  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.
