Current Relaxation Research Articles

Unmasking coupling between channel gating and ion permeation in the muscle nicotinic receptor.

Whether ion channel gating is independent of ion permeation has been an enduring, unresolved question. Here, applying single channel recording to the archetypal muscle nicotinic receptor, we unmask coupling between channel gating and ion permeation by structural perturbation of a conserved intramembrane salt bridge. A charge-neutralizing mutation suppresses channel gating, reduces unitary current amplitude, and increases fluctuations of the open channel current. Power spectra of the current fluctuations exhibit low- and high-frequency Lorentzian components, which increase in charge-neutralized mutant receptors. After aligning channel openings and closings at the time of transition, the average unitary current exhibits asymmetric relaxations just after channel opening and before channel closing. A theory in which structural motions contribute jointly to channel gating and ion conduction describes both the power spectrum and the current relaxations. Coupling manifests as a transient increase in the open channel current upon channel opening and a decrease upon channel closing.

Interfacial Charge Dynamics of XLPE/EPDM Double Layers by Simultaneous Measurement of Space Charge and Relaxation Current

Simultaneous measurement at various temperatures of space charge and relaxation current is performed on cross-linked polyethylene (XLPE)/ethylene-propylene-diene monomer (EPDM) double layers with and without silicone grease. The simultaneous measurement correlates the charge distribution and current information for the same sample and avoids discrepancy, due to the separate measurement on different samples. It is found that the polarity of interfacial charges change to the opposite with the increase in temperature in most samples, which is related to both Maxwell-Wagner polarization and enhanced charge injection and migration at higher temperatures. Bipolar charges appear in the vicinity of interface in some XLPE/EPDM double layers with silicone grease. This phenomenon can be explained by Maxwell-Wagner polarization at two interfaces of XLPE/silicone grease and EPDM/silicone grease. The addition of silicone grease decreases the conductivity of XLPE/EPDM double layers. Based on the proposed three-component exponential decay model, it is found that the interfacial charge movement affects the polarity of induced relaxation current, and the addition of silicone grease decreases the interface trap depth.

Realizing Quantitative Evaluation of Trap Energy Distribution in Polymers with an Electric Field Stimulated Depolarization Current Method

In this paper, we propose a method for characterizing the trap energy distribution of polymers. The method can accurately extract the de-trapping current from the polarization current of the sample, thus overcoming the accuracy problem of the traditional isothermal relaxation current (IRC) method. Further application of this method on crosslinked polyethylene (XLPE) sheet samples revealed the effect of thermal degradation degree on the trap distribution of XLPE. At the recrystallization stage as polyethylene becomes re-crosslinked, the number of traps decreases. At the thermal-oxidative aging stage as the sample is seriously damaged, the number of traps increases. The de-trapping current of the sample thus exhibits a first decreasing and then increasing trend.

Behaviour prediction of closed-loop HTS coils in non-uniform AC fields

Field decay rate is the key characteristic of superconducting magnets based on closed-loop coils. However, in Maglev trains or rotating machines, closed-loop magnets work in external AC fields and will exhibit an evidently accelerated field decay resulting from dynamic resistances, which are usually much larger than joint resistance. Nevertheless, there has not been a numerical model capable of systematically studying this behaviour, which is the main topic of this work. The field decay curves of a closed-loop high-temperature-superconducting (HTS) coil in various AC fields are simulated based on H-formulation. A non-uniform external field generated by armature coils is considered. Reasonable consistence is found between experimental and simulation results. In our numerical model, the impact of current relaxation, which is a historical challenge, is analysed and subsequently eliminated with acceptable precision. Our simulation results suggest that most proportion of the field decay rate is from the innermost and outermost turns. Based on this observation, a magnetic shielding pattern is designed to reduce the field decay rate efficiently. This work has provided magnet designers with an effective method to predict the field decay rate of closed-loop HTS coils in external AC fields, and explore various shielding designs.

Stable heat and particle flux detachment with efficient particle exhaust in the island divertor of Wendelstein 7-X

The island divertor concept is an innovative and promising idea to handle heat and particle exhaust in stellarators. At the Wendelstein 7-X (W7-X) stellarator, this divertor concept plays a central role in the device mission to demonstrate reactor relevant plasma confinement for steady-state time scales of up to 30 minutes in the high-performance campaign (OP2) starting in 2022. During the recently concluded first campaign with the inertially cooled island divertor, a large step in the experimental qualification of this divertor concept has been made. In discharges heated with electron cylotron resonance heating of 5−6 MW, central densities in the range of 0.7−1.2 × 1020 m−3 have been reached in combination with full divertor heat flux detachment. Also, significant neutral gas pressures and neutral compression ratios were shown for the first time in combination with reduced divertor particle flux. The divertor heat loads drop by an order of magnitude from >5 MW m−2 to below 0.5 MW m−2 with increasing density, and substantial compression of neutrals reaching neutral pressure in the sub-divertor volume of >6.0 × 10−4 mbar was seen. These elevated neutral pressure levels can be obtained and maintained with an up to 80% reduction of the particle fluxes onto the divertor target tiles. This discharge scenario was held stably detached for up to 28 seconds, which is equivalent to several hundred energy confinement times τ E and longer than the time scales for current relaxation. No impurity accumulation was seen at constant Z eff ≈ 1.5 and the stored energy stayed constant at levels of W dia > 600 kJ. The level of neutral pressure and compression reached in this scenario extrapolates well to the steady-state particle exhaust requirements for high-performance steady-state operation in OP2, in which the fully actively cooled high-heat-flux divertor will be available. An overview of this recently discovered divertor regime is given and the status ofthe physics understanding based on modeling of these regimes with the EMC3-EIRENE code is presented.

Study on Aging Characteristics of XLPE Cable Insulation Based on Quantum Chemical Calculation

In this paper, quantum chemical calculation and isothermal relaxation current (IRC) measurement are performed on thermally aged XLPE cables to investigate the effect of defects on dielectric properties and trap energy distribution of XLPE insulation. Samples of XLPE cables are thermally aged at 135 °C up to 45 days. Microscopic experiments including gel permeation chromatography, differential scanning calorimetry, and Fourier transform infrared spectrometer show that the molecular weight and crystallinity of XLPE decreases with aging time. The IRC results show that both the trap density and the trap depth increase with aging. To quantitatively calculate the trap depth variation due to physical and chemical defects by thermal aging, a quantum chemical calculation is carried out based on Gaussian 09. It is found that the broken cross-linking bonds between molecular chains weaken the stability of the molecular network structure, introducing the shallow physical trap (0.3 eV) in the aged XLPE. The molecular chain is thermally oxidized, the carbonyl group is introduced as a deep chemical trap (1.44 eV) in the XLPE. The chemical defect not only changes the localised energy state, but also significantly decreases the intrinsic breakdown strength of XLPE molecule from 0.62 to 0.51 V/Angstrom in quantum chemistry calculation.

Miscibility of Poly(Methyl Methacrylate) and Poly(Vinylidene Fluoride) Examined by Thermally Stimulated Depolarization Current and Dielectric Relaxation Spectroscopy

Samples of poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF) and melt blended PMMA/PVDF with weight ratios 4/1 and 3/2 were characterized by thermally stimulated depolarization currents (TSDC) in the temperature interval 123-303 K and by dielectric relaxation spectroscopy (DRS) in the frequency window 10−2 − 106 Hz in the temperature interval 153-423 K. The TSDC thermograms and DRS profiles of the samples showed the two types of conducnivity – non-cooperative and cooperative. The experimental DRS data in the vicinity of the glass transition temperature, Tg, were quantitatively fitted to the Havriliak-Negami function and values of the maximum relaxation time were fitted to the Vogel-Tammann-Fulcher equation. Higher values of the strength index D, of the dielectric relaxation strength Δε and of the σ ac conductivity for the binary blends were considered as experimental evidence for nanoscale composition heterogeneities originating from strong specific interactions between the PMMA and PVDF. Therefore, we suggest the PMMA/PVDF blends will be usefull as effective materials for the manufacture of lithium-ion batteries, electrostatic capacitors with superior energy storage performance, polymer composite electrolytes for solar cells, etc.

The effects of a 6-Week balance training in addition to conventional physiotherapy on pain, postural control, and balance confidence in patients with cervical disc herniation: a randomized controlled trial

Purpose Impaired postural control and fear of falling are common symptoms in patients with neck disorders, but the effects of balance training have not yet been sufficiently investigated. The aim of this study was to determine the effects of balance training on neck pain, postural control, and balance confidence in patients with cervical disc herniation (CDH). Materials and methods Thirty patients (21 females and 9 males) aged 18–55 years were randomly assigned into two groups and followed up for 6 weeks, with a 3-day weekly treatment program. Both groups received hot pack therapy, interferential current, and myofascial relaxation and cervical dynamic isometric exercises. The training group also received additional balance training on a force platform with the visual feedback technique. All patients were evaluated using the visual analogue scale for pain intensity, activity specific balance confidence scale (ABC) for balance confidence, centre of pressure with eyes closed-eyes open (CoP-EO/CoP-EC), and the limits of stability tests (LoS) for postural control with a monoaxial bipodal stabilometric force platform pre- and post-intervention. Results The pain intensity was significantly reduced in both groups (p = 0.01). CoP-EO/CoP-EC and LoS results improved in the training group (p = 0.036, p = 0.020, p = 0.23, respectively), whereas no effects were observed in the control group (p = 0.955, p = 0.307, p = 0.363, respectively). Although ABC scores increased significantly in both groups (p = 0.001, p = 0.009, respectively), the increase in the training group was significantly higher than that of the control group (p = 0.003). Conclusions The results of our study revealed that balance training added to the treatment protocol of CDH may have positive results in the treatment of neck problems. Accordingly, balance training in the treatment of chronic neck problems should also be considered for a comprehensive management of CDH

Investigations on the effect of ageing on charge de‐trapping processes of epoxy–alumina nanocomposites based on isothermal relaxation current measurements

In this study, the relationship between thermal ageing and charge trapping properties of epoxy-based nanocomposites has been investigated. With ageing, any dielectric material undergoes thorough degradation. This degradation significantly affects the space charge accumulation and charge trapping behaviour of the dielectric, which are very important parameters for insulation health under high-voltage direct current (HVDC) environment. In this work, an improved model based on the isothermal relaxation current (IRC) has been developed to study the charge trapping behaviour of pure epoxy and epoxy alumina (Al2 O3) nano-composites at different ageing conditions. A methodology based on polarisation–depolarisation current (PDC) measurements has been proposed to identify the current component due to a dipolar relaxation in measured total IRC. This will help to identify the trap distribution characteristics more accurately compared to conventional IRC measurements. It was experimentally observed that the addition of nanoparticles significantly reduces trapped charge formation and reduces thermal degradation. It is observed that aging leads to the generation of deeper traps, while the addition of Al2 O3 nanoparticles mainly enhances the density of shallow traps. Results presented in this work indicate that epoxy–alumina nanocomposites are very much suitable in HVDC applications from the perspective of trapped charge accumulation.

Lipophilic ion aromaticity is not important for permeability across lipid membranes

To clarify the contribution of charge delocalization in a lipophilic ion to the efficacy of its permeation through a lipid membrane, we compared the behavior of alkyl derivatives of triphenylphosphonium, tricyclohexylphosphonium and trihexylphosphonium both in natural and artificial membranes. Exploring accumulation of the lipophilic cations in response to inside-negative membrane potential generation in mitochondria by using an ion-selective electrode revealed similar mitochondrial uptake of butyltricyclohexylphosphonium (C4TCHP) and butyltriphenylphosphonium (C4TPP). Fluorescence correlation spectroscopy also demonstrated similar membrane potential-dependent accumulation of fluorescein derivatives of tricyclohexyldecylphosphonium and decyltriphenylphosphonium in mitochondria. The rate constant of lipophilic cation translocation across the bilayer lipid membrane (BLM), measured by the current relaxation method, moderately increased in the following sequence: trihexyltetradecylphosphonium ([P6,6,6,14]) < triphenyltetradecylphosphonium (C14TPP) < tricyclohexyldodecylphosphonium (C12TCHP). In line with these results, measurements of the BLM stationary conductance indicated that membrane permeability for C4TCHP is 2.5 times higher than that for C4TPP. Values of the difference in the free energy of ion solvation in water and octane calculated using the density functional theory and the polarizable continuum solvent model were similar for methyltriphenylphosphonium, tricyclohexylmethylphosphonium and trihexylmethylphosphonium. Our results prove that both cyclic and aromatic moieties are not necessary for lipophilic ions to effectively permeate through lipid membranes.

A suspension of conducting particles in a magnetic field – the particle stress

Abstract

Kinetic simulations of anomalous resistivity in high-temperature current carrying plasmas

Anomalous resistivity caused by lower hybrid drift instabilities (LHDIs) has been extensively studied in the literature, and has been invoked to explain the rates of magnetic diffusion and plasma profile evolution in the low-density plasma periphery of theta and screw pinch plasmas. This article presents a fully kinetic simulation study of LHDIs with a realistic mass ratio using the particle-in-cell code Chicago over a large range of drift speeds, 0.5&amp;lt;vdi2/vti2&amp;lt;14.5, plasma β, β≥2×10−5, and temperature ratios, 0.1&amp;lt;Te/Ti&amp;lt;10. The resistivity quantified from the simulations is compared with the analytic estimates from the literature and a generalized resistivity expression is presented, which can be applied over the entire range of plasma environments that were simulated in this study. This expression uses the amplitude of the LHDI fluctuations resulting from instability saturation by electron resonance broadening rather than plateau formation, current relaxation, or ion trapping. The generalized resistivity expression is appropriate for full-scale fluid simulations that encounter a large range of plasma conditions, and has better agreement with the resistivity quantified from kinetic simulations than previous expressions in the literature, particularly at low plasma β. Anomalous resistivity has also been considered as an explanation of the rapid magnetic field dissipation during magnetic reconnection events at large plasma β. At a large β, the magnetic field fluctuations of LHDIs can dominate the electrostatic fluctuations and this article investigates enhanced collisionality due to the nonlinear magnetic force, δJ×δB/c.

BT-30 Ceramic Electrophysical Properties

A total complex resistance (impedance) method is used to study the electrophysical characteristics of (BeO + TiO2)-ceramics modified with TiO2 micro- and nanoparticles in an amount of 30 wt.% (BT-30). Dispersion of the actual e′ and imaginary e′′ components of the dielectric permittivity component and specific conductivity in the frequency range from 100 Hz to 100 MHz from room temperature to the boiling point of liquid nitrogen are determined. High values of e′ and e′′ in the low-frequency range are typical for structurally inhomogeneous materials due to the accumulation of electric charges at the surface and within microcrystals. Two dielectric relaxation processes associated with electrical conductivity within the body and at the surface of ceramics are detected for the first time. An increase in conductivity with an increase in the microwave field frequency above 1 MHz is explained by the appearance of a current relaxation component. The activation energy of the static resistance of ceramic specimens is determined as a function of the reciprocal temperature that depends little on the weight content of TiO2 nanoparticles and varies in the range of 0.024 – 0.10 eV This also confirms the existence of two independent conduction processes, weakly dependent on the nanoparticle content in the ceramic composition. With placement of ceramic in a high-frequency electric field, spatial charges are formed, the field of which contributes to creation of additional polarization and dielectric losses.

AC conductivity and dielectric relaxation of chitosan/poly(vinyl alcohol) biopolymer polyblend

Polyblend samples of chitosan/poly(vinyl alcohol) (PVA) have been prepared using a casting technique. Scanning electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis measurements revealed that chitosan and PVA are compatible with each other. Alternate current (AC) conductivity and dielectric relaxation features of pure and polyblend samples are analysed in the frequency range of 0.1 Hz to 100 kHz covering a broad temperature range from room temperature to 423 K. Variation of AC conductivity, σAC, of pure and chitosan/PVA polyblend samples is found to be characterized by a plateau region at low frequency and high temperature, and this plateau region increases with increase in temperature. Based on the behaviour of the exponent s vs. temperature, AC conductivity dependence on frequency is found to be correlated with overlapping-large polaron tunnelling (OLPT) model. The polyblend samples showed an improvement in their dielectric properties compared to the pure materials. The dielectric constant, e′, of polyblend samples was increased by increasing the content of PVA. The dielectric dispersion was observed in the variation of e′ against frequency for all samples. The high values of e′ for all samples at high temperature and low frequency are attributed to space charge polarization. Also, loss tangent-frequency behaviour of pure chitosan, PVA and all polyblend samples showed two distinguished relaxation peaks with different values of activation energies. The first relaxation peak is termed as interfacial polarization or Maxwell–Wagner–Sillars polarization due to heterogeneity of the polyblend samples, whereas, the second relaxation peak is termed as δ-relaxation and α-relaxation, for pure chitosan and PVA, respectively.

Thermally Stimulated Current Study and Relaxation Behaviour of Annealed Copolymer P(VDF-TrFE) Films for Potential Pyroelectric Energy Harvesting

The effects of annealing treatment on the polarization of copolymer P(VDF-TrFE) free-standing films were thoroughly studied by the thermally stimulated current (TSC) method using decomposition analysis and first-order kinetic theory. The TSC measurement mainly revealed three depolarization peaks, which are known as the β, α and Curie mode (ρ) peaks in P(VDF-TrFE). The origin of the TSC peaks and their relaxation behaviour were correlated with structural, morphological and electrical properties such as ferroelectricity, dielectricity and pyroelectricity. Spin-coated copolymer thin films with thicknesses of 300 nm were also prepared from 5 wt.% solutions. Both free-standing and thin films were initially annealed in the range of their Curie temperature up to the melting point [80–140°C] to increase their crystallinity. A remnant polarization current of 76.7 mC m−2, which gives a pyroelectric coefficient of 31 μC m−2K−1 and a figure of merit FD of 86 μC m−2K−1 (1 kHz), was obtained from the sample annealed at 100°C, where the percentage of crystallinity was maximum and the phase was about to transition from ferroelectric to paraelectric.

Hole injection in perovskite light-emitting device with PEDOT:PSS/perovskite interface via MS contact

The hole-injection mechanism of FAPbBr3-based lead halide perovskite light-emitting diodes with PEDOT:PSS as the hole injection layer is revealed as the metal-semiconductor contact at the PEDOT:PSS/perovskite interface. The accumulation of ionic charges at the interface at the external voltage is responsible for the enhancement of hole injection. It is discovered that the strong binding between the positively charged PEDOT chain and the negatively charged Br− gives rise to the slowest charge relaxation. Moreover, the charge concentration of FAPbBr3 perovskite is estimated to be around 3.0 × 1018 cm−3 by saturating the relaxation current.

Quantum dot semiconductor optical amplifier: investigation of ultra-fast cross gain modulation in the presence of a second excited state

In this paper, a theoretical model is proposed to investigate and analyze an ultra-fast cross gain modulation (XGM) in quantum dot semiconductor optical amplifiers (QDSOAs). The proposed model is based on a set of carrier’s rate equations, propagation equations for pump and probe signals and phase equations. To investigate the XGM mechanism, nonreturn-to-zero (NRZ) pulse trains with various bitrates are supposed to propagate as the input pump signals through the QDSOA’s active layer. The probe signal is assumed to be a continuous wave (CW) signal. Furthermore, optical gain of quantum dots (QDs) is calculated using a density matrix approach. For the first time, to the best of our knowledge, a second excited state is considered in the band diagram of QDs, to obtain the ultra-fast XGM in QDSOAs. Therefore, in the presented model, the rate equations are written for the carriers in the ground state, first and second excited states (ES2), continuum state and the wetting layer. In the presence of ES2, gain recovery time and gain saturation is reduced and, therefore, ultra-high bitrate pulse trains can be modulated with negligible pattern effects and wave distortion. The effect of the injection current density increment and carrier’s relaxation lifetime decrement are illustrated to show the improvement in the XGM mechanism in the QDSOA. It is shown theoretically that ultra-high bitrate XGM up to 450 Gbps is possible due to the reduced gain recovery time in the presence of ES2. It is demonstrated that at the same pump power for higher injection current densities and lower carrier relaxation lifetimes, the pattern effect and wave distortion has almost vanished. Although, in this case, the extinction ratio (ER) is decreased. It is shown that to recover the ER, the input pump power should be increased. Furthermore, based on the results in the presence of ES2, the ER is decreased by 0.5 dB.

Blue straggler populations of seven open clusters with Gaia DR2

ABSTRACT Blue straggler stars (BSS) are well studied in globular clusters but their systematic study with secure membership determination is lacking in open clusters. We use Gaia DR2 data to determine accurate stellar membership for four intermediate-age open clusters, namely Melotte 66, NGC 2158, NGC 2506, and NGC 6819, and three old open clusters, namely, Berkeley 39, NGC 188, and NGC 6791, to subsequently study their BSS populations. The BSS radial distributions of five clusters, namely Melotte 66, NGC 188, NGC 2158, NGC 2506, and NGC 6791, show bimodal distributions, placing them with Family II globular clusters that are of intermediate dynamical ages. The location of minima, rmin, in the bimodal BSS radial distributions, varies from 1.5rc to 4.0rc, where rc is the core radius of the clusters. We find a positive correlation between rmin and Nrelax, the ratio of cluster age to the current central relaxation time of the cluster. We further report that this correlation is consistent in its slope, within the errors, with the slope of the globular cluster correlation between the same quantities, but with a slightly higher intercept. This is the first example in open clusters that shows BSS radial distributions as efficient probes of dynamical age. The BSS radial distributions of the remaining two clusters, Berkeley 39 and NGC 6819, are flat. The estimated Nrelax values of these two clusters, however, indicate that they are dynamically evolved. Berkeley 39 especially has its entire BSS population completely segregated to the inner regions of the cluster.

Thermal History-Dependent Current Relaxation in hBN/MoS2 van der Waals Dimers.

Combining atomically thin layers of van der Waals (vdW) materials in a chosen vertical sequence is an emerging route to create devices with desired functionalities. While this method aims to exploit the individual properties of partnering layers, strong interlayer coupling can significantly alter their electronic and optical properties. Here we explored the impact of the vdW epitaxy on electrical transport in atomically thin molybdenum disulfide (MoS2) when it forms a vdW dimer with crystalline films of hexagonal boron nitride (hBN). We observe a thermal history-dependent long-term (over ∼40 h) current relaxation in the overlap region of MoS2/hBN heterostructures, which is absent in bare MoS2 layers (or homoepitaxial MoS2/MoS2 dimers) on the same substrate. Concurrent relaxation in the low-frequency Raman modes in MoS2 in the heterostructure region suggests a slow structural relaxation between trigonal and octahedral polymorphs of MoS2 as a likely driving mechanism that also results in inhomogeneous charge distribution in the MoS2 layer. Our experiment yields an aspect of vdW heteroepitaxy that can be generic to electrical devices with atomically thin transition-metal dichalcogenides.

Resilient Model Predictive Control (RMPC) Technique Based Induction Motor Monitoring and Control using Labview

Nowadays induction motor is the most popular type of motor for industrial applications. The main advantage of the induction motor is its straightforward rotor construction leading to low cost, hard work, and low maintenance requirements. This work presents a remote control and monitoring the electrical and mechanical faults of an induction motor based on Labview for safe and economic data communication in industrial fields. In this work, the utilization of the Resilient Model Predictive Control (RMPC) to tackle the electrical and mechanical issue in an induction motor is proposed. This strategy allows for synchronization and active fault detection for the important and exclusion of a particular setting. Different importance levels of different control models of performance are evaluated. We continuously monitor technical motor parameters such as the sensor's peak timing, increment duration, peak overflow, and the load current and voltage relaxation error in an induction motor. The measured values are then sent to the processing unit, which displays the processing and displaying parameters that the Gateway module communicates with the Gateway module to send information to the remote monitoring cloud. The system also presents automatic and manual control methods to stop or start the induction motor using Lab view to avoid system failure. The applicability of the proposed framework is to use MATLAB in the creation of simulation results and experiments to approve MATLAB2017a programming and its execution evaluation results show that under actual operating conditions.