Tunable Negative Permittivity Research Articles

Kapok-derived carbon/fly ash ceramic metacomposites with tunable negative permittivity behavior

In this paper, kapok-derived carbon/fly ash (KDC/FA) ceramic metacomposites were fabricated by vacuum hot-press sintering with FA and kapok fiber as raw materials, and their conductivity, dielectric properties, and impedance response were researched. The composition and microstructure of the metacomposites had a significantly impact on the electrical characteristics. With the increase of KDC content, a 3D conductive network was formed inside the composite, accompanied by a shift in the conductive mechanism. The tunable plasma-type negative permittivity behavior was observed. The absolute value of negative permittivity and the plasma oscillation frequency became larger as the KDC contents increased, which were consistent with the analytical results of the Drude model. This study designed and prepared a new environmental metacomposite with adjustable negative permittivity using biomass materials (kapok fibers) and solid waste (FA), which had a great significance for development of metamaterials in the future.

Flexible MWCNT/PVA/Mn Ferrite/AC Metacomposites with Tunable Negative Permittivity for Low-Frequency Reflectors and Optical Sensors

Flexible MWCNT/PVA/Mn Ferrite/AC Metacomposites with Tunable Negative Permittivity for Low-Frequency Reflectors and Optical Sensors

Tunable negative permittivity performance of carbon/silicon dioxide ceramic metacomposites under external DC bias voltage

Metacomposites with negative permittivity have gained increasing momentum in recent years due to their unique electromagnetic characteristics and considerable potential application. However, how to adjust negative permittivity is still a big technical challenge to overcome. In this paper, silicon dioxide (SiO2) ceramic metacomposites consisting of carbon fibers (CFs) were fabricated by vacuum hot press sintering, and their negative permittivity might be modified by adjusting the filler content and applying external direct current (DC) bias voltages. As the CF content increased, the electrical conductivity of composites increased markedly, and the conduction mechanism transformed to a metal-like conductivity from hopping conductivity. The permittivity values of composites with high CF contents were negative at the test frequency range owing to the appearance of a low plasmonic state of unbound electrons in the percolating carbon networks. The conductivity of the composites increased when DC bias voltage was applied, because some of the accumulated local electrons at the CF-SiO2 interface could absorb energy and become free electrons. Larger bias voltages led to increased concentration of free electrons, so the absolute values of the negative permittivity raised according to the Drude model and the frequency band of negative permittivity became wider. This work presents an effective approach for adjusting the negative permittivity value and frequency band, which is advantageous in clarifying the realization and regulation mechanisms of negative permittivity.

Tunable Negative Permittivity in Graphene/Poly(Vinylidene Fluoride) Composites with Low Percolation Threshold

Recently, metamaterials with fascinating dielectric behavior have drawn much attention for their potential applications. Herein, percolating graphene/poly(vinylidene fluoride) (GR/PVDF) composites consisting of GR sheets homogeneously dispersed in PVDF matrix are fabricated via hot‐pressing method in 20 Hz–1 MHz frequency regions, of which simultaneously realizing negative permittivity with low percolation threshold. It is demonstrated that the negative permittivity of GR/PVDF composites is resulted from the low‐frequency plasma oscillation of free electrons, and a universal regulatory mechanism of the Drude model is analyzed. At the same time, rapid formative 3D GR conductive networks contributed to the low percolation threshold (7.78 wt%) of composites. Moreover, it is shown that electrical percolation occurs in samples as GR content exceeds percolation threshold, indicating a typical metal‐like behavior. In addition, the reactance is discussed according to the equivalent circuit analysis. This work constructed the metacomposites with low filler content, which is a benefit for practical applications in electromagnetic devices.

Fine-tunable ε′-negative response derived from low-frequency plasma oscillation in graphene/polyaniline metacomposites

Remarkable ε′-negative responses at radio-frequency endow metacomposites with revolutionary applications in electromagnetic (EM) fields. However, uncovering its regulation mechanism and subsequently achieving fine-tunable ε′-negative response remains challenging. Here, we meticulously designed graphene/polyaniline (GR/PANI) metacomposites above percolation, which directly enable tunable negative permittivity at an order of magnitude of −103. By rationally adjusting the three-dimensional GR network, we were able to form collective low-frequency plasma oscillation of free electrons in composites, which intrinsically contributes to the tunable ε′-negative response. Moreover, an ε′-near-zero (ENZ) response was surprisingly observed at ∼920 MHz due to the ineffectiveness of the plasmonic state at high frequencies. In addition, we carried out equivalent circuit analysis to demonstrate the phase relationship and electrical characteristics of these metacomposites. This work could contribute to a new category of polymer-matrix composites and enable exotic properties beyond their natural and bulk counterparts.

High temperature study of dielectric and electrical conduction behaviour of La2NiO4

Materials showing negative permittivity have attracted considerable attention from researchers on account of their notable applications in electronic and electromagnetic devices. Discovering materials exhibiting negative permittivity is a challenging task. Very few single-phase oxides have been reported as negative permittivity materials. In this work, we have attempted to investigate the dielectric properties of La2NiO4 to explore the possibility of negative permittivity. To achieve the goal, powder, and ceramic of La2NiO4 have been synthesized by the conventional solid-state reaction method. Rietveld refinement of the X-ray diffraction data studies has confirmed the tetragonal structure and space group I4/mmm. The Dielectric properties and AC conductivity have been studied in the frequency range of 20 Hz–2 MHz and over a wide temperature range (30 °C–600 °C). The permittivity of La2NiO4 is found to be negative at all the measured frequencies and temperatures. The Drude model fitted the experimental data very well, suggesting that negative permittivity behavior can be assigned to the plasma oscillation of conducting electrons. The trend of AC conductivity with frequency is similar to the skin effect observed in metals. The variation of DC conductivity with temperature indicated small polaron hopping conduction mechanism. Analysis of reactance (Z′′) suggested inductive electrical character in the tested frequency and temperature range. Room temperature negative permittivity behavior of La2NiO4 makes it a potential candidate for practical electronic and electromagnetic devices that work in the Radio-frequency range. Furthermore, this work will add a new member to the family of Perovskite oxides showing tunable negative permittivity.

MOF-derived ZrO2/C-polyvinylidene fluoride composite towards negative permittivity regulation mechanism

Composites featuring negative permittivity have garnered significant attention for their potential in novel capacitance designs, coil-less electrical inductors, and electromagnetic shielding applications. In this study, we prepared polyvinylidene fluoride (PVDF) matrix composites filled with ZrO2/C nanoparticles derived from metal-organic frameworks (MOFs) via a hot-pressing method. With an increase in the ZrO2/C content to 30 wt.%, electrical percolation was observed, accompanied by a transition mechanism from hopping conduction to metal-like conduction. This enabled the realization of ZrO2/C/PVDF composites with tailorable negative permittivity properties, attributed to the plasmonic oscillation of free electrons in the composites beyond the percolation threshold (30 wt.%). Furthermore, the permittivity transition along to a shift in the electrical behavior of the percolative composites from capacitive to inductive. We explored the regulatory mechanism behind the negative permittivity in this random composite system, and our findings highlight the potential of these tunable negative permittivity media as promising candidates for diverse electromagnetic applications.

Carbon nanotube-carbon black/CaCu3Ti4O12 ternary metacomposites with tunable negative permittivity and thermal conductivity fabricated by spark plasma sintering

Carbon nanotube-carbon black/CaCu3Ti4O12 ternary metacomposites with tunable negative permittivity and thermal conductivity fabricated by spark plasma sintering

Cost efficient fabrication of flexible polymer metacomposites: Impact of carbon in achieving tunable negative permittivity at low radio frequency range

Cost efficient fabrication of flexible polymer metacomposites: Impact of carbon in achieving tunable negative permittivity at low radio frequency range

Low‐loss percolative epsilon‐negative composites derived from the ultra‐low percolation threshold

AbstractIn this work, the mechanism of negative permittivity response with low losses in the silver nanowires/polyvinylidene fluoride composites were demonstrated. Due to the large aspect ratios of silver nanowires, ultra‐low percolation threshold () between 0.7 and 0.8 wt% was achieved with the losses effectively decreased by several orders of magnitude down to a low level of about 0.2. The negative permittivity response from Lorentz type and Drude type presented the transition from insulator to conductor. The tunable Drude type negative permittivity was ascribed to the plasma oscillation of free electrons, which leads to an inductive property. The conduction mechanism was switched from hopping to metallic conduction with the observation of skin effect. The epsilon‐negative materials with lower losses have potential applications in communication devices, such as novel capacitors and antennas.

Magnetic NiFe2O4/Polypyrrole nanocomposites with enhanced electromagnetic wave absorption

• NiFe 2 O 4 /PPy metacomposites with tunable negative permittivity are prepared by a simple surface-initiated polymerization method. • Minimum reflection loss of NiFe 2 O 4 -PPy metacomposites is −40.8 dB. • Effective absorption bandwidth blew −10 and −20 dB is 6.08 and 2.08 GHz, respectively. • Excellent EMW absorption performance benefits from improved independence matching, EMW attenuation capacity and synergistic effects of conduction loss, dielectric loss and magnetic loss. NiFe 2 O 4 /polypyrrole (NiFe 2 O 4 /PPy) nanocomposites are prepared by a simple surface-initiated polymerization method and demonstrate negative permittivity in the low frequency regions. These nanocomposites also exhibit significantly enhanced electromagnetic wave (EMW) absorption property in the high frequency regions. Compared with pure PPy, the enhanced negative permittivity is observed in the NiFe 2 O 4 /PPy nanocomposites with a NiFe 2 O 4 loading of 5.0, 10.0, 20.0 and 40.0 wt%, indicating the formation of metal-like electrical conducting network in NiFe 2 O 4 /PPy nanocomposites. Moreover, the negative permittivity could be tuned by changing the NiFe 2 O 4 loading. The minimum reflection loss ( RL ) of -40.8 dB is observed in the 40.0 wt% NiFe 2 O 4 /PPy composites with a thickness of only 1.9 mm. The effective absorption bandwidth below -10.0 and -20.0 dB reaches 6.08 and 2.08 GHz, respectively. The enhanced EMW absorption performance benefits from the improved independence matching, EMW attenuation capacity, and synergistic effects of conduction loss, dielectric loss (interfacial and dipole polarizations) and magnetic loss (exchange and natural resonances). This research work provides a guidance for the fabrication of nanocomposites with an excellent EMW absorption.

Coassembly of elastomeric microfibers and silver nanowires for fabricating ultra-stretchable microtextiles with weakly and tunable negative permittivity

In this work, a type of unique flexible and ultra-stretchable microtextiles was fabricated by coassembly of Ag nanowires (AgNWs) and thermoplastic polyurethane (TPU) microfibers. The microtexiles show tunable and weakly negative permittivity at a low filler content of 1.0–2.4 wt%. The resonance-induced negative permittivity observed in the AgNW/TPU microtextiles below the percolation threshold (fc) is attributed to the electric dipole moment of the isolated AgNWs, and this phenomenon can be explained by the Lorentz model. When the AgNW content exceeds the fc, the low-frequency collective plasma oscillation of the AgNW networks in the composites triggers the tunable plasma-type negative permittivity, which agrees with the Drude model. Additionally, the inductive character of the composites was assessed with the reactance spectra. The as-prepared Ag/TPU microtextiles could be a good candidate for the applications of garment integrated electronic, novel sensors, electromagnetic cloaking, and wearable electronic devices.

Tunable negative permittivity behavior in percolated MWCNTs/PVDF composites

Composites with negative permittivity behavior have brought about widespread attention for their broad application prospects in electromagnetic field or being used for novel capacitance. In this work, multiwall carbon nanotubes/polyvinylidene fluoride (MWCNTs/PVDF) composites with gradually increasing MWCNT filling content were prepared by hot-pressing in order to acquire negative permittivity. The microstructure, ac conductivity, and real permittivity of the samples were elaborated. As MWCNTs content increases, the percolation phenomenon occurs accompanied by the conversion of conductive mechanism from hopping conduction to free electron conduction. It’s found that negative permittivity property was obtained at a lower filler content and can be regulated by controlling MWCNTs content, which is related to the plasma oscillation state of free electrons in MWCNTs.

Low-frequency plasmonic state and tunable negative permittivity in percolative graphite / barium titanate composites

Percolative composites with negative permittivity have attracted widespread attentions due to their great potential in electromagnetic shielding and microwave devices. Targeting at achieving epsilon-negative properties, the percolative graphite/barium titanate (GR/BaTiO 3 ) composite is herein designed and prepared using hot-pressing sintered process. It's found that the plasma oscillations of delocalized electrons result in the epsilon negative permittivity behaviors when the GR contents exceeds the 2 wt% (percolation threshold), and frequency dependence of the negative permittivity which is in well agreement with the Drude model. Meanwhile, it's demonstrated that the ac conductivity represents a typical metal-like behavior as the conductive networks formed within the composites by the increasing GR loadings. Moreover, the equivalent circuit analysis reveals the relationships between capacitive-inductive transition and the conversion of permittivity changing from positive to negative. This work provides effective possibility for developing excellent dielectric properties of percolative GR/BaTiO 3 composite for capacitors and coil-less electrical inductors applications.

Cost Efficient Fabrication of Flexible Polymer Metacomposites: Impact of Carbon in Achieving Tunable Negative Permittivity at Low Radio Frequency Range

Cost Efficient Fabrication of Flexible Polymer Metacomposites: Impact of Carbon in Achieving Tunable Negative Permittivity at Low Radio Frequency Range

Cost Efficient Fabrication of Flexible Polymer Metacomposites: Impact of Carbon in Achieving Tunable Negative Permittivity at Low Radio Frequency Range

Cost Efficient Fabrication of Flexible Polymer Metacomposites: Impact of Carbon in Achieving Tunable Negative Permittivity at Low Radio Frequency Range

Communication—Tunable Negative Permittivity of Cobalt and Epoxy Composites at 3 kHz ∼ 1 MHz Frequency Regions

Negative permittivity in percolation composites has drawn much attention owing to their potential application. This study set out to explore whether tunable negative permittivity can be realized in metal composites at 3 kHz ∼ 1 MHz frequency regions. When the content of cobalt is 80 wt%, the permittivity turns to negative in the test band. These findings show that the Co conductive network has formed and provides large numbers of free electrons. Meanwhile, the percolation composites with negative permittivity show inductive characteristic. This research supports the idea that tunable low-frequency negative permittivity can be realized in metal composites.

Negative permittivity behavior in carbon fibre/silicon nitride ceramic composites prepared by spark plasma sintering

Carbon/ceramic composites with adjustable electrical properties can be promising candidates for electromagnetic metamaterials with negative permittivity. Herein, silicon nitride (Si3N4) ceramic composites incorporated with carbon fibres (CFs) were prepared via spark plasma sintering. The fibres were randomly distributed in the ceramic matrixes, and their amounts had a great influence on the microstructure, electrical conductivity, dielectric behaviour, and impedance response of the CF/Si3N4 composites. As the CF content was low, the ceramics with isolated CFs showed a dielectric type of conductivity, along with a capacitive character and positive permittivity. When the CFs were more sufficient to contact or overlap each other in the ceramics, the formation of CF networks endowed the ceramics with a metallic type of conductivity and inductive character. A plasma-type negative permittivity behaviour was observed in these ceramics and was imputed to the low-frequency plasmonic state of free electrons in the formed CF networks. The Drude model was applied to make a precise description of the negative permittivity behaviour. As the CF content increased, the plasma frequency (negative-positive transformation frequency of permittivity) of the ceramics shifted to a higher frequency, and their absolute magnitudes of negative permittivity also increased. Furthermore, equivalent circuit models were utilized to interpret the impedance response of the ceramics, which demonstrated that the negative permittivity was linked to the inductive character. This work provides support for developing ceramic-based metamaterials with tunable negative permittivity that is desirable for many applications.

Tailorable high-k and negative-k percolation behaviors in PPy/P(VDF-HFP) composites

Polymer-based composites with high permittivity and low loss are highly desirable for many applications such as embedded capacitors and pulse-power. In the meantime, percolating composites with negative permittivity can be promising candidates for metamaterials. In this paper, nanocomposites using conductive polypyrrole nanowires as fillers and poly(vinylidenefluoride-hexafluoropropylene) (P(VDF-HFP)) as matrix were prepared by a solution-casting method. The electrical properties were investigated in detail. Two types of percolation behavior were observed in the composites with increasing PPy content. The permittivity significantly enhanced when the PPy content exceeded high-dielectric percolation threshold (i.e. fhigh-k) due to the large number of microcapacitors formed by PPy fillers and P(VDF-HFP) matrix. With further increasing PPy content, tunable negative permittivity behavior was achieved due to the formation of conductive networks, this phenomenon can be regarded as a negative-dielectric percolation (i.e. negative-k percolation), which can be well described by Drude-Lorentz model. The realization of high permittivity and tunable negative permittivity in PPy/P(VDF-HFP) nanocomposites gives a new and efficient strategy toward high-k materials and random metamaterials.

Interfacial polymerized copolymers of aniline and phenylenediamine with tunable magnetoresistance and negative permittivity

With the purpose of ameliorating the electrical property, permittivity and magnetoresistance of polyaniline (PANI) nanofibers, three types of phenylenediamine (PDA), including o-PDA, m-PDA, and p-PDA, and different contents of PDA have been incorporated into the PANI polymer backbone by the interfacial copolymerization of aniline with PDA. The results demonstrate that different types and contents of PDAs could disclose the distinct effects on the chemical structures, crystalline structures, and morphologies for copolymers of aniline with PDA affirmed by Fourier transform infrared (FTIR) spectroscopy, solid-state 13C nuclear magnetic resonance (13C NMR), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The molecular weight of all copolymers is reduced by several orders of magnitude relative to pure PANI nanofibers confirmed by gel permeation chromatography (GPC). The resistivity variations for the copolymers of aniline with different types of PDAs are well explained from the point view of their molecular structures based on the first-principle simulations (i.e. intrachain electrical conductivity) and degree of crystallinity gained from XRD (i.e. interchain electrical conductivity). A tunable room-temperature magnetoresistance (MR) and a controllable negative permittivity are discovered in the copolymers of aniline with different types and contents of PDAs. This work provides a new insight for the manipulation of electrical transport for PANI nanofibers and helps seek the new applications of conductive polymer.