Polycrystalline samples of multi-walled C nanotube doped MgB2 superconductors were synthesized by solid-state reaction. The C nanotube substitution resulted in a small decrease in lattice parameter and TC, as compared to ordinary C substitution. Enhancements of HC2 and Hirr shown by C nanotube-doped samples showed that the effect of CNT doping was 2-fold. For sintering temperatures higher than 900C, the C nanotubes partially decomposed, allowing substitution of B for C. This results in an increased HC2. However, remains of the C nanotubes were incorporated into the crystal matrix of MgB2, and they acted as pinning centers. For lower sintering temperatures, there was little substitution of B for C, and C nanotubes were incorporated into the crystal matrix as a whole, acting as pinning centers without affecting HC2.

Strong Pinning and High Critical Current Density in Carbon Nanotube Doped MgB2. W.K.Yeoh, J.Horvat, S.X.Dou, V.Keast: Superconductor Science and Technology, 2004, 17, S572-7