Effect of Diffusion Annealing and Design of Internal Tin Wires on the Structure and Morphology of Superconducting Nb3Sn Layers

The structure and morphology of superconducting Nb3Sn layers in multifilamentary internal tin (IT) wires have been studied by transmission electron microscopy and scanning electron microscopy. The composites under study differed in the initial Sn concentration, the presence of strengthening Cu-Nb inserts, and the number of Nb filaments and their diameter in the final wire. In all the analyzed wires, columnar Nb3Sn grains were always present. The Nb3Sn grains are coarser and their size distribution is wider in the filaments of IT strands compared with the bronze-processed wires, the Jcin the former being as high as 2276 A/mm2. Much lower Jcvalues (850 A/mm2) were observed in a wire with a smaller diameter and a higher number of Nb filaments, which is not only due to the lower Sn concentration in the matrix but also to larger sizes and size scattering of equiaxed Nb3Sn grains, as well as to wider zones of columnar grains.