The critical current density and microstructural state of an internal tin multifilamentary superconducting wire

The critical current density (Jc) of internal tin wires is increased when low-temperature diffusion heat treatments are performed prior to a high temperature reaction. To determine the variation of Jcwith pre-reaction heat treatments a copper-stabilized IGC internal tin wire with an outside diameter of 0.267mm was studied. The wire has 2 to 2.5 μm diameter filaments, and within the Ta barrier, the area ratio of the copper matrix and Sn core to Nb is about 2.2. Due to the character of the Cu-Sn phase diagram, heat treatments at a series of temperatures below the Nb3Sn reaction temperature affect the local Sn concentration in the matrix about the Nb filaments. The variation in Jcresulting from these heat treatments is a consequence of the microstructural state of the conductor and the morphology of the Nb3Sn layer produced. The results of this work show that the internal tin and bronze-processed wires have different Jc(H) characteristics. The two processes have comparable critical currents at high fields, suggesting the same Hc2, while at low fields the internal tin wire is superior, suggesting a better grain morphology.