  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 600-950C for
1 h. Higher values of Jc and Birr were seen for 5 at.%C-doped MgB2 tapes at
higher sintering temperatures, where substantial substitution of boron for
carbon occurred. The C-doped samples sintered at 950C 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 nano-sized C particles can
be very beneficial in the fabrication of MgB2 tapes for magnetic resonance
imaging applications at 20 K.
