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

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