Abstract Body

The practical order of critical current density J_c, which is 10⁵ A/cm² at 10 T and 4.2 K, for superconducting wires and tapes towards the high-field applications, can be achieved by the iron-based superconductors (IBSs) like hot-pressed Ag-sheathed Ba_1-xK_xFe₂As₂ (Ag-Ba122) [1] and hot-isostatic-pressed Cu/Ag composite-sheathed Ba_1-xK_xFe₂As₂ (Cu/Ag-Ba122) [2] tapes. They prove the development potential and advantages for the IBS tapes fabricating by the low-cost powder-in-tube (PIT) method. However, the extra techniques of hot pressing and hot isostatic pressing during the PIT process increase the complexity and bring the restrictions on fabricating long wires and tapes while greatly improving the J_c. Fortunately, the development of stainless steel/Ag composite-sheathed Ba_1-xK_xFe₂As₂ (SS/Ag-Ba122) tapes, which are led by an innovative fabrication strategy and have made a new record of 2.2 × 10⁵ A/cm² at 10 T and 4.2 K for IBSs [3], give another possibility of large J_c without using hot pressing or hot isostatic pressing. This study investigated the magnetic field and angular dependence of J_c, J_c-B and J_c-θ, for the high-performance SS/Ag-Ba122 tapes, trying to find out the origin of J_c improvement and give suggestions to the future fabrication through showing their special J_c properties comparing to the flat-rolled Ag- and Cu/Ag-sheathed ones.

The J_c-B was measured from 0 T to 18 T (B^inc) and from 18 T to 0 T (B^dec) at both B⊥tape (θ = 0°) and B//tape surface (θ = 90°) as shown in Fig. 1. The magnetic field hysteresis that J_c at B^dec is larger than J_c at B^inc below certain magnetic field B_equal is observed like that in the Ag- and Cu/Ag-Ba122 tapes, but the value of B_equal is only 2~3 T, much lower than 8~10 T for the previous flat-rolled Ba122 tapes. It shows that SS/Ag-Ba122 tapes have relatively small magnetic field hysteresis in J_c, indicating the improved grain boundaries. In case of decreasing B for both angles, J_c drops when B≈ 0 T. In case of increasing B, the J_c behaviors are different and more complicated. At θ = 0°, J_c increases from 0 T to 0.2 T, then turns to reduce until 18 T. At θ = 90°, J_c reversely reduces from 0 T to 0.2 T, then turns to increase to get a peak at 2 T (B_peak) and finally back to reduce until 18 T. The low field J_c behaviors can be explained by Gurevich and Cooley’s theory [4] and have been widely observed in the Cu/Ag-Ba122 tapes in our previous studies, although  the B_peak (~ 6.5 T) is higher. Lower B_peak of SS/Ag-Ba122 also suggests the better connection between grains and a better grain texture.

The J_c-θ was measured from 0° to 90° at magnetic fields 1 ~ 18 T. The angular hysteresis in J_c is negligibly small even at 1 T. The J_c anisotropy, J_c(90°)/ J_c(0°) < 1 is similar to that for the previous high-performance Ba122 tapes. The J_c shows single peak around θ = 0° at all the measured magnetic fields up to 18 T, which are different in the Cu/Ag-sheathed Ba122 tapes [6].

Considering the extremely high-J_c for SS/Ag-Ba122 comparing to the previous Ba122 tapes, the differences and changes in the J_c properties may be related to the factors of the J_c improvement. In the presentation, the comparison among different Ba122 tapes and the detailed analysis will be made and discussed.

References

[1] H. Huang et al., Supercond. Sci. Technol., 31, 015017, 2018.
[2] S. F. Liu et al., Sci. China Mater., 64, 2530-2540, 2021.
[3] Y. W. Ma, EUCAS2023, 4-MO-FM2-03S, 2023. Paper to be published.
[4] A. Gurevich and L.D. Cooley, Phys. Rev. B., 50, 13563, 1994.
[5] J. Y. Luo et al., to be submitted.
[6] J. Y. Luo et al., IEEE Trans. Appl. Supercond. 33, 8200405, 2023.

Figure 1. The magnetic field dependence of J_c at (a) B⊥tape (θ = 0°) and (b) B//tape (θ = 90°) for the SS/Ag-Ba122 tape.

Keywords: Iron-based superconductor, critical current density, anisotropy, hysteresis