Demands for new generation superconducting wires are growing rapidly. One of good candidates to meet such demands is wires of cuprate superconductors. However, they face several issues such as strong magnetic field of critical current density (J_c) and delamination. In addition, due to complicated fabrication process, fabrication cost of wire continues to be high. In such a situation, wires and tapes of iron-based superconductors (IBSs) attract much attention, since they have sufficiently high T_c and H_c2. In addition, IBSs have sufficiently high J_c in single crystals, and it can be enhanced by the introduction of artificial defects [1]. Much effort is devoted to the development of tapes and wires of IBSs, in particular to those based on 122-type IBSs. Actually, J_c over the critical level of 1x10⁵ A/cm² under the field of 100 kOe has been achieved in 2014 in tapes [2], and J_c in wires also exceeded a half of that level in 2021 [3].

We have started the preparation of long-length (Ba,Na)Fe₂As₂ wire over 10 m, and constructed small coils to generate magnetic field [3]. Critical current (I_c) of the long wire is much smaller than that in short segments picked up from the long wire, and varies considerably along the length as shown in Fig. 1(a). It is important to know the reason for the suppression of I_c and parameters that affect it most. We have employed X-ray characterizations for such purpose. Magnetic field dependence of I_c for the best segment is shown in Fig. 1(b). The largest value of J_c calculated from the smallest cross section of this segment evaluated using X-ray tomography turn out to be record high, 6.7x10⁴ A/cm² at 100 kOe. In addition, X-ray diffraction measurements on the longitudinal and transverse cross sections of the wire has proven that the core of the wire is concentrically textured. It demonstrates the importance of texturing in enhancing J_c.

Fig. 1 (a) Transport Ic at several magnetic fields for short segments picked up from different positions of the long wire used for a small coil. (b) Magnetic field dependence of transport I_c for the short segment with the largest I_c. The inset shows a representative X-ray tomography image of the transverse cross section

[1] T. Tamegai et al., Supercond. Sci. Technol. 25, 084008 (2012).
[2] X. P. Zhang et al., Appl. Phys. Lett. 104, 202601 (2014).
[3] S. Pyon et al., Supercond. Sci. Technol. 34, 105008 (2021).

Keywords: iron-based superconductor, superconducting wire, critical current density, X-ray tomography