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

The critical current density (J sub c) 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 J sub c with prereaction 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 micron diameter filameters, 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 J sub c resulting 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 J sub c (H) characteristics. The two processes have comparable critical currents at high fields, suggesting the same H sub c2, while at low fields the internal tin wire is superior, suggesting a better grain morphology.