Low Frequency Plasmonic State Research Articles

Three-Dimensional random network of metacomposites by synergizing Multi-Walled carbon Nanotube-Carbon black for tunable Epsilon-Negative and Epsilon-Near-Zero responses

Epsilon-near-zero (ENZ) and epsilon-negative (EN) responses are remarkable properties of electromagnetic (EM) metamaterials that have sparked considerable interest. This paper introduces an innovative strategy using ternary metacomposites that achieve excellently tunable ENZ (|ε’| < 1) and EN (ε’ < 0) parameters within the radio-frequency band. The approach leverages a synergistic effect of multi-walled carbon nanotube-carbon black (MWCNT-CB) composites integrated into a polyaniline (PANI) matrix to construct three-dimensional (3D) carbon networks. As the loading content of MWCNT-CB increases, these networks evolve from clusters, enabling a finely tunable range of EN parameters from 100 to 103. The ENZ response occurs at approximately 265 MHz and 830 MHz, triggered by dielectric resonance due to electric dipoles at MWCNT-CB/PANI interfaces and a low-frequency plasmonic state in the 3D MWCNT-CB networks, respectively. This research establishes a foundation for tunable ENZ and EN responses by introducing a new class of ternary metacomposites.

Extremely low-frequency plasmonic state achieved in AC-ZnO polymer composite for tuned negative permittivity applications

Flexible composites with negative permittivity behaviour have generated a lot of attention due to their potential applications in the electromagnetic field and novel capacitance. This work aimed to design and explore surface microstructure-induced negative permittivity flexible composites for their metamaterial properties. The composites were synthesized using poly (methyl methacrylate) or PMMA, activated carbon (AC), and zinc oxide (ZnO) through an in-situ polymerization process. The composite was designed to acquire negative permittivity in the extremely low-frequency regime by increasing the AC and ZnO filling content. The microstructure formation, real permittivity, and ac conductivity of the composites were investigated, and it was observed that the negative permittivity and its magnitude could be controlled by changing the amount of the filler, AC_ ZnO. Flexible composites with suitable permittivity values for ELF applications can create heavy industrial demand.

Spark plasma sintered graphene/copper calcium titanate ceramic composites with negative permittivity and enhanced thermal conductivity

Ceramic composites with negative permittivity have provoked considerable interests of researchers in electronic and dielectric devices due to the extraordinary electromagnetic performance in radio-frequency (RF) region. Herein, graphene/CaCu3Ti4O12 (GR/CCTO) ceramic composites were spark plasma sintered, of which the dielectric and thermal properties were demonstrated at RF region. An electrical percolation was identified with GR content varying from 10 wt% to 14 wt% which presenting as a dramatic increase of ac conductivity. The conduction mechanism changed from hopping conductivity to metal-like conductivity. Meanwhile, the real permittivity (ε′) turned from positive to negative which indicating an intrinsic transition of dielectric response mechanism. Therefore, Drude model was applied to elucidate the RF negative permittivity (ε' < 0) which manifesting the low-frequency plasmonic state of delocalized electrons in composites. The constructed GR networks in composites also leaded to the enhanced thermal conductivity due to the dominating contribution of phonon vibration in GR sheets. Besides, theoretical models of capacitive and inductive equivalent circuits were used on impedance spectra which successfully clarified the inductive character of negative permittivity. This work benefits expounding the generation and regulation mechanism of negative permittivity and will be favorable to exploring brand-new applications of ceramic composites.

Defect-induced insulator-metal transition and negative permittivity in La1-Ba CoO3 perovskite structure

• Low-frequency plasmonic state and temperature-stable negative permittivity in La 1- x Ba x CoO 3 perovskite structure is achieved. • The partial replacement of La 3+ by Ba 2+ in La 1- x Ba x CoO 3 system can create oxygen vacancies and Co element with higher oxidation states. • Samples with x ≥ 0.12 manifest metal-like character described by the Drude model approximat. • Zero-crossing behavior of real permittivity is observed in La 0.9 Ba 0.1 CoO 3 sample. The development of negative permittivity materials in multifunctional applications requests expansion of their operating frequency and improvement of stability of negative permittivity. Low electron density is beneficial to reduce plasma frequency so that negative permittivity is achieved in kHz region. Negative permittivity achieved by percolating composites is restricted in practicality due to its instability nature at high temperatures.To achieve temperature-stable negative permittivity in kHz region, mono-phase La 1- x Ba x CoO 3 ceramics were prepared, and the transition from dielectric to metal was elaborated in the perspective of electrical conductivity and negative permittivity. The plasma-like negative permittivity is attained in kHz region, which is interpreted by the collective oscillation of low electron density. The temperature-stable negative permittivity is based on the fact that the plasmonic state will not be undermined at high temperatures. In addition, zero-crossing behavior of real permittivity is observed in La 0.9 Ba 0.1 CoO 3 sample, which provides a promising alternative to designing epsilon-near-zero materials. This work makes the La 1- x Ba x CoO 3 system a source material for achieving effective negative permittivity.

Communication—Function-Oriented Design of 3D Carbon Networks Toward Negative Permittivity at kHz Frequencies

Three-dimensional (3D) carbon networks composed of graphene (GR) and carbon nanotube (CNT) were constructed in copper calcium titanate (CCTO) in order to realize negative permittivity behavior. The results show that negative permittivity can be obtained at kHz frequencies above percolation threshold when 3D carbon networks are successfully constructed. Negative permittivity originates from the low-frequency plasmonic state which is explained by Drude model. The magnitude of negative permittivity was tuned between 105 and 106 which significantly correlates with concentration of free carriers. Moreover, the reactance spectra clarify the inductive character of negative permittivity materials.

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.

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.

Radio-frequency epsilon-negative property and diamagnetic response of percolative Ag/CCTO metacomposites

Here, Ag/CaCu3Ti4O12 (Ag/CCTO) percolative nanocomposites were fabricated by an in-situ synthesis, where the percolation-related transitions of dielectric and magnetic properties were investigated at radio-frequency band. Epsilon-negative property (ε′ < 0) and diamagnetic responses (μ′ < 1) were observed in percolated composites, owing to low-frequency plasmonic state and induced eddy currents within formed conductive Ag networks respectively. The competitive effect between polarization response of ε′-positive CCTO matrix and collective plasma oscillation of ε′-negative Ag networks leads to the tunable negative permittivity. The clarification of inductive character of ε′-negative materials benefits constructing effective strategy for adjusting negative value of dielectric constant.

Communication—Tunable Epsilon-Negative Property of Nickel/Copper Calcium Titanate Cermets

Nickel/copper calcium titanate (Ni/CCTO) cermets with variated Ni content were prepared. The abrupt shift of ac conductivity, reactance and complex permittivity occurred in composites. Epsilon-negative property is an intrinsically physical performance of inductive character which was explained by low-frequency plasmonic state of Ni networks. Negative value of real permittivity in metacomposites reached 109 and was tuned by adjusting the concentration of free carriers in Ni networks. The epsilon-negative materials are accompanied by strongly dielectric loss and high reactance value. This work expands the applying scope of CCTO into metacomposites and meanwhile facilitates to clarify the regulation mechanism of epsilon-negative.

Low-frequency plasmonic state and negative permittivity in copper/titanium dioxide percolating composites

The titanium dioxide-based varistor is generally equivalent to a combination of resistor and capacitor, whereas, for the titanium dioxide-based composite with negative permittivity, it can be composed of a resistor, capacitor, and inductor. In this work, ac conductivity, real permittivity, and reactance of copper/titanium dioxide (Cu/TiO2) percolating composites are investigated in detail in the frequency of 10 kHz - 1 MHz. As Cu content increases, percolation occurs in composites, accompanied by the transformation of electron transport mechanism from localization to delocalization. Furthermore, the regulation of permittivity by compositions and microstructures is specifically analyzed. It suggests that the enhanced permittivity before percolation ascribes to the enhancement of interfacial polarization, when reaching the excessively percolating state, plasma-like negative permittivity is achieved, consequently, revealing the low-frequency plasmonic state. Moreover, reactance analyses indicate the negative permittivity is ascribed to the inductive characteristics, and the stronger the inductance, the greater the contribution to the negative permittivity.

Communication—Tunable Epsilon-Negative Property in FeCrNi/CaCu3Ti4O12 Metacomposites

In this paper, FeCrNi/CaCu3Ti4O12 metacomposites with different FeCrNi content were fabricated by low-temperature presureless sintering process. With increasing FeCrNi content above percolation threshold, the conduction mechanism changed from hopping conduction to metal-like conduction. Meanwhile, epsilon-negative property was obtained at 70 wt%–90 wt% content of FeCrNi, ascribing to the low frequency plasmonic state from three-dimensional conductive FeCrNi networks. The correlation between epsilon-negative property and inductive character was clarified by reactance spectrum. This work benefited the exploration of metal/ceramic metacomposites which could facilitate their promising applications in invisibility, high-k capacitors, novel antennae and sensors.

Tunneling-induced negative permittivity in Ni/MnO nanocomposites by a bio-gel derived strategy

Tunneling-induced negative permittivity is attributed to the low frequency plasmonic state in tunneling networks, where nickel nanoparticles are still isolated geometrically but connected electrically.

Tunable negative permittivity behavior and electromagnetic shielding performance of silver/silicon nitride metacomposites

The recent rise of metacomposites offered a new research strategy for electromagnetic shielding materials owing to their negative electromagnetic parameters, such as negative permittivity. Herein, we prepared silver/silicon nitride (Ag/Si3N4) metacomposites with tunable negative permittivity by a facile impregnation-calcination process, and explored their electrical conductivity, permittivity and electromagnetic shielding properties. As the Ag content increased, formative metal networks in the composites rendered their conductivity characteristic changing from a hopping conductivity to a metal-like conductivity. Tunable negative permittivity behavior combined with enhanced shielding effectiveness (SE) was observed at 2–18 GHz in the metacomposites with high Ag contents. The plasma-like negative permittivity was accounted for by a low frequency plasmonic state of free electrons in the inductive Ag networks, and the frequency band and absolute magnitude of negative permittivity could be adjusted by controlling Ag content, which was well described by Drude model. The average total SE of our obtained metacomposite could reach ~30 dB, and the reflection was the primary shielding mechanism, which was attributed to the intense impedance mismatching stemmed from the negative permittivity. Our work opens up the possibility of designing metacomposites for promising electromagnetic shielding materials, promoting their application in microwave field.

Fine-tuning of negative permittivity behavior in amorphous carbon/alumina metacomposites

In this work, amorphous carbon/alumina (C/Al2O3) metacomposites with tunable negative permittivity were synthesized by a facile impregnation method. We analyzed the influence of carbon content and the carbonization temperature on the microstructure and electrical properties of the metacomposites. It was found that the conductive carbon networks were formed in the composites with the higher carbon contents and carbonization temperatures, which lead to the appearance of a metal-like conduction and negative permittivity behavior. Low-frequency plasmonic state provided by plenty of free electrons in the carbon networks resulted in the negative permittivity, and its numerical values could be effectively adjusted by controlling the carbon content and carbonization temperature, which could be well explained by Drude model. This work provides a feasible way to prepare metacomposites with tunable negative permittivity, which will expand their potential applications in shielding, absorbing and novel capacitor fields.

Radio-frequency negative permittivity of carbon nanotube/copper calcium titanate ceramic nanocomposites fabricated by spark plasma sintering

Ceramic nanocomposites have been developed to provide a feasible and effective strategy in realizing extraordinary electromagnetic properties, such as negative permittivity. Here, copper calcium titanate CaCu3Ti4O12 (CCTO) nanocomposites incorporated with multiwall carbon nanotube (MWCNT) were prepared by spark plasma sintering (SPS) technology. It was found that MWCNT clusters were randomly embedded in CCTO matrix, which destroyed the microstructures of internal barrier layer capacitors in CCTO. The composites with low MWCNT content presented a hopping conduction behavior, while a metal-like conduction behavior was observed in the ceramic with 18 wt% MWCNT content. Interestingly, weakly negative permittivity (~ -103) was obtained in the ceramic consisting of interconnected MWCNT networks, as a great number of free electrons in the MWCNTs formed significantly collective oscillation state over 10 MHz-1 GHz range. Macroscopically, leakage current among MWCNT clusters or networks caused strong conduction loss at low frequencies. Equivalent circuit analysis manifested the correlation between low-frequency plasmonic state and inductive character in the composite with negative permittivity. This work could extend the potential applications of ceramic nanocomposites to metamaterials.

MWCNTs/BaTiO3 metacomposite with negative permittivity behavior and electric percolation phenomenon in radio frequency

Negative permittivity has taken off as a research topic in percolative composites recent years. Comparing with the common conductive constituent element in the preparation of metamaterials such as metal, one-dimensional conductive carbon nanotubes (CNTs) provide another choice of functional phase. In this paper, barium titanate (BaTiO3) with multi-walled carbon nanotubes (MWCNTs) uniformly dispersed in the ceramic matrix were prepared by traditional ceramic sintering technology. Field emission scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize microstructures and phase composition of the metacomposites. With the increasing of CNTs content, the percolation phenomenon of electrical conductivity (9.95 wt%) was found and the conductive mechanism changed because of the formation of conductive MWCNTs networks. Moreover, the negative permittivity behavior was observed when the MWCNTs content reached 7 wt% and the plasma-like negative permittivity behavior could be well explained by the low frequency plasmonic state generated from conductive nanotube networks using the Drude model.

Reverse design of negative permittivity property in Nickel-Network/Epoxy composites

Epsilon-negative materials are usually prepared by additive manufacturing, while a reverse design method needs to be developed to construct the epsilon-negative materials by subtractive manufacturing. Here the Nickel-Network/Epoxy composites were prepared via a chemical corrosion and curing process. The connectivity of nickel network was weakened by chemical corrosion, which was companied with a change of conductivity mechanism from metal-type conductivity to the hopping conductivity. The negative permittivity value could be effectively adjusted by corroding the nickel network, which was attributed to low-frequency plasmonic state of nickel network. The reverse design method provided a novel and more flexible method to construct the desired the epsilon-negative materials.

Electromagnetic Properties of Fe50Co50/Cu Granular Composite Materials Containing Flaky Particles

Electromagnetic properties of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu hybrid granular composite materials containing flaky Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> particles and flaky Cu ones have been studied in the RF to microwave frequency range to realize the double negative (DNG) material. The electrical percolation along the dispersed Cu particle chains takes place at the critical Cu particle volume fraction φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c_Cu</sub> = 0.07; the σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> value of composites with φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c_Cu</sub> and above was more than 10-1 S/cm. The negative permittivity caused by the low-frequency plasma oscillation of conduction electron in percolated Cu particle chains was observed above φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> = 0.08; the hybrid composites with φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> 0.08 are in the low-frequency plasmonic state. Meanwhile, the negative permeability due to the magnetic resonances was obtained above φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> = 0.10. Therefore, in the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu flaky particle composites with φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> 0.10, the DNG characteristics were realized by the low-frequency plasma oscillation and magnetic resonances in the RF to microwave frequency range. In the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu flaky particle composites, the DNG spectrum was observed at relatively low Cu particle content and the DNG frequency band becomes broad compared to the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu hybrid composites containing spherical or arborized shape particles.

Electromagnetic Properties of Au/Fe53Ni47 Hybrid Granular Composite Materials

Complex permittivity and permeability spectra of Au/Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">53</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">47</sub> hybrid granular composite materials containing spherical Au and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">53</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">47</sub> microparticles have been investigated from RF to microwave frequency range. In the 80 vol.% composite containing Au/Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">53</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">47</sub> hybrid particles, the metallic property was achieved at the critical volume fraction of Au, φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_Au</sub> = 0.04 by the electrical percolation of hybrid metallic particles; electrical conductivity at 1 kHz becomes more than 0.1 S/cm. A low-frequency plasmonic state with negative permittivity spectrum was observed above φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_Au</sub> . Meanwhile, the complex permeability of the hybrid composites showed a Lorentz-type frequency dispersion with the negative permeability spectrum caused by the magnetic resonance at several hybrid particle contents. Consequently, double negative permittivity and permeability spectra have been realized from about 100 MHz to several gigahertz.

Communication—Epsilon-Negative Metacomposite Realized by Titanium Carbide Alumina Binary Ceramics in Radio Frequency

Titanium carbide alumina (TiC-Al2O3) binary ceramics were fabricated and the distributions of TiC within the ceramic composites mattered most the dielectric performances and electrical conductivity. There have taken on a low frequency plasmonic state when TiC particles connected with each other and formed a three-dimensional conductive pathway within Al2O3 matrix, thus plasma-like negative permittivity appeared. The conduction mechanism changed from hopping conduction to metal-like conduction. The reactance spectrum suggested negative permittivity media showed inductive character. This work could contribute to the integration of structure and function for TiC-Al2O3 binary ceramics and the obtained ceramics have promising applications for electromagnetic metamaterials.