In-situ studies of Fe₂B phase formation in MgB₂wires and tapes by means of high-energy x-ray diffraction

Abstract

The phase transformations occurring in the ceramic core of Fe-sheathed MgB 2 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 Fe 2 B phase, forming at the interface between the sheath and the ceramic, was studied at different sintering temperatures. The reactivity of the sheath towards Fe 2 B formation is strongly dependent on powder pre-treatment. In wires produced with commercial Mg and B powders without additional mechanical activation, the Fe 2 B phase starts forming around 650C. In contrast, in tapes produced from a mixture of Mg and B powders subjected to high-energy ball milling, the interfacial Fe 2 B layer forms readily at 600C. The increase of Fe 2 B 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 MgB 2 at a temperature, which is low enough to avoid Fe 2 B formation, the interface is stable during further annealing at temperatures up to 700C at least. However, too high annealing temperatures (T > 800C), would result in formation of Fe 2 B, probably following the partial decomposition of MgB 2 .