Improved critical current in MgB₂ tapes sheathed with carbon steels

We fabricated MgB₂ tapes by a powder-in-tube method using sheath materials of Fe and carbon steel (CS). In the as-rolled state, the CS-sheathed tapes showed higher transport critical current density (J_c) values than the Fe-sheathed tape did. This is due to the higher-density MgB₂ layer associated with the high mechanical strength of the CS. Furthermore, heat treatment above 800 °C was very effective in increasing the J_c of these tapes. The heat-treated CS-sheathed tapes showed J_c values of 10 kA/cm² at 4.2 K and 7.5 T and still above 2 kA/cm² at 10 T. These values are the highest ever reported for MgB₂ tapes. An extrapolation to 0 T in the J_c-B data gave about 1 MA/cm², which was independent of the sheath materials. Microstructural observations suggest that the high J_c and the small field dependence of the J_c properties of the CS-sheathed tapes can be ascribed to the improved grain connectivity and grain alignment produced by the high mechanical strength of CS.