Structure and high-field performance of Nb3Sn superconductors prepared from Ta-Sn core

Nb3Sn superconductors with excellent high-field performance are prepared by the reaction between the Ta-Sn core and the Nb (Nb-Ta) sheath. The mechanism of the formation of thick Nb3Sn layers by the reaction has been studied. No stable compound richer in Sn exists in the Ta-Sn system, and Sn has stronger affinity to Nb than to Ta. Furthermore, Nb in the sheath diffuses into the core at the reaction. These seem to be possible origins for the enhancement of the diffusion of Sn from the core to the sheath synthesizing thick Nb3Sn layers. Ta in the core is incorporated into the Nb3Sn layer at the reaction. Subsequently, the effect of the Cu addition to the Ta-Sn core of the composite has been studied. The Cu addition appreciably decreases the reaction temperature, and reduces the grain size of Nb3Sn. The Cu addition enhances the J(c) in lower field, while it reduces the J(c) in higher fields. A variety of J(c) versus magnetic field performance can be obtained by adjusting the amount of Cu addition to the core. The amount of Cu addition and the Ta : Sn ratio in the core, as well as the reaction temperature of the specimen have been optimized. The development of Nb3Sn conductors capable of generating fields over 20 T at 4.2 K seems to be promising through the present process.