Niobium aluminide as a source of high-current superconductors

The development of high-critical-current superconducting A15 conductors able to carry current in very high magnetic fields (25–30 T) is recognised as an enabling technology for the construction of second-generation NMR magnets operating at frequencies well above 1 GHz. This paper highlights the current status of development of the niobium-aluminide intermetallics with special attention to Nb₃Al, and Nb₃(Al_1-x,Ge_x). Discussion is focused on the materials science aspects of conductor manufacture, such as β-phase (A15) formation, with particular emphasis on the maximisation of the superconducting parameters, such as critical current density, J_c, critical temperature, T_c, and upper critical field, H_c2. Many successful manufacturing techniques of the potential niobium-aluminide intermetallic superconducting conductors, such as solid-state processing, liquid-solid processing, rapid heating/cooling processes, are described, compared and assessed. Special emphasis has been laid on conditions under which the J_c(B) peak effect occurs in some of the Nb₃Al wires. The case is made that mechanical alloying during final wire preparation plays an important role in diminishing the peak effect, except in the case of A15 formation from a mixture of ‘σ-phase’ and Nb. Further study of the influence of mechanical alloying on the maximisation of the critical current density at high magnetic fields is crucial to the understanding of the peak effect formation in tapes formed from ‘σ-phase’. Additionally, many aspects of conductor design requirements such as stress, strain, a.c. losses, thermal and electromagnetic stabilisation, are discussed with reference to literature sources.