Due to its short coherence length, two-dimensional anisotropy and large thermal fluctuation at elevated operating temperatures, high Tc superconductors (HTS) show quite different electromagnetic properties from those of conventional lowtemperature metallic superconductors. In this paper, the peculiarity of critical currents and related phenomena in HTS, such as grain connectivity, spatial non-uniformity and large flux creep, are described in terms of the relationship to in-field currentvoltage characteristics, vortex glass-liquid phase transition and irreversibility field.
Characteristics such as the critical current density (Jc) and mechanical property of bronze-processed Nb 3 Sn superconducting wires have improved with the progress on high-field superconducting magnets such as nuclear magnetic resonance (NMR) magnets. On the other hand, for Nb 3 Sn wiresused in intermediate magnetic-field applications such as fusion and particle accelerator magnets, the wire parameters have been optimized to realize high Jc around 12 T. In this article, improvements in the Jc and mechanical property of bronze-processed Nb 3Sn wires based on the activities of Japan Superconductor Technology, Inc. (JASTEC) and Kobe Steel, Ltd. are described.
Bronze-processed Nb 3 Sn wires have been used in practical applications of high-field magnets and large-scale magnets. Since the 1980s, Furukawa Electric Co., Ltd. has been developing and manufacturing many high-performance bronzeprocessed Nb 3 Sn wires. This paper describes our recent activity in development of the bronze-processed Nb 3 Sn wires. The nonCu critical current density of the ITER-type strand was 1,150 A/mm2 at 12 T and 4.2 K, which is 1.8 times higher than that of the ITER-CS model coil 15 years ago. High-strength Nb 3Sn wires reinforced with Cu-Ni/Nb-Ti or Cu-Nb composites have been successfully fabricated for high-field magnets. Enhancements of the superconducting properties resulting from pre-bending effects were demonstrated with Cu-Nb reinforced Nb 3 Sn wires. Mass production of practical cables for JT-60SA CS coils have been completed in exact accordance with the specifications. In addition, experimental manufacture of ITER-CSJA1 cables was properly carried out. Furthermore, a novel type of aluminum-alloy jacketed Nb 3Sn conductor has been developed using the friction stir welding (FSW) technique.
Nb 3 Sn superconducting wires are very important for superconducting magnets generating high fields of more than 10 T. From the viewpoint of practical use, one of the serious problems for Nb 3Sn wires is enumerated that their superconducting properties are sensitively deteriorated due to the applied strain, when a large electromagnetic force is added to the high field superconducting magnet. Recent experimental results suggest that the three-dimensional strain state has to be evaluated to understand the strain dependence of superconducting properties for Nb 3 Sn wires. In this article, the strain effects for Nb 3 Sn wiresin high magnetic fields are introduced. The physical background and a three-dimensional strain model for the strain effect on Nb 3 Sn wires are explained in order to understand the strain dependence of superconducting properties for Nb 3Sn.
Vanadium-gallium (V 3 Ga) superconducting wire is an “old superconducting material", and was one of the original materials famed for the “Cu additive effect" that was extended to the “Bronze route process". The “Cu additive effect" in A15 phase compounds promotes A15 phase formation via diffusion reaction. The V 3 Ga compound has interesting properties for an advanced magnetic confinement fusion reactor beyond ITER. The decay time of induced radio activity for V 3Ga is within 1 month and is much shorter than that of Nb-based superconductors such as Nb-Ti, Nb 3 Sn and Nb 3 Al. We thought that V 3 Ga wire was one of the candidate materials for “Low activation superconducting wires" to operate under a neutron irradiation environment such as in a fusion reactor. However, the Jc and Hc2 properties of V 3 Ga wire are insufficient to realize this feature in fusion application. In previous studies, V 3 Ga wire was mainly investigated in term of the “Diffusion process" between Cu-Ga within a 20 at% Ga composition and V filament. For further Jc and Hc2 enhancements, we investigated the fabrication of V 3 Ga compound multi-filamentary wires using a high Ga content Cu-Ga compound applying the powder-in-tube process. Thicker V 3 Ga layers formed along the boundary between the Cu-Ga powder filaments and V matrix, and the volume fraction of V 3 Ga increased compared to previous diffusion processed samples. We also found that the new route PIT process using a high Ga content Cu-Ga compound is effective for improving the superconducting properties of the V 3 Ga compound wire.
This article describes the present status and R&D issues related to the rapid heating/quenching and transformation (RHQT)-processed Nb 3 Al superconducting wires. The RHQT-processed Nb 3 Al wires were first proposed by Y. Iijima, et al. in 1994. Eighteen years have already passed, and this wire is still undergoing R&D towards the realization of practical superconducting wires. The RHQT-processed Nb 3 Al wires are made utilizing three unique fabrication stages: jellyrolled precursor fabrication, rapid heating/quenching treatment and electroplated copper stabilizer fabrication. Recently, even though a RHQT-processed Nb 3Al wire with copper stabilizer of over 1 km in length was fabricated, several difficult problems still remain to be resolved before industrial mass production is possible. In the stage of precursor fabrication, much improvement in the mechanical cold-workability of tantalum raw materials is required in order to avoid wire breakage. Additionally, the homogeneity of the cross-section of the precursor wires in the longitudinal direction needs to be improved for carrying out the homogeneous rapid heating/quenching treatment. The copper stabilizer for the RHQT-processed Nb 3 Al wires is fabricated using high-speed reel-to-reel electroplating. Furthermore, reel-to-reel copper ion-plating with a thickness of 1 μm is performed before the thick electroplating, thereby obtaining good bonding between the Nb 3Al wire and copper stabilizer. The maximum current density during electroplating is currently 40 A/dm2, and approximately one week is needed to electroplate 1 km of Nb 3Al wire with a 1.0 mm outer diameter and 1.0 Cu/non Cu ratio. Future R&D will focus on significantly shortening the electroplating time.
The transformation-processed Nb 3Al superconductor is a promising alternative for large-magnet applications; for example, for nuclear fusion magnets and accelerator magnets. In this review, some issues that should be solved for industrialization and their possible solutions are introduced. The review focuses on improvement of the longitudinal uniformity of the conductor, a facile stabilization method, minimization of magnetization compatible with the high critical current density and the characteristic microstructure of transformation-processed Nb 3Al superconductors related to the pinning property.
The internal-tin technique is an excellent method for fabricating Nb 3 Sn superconducting wire with a high critical current density. We have developed a new type of internal-tin Nb3Sn wire in which the multi-filamentary wire is fabricated by combining mono-filamentary Nb elements and mono-filamentary Sn elements. The simple fabrication process will enable the fabrication cost of the wires to be reduced. By optimizing the spacing of the Nb 3 Sn filaments, we have obtained an internal-tin wire with high critical current density and low filament coupling. Additionally, a long wire, 5 km in length, has been fabricated, which indicates that manufacturing of mass-production-scale wires is possible using the newly developed internal-tin process.
Copper-Tin (Cu-Sn) bronze alloy is the key material for bronze-processed Nb 3 Sn superconducting wires. The Osaka Alloying Works in Japan has established a unique melting process for large-scale bronze alloys called the “Mizuta method". In this process, the graphite crucible containing hot molten metal is cooled by passing it directly through a water shower. A general mold casting is not needed, so only slight oxidation of the hot molten metal occurs. Since a uni-directional solidification process is used, a very homogeneous tin concentration without severe inverse segregation in the longitudinal direction of the ingots is obtained. In this paper, we investigate the microstructure and mechanical performance details of the practical bronze having Sn concentrations of 14, 15 and 16 mass%. Elongation, 0.2% toughness, Vickers hardness at room temperature, and the limit of continuous cold-drawability as a function of intermediate annealing were studied. Degradation of cold-drawability was excessive, with a rather low annealing temperature of 400oC. We found that numerous fine precipitates appeared at the intra- and inter-alpha grains after low-temperature annealing. The transmission electron microstructure analysis revealed that those precipitates were the delta phase (Cu41Sn11). The delta precipitates may behave as a pinning site of the slip motion for the plastic deformation of bronze alloys. We have to re-recognize that the intermediate annealing temperature is a very important parameter for avoiding wire breakage during the industrial production of bronze-processed Nb 3 Sn superconductors.
Although the rapid heating and quenching (RHQ) optimization process is a key to achieving high critical current density (Jc) through controlling the microchemistry and grain structure of a bcc supersaturated-solid solution and subsequently transformed (T) Nb 3 Al phases, only the RHQ parameters of wire diameter (dwir e ), wire speed (vwi re ) and heating current (IRHQ) for a given electrode spacing (Lelectrode) have been investigated. A heating time (τ) of a focused wire position was given by Lelectrode/vwir e. Thus, a larger τneedes a smaller vwir e, which also causes a slower wire cooling rate that partly forms the undesirable A15 phase during RHQ treatment. In the present study, an attempt has been made to extend the Lelectrode from 100 mm to 470 mm. This enables both a large τand a relatively large vwir e , which ensures a cooling rate sufficient to obtain the bcc supersaturated solid solution. The Nb/Al precursor used is a single-filament jelly roll (JR) wire in which the matrix (sheath) species is Ta and the Al layer thickness is 600 nm (3-6 times thicker than conventional multifilament JR precursors). A large τresults in an increase in the volume fraction of the bcc supersaturated solid solution. The critical temperature (Tc) and Jc optimized by RHQ treatment for each τare found to increase in proportion to τ. The plastic deformation carried out between RHQ and transformation treatments is effective for enhancing the Jc properties, even for RHQ parameters of large Lelectrod e and τ .