Frequency Dispersion Research Articles

Comparison of free‐space VNA and TDS measurements using transmission‐type frequency selective surfaces

AbstractA method for directly comparing vector network analyzer (VNA) and time‐domain spectrometer (TDS) measurements in free space using transmission‐type frequency selective surfaces (FSSs) is proposed. The transmission characteristics of FSSs with artificially controlled frequency dispersions facilitate flexible and detailed comparisons of VNA and TDS measurements. Four FSSs were designed as verification devices of free space measurements. The proposed approach was demonstrated in the D and J bands by comparing the measurement results of the transmission amplitude and phase characteristics of the FSSs obtained using VNA and TDS systems. The amplitude and phase results obtained using both systems were in good agreement, confirming the consistency and validity of the VNA and TDS measurements.

Multiscale structure and interface engineering of Fe/Fe3C in situ encapsulated in nitrogen-doped carbon for stable and efficient multi-band electromagnetic wave absorption

Most reported electromagnetic wave absorption (EWA) materials show significant effective absorption in a certain frequency range, but their performances deteriorate dramatically as the frequency changes. As the range of working frequencies for electronic devices is gradually widening, it is of great interest to explore frequency-insensitive EWA materials that can achieve efficient absorption in every waveband by simply changing the absorption thickness. To this end, a multi-scale absorber (Fe/Fe3C@NC) is rationally synthesized by chemical foaming and in-situ growth strategy. By controlling the growth of carbon nanotubes, the Fe/Fe3C@NC-2 exhibits a well-constructed 3D multi-scale architecture. Thanks to dipole polarization, interface polarization and magnetic-dielectric energy conversion, the Fe/Fe3C@NC-2 overcomes the frequency dispersion behavior and keeps a stable dielectric attenuation capability across the entire frequency range. Consequently, it delivers a superb full-band absorption of -50.1, -59.83, -55.87 and -51.91 dB in the S, C, X and Ku bands, respectively. The maximum radar cross-sectional reduction reaches 35.44 dB m−2 when the incident θ is 20°, testifying its impressive performance. Surprisingly, this EWA material also shows a remarkable resistance to oxidation and corrosion derived from the tightly coated carbon layers. This work provides new insight into the design of multi-band and stable EWA materials for practical application.

Frequency of olfactory dysfunction in pregnant women with SARS-CoV-2 infection.

In patients with COVID-19, olfactory dysfunction and anosmia have been reported, which in pregnant women occur in up to 24.2 %. To know the frequency at which pregnant women with SARS-CoV-2 infection have olfactory dysfunction. Age, gestational age, temperature, presence of nasal constipation or rhinorrhea, myalgia, headache, cough or chest pain were asked. Whether patients perceived and identified the scent of grape juice, coffee powder and menthol was evaluated. Central tendency and dispersion measures, frequencies and percentages were used. Sensitivity, specificity, positive and negative predictive value were calculated. Mann-Whitney's U-test and contrast of proportions were used for comparisons between groups. There was a higher proportion of women with cough, headache, dyspnea, myalgia, odynophagia, rhinorrhea, chest pain, and anosmia in SARS-CoV-2-positive women. In patients without COVID-19, 88.9 % detected each one of the scents; only 31.8 % of the positive group detected grapes scent, 47.7 % coffee and 59.1 % menthol, which had the highest percentages of sensitivity (40 %), specificity (21 %), positive predictive value (59 %) and negative predictive value (11 %). Olfactory dysfunction occurs in a significant percentage of pregnant women with COVID-19.

Evolution of tsunami-like isolated waves shoaling over a plane slope

Numerical experiments are conducted to simulate Gaussian-shaped waves shoaling over a uniformly sloping bed. A higher-order pseudo-spectral method is implemented to solve the fully nonlinear Euler water-wave problem for a variable depth. Unlike a solitary wave, the amplitude and wave breadth of a Gaussian waveform can be assigned independently. Therefore, it is convenient to use a Gaussian as a generic tsunami waveform. Although the rate of shoaling is consistent with Green's law only for small-amplitude waves propagating over intermediate sloping beds, the shoaling processes, in general, are complex and the behaviors cannot be accurately predicted by a single law. Several factors are involved in the complexity. When the slope is sufficiently mild, a Gaussian wave transforms into a series of solitons, hence leading to the adiabatic shoaling process. In some instances, the initial rate of shoaling even surpasses the adiabatic rate. This occurs when a soliton emerges from the initial conditions with an amplitude greater than the original Gaussian wave amplitude. In general, there is a trend of a faster shoaling rate when the nonlinearity effect of the Gaussian is greater than that of the corresponding solitary wave, whereas the rate of shoaling for Gaussian waves is slower when the frequency dispersion is greater than that of the solitary wave. The model is further applied to simulate the 2011 Heisei tsunami event, and the shoaling behavior is analyzed by comparing the numerical results with the recorded data in the field.

Analysis and Design of Broadband Outphasing Power Amplifier Based on Complex Combining Impedance

Though Outphasing power amplifiers (OPAs) can maintain high efficiency over a large dynamic range, their bandwidth should be further extended to satisfy the wireless communication systems. This paper presents a complex combining method for designing wideband OPAs. Complex combining impedance is adopted to compensate the frequency dispersion of the active load modulation in the OPA, leading to an extended bandwidth. It is illustrated for the first time that the load modulation trajectories of the OPA can be corrected over a wide bandwidth by elaborately designing the complex combining impedance. Furthermore, a comprehensive theoretical analysis is conducted on the transmission-line based combiner to derive the complex load impedance needed at different frequency points, recovering the back-off efficiency of a broadband OPA. As a proof of concept, a broadband OPA operating over 1.7-2.2 GHz is designed and fabricated. The experimental results show the fabricated OPA delivers a maximum output power of 43.1-44.3 dBm with a drain efficiency of 63.4%-78.4%. Meanwhile, a 6-dB back-off drain efficiency of 45.5%-70.4% can also be achieved by the fabricated OPA.

Design of an Outphasing Power Amplifier Using Complex Combining Method for Broadband Application

This letter presents the design of a broadband outphasing power amplifier (OPA) based on complex combining impedance. The complex combining impedance is realized using a post-matching network (PMN) and is utilized to compensate for the frequency dispersion of the active load modulation in the OPA. It is illustrated for the first time that the bandwidth of an OPA can be extended by elaborately designing the complex combining impedance. To validate the proposed method, a broadband OPA operating over 1.7–2.2 GHz is designed and implemented. Measurement results show the fabricated OPA achieves a saturation power of 43.1–44.3 dBm, a saturation drain efficiency (DE) of 63.4%–78.4%, and a 6-dB back-off DE of 45.5%–70.4% in the interested frequency band.

Modelling of impedance dispersion in lateral β-Ga2O3 MOSFETs due to parallel conductive Si-accumulation layer

Frequency dispersion of impedance in lateral β-Ga2O3 MOSFETs has been characterized and a model has been established to explain the behavior. The dispersion occurs due to resistive and capacitive coupling between terminal contact pads and the buried conducting layer associated with the accumulation of Si contaminant at the unintentionally-doped epitaxy/substrate interface, which is also responsible for a buried parallel leakage path. Particularly, it is demonstrated that the dispersion is not associated with gate dielectric traps as would often be assumed. A generalized equivalent circuit model, which is capable of reproducing the experimental results, is proposed to explain the observations.

Use of two-dimensional refined composite multi-scale time-frequency dispersion entropy for rolling bearing condition monitoring

Rolling bearing vibration signals with local failure usually contain rich fault characteristic in the time and frequency domains. Two-dimensional multi-scale dispersion entropy (MDE2D) is a nonlinear dynamic analysis method widely used in image signal processing. However, the multi-scale coarse-grained process of MDE2D is relatively complex and the computational cost is high. Therefore, this paper develops a two-dimensional multi-scale time-frequency dispersion entropy (MTFDE2D) to solve the problem of high calculation cost of MDE2D. Unfortunately, the length of the coarse-grained sequence used in MTFDE2D will become shorter as the scale factor increases, resulting in a loss of potential useful information. To address this, a two-dimensional refined composite multi-scale time–frequency dispersion entropy (RCMTFDE2D) is proposed. In RCMTFDE2D, the utilization of information contained in the original date is first effectively improved through the coarse-grained process of sliding values of adjacent elements, and then the refined process is used to avoid the fluctuation of entropy on large scale factors. Furthermore, the rolling bearing test data under two different operating conditions are analyzed. The results illustrate that RCMTFDE2D can successfully extract the rolling bearing fault information in time–frequency domain, and the condition monitoring method based on RCMTFDE2D can effectively identify the rolling bearings in different health states and has the highest recognition rate.

Study of optoelectronic and thermoelectric properties of double perovskites X2HfI6 (X = Ga, In, Tl) for renewable energy

The double perovskites (DPs) are outstanding materials for renewable energy and other optoelectronic applications. Here in this paper, the thermoelectric and optical behavior of X2HfI6 (X = Ga, In, Tl) have been explained systematically by DFT. The structural stability has been certified using ionic radii in terms of tolerance factor, and dynamic stability has been confirmed by positive phonon dispersion frequencies in acoustic modes. The band structure findings show the direct band gaps for (Ga, In, Tl) based DPs. The optical properties have been investigated by dispersion, absorption, refection, and related other optical characteristics. The absorption bands are existing in ultraviolet region and shifted to higher energy (blue shift). The reflection and optical loss of light are noted less than 0.3 which increases their potential for optoelectronic devices. The high Seebeck coefficient and electrical conductivity along with ultralow lattice thermal conductivity confirm the high figure of merit at and below the room temperature.

Properties of the underwater sound fields during some well documented beaked whale mass stranding events

Recent mass strandings of marine mammals, mostly Cuvier’s beaked whales (Ziphius cavirostris) from the family of ziphiidae, have occurred coincident in space and time with human production of high levels of underwater sound. Three of these events, the May 1996 mass stranding along the Greek coast, the Bahamas mass stranding event in March 2000 and the September 2002 event in the Canary Islands, were selected for consideration here since pertinent information was readily available. The purpose of this paper is to summarise the probable characteristics of the sound fields during these events and to search for common features. The acoustic sources in all three cases moved at speeds of 5 knots or greater and generated periodic sequences of high amplitude, transient pulses 15-60s apart that contained significant energy in the 1-10kHz frequency band. The environmental conditions included water depths exceeding 1km close to land. In addition, the depth dependence of the ocean sound speed created an acoustic waveguide whose lower boundary was formed by refraction within the water column. The anthropogenic sources in all cases were located within such waveguides. Under these conditions, sound levels decrease more slowly with increasing range after a certain transition range than otherwise, due to sound focusing and to decreased attenuation because of isolation over extended ranges from the ocean bottom. In addition, the frequency dispersion is such that pulses tend to remain as pulses during propagation. For those events involving near-surface sources in surface ducts, weather conditions were calm leading to minimal sound attenuation and scattering by near-surface bubbles and ocean surface roughness. Quantitative prediction of the actual sound field properties during these events is limited primarily by the lack of knowledge of prevailing environmental conditions. Results from simple numerical modelling show that received sound level increases of up to 20dB occur after the transition range for sources and receivers within refractive waveguides. Data-based semi-empirical models of surface duct propagation provide simple, realistic, quantitative estimates of the mean acoustic field in the duct and the effects of changes in environmental conditions. Numerical modelling of total sound exposure (pressure squared integrated with respect to time) illustrates the importance of the relative velocity and minimum range between source and receiver, indicating that realistic animal motion models are required to obtain representative results. Although several features of the sound fields during these three mass stranding events are very similar, their actual relationship to the strandings is unknown.

Structure, dielectric, and relaxor properties of BaTiO3-modified high-entropy (Bi0.2Na0.2K0.2Ba0.2Ca0.2)TiO3 ceramics for energy storage applications

High-entropy (1−x)Bi0.5Na0.5K0.5Ba0.5Ca0.5TiO3-xBaTiO3 (BNKBCT-BT, x = 0.02, 0.06, 0.10, 0.14 and 0.20) ceramics are successfully prepared via the solid-state reaction method. The XRD and Raman analyses reveal that all samples exhibit a pseudo-cubic phase structure. With increasing x, the a is gradually increased, resulting in the expansion of unit cell volume. A strong frequency dispersion appeared around Tm, demonstrating that the as-prepared ceramics is a relaxor ferroelectric. Owing to the increase in the degree of lattice disorder, as well as relaxation and thermal evolution of PUs, the as-prepared ceramics exhibit a broad relaxor behavior. The ΔP is effectively increased by doping BT, which is conducive to achieving excellent energy storage properties. Moreover, Wrec of all samples is more than 1.31 J/cm3 and, at x = 0.20, Wrec reaches the maximum value of 2.24 J/cm3 at 140 kV/cm. These results indicate that high-entropy (1−x)BNKBCT-xBT ceramics are promising materials for energy storage applications.

Dispersion properties of glide-symmetric corrugated metasurface waveguides

Abstract In this paper, a mode-matching procedure is used to prove for the first time the dispersionless behavior of glide symmetry (GS) in corrugated metasurface waveguides. Depending on their field symmetry, the Floquet harmonics have a different impact onto the frequency dispersion, which is reduced with GS when the gap between the surfaces is small. Indeed, the glide-symmetric waveguide is shown to have the same effective propagation features as a scaled mirror-symmetric waveguide with half the period and a doubled gap. We propose closed-form formulas for the effective refractive index of the glide-symmetric waveguide in the first Brillouin zone, having a small groove compared to the period. The closed-form expressions analytically prove the dispersionless behavior of GS.

SBLMD–ANN–MOPSO-based hybrid approach for determining optimum parameter in CNC milling

Chatter is a type of self-excited vibration that expresses variations in frequency and energy dispersion during the milling process and invariably results in poor part quality and a lower material removal rate. An efficacious chatter detection approach is necessary to anticipate the chatter's nascent stage. Feature extraction is an important step in identifying chatter. In this paper, an efficient PF-based multi-mode signal processing method, i.e., Spline-Based Local Mean Decomposition (SBLMD), has been used to decompose the experimentally acquired sound signals into a series of PFs and then, selected PFs have been used to reconstruct the new chatter signal, which is rich in information. Further, two three-layer ANN-based prediction models are established to predict the CI and MRR. Three process-related variables, such as tool speed, feed speed, and cut depth, have been used to develop ANN models. Statistical comparisons have been conducted to obtain the optimal training algorithm and have found that the LM-based training algorithm was the best among the five other training algorithms. For the estimates of CI and MRR, the neurons in the hidden layer are optimized at 5 and 7 with the least mean squared errors (MSE), respectively. After applying MOPSO to ANN-based prediction models, the MOPSO-optimized ANNs are very sensitive to optimizing input layers. The optimal ranges for the Cut Depth (CD), Feed Speed (FS) and Tool Speed (TS) are 1.56–1.77 mm, 77–97 mm/min, and 2380–2880 rpm, respectively.

Rayleigh wave propagation in isotropic sandy layer sliding over isotropic sandy semi-infinite medium with sliding contact

The present study aims to investigate Rayleigh wave propagation in an isotropic sandy layer overlying an isotropic sandy semi-infinite medium, with interface considered to be imperfect (slide contact and dislocation like model). Expressions for displacement components are obtained using the variable separation method. The dispersion frequency equation for the Rayleigh wave propagating in sandy media is derived using suitable boundary conditions. Particular cases, such as when the interface is in smooth contact and when sandy media are replaced by elastic media, are also discussed. Using MATLAB software, the effects of the imperfectness parameter (slide contact and dislocation like model) and sandy parameter on the Rayleigh waves’ phase velocity are investigated and compared with the already obtained results of the dislocation like model. The present study may find useful applications in geophysics, civil engineering and soil mechanics.

Microstructures and physical properties of sol–gel derived Ba2FeMnO6 double perovskite

AbstractWe report microstructures and physical (dielectric, magnetic, and optical) properties of sol–gel derived Ba2FeMnO6 (BFMO) double perovskite powders. The BFMO powders belong to hexagonal crystal structure with P‐6m2 space group. SEM images reveal the powders are nearly homogeneous distribution with spherical morphology and average particle size of 300 nm. Energy‐dispersive spectra gave out the molar ratio of the Ba:Fe:Mn elements equal to 2.16:1.00:1.00. FTIR spectrum verified the [FeO6] and [MnO6] octahedra present in the powders. X‐ray photoelectron spectroscopy spectra identified the chemical valence states of the constituent elements. The BFMO ceramics displayed a strong frequency dispersion dielectric behavior. A relaxor‐like dielectric behavior appeared around ∼475 K due to the contributions of oxygen vacancies of and the defect dipoles. A ferrimagnetic behavior was observed in the powders at 5 K with MS = 0.14 μB/f.u. and HC = 1.51 kOe. The magnetic Curie temperature (TC) was 381 K, whereas the Neel temperature (TN) was 31 K. The ferrimagnetic behavior is governed by the Mn3+–Fe4+ double‐exchange interaction via the mediated oxygen between them. The BFMO powders have a direct optical bandgap of 1.65 eV, which originates from the electron transferring from O 2p to Mn 3d (and/or Fe 3d) levels. The unique combination of high temperature ferrimagnetism and semiconductivity in the BFMO powders makes them particularly appealing for magnetic spintronics and photovoltaics.

Admittance frequency dispersion in lateral AlGaN/GaN Schottky barrier diodes: Other origins of two Gp/ω peaks

The frequency dispersion in admittance measurements in AlGaN/GaN high-electron-mobility transistors, which is typically interpreted to result from an interface trap density Dit, is also known to be caused by effects other than Dit. To study the origin of two peaks of conductance component “Gp/ω” as a function of frequency in lateral gated diodes using AlGaN/GaN structures on Si wafers grown by metal–organic chemical vapor deposition, we measured capacitance C and conductance Gp/ω–voltage V as a function of angular frequency ω (C–ω–V and Gp/ω–ω–V, respectively) of long-gate field-effect transistors with a varied gate length. We also simulated the C–ω–V and Gp/ω–ω–V curves using an equivalent circuit that consisted of actually measured component parameters without Dit. We confirmed that the Gp/ω–ω curves show two peaks caused by the two-dimensional electron gas channel resistance and the gate current leakage and quantitatively determined the two ωpeak positions and intensities. We also discussed the effect of acceptor concentration in the GaN channel-layer on lowering of ωpeak at weak inversion.

Enhanced magnetoelectric effect in Lead-Free piezoelectric BaZr0.2Ti0.8O3 − 0.5 Ba0.7Ca0.3TiO3 and Fe-rich magnetostrictive Co0.8Fe2.2-xDyxO4 nanocomposites for energy harvesting applications

The design, discovery and development of magnetoelectric (ME) composites with enhanced properties and performance opens up further opportunities to explore new scientific aspects and to develop next-generation electromagnetic devices. Nevertheless, the engineered complex ME composites with multi-phase interfacial chemistry have rarely been reported. In this context, in the present work, enhanced magnetoelectric phenomenon reported for the ME-composites made using lead-free ferroelectric phase BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 (BZT-BCT) and magnetostrictive phase Co0.8Fe(2.2-x)DyxO4 (CFDO). The ME composites were synthesized by varying the magnetostrictive CFDO phase, where Dy(x) varied as x = 0.000(ME1), 0.025 (ME2), 0.050 (ME3) and 0.075 (ME4). The effect of varying CFDO phase on the structure, morphology, microstructure, density, piezoelectric, dielectric, ferroelectric and magnetoelectric properties of the ME-composites is evaluated. X-ray diffraction analyses of all ME-composites confirm the tetragonal structure of the BZT-BCT ferroelectric phase and spinel cubic structure of the CFDO ferrimagnetic phase. Raman spectroscopic analyses also confirm and validate the respective phases with a clear signature presence of the characteristics modes. Granular, dense microstructure with a uniform particle size distribution evidenced in scanning electron microscopy imaging analyses. The physical density measurements indicate that the ME composites attain a density in the range of 6.0–6.4 g/cm3; density increases with increasing CFDO phase. Frequency dispersion profiles of the dielectric constant exhibit a maximum dielectric constant for ME1 composite while a sharp decrease in dielectric constant at higher frequencies is noted with increasing CFDO phase in the ME-composites. Ferroelectric properties evaluated by measuring the polarization (P) – electric field (E) hysteresis loops at 300 K indicate the presence of typical P-E loops in all ME composites confirming that the ferroelectric properties are retained even after combining with the magnetic phase. The ME4 composite demonstrate a 0.573 squareness ratio, which is useful for multistate memory device applications. The ME4 composites further characterized by a moderately higher remnant polarization (Pr = 2.24 µC/cm2). All the synthesized ME-composites are promising for permanent memory storage device (FeRAM) applications. Piezoelectric coefficient measurements indicate a maximum piezoelectric coefficient of 116 pC/N for the ME4 composite while the ME-coefficient measurements demonstrate higher ME-coefficients of 250 mV/(cm-Oe) for ME2 composite. Based on the comprehensive and comparative studies of the ME-composites with variable magnetostrictive phase, the structure-chemistry-property correlation in these ME-composites established for possible applications in electromagnetic devices and energy harvesting applications.

Surface Effects on the Frequency Dispersion of Flexural Waves in Timoshenko Nanobeams

The surface effects play an important role in nanobeams. Based on a recently developed theory of surface elastodynamics, a model of the flexural wave propagation in Timoshenko nanobeams is established, in which the surface effects characterized by surface energy and surface inertia are introduced. It is found that when the size of a beam is comparable with nanometers, the surface energy effect would enhance the wave speed, while the surface inertial one would reduce it. An interesting phenomenon discovered is that with an increasing wave frequency, the dominant role transits from the surface energy effect to the surface inertial effect. The two kinds of surface effects exhibit a frequency-dependent competitive mechanism. In contrast to the macroscopic beams, due to the surface effects, the frequency dispersion of flexural waves in nanobeams becomes size-dependent. Furthermore, a comparison of the Timoshenko nanobeam and the Euler one indicates that the shear deformation effect and rotary inertial one cannot be neglected for a large wave number, which would prominently decrease the wave speed. Besides, when the size of the beam is large enough, the surface effects can be neglected and the present results can degenerate to the classical Timoshenko ones. The present results should be helpful not only for deep understanding of the dispersive mechanism of flexural waves in nanobeams, but also for optimal design of nanobeam-based acoustic wave devices.

Dispersion‐Enabled Symmetry Switching of Photonic Angular‐Momentum Coupling

AbstractPhotonic spin‐orbit interactions describe the interactions between spin angular momentum and orbital angular momentum of photons, which play essential roles in subwavelength optics. However, the influence of frequency dispersion on photonic angular‐momentum coupling is rarely studied. Here, by elaborately designing the contribution of the geometric phase and waveguide propagation phase, the dispersion‐enabled symmetry switching of photonic angular‐momentum coupling is experimentally demonstrated. This notion may induce many exotic phenomena and be found in enormous applications, such as the spin‐Hall effect, optical calculation, and wavelength division multiplexing systems. As a proof‐of‐concept demonstration, two metadevices, a multi‐channel vectorial vortex beam generator and a phase‐only hologram, are applied to experimentally display optical double convolution, which may offer additional degrees of freedom to accelerate computing and a miniaturization configuration for optical convolution without collimation operation. These results may provide a new opportunity for complex vector optical field manipulation and calculation, optical information coding, light‐matter interaction manipulation, and optical communication.

Dielectric and ferroelectric characteristics of Ba(Ti0.25Zr0.25Hf0.25Sn0.25)O3 high-entropy ceramics

Ba(Ti0.25Zr0.25Hf0.25Sn0.25)O3 high-entropy ceramics were prepared by a standard solid state reaction process, and the dielectric and ferroelectric characteristics were investigated together with the structures. Both X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis demonstrated a single-phase perovskite structure in the present ceramics. A broad dielectric peak with strong frequency dispersion feature was determined, which indicated the typical relaxor nature originating from the nanoscale ferroelectric domain structures. These resulted from the structural distortion and chemical disorder due to high-entropy, where the long-range order of ferroelectric domains was destroyed. The homogeneous microstructure led to the reduced leakage current density and significantly improved dielectric strength, which was desired for the practical applications. Compared with the similar systems of Ba(Ti1-xZrx)O3 & Ba(Ti1-xSnx)O3, the present high-entropy ceramics indicated better relaxor ferroelectric characteristics.