Total Current Density Research Articles

P12-F Blinding of MRI based-individualized electrode montages for modulating excitability of the somatosensory cortex by tACS

Background Most transcranial electrical stimulation studies have been performed at the motor cortex due to easy mapping and easy excitability outread both by THere we target the sensory cortex with a multichannel stimulation array for the purpose of excitability modulation by tACS at the source level. We assume that stimulation can be optimized with precise location of the targeted source and using this information for guiding current flow into the cortex. Methods EEG and MRI data were collected from seven participants to reconstruct the P20/N20 source in the Brodmann area 3b. Real stimulation of tACS was optimized to the source using 140 Hz at 1 mA for 10minutes with 8 mm diameter EGI electrodes. A new sham protocol was developed with 1 ms pulse stimulation for 10 s (0.5 Hz) at 0.2 mA. FDM and eLORETA were used to generate an optimized source based tACS electrode montage. Results Total current density at the skin in the real stimulation montage varies from 0.365 mA/cm2 to 0.539 mA/cm2 which is about 10 times higher than standard 35 cm2 (0.029 mA/cm2) and 16 cm2 (0.062 mA/cm2) electrodes. On the other hand, the current density of the sham protocol was 0.0077 mA/cm2 to 0.0175 mA/cm2 which is 1.4 to 3.1 times higher than with the 35 cm2 electrode (0.0057 mA/cm2). Even with higher current density, with multiple electrodes used here a lesser amount of discomfort feeling was reported (real stimulation = 67% of participants reported with mild to moderate itching and warm; sham = 60% claimed to feel mildly warm and itchy). Conclusion Multichannel electrodes with higher current densities per electrode induce a more focal stimulation as compared to standard rubber electrodes. Stimulation was tolerable and provided better blinding.

Simulation of the Charge Process Incorporating Vanadium Crossover in Vanadium Redox Flow Battery

The negative electrode reactant of VRFB easily crosses the separator and causes electrolyte imbalance, because of its small ion size. In this study, a 3-D isothermal model of a redox reaction in a porous electrode is formulated. The simulation is performed for the charge current voltage and the ion diffusion phenomenon of the negative electrode. The total current density is affected by the mass transfer to the surface, species crossover, and the electrode kinetics. The simulated battery charging curve was compared with the experimental results. The numerical result is validated against the experiment data. Since the ions of negative electrode easier diffuses to the other electrode, the phenomena is simulated by the overpotential that is related to SOC. Additionally, the components allow the visualization and the organization of the output to make it easily understandable whether the result is acceptable. The simulation results indicate that the edge effect of the concentration of V(II) ion is obvious at the lower SOC. Due to vanadium crossover, the concentration distribution of V(II) ion is near the current collector at the higher SOC. The positive potential distribution is closer the negative. The potential difference is about 0.9 V across the ion-exchange membrane.

Modelling Anion-Exchange Membrane-Electrode Assembly Systems for CO2 Reduction

Gas diffusion electrodes (GDEs) have drastically increased the achievable CO2 reduction (CO2R) current density by overcoming the significant mass transport limitations observed in aqueous planar electrodes.1-3 The thin diffusion layer in GDEs, typically in the order of 10-100 nm, also allows operation of CO2R in alkaline conditions, which have been shown to suppress the side reaction, hydrogen evolution reaction (HER), and reduce the onset potentials of CO2R products such as CO and C2H4.4-7 With this increase in total current density, conventional cell designs for planar electrodes become severely limited by the ohmic drop across the cell, making membrane-electrode assemblies (MEAs) an attractive alternative. MEAs have smaller cell resistances as they do not have aqueous electrolyte compartments and can greatly reduce the distance between the two electrodes. In this talk, we explore the mechanisms and limitations through multiphysics modeling of two MEA designs: one with gaseous feeds at both the anode and cathode (full-MEA); the other with an aqueous anode feed (KHCO­3 or KOH exchange solution) and a gaseous cathode feed (exchange-MEA). We discuss important cell design considerations such as the distribution of the applied voltage, water management, and CO2 utilization efficiencies. Finally, we examine the effects of temperature and the trade-off between CO2 solubility and water supply for a full-MEA. R. L. Cook, R. C. Macduff and A. F. Sammells, J. Electrochem. Soc., 1990, 137, 607-608.B. Kim, S. Ma, H.-R. Molly Jhong and P. J. A. Kenis, Electrochim. Acta, 2015, 166, 271-276.L. C. Weng, A. T. Bell and A. Z. Weber, Phys. Chem. Chem. Phys., 2018, 20, 16973-16984.S. Verma, X. Lu, S. Ma, R. I. Masel and P. J. Kenis, Phys. Chem. Chem. Phys., 2016, 18, 7075-7084.C. T. Dinh, T. Burdyny, M. G. Kibria, A. Seifitokaldani, C. M. Gabardo, F. P. G. de Arquer, A. Kiani, J. P. Edwards, P. De Luna, O. S. Bushuyev, C. Q. Zou, R. Quintero-Bermudez, Y. J. Pang, D. Sinton and E. H. Sargent, Science, 2018, 360, 783-787.C.-T. Dinh, F. P. García de Arquer, D. Sinton and E. H. Sargent, ACS Energy Lett., 2018, 3, 2835-2840.D. Raciti, M. Mao, J. H. Park and C. Wang, J. Electrochem. Soc., 2018, 165, F799-F804.

Effect of the Electrode Shape on CO2 Electroreduction Product Selectivity over Ag and Cu Catalysts

CO2 emissions from fossil fuels and industrial processes are impacting climate change. Electroreduction of CO2 into valuable chemicals and fuels could be a promising technology to offset CO2 emissions 1, 2. Product selectivity in CO2 electroreduction is impacted by parameters such as catalyst roughness, crystal orientation, and pH2, 3. Altering the current distribution and consequently the electric field in the CO2 electroreduction cell can also alter the product distribution. Enhancing the electric field close to the electrode surface can increase the CO2 molecules adsorbed on the surface due to the high concentration of cations on the surface (FIRC effect) 4. Moreover, it can more stabilize intermediates on the surface and enhance the formation of C+2 products 5, 6. The shape of electrode is an important parameter that can alter the current distribution in the electrochemical cell and thus have an impact on product selectivity. We have studied the impact of electrode shape on the product selectivity in CO2 electroreduction. Ag and Cu electrodes with two different shapes - wire and flag were used. The results showed an enhancement in C2 products formation for both Cu wire-shaped and Ag wire-shaped electrodes compared to the flag electrodes. The Ag wire-shaped electrodes had the lower onset potentials for hydrocarbon products and had 40% increase in CO and 75% increase in CH4partial current density at -1.3V. Ethylene was not observed on Ag flag even at high potentials (up to -1.3 V), while ethylene was observed on the Ag wire starting at -1.2 V. By changing the electrode from flag to wire, the total current density increased up to 60% on Ag, while current densities for Cu flag and Cu wire were comparable. Moreover, the results indicate that the partial current density and %FEs for ethylene and ethanol on Cu wire were higher than those on Cu flag at all studied potentials. The Cu wire-shaped electrodes had lower onset potentials for ethanol formation (-0.9 V). The main hydrocarbon on Cu flag was methane, while the main hydrocarbon on Cu wire was ethylene. This superior catalytic performance can be attributed to the higher surface current intensities on the wire-shaped electrodes due to their geometry. These results indicated that the shape of electrode is an important parameter that affects the product selectivity and partial current density during CO2 electroreduction. References Cave, E. R.; Shi, C.; Kuhl, K. P.; Hatsukade, T.; Abram, D. N.; Hahn, C.; Chan, K.; Jaramillo, T. F., Trends in the catalytic activity of hydrogen evolution during CO2 electroreduction on transition metals. ACS Catalysis 2018, 8(4), 3035-3040.Karaiskakis, A. N.; Biddinger, E. J., Evaluation of the impact of surface reconstruction on rough electrodeposited copper-based catalysts for carbon dioxide electroreduction. Energy Technology 2017, 5(6), 901-910.Gattrell, M.; Gupta, N.; Co, A., A review of the aqueous electrochemical reduction of CO2to hydrocarbons at copper. J. Electroanal. Chem. 2006, 594(1), 1-19.Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P. G.; Safaei, T. S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.; Kelley, S. O.; Sargent, E. H., Enhanced electrocatalytic CO2reduction via field-induced reagent concentration. Nature 2016, 537(7620), 382-386.Montoya, J. H.; Shi, C.; Chan, K.; Nørskov, J. K., Theoretical insights into a CO dimerization mechanism in CO2electroreduction. J. Phys. Chem. Lett. 2015, 6(11), 2032-2037.Chen, L. D.; Urushihara, M.; Chan, K.; Nørskov, J. K., Electric field effects in electrochemical CO2reduction. ACS Catal. 2016, 6(10), 7133-7139.

Electroreduction of CO2 to High-Value Products Using Selenide Based Electrocatalysts

Electrochemical conversion of carbon dioxide (CO2) to useful chemicals presents both the potential for advantageous processes to the industry and for environmental benefits. Anthropogenic gas emissions has increased to a new level (413 ppm) in past few years. Concerns are high about impacts on climate due to high CO2 gas levels. Intergovernmental Panel on Climate Change (IPCC) report has indicated a strong risk of crisis as early as 2040 which includes worsening food shortages and wildfires, and a mass die-off of coral reefs.3 Fossil fuels has been predicted to last only for 100 years. Also the common renewable resources like wind energy or solar energy depends on the whim of weather. CO2 has been believed to cause the rise in temperature of earth’s atmosphere by 1°C. If we can use CO2 to produce energy then it can prove an everlasting energy source. Copper based catalysts are mostly active for the electrochemical reduction of CO2. Cu nanocrystals when used for CO2 electrochemical reduction give some C2 products which contain a C-C bond. Cu nanowires also reduced CO2 to CO with a faradaic efficiency of ∼50% at a moderate overpotential of 490 mV. Electrochemical reduction of CO2 if incorporated in the modern day energy resources can serve two purpose: make environment clean and provide energy too. In this work we have shown for the first time, electrocatalytic activity of Cu2Se for CO2 electroreduction to value-added chemicals. The Cu­2Se catalyst was synthesized hydrothermally and was dropcasted on Carbon Fiber paper substrate (CFP) using very low quantity of nafion as binder. The Cu2Se modified CFP served as the working electrode for CO2 electroreduction and the electrochemical measurement was measured in KHCO3 electrolyte under flowing CO2 using a two-compartment electrochemical cell. The electrocatalyst was characterized by XRD, XPS, SEM, TEM, and EDS for morphology, and elemental chemical compositions. It was observed that the reduction products could be controlled by tuning the reduction potential, and a highly selective formate production was obtained at a higher potential with a high total current density approximately 30 mA cm-2. C2 products such as ethanol and acetic acid were obtained as reduction products. The details of catalyst synthesis, characterization and activity will be presented in the meeting.

Non-equilibrium modelling of free-burning argon arc in different anode sheath regimes

In this paper, an anode sheath model coupled with two-dimension arc column model is performed to study the physical process in the near-anode region. In the anode sheath model, two different physical mechanisms of ion and electron dynamics, for positive and negative anode sheath voltages, are considered. The effect of the arc column on the anode is also considered. The arc column model accounts for the thermal non-equilibrium and ionization non-equilibrium. The diffusion equations and generalized Ohm’s law are used to analyze the effect of diffusion of plasma species near the anode region. The simulation results from this model are presented for an atmospheric free-burning argon arc with arc currents of 50 A and 150 A. The simulation shows that: (1) both positive and negative anode sheath voltages are obtained for I = 50 and 150 A. The distributions of electric potential in front of the anode surface predicted by the model are in good agreement with the experimental data. (2) In the negative anode sheath regime, the diffusion current density exceeds the total current density, which makes the electric field reverse in front of anode surface. In the positive anode sheath regime, the diffusion current becomes negative in the near-anode region, which makes the ohmic current increase rapidly in the near-anode region. (3) There is a significant interaction, which is positive feedback, between the diffusion current and anode sheath voltage in both positive and negative anode sheath modes. (4) The thermal conduction from the arc column plays a major role in the heat transfer to the anode for the negative anode sheath mode while it becomes much smaller for the positive anode sheath mode.

A bio-inspired O2-tolerant catalytic CO2 reduction electrode

The electrochemical reduction of CO2 to give CO in the presence of O2 would allow the direct valorization of flue gases from fossil fuel combustion and of CO2 captured from air. However, it is a challenging task because O2 reduction is thermodynamically favored over that of CO2. 5% O2 in CO2 near catalyst surface is sufficient to completely inhibit the CO2 reduction reaction. Here we report an O2-tolerant catalytic CO2 reduction electrode inspired by part of the natural photosynthesis unit. The electrode comprises of heterogenized cobalt phthalocyanine molecules serving as the cathode catalyst with >95% Faradaic efficiency (FE) for CO2 reduction to CO coated with a polymer of intrinsic microporosity that works as a CO2-selective layer with a CO2/O2 selectivity of ∼20. Integrated into a flow electrolytic cell, the hybrid electrode operating with a CO2 feed gas containing 5% O2 exhibits a FECO of 75.9% with a total current density of 27.3 mA/cm2 at a cell voltage of 3.1 V. A FECO of 49.7% can be retained when the O2 fraction increases to 20%. Stable operation for 18 h is demonstrated. The electrochemical performance and O2 tolerance can be further enhanced by introducing cyano and nitro substituents to the phthalocyanine ligand.

Theoretical modeling of the gas humidification effect on the characteristics of proton ceramic fuel cells

Proton ceramic fuel cells are promising electrochemical converters that produce electrical energy by oxidizing hydrogen. Understanding the impact of gas humidification on the characteristics of such fuel cells is pivotal for the development of high-performance devices. This study develops a theory elucidating the effect of hydrogen and air humidity on the output characteristics of fuel cells based on proton-conducting oxide membrane with mixed (proton and hole) conductivity. The theory is based on a self-consistent description of the strong non-uniform distribution of charge carriers inside the membrane and the variation of the gas phase composition along the fuel and air channels. It is shown that the total current density significantly and nonmonotonously depends on humidity of the inlet hydrogen fuel, while humidification of the inlet air has minor effect on the fuel cell characteristics. The influence of the anode and cathode gas humidity on the distribution of the various fuel cell parameters (current densities, resistivity, etc.) along the membrane is also determined. The obtained results reveal the possibility of enhancing the performance of the proton ceramic fuel cells with low polarization losses by optimizing the composition of the inlet hydrogen fuel.

A microscopic physical description of electrothermal-induced flow for control of ion current transport in microfluidics interfacing nanofluidics.

The phenomenon of electrothermal (ET) convection has recently captured great attention for transporting fluidic samples in microchannels embedding simple electrode structures. In the classical model of ET-induced flow, a conductivity gradient of buffer medium is supposed to arise from temperature-dependent electrophoretic mobility of ionic species under uniform salt concentrations, so it may not work well in the presence of evident concentration perturbation within the background electrolyte. To solve this problem, we develop herein a microscopic physical description of ET streaming by fully coupling a set of Poisson-Nernst-Planck-Navier-Stokes equations and temperature-dependent fluid physicochemical properties. A comparative study on a standard electrokinetic micropump exploiting asymmetric electrode arrays indicates that, our microscopic model always predicts a lower ET pump flow rate than the classical macroscopic model even with trivial temperature elevation in the liquid. Considering a continuity of total current density in liquids of inhomogeneous polarizability, a moderate degree of fluctuation in ion concentrations on top of the electrode array is enough to exert a significant influence on the induction of free ionic charges, rendering the enhanced numerical treatment much closer to realistic experimental measurement. Then, by placing a pair of thin-film resistive heaters on the bottom of an anodic channel interfacing a cation-exchange medium, we further provide a vivid demonstration of the enhanced model's feasibility in accurately resolving the combined Coulomb force due to the coexistence of an extended space charge layer and smeared interfacial polarizations in an externally-imposed temperature gradient, while this is impossible with conventional linear approximation. This leads to a reliable method to achieve a flexible regulation on spatial-temporal evolution of ion-depletion layer by electroconvective mixing. These results provide useful insights into ET-based flexible control of micro/nanoscale solid entities in modern micro-total-analytical systems.

Infrared Light Management Using a Nanocrystalline Silicon Oxide Interlayer in Monolithic Perovskite/Silicon Heterojunction Tandem Solar Cells with Efficiency above 25%

AbstractPerovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single‐junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, results in significant reflection losses at the internal junction between the cells in monolithic (two‐terminal) devices. Therefore, light management is crucial to improve photocurrent absorption in the Si bottom cell. Here it is shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate can be significantly reduced by using an optical interlayer consisting of nanocrystalline silicon oxide. It is demonstrated that 110 nm thick interlayers with a refractive index of 2.6 (at 800 nm) result in 1.4 mA cm−² current gain in the silicon bottom cell. Under AM1.5G irradiation, the champion 1 cm2 perovskite/silicon monolithic tandem cell exhibits a top cell + bottom cell total current density of 38.7 mA cm−2 and a certified stabilized power conversion efficiency of 25.2%.

Improved <italic>RLMC</italic>-Circuit HF-Dependent Parameters Using FE-EM Computation Dedicated to Predict Fast Transient Voltage Along Insulated Windings

The aim of the present paper is to develop methods that would be able to predict transient voltage distribution along the insulated-multiconductor windings of electric equipment fed by frequency converters. Several electromagnetic (EM) challenges are presented with this desire. First, the resistance ( R ) and self-/mutual inductances ( L , M ) frequency-dependent parameters are accurately computed using the enforced electric total current process based on the finite-element (FE) formulation of the strongly coupled time-harmonic magnetic field and total current density equations model including skin and proximity effects. Second, the coupling capacitances ( C ) are computed from the FE electrostatic analysis using the floating electric scalar potential process. Finally, a realistic high-frequency electric circuit of the distributed ( R, L, M, C ) EM parameter matrices is formed in order to predict the turn-to-ground and turn-to-turn voltage transients. The numerical results obtained from the proposed models implemented under the MATLAB code are successfully verified by the corresponding laboratory test measurements.

Study of deep traps in AlGaN/GaN high-electron mobility transistors by electrical characterization and simulation

The localization of deep traps in normally-off AlGaN/GaN metal-oxide-semiconductor channel high-electron mobility transistors has been established by means of capacitance and current deep level transient spectroscopies (DLTS). Electrical simulations of the total current density between the drain and source contacts, the electron density, and the equipotential line distribution helped to understand the transport mechanisms into the device and to determine the zone probed by DLTS measurements. By changing the drain-source voltage in current DLTS or the reverse bias in capacitance DLTS, we demonstrated that we can choose to probe either the region below the gate or the region between the gate and drain electrodes. We could then see that defects related to reactive ion etching induced surface damage, expected to be formed during the gate recess process, were located only under the gate contact whereas native defects were found everywhere in the GaN layer. Thanks to this method of localization, we assigned a trap with an EC – 0.5 eV to ion etching induced damage.

Steady-state thermal-electric analysis of a π-shaped 8-pair thermoelectric generator

In this study, steady-state thermal-electric analysis of a π-shaped 8-pair TEG was performed by finite element method using thermal-electric module in Ansys Workbench for high temperature applications. TE powders of Ca2.7Ag0.3Co4O9 and Zn0.94Al0.04In0.02O were synthesized by sol-gel method followed by cold pressing (CP) to obtain bulk samples. High temperature thermoelectric properties of the bulk samples were used for p- and n-type legs, respectively. In the model, a ΔT temperature difference of 400 °C was applied to obtain a high temperature power output. As a result of the thermal-electric analysis, temperature distribution and total current density along the TEG were evaluated.

Concentration Effects on the Spatial Interaction of Corrosion Pits Occurring on Zinc in Dilute Aqueous Sodium Chloride

The scanning vibrating electrode technique is used to study the localized corrosion of unpolarized zinc in near-neutral aqueous sodium chloride electrolyte of varying concentration [NaCl]. As [NaCl] is reduced from 1% (w/v) to 0.0005% (w/v) the morphology of attack changes from large, irregular, areas of anodic zinc dissolution and cathodic oxygen reduction, to a regular array of anodic pits set in an otherwise cathodic surface. In dilute electrolyte ([NaCl] < 0.01%) the total (area averaged) corrosion current density, J, (A·m−2), obtained from a numerical area integral of SVET anodic current density data, is shown to vary with approximately the square root of [NaCl]. The number density of pits, n (m−2) is shown to decrease with increasing [NaCl]. The mean individual pit current, , (A = J/n) varies with approximately the square root of [NaCl]. A simple geometric analysis, based on a calculation of the radial dependence of electrical resistance exhibited by a hemispherical shell of electrolyte concentric with the pit, is used to explain the relationship between ohmic potential drop in the external solution, solution conductivity, and the radial distance away from an existing active pit.

Macrocell corrosion in carbonated Portland and Portland-fly ash concrete - Contribution and mechanism

The corrosion of reinforcement in carbonated concrete with high moisture state was measured with and without electrical connection to reinforcement in non-carbonated concrete. The impact of the fly ash content and the cathode-to-anode ratio (C/A) was studied. A model was proposed and applied to quantify the contribution of the anodic, cathodic and ohmic partial processes to the macrocell corrosion and the impact of C/A.The total current density for the high moisture state investigated was high in all cases regardless the amount of fly ash replacement. The governing partial process depended on the cathode-to-anode ratio.

Study of an argon dielectric barrier discharge reactor with atmospheric pressure for material treatment

In this study, effects of the variation in the dielectric barrier discharge’s (DBD) gap distance and the nature of dielectric layers which cover both of the reactor electrodes on the electron density, mass fraction of excited argon atoms across the discharge gap, mean electron energy, ion and electron current density, and electron temperature are investigated at atmospheric pressure. In order to find the optimal reactor gap, the DBD’s average power consumption is studied. The achievements show that when the value of dielectric constant is increased from 7.6 to 10, discharge gap of 1 mm still demonstrates the maximum power consumption, which can be considered as the optimum discharge gap. To optimize the characteristics of one-dimensional modeling of DBD system for material treatment, various types of materials with different values of the permittivity [aluminum, glass (quartz) and silicon] are embedded in the discharge gap between the two electrodes. In this case, the reactor gap is changed from 0.5 mm to 2 mm, while the dielectric constant of the dielectric layers which cover both of the metallic electrodes is assumed to be 10. Compared to the other examined materials, our numerical results illustrate that the treated material with higher value of the relative permittivity (silicon) has greater influences on the variations in the electron density, argon ion density and also total plasma current density than in the values of excited argon atom density, mass fraction of excited argon atoms and also average power consumption.

Calculation and Analysis of Electromagnetic Environment Indexes and Influence Factors of ±1100 kV UHVDC Transmission Lines

In order to study the influence of electromagnetic environment of ±1100 kV Changji-Guquan the UHVDC lines, the ground total electric field and ion current density of the HVDC line is studied in this paper by using the charge simulation method and Deutsch’s hypothesis-based analytical method. And the correctness of the method is verified by the measured results. The EPRI formula and CISPR formula are applied to study the audible noise and radio interference. At last, the influence of UHVDC transmission line parameters on the electromagnetic environment is analyzed, and the influence factors that restrict the construction of the UHVDC transmission lines are also studied. The results show that the minimum height of the pole is limited by the total electric field strength, and the influence of the roughness coefficient should also be taken into account when calculating the minimum height of the wire, while the selection of the diameter of the wire is constrained by both the ground total electric field and the audible noise.

Analysis of Electromagnetic Environment and Influencing Factors of Yihua DC Transmission Lines

The electromagnetic environment of transmission lines is one of the major constraints to the design and construction of HVDC transmission lines. In this paper, the charge simulation method and the analytic method based on the Deutsch hypothesis are used to calculate the total electric field strength and ion current density of the 500 kV Yihua DC transmission lines, and the correctness of the calculation is verified by the actual measurement results. Then the EPRI formula and CISPR formula are applied to analyse the audible noise and radio interference of HVDC transmission lines. At last, the influence of factors such as the pole spacing of HVDC lines, the conductor height to the ground, the splitting spacing and the roughness coefficient on the electromagnetic environment of HVDC transmission lines are analysed. Among them, the height of conductor has effective impact on the total electric field strength, while the control of polar spacing is limited. The height of the conductor and the distance between the poles affect the level of audible noise and radio interference mostly, but the control effect of the two is opposite.

Hall Effect in Laboratory and Space Current Sheets

The role of the Hall effect in the formation of thin current sheets (CSs) is analyzed by comparing results of laboratory and satellite experiments. In spite of the strong difference in the plasma parameters, both laboratory and space CSs are characterized by the hot ion component and cold isotropic electron component. Such a relation between the ion and electron components would have to guarantee the predominant contribution of the ion diamagnetic drift current to the total current density. However, the Cluster spacecraft observations of thin CSs in the Earth’s magnetotail have revealed the predominance of a strong electron (rather than ion) current. This may be due to the Hall effect, which causes generation of electric fields, which redistribute the CS currents in favor of the electron currents at the expense of the ion currents. Observations demonstrate that the thinner the CS, the stronger this effect. A similar tendency was also observed in laboratory experiments on CS formation in plasma with ions of different mass, which made it possible to vary the CS relative thickness and monitor the consequences of the Hall effect. Comparison of results of satellite and laboratory experiments indicates that the Hall effect plays an important role in the formation of thin CSs in both magnetospheric and laboratory plasmas.

Investigation of surface leakage current in MWIR HgCdTe and InAsSb barrier detectors

In this paper we have investigated dark current sources in high-operating temperature HgCdTe and InAsSb and barrier infrared detectors fabricated using metal organic chemical vapour deposition and molecular beam epitaxy, respectively. The bulk leakage and surface leakage components of the dark current were determined for unpassivated devices fabricated in a round mesa geometry and in different sizes, from 100 to 500 μm diameters. Results show that the surface leakage current depends on both the conductivity types of the active layers and the semiconductor surfaces. In the InAsSb detector the bulk component constitutes 100% of the total dark current flowing through the heterostructure. Since the absorber region has the same conductivity type in its bulk and at its surface, the unipolar barrier blocks the current in the bulk, and also blocks the current along the n-type surface of the device. For the HgCdTe detector, the bulk component constitutes about 50% of the total dark current density in the 200 μm device operating at 230 K and −0.1 V, whereas for the InAsSb detector this share is much higher and amounts to 98%. In the HgCdTe detector, the surface leakage current is attributed to the sidewall surfaces along the absorber, as well as depleted surface. In InAsSb, the surface states are pinned into the conduction band, thus the surface current is limited and comes mainly from the depleted area.