The critical parameters in in-situ MgB_2 wires and tapes with ex-situ MgB_2 barrier after hot isostatic pressure, cold drawing, cold rolling and doping

摘要

MgB_2 precursor wires were prepared using powder in tube technique by Institute of High Pressure PAS in Warsaw. All samples were annealed under isostatic pressure generated by liquid Argon in the range from 0.3 GPa to 1 GPa. In this paper, we show the effects of different processing routes, namely, cold drawing (CD), cold rolling (CR), hot isostatic pressure (HIP) and doping on critical current density (J_c), pinning force (F_p), irreversible magnetic-field (B_(irr)), critical temperature (T_c), n value, and dominant pinning mechanism in MgB_2/Fe wires with ex situ MgB_2 barrier. The results show that medium pressures (～0.35 GPa) lead to high J_c in low and medium magnetic fields (0T -9T). On the other hand, higher pressures (～1 GPa) lead to enhanced J_c in high magnetic fields (above 9T). Transport measurements show that CD, CR, and HIP have small effects on B_(irr) and T_c, but CD, CR, HIP, and doping enhance J_c and F_p in in situ MgB_2 wires with ex situ MgB_2 barrier. Transport measurements on in situ undoped MgB_2 wire with ex situ MgB_2 barrier yield a J_c of about 100 A/mm~2 at 4.2 K in 6 T, at 10 K in 4 T and at 20 K in 2 T. The results also show that cold drawing causes increase of n value.