Optimization of internal-tin niobium-tin composites.

摘要

The heat treatment of internal-Sn wires has two functions: (1) to mix the interfilamentary Cu with Sn from the core, and (2) to react the Sri with Nb to form the superconducting Nb-Sn A15 phase. To study the effectiveness of CU-Sn mixing, the Cu-Sn microstructures were examined in two very different internal-Sn composites after heat treatments of 24 and 150 hours at each of 10 different temperatures (up to 600°C). None of the heat treatments generated a single-phase Cu-Sn microstructure. It was concluded that complete mixing of the Cu and Sn is highly unlikely in commercial wires. However, it was found that Cu-Sn microstructure inhomogeneity has no effect on the critical temperature (T_c), irreversibility field (H*) or inductively measured critical current density (J_c) of fully reacted Nb₃Sn wire. Thus control of the superconducting properties lies with the composite design and the A15 reaction heat treatment. However, a Cu-Nb-Sn ternary intermetallic was discovered, which forms as a result of dissolution of Nb and therefore has deleterious effects on J_c. As the Cu fraction within the filament bundle decreases, lengthy Cu-Sn mixing heat treatments must be employed to limit formation of this ternary phase. A direct correlation was found between T _c and H*_Kramer that is strongly dependent on the Sn concentration in the A15 layer, emphasizing the need to provide proper Sn stoichiometry in the Cu-Nb-Sn package and to effectively contain the Sn within the bundle. Commonly used Nb diffusion barriers can react completely through locally, resulting in Sn loss from the A15 layer and degradation of the superconducting properties. To generate non-Cu J_c(12 T, 4.2 K) = 3000 A/mm2, the required J_c within the A15 layer is 4610 A/mm2, equal to the best literature values. In the best performing internal-Sn conductor to date, it is shown that non-Cu J_c(12 T, 4.2 K) of 3000 A/mm2 should be produced by replacing 5 vol.% of the Sn core with Nb or by replacing 3 vol.% of interfilamentary Cu with Nb and Sn.