Complex Permittivity Research Articles

Enhancement in the Electrical Properties and Electromagnetic Interference Shielding Performance of Acrylonitrile–Butadiene–Styrene/Carbon Nanotubes Foams via the Introduction of Bimodal Cell Structures

The cell structures of conductive polymer‐based composite foams significantly influence their electrical properties and electromagnetic interference (EMI) shielding effectiveness (SE), necessitating a thorough understanding of how these properties improve with evolving cell structures. In this study, supercritical CO2 foaming technique was manipulated to fabricate acrylonitrile–butadiene–styrene (ABS)/carbon nanotubes (CNTs) foams. The constant‐temperature mode was used to prepare unimodal foams (UF), while the bimodal foams (BF) were produced by varying‐temperature mode. The foaming properties, electrical conductivity, complex permittivity, and EMI SE of ABS/CNTs foams with various cell structures are methodically investigated at identical volume expansion ratio and CNTs content. The electrical conductivity of bimodal ABS/CNTs foam with CNTs content of 20% (BF‐C20) is 0.2191 S cm−1, higher than that of unimodal ABS/CNTs foam with CNTs content of 20% (UF‐C20) (0.1765 S cm−1) owing to the introduction of bimodal cell structures. Complex permittivity results manifest that at 8.2 GHz, the ε′ and ε″ of BF‐C20 are 67.7 and 74.5, respectively, which are higher than 57.9 and 52.9 of UF‐C20. Among all ABS/CNTs foams, the total EMI SE of BF‐C20 attains the highest EMI shielding value, which reaches 30.2 dB. Furthermore, the absolute shielding effectiveness of BF‐C20 is 188.5 dB (g cm−2)−1, which is 17.3% higher than that of UF‐C20.

Measuring and Extracting the Complex Permittivity of Porous Ceramic Materials in the Y-Band

ABSTRACT Porous ceramics find extensive applications in aerospace and other fields, and the measurement of their electromagnetic parameters is helpful in optimization of material properties and device designs. In this paper, a free-space 8f quasi-optical measurement system consisting of a vector network analyzer, a 325–500 GHz frequency expansion module, and four off-axis parabolic mirrors have been established to measure the transmission parameter S21 of porous ceramics. The complex permittivity was extracted according to the Newton-Raphson iterative method utilizing its measured S21 parameters based on flat mode. The porous ceramics with different porosity and density were measured and the relationships between their permittivity and porosity and density was established, respectively. These results are beneficial to the quality evaluation and application design of porous ceramic materials in the aerospace field.

Weakly negative permittivity from three-dimensional printed polydimethylsiloxane/graphene films in the radio frequency band

Epsilon-negative materials (ENMs) has become a research hotspot due to their unique physical properties and practical applications in fields such as directional radiation and tunneling effects. In this work, weakly negative permittivity (from -50 at 10 kHz to -25 at 1 MHz) was realized from three-dimensional printed polydimethylsiloxane/graphene films. The dielectric properties including complex permittivity (ε', ε″), complex impedance (Z', Z″), alternating current conductivity (σac), and dielectric loss tangent (tanδ) were investigated. When graphene content reached 15 wt%, the interconnected graphene network was formed, resulting in negative permittivity. Theory calculations were further used to explore the mechanism. The electrons around carbon atoms showed significant delocalization. With the effect of the external electromagnetic field, the electrons on the surface of graphene could easily oscillate collectively, leading to negative permittivity. Differential charge density exhibited that interfacial electric fields were formed between polydimethylsiloxane and graphene, enhancing positive permittivity. Therefore, when the graphene content reached 15 wt%, the film exhibited weakly negative permittivity, which was the result of the mutual balance between the graphene conductive network and the interface electric fields.

Electric Response of a Negative Dielectric Anisotropy Nematic Liquid Crystal Doped with Ionic Dopant

The electrical properties measurements have been performed in a homogeneous alignment parallelepiped cell containing 4-methoxy benzylidene- 4-butylaniline (MBBA) liquid crystal doped with 0.02%wt tetrabutylammonium bromide (TBAB). The measurement of the complex permittivity was conducted in the nematic phase, covering a frequency range of 42 Hz to 5 MHz. A new relaxation mode was observed in the low-frequency region, which was not present in pure MBBA. The obtained dielectric dispersion could be fitted using the double Cole–Cole formula to determine the relaxation frequencies. The steady-state current exhibited a nonlinear dependence on the applied voltage, and hysteresis was observed in the transient current-voltage characteristic curve.

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.

Dielectric relaxation study of pyridine-n-propyl alcohol mixture using time domain reflectometry technique

ABSTRACT The Time Domain Reflectometry (TDR) technique was used to analyse the complex permittivity spectra of the Pyridine-n-propyl alcohol mixture across the concentration and temperature range of 10–25°C. The Cole-Davidson model was used to match the Pyridine- n-propyl alcohol complex permittivity spectra. The nonlinear least squares fit approach has been used to obtain the Static dielectric constant (ε0), Relaxation time (τ), Effective Kirkwood correlation factor (geff) Excess permittivity ε 0 E , Thermodynamic parameters (activation enthalpy and activation entropy), and Bruggeman factor (fB). Furthermore, the analysis of excess characteristics and the Bruggeman factor points towards the presence of hydrogen bonding between the pyridine and n-propyl alcohol molecules.

Magneto-optical properties of heavily Fe-doped GaAs: a density functional approach.

The magneto-optical properties of heavily iron-doped GaAs are investigated. Complex permittivity, optical conductivity and absorption coefficients are mainly discussed using spin-polarized electronic band structures close to the Fermi energy level. Furthermore, prominent magneto-optical properties in visible light and ultraviolet regions, including magnetic circular dichroism as well as complex Kerr and Faraday rotation angles, are presented and discussed. A density functional approach corroborated by some experimental results indicates that GaAs with 25% Fe dopants either in As or Ga sites is a promising ferromagnetic material and a useful platform for optoelectronic and spintronic device applications.

Design and development of low‐weight and high‐strength electromagnetic shielding nanocomposite in a microwave frequency range

AbstractIn this novel work, we fabricated the multiwalled carbon nanotubes (MWCNTs)/polypyrrole (PPy) nanocomposite by employing the novel ultrasonic dual mixing (UDM) method. Subsequently, the atomistics and structural characterizations of the fabricated MWCNT/PPy nanocomposite (PM) were performed using SEM, Raman, and XRD. Furthermore, we experimentally evaluated the mechanical properties of the fabricated PM by reinforcing the PPy matrix with MWCNTs at different wt.%. The experimental study of PM shows that the mechanical properties depend upon the amount and dispersion quality of MWCNTs in the PPy matrix. In order to validate the experimental outcomes, the conservative fully coupled finite element (FE) models were developed using the ANSYS Workbench and novel FEAST software. The electromagnetic interference (EMI) shielding effectiveness (SE) of developed PM was also calculated at different thicknesses of PM by taking the effect of SE due to absorption (SEA) and SE reflection (SER) in the microwave frequency range of the C‐band (5.37–8.2 GHz) into consideration. The total SE (SET), the cumulative sum of SEA and SER, is the strong function of the thickness of the PM. Additionally, our study's outcomes reveal that as the thickness of the PM increased, the SET also increased by dominating the SEA rather than the SER. The SET value of 75 dB was observed for a sample with a thickness of 3 mm, indicating a high level of shielding. Thus, material becomes a highly attractive option for achieving high‐performance EMI SE in the C‐band for satellite communications, Wi‐Fi devices, weather radar systems, surveillance, and cordless telephones. A complex permittivity and permeability were also studied using the Nicolson–Ross–Weir (NRW) method to understand the mechanism behind the EMI SE.Highlights Nanocomposite fabricated using the novel ultrasonic dual mixing method. FE models were developed to validate experimental results. The EC and EMI SE are improved with the increase in the sample thickness. Absorption, rather than reflection, dominated the shielding mechanism. SE was observed dB with good mechanical properties.

Influence of bed temperature on the final properties of PLA parts manufactured by material extrusion

Purpose This paper aims to study the influence of bed temperature on the properties of printed parts and their structural stability. Design/methodology/approach Material extrusion is a manufacturing technique in which a part is completed layer by layer with molten filament. The first layer is deposited on a build platform called bed, which is usually at a controlled temperature above room temperature. The density, coefficient of thermal expansion and Young’s modulus were determined as a function of the bed temperature. The complex permittivity was determined at different temperatures, with the aim of analyzing the influence of the bed temperature and isothermal treatments on the characteristics of the amorphous phase. Findings The Young’s modulus presented a non-monotonic behavior, while the coefficient of thermal expansion did not present a clear dependence on the bed temperature. However, a contraction of the dimensions of the parts was observed after heating at temperatures above the glass transition. With treatments at temperatures lower than the glass transition temperature, no changes were observed. However, with treatments at temperatures higher than this, changes in the mobile amorphous region were inferred. Originality/value Issues related to the use of parts manufactured by 3D printing after a posterior heating were analyzed: an improvement in the Young’s modulus and a slight variation of the coefficient of thermal expansion were observed. However, a significant variation in dimensions was detected, mainly for the lowest bed temperatures. This is important for possible applications at temperatures above 60°C.

Determining Bandgaps in the Layered Group‐10 2D Transition Metal Dichalcogenide PtSe2

AbstractUnlike traditional group‐6 transition metal dichalcogenides (TMDs), group‐10 TMDs such as PtSe2 and PdTe2 possess highly tuneable indirect bandgaps, transitioning from semiconducting in the near‐infrared to semimetal behavior with a number of monolayers (MLs). This opens up the possibility of TMD‐based mid‐infrared and terahertz optoelectronics. Despite this large potential, the optical properties of such materials have shown an extremely large disparity between that predicted and measured. For example, simulations show that a few MLs is required for the semiconductor–semimetal transition, whilst tens of MLs is found experimentally. This is a result of widely used optical extrapolation methods to determine bandgaps, such as the Tauc plot approach, that are not adapted here owing to i) nearby direct transitions, ii) the material dimensionality and iii) large changes in the non‐parabolic bandstructure with MLs. Here, uniquely combining optical ellipsometry to determine the complex permittivity, terahertz time resolved spectroscopy for the complex conductivity and in‐depth density functional theory (DFT) simulations, it is shown that the optical properties and bandstructure can be determined reliably and demonstrate clearly that the semiconductor‐semimetal transition occurs for PtSe2 layers ≈5 MLs. The microscopic origins of the observed transitions and the crucial role of the Coulomb interaction for thin semiconducting layers, and that of interlayer van der Waals forces for multilayer semimetallic samples are also demonstrated. This work of combining complimentary experimental techniques and extensive simulations avoids the application of constrained extrapolation methods to determine the optical properties of group‐10 TMDs, and will be of importance for future mid‐infrared and terahertz applications.

Rapid synthesis and enhanced microwave absorption of carbon-supported high-entropy metal oxide

Complex preparation process and long annealing time of high-entropy metal oxide (HEO) limit its extensive application as microwave absorber at civil and military fields. Herein, a rapid synthesis method of mesocarbon microbeads (MCMB)-loaded spinel structure (FeCoCrNiMnCu)O is studied via plasma flow. In the presence of extreme plasma heating rate, a nanoscale high-entropy phase can be formed while modifying the surface activity of carbon. Simultaneously, due to the short synthesis time, gaseous conversion of carbon under high temperature aerobic conditions is avoided. By appending of carbon, the dual-media absorber exhibits better absorption performance with a maximum peak (-44.7 dB), which achieves over 90 % absorption in 11.2–15.5 GHz with a thickness of only 1.9 mm. Interestingly, the real and imaginary parts of complex permittivity increase remarkably. The former change is attributed to the expansion of heterogeneous hole carrier region and the latter dues to the interfacial polarization of the strongly coupled interface. The increase of dielectric loss and conductance loss leads to great improvement of microwave absorption. Our study injects new vitality into the preparation of HEO for advanced applications and the as-prepared HEOM is promised to be high-efficient microwave absorber based on robust stability and excellent absorbing performance.

Complex Permittivity Spectra of Granular Polymer Composites with Dispersed Ag-Coated Cu Flakes

AbstractConductive particle-containing granular composites with tuneable negative permittivity are being studied to improve the performance of electromagnetic devices, such as shielding materials. In this study, we investigated the relative complex permittivity and electrical conductivity of granular composites of polyphenylene sulfide (PPS) resin and Ag-coated Cu flakes in the radio- to microwave-frequency range and compared them with those of PPS/bare Cu flake composites. Electrical conductivity measurements revealed that the PPS/Ag-coated Cu flake composites have a lower percolation threshold (φc) than the PPS/bare Cu flake composites, whereas the electrical conductivity of the PPS/Ag-coated Cu flake composites in the percolated particle state was higher at the same particle volume fraction. At particle contents above φc, a low-frequency plasmonic state of conduction electrons was achieved in the percolated particle chains in both composites, and negative permittivity spectra were obtained. The percolated PPS/Ag-coated Cu flake composites had a negative permittivity up to a higher frequency than the percolated PPS/bare Cu flake composites. Furthermore, the Drude model was used to analyze the negative permittivity spectra of the composites in the percolated particle state. The plasma frequency of the composites with percolated Ag-coated Cu flakes was higher than that of the composites with percolated bare Cu flakes. Thus, coating Ag on Cu particles improved the conductivity of the composite, leading to negative permittivity up to higher frequencies. This study contributes to the enhancement of the negative permittivity achieved by granular composites, which is useful for microwave technology applications. Graphical Abstract

Self-assembly core–shell hollow fly ash sphere-bridged Ti3C2Tx MXene 3D spherical/ lamellar composites with specific electromagnetic wave absorption performance

A new A/B spherical/lamellar electromagnetic wave absorption (EWA) model was established, in which A phase was a spherical material with low dielectric constant and B phase was a lamellar 2D material with high dielectric constant. Insulative fly ash cenospheres (FA) and conductive Ti3C2Tx flakes were selected as the A phase and the B phase which acted as the framework (as well as core) and the interface builder (as well as shell) respectively to construct the novel 3D connected structure system via simple self-assembly. FA, a cheap power plants solid waste, was used as an insulating bridge for Ti3C2Tx to help to adjust the conductivity, the electromagnetic parameters and the impedance matching. The synergistic effect of the special heterogeneous structures and the moderate composition of FA and Ti3C2Tx endowed the composites with controllable complex permittivity and excellent EWA performances. FA@T0.35 exhibited a RLmin of − 50.94 dB at 3.5 mm and its widest EAB was 4.25 GHz at 2.0 mm. FA@T0.40 reached a RLmin of − 45.13 dB with an EAB of 4.03 GHz at 2.0 mm. The widest EAB was 4.19 GHz at only 1.5 mm. The work offers a new guideline for designing a high-performance EWA material by using waste materials.

Control of dielectric properties of composite materials by editing low dielectric ceramics with anchored structure

Extensive research over the impact of the content of a particular constituent in composites on their electromagnetic wave-absorbing performance has been witnessed. However, an accurate summary regarding the impact of ceramics’ dielectric constant on electromagnetic absorption performance of ceramic/graphene composites is still ad referendum. In this paper, a series of low-dielectric ceramics were synthesized by precisely controlling the proportion of boric acid and subsequently integrated with reduced Graphene nanosheets (RGNSs) to fabricate ceramic/RGNSs composites for efficient electromagnetic wave absorption. The amorphous range of ceramics, which is affected by the boric acid content, controls the dielectric properties that subsequently affect the absorption performance. Increasing the amorphous range of ceramics was found to result in an increase in both maximum value and stability of the actual component of complex permittivity for SiBON/RGNSs composite material. Additionally, tangent values at X and Ku bands also exhibited an increase, leading to an augmented microwave absorption performance (maximum absorption bandwidth: 4.94 GHz; minimum reflection loss: −50.45 dB).

Rapid fabrication of Ba1–xSrxTiO3 ceramics via reactive flash sintering

Ba1–xSrxTiO3 (BST) ceramics are promising dielectric materials. However, their fabrication is usually laborious, requiring long preparation stages and high temperatures, which are not favorable for tuning the grain size and dielectric properties. In this study, (Ba1–xSrx)TiO3 (x = 0.1, 0.3, 0.5) ceramics were produced via reactive flash sintering (RFS) of a mixture of BaCO3, SrCO3 and TiO2 powders at 900 °C and relatively short times (15–90 s). The electric field (E-field) with a strength of 400 V/cm and the current densities of 25–75 mA mm−2 were applied during RFS. Once the Sr2+ content increased, the incubation time for the RFS decreased, suggesting that the addition of Sr2+ facilitated the RFS process. At the RFS time above 15 s, a single-phase Ba0.6Sr0.4TiO3 perovskite structure was formed. When the time and the applied current density increased, both the densities and grain sizes within the ceramics increased. This increased the real part (ε’) of the complex permittivity compared to that of the conventionally sintered (CS) ceramic. Moreover, the enhancement of mass transport by E-field-induced oxygen vacancies was considered the predominant mechanism of RFS in the production of BST ceramics.

Collective effect of reduced graphene oxide and Zn2U hexaferrite filler based epoxy composites on microwave absorption with improved mechanical properties

Herein, we have synthesized Ba4Zn2Fe36O60 (Zn2U) and have produced reduced graphene oxide (rGO) wrapped filler material for the preparation of epoxy composites of different weight percentages (40, 50 and 60 wt %). These rGO wrapped Zn2U hexaferrite filler was analyzed by XRD and SEM to examine the crystal structure and distribution of Zn2U hexaferrite in between/on the surface of rGO respectively. The magnetic and mechanical properties of prepared rGO@Zn2U@epoxy composites were evaluated by employing vibrating sample magnetometer and universal testing machine respectively. The investigation involved analyzing the complex permittivity and permeability, and based on this information, an assessment of the reflection loss was conducted. The results obtained indicate that epoxy composites with a filler content of 60 % rGO@Zn2U have significant promise as an advantageous choice for EMI shielding and microwave absorption applications.

Ionic Conductivity Studies on Proton Conducting Solid Biopolymer Electrolyte based on 2-Hydroxyethyl Cellulose (2HEC) Doped with Ammonium Chloride (NH4CL)

Solution casting techniques have been used to create a solid biopolymer electrolyte (SBEs) based on 2-hydroxyethyl cellulose (2HEC) doped with varying amounts of ammonium chloride (NH4Cl) to investigate the potential of NH4Cl in enhancing the ionic conductivity of SBE. Electrical impedance spectroscopy (EIS) was used to measure the impedance of the electrolytes between 50 Hz and 1 MHz in frequency. At room temperature, ionic conductivity was at its highest for 16% weight percent of NH4Cl which was 1.74 x 10-3 Scm-1. The temperature dependence data of 2HEC-NH4Cl SBEs abides Arrhenius behaviour. Complex electrical modulus and complex permittivity were used to assess the dielectric data for the sample at various temperatures.

Designing Tb-doped Mn-Ni-Cu ferrite meta-absorbers for a broad frequency range: Magnetodielectric, physical, reflection loss, and absorptivity characteristics

Developing meta-absorbers with exceptional EM wave absorption, shielding capacity, and superior dissipation performance remains worthwhile in the gigahertz (GHz) range. Electromagnetic pollution has a detrimental impact on the health of living organisms due to the emergence of high-frequency waves from communication devices. The challenges were addressed by preparing, designing, and simulating ferrite-based meta-absorbers. The nanoferrites of Tb-doped Mn-Ni-Cu (Mn0.2Ni0.3Cu0.5TbxFe2-xO4) with different levels of Tb (x = 0.00, 0.025, 0.05, 0.075) were prepared utilizing the sol-gel auto combustion technique. The phase, structural, elastic, and magnetic features of the Tb-doped Mn-Ni-Cu ferrites were examined using XRD, FESEM, FTIR, and VSM techniques, respectively. The MAUD software was utilized to obtain refinement and precise structural characteristics. The force constant, phase, and elastic properties were also assessed. The magnetization and coercivity exhibited a reduction. An assessment was conducted on the performance of field switching and high-frequency response. At high frequencies, the dielectric characteristics, such as complex permeability and permittivity, exhibit greater values, while losses diminish. At a frequency of 4.6 GHz, a reflection loss (RL) of −54.21 dB was measured for x = 0.05, whereas a reflection loss of −54.03 dB was measured for x = 0.075. The meta-absorbers of Tb-doped Mn-Ni-Cu ferrites were designed and simulated. The absorptivity of the Tb-doped Mn-Ni-Cu ferrite substrate material is illustrated. The meta-absorber exhibited three distinct absorption peaks at frequencies of 1.5, 2.5, and 4.5–6 GHz, respectively. The absorptivity of the material likewise rises with an increase in the incidence angle; the material's absorptivity also increases. The TM mode exhibited the highest values at an angle of 60 degrees, while the TE mode displayed the highest values at an angle of 0 degrees. Tb-doped Mn-Ni-Cu ferrites are effective for electromagnetic interference (EMI), multilayer ceramic integrated circuits (MLCIs), shielding, and absorbing high-frequency signals in 5 G applications.

A novel microwave microfluidic sensor for purity detection of Medicines’ raw materials

A novel microwave microfluidic sensor is offered for purity detection of medicine’s raw materials which is composed of two radial components and a PDMS microfluidic holder to apply the test samples. The basis of the sensing method is the shift in the resonance frequency and quality factor of the resonator in the presence of liquids with different complex permittivity. To investigate the performance of the proposed sensor, various solvents, binary mixtures of ethanol–water, and raw materials of 3 different medicines have been checked and a 980 MHz variation of resonance frequency, which provided a very high sensitivity equal to 0.53, was achieved for a complex permittivity range of 1–78.4. The proposed sensor has a small size, simple design, easy fabrication process, and very high sensitivity while it is fabricated on an FR4 substrate which decreases the total cost significantly.

Electrical Quantum Coupling of Subsurface-Nanolayer Quasipolarons.

We perform dielectric and impedance spectrums on the compressively-strained ceramics of multiferroic bismuth ferrite. The subsurface-nanolayer quasipolarons manifest the step-like characteristic of pressure-dependent transient frequency and, furthermore, pressure-dependency fails in the transformation between complex permittivity and electrical impedance, which is well-known in classic dielectric physics, as well as the bulk dipole chain at the end of the dissipation peak.