Strong pinning and high critical current density in carbon nanotube doped MgB2

Polycrystalline samples of multi-walled carbon nanotube (CNT) doped MgB2 superconductors were synthesized by solid state reaction. The carbon nanotube substitution results in a small decrease in lattice parameter and Tc, as compared to ordinary carbon substitution. Enhancement of Hc2 and Hirr shown by CNT doped samples show that the effect of CNT doping is two-fold. For sintering temperatures higher than 900 deg. C, the CNTs partially decompose, allowing substitution of boron for carbon. This results in an increased Hc2. However, remains of the CNTs are incorporated into the crystal matrix of MgB2, and they act as pinning centres. For lower sintering temperatures, there is little substitution of boron for carbon, and CNTs are incorporated into the crystal matrix as a whole, acting as pinning centres without affecting Hc2