Fabrication and properties of powder-in-tube-processed MgB2 tape conductors

MgB2 tapes are now fabricated by two powder-in-tube (PIT) methods. One is an in situ method, in which a powder mixture of Mg and B is used as a starting material. The other is an ex situ method, in which reacted MgB2 powder is used. Here, we report the recent progress of our PIT-processed MgB2 tapes. The superconducting properties of PIT-processed MgB2 tapes are sensitive to the quality of the starting powder, the porosity of the MgB2 core, the heat-treatment temperature, and impurity additions. In the case of the in situ method, a sub-micrometer Mg starting powder is quite effective for enhancing Jc. Some kinds of carbon compound additions to the starting powder introduce a carbon substitution for boron in MgB2 crystal and enhance Birr. At 20 K, Birr reached ∼10 T, a value which is nearly equal to that of commercial Nb–Ti at 4.2 K. This result clearly indicates that MgB2 tapes are promising as conductors of cryogen-free magnets. In general, ex situ-processed MgB2 tapes show lower Jc than in situ-processed tapes. The key factor to obtain a high Jc for an ex situ tape is the high quality of the MgB2 starting powder. Recently, we succeeded in the fabrication of high Jcex situ MgB2/Fe tapes using MgB2 cores removed from in situ MgB2 tapes. Furthermore, the Jc of this ex situ-processed tape was less sensitive to the magnetic field than that of the in situ tape because of the higher Birr of the ex situ tape. Al-sheathed ex situ-processed tapes are interesting due to their light weight and the high thermal stability of conductor. By improving the tape fabrication process mentioned above, we have obtained a large increase of Jc values. The highest Jc values obtained so far are 27 kA/cm2 at 4.2 K and 10 T and higher than 10 kA/cm2 at 20 K and 5 T. However, these Jc values are still below a practical level. Further improvement of the processing of MgB2 tapes is essential in order to obtain a practical level of Jc values.