Abstract
Polycrystalline MgB 2− x C x 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 MgB 2. X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg 2C 3 and MgB 2C 2 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.
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