Properties and flux pinning of Cu-Nb superconductors with nanometric-scale pinning centers

The introduction of normal phases into the superconducting phases is one of the most efficient forms to improve the pinning center densities and the critical current densities J_c of superconductors. Controlled generation of pinning centers with a projected distribution can contribute to estimate the pinning forces and mechanisms acting on the flux lines and to determine procedures to improve J_c. The introduction of nanometric-scale Cu regions into the Nb filaments region during the fabrication of Cu-Nb (or Cu-Nb₃Sn) composite superconductors changes the properties of the superconducting phase, mainly due to the proximity effect. The present work shows the development and characterization of Cu-Nb composites prepared by successive bundlings followed by swaging and wire drawing, leading to dimensions of the Cu regions as small as 43 nm. The samples were characterized by SEM and by the measurements of electrical resistivity as a function of temperature, critical temperatures T_c, J_c vs. H, and F_p vs. H, in different steps of deformation and different dimensions of Cu and Nb, in comparison to the superconducting coherence length ξ. The results helped to determine the influences of the Cu presence and of the proximity effect on the superconducting properties, leading to important conclusions useful to the optimization of J_c in superconductors.