Dielectric Loss Characteristics Research Articles

Micro-helical Ni3Fe chain encapsulated in ultralight MXene/C aerogel to realize multi-functionality: Radar stealth, thermal insulation, fire resistance, and mechanical properties

Magnetic carbon aerogels with high porosity, ultralow density, and excellent impedance matching are considered to be a promising microwave absorption (MA) material. In this study, ethylene glycol polymerization is used to assemble Ni3Fe particles into a micro-helical chain, which is further wrapped by MXene and then encapsulated in poly-Schiff-base aerogels through the carbonyl–amine condensation reaction. Finally, ultralight TiO2/Ni3Fe/MXene/C (NFMC) aerogels are synthesized through calcination treatment. Benefiting from the electromagnetic synergistic effect of multi-dimensional and multi-component materials, the NFMC-7 aerogel exhibits excellent MA performance with a minimum reflection loss of −61.6 dB at 3.0-mm thickness and an effective absorption bandwidth of 8.16 GHz at 2.77-mm thickness under a low filler loading of 7.6 wt%. Commercial computer simulation technology is used to verify the strong magnetic and dielectric loss characteristics of Ni3Fe chains. At the same time, the NFMC-7 aerogel exhibits excellent radar stealth, heat insulation, compression resistance, and fire resistance performance, which verifies its practical applicability. Overall, this work establishes an effective strategy to fabricate multi-functional magnetic aerogels, which can serve as lightweight, thin, and strong microwave absorbers with broad absorption bandwidth.

A study on the effect of temperature rise on the effectiveness of detecting typical defects in rubber-impregnated paper sleeves

Epoxy rubber-impregnated paper casing has the advantages of stable electrical performance, explosion-proof function, small volume, lightweight, etc., and has been widely used in transformer casing and through-wall casing. However, its manufacturing process is complex, which makes it easy to produce defects in the production process, and it is difficult to detect certain defects in the test at room temperature. To study the partial discharge and dielectric loss characteristics of typical defects of rubber-impregnated paper bushings after temperature rise, we designed and prepared defective bushings with moisture, metal particles, and core cracks, and measured the partial discharge pattern and dielectric loss factor of the bushings after they were heated to a certain temperature. The results show that the core crack defective casing discharge increases more after heating, followed by moisture defective casing, and metal particles defective casing has the smallest impact, indicating that for the temperature rise test in the thermal state of the core crack, moisture defective casing detection effect is better. The results of the study can provide certain references and guidance for the factory inspection and troubleshooting of rubber-impregnated paper casing.

Tunable Impedance of Cobalt Loaded Carbon for Wide-Range Electromagnetic Wave Absorption.

Impedance matching modulation of the electromagnetic wave (EMW) absorbers toward broad effective absorption bandwidth (EAB) is the ultimate aim in EMW attenuation applications. Here, a Joule heating strategy is reported for preparation of the Co-loaded carbon (Co/C) absorber with tunable impedance characteristics. Typically, the size of the Co can be regulated to range from single-atoms to clusters, and to nanocrystals. The varied sizes of the Co combined with different graphitization degrees of carbon can result in different relative input impedances and electromagnetic loss, leading to the tunable EMW absorption properties of the Co/C absorber. By meticulously coalescing the different prepared Co/C, the working frequency can be easily tuned, covering Ku, X, and C bands. Furthermore, the Co/C demonstrates a high EMW attenuation due to its unique dielectric loss capability and magnetic loss characteristics. The abundant interfaces of Co/C can also contribute to the enhanced interfacial polarization for improving EMW attenuation. This work demonstrates the importance of optimizing the metal and carbon interaction to the impedance matching toward wide EAB of the EMW absorbers.

Featuring heterogeneous composite of W-type hexagonal ferrite with 2D vanadium carbide MXene for wideband microwave absorption

In this study, W-type hexagonal ferrite BaCo1.2Zn0.8Fe16O27 (BCZF) was synthesized via solid-state sintering and V2CTx-MXene was chemically etched from its V2AlC MAX phase. Subsequently, BCZF and V2CTx composites were synthesized by hydrothermal integration of V2CTx on the surface of BCZF bonded by electrostatic forces at varying mass ratios. The extensive heterostructure interfacial morphology of two dimensional V2CTx and BCZF plays an explicit role in tuning the dielectric and magnetic loss characteristics of the synthesized composites. The dielectric-magnetic synergistic loss mechanism caused by interfacial polarization, strong multi-reflections, scattering between MXene multilayer structures, conduction loss, and magnetic resonance effectively optimized impedance matching to improve the attenuation ability of the synthesized composites. Remarkably, the BCZF@10%V2CTx composite achieved strong electromagnetic wave absorption ability with an effective absorption bandwidth (RL < −10 dB) of 8.0 GHz (10–18 GHz) which is superior to that of contemporary magnetic–dielectric hybrid composites. These results present a novel perspective on magnetic V2CTx-MXene composites for microwave absorption applications, and provide a basis for the design and development of high-performance cloaking materials.

A "Win-Win" Strategy to Modify Co/C Foam with Carbon Microspheres for Enhanced Dielectric Loss and Microwave Absorption Characteristics.

3D carbon foams have demonstrated their superiority in the field of microwave absorption recently, but the preparation processes of traditional graphene foams are complicated, while some novel carbon foams usually suffer from inadequate dielectric property. Herein, a simple "win-win" strategy is demonstrated to synchronously realize the construction of 3D Co/C foam and its surface decoration with carbon microspheres. Therein, the host Co/C foams and guest carbon microspheres interact with each other, resulting in the improvement of the dispersity of carbon microspheres and Co nanoparticles. The bilaterally synergistic effect can effectively enhance the interfacial polarization and conductive loss of these obtained samples. Electromagnetic analysis reveals that the optimized sample with moderate carbon microsphere content (about 33.5 wt%) displays a widened maximum effective absorption bandwidth of 5.2GHz and a consolidated reflection loss intensity of -67.6dB. Besides, the microwave absorption enhancement mechanisms are investigated and discussed in detail. It is believed that this work provides valuable ideas for the development of 3D-foam-based microwave absorbing materials for practical applications.

Dual template induced assembly of 2D nanosheets to 3D porous Mo2C/NiFe-NC networkers for electromagnetic wave absorption

Developing high-performance materials with ultrathin matching thickness for electromagnetic wave absorption (EMW) is essential for the commercialization of electromagnetic devices, but it is still formidably challenging. In this work, Mo2C/NiFe-NC networker with small Mo2C grains, NiFe alloy coated by graphitized carbon (NiFe-NC), and N-doped carbon matrix are constructed by free-drying and dual template strategy. With the assistance of hard template (salt, NaCl)) and soft template (urea), the ultrathin 2D layered carbon nanosheets produced by pyrolysis are assembled into a 3D hierarchical porous network. The designed Mo2C/NiFe-NC networker exhibits a hierarchical porous structure, 3D network skeleton, heterogeneous interface, abundant heteroatom doping, and magnetic NiFe alloy, which inherits good dielectric loss (dipole polarization, interfacial polarization, and conduction loss), magnetic loss (natural resonance and exchange resonance) and considerable impedance matching characteristics. Impressively, the well-designed Mo2C/NiFe-NC networker enables a reflection loss (RL) value of −51.56 dB at an ultrathin matching thickness of 1.4 mm. Furthermore, the effective absorption bandwidth (EAB, RL ≤ −10 dB) is up to 3.7 GHz. Importantly, the EAB in the matching thickness range of 1–5 mm is as high as 14.6 GHz, covering the entire C-band, X-band, and Ku-band. This work may open a key avenue for high-performance EMW absorption materials via a dual template assistance strategy.

Electrical characteristics of multiferroic BiFeO3 electronic system

The structural, dielectric and conductivity properties of bismuth ferrite oxide (BiFeO3) electronic system which was synthesized by means of a solid-state reaction route are explained in this literature. For material fabrication, the exact proportion of 99.9 % pure Fe2O3 and Bi2O3 was taken and processed. The resulting material is extremely pure and stable in nature. The X-ray Diffraction (XRD) on calcined powder confirms the prepared material's structure as a single-phase rhombohedral crystal system. To understand the electronic behavior in the frequency range of 1 kHz-1 MHz with a temperature range of 35–450 °C, the prepared material was characterized in the form of dielectric and conductivity properties. The synthesized material's normal dielectric behavior has been described. It has a high dielectric constant at high temperatures and a low tangent loss at low temperatures. The tangent loss exhibits lower value at lower temperatures, but as the temperature rises, the plot rises abruptly and reaches a peak around 380 °C. This dielectric loss characteristic is caused by inadequate thermal carriers and oxygen vacancies. The significant grain and grain boundary effect of resistive characteristics of synthesized BFO has been observed at this higher temperature. The frequency-dependent dielectric plots show that the dielectric constant and dielectric loss of the composed material decrease with increasing frequency, regardless of temperature variation. The AC conductivity value decreases with increasing frequency at low temperatures. With the increase in frequency up to 10 kHz, the calculated activation energy of synthesized BFO at a specific temperature region rises. The Fourier transform infrared spectroscopy (FTIR) analysis of the processed material reveals the formation of octahedral FeO6 properties in perovskites mode and many other intriguing results regarding BFO structure. Electronic material may act as a budding contender for designing electronic devices for many useful applications due to all these intriguing characteristics and findings.

Effect of Dielectric Relaxation of Epoxy Resin on Dielectric Loss of Medium-Frequency Transformer

Epoxy resin (EP) is a typical and widely used insulating material for medium-frequency transformers (MFTs). The dielectric loss of EP is one of the key variables in the design and operation of MFT. In this article, six types of epoxy/anhydride systems are prepared, and the dielectric loss of MFT is calculated by finite element method (FEM) analysis based on broadband dielectric spectroscopy measurements. The material dependence of the dielectric loss of MFT is investigated using dielectric spectroscopy and molecular dynamics simulations over a wide temperature and frequency range. The results show that the variation of dielectric loss with output power is attributed to the temperature characteristics of the imaginary part ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon ^{\prime \prime }$ </tex-math></inline-formula> ) of complex permittivity. Compared to the dielectric loss at no-load state, the one at rated power is reduced by 5–10 W. The anhydride hardener containing benzene ring or a methyl substituent has an inhibitory effect on dielectric loss, and the EP cured by methyl nadic anhydride (MeNA) has the lowest dielectric loss (16.2 W) at rated power. The dielectric loss and overload characteristics of MFT are closely related to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -relaxation and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> -relaxation of EP, respectively. The acid anhydride curing agent containing benzene ring or a methyl substituent reduces the temperature corresponding to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -relaxation peak by more than 50 °C compared to the one without. The increased rigidity of the chain segment of EP makes the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -relaxation move to the low-temperature side, which is beneficial to suppress the dielectric loss. This article not only serves as a bridge among dielectric loss of MFT, dielectric spectroscopy, and molecular structure of EP but also lays a solid foundation for the selection of insulating materials for MFT.

Controllable seeding of nitrogen-doped carbon nanotubes on three-dimensional Co/C foam for enhanced dielectric loss and microwave absorption characteristics

Carbon-based composites with three-dimensional (3D) foam structure are widely considered as a kind of prospective functional materials, and more and more researchers are focusing on their application in the field of microwave absorption. Herein, we demonstrate a controllable seeding of nitrogen-doped carbon nanotubes (NCNTs) on the surface of Co/C foams through in-situ growth. By manipulating the weight ratio of melamine to pre-prepared Co/C foams, the chemical composition and electromagnetic (EM) properties of these samples can be easily controlled. EM analysis reveals that NCNTs will create sufficient polarization effects and induce multiple reflection of incident EM waves. Moreover, the migration of electrons along NCNTs also promotes conductive loss significantly. The optimized sample with moderate NCNTs loading (ca. 23.5 wt%) exhibits a wide effective absorption bandwidth (EAB) of 6.3 GHz and the strongest reflection loss (RL) intensity of −97.3 dB at 16.2 GHz, which are superior to those of pristine Co/C foam and many other common magnetic metal/carbon composites. The unique composition and microstructure ensure that such a composite can produce good microwave absorption performance under a low filling content (15 wt%). It is believed that this study may inspire the fabrication and function enhancement of microwave absorbing materials (MAMs) based on carbon foam.

Development and Performance Analysis of High Thermal Conductivity Epoxy Resin for Dry-type Transformer Casting

The purpose of this paper is to provide a preparation method of epoxy resin with high thermal conductivity transformer casting. Adding metal oxide and nitride such as silicon nitride, boron nitride, aluminum nitride and so on into epoxy resin, so that the interface structure of the epoxy resin can be effectively changed, the thermal conductivity of the filling material is greatly improved. It greatly avoids the influence of temperature on its internal parameters, so that the transformer can works smoothly. However, the addition of thermal conductive fillers will affect its dielectric properties (such as Insulation strength, dielectric loss and conductivity characteristics) and mechanical stability. This paper solves the problem of how to improve the thermal conductivity and ensure the dielectric and mechanical properties.

Three-dimensional network FeNi/C composites with excellent microwave-absorbing properties

In this paper, the carbon coated iron nickel nanoparticles were obtained by calcining PVP crosslinked iron nickel oxalate. The morphology, structural composition and microwave-absorbing (MA) properties of the composites were analyzed by various characterization methods. The results show that FeNi/C composites have excellent impedance matching characteristics and MA performance in multiple frequency by virtue of their microscopic network structure. At the same time, through the analysis of loss mechanism, we found that the material has high dielectric loss and magnetic loss characteristics. Under the joint action of various loss mechanisms such as interface polarization and eddy current loss, the material has a high reflection loss (RL) value at different frequency. At a thickness of 3 mm, the sample FNC12 has a minimum RL of − 52.99 dB at 3.6 GHz. The effective absorption bandwidth (EAB) (RL ≤ −10 dB) is 1.81 GHz.

Analysis on dielectric loss characteristics of polyvinylidene fluoride and its composites

Analysis on dielectric loss characteristics of polyvinylidene fluoride and its composites

Controllable morphology, dielectric, magnetic and reflection loss characteristics of ferrite/wax composites for low-loss applications

In this work, composites of M-type Ba(1−x)SrxFe12O19 hexagonal ferrite (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) and paraffin wax have been prepared. Microwave characterization of the prepared composites was performed with a vector network analyzer from 0.13 GHz to 3 GHz. The dependence of Sr2+ ion content on different compositions was determined in terms of grain morphology, dielectric, magnetic, conductivity, and reflection loss (RL) properties. The grain size varied with dopants and rendered a significant effect on material parameters. The dielectric and magnetic loss tangent were reduced with the increase in frequency and content of Sr2+ ions. The AC conductivity followed the trend of gradual increase with frequency and x=1.0 composition attained the lowest value encompassing the frequency spectrum mostly. The observed RL peaks have been narrow in Ba0.8Sr0.2Fe12O19/wax composite and broad in SrFe12O19/wax composite along with the low frequency spectrum. x=0.6 composition has-0.35×10−3 dB reflection loss at 0.15 GHz. All compositions revealed more than 99.77% unattenuated (low-loss) microwave signal, with −0.01 dB bandwidth ≥1.50 GHz, and owe good scope for their application in low-loss microwave devices.

Analysis of FDS Characteristics of Oil-impregnated Paper Insulation under High Electric Field Strength Based on the Motion of Charge Carriers

Frequency Domain Spectroscopy (FDS) is a powerful tool to study the dielectric properties of oil-paper insulation, which is widely used under high electric field strength. The motion of charge carriers under high electric field strength will cause new dielectric characteristics in FDS curves, from which more information could be extracted for insulation state assessment. In this paper, FD spectra are measured under high electric field strength. The electric field dependence of the loss peak in middle frequency and the characteristics of dielectric loss in low frequency are studied. With the influence of trapping on the motion of charge carriers under electric field being considered, charge carriers hopping polarization model and charge carriers motion dependent conductance model are proposed. The trap energy level of oil-paper insulation decreases when the moisture content increases, thus, the motion characteristics of charge carriers change, leading to decreasing loss peak in middle frequency and inverse dielectric loss characteristics in low frequency. These two models introduce the influence of traps on carrier motion into the dielectric response and enable one to gain a better understanding of the dielectric properties of oil-paper insulation under high electric field strength.

Review on Shielding Mechanism and Structural Design of Electromagnetic Interference Shielding Composites

AbstractConductive polymer composites (CPCs) for electromagnetic interference shielding have received significant attention and shown rapid development. According to the electromagnetic wave interface conduction theory of Schelkunoff, excellent conductive performance and perfect conductive network structure are prerequisites for high shielding efficiency of electromagnetic interference shielding composites. Effective multiple interface reflection absorption, dielectric loss, and hysteresis loss characteristics of the materials are crucial for realizing the regulation of the electromagnetic interference shielding performance of CPCs. Therefore, the structural design of conductive and magnetic network for CPCs is crucial for achieving high shielding performance. In this study, it is established that an electromagnetic shielding composite with a uniform structure is widely used because of its simple preparation process, but its inefficient conductive network causes a high percolation threshold. The inefficiency can be solved by designing a composite structure and improving the efficiency of the conductive network. Currently, common structural designs include segregated structural, layered structural, and foam structural designs. These structural designs effectively solve the problem of high percolation threshold of CPCs and coordinate the contradiction between the performance of electromagnetic interference shielding and other advantages.

Investigation on microstructures, electronic structures, electromagnetic properties and microwave absorption properties of Fe3Si/PPy composites

Microstructures, electronic structures, and microwave absorption properties of Fe3Si/PPy composites have been investigated through experiments and first principal method. The calculated results show that Fe3Si/PPy composites are ferromagnets, and the total magnetic moment is 1.87μB. Charge transfer between atoms is conducive to dielectric loss. The Fe3Si/PPy composites were prepared with the one-pot method. The presence of Fe3Si and PPy was confirmed by XRD, FTIR, EDS and TEM. The Fe3Si/PPy composites showed good wave-absorbing ability in the X-band and Ku-band in contrast to pure Fe3Si. The RL value was −13.73 dB at 16.23 GHz in 1.5 mm-thick Fe3Si/PPy composites, indicating that only 10% EM waves are reflected and 90% EM waves were absorbed. The maximum RL value of the 5.0 mm-thick Fe3Si/PPy composites was −33.1 dB at 13.05 GHz. The effective bandwidth was 2.88 GHz. The dielectric loss and magnetic loss characteristics of the Fe3Si/PPy composites greatly contributed to their excellent wave absorption performance.

Reduced graphene oxide-supported boron and nitrogen co-doped carbon nanotubes with embedded cobalt nanoparticles for absorption of electromagnetic wave

We develop a facile method to fabricate Co nanoparticles-embedded N, B co-doped carbon nanotubes on reduced graphene oxide sheets (Co@BNCNT/rGO). The prepared Co@BNCNT/rGO has high electrical conductivity, a large number of defects, and a three-dimensional structure with multiple interfaces. The experimental results show that the introduction of rGO can adjust the dielectric loss and impedance matching characteristics of the Co@BNCNT/rGO. Density functional theory calculations indicate that the electrons transfer from metallic Co to N,B-doped CNT, leading to interfacial polarization relaxation that favors the improvement of the electromagnetic wave absorption property of the Co@BNCNT/rGO. As a result, the Co@BNCNT/rGO exhibits excellent electromagnetic wave absorption properties with a minimum reflection loss exceeding −20 dB even if the matching thickness is lower than 2.0 mm. Moreover, the filler mass load of the Co@BNCNT/rGO is merely 20 wt%. Our results highlight a new method for synthesizing high-performance electromagnetic wave absorbers.

Determination of the loss tangent for solid dielectrics at high frequencies

Cells with a micrometer screw are still used for testing solid dielectrics at frequencies from units to hundreds of megahertz despite the development of dielectric measurement technology. We present the results regarding elimination of the negative impact of successive stray inductances (Ln) and active resistance (rn) of such a cell on the accuracy of the dielectric loss tangent (tanδ) determination with allowance for the final value of the Q-factor (Qk) of the circuit inductor in resonant measurements. Tests with and without a dielectric sample were carried out under the condition of maintaining the cell capacity value. No information is available in the regulatory documents and technical literature about taking the aforementioned parameters into consideration. It is shown that the impact of Ln consists in the overestimation of measured tanδ values. To eliminate this effect, we proposed to introduce an additional factor which depends on Ln, frequency and capacity of the sample into existing formulas used for tanδ calculation. It is also shown that the tanδ error increases significantly with a decrease in the ratio of the dielectric loss characteristics of the measuring circuit with and without a sample which can be attributed to rn and Qk. The requirements for rn and circuit damping without a sample are formulated to ensure the desired accuracy. Conditions providing the possibility of assessing the suitability of a particular equipment for tanδ measurements are specified on basis of the developed requirements. The results experimentally proved in resonance measurements with a Q-meter can be used in high-frequency tanδ measurements to provide the required accuracy.

Efficient magnetoelectric dispersion in Ni and Co co-doped BiFeO3 multiferroics

Pure and Ni/Co co-doped BiFeO3 samples were prepared through sol‒gel auto-combustion technique. The synthesized samples were calcined at 500 °C for 1 h before further characterizations. The rhombohedrally distorted cubic perovskite structure was confirmed for all samples through X-ray diffraction analysis. The surface morphology of each sample was visualized using scanning electron microscopy which elaborated the formation of spherical grains along with some agglomerations. Energy dispersive X-ray spectra revealed the presence of Bi, Fe, O, Ni and Co atoms in the same atomic and weight percentages as expected from their empirical formulae. The frequency dependent dielectric constant, tangent loss and impedance characteristics were investigated using impedance analyzer. It was also noticed that the ac conductivity increased when Ni and Co were substituted at equal amounts. The magneto-electric analysis witnessed the coupling between different ferroic orders in these samples.

Complex permittivity and complex permeability characteristics of Co–Ti doped barium strontium hexaferrite/paraffin wax composites for application in microwave devices

The microwave characteristics of composites prepared from Ba0.5Sr0.5CoxTixFe(12-2x)O19/paraffin wax with 10% and 20% volume fraction of ferrite powder in wax have been examined in the test frequency range of 0.13–4 GHz. The purpose was to determine an optimum composition that could be used in the design of microwave devices for low loss wireless communication. The effect of volume fraction, doping, and thickness on dielectric, magnetic, conductivity, and reflection loss characteristics was explored to ascertain microwave low loss/unattenuated transmission. The results reveal that the dielectric and magnetic properties can be tuned by Co2+ and Ti4+ ion doping and the percentage volume fraction ratio of the ferrite in the ferrite/wax composite. The height of reflection loss dips increased with thickness and they narrowed for higher doping with Co2+ and Ti4+ ions. All compositions render 98.8% unattenuated transmission of the microwave signal encompassing the majority of the investigated frequency regime. Composition x = 0.6 has the maximum reflection loss of − 0.35 × 10–3 dB at 0.15 GHz. The obtained results for these compositions could be considered for designing microwave devices for low loss wireless communication with 0.05 dB bandwidth of 3 GHz in the explored microwave range.