Studies on in situ Cu-Nb composites

As part of a program of developing Cu-Nb3Sn composite superconductors through in situ techniques, systematic investigations have been performed on Cu-Nb composites containing niobium up to 30 at %. These alloys, prepared by an arc melting technique, show excellent filamentary morphology when reduced to fine wires. The T /SUB c/ (midpoint) of these composite wires is 8.8 K with a ΔT /SUB c/ = 0.12 K. The self-field overall J /SUB c/ of 0.21-mm-diameter wires of these composites at 4.2 K IS 2.5 X 105 a cm -2. The J /SUB C/ increases with area red;uction ratio up to a value of 2000 and has not attained saturation. Kramer plots (J /SUB c/ /SUP 1/2/ H /SUB c/ /SUP 1/4/ versus H) of the compsite wire yield a value of effective upper critical field H /SUB c2/ of 1.1 T, significantly higher than for pure niobium wire. Enhancement of H /SUB c2/ is attributed to the increase of the normal resistivity of the niobium filaments. In situ tapes show large critical current anisotropy with applied field direction, strongly indicating that surface flux pinning at the interface boundaries between the supe4rconducting filament and matrix is important in these materials. The peak in the volume pinning force, versus reduced field plots occurs for Cu-Nb (20 at %) composites consistently at 0.25, very close to the predicted value on the basis of surface flux pinning