  The effect of nano-C doping on the microstructure and superconducting
properties of Fe-sheathed MgB2 tapes prepared through the in-situ
powder-in-tube method was studied. Heat treatment was performed at a low
temperature of 650C for 1 h. Scanning electron microscopy investigation
revealed that the smaller grain size of MgB2 in the samples with the C-doping.
Further, the a-axis lattice parameter and transition temperature decreased
monotonically with increasing doping level, which is due to the C substitution
for B. High critical current density Jc values in magnetic fields were achieved
in the doped samples because of the very fine-grained microstructure of the
superconducting phase obtained with C doping.
