Permittivity Of Material Research Articles

Ultra-wideband Luneburg lens with high performance based on gradient metamaterials

Based on the perspective of a wide scanning range and ultra-broad bandwidth, Luneburg lenses are highly anticipated to be an outstanding option for multibeam radiation. However, owing to the lack of low-loss continuously varying permittivity materials, the practical application of Luneburg lenses is far below the expected level. In this paper, an ultra-wideband planar Luneburg lens (PLL) is proposed. Due to the novel design of an all dielectric lightweight radially symmetric periodic gradient metamaterial, the presented lens is able to yield highly directional emission with side lobes all below −8 dB and achromatic sub-diffraction focusing with full width at half maximum about 0.4λ from 4 GHz to 22 GHz. The prototype of the lens is manufactured by computer-numerical controlled machining. The measured data of the near field and far field agree well with that of the simulated data, verifying the effectiveness of the proposed design methodology. The superiority of the presented approach to design a Luneburg lens is demonstrated. Therefore, the PLL has the advantages of being lightweight, with a compact structure, low profile, ultrabroadband function, high resolution, and convenient fabrication, giving it great potential to be practically deployed.

A Multiple Resonant Frequencies Sensor for Measuring Permittivity Based on Stepped Impedance Resonator

This paper presented a novel sensor based on a stepped impedance resonator, which can measure the permittivities of materials at different resonant frequencies simultaneously. The multiple resonant frequencies are generated by the fundamental frequency and harmonics. The sensitivity of designed sensor at different resonant frequencies is also analyzed in detail. For verifying the feasibility of the sensor, three different kinds of materials under test are measured. The experimental results demonstrated that the tested resonant frequencies show a multiplicative growth and are consistent with our theoretical analysis. Besides, the higher the tested resonant frequency, the greater the sensitivity. The permittivity of the material under test is derived from these measured resonant frequencies by simulation and calculation. The maximum relative error through simulation and calculation are 3.4% and 26.67%, respectively. Moreover, the proposed sensor also has the advantages of being simple to fabricate, inexpensive to manufacture, and compact in volume.

Dielectric properties and negative permittivity performance modulated by dual fillers in CNTs/TiN/CaCu3Ti4O12 ternary composites

Ceramic composites with negative permittivity have attracted widespread attention during the development of novel electromagnetic metamaterials. Herein, carbon nanotubes/TiN/CaCu3Ti4O12 (CNTs/TiN/CaCu3Ti4O12) ternary composites were prepared, and the effects of dual fillers on the dielectric properties of the ternary composites are discussed. The electrical conductivity increased markedly, and the conduction mechanism changed owing to the percolation of the dual conductor fillers. The switching of the reactance, from negative to positive, suggests that the ternary composites underwent a capacitive-inductive transition. The dielectric permittivity was negative owing to the plasmonic state of the free electrons in the percolating conductor fillers. Regulating the electron concentration by changing the filler content or adjusting the microstructure by changing the relative content of the dual fillers proved to be an effective way to tailor the negative permittivity performance of ternary composites. In addition, conduction loss dominated the dielectric loss mechanism of the negative permittivity composites. The realization and regulation mechanism of the negative permittivity of ternary composites are clarified, shedding light on the preparation and development of negative permittivity materials for various electromagnetic applications.

Magnetospectroscopy of terahertz surface plasmons in subwavelength perforated superlattice thin-films

Surface plasmons, the resonant oscillations of conducting electrons at the interface of negative and positive permittivity materials, pave the way for enhanced electromagnetic wave–matter interactions at a subwavelength scale. On the other hand, spin-dependent magnetotransport ushers an ingenious technology by inculcating electron spin to realize miniaturized, energy-efficient electromagnetic devices. Generally, magneto-resistive devices (viz., multilayer un-patterned magnetic–non-magnetic thin films) relying on magnetotransport mechanisms are not recognized for supporting surface plasmons toward enhanced electromagnetic interactions. However, an amalgamation of surface plasmons with spin-dependent magnetotransport can exploit magnetic (spintronic) degree of freedom in plasmonic devices. In this work, we propose a patterned superlattice (non-magnetic/ferromagnetic thin films) terahertz (THz) magneto-resistive device for supporting surface plasmons toward enhanced electromagnetic interactions. Magnetotransport dependent enhancement and dynamic magnetic modulation of resonant THz transmissions are experimentally demonstrated in subwavelength superlattice (Al/Ni) hole arrays for varying lattice parameters. Our experiments reveal that typical non-magnetic electromagnetic phenomena like surface plasmon resonances can be tweaked by externally applied low intensity magnetic fields [∼few tens of milli-tesla (0–30 mT)]. Experimental outcomes are explicated by spin-dependent terahertz magnetotransport theory in perforated superlattice metal sheets and, therefore, can stimulate a paragon for spin-based integrated photonic technology.

A Compact CSRR-Based Sensor for Characterization of the Complex Permittivity of Dielectric Materials

A sensor is proposed to characterize the complex permittivity of dielectric materials in a non-destructive and non-invasive way. The proposed sensor is based on a rectangular patch microstrip two-port circuit with a complementary split-ring resonator (CSRR) element. The slotted CSRR element of the sensor plays a key role in determining the electrical properties of the materials under test (MUT). The sensitivity analysis is determined by varying the permittivity of the MUT. The proposed sensor is simulated and analyzed using Ansoft HFSS software. A prototype was fabricated and measurements were made on two different samples of dielectric materials with complex permittivity values available in the literature. The simulated and measured results showed good agreement.

Complex Permittivity Measurement of Low-Loss Anisotropic Dielectric Materials at Hundreds of Megahertz

Resonant methods are well known for their accuracy in determining the complex permittivity of materials. However, to guarantee the accuracy, interference modes must be suppressed in the measurement band. In this paper, a multi-mode split rectangular cavity is designed and fabricated to measure low-loss anisotropic dielectric materials in the frequency range from 300 to 1000 MHz. Details of the cavity design are presented. Simulation results for a uniaxially anisotropic material validate both the setup and the processing. Moreover, validation measurements on a sheet of polytetrafluoroethylene (PTFE) are in agreement with the literature data, which indicate that the proposed method in this paper is reliable and accurate for low-loss complex permittivity characterization at hundreds of megahertz.

Real and imaginary permittivity measured by thermal noise dielectric spectroscopy

This paper demonstrates that both real and imaginary parts of dielectric permittivity in a kHz frequency range can be determined from the thermal noise voltage spectra. The proposed method, termed as thermal noise dielectric spectroscopy, relies on the set of calibration measurements of gauge resistors and capacitors. This method has a great potential for investigations of strongly nonlinear dielectric materials whenever the initial permittivity is of interest because the thermal noise method probes the permittivity by an almost zero electric field. Here, we tested this method by measuring dielectric spectra of 100 nm thin film of paraelectric SrTiO3 in a thermal noise setup in which the film is effectively probed by voltages of the order of nanovolts. For the sake of comparison, the permittivity of the same SrTiO3 thin film has been also measured using the standard impedance analyzer with the probing AC voltage of 10 mV. Agreement between methods is remarkable for both real and imaginary parts of the permittivity. Results strongly suggest that the proposed technique is broadly applicable for investigation of the frequency and temperature dependence of the complex permittivity of nanoscale dielectric materials.

Interdigitated Planar Microwave Sensor for Characterizing Single/Multilayers Magnetodielectric Material

Simultaneous characterization of permittivity and permeability of magnetodielectric materials using a single resonator-based microwave sensor is challenging due to their similar impacts on the sensor’s response. This work presents an interdigital structure-based planar microwave resonator sensor featuring high-intensity electric (E)-field zones and magnetic (M)-field zones to characterize permittivity and permeability. The proposed sensor’s sensing E-field and M-field zones were implemented using the interdigital fingers configured as a spur line structure and interdigital electrodes, respectively, generating a 5.9-GHz resonance frequency with a high Q-factor (1700). The resonance frequency of the designed sensor exhibited high sensitivities (324 MHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \varepsilon _{r}$ </tex-math></inline-formula> for permittivity and 825 MHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \mu _{r}$ </tex-math></inline-formula> for permeability) for characterizing single/multilayers magnetodielectric materials with excellent resolutions (0.324 for permittivity and 0.825 for permeability). The demonstrated high sensitivity and resolution results validate the performance of the interdigital structure-based microwave resonator sensor for simultaneous characterization of multilayer magnetodielectric materials.

Defect-induced insulator-metal transition and negative permittivity in La1-Ba CoO3 perovskite structure

• Low-frequency plasmonic state and temperature-stable negative permittivity in La 1- x Ba x CoO 3 perovskite structure is achieved. • The partial replacement of La 3+ by Ba 2+ in La 1- x Ba x CoO 3 system can create oxygen vacancies and Co element with higher oxidation states. • Samples with x ≥ 0.12 manifest metal-like character described by the Drude model approximat. • Zero-crossing behavior of real permittivity is observed in La 0.9 Ba 0.1 CoO 3 sample. The development of negative permittivity materials in multifunctional applications requests expansion of their operating frequency and improvement of stability of negative permittivity. Low electron density is beneficial to reduce plasma frequency so that negative permittivity is achieved in kHz region. Negative permittivity achieved by percolating composites is restricted in practicality due to its instability nature at high temperatures.To achieve temperature-stable negative permittivity in kHz region, mono-phase La 1- x Ba x CoO 3 ceramics were prepared, and the transition from dielectric to metal was elaborated in the perspective of electrical conductivity and negative permittivity. The plasma-like negative permittivity is attained in kHz region, which is interpreted by the collective oscillation of low electron density. The temperature-stable negative permittivity is based on the fact that the plasmonic state will not be undermined at high temperatures. In addition, zero-crossing behavior of real permittivity is observed in La 0.9 Ba 0.1 CoO 3 sample, which provides a promising alternative to designing epsilon-near-zero materials. This work makes the La 1- x Ba x CoO 3 system a source material for achieving effective negative permittivity.

Reconstructing unseen spaces in collapsed structures for search and rescue via deep learning based radargram inversion

This paper developed a novel deep learning-based approach to processing ground-penetrating radar (GPR) radargrams to reconstruct the occluded interior spaces of collapsed structures and extract essential information such as survivable void spaces to assist search-and-rescue operations. The proposed method innovatively exploits a generative adversarial network (GAN) to augment synthetic GPR training data and an end-to-end deep learning model to invert a GPR radargram to a permittivity map of the cross-section that can be further interpreted to reconstruct the interior scenarios of collapsed structures. First, to address the lack of training data with correct labels, synthetic GPR radargrams were generated from simulated scenarios of collapsed structures. The GAN was applied to augment the realism of synthetic GPR radargrams for training, providing a new mechanism for preparing and augmenting data that is difficult to collect from a real disaster site. Second, instead of detecting and segmenting nonintuitive features in GPR radargrams, a new encoder-decoder structure was trained using the augmented GPR radargrams to directly reconstruct permittivity maps corresponding to the cross-sections of collapsed structures. The visual Turing test indicated that the GAN substantially improved the realism of synthetic GPR radargrams. The proposed GPR inversion method achieved an R2 value of 0.93, a Mean Absolute Error (MAE) of 0.73, and a Structural Similarity Index Measure (SSIM) of 0.95 in inferring the material permittivity from synthetic radargrams. The predicted permittivity map was further used to identify void spaces, achieving an F1 score of 64.34%, a precision of 63.06%, and a recall of 71.84% at the pixel level. On the augmented radargrams, the network achieved an R2 value of 0.76, an MAE of 1.49, and an SSIM of 0.89 in predicting the permittivity map. The void detection on augmented radargrams achieved an F1 score of 45.20%, a precision of 45.35%, and a recall of 53.99% at the pixel level. The feasibility of using the proposed inversion network to reconstruct permittivity maps from radargrams is also experimentally tested and demonstrated in simulations of two multistory building collapses.

Effective parameters of dielectric absorption of polymeric insulation with semiconductor coatings of power high voltage cables

Introduction. The presence of semiconductor shields leads to additional dielectric losses compared to polymer insulation without shields. Losses in cables in the presence of semiconductor coatings depend on the dielectric permittivity and resistivity of the composite polymeric material, which are frequency-dependent characteristics. Purpose. To determine in a wide range of frequencies, taking into account the variance of electrophysical characteristics and thickness of semiconductor shields effective electric capacitance and tangent of dielectric losses angle of high-voltage power cables with polymer insulation. Methodology. Serial-parallel nonlinear circuit replacement of semiconductor coatings and linear polymer insulation to determine in a wide range of frequency the effective parameters of the dielectric absorption of a three-layer composite system of high-voltage power cables of single core. Practical value. The obtained relations are the basis for the development of practical recommendations for substantiating the thickness and electrophysical parameters of semiconductor shields to reduce the impact on the effective tangent of the dielectric losses angle of a three-layer composite system of high-voltage power cables.

A circular monopole antenna with uniquely packed quad T-shaped strips for WLAN/WiMAX application

Abstract This paper presents a circular monopole antenna with uniquely packed quad T-shaped strips etched on FR4 epoxy substrate having material permittivity 4.4, loss tangent 0.02, and thickness 1.6 mm. The 3.5 GHz band is achieved through this arrangement, which has return loss less than −10 dB while maintaining VSWR &lt;2. It provides a peak gain of 2.65 dBi at 3.5 GHz and average gain of 2.52 dBi in the entire operating frequency band. The radiation pattern is almost omnidirectional in E plane and bidirectional in H plane. The proposed antenna has a compact size of 41.25 mm × 30.55 mm. The simulated results of S11 parameters, gain, VSWR, radiation pattern, and efficiency are studied and verified with the measured results of fabricated antenna as well as its equivalent lumped circuit model. Investigation shows that the proposed antenna can be employed for WLAN/WiMAX applications efficiently.

Nanofocusing performance of plasmonic probes based on gradient permittivity materials

Probe is the core component of an optical scanning probe microscope such as scattering-type scanning near-field optical microscopy (s-SNOM). Its ability of concentrating and localizing light determines the detection sensitivity of nanoscale spectroscopy. In this paper, a novel plasmonic probe made of a gradient permittivity material (GPM) is proposed and its nanofocusing performance is studied theoretically and numerically. Compared with conventional plasmonic probes, this probe has at least two outstanding advantages: first, it does not need extra structures for surface plasmon polaritons excitation or localized surface plasmon resonance, simplifying the probe system; second, the inherent nanofocusing effects of the conical probe structure can be further reinforced dramatically by designing the distribution of the probe permittivity. As a result, the strong near-field enhancement and localization at the tip apex improve both spectral sensitivity and spatial resolution of a s-SNOM. We also numerically demonstrate that a GPM probe as well as its enhanced nanofocusing effects can be realized by conventional semiconductor materials with designed doping distributions. The proposed novel plasmonic probe promises to facilitate subsequent nanoscale spectroscopy applications.

Measurement of Uniaxially Anisotropic Dielectrics With a Fabry–Perot Open Resonator in the 20–50 GHz Range

Measurement results of uniaxially anisotropic materials performed with the aid of a plano-concave Fabry–Perot open resonator in the 20–50 GHz frequency range are presented and discussed in this letter. It is shown that a linear polarization of Gaussian modes applied in the characterization allows extracting in-plane complex permittivity of such materials as crystals and foils in a single measurement cycle. It is also shown that dielectric losses exhibit anisotropy, which can be quite significant, like in the sapphire, or relatively small, like in polymer foils.

Proposal to repeat the Abraham force experiment using giant permittivity materials

In the more than 100 years-old Abraham–Minkowski problem in macroscopic electrodynamics, the issue of how to observe the so-called Abraham term [Formula: see text] has been a main point. Such a measurement is rather delicate. Recent years have seen a number of beautiful experiments in radiation optics, but these experiments usually give no information about the Abraham term as this term simply fluctuates out. So, one is left with somewhat indirect verifications of this force, as in the radiation pressure of Jones et al. in the 1950s, testing the radiation pressure on a mirror immersed in a dielectric liquid. Now, there is a different way to test the existence of [Formula: see text], namely to work with low (quasi-stationary) frequencies enabling one to observe the sinusoidal variation of the force directly. These kind of experiments were actually done by Walker et al. in the 1970s, using BaTiO3 as a high-frequency dielectric (permittivity [Formula: see text]). The outcome of these experiments was convincing. Now, in recent years there have appeared dielectric materials with giant permittivities, of order 105 or even higher. It is therefore natural to consider the idea of Walker et al. anew, in order to test if this demanding experiment can be facilitated and give better accuracy. That is the main topic of this paper. The positive outcome of these kinds of experiments clearly supports the Abraham energy–momentum tensor at low frequencies. The Minkowski tensor is unable to predict a torque at all. The low-frequency and the high-frequency regimes are in this way highly contrasted, as it is obvious that in optical experiments the Minkowski tensor is by far the simplest and most convenient one to use. We end this note by commenting upon the use of the Einstein–Laub tensor (1908) in explaining this experiment, and discuss also the influence from air friction.

Microstructure, low loss tangent, and excellent temperature stability of Tb+Sb-doped TiO2 with high dielectric permittivity

The study of colossal permittivity (CP) materials possessing very high dielectric constants (ε′ > 104) has gained traction due to their suitability for application in microelectronic devices, which are evolving rapidly. In addition, the loss tangent (tanδ) and temperature stability of ε′ are crucial factors to consider for actual applications. In this study, (Tb3+/4++Sb5+) co-doped TiO2 (TSTO) ceramic with an appropriate co-dopant content presented an extremely high ε′ value of ∼9.31 × 104 and ultra-low tanδ (∼0.013) at 1 kHz. Moreover, its temperature-dependent coefficient of permittivity deviation (Δε'(T)/ε'30°C) was lower than |±15%| over the temperature range from −60 ℃ to 210 ℃. The TSTO ceramics exhibited highly compact microstructures. The grains, grain boundaries, and microwave dielectric phases (i.e., Tb2Ti2O7) were detected, and their presence is considered to determine the dielectric behavior of the TSTO ceramic. The observation of Ti3+ induced by Sb5+ via X-ray photoelectron spectroscopy explains the appearance of semiconducting grains, as confirmed by impedance spectroscopy (IS). The high resistivity resulting from the grain boundaries and microwave dielectric phase was also confirmed by IS. The findings describe the CP properties of TSTO ceramics via the interfacial polarization process and demonstrate that the existence of microwave dielectric phase particles with appropriate content affects the dielectric relaxation, resulting in the reduction of tanδ.

Structure and circuit modeling of frequency domain polarization characteristics for porous composite material

The relationship between the permittivity of a two-phase porous composite material and its structure forms the basis for the adjustment and analysis of the permittivity of the porous material. However, existing calculation models exhibit significant errors at low frequencies. A wide-frequency equivalent model is proposed in this paper based on the geometrical structural characteristics of porous materials, considering the effects of interface polarization and dielectric relaxation process. The relationship between the two-phase porosity material and the dielectric response of its framework and filling medium in a wide-frequency range can be calculated. Furthermore, the modified Cole–Cole model is adopted to analyze the dielectric spectrum characteristics of cellulose and aramid, thereby exhibiting the effects of polar bonds in the molecular structure and impurity ions in the material on the polarization strength and polarization process. The results obtained from this study can provide key parameters and a basis for the analysis of the polarization process of two-phase combined porous materials and the design of low-permittivity insulating paper.

A Substrate Integrated Waveguide Resonator Sensor for Dual-Band Complex Permittivity Measurement

This paper presents a novel dual-band substrate integrated waveguide (SIW) sensor that is designed to measure the complex permittivities of liquids or solid powders at two industrial, scientific, and medical (ISM) frequencies simultaneously. Resonant frequencies and quality factors are obtained from S-parameter measurements with the proposed SIW sensor, and applied to reconstructing the permittivities of materials under test through an artificial neural network. The water–ethanol mixed liquids were measured with the proposed sensor. The maximum deviations of the measured permittivities at 2.45 and 5.8 GHz are within 3% of literature results. The measurement by the proposed SIW sensor with artificial neural network reconstruction is accurate and efficient.

Fast and Robust Characterization of Lossy Dielectric Slabs Using Rectangular Waveguides

Waveguide characterization of dielectric materials is a convenient and broadband approach for measuring dielectric constant. In conventional microwave measurements, material samples are usually mechanically shaped to fit the waveguide opening and measured in closed waveguides. This method is not practical for millimeter-wave and sub-millimeter-wave measurements where the waveguide openings become tiny, and it is rather difficult to shape the sample to exactly the same dimensions as the waveguide cross-section. In this paper, we present a method that allows one to measure arbitrarily shaped dielectric slabs that extend outside waveguides. In this method, the measured sample is placed between two waveguide flanges, creating a discontinuity. The measurement system is characterized as an equivalent Pi-circuit, and the circuit elements of the Pi-circuit are extracted from the scattering parameters. We have found that the equivalent shunt impedance of the measured sample is only determined by the material permittivity and is rather insensitive to the sample shape, position, sizes, and other structural details of the discontinuity. This feature can be leveraged for accurate measurements of permittivity. The proposed method is very useful for measuring the permittivity of medium-loss and high-loss dielectrics from microwave to sub-terahertz frequencies.

A More Accurate Measurement Method of the Dielectric Material Properties With High Tolerance Using an Overmoded Waveguide

This article proposed an overmoded waveguide method (OWM) for more accurate measurement of dielectric properties in the subterahertz and terahertz band. Compared with the traditional standard waveguide method (SWM), the OWM is beneficial to alleviate the significant error caused by the air gap. Simulation predicts that for typical medium permittivity and loss materials, the relative errors of <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula> and tan<inline-formula> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> can be reduced from 10&#x0025; to below 3&#x0025; using the OWM with an overmoded factor of 3 when the air gap is 0.02 mm at W-band. When extending the overmoded factor over 5, the relative error can be reduced below 1.5&#x0025;. The tolerance improvement was verified by measuring a polytetrafluoroethylene (PTFE) sample through SWM and OWM. Experimental results show the relative error of <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula> is lower than 5&#x0025; even when there exists a 0.24-mm air gap in the OWM.