Superconducting composite of copper and niobium—a metallurgical approach

Abstract

Superconducting composites of niobium filaments in copper have been prepared by casting and subsequent cold working. In the cast copper-rich Nb alloys, there is a somewhat uniform distribution of Nb particles (∼ 10μm average diameter) resulting from liquid-state precipitation. After cold rolling or drawing, these Nb primary dendrites are elongated and aligned, and they eventually form quasicontinuous superconducting paths. Alloys containing more that 5 at.% Nb and with a cross-sectional area reduction ratio of ∼400 to :1 show a sharp resistive superconducting transition around 9°K. As an example, an alloy of composition Cu90Nb10 gives a critical current density Jc at 4.2°K and zero magnetic field of the order of 2×104 A/cm2 and a critical field of ∼10 kG. It is found that the addition of a small amount (a few at%) of Al to the basic Cu–Nb alloys greatly improves the continuity of the Nb filaments. This in turn results in more uniform superconducting properties in various sections of a long wire. No degradation of the superconducting properties were observed after a 0.25-mm wire sample was bent and straightened more than 50 times around a 1-in. mandrel. In fact, the critical current density increases with the amount of bending. For example, Jc at 4.2°K and zero magnetic field of a 0.25-mm wire of Cu90Nb10 increases from 1.5×104 to 105 A/cm2 after 20 bends.