cu2+ < Ca2+ < Al3+ < Cr3+, and shows a consistent trend as the adsorbed metal ion valence (monovalent < divalent < trivalent). Three types of stable adsorption configurations are observed, corresponding to three different bonding mechanisms. Na+, K+ and Ca2+ ions with a large radius can form two ionic bonds and one weak covalent bond with the O and H atoms respectively. In addition, the medium-sized ion of Cu2+ forms two covalent bonds with the O and H atoms. Furthermore, Al3+ and Cr3+ ions with the smallest radius form two metal-oxygen and one metal-hydrogen covalent bonds with the surface, making one H-O bond broken. Compared with other metal ions, Al3+ and Cr3+ have the strongest interactions with the surface, which can be explained by the significant electron transfer and more stable covalent bond formations between these two ions and the surface.

Insulating Technology Control of Soft Magnetic Composites

In this paper, fabrication procedures were adjusted and investigated in soft magnetic composite (SMCs) when the micro-scale iron powder was served as matrix with amorphous silica as the insulating coating layer. The coating situations were alternated by transforming the TEOS (tetraethoxysilane) hydrolysis parameters such as temperature, PH value, and TEOS reagent dosage. It was found that the silica deposited on raw iron particles which is produced by the hydrolysis reaction from the increasing addition of TEOS reagent would make the coating layer too thick. Over-high pH value would has a great negative influence on the silica coating layers, over-high temperature would speed the hydrolysis reaction up, while silica derived from hydrolysis reaction were self-assembled to silica particles and the deposition amounts on iron particles were less. Moreover, the circumstances of insulation coating layer were characterized by introducing low frequency complex dielectric parameters as indirect assessments. The magnetic properties including saturation flux density (Bs), hysteresis loss were investigated. SEM images were presented to directly exhibit the morphology of coated iron particles. The thorough investigation of coated particles’ fabrication procedures greatly facilitates the applications of soft magnetic materials.

Dielectric Properties of Cu/Epoxy Random Composites at Radio-Frequency Range

Cu/epoxy composites with the different volume fraction of Cu were prepared through hot press process. The conductivity and dielectric properties of the Cu/epoxy composite were investigated. The analyze of the results indicated that the real part of permittivity \( \varepsilon _{r}^{{\prime }} \) of Cu/epoxy composites increased when copper content increased, which is caused by the increasing interface connect between Cu fillers and epoxy partials. The frequency dispersions of ac conductivity follow the power law and thus indicates the hopping conduction behavior in composites.