Prospects for MgB₂ superconductors for magnet application

Superconducting magnets have a variety of industrial, medical and research applications. This review discusses the prospects for MgB₂ superconductor for practical magnet applications vis-à-vis the intermetallic low (LTS) and high temperature superconductor (HTS) cuprates. It has high T_C (39 K), high J_C (10⁵–10⁶ A cm⁻² at 4.2 K and in the self-field) and H_C2 (15–20 T at 4.2 K) in wire/tape geometry, with great scope for further improvement in the coming years, making it a promising candidate for practical applications. The superconducting properties of MgB₂ differ from those of LTS and HTS in many ways. Besides the unusually high T_C, MgB₂ has a large coherence length, low anisotropy and transparent grain boundaries. The most important difference between MgB₂ and other practical superconductors is that it has two superconducting gaps originating from two different bands. Tuning the scattering rates between the two bands improves the superconducting properties and the practical applicability of MgB₂. The different methods of fabrication of MgB₂ conductors are described and compared. Fabrication of long length MgB₂ conductors is relatively easy and less expensive as compared to HTS and the method allows the use of a variety of sheath materials with suitable barriers or reinforcement. The conductors have much better mechanical properties for practical applications. The critical issues and the challenges to be addressed for realization of MgB₂ superconductors as the first choice for high field magnet applications are discussed. At present MgB₂ is most suited for 20–25 K operation in fields of 1–2 T.