Surface Current Density Research Articles

On the Application of the Langevin and Gorter—Casimir Two-Component Theory to Superconductor Surface Currents

Within the framework of classical electrodynamics, the model problem of current distribution on the surface of a superconducting sphere and of magnetic induction are analyzed in the presence of an external current flowing onto a given superconducting sphere. The possibility of using the Langevin and Gorter—Kasimir two-component theory to surface currents produced in superconducting structures is considered. Within the framework of the formulated problem, the current distributions are calculated in detail in the case where electrons flow onto a superconducting sphere. Theoretical analysis of the main electrodynamic and London equations shows that a constant current component, which is related to the external flowing current, appears. The boundary conditions are analyzed in detail. Using them, we conclude that there is no tangential component near the sphere surface. A part of the boundary conditions is written in terms of the surface current density, which is also fundamentally new in the theoretical presentation of this physical problem.

Perfect metamaterial absorber based on the circular or elliptical cavity with one side open in the terahertz region

ABSTRACT In this work, we present the analysis and design of a perfect metamaterial absorber (MA) based on the circular cavity with one side open in the terahertz region. This structure consists of a metal plate where one circular cavity with one side open is vertically placed on. Based on the circular-cavity-theory, a formula can be used to calculate the resonant frequency. In terms of normal incidence, the simulated results show that the MA has one resonance point on 6.674 THz, which is consistent with the calculated results. What’s more, we present the distribution of E-field, surface current distribution and power loss density distribution in the cavity, and use the standing wave theory and electric dipole to prove the physical mechanism of the perfect absorption. Finally, an MA based on elliptical cavity with one side open is also studied. These results verify the application of resonant cavities in the field of MA.

Specimen representation on the prediction of artificial test lightning plasma, resulting specimen loading and subsequent composite material damage

Preceding work has established that artificial test lightning plasma and composite test specimen damage can be modelled. However, no work has studied the impact of specimen representation in the modelling of the plasma and the resulting impact on specimen composite damage. Herein four distinct specimen designs have been modelled, with a magnetohydrodynamic FE multiphysics model employed to simulate the plasma and a FE thermal-electric modelling approach used to predict composite material damage. For the test arrangements modelled herein it has been found that specimen representation has limited impact on plasma global structure, even with significant change in specimen properties (e.g. from copper to epoxy). However, noteworthy variation in the local specimen surface loading is witnessed with specimen property change (e.g. epoxy to carbon reinforced epoxy), with peak magnitudes for surface pressure, velocity, current density and temperature changing by up to 88%. Such variation in local specimen surface loading does significantly vary the prediction of composite material thermal damage depth (up to 1200%) and surface damage area (up to 1314%). Moreover, this work, for the first time, provides predictions for the thermal damage suffered by both protected and unprotected composite specimens exposed to test standard Waveform B.

Frontal theta and posterior alpha in resting EEG: A critical examination of convergent and discriminant validity.

Prior research has identified two resting EEG biomarkers with potential for predicting functional outcomes in depression: theta current density in frontal brain regions (especially rostral anterior cingulate cortex) and alpha power over posterior scalp regions. As little is known about the discriminant and convergent validity of these putative biomarkers, a thorough evaluation of these psychometric properties was conducted toward the goal of improving clinical utility of these markers. Resting 71-channel EEG recorded from 35 healthy adults at two sessions (1-week retest) were used to systematically compare different quantification techniques for theta and alpha sources at scalp (surface Laplacian or current source density [CSD]) and brain (distributed inverse; exact low resolution electromagnetic tomography [eLORETA]) level. Signal quality was evaluated with signal-to-noise ratio, participant-level spectra, and frequency PCA covariance decomposition. Convergent and discriminant validity were assessed within a multitrait-multimethod framework. Posterior alpha was reliably identified as two spectral components, each with unique spatial patterns and condition effects (eyes open/closed), high signal quality, and good convergent and discriminant validity. In contrast, frontal theta was characterized by one low-variance component, low signal quality, lack of a distinct spectral peak, and mixed validity. Correlations between candidate biomarkers suggest that posterior alpha components constitute reliable, convergent, and discriminant biometrics in healthy adults. Component-based identification of spectral activity (CSD/eLORETA-fPCA) was superior to fixed, a priori frequency bands. Improved quantification and conceptualization of frontal theta is necessary to determine clinical utility.

Novel square spiral Coil for achieving uniform Distribution of magnetic Field

In view of the problems existing in the existing wireless charging technology, the anti-offset performance of the charging region and the system was studied. A primary transmitting coil capable of achieving uniform magnetic field distribution was proposed. The magnetic field strength at a point on the coil axis and above the coil surface was calculated, and the surface current density of the transmitting coil was analyzed to calculate the radius of each turn of the spiral winding coil. Optimized the structure of the transmitting coil, the number of turns and the arrangement of the coils, A square spiral coil having a size of 20 cm η 20 cm and a number of turns of 11 inches was determined. The optimized square spiral coil was modeled by the software comsol5.3 RF module, and the strength and uniformity of the coil surface and axial space magnetic field were simulated. The axial magnetic field on the surface of the coil was measured by a multi-dimensional visual electromagnetic measuring device, and compared with the simulation data, it was proved that the axial magnetic field of the square spiral coil surface satisfied the uniformity requirement.

Comptibility of optical transparency and microwave absorption in C-band for the metamaterial with second-order cross fractal structure

Abstract An optically transparent metamaterial absorber (MMA) operated in the C-band (4–8 GHz) is optimally designed based on the second-order cross fractal structure. The experimental results show that the reflection loss of the proposed MMA is lower than −10 dB from 3.98 GHz to 8.68 GHz. Two overlapped resonances located at 4.77 GHz and 7.33 GHz, respectively, are caused by the second-order cross fractal structure. Distributions of surface current, electromagnetic field and power loss density are investigated to illustrate the absorptive characteristics and electromagnetic loss mechanisms of the proposed MMA. Moreover, the average optical transmittance of the MMA is close to 75% from 390 nm to 780 nm. The MMA possesses simultaneously strong absorptive capacity in the C-band and high transmittance in the visible light range, showing potential application in special environment.

Electrical properties of nBn structures based on HgCdTe grown by molecular beam epitaxy on GaAs substrates

The unipolar MWIR nBn structures based on HgCdTe grown by molecular beam epitaxy on GaAs (0 1 3) substrates were created. The CdTe content in the barrier layer varied from 0.67 to 0.84 for various samples. Aluminum oxide (Al2O3) films were used to passivate the surface of the mesa structure. For fabricated samples, dark current density values were measured at various voltages in a wide temperature range. It is shown that at lower compositions in the barrier layer the dark current is limited by surface leakage. With an increase in the barrier composition, the number of structures with a bulk dark current was increased. The voltage dependences of surface and bulk current density are constructed. The activation energies for structures with the dominance of the bulk dark current are determined. It is shown that for a considerable part of structures with a large barrier composition (x = 0.84) the bulk dark current is determined by diffusion processes.

In-Situ Diagnostics of Coupled Electrochemical-Mechanical Properties of Solid Electrolyte Interphases on Lithium Metal for Rechargeable Batteries

Low cycle efficiency and dendrite growth are two critical barriers for rechargeable batteries using Li metal as negative electrodes, mainly due to the coupled mechanical/chemical degradation of the SEI layer formed on Li metal surface. We have developed a comprehensive set of in situ diagnostic techniques combined with atomic/continuum modeling schemes to investigate and understand the coupled mechanical/chemical degradation of the SEI layer/Li system including fundamentally understanding the surface and interface phenomena. We have found that the mechanical incompatibility between SEI and soft Li leads to the complicated mechanical behaviors of the lithium metal electrode during the plating and stripping process. We systematically investigated the relationship between surface morphology and current density distribution which results in an inhomogeneous Li plating/stripping process. Based on the fundamental understanding from the integrated in-situ diagnostics, atomic simulation, and continuum frame work, we have developed a new coating design strategy to achieve high cycle efficiency/dendrite free and extend the cycle life of lithium rechargeable batteries.

Arc discharges during disruptions in the T-10 tokamak

Arc discharges are considered as a possible mechanism determining transition from relatively slow growth of the magnetohydrodynamic (MHD) perturbations to thermal quench (minor disruption) and subsequent transition to the major disruption with current collapse in the T-10 tokamak. The effect of the arc discharges is evaluated using analysis of the fast-scale (0.2–1.5 MHz) electromagnetic oscillations measured by movable magnetic and electric probes located near the plasma boundary at multiple positions inside the vacuum vessel and by postoperational inspection of the in-vessel components. Experiments using a special “arc” probe with castellated surface stimulating arc discharges indicated that transition to the major disruption is associated with enhanced electron emission from the plasma facing components. The role of the arcs is confirmed in experiments with initiation of the currents in the peripheral plasma using biasing (Ub ∼ 0–400 V) between the rail limiter and the movable tungsten electrode. Experiments indicated that critical surface current density is required for the arc initiation. The radial decay rate and frequencies of the electromagnetic oscillations measured in the T-10 experiments are found to be similar to the ones during arc discharges at the laboratory bench. Present experiments in the T-10 tokamak indicate that arc discharges could determine plasma-wall interaction during growth of the MHD modes and disruptions.

Structural Effect of 10um Depth Surface Relief on the Cycling Behavior of Lithium Metal Anode

Lithium metal anodes (LMAs) are required to fulfill higher energy density battery system beyond lithium ion batteries (LIBs). However, practical application of LMAs has been hindered from poor cycle life and safety issue caused by lithium (Li) dendrite growth and inhomogeneous Li plating/stripping. Recently, the Li surface patterning technique was developed for the lower surface current density and stable Li plating/stripping for LMB. Nevertheless, the impact of various factors such as the pattern morphology and charge-discharge condition on Li anode was much unexplored. Here, we report the comparison of the cycling behavior of 10 μm depth continuous and discontinuous surface relief on lithium anode under the Li overflow conditon with varying induced current density. Both continuous and discontinuous surface relief efficiently reduced dendritic Li growth and improved Li plating/stripping behavior than the bare Li anode. The most controlled Li plating/stripping was observed along the discontinuous surface relief. This result affects the improved cycle performance and lowered overpotential of surface patterned lithium metal anode. Furthermore, it was revealed that the 10 μm depth surface relief can not only be applied on the thin (20 μm) Li anode but also enhance cycling stability.

A low profile miniature RFID tag antenna dedicated to IoT applications

ABSTRACTRFID tag antennas with stable performance on the diverse electromagnetic mounting platforms are an integral part of the ubiquitous RFID systems. This research article presents a novel tag antenna design that facilitates the said objective. The proposed antenna consists of a modified H-shaped slot structure that ensures considerable robustness from the application environment through confining the surface current density within the antenna structure. The antenna offers a tunable bandwidth of 40 MHz within the microwave band of (2.4–2.5) GHz. The proposed tag antenna exhibits excellent response on metallic platforms of different sizes and thicknesses with an effective gain of almost four times of that in free space. Furthermore, the designed tag antenna performs adequately well on low–medium permittivity dielectrics (glass, paper, and plastic) and RF absorbers (water). The free space and on-metal performance of the proposed tag antenna are verified by testing a prototype realized on the FR4 substrate.

Upper Bounds and Design Guidelines for Realizing Uncorrelated Ports on Multimode Antennas Based on Symmetry Analysis of Characteristic Modes

The design of multiport antennas for multiple-input multiple-output (MIMO) applications utilizing characteristic modes is investigated. For good MIMO performance, uncorrelated antenna ports are generally required. In order to analyze the port correlation, symmetry analysis based on group theory and matrix representations is applied to the theory of characteristic modes. The characteristic surface current densities act as basis functions of the irreducible representations of the symmetry group of the antenna. Current densities belonging to different representations or belonging to different rows of the same representation are orthogonal to each other and can, thus, be excited separately. Therefore, an upper bound for the number of uncorrelated antenna ports can be derived for a given antenna structure based on the symmetry analysis of the characteristic modes. Furthermore, design guidelines on which antenna geometry to choose in order to realize a given number of uncorrelated antenna ports and on how to implement these ports can be deduced. The concept is illustrated by means of examples.

Five-Band Terahertz Perfect Absorber Based on Metal Layer–Coupled Dielectric Metamaterial

A five-band tunable ideal terahertz metamaterial absorber based on subwavelength range is proposed. It consists of a reflective layer, a dielectric layer, and an absorbing layer consisting of an internal closed square ring and an outer open square circle. We have found that the five absorption peaks can reach more than 97% on average. Meanwhile, in order to analyze the absorption mechanism, we studied the different structures and geometric parameters of the absorber and drew the electric field intensity diagram and surface current density diagram at different peaks. Moreover, we also studied the effect of similar opening structures based on this structure on absorption. Therefore, the designed absorber has good frequency selectivity and can be applied to terahertz imaging, detection, and so on.

Dual band omnidirectional millimeter wave antenna for 5G communications

ABSTRACTA planar dual band millimeter wave printed monopole antenna and a MIMO antenna are presented for 28/38 GHz future 5G wireless communications. The shape of the radiating element of the proposed antenna is derived from a 28 GHz rectangular monopole. The radiating element isproducing a second resonance at 38 laterally slimmed in a hemispherical manner to intensify the surface current density along the hemispherical contour, producing a second resonance at 38 GHz. The measured impedance bandwidth confirms its dual band characteristics, covering 26.65–29.2 and 36.95–39.05 GHz where . The measured radiation patterns are omnidirectional, with a peak gain value of 1.27 and 1.83 dBi at 28 and 38 GHz, respectively. Moreover, diversity performance of the proposed MIMO antenna is evaluated in terms of MIMO performance metrics. such as envelope correlation coefficient, diversity gain, isolation and channel loss capacity.

Spectrum manipulation of a dual-stop-band meta material terahertz filter based on unit cell dimension changes

We numerically and experimentally proposed a dual-stop-band terahertz filter based on standard microelectronic fabrication method. The stop bands locate at 0.32 THz and 1.02 THz with 3 dB bandwidths of 0.26 THz and 0.55 THz, respectively. The resonance characteristics of the proposed device were discussed with the help of surface current maps and field density maps extracted from computer simulation software to better understand the working principle of the proposed device. On top of that, a total of seven devices with different dimensions were fabricated to fully discuss the dimension effects on the resonant frequency shift and bandwidth changes. This fabrication process is applicable for related integrated metamaterial devices and provides essential experiment evidences for effective ways of manipulating the transmission spectrum of the proposed filter.

Chemical Mechanical Polishing Implementation for Corrosion Prevention of Titanium Bio-Implants

Corrosion and surface film dissolution are two mechanisms for titanium-based bioimplants to release ions into the human body and hence degrading the biocompatibility. Electrochemical methods are commonly practiced in testing metallic biomaterials in-vitro measurements[1] (Kuhn, Neufeld and Rae, 1988). In an attempt to prevent the in-vivo corrosion of the titanium implants, we have introduced a new approach by utilizing chemical mechanical polishing (CMP) as a synergistic approach to form a protective surface film and as a tool to control the surface nanostructure [2] to promote or demote cell attachment. In this paper, application of CMP on titanium implant surfaces is studied through electrochemical analyses in the absence and presence of an oxidizer. Corrosion rates are extrapolated from anodic polarization of the potentiodynamic curves (Figure 1) with and without CMP implementation to compare the affect of surface engineering. It was observed that the higher corrosion (Table 1) rates correlated to the higher material removal during polishing, which can be controlled through oxidizer concentration that affects the surface TiO2 film growth. Furthermore, potentiostatic measurements showed that the addition of oxidizer increased the rate of oxide growth and decreased the surface current density levels indicating that the surface self-passivation took place. Post CMP contact angle (wettability) and AFM measurements confirmed an improved surface topography and surface homogeneity by CMP treatment as better surface smoothness and wettability control was achieved. [1] Kuhn, A., Neufeld, P. and Rae, T. (n.d.). Synthetic Environments for the Testing of Metallic Biomaterials. The Use of Synthetic Environments for Corrosion Testing, 1988, pp.79-79-19. [2] Ozdemir, Z., Ozdemir, A. and Basim, G. (2016). Application of chemical mechanical polishing process on titanium based implants. Materials Science and Engineering: C, 68, pp.383-396. Figure 1

Electromagnetic modes and resonances of two-dimensional bodies

The electromagnetic modes and the resonances of homogeneous, finite size, two-dimensional bodies are examined in the frequency domain by a rigorous full wave approach based on an integro-differential formulation of the electromagnetic scattering problem. Using a modal expansion for the current density that disentangles the geometric and material properties of the body the integro-differential equation for the induced surface (free or polarization) current density field is solved. The current modes and the corresponding resonant values of the surface conductivity (eigen-conductivities) are evaluated by solving a linear eigenvalue problem with a non-Hermitian operator. They are inherent properties of the body geometry and do not depend on the body material. The material only determines the coefficients of the modal expansion and hence the frequencies at which their amplitudes are maximum (resonance frequencies). The eigen-conductivities and the current modes are studied in detail as the frequency, the shape and the size of the body vary. Open and closed surfaces are considered. The presence of vortex current modes, in addition to the source-sink current modes (no whirling modes), which characterize plasmonic oscillations, is shown. Important topological features of the current modes, such as the number of sources and sinks, the number of vortexes, the direction of the vortexes are preserved as the size of the body and the frequency vary. Unlike the source-sink current modes, in open surfaces the vortex current modes can be resonantly excited only in materials with positive imaginary part of the surface conductivity. Eventually, as examples, the scattering by two-dimensional bodies with either positive or negative imaginary part of the surface conductivity is analyzed and the contributions of the different modes are examined.

Design and analysis of polarization independent conformal wideband metamaterial absorber using resistor loaded sector shaped resonators

A polarization independent conformal wideband metamaterial absorber has been proposed in this study. The proposed absorber unit cell consists of a circle and a slotted sector, loaded with four lumped resistors between them to improve the absorption bandwidth. Measured and simulated results exhibit more than 90% absorption in the frequency band from 3.90 GHz to 10.5 GHz under the normal incidence angle, and the fractional bandwidth is 91.6%. The proposed absorber is polarization independent due to the symmetrical nature. The absorber unit cell is compact in configuration with a unit cell size of 0.16 λo × 0.16 λo, where λo is the wavelength at 3.9 GHz and the substrate thickness is 0.098 λo. Once the proposed absorber is wrapped on the cylindrical surface, it shows good absorptivity in the absorption frequency band. The effects of many designed parameters have been studied to understand the performance of the wideband absorber. To recognize the absorption phenomenon of the absorber, normalized impedance, electric field distribution, and surface current density have been demonstrated. Finally, the proposed conformal absorber fabricated and measured the absorption for flat and curved surfaces, which exhibits good agreement between the simulated and the experimental results.

Early stage of the discharge in ablative pulsed plasma thrusters

Currently the renewed interest in ablative pulsed plasma thrusters (APPTs) mostly results from the needs of satellites of the nano and micro sectors. However, the efficiency of APPTs significantly decreases when they are scaled down. A better understanding of the physical processes involved in plasma generation and maintenance in this type of thruster may help to minimize APPTs’ performance degradation, when they are miniaturized. One of the issues, which up to now seems to be only marginally clarified, is an initial phase of discharge between the APPT’s electrodes. In this paper a novel analytical model of a flashover development is proposed. In the model, movement of electrons between the PPT’s electrodes is described relying on the concept of a saturated secondary electron avalanche. Two analytical approaches are considered. In the simplest approach, secondary electrons are assumed to be monoenergetic, whereas in the second, the Maxwellian distribution function is assumed. Simple, analytical formulas for surface current density, electron density and the thickness of the electron layer are derived. The predicted values are in a good agreement with measurements and computer simulations of other authors. The work was inspired by experimental examination of a micro APPT fed with polymeric nonvolatile propellant. For completeness, the thruster, known as an LμPPT (liquid micro pulsed plasma thruster), is described and its performance is presented.

Deposition and Stripping Behavior of Lithium Metal in Electrochemical System: Continuum Mechanics Study

Metallic lithium (Li) is a promising anode candidate for high-energy-density rechargeable batteries because of its low redox potential and high theoretical capacity. However, its practical application is not imminent because of issues related to the dendritic growth of Li metal with repeated battery operation, which presents a serious safety concern. Herein, various aspects of the electrochemical deposition and stripping of Li metal are investigated with consideration of the reaction rate/current density, electrode morphology, and solid electrolyte interphase (SEI) layer properties to understand the conditions inducing abnormal Li growth. It is demonstrated that the irregular (i.e., filamentary or dendritic) growth of Li metal mostly originates from local perturbation of the surface current density, which stems from surface irregularities arising from the morphology, defective nature of the SEI, and relative asymmetry in the deposition/stripping rates. Importantly, we find that the use of a stripping rate of Li metal that is slower than the deposition rate seriously aggravates the formation of disconnected Li debris from the irregularly grown Li metal. This finding challenges the conventional belief that high-rate stripping/plating of Li in an electrochemical cell generally results in more rapid cell failure because of the faster growth of Li metal dendrites. Figure 1