High-performance MgB₂ superconducting wires for use under liquid-helium-free conditions fabricated using an internal Mg diffusion process

MgB₂ has a superconducting transition temperature (T _c) of 39 K, which is much higher than that for practical metallic superconductors. Thus, it is hoped that MgB₂ can not only replace metallic superconductors, but can be used under liquid-helium-free conditions, for example, at temperatures of 10-20 K that can easily be achieved using cryocooling systems. However, to date, the reported critical current density (J_c) for MgB₂ wires is not high enough for large-scale applications in liquid-helium-free conditions. In the present study, successful fabrication of high-performance MgB₂ superconducting wires was carried out using an internal Mg diffusion (IMD) process, involving a p-dimethylbenzene (C₈H₁₀) pre-treatment of carbon-coated B powder with nanometer-sized particles. The resulting wires exhibited the highest ever J_c of 1.2 × 10⁵ A cm^-2 at 4.2 K and 10 T, and an engineering critical current density (J_e) of about 1 × 10⁴ A cm^-2. Not only in 4.2 K, but also in 10 K, the J_c values for the wires fabricated in the present study are in fact higher than that for Nb-Ti wires at 4.2 K for the magnetic fields at which the measurements were carried out. At 20 K and 5 T, the J_c and Je were about 7.6 × 10⁵ A cm^-2 and 5.3 × 10³ A cm^-2, respectively, which are the highest values reported for MgB₂ wires to date. The results of a detailed microstructural analysis suggested that the main reason for the superior electrical performance was the high density of the MgB₂ layer rather than just the small grain size, and that the critical current could be further increased by suitable control of the microstructure. These high-performance IMD-processed MgB₂ wires are thus promising superconductors for applications such as magnetic resonance imaging and maglev trains that can operate under liquid-helium-free conditions.