Current Density Gradient Research Articles

Mechanism of Aluminium Electrochemical Oxidation and Alumina Deposition Using a Carbon Sphere Electrode

Using electromagnetic and electrochemical theories as a framework, this study examines the influence of carbon sphere electrodes on the distribution patterns of anodic oxidation and deposition current densities in metallic aluminium and porous anodic alumina. Theoretical calculations show that the current density symmetrically decreases from the centre outward under the effect of carbon sphere electrodes. Increasing the electrode distance improves the uniformity of the current distribution across the film, while decreasing the distance increases the rate of gradient change in current density. Simulation results reveal that at electrode spacings of 15 cm and 1 cm, the oxidation current density at the film centre is 1333 A/m2 and 2.9 × 105 A/m2, respectively. The current density gradually decreases outward along the radius, reaching 1330 A/m2 and 1.8 × 105 A/m2 at the edges, with observed current density gradient change rates of 500 A/m3 and 1.83 × 107 A/m3, respectively. Experimental results confirm that carbon sphere counter electrodes can create non-uniform oxidation and deposition electric fields. Microstructures with gradually varying symmetry can be generated by adjusting the electrode spacing, resulting in porous anodic alumina and composite films exhibiting iridescent, ring-like structural colours. The experimental findings align well with theoretical calculations and simulation results.

Operando Quantification of Dynamic Lithium Active Area Growth in Zero-Excess-Lithium Solid-State Batteries

Growing demands for electric vehicles motivate the need for energy-dense battery technologies that can enable enhanced driving ranges on a single charge [1]. Zero-excess-lithium solid-state batteries operate with no excess lithium at the anode, instead fully cycling all the lithium within the cell during each cycle. Removing excess lithium significantly increases the specific and volumetric energy density, improves battery safety, and reduces manufacturing costs [2]. However, zero-excess-lithium solid-state batteries suffer from poor coulombic efficiency and lithium dendrite formation [3] resulting from non-uniform lithium deposition onto the anodic current collector [4]. Optical microscopy is a promising operando technique for examining the electro-chemo-mechanics of lithium nucleation and growth during lithium deposition in a zero-excess-lithium solid-state battery [4]. In this work, we investigate the lithium nucleation and growth behavior on a current collector substrate using a custom cell conducive to operando optical microscopy. The transparent components of the cell allow for direct, real-time observation of lithium plating behavior. In particular, quantifying the dynamic in-plane lithium growth as a function of operating conditions such as current density, temperature, and thermal gradients provides crucial insight into the lithium growth mechanism. Optical data is complemented with mapped synchrotron diffraction data that elucidate the vertical growth of lithium into the solid electrolyte pellet by measuring the relative intensities of lithium. These results enable comprehensive characterization of lithium growth regimes, including in-plane-growth dominant and vertical-growth dominant regimes, at successive capacities of lithium plated. Understanding these lithium growth regimes guides strategies to attain more lateral, uniform lithium deposition, with long-term implications in achieving stable, high-capacity zero-excess-lithium solid-state battery operation.

Linear and nonlinear dynamics of self-consistent collisionless tearing modes in toroidal gyrokinetic simulations

We investigate tearing modes (TM) driven by current density gradient in collisionless tokamak plasmas by using the electromagnetic gyrokinetic simulation code ORB5. We elucidate the TM width by simulations for flat profiles, as the absence of background diamagnetic flows implies a small rotation speed, while finite gradients are included to investigate the TM rotation. For flat profiles, the initial saturation width of nonlinearly driven magnetic islands is related to the TM linear growth rate; however, large islands in the initial saturation phase are prone to current density redistribution that reduces the island width in the following evolution. Island-induced E×B and diamagnetic sheared flows develop at the separatrix, able to destabilize the Kelvin–Helmholtz instability (KHI). The KHI turbulence enhances a strong quadrupole vortex flow that reinforces the island decay, resulting in a strong reduction of the island width in an eventual steady state. This process is enhanced by trapped electrons. For finite gradients profile, the TM usually rotates in the electron diamagnetic direction but can change direction when the ion temperature gradient dominates the other gradients. The reduced growth of the TM by diamagnetic effects results in a moderate island size, which remains almost unchanged after the initial saturation. At steady state, strong zonal flows are nonlinearly excited and dominate the island rotation, as expected from previous theoretical and numerical studies. When β is increased, the TM mode is suppressed and a mode with the same helicity but with twisting parity, coupled with the neighboring poloidal harmonics, is destabilized, similar to the kinetic ballooning mode.

Thermal gradient-induced critical current degradation in mesoscopic superconducting thin film

Abstract Superconducting materials inevitably suffer from the sudden change of temperature in localized areas in practical applications, and the concomitant thermal gradient may be detrimental to their performance. Critical current density is a key factor affecting the performance of superconductors. However, the effect of thermal gradient on the critical current density has not been identified. Here, by combining the time-dependent Ginzburg–Landau equations and the heat transfer equation, the thermal gradient and magnetic field dependence of the critical current density are systematically investigated and rationalized by exploring the behavior of vortex and magnetization. For lower magnetic fields, it is found that the thermal gradients strongly reduce the local surface barriers, which inhibits vortex entry and movement, leading to a rapid deterioration of the current-carrying capability. Under moderate magnetic fields, the critical current density corresponding to higher thermal gradients decreases more slowly with increasing magnetic field, which results from the thermal gradient-induced entry and moving of vortices along the current direction. As the magnetic field continues to increase, the variation of the critical current density transitions into a platform period and even slightly rises. The enhanced critical current is primarily attributed to the excess entry of vortices, which increases the surface barrier of the sample. With the further increase in the magnetic field, the critical current density continues to decrease due to increased magnetic field penetration. These results unveil the fundamental interplay between thermal gradients, external magnetic field, vortex, magnetization and critical current density, and provide a theoretical basis for understanding the heat-induced quenching of mesoscopic superconducting thin films in practical applications.

Comparison and optimization of electrochemical and thermal performances of SOFC with different configurations of a metal foam flow field

The effects of different configurations of a metal foam flow field on current density and temperature gradient of a single channel solid oxide fuel cell (SOFC) are investigated, and the overall performance of the optimum configuration is optimized. First, model A (conventional channel-rib flow field) is compared with three different configurations (i.e., model B with metal foam flow field only at anode, model C with that only at cathode, and model D at both). Although model C achieves the highest current density, its temperature is also fairly high. Although model B achieves the lowest temperature gradient, its current density is also fairly low. Model D performs well in both current density and temperature gradient. Then, the effect of electrode thickness and metal foam thickness on the performance of model D is investigated. The Ohmic polarization of model D remains almost constant with different electrode thicknesses, and its concentration polarization decreases as the electrode thickness decreases, which is totally different from the channel-rib flow field. Moreover, a thinner cathode causes lower activation overpotential, whereas a thinner anode causes higher activation overpotential. In general, a thick anode, a thin cathode, and thin metal foam can maximize the current density of model D, while a thick electrode and metal foam can always reduce the temperature gradient. Finally, the Taguchi method and gray relational analysis are used to optimize the current density and temperature gradient of model D. The current density of an optimized model is 3.27% higher than that of the original model with a temperature gradient of 9.05 K/cm.

Advancing Towards Commercialization with Enhanced Stability and Efficiency in Lithium Metal Batteries Using TTE as a Co-Solvent in Carbonate Electrolytes

Lithium metal batteries are considered ideal anode materials for next-generation battery systems due to their high theoretical specific capacity (3,860 mAh g-1) and low negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode), which are essential for high energy density. However, the fatal dendritic growth on the lithium metal surface, which occurs during repetitive cycling, acts as a significant barrier to commercially successful lithium metal battery application. Therefore, regulating the morphology of lithium plating in commercial carbonate-based electrolytes is a critical challenge for the stable operation of lithium metal batteries. Additionally, this leads to the formation of inhomogeneous lithium dendrites in carbonate-based electrolytes, manifesting as uneven current density and lithium-ion concentration gradients. The uneven current leads to inconsistent charging/discharging rates in the corresponding Ni-rich cathode, which in turn causes overpotentials and deteriorates cell safety, culminating in a reduction of capacity. this has been clarified through Kelvin probe force microscopy analysis to demonstrate an uneven state of charge distribution in the Ni-rich cathode.In this work, we propose a new strategy for high-performance lithium metal batteries, utilizing 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a co-solvent with carbonate electrolytes. By using TTE as a co-solvent, we observed a reduction in the viscosity of the electrolyte, leading to improved ionic conductivity and a decrease in the overpotential during the lithium plating/stripping process via in-situ electrochemical atomic force microscopy. This resulted in more uniform Li deposition compared to conventional carbonate-based electrolytes, forming a dense and stable Li metal layer. Furthermore, by leveraging our control over stable lithium plating morphologies via TTE co-solvent system, we observed long-term cycling stability (capacity retention of 78% at 300 cycles) with the use of an ultra-thin Li layer (40 µm). Through these results, we have effectively enhanced the stability and efficiency of high-energy-density (380 Wh kg-1) electrodes based on carbonate electrolytes, and we believe that this study can contribute to the commercialization of lithium metal batteries. Figure 1

Study of the influence of magnetic shear on the linear MHD instabilities in the pedestal of elongated divertor configurations using CLT code

AbstractEdge localized modes resulted from magnetohydrodynamic (MHD) instabilities in the pedestal region are a significant concern for future tokamaks. In this work, Ci‐Liu‐Ti (CLT), an MHD code in the three‐dimensional toroidal geometry, is applied for the linear simulation of the ideal pedestal MHD instabilities. The simulations are performed for the experimental advanced superconducting tokamak‐like elongated divertor configuration with large triangularity, which is generated by the high‐accuracy free‐boundary equilibrium solver (CLT‐EQuilibrium, i.e., CLT‐EQ) developed recently. The present work focuses on the influence of the magnetic shear, which is scanned by adjusting the pedestal current with a fixed pedestal pressure profile. As the pedestal current increases, both the local (Slocal) and global (Sglobal) magnetic shear decrease. The ballooning mode is destabilized along with the decrease of Slocal, and stabilized when Slocal is negative for the whole region of bad curvature, which implies the access of the second stable region. Further increase of the pedestal current leads to the destabilization of the kink mode, which is stabilized again until Sglobal is negative at the location of significant gradient of current density. The simulated results are consistent with the findings in Radovanovic et al. Nucl. Fusion 62 (2022) 086004 and C. K. Sun et al. Phys. Plasmas 25 (2018) 082106, which indicates the availability of CLT in the linear simulation of the ideal pedestal instabilities.

High Spatial Resolution 2D Imaging of Current Density and Pressure for Graphene Devices under High Pressure Using Nitrogen-Vacancy Centers in Diamond.

Current density imaging is helpful for discovering interesting electronic phenomena and understanding carrier dynamics, and by combining pressure distributions, several pressure-induced novel physics may be comprehended. In this work, noninvasive, high-resolution two-dimensional images of the current density and pressure gradient for graphene ribbon and hBN-graphene-hBN devices are explored using nitrogen-vacancy (NV) centers in diamond under high pressure. The two-dimensional vector current density is reconstructed by the vector magnetic field mapped by the near-surface NV center layer in the diamond. The current density images accurately and clearly reproduce the complicated structure and current flow of graphene under high pressure. Additionally, the spatial distribution of the pressure is simultaneously mapped, rationalizing the nonuniformity of the current density under high pressure. The current method opens a significant new avenue to investigate electronic transport and conductance variations in two-dimensional materials and electrical devices under high pressure as well as for nondestructive evaluation of semiconductor circuits.

Study on Phase Electromigration and Segregation Behavior of Cu-Cored Sn-58Bi Solder Interconnects under Electric Current Stressing

Cu-cored solder interconnects have been demonstrated to increase the performance of interconnect structures, while the quantitative understanding of the effect of the Cu-cored structure on microstructure evolution and atomic migration in solder interconnects is still limited. In this work, the effect of the Cu-cored structure on phase migration and segregation behavior of Sn-58Bi solder interconnects under electric current stressing is quantitatively studied using a developed phase field model. Severe phase segregation and redistribution of Bi-rich phase are observed in the Cu-cored Sn-58Bi interconnects due to the more pronounced current crowding effect near the Cu core periphery. The average current density and temperature gradient in Sn-rich phase and Bi-rich phase decrease with an increase in the diameter of the Cu core. The temperature gradient caused by Joule heating is significantly reduced owing to the presence of the Cu core. Embedding of the Cu core in the solder matrix could weaken the directional diffusion flux of Bi atoms, so that the enrichment and segregation of the Bi phase towards the anode side are significantly reduced. Furthermore, the voltage across the solder interconnects is correspondingly changed due to the phase migration and redistribution.

Field‐Free Switching of Spin Crossbar Arrays by Asymmetric Spin Current Gradient

AbstractSpin orbit torque (SOT) devices with the advantages of high speed, low power consumption, and high stability have wide application prospects in the field of spintronics. The SOT‐based crossbar array device is an important extension of SOT devices, but it is not reported so far. Here, the all electrical magnetization switching of Hall crossings based on SOT crossbar array devices is realized. Through analyzing the current distribution and micromagnetic simulations, it is found that this field‐free SOT switching in the array devices comes from the asymmetric current density gradient distribution at the Hall‐crossings due to the shunt effect of grid circuits. All electrical tristate magnetization switching and the write protection of spin crossbar array devices are demonstrated. This work will further promote the application of the efficient memory or edge computing based on spin crossbar array devices.

Minor Ag inducedshear performance alternation in BGA structure Cu/SnBi/Cu solder joints under electric current stressing

In this work, the effect of minor Ag addition on shear performance of ball grid array (BGA) structure Cu/SnBi/Cu solder joints under electric current stressing was investigated. The results show that the shear strength of solder joints decreased with increasing current density, and the effect of Ag addition on shear strength changed from an elevating state to a deteriorating state. The elevating effect of Ag addition was due to the dominance of Sn/Bi phase refinement and Ag3Sn dispersion, which was weakened gradually with the increase in current density. The deteriorating effect of Ag addition was attributed to the more inhomogeneous distribution of current density and higher thermal gradient induced by the finer phase of the solder matrix as well as the consequent severer strain mismatch at the Sn/Bi phase interface. The finer Sn/Bi phase was also conductive to easier lattice atom migration under current stressing, resulting in a rapid decrease in shear strength. Moreover, the dominant factor on the shear strength reduction tended to change from Joule heating to athermal effect of current stressing. The fracture of solder joints indicated an insignificant influence by the Ag addition, which all occurred in the solder matrix in a ductile mode.

Induced magnetic transportation of Soret and dissipative effects on Casson fluid flow towards a vertical plate with thermal and species flux conditions

This research offers an analysis of the mixed convective transient boundary film Casson fluid stream and thermal distribution through a vertical sheet. Viscous dissipation and the Soret effect are introduced to support the flow in stimulating heat capacity. Unlike typical investigations, the present flow-formulated model is done to capture an induced magnetic field. The Boussinesq approximation is used to describe the nonlinear formulated partial derivatives governing the heat transfer fluid that is non-dimensionalized using suitable dimensionless quantities. The transformed partial derivative model is numerically solved via the spectral Chebyshev technique and the results of shear stress, current density, Nusselt number, and mass gradient are tabulated. The role of numerous terms on dimensionless flow rate, induced velocity, and heat transfer with species distribution is discussed in a very effective way. Velocity and temperature of liquid decline as boosting the material parameter. Enhancing values [Formula: see text] favor the flow momentum and oppose the temperature profile.

Current-driven magnetic skyrmion diodes controlled by voltage gates in synthetic antiferromagnets

Magnetic skyrmions, as promising candidates in various spintronic devices, have been widely studied owing to their particle-like properties, nanoscale size, and low driving current density. Here, we numerically and theoretically investigate the dynamics of current-driven skyrmion passing through a voltage gate in a synthetic antiferromagnetic racetrack. It is found that the critical current required for skyrmion to pass through the voltage gate positively is much less than that for skyrmion to pass through the gate negatively. Furthermore, we systematically study the linear dependence of the minimum velocity of skyrmion on the driving current density and perpendicular magnetic anisotropy (PMA) gradient, and the calculation results are quite consistent with the simulation results. Finally, we find that the variation of the PMA energy with the position of skyrmion can help us to compare the magnitude of resistance force when the skyrmion passes through different voltage gates. Our results can be beneficial for the design and development of skyrmion diodes.

Electropulse-induced texture elimination and texture formation in hot-rolled annealed 35CrMo steel

Rolling texture impacts the formability and usability of steel due to the anisotropy, but it is difficult to remove even after annealing. This work found that the electropulsing treatment can eliminate the hot-rolled and annealed texture in 35CrMo steel (<111>//ND, normal direction, and <110>//RD, rolling direction) at a relatively lower current density and produce a new <100> texture along the current direction via higher current density. Furthermore, the electropulse-induced texture evolution law was established by preparing gradient current density in steel. Electron backscattered diffraction and micro-region X-ray diffraction were used to study texture evolution. By analyzing the texture evolution at different stages of the phase transition, it is concluded that the electropulse-induced texture change mainly occurs at the grain growth stage. The suppression and preferential grain growth caused by different scattering degrees of electron flow with various grain orientations are thought to be responsible for the observed phenomenon.

Local electropulse-induced gradient and hierarchical architecture of soft-hard phase in 35CrMo steel

To improve the comprehensive mechanical properties while avoiding the overall heat treatment, this study provided a method to prepare structural steel with soft-hard phases arranged in complex gradient, hierarchical structures via processing a homogenous material using local electropulsing treatment (LEPT). The results show that ultimate tensile strength, elongation-to-failure, and curves area of the structural steel plate are about 1400 MPa, 15.93%, and 19,100 MPa·%, respectively, which are 16.5%, 34.7%, and 47.5% higher than the conventional quenched and tempered steel. The excellent mechanical properties of LEPTed specimens are mainly due to the local strengthening caused by the effects of pulse current, and the synergistic effect of hard and soft phases. The designed hardened units produced by LEPT exhibit hierarchical structure and gradient variations in texture and grain size. There also exists gradient hardness distribution in hardened units. These structures are produced due to the multi-scale effects of current and the gradient current density distribution. Moreover, due to the introduction of hardened units at another scale, there were four stages in the strain-hardening behavior of LEPT based on differential Crussard-Jaoul (DC-J) analysis, which deviates from the three stages of two-phase or multi-phase steels.

Purification of Cu–Zn melt based on the migration behavior of lead and bismuth under pulsed electric current

The recovery and regeneration of waste brass can alleviate the pressure of ore resource demand, and can also obtain considerable economic profits from the regenerated alloy products. However, excessive lead and bismuth residues in brass will lead to severe high temperature brittleness of the alloy, and chemical separation will complicate the separation process due to the difference in activity between elements. In this study, the physical separation of Pb and Bi from brass was realized by applying pulse current based on the electrical conductivity difference between Pb, Bi and brass melt, overcoming the limitation of using metal activity difference for oxidation refining. For 10 kg melt, the distribution of current line is adjusted by designing current intensity, frequency and electrode position to explore the migration behavior of Pb and Bi in brass melt under the action of pulse current. After pulse current treatment, Pb and Bi accumulate on the surface of the melt, especially at the bottom of the melt, forming a region rich in Pb or Bi. Based on the numerical calculation, Pb and Bi migrate in the direction perpendicular to the current line driven by the current density gradient, and the migration direction is the direction of the current density gradient decreasing. Pulse current separation technology has the characteristics of shortening the recovery steps, green energy saving, which provides a new solution for the difficult recycling of scrap copper and has great commercial value.

Fast hardening response of martensitic stainless steel by copper-rich cluster formation under pulsed electric current

The pursuit of excellent comprehensive mechanical properties using a rapid aging process is of practical significance in the field of aging strengthening. A pulsed electric current assisted aging process can effectively accelerate aging and reduce the energy consumption caused by long-term thermal aging. The mechanism of a Cu-rich cluster formation promoted by a pulsed electric current was investigated to better understand the fast-hardening response process. The size and structure of the Cu-rich cluster were characterized using transmission electron microscopy (TEM); the results indicated that the pulsed electric current did not change the mechanism of aging strengthening but accelerated the process. The time required to achieve peak aging decreased owing to the athermal effect (interaction between atoms and electrons) induced by the pulsed electric current. The athermal effect decreased as the temperature increased, and a relationship between the athermal effect and temperature was obtained. When a current density of 7 × 107 A/m2 was applied, the precipitation activation energy of the Cu-rich clusters decreased by 91 kJ mol−1; this was owing to the following two aspects. (1) The pulsed electric current lowered the vacancy formation energy and resulted in an increase in vacancy concentration and (2) the current density gradient caused by the difference in conductivity between the matrix and Cu-rich clusters accelerated the substitution of the Cu atoms and vacancies and reduced the atomic migration energy. These results provide insights into the effect of a pulsed electric current on the kinetics of precipitation in the martensitic stainless steel and a deduction for the mechanism of the rapid growth of Cu-rich clusters.

Effects of thermal instability on density limit disruption in J-TEXT

As an important precursor of density limit disruption, thermal instability under J-TEXT high-density discharges is studied in this paper. An extended MHD code called NIMROD [Sovinec et al., J. Comput. Phys. 195, 355 (2004)] is used to explore the intrinsic relationship between density limit disruption and thermal instability. The experimental and simulation results show that radiation from the boundary impurity can cause thermal instability and impurity radiation increases rapidly when the plasma temperature decreases to the nonlinear range of carbon cooling rates, which cools down the plasma and enhances impurity radiation. Further investigations show that the local reduction in thermal instability at the plasma edge shrinks the local current channel and increases the internal current density gradient, which triggers the 2/1 mode and destabilizes the 3/1 and other higher-order modes. Finally, a rapid increase in the MHD instability can cause density limit disruption.

Theoretical and Experimental Study of Joint Osmotic and Electroosmotic Water Transfer through a Cation-Exchange Membrane.

Using the previously developed cell model of a charged membrane and the principles of linear thermodynamics of irreversible processes (the Onsager approach), exact and approximate (in the case of an ideally selective membrane) analytical formulae for calculating the osmotic and electroosmotic permeability of the membrane in aqueous solutions of 1:1 electrolyte at constant electric current density and concentration gradient were suggested. The formulae have been successfully verified by our own experimental data for the extrusion cation-exchange membrane MF-4SC p.29 in NaCl solution up to concentrations of 3 M. The contribution of electroosmotic and osmotic water fluxes to the total water transport through the mentioned individual perfluorinated ion-exchange membrane under conditions close to the process of electrodialysis concentrating was experimentally estimated. The cases of co- and counter-directed osmotic and electroosmotic water fluxes are studied. A good correspondence between theoretical and experimental results was obtained, which made it possible to determine the physicochemical parameters of the electromembrane system (the diffusion coefficients of individual ions and the coefficient of equilibrium distribution of electrolyte molecules in the membrane matrix, the characteristic exchange capacity of the cell model). The achieved results make it possible to fully characterize existing and promising types of ion-exchange membranes based on the developed cell model of a charged membrane.

Stabilization of double tearing mode growth by resonant magnetic perturbations

It is well known that for non-monotonic profiles of the safety factor q with two q = m/n resonant surfaces inside the plasma (m/n being the poloidal/toroidal mode numbers), the low-m double tearing modes (DTMs) are usually unstable, especially for plasmas with a high bootstrap current fraction as required for the steady operation of advanced scenarios. The effect of applied resonant magnetic perturbations (RMPs) on the m/n = 2/1 DTM growth is investigated numerically in this paper using two-fluid equations. The DTM growth is found to be stabilized by moderate static m/n = 2/1, 4/2 or 6/3 RMPs below their penetration threshold if the distance between the two resonant surfaces and the local plasma rotation velocity at the outer resonant surface are sufficiently large. The outer magnetic island is stabilized due to the change of the local plasma current density gradient around the outer resonant surface caused by RMPs, while the inner island growth is stabilized by the bootstrap current perturbation in the negative magnetic shear region. The mode stabilization is more effective for a higher electron temperature, indicating a possible method to improve the DTM stability in a fusion reactor.