Flux-pinning of Bi_2Sr_2CaCu_2O_(8+δ) high T_c superconducting tapes utilizing (Sr, Ca)_(14) Cu_(24)O_(41+δ) and Sr_2CaAl_2O_6 defects

摘要

Efforts to improve the magnetic flux-pinning properties of Bi_2Sr_2CaCu_2O_(8+δ)/Ag (2212/Ag) tape conductors utilizing (Sr_(1-x)Ca_x)_(14)Cu_(24)O_(41+δ) and Sr_2CaAl_2O_6 defects are described. Precursor powders with composition (2212 + N% volume fraction Sr_(10)Ca_4Cu_(24)O_(41+δ); N = 0, 7, 15) were prepared by solid-state reaction to obtain subsolidus phase equilibrium at 860 °C, as measured by X-ray diffraction (XRD). Nanophase (15 ± 5 nm) Al_2O_3 was added (1.1% mass fraction) to N = 0 and 15 fully reacted powders. Brush-on coated tapes (13 μm to 17 μm 2212 thickness) were processed by a partial-=melt growth method in air with variable melting from 865 °C to 890 °C, and slow-cool recrystallization from 856 °C to 847 °C. The effect of different melt temperatures and compositions on film properties (phase assemblages, orientations, and compositions, and defect sizes) was studied by XRD, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Higher N increased the amount of (Sr_(1-x)Ca_x)CuO_(2+δ) and (Sr_(1-x)Ca_x)_(14)Cu_(24)O_(41+δ) defects observed in processed films. Addition of 1.1% mass fraction Al_2O_3 for N = 0, 15 powders and melt temperatures 870 °C to 900 °C had two main effects: (1)Al_2O_3 reacted within ≤ 0-4 minutes of melting to produce defects with primary composition Sr_2CaAl_2O_(6+δ), as verified by XRD and EDS, and (2) dramatically shifted the phase assemblages toward increasing (Sr_(1-x)Ca_x)_(14)Cu_(24)O_(41+δ) and decreasing (Sr_(1-x)Ca_x)CuO_(2+δ) defects. The Sr_2CaAl_2O_6 defects increased to (1-5) μm size by coarsening with increasing melting temperatures ≈ (880 to 895) °C. Magnetic critical current density (J_c) of (N = 15 + alum powder) tapes showed improvement for 1T applied fields in the (20 to 30) K range. Transport J_c (4.2 K, OT) of N = 15 and N = 15 + alum composition tapes were ≈ 30% and ≈ 5%, respectively, of N = 0 TAPES.