Enhancement of in-field Jc in MgB2/Fe wire using single and multi-walled   nanotubes
  We investigated the doping effects of SWCNTs and MWCNTs on the Tc, lattice parameters, Jc(B), microstructure, and Hc2 of MgB2/Fe wire. These effects systematically showed the following sequence for Tc and the a-axis: the SWCNT doped wire < the MWshortCNT doped wire < the MWlongCNT doped wire < un-doped wire, while Jc(B) followed the sequence of the SWCNT doped wire > the MWshortCNT doped wire > the MWlongCNT doped wire > un-doped wire. A dominating mechanism behind all these findings is the level of C substitution for B in the lattice. The best Jc(B) and Hc2 were obtained on SWCNT doped wire because the level of C substitution for B in this wire is higher than all others. 
J. H. Kim, W. K. Yeoh, M. J. Qin, X. Xu, S. X. Dou, P. Munroe, H.   Kumakura, T. Nakane, C. H. Jiang