Magnetic Permittivity Research Articles

Радиопоглощающие и радиоэкранирующие характеристики феррит-полимерных композитов Mn-Zn феррит/П (ТФЭ-ВДФ)

The article discusses the electromagnetic absorbing and shielding properties of ferrite-polymer composites of the composition Mn-Zn ferrite/fluoroplast-42, obtained by pressing a mixture of powders with heating. The measurement of the complex magnetic and dielectric permittivity spectra, as well as the reflection coefficient spectra was carried out in the frequency range 0.1 - 7 GHz. Using the obtained spectra, a comprehensive analysis of the absorbing characteristics of the composites was carried out, and the factors responsible for the absorption were determined. Fitting of the composites magnetic permeability spectra show that the process of natural ferromagnetic resonance prevails over the resonance of domain walls, and a decrease in the concentration of ferrite inclusions leads to a significant shift in the frequency of natural ferromagnetic resonance to high frequencies. It was found that for composites with a thickness of 5 - 10 mm, compositions with a mass fraction of ferrite ≤ 0.4 show radio-absorbing properties, while compositions with a fraction of ≥ 0.6 show shielding properties.

Phantom Construction and Equipment Configurations for Characterizing Electrical Properties Using MRI.

Phantom objects are commonly employed in MRI systems as stable substitutes for biological tissues to ensure systems for measuring images are operating correctly and safely. For magnetic resonance electrical impedance tomography (MREIT) and magnetic resonance electrical property tomography (MREPT), conductivity or permittivity phantoms play an important role in checking MRI pulse sequences, MREIT equipment performance, and algorithm validation. The construction of these phantoms is explained in this chapter. In the first part, materials used for phantom construction are introduced. Ingredients for modifying the electromagnetic properties and relaxation times are presented, and the advantages and disadvantages of aqueous, gel, and hybrid conductivity phantoms are explained. The devices and methods used to confirm phantom electromagnetic properties are explained. Next, different types of MREIT electrode materials and the constant current sources used for MREIT studies are discussed. In the last section, we present the results of previous MREIT and MREPT studies.

Поглотители электромагнитного излучения СВЧ-диапазона на основе частиц Fe и Fe – Al, полученные в поле вращающихся магнитных диполей

The purpose of the work is to obtain microwave absorber materials based on Fe – Al particles and to study its absorbing characteristics. Composite materials consisting of Fe, Fe – Al particles and a paraffin matrix were obtained by rotational magnetic field method. The mechanism of the field of rotational magnetic dipoles usage for Fe particles coating with Al nanoparticles and further composite material synthesis was described. Microstructure, mechanical, magnetic and dielectric properties and parameters of microwave absorption of obtained composite materials were investigate. Magnetic and dielectric permittivity in the frequency range 8 – 12 GHz were obtained. The influence of the field of rotational magnetic dipoles in combination with Al nanoparticles allows to obtain more durable structure in comparison with composite materials without Al nanoparticles. It was found that composite materials have a good radio-absorbing properties in the studied frequency range. The maximal value of S21 = –27,9 dB was obtained on 12 GHz frequency for a Fe – Al composite material with the field of rotational magnetic dipoles application.

Investigation of the parameters of the reflected wave near the Brewster angle

In free space, the relative permittivity is determined by the Brewster formula without taking into account dielectric and magnetic losses. In experimental studies, discrepancies in the angular position of the minimum of the reflected wave from dielectric materials are observed in comparison with calculations, which are known as deviations from Fresnel’s laws. By solving the task of inclined falling wave on an plate made of a dielectric material with complex of the dielectric and magnetic permittivity, the parameters of the reflected wave were calculated, according to which the angles corresponding to the minimum reflection were determined, depending on the dielectric losses of the material. From the condition that the reflected wave is equal to zero, a formula for determining the Brewster angle for a material with dielectric and magnetic losses was analytically obtained, the results of calculations for which coincided with the calculations for the reflected wave in the context of geometric optics. It is determined that in the general case, the conditions for determining the position of the minimum of the complex amplitude and the phase jump by 180° of electromagnetic waves do not coincide and can be found only when solving the task an falling wave on a plate with complex electrodynamic parameters of the material in the context of geometric optics.

Determination of the Brewster angle of the wave reflected from a plate with dielectric losses

In free space, the permittivity of materials is usually determined by the value of the Brewster angle using the angular dependences of the amplitude and phase of the wave reflected from the material plate. An expression corresponding to materials without dielectric and magnetic losses is used as a calculation model. Experimental studies of the parameters of the wave reflected from dielectric materials show the discrepancies with theoretical calculations known as deviations from the Fresnel laws. We present the results of determining the Brewster angle of the wave reflected from a plate made of a material with dielectric losses. The angular dependences of the amplitude and phase of the reflected wave were calculated using the numerical solution of the problem of falling at an arbitrary angle of a plane linearly polarized wave with an electric field vector lying in the plane of incidence on a plate of a dielectric material with complex values of the dielectric and magnetic permittivity. They were used to determine the angles corresponding to the minimum reflection coefficient depending on the dielectric losses of the plate material. The differences between the numerical calculations and the data obtained using the Brewster angle formula were noted, which increased with increasing dielectric losses of the material. From the condition that the modulus of the reflected wave amplitude is equal to zero, a different formula for calculating the Brewster angle for a material with losses is analytically obtained. The results of calculations using this formula coincided with the calculations for the reflected wave when solving the classical problem of the inclined incidence of a plane wave on a plate of a dielectric material in the framework of geometric optics. The results obtained can be used to determine the Brewster angle for a wave reflected from a plate with magnetic and dielectric losses.

Microstructural, magnetic and dielectric performance of rare earth ion (Sm3+)-doped MgCd ferrites

The combined effects of Sm3+ substitution together with the addition of 3 wt% Bi2O3 endow MgCd ferrites with excellent magnetic permeability and dielectric permittivity. Various concentrations of Sm3+ (x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) were employed to modify the permeability (μ′) and permittivity (ε′) of the MgCd ferrites. X-ray diffraction, scanning electron microscopy (SEM), vibrating sample magnetometry and vector network analysis techniques were used to characterize the samples. The measurement results reveal that the ferrites processed a saturation magnetization of up to 36.8 emu/g and coercivity of up to 29.2 Oe via the conventional solid-state reaction method. The surface morphology SEM confirms that with increasing Sm3+ concentration, the grain shape changes from a polygon to a circle. Moreover, the dielectric permittivity can reach a value of 23. The excellent properties obtained in Sm3+-substituted Mg ferrites suggest that they could be promising candidates for modern high-frequency antenna substrates or multilayer devices.

Electromagnetic characteristics of materials based on ultrafine hexaferrite powders

The electromagnetic characteristics of composite materials based on nanosized powders of W-type hexaferrites are considered in the article. It is shown that not only the composition, but also the mechanical treatment affects the electromagnetic parameters. This article presents the frequency dependence of the complex magnetic and dielectric permittivity of a system of W-type hexafferites. The studies were carried out on a universal wide-band measuring complex based on the Agilent PNA-X N4257A Vector Network Analyzer. The results are presented in the frequency range from 2 to 14 GHz.

Enhancement of magnetic and dielectric properties of low temperature sintered NiCuZn ferrite by Bi2O3–CuO additives

With a series of 1.0 wt%Bi2O3–x wt% CuO (x = 0.0, 0.2, 0.4, 0.6, and 0.8) serving as sintering additives, Ni0.23Cu0.32Zn0.45Fe2O4 ferrites are successfully synthesized at a low temperature (900 °C) by using the solid state reaction method. The effects of the additives on the phase formation, magnetic and dielectric properties as well as the structural and gyromagnetic properties are investigated. The x-ray diffraction (XRD) results indicate that the added Bi2O3–CuO can lower the synthesis temperature significantly without the appearing of the second phase. The scanning electron microscope (SEM) images confirm that Bi2O3 is an important factor that determines the sintering behaviors, while CuO affects the grain size and densification. With CuO content x = 0.4 or 0.6, the sample shows high saturation magnetization, low coercivity, high real part of magnetic permeability, dielectric permittivity, and small ferromagnetic resonance linewidth (ΔH). The NiCuZn ferrites are a promising new generation of high-performance microwave devices, such as phase shifters and isolators.

Effects of sintering temperature on the microstructure, electrical and magnetic characteristics of copper-zinc spinel ferrite with possibility use as humidity sensors

In this paper was investigated the effect of sintering temperature on a copper-zinc spinel ferrite, Cu0.5Zn0.5Fe2O4, synthesised by using a sol-gel self-combustion method, as material with possible to application in the field of humidity sensors. In order to highlight the influence of temperature treatment on the structure, electrical and magnetic properties, the powders of ferrite were annealed at various temperatures: 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C. For the analyzed samples, the X-ray diffraction investigations have revealed the phase composition and the average crystallite size. By using techniques scanning electron microscopy (SEM) and atomic force microscopy (AFM) were find the influence of the sintering temperature on some parameters like the porosity, homogeneity, shape and size of the crystallites. We have also investigated the temperature variation of the electrical conductivity, relative magnetic permeability and relative permittivity variation with frequency at room temperature. Finally, we analyzed the time response, variations of electrical conductivity and relative permittivity for various humidity levels and the perspectives of applicability copper-zinc spinel ferrite as humidity sensor.

Broadband and tunable high-performance microwave absorption properties by Ni-coated carbon fibers

Composites of carbon fibers (CFs) and magnetic metals have concerned attention for tunable microwave absorption materials (MAMs). In this work, magnetic nickel (Ni) particles were coated on the CFs surface by an electroplating technique. Different morphology of Ni-coated CFs was established by setting the plating time. The microstructure, electric and magnetic properties, complex permittivity and permeability, and reflection loss (RL) of the Ni-coated CFs were specified as a function of the surface morphology of composites. SEM and EDX results displayed that Ni particles are uniformly deposited on the surface of CFs. Moreover, the Ni-coated CFs with different coating morphologies exhibit different microstructure. The creation of a magnetic Ni layer onto CFs improved composite properties, not only raising the saturation magnetization (17–46 emu/g) but also enhancing electrical conductivity (361–594 S/cm). The electromagnetic parameters and RL values of the Ni-coated CFs were investigated over the 2.0–18.0 GHz frequencies. The minimum RL could reach up to −64.8 dB at 14.9 GHz with 2.0 mm matching thickness and high effective bandwidth of 2.2–18.0 GHz. It has been displayed that the properties of Ni-coated CFs can be effectively controlled by changing the morphology of the Ni layer, which provides a novel route to fabricate Ni-coated CFs with tunable and broadband microwave absorption properties.

Nanocomposites FeCoNi/C based on polyvinyl alcohol: synthesis and electromagnetic properties

Triple FeCoNi nanoparticles distributed and stabilized in the carbon matrix of FeCoNi/C metal-carbon nanocomposites were synthesized. The synthesis of nanocomposites was carried out by controlled IR pyrolysis of precursors of the "polymer-metal nitrates" type, obtained by joint dissolution of the components with subsequent removal of the solvent. The effect of the synthesis temperature on the structure, composition, and electromagnetic properties of nanocomposites has been studied. It was shown by XRD that the formation of ternary FeCoNi nanoparticles occurs through the dissolution of Fe in the nanoparticles of the NiCo solid solution. With an increase in the synthesis temperature, the size of metal nanoparticles increases, which is determined by the processes of their agglomeration and coalescence during matrix rearrangement. Also, depending on the synthesis temperature, nanoparticles of a ternary alloy with different compositions can be formed, and the ratio of metals specified in the precursor is achieved at 700 °C. By Raman spectroscopy was shown that, with an increase in the synthesis temperature, the degree of crystallinity of the carbon matrix of nanocomposites increases, and graphene structures consisting of several layers can be formed. The frequency dependences of the relative complex dielectric and magnetic permeabilities of nanocomposites in the range of 3–13 GHz were studied. It is shown that an increase in the synthesis temperature causes a significant increase in both dielectric and magnetic losses (~ 2 times). The former are associated with the formation of a complex nanostructure of the carbon matrix of the nanocomposite, while the latter are determined by an increase in the size of nanoparticles and a shift of the EFMR frequency to the low-frequency region. Reflection loss (RL) calculations were performed according to the standard procedure based on experimental data of the frequency dependences of the complex magnetic and dielectric permittivity. It was shown that control of the frequency range and absorption value of electromagnetic waves (from 50 to 94%) can be carried out by changing the temperature of synthesis of nanocomposites.

The effect of alloying additives on the magnetic permeability and permittivity of ferrite spinel

Coatings made of the materials that effectively absorb radiation, e.g., ferrite materials, are used to reduce the level of electromagnetic radiation in rooms containing household or industrial equipment. It is known that significant dissipation of the radiation energy is provided by the thickness of the shielding coating which should be comparable to the length of the electromagnetic wave in the material which, in turn, significantly decreases at high values of the magnetic permeability and permittivity of the radio-absorbing material. Ferrite radio-absorbing coatings are characterized by the high heat resistance, low flammability and small (10 – 20 mm) thickness. However, at frequencies less than 40 MHz, plates with a thickness of more than 30 mm are to be used to provide the effective absorption, and the weight and cost of the coatings increase significantly. The results of studying the effect of the sintering temperature and micro-additives of titanium, calcium and bismuth oxides on the dielectric constant of Ni- and Mn-Zn radio-absorbing ferrites are presented. Reactively pure starting oxide components with a basic substance content of more than 99.6 % wt. were used to synthesize samples using traditional oxide technology. It is shown that alloying with bismuth and titanium oxides is rather effective for obtaining radio-absorbing ferrites with a combination of high values of the magnetic permeability and dielectric permittivity. The obtained results can be used in production of ferrite radio- absorbing materials operating in the megahertz range.

Properties of Nickel Zinc Ferrite Nanopowders Prepared by Different Methods

We have studied the electronic and electromagnetic properties of synthesized Ni0.5Zn0.5Fe2O4 nickel-zinc ferrite nanopowders. The Ni0.5Zn0.5Fe2O4 materials prepared by a pyrochemical nitrate–urea process and a sol–gel nitrate–citrate process have been shown to differ in microstructural, electronic, and electromagnetic characteristics. In the frequency range 1–7 GHz, the Ni0.5Zn0.5Fe2O4 ferrite prepared by the pyrochemical nitrate–urea process, with an average nanoparticle size of 33 nm, has a higher magnetic permeability, dielectric permittivity, and magnetic loss tangent than does the Ni0.5Zn0.5Fe2O4 material with an average nanoparticle size of 46 nm, prepared by the sol–gel nitrate–citrate process.

Prospects for the use of ferrites with high magnetic permeability and permittivity as radio-absorbing materials

The paper presents an overview of studies on the influence of the main parameters of ferrites on their ability to absorb electromagnetic radiation in the megahertz range (more than 10 dB). The main advantage of ferrites is a high refractive index (more than 1000) in the megahertz range due to the combination of high values of magnetic and dielectric permeability, which makes it possible to produce radio-absorbing coatings of small thickness (less than 10 mm) that effectively absorb electromagnetic radiation. Studies show that the attenuation of the power of electromagnetic radiation reflected from the surface of the ferrite is due to both interference processes and the processes of dissipation of electromagnetic energy during propagation in the ferrite. The inversely proportional relationship between the refractive index and the frequency of electromagnetic radiation in the megahertz range provides the condition for the interference minimum of the reflected radiation at a constant thickness of the ferrite coatings. A high refractive index slows down the speed of propagation of electromagnetic waves in ferrites, which enhances the processes of dissipation of their energy. The paper presents studies on the influence of the basic chemical composition of ferrites, alloying additives, microstructure parameters and technological modes on their magnetic and dielectric permeability. Studies have shown that an excess of iron oxide in excess of stoichiometry, which provides the semiconducting properties of ferrite grains, significantly increases the dielectric constant. High values of the dielectric constant of ferrites are provided by a combination of the dielectric properties of grain-boundary layers and the semiconducting properties of the grains themselves, which form the barrier capacity of grain boundaries according to the Okazaki mechanism. Alloying ferrites with oxides TiO 2 , Bi 2 O 3 , CaO in an amount of up to 1 wt.% allows increasing the electrical resistance and dielectric constant of grain-boundary layers, providing an increase in the dielectric constant of ferrite as a whole. It has been established that the formation of a dense coarse-grained structure provides an increase in both the magnetic permeability and permittivity.

Electromagnetic Properties of Composite Materials Based on Nd-Compound Modified Fe-Si-Al Alloy

The investigation of microstructure, optical and electromagnetic properties of powder prepared from Nd-compound doped Al6Si9Fe85 (Alsifer) alloy. The result of energy-dispersive X-ray spectroscopy is an evidence that the complementary Nd-containing Nd-Fe phase is on the surface of Alsifer particles. The magnetic particles distribution histogram in powder after mechanical treatment was plotted and analyzed. The electromagnetic properties (components magnetic permeability and dielectric permittivity) for produced composites with Nd-compound doped Alsifer powder with 50-90% (wt.) was studied. The nonlinear concentration dependence of magnetic and dielectric permittivity versus concentration Nd-compound doped Alsifer particles was observed. The magnetic resonance in composites was detected that shifted in low frequencies with increased magnetic filler concentration. The resonance field, absorption linewidth, g-factor and microwave absorption ability of Nd-compound doped Alsifer powder were determined from FMR experiment.

Propylene carbonate-nitrile solvent blends for thermally stable gel polymer lithium ion battery electrolytes

Gel polymer electrolytes (GPEs) based on poly (vinylidene difluoride-co-hexafluoropropylene) (PVdF-HFP) containing propylene carbonate (PC), isobutyronitrile (IBN) and trimethyl acetonitrile (TMAN) solvent blend electrolytes were developed to enhance high temperature electrochemical performance and to improve safety of LiNi3/5Mn1/5Co1/5O2 (NMC622)‖graphite lithium ion battery (LIB) cells. These liquid electrolytes (LE) consist of lithium bis(trifluoro-methane)sulfonimide (LiTFSI) as conducting lithium salt and PC/nitrile (1:1, v/v) solvent blends. Ethylene sulfite (ES) and vinylene carbonate (VC) were used as solid electrolyte interphase (SEI) forming additives and contributing to an excellent cycling stability of electrolytes comprising nitrile solvents in NMC622‖graphite cells. Electrochemical impedance spectroscopy (EIS), pulsed field gradient nuclear magnetic resonance spectroscopy (PFG NMR) and relative permittivity determination reveal remarkable ion conducting properties of IBN and TMAN solvents. Since their electrolytes are incompatible with lithium metal electrodes, their electrochemical stability window (ESW) was determined using lithiated lithium titanate (Li7Ti5O12) (LTO) as counter and reference electrodes and the obtained values were confirmed by quantum mechanical computation. Differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC) confirmed significantly improved safety of IBN and TMAN containing electrolytes. Therefore, PVdF-HFP-based GPEs containing PC/nitrile solvent blends are a promising alternative to SOTA electrolyte for battery cycling at elevated temperature (60 °C) in NMC622‖graphite cells.

Effect of magnetic-field-induced restructuring on the elastic properties of magnetoactive elastomers

Composite materials where magnetic micrometer-sized particles are embedded into a compliant polymer matrix are known as magnetorheological (or magnetoactive) elastomers (MAEs). They are distinguished by huge variations in their physical properties, when in a magnetic field, which is commonly attributed to the restructuring of the filler. The process of the magnetic-field-induced restructuring in a magnetorheological elastomer is interpreted as progression towards percolation. Such a physical model was previously used to explain the dependence of the magnetic permeability and dielectric permittivity of MAEs on the magnetic field strength. Based on this hypothesis, the magnetorheological effect in MAEs is considered theoretically. The theoretical approach is built upon a self-consistent effective-medium theory for the elastic properties, extended to the variable (field dependent) percolation threshold. The proposed model allows one to describe the large variations (over several orders of magnitude) of the effective elastic moduli of these composite materials, known as the giant magnetorheological (MR) and field-stiffening effects. The existence of a giant magnetic Poisson effect is predicted. The relation of the proposed model to the existing theories of the MR effect in MAEs is discussed. The results can be useful for applications of MAEs in magnetic-field-controlled vibration dampers and isolators.

Development of a new type of metamaterial and its application in antenna technology

In this paper, we consider a method for creating left-sided metamaterials. We select a method for creating metamaterials from a system with distributed parameters that are non-resonant structures. The equation of a discrete transmission line is solved taking into account the conditions for creating a metamaterial. The resulting solution in the form of effective permittivity shows that such a medium really has a double negative value of both the dielectric and magnetic permittivity. A generalized scheme of a universal method for searching for creating metamaterials is considered.

An indirect measurement of the speed of light in a General Physics Laboratory

This paper features an indirect method to measure the speed of light. First, the electrical permittivity of air ε0, is obtained, by using a capacitance meter to measure the capacitance of a parallel-plate capacitor, by varying the separation between its plates. By means of a least squares adjustment, the slope of the straight line is calculated which is related to ε0.Next, the magnetic permittivity of air μ0 is obtained by using a solenoid through which different currents are circulated and the magnetic field is measured in its centre using the Hall sensor of a Smartphone. By means of a least squares adjustment, the slope of the straight line is calculated which is related to μ0.Once ε0 and μ0 have been obtained, the speed of light is calculated by the expression c=1ε0μ0 with its corresponding absolute and relative errors, to verify if the obtained value is compatible with the exact value of c.

Development of high frequency equivalent electric circuits for capacitors and chokes taking into account frequency-dependent dielectric and magnetic permittivity properties of dielectric and magnetic materials

Switching of power semiconductor devices in secondary power supplies is the main source of electromagnetic interference. The level of conductive interference must be lowered to meet standards of electromagnetic compatibility. This lowering can be achieved by various methods, including radio interference filters, that are built mainly from inductor coils (chokes) and capacitors. Chokes and capacitors are key elements to design radio interference filters and knowledge of their accurate high-frequency models is required for a wide range of frequencies up to 100 MHz and higher. The article is devoted to development of high-frequency models of capacitors and chokes based on equivalent electric circuits, which parameters are expressed either through electrical-physical values, or through frequency-dependent dielectric permittivity of the dielectric material and magnetic permittivity of the core. Results of modelling are confirmed by impedance measurements of a capacitor with ferroelectric and pyroelectric material used as capacitor’s dielectric and a choke with a core made of nanocrystalline alloy based on GM414 iron. An example of radio interference filter for attenuation of balanced and unbalanced conductive interference is used to consider the negative impact of «spray inductance» of capacitors and «spray capacitance» of chokes.