High Complex Permittivity Research Articles

The synergistic effects on the magnetic CoNi and Fe3O4 nanoparticles co-embedded porous carbon toward enhanced microwave absorbing performance

Abstract Lightweight and environmentally friendly three-dimensional biomass-derived porous carbon (3D BPC) holds great potential as a highly effective material for microwave absorption (MA) applications. However, the high complex permittivity and lack of magnetic loss in a single 3D BPC lead to impedance mismatching and a limited absorption bandwidth. Developing 3D BPC–based absorbers with multiple loss mechanisms and excellent impedance matching remains a notable yet challenging task. In this study, inspired by a synergistic composition and microstructure design, 3D BPC co-embedded with magnetic cobalt–nickel (CoNi) and iron(III) oxide (Fe3O4) nanoparticles (3D BPC@CoNi@Fe3O4) was developed. 3D BPC@CoNi@Fe3O4 exhibited consistently low complex permittivity, primarily due to the suppression of conductive loss, whereas the introduction of magnetic phases (CoNi and Fe3O4) enhanced the magnetic loss. Optimised impedance matching, achieved through the synergistic regulation of the electromagnetic parameters, emerged as the key mechanism for improving the MA properties. The as-synthesised 3D BPC@CoNi@Fe3O4 composite, with only 20 wt.% loading demonstrated a wide effective absorption bandwidth (reflection loss [RL] < −10 dB) of 6.08 GHz and an impressive minimum RL value of −40.08 dB. These findings offer valuable insights into the development of high-performance 3D BPC–based microwave absorbers through composition and microstructure optimisation.

Zirconium carbide modified SiOC composites with porous lamellar structures for broadband microwave absorption and thermal stability

The inherent dielectric properties and thermal stability of transition metal carbides (TMCs) have generated significant interest because of their potential use in electromagnetic (EM) wave applications at various temperatures. However, the high relative complex permittivity and conductivity of TMCs can cause impedance mismatch issues when utilized as absorbers, thereby limiting their effectiveness in absorbing EM waves. To solve these problems, ZrC/ZrO2–SiOC composites with a porous lamellar structure were prepared using polymer-derived ceramics (PDC) technology, which involved dispersing zirconium carbide (ZrC) into modified polysiloxane (PSO). The resulting ZrC/ZrO2–SiOC composite (with 5 wt% ZrC content sintered at 900 °C) achieved remarkable EM wave absorption performance, with a minimum reflection loss (RLmin) of −33.52 dB at 13.0 GHz and an effective absorption bandwidth (EAB) extending up to 7.75 GHz. This exceptional absorption of EM waves is due to the synergistic interplay of various factors such as interface polarization, dipole polarization, and conduction loss. Moreover, the composites exhibit a minimal weight loss of less than 2 % when exposed to air at temperatures ranging from 25 to 1200 °C. This study not only broadens the application possibilities of TMC materials in microwave absorption but also highlights their potential for further improvement and deployment in high-temperature environments.

A flow through coaxial cell to investigate high frequency broadband complex permittivity: Design, calibration and validation

This paper presents a new apparatus: a large flow through coaxial cell designed for broadband dielectric characterization of material under controlled hydraulic and chemical boundaries. The cell is calibrated with a single measurement made on a perfectly known dielectric liquid (deionized water) together with an optimization procedure. Then, two methods are proposed to compute the dielectric characteristics: an iterative solver and an optimization method. These two methods are systematically investigated for two reference liquids: a low loss dielectric material (ethylene glycol) and high loss dielectric liquid (saline solutions with different concentration). Tabulated data were used to perform a quantitative error analysis in terms of estimated complex permittivity. The results have shown high performance in terms of real part and good performance in terms of real part. The measurements on saline solution also highlight the impact of electrode polarization with dramatic effect in the lower part of the frequency range.

Suppressed conductive loss of flower-like NiCo2O4-wrapped flaky FeSiAl for excellent low-frequency microwave absorption

Flaky FeSiAl is a high potential absorbent for low-frequency microwave applications. However, impedance mismatching due to its excessively high complex permittivity presents a challenge. Herein, inspired by the surface modification strategy, flaky FeSiAl wrapped with flower-like NiCo2O4 (NiCo2O4@flaky FeSiAl) was developed via a hydrothermal method. The introduction of a surface NiCo2O4 insulating layer was expected to reduce the complex permittivity and increase the high complex permeability in the NiCo2O4@flaky FeSiAl composite. Subsequently, an excellent low-frequency microwave absorption performance was achieved. The designed NiCo2O4@flaky FeSiAl displayed a robust minimum refection loss (RL min) of −33.41 dB at 6.32 GHz and increased the effective absorption bandwidth (EAB, RL < −10 dB) to 4.08 GHz when the matching thickness was 1.6 mm. The enhanced low-frequency microwave absorption performance was mainly attributed to the improvement in impedance matching, which originated from the collaborative regulation of electromagnetic parameters. This study provides a promising approach for designing flaky FeSiAl-based composites and high-performance low-frequency microwave absorbents.

Continuous flowing polarization system containing absorbents for acceleration of absorption of oil-in-water micelles in water by electric dipole moments

The concentration gradient between solution to absorbent predominately determines the absorption and desorption rates, limited the efficiency of wastewater treatment at low pollutant concentration. Dielectrophoresis force (FDEP) is generated with polarization to assemble the absorbents and oil-in-water micelles (OWM) in a home-made parallel plate capacitor (PPC) full of adsorbent solution and enhance the efficiency of pollutant removal. Graphene oxide (GO) and Fe3O4 nanoparticles (FN) are coated on polystyrene microspheres (PSs) as the core/shell PS/GO/FN absorbents that were encapsulated within the PPC to treat the solution of OWM. The FDEP enhances the adsorption rate ca. 2–4 times with polarization at 14 V and 100 kHz. 50 % volume reducing of pure water through the PPC for the recycling of the absorbents could achieve at 1200 kHz. The high complex permittivity of the GO/FN coating facilitates the response to the alternative electric field to induce the dipoles in the object without significant damage. The PS/GO/FNs was reused ca. 10 times at 25 mL min−1 of flow rate through the PPC between 100 and 1200 kHz. The continuous dielectric polarization system improves the absorption rates and efficiency of absorbent recycling with low secondary pollution.

Surface modification of carbon fibers for enhanced electromagnetic absorption

The exceptional chemical and physical properties of carbon fiber make it highly promising for utilization in the field of wave-absorbing materials. However, the low magnetic loss and high complex permittivity of conventional carbon fiber materials limit their use in wave-absorbing materials. Hence, this study presents the synthesis of a FeCoNi/N-doped carbon coating/SCF composite with a distinctive microstructure by an in-situ reduction process. The successful formation of this unique architecture was confirmed through various characterization techniques. The composite material displayed both magnetic and dielectric characteristics. It showed an effective bandwidth spanning 14.77 GHz with a reflection loss (RL) of − 10 dB or less. In addition, it attained the minimum RL value of − 50.5 dB at a distance of 3.5 mm. The composite material is hypothesized to show great potential in electromagnetic wave absorption.

Fabrication of sandwich-structured capacitor containing core@shell polystyrene@graphene oxide microspheres for switchable removal of dyes from water by dielectrophoresis force

Absorption rates for traditional adsorbents were predominately determined by the concentration gradient from bulk to absorbents and surface area. A novel sandwich-structured capacitor (SSC) containing adsorbents is designed to reduce the time consuming of absorption of pollutants by dielectrophoresis force (FDEP). Graphene oxide (GO) are coated on polystyrene microspheres (PSs) as the core/shell PS@GO adsorbents that were encapsulated within the SSC to treat the solution of dye-emulsified micells (DEM). The FDEP results the assembling of PS@GOs and DEMs under polarization and enhance the adsorption rate ca. 1 ∼ 2 times with polarization at 3 V and 200 kHz. Additionally, most adsorbents are recycled with secondary pollution. Only two-fifth pure water volume of the wastewater through the SSC could achieve the recycling of the adsorbents at 900 kHz. The high complex permittivity of GO coating at high frequency allows ultrafast alternative spinning of the induced dipoles in the PS@GOs facilitates desorption and hinders the re-adsorption of the DEM during the recycling of PS@GOs. The PS@GOs was reused ca. 10 times at 20 mL min−1 of flow rate through the SSC without damages between 200 and 900 kHz. The designed SSC containing PS@GOs accomplishes either reusing of adsorbents or concentrating of dyes with low secondary pollution in practice through frequency manipulation.

Ultrafast absorption mechanism of oil-emulsified micelles onto ferrous absorbents with dielectrophoresis force in the presence of polarization

Absorption and desorption rates were generally dependent on the concentration gradient from bulk to absorbents. A novel methodology based on a capacitor with an alternating electric field (AEF) is developed to accelerate the absorption and desorption rates with the frequency manipulation. Ferrous polystyrene microspheres (PISMs) are synthesized as absorbents, which could enhance the complex permittivities as well as dielectric properties. Theoretically, the attractive force and viscous force predominately determine the particle and micelles movement in the medium under an AEF. Oil-emulsified micelles (OEM) with various viscosities were selected as absorbates. Both the OEM and microspherical absorbents assembled through the external attractive force in the presence of the AEF. When the attractive force is equal to viscous force in the medium at the characteristic frequency, the optimal absorption rate could be obtained. The absorption rate constants of pseudo-first-order for OEMs under the polarization at 50 V and 120 kHz of frequency are ca. 10 times higher than that in absence of the polarization. The desorption rate as well as recycling efficiency could be also improved at 800 kHz. The ferrous PISMs with high complex permittivity prevented the damage from the AEF, which could be recycled 10 times of absorption and desorption with frequency manipulation under the AEF. Our methodology provides novel insights for ultrafast wastewater treatment.

Optimizing electromagnetic parameters to enhance the microwave absorbing properties of core–shell FeCo2O4@flaky FeSiAl composites

The high complex permittivity of flaky FeSiAl in microwave-absorbing materials leads to poor electromagnetic impedance matching and a narrow effective absorption bandwidth. To overcome this issue, we developed a novel approach by wrapping magnetic FeCo2O4 on the surface of flaky FeSiAl using a hydrothermal method, followed by calcination. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were used to confirm the formation of a FeCo2O4@flaky FeSiAl composite with a well-defined core–shell microstructure. The prepared FeCo2O4@flaky FeSiAl composite exhibited a significantly reduced complex permittivity, which improved its electromagnetic impedance matching performance. When filled in paraffin with a thickness of only 1.6 mm, the composite demonstrated excellent electromagnetic wave absorption performance, with an effective absorption bandwidth of 5.60 GHz (RL < − 10 dB), indicating its potential in the field of microwave-absorbing materials. These results were achieved owing to the unique properties of the FeCo2O4@flaky FeSiAl composite, demonstrating its potential as a high-performance microwave absorber. This study may be of significant interest to researchers and engineers in the fields of magnetism and magnetic materials, especially those in the microwave field.

On a Low-Frequency and Contrast-Stabilized Full-Wave Volume Integral Equation Solver for Lossy Media

In this article, we present a new regularized electric flux volume integral equation (D-VIE) for modeling high-contrast conductive dielectric objects in a broad frequency range. This new formulation is particularly suitable for modeling biological tissues at low frequencies, as it is required by brain epileptogenic area imaging, but also at higher ones, as it is required by several applications, including, but not limited to, deep brain stimulation (DBS). When modeling inhomogeneous objects with high complex permittivities at low frequencies, the traditional D-VIE is ill-conditioned and suffers from numerical instabilities that result in slower convergence and less accurate solutions. In this work, we address these shortcomings by leveraging a new set of volume quasi-Helmholtz projectors. Their scaling by the material permittivity matrix allows for the rebalancing of the equation when applied to inhomogeneous scatterers and, thereby, makes the proposed method accurate and stable even for high complex permittivity objects until arbitrarily low frequencies. Numerical results, canonical and realistic, corroborate the theory and confirm the stability and the accuracy of this new method both in the quasi-static regime and at higher frequencies.

A novel synthesis of Porous Fe4N/carbon hollow microspheres for thin and efficient electromagnetic wave absorbers

Modulating the structure and morphology is essential in fabricating high-performance electromagnetic absorbing materials. Herein, we obtained porous Fe3O4/carbon hollow microspheres and porous Fe4N/carbon hollow microspheres derived from Fe-glycerol hollow microspheres. Through structure and morphology analysis, we proved the existence of porous and hollow features. By comparison, it can be found that the porous Fe4N/carbon hollow microspheres have electromagnetic wave absorption performance superior to that of porous Fe3O4/carbon hollow microspheres. The reflection loss value of porous Fe4N/carbon hollow microspheres reaches −42.2 dB at a matching thickness of merely 1.4 mm, and its effective absorbing bandwidth approaches 4.5 GHz, whereas the reflection loss of porous Fe3O4/carbon hollow microspheres in the 2–18 GHz range is over −10 dB. Reasons for the better electromagnetic wave absorption performance are revealed to be that the magnetic Fe4N has higher complex permittivity and complex permeability, and the porous hollow microspherical structure increases the multiple scattering and reflection of electromagnetic waves. Meanwhile, the impedance matching and attenuation constant are optimized together through the synergy of dielectric and magnetic loss. This research can provide instructive findings for thin-thickness electromagnetic wave absorbing materials based on Fe4N with an appropriate microstructure.

A New Correction to the Rytov Approximation for Strongly Scattering Lossy Media

We propose a correction to the conventional Rytov approximation (RA) and investigate its performance for predicting wave scattering under strong scattering conditions. An important motivation for the correction and investigation is to help in the development of better models for inverse scattering. The correction is based upon incorporating the high frequency theory of inhomogeneous wave propagation for lossy media into the RA formulation. We denote the technique as the extended Rytov approximation for lossy media (xRA-LM). xRA-LM significantly improves upon existing non-iterative linear scattering approximations such as RA and the Born approximation (BA) by providing a validity range for the permittivity of the objects of up to 50 times greater than RA. We demonstrate the technique by providing results for predicting wave scattering from piece-wise homogeneous scatterers in a two-dimensional (2D) region. Numerical investigation of the performance of xRA-LM for solving direct problem show that xRA-LM can accurately predict wave scattering by electrically large, low-loss scatterers with high complex permittivity ($\epsilon_r> 50+5j$). To the best of our knowledge, this is the first non-iterative, linear approximate wave scattering model which has a large validity range in terms of both permittivity and electrical size.

Comparative study of microwave absorption properties of Ni–Zn ferrites obtained from different synthesis technologies

Spinel ferrites are widely used for electromagnetic wave (EMW) absorption applications. In this study, spinel Ni–Zn ferrites with excellent microwave absorption properties were synthesized. Their EMW absorption characteristics and interaction mechanisms were studied to lay the foundation for the study of the role of Ni–Zn ferrite as a magnetic substrate for composites. Herein, Ni0·5Zn0·5Fe2O4 was prepared by the hydrothermal method (H-NZFO) and the sol–gel auto-combustion method (S-NZFO); both samples exhibited distinct microwave absorption properties. The S-NZFO absorber (thickness = 3.72 mm) demonstrated the best dual-zone microwave absorption with two strong reflection loss peaks at 5.1 and 10.5 GHz. The corresponding effective absorption bandwidth (EAB) reached 9.0 GHz, which covered part of the S-band and all of the C- and X-bands. These results were attributed to the high saturation magnetization, outstanding complex permeability, and multiple magnetic loss channels of S-NZFO. The H-NZFO sample exhibited excellent absorption capability and matching thickness. At a thickness as low as 1.71 mm, the minimum reflection loss (RLmin) of the H-NZFO absorber reached -60.2 dB at 13.1 GHz. The maximum bandwidth corresponding to RL below -10 dB was 4.6 GHz. These results can be attributed to small particle size, high complex permittivity, and multiple dielectric loss channels of H-NZFO. The observed wide effective absorption bandwidth of S-NZFO and strong microwave absorption capability of H-NZFO suggest the potential of both materials as substrates for efficient microwave absorbers in military as well as civilian absorption applications.

Lightweight TiO2@C/Carbon Fiber Aerogels Prepared from Ti3C2Tx/Cotton for High-Efficiency Microwave Absorption.

Carbon fiber aerogel (CFA) derived from cotton wool as a potential microwave absorbing material has received intensive attention owing to the low density, high conductivity, large surface area, and low cost, but its applications are limited by the relatively high complex permittivity. To solve this problem, TiO2@C (derived from Ti3C2Tx) is introduced into CFA to prepare lightweight TiO2@C/CFA composites based on electromagnetic (EM) parameter optimization and enhanced EM wave attenuation performance. The microwave absorption capacity of TiO2@C/CFA-2 composite is obviously better than that of CFA. It is confirmed that good impedance matching derived from the combination of TiO2@C and CFA is the main factor to achieve excellent microwave absorption. Moreover, the improved microwave absorption capabilities are closely related to multiple EM wave absorbing mechanisms including multiple reflections and scattering, dipolar and interfacial polarization, and conductivity loss. TiO2@C/CFA-2 possesses a maximum reflection loss (RL) of -43.18 dB at a low response frequency of 6.0 GHz. As the matching thickness is less than 2.0 mm, the maximum RL values can still exceed -20 dB, and at the same time, the wide effective absorption bandwidth (EAB) below -10 dB achieves 4.36 GHz at only 1.9 mm thickness. Our work confirms that the lightweight and high-performance TiO2@C/CFA composites are promising choices and offer a new approach to design and construct carbon-based microwave absorbents derived from biomass.

High temperature W-band complex permittivity measurements of thermally cycled ceramic-metal composites: AlN:Mo with 0.25 to 4.0 vol% Mo from 25 °C to 1000 °C in air

An apparatus for measuring the W-band (75–110 GHz) complex permittivity of dielectrics at 1000 °C was developed. This apparatus allows for measurements at approximately twice the temperature of previously published high temperature free-space measurement systems while maintaining similar precision. Challenges were addressed related to high temperature measurements, including temperature uniformity, the accuracy of temperature measurements, and preventing temperature related changes to mm-wave measurement systems. The details of complex permittivity extraction from the measured S-parameters are discussed. Sources of error related to permittivity measurement and mathematical models were identified and are discussed in detail herein. Thermally-cycled, mm-wave absorbing, aluminum nitride ceramic composites containing varying levels of molybdenum additives were measured over the range of 25 °C–1000 °C. These measurements were compared to the same composites before thermal cycling. It was found that ceramic composites are largely stable after thermal cycling in terms of dielectric properties despite the presence of visible surface modifications.

Effects of La3+ or Ti4+ doping on dielectric and microwave absorption performance of CaMnO3 in the 8.2–18 GHz

In this paper, CaMnO3 and Ca0.97La (or Ti)0.03MnO3 powders were synthesized using mechanical ball milling technique and sintered at 1100°C for 12 h in an air atmosphere. Results showed that all the samples are single phase with an orthorhombic symmetry. Compared with the pure CaMnO3 sample, the Ca0.97La0.03MnO3 and Ca0.97Ti0.03MnO3 samples have higher complex permittivity and better microwave absorbing properties. These changes are caused by the doping-ion incorporating in CaMnO3 lattice and Ca vacancies, which could result in abundant dipoles and the local charge accumulation. The Ca0.97Ti0.03MnO3 sample exhibited the best microwave absorption performance, i.e. the minimum reflection loss was -46.9 dB with a matching thickness of 2.2 mm and effective absorption bandwidth reached 5.2 GHz with a thickness of only 1.8 mm. This study demonstrates that the CaMnO3 would be a potential material in the microwave absorbing field and needs further research.

Oriented flaky carbonyl iron and MoS2/polyurethane composite with improved microwave absorption at thin thickness by shear force

Oriented flaky carbonyl iron and MoS2/polyurethane (FCI & MoS2/PU) composite with wide absorption bandwidth at thin thickness was successfully prepared by shear force. The morphology, electromagnetic, and microwave absorption properties in the frequency of 2.6–18 GHz of the composites were investigated. It was observed that the flaky FCI and MoS2 particles became orienting along the tape-casting direction under shear force and parallel to each other. Compared with the un-oriented FCI & MoS2/PU composite, the higher complex permittivity and permeability were obtained after oriented, which was superior in achieving improved microwave absorption performance at thin thickness. Wider absorption bandwidths of reflection loss (RL) values below − 5 dB were obtained at the thin thicknesses of 0.5–1.2 mm for the oriented FCI & MoS2/PU composite. It suggests that the orientation by shear force is a promising approach for the preparation of materials with good microwave absorption property at thin thickness.

PVP-assisted transformation of ZIF-67 into cobalt layered double hydroxide/carbon fiber as electromagnetic wave absorber

A high efficiency and ultra-light electromagnetic wave (EW) absorber is proposed without precedent. A series of hierarchical short carbon fibers (SCFs) based composites with in-situ grown cobalt layered double hydroxides (Co-LDHs) can be successfully synthesized via a simple PVP-assisted solvothermal method. The synthesized hierarchical structure is composed of 1D SCFs skeleton, 3D hydrangea-like Co-LDHs-1 and 1D rod-like Co-LDHs-2. Notably, ultralight (5 wt% in paraffin matrix), ultrathin (2.1 mm) and ultrawide (6.5 GHz) features are achieved in our composites by simply tuning the doping amount of PVP. The mechanism is explained by the tunable attenuation constant and impedance matching, that is, Co-LDHs-1 and Co-LDHs-2 with different conductivities and morphologies are coordinated to optimize the excessively high complex permittivity of SCFs. This contribution will shed the light on design of LDHs-based materials for light-weight, tuning, and high efficiency EW absorbers.

High loss liquid dielectric characterization: Comparison of microwave waveguide and resonator measurement techniques

AbstractThe microwave waveguide and resonator methods are compared as applied to the experimental determination of the dielectric properties of high loss liquids. A differential microwave waveguide cavity for measuring high loss liquids complex permittivity in a small volume has been designed and studied. This cavity consists of two circular waveguide cells with central rods made of quartz and surrounded by high loss liquid tested. The cells have different lengths to eliminate complex propagation coefficient measurement errors due to the diffraction effect on the ends of the layered waveguide cells. We have measured the wave amplitude and phase coefficients for the waveguide cavity to estimate physical properties of a high loss liquid under test. The resonant frequencies and the Q-factor of a semi-disk dielectric resonator with high loss liquid filling a capillary have been measured. We have selected water-ethanol solutions as a high loss liquid under test for both techniques. We have determined the measurement sensitivity for these two techniques. The measuring results are discussed. Both the waveguide and resonator methods provide comparable sensitivity and can be successfully used for the complex permittivity characterization of high loss liquids in small volumes.

Bamboo-like short carbon fibers@Fe3O4@phenolic resin and honeycomb-like short carbon fibers@Fe3O4@FeO composites as high-performance electromagnetic wave absorbing materials

In recent years, the development of an electromagnetic wave (EMW) absorbing material with low cost, wide bandwidth and strong absorption strength has been widely explored. In this study, SCFs@Fe3O4 was used as the precursor (S1), and two methods are explored to ameliorate the ultra-high complex permittivity. The method of adding phenolic resin has achieved ideal results, and its bamboo-like SCFs@Fe3O4@phenolic resin (PR) (S5) has excellent property. In addition, the electromagnetic properties can be improved by calcining the precursor at 700 °C in a tubular furnace protected by Ar gas, and the obtained honeycomb-like porous SCFs@Fe3O4@FeO (S4) composite has superior EMW absorption performance. The excellent EMW absorption performance comes from its unique porous structure. The sample also has a dual loss mechanism of dielectric and magnetic loss. Among them, conduction loss, interfacial polarization, Debye relaxation, hysteresis loss, natural ferromagnetic resonance and exchange resonance play an important role in the process of EMW absorption. It exhibits an effective absorption bandwidth (EAB) of 6.1 GHz with a thin thickness of 1.9 mm. Noting that, by adjusting the thickness (1–5 mm), reflection loss (RL) lower than −10 dB can be achieved in the range of 4–18 GHz, which covers the entire C, X and Ku bands. In this study, we not only successfully prepared honeycomb-like porous SCFs@Fe3O4@FeO and bamboo-like SCFs@Fe3O4@PR, but also proposed two simple methods to solve the excessively high complex permittivity of carbon materials. This has important reference value for the subsequent research of EMW absorbing materials.