Akiyoshi Matsumoto and Hiroaki Kumakura
National Institute for Materials Science, Tsukuba 305-0047
MgB2 wires are now fabricated by two (in situ and ex situ) powder-in-tube (PIT) methods. The superconducting properties of PIT-processed MgB2 wires are sensitive to the quality of the starting powder in both methods, the heat-treatment temperature, the porosity of the MgB2 core, 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 both Bc2 and Jc in high magnetic fields. In general, ex situ-processed MgB2 wires show lower Jc than in situ-processed wires. The key factor to obtain a high Jc for an ex situ tape is also the high quality of the MgB2 starting powder. Recently, we succeeded in the fabrication of high Jc ex situ MgB2/Fe wires using MgB2 cores removed from in situ MgB2 wires. Al-sheathed ex situ-processed wires are interesting due to their light weight and the high thermal stability of conductor. At the present stage, however, Jc values of MgB2 wires are still below a practical level. Further improvement of the processing of MgB2 wires is essential in order to obtain a practical level of Jc values.
critical current density, magnesium diboride, powder in tube
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