Non-uniform Current Density Distribution Research Articles

A Phenomenological Model of Resistance Spot Welding on Liquid Metal Embrittlement Severity Using Dynamic Resistance Measurement

Abstract Liquid metal embrittlement (LME) is one of the severe problems of Zn-coated steel in resistance spot welding (RSW). Hence, proper welding schedules for Zn-coated steel are of practical interest. RSW involves a complex interaction between electrical, thermal, and mechanical phenomena. Identification and integration of all these governing physics are almost impossible by performing simple experiments. Hence, phenomenological modeling of RSW has gained a significant attention in the recent past. The complexity of the physical process introduced by the dynamic nature of contact resistance brings challenges for the model. A simplified but effective modeling approach of RSW is proposed where attention is focused on the evaluation of the thermal field using the finite element (FE) method. The interaction of the mechanical and electrical field is performed by the dynamic variation of the contact area, stress concentration, and non-uniform current density distribution in a semi-analytical model. These internal variables of the model are incorporated through the scaling of the governing parameters by the dependence of the transient and converged temperature field within a time step. The transient-dynamic contact resistance is detached from the measurement of total resistance and mapped adaptively by the implicit scheme within a time step of the numerical model. The transient development of the nugget is investigated for dual-phase steel (DP980) with an interrupted test of the dynamic resistance curve. The FE model is validated with experimentally measured results at different process conditions. The characterization of the thermal history from the model relatively identifies the LME phenomena and suggests corresponding modification of the welding schedule.

Three-dimensional simulation of a new cooling strategy for proton exchange membrane fuel cell stack using a non-isothermal multiphase model

In this study, a new cooling strategy for a proton exchange membrane (PEM) fuel cell stack is investigated using a three-dimensional (3D) multiphase non-isothermal model. The new cooling strategy follows that of the Honda’s Clarity design and further extends to a cooling unit every five cells in stacks. The stack consists of 5 fuel cells sharing the inlet and outlet manifolds for reactant gas flows. Each cell has 7-path serpentine flow fields with a counter-flow configuration arranged for hydrogen and air streams. The coolant flow fields are set at the two sides of the stack and are simplified as the convective heat transfer thermal boundary conditions. This study also compares two thermal boundary conditions, namely limited and infinite coolant flow rates, and their impacts on the distributions of oxygen, liquid water, current density and membrane hydration. The difference of local temperature between these two cooling conditions is as much as 6.9 K in the 5-cell stack, while it is only 1.7 K in a single cell. In addition, the increased vapor concentration at high temperature (and hence water saturation pressure) dilutes the oxygen content in the air flow, reducing local oxygen concentration. The higher temperature in the stack also causes low membrane hydration, and consequently poor cell performance and non-uniform current density distribution, as disclosed by the simulation. The work indicates the new cooling strategy can be optimized by increasing the heat transfer coefficient between the stack and coolant to mitigate local overheating and cell performance reduction.

The Interplay of Electrode Heterogeneity, SEI Growth, and Li Plating in Li-Ion Batteries

Newman-type or macrohomogeneous cell models have had a significant contribution toward designing and optimizing Li-ion batteries. In order to lower the computational cost and accommodate limited experimental information, this model assumes that battery electrodes are functionally homogenous. However, this assumption can be far from reality even for commercial-quality electrodes, due to variability in microstructure that arises during the manufacturing process. Previous work in our group showed that electrode heterogeneity can lead to non-uniform distribution of current density, positive and negative electrode states of charge, and charge/discharge capacities [1]. Consequently, heterogeneity can cause faster degradation of materials, prevent maximum utilization of active materials, and even lead to catastrophic failure of the cell. Among the different modes of cell aging, solid electrolyte interphase (SEI) growth and Li plating are considered mainly responsible for capacity fade due to loss of active Li. In this study, we will discuss how electrode heterogeneity can lead to non-uniform SEI growth and Li plating, and how the growth of this resistant layer affects the level of heterogeneity, performance, and lifetime of the cell. To better understand the interplay of the electrode’s heterogeneity and cell aging, we use a combination of a Newman-type model and an equivalent-circuit model for an NMC cathode and graphite anode. The equivalent-circuit model consists of three parallel resistances, each represent high, medium, and low conductivity regions in an electrode film. This research was funded by the BMR program of the US Department of Energy. [1] Forouzan et al., J. Electrochem. Soc. 165, A2127-A2144 (2018).

In Situ Current Distribution Measurements in C-Shaped Armatures

The knowledge of the current density distribution in C-shaped armatures during a railgun experiment can be of crucial importance in the understanding of transition mechanisms and optimization of the electrical contact between rails and armature. That is why we applied during single-shot experiments with C-shaped armatures the measurement technique described, in order to deduce the spatial and temporal current density distribution of C-shaped armatures during the experiment. Based on our experimental results, we observe nonuniform current density distributions in the armature, including areas with high current densities and very low current densities. We compare our findings with the state of the post shot rail surfaces and to strengthen our interpretation we performed 3-D simulations using the COMSOL Multiphysics program.

Properties of hens’ eggs after surface irradiation by nanosecond electron beam

There are presented some details of the technology of surface disinfection of the fresh eggs. Irradiation experiments were carried out by means of the pulsed repetitive nanosecond accelerator URT-0.5 (electron energy of up to 500 keV, a pulse duration of 50 ns, pulse repetition rate up to 200 pps, which significantly reduce the costs of the irradiation in comparison with high-energy electrons (several MeV) or γ-radiation.The determination of the distribution of the absorbed dose in the depth of the polyethylene was conducted by the gray wedge method. Measurement of the electron beam dose on the surface of the shell and under the shell, as well as beneath the absorber layer (polyethylene 80 μm thick) was also performed by a film dosimeter. The electron average dose rate on the egg surface was 200 Gy/pulse and the instantaneous dose rate was 4 GGy/s. The non-uniform distribution of the electron beam current density on the output foil was ±15%.Thermoluminescent dosimeters, TLD-500, were used to determine the distribution of the bremsstrahlung dose inside hens’ eggs. It has been established that for nanosecond electron beam irradiation with dose higher than 5 kGy sufficient for complete disinfection on the hens’ egg surface, due to bremsstrahlung, the dose inside it will not exceed 8 cGy. This dose value should not lead to biological changes in the white or yolk.Dose 5 kGy does not influence the observed shell surface; there are no changes on the egg surface according to scanning electron microscope data. For heavy doses, rounded areas of the surface relief are observed.Data from energy-dispersive X-ray spectroscopy analysis for high dose show a significant increase in the amount of phosphorus and especially calcium in the shell and some reduction in the amount of oxygen, most likely due to change in the surface distribution of elements in the shell.

Coupled stress–strain and transport in proton exchange membrane fuel cell with metallic bipolar plates

Metallic bipolar plates (BPPs) for proton-exchange membrane fuel cells (PEMFCs) are desirable in automotive applications because they (i) offer good mechanical properties and manufacturability, (ii) reduce costs compared with graphite-based BPPs, and (iii) allow flexible flow-channel designs that increase power density. In this study, the relatively unexplored couplings between the mechanical and electrochemical effects due to stack compression were analyzed using a model that accounts for the transport, electrochemical reaction, heat transfer, and stress mechanics. The present model is aimed to be employed into simulation tools for PEMFC design and application. Both the tilt angle and flow-channel width of the BPPs were found to affect the stress distribution in the gas-diffusion layer (GDL) and BPP, as well as the contact resistance. The coolant pressure affected the stress distribution in the BPP, particularly at the welded joint between two adjacent plates. Stack compression not only increased the mass-transfer resistance of the GDL, particularly under the rib region, but also resulted in improved heat transfer, which reduced the PEMFC temperature and improved the uniform temperature distribution. Although the impacts of compression on the heat and mass transfer became more pronounced at higher current densities, the combined effect with the reduced membrane temperature and contact resistance between the GDL and BPP resulted in improved PEMFC performance. Applying the model to investigate a range of mechano-electrochemical conditions revealed that higher stress–strain concentrations resulted in a more nonuniform current–density distribution at the interface between the microporous layer and catalyst layer.

Effects of critical current density on mechanical properties of cylindrical superconductors

High-temperature superconductor has high critical temperature, high transport current capacity and low energy consumption, which correspondingly offer the wide applications in the field of electric power. As an important concern, the mechanical properties of superconductor carried with transport current have received extensive attention. Still, its mechanical properties in various electromagnetic environments are under study. Most of previous studies are based on the assumption of uniform distribution of critical current density, and only few researches based on the non-uniform distribution of critical current density are carried out. In this work, the mechanical flux pinning response of cylindrical superconducting structures is studied. Considering the non-uniform features of critical current density along the radial direction, the distribution law of induced magnetic field and current for the cylindrical superconducting structure is obtained based on the Bean model. Combined with the plane strain method, the analytical expression of magnetic flux pinning force, stress and magnetostriction in the superconducting structure are obtained. The results show that the uneven distribution of critical current density causes the flux pinning force to change, which further leads the superconductor`s local radial stress to vary with the critical current density. When the transport current flowing through the superconductor is increased in the ascending field, the radial stress and the hoop stress both appear compressive. The non-uniform distribution of critical current density has no significant effect on the overall trend of the internal stress, but displays an obvious influence on the stress distribution, and the superconducting structure is compressed and deformed. The results are consistent with those in the uniform case. When the transport current decreases during field descending, the critical current starts to reverse from the outermost part, then the compressive stress and tensile stress exist simultaneously. The hoop stress has a discontinuous point at the discontinuous portion of the critical current density, thus the damage probability is higher than that of the uniform distribution. In other words, the shear strength of superconductor is required to be higher for application. Also, the degree of magnetostriction is higher when the distribution of critical current density is set to be uniform, that is, the non-uniform distribution of the critical current density causes the superconducting structure to undergo greater deformation. Therefore, in engineering applications, the structural strength of the superconducting material must be enhanced to cope with the challenge from the uneven distribution of critical current density.

Design and analysis of a high quality factor multipath spiral inductor

Recently high quality factor and small-occupying-area inductors are prerequisite for wireless communication products such as mobile phones and wireless network, low cost and high performance on chip radio-frequency devices and microwave integrated circuit applications. In high frequencies, non-uniform current density distribution in the edge of the coil due to skin effect and proximity effect increases effective resistance of inductor and reduces the quality factor. This paper presented a multipath circular spiral inductor. The multipath technique reduces the skin and proximity effects, therefore increases the quality factor of the inductors. Simulation has been taken using COMSOL Multiphysics software. The results show maximum quality factor of proposed inductor have been achieved about 79 at 230 MHz that have been improved around 73% compared with conventional one. The dimension of the inductor is 2300 µm × 2300 µm. The small size proposed structure with high Q-factor is sufficient for the wireless applications.

Design and analysis of a novel MEMS spiral inductor with high quality factor

In this paper a novel spiral inductor with high quality (Q)-factor is presented. Non-uniform current density distribution, especially in inner turn, increases the effective metal resistance due to skin and proximity effect. In order to overcome this problem, an improved inductor layout with non-uniform metal width and non-uniform spacing is proposed to increase the quality factor. For this inductor layout, from outer coil to inner coil, the metal widths are reduced but spacing between strips are increased. Mainly due to the decrease of eddy current loss by weakening the current crowding effect in the center of the spiral inductor. By reducing the current crowding effect, the effective resistance is minimized, thereby increase the quality factor. Simulation has been taken using COMSOL Multiphysics software. The results indicate that maximum value of quality factor and self-inductance of the novel inductor have been obtained about 80.34 and 324 nH, respectively. Which Q-factor improves 27% more than conventional inductors with uniform width. The dimension of the inductor is 1700 × 1660 μm.

Non-uniform effect on the thermal/aging performance of Lithium-ion pouch battery

Non-uniform distribution of current density and temperature is inevitable especially in high C-rate and it can lead to bad performance of battery. Therefore, the non-uniform effect (non-uniform temperature, current density and aging) on the pouch battery performance is studied with experiment and simulation. A new method, which measures the direct current resistance (DCR) based on the discharge curve, is proposed to get more detail of resistance variation. The measurement shows that the resistance of Lithium-ion pouch battery with non-uniform temperature is similar to that of average temperature. Then, effect of non-uniform aging is simulated based on the electro-thermal coupled model. It is found that battery suffering non-uniform aging has smaller discharging capacity relatively. The main cause for the discharge capacity reduction between the uniform and non-uniform aging battery is the big difference of local stoichiometry of cathode electrode θLiFePO4. The capacity reduction in 1C rate occupies about 6% of the permissible capacity loss. Finally, tabs of battery are changed in order to acquire uniform temperature distribution. Battery whose tabs are put on the middle of the top side and the bottom side has a better performance in the opinion of thermal analysis.

Repetitive nanosecond electron accelerators type URT-1 for radiation technology

The electron accelerator URT-1М-300 for mobile installation was created for radiation disinfecting to correct drawbacks that were found the URT-1M electron accelerator operation (the accelerating voltage up to 1 МV, repetition rate up to 300 pps, electron beam size 400 × 100mm, the pulse width about 100ns). Accelerator configuration was changed that allowed to reduce significantly by 20% tank volume with oil where is placed the system of formation high-voltage pulses, thus the average power of the accelerator is increased by 6 times at the expense of increase in pulses repetition rate. Was created the system of the computerized monitoring parameters (output parameters and thermal mode) and remote control of the accelerator (charge voltage, pulse repetition rate), its elements and auxiliary systems (heat of the thyratron, vacuum system), the remote control panel is connected to the installation by the fiber-optical channel, what lightens the work for service personnel. For generating an electron beam up to 400mm wide there are used metal- ceramic] and metal-dielectric cold cathodes of several emission elements (plates) with a non-uniform distribution of the electron beam current density on the output foil ± 15%. It was found that emission drop of both type of cathodes, during the operation at the high repetition rate (100pps) is substantial at the beginning of the process, and then proceeds rather slowly that allows for continuous operation up to 40h. Experiments showed that linear dependence of the voltage and a signal from the pin-diode remains within the range of the charge voltage 45–65kV. Thus, voltage increases from 690 to 950kV, and the signal from the pin-diode – from (2,8–4,6)*104Gy/s. It allows to select electron energy quite precisely with consideration of the radiation technology requirements.

High performance planar micro-transformer using novel crossover connection

In this paper a novel planar micro-transformer with high performance is presented. Non-uniform current density distribution, especially in inner turn, increases the effective metal resistance due to skin and proximity effect. In order to overcome this problem, a novel crossover connection between turns has been used to make equal current paths in the turns and uniform the current distribution. By reducing the current crowding effects, the effective resistance is minimized, thereby the transformer performance characteristics can be increased. Simulation has been taken using ADS Momentum and HFSS software. The results show that the quality factor, self-inductance, mutual inductance and coupling factor are about 42.6, 4.9, 3.3 nH, 0.747 respectively, at 3.5 GHz frequency. The footprint of proposed structure is 860 μm × 860 μm. Compare with conventional micro-transformer, 17% in quality factor, 5% in coupling coefficient and more than 20% in self and mutual inductance improvement has been achieved.

Rail Heating in a Rapid-Fire Multirail Launcher Powered by a Pulsed MHD Generator

The operation of electromagnetic multirail launchers of solids in the mode of rapid-fire sequential launching of several projectiles has been studied by a combined 2-D and 3-D nonstationary numerical simulation. A Sakhalin type pulsed magnetohydrodynamic generator is used in the calculations as a power supply for the launchers. Launchers with three or five pairs of parallel rails connected into a series electrical circuit are considered. The acceleration of different numbers of projectiles in a burst and for different masses of projectiles is simulated. It has been found that the heating of the rails is a major factor that limits the operation of the launchers in these modes. An essential feature that determines the rate of rail heating is the nonuniform current density distribution over the rail cross section due to the nonstationary diffusion of the magnetic field into the rails. The calculations taking into account the nonstationary distribution of currents in the rails of a multirail launcher have shown that an appropriate choice of the mass of projectiles, their number in a bursts on the order of five projectiles with a firing frequency on the order of 200 Hz, it is possible to accelerate projectiles weighing up to 800 g to velocities 1.8–2.5 km/s without the rails melting.

Fabrication, Testing, and Simulation of All-Solid-State Three-Dimensional Li-Ion Batteries.

Demonstration of three-dimensional all-solid-state Li-ion batteries (3D SSLIBs) has been a long-standing goal for numerous researchers in the battery community interested in developing high power and high areal energy density storage solutions for a variety of applications. Ideally, the 3D geometry maximizes the volume of active material per unit area, while keeping its thickness small to allow for fast Li diffusion. In this paper, we describe experimental testing and simulation of 3D SSLIBs fabricated using materials and thin-film deposition methods compatible with semiconductor device processing. These 3D SSLIBs consist of Si microcolumns onto which the battery layers are sequentially deposited using physical vapor deposition. The power performance of the 3D SSLIBs lags significantly behind that of similarly prepared planar SSLIBs. Analysis of the experimental results using finite element modeling indicates that the origin of the poor power performance is the structural inhomogeneity of the 3D SSLIB, coupled with low electrolyte ionic conductivity and diffusion rate in the cathode, which lead to highly nonuniform internal current density distribution and poor cathode utilization.

Dynamic thermoelectric model of a light-emitting structure with a current spreading layer

The non-stationary thermoelectric model of the axisymmetric heterostructure of a light-emitting device is considered taking into account positive feedback mechanisms and the effect of the current-spreading-layer resistance. Taking into account the current localization effect, the nonuniform distribution of the heterojunction current density over the heterostructure area is determined. The non-stationary thermal conductivity equation with temperature-dependent current density flowing into the heterojunction is solved by the numerical–analytical iterative method. Based on the developed model, the current density, temperature, and thermomechanical stress distributions for the heterojunction plane are determined.

Determination of Source-and-Drain Series Resistance in 16-nm-Gate FinFET Devices

Source/drain (S/D) series resistance is difficult to extract, owing to poor epigrowth and nonuniform distribution of current density in S/D, critical limitation of restrictive design rule, ultrathin contact film, and complicated 3-D FinFET structure. In this brief, we, for the first time, propose a novel test structure for the measurement of the S/D series resistance. This technique enables us to determine the individual value of the S/D series resistance resulting from the S/D contact, the S/D epigrowth fin, and the channel gate, respectively. Each device’s S/D series resistance on different layout locations is characterized on the basis of its connection with specified S/D contact. The test structure and extraction method can be applied to monitor the process development of sub-16-nm-gate multifin bulk FinFET devices, such as the channel fin doping, the S/D epigrowth, and the S/D contact size formation.

Source/Drain Series Resistance Extraction in HKMG Multifin Bulk FinFET Devices

Effective extraction of source/drain (S/D) series resistance is a challenging task owing to poor epi-growth and nonuniform distribution of current density in S/D, critical limitation of restrictive design rule, ultra thin contact film, and complicated 3-D fin-type field effect transistor (FinFET) structure. In this paper, we report a test structure for measurement of linear and nonlinear S/D series resistances. This technique enables us to evaluate each component of S/D series resistance resulting from the S/D contact, the S/D epi-growth fin, the S/D extension, and the channel gate, respectively. The S/D series resistance for fins on different layout location of the same diffusion is characterized and modeled by connection with a specified S/D contact on it. Furthermore, the S/D series resistance of each fin can be analytically calculated, respectively, by swapping the S/D bias condition. The proposed test structure and extraction technique provides a robust monitoring tool to diagnose a process weak point of the 16-nm multifin high-k/metal gate bulk FinFET devices.

The Influence of Eddy Effect of Coils on Flow and Temperature Fields of Molten Droplet in Electromagnetic Levitation Device

In this work, the influence of eddy effect of coils on magnetic, flow, and temperature fields in an electromagnetically levitated molten droplet was investigated by a serial of axisymmetric numerical simulations. In an electromagnetic levitation device, both metal droplet and coils are conductive materials, therefore the distributions of current density in them should be non-uniform as a result of the eddy effect. However, in previous works, the eddy effect was considered alone in metal droplet but ignored in coils usually. As the distance of coils and metal droplet is several millimetres in general, the non-uniform distribution of current density in coils actually gives important influences on calculations of magnetic, flow, and temperature fields. Here, we consider the eddy effect both in metal droplet as well as that in coils simultaneously. Lifting force, absorbed power, fluid flow, and temperature field inside a 4-mm radius molten copper droplet as a typical example are then calculated and analyzed carefully under such condition. The results show that eddy effect leads to higher magnetic force, velocity, and temperature in both levitating and melting processes than those when the eddy effect is ignored. What is more, such influence increases as the distance of droplet and coils becomes closer, which corresponds to experimental measurement. Therefore, we suggest that eddy effect of coils should be considered in numerical simulation on this topic to obtain more reliable result.

Towards Long Cycle Life of Soluble Lead-Acid Redox Flow Batteries

A redox flow battery (RFB), with the freedom to design its power and energy capacity separately, is one of the most promising candidates for grid power management. The soluble lead acid RFB, which possesses characteristics of simple structure and inexpensive comprising materials, stands out for large-scale stationary storage. However, state-of-the-art lead-acid RFBs have limited cycle life and are not applicable to power grid applications. This study focuses on developing a single-flow, membrane-less, soluble lead acid RFB for grid-scale energy storage. First, a pilot RFB based on soluble lead-acid chemistry and with a 2 cm2 electrode area is designed and constructed. A parasitic pump with controllable pumping rate is utilized to circulate the electrolyte through the cell. Composite electrodes comprised of carbon and adhesive polymers are fabricated through hot pressing and slurry drying techniques. Different routes have been tried to prepare methanesulfonic acid solution, which serves to dissolve lead, to achieve desired concentrations. Various materials, including 316 stainless steel and graphite, are tested for their long-term stability when they serve as positive and negative current collectors. The performance of lead-acid RFBs with different combination of components is compared. We have achieved voltage efficiency and columbic efficiency both of over 80%, and energy efficiency of over 60% for over 100 cycles of deep charge and discharge. In addition, activation of electrodes at low current density before cycling is found to be necessary for higher charge efficiency and longer cycle life. Although the mechanism is not completely understood, we find that the activation procedure able to enhance the attachment of lead dioxide on the positive electrode surface, which in turn leads to improved electrode stability. Electrochemical impedance spectroscopy is conducted and the charge transfer resistance after activation is found to be significantly reduced. Other pertinent thermophysical properties are acquired through electrochemical measurements. Based on our optimization experience of the pilot cell, a scaled-up, soluble lead acid RFB with a 36 cm2 area electrode is built. A magnetic drive pump is used to circulate the electrolyte through the electrolyte storage tank and the battery. The attached figure demonstrates the performance of lead acid redox flow battery with a 36cm2 of electrode area, under an operation current density of 12mA/cm2. New challenges facing scaled up lead-acid RFBs are investigated. For example, after the scaled-up lead-acid RFB is cycled for a long time, we often find certain yellow color byproduct resides at the corner of the electrode, which was rarely seen in the 2 cm2 pilot cell. We believe the byproduct is induced by non-uniform current density distribution in a larger cell. Other factors that could facilitate extension of cycle life and improvement of charge/discharge efficiency will be discussed. Figure 1

透明导电薄膜ITO对浅面浮雕VCSEL的影响

The mode stability of shallow surface relief vertical cavity surface emitting laser( VCSEL)is constrained by the nonuniform current density distribution in the active region. In order to solve this problem,a new type of structure was put forward,in which indium tin oxide( ITO) transparent conductive layer was employed in shallow surface relief VCSEL. This structure not only can increase the threshold gain of the higher-order modes but also can improve the gain of fundamental model and strengthen the suppressive effect of fundamental mode on higher order mode. The influence of the ITO thickness on the threshold gain and the effect of ITO on the current density distribution in active region are studied. It is found that the suppressive effect of fundamental mode on higher order mode is strongest when the thickness of ITO is integral multiples of the half wavelength. The gain of fundamental mode increases and the gain of higher order decreases with the improving of the current density distribution in the active region of VCSEL,and at the same time,the series resistance and the voltage are also reduced.