A15 Multifilamentary Superconductors by the Infiltration Process

Abstract

The morphology of practical superconductors is an essential consideration that is directly related to the requirement of conductor stability. An extensive treatment of stabilization criteria has been provided by the Rutherford Laboratory Superconducting Applications Group.¹ The adiabatic criterion is perhaps the most basic. For intrinsic stability, it requires that a superconductor should not exceed a critical size that is determined by certain properties of the superconducting material. In view of the adiabatic and the other criteria, the optimal size is generally considered to be less than 10 µm. From this requirement for such small sizes stems the preferred morphology for an intrinsically stable conductor: a large number of superconducting filaments arrayed in a normal matrix. Multifilamentary conductors were readily achieved with the ductile niobium-titanium superconducting alloy, and these have been developed to a high degree of sophistication, involving twisting, braiding and cabling. However, as shown in Table 1, the superconducting properties of niobium-titanium do not compare favorably with those of the A15 compounds. Consequently a great deal of current research effort is devoted to the development of multifilamentary conductors based on the intrinsically brittle intermetallic compounds of the Nb₃Sn type.