Real Permittivity Research Articles

MWCNT induced negative real permittivity in a copolyester of Bisphenol-A with terephthalic and isophthalic acids

In the present study, the negative real permittivity behavior of a copolyester of bisphenol-A with terephthalic acid and isophthalic acid (PAr) containing 1.5 to 7.5 wt% multi-walled carbon nanotubes (MWCNTs) have been investigated in detail. The structural and morphological analysis of the melt-mixed composites was performed by Fourier transform infrared spectroscopy using attenuated total reflection (FTIR-ATR), atomic force microscopy (AFM), x-ray diffraction (XRD), and light microscopy. The influences of the MWCNT filler on the AC impedance, complex permittivity, and AC conductivity of the PAr polymer matrix were investigated at different operating temperatures varied between 296 K and 373 K. The transition from a negative to positive real permittivity was observed at different crossover frequencies depending on the MWCNT content of the composites whereas pure PAr showed positive values at all frequencies. The negative real permittivity characteristic of the composites was discussed in the context of Drude model.

Methodology for High Purity Broadband Near-Unity THz Linear Polarization Converter and its Switching Characteristics

Terahertz (THz) polarization converter with high purity of linear polarization conversion, broader conversion band and near-unity conversion efficiency is one of the main research interests in the photonic field. Based on a tri-layer structure of Au, parylene-C and Au, a high purity broadband near-unity THz converter is successfully achieved, and the relative bandwidth (RBW) with polarization conversion ratio (PCR) greater than 98% (90%) can reach about 64.782% (73.094%). The ellipticity is close to zero and the polarization azimuth angle is almost equal to ±90° within the frequency range of 1.818~2.631 THz. These performances originate mainly from the hybridization of the electric excitations and strong coupling effect among the three resonant modes. Besides, the dependencies of conversion performances on the material's electromagnetic parameters and structural parameters are discussed in detail. By selecting metal material with high conductivity and dielectric material with small real and imaginary permittivities, the conversion performances of the proposed converter can be well improved. Meanwhile, the conversion performances are highly robust when the structural parameters change slightly near the optimized value. Finally, the influences of the incident angle and the polarization angle on the conversion performances are also investigated, and the results show that the proposed converter has the characteristics of wide incident angle and polarization angle dependent switching performance. The simple geometry and the better performances make the proposed linear polarization converter a potential candidate in the area of sub-wavelength scale polarization converters, filters, sensors, switches, and so on.

Investigation of structural, magnetic and dielectric properties of gallium substituted Z-type Sr3Co2-xGaxFe24O41 hexaferrites for microwave absorbers

Gallium substituted Z-type hexagonal ferrites with chemical composition Sr3Co2-xGaxFe24O41 (x = 0.0,0.4, 0.8, 1.2, 1.6, and 2.0) were successfully synthesised in air at 1200 °C for 5 h using the sol-gel auto-combustion technique, in order to investigate the effect of gallium substitution on structural, magnetic and dielectric properties. X-ray Diffraction (XRD) analysis of all samples reveals the formation of mixed hexaferrite phases, with Z ferrite as the major phase (72–90%).The average crystallite size of heated powders was found to be in the range of 21–40 nm. The saturation magnetisation decreases after gallium substitution, with the lowest values of 64 Am2 kg−1 for composition x = 1.6, which also hasthe highest value of coercivity (28.3 kA m−1). Nevertheless, all were soft ferrites, with Hc between 3.4 and 28.3 kA m−1.The Mr/MS ratio of all samples was found to be less than 0.5, suggesting that all the compositions possess multi-domain microstructures. Mössbauer spectroscopic analysis confirmed that the Fe ions were found in the 3 + high spin state for compositions below x ≤ 0.4, whereas ∼1.5% of the Fe ions were converted into Fe2+ high spin state beyond x ≥ 0.8 compositions, as Ga3+ began to substitute for Fe3+, forming Fe2+ in the cobalt positions. The average hyperfine magnetic field (<Hhf>) was found to be decreased with Ga-substitution. Dielectric parameters such as dielectric constant and loss factor were studied as a function of frequency, and their results show normal behaviour for ferrimagnetic materials. In complex measurements at microwave frequencies (8 GHz–12.5 GHz, the X-band), all samples had a real permittivity of around 8–14. For sample x = 2.0, a dielectric resonance peak was observed around 12.15 GHz. All showed a real permeability of around 1.0–1.4 over the frequency of 8 GHz–12.5 GHz range, and ferromagnetic resonance (FMR) was observed in x = 0.0 and 2.0 samples, at around 11 and 12 GHz, respectively. This suggests that the prepared samples can be used as microwave absorbers/EMI shielding at specific microwave frequencies. The co-existence of FMR and dielectric resonance at the same frequency of 12.15 GHz for x = 2.0 could lead to the coupling of these resonances and the development of potential metamaterials.

Tailoring the optical and dielectric properties of PVC/CuO nanocomposites

The solvent casting method has been applied to prepare PVC/CuO nanocomposite films with CuO ratios (0, 2.5, 5.0, 10.0 and 15.0 wt%). X-ray diffraction (XRD), Fourier transform infrared spectroscopy, optical absorption and dielectric experimental analysis are applied to these nanocomposite films. The CuO nanoparticles have been prepared by using sol–gel method. XRD pattern analysis revealed the formation of CuO (30 nm) single-phase with monoclinic structure and space group C2/c and the amorphous structure of polyvinyl chloride (PVC) matrix. Energy-dispersive X-ray data revealed the increase in CuO content, and scanning electron microscopy morphology analysis approved the dispersion of CuO nanoparticles on the PVC film surface. The optical absorption increased with CuO content, while the transmission decreased. Both direct and indirect energy gaps decrease, while the Urbach energy increases as CuO content increases. The real dielectric permittivity enhanced as the concentration of CuO increased and the imaginary permittivity showed single relaxation at high frequency. The estimated values of both static and high-frequency dielectric constant increased as CuO percentage increases. The presence of CuO causes increase in the loss tangent values for the PVC/CuO polymer films.

Improving the electromagnetic shielding of fabricated NdFeB particles by a coating thin carbonaceous layer

Fabrication and improvement of electromagnetic interference (EMI) shielding products have recently been the subject of intense research. In the present study, the NdFeB magnetic powder was used in prolonged high-energy wet ball milling to synthesize and enhance properties of the magnetic filler of EMI shielding composites. The physicochemical changes induced in particles prepared by the process confirmed that the applied oleic acid surfactant was simply converted into a thin amorphous carbon layer on NdFeB nanoparticles. Increasing the amount of surfactant in the ball milling procedure caused a thicker layer of the carbonaceous blend to be formed on magnetic nanoparticles. We found that the thicker layer of the amorphous carbon covering the synthesized magnetic particles increased the real and imaginary dielectric permittivity of the resulting product about 15% and 95%, respectively. This also increased the shielding absorption efficiency (SEA) about 40% and the total shielding efficiency (SET) about 30% at some frequencies. Furthermore, more carbonaceous surfactant in composite (II) improved impedance matching, leading to a decrease in reflection loss and an increase in microwave absorption of the composite.

Multiple polarization effect of shell evolution on hierarchical hollow C@MnO2 composites and their wideband electromagnetic wave absorption properties

Abstract Nanoengineering technology has made great achievements, through the design of useful nanostructures and components, to provide us with more functional materials. Therefore, the ideal performance can be achieved by maximizing the contribution of nanostructures and components. As a typical material, core-shell material has been widely studied in order to find more possibilities in the nanoengineering applications. In this work, a designable core-shell composite material with graphitized tunable hollow carbon sphere as the core and sheet-like MnO2 particles as the shell is developed. The size and shell thickness of the core-shell shaped C@MnO2 was nearly 230, 40 nm, respectively. The correlation among shell-evolution, graphitized-tuned carbon core and their corresponding dielectric response ability has been well analyzed. The hollow carbon nanosphere@MnO2 with desirable real and imaginary permittivity (e'/e'') can be obtained in order to achieve the optimized electromagnetic (EM) absorption performance. One optimal reflection loss of −53.5 dB is gained at the thickness of 1.5 mm and the corresponding frequency region with RL

Layer-dependent dielectric permittivity of topological insulator Bi2Se3 thin films

Dielectric properties and their evolutions are fundamental to reveal the optical responses of topological insulators and to guide the design of related devices. Here, we systematically investigate the layer-dependent dielectric permittivity of Bi2Se3 over an ultra-broad spectral range (0.73–6.43 eV) by combining the spectroscopic ellipsometry, energy-loss function (ELF), critical point (CP) analysis, and density function theory. Results demonstrate that the epsilon-near-zero point of the real permittivity and the ELF peak exhibit blue shifts, indicating the resonance frequencies of surface plasmon modes move towards the higher energy, due to the enhanced hybridization between the opposite surfaces. The joint density of states dominates the layer-dependent increase in imaginary permittivity as the thickness increases. We also find that the CP transition positions in band structures are layer-independent, and the CP center energies exhibit red shifts resulted by the reduced surface state gap with the thickness increasing.

Utilization of soft magnetic nanoparticles for ultra-high frequency range

In the communication sector especially at ultra-high frequency range, a major challenge is to maintain the real permeability (μ′) and real permittivity (ε′) with controlled electromagnetic losses. To fulfill this demand, the present work reported electromagnetic studies over the frequency range of 2–3.4 GHz for magnetic nanoparticles of Sr2+-doped Z-type hexaferrites of compositions Ba3-xSrxCo2Fe24O41(x = 0, 0.5, 1.0, 1.5 and 2.0) synthesized by sol-gel auto combustion soft chemical route. The most desirable results obtained at 2 GHz for composition Ba1·5Sr1·5Co2Fe24O41 hexaferrite were μ′ = 5.002 with magnetic loss tangent of 0.003, and ε′ = 4.734 with electric loss tangent of 0.005. The multiple manifestations of electromagnetic studies are reported which are explained on the basis of a suitable Globus model, a theoretical model of Herzer, Snoek's law, a Maxwell–Wagner model together with Koop's model, and pertinent existing theories. Such magneto-dielectric substrate material could be favorable for fabrication of antennas with reduced physical dimensions and, also, almost equal values of μ′ and ε′ led to good impedance-match to free space in the gigahertz range for communication applications.

Preparation and electromagnetic properties of Fe80.7Si4B13Cu2.3 nanocrystalline alloy powders for electromagnetic wave absorbers in X-band

Fe80.7Si4B13Cu2.3 nanocrystalline soft magnetic alloy powders with outstanding electromagnetic wave absorption properties in X-band (8–12 GHz) have been successfully synthesized by a ball-milling method. As milling time increases from 40 to 160 h, the mean particle size of the near spherical powders with a bcc-Fe nanocrystalline/amorphous composite structure decreases from 9 to 6 μm, while the average nanocrystalline grain size stabilizes at around 10 nm. The alloy powders possess high saturation magnetization of 178.2–183.4 emu/g. The alloy powders/paraffin (3:1 in mass ratio) composites exhibit real permeability and permittivity of 2.18–2.42 and 7.3–9.7 at 2 GHz, respectively, and imaginary permeability of 0.68–0.77 at resonance frequencies of around 12.5 GHz. The composites with thicknesses of 2.1–2.3 mm exhibit large reflection losses (RL) of −39.1 to −50.5 dB with effective absorption bandwidths (RL < −10 dB) of 4.5–5.0 GHz covering the X-band. The excellent absorption properties are benefited from the high permeability and matchable permittivity of the nanocrystalline alloy powders in GHz range.

Interplay between nanoparticles and water on dielectric properties of nanofluids

This paper deals with the interaction of nanoparticles (NPs) in transformer oil and water in the oil and its effect on the electrical properties of the nanoffuid (NF). The breakdown strength and dielectric spectrum measurements under AC for various concentrations of natural SiO 2 and water in mineral oil were studied. Mineral oil-based SiO 2 NFs exhibit significantly higher breakdown voltage than that of base oil under high water content. However, there is an optimal range of NP concentration and water content for maximum breakdown voltage. The results of dielectric spectrum indicate that the dielectric constants of NFs and base oil are greatly affected by water content. The real relative permittivity of oil samples increases exponentially with water content at decreasing frequency (f≤10 Hz). The imaginary relative permittivity of NFs is less than that of the base oil in all cases. Based the experimental results reported, a model on the interplay between NPs and water in transformer oil is presented. The model can broadly explain the influence of the coexistence of NPs and water on the dielectric properties of transformer oil.

Structure and dielectric properties of polyethylene nanocomposites containing calcined zirconia

The current work investigates the effects of filler calcination on the structure and dielectric properties of polyethylene/zirconia (ZrO 2 ) nanocomposites. Calcination temperatures of 600 and 900 °C have been applied to ZrO 2 in an attempt to modify its surface chemistry and structure. Calcination results in a slight reduction in the concentration of physically adsorbed water on ZrO 2 and, in addition, structural reorganization of the ZrO 2 nanopowder. Consequently, the real permittivity of nanocomposites containing calcined ZrO 2 is reduced compared to nanocomposites containing uncalcined ZrO 2 . Furthermore, the DC breakdown strength of nanocomposites containing calcined ZrO 2 improves compared to nanocomposites containing uncalcined ZrO 2 . Since the removed water content is minimal, we primarily attribute the observed dielectric results to structural changes in ZrO 2 rather than water-related effects. As such, calcining can have positive effects in enhancing the dielectric properties of ZrO 2 -based nanocomposites.

CdSe/V2O5 core/shell quantum dots decorated reduced graphene oxide nanocomposite for high-performance electromagnetic interference shielding application

The present work reports nanocomposite of CdSe/V2O5 core–shell quantum dots with reduced graphene oxide (rGO-V-CdSe), as an efficient lightweight electromagnetic wave shielding material, synthesized by a simplistic solvothermal approach. The as-synthesized nanocomposite was analyzed for its structural, compositional and morphological features by x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). The measurement of complex permittivity/permeability and total shielding efficiency of the as-synthesized samples has been done in a wide frequency range of 8–12 GHz (X-band). Compared to rGO and rGO-CdSe, rGO-V-CdSe nanocomposite exhibits significantly enhanced EMI shielding properties in terms of both dielectric loss and total shielding SET. The high value of real permittivity (average ε′∼70) and the overall shielding effectiveness up to ∼38 dB have been recorded for rGO-V-CdSe nanocomposite. The studies also infer that the absorption contributes more in total shielding than reflection. The high value of dielectric loss and shielding effectiveness could also be attributed to the presence of various defects leading to dipolar and interfacial polarizations. The excellent EMI shielding properties of the nanocomposite in GHz frequency range (X-band) pave an intuitive way for fabricating a versatile EMI shielding nanocomposite material for applications.

Reduced graphene/nanostructured cobalt oxide nanocomposite for enhanced electrochemical performance of supercapacitor applications

We demonstrate the preparation of nanostructures cobalt oxide/reduced graphene oxide (Co3O4/rGO) nanocomposites by a simple one-step cost-effective hydrothermal technique for possible electrode materials in supercapacitor application. The X-ray diffraction patterns were employed to confirm the nanocomposite crystal system of Co3O4/rGO by demonstrating the existence of normal cubic spinel structure of Co3O4 in the matrix of Co3O4/rGO nanocomposite. FTIR and FT-Raman studies manifested the structural behaviour and quality of prepared Co3O4/rGO nanocomposite. The optical properties of the nanocomposite Co3O4/rGO have been investigated by UV absorption spectra. The SEM/TEM images showed that the Co3O4 nanoparticles in the Co3O4/rGO nanocomposites were covered over the surface of the rGO sheets. The electrical properties were analyzed in terms of real and imaginary permittivity, dielectric loss and AC conductivity. The electrocatalytic activities of synthesized Co3O4/rGO nanocomposites were determined by cyclic voltammetry and charge-discharge cycle to evaluate the supercapacitive performance. The specific capacitance of 754 Fg−1 was recorded for Co3O4/rGO nanocomposite based electrode in three electrode cell system. The electrode material exhibited an acceptable capability and excellent long-term cyclic stability by maintaining 96% after 1000 continuous cycles. These results showed that the prepared sample could be an ideal candidate for high-energy application as electrode materials. The synthesized Co3O4/rGO nanocomposite is a versatile material and can be used in various application such as fuel cells, electrochemical sensors, gas sensors, solar cells, and photocatalysis.

A systematic Mössbauer spectroscopy study of Y3Fe5O12 samples displaying different magnetic ac-susceptibility and electric permittivity spectra

A comparative study of dielectric, specific heat, and magnetic properties has been carried out between bulk Y3Fe5O12 (YIG) samples which display different behavior regarding the dielectric and ac-susceptibility spectra. Experimental results are presented from two categories of YIG samples (called sample-A and sample-B). Sample-A exhibits zero imaginary part χ″(T) of the magnetic ac susceptibility whereas sample-B exhibits χ″(T) curves with two well defined broad maxima in the kHz region. These two maxima can be assigned to two different relaxation regimes. The frequency dependence of real (ε′) and imaginary (ε″) dielectric permittivity of sample-B reveals step-like transitions in ε′ and local maxima in ε″ at the kHz region, which are indicative for several relaxation processes. These processes are not observed in the electric permittivity of sample-A, indicating that dielectric and magnetic relaxation effects depend strongly on the preparation conditions of YIG. Such a completely different spectral behavior among YIG samples corresponds to a direct shape change of absorption sextets in Mössbauer spectra (MS). It was discovered that samples without dispersion in magnetic susceptibility and electric permittivity exhibit MS with distinct broadening of absorption lines in tetrahedral sites, as compared with samples that display dispersion. Moreover, Mössbauer spectroscopy for both categories of samples does not provide evidence for the presence of Fe2+.

Permittivity measurement of cementitious materials with an open-ended coaxial probe

This study presents a methodology to investigate the complex permittivity of heterogeneous samples like concrete, mortars and cement pastes by using an open-ended coaxial probe. The probe is associated with portable Vector Network Analyser (VNA) allowing in-situ measurements. Another advantage of the proposed methodology is that the complex permittivity of concrete can be assessed from the measurement of the reflection parameter S11 alone which is innovating in comparison with common devices. This can be achieved by modelling the electrical equivalent circuit of the coaxial probe and the dielectric sample with a capacitive model briefly presented in this paper. A calibration procedure on materials with known permittivity developed to estimate the input parameters of the capacitive model is presented in this paper meanwhile all the details of measurements. Measurements performed on various cementitious materials (cement paste, mortar and concrete) at different degrees of water saturation (from almost dry to fully saturated) show that the probe is able to characterize their dispersive behavior. It is also demonstrated that the probe is able to measure both real and imaginary permittivity of cementitious materials in an extended range of moisture. Relationships are proposed between real or imaginary permittivity and volume water content.

Dual functionality of hierarchical hybrid networks of multiwall carbon nanotubes anchored magnetite particles in soft polymer nanocomposites: Simultaneous enhancement in charge storage and microwave absorption

We demonstrated, for the first time, the use of submicron size insulative junctions of magnetite (Fe3O4) in conductive networks of multiwall carbon nanotubes (MWNTs) for simultaneously achieving enhanced charge storage and superior microwave absorption in X-band (8.2–12.4 GHz) frequency range. Herein, electrical conductivity in fluoroelastomer matrix was attained by dispersion of MWNTs, whereas low dielectric loss was achieved by employing Fe3O4 particles as insulative spacers between MWNT networks. High-performance charge-storing nanocomposites were developed using unique approach where relatively large insulative Fe3O4 spacers were anchored to MWNT networks leading to disrupted conductive pathways. The size of Fe3O4 particles was controlled in such a way that the distance between discrete MWNTs at the insulative junction is larger than the critical distance required for hopping and tunneling of nomadic charges. This also resulted in optimum impedance matching and additional magnetic loss associated with Fe3O4, which led to the synergistic absorption of microwaves. Taken together, the anchoring of sub-micron size Fe3O4 particles with MWNTs provided dual functionality of high real permittivity (ε') along with decreased dielectric loss which are both favorable for charge storage. Furthermore, it led to improved impedance matching and we were able to achieve maximum microwave absorption.

Effects of nanoparticles size and interactions on dielectric properties of polymer matrix flexible dielectric nanocomposites

Effects of particle size on the dielectric properties of polymer matrix flexible nanocomposites were studied. Synthesized barium titanate (BaTiO3; BT) nanoparticles (NPs) with different sizes were effectively dispersed in the matrix polymer using a simple phase separation method. The dielectric responses of free-standing thin composite films were studied by impedance spectroscopy. The interactions between nanoparticles were also studied by finite element analysis and compared with experimental data using a time harmonic-electric current solver in the AC/DC module of COMSOL Multiphysics® 5.2. The dielectric permittivity of polymer matrix nanocomposites was remarkably affected by the BT particle size, while the dielectric loss did not show any noticeable changes. The real permittivity exhibited the highest value when the BT particle size was near 100 nm. The calculated energy density based on measured real permittivity and breakdown strength was as high as 17.24 J/cm3. The dielectric properties of polymer matrix nanocomposites can be improved by controlling the particle size of nanofillers in conjunction with surface treatment of fillers.

Exploring the Structural, Thermal and Dielectric Properties of PVA/Ni0.5Zn0.5Fe2O4 Composites

Nano-Ni0.5Zn0.5Fe2O4 embeded in polyvinyl alcohol (PVA) with different weight ratios (5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, 30 wt.%, 40 wt.% and 50 wt.%) were prepared by solution casting. The formation of PVA/Ni0.5Zn0.5Fe2O4 hyprid composite is confirmed through x-ray diffraction, scanning electron micrographs, transmission electron microscopy, and Fourier transform infrared spectroscopy analysis. Thermogravimetric analysis showed that the composite films have thermal stability higher than pure PVA. The complex dielectric constant (e*), electrical conductivity, electric modulus and impedance spectra of these polymer nanocompsites have been studied over the frequency range 1 Hz–20 MHz. The real permittivity (e′) of PVA polymer film enhanced while the real electric modulus (M′) decreased by the addition of Ni0.5Zn0.5Fe2O4 nanoparticles. The imaginary part of permittivity has the same trendline of e′ leading to the increase in its values. These aspects of PVA composites making it a promising material to be used in microwave absorbers depending on the dielectric loss.

Near-Zero Negative Real Permittivity in Far Ultraviolet: Extending Plasmonics and Photonics with B1-MoNx

CMOS-compatible, refractory conductors are emerging as the materials that will advance novel concepts into real, practical plasmonic technologies. From the available pallet of materials, those with negative real permittivity at very short wavelengths are extremely rare; importantly, they are vulnerable to oxidation—upon exposure to far-UV radiation—and nonrefractory. Epitaxial, substoichiometric, cubic MoN (B1-MoNx) films exhibit resistivity as low as 250 μΩ cm and negative real permittivity for experimental wavelengths as short as 155 nm, accompanied with unparalleled chemical and thermal stabilities, which are reported herein. Finite-difference time domain calculations suggest that B1-MoNx operates as an active plasmonic element deeper in the UV (100–200 nm) than any other known material, apart from Al, while being by far more stable and abundant than any other UV plasmonic conductor. Unexpectedly, the unique optical performance of B1-MoNx is promoted by nitrogen vacancies, thus changing the common perception on the role of defects in plasmonic materials.

Characterization of AlN-based ceramic composites for use as millimeter-wave susceptor materials at high temperature: Dielectric properties of AlN:Mo with 0.25 vol% to 4.0 vol% Mo from 25 to 550 \xb0C

Microstructural analysis and bulk dielectric property analysis (real and imaginary permittivity at 95 GHz) were performed at temperatures ranging from 25 to 550 °C for ceramic composites comprising a hot-pressed aluminum nitride matrix (containing yttria and trace carbon as sintering additives) with molybdenum powder as a millimeter-wave radiation-absorbing additive. Loading percentages in the range of 0.25 vol% to 4.0 vol% Mo were characterized. For the temperature regime evaluated, the temperature-related changes in real and imaginary components of permittivity were found to be relatively modest compared with those driven by Mo loading. Energy-dispersive X-ray spectroscopic analysis of Mo grains and surrounding regions showed the presence of a mixed-phase layer, containing Mo2C, at the AlN–Mo interface. The Mo2C-containing mixed-phase layer, typically a few micrometers thick, surrounded the Mo grains. Further characterization of this mixed-phase layer is required to determine its contribution to the dielectric properties of the composite.