Multifilamentary Wires Research Articles

Development of design for large scale conductors and coils using MgB2 for superconducting magnetic energy storage device

MgB2 wires are commercially available, and their superconducting characteristics have been continuously developed in the last decade. The relatively high critical temperature of these wires has attracted the attention of researchers, especially in the field of superconducting magnetic energy storage (SMES) coil applications in terms of its relatively high critical temperature, as it enables the use of liquid hydrogen for cooling the coils. The sensitivity of multi-filament MgB2 wires to bending strain makes the design of large-scale conductors and coils for an SMES system technologically difficult, and the careful investigation of the applied strains during manufacturing is required. Two-conventional methods have been introduced for the fabrication of the coils: wind-and-react (W&R) and react-and-wind (R&W). These methods have been demonstrated to be suitable for the production of large-scale MgB2 magnets to maximize the coil performance. The W&R and R&W methods have been successfully applied to the designs of conductors and coils, and small W&R test coil fabrication, as well as stability demonstrations are performed in this study. Our study is the first to demonstrate the feeding of hundreds of amperes of transport current using multifilamentary MgB2 wires at around liquid hydrogen temperature in the practical background magnetic field of 2 T. The minimum quench energy and normal zone propagation velocity are also experimentally investigated for the protection of the actual coils for SMES application.

Design, Fabrication, and Testing of MgB2/Fe Racetrack Coils

We fabricated four superconducting racetrack coils wound by bare in situ MgB 2 /Fe mono and multifilamentary wires produced in our laboratory by using the wind-and-react method. Transport measurements in self-field were performed in a liquid helium dewar. The magnetic field flux density B = 25 mT for I = 92 A was measured to verify how the current flowed inside the coil for one of the coils by means of a cryogenic axial Hall sensor placed into the central bore region of the coil. The coil with 36 turns wound by mono MgB 2 /Fe wire of l = 24 m has a highest current-carrying capacity of more than 150 A at T = 4.2 K and self-field among the coils. The coils fabricated by multifilamentary wires have relatively low engineering critical current values of I ce = 67 A for 18 + 1 and I ce = 57 A for 4 + 5 superconducting/copper filament wires at T = 4.2 K in self-field. Our results form essential contribution toward determination of the possible use of low-cost MgB 2 /Fe wires in low-field superconductor coil applications.

Study on Cold-Drawing in Powder-in-Tube Process in Fabricating Silver-Sheathed (Bi,Pb)-Sr-Ca-Cu-O Tapes

Since Cu oxide superconductors have been discovered, the application for this type of superconductor especially in the form of superconducting tape to be used as electric cable to transport the electricity. The aim to this study is to analyse the manufacture of superconducting wire with superconducting pellets as its precursor. A pellet disc of High Temperature Superconducting (HTS) Bi,Pb-Sr-Ca-Cu-O has been made by using solid state method. This pellet has been prepared by mixing each of oxide powders together, and then sintered at 865°C for 90 hours. We have analysed the samples using X-Ray Diffractometer (XRD), Energy Dispersive x-ray Spectroscopy (EDS), Scanning Electron Microscopy (SEM) and also resistivity measurement to confirm its superconductivity. After confirmed the superconducting properties of the pellet, it was grinded into powder and inserted into silver tube with outer diameter of 10 mm and thickness of 1 mm. And then, drawn to make monofilament wire. It were then cut and inserted into another silver tube, and then re-drawing and rolled into multifilament tapes using Powder-in-Tube (PIT) method. The multifilament tapes was heated at 850°C for 12 hours. We confirmed its phase crystalline and superconductivity at pellet disc. However, when the pellet disc was made into wire and sintered at 850°C, it became amorphous and conductivity behaviour was decline. According to SEM analysis and resistivity measurement result, silver also penetrated inside the sample region of multifilament wire.

In situ x-ray and neutron diffraction investigation of Bi-2212 in multifilamentary wires during thermal treatment

<i>In situ</i> x-ray and neutron diffraction investigation of Bi-2212 in multifilamentary wires during thermal treatment

Numerical Simulation on Coupling Current for Multifilamentary HTS Wire

Since a high temperature superconducting (HTS) wire such as Bi-2223 (Bi2Sr2Ca2Cu3O) and REBCO((RE)Ba2Cu3O) tapes indicates good superconducting characteristics under high magnetic field, ultra-high field magnets wound HTS wire are applicable to a nuclear magnetic resonance (NMR) spectrometer and magnetic resonance imaging (MRI). The large and long-time-constant screening current is induced in the HTS wire, which is the tape shape and not twisted, and the magnetic field generated by screening current deteriorated the field quality such as temporal stability and spatial homogeneity. Because NMR and MRI requires highly accurate field on temporal stability and spatial homogeneity, it is necessary to investigate the influence of the screening current-induced field. In REBCO tape, the screening current can be reduced by dividing the superconductor layer. However, filaments are electrically connected because they are covered with copper due to strength and thermal stability. On the other hand, a Bi-2223 is wire which multiple superconducting filaments are covered with silver or a silver alloy, therefore, the screening current is smaller than that in a REBCO tape. However, in a Bi-2223 tape, a coupling current flows because of electrical bridge between the filaments. In this study, we discuss coupling current distribution from numerical simulation on the multifilamentary HTS coil which is given the local electrical contact between filaments. The screening-current field decreases with increasing the interval distance. In addition, the current distribution is different depending on the interval distance.

Development of Superconducting Soldered Joints Between Bi-2212/Ag Wires

Bi-2212/Ag superconducting wires are being considered as key conductor candidates for the development of high-field magnets (up to 25 T), but it is recognized that a reliable joining process between these wires is a critical challenge for most high-field applications. This work focuses on the design and testing of jointing processes between multifilamentary Bi-2212 wires to make persistent mode joints using soldering techniques. The ability of several different superconducting solder alloys from the PbBi and SnInBi systems to remove and replace the Ag matrix in fully reacted Bi-2212 wires has been explored. The SnInBi lead-free solder was found to be relatively effective at removing the Ag matrix, but does not make good contact with the Bi-2212 filaments. PbBi solder wets the Bi-2212 filaments more effectively, but is slower at removing the Ag matrix. Therefore, a two-stage process has been developed using molten Sn to first remove the Ag matrix followed by PbBi to replace the nonsuperconducting Sn. Joints made using this two-stage process have higher critical currents (133 A in self-field at 4 K) than joints made using PbBi alone (120 A in self-field at 4 K).

A fast response, low heat generating activation method for LHe level sensors.

A superconducting liquid helium (LHe) level sensor of length 300 mm has been fabricated based on the principle of differential heat transfer characteristic in helium gas compared to that in liquid. The sensor wire used has a diameter of 38 μm, and the wire was obtained from a pack of multifilament wires. A full thermo-electrical characterisation of the sensor was carried out in a dedicated setup. Its dynamic thermal response was also studied to understand its timing characteristics at different liquid levels and excitation currents. Based on the sensor characterisation, a new level measurement technique is evaluated which can reduce the heat load going to LHe during sensor activation without compromising on its sensitivity or accuracy. The timing response with this technique will make the level detection faster compared to the conventional techniques.

Improved superconducting properties for multifilament graphene-doped MgB2 wires by an internal Mg diffusion process

We have fabricated 6 + 1-filament MgB2 wires using an internal Mg diffusion (IMD) process. For the 3 at% graphene (G) doped long wire with 1.4 mm diameter, the critical current is about 530 A at 4 T, which is 45% higher than that of the undoped sample. Moreover, the G doped wire has higher irreversible strain of 38% in comparison to the undoped one of 30% after 650 °C × 2 h annealing. The obtained results show that the G doped IMD wires with excellent superconductivity and mechanical property can compete with the conventional PIT wires in practical application.

Changes of Superconducting Properties Due to the Unidirectional Tensile Deformation on Bronze-Processed Nb3Sn Multifilamentary Wires Using Various Cu–Sn–Zn Ternary Alloy Matrices

The degradation of transport current property by the mechanical strain on the practical Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire is a serious problem for the future fusion magnet operated under higher electromagnetic force. Recently, we developed various Zinc (Zn) solid solution ternary Cu-Sn alloy (Cu-Sn-Zn) matrices for the internal strengthened matrix and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn multifilamentary precursor wires using various Cu-Sn-Zn matrices through the conventional bronze process. After the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn synthesis heat treatment, the additional Zn remained in the matrix and it may act as a solid solution strength factor of the matrix. We carried out the unidirectional tensile test under 4.2 K and 15 T on the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires using various Cu-Sn-Zn matrices in order to investigate the remaining Zn effect on the mechanical property. The peak tensile stress in the maximum critical current density on the 10 mass%Zn additional sample was obtained to approximately 200 MPa and it became remarkably higher compared with conventional bronze processed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire. This tensile stress was similar to the CuNb and ODS-Cu reinforced Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires. The internal strengthened matrix due to the solid solution was one of the most effective methods for improving mechanical property of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire.

Superconducting Properties of Reacted Mono- and Multifilament MgB2 Wires With Respect to Bending Diameters Using a Custom-Made Bending Test Probe

In this paper, the superconducting properties of reacted mono- and multifilament MgB2 wires were investigated with regards to the bending diameter via a three-point bending test using a lab-made bending test probe. The wires were manufactured by Kiswire Advanced Technology Co., Ltd. According to the results of the bending test, the tolerable bending diameters for mono- and multifilament MgB2 wires were 30 and 160 mm, respectively. In addition, the multifilament wire at a bending diameter of 150 mm showed the existence of a resistive zone caused by the “disconnection” of the MgB2 filaments due to microcracks developed across the entire wire along the transverse direction of the filament. Monofilament wires have the potential to be utilized in most superconducting applications in terms of bending diameter, whereas the bending properties of multifilament wires require improvement to facilitate the design and fabrication of multifilament MgB2 magnets.

3-D Equivalent Model to Compute the Electro-Magnetic Behavior of Twisted Multi-Filamentary Superconductors Wires

In this paper, we investigate new methods to model in 3-D, efficiently and simply, twisted multi-filamentary superconductors subjected to an external magnetic field. Several simulation cases, based on the H-formulation implemented in GetDP, will help derive through appropriate methods, such as changing the reference from where the problem is seen and described to a reference where everything becomes simple to deal with, a simple equivalent 3-D problem to solve efficiently and quickly.

Improvement of stability of Nb3Sn superconductors by introducing high specific heat substances

High-Jc Nb3Sn conductors have low stability against perturbations, which accounts for the slow training rates of high-field Nb3Sn magnets. While it is known that adding substances with high specific heat (C) into Nb3Sn wires can increase their overall specific heat and thus improve their stability, there has not been a practical method that is compatible with the fabrication of long-length conductors. In this work, we put forward a scheme to introduce such substances to distributed-barrier Nb3Sn wires, which adds minimum difficulty to the wire manufacturing process. Multifilamentary wires using a mixture of Cu and high-C Gd2O3 powders have been successfully fabricated along this line. Measurements showed that addition of Gd2O3 had no negative effects on residual resitivity ratio or non-Cu Jc, and that flux jumps were remarkably reduced, and minimum quench energy values at 4.2 K, 14 T were increased by a factor of three, indicating that stability was significantly improved. We also discussed the influences of the positioning of high-C substances and their thermal diffusivity on their effectiveness in reducing the superconductor temperature rise against perturbations. Based on these results, we proposed an optimized conductor architecture to maximize the effectiveness of this approach.

Recent progress of 122-type iron-based superconducting wires and tapes

With high transition temperature Tc (~38 K), high upper critical field Hc2 ( 100 T), superior transport Jc (~106 A/cm2) and extremely small anisotropy (1.5-2.0), the 122-type iron-based superconductors show great promise in high-field applications such as next-generation high energy physics accelerator and high-field magnetic resonance imaging (MRI). Power-in-tube (PIT) method is widely adopted to fabricate the iron-based superconducting wires and tapes due to low cost and easiness of large-scale fabrication. In the past few years, substantial efforts have been made to improve the transport performances of 122-type iron-based superconducting wires and tapes by ex-situ PIT technique. In this review, the recent progress of 122-type iron-based superconducting wires and tapes is presented. Firstly, we focus on the techniques for fabricating high-performance 122-type wires and tapes. We also discuss the key factors affecting the final performances of wires and tapes during the PIT process, including the preparation of high-quality precursor, the effect of chemical doping, the improvement of core density and grain connection. Recently, due to the improving of degree of c-axis texture and connectivity of grains, the transport Jc value of 122/Ag tapes reached 1.5105 A/cm2 at 4.2 K and 10 T, which exceeds the practical level of 105 A/cm2 and demonstrates their promise in high-field applications. Then, the progress of practical application of 122-type wires and tapes is summarized. In order to reduce the fabrication cost and improve the mechanical strengths of superconducting wires and tapes, an additional outer sheath such as Fe, Cu and stainless steel was used in combination with Ag. Besides, a favourable transport Jc was also obtained in the Cu-, or Fe-sheathed 122 tapes. For round wires, the highest Jc value reached 3.8104 A/cm2 in Cu/Ag composite sheathed wires at 4.2 K and 10 T, obtained by the hot-isostatic-press technology. From the viewpoint of practicality, the fabrication of multifilamentary wires and tapes is an indispensable step. The 7-, 19-and 114-filament 122 wires and tapes were successfully fabricated by the PIT method, and these multifilamentary tapes exhibited weak field dependence of Jc. Based on the experience of high-performance short samples and multifilamentary wires process, the scalable rolling process has been used to produce the first 115-m-long 7-filament Sr1-xKxFe2As2/Ag superconducting tape, confirming the great potential for large-scale manufacture. Moreover, the mechanical property, anisotropy and superconducting joint of 122 tapes are also studied. Finally, a perspective for the future development of 122-type wires and tapes in practical applications is given.

Microstructural study of brass matrix internal tin multifilamentary Nb3Sn superconductors

Zn addition to the Cu matrix in internal-tin-processed Nb3Sn superconductors is attractive in terms of the growth kinetics of the Nb3Sn layers. Sn activity is enhanced in the Cu–Zn (brass) matrix, which accelerates Nb3Sn layer formation. Here, we prepared multifilamentary wires using a brass matrix with a Nb core diameter of less than 10 µm and investigated the potential for further Jc improvement through microstructural and microchemical studies. Ti was added into the Sn cores in the precursor wire. Microchemical analysis showed that Ti accumulates between subelements consisting of Nb cores, which blocks Sn diffusion through this region when the spacing between the subelements in the precursor wire is a few microns. The average grain size was found to be about 230 nm through image analysis. To date, matrix Jc values of 1470 and 640 A/mm−2 have been obtained at 12 and 16 T, respectively. The area fraction of Nb cores in the filamentary region of the precursor wire was about 36.3%. There was still some unreacted Nb core area after heat treatment. Insufficient Ti diffusion into the Nb3Sn layers was identified in the outer subelements. These findings suggest that there is still room for improvement in Jc.

Computational homogenization of the elastic and thermal properties of superconducting composite MgB2 wire

Magnetic Resonance Imaging (MRI) background magnets are made from superconducting composite wires that are a metal matrix composite (MMC). Thermal and mechanical properties of the specific wire matrix must be known to optimize the wire configuration for MRI magnet designs. Computational analysis techniques based on numerical homogenization may provide an accurate characterization of a multifilament MMC wire and can reduce the expenses and time required for experimental tests. Recent developments in magnesium diboride (MgB2) superconducting wire has demonstrated their feasibility in a liquid helium (LHe) free conduction-cooled MRI magnet. Computational analysis of an entire superconducting magnet design requires the elastic and thermal properties of these wires. In this work, the temperature dependent elastic modulus, Poisson’s ratio, thermal expansion coefficients, thermal conductivity, and specific heat are estimated using Finite Element Analysis (FEA) and compared to analytical approaches. The orthotropic elastic and thermal properties of four different superconducting composite wire configurations are evaluated. The experimental thermal conductivity and specific heat at room temperature agree well with its corresponding computational results. FEA based computational homogenization has been demonstrated as an acceptable method to estimate the required material properties of the MMC superconducting wire for magnet analysis of MRI manufacturing and operation.

Efficacy of Tantalum Tungsten Alloys for Diffusion Barrier Applications

Traditionally either Niobium, Tantalum or a combination of both have been used as diffusion barriers in Nb3Sn Multi-filament wire. Vanadium has also been used successfully but the ultimate RRR of the copper is limited unless an external shell of Niobium is included. Niobium is preferred over Tantalum when alternating current losses are not an issue as the Niobium will react to form Nb3Sn. Pure Tantalum tends to deform irregularly requiring extra starting thickness to ensure good barrier qualities. Our evaluations showed Tantalum lightly alloyed with 3 wt% Tungsten is compatible with the wire drawing process while deforming as well as or better than pure Niobium. Ta3wt%W has been processed as a single barrier and as a distributed barrier to fine dimensions. In addition, the higher modulus and strength of the Tantalum Tungsten alloy improves the overall tensile properties of the wire.

Influences of the Bi2Sr2CaCu2Ox/Ag interface and interfilamentary bridge connections on AC loss of composite wires

The microstructure of Bi2Sr2CaCu2Ox (Bi2212) filamentary composite round wires (RWs) changes to be extremely complicated after heat treatment, with the interface between Bi2212 filaments and Ag matrix being meandering and a lot of bridge connections (BCs) generating between filaments. The influence of such the unique microstructure on the electromagnetic behavior and especially the AC loss has not been far from completely understanding. In this paper, according to the H formulation and on the nonlinear E–J relationship, we developed a finite element model, and further on this model evaluated the current and AC loss of a Bi2212 multifilamentary wire with the rough Bi2212/Ag interface and several BCs between filaments under the alternating magnetic field (Hex(t) = H0sin2πft). Through the comparative study, we found that the penetration magnetic field Hp is a critical turning point and on this base the influences of such the complex microstructure are quite different when the amplitude of applied magnetic field H0 < Hp and when H0 > Hp. The rough interface impacts the current distribution and causes the local heat concentrated at the concave locations. For BCs between Bi2212 filaments, it is most significant that they can share the current and make the current redistribute between filaments. Further for the wire with various types of BCs, the current distribution and the AC loss relative to the wire without BCs (Q/QmagnoBCs) were comprehensively investigated.

Status of MgB2 wire and cable applications in Europe

Since its discovery in 2001, MgB2 has generated interest for practical applications. Its availability in the form of multifilamentary round wire makes it suitable for production of cables. Together with relatively high critical temperature and potential low-cost, this renders it appealing for use in superconducting devices where its limited in-field performance can be tolerated. The state-of-the-art properties of commercially available wire and the potential of MgB2 conductors for use in superconducting systems are discussed. An overview of high-current electrical transmission projects where MgB2 has been proposed as an alternative to conventional Nb-Ti or High Temperature Superconductors is presented.

Properties of Multifilamentary MgB2 Wires Fabricated by Internal Magnesium Diffusion Using Amorphous and Crystalline Boron Powders

MgB 2 /Nb/Monel multifilamentary wires were fabricated using amorphous and crystalline boron powders by an internal Mg diffusion (IMD) process. The microstructure, morphology, and the critical current density (J c ) of the reacted MgB 2 layers in the multifilamentary wires made using different boron powders were analyzed and compared. The purity of boron powder influences greatly the transport properties of MgB 2 wires. The J c as high as 6.1 × 10 4 A cm -2 at 4.2 K, 10 T is obtained in the seven-core MgB 2 wire made using amorphous boron powder. The IMD-processed seven-core MgB 2 wire fabricated using high-purity crystalline boron powder has also a high J c of 4.9 × 10 4 A cm -2 at 4.2 K and 10 T. However, the seven-core MgB 2 wire prepared using low-purity crystalline boron powder has a relatively low J c compared with the others, which is expected to allow the fabrication of large-scale and low-cost superconducting wires for practical application. The further improvement of layer J c , nonbarrier J c and engineering critical current density J e is necessary because of the irregular subfilaments and the inhomogeneous thickness of the reacted MgB 2 layer in multifilamentary wires.

Synchrotron Microtomography Investigation of the Filament Microstructure in Differently Processed Bi-2212 Wires

The Bi-2212 microstructure and porosity distribution in entire state-of-the-art multifilament wires was characterized by high-energy synchrotron X-ray absorption microtomography after meltprocessing at ambient pressure, at 100 bar overpressure, and at ambient pressure after prior cold isostatic pressing of initially identical as-drawn wires. Beyond the visualization of the Bi-2212 microstructure, the porosity distribution in the different filament bundles has been quantified. Spherical granulometry and finite percolation probability calculations have been performed in order to provide a measure for the porosity distribution and to compare filament connectivity in the axial wire direction. Possible artifacts of the microtomography (μ-CT) experiment and the possibility to estimate the effect of the Bi-2212 microstructure as seen in the μ-CT images on critical current density are discussed.