Transport properties and flux pinning analysis of high-performance FeAs122 superconducting wires

Iron-based superconductors (IBS), especially 122 type, are very promising candidates for high-field applications because of its ultra-high H_c2 > 70 T at 20 K, low anisotropy (g < 2 for 122), and ease of fabrication. In recent years, tremendous progress has been made on the critical current density (J_c) of the 122-type IBS wires based on a powder-in-tube technique. Encouraging breakthroughs were made, including a high transport J_c exceeding the practical level of 10⁵ A cm⁻² (at 4.2 K, 10 T), the first 100 meter-class wire and the first performance test of a 30 mm IBS inserted coil under a 24 T background field. More recently, the highest transport J_c value has achieved 0.15 MA/cm² (I_c = 437 A) at 4.2 K and 10 T in densified and textured 122 tapes made by hot pressing. The transport J_c measured at 4.2 K under high magnetic field of 27 T is still on the level of 55 kA/cm². Herein we compared the T_c and J_c distributions of the K-doped FeAs122 tapes by a calorimetric method. We found that hot-pressing provides a better environment for a complete chemical reaction and a more homogenous dopant distribution, which are beneficial to the global current of a superconductor. We further study the vortex dynamics of the hot-pressed high-J_c tapes. We found that the magnetization relaxation rate below 10 K shows a temperature insensitive plateau with a value comparable to that of low temperature superconductors. Moreover, the relaxation rate below 20 K tends to saturate with the increasing field which is beneficial for high field application. We also highlight some remarkable advances relevant to practical applications, including mechanical strain properties, copper sheaths, multifilamentary fabrication, and superconducting joints.