Microstructure and Critical Current Density of MgB2 and (Ba(Sr), K)Fe2As2 Wires and Tapes

MgB2 is expected to operate in helium free-condition to replace the practical metal superconducting wires in liquid helium condition. But the critical current properties for MgB2 wires are lower for large-scale applications at present. Internal Mg diffusion (IMD) method is promising to fabricate high performance MgB2 wires. Recently, we fabricated high J(c) thin MgB2 wires by applying IMD method and using 4%C-coated high quality B powder. During the heat treatment most of the B layer was reacted with Mg to form MgB2. Highest J(c) of 1100 A/mm(2) and 760 A/mm(2) were obtained at (4.2 K, 10 T) and (20 K, 5 T), respectively for the wire fabricated with the C-coated B powder. Engineering J(c)(J(e)) was higher than 100 A/mm(2) at 4.2 K and 10 T. These values of J(c) and J(e) are higher than those of well-known SiC-added wire. SiC addition brings about Mg2Si precipitates which act as barriers of superconducting currents and decrease J(c), while the wire fabricated with C-coated B powder contains no such precipitates. Among many iron arsenide superconductors, K-doped BaFe2As2(Ba-122) and SrFe2As2(Sr-122) are the most interesting superconductors for high field magnet applications due to their high B-c2 of over 50 T and relatively small anisotropy. We fabricated Ba (Sr)-122 tapes by applying ex situ powder-in-tube (PIT) technique. We found that the uniaxial pressing brings about a dramatic improvement of transport J(c) for ex situ PIT processed Ba(Sr)-122 tape when it is properly combined with flat rolling and heat treatment. The pressure was changed between 0.4 GPa to 4 GPa. The increase of uniaxial pressing from 0.4 GPa to 4 GPa significantly enhanced J(c) values from 210 A/mm(2) to 860 A/mm(2) at 4.2 K and 10 T. Microstructure analysis with SEM indicates that the packing density of Ba-122 core pressed under 4 GPa is higher than that of the tape pressed under 0.4 GPa. This suggests that the packing density is one of the most important parameters that influence J(c) of ex situ PIT processed Ba-122 superconducting tapes.