Enhancement of critical current density and irreversibility field by nano carbon substitution in MgB2

We have investigated the effect of nano-carbon doping on phase formation, critical current density, flux pinning and irreversibility field in MgB2-x bulk sample where x = 0, 0.05, 0.1, 0.2 and 0.3. XRD results showed a systematic shift of lattice parameter a, which is attributed to carbon substitution, while the lattice parameter c almost did not change as the carbon content increased. Enhancement of magnetic J(c) by two orders at 5 K and 8 T was observed for the sample of x = 0.1, as the J(c)(H) reached the value of > 10(4) at 8 T. In addition, the H-irr of the 5% doped carbon was enhanced from 4.9 T to 5.8 T as compared to the undoped sample. Boron substitution for carbon is proposed to be responsible for the enhancement of flux pinning. (c) 2007 Elsevier B.V. All rights reserved.