Quantitative phase analyses have been performed on Fe-sheathed ex situ MgB 2 tapes after the deformation process and after annealing at 700, 850 and 920 °C in vacuum. The techniques used were x-ray diffraction, differential thermal analysis and scanning electron microscopy. All superconducting cores were found to contain certain amounts of MgO, the largest amounts being observed near the sheath, and after annealing at the highest temperature. Small amounts of MgB 4 were detected in the inner part of the core, presumably formed by the decomposition of MgB 2 . At the MgB 2 /Fe interface, a reaction layer was observed, which was identified as the boride Fe 2 B. In addition, tapes annealed at 700 °C revealed the presence of the metastable phase Fe 23 B 6 . The interaction of the MgB 2 filament with the Fe sheath and the presence of trapped oxygen limit the possibilities of the current tape fabrication techniques.
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Lezza et al. (2005) studied this question.
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