Upper critical field, irreversibility field, and critical current density of powder-in-tube-processed MgB_2/Fe tapes

摘要

We fabricated pure and SiC-added MgB_2/Fe composite tapes using a MgH_2 starting powder and applying heat treatments at 600-900 deg C and systematically investigated their superconducting properties. For both the pure and SiC-added tapes, the critical temperature (T_c) increased with increasing heat-treatment temperature due to the improved crystallinity of MgB_2. The SiC addition decreased the T_c but increased the slope of the B2~f and B_(irr)-T curves, dSC2/dT and d B_(irr)/dT, for all heat-treatment temperatures. The d B_(irr)/dT and d B_(irr)/dT of the pure tape decreased with increasing heat-treatment temperature from 600 to 700 deg C because of the longer coherence length associated with the improved crystallinity. However, the SiC addition significantly decreased the heat-treatment temperature dependences of dS2/dr and d B_(irr)/dT. At a temperature of ~20 K, which is easily obtained using a cryocooler, the B_(irr) is governed by both the T_c and d B_(irr)/d T. The B_(irr) of a pure tape at 20 K decreased with increasing heat-treatment temperature from 600 to 700 deg C, but the B_(irr) of the 10 mol percent SiC-added tape increased with the temperature. These behaviours can be explained by the heat-treatment temperature dependence of the T_c and d B_(irr)/dT. At 20 K the highest B_(irr) of 10 T was obtained under the conditions of a 10 mol percent SiC addition and heat-treatment temperature of 900 deg C. This B_(irr) at 20 K is comparable to that of commercial Nb-Ti at 4.2 K. The 10 mol percent SiC-added tape heat treated at 900 deg C and the 5 at: percent SiC-added tape heat treated at 800 deg C showed J_c (MgB_2 core) values higher than 10~4 A cm~(-2) at 20 K in 5 T.