Properties of niobium–titanium superconducting wires with Nb artificial pinning centers

Artificial pinning center (APC) Nb 47 wt % Ti composite wires containing 24 vol % Nb pins were characterized as the wire diameter was reduced. As the nominal pin diameter dp was reduced from 165 to 15 nm, the nominally round pins transformed into ribbons with a relatively wide distribution in thickness, as compared to the α-Ti ribbons in optimized, conventionally processed Nb 47 wt % Ti. The maximum in the bulk flux pinning force Fp of 25 GN/m3 (4.2 K, 2.5 T) occurred at dp=40 nm, for which the measured Nb ribbon thickness ranged from 1 to 16 nm. This Fp value was about one third higher than that found in the best conventional Nb 47 wt % Ti. The upper critical magnetic field Hc2, measured by magnetization, decreased from 10.3 to 9 T as the Nb pins became proximity-effect coupled to the matrix. To better compare APC and conventional wires, we measured the properties of the best APC wire at a reduced temperature so that its Hc2 was the same as Hc2(4.2 K) for Nb 47 wt % Ti. The peak value of Fp was then 36 GN/m3 at 3 T, almost twice the maximum value yet reported for conventional Nb–Ti (∼19 GN/m3 at 5 T). In spite of the pinning force curve remaining sharply peaked at lower fields, the equalized 5 T critical current density was 4600 A/mm2, some 25% higher than the best values of conventional Nb–Ti. These properties demonstrate the strong potential of APC composites.