Improved critical current in MgB 2 tapes sheathed with carbon steels
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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 (Jc) 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 Jc of these tapes. The heat-treated CS-sheathed tapes showed Jc 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 Jc-B data gave about 1 MA/cm2, which was independent of the sheath materials. Microstructural observations suggest that the high Jc and the small field dependence of the Jc 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.
I. INTRODUCTION
The discovery of superconductivity with a transition temperature (Tc) of 39 K in a simple binary compound magnesium diboride MgB2 was recently reported.1 The Tc is the highest among intermetallic superconductors. This newly discovered superconductor has been extensively studied from theory to application. From the point of view of application, MgB2 has several advantages such as low cost and light weight. Furthermore, the Tc of 39 K is high enough for usage at elevated temperatures at which a cryogen-free cooling system is readily available. Concerning its applications to wires and tapes, MgB2 has advantageous characteristics. Studies on singlecrystalline samples revealed that MgB2 was nearly isotropic, with an upper critical field ratio of about 2–3.2,3 Furthermore, the weak-link behavior of grain connectivity was not observed for bulk compounds.4 Their characteristics of these bulk compounds are largely different from those observed in high-Tc cuprates. This suggests that the grain alignment required for high-Tc cuprates is not necessary in MgB2. This is of great advantage in reducing manufacturing costs in the fabrication of longlength conductors. MgB2 wires were first produced by the reaction of B fibers with Mg sealed in a Ta tube.5 However, a more widely used method for the fabrication of wires and tapes is the so-called powder-in-tube (PIT) method. A mixture of Mg and B powders is packed into a metallic tube, such as Fe, which does not react with Mg.6 After packing, the tube is deformed into a wire or tape, and a heat treatment is finally performed to obtain MgB2. Alternatively, J. Mater. Res., Vol. 17, No. 9, Sep 2002
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MgB2 powder is packed into a tube and heat-treated at 900–1000 °C for a short period to improve the connectivity of the MgB2 grains.7,8 Recently, it was found that critical current density (Jc) as high as 100 kA/cm2 at 4.2 K and 0 T was obtained for Ni-sheathed MgB2 tapes fabricated by a PIT method without heat treatment.9 This process is very attractive because
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