Doping Effects of and on Critical Current Properties of Tapes

Doping effects of TiC and Mo₂C on critical current properties and microstructures of MgB₂ short tapes were systematically studied. Reduction of T_c and shortening of a-axis length were found to be derived from the substitution of carbon, independent of carbon source. Enhanced critical current properties under high magnetic fields were observed in the TiC, Mo₂C doped samples. Grain growth of MgB₂, which means a decrease of grain boundaries, i.e. reduction of effective pinning center density, was suppressed by TiC or Mo₂C doping. Moreover, field dependence of J_c at low temperatures ~5 K was found to be strongly correlated with the actual carbon content of the MgB₂ lattice. Our results suggest that control of actual carbon content in the MgB₂ bulk is crucially important for improvement of Jc under magnetic fields