Effect of annealing regimes on the structure of Nb3Sn superconducting layers in composites with internal tin sources

Abstract

The structure and morphology of Nb₃Sn superconducting layers that are formed in multifilamentary conductors with distributed tin sources in different diffusion-annealing regimes have been investigated by transmission and scanning electron microscopy. The composites studied differ in the design (the number of Nb filaments and their final diameter, initial tin concentration, the presence of reinforcing Cu‒Nb inserts in the stabilizing copper sheath, alloying of the copper matrix with manganese) and diffusion-annealing regime. In all of the conductors, superconducting layers arise with zones of different morphologies, namely, columnar grains are present along with fine equiaxed Nb₃Sn grains. Compared to Nb₃Sn-based conductors produced by the so-called bronze method, composites with internal tin layers are characterized by coarser Nb₃Sn grains with a greater spread of sizes. Nevertheless, the critical current density J_c of these latter can reach values 2276 A/mm² due to a higher Sn concentration in the superconducting phase and a larger relative amount of this phase in the conductor. Lower values of the critical current density (J_c = 850 А/mm²) were obtained in the conductor with a reduced tin concentration in the matrix and an enhanced number of Nb filaments with a smaller diameter, in which coarser Nb₃Sn grains with a wide spread of sizes and wider zones of columnar grains are formed.