Abstract
In high magnetic field magnets, especially those with high stability requirements, superconducting joints with ultra-low resistances play an important role in magnet fabrication. The Nb3Sn and NbTi joints were fabricated by the resistive welding technique. The Nb3Sn joint was fabricated by sintering the Nb-Sn precursors after mechanical alloying. Then the Nb3Sn bulk in the Nb3Sn joint was welded to the NbTi conductor. The microstructure and grain sizes of Nb3Sn bulk after long periods of time at high temperature were investigated. After welding, the joint was analyzed using X-ray nano-CT technology to display the connecting status and the defects between the Nb3Sn and NbTi conductor without destroying the joint. The electrical characteristics of the joints were measured under background fields through the field decay method. Results show that compared with the ones prepared by the commonly used solid matrix replacement technique, the prepared joint exhibits better magnetic field resistance and lower resistance at 1.5 T background field.
Superconducting joint is one of the most important components in the magnet system because superconducting wires are not long enough to wound the whole magnet due to limitations of process conditions. Typically, a superconducting magnet consists of several joints of the same (NbTi to NbTi, Nb3Sn to Nb3Sn) and different materials (Nb3Sn to NbTi). For large scale magnets, the numbers of joints reach dozens or even hundreds. The joints have different physical and chemical properties of the original materials, which results in joint resistance. The superconducting current flowing in the magnet generates loss when passing through joints and generates heat in joints, which causes further deterioration in the cooling operating environment. The superconducting joint significantly influences the magnet operation, especially for a high stability magnet operated in a closed loop mode. Every superconducting joint resistance and their sum in the magnet must be low enough to meet the requirements of current decay rate.
Two of the most commonly used low-temperature superconducting materials are Nb3Sn and NbTi. Nb3Sn has a A-15 intermetallic structure and can be fabricated either above 930 °C in the presence of a Nb-Sn melt or by solid state reaction
Various methods have been proposed for fabricating the superconducting joints such as solid matrix replacemen
In this study, the resistive welding technique was applied to the Nb3Sn and NbTi superconducting joints. Using this method, Nb3Sn wires can be connected with NbTi wires, providing more convenience and flexibility of NbTi material. The Nb3Sn and NbTi superconducting joints with ultra-low resistance were fabricated by the method and the obtained joins can work in high magnetic field above 1.5 T.
The ends of the Nb3Sn wires were immersed in nitric acid to remove their stable outer layers. The exposed filaments were then immersed in a mixed acid solution to etch the surfaces, and then cleaned by water and alcohol in sequence. The loose filaments were twisted to increase their contact areas and the reliability of the joint. Using a high energy mechanical alloying method, the Nb and Sn powders with a stoichiometric ratio of 3:1 were blended and alloyed to form the Nb-Sn precursors. The ends of the Nb3Sn wires were inserted into a mold that can join the filaments with the Nb-Sn precursors. Through the mold, the Nb3Sn wires and the Nb-Sn precursors were pressed into a block. Similarly, the other ends of the Nb3Sn wires were fabricated into another block. Then, the Nb3Sn wires with two blocks on their two ends were heat treated in the furnace. We adopted a heat treatment schedule the same as that for the original wire. The five dwell stages were 210 °C for 100 h, 340 °C for 50 h, 450 °C for 50 h, 575 °C for 100 h, and 650 °C for 150 h. After heat treatment, the Nb3Sn wires were connected with two Nb3Sn bulks on both ends. Due to its high plasticity, NbTi superconducting wire is more easily manipulated compared with Nb3Sn. The copper layer on the NbTi wires was removed by nitric acid to expose the NbTi filaments. Similarly, the NbTi filaments were etched and cleaned. The NbTi filaments were welded together with the Nb3Sn bulk through resistive welding technology. The current capacity was 10–20 A. Six spots were welded on both joints of NbTi and Nb3Sn.
The microstructure was characterized by scanning electron microscopy (SEM) and the sample composition was identified by energy dispersive spectroscopy (EDS) system. Phase identification of the samples was characterized by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation. The grain orientations and phase maps of the sample were examined by electron backscatter diffraction (EBSD). The sample plane was defined as the RD-TD plane (RD: roll direction; TD: transverse direction). ND (normal direction) was the direction perpendicular to the RD-TD plane. The Nb3Sn bulk after heat treatment at 650 °C for 150 h processed by focused ion beam (FIB) technique was submitted to transmission electron microscopy (TEM) examination. X-ray 3D computed tomography (CT) was utilized to characterize the connecting status and internal defects of the Nb3Sn-NbTi joints. The superconducting property of the sample was examined by a physical property measurement system (PPMS) up to 9 T. The Tc value was determined by choosing the first deviation point from linearity that signifies the transition from normal to superconducting state. The Nb3Sn-NbTi joint resistance was tested using the resistance measuring device based on the current decay method. The joint was immersed in liquid helium to achieve the superconducting state.
Fig.1 SEM image (a) and EDS results (b‒d) of Nb-Sn powders with a ratio of 3:1 after mechanical alloying of 5 h
Fig.2 SEM images of the samples after mechanical alloying (a) and heat treatment (b)
The XRD pattern of the obtained Nb3Sn bulk heat treated at 650 °C for 150 h shows that most of peaks can be indexed to the Nb3Sn structure (
Fig.3 XRD pattern of the bulk sample after heat treatment (a) and temperature dependence of the DC magnetic susceptibility curve in an applied field of 2388 A/m (b)
Fig.4 EBSD results of the sample heat treated at 650 °C for 15 h: (a) band contrast map, (b) phase map, and (c) grain orientation map in ND
The grain orientation map in ND indicates that the Nb3Sn grains have random orientations. The Nb3Sn grains have almost the same growth velocity, and equiaxed grains with crystallizing morphology. The small sizes of the Nb3Sn grains indicate that the grain growth is not severely affected by the long periods of time at high temperatures. After mechanical alloying, nanocrystalline grains are formed and have finer grain sizes and large amounts of boundaries that provide many evenly distributed nucleation sites. In addition, Nb and Sn atoms are uniformly mixed with 3:1 stoichiometry on the microscale.
Fig.5 Pole figures and inverse pole figures of the sample
To further investigate the microstructure of the Nb3Sn joint, the Nb3Sn bulk was subjected to TEM examination. The grain size of Nb3Sn has a significant influence on the superconducting properties of Nb3Sn conductors. Fine Nb3Sn grains produce a high density of grain boundaries, which can act as pinning on the centers of the flux and improve the critical current densit
Fig.6 TEM results of the sample heat treated at 650 °C for 150 h: (a) TEM image of Nb3Sn grains; (b) TEM image and EDS map of Cu, Nb and Sn overlays; (c) EDS mappings of Nb and Sn; (d) EDS spectrum of the sample surface
Fig.7 TEM results of Nb3Sn grains heat treated at 650 °C for 150 h: (a) TEM image, (b) SAED pattern, and (c‒d) HRTEM images
X-ray CT is a nondestructive testing method and can realize the online analysis without destroying the joint.
Fig.8 Nano-CT scanning images of cross section (a) and six spots (b) for NbTi-Nb3Sn joint
Fig.9 Nano-CT scanning images of welding spots of longitudinal section (a) and obvious crack (b) for the NbTi-Nb3Sn joint
The joint resistance of Nb3Sn and NbTi was measured at 4.2 K under 1.5 T background field. The joint loop was charged to 100 A using a superconducting powder supply.
Fig.10 Magnetic field variation in the closed loop against time under 1.5 T background field
1) Superconducting joints between Nb3Sn and NbTi conductors can be fabricated by the resistive welding technology. The Nb3Sn joints are first synthesized by sintering the Nb-Sn precursors after mechanical alloying.
2) The Nb3Sn joints have small grain sizes without excessive growth after long periods of time at high temperature. The Nb3Sn joint is successfully welded together with the NbTi conductor, although several cracks appear below one spot in Nb3Sn.
3) The joint of Nb3Sn and NbTi has ultra-low resistances of 1.3×1
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