Developments of (Ba,Na)Fe2As2and CaKFe4As4HIP round wires

With excellent superconducting properties such as highT(c), largeH(c2), small anisotropy, and largeJ(c), developments of superconducting wires and tapes of iron-based superconductors (IBSs) have been undertaken with promising results. So far, K-doped 122-type IBSs have been mainly developed for such purposes. Here, efforts to fabricate Na-doped 122-type IBS, (Ba,Na)Fe2As2, are presented. With the help of hot isostatic pressing (HIP) process, excellentJ(c)characteristics of up to 44 kA cm(-2)at 4.2 K under 100 kOe have been achieved. Detailed characterizations of (Ba,Na)Fe2As2HIP round wire are presented. On the other hand, discoveries of new IBSs continue, with CaKFe(4)As(4)as one of recent examples. CaKFe(4)As(4)belongs to a new family of IBSs with 1144 structure, where Ca and K occupy the Ba site in BaFe(2)As(2)alternately along thec-axis. Studies on CaKFe(4)As(4)single crystals have demonstrated moderately largeJ(c)with peculiar anisotropic properties related to the presence of novel planar defects. We have also fabricated HIP round wires of CaKFe4As4, and reported their basic characterizations. Despite the instability of CaKFe(4)As(4)at low temperatures,J(c)values at 4.2 K close to 100 kA cm(-2)at self-field and 7.6 kA cm(-2)at 100 kOe are achieved. Attempts to introduce defects into CaKFe(4)As(4)single crystals, chemically by Co substitution and physically by proton irradiation, are attempted, and the effects are compared. Attempts to fabricate Co-doped CaKFe4As4HIP round wires with limited success are also reported.