Formation and structure of Nb3Sn layers in bronze-processed superconductors under various intermediate heat treatments

Multifilamentary Nb3Sn-based superconducting wires are created and used for various superconducting magnetic systems such as thermonuclear reactors and future large accelerator projects. The crucial role in achieving high superconducting characteristics is played by the structure of Nb3Sn layers, which, in its turn, is determined by a number of factors, such as fabrication route, composition, annealing schedules, etc. In the present study the superconducting layers formation and structure have been studied by SEM and TEM after various regimes of intermediate heat treatments and two-staged diffusion annealing. Optimal heat treatment schedules for the formation of perfect nanocrystalline structure of Nb3Sn layers for an achievement of highest critical current densities have been revealed. (paper)