Non-uniform Current Density Research Articles

Effect of Intermetallic on Electromigration and Atomic Diffusion in Cu/SnAg3.0Cu0.5/Cu Joints: Experimental and First-Principles Study

Electromigration phenomena in a one-dimensional Cu/SnAg3.0Cu0.5/Cu joint were investigated with current stressing. The special effect of intermetallic compound (IMC) layers on the formation of serious electromigration damage induced by nonuniform current density distribution was discussed based on experimental results. Meanwhile, hillocks were observed both at the anode and near the cathode of the joint, and they were described as the result of diffusion of atoms and compressive stress released along grain boundaries to the relatively free surface. Moreover, the diffusion behavior of Cu at the cathode was analyzed with the electromigration equation, and the stability of Ag atoms in the solder during electromigration was evaluated with a first-principles method.

Modelling of the reactive sputtering process with non-uniform discharge current density and different temperature conditions

The majority of current models of the reactive magnetron sputtering assume a uniform shape of the discharge current density and the same temperature near the target and the substrate. However, in the real experimental set-up, the presence of the magnetic field causes high density plasma to form in front of the cathode in the shape of a toroid. Consequently, the discharge current density is laterally non-uniform. In addition to this, the heating of the background gas by sputtered particles, which is usually referred to as the gas rarefaction, plays an important role. This paper presents an extended model of the reactive magnetron sputtering that assumes the non-uniform discharge current density and which accommodates the gas rarefaction effect. It is devoted mainly to the study of the behaviour of the reactive sputtering rather that to the prediction of the coating properties. Outputs of this model are compared with those that assume uniform discharge current density and uniform temperature profile in the deposition chamber. Particular attention is paid to the modelling of the radial variation of the target composition near transitions from the metallic to the compound mode and vice versa. A study of the target utilization in the metallic and compound mode is performed for two different discharge current density profiles corresponding to typical two pole and multipole magnetics available on the market now. Different shapes of the discharge current density were tested. Finally, hysteresis curves are plotted for various temperature conditions in the reactor.

Magnetic vortex driven by non-uniform injection of spin-polarized current in nano-scale spin valves

We demonstrate that a current pulse of a non-uniform spin-polarized current density in a nanomagnet can drive, apart from magnetization reversal a static magnetic vortex. This vortex configuration can be achieved in low shape anisotropy spin valves of elliptical cross-sectional area. These non-uniform configurations exist also in presence of either ion mill damages below the nano-aperture or thermal effects at low temperature. We performed a numerical experiment of spin-torque driven ferromagnetic resonance in a magnetic vortex configuration, our results predict a frequency response with a few maxima and minima related to small oscillation of the vortex state around its equilibrium configuration.

Computer simulation of three-dimensional heavy ion beam trajectory imaging techniques used for magnetic field estimation

A magnetic field mapping technique via heavy ion beam trajectory imaging is being developed on the Madison Symmetric Torus reversed field pinch. This paper describes the computational tools created to model camera images of the light emitted from a simulated ion beam, reconstruct a three-dimensional trajectory, and estimate the accuracy of the reconstruction. First, a computer model is used to create images of the torus interior from any candidate camera location. It is used to explore the visual field of the camera and thus to guide camera parameters and placement. Second, it is shown that a three-dimensional ion beam trajectory can be recovered from a pair of perspectively projected trajectory images. The reconstruction considers effects due to finite beam size, nonuniform beam current density, and image background noise. Third, it is demonstrated that the trajectory reconstructed from camera images can help compute magnetic field profiles, and might be used as an additional constraint to an equilibrium reconstruction code, such as MSTFit.

Vortex deformation and breaking in superconductors: a microscopic description

Vortex breaking has traditionally been studied for non-uniform critical current densities, although it may also appear due to non-uniform pinning force distributions. In this article we study the case of a high-pinning/low-pinning/high-pinning layered structure. We have developed an elastic model for describing the deformation of a vortex in these systems in the presence of a uniform transport current density J for any arbitrary orientation of the transport current and the magnetic field. If J is above a certain critical value, Jc , the vortex breaks and a finite effective resistance appears. Our model can be applied to some experimental configurations where vortex breaking naturally exists. This is the case for YBa2Cu3O7−δ (YBCO) low-angle grain boundaries and films on vicinal substrates, where the breaking is experienced by Abrikosov–Josephson vortices (AJV) and Josephson string vortices (SV), respectively. With our model, we have experimentally extracted some intrinsic parameters of the AJV and SV, such as the line tension ϵ l and compared it to existing predictions based on the vortex structure.

Combined neutron radiography and locally resolved current density measurements of operating PEM fuel cells

Neutron radiographic imaging is combined with locally resolved current density measurements to study the effects of local water content on the performance of the corresponding electrochemical active area in an operating PEM fuel cell. Liquid water agglomerates are detected, quantified and correlated with the activity of the respective area. At low currents, depletion of the reactant gas leads to a decreasing performance along the anodic flowfield channel. At high currents, an optimum humidification is reached in the central part of the fuel cell; close to the inlets respectively outlets, flooding and drying can be observed concurrently and cause a non-uniform current density distribution across the reactive area. The fast response of the local performance on water droplets migrating in the gas channel is tracked by short-term imaging taking place on a timescale of several seconds.

Current Limiting Properties of MOD-YBCO Thin Films Stabilized With High-Resistivity Alloy Shunt Layer

Switching power densities of current limiting elements consisting of metal organic deposition YBa2Cu3O7 (MOD-YBCO) thin films with high-resistivity Au-Ag alloy shunt layers and non-inductive Manganin resistors are evaluated. Despite the non-uniform critical current density distribution in MOD-YBCO films, the use of a high-resistivity Au-Ag shunt layer allows MOD-YBCO-based elements to achieve switching power densities comparable to those obtained for elements based on co-evaporated YBCO thin films. The present MOD-YBCO-based elements are capable of withstanding electric fields of 46 V/cm with a switching power density of 2.0 kVA/cm2. These results indicate that low cost MOD-YBCO thin films are promising for practical fault current limiters. In addition, relationships between the uniformity of Jc distribution and current limiting properties are also discussed.

Estimation of the temperature rise in cylindrical conductors subjected to heavy 10/350 μs lightning current impulses

Lightning is a very high-energy phenomenon, which can pose serious dangers for human beings and the environment. Although much progress has been made in the area of lightning protection, there are still many cases of fires caused by lightning and power and telecommunication system outages. We have carried out an experimental and mathematical investigation into the temperature rise in cylindrical conductors subjected to heavy 10/350 μs lightning current impulses. The temperature rise measurements took place in one of the most reliable laboratories in Europe (BET, Menden, in Germany). The proposed mathematical models used to estimate the temperature rise in cylindrical conductors assumed both a uniform and non-uniform current density distribution by taking into account the transient skin effect. In addition, verification of the mathematical results were studied. Suggestions for further investigations are also provided. The experimental results are consistent with the results from the mathematical models. It is convincingly shown that copper conductors can be influenced by the transient skin effect. The additional temperature rise however, caused by the transient skin effect, is minimal, and can be neglected.

The Alfvén limit revisited and its relevance to laser-plasma interactions

Alfvén's derivation of his current limit is given. It demonstrates that it does not give the maximum possible current of a beam, but the maximum current that can propagate for an indefinite distance and time, from a source, in a charge neutral beam. Furthermore, the value Alfvén obtained applies to a uniform current density and to particles initially moving in the direction of the beam. It is also shown that Alfvén predicted that beams which exceed the limit will filament as a result of the particles that are turned back by the magnetic field. His work is extended to beams with particles that have transverse momentum, to beams with non-uniform current densities, to beams that are not charge neutral and to the time dependent case. These extensions of Alfvén's work are found to require numerical calculations in most cases and to give ambiguous results in some cases. A general formula for the current limit is given based on the conservation of energy. It is calculated for the cases considered previously and found to confirm the accuracy of Alfvén's original estimate. The relevance of the current limit to high intensity laser-solid interactions and fast ignition is then discussed.

The design of space-charge limited magnetron injection guns with control electrodes for gyroklystron applications

Temperature-limited (TL) magnetron injection guns (MIGs) suffer from nonuniform current density distributions due to thermal and work-function variations. A preliminary design of a space-charge-limited (SCL) MIG which could overcome this disadvantage was presented earlier. The design indicated that comparable beam quality as the TL MIG could be achieved, but with a slight cost of increased peak electric fields and cathode loading densities. One disadvantage of the SCL design is that the ability to control the beam current by varying the cathode temperature is lost. In this paper, we present two designs that use nonintercepting control electrodes to address this issue. Both designs have been developed with the basic goal of achieving a 500-kV, 500-A beam for the gyroklystron experiment in our laboratory. The first design has small electrodes that can be adjusted by +/-2% of the main beam voltage to adjust the current by +/-60%. The second design has larger control electrodes that can be biased sufficiently to cut off the beam flow even when the full beam voltage is applied. The design details and parametric studies are presented.

Mixed-Potential Volume Integral-Equation Approach for Circular Spiral Inductors

The electromagnetic behavior of circular spiral inductors on layered substrates is analyzed by using the concentric ring approximation. The problem is formulated in terms of mixed-potential volume integral equations. The latter are semianalytically solved within the quasi-one-dimensional approximation. The result is a fast computationally efficient model, which takes into account substrate RF loss, as well as nonuniform current density distribution across inductor windings. The results are in good agreement with experimental data from various inductors on both low- and high-resistivity silicon substrates.

Influence of nonuniform critical current density profile on magnetic field behavior of AC susceptibility in 2D Josephson junction arrays

Employing mutual-inductance measurements, we study the magnetic field dependence of complex AC susceptibility of artificially prepared highly ordered (periodic) two-dimensional Josephson junction arrays of unshunted Nb–AlO x –Nb junctions. The observed behavior can be explained assuming single-plaquette approximation of the overdamped model with an inhomogeneous critical current distribution within a single junction.

Evolution of an Electron Beam With Azimuthal Density Nonuniformity in a Cylindrical Beam Tunnel

We present the results on the deformation of the cross section of an electron beam in a cylindrical beam tunnel with /spl part///spl part/z=0 and homogeneous magnetic field, due to azimuthally nonuniform current density distribution. For this purpose, a new three-dimensional, self-consistent, trajectory code has been used. In addition, a simple theoretical model has been seen to be adequate for the qualitative and quantitative interpretation of the numerical results. It has been found, that the azimuthal nonuniformity causes effects clearly associated with/spl I.oarr/E/spl times//spl I.oarr/B drifts, but in absolute terms these effects are too small to affect the performance of a gyrotron.

3-D Microbatteries

Batteries based on three-dimensional (3-D) microstructures are shown to offer significant advantages (e.g., small areal footprint, short diffusion lengths) in comparison to thin film devices for powering microelectromechanical systems and miniaturized electronic devices. A key limitation in all 3-D periodic cell architectures is the inherent non-uniform current density. Finite-element simulations of the current and potential in several cathode/anode array configurations are presented to illustrate the difficulty in obtaining relatively uniform current densities in 3-D batteries based on periodic elements.

Finite element modelling of resistance spot welding of aluminium with spherical tip electrodes

A FEM based simulation model of the resistance spot welding process has been developed. The current simulation accounts for the non-uniform current density distribution in the sheet-electrode geometry and the elastoplastic deformation of the sheet due to electrode force, especially with spherical tip electrodes, since these effects are driven by and contribute to the main heat transfer analysis, which is governed by the internal heat generation within the sheet-electrode geometry as well as along the contact surfaces. The latent heat of transformation during melting or solidification, the variation of sheet/sheet contact resistance with temperature, and temperature dependent thermophysical material properties have been incorporated. The model also calculates the local time-temperature history of the sheet-electrode geometry that can be coupled with appropriate metallurgical reactions to determine metallurgical changes and subsequent mechanical properties in both the fusion zone and the HAZ.

Modelling the two-phase flow and current distribution along a vertical gas-evolving electrode

The bubbly two-phase flow and electric current density distribution along a single, vertical, gas-evolving electrode are modelled and the results of a boundary layer analysis are presented. Existing empirical models for particle transport in sheared and sedimenting suspensions are adopted for the bubble mixture to close the two-phase model. Ionic species concentrations are shown to be essentially homogeneous as the mixing effect of the bubble suspension usually is much larger than dispersion by molecular diffusion even at laminar flow conditions. The non-uniformity of the bubble distribution along the electrode results in a non-uniform current density distribution, which agrees well with existing experimental findings in the literature.

Attractive force between a YBCO ring and rod

In this paper, we examine the force between two YBCO trapped field magnets which initially attract each other. Two different cases have been studied: a rod and a ring, and, for comparison, the simpler geometry of two disks. In both cases, the magnets were first field cooled separately, and then the force was measured as one was moved toward the other. Nonuniform current density was found to have a strong effect on the force-distance curve. A model has been developed which includes computation of the current distribution within the magnets.

Aggregation of pairs of particles on electrodes during electrophoretic deposition

During electrophoretic deposition (EPD) of colloidal particles, the particles that have been deposited on the electrode surface move laterally along the electrode and form larger clusters. Models based on fluid convection have been advanced to explain this phenomenon. The models fall into two categories: (1) electrokinetic — electroosmotic flow is generated by the interaction between the applied electric field and the charged double layer on the surface of each particle, and (2) polarization — flow is generated by nonuniform current densities on the electrode. The electrokinetic model predicts that the aggregation rate is proportional to the applied electric field E ∞ and the zeta potential of the particles, while the polarization model predicts that the rate goes as E ∞ 2 and is independent of the zeta potential. We report experimental data for the dynamics of interactions between two deposited particles in a steady electric field in the range 20–100 V/m. The particles were polystyrene latex and their size was between 2.5 and 10 μm diameter. The direction of the electric field was also changed, and two different concentrations of supporting electrolyte were used. The aggregation trajectories of sets of two deposited particles from an initial separation of about a particle diameter were recorded using optical microscopy, and the video frames were analyzed to compute separation vs. time for each set of particles. The data show that the relative velocity between the particles is proportional to the electric field and the zeta potential of the particles, and scale with the particle size as predicted by the electrokinetic theory.

磁性材料・半導体 低コストプロセスにより作製した集光用Ｓｉ太陽電池

Obviously, concentrator solar cells reduce the total area of cells required, which provides lower system costs. For non-concentrator solar panels, the solar cells represent 80% of the system cost. The use of concentrator cells provides a cost reduction for cells inversely proportional to the concentration ratio. However conventional Si concentrator cells normally require more complex photolithography which increases the cost of manufacture. In this novel approach a single step photolithography process is described which includes superfine random pyramid texture, with the electrode directly defined onto the textured surface. Additionally, improved electrode design was applied, allowing for operation under non-uniform surface voltage and current density, experienced during high concentrator conditions. This has lead to energy conversion efficiencies in excess of 19%. Experimental results on the conversion efficiency are in good agreement with the results calculated by the proposed equations taking into account of the distributed diode effect at high concentration ratio.

A mathematical model for the internal cathodic protection of cylindrical structures by wire anodes

The applicability of a two-dimensional mathematical model of the impressed current cathodic protection (CP) system of the interior wall of a cylindrical structure was investigated. The modelled system was an axisymmetric cylindrical column filled with electrolyte with a wire anode running along its entire length and displaced from the column axis. Model inputs included anode and cathode dimensions, the anode-to-cathode distance, the electrolyte conductivity and limiting current density of oxygen reduction. A semi-analytical solution to the Laplace equation was used to compute the distribution of electrical potential. From these results, the nonuniform current density distribution was obtained to a first approximation by entering the computed values of the position-dependent cathode polarization potential into the nonlinear experimentally obtained polarization expression. The experimental electrical potential distribution compared well with that predicted by the model. Results showed the possibility of overprotection occurring in the system. The mathematical analysis was also extended to the case of a system with two wire anodes.