Flux Pinning Behavior of Nb3Sn Superconductors With Nanostructured Pinning Centers

Flux pinning in superconductors must be optimized in order to improve their critical current densities J_c. The mixture of pinning behaviors is seen as one efficient way to increase pinning, leading to materials with grain boundaries, structural defects and phases with different physical properties, in comparison to the superconducting phase, which contribute efficiently to the overall pinning behavior of the composite. This type of composite can be created with a carefully chosen methodology where nanometric-scale pinning centers are introduced into the superconducting matrix, followed by formation and reaction heat treatments. The present work describes the flux pinning analysis of such kind of composite. The introduction of Cu(Sn) nanometric pinning centers into the Nb₃Sn phase was performed through successive bundlings and mechanical deformation, leading to Cu(Sn) pinning regions as small as 40 nm. The properties of the superconducting phase were drastically changed, leading to materials with lower upper critical fields than the conventional Nb₃Sn stoichiometric phase, but with the peak of pinning force shifted to fields around 10 T, which means b = B/B_c2 = 0.5, similar to materials with normal phases as major pinning centers. The microstructural and electromagnetic characterization of the samples lead to the partial understanding of the new composite behavior, suggesting the probable microstructure acting on the flux line pinning.