Modelling of Resistivity Changes in MultifilamentaryCu-Ta-Nb-Sn Conductors for Monitoring theDevelopment of Superconducting Nb3Sn Layers DuringIsothermal Heat Treatment

摘要

During the manufacture of multifilamentary Nb3Snsuperconducting wires by reaction diffusion processes, the thickness andmicrostructure of the Nb3Sn layer changes as a function of time,temperature, local tin concentration and wire geometry. Thesuperconducting performance of these wires depends sensitively onfeatures of the A15 phase and the cryo-magnetic stabilisation. Normalstateelectrical resistivity is also strongly dependent on many of thesefeatures. Models have been developed to predict the electrical resistivityof several bronze-process multifilamentary Cu-Ta-Nb-Sn wires as afunction of temperature and the degree of reaction. Experimentalcharacterisation of these composite wires after heat treatment has beenused with diffusion modelling to permit the prediction of resistivity bothafter and during heat treatment, which is of great importance forconductor optimisation. The predictions of the diffusion model have beencompared with experimental composition profiles, and good agreementhas been found. Predicted resistivities compare well with measurementsmade in situ during cooling after isothermal annealing when the extent ofreaction is constant and known, but predictions and experiment cannot yetbe directly correlated during isothermal reaction because low-temperaturereaction (during initial heating) has not been modelled. When thisrefinement is complete, this approach should permit monitoring, controland optimisation of the design and manufacturing process forsuperconducting wires, ultimately including control of the microstructuraldevelopment of Nb3Sn layers, especially the grain size.