Frequency Dispersion Behavior Research Articles

Effective medium approximation for the electromagnetic properties of rocks with surface conductivity

We present an approach to calculate the complex dielectric permittivity of a microheterogeneous rock composed of non-conductive solid grains with surface conductivity and a conductive liquid.We have calculated the effective electrical properties of a rock using the model that consider the complex structure of the conducting double layer between a solid grain and the electrolyte in the pores. The influence of two parts of double layer: the Stern (inner) layer on the solid surface and the diffuse (outer) layer was considered.Previously, the Differential Effective Medium (DEM) scheme was used to calculate the effective conductivity and dielectric permittivity. In contrast, we have adopted the Effective Medium Approximation (EMA) method for calculation of the effective electromagnetic properties of a rock with high inclusion concentration. This method allows one to describe both elastic and electromagnetic properties of the rock based on the unified model of the pore space.The calculations were performed both for the rock model with a fixed grain size and for the model with a fractal distribution of grain sizes.Our calculations have shown that the value of the dielectric permittivity in the low frequency range depends on the concentration and dimension of solid grains. However, the frequency-dispersion behavior is a function of the inclusion size only and it does not relate to the inclusion concentration in the porosity range typical for sedimentary rocks. This effect confirms the feasibility of the determination of the inclusion concentration and dimension by using the dielectric permeability and electrical conductivity dispersion curves.

Dielectric Terahertz Characterization of Microwave Substrates and Dry Resist

Microwave fabrication and design techniques are commonly employed in the terahertz (THz) domain. However, a characterization of commercially available microwave dielectric materials is usually lacking at sub-THz and THz frequencies. In this work, we characterized four substrates by Rogers and an Ordyl dry resist between 0.2 and 2 THz, in terms of relative permittivity and loss tangent. The reflectance spectra of the investigated materials were retrieved by means of THz time-domain spectroscopy in reflection mode and post-processed according to a transmission-line model in which the materials’ parameters are fit by means of the Havriliak–Negami variation of the Debye model. The relative permittivity of the investigated materials showed negligible frequency dispersion in the sub-THz and in the THz range. In terms of the loss tangent, the Rogers substrates revealed a more pronounced frequency-dispersive behavior among different materials, as dictated by the Havriliak–Negami model. The Ordyl resist was dispersive in the 0.2–1.2 THz range and presented a nearly constant loss tangent value between 1.2 and 2 THz. These results may represent a reference for the development of innovative components for THz and sub-THz emerging applications.

Bidirectional periodic pore structure Si-C-N multiphase ceramic with high thermostability and excellent microwave absorption properties over a wide temperature range

High-temperature electromagnetic wave (EMW) absorption materials are urgently needed for high thrust-to-weight ratio aero-engine. A novel EMW absorbing Si-C-N ceramic at elevated temperatures was fabricated by chemical vapor infiltration (CVI) Si3N4 and SiC on 2D plain carbon fiber preform forming periodic framing and then oxidation to remove carbon fiber. The three-level gradient periodic microstructure was in-situ constructed in the Si-C-N ceramics to meet the impedance matching characteristics and effectively attenuate as much as EMWs. Compared with traditional EMW absorption materials, the Si-C-N ceramics with bidirectional periodic pore had a lower density of 1.2 g/cm3 and better frequency dispersion behavior which could obtain an effective absorption bandwidth (EAB) covering the whole X-band at 2.45 mm and the lowest reflection coefficient (RCmin) of − 40 dB at 3.5 mm. The optimal Si-C-N ceramics (S2) show excellent EMW absorption performance over a wide temperature range. The Si-C-N ceramic had a promising applications at high temperatures.

Magnetic‐Driven Broadband Epsilon‐Near‐Zero Materials at Radio Frequency

AbstractEpsilon‐near‐zero (ENZ) materials, exhibiting unique physical characteristics such as near‐zero refraction, have aroused extensive interest and exhibit great potentials in novel applications of perfect absorbers, high‐harmonic generation, and nonlinear optical response. Here, for the first time, magnetic‐driven broadband ENZ materials are designed by fabricating polyvinyl alcohol (PVA)/Ni@carbon nanotubes (CNTs) films. Dielectric properties including real permittivity (ɛ′), imaginary permittivity (ɛ″), dielectric loss (tanδ), and impedance (Z) are investigated. When Ni@CNTs content reached 30 wt.%, negative permittivity transferred to positive permittivity at ≈11.5 MHz, and epsilon‐near‐zero (|ɛ′| < 1) is realized from ≈9 to 14 MHz, exhibiting broad ENZ bandwidth of ≈5 MHz. Theory calculations confirm that delocalized electrons are introduced from CNTs, which improve the carrier mobility and achieve low frequency dispersion behavior. Longer interfacial polarization electric fields between PVA and CNTs are also demonstrated by theory calculations, enhancing the positive permittivity response to offset negative permittivity response from Ni@CNTs. These two mechanisms result in broadband ENZ at radio frequency. This film also exhibits excellent magnetic actuation ability under magnetic field, broadening applications from ENZ materials to novel fields such as magnetically actuated robots with perfect absorption, magnetic‐driven biomimetic aircrafts with shielding ability, magnetic‐driven photodetectors, etc.

Observation of relaxor ferroelectric behavior and energy storage performances in Sr1.25Bi2.75Nb1.25Ti1.75O12 aurivillius ceramic synthesized by molten salt method

In this work, a novel three-layer Aurivillius phase Sr1.25Bi2.75Nb1.25Ti1.75O12 was synthesized using the molten salt method, and the crystal structure, morphology, electrical, optical, and energy storage properties were systematically investigated. XRD data supported by the Le Bail refinement technique shows that the product adopts a B2cb orthorhombic crystal structure. FTIR analysis indicates that the Nb5+/Ti4+ ions occupy the perovskite B-site and suggests that the two local environments on the B-site as Ti–O–Ti and Nb–O–Nb linkages. Ferroelectric ordering takes place below the ferroelectric Curie temperature, Tc of 100 °C, which is also revealed by the open ferroelectric hysteresis loops. The characteristic of relaxor-ferroelectric behavior is characterized by the broadened phase transition peaks and the frequency-dispersive behavior. The energy storage density of 12.69 mJ/cm3 with an energy efficiency (η) of 42.1% was obtained in the ceramic sample at applied electric field of 75 kV/cm and room temperature. These findings will provide an important contribution to further research to explore new Aurivillius compounds as potential energy storage materials.

Multiscale structure and interface engineering of Fe/Fe3C in situ encapsulated in nitrogen-doped carbon for stable and efficient multi-band electromagnetic wave absorption

Most reported electromagnetic wave absorption (EWA) materials show significant effective absorption in a certain frequency range, but their performances deteriorate dramatically as the frequency changes. As the range of working frequencies for electronic devices is gradually widening, it is of great interest to explore frequency-insensitive EWA materials that can achieve efficient absorption in every waveband by simply changing the absorption thickness. To this end, a multi-scale absorber (Fe/Fe3C@NC) is rationally synthesized by chemical foaming and in-situ growth strategy. By controlling the growth of carbon nanotubes, the Fe/Fe3C@NC-2 exhibits a well-constructed 3D multi-scale architecture. Thanks to dipole polarization, interface polarization and magnetic-dielectric energy conversion, the Fe/Fe3C@NC-2 overcomes the frequency dispersion behavior and keeps a stable dielectric attenuation capability across the entire frequency range. Consequently, it delivers a superb full-band absorption of -50.1, -59.83, -55.87 and -51.91 dB in the S, C, X and Ku bands, respectively. The maximum radar cross-sectional reduction reaches 35.44 dB m−2 when the incident θ is 20°, testifying its impressive performance. Surprisingly, this EWA material also shows a remarkable resistance to oxidation and corrosion derived from the tightly coated carbon layers. This work provides new insight into the design of multi-band and stable EWA materials for practical application.

Temporal rainbow scattering at boundary-induced time interfaces

Since the dawn of modern optics and electromagnetics, the optical prism is one of the most fascinating optical elements for refracting light. Exploiting its frequency dispersive behavior, a prism is able to refract different frequencies in different directions, realizing polychromatic light rainbows. Recently, thanks to their engineerable electromagnetic response, metamaterials have been exploited for achieving novel refractive scattering processes, going beyond the classical prism effects. In this Letter, we report on a rainbow-like scattering process taking place at the interface of a boundary-induced temporal metamaterial realized by instantaneously opening the boundary conditions of a parallel plate waveguide. Changing abruptly the conductivity of one of the two metallic plates, we demonstrate that an equivalent temporal interface between two different media is realized, and the monochromatic wave propagating into the waveguide gets scattered into a polychromatic rainbow in free space. We derive the relationships between the waveguide mode and the raising rainbow in terms of scattered amplitude and frequencies as a function of the elevation angle with respect to the waveguide axis. We apply the underlying physics to control the temporal rainbow by imposing a principal direction of scattering by design. Full-wave numerical simulations are performed for computing the rainbow temporal scattering and verifying the design guidelines for achieving controlled temporal rainbow scattering.

Dielectric Characterization and Modeling of Composite Materials Based on Epoxy Resin/Black Iron Oxide/Titanates in Several Frequency Ranges

The purpose of this article is to study the dielectric dispersion of two different ternary composites made of epoxy resin (RE), black iron oxide (Fe3O4) with one of the two titanates (calcium titanate (CaTiO3) or strontium (SrTiO3)) on several frequency bands. Additionally, the effect of the two titanates and the (Fe3O4) on the permittivity and conductivity of these ternary composites is investigated. These composite materials were characterized using time domain spectroscopy (TDS). The inclusion of the two titanates increased the real permittivity and conductivity of the two composites, shifting the resonance frequency (ƒR) towards the low frequency range and causing the opposite phenomenon for the static permittivity (ε s). The frequency dispersion behavior model for the complex permittivity has been proposed to improve the effectiveness of the predictive frequency model through a better choice of the damping factor and to bring theoretical and experimental findings closer together. Comparing these data shows that the proposed model is applicable to ternary combinations with high accuracy. The values of the quality factor (Q) obtained are encouraging in microwave applications. These composite materials witch containing CaTiO3 and SrTiO3 inclusions have contributed to the development of dielectric permittivity that suits very well frequency communication systems.

ENZ-metamaterial–induced perfect transparency for the highly directive plano-concave lens

We propose an ENZ-metamaterial–based transparent sandwich design consisting of periodic slits in a metal screen integrated with ENZ metamaterials for the exploration of superior transmissions. An equivalent circuit is proposed to reveal the interactions between the slit array and ENZ metamaterials when electromagnetic fields are penetrating through, and indicates that the involvement of ENZ metamaterials with the frequency dispersion behaviors can create the perfect match for total transmissions. On this basis, we further propose the design of plano-concave lens with the non-uniform ENZ metamaterial interlayer and demonstrate the superior collimation and focusing of electromagnetic fields at specific frequency points with perfect transparency. These findings may initiate the quest for building up more advanced electromagnetic devices tailored to specific applications with extreme parameter metamaterials.

Defect‐Engineered Graphene/Si3N4Multilayer Alternating Core‐Shell Nanowire Membrane: A Plainified Hybrid for Broadband Electromagnetic Wave Absorption

AbstractTo tackle the increasing electromagnetic pollution, broadband electromagnetic wave (EMW) absorption materials are urgently needed. Toward this goal, traditional strategies resort to the construction of multicomponent dielectric/magnetic hybrid materials, including ternary, quaternary, or even more complicated systems. However, they always suffer from many intrinsic drawbacks in practical applications. Herein, a theory‐directed strategy is presented to design plainified EMW absorption materials (binary hybrids) via amplified interface effects, which are based on well‐designed multilayer alternating core‐shell nanostructures by chemical vapor deposition (CVD). A defect‐engineered CVD graphene (DG) core composed of graphitic open edges and in‐plane defects is used as a lossy phase. Correspondingly, a CVD Si3N4layer with nanometer thickness is used as an impedance matching shell. By optimizing the alternating numbers of DG/Si3N4units, enhanced interface polarization and strong frequency dispersion behavior of permittivity (especially the real part, ε′) are obtained, which helps the plainified binary hybrids to reach an effective absorption bandwidth (EAB) of 8.0 GHz at a thickness of 2.7 mm. Moreover, these plainified hybrids show excellent thermal and pH stability. Even after 1000 °C oxidation, for example, an EAB of 7.44 GHz coupling with a minimum reflection coefficient of −77.3 dB is still achieved.

Flexible multi-walled carbon nanotubes/polyvinylidene fluoride membranous composites with weakly negative permittivity and low frequency dispersion

Flexible composites with negative permittivity have great potentials in flexible electronics and wearable devices. To improve their working performance, it is critical to search for such materials that possess weakly and stable negative permittivity in a wide frequency range. In this work, the polyvinylidene fluoride (PVDF) was selected as the flexible matrix and the modified multi-walled carbon nanotubes (MWCNTs) were used as functional fillers to prepare MWCNTs/PVDF membranous composites with a weakly negative permittivity. When the content of MWCNTs exceeded 13 wt%, the negative permittivity conforming to Drude model appeared. The absolute value of negative permittivity can be as low as approximately 10 in the whole test frequency range, presenting a low frequency dispersive behavior. It demonstrated that the weakly negative permittivity was ascribed to the dilution of electrons in the resulting composites, and the high electron mobility in MWCNTs was responsible for the low frequency dispersion. The MWCNTs/PVDF membranous metacomposites with a twisted and foldable flexibility can contribute to the development of flexible metamaterials and enrich their significant applications.

Modulation in Electric Conduction of PVK and Ferrocene-Doped PVK Thin Films

In this article, the dielectric properties of poly (9-vinylcarbazole) (PVK) and ferrocene-doped PVK thin films are studied. The thin films were grown by the isothermal solution casting technique. Dielectric properties of grown films were studied as function of ferrocene concentration, frequency, and temperature. The relative permittivity (ε′) is increased with increasing ferrocene percentage (~1%) due to the free charge carriers. The relative permittivity decreases for higher ferrocene percentage (~2%). However, the relative permittivity of PVK and ferrocene-doped PVK samples remains almost constant for studied temperature range (313–413 K). The frequency dependence of tan δ for all samples is studied. The frequency dependence of dielectric parameter exhibits frequency dispersion behavior, which suggests all types of polarization present in the lower frequency range. The loss tangent (tanδ) values are larger at higher temperatures in the low frequency region. However, the tan δ values at different temperatures are almost similar in the high frequency region. It is observed that the relative permittivity is maximum, dielectric loss is minimum, and AC conductivity is minimum for 1% ferrocene doped PVK as compared to pure PVK and 2% ferrocene doped PVK samples.

Efficientenergy‐conservativedispersive transistor modelling usingdiscrete‐timeconvolution and artificial neural networks

AbstractThis paper describes an efficient simulation technique for field‐effect transistor devices exhibiting significant frequency dispersive behavior. The device model is formulated such that a single energy function completely describes the channel displacement current, thus imposing charge and energy conservations principles from the outset. It is shown that adherence to energy conservation necessarily implies a reciprocal intrinsic displacement network, which is typically too rigid to follow measured device data. This problem is overcome by considering a frequency dispersive time‐delay parameter, which has the effect of allowing nonreciprocal susceptance behavior to be captured by the overall device model. This dispersive time‐delay parameter is included within the nonlinear device simulation via an efficient discrete‐time convolution.

Optical and dielectric spectroscopic analysis of SmMn2O5

Polycrystalline SmMn2O5 has been synthesized by using sol-gel auto combustion tecnique. X-ray diffraction (XRD) pattern reflects the single phase formation of orthorhombic structure with Pbam space group. In the frequency range of (30–1000 cm−1) reflectivity spectrum reveals the 14 infrared active phonons out of 36, which are already pridicted theoretically. We have estimated the optical energy band gap (E g ) ≃ 1.5 eV from UV–visible absorption spectrum. Temperature dependent (293-533 K) DC electrical characterization up to 40 V suggests semiconducting nature of the material. The temperature dependent impedance spectroscopy within a broader frequency range of (20 Hz—3 GHz) shows decreasing behavior of dielectric constant and tangent loss while AC conductivity increasing with the rise in frequency and temperature suggest the strong frequency dispersion and relaxational behavior of the material. The activation energy calculation from AC conductivity is in good agreement with semiconducting material energy band gap.

Fractional-order electric double-layer capacitors with tunable low-frequency impedance phase angle and energy storage capabilities

In this study, we report the electrical response of two sets of solid-state fractional-order electrochemical capacitors for which the low-frequency impedance phase angle can be tuned from −69° to −7°. The configuration makes use of a gel electrolyte in which carbonaceous additives (graphite or reduced graphene oxide) are dispersed at different proportions. Such a disordered electrolyte structure results in subdiffusive charge transport and thus a frequency dispersive capacitive-resistive behavior typical of a constant phase element, which can be useful for both frequency applications and energy storage purposes.

Two‐dimensional Ti3C2Tx/poly(vinylidene fluoride) metacomposites with weakly negative permittivity

AbstractIn recent years, random metacomposites with tunable negative permittivity have aroused widespread attention because of their distinctive properties and broad potential prospects. It is indicated that an enormously negative permittivity with a large value is not conducive to performance optimizing and impedance matching. In this article, the 2D Mxene Ti3C2Tx was selected as the conductive phase instead of metal and carbon materials to fabricate Ti3C2Tx/poly(vinylidene fluoride) metacomposites. The results of dielectric properties indicated that the conductive Ti3C2Tx networks were formed by Ti3C2Tx contacting with each other near the percolation threshold (ƒc). The as‐obtained metacomposites containing 75 wt% Ti3C2Tx generated percolating phenomenon. Additionally, when the fraction of Ti3C2Tx was below ƒc, the frequency dispersion behaviors of conductivity were in accordance with the Jonscher's power law, which proved that conducting mechanism was dependent on hopping conduction. While above ƒc, the conductivity conformed to the Drude model and Lorentz model, which demonstrated a conductive behavior. Furthermore, the real part of permittivity transformed to negative value at 75 wt%, which can be explained as the existence of the low‐frequency plasma oscillation and electric‐dipole‐induced resonance. The dielectric loss of the metacomposite was also analyzed by the imaginary permittivity spectra. Small negative permittivity values in the range of −550 to −400 were observed, owing to the low free carrier concentration in composites. The appearance of weakly negative permittivity opened up a new direction in negative dielectric materials by 2D layered Ti3C2Tx nanomaterials.

Study of the relaxation and resonance behaviors of ternary composites: Epoxy–strontium titanate–carbon black

The principal subject of the present article is the study of the phenomenon of dispersion as well as the effect of the concentration of strontium titanate (SrTiO3) and carbon black on the complex permittivity of ternary composites: epoxy resin–SrTiO3 –black carbon. The relative permittivity of the mixtures as a function of volume fraction of SrTiO3 was modeled by the modified Lichtenecker mixing law (MLL). A new dispersion model, based on the Lorentzian resonance model, has been proposed to describe the frequency behavior of complex permittivity. For these ternary composites, the frequency dispersion behavior of the complex permittivity that exhibits both relaxation and resonance spectra with increasing SrTiO3 concentration has been showed. The new empirical equation proposed in our work has been well describing the complex permittivity of resonance type for the SrTiO3 and carbon black composites. The effects of SrTiO3 content on the electromagnetic properties and absorption characteristics of electromagnetic waves of epoxy resin composites were studied. As the volume fraction of SrTiO3 increases, it was confirmed that the complex permittivity of the composites follows the MLL and the resonant frequency shifted toward the high frequency range. The resonance frequency of the composites was estimated in good agreement with the theoretical values calculated by the second new equation proposed in this article. Complex permittivity is measured using time domain spectroscopy in the frequency range direct current (DC) to 30 GHz.

Ferroelectric and Multiferroic Properties of Single-Phase Yb2NiMnO6 Double-Perovskites

Here we present the potential for single-phase Yb2NiMnO6 as a multiferroic 2D material with coupled ferroelectric/magnetic orders synthesized via solid-state reaction. Realistically large room- to elevated-temperature stable switching polarization reversal loops confirm the elevated temperature dense data bit storage material. Temperature-stable ferroelectricity and fine polarization recorded at room temperature and read at 200°C confirm elevated temperature dense data storage. The physics behind the coexistence of coupled orders is the focus of this work. Realistically large ferroelectric polarization loops produced at ±400 kV/cm confirm saturation polarization (PS) at around 20 μC/cm2. Microstructural and compositional analysis reveals that the ferromagnetic signature is generated from single-phase Yb2NiMnO6 with a confidence level of 99.95%. Rietveld refined x-ray diffraction reveals that Yb2NiMnO6 is a single-phase material displaying a GdFeO3-type orthorhombic structure with distorted (Ni/Mn)O6 (pnma; a = 5.5519 Ǻ; b = 5.1851 Ǻ; c = 7.4396 Ǻ; V = 214.18 Ǻ3). X-ray photoelectron spectroscopy confirms that Yb2NiMnO6 constituting the Mn2P3/2 core level appears at ∼642.2 eV and an Ni2P3/2 peak appears at ∼855.3 eV, displaying +4 and +2 valence states. Frequency dispersion and scaling behaviour in dielectric (e′) and dielectric loss (tanσ) indicate that the relaxation mechanism is independent of temperature. The σDC versus 1/T slope confirms that the hopping mechanism is responsible for charge carrier transport. The complex impedance reveals that the relaxation/conduction is the bulk effect arising from the semiconductor grains displaying grain resistance and capacitance with respect to temperature (113–363 K) and frequency (1–1 × 107 Hz). We have correlated and presented the effect of the microstructure on the electrical properties using an equivalent circuit (RgQg) (RgbQgb) (RcQc) at low temperature and changing to (RgbQgb) (RcQc) on high temperature. MS, HC and MR extracted from the ferromagnetic hysteresis loop are 11.75 emu/g, 180 Oe and 4.15 emu/g at room temperature (300 K). The zero-field-cooled and field-cooled magnetization (ZFC–FC) curve confirms the ferromagnetic nature. From these results, we show that Yb2NiMnO6 is multiferroic, with potential for application in four-state logic memory, ferroelectric random access memory and ferroelectric/magnetic sensors.

Weakly negative permittivity and low frequency dispersive behavior in graphene/epoxy metacomposites

As a promising substitute for periodical metamaterials, random composites with tunable negative electromagnetic parameters, which are termed as metacomposites, have drawn extensive attention owing to their unique properties and great potential applications. It is significant to decrease the values of negative permittivity and attain weakly negative permittivity with low frequency dispersive behavior, when searching for substitutable metacomposites. In this paper, the absolute value of negative permittivity in graphene/epoxy resin composites was dramatically decreased to approximately 6 by controlling the content of graphene and the microstructure of the resultant composites. Meanwhile, the permittivity spectrum showed low frequency dispersive behavior, which kept steady in a wide scope of frequency. It is indicated that the independence of negative permittivity on frequency results from the outstanding electron mobility of graphene. Moreover, it is demonstrated that the positive permittivity is ascribed to capacitive character, and the inductive character results in negative permittivity.

Sparse Wavenumber Recovery and Prediction of Anisotropic Guided Waves in Composites: A Comparative Study.

Guided wave methodologies are among the established approaches for structural health monitoring (SHM). For guided wave data, being able to accurately estimate wave properties in the absence of ample measurements can greatly facilitate the often time-consuming and potentially expensive data acquisition procedure. Nevertheless, inherent complexities of the guided waves, including their multimodal and frequency dispersive nature, hinder processing, analysis, and behavior prediction. The severity of these complexities is even higher in anisotropic media, such as composites. Several methods, including sparse wavenumber analysis (SWA), have been proposed in the literature to characterize guided wave propagation by extracting wave characteristics in a particular medium from the information contained in a few measurements, and subsequently using this information for full wavefield prediction. In this paper, we investigate the efficacy of guided wave reconstruction techniques, based on SWA, for predicting the behavior of guided waves in composite materials. We implement these techniques on several experimental and simulation data sets. We study their performance in estimating the frequency-dependent (dispersive) and anisotropic velocities of guided waves and in reconstructing full wavefields from limited available information.