Multifilamentary Wires Research Articles

High critical current properties of multi-filamentary MgB2 superconducting wires fabricated using an internal Mg diffusion method

Although both the mass density and grain connectivity of MgB2 superconducting layers can be greatly improved via an internal Mg diffusion (IMD) process, the poor structural uniformity and low MgB2 filling factor of IMD wires limit further enhancement of their superconducting performance. Herein, we prepared 19-filament and 37-filament IMD-MgB2 superconducting wires using a combination of optimization of the component structure and the introduction of an intermediate annealing process. Microstructure analysis suggests that good structural uniformity and high layer density have been achieved in the multi-filamentary MgB2 wires, and the MgB2 filling factor reaches 9.3%–11.0%. The magnetic superconducting transition of MgB2 wires is relatively sharp, and the onset T c is around 37 K. Remarkably, there is no magnetic flux jump for 37-filament wires in the low-field region at 5 K. At 4.2 K and 4 T, the transport layer J c values of 19-filament and 37-filament MgB2 wires are as high as 1.5 × 105 A cm−2 and 2.2 × 105 A cm−2, respectively, with, accordingly, engineering J e values of 1.7 × 104 A cm−2 and 2.0 × 104 A cm−2. These results indicate that the performance of multi-filamentary IMD-MgB2 wires can compete with traditional powder-in-tube-MgB2 wires applied in industry.

Density dependent constitutive model for Bi-2212 powder compression deformation

Bi-2212 HTS materials are fabricated into multi-filamentary wires via powder-in-tube (PIT) method followed by proper heat treatment to obtain superconductivity, but how to predict the large compression deformation behaviors of the Bi-2212 powder is critical to design the processing of the Bi-2212 HTS wire. Drucker Prager/Cap (DPC) model was the most commonly used model for powders including Bi-2212 with soil-like mechanical behavior to consider its shear failure as well as hydrostatic compression. However, the parameters for DPC Cap evolve with densities change and the original model is inadequate to precisely describe the densification process of Bi-2212 powder with large strain. In this study, the modified DPC model with density dependent parameters was introduced for Bi-2212 powder compressions by measuring the failure strength and hydrostatic compressive behavior under different density states. The DPC yield surface was plotted with an evolution trend of non-linear outward expansion with density increased. FEM model of uniaxial compression based on the as-introduced model was built with subroutine VUSDFLD applied. The distribution of Mises stress and relative density were analyzed. The axial stress-density curve for FEM and experimental results were normalized and quantitively evaluated by Mean Square Error (MSE). The introduced model shows good convergence and could match the experimental results well with normalized MSE of 0.000207 and Root Mean Square Error (RMSE) of 0.0144, indicating the mean error percentage of 1.44%. The model introduced in this article provides supports toward large strain deformation simulation of Bi-2212 powder.

The Structural Examination of Fe/(Cu/Nb)/MgB2 Multifilament Wires During Cold Forming Process

In this study, we have successfully produced a Fe-sheathed 6+1 multifilament wire using Cu/Nb/MgB2 monocore wires. The mono filament wire was prepared using Mg+2B powder mixture by powder-in-tube method without any intermediate heat treatment. The powder mixture of the amorphous nano boron (PVZ Nano Boron, purity of 98.5%, particle sizes < 250 nm) and high purity Mg powder (PVZ Mg, purity of 99%, particle size 74µm) were used. The multifilament wire was produced using groove rolling and cold drawing machines. The geometrical form of the filaments was examined using wire pieces taken from the wire at different steps throughout the production process. Finally, the multifilament wires produced in two different diameters of 1.02 mm and 0.82 mm were investigated in terms of filament uniformity, crack formation, surface roughness, and electrical transport properties. The structural examination was done on Nb filaments after the Fe and Cu sheaths were etched using HCl and HNO3 solution. The I – V measurements of the multifilament wires heat treated at 650 °C for 15, 30, 45, 60, and 240 minutes, and 700 °C for 60 minutes were carried out for the applied current up to 1 A at 25 K under various external magnetic field.

Development of persistent joints for superconducting Bi-2212 coils

Bi2Sr2CaCu2O8+x (Bi-2212) multifilamentary wire is the only high-temperature superconductor manufactured in the form of an isotropic round wire, and so offers a number of advantages for the designers of high field magnets. However, for high-field (>25 T), high-stability magnet applications, ultra-low resistance superconducting joints (R < 10−12 Ω) will be needed to take advantage of the excellent properties of the Bi-2212 wire. This study focuses on the fabrication of compact melt processed joints in small coils of Bi-2212/Ag multifilamentary round wires and the testing of their superconducting performance by inductive resistance measurements. Microstructural analysis is carried out to correlate the microstructure to the superconducting performance of the joints. Our optimized technique led to a reliable process for the preparation of small coils with melt processed joints that occupy very small volumes but can still carry the highest persistent currents reported so far for Bi-2212.

Revealing the effect of bridges in the multi-filamentary Bi-2212 wires

As the excellent current carrying capacity in ultra-high magnetic fields, Bi-2212(Bi2Sr2CaCu2O8+x ) has attracted much attention. In view of the unique nature of Bi-2212, high-temperature heat treatment is highly needed for the formation of the continuous textured structure in the multi-filamentary Bi-2212 wire. In the meanwhile, bridges between filaments can also be produced in the heat treatment process, which brings structural changes in the wires. However, the mechanism of the effect of the bridges is yet to be understood. In this work, systematical research was carried out on the multi-filamentary Bi-2212 wire for the effect mechanism. Based on the results of the four-probe method and the magnetic moment results, inferiority in Ic combined with a larger magnetic moment at high magnetic fields was detected in samples that have a large number of bridges. Further analysis indicates the inferiority in Ic can be attributed to the reduced cross-sectional area of the filaments and the inferiority in the quality of texture on the basis of results of a series of structural characterizations.

High transport Jc performance by enhancing grain coupling in (Ba,K)Fe2As2 multi-filamentary wires

High transport Jc performance by enhancing grain coupling in (Ba,K)Fe2As2 multi-filamentary wires

Ic measurement of twisted multifilamentary MgB2 wires with non-magnetic sheath over a wide range of temperatures and fields

All-superconducting rotating machines have the potential for meeting the high power density and high efficiency required for electrical aircraft applications. However, very high AC loss encountered in superconducting armature windings could hinder their development. Multifilamentary MgB2 wires are one of the promising candidates for the stator windings, due to their potentially low AC loss properties with small filament size and twist pitches. As the first step, the dependence of critical current and n-value on magnetic fields and temperatures Ic(B, T) and n(B,T), which are basic input parameters for AC loss simulation, needs to be measured. In this work, we present transport Ic measurements in three non-magnetic multifilamentary MgB2 wires (MgB2/Nb/CuNi/CuZn): one large wire with a 0.70 mm diameter and 25 mm twist pitch, and two small wires with a 0.48 mm diameter each and a 10 mm and 30 mm twist pitch respectively. A four-probe direct current method is used to measure Ic of the MgB2 wires with variations in temperature (15 – 35 K) and magnetic field (0 – 5.5 T). Full Ic data for the small wire with 10 mm twist pitch was obtained, and the n-values were mostly less than 20. While the Ic data for the large wire at low fields was more limited due to heating, the n-values were higher and could be up to around 100. The difference is attributed to the different filament sizes. Experiments also found that there is no significant hysteresis in the transport critical current measured by decreasing or increasing magnetic fields due to the non-magnetic sheaths. This non-hysteretic characteristic is critical for lowering AC loss because the additional losses from magnetic sheaths can be eliminated. From the magnetic-field dependence of critical current density, an empirical expression has been developed that provides suitable extrapolations to lower fields for the large wire.

Phase formation and transport properties of vapor-solid reacted AIMI MgB2 superconductors

Vapor-solid reactions were applied to monofilament and 6-filament AIMI MgB2 wires. A layer Jc of ∼ 105 A cm−2 was achieved by the generation of a highly dense MgB2 layer after reaction at 625 °C for 8 to 16 h. It was found that the critical current, Ic, and engineering current density, Je, reached maximum values of 92.3 A and 1.15 × 104 A cm−2 at 4.2 K, 10 T when the multifilament AIMI wire was heat-treated at 625 °C for 14 h. Transport Ics were also measured at 10–20 K for the wire heat-treated at 625 °C for 16 h. Phase formation investigations suggested that even better performance of Ic and Je could be attained in multifilamentary AIMI MgB2 wires after further conductor optimization.

Dominant effect of residual secondary phase of powders on Jc and microstructure of Bi-2212 superconducting wires

Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting round wires exhibited great potential for use in high-field applications. The purity of the precursor powders is critical for the transport current of the wires. However, the role of the residual secondary phase in the precursor powders is not fully understood. Here, the origin of the secondary phase was investigated in precursor powders that were prepared using ultrasonic spray pyrolysis (USP) and calcination processing. The microstructure and phase evolution of the precursor powders during the crystallization process were analyzed. Moreover, the effects that the residual secondary phase has on melting behavior, morphology properties, and the supercurrent flow of Bi-2212 multi-filamentary wires are systematically discussed. The residual secondary phase in the filament caused further crystallization, and this led to the formation of more and larger Bi-2201 grains at the onset of the melting process. The poor microstructure and low critical current of the final Bi-2212 wires can be attributed to the presence of the residual copper-rich phase. Bi-2212 wires that were prepared with fully crystallized powders had a high critical current density (Jc) of 6773 A/mm2 at 4.2 K, self-field. It was revealed that control of the secondary phases in precursor powders is greatly significant for achieving superior values of Jc.

Cryogenic Electrical Properties of an Aluminum-Beryllium Nanocomposite

Many power distribution networks are desiring lightweight electrical conductors. Cryogenic hyperconducting aluminum (99.999%+ pure) is a competitive option to HTS cables at 20 K, but these cables require mechanical reinforcement for many applications, and this strengthening must be compatible with aluminum's annealing schedule to prevent impurity diffusion. AlBeMet AM162H is a lightweight Al-Be nanocomposite which can be processed similar to aluminum alloys, and similar to hyperconducting aluminum, does not experience the extreme quench characteristics seen in superconducting composites. In this research, we shall investigate the electrical conductivity of cryogenic AlBeMet AM162H, and compare its mass specific engineering current carrying capacity with hyperconducting aluminum composites and high purity copper. Additionally, the AC losses of AlBeMet AM162H will be compared to other cryoresistive metals and state-of-the-art low AC loss BSCCO-2212 and MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> multifilamentary wire.

Fast time-varying current triggering flux jumps of multi-filamentary Nb3Sn wire exposed to oblique magnetic field

Due to the oblique magnetic field and rapid time-varying current, the electromagnetic response and thermomagnetic instabilities of high-field superconducting dipole magnets are quite different from the solenoid coils. In this work, we theoretically investigate the flux jump of composite multi-filamentary Nb3Sn wire with high critical current density exposed to an oblique magnetic field and fast transport current. The thermomagnetic stability/instability regions are obtained with respect to flux creep factor and oblique ratios of magnetic field. It is found that the parallel component of the oblique magnetic field can suppress the flux jump. Unlike slow current, it is interesting to find that the fast variations of self-field by high ramp current can trigger flux jumps in Nb3Sn wire. The fast current triggering flux jumps can be tuned by the static oblique magnetic field. Furthermore, we demonstrate that current-like distribution is more likely to trigger quenches, while the magnetic field-like distribution is more likely to trigger flux jumps. The findings of this work are helpful for the optimization of the superconducting coils exposed to oblique magnetic field and fast time-varying current.

Ac susceptibility studies of intra- and intergrain properties of high- Jc Bi-2212 wires

${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{CaCu}{\mathrm{O}}_{x}$ (Bi-2212) is the only high-${T}_{c}$ superconductor (HTS) available as a multifilamentary round wire with multiple architectures and it is a very promising conductor for the realization of high-field applications. Despite their relatively simple wire fabrication by the powder-in-tube technique, Bi-2212 wires require a tightly controlled overpressure heat treatment (HT) with a multiparameter time-temperature schedule to achieve high critical current density, ${J}_{c}$. The variation of these HT parameters, changes in the wire design, wire diameter, and powder quality can lead to variations in both the microstructure and the superconducting performance. Particularly noticeable are variations in ${J}_{c}$ performance and degree of filament bridging. In this work, we focus on the use of different magnetic characterization techniques to estimate the bridging level and assess the balance of intergrain and intragrain superconducting properties including the irreversibility field (${H}_{\mathrm{irr}}$) and the pinning energy (${U}_{o}$) in differently processed wires. Regardless of the actual bridging level, we find that the supercurrent flows at the filament bundle level, not just at the individual filament level. Moreover, using ac susceptibility we identify two distinct supercurrent contributions, one related to the intragrain and one to the intergrain properties, whose irreversibility fields are different but without large sample-to-sample variation. Moreover, an additional component of intragrain pinning mechanism becomes effective at low temperatures with positive effects also on the intergrain performance. The work clearly shows that detailed magnetic characterizations can become valuable tools to investigate the performance of differently processed Bi-2212 wires, correlating their microstructure and overall transport ${J}_{c}$, to obtain a deeper understanding of the causes of performance variation and paths to achieve further improvement.

Improvement of the multifilament wire lager for cable production

The article discusses a possible way to solve the urgent problem of cable production - increasing the efficiency of the technological process by modernizing the control system of technological equipment by developing an intelligent system that allows to improve the reliability of the technology as a whole. The stranding equipment for the implementation of the stranding technology has been chosen as the object of the study, namely, a multifilament wire lager. This equipment is used in the production process of manufacturing cable and wire products, in particular the production of cable semi-finished products – strands. The work done on the modernization of the multifilament wire lager due to the introduction of the intelligent control system has resulted in outputs that provide high flexibility of the technology, which shall meet modern quality requirements of finished cable products.

Mechanical strain triggering flux jumps of multi-filamentary Nb3Sn wires

The composite multi-filamentary Nb 3 Sn wire with a high critical current density is a preferred option for fabricating the superconducting magnet beyond the limit of NbTi wire (9-16 T). However, one crucial issue stems from the fact that electromagnetic force in superconducting coils is very strong, and the critical physical properties of Nb 3 Sn, such as J c , are more sensitive to mechanical strain than those of other possible low-temperature superconductors. We theoretically investigated the impact of mechanical strain on the thermomagnetic instabilities such as the flux jump (FJ) and quenching of Nb 3 Sn wire exposed to a static magnetic field and transport current. The good agreements with H formulation or H-φ formulation implemented on COMSOL software confirm the validity of our numerical simulations using home-made codes. It is discovered that mechanical strain can trigger flux jumps even in a static magnetic field. Furthermore, the threshold value of mechanical strain to trigger the first flux jump is a monotonic function of the static magnetic field in the case of high transport current, while it is a non-monotonic function in the case of low transport currents. It is attributed to the fact that flux can be released by the mechanical strain first, causing smooth flux penetration before triggering the flux jump. We also present the stable/unstable regions by applying mechanical strain by varying transport current, magnetic field, and working temperature, which helps in avoiding thermomagnetic instabilities while designing the superconducting magnet.

Modulation of superconducting grain structure to achieve high critical current in Ba0.6K0.4Fe2As2 multifilament round wires

Iron-based superconducting wires fabricated by the powder-in-tube method are promising candidates for practical applications due to their prominent high-field performance. The morphology, compactness and alignment of the superconducting grains, which are mainly determined by the deformation and post-sintering strategy, have a large significance for the current carrying ability. Herein, we apply a stainless steel/AgSn/Ag composite architecture combined with a scalable groove rolling process to fabricate high-performance iron-based superconducting multifilamentary round wires. Highly dense superconducting filaments are obtained without hot-isostatic-pressing. A critical current density Jc= 3.02 × 104 A/cm2 at 4.2 K and 10 T is achieved, which is the highest value ever reported in iron-based multifilament wires. Through detailed microscopic and theoretical analysis, we conclude the deformation mechanism of Ba0.6K0.4Fe2As2 grains and discuss its influence on the supercurrent transport properties. We suggest that higher Jc can be achieved by introducing a better grain texture. Our method provides a simple and cost-effective way to densify practical superconducting wires and is very promising to be scaled up to long wire productions.

Influence of annealing temperature and isostatic pressure on microstructure and superconducting properties of isotopic Mg11B2 wires fabricated by internal Mg diffusion method

Here, we report superconducting Mg11B2 wires made by using the internal Mg diffusion technique with isotopic amorphous nano boron (11B) as the precursor material. We show the influence of annealing temperature and isostatic pressure of 0.1 MPa and 1.1 GPa on Mg diffusion into 11B layer, microstructure of superconducting filament, critical current density (Jc) at 20 K and 25 K, critical temperature (Tc) and irreversible magnetic induction (Birr) in mono (single-core) - and multi-filament Mg11B2 wires. Our research shows that thermal treatment at 700 °C and 0.1 MPa for 60 min yields a superconducting phase with low Tc, Birr and Jc in single-core Mg11B2 wire. A higher annealing temperature (740 °C and 0.1 MPa for 60 min) significantly accelerates the diffusion of Mg into the 11B layer and increases the Tc, Birr and Jc. However, the distribution of Mg in 11B layer is very heterogeneous (places with high and low Mg concentration). This leads to heterogeneity in the superconducting material and inhomogeneous Tc, Birr and Jc. Further studies showed that higher annealing temperatures at 770 °C and 800 °C significantly accelerates the diffusion of Mg into the 11B layer and leads to the increase of Tc, Birr and Jc in single-core Mg11B2 wire. In single-core wire annealing treatment under isostatic pressure of 1.1 GPa for 60 min at temperature of 800 °C it causes Mg penetrates into the 11B layer very heterogeneously and large amount of 11B remains unreacted. However, the same heat treatment at 800 °C and 1.1 GPa for 60 min in multi-filament wires leads to a complete Mg synthesis reaction with the 11B layer. This leads to increase Birr and Jc. This work shows that the formation and accumulation of pinning centers is essential to further increase Jc in Mg11B2 wires.

Microstructure and Critical Current Density of Bi-2212 Single- and Multi-filamentary Wires with Overpressure Heat Treatment Processing

Microstructure and Critical Current Density of Bi-2212 Single- and Multi-filamentary Wires with Overpressure Heat Treatment Processing

Jc Performance Under Transverse Compressive Stress of Internal Matrix Reinforced Nb3Sn Multifilamentary Wires Using Various Ternary Bronze Alloys

We approached internal matrix reinforcement using Cu-Sn-Zn and Cu-Sn-In ternary bronze alloy matrices for enhancing the effective mechanical strength of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire. In this study, we evaluated the critical current density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> ) degradation due to transverse compressive stress on internal matrix reinforced Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn multifilamentary wires using these ternary bronze alloy matrices, and the effects of both Zn and In solute element on the changes of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> property under compressive stress was investigated. In the cases of internal matrix reinforced Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires using the Cu-Sn-Zn and Cu-Sn-In ternary alloy matrices, no <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> degradation was observed within 100 MPa of the transverse compressive stress. The (Cu, Zn) and (Cu, In) solid solution matrices contributed to suppressing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> degradation by the compressive stress. In addition, the In element was the more effective solute element in the ternary alloy matrix on the internal matrix reinforcement based on the irreversible compressive stress estimated by the loading-unloading test.

Novel High-Tin Bronze and Nb3Sn Multifilamentary Wires

We developed a method for manufacturing a high Sn concentration bronze material with high ductility. The high-Sn bronze was used to produce multifilamentary wires, and their characteristics were evaluated. To increase the Sn content of the bronze and ensure ductility, the δ-phase (Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">41</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> ), which is a eutectoid when the Sn content is greater than the Cu-Sn solid-solution limit, was replaced with a ternary compound by the addition of Ti. The compound was further refined by hot forging to develop ductility. In this study, 1615-filament wires were produced using the high Sn concentration bronze and a commercial bronze for comparison. Each wire was heat treated with different conditions, and their superconducting properties were evaluated. The area ratio of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn phase was higher with the high-Sn bronze alloy irrespective of the heat-treatment temperature. The critical current density was improved for the high-Sn bronze for heat treatment at 625 °C or lower, but when the treatment was performed at 650 °C, the Ti concentration in the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn phase exceeded 2.0at% and the upper critical magnetic field deteriorated dramatically.

Relation between constituent material fraction in multifilamentary MgB2 wires and requirements for MRI magnets

Magnetic resonance imaging (MRI) occupies the largest segment of the commercial applications of superconductivity. The NbTi wire is typically applied to MRI magnets and fulfils their strict requirements. On the other hand, the dramatically large energy margin in the MgB2 wire is attractive for liquid helium-saving MRI magnets. However, there are many types of cross-sections in the MgB2 wires. This makes it difficult to analyse the applicability of the MgB2 wires to the MRI magnets systematically. This paper focuses on the in situ MgB2 wires with an iron matrix and a Monel reinforced member. Multiple evaluations are conducted for several types of MgB2 wires, and their applicability to the MRI magnets is discussed. Because the critical current density of the superconducting filaments does not largely depend on the cross-section of the wires, the engineering critical current density (J e) is roughly proportional to the superconducting fraction (λ sc). The acceptable bending strain of the heat-treated wires increases with the Monel fraction and is in the range of 0.3%–0.65%, which is larger than the value required for coil winding of the MRI magnets. Two types of protection approaches of the magnet are considered. One is an active protection. This approach requires a large fraction of the copper stabilizer in the cross-section of the wire and relatively reduces λ sc and J e. The other is the avoidance of quenches over the product lifetime using quick ramp-down of the magnet for unfortunate events, such as cooling system failure and emergency rundown. This approach requires no copper stabilizer and increases λ sc and J e thus widens the acceptable operational temperature range. The cross-section of the MgB2 wire can be designed with a certain level of freedom depending on its functional requirements.