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

We fabricated pure and SiC-added MgB 2 /Fe composite tapes using a MgH 2 starting powder and applying heat treatments at 600–900 °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 B c2 –T and B irr –T curves, d B c2 /d T and d B irr /d T , for all heat-treatment temperatures. The d B c2 /d T and d B irr /d T of the pure tape decreased with increasing heat-treatment temperature from 600 to 700 °C because of the longer coherence length associated with the improved crystallinity. However, the SiC addition significantly decreased the heat-treatment temperature dependences of d B c2 /d T and d B irr /d T . 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 °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 T c and d B irr /d T . 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 °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 °C showed J c (MgB 2 core) values higher than 10 4 A cm −2 at 20 K in 5 T.