[Show abstract][Hide abstract] ABSTRACT: MgB2 wires, tapes and bulk samples produced within the EU-funded HIPERMAG project have been studied by a combination of X-ray diffraction and electron microscopy. The reaction layers forming at the interface between the ceramic core and Fe or Ni sheaths can be studied with both methods. The complementary techniques enable to study both the microstructure and the formation kinetics of the interface layers. Grain sizes can be determined either by direct observation or by analysis of the shape of X-ray diffraction peaks. Electron microscopy can detect B-rich secondary phases and phases present in small fractions that are not accessible by x-ray diffraction. On the other hand, synchrotron diffraction provides a fast and non-destructive method for the study of the main phases and their development during in-situ, high-temperature investigations. The combination of the two techniques is a very valuable tool for the optimisation of MgB2-based superconducting material.
Full-text · Article · Jul 2006 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: The phase transformations occurring in the ceramic core of Fe-sheathed MgB2 wires and tapes prepared by in-situ reaction of Mg and B precursor powders, have been studied by means of high-energy x-ray diffraction. In particular, the time evolution of the Fe2B phase, forming at the interface between the sheath and the ceramic, was studied at different sintering temperatures. The reactivity of the sheath towards Fe2B formation is strongly dependent on powder pre-treatment. In wires produced with commercial Mg and B powders without additional mechanical activation, the Fe2B phase starts forming around 650°C. In contrast, in tapes produced from a mixture of Mg and B powders subjected to high-energy ball milling, the interfacial Fe2B layer forms readily at 600°C. The increase of Fe2B volume fraction is linear to first approximation, showing that the interfacial layer does not act as a diffusion barrier against further reaction between the sheath and the ceramic core. If the ceramic core is converted to MgB2 at a temperature, which is low enough to avoid Fe2B formation, the interface is stable during further annealing at temperatures up to 700°C at least. However, too high annealing temperatures (T > 800°C), would result in formation of Fe2B, probably following the partial decomposition of MgB2.
Full-text · Article · Jun 2006 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: The reduction of mono-O-methyl-d-glucoses to their corresponding mono-O-methyl-d-glucitols and gas chromatography of the trifluoroaceyl derivatives of the latter resulted in an excellent separation of the compounds under investigation. Trimethylsilyl derivatives of the same compounds were not satisfactorily separated. The retention times of the mono-O-methyl-per-O-trifluoroacetyl-d-glucitols were comparable with those of trimethylsilyl derivatives. Trifluoroaetylation of mono-O-methyl-d-glucitols was found to be sufficiently fast for the procedure to be used for routine analyses.