Large irreversibility field in nanoscale C-doped MgB2/Fe tape conductors

We investigated the effect of nanoscale C doping on the critical current density Jc and irreversibility field Birr of Fe-sheathed MgB2 tapes prepared by the in situ powder-in-tube method. The tapes were heat treated at 600-950 deg. C for 1 h. Higher values of Jc and Birr were seen for 5 at.% C-doped MgB2 tapes at higher sintering temperatures. The C-doped samples sintered at 950 deg. C showed the highest Birr, for example at 4.2 K the Birr reached 22.9 T. In particular, at 20 K, Birr for the C-doped tape achieved 9 T, which is comparable to the upper critical field of the commercial NbTi at 4.2 K. This role of nanosized C particles can be very beneficial in the fabrication of MgB2 tapes for magnetic resonance imaging applications at 20 K. (rapid communication)