2017 年 52 巻 6 号 p. 405-414
The iron-based superconductors of K-doped Ba(Sr)Fe2As2(Ba(Sr)-122) are most potentially useful for high-field applications as a result of their high upper critical fields of over 50T and their small anisotropies. For Ba(Sr)-122, a powder-intube(PIT) method using an Ag sheath has been developed for tape and wire fabrications. Two important factors that greatly influence the transport Jc of PIT Ba(Sr)-122 tape are the density and c-axis grain orientation of the Ba(Sr)-122 core. The Jc of our PIT Ba-122/Ag tapes approach 4×104 A/cm2 at 4.2 K and 10 T for flat-rolled thin tapes. Alloying Ag with Sn is effective for improving the density and grain orientation of the Ba-122 core, and hence the Jc values. The application of uniaxial pressing in the final stage of deformation significantly enhanced Jc values up to ~7×104 A/cm2. This is also due to improving the density and grain orientation of the Ba-122 core. Another effective method to increase the density and grain orientation is double sheathing with an Ag or Ag-Sn inner sheath and stainless steel (SUS) outer sheath. This double-sheath architecture brings about much improved packing density and c-axis grain orientation in the Ba-122 core. The flat-rolled SUS/(Ag-Sn) sheathed Ba-122 tapes show Jc values of 1×105 A/cm2 at 4.2 K and 10 T with high homogeneity. The application of uniaxial pressing to this doublesheathed tape further increased Jc up to 1.4×105 A/cm2. The Jc values of both rolled and pressed Ba-122 tapes show extremely small magnetic field dependence up to 28 T, indicating that Ba-122 is a promising candidate as a conductor for high-field magnets.