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Since the electromagnetic metamaterials are artificial materials with subwavelength‐scale periodic architectures, they can manipulate electromagnetic waves by exhibiting unusual properties of characteristic negative permittivity ε and permeability μ. However, their fabrication toward reducing the anisotropy for the realization of practical metamaterials is still a challenge. A homogeneous La0.5Sr0.5MnO3 with negative properties synthesized by a facile method consisting of the sol–gel method and pressure‐less sintering is proposed in this paper. By appropriately tuning eg electrons concentration, lattice symmetry, and magnetic resonance, the La0.5Sr0.5MnO3 can simultaneously achieve negative ε and μ at the frequency range of 575–650 MHz. Experimental results and theoretical simulation suggest that negative ε is attributed to the plasma oscillation of eg electrons and high lattice symmetry, whereas the magnetic resonance causes the negative μ. The enhancement is due to the reduction of the eddy current and the effective magnetic anisotropy. An important step toward the development of natural compound metamaterials is represented in this paper. [ABSTRACT FROM AUTHOR]

Metamaterials with metal conductive phase always suffer from their high negative permittivity (epsilon-negative), the mechanism of negative permittivity is still need to be explored. Herein, we prepared iron (Fe)/epoxy composites by mechanical mixing and pressure forming. With the increase of Fe content, the conductive behavior changed from hopping conduction to metal-like conduction. The permittivity became negative when the iron content exceeded the percolation threshold (0.425), the negative permittivity was caused by the plasma oscillation of delocalized electrons in the iron conductive network. Besides, when the iron content was 50, 55, 60 or 70 vol%, the permittivity changed from negative to positive at certain frequency points, which is called epsilon-near-zero property. We used the Debye-Drude model to analyze this special dielectric behavior, and the fitting results are in good agreement with the experimental results. A transformation from capacitive to inductive was founded by using equivalent circuit analysis model, this suggests that the occurrence of negative permittivity is accompanied by the appearance of inductance and the negative permittivity behavior is inductance. The epsilon-near-zero and epsilon-negative property could be adjusted effectively by simply changing iron content. Our work provides an understanding guidance on the mechanism of epsilon-negative and epsilon-near-zero property and offers an efficient way to regulate the dielectric property. [ABSTRACT FROM AUTHOR]

Aerogels with negative parameters have become a research hotspot since the discovery of negative Poisson's ratio and negative thermal expansion coefficient in aerogels. The negative parameters are originally brought up and studied in electromagnetic metamaterials, leaving a question that whether aerogels can be used to construct metamaterials with negative electromagnetic parameters. Here, the authors show that negative permittivity (epsilon‐negative) property can be achieved in carbon aerogels, and its value is adjusted continuously by changing the composition of carbon aerogels but keeping their microstructure unchanged. This is a novel strategy different from the typical array‐structure‐containing metamaterials. Moreover, a physical state transition is found when increasing the carbonization temperature, where the initial low‐loss dielectric state evolves into a high‐loss one upon increasing the temperature from 400 to 700 °C followed by a final degenerate semiconductor state when increasing the temperature to 800 °C and above. This work can provide a new method toward the design of electromagnetic metamaterials, starting by choosing the component materials with optimal composition. [ABSTRACT FROM AUTHOR]

A comprehensive review of the fundamentals and applications of epsilon-negative materials is presented in this paper. Percolative composites, as well as homogeneous ceramics or polymers, have been investigated to obtain the tailorable epsilon-negative properties. It's confirmed the anomalous epsilon-negative property can be realized in conventional materials. Meanwhile, from the perspective of materials science, the relationship between the negative permittivity and the composition and microstructure of materials has been clarified. It's demonstrated that the epsilon-negative performance is attributed to the plasmonic response of delocalized electrons within the materials and can be modulated by it. Moreover, the potential applications of epsilon-negative materials in electromagnetic interference shielding, laminated composites for multilayered capacitance, coil-less electric inductors, and epsilon-near-zero metamaterials are reviewed. The development of epsilon-negative materials has enriched the connotation of metamaterials and advanced functional materials, and has accelerated the integration of metamaterials and natural materials. [ABSTRACT FROM AUTHOR]

Herein, the conception of 2.5D metamaterial is proposed. The basic idea of the 2.5D metamaterial is to build "3D" metamaterial by stacking 2D metasurfaces layer‐by‐layer. In this design, the trapezoidal prism units (3D) is built up by stacking a series of rectangle units (2D) together. This means, in the conception of 2.5D metamaterial, any kind of 3D structure with complex geometrical shape can be easily built up by controlling the 2D shape in each layer, which provide a new way for the fabrication of 3D complex units. Furthermore, paper is chosen as the substrate of 2D layer, because the interconnected lignocellulosic microfibers in paper will not only bring in flexibility to the 2.5D metamaterial but also strong mechanical strength. The results show that the 2.5D metamaterial still maintain a good filtering performance at 11 GHz when the pressure loading rise up to 7 MPa, whereas the classic 3D metamaterial (built by copper‐clad plate) undergoes a fracture failure. The conception and demonstration of 2.5D metamaterial with high mechanical property not only overcome the lacking of the basic structural carrying capacity in 3D electromagnetic metamaterial but also give new idea to the design of multifunctional metamaterial. [ABSTRACT FROM AUTHOR]

In this paper, Ag/paper-based composites with low dielectric loss were obtained by introduce the concept of metamaterial into the fabrication process. In this kind of metacomposite, single-layer paper was used as basic block to build up a matrix with sandwich structure, in which Ag particles are embedded and silver particles are evenly distributed. The microstructure and phase composition were characterized by scanning electron microscope and X-ray diffractometer. The impedance spectrum of the composites in 20 Hz–1 MHz region was measured by using LCR Meter, which can be well fitted by equivalent circuit method. The results show that, compared to the composite in which Ag particles disorder distributed, the dielectric loss of the layer-by-layer structured metacomposite can be effectively reduced (tan θ ≤ 0.1), without negative effect on the dielectric constant. Moreover, the dielectric properties of metacomposites show obvious anisotropy in the horizontal and vertical directions, which might give a feasible way for flexible smart material in its design. [ABSTRACT FROM AUTHOR]

Metamaterials are artificial periodic structures with negative permittivity and permeability. Several interesting properties can be obtained in metamaterials, such as negative index behavior, which can be used for building perfect lenses, cloaking, antennas, etc. As the metamaterial's properties are determined by its structure, the key challenge is to reduce the fabrication cost of the periodic structure on the micrometer or nanometer scale for realistic applications. In this paper, we experimentally demonstrate a new one-step method for the fabrication of a large-area infrared metamaterial at extremely low cost. A metallic mesh is used as a shadow mask during the pulsed laser deposition (PLD) process to fabricate a FeNi/SiC/FeNi multilayer sandwich structure on Si substrate (cm level). The sample shows a strong absorption peak in the infrared frequency range, and the absorption intensity changes with the sample's geometry. [ABSTRACT FROM AUTHOR]

Negative capacitance in ferroelectrics has seem remarkable progress in recent years. However, direct measurement of negative capacitance has so far been a challenge due to its instable characteristic in the electrostatic field. An ingenious method to design stable negative capacitance by dielectric resonance under high‐frequency alternating electric fields is demonstrated. The well‐pronounced dielectric resonance and electrical property in BaTiO3/SrTiO3 (BT/ST) bulk ceramics consisted of epitaxial core–shell particles are studied. As the negative capacitance can be considered as an inductive behavior, the dielectric resonance of permittivity induced by a LC resonance can be occurred. The dielectric resonance can be described by the Lorentz model and explained by Fano‐like resonance of TO2 phonon in BT. In addition, lowest resonant frequency is observed at effective Curie temperature. Benefiting from negative capacitance in the BT/ST ceramics, both high permittivity and negative permittivity are accompanied with low losses around the resonant frequency, which is useful for developing capacitors and metamaterials. Significantly, this work will provide an initiative avenue to obtain explicit negative capacitance of ferroelectrics in the frequency spectrum. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 (SiO 2) 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-SiO 2 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. [ABSTRACT FROM AUTHOR]

Although metals are the most common conductive functional phase in metacomposites preparation, one-dimensional carbon fibre (CF) is an ideal alternative. In this paper, ceramic composites with CFs homogeneously dispersed in an alumina matrix were prepared using a hot press sintering process. The impact of CF contents on the microscopic morphology, electrical conductivity and negative permittivity behavior of the composites was systematically investigated. The conductivity of the composites increased with increasing CF contents, and their impedance modulus subsequently decreased. The conductivity mechanism of the composites transformed from a hopping conductivity to a metal-like conductivity. With the increase in CF contents above the percolation threshold, the composites showed a negative permittivity behavior at the frequency range from 1 kHz to 10 MHz. The negative permittivity of the metacomposites was attributed to the plasma oscillations of free electrons in the three-dimensional CF networks, and the corresponding dispersion characteristics were in agreement with the Drude model. The plasma oscillation frequencies of ceramic composites moved to higher frequencies with increasing CF content in the composites, and the absolute value of negative permittivity also increased. These findings can be useful for clarifying the realization and regulation mechanism of negative permittivity, which greatly facilitates the development of ceramic-based metacomposites. [ABSTRACT FROM AUTHOR]

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/TiO 2) 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. [ABSTRACT FROM AUTHOR]

Epsilon-negative behavior in metamaterials has attracted increasing attention recently. In this work, excessively percolating BaTiO 3 /Ag composites were synthesized by an in-situ preparation method to achieve negative permittivity. The epsilon-negative behavior is explained by low-frequency plasma oscillation of free electrons, which can be well fitted by the Drude model. Moreover, the negative permittivity of composites can be tuned by adjusting the content of randomly distributed Ag particles. The ac conductivity of excessively percolating BaTiO 3 /Ag composites shows a typical metal-like behavior. Furthermore, the investigation of reactance reveals that the inductive character derived from the conductive network is the main contributor to negative permittivity, as confirmed by the equivalent circuit analysis. This work provides a theoretical recipe for designing negative permittivity, supplements the family of metamaterials, and would further facilitate the practical applications of epsilon-negative materials in electromagnetic devices. [ABSTRACT FROM AUTHOR]

Abstract Since negative permittivity demonstrated in metamaterials, how to effectively tune negative permittivity still the challenge to satisfy practical applications. Meta-composites could provide alternative routes to tunable negative permittivity. In this paper, Graphene/CaCu 3 Ti 4 O 12 (GR/CCTO) meta-composites with different GR content were fabricated by low-temperature pressureless sintering. Microstructures and compositions of GR/CCTO were investigated in detail. The electrical and dielectric properties of GR/CCTO nanocomposites were investigated at 10 kHz −1 MHz and 20 MHz - 1 GHz region respectively. AC conductivity spectra showed different variation trends which were explained by Jonscher's power law or Drude model, indicating conductive mechanism transformation from hopping conduction to metal-like conduction. Lorentz-type and/or Drude-type negative permittivity behaviors were observed at radio-frequency region. Tunable negative permittivity were realized by changing GR content in GR/CCTO nanocomposites. Correspondence between inductive characteristic and negative permittivity was manifested by equivalent circuit analysis of impedance response of GR/CCTO composites. This work not only presents novel routes to tune negative permittivity, but also further clarifies negative permittivity generation mechanism, which will greatly facilitate applications in impedance matching, electromagnetic shielding and multi-layer high- k capacitors etc. Graphical abstract Image 1 Highlights • GR/CCTO nanocomposites were fabricated by low-temperature sintering. • GR/CCTO composites were characterized by TEM, SEM, XRD, Raman and FT-IR. • Lorentz-type and/or Drude-type negative permittivity behaviors were observed. • Negative permittivity were tuned at 10 kHz - 1 MHz and 20 MHz - 1 GHz region. • Equivalent circuit analysis of impedance response were performed. [ABSTRACT FROM AUTHOR]

Abstract Metacomposites with conductive polymer matrix toward negative permittivity have promising applications in energy storage devices and electromagnetic shielding. In this paper, carbon nanotubes (CNTs)/polypyrrole (PPy) nanocomposites with different loadings of CNTs have been synthesized using a surface-initiated polymerization (SIP) method. Scanning electron microscopy (SEM) results indicate that conductive networks have constructed inside the nanocomposites by CNTs and PPy granules. A frequency switch of permittivity from positive to negative in pure PPy and CNTs/PPy nanocomposites with low CNTs loading (1 wt%) is observed, which can be analyzed by the Lorentz model. Besides, all negative permittivity behavior in the test frequency range (10–1000 MHz) is found in CNTs/PPy nanocomposites with higher CNTs content which can be easily tuned by adjusting the CNTs loading. Negative permittivity is considered arising from the plasma oscillation of delocalized electrons in conductive networks and could be fitted by Drude model. Furthermore, the equivalent circuit models, ac conductivity and dielectric loss are discussed. Graphical abstract Image Highlights • A frequency switch of permittivity from positive to negative in pure PPy and 1 wt% CNTs/PPy nanocomposite was observed. • Negative permittivity behaviors were analyzed by Lorentz model and Drude model. • Equivalent circuit analysis was applied to manifest the existence of conductive networks inside the materials. [ABSTRACT FROM AUTHOR]

Carbonaceous materials served as functional units have trigged tremendous fundamental and practical interest in preparing electronic composites especially in meta-composites. In this paper, carbon functional nano-units were built by electrostatic self-assembly of carbon nanotubes (CNTs) and carbon black (CB). Three-dimension carbon networks hosted in CaCu 3 Ti 4 O 12 (CCTO) matrix were constructed by randomizing CNTs/CB nano-units. Negative permittivity was observed when increasing CNTs/CB nano-units contents exceeded the percolation threshold. The negative permittivity behavior considered as deriving from the low frequency plasma oscillation of delocalized electrons in carbon networks was analyzed by Drude model and equivalent circuit model. Besides, the electrical properties influenced by percolating phenomenon including ac conductivity, dielectric loss, and impedance were investigated. This work presents a novel meta-composites with negative permittivity built by CNTs/CB nano-units filling in low-temperature sintering CCTO matrix which extends the fields of meta-composites and will greatly facilitate its practical applications. [ABSTRACT FROM AUTHOR]

While metal is commonly used to construct electromagnetic metamaterials, herein carbon aerogel constitutes the first example of application as building blocks for metamaterials of low density. The carbon aerogel is synthesized by the sol-gel polymerization of resorcinol with formaldehyde followed by supercritical fulid drying and carbonization. The resulting nanostructure of carbon aerogel is adjusted by the concentrations of the reactants and catalysts in precursor solutions. Negative permittivity and negative permeability are simultaneously achieved in the low density carbon aerogel. The plasma-like negative permittivity is explained well by the low frequency plasmonic state in carbon networks. Equivalent circuit models and conductivity property indicate that permittivity values have heavy dependence on the framework and micropores of carbon aerogel. Negative permeability is attributed to the high-frequency magnetic response of induced circulating currents. The calculation analysis indicates that permeability values have a linear relation with ( ω ) 0.5 , showing a relaxation-type spectra unlike “magnetic plasma” of typical metamaterials. This work offers a novel and potentially powerful route for constructing metamaterials with ultralow density at the nanoscale, and can expand the potential applications of metamaterials especially under extreme conditions. [ABSTRACT FROM AUTHOR]

Metamaterials usually require that the unit size a should be comparable to the wavelength λ . Although the λ / a relationship could tell us what size of unit we need and which method we should choose for the fabrication, it limits the application of metamaterials in the kHz and MHz frequency range, as the unit size would be on the order of 10 2 m, making the overall size of the metamaterial too large for practical application. In this paper, we firstly demonstrate that the λ / a relationship limitation could be overcome by a new kind of composite, which we have called carbon-based ‘meta-composite’. A SiO 2 matrix with periodic microstructure is fabricated by using self-assembly of SiO 2 microspheres, and amorphous carbon fills in the gaps to form a three-dimensional periodic carbon network. The experimental results indicate that the carbon network will introduce tunable electromagnetic properties, which could be precisely tailored by controlling the geometric size of the carbon network. It is worth pointing out that the unit size of the periodic carbon network is on the sub-micrometre level, but the wavelength is on the order of 10 2 m. This means that the meta-composite overcomes the λ / a relationship limitation in the kHz and MHz frequency range, which shows great potential for the miniaturization of electronic devices. [ABSTRACT FROM AUTHOR]