High Complex Permittivity Research Articles

Debye Frequency-Extended Waveguide Permittivity Extraction for High Complex Permittivity Materials: Concrete Setting Process Characterization

A waveguide with central frequency 868 MHz is used in the transmission/reflection operation regime to accurately measure the behavior of the complex permittivity of high complex permittivity granular materials, and it has been frequency-extended up to 3 GHz using the Debye fit relaxation model. It is shown that for highly granular high permittivity materials, a waveguide-based transmission/reflection technique is necessary to reduce the uncertainty of the extracted permittivity values. The technique is first described and validated with isopropyl alcohol and then applied to the characterization of cement-based materials. This article provides accurate data on the evolution of the complex permittivity of concrete and mortar from the moment of pouring until air-dried condition is achieved.

Preparation of Carbon Encapsulated Magnetic FeCo Alloy Nanoparticles Supported on Carbon Nanotubes for Enhanced Microwave Absorption

A mixture of multi-walled carbon nanotubes in an ethanol solution of Fe(NO3)3.9H2O and Co(NO3)2.6H2O, were used to prepare an Fe2O3-CoO catalyst supported on carbon nanotubes by a supercritical fluid drying method. The carbon encapsulated FeCo nanoparticles were prepared by the catalytic decomposition of methane over the catalyst at 850°C for 30 mins. The morphology and microstructure of the catalyst and carbon encapsulated nanoparticles (CEP) were characterized by XRD, FESEM, EDS and TEM analyses. The electromagnetic characteristics and microwave absorption properties of the CEP product were also studied. Results show that the CEP product possesses high complex permittivity and good permeability in the microwave frequency range from 2 to 18 GHz. The maximum absorption peak was observed at 9.44 GHz for a large reflection loss (R) of 13.3 dB, with a band width of 7.4 GHz and 2.56 GHz, at R <- 5 dB and R <- 10 dB, respectively.

Magnetic aligned Fe3O4-reduced graphene oxide/waterborne polyurethane composites with controllable structure for high microwave absorption capacity

Fe3O4-reduced graphene oxide (FRGO)/waterborne polyurethane (WPU) composites with controllable structure and high microwave absorption capacity were developed. The aligned structure of FRGO in the FRGO/WPU composites was realized by a magnetic-field-induced method. WPU dispersion with nano-scaled particles could suppress the aggregation of FRGO during the drying process. The FRGO/WPU composites have much high complex permittivity, dielectric loss tangent, and microwave absorption capacity owing to the magnetic alignment. WPU with 2% FRGO achieved the minimum reflection loss value of −67.8 dB, which can be attributed to the orientation and three-dimensional structure of FRGO. The effective bandwidth wider than 14 GHz can be reached by a combination of two layers of composites with different permittivity through theoretical calculation. The aligned FRGO/WPU composites have a great future in the application of microwave absorption materials.

Multi-Walled Carbon Nanotubes Composites for Microwave Absorbing Applications

The response of materials to impinging electromagnetic waves is mainly determined by their dielectric (complex permittivity) and magnetic (complex permeability). In particular, radar absorbing materials are characterized by high complex permittivity (and eventually large values of magnetic permeability), Indeed, energy dissipation by dielectric relaxation and carrier conduction are principally responsible for diminishing microwave radiation reflection and transmission in non-magnetic materials. Therefore, the scientific and technological community has been investigating lightweight composites with high dielectric permittivity in order to improve the microwave absorption (i.e., radar cross-section reduction) in structural materials for the aerospace industry. Multiwalled carbon nanotubes films and their composites with different kind of polymeric resins are regarded as promising materials for radar absorbing applications because of their high permittivity. Nanocomposites based on commercial multi-wall carbon nano-tube (MWCNT) fillers dispersed in an epoxy resin matrix were fabricated. The morphology of the filler was analyzed by Field emission scanning electron microscopy (FESEM) and Raman spectroscopy, while the complex permittivity and the radiation reflection coefficient of the composites was measured in the radio frequency range. The reflection coefficient of a single-layer structure backed by a metallic plate was simulated based on the measured permittivity. Simulation achievements were compared to the measured reflection coefficient. Besides, the influence of morphological MWCNT parameters (i.e., aspect ratio and specific surface area) on the reflection coefficient was evaluated. Results verify that relatively low weight percent of MWCNTs are suitable for microwave absorption applications when incorporated into polymer matrixes (i.e., epoxy resin).

Enhanced Electromagnetic and Microwave Absorption Properties of Hybrid Ti3SiC2/BaFe12O19 Powders

In this work, Ti3SiC2 powders were composited with BaFe12O19 to improve electromagnetic and microwave absorption properties. The hybrid Ti3SiC2/BaFe12O19 powders were prepared in two different ways: mechanical mixing and auto-combustion. The electromagnetic and microwave absorption properties of the powders were investigated in the frequency range of 8.2–12.4 gigahertz (GHz). For the hybrid Ti3SiC2/BaFe12O19 powders, the complex permittivity was higher while the complex permeability was lower than individual BaFe12O19 powders. In addition, the hybrid powders prepared by auto-combustion had higher complex permittivity and permeability than the mechanical mixing powders. The most favorable absorption performance was obtained for the sample with a thickness of 2.4 mm. The reflection loss value below − 5 decibels (dB) was in the frequency range of 9.3–12.4 GHz, with a minimum reflection loss value of − 14.6 dB at 10.9 GHz.

Dielectric and microwave absorption properties of ZrB2/Al2O3 composite ceramics

ZrB2/Al2O3 composite ceramics are prepared by hot-pressing sintering process. Influences of ZrB2 content and particle size on dielectric and microwave absorption properties of the ceramics are investigated. There are no chemical reactions between the ZrB2 and Al2O3 powders during the sintering process at high temperature. The bulk densities of the ceramics decrease with the rising ZrB2 content and decreasing particle size. The ceramic with finer ZrB2 owns higher complex permittivity when the content of ZrB2 is fixed. As the ZrB2 content increases from 0 to 15 wt.%, the real part increases from 6.53 to 6.64 to 21.22–21.67 and the imaginary part rises from 0.61 to 0.7 to 4.83–4.49 in the frequency range of 8.2–12.4 GHz. The ceramic containing 15 wt.% ZrB2 presents the most favorable microwave absorption performance. For the ceramic with 1.4 mm thickness, the effective absorption bandwidth (≤ 10 dB) is achieved in the frequency range of 10.3–12 GHz, with a minimum RL value of − 15.6 dB at 10.8 GHz.

Investigation on microwave dielectric behavior of flaky carbonyl iron composites

The corresponding mechanism of high complex permittivity for micro flaky Fe-filled composites was still not apparent. In this letter, the dielectric behaviors of flaky carbonyl iron/epoxy resin composites within 2–18 GHz were studied experimentally and theoretically. Results show that an obvious increase both in real and imaginary part of permittivity can be observed as the volume fractions of FCI is up to the percolation threshold. Considering the influence of conduction current, a revised Debye model of permittivity has been proposed to reveal the mechanism of dielectric behavior in composites. The best fitting result demonstrate that conduction loss played an important role in increasing the permittivity after percolation. Fe3O4 was used as encapsulated shell on surface of the FCI to control the conduction and enhance the percolation threshold, resulting in better microwave absorption from better balance between the complex permittivity and permeability.

Improved microwave biosensor for non-invasive dielectric characterization of biological tissues

Microwave planar sensors have a great interest in the medical environment due to their ability to measure the bulk dielectric parameters of biological tissues through non-invasive and contact-less sensing properties. Changes of these parameters, which are frequency dependent, can be representative of the pathological state of biological tissues. In this work, an improved prototype of planar sensor based on a microwave ring resonator operating at 1 GHz for the fundamental mode is presented. The objective is to obtain a better sensitivity for measuring high complex permittivity values of materials such as biological tissues, and to obtain higher precision in parameters determination. The performances of two sensors optimized on two different substrates were measured in a frequency range 1–10 GHz; an accurate equivalent electrical model is proposed to reproduce the frequency dependence of the resonators. Characterization of liquids and ex-vivo animal tissues is achieved to evaluate the effectiveness and the performances of the resonator sensor, and results are compared with electromagnetic simulations.

NiFe2O4 nanoparticles filled BaTiO3 ceramics for high-performance electromagnetic interference shielding applications

The complex permittivity, complex permeability and electromagnetic interference (EMI) shielding properties of the BaTiO3 ceramic filled with NiFe2O4 micron-grade particle or nanoparticles were investigated. As the micron-grade NiFe2O4 particles decreased to about 30 nm, NiFe2O4 nanoparticles filled BaTiO3 ceramics with high complex permittivity can be obtained in the X–band. The total EMI shielding effectiveness of 30 wt% NiFe2O4 nanoparticles filled BaTiO3 ceramics is greater than 34 dB with a thickness of 1.2 mm. The results of this experiments demonstrated that such ceramics can be used to produce high complex permittivity materials for high-performance EMI shielding applications.

Near surface bulk density estimates of NEAs from radar observations and permittivity measurements of powdered geologic material

The variations in near-surface properties and regolith structure of asteroids are currently not well constrained by remote sensing techniques. Radar is a useful tool for such determinations of Near-Earth Asteroids (NEAs) as the power of the reflected signal from the surface is dependent on the bulk density, ρbd, and dielectric permittivity. In this study, high precision complex permittivity measurements of powdered aluminum oxide and dunite samples are used to characterize the change in the real part of the permittivity with the bulk density of the sample. In this work, we use silica aerogel for the first time to increase the void space in the samples (and decrease the bulk density) without significantly altering the electrical properties. We fit various mixing equations to the experimental results. The Looyenga–Landau--Lifshitz mixing formula has the best fit and the Lichtenecker mixing formula, which is typically used to approximate planetary regolith, does not model the results well. We find that the Looyenga--Landau–Lifshitz formula adequately matches Lunar regolith permittivity measurements, and we incorporate it into an existing model for obtaining asteroid regolith bulk density from radar returns which is then used to estimate the bulk density in the near surface of NEA’s (101955) Bennu and (25143) Itokawa. Constraints on the material properties appropriate for either asteroid give average estimates of ρbd=1.27±0.33g/cm3 for Bennu and ρbd=1.68±0.53g/cm3 for Itokawa. We conclude that our data suggest that the Looyenga-Landau-Lifshitz mixing model, in tandem with an appropriate radar scattering model, is the best method for estimating bulk densities of regoliths from radar observations of airless bodies.

High-temperature electromagnetic interference shielding of layered Ti3AlC2 ceramics

The electromagnetic interference (EMI) shielding of dense Ti3AlC2 ceramics with distinct microstructures were investigated up to 800°C. Ti3AlC2 exhibits high EMI shielding effectiveness (SE) around 30dB which was mainly attributed to high electrical conductivity and complex permittivity. Besides, the EMI shielding were sensitive to the microstructure. Ti3AlC2 with high aspect ratio grains and certain degree of texture were preferred for high SE indicating that the typical layered structure also contributes to the high shielding capability. These results indicate that layered Ti3AlC2 ceramics could be considered as promising structural EMI shielding materials at high temperatures.

Improvement dielectric and microwave properties of SiCf/SiC-AlPO4 composites prepared by precursor infiltration and pyrolysis process

SiCf/SiC-AlPO4 composites were prepared with dual matrix precursor of polycarbosilane and aluminum dihydrogen phosphate during the PIP process to optimize the dielectric and microwave absorption properties at 8.2–12.4 GHz. Both complex permittivity and tanδ of the composites with simplex SiC and dual SiC-AlPO4 matrix show obvious sensitive behavior with the test frequency. The SiCf/SiC composites exhibit poor reflection loss values owing to the deviation of impedance matching caused by the high complex permittivity. With introduction of AlPO4 into SiCf/SiC composites, the complex permittivity decreased to proper levels and the microwave absorption properties are significantly improved over the whole test frequency ranges. The results indicates that the minimum RL values and bandwidth below −10 dB for the SiCf/SiC-AlPO4 composites reach-22.8 dB and 4.2 GHz, respectively.

Precursor-directed synthesis of porous cobalt assemblies with tunable close-packed hexagonal and face-centered cubic phases for the effective enhancement in microwave absorption

Metal cobalt is one of the most promising candidates for high-performance microwave absorbers due to its compatible dielectric loss and magnetic loss abilities. Rational design on the microstructure of metal cobalt became a popular way to upgrade its microwave absorption performance in the past decade, while much less attention has been paid to the electromagnetic functions derived from its different crystal structures. Herein, we report the microwave absorption of porous cobalt assemblies with varied composition of close-packed hexagonal (hcp) and face-centered cubic (fcc) phases. Electromagnetic analysis reveals that the change of phase composition can significantly impact the complex permittivity and complex permeability of metal cobalt, where hcp-cobalt favors high complex permittivity and fcc-cobalt produces high complex permeability. The optimum phase composition in these porous cobalt assemblies will promise well-matched characteristic impedance and good performance in strong reflection loss (−41.0 dB at 9.4 GHz) and wide response bandwidth (4.0–17.4 GHz over −10.0 dB). The enhanced microwave absorption is superior to many cobalt absorbers ever reported. It is believed that these results will provide a new pathway to the design and preparation of highly effective metal cobalt and cobalt-based composites as novel microwave absorbers in the future.

Enhanced dielectric and microwave absorption properties of Cr/Al2O3 coatings deposited by low‐power plasma spraying

AbstractLow‐power plasma‐sprayed Cr/Al2O3 coatings have been developed for their potential application as broad bandwidth, thin thickness, lightweight, and strong microwave‐absorbing materials. The dielectric and microwave absorption properties of the as‐sprayed coatings were studied in the X‐band (from 8.2 to 12.4 GHz). High complex permittivity of the coatings was obtained because of a large number of internal boundaries and the conductive networks. Meanwhile, a significant enhancement of microwave absorption properties of the coating was achieved due to the enhanced interfacial polarization and conductance loss. The reflection loss (RL) &lt;−10 dB of the Al2O3–15Cr coating was obtained from 9.8 to 11.4 GHz by choosing an appropriate coating thickness, and an optimal minimum reflection loss (RLmin) of −45.35 dB was achieved at 10.3 GHz with a thin thickness of 1.32 mm.

Titanium carbide (MXene) nanosheets as promising microwave absorbers

The electromagnetic properties of Ti3C2 nanosheets, prepared by immersing Ti3AlC2 powders in HF and subsequent ultrasonication, were first investigated in the frequency range of 12.4–18GHz. The Ti3C2 nanosheet filled composites showed high relative complex permittivity and microwave absorption as compared to those filled with the original Ti3AlC2 powders under the same content. Reflection loss values of the Ti3C2 nanosheets filled composites exceeded −11dB (more than 92% absorption) in the frequency range of 12.4–18GHz with a thickness of 1.4mm. These results suggest that the Ti3C2 nanosheets with unique dielectric properties are highly promising fillers for dielectric and electromagnetic applications.

Enhanced dielectric and electromagnetic interference shielding properties of FeSiAl/Al2O3 ceramics by plasma spraying

FeSiAl/Al2O3 ceramics with flaky metallic FeSiAl were fabricated by low power plasma spraying. High complex permittivity of the flaky FeSiAl/Al2O3 ceramics can be obtained in the X-band due to the larger internal boundary layer capacitance and conductive network. The reflection loss (RL) values less than −7 dB (larger than 80% absorption) can be obtained in the whole X-band for the FeSiAl/Al2O3 ceramics with 20 wt% FeSiAl and a thickness of 1.18 mm. The total electromagnetic interference (EMI) shielding effectiveness is over 36 dB in the whole X-band for the FeSiAl/Al2O3 ceramics with 40 wt% FeSiAl and a thickness of 2 mm. These results indicated that such ceramics with conductive networks and high complex permittivity can be used to produce effective shielding materials for EM application.

Magnetic Properties of Amorphous Hydrogenated Carbon Thin Films Doped by Ni

Amorphous hydrogenated carbon thin films were deposited on Si and glassceramics substrates by reactive ion-plasma magnetron sputtering. Thin films were without (a-C:H) and with Ni nanoparticles (a-C:H:Ni). Measurement of absorption coefficient in 220-850 nm spectral range using spectrophotometer and following approximation applying of Tauc method shows that obtained a-C:H thin films material has near 3.6 eV optical band gap and confirmed that this material is amorphous semiconductor. Thin films with Ni nanoparticles have a high complex permittivity ε* and permeability μ* (ε′ ≈ 1000–10000, ε′′ ≈ 100–1500, μ′ ≈ 10–70, μ′′ ≈ 0.4-10). Some magnetic parameters of thin films were determined.

Synthesis, magnetic and microwave electromagnetic properties of dendritic iron

Iron dendritic micropines are synthesized by a hydrogen reduction, where the hematite dendritic micropines prepared by a hydrothermal method are used as starting materials. The as-obtained dendritic iron exhibits enhanced coercivity and remanent magnetization at room temperature and high complex permittivity at 2–18 GHz due to the peculiar shape anisotropy and good crystallinity. The negative imaginary permeability is observed at 14.5–18.0 GHz, suggesting it has a potential as a left-handed material. The paraffin-based composites containing 30 wt% dendritic irons show large permittivity resulting from the charge polarization and the conductivity and have a minimal reflection loss (RL) of −37.4 dB at 7.4 GHz when the thickness (d) is 2.0 mm. The RL values less than −20 dB are obtained in the frequency range of 5.5–12.9 GHz when d increases from 0.9 to 3.0 mm.

Temperature-dependent dielectric and microwave absorption properties of SiCf/SiC–Al2O3 composites modified by thermal cross-linking procedure

SiCf/SiC–Al2O3 composites were modified by thermal cross-linking procedure during the PIP process to optimize the dielectric and microwave absorption properties in the temperatures of 25–700°C at 8.2–12.4GHz. Both complex permittivity and tanδ of the composites without and after cross-linking show obvious temperature-dependent behavior and increase with the increasing temperatures. The mechanisms for these corresponding enhancements are also proposed. The composites without cross-linking exhibit poor RL values in the experimental temperatures owing to the deviation of impedance matching caused by the high complex permittivity. After cross-linking, the complex permittivity of the composites is reduced to suitable levels and shows low increasing tendency and amplitude with the increasing temperatures. Thus, the microwave absorption properties of the composites are significantly enhanced over the whole temperature and frequency ranges. Particularly, results indicate that minimum RL values and bandwidth below −10dB reach −41.71dB and 3.4GHz at 300°C, and −22.76dB and 4.1GHz at 400°C, respectively.

A new triple-layered composite for high-performance broadband microwave absorption

We report a novel, triple-layered composite composed of multi-walled nanotubes (MWNTs) dispersed in an epoxy/cotton fabric resistive layer that is sandwiched between two MWNT-dispersed E-glass fiber/epoxy dielectric absorbing layers. This structure efficiently absorbs microwave radiation over a frequency range of 2–18GHz. This layered, composite structure is thin relative to previously developed, multi-layer systems with a resistive layer/low dielectric layer/resistive layer/low-dielectric layer structure. The paper-like resistive layer facilitates broadband microwave absorption by matching the impedance between the two absorbing layers. The E-glass fiber/epoxy laminate composites provide structural and mechanical support. The conductive MWNT filler controls dielectric losses through the formation of a network that allows high complex permittivity at low concentrations. Free-space measurements showed that the resulting triple-layered structure boasts an outstandingly broad bandwidth of reflection loss below −10dB from 4.7 to 13.7GHz. This new multi-layer structure offers a promising solution for attenuating electromagnetic waves over broad frequency ranges.