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In this study, the evolution of the electrical resistivity of metal powders during densification and the resulting current flow through punch, powder compact, and die is investigated. The e...

International audience; Experimental results based on in-situ measurements at the interface between the catalyst layer and the gas diffusion layer (GDL) on the cathode side at the channel e rib scale show an interesting variation of the current density distribution as the mean current density is increased. It is found that the local current density below the rib median axis corresponds to a maximum at low to intermediate mean current densities and to a minimum when the mean current density is sufficiently high. Also, the higher is the current density, the more marked the minimum. From numerical simulations, it is shown that the current density distribution inversion phenomenon is strongly correlated to the liquid water zone development within the GDL.

Water and nitrogen can accumulate in the anode channel in proton exchange membrane fuel cells (PEMFCs) with dead-ended anode (DEA) and can affect cell performance significantly. In this paper, the cell performance characteristics in DEA PEMFCs with three different anode flow fields under two operating modes are studied through measuring the cell voltages and local current densities. The effect of the anode exit reservoir is also studied for the three different anode flow fields. The experimental results show that the interdigitated flow field has the most stable cell performance under both constant pressure and pressure swing supply modes. Parallel and serpentine flow fields lead to very non-uniform local current distributions under constant pressure supply mode and experience severe fluctuations and spikes in local current densities under pressure swing supply mode. The results also show that anode pressure swing supply mode can achieve more stable cell performance than anode constant pressure supply mode for parallel and serpentine anode flow fields. The anode exit reservoir can significantly improve cell performance stability for parallel and serpentine flow fields, but has no significant effect on interdigitated flow fields. Besides, the results further show that PEMFCs with DEA can maintain very stable operation with anode serpentine flow field and an anode exit reservoir under pressure swing operation.

Consistent performance of Li metal electrode (LME) relies on well-distributed current across the electrode surface. Lithium plating and dendrite growth are challenging issues for LME performance in high-energy rechargeable Li battery (RLB) development. The morphology of Li plating is affected by local current density variations on LME. A three-dimensional RLB cell model is used to study current density variations induced by cathode particle size. Smaller cathode particles show lower variances in the current density distribution throughout the separator. The results suggest particle size could affect uniformity of Li plating, propensity for dendrite formation and ultimately cycle life of RLB.

The discontinuity of a spin-current through an interface caused by spin-orbit coupling is characterized by the spin memory loss (SML) parameter {\delta}. We use first-principles scattering theory and a recently developed local current scheme to study the SML for Au|Pt, Au|Pd, Py|Pt and Co|Pt interfaces. We find a minimal temperature dependence for nonmagnetic interfaces and a strong dependence for interfaces involving ferromagnets that we attribute to the spin disorder. The SML is larger for Co|Pt than for Py|Pt because the interface is more abrupt. Lattice mismatch and interface alloying strongly enhance the SML that is larger for a Au|Pt than for a Au|Pd interface. The effect of the proximity induced magnetization of Pt is negligible.

Harmonic analysis has been used in corrosion measurements for several decades. During this period the method has been refined and developed. Nevertheless, the technique has not been applied to properties determination in local scale yet. This paper is focused on local corrosion current measurements based on harmonic analysis. For this purpose, a non-linear nature of electrode processes is used and measured in local scale applying Harmonic Analysis Microscope (HAM).

This paper presents analytical studies on the charging characteristics of a no-insulation (NI) REBCO pancake coil. A partial element equivalent circuit (PEEC) model has been developed to evaluate the local current paths in NI REBCO pancake coils. The PEEC model is able to consider the asymmetry of the coil winding, the partial turn-to-turn contact electrical resistance, and the partial self and mutual inductances. The analysis results are compared with previous charging experimental results and simulations using a simplified parallel equivalent circuit (SPEC) model to validate the proposed approach with the PEEC model. To investigate the charging characteristics of a meter-class NI REBCO pancake coil, numerical simulations were performed by using the PEEC and SPEC models as a parameter of the turn-to-turn contact electrical resistance. In addition, the magnetic field in a 3-T NI REBCO coil was calculated by the SPEC model to clarify the charging delay of a practical scale magnet.

We have investigated the local electrical and structural properties of threading dislocations (TDs) in Na-flux-grown GaN bulk single crystals that consist entirely of c-plane growth sectors. Conductive atomic force microscopy analysis of Schottky-like contacts locally fabricated on dislocation-related etch pits showed a variety of leakage current under reverse bias conditions. Microstructural analysis around TDs revealed a correlation between the current leakage and the structure of the TDs and their surroundings. Microscale faceted growth sectors with a higher oxygen impurity concentration locally formed in the vicinity of TDs had a larger influence on the current leakage than the structure of TDs.

Reactant starvation during proton exchange membrane fuel cell (PEMFC) operation can cause serious irreversible damages. In order to study the detailed local characteristics of starvations, simultaneous measurements of the dynamic variation of local current densities and temperatures in an experimental PEMFC with single serpentine flow field have been performed during both air and hydrogen starvations. These studies have been performed under both current controlled and cell voltage controlled operations. It is found that under current controlled operations cell voltage can decrease very quickly during reactant starvation. Besides, even though the average current is kept constant, local current densities as well as local temperatures can change dramatically. Furthermore, the variation characteristics of local current density and temperature strongly depend on the locations along the flow channel. Local current densities and temperatures near the channel inlet can become very high, especially during hydrogen starvation, posing serious threats for the membrane and catalyst layers near the inlet. When operating in a constant voltage mode, no obvious damaging phenomena were observed except very low and unstable current densities and unstable temperatures near the channel outlet during hydrogen starvation. It is demonstrated that measuring local temperatures can be effective in exploring local dynamic performance of PEMFC and the thermal failure mechanism of MEA during reactants starvations.

We show that local currents observed by the Conductive Atomic Force Microscopy (C-AFM) of silicon thin films measured in ambient atmosphere are generally limited by surface oxide, either native or created by the measurement itself in a process of Local Anodic Oxidation (LAO), as evidenced by observed topographic changes of Si surface. The tip-induced LAO changes the character of the local current maps in repeated scans or even in the first scan of a pristine surface. In particular, the oxidation of the neighboring scan lines leads to the appearance of grain edges as conductive rings. Finally, we have used brief HF acid etch to strip the oxide in order to restore the contrast in the C-AFM maps of aged samples and we compare the observed local current levels to those observed in ultra-high vacuum C-AFM on in-situ deposited samples (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of voltage exceeding Vmin, the voltage corresponding to the local current minimum, on organic bistable devices is investigated. It is found that when the termination voltage VT exceeds Vmin, with the increasing VT, the bistable effect gets worse and finally lost at VT about 2Vmin. However, the bistable effect can be recovered by applying a voltage pulse with amplitude near Vmin for milliseconds or longer. The existence of localized conducting micro-channels is suggested to account for the bistable effect.

For theoretical clarification of mechanism of a pulsed-field magnetization (PFM) and development of more efficient PFM method, we carried out a detailed computer simulation in consideration of the difference in the superconducting characteristics between the growth sector boundary (GSB) and the growth sector region (GSR) parts of the material. The superconducting current was defined in each part and the magnetic field induced by the local current was calculated. The entire magnetic field distribution was obtained by adding individual distributions. The experiment with a Sm123 bulk superconductor was performed and it was confirmed that the calculated distribution reproduces the experimental one.

The micro electro-luminescence (EL) investigation of high-power blue flip-chip LEDs at high currents has been performed. The studied distribution of the spectral EL characteristics over the LED emitted area allowed evaluating the current crowding effect and revealed the local overheating of the p-n junction. The current crowding effect lead to the two times higher local EL intensity. The local p-n junction temperature distribution was calculated from FWHM map using obtained value a FWHM temperature coefficient. Maximum local junction temperature was by 50 K higher than average value over the LED area. The high local temperature and high local current density can lead to device failure or drastically decreases the LED lifetime. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The irreversible magnetic behaviour at different temperatures of an ex situ Fe-alloy/MgB2 wire, exhibiting a granular compositional distribution, was studied using an in-field, high resolution Hall probe imaging system. Quantitative information about the local current density was obtained by solving the Biot–Savart inversion problem. The flux penetration and current distribution maps obtained can be attributed to a inhomogeneous compositional 'plum-cake-like' system, consisting of large, isolated MgB2 agglomerations embedded in a matrix of finely distributed MgB2+MgO. The critical current densities within the grains and their evolution with the applied magnetic field and temperature have been obtained, and compared to the mean Jc(H,T) in the matrix.

Magnetic flux penetration and trapped flux patterns formed into a MgB2 film during the increase and the subsequent decrease of the applied external magnetic field were studied using magneto-optical imaging. The film was grown by electron-beam evaporation and exhibits a granular structure. The non-homogeneous fan-like shaped penetration, already pointed out in the literature, was observed. To investigate the origin of this kind of penetration, a quantitative approach was chosen. The induction magnetic field map and the corresponding contour map of a framed zone show many isolated loops originating from the granular nature of the sample. Inside such loops, an estimation of the local current density was made through the inversion of the Biot–Savart law. All the results point towards the conclusions that the fan-like shaped patterns do mirror the percolation of dissipation paths. Along these paths, some interfaces, distributed in a hierarchical order, play the role of either pinning barriers or easy-flow channels.

The local current density distribution of grain boundaries in differently processed melt-textured YBa2Cu3O7-δ (YBCO) is investigated by a magneto-optical technique and inversion of Biot-Savart's law. By correlating the local current density with the microstructure revealed by a scanning electron microscope, we are able to distinguish the depression of the critical current density jc at clean grain boundaries (GBs) from that occurring at normal phases and cracks at and in the vicinity of GBs. The GBs are formed by different multi-seeding melt-growth processes using a non-stoichiometric precursor which is optimized with respect to the bulk critical current. In order to obtain GBs with extended regions free of normal phases and with large critical current densities the amount of liquid phase and the density of Y2BaCuO5 particles has to be controlled. The properties of GBs can be tailored by both controlling the amount of liquid phase and the seed distance. The related changes in the growth and microstructure are analysed and we show how to grow extended GBs almost free of normal phases with optimized critical current densities in multiple-seeded melt-growth YBCO.

Magneto-optical imaging, electromagnetic measurements and SEM have been used for the characterization of a monocore BSCCO tape made by oxide powder-in-tube process which had a transport J/sub c/ (77 K, 0 T) of 37 kA/cm/sup 2/. Our study evaluates the connectivity between superconducting grains and reconstructs the local current distribution at

The reasons for the heavy mass losses usually observed to occur from various parts of recovered solid armatures, even in cases where there was no transition, remain unclear. We earlier suggested that the losses could result from strong non-uniformities in the distribution of thermodynamic, electrical and dynamic parameters in the body of the accelerating armature, the so-called TED effects. As a consequence, some surface areas of the armature can break away from the main body to be either ejected forward or to lag behind by inertia. This paper presents ultra-high-speed camera sequences demonstrating continuous inertial mass loss from the root of a C-shaped armature, where local current concentration gives rise to a strong heating of the material. The mass loss is particularly intensive when launched on the rails coated by an In-Ga eutectic. This may imply that the current concentration is stronger in the rear part of the armature contact surfaces.

The correlation between the local current distribution and the microstructure of different types of grain boundaries in YBaCuO monoliths is investigated by magneto-optical technique and inversion of Biot-Savart's law. The grain boundaries (GB's) are formed by a multi-seeding melt-growth process and bonding of monoliths by annealing two monoliths at an external pressure with and without chemical additions.

We introduce a fast and easy to apply method for determining the local series resistance of standard silicon solar cells. For this method only two electroluminescence images taken at different voltages are needed. From these two images, the local voltage and the local current density through the device can be calculated. Knowing these parameters for each pixel yields the local series resistance. By calculating the cell's dark saturation current from the lower voltage image, the method also works with multicrystalline material. We show images, acquired in only 300 ms and compare them with other luminescence based series resistance images. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A phase-transition–like change from saturating to non-saturating behavior is found in the longitudinal bulk effective magnetoresistivity ρ(e)∥ along some fixed directions of a two-dimensional periodic array of long, parallel, perfectly insulating inclusions embedded in a simple, free-electron conducting host. Near the transition, ρ(e)∥ exhibits scaling behavior, with a scaling variable that is the ratio of two physical lengths which are tending to 0. At the transition, the local current is constrained to flow in some very narrow boundary layers.

We have studied the breakdown of the quantum Hall effect in GaAs/AlxGa1-xAs heterostructures with antidot arrays as a function of the density and distribution of antidots. For periodic arrays (lithographic antidot diameter 100 nm) and periods from 400 to 1000 nm, the breakdown current is systematically reduced with increasing antidot density and determined by the peak value of the local current density. In aperiodic arrays, the breakdown current is markedly lower than in periodic arrays of the same antidot density due to higher local current densities at the same total current.

Using first-principles methods, we investigate the electronic transport properties of zigzag MoS2 nanoribbons (Z-MoS2NRs). The current?voltage (I?V) curves of Z-MoS2NRs show a negative differential resistive (NDR) effect, and are independent of nanoribbon width. The current flowing through the nanoribbon is mainly along the Mo-edge, with two different local current channels (Mo?????Mo hop current and S?????Mo?????S bond current). The current will be suppressed when introducing a Mo vacancy-defect at the Mo-edge under low biases?while, under high biases, the current through the defected Z-MoS2NRs will increase a little, due to the other S-edge channel being opened.

Positive and negative magnetostrictive amorphous ribbons have been locally current annealed under local stress to induce different anisotropies, which have been studied by observing the magnetic domains. In annealed samples under low applied stresses, the induced anisotropy is caused by the internal mechanical stresses produced during the thermal treatment due to the change in volume of the annealed zones. In annealed samples under high stresses, the main origin of the induced anisotropy is in the stresses produced by plastic deformation. After current annealing, local changes in the magnetostriction constant have been found in Co-based amorphous ribbons with low magnetostriction constant.

We analyze phase slip phenomena in one and two dimensional superconducting rings by solving the time-dependent Ginzburg-Landau equation. In the one dimensional case we show that the phase slipkinetics occurs simultaneously and consecutively depending on the dimensionless parameter u in the equation. In two dimensions there are two values ofcritical currents j c1 and j c2. When the local current is larger then j c1 the phase slip is similar to the one dimensional case.Kinetics is governed by kinematicvortices. When the local current exceeds j c2 value the vortex generation is governed by the Kibble-Zurek quench mechanism.

We have applied local current annealing under stress in amorphous ribbons to induce different anisotropies. The magnetic structures of the samples were observed by Bitter technique and a theoretical model of the induced stresses effect by the local current annealing has been done. From this model we have obtained that the calculated induced stresses are in good agreement with the magnetic structures observed in the samples.

The stability of tearing modes is analysed for current density profiles with a small, well localized, additional current around the resonant mode rational surface. The stability parameter Δ' is calculated with a perturbative treatment of the additional current. For the case of a Gaussian profile of the additional current, this is shown to lead to a simple and illustrative condition for the amount of additional current required for stabilization. The consequences of this condition are analysed. In particular, the dependence of the effect of the additional current on its exact location is clearly shown. In the case of a co-driven additional current, far less current is required than in the case of a counter-driven current.

We studied local leakage currents induced in stressed gate SiO2 films and their time dependence by conductive atomic force microscopy (C-AFM). The current-voltage characteristics of the leakage currents detected in the C-AFM observations indicate Fowler-Nordheim tunneling currents enhanced by holes trapped in the stressed SiO2 films. By repeated C-AFM observations at the same area, it was found that individual spot currents decrease at different rates. This result indicates hole detrapping with different time constants from the stress-induced defects that have different features.

The local and transport critical current densities in a melt-processed Y-Ba-Cu-O were estimated by analyzing the results of the Campbell method and DC magnetization measurement in terms of the Campbell model in which the reversible fluxoid motion is treated. The factor of overestimation of the local current density was evaluated from the slope of minor DC magnetization curve. The obtained local critical current density was about one order of magnitude larger than the transport critical current density and was comparable to a theoretical value where normal particles of the 211 phase are assumed to act as dominant pinning centers.

It is shown that a presence of a magnetic field gives an extra structure to the wavefunction, which has to be complex. The authors study the phase of the wavefunction to construct a gauge-invariant vector related to the local current density. A numerical study for the disordered system in a strong magnetic field in two dimensions reveals a structure of the current with vortices, which behaves quite differently for localised and delocalised states.

By means of a grid of point like electrodes the homogeneity of ZnO varistors is investigated. The local distribution of voltage drops on a varistor disk at a fixed local current can be described by a normal distribution, whereas the local distribution of current densities at constant voltage along the whole varistor disk (usual operating case) is given by a log-normal distribution. Deviations from these distributions are caused by strong inhomogeneities. Degradation of ZnO varistors by electric load leads to initially increasing inhomogeneity of the local current and voltage distributions. Proportionality between degradation rate and passed charge rate is also valid in microscopic ranges in the case of small charge rates. For higher current densities and longer loading time saturation phenomena are observed. Mit rasterformig angeordneten Punktelektroden wird die elektrische Homogenitat von ZnO-Varistoren untersucht. Die lokale Verteilung der Spannungsabfalle uber einer Varistorscheibe bei gegebenem lokalen Strom last sich durch eine Normalverteilung beschreiben, wahrend die lokale Verteilung der Stromdichte bei konstanter Spannung uber die gesamte Varistorscheibe (ublicher Betriebsfall) einer logarithmisch-normalen Verteilung entspricht. Abweichungen von diesen Verteilungen sind durch ausgedehnte Probeninhomogenitaten bedingt. Die Alterung der ZnO-Varistoren durch die elektrische Belastung fuhrt zu anfanglich wachsender Inhomogenitat in den lokalen Strom- und Spannungsverteilungen. Die Proportionalititat der Schadigungsrate zur durchgeflossenen Ladungsmenge ist fur kleine Ladungsmengen auch in submikroskopischen Bereichen gultig. Fur groser Stromdichten und langere Belastungszeiten werden Sattigungserscheinungen beobachtet.

In the present paper the analysis of VA characteristics of the leakage current determined experimentally for the Al-Al2O3-Al system is discussed. It is shown that the total leakage current is given by a sum of several local currents from the single places of the dielectric given by various mechanisms acting simultaneously. The tunnel component of the leakage current passes through a part of the dielectric area only. This current component obeys the same regularities holding for the case of emission current of electrons into vacuum. Comparison of the values of applied field intensity obtained for this tunnel component leakage current with the values corresponding to the Fowler-Nordheim equation suggests the assumption of a nonuniform potential distribution in the dielectric, i.e. the existence of a region where the potential drop agrees with the theoretical values and the region of a lower potential drop. Further, the influence of the position of potential barrier and the transmission probability over the top electrode on the emission VA characteristics is shown.

When a magnetically reversible type II superconductor is placed in a uniform magnetic field there is established within the superconductor an Abrikosov1 flux line lattice of uniform density. If such a superconductor is subjected to an applied magnetic field and current in a parallel geometry, there is set up within the superconductor a flux and current structure which is Lorentz-force free.2–4 Everywhere within the superconductor the local current density J and field B are parallel.

Several fundamental problems of transport phenomena are discussed: First, the definition of the transport current density is examined. Two different definitions of the transport current density are distinguished and their explicit expressions are derived. One is the response-current density ${\mathit{j}}_{\mathit{R}}$, which refers to the local change of the current density induced by the presence of transport. The other is the Fermi-surface current density ${\mathit{j}}_{\mathit{F}}$, which represents the fraction of the current density that arises from the electrons around the Fermi surface in the presence of transport. ${\mathit{j}}_{\mathit{R}}$ and ${\mathit{j}}_{\mathit{F}}$ are equal to each other in the zero magnetic field, but different in magnetic fields. The controversy about the local current distribution in the integer quantum Hall effects substantially arises from the confusion between ${\mathit{j}}_{\mathit{R}}$ and ${\mathit{j}}_{\mathit{F}}$, viz., ${\mathit{j}}_{\mathit{F}}$ is finite only at the edge state, while ${\mathit{j}}_{\mathit{R}}$ spreads out into the interior region of a conductor. Second, the scattering-theoretic approach to transport is reexamined. The B\"uttiker formula is rederived through straightforward calculations from the expression of ${\mathit{j}}_{\mathit{F}}$ to unambiguously establish the validity of the formula in the presence of electric fields. Third, apart from the distinction between ${\mathit{j}}_{\mathit{R}}$ and ${\mathit{j}}_{\mathit{F}}$, there are two contributions to the transport current. One is the component driven by an external electric field, which is referred to as the electrostatic-potential current ${\mathit{j}}_{\mathit{E}}$. The other is the component carried by the extra electrons and holes added to the conductor, referred to as the chemical-potential current ${\mathit{j}}_{\mathit{c}}$. Both the response-current density ${\mathit{j}}_{\mathit{R}}$ and the Fermi-surface current density ${\mathit{j}}_{\mathit{F}}$ are, respectively, the sum of these two contributions, ${\mathit{j}}_{\mathit{R}}$=${\mathit{j}}_{\mathrm{RE}}$+${\mathit{j}}_{\mathit{c}}$ and ${\mathit{j}}_{\mathit{F}}$=${\mathit{j}}_{\mathrm{FE}}$+${\mathit{j}}_{\mathit{c}}$, where the electrostatic-potential current has different expressions ${\mathit{j}}_{\mathrm{RE}}$ and ${\mathit{j}}_{\mathrm{FE}}$. The electrostatic-potential current, ${\mathit{j}}_{\mathrm{RE}}$ or ${\mathit{j}}_{\mathrm{FE}}$, and the chemical-potential current, ${\mathit{j}}_{\mathit{c}}$, are, respectively, reduced to the Hall current, ${\mathit{j}}_{\mathrm{RH}}$ or ${\mathit{j}}_{\mathrm{FH}}$, and the chemical-potential edge-current ${\mathit{j}}_{\mathrm{CE}}$ in strong magnetic fields. In realistic conductors, the contribution from the chemical-potential edge current ${\mathit{j}}_{\mathrm{CE}}$ is shown to be only a small fraction compared to the Hall-current contribution. Finally, the relation between the standard linear-response-theoretic approach and the scattering-theoretic approach is discussed. \textcopyright{} 1996 The American Physical Society.

The fermion current induced by slow variations in background scalar and gauge fields are computed for a class of 2 + 1 dimensional σ-like models. Local current densities proportional to topological currents in the background fields are found. The coefficient depends discontinuously on certain field ratios. The induced fermion numbers we find, mesh nicely with recent results on induced angular momentum and induced statistics. In particular, the spin and statistics is intimately related to the global parity anomaly. Lattice realizations are suggested.