Systematic study of the microstructures and electrical and superconducting properties of nanocarbon doped MgB2 tapes

A series of C doped and undoped MgB2 tapes sintered at 600 -950 degrees C was fabricated. The relationships between the critical current properties, crystallinity and microstructures were systematically studied as functions of the C doping level and heating conditions. A decrease in the a-axis length due to carbon substitution was observed in the C doped samples. The full width at half maximum (FWHM) of the MgB2 (101) peak increased with increasing C doping level, suggesting that the introduction of lattice defects and/or the decrease of the grain size of MgB2 occurred on C doping. Due to the substitution of C for B, the upper critical field was improved in C doped MgB2 samples. Substantially improved critical current density, J(c), was found in all of the C doped samples. At 4.2 K, 10 T, J(c) reached 2.2 x 104 A cm(-2) in the 8% C doped samples.