Doping Effects of$rm TiC$and$rm Mo_2rm C$on Critical Current Properties of$rm MgB_2$Tapes

Doping effects of TiC and Mo
₂
C on critical current properties and microstructures of MgB
₂
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, Mo
₂
C doped samples. Grain growth of MgB
₂
, which means a decrease of grain boundaries, i.e. reduction of effective pinning center density, was suppressed by TiC or Mo
₂
C 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 MgB
₂
lattice. Our results suggest that control of actual carbon content in the MgB
₂
bulk is crucially important for improvement of Jc under magnetic fields