Effect of Carbon Substitution on the Superconducting Properties of ${\rm MgB}_{2}$ Doped With Multi-Walled Carbon Nanotubes and Nano Carbon

We have fabricated the multiwalled carbon nanotube (MWCNT) and nano-C doped MgB
₂
bulk and wire to investigate the effect of carbon substitution in the MgB
₂
. Carbon substitution into boron site was confirmed by the shrinkage of the lattice parameter a with the increasing of the doping level. It was found that the lattice distortion and optimum doping level are different in the CNT and nano-C samples. This can be explained by carbon substitution, which is a more direct substitution process with the nano-C precursor. With CNT doping, carbon substitution only occurred after break out of carbon atoms from CNT. Both dopants showed significant enhancement in both magnetic critical current density, J
_c
(H), and transport critical current at high sintering temperature like 900degC . Transport I
_c
of magnitude 14 A and 30 A was observed at 4.2 K and 12 T for CNT and nano-C doping, respectively.