Fabrication and properties of powder-in-tube-processed MgB 2 tape conductors

Abstract

MgB 2 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 MgB 2 powder is used. Here, we report the recent progress of our PIT-processed MgB 2 tapes. The superconducting properties of PIT-processed MgB 2 tapes are sensitive to the quality of the starting powder, the porosity of the MgB 2 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 J c. Some kinds of carbon compound additions to the starting powder introduce a carbon substitution for boron in MgB 2 crystal and enhance B irr. At 20 K, B irr reached ∼10 T, a value which is nearly equal to that of commercial Nb–Ti at 4.2 K. This result clearly indicates that MgB 2 tapes are promising as conductors of cryogen-free magnets. In general, ex situ-processed MgB 2 tapes show lower J c than in situ-processed tapes. The key factor to obtain a high J c for an ex situ tape is the high quality of the MgB 2 starting powder. Recently, we succeeded in the fabrication of high J c ex situ MgB 2/Fe tapes using MgB 2 cores removed from in situ MgB 2 tapes. Furthermore, the J c of this ex situ-processed tape was less sensitive to the magnetic field than that of the in situ tape because of the higher B irr 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 J c values. The highest J c values obtained so far are 27 kA/cm 2 at 4.2 K and 10 T and higher than 10 kA/cm 2 at 20 K and 5 T. However, these J c values are still below a practical level. Further improvement of the processing of MgB 2 tapes is essential in order to obtain a practical level of J c values.