Effect of nano-carbon particle doping on the flux pinning properties of MgB2 superconductor

Polycrystalline MgB2-xCx samples with x = 0.05, 0.1, 0.2, 0.3 and 0.4 nano-particle carbon powder were prepared using an in situ reaction method under well-controlled conditions to limit the extent of C substitution. The phases, lattice parameters, microstructures, superconductivity and flux pinning were characterized by XRD, TEM, and magnetic measurements. It was found that both the a-axis lattice parameter and the T-c decreased monotonically with increasing doping level. For the sample doped with the highest nominal composition of x = 0.4 the T-c dropped only 2.7 K. The nano-C-doped samples showed an improved field dependence of the J(c) compared with the undoped sample over a wide temperature range. The enhancement by C doping is similar to that of Si doping but not as strong as for nano-SiC-doped MgB2. X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2 particles. Nano-particle inclusions and substitution, both observed by transmission electron microscopy, are proposed to be responsible for the enhancement of flux pinning in high fields. (C) 2003 Elsevier Science B.V. All rights reserved.