Local Critical Current Density Research Articles

Correlation between microstructure and superconducting properties of MgB2 bulk samples with Mg addition and Mg/hBN co-additions

Abstract The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (J c) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hexagonal boron nitride (hBN) doping improves intrinsic J c by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic J c by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1 wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature.

Flux penetration of an HTS coated-conductor tape by an approaching permanent magnet

Interactions between inhomogeneous magnetic fields and a superconductor lie at the heart of several high-temperature superconducting (HTS) devices. An example is the situation where the magnetic field from a small permanent magnet (PM) is applied to a coated-conductor HTS tape, such that flux penetrates the superconductor. The PM field is highly spatially inhomogeneous, such that there is significant variation in the applied field magnitude and direction across the tape. This leads to a varying local critical current density, Jc(B , θ ), across the tape, and consequently different ‘shielding’ (magnetic flux penetration) behaviour than is the case for a uniform applied field. Here, we report results from measurements and numerical simulations of the penetrating magnetic field within an HTS coated-conductor tape which occur as a PM dipole approaches from a distance. Electromagnetic simulations were performed using a finite-element model based on the H -formulation, and which incorporated experimentally-measured anisotropic Jc(B , θ ) properties from a coated-conductor tape. This showed good agreement with experimental measurements. The effects of varying key modelling assumptions and parameters were then studied, including changing the widths of the PM and HTS tape and the magnitude of Jc(B , θ ). The inhomogeneity of the PM field leads to a characteristic gull-wing distribution of magnetic flux across the superconductor at elevated applied fields. Close approach of the PM to the tape suppresses Jc in the centre of the tape, and this results in the observation of a characteristic maxima for the total shielding currents circulating in the tape. A figure of merit is introduced which uses this effect to provide a threshold definition for ‘full flux penetration’ of an HTS tape by an inhomogeneous dipole field.

Reduced granularity in BHO-doped YBCO films on RABiTS templates

REBCO based coated conductors (CCs) are a viable alternative to conventional superconductors for many applications, therefore the optimization of their current carrying capacity is an ongoing process. A promising route for the increase in performance is the introduction of artificial pinning centers such as BaHfO3 (BHO) nanoparticles. However, granularity still imposes a substantial performance limitation, especially in REBCO CCs deposited on RABiTS based templates, as the critical current density is severely reduced by moderate misalignment angles of adjacent grains. A combined study of scanning Hall probe microscopy and electron microscopy of undoped and BHO-doped YBa2Cu3O7-δ (YBCO) films on technical templates shows that BHO-doping leads to a denser microstructure of the superconducting layer and higher global and local critical current densities. The statistical evaluation of local current maps allows for a quantification of the magnetic granularity where a reduction of granularity with increasing film thickness, doping and increasing temperature is found. In particular, the dependence of granularity on the film thickness and enhanced film growth through BHO-doping shows the potential for further optimization of YBCO films on RABiTS based templates.

Current redistribution during inhomogeneous quench of 2G HTS tapes

Inhomogeneous temperature elevations, also called hot spots, occurring in second generation high-temperature superconductor (2G HTS) tapes may lead to their destruction. A better understanding of inhomogeneous quenches would contribute to develop strategies to better protect superconducting devices based on 2G HTS tapes against hot spots. To do so, we investigated the current redistribution around a dissipative zone in a 2G HTS tape with a combination of experiments and numerical simulations based on the finite element method (FEM). Firstly, the inhomogeneous heat generation in a commercial 2G HTS bare tape (without copper cladding) was observed through the visualization of bubble generation. Secondly, the current redistribution around a dissipative zone in commercial 2G HTS bare tapes from two different manufacturers was investigated using voltage taps on both sides of the tape. The measured voltages showed that the current redistribution around the dissipative area in the top stabilizer layer of the tape is different from that in the bottom stabilizer layer. Using a 3D electro-thermal FEM model, we reproduced these behaviors, assuming a HTS tape architecture with an inhomogeneous local critical current density. Finally, using the same FEM model, we explored the impact of a lack of silver on one lateral side of a 2G HTS tape. Our results indicate that such a lack of silver does not critically affect the quench dynamics.

Microstructure and superconducting properties of bulk EuBCO-Ag with and without holes

Bulk single-grain EuBCO-Ag superconductors with holes and without holes grown using a top-seeded melt-grow process are analysed. It turns out that the levitation force and the trapped magnetic field at 77 K are higher for the sample with holes (0,85 N and 1 T for the sample with a diameter of 28 mm and a height of 10 mm). Careful complex microstructural analyses undertaken by polarised light optical microscopy and scanning electron microscopy show that the reason for this behaviour is mainly the lower porosity of the sample with holes and the lower density of the of a/c-oxygenation cracks in non-porous areas along the holes. It also appears that other microstructural features, such as higher volume fraction of Eu2BaCuO5 particles in the a-growth sector than in the c-growth sector, uniform distribution of Ag particles in samples, presence of tetragonal EuBa2Cu3O7−x regions are similar on both types of samples and cannot significantly affect macroscopic superconducting properties. The influence of porosity on the local critical current density and transition temperature is also illustrated.

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.

Characterization of Spatial Distribution of Local Critical Current Density in a Co-Doped BaFe2As2 Film Based on Magnetic Microscopy

We have investigated spatial distribution of local critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) in a Co-doped BaFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Ba122:Co) film based on scanning Hall-probe microscopy (SHPM). In-plane J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> distribution was derived from the magnetic field distribution. Thanks to the high-resolution characterization, local obstacle can be detected clearly, as well as a local spot with extremely high J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> over 7 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is among the highest value at remanent state reported for this series of films so far. The SHPM allows us to separate the influence of current blocking obstacles and intrinsic flux pinning properties on J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . Validity of the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> analysis was checked by a magnetization measurement based on a SQUID magnetometer. Moreover, we investigated the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> distributions at different temperatures by SHPM and studied their probability distribution functions (PDFs). PDFs are found to be scaled by a Weibull function with the same shape parameter, while J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> s are decreasing with higher temperature. These results indicate that the intrinsic in-plane pinning is controlled by the same mechanism.

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

High-${T}_{c}$ superconducting (HTS) dynamos are experimentally proven devices that can produce large (more than a kiloamp) dc currents in superconducting circuits, without the thermal leak associated with copper current leads. However, these dc currents are theoretically controversial, as it is not immediately apparent why a device that is topologically identical to an ac alternator should give a dc output at all. Here, we present a finite-element model and a comparison of it with experiment that fully explain this effect. It is shown that the dc output arises naturally from Maxwell's laws when time-varying overcritical eddy currents are induced to circulate in a HTS sheet. We first show that our finite-element model replicates all of the experimental electrical behavior reported so far for these devices, including the dc output characteristics and transient electrical waveforms. Direct experimental evidence for the presence of circulating eddy currents is also obtained through measurements of the transient magnetic field profile across the HTS tape, using a linear Hall array. These results are also found to agree closely with predictions from the finite-element model. Following this experimental validation, calculated sheet current densities and the associated local electric fields are examined for a range of frequencies and net transport currents. We find that the electrical output from a HTS dynamo is governed by the competition between transport and eddy currents induced as the magnet transits across the HTS tape. The eddy currents are significantly higher (approximately 1.5 times) than the local critical current density, and hence experience a highly nonlinear local resistivity. This nonlinearity breaks the symmetry observed in a normal ohmic material, which usually requires the net transport current to vary linearly with the average electric field. The interplay between local current densities and nonlinear resistivities (which both vary in time and space) is shown to systematically give rise to the key observed parameters of experimental HTS dynamo devices: the open-circuit voltage, the internal resistance, and the short-circuit current. Finally, we identify that the spatial boundaries formed by each edge of the HTS stator tape play a vital role in determining the total dc output. This offers the potential to develop alternative designs for HTS dynamo devices, in which the internal resistance is greatly reduced and the short-circuit current is substantially increased.

Magnetometry Diagnostics of Defects in High-Temperature Superconducting Tapes in a Gradient Magnetic Field

A new technique for contactless magnetometric determination of the local critical current in high-temperature superconducting tapes is proposed. In contrast to conventional approaches, where currents in a superconductor are induced by a uniform magnetic field of an external source, in our variant the tape is magnetized by a series of bipolar permanent magnets. It is shown that, for solving a number of technical problems of defect diagnostics, the proposed approach is more effective than those used earlier. Two variants of diagnostics are discussed. The first variant is intended for express diagnostics of local defects (first of all, transverse cracks) in a separate tape and in a tape stack and the second variant is developed for the case of a smooth critical current variation in a separate tape. The proposed method can significantly improve the accuracy of determining the local critical current density.

Магнитометрическая диагностика дефектов высокотемпературных сверхпроводящих лент в градиентном магнитном поле

AbstractA new technique for contactless magnetometric determination of the local critical current in high-temperature superconducting tapes is proposed. In contrast to conventional approaches, where currents in a superconductor are induced by a uniform magnetic field of an external source, in our variant the tape is magnetized by a series of bipolar permanent magnets. It is shown that, for solving a number of technical problems of defect diagnostics, the proposed approach is more effective than those used earlier. Two variants of diagnostics are discussed. The first variant is intended for express diagnostics of local defects (first of all, transverse cracks) in a separate tape and in a tape stack and the second variant is developed for the case of a smooth critical current variation in a separate tape. The proposed method can significantly improve the accuracy of determining the local critical current density.

Use of a Sigmoid Function to Describe Second Peak in Magnetization Loops

Order-disorder transitions of a vortex lattice transfer type-II superconductors from a low critical current state to a high one. The similar transition between different current states can be caused by electromagnetic granularity. A sigmoid curve is proposed to describe the corresponding peak in a field dependence of the macroscopic critical density. Using the extended critical state model, analytic expressions are obtained for the field dependencies of the local critical current density, the depth of equilibrium surface region, and the macroscopic critical current density. The expressions are well fit to published data.

Correlation of local structure peculiarities and critical current density of 2G MOCVD YBCO tapes with BaZrO3 nanoinclusions

We have studied the influence of BaZrO3 nanoinclusions on the local structure and critical current density of second-generation high temperature superconducting tapes based on YBa2Cu3O7−δ (YBCO) films. The films were made by metal–organic chemical vapor deposition (MOCVD). The crystal and local structure of the materials under study were analysed by x-ray diffraction and x-ray absorption spectroscopy (EXAFS + XANES). We have found that, being added at MOCVD process, Zr forms BaZrO3 nanoinclusions in YBCO matrix. The distance between Zr and the neighboring atoms is shorter than that one in a bulk crystalline BaZrO3, so we conclude that the nanoinclusions are in compressed state. The incorporation of 5 mol% BaZrO3 minimizes the static disorder of Cu–O bonds and maximizes their stiffness in YBCO. We show that the local structure peculiarities correlate well with the observed critical current behavior and consider this to be additional evidence in favor of small amounts of BaZrO3 nanoinclusions as efficient pinning centers.

A new method of reconstructing current paths in HTS tapes with defects

We propose a new method for calculating current paths in high-temperature superconductive (HTS) tapes with various defects including cracks, non-superconducting inclusions, and superconducting inclusions with lower local critical current density. The calculation method is based on a model of critical state which takes into account the dependence of the critical current on the magnetic field. The method allows us to calculate the spatial distribution of currents flowing through the defective HTS tape for both currents induced by the external magnetic field and transport currents from an external source. For both cases, we performed simulations of the current distributions in these tapes with different types of defects and have shown that the combination of the action of the magnetic field and transport current leads to a more detailed identification of the boundaries and shape of the defects. The proposed method is adapted for calculating modern superconductors in real superconducting devices and may be more useful as compared to the conventional magnetometric diagnostic studies, when the tape is affected by the magnetic field only.

Returning to Zinc to Meet the Challenges for Next-Generation Energy Storage

The number of battery chemistries that check the boxes of high specific energy (>100 W h kg–1), rechargeability, power performance, inherent safety, low cost, and worldwide abundance is a short list. While lithium-based batteries continue to permeate the energy-storage landscape, many safety and supply concerns remain. Aqueous-electrolyte batteries meet many of the requirements for next-generation energy storage devices, but depending on the chemistry (Pb acid, Ni–MH, Ni–Zn, Zn–Air) often fail when it comes to safety, specific energy, and/or rechargeability. The radar chart below reveals the advantages and disadvantages of several battery technologies and especially highlights that improvements to rechargeable Zn-based batteries will propel them to the forefront of the energy landscape. The historical hindrance to broader use of Zn-based batteries is the poor rechargeability of the Zn electrode, especially when formulated as a powder-bed electrode that is too prone to launch dendrites upon cycling, which then short-circuit battery operation. The U.S. Naval Research Laboratory has redesigned the Zn electrode into a monolithic, metallically interconnected three-dimensional sponge form factor that supports a more uniform current distribution, thereby allowing nearly complete utilization of the Zn active material (>90%; projecting to ~450 W h kg–1) and avoiding the local critical current density required to spawn dendrites upon cycling. We established the reversibility of our 3D Zn sponge electrodes by cycling in Ni–3D Zn coin cells with particular emphasis on suitability in next-generation electric and start–stop vehicles. The lessons derived from studying Ni–3D Zn deep cycling allow us to demonstrate preliminary cycling performance of 3D Zn–air cells and offer prescriptions on appropriate electrolytes, current collectors, cell-fabrication particulars, and the future of Zn-based batteries. Figure 1

Evaluation of global and local critical current densities in 122-type iron-based superconducting tapes

We demonstrate the fabrication of (Ba,K)Fe2As2 superconducting pressed tapes through a powder-in-tube method using uniaxial cold pressing technique, and evaluate global and local Jc by several physical measurements. We found an intriguing anisotropy in Jc determined by magnetic measurements, which is related to microcrack structures affecting the local Jc. The maximum Jc estimated from magnetization measurements has reached ∼1.9×105A/cm2 under self-field at 4.2K. Furthermore, microcrack structures are directly observed by magneto-optical imaging technique.

Superconductor Stability Against Quench and Its Correlation with Current Propagation and Limiting

This paper presents a numerical (finite element) analysis of superconductor stability and current propagation under random variations of critical superconductor parameters. Instead of using singular (homogeneous) values, random variations potentially are appropriate to take into account any conductor inhomogeneity that can be considered as an obstacle to current propagation. Traditional assumptions like homogeneous current distribution, critical temperature, critical current density and critical magnetic fields are not justified in general; a local disturbance (for example, release of mechanical stress energy), if not immediately distributed by solid conduction, would generate a transient increase of local conductor temperature. Local critical current density and magnetic field then will be reduced, and current distribution will change. Disturbances may arise also from transport currents that locally exceed the critical current of the superconductor. Disturbances of all kinds may increase the conductor temperature above its critical value. A local analysis of all superconductor states thus is mandatory to safely avoid a quench. As an extension of standard stability models, also flux flow resistive states are taken into account. We will try to find a possibly existing correlation between current propagation and superconductor stability. Fault current limiting is discussed as a special case of current propagation. The analysis is applied to a bundle of high-temperature superconductor (HTSC) filaments. As will be shown, temperature profiles in a superconductor do not allow a clear distinction between Ohmic resistive or flux flow resistive fault current limiting. Though frequently made in the literature, this separation is highly questionable, because Ohmic resistive and flux flow resistive states may locally coexist, side by side, but are not very stable in the superconductor volume.

Influence of Internal Magnetic Field Distribution on Critical Currents in a Single and Assembled Bi-2223 Tapes

We have investigated the influence of self-field on the critical current density distribution in a Bi-2223 tape and its effect on the current capacity of an assembled conductor. Critical current of a tape is usually measured by four-probe transport method under self-field. On the other hand, the critical current of an assembled conductor cannot be simply summed because the tape experiences different condition of magnetic field due to the interaction among the tapes. To quantify the situation, it is necessary to clarify first how the critical current of a tape is determined at the self-field and then to consider the interaction among the tapes. In this study, it was found that the distribution of critical current density was largely influenced by local magnetic field inside the tape. The results were quantitatively described by a model considering the position dependence and magnetic field dependence of local critical current density. Using such a quantitative model, we numerically investigated the current capacities of assembled conductors with different arrangements of the tapes.

Magnetic-field dependence of the local critical current density in the second-generation HTSC tapes

An experimental-computational algorithm of precisely determining the dependence of the local critical current density in a superconducting film on the local magnetic field is developed. The determination is performed proceeding from the dependence of the transport critical current through a wide superconducting tape on the external magnetic field. It is shown that the existing formulas approximating this dependence do not allow the solution of this problem with an accuracy provided by modern experimental techniques. To provide a necessary accuracy of the solution, a four-parametric approximation formula is proposed and the parameters of this dependence are determined by the example of the second-generation HTSC tape.

Grain refinement influence on the critical current density of the MgB2 superconductor sample

There exist two kinds of critical current densities in polycrystalline bulk of MgB2, i.e., the large local critical current density corresponding to the shielding current in inductive measurements, which flows inside the grains, and the small global critical current density that flows through the grains for whole sample. This behavior is considered to be mainly caused by the significant granularity in polycrystalline bulk. In this work, MgB2 superconductors are prepared under different Spark plasma sintering (SPS) heating rates. The microstructures of the samples are investigated, and their critical current densities are measured by Campbell method from the penetrating AC flux profile and the AC magnetic field versus penetration depth. It is found that an extremely high global critical current flows through the whole sample with a bigger grain size, which is prepared by a low heating rate during SPS sintering. That is to say, the grain refinement only increases the local critical current density of the sample. These results imply that the global current is reduced due to the existence of various defects and the poor electrical connectivity in MgB2 sample.

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This article reviews a method for characterizing the local critical current density (Jc) distribution in superconducting tapes and wires based on scanning Hall-probe microscopy (SHPM). This method is very powerful for (1) finding the bottleneck that limits the global performance of a superconductor, (2) investigating the local inhomogeneity that may become the origin of local quench, especially in HTS applications, (3) establishing the processes for narrow and/or multifilamentary HTS conductors for reducing the magnetization in magnet applications and the AC losses in power applications. The principle and the functions of this technique are introduced by referring to characterization examples of REBCO coated conductors and an MgB2 wire.