Effect of interface angle, added powder and applied deformation on the transport current and structure of scarf joints of single- and multi-core unreacted MgB2 wires

Magnesium diboride (MgB2) is a promising superconductor for many applications. One of the advantages of this material, is that it is possible to join single or multi-core MgB2 wires using MgB2 joint, obtaining a connection with very low or zero electrical resistance suitable for persistent mode operation of devices, such as high field superconductor magnets. The wires are joined by a scarf joint, which can be minimized to comparable size as the wires, making it possible to use it directly in magnet windings. In Figure 1. is depicted the scarf joint fabrication process – first, the wires to be joined in the scarf joint are polished to shallow angle. Afterwards, one wire is inserted into a metallic capillary and Mg+B powder mixture is added from the other end. The structure is then enclosed by inserting the second wire. The whole joint is pressed together and heat treated to form MgB2 phase from the Mg+B powder to obtain superconducting connection between the wires.

Figure 1: Left) Scarf joint fabrication process Right) Cross-sections of the MgB2 wires used in the joints

A single-core MgB2/Fe wire and six-filament MgB2/Nb/Fe wire joints were prepared and studied in this study. The wires were cut and polished at an angle of 10° – 15°. The joint was covered in split-dies and placed into 100 kN hydraulic press. The resulting deformation is controlled by changing the wall thickness of the joint capillary. The transport properties of the wires and joints were measured at 4.2 K and at 2-8 T external magnetic field. The joint’s resistive transition was measured at temperatures between 27 – 42 K.  Optical microscopy studies provided necessary information about the interface between the wires (Fig. 2). The interface was optimized by adjusting the deformation of the joint, particle size of the Mg+B powder and by increasing the area between the joined wires. After these optimizations, Imrich Hušek and his team made single-wire joint with 73 % of the wire’s critical current and in the case of 6-filament wires the joint had 53 % of the wire’s critical current. The results show that the presented joint technique could be used for superconducting MgB2 coils in a persistent mode.

Figure 2: Optical microscopy study of the joint interface

Authors: Imrich Hušek, Pavol Kováč, Tibor Melišek, Dušan Berek, Ľubomír Kopera

Link: https://iopscience.iop.org/article/10.1088/1361-6668/ad5113