Analysis of interdiffusion between SmFeAsO_0.92F_0.08 and metals for exsitu fabrication of superconducting wire

To find good sheath materials that react minimally with the superconducting core of iron-based superconducting wires, we investigated the reaction between polycrystalline SmFeAsO_0.92F_0.08 and the following seven metals: Cu, Fe, Ni, Ta, Nb, Cr and Ti. Each of the seven metals was prepared as a sheath-material candidate. The interfacial microstructures of SmFeAsO _0.92F_0.08 and these metal-sheath samples were analysed by an electron probe microanalyzer after annealing at 1000 °C for 20h. Amongst the seven metal-sheath samples, we found that Cu was the best, because it reacted only very weakly with polycrystalline SmFeAsO_0.92F _0.08. Moreover, Cu is essential for superconducting wires as a stabilizing material. Metal sheaths made of Fe and Ni do not give rise to reaction layers, but large interdiffusion between these metals and polycrystalline SmFeAsO_0.92F_0.08 occurs. In contrast, metal sheaths made of Ta, Nb, Cr and Ti do form reaction layers. Their reaction layers apparently prevent electric current from flowing from the sheath material to the superconducting core. In general, through this research, Cu will be expected to be suitable not only as a stabilizing material but also as a sheath material for superconducting Sm-1111 wire fabricated by the exsitu PIT method.