Transient Electric Currents Research Articles

Glassy-like Transients in Semiconductor Nanomaterials.

Glassy behavior is manifested by three time-dependent characteristics of a dynamic physical property. Such behaviors have been found in the electrical conductivity transients of various disordered systems, but the mechanisms that yield the glassy behavior are still under intensive debate. The focus of the present work is on the effect of the quantum confinement (QC) and the Coulomb blockade (CB) effects on the experimentally observed glassy-like behavior in semiconductor nanomaterials. Correspondingly, we studied the transient electrical currents in semiconductor systems that contain CdSe or Si nanosize crystallites, as a function of that size and the ambient temperature. In particular, in contrast to the more commonly studied post-excitation behavior in electronic glassy systems, we have also examined the current transients during the excitation. This has enabled us to show that the glassy behavior is a result of the nanosize nature of the studied systems and thus to conclude that the observed characteristics are sensitive to the above effects. Following this and the temperature dependence of the transients, we derived a more detailed macroscopic and microscopic understanding of the corresponding transport mechanisms and their glassy manifestations. We concluded that the observed electrical transients must be explained not only by the commonly suggested principle of the minimization of energy upon the approach to equilibrium, as in the mechanical (say, viscose) glass, but also by the principle of minimal energy dissipation by the electrical current which determines the percolation network of the electrical conductivity. We further suggest that the deep reason for the glassy-like behavior that is observed in the electrical transients of the nanomaterials studied is the close similarity between the localization range of electrons due to the Coulomb blockade and the caging range of the uncharged atomic-size particles in the classical mechanical glass. These considerations are expected to be useful for the understanding and planning of semiconductor nanodevices such as corresponding quantum dot memories and quantum well MOSFETs.

Experimental Study on the Transient Electric Shock Characteristics Between Metal Sheds and the Human Body Near Ultrahigh Voltage Alternating Current Transmission Lines

With the development of ultrahigh voltage (UHV) transmission lines in China, line corridors are becoming increasingly limited, and the lines pass closer to inhabited areas and the occurrence probability of induced electric shocks are increased. The research on transient electric shock and human sensory is essential due to the displeasure and complaints of people on electric shock. In this article, a real-size metal shed was established near a 1000-kV UHV alternating current (ac) transmission line, and the induced voltage and transient current characteristics were studied when the human body was in contact with the shed. Then, the amplitude and pulsewidth of the transient electric shock current were investigated, and the associations between discharge energy and sensation were established. The research found that transient electric shock is the most serious when the grounded human body contacts the insulated shed, and the imparted electric shock energy reaches 25 mJ. When the body and the shed are grounded, there is no electric shock sensation. When the insulated human body is in contact with the insulated shed, the electric shock sensation is intermediate, and women's perceptions of pain are more intense. The results provide references to transient electric shock protection for power transmission lines.

Measurement and interpretation of electrical signals in transient electrical discharges* *Also at ETSII C.Real, University of Castilla-la Mancha, Spain.

Electrical discharges in experiments like Exploding Wire, Plasma Focus, or Z-pinch, involve regions where strong transient electrical currents generates magnetic flux variations within the limits of experiment and diagnostics regions. Due to different experimental conditions, time duration of the transient phase may vary from negligible to play an important role in the explanation of the measured signal of the experiment, in which case Faraday’s law of induction cannot be neglected when analyzing the electrical signals. In this work the effects of circuit modeling taking into account Faraday’s law will be discussed for the exploding wire experiment in a more detailed way than previous works.

Comparison of Machine Learning Algorithms for the Prediction of Mechanical Stress in Three-Phase Power Transformer Winding Conductors

This research compares four machine learning techniques: linear regression, support vector regression, random forests, and artificial neural networks, with regard to the determination of mechanical stress in power transformer winding conductors due to three-phase electrical faults. The accuracy compared with finite element results was evaluated for each model. The input data were the transient electrical fault currents of power system equivalents with impedances from low to high values. The output data were the mechanical stress in the conductors located in the middle of the winding. To simplify the design, only one hyperparameter was varied on each machine learning technique. The random forests technique had the most accurate results. The highest errors were found for low-stress values, mainly due to the high difference between maximum and minimum stresses, which made the training of the machine learning models difficult. In the end, an accurate model that could be used in the continuous monitoring of mechanical stress was obtained.

Frequency Upshift of the Transverse Optical Phonon Resonance in GaAs by Femtosecond Electron-Hole Excitation.

The impact of transient electric currents on the transverse optical (TO) phonon resonance is studied after excitation by two femtosecond near-infrared pulses via the fourth-order nonlinear terahertz emission. Nonlinear signals due to interband shift currents and heavy-hole-light-hole polarizations are separated from Raman-induced TO phonon coherences. The latter display a frequency upshift by some 100GHz upon interband excitation of an electron-hole plasma. The frequency shift is caused by transverse electronic shift currents, which modify the dielectric function. A local-field model based on microscopic current densities reproduces the observed frequency upshift.

Time-dependent thermoelectric transport in mesoscopic systems under a quantum quench

We investigate the transient behavior of the quantum transport in mesoscopic systems under a quantum quench within the Caroli scheme. Using the nonequilibrium Green's function approach, an exact solution of the transient electric current, energy current, and their fluctuations in the presence of both external bias and temperature gradient are presented that goes beyond the wide-band limit. The exact solution of the time-dependent Seebeck coefficient in the linear response regime is also obtained. This formalism is applied to study the transient behavior of a single-level quantum dot with Lorentzian linewidth induced by the temperature gradient. The damped oscillatory behavior is found in the transient electric and energy currents, as well as their fluctuations. The oscillation frequency of electric and energy currents increases with the increasing energy level of quantum dot and the decay rate of oscillation decreases as the bandwidth increases. A significantly enhanced Seebeck coefficient is generated in the transient regime. We find the maximum value of the time-dependent Seebeck coefficient can be enhanced by increasing the energy level of the quantum dot and the reference temperature of leads.

Crustal and time-varying magnetic fields at the InSight landing site on Mars

Magnetic fields provide a window into a planet’s interior structure and evolution, including its atmospheric and space environments. Satellites at Mars have measured crustal magnetic fields indicating an ancient dynamo. These crustal fields interact with the solar wind to generate transient fields and electric currents in Mars’s upper atmosphere. Surface magnetic field data play a key role in understanding these effects and the dynamo. Here we report measurements of magnetic field strength and direction at the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) landing site on Mars. We find that the field is ten times stronger than predicted by satellite-based models. We infer magnetized rocks beneath the surface, within ~150 km of the landing site, consistent with a past dynamo with Earth-like strength. Geological mapping and InSight seismic data suggest that much or all of the magnetization sources are carried in basement rocks, which are at least 3.9 billion years old and are overlain by between 200 m and ~10 km of lava flows and modified ancient terrain. Daily variations in the magnetic field indicate contributions from ionospheric currents at 120 km to 180 km altitude. Higher-frequency variations are also observed; their origin is unknown, but they probably propagate from even higher altitudes to the surface. We propose that the time-varying fields can be used to investigate the electrical conductivity structure of the martian interior. The magnetic field measured by the InSight lander on Mars varies daily and is ten times stronger than expected. The field is inferred to originate from components of basement rocks magnetized by an ancient dynamo of Earth-like strength.

Coupled simulation of transient heat flow and electric currents in thin wires: Application to bond wires in microelectronic chip packaging

This work addresses the simulation of heat flow and electric currents in thin wires. An important application is the use of bond wires in microelectronic chip packaging. The heat distribution is modeled by an electrothermal coupled problem, which poses numerical challenges due to the presence of different geometric scales. The necessity of very fine grids is relaxed by solving and embedding a 1D sub-problem along the wire into the surrounding 3D geometry. The arising singularities are described using de Rham currents. It is shown that the problem is related to fluid flow in porous 3D media with 1D fractures (C. D’Angelo, SIAM Journal of Numerical Analysis 50.1, pp. 194–215, 2012). A careful formulation of the 1D–3D coupling condition is essential to obtain a stable scheme that yields a physical solution. Elliptic model problems are used to investigate the numerical errors and the corresponding convergence rates. Additionally, the transient electrothermal simulation of a simplified microelectronic chip package as used in industrial applications is presented.

Hole mobility in thieno[3,2-b]thiophene oligomers

A study of transient and steady state electrical currents in the solution-processed thin films of thieno[3,2-b]thiophene oligomers has demonstrated charge carrier mobility of about 10−2 cm2 V−1 s−1. Thieno[3,2-b]thiophene moieties serve as reasonable charge transport sites providing the high mobility. Varying end alkyl chains in such oligomers is suggested as a viable tool to tune the mobility.

Analyzing Temperature Rise and Fluid Flow of High-Power-Density and High-Voltage Induction Motor in the Starting Process

Transient characteristics in the starting process play an important role in the design and operation of high-power-density and high-voltage induction motor. A field–circuit coupling model is proposed to analyze the transient fluid flow and temperature rise of a YJKK 500-4 2500 kW HPDHV induction motor in the starting process. The wind resistance network model is built to investigate the transient fluid flow and is used to obtain the heat dissipation boundary condition of the transient temperature calculation. The transient electric current is the key to the heat source of the temperature distribution and is calculated by the dynamic mathematical model. According to the obtained heat dissipation boundary condition and heat source, the 3-D fluid–solid coupling model is solved to obtain the transient temperature distribution. Moreover, the highest temperature rise in the starting process is greatly affected by the load. The simulation results show that the smaller fluid flow and the starting current make the winding temperature rise rapidly before the rotating speed of the motor reaches the rated value. When the load is much heavier, the starting time becomes longer and the winding temperature in the starting process will rise rapidly. The experimental results indicate that the proposed model is validated.

Study of electromagnetic disruption forces for plasma detachment measurements in DEMO

Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) can generate huge transient electrical currents in the conductive parts of the divertor. The interaction of the currents with the magnetic field induces Lorentz force loads. Particularly, the force induced by a VDE can impose a critical impact on the divertor water-cooling pipes due to high current density. A recently proposed isolated-target design solution for the DEMO divertor introduces a shunt element between the target plate and the cassette body to divert electrical current towards more massive cassette body. Such a design solution is also considered as a diagnostic tool for measuring thermo currents (voltages) through a shunt resistor to control plasma detachment.In this study, a finite-element analysis is performed to provide a conservative upper value for the shunt resistance by investigating the impact of the electromagnetic disruption forces on the integrity of water-cooling pipes in the isolated-target design solution of the DEMO divertor. In the analysis, a maximum allowed pipe current is estimated assuming realistic material properties and magnetic fields, and true geometries of the divertor cassette and water-cooling pipes fixed to the cassette. The results show that local pipe yielding (representing damage initiation) develops for electrical currents larger than ∼3.5 kA, which sets a conservative (assuming extreme VDE halo currents) upper value for the shunt resistance to ∼40 μΩ.

Shaping the Effects of Associative Brain Stimulation by Contractions of the Opposite Limb.

There has been an explosion of interest in methods that may promote neural plasticity by indirectly stimulating tissue in damaged brains using transient magnetic fields or weak electrical currents. A major limitation of these approaches is that the induced variations in brain activity tend to be diffuse. Thus far it has proved extremely difficult to target pathways from the brain to specific muscles. This is a particular challenge for applications in rehabilitation. Stroke survivors often exhibit abnormal patterns of muscle activation, including diminished specificity and high levels of co-contraction. For the clinical relevance of brain stimulation to be enhanced, it is desirable that the effects can be restricted to pathways controlling muscles that are the specific targets of movement therapy. We have demonstrated previously that increases in the excitability of corticospinal projections to forearm muscles generated by paired associative stimulation (PAS), are modulated by contractions ipsilateral to the site of the cortical stimulus. The current aim was to determine whether in chronic stroke survivors, simultaneous contractions performed by the non-paretic limb increase the muscle specificity of changes in the excitability of projections to the impaired limb induced by PAS. Ten chronic stroke survivors, 13 age-equivalent and 27 younger healthy controls, completed two separate sessions/conditions. In one (PAS+CONT), isometric wrist flexion contractions of the non-impaired limb were made simultaneously with PAS. In the other (PAS), associative stimulation only was applied. In all groups, PAS alone gave rise to large increases in the excitability of projections to a wrist extensor muscle (extensor carpi radialis – ECR) that was not the target of stimulation. In marked contrast, for the stroke survivors, following combined PAS and flexion contractions of the non-impaired limb, there was no corresponding elevation in the excitability of corticospinal projections to the ECR of the paretic limb. A similar effect was present for the healthy young adults, but not expressed clearly for the age-equivalent controls. The implications of these findings with respect to the clinical deployment of non-invasive brain stimulation in movement rehabilitation are discussed.

Impulse electromagnetic wave propagation in Kerr medium

Background: Propagating ultrashort impulse fields concentrate energy in a small time interval in a limited space that can cause nonlinear phenomena. The study of the influence of nonlinear phenomena on the shape of pulses during propagation will help to avoid the difficulties in the signal identifying by their shape, such as the Kalman filter method. Objectives: The aim of the work is to study the effect of Kerr nonlinearity on the step response of the antenna with a circular aperture and uniform current distribution. Given that the shape of the pulse depends on the coordinates of the 9observation point, to simplify the problem, consider the case when the observation point is on the longitudinal axis of the radiator. Materials and methods: The object of the study is an electric field radiated from the antenna with the circular aperture and propagating in a Kerr nonlinear medium. The solution of the direct electromagnetic problem of electrodynamics, namely, the generation of a field by transient electric current, is carried out by the method of evolutionary equations. The nonlinear radiation problem is solved using the perturbation theory. Numerical integration of improper integrals to obtain the nonlinear correction for the electric field is carried out by quadrature methods, taking into account the unbounded integration interval. A cubic model of nonlinearity is considered as an example of a nonlinear medium. Results: The initial source of the field is a flat disk with a uniform distribution of transient current. The method of evolutionary equations is applied to solve the problem of radiation of such a source. The approach that consists of the analyzing of the linear part of the solution to effectively limit the calculation domain, which significantly affects on the formation of the resulting field was verified. The influence of weak nonlinearity of the Kerr medium on the shape of the radiated transient pulse is estimated. Conclusion: the ultrawideband signal propagating through the Kerr medium shows significant self-influence changing its shape. The additional components form a delay in the arrival time and have a shape resembling the first derivative of the exciting current.

Galvanic microparticles increase migration of human dermal fibroblasts in a wound-healing model via reactive oxygen species pathway

Electrical signals have been implied in many biological mechanisms, including wound healing, which has been associated with transient electrical currents not present in intact skin. One method to generate electrical signals similar to those naturally occurring in wounds is by supplementation of galvanic particles dispersed in a cream or gel. We constructed a three-layered model of skin consisting of human dermal fibroblasts in hydrogel (mimic of dermis), a hydrogel barrier layer (mimic of epidermis) and galvanic microparticles in hydrogel (mimic of a cream containing galvanic particles applied to skin). Using this model, we investigated the effects of the properties and amounts of Cu/Zn galvanic particles on adult human dermal fibroblasts in terms of the speed of wound closing and gene expression. The collected data suggest that the effects on wound closing are due to the ROS-mediated enhancement of fibroblast migration, which is in turn mediated by the BMP/SMAD signaling pathway. These results imply that topical low-grade electric currents via microparticles could enhance wound healing.

Inelastic transient electrical currents and phonon heating in a single-level quantum dot system

We present a theoretical study on inelastic transient electrical currents and the effects of phonon heating in a single-level quantum dot system weakly coupled to a localized vibration degree of freedom, using the nonequilibrium Green's function method under the wide-band-limit and the lowest-order-expansion approximations. The energy transfer between electron and phonon systems is evaluated using both approximations, which separately are exact in the limits of the equilibrium state (t < 0) and steady state (t→∞). The time-dependent phonon number, which determines the system temperature and heating effects on the inelastic current, is calculated using a phenomenological method employing the time-dependent power transfer. The two approximations are shown to provide qualitatively similar dynamical behaviors for the system temperature, which can be grouped under two responses: if the energy corresponding to the applied bias voltage is smaller than or equal to the phonon energy, the temperature first increases because of phonon emission, and then decreases because of phonon absorption; alternatively, if the energy corresponding to the bias voltage is larger than the phonon energy, the temperature increases monotonically until a steady state is reached. The total electrical current is suppressed by phonon heating, while heat transferring between dot and environment mitigates the effects of such heating. Furthermore, the relaxation time of the current is extended by phonon scattering and heating.

AXIALLY SYMMETRIC TRANSIENT ELECTROMAGNETIC FIELDS IN A RADIALLY INHOMOGENEOUS BICONICAL TRANSMISSION LINE

In the present paper a novel mathematical model of physical processes of transient electromagnetic waves excitation and propagation in a biconical transmission line with radially inhomogeneous magneto-dielectric fllling is proposed. The model is based on time domain mode expansions over spherical waves. The basis functions of the mode expansions are calculated analytically. The mode expansion coe-cients are governed by Klein-Gordon-Fock equation with coe-cients depending on a radial spatial coordinate. The explicit flnite difierence time domain computational scheme is derived to calculate the mode expansion coe-cients. Dependences of cutofi frequencies of higher modes of TE and TM waves on the line geometry and dielectric fllling are studied. In order to calculate electromagnetic fleld in the line with higher accuracy, just flnite number of terms in the mode expansions is required. Electromagnetic fleld excited by the transient electric ring current is calculated in both homogeneous and radially inhomogeneous biconical transmission line. It is shown that there is a possibility to increase the bandwidth of the line via introduction of partial dielectric fllling without changing the line geometrical size.

Electromagnetic fields at the sea bottom induced by a line of immersed electric dipoles

The analysis of electromagnetic fields caused by alternate or transient electric currents flowing along a cable in sea water has several applications. It supports the interpretation of electromagnetic geophysical data and safety procedures against the threat of sea mines. The approach to the problem employs a magnetic vector potential in the frequency domain due to a pulse source electric dipole, and performs Laplace and Hankel transforms and integration along the cable, to describe the variation of the magnetic induction field due to an electric dipole of finite length. The result is applicable to shallow or deep sea water environments, adaptable to any transmitting current waveform and useful for wave-field separation. The prospects relate to a horizontal receiving coil at the sea bottom and simulate: a minesweeper campaign with a current source at the sea surface or a geophysical survey with a current source close to the sea floor. Therefore, the present analysis may serve: to define parameters in counter-sweeping of submarine mines; to map the conductivity of sediments under shallow waters for the prevention and control of contamination; and as a first approach in the characterization of offshore mineral and oil economic deposits.

DC Current in 4-N-Pentyl-4′-Cyanobiphenyl Liquid Crystal Cells

Transient and steady electric currents through a pentyl-cyano-biphenyl nematic liquid crystal cell have been studied under and without external photo-irradiation. The importance of space-charge effects has been demonstrated.

Transient electrophoretic current in a nonpolar solvent

The transient electric current of surfactants dissolved in a nonpolar solvent is investigated both experimentally and theoretically in the parallel-plate geometry. Due to a low concentration of free charges the cell can be completely polarized by an external voltage of several volts. In this state, all the charged micelles are compacted against the electrodes. After the voltage is set to zero the reverse current features a sharp discharge spike and a broad peak. This shape and its variation with the compacting voltage are reproduced in a one-dimensional drift-diffusion model. The model reveals that the broad peak is formed by a competition between an increasing number of charges drifting back to the middle of the cell and a decreasing electric field that drives the motion. After complete polarization is achieved, the shape of the peak stops evolving with the further increase of the compacting voltage. The spike-peak separation time grows logarithmically with the charge content in the bulk. The time peak is a useful measure of the micelle mobility. Time integration of the peak yields the total charge in the system. By measuring its variation with temperature, the activation energy of bulk charge generation has been found to be 0.126 eV.

Transient and steady electric currents through a liquid crystal cell

This paper presents a study of the transient and steady electric currents through a pentyl-cyano-biphenyl (5CB) nematic liquid crystal (LC) cell. It is demonstrated that space-charge effects are important, contrary to what has been observed before. Specifically, the nematic interlayer can be considered as an inhomogeneous medium, with near-electrode double layers emerging due to adsorption of ions present in the LC. The bulk of the LC contains charge carriers of both signs, which might be either extrinsic (impurity-induced) or intrinsic. In the latter case, they could consist of heavy fragments of LC molecules or anion and cation radicals, induced by redox processes near the electrodes. In the steady-current regime, the current–voltage characteristics are, most probably, governed by a space charge distributed in the nematic. For large enough biases U, the steady current approaches the Ohmic regime due to the depletion of the space-charge cloud. The observed transient-current phenomena suggest at least two types of charge carriers.