Flux pinning centers in superconducting Nb3Sn

Abstract

Very high critical current densities (J_c=8×10⁵ A/cm² at 4.2 K and 6 T) have been achieved in commercial Nb₃Sn tape and more recently in multifilamentary Nb₃Sn superconductors. The relative contributions of two types of pinning centers, grain boundaries and second phase particles, to flux pinning have been determined by correlating J_c with microstructure. It was found that grain boundaries were the only defect present in sufficient density to account for flux pinning in the multifilamentary Nb₃Sn material. The dependence of J_c on grain size was measured, and the relationship between pinning force and grain size is discussed in terms of current theories for flux pinning in type‐II superconductors.