Optimized superconducting MgB2 joint made by IMD technique

Abstract

A novel type of superconducting joining technique has been introduced to join unreacted internal Mg diffusion (IMD) single-core MgB2 wires. Our method is based on fabricating a small diameter joint mould obtained by deforming an Nb/Cu composite tube with a longitudinal semi-cylindrical Mg and B core into a thick round wire. The small diameter of the joint provided advantages such as rapid cooling, low resistance, and the unique core design inside the joint ensured a uniform MgB2 phase formation. Scanning electron microscope analysis revealed that the IMD MgB2 wires had excellent contact with the superconducting MgB2 bulk material inside the joint. The joint resistance, calculated from the decay of the trapped magnetic field over time, is a quite low value of 6.44 × 10−16 Ω at 20 K. The transport critical current (I c) of the joint is 62 A at 20 K under a self-magnetic field, and the n-value of the joint is 66 at 20 K under 1.5 T. The results showed that the I c of our joint can be determined precisely, regardless of whether the magnetic field is applied from low to high or from high to low value during I–V measurements.