A series of C doped and undoped MgB 2 tapes sintered at 600-950 • 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 MgB 2 (101) peak increased with increasing C doping level, suggesting that the introduction of lattice defects and/or the decrease of the grain size of MgB 2 occurred on C doping. Due to the substitution of C for B, the upper critical field was improved in C doped MgB 2 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 × 10 4 A cm −2 in the 8% C doped samples.

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