Effective Dielectric Permeability Research Articles

Analysis of microwave heating of copper powder compacts

The influence of the native oxide films, which is essential for the understanding of the microwave heating of metal powder, is analyzed numerically based on the experimental data on temperature, dilatation and resistance of compacted copper powder samples. The indicative thickness of the oxide films, determined from the resistivity data using an effective medium approximation, is shown to decrease from ≥1 μm to below 1 nm. The effective complex dielectric permittivity and magnetic permeability of the copper powder with oxide films on the particles are calculated within the recently developed models. The dielectric permittivity exhibits a percolation behavior in the temperature range of the oxide decomposition, viz. 210–250 °C when microwave heating is carried out in nitrogen and 330–375 °C – in argon. The efficiency of absorption of the incident 30 GHz microwave radiation in a slab of powder is assessed separately for the electric and magnetic-type losses, the contribution of the latter being predominant until the percolation transition. The energy flux density of the incident microwave radiation required to sustain the prescribed heating rate is shown to increase during the microwave heating process from ∼20 W/cm2 to >1 kW/cm2 due to increasing reflection. The additional microwave energy input required for the completion of the endothermic oxide decomposition reaction is shown to be significant at the initial stage of the process (below 200 °C).

Designing finishing materials with a gradient of electrophysical properties

Тhe article discusses the basics of development and production of materials for shielding electromagnetic fields in a wide range of frequencies. The purpose of these materials is to cover large surface areas. The basic requirements for these types of materials have been established. The main ones are: the front surface must have electrophysical properties (dielectric and magnetic permeability) to ensure the lowest possible reflection coefficient of electromagnetic waves. At the same time, it is mandatory to simultaneously ensure the characteristics of strength, fire resistance, non-toxicity, etc. The content of radio-absorbing particles and effective dielectric (magnetic) permeability in the interlayer of the layered structure in the direction of growth of the substrate should ensure broadband and efficiency of the material. The dispersion dependence should ensure uniform absorption of electromagnetic energy and its passage from the input surface to the substrate in a given frequency range. Based on the maximum and minimum wavelengths of the shielding field, permeability and thickness of individual layers, calculations of the required thickness of the gradient material for a given reflection coefficient are given. Thanks to heat treatment of the surface of the material in the manufacturing process, the possibility of manufacturing monolithic metal-polymer screens with surface layers of low dielectric permeability is shown. The possibility of creating monolithic metal-polymer screens from ferromagnetic finely dispersed substances with an adjustable gradient in the direction from the front surface to the bottom is demonstrated, and a technical solution is given. This material can be used to control the ratio of shielding factors for high-frequency electromagnetic fields, ultra-low-frequency electric and magnetic fields, as well as concomitant shielding from natural magnetic fields.

Effect of chemical modifications and coating with Cu-based nanoparticles on the electro-physical properties of jute fabrics in a condition of high humidity

The electro-physical properties of raw, alkali and oxidatively modified jute fabrics were investigated as complex phenomena of the interactions between the fabrics’ chemical composition, carboxyl group content, crystallinity, structural characteristics, moisture sorption, and frequency of the electric field. At 80% relative air humidity, chemically modified jute fabrics have 38–179% and 1.7–5.4 times higher dielectric loss tangent and effective relative dielectric permeability compared to unmodified. To further improve these properties, fabrics were treated with CuSO4 and Cu-based nanoparticles were in situ synthesized on their surfaces by reduction. A few smaller agglomerates of Cu-based nanoparticles were observed across the alkali modified fabric’s surface, while smaller irregularly shaped and larger agglomerates were distributed over the oxidatively modified fabric’s surface. Regardless of the type of synthesized nanostructures (metallic Cu, copper oxide (Cu2O or CuO) or their mixtures), excellent fabrics’ effective relative dielectric permeability is guaranteed. Precisely, exploitation in specific conditions that contribute to copper reduction will make jute fabrics able to store 21–163 times more energy from an external electric field than before the exploitation, which will extend their lifetime. On the other hand, with increasing the total content of Cu after the reduction and formation of agglomerates of Cu-based nanoparticles, the movements of cellulose, hemicelluloses, and lignin molecules become difficult resulting in lower energy dissipation within the chemically modified than within unmodified fabric. Applied chemical modification and coating with Cu-based nanoparticles enables designing fabrics with predictable electro-physical properties, which is very important from the application point of view.

Effective Dielectric Permeability of a Composite with Matrix Ellipsoidal Inclusions

A composite material based on ellipsoidal inclusions distributed in a bonding medium is considered. A formula is obtained for calculating the effective dielectric constant of such a material, taking into account the presence of a layer of a binder on the inclusions, which prevents their direct contact. It is shown that the calculated values of the effective dielectric constant correspond to the experimental values for a composite based on sections of conductive fibers. Keywords: composite material, radar absorbing structure, diffraction of electromagnetic waves.

Effective dielectric permeability and electrical conductivity of polycrystalline PbTe films with disturbed stochiometry

This paper presents the study of the effective dielectric constant and electrical conductivity of polycrystalline PbTe films with disturbed stoichiometry. It is shown that superstoichiometric additions of Te and Pb within the limits of solubility by doping with PbTe contribute to a change in a wide range of concentrations of electrically active impurities and, consequently, to an increase in the dielectric constant (ɛ), electrical conductivity (σ) and absorption coefficient (α), in addition, the excess of these components strongly affects the amount of polarization.

Design of liquid composite materials for shielding electromagnetic fields

This paper reports the principles of design and the examined protective properties of liquid materials for shielding the electric, magnetic, and electromagnetic fields over a wide frequency range. The materials were made on the basis of iron ore concentrate and a pigment additive, with water-dispersed and geopolymer paints used as a matrix. The tests of protective properties for the electrical and magnetic components of the electromagnetic field of industrial frequency showed that the electric field shielding coefficients at a concentration of the screening substance of 15−60 % (by weight) equaled 1.1−8.6; magnetic field – 1.2−5.3. The shielding coefficients of the material based on a water-dispersed paint are lower than those of a geopolymer one, which can be explained by the oxidation of an iron-containing component and a decrease in electrical conductivity. The shielding coefficients of the electromagnetic field with a frequency of 2.45 GHz are 1.2−7.9. The highest coefficients are inherent in the material with filler made of iron ore concentrate and titanium-containing pigment powder in a ratio of 1:1. To design materials with the required (predictable) protective properties, the relative magnetic, dielectric permeability of materials was calculated. It is shown that the obtained data are acceptably the same as the results from direct measurements of magnetic and dielectric permeability and could be used to calculate the wave resistance of the material and the predicted reflection coefficient of electromagnetic waves. Thus, there is reason to assert the need to build a database on the frequency dependence of effective magnetic and dielectric permeability in order to automate the design processes of composite materials with predefined protective properties.

Generalized Formula for Effective Dielectric Permeability of the Medium with Ellipsoidal Inclusions

Generalized Formula for Effective Dielectric Permeability of the Medium with Ellipsoidal Inclusions

Homogenization of Maxwell\u2019s equations in a layered system beyond the static approximation

The propagation of electromagnetic waves through a disordered layered system is considered in the paradigm of the homogenization of Maxwell’s equations. Although the accuracy of the effective dielectric permittivity and/or magnetic permeability is still unclear outside the static approximation, we show that the effective wave vector can be correctly introduced even in high-frequency cases. It is demonstrated that both the real and imaginary parts of the effective wave vector are self-averaging quantities connected by the Kramers–Kronig relations. We provide a unified approach to describe the propagation and localization of electromagnetic waves in terms of the effective wave vector. We show that the effective wave vector plays the same role in describing composite materials in electrodynamics as the effective dielectric permittivity does in statics.

Homogenization method for one-dimensional photonic crystals with magnetic and chiral inclusions

We have theoretically investigated the electromagnetic properties for one-dimensional (1D) photonic crystals with magnetic and artificial chiral inclusions in the quasi-static limit, that is when the size of the unit cell of the crystal is small with respect to the wavelength of the operating wave. We suggest a homogenization theory to determine the effective tensors of the optical response, to achieve this objective, we apply the Bloch’s plane waves method to describe the electromagnetic modes that can propagate in the periodic structure under consideration. Subsequently, the Maxwell’s “microscopic” equations are homogenized replacing the Bloch waves by plane waves that attenuate the fast oscillations of the electromagnetic fields within the unit cell (macroscopic level), i.e., the average-macroscopic electromagnetic fields are determined by the component corresponding to the null reciprocal lattice vector in the expansion in plane waves. The numerical implementation of our theory of homogenization allow us to study the effective bianisotropic electromagnetic response (effective dielectric permittivity, magnetic permeability and electric-magnetic coupling tensors, this last is described by an effective chiral parameter) for 1D photonic crystals whose constituents in its unit cell are a dielectric layer and another magnetic (both isotropic and anisotropic) or chiral inclusion layer. The results are illustrated and discussed by the effective parameters as a function of the filling fraction of the inclusion and show for each case of homogeneous effective medium different components of anisotropy in the electromagnetic response of permittivity, permeability and chirality with the increase of the filling fraction. Besides, the behaviors of the obtained graphs agree well with Rytov’s formulas of effective medium. The relevant results of this theory will be very useful for the study and better understanding of the nature and design of metamaterials with predetermined anisotropic optical properties.

Microwave refraction coefficient in nanocomposites and nanostructures in magnetic field

Microwave refraction coefficient is studied for two types of composites and for metallic (CoFe)/Cu nanostructures. Nanocomposites of the first type are prepared on the base of artificial opal matrices with embedded particles of nickel-zinc ferrite and metallic cobalt. The introduced particles are sized from 5 to 60 nm. The composite material of the second type is prepared of permalloy flakes sized about 2 – 50 μm placed into epoxy amine matrix. At frequencies of centimeter and millimeter waveband the magnetic field dependences of transmission and reflection coefficients have been measured. The complex refractive coefficients are obtained as a function of magnetic field. The complex refraction coefficient can be expressed via the effective dielectric permittivity and magnetic permeability. Analysis of magnetic field dependences of refraction coefficient shows that strong variations are observed near ferromagnetic resonance. For metallic nanostructures there are also variations of the refraction coefficient caused by giant magnetoresistive effect.

On the formation of gap solitons in nonlinear electromagnetic metamaterials

A nonlinear (Kerr‐type) electromagnetic metamaterial, characterized by a double‐Lorentz model of its frequency‐dependent linear effective dielectric permittivity and magnetic permeability, is considered. The formation of gap solitons in the low‐ and high‐frequency band gaps of this metamaterial is investigated analytically. Evolution equations governing the gap solitons, of the form of a nonlinear Klein‐Gordon and a nonlinear Schrödinger equation, are obtained, and the structure of their solutions is discussed.

Research of electrodynamics characteristics of antennas in the form of an open end of a rectangular waveguide with partial dielectric filling and excitation pin

The electrodynamics characteristics of the antenna in the form of an open end of a rectangular waveguide with partial dielectric filling and a pin of excitation are studied. In the antennas in the form of an open end of a rectangular waveguide with a pulse drive and a pin excitation there is insufficient coordination of the input resistance of the pin with the wave impedance of the coaxial power line. It is proposed to reduce the influence of this disadvantage by taking into account the effective dielectric permeability, the size of the excitation pin and its displacement relative to the waveguide axis with the help of the obtained improved expression for calculating the conductivity of the coaxial-waveguide transition from the side of the rectangular waveguide. In accordance with the given method, the graphs of the active and reactive resistance of the excitation pin on its geometric dimensions and location are constructed, the normalized far field pattern of the antenna in planes E and H, which show the possibility of reducing the dimensions of the cross section of the waveguide for its constant electric size, as well as the improvement of coordination antennas in a defined frequency band.

Design and Analysis of a Novel LHM Structure Realized in Low-Frequency Band

The left-handed material (LHM) composed of a combination of magnetic and electrical resonator is an important method for designing the structure of the LHM. In this paper, a novel structure of LHM is designed and analyzed based on the traditional LHM structure. The parameter extraction method is applied to calculate the effective dielectric constant and effective magnetic permeability. The simulation results indicate that the effective dielectric constant and effective magnetic permeability can achieve negative value in a specific frequency band with about 200 MHz. It will provide important theoretical and practical guiding for designing the novel LHM structure with low-frequency band.

Influence of the alkali treatment on the sorption and dielectric properties of woven jute fabric

Woven jute fabric was treated with sodium hydroxide solution of different concentrations at room temperature, for different periods of time. After that, jute fabrics with gradually decreased content of hemicelluloses were obtained. The changes of the sorption properties (moisture sorption, water retention power and degree of fiber swelling) and dielectric properties (effective relative dielectric permeability, AC specific electrical conductivity and dielectric loss tangent) of alkali treated jute fabrics were investigated. After the alkali treatments, the degree of accessibility of the cell wall components to water vapor increased with increased severity of the alkali treatment. In parallel, the degree of fiber swelling and total water holding capacity of the fabrics were increased. The dielectric properties are very sensitive to fabric structural characteristics, chemical composition and its ability for moisture sorption. Thus, the obtained increase of the effective relative dielectric permeability after the alkali treatments can be attributed to the changes in the structural characteristics and decrease in the content of hemicelluloses, which further contributed to an increased ability for moisture sorption. The changes in the AC specific electrical conductivity can be explained by the fact that the hemicelluloses not only restrict the freedom of the water molecules to take part in the polarization process, they also change the structure in such a way that the mobility of the ions in the electric field is restricted. The values of dielectric loss tangent increased after the alkali treatments due to the increase in the number of polar groups.

Heterogeneity of microwave electromagnetic field inside of artificial magnetic crystal

The nanocomposite artificial crystals with embedded magnetic nanoparticles are obtained from opal matrices composed of the submicron SiO2 spheres. The introduced particles are sized from 5 to 60 nm. These artificial crystals contain metallic cobalt, nickel and iron particles, nickel-zinc ferrite and silver particles. The complex refractive coefficients at frequencies of centimeter and millimeter waveband are obtained as a function of magnetic field. A parameter, characterizing the heterogeneity of electromagnetic fields at different distances from a magnetic particle, has been calculated. The complex refraction coefficient can be expressed via the effective dielectric permittivity and magnetic permeability. Analysis of the space distribution of the heterogeneity parameter for the artificial crystal with metallic cobalt particles at 14 GHz in external magnetic field shows that very strong variations of the nonuniformity parameter are observed near ferromagnetic resonance (FMR), and much smaller variations for the fields exceeding FMR. In all cases the averaging over the distance in 2 – 3 times exceeding the lattice parameter of the artificial opal crystal leads to damping of strong oscillations.

A high performance and temperature sensitive absorber based on hybrid dielectric layers metamaterial

In this paper, a high performance hybrid metamaterial absorber, is proposed experimental preparation with two absorption peaks (80.1% and 82.2%). One of absorption peaks is excited by electrical mode resonance, and another absorption peak is originated from cavity mode resonance. Based on the S-parameter extraction method, the effective dielectric constant and permeability are calculated. It is seen that near perfect impedance matching is achieved at resonance wavelength 14.6 μm and 17.1 μm, respectively. In order to elucidate the effects of ambient temperature on the resonant absorption behaviors, a group of experiments is carried out. It is found that the absorption property of the metamaterial absorber is sensitive to ambient temperature change. The electrical peak is reduced from 80.1% to 22.3% when the measured ambient temperature reaches 92 °C, while the cavity resonance peak is reduced from 82.2% to 23.1%. Moreover, when the ambient temperature is higher than the critical temperature of the phase transition, a new electrical mode resonance is found between VO2 and metal layers, which results n a new absorption peak (43%) at resonance wavelength 13.4 μm.

Multifunctionality of particulate composites via cross-property maps

Closed-form analytical expressions for the effective electrical (thermal) conductivity and elastic moduli of a wide class of three-dimensional isotropic particulate composites (dispersions) for all phase contrasts and volume fractions have been previously formulated. These property estimates were extracted from exact strong-contrast expansions that incorporate complete microstructural information. In this paper, we employ these analytical expressions to derive and apply ``cross-property maps'' that connect combinations of pairs of the aforementioned effective transport and elastic properties to one another for a wide class of dispersions in which the inclusions can have different shapes and sizes and are spatially distributed in a matrix with varying degrees of order/disorder. We illustrate cross-property maps for a variety of cases (e.g., incompressible or compressible composites with conducting/insulating inclusions that may be rigid or compliant or auxetic) for high inclusion volume fractions. All of the cross-property maps that involve the effective conductivity translate immediately into equivalent results for the effective dielectric constant, magnetic permeability, or diffusion coefficient because they are mathematically analogous. We discuss an example that enables us to design a disordered dispersion with desired values of the effective dielectric constant and effective Young's modulus. Cross-property maps and their extensions will facilitate the rational design of particulate media with different desirable multifunctional characteristics. Moreover, our investigation has implications for the application of machine-learning and other data-driven approaches for multifunctional materials discovery.

Nonexponentially Decaying Electromagnetic Waves Localized at the Boundary of Anisotropic Metal–Dielectric Structures

The possibility of the existence of a special class of surface electromagnetic waves with amplitude decreasing nonexponentially as they pass into an anisotropic metal–dielectric structure that borders the isotropic dielectric has been shown. The dispersion equation describing the propagation of such special surface waves has been obtained, and their attenuation coefficients and formula for electric and magnetic fields in contacting media have also been found. The expressions for the longitudinal and transverse components of the Poynting vector and volume density of electric and magnetic energy of special surface waves have been obtained. Numerical calculations for the layered metal–dielectric nanostructure that is described by a uniaxial tensor of effective dielectric permeability have been carried out Decaying.

Modelling of electromagnetic properties of multicomponent material

Current theories of heterogeneous media consider non-uniform materials as natural and artificially synthesizable structures. Nowadays, synthesis of the non-uniform multicomponent materials with given electrodynamic properties and characterized by magnetic and dielectric permeability, is gaining increasing development. When modeling a multicomponent structure as a uniform material with effective dielectric permeability (ignoring the magnetic properties) using the developed models for the components with known dielectric permeability, the errors arise in calculation of the transmission coefficient of a plane wave through the antenna dome wall. We present a heuristic model based on the laws of optics which is intended for simultaneous determination of the effective magnetic and dielectric permeability of multicomponent material in contrast to known models describing statistically non-uniform media only for one electrodynamic parameter. The electrodynamic model developed for description of the effective magnetic and dielectric permeability of non-uniform material suggests a possibility of characterizing a polarized material with the total dipole moment arising in alternating field and expressing the Brewster angle as a the sum of the polarization angles proportional to volume content the mixture components.

Multipurpose nonwoven viscose/polypropylene fabrics: Effect of fabric characteristics on sorption and dielectric properties

ABSTRACTIn this work, sorption and dielectric properties of viscose/polypropylene multipurpose nonwoven fabrics were examined. The analysis of sorption behavior showed that the changes of the water absorptive capacity, the height of capillary rise and water retention value are in a function of viscose fiber content, total porosity, the pore size and used web bonding process. It is observed that dielectric properties at frequencies from 30 Hz to 140 kHz, for samples exposed to different relative air humidity and wet samples, are dependent on viscose fiber content, web bonding process, frequency of electric field and bulk free water content. The effective dielectric permeability of wet samples rapidly decreases with an increase in frequency up to 3 kHz while spectra of the AC specific electrical conductivity showed a plateau above 13 kHz. It is also observed that the dielectric properties of wet samples increase by several orders of magnitude compared to dry samples. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 947–957