Superconductivity of copper containing small amounts of niobium

Superconductivity of copper containing small amounts of niobium has been investigated by measuring the electrical resistivity, superconducting volume fraction and by metallographic studies. Small amounts of niobium added to copper has a drastic effect on the low temperature resistivity of the alloys, The annealed alloy Cu_99.5Nb_0.5 shows zero resistance at a current density of 200 A cm^–2 below 3K. The estimated superconducting volume fraction of this alloy at 2K is about fifty times the physical volume fraction of the Nb in the alloy.When more Nb is added these effects unexpectedly become much smaller than those observed in the dilute alloys (< 1.5 at. % Nb). Metallographic results indicate that in all the Cu-Nb alloys studied there are two distinct types of Nb particles in the Cu matrix. The large particles (average size 10 m) randomly distributed in the alloy are probably formed at high temperature when the bulk of the alloy is still in the liquid state. The small Nb particles (size 1 m, interparticle distances < 0.2 m) probably form through a solid state precipitation. It has been found that the large precipitates are more abundant in the alloy containing more than 1.5 at. % Nb, than in alloys containing less than 1.5 at. % Nb. The observed superconducting properties of alloys Cu_99.8Nb_0.2 and Cu_99.5Nb_0.5 have been attributed to the proximity effect of the small Nb particles whose interparticle distances are compatible with the coherence length in the Cu matrix. The very wide superconducting transition shown in both the resistivity and the inductance measurements suggested a distribution in the Nb particle sizes as well as in the interparticle distances.