Iron-based superconductors have received extensive attention in superconductivity mechanisms and high-field applications due to their ultra-high upper critical fields and low anisotropy. A major goal in the application research of iron-based superconductors is the fabrication of superconducting wires and tapes with high critical current density (J_c) and low cost. The use of Cu/Ag composite sheath can reduce the use of expensive silver, thereby reducing the cost of the tapes. In practical research, it was found that the content of K element has a great influence on the transport J_c of Cu/Ag composite tapes. In this study, the effects of K content on the transport J_c were systematically studied by adding additional K element in the preparation of (Ba, K) Fe₂As₂ precursor powder. The highest transport J_c was measured for Cu/Ag composite sheathed tapes prepared using precursor powders with a K ratio of 0.46 (K-0.46) at initial K ratios ranging from 0.42 to 0.54. K-0.46 tape sintered at 740 °C for 3 h achieved a critical current density of 5.8 × 10⁴ A cm^-2 at 10 T and 4.2 K. The properties of tapes with different K contents were characterized, including grain crystallinity, core density, c-axis texture and elemental distribution. The results show that that the precursor powder with a K ratio of 0.46 is the best choice for the preparation of high-performance Cu/Ag composite tapes.

Figure 1. Flow chart for tape preparation

Keywords: Iron-based superconductors, content of K element, critical current density, pre-composite, Cu/Ag composite tapes