Effect of processing temperature on high field critical current density and upper critical field of nanocarbon doped MgB_2

摘要

Correlation of upper critical field (H_(c2)) and critical current density (J_c) with processing temperature of nano-C doped MgB_2 has been studied in comparison to SiC and pure MgB_2. SiC and C doped MgB_2 exhibit opposite trends in the dependence of J_c and H_(c2) on sintering temperature. This is explained by different reactivities of carbon available upon creation of MgB_2 for the two types of doping. Nanocarbon doped MgB_2 requires sintering temperatures in excess of 900℃ to obtain high boron substitution for carbon, enhancing the vortex pinning and impurity scattering of charge carriers. However, carbon substitution in nano-SiC doped MgB_2 occurs at less than 650℃, allowing lower sintering temperature and high degree of carbon substitution. Both pure and SiC doped MgB_2 benefit from low sintering temperature, which results in more grain boundary defects. Substantial carbon substitution can compensate for the disadvantage of sintering at high temperature of nano-C doped MgB_2, giving the best J_c of 4.8 x 10~3 A/cm~2 at 4.5 K and 12 T. This is comparable to the low-temperature sintered nano-SiC doped MgB_2 wires.