Upper critical field, irreversibility field, and critical current density of powder-in-tube-processed MgB2/Fe tapes

We fabricated pure and SiC-added MgB2/Fe composite tapes using a MgH2 starting powder and applying heat treatments at 600-900 degrees 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 MgB2. The SiC addition decreased the Tc but increased the slope of the B-c2-T and B-irr-T curves, dB(c2)/dT and dB(irr)/dT, for all heat-treatment temperatures. The dB(c2)/dT and dB(irr)/dT of the pure tape decreased with increasing heat-treatment temperature from 600 to 700 degrees C because of the longer coherence length associated with the improved crystallinity. However, the SiC addition significantly decreased the heat-treatment temperature dependences of dB(c2)/dT and dB(irr)/dT. At a temperature of similar to 20 K, which is easily obtained using a cryocooler, the Bin. is governed by both the T-c and dB(irr)/dT. The B-irr of a pure tape at 20 K decreased with increasing heat-treatment temperature from 600 to 700 degrees C, but the B-irr of the 10 mol% SiC-added tape increased with the temperature. These behaviours can be explained by the heat-treatment temperature dependence of the Tc and dB(irr)/dT. At 20 K the highest B-irr of 10 T was obtained under the conditions of a 10 mol% SiC addition and heat-treatment temperature of 900 degrees C. This B-irr at 20 K is comparable to that of commercial Nb-Ti at 4.2 K. The 10 mol% SiC-added tape heat treated at 900'C and the 5 at.% SiC-added tape heat treated at 800 degrees C showed J(c) (MgB2 core) values higher than 10(4) A cm(-2) at 20 K in 5 T.