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 600-950C 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. 
Yanwei Ma, Xianping Zhang, S. Awaji, Zhaoshun Gao, Dongliang Wang,   Zhengguang Yu, G. Nishijima, K. Watanabe