Local Current Density Distribution Research Articles

Design of Lithium Metal Anode and Other High-Capacity Alloy Anodes Solid-State Batteries in Context of Contact Loss between Anode and Solid Electrolyte Separator

Cyclic volume changes and non-uniform electrodeposition/stripping, among other cycling-induced chemo-mechanical degradation of lithium metal and lithium-alloy solid state batteries, lead to contact loss between the anode and the solid electrolyte separator[1, 2]. In-operando experiments have shown ac-celerated short-circuiting behavior due to contact loss in “anode-free” solid-state batteries[2]. Simulations reveal the relationship between active area fraction and the ratio of effective conductivities in made-up active area configurations[3]. Through modeling experiments using imputed active con-tact area of lithium-metal anode batteries, we quantify the effects of this contact loss in terms of accelerated chemo-mechanical degradation and de-creased rate capability. Specifically, we (1) quantify the interfacial resistance due to this contact loss, (2) show non-uniform local current density distribution such that local current densities could exceed lithium filament growth critical current densities, (3) show uniaxial stress inhomogeneity due to non-uniform reaction at the anode/separator interface, and (4) show non-uniform reaction distribution at the positive electrode.Besides allowing for the optimization of designs to minimize contact loss, our work sheds light on tradeoffs in the design of solid electrolyte separators.

A Large-Scale Fabrication of Flexible, Ultrathin, and Robust Solid Electrolyte for Solid-State Lithium-Sulfur Batteries.

All-solid-state lithium metal batteries (ASSLMBs) are considered as the most promising candidates for the next-generation high-safety batteries. To achieve high energy density in ASSLMBs, it is essential that the solid-state electrolytes (SSEs) are lightweight, thin, and possess superior electrochemical stability. In this study, a feasible and scalable fabrication approach to construct 3D supporting skeleton using an electro-blown spinning technique is proposed. This skeleton not only enhances the mechanical strength but also hinders the migration of Li-salt anions, improving the lithium-ion transference number of the SSE. This provides a homogeneous distribution of Li-ion flux and local current density, promoting uniform Li deposition. As a result, based on the mechanically robust and thin SSEs, the Li symmetric cells show outstanding Li plating/stripping reversibility. Besides, a stableinterfacecontact between SSE and Li anode has been established with the formation of an F-enriched solid electrolyte interface layer. The solid-state Li|sulfurized polyacrylonitrile (Li|SPAN) cell achieves a capacity retention ratio of 94.0% after 350 cycles at 0.5 C. Also, the high-voltage Li|LCO cell shows a capacity retention of 92.4% at 0.5 C after 500 cycles. This fabrication approach for SSEs is applicable for commercially large-scale production and application in high-energy-density and high-safety ASSLMBs.

Multiscale study of reactive transport and multiphase heat transfer processes in catalyst layers of proton exchange membrane fuel cells

Improving the performance of proton exchange membrane fuel cells (PEMFCs) requires deep understanding of the reactive transport processes inside the catalyst layers (CLs). In this study, a particle-overlapping model is developed for accurately describing the hierarchical structures and oxygen reactive transport processes in CLs. The analytical solutions derived from this model indicate that carbon particle overlap increases ionomer thickness, reduces specific surface areas of ionomer and carbon, and further intensifies the local oxygen transport resistance (Rother). The relationship between Rother and roughness factor predicted by the model in the range of 800-1600 s m-1 agrees well with the experiments. Then, a multiscale model is developed by coupling the particle-overlapping model with cell-scale models, which is validated by comparing with the polarization curves and local current density distribution obtained in experiments. The relative error of local current density distribution is below 15% in the ohmic polarization region. Finally, the multiscale model is employed to explore effects of CL structural parameters including Pt loading, I/C, ionomer coverage and carbon particle radius on the cell performance as well as the phase-change-induced (PCI) flow and capillary-driven (CD) flow in CL. The result demonstrates that the CL structural parameters have significant effects on the cell performance as well as the PCI and CD flows. Optimizing the CL structure can increase the current density and further enhance the heat-pipe effect within the CL, leading to overall higher PCI and CD rates. The maximum increase of PCI and CD rates can exceed 145%. Besides, the enhanced heat-pipe effect causes the reverse flow regions of PCI and CD near the CL/PEM interface, which can occupy about 30% of the CL. The multiscale model significantly contributes to a deep understanding of reactive transport and multiphase heat transfer processes inside PEMFCs.

Towards reusable 3D-printed graphite framework for zinc anode in aqueous zinc battery

Aqueous zinc ion batteries (AZIBs) are an increasingly popular high-safety and eco-friendly energy storage solution. However, the development of high-performance zinc (Zn) anodes remains a formidable challenge, primarily due to dendrite formation and poor reversibility. To address these challenges, this study harnesses the potential of digital light process (DLP) 3D printing technology to fabricate reduced graphite oxide-based 3D gyroid structure (3DP-rGG) as zinc anode frameworks for aqueous zinc batteries. The 3D-printed structure effectively regulates local current density distribution, offering ample nucleation sites and free space for inducing uniform zinc deposition and accommodating diminutive zinc nodules. Consequently, the 3D-printed graphite framework demonstrates remarkable reversibility in zinc plating and stripping processes, resulting in commendable coulombic efficiency and low voltage hysteresis. The full battery with a 3D-printed zinc anode, incorporating a polyaniline-intercalated vanadium oxide cathode (PVO), exhibits high specific capacity and superior long-term cycling stability. Remarkably, the robust 3DP-rGG structure can be reused over ten times without any discernible impact on its electrochemical performance, thereby underscoring the potential of this controllable and efficient fabrication of 3D graphite current collectors as a promising solution to develop reusable 3D frameworks for high-performance metal batteries.

Study on the effect of NO2 impurity gas on the performance and local current density distribution of PEMFC

The performance of proton exchange membrane fuel cells (PEMFCs) is affected by impurities, and airborne NO2 is a potential factor in the degradation of the performance and durability of PEMFCs. Herein, the effect of 50 ppb ∼ 5 ppm NO2 on PEMFC was investigated by the poisoning strategy of increasing concentration gradient, and the effect of NO2 on the local current density distribution was explored in both galvanostatic and potentiostatic states. The results showed that 2 ppm NO2 caused a significant voltage decay accompanied by an increase in activation impedance and mass transfer impedance. NO2 was decomposed into oxygen atoms and adsorbed NO on the Pt surface, occupying active sites, impeding oxygen reduction reaction, and further hindering mass transfer. Polarization curve results showed more pronounced voltage decay at high current densities. Polarization curve testing and clean air purging were effective in restoring the poisoned PEMFC. Moreover, it was proved that the poisoning effect is more severe in the inlet region, especially in potentiostatic state. Hence, it is suggested to operate in galvanostatic state, which has a positive impact on reducing the uneven performance caused by NO2. This study provides a reference for the prevention of PEMFC poisoning and performance recovery after poisoning.

Fault Detection and Identification for Polymer Electrolyte Membrane Fuel Cell Stack by External Magnetic Field

Many technological locks prevent the deployment of Proton Exchange Membrane Fuel Cell (PEMFC). Particularly for vehicle applications, cost and durability represent two of the most significant challenges to achieve clean, reliable, and cost-effective PEMFC systems. Solving these shortcomings will be a great opportunity for a decisive breakthrough towards mass-diffusion of the current PEMFC technology. The long-term electrical performance losses are associated to either the degradation of the MEA constitutive materials or due to specific operating conditions (flooding, drying, etc.). In perspective, local current density distribution may support stack-level degradation analyzes as well as effective stack control to mitigate the consequences of degradation due to load cycles (electrochemical, thermal, mechanical and humidity effects) or faults.External magnetic field measurement (MFM) shows a high potential in PEMFC non-invasive diagnosis. Thus, 2D and 3D current distributions and their anomalies in the fuel cell can be identified independent of the size of the causing fault.To perform this local characterization, the external magnetic field measurement will enable to model the current density in stationary conditions over the cell surface and for the different cells of the stack. This methodology requires the complementary development of magnetic field techniques and inverse models.In this work, this challenge is tackled by using tailored defective MEAs and specific operating conditions (flooding, drying, ...) to characterize, how local and overall performances of the MEA are affected and to identify the signatures of the various anomalies. Figure 1

Using Magnetic Field Analysis for Localizing Defects of Fuel Cell & Electrolyzer Components

Production capacities of electrolysis and fuel cell technologies are expected to increase significantly in the upcoming years, reducing costs of stack components through numbering-up. To ensure reliability and durability and reduce costs along the manufacturing process, developing suitable in-line non-destructive diagnostic methods for quality control becomes essential for their industrialization.In both fuel cell and electrolyzer stacks, the main components' electrical resistance and current density distribution greatly influence stack efficiency and longevity. Here, the flowing electric current generates a magnetic field depending on its strength and direction. Potential defects caused by the manufacturing process, including cracks, defective welding spots, coating adhesion issues, delamination, and contamination, influence the components' electric conductivity and current density distribution and hence the magnetic field distribution.This work investigates non-destructive diagnostic methods using magnetic field analysis (MFA) to identify changes in the magnetic field distribution and, thus, local electrical resistance and current density distributions of fuel cell and electrolyzer components. The methods used are magnetic field imaging (MFI) and Eddy Current Testing (ECT). MFI analyzes the magnetic field distribution in all three spatial directions to trace electrical currents and identify electrical defects, creating a magnetic field signature that can potentially distinguish between functional and non-functional components along the manufacturing process. ECT measures variations in local conductivity by inducing eddy currents in the sample through an alternating magnetic field generated by a sender coil. These eddy currents create a detectable magnetic field in a receiver and are affected by defects and changes in electrical conductivity. Therefore, the method can characterize sheet resistance and material homogeneity through electrical anisotropy in electrically conductive components.In initial experiments, ECT measurements on coated and uncoated hollow embossed stainless steel bipolar half plates showed systematic differences depending on quality characteristics resulting in unique patterns and inhomogeneities which require further investigation. Furthermore, experiments with both methods on other stack components will be presented and discussed. In addition, their in-line capability will be assessed and evaluated. However, these first promising results imply that MFA is potentially useful as an in-line, non-destructive diagnostic method for quality control.

Unraveling the Coupling Effect between Cathode and Anode toward Practical Lithium-Sulfur Batteries.

The localized reaction heterogeneity of the sulfur cathode and the uneven Li deposition on the Li anode are intractable issues for lithium-sulfur (Li-S) batteries under practical operation. Despite impressive progress in separately optimizing the sulfur cathode or Li anode, a comprehensive understanding of the highly coupled relationship between the cathode and anode is still lacking. In this work, inspired by the Butler-Volmer equation, a binary descriptor (IBD ) assisting the rational structural design of sulfur cathode by simultaneously considering the mass-transport index (Imass ) and the charge-transfer index (Icharge ) is identified, and subsequently the relationship between IBD and the morphological evolution of Li anode is established. Guided by the IBD , a scalable electrode providing interpenetrated flow channels for efficient mass/charge transfer, full utilization of active sulfur, and mechanically elastic support for aggressive electrochemical reactions under practical conditions is reported. These characteristics induce a homogenous distribution of local current densities and reduced reaction heterogeneity on both sides of the cathode and anode. Impressive energy density of 318Wh kg-1 and 473Wh L-1 in an Ah-level pouch cell can be achieved by the design concept. This work offers a promising paradigm for unlocking the interaction between cathode and anode and designing high-energy practical Li-S batteries.

Interaction phenomena of electrically coupled cells under local reactant starvation in automotive PEMFC stacks

The local current density distribution of polymer electrolyte membrane fuel cells (PEMFCs) can be distorted by various error states. Differences in current density distributions (CDDs) of adjacent cells in a stack are equilibrized by in-plane currents within the sandwiched bipolar plates. Degradation stressors such as detrimental differences in local cell voltage and current density maxima can thus be generated. A novel method was therefore developed to intentionally manipulate CDD profiles by integrating local artificial starvation into only one fuel cell in an assembly. This technique is applied to automotive-sized PEMFCs single cells as well as in 20 cell short-stack to analyze such voltage and current redistribution phenomena. A drastic distortion of local cell voltage is only observed for stacks, which is explained by a supplementary simulation. The local voltage distribution of an electrically coupled fuel cell is therefore calculated by combining CDD measurements with a spatially resolved polarization curve model. The capabilities and limits of a multipoint cell voltage monitoring measurement device are discussed on this basis. The inspected correlation between these two independent online measurement techniques allows to localize such error states with considerable accuracy during operation of automotive sized PEMFC stacks.

Investigating Different Local Polyurethane Coatings Degradation Effects and Corrosion Behaivors by Talaromyces funiculosus via Wire Beam Electrodes.

The degradation effect of mold on the coating in a hot and humid environment is one of the important factors that cause layer failure. Combined with the wire beam electrode (WBE) and the traditional surface analysis technique, the local biodegradation of the coatings and the corrosion behaviors of metal substrates can be characterized accurately by a WBE. Herein, a WBE was used to study the degradation impact of Talaromyces funiculosus (T. funiculosus) isolated from a tropical rainforest environment on the corrosion of polyurethane (PU) coating. After immersion for 14 days, the local current density distribution of the WBE surface can reach ~10-3 A/cm2 in the fungal liquid mediums but maintains ~10-7 A/cm2 in sterile liquid mediums. The |Z|0.01Hz value of the high current densities area (#85 electrode) was 1.06 × 109 Ω cm2 in a fungal liquid medium after 14 days of immersion. After being attacked by T. funiculosus, the degradation of the PU was more severe, and there were wrinkles, cracks, blisters, and even micro-holes distributed randomly on the surface of electrodes. This resulted from the self-corrosion caused by the T. funiculosus degradation of the coating; the corrosion caused by the electric coupling effect of the coating was introduced. Energy dispersive spectroscopy (EDS) and Raman spectra results showed that the corrosion products were flakey and globular, which consisted of γ-FeOOH, γ-Fe2O3, and α-FeOOH.

Experimentally investigation on current density distribution characteristics of hydrogen-oxygen proton exchange membrane fuel cells under dynamic loading

The performance improvement and mitigate degradation of hydrogen-oxygen proton exchange membrane fuel cells (PEMFCs) are of great significance for accelerating the practical application of clean energy and alleviating the energy crisis. In this work, a PEMFC with an active area of 25 cm2 was designed based on multilayer printed circuit board technology for high-resolution current density mapping. The characteristics of uneven distribution of local current density under dynamic loading and its influence on performance degradation are further analyzed. The results show that the uneven density distribution at low current density is caused by membrane dehydration and gas starvation, while at high current density, it is caused by the combined effect of gas starvation, and the uneven current density distribution is even worse at high current density. In addition, oxygen starvation causes more worse uneven density distribution than that of hydrogen starvation, and the lowest current density is concentrated near the cathode outlet. What's more, the uneven distribution of local current density led to serious performance degradation, the performance degradation reached 40.03%, the electrochemical active surface area decreased by 31.87% and charge transfer resistance increased by 22.08%. The scanning electron microscope and energy dispersive X-ray spectrometry show that performance degradation is due to the loss of catalysts, especially near the cathode outlet.

Locally regulating Li+ distribution on an electrode surface with Li-Sn alloying nanoparticles for stable lithium metal anodes.

Despite the fact that lithium metal batteries (LMBs) have the advantage of higher energy density than traditional lithium-ion batteries (LIBs), the development of Li anodes is hindered by the issues of dendritic Li growth and parasitic reactions during cycling, which can cause a coulombic efficiency decrease and capacity decay. Herein, a Li-Sn composite anode is developed by a facile rolling method. The in situ generated Li22Sn5 nanoparticles are uniformly distributed in the Li-Sn anode after the rolling process. The Li22Sn5 nanoparticles on the surface of the electrode exhibit excellent lithiophilicity, reducing the Li nucleation barrier. Multiphysics phase simulation discloses the distribution of local current density around the holes, guiding Li preferentially to deposit back onto the sites of previous Li stripping, and then realizing a controllable plating/stripping behavior of Li on the Li-Sn composite anode. Consequently, the symmetrical cell of Li-Sn||Li-Sn achieves a stable cycling lifetime of more than 1200 h at a current density of 1 mA cm-2 with a fixed capacity of 1 mA h cm-2. Besides, the full cell pairing with the LiFePO4 cathode delivers excellent rate performance and capacity retention after long cycles. This work provides new insight to modify the Li metal for preparing dendrite-free anodes.

Electrodeposition model with dynamic ion diffusion coefficients for predicting void defects in electroformed microcolumn arrays.

Due to the confined mass transfer capability in microchannels, void defects are easily formed in electroformed microcolumn arrays with a high depth/width ratio, which seriously affects the life and performance of micro-devices. The width of the microchannel constantly decreases during electrodeposition, which further deteriorates the mass transfer capability inside the microchannel at the cathode. In the traditional micro-electroforming simulation model, the change of the ion diffusion coefficient is always ignored, making it difficult to accurately predict the size of void defects prior to electroforming experiments. In this study, nickel ion diffusion coefficients in microchannels are tested based on the electrochemical experiments. The measured diffusion coefficients decrease from 4.74 × 10-9 to 1.27 × 10-9 m2 s-1, corresponding to microchannels with a width from 120 to 24 μm. The simulation models of both constant and dynamic diffusion coefficients are established, and the corresponding simulation results are compared with the void defects obtained using micro-electroforming experiments. The results show that when the cathode current densities are 1, 2 and 4 A dm-2, the size of void defects obtained with the dynamic diffusion coefficient model is closer to the experimental results. In the dynamic diffusion coefficient model, the local current density and ion concentration distribution proves to be more inhomogeneous, leading to a big difference in the deposition rate of nickel between the bottom and the opening of a microchannel, and consequently a larger void defect in the electroformed microcolumn arrays. In brief, the ion diffusion coefficient inside microchannels with a different width is tested experimentally, which provides a reference for developing reliable micro-electroforming simulation models.

Electrodeposited 3D lithiophilic Ni microvia host for long cycling Li metal anode at high current density

The uneven deposition of Li metal is the main reason leading to Li dendrite growth, dead Li formation and infinite volume change, shortening the Li metal battery lifetime and causing catastrophic safety hazards. The widely studied 3D micro-nanostructured lithiophilic Li metal anode hosts can suppress the Li dendrite growth, however, it could barely endure high-rate cycling due to the exacerbated uneven deposition of Li metal at a high current density. Herein, we report an electrodeposited 3D microvia structured lithiophilic Ni skeleton on planar Cu with a nanoscale surface roughness as Li metal anode current collector. The high specific surface area and nanoscale rough surface can achieve both low local current density and uniform electric field distribution, which contribute to the even deposition of Li metal at a high current density. Additionally, the in-situ formed Ni(OH)2 thin layer during electrodeposition is lithiophilic, which could induce the uniform Li nucleation. This 3D lithiophilic Ni microvia (3D NMV) host endow the Li metal anode with long cycling performance for over 830 cycles at an ultra-high current density of 10 mA cm−2. Our work illustrates the synergic effect of 3D structure and surface flatness in achieving high-rate cycling performance.

Local ageing effects of polymer electrolyte fuel cell membrane electrode assemblies due to accelerated durability testing

AbstractAccelerated durability test (ADT) protocols are useful tools to reduce testing time and development costs of automotive polymer electrolyte fuel cell stacks. Established accelerated stress tests allow comparing individual cell components. However, such tests in general do not allow drawing reliable conclusions on the expected lifetime for a complete stack. In this work, we examine the influence of combined stressors on the ageing behavior of individual cell components operated on stack level. We combine known main stressors for mobile fuel‐cell operation such as dynamic load, temperature and humidity cycling or hydrogen/air fronts during start‐up to develop a new accelerated test protocol. It was applied to an automotive 5‐cell short stack for 460 operating hours (OpH). A second stack was operated in a reference long‐term test for 2300 OpH. Several in situ characterization techniques, such as cyclic voltammetry, and recording the local current density distribution were employed. In addition, post‐mortem analyses, such as focused ion beam scanning electron microscopy imaging, was applied in order to better understand the degradation mechanisms. The results presented here provide an excellent basis for the further development of a new ADT protocol, which will accelerate ageing processes in the fuel cell in a realistic way, i.e. leading to similar degradation characteristics as in long term testing.

Study on the effect of purging time on the performance of PEMFC with dead-ended anode under gravity

In the working process of proton exchange membrane fuel cell (PEMFC) with dead-ended anode (DEA), the accumulation of water and nitrogen in the anode flow channel will cause its transient working performance to degrade. Generally, the transient working performance is recovered by periodically opening the anode outlet for gas purging. However, the purging time under gravity will change the distribution state of water and gas in the anode flow channel, thus seriously affecting the working performance of PEMFC with DEA. In this paper, the influence mechanism of purging time on the performance of PEMFC with DEA under gravity is investigated by theoretical analysis and experimental method. The results show that if the purging time under gravity is shorter, the faster the transient working performance of PEMFC with DEA decreases in the next working cycle, and the area with high current density distribution is always close to the gas inlet of the flow channel, as well as the area with low current density distribution is always close to the gas outlet of the flow channel. Moreover, the effect of different purging times on the local current density distribution will lasts for a long time, and also have significant effects on the accumulation and distribution status of water at the anode side under gravity. The results can provide an important guidance for optimizing the purging strategy of PEMFC with DEA.

Influence of cathode channel blockages on the cold start performance of proton exchange membrane fuel cell: A numerical study

Proton exchange membrane fuel cell (PEMFC) performance is mainly limited by the oxygen transport to the cathode catalyst layer. Using gas flow channel blockages can effectively enhance oxygen transport. Previous research efforts demonstrated the beneficial effect of blockages on the performance of PEMFC. However, to date, the impact of flow channel blockages on the cold start performance of PEMFC has not been studied. In this work, a three-dimensional, transient, non-isothermal cold start model is developed to investigate the influence of adding staggered blocks in the cathode gas channels of a parallel flow field. The model is validated by previous experimental data. It is revealed by simulations that the addition of blockages helps to mitigate the oxygen transport limitation over the cold start duration, thus a better cold start performance is achieved. The simulated results show that the cathode full-blockage placement enhances the cold start performance much better than the partial blockage one. The local current density distribution in the full-blockage configuration case has greatly advanced, especially under the land region, due to the induced strong forced convection. More importantly, the existence of full blockages drives more oxygen into the CL for the reaction; therefore, more water correspondingly is generated, resulting in more amount of ice formation in the cathode CL, which is not favorable to the cold start process. However, despite the relatively higher ice fraction of the full-blockage case, the enhanced oxygen transport plays more dominant role at the late stages of the cold process by improving the uniformity of ice distribution in the flow direction and significantly boosting the local current density under the land region. The present article provides helpful insight in the possible utilization of the cathode channel blockage approach for assisting the design optimization of cold start process of PEMFCs.

Relationship of local current and two-phase flow in proton exchange membrane electrolyzer cells

A macro segmented Proton exchange membrane electrolyzer cell (PEMEC) with visualized endplates is designed and assembled to investigate the internal performance inhomogeneities. The local current density distribution (CDD) of the four segments of the PEMEC is measured, and the flow in the channel is observed and recorded by visualizing the endplates. The effect of varying operating conditions on the two-phase flow type distribution and performance are analyzed. It is found that the variations of the two-phase flow type from the inlet to the outlet of the electrolyzer cell lead to significant differences in the internal local performance. Large bubbles have a positive effect on the performance improvement, due to they are able to carry bubbles moved backwards along the flow direction, resulting in the region with the best performance moving from the inlet segment to the middle segment. In addition, the electrolytic current density increases as the increase of voltage, and the variation of the two-phase flow type lead to the increase of inhomogeneity. The above investigation provides a better understanding on the internal inhomogenrity of the electrolyzer cell, which is related to the two-phase flow behavior.

A 1 + 1-D Multiphase Proton Exchange Membrane Fuel Cell Model for Real-Time Simulation

A 1 + 1-dimensional (1 + 1-D) multiphase model of proton exchange membrane fuel cell (PEMFC) is developed to simulate the dynamic behaviors of the fuel cell under various operating conditions. Electrochemical, fluidic, and thermal physical phenomena are considered, and the model can accurately describe the multiphysical processes inside the PEMFC. Meanwhile, the water phase change and nitrogen crossover are also considered in this model. The developed model can simulate the fuel cell behaviors in both normal temperature and cold start conditions. Selection of the time step size is discussed, and the solution method of the real-time model is proposed. Computational efficiency is greatly improved by simplifying the multicomponent diffusion inside the membrane electrode assembly (MEA), which results in the shorter calculation time than the simulated physical time. The proposed model is suitable for embedded applications, such as real-time simulation or online diagnostic control. The comprehensive model validation is carried out by comparing the simulation results, including polarization curves, ohmic resistance, local current density distribution, and voltage evolution, under various working conditions of steady state, load change, and cold start, and a good agreement is achieved. Finally, some simulation cases are studied to further demonstrate the simulation ability of the model.

J c and Composition Distribution of YBCO Coated Conductors Fabricated by the TFA-MOD Process

The Trifluoroacetate-Metal Organic Deposition (TFA-MOD) process is one of the most promising fabrication processes for low-cost superconducting coated conductors (CCs) because of its high yield and non-vacuum system. However, in the doping and multi-coating process for the enhancement of critical current (<i>I</i>_c), degradation of <i>I</i>_c often occurs. In this study, to clarify the origin of <i>I</i>_c degradation, we have investigated the local critical current density (<i>J</i>_c) distribution, in-field <i>J</i>_c properties and the variation of composition for TFA-MOD processed YBa₂Cu₃O_y (YBCO) CCs. The local <i>J</i>_c distributions were measured by Scanning Hall-Probe Microscopy for confirming the location of the high&#x002F;low-<i>J</i>_c regions. The CCs were then cut into 3 mm&#x00D7;3 mm or 3 mm&#x00D7;4 mm for dc magnetization measurement. The <i>T</i>_c of the all samples were about 87 K to 90 K. In-field <i>J</i>_c at 77 K was obtained from <i>M</i>-<i>H</i> measurements and all the samples were compared with each other. The in-field <i>J</i>_c of high-<i>J</i>_c sample showed about 100 times higher value than that of the low-<i>J</i>_c sample. Then we have observed the microchemistry and the variation of composition by SEM-EDS. In the high-<i>J</i>_c sample, the grains of the superconducting phase were large in size and were connected to each other. This means that superconducting current could form through the whole sample area and generate a large magnetic moment. On the other hand, in the low-<i>J</i>_c sample, though the superconducting phase could be observed, the grains were small in size and sparse. Therefore, the magnetic moment measured was small. However, it was also suggested that the intra-grain <i>J</i>_c would be almost the same as that of high-<i>J</i>_c sample.