Doping effects on critical current properties of MgB/sub 2/ bulks synthesized by modified powder-in-tube method

Doping effects of B/sub 4/C and SiC on the superconducting properties of MgB/sub 2/ bulks synthesized by low-temperature solid-solid reaction and high-temperature liquid-solid reaction were systematically studied. Reduction of T/sub 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/sub c/ at 5 K is strongly correlated with lowered T/sub c/ by doping. Moreover, our results indicate that the B/sub 4/C is also an effective dopant for the improving J/sub 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 MgB/sub 2/ bulk even through the low-temperature reaction, /spl sim/600/spl deg/C.