Doping effects on critical current properties of MgB2 bulks synthesized by modified powder-in-tube method

Doping effects of B₄C and SiC on the superconducting properties of MgB₂ bulks synthesized by low-temperature solid-solid reaction and high-temperature liquid-solid reaction were systematically studied. Reduction of T_c and shortening of a-axis length are found to be derived from the substitution of carbon, not depending on carbon source. Field dependence of J_c at 5 K is strongly correlated with lowered T_c by doping. Moreover, our results indicate that the B₄C is also an effective dopant for the improving J_c under magnetic fields besides SiC, if the carbon substitution level is suitable. On the other hand, SiC was found to have advantageous points as a carbon source, because a moderate amount of carbon can incorporate into MgB₂ bulk even through the low-temperature reaction, ∼600°C.