Morphology and structure of Nb3Sn diffusion layers in superconductors with tubular Nb filaments

The structure and morphology of Nb3Sn layers in superconducting Nb/Cu-Sn composites with ring (tubular) Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy after various regimes of diffusion annealing. It is demonstrated that the tubular geometry of Nb filaments, in which Sn diffuses from the bronze matrix both from inside and outside, ensures practically complete transformation of Nb into the superconducting Nb3Sn phase. Besides, at certain regimes of the diffusion annealing this geometry enables the improvement of the superconducting layers morphology compared to that of wires with continuous filaments, namely, to obtain wide Nb3Sn layers with fine equiaxed grains and to avoid the formation of columnar grains, which promotes enhanced current-carrying capacities of the wires. © (2015) Trans Tech Publications, Switzerland.