  We evaluated the doping effect of two types of multiwall carbon nanotube
(CNT) with different aspect ratios on MgB2/Fe monofilament wires. Relationships
between microstructure, magnetic critical current density (Jc), critical
temperature (Tc), upper critical field (Hc2), and irreversibility field (Hirr)
for pure and CNT doped wires were systematically studied for sintering
temperature from 650oC to 1000oC. As the sintering temperature increased, Tc
for short CNT doped sample slightly decreased, while Tc for long CNT doped
sample increased. This indicates better reactivity between MgB2 and short CNT
due to its small aspect ratio and substitution of carbon (C) from short CNT for
boron (B) occurs. In addition, short CNT doped samples sintered at the high
temperatures of 900oC and 1000oC exhibited excellent Jc, and that value was
approximately 104 A/cm2 in fields up to 8 T at 5 K. This suggests that short
CNT is a promising carbon source for MgB2 superconductor with excellent Jc. In
particular, inclusion of nanosized MgO particles and substitution of C into the
MgB2 lattice could result in strong flux pinning centers.
