Study of Magnetic Flux Pinning in Nb-Ti Superconductors Using Artificial Pinning Center Technology

Multifilamentary Nb-Ti superconducting wires with heterogenous flux pinning nano-structures have been fabricated using artificial pinning center (APC) technology. Pinning centers were introduced into Nb-Ti filaments, consisting of Cu-Ni, Nb and Nb+Ti rods with the triangular arrangement and the volume fraction of 17 %. After final hot extrusions, the APC wires were drawn down over a high strain range of εf ~9. In the Cu-Ni APC, Kirkendall voids were produced at the interface of pins by diffusion during drawing processes. Typical diameters of the voids were less than 20 nm and they formed with remarkable pinning array. It works out high Jc at low fields. On the other hand, in the Nb and Nb+Ti APC’s, the plane-strain induced folded sheet nano-structures were developed with reducing the wire diameter. When the nano-stracture of pins is developed, proximity-induced superconductivity comes into pins, therefore Nb pins act as repulsive ones while Ti pins act as attractive pins. For Nb+Ti mixed APC, the Fp was less than half of that for Nb APC. Furthermore the saturation of Fp was observed with further reducing pin size. The numerical simulations of Fp for Nb+Ti pins were in agreement with the experimental results. This reveals that for the Nb+Ti APC, a destructive interference between heterogenous pinning nano-structure takes place when the density of pinning site increases further.