It was recalled that the preparation of MgB2 starting from the elements (in situ process), when performed in conjunction with the Mg liquid infiltration process, gave rise to very dense products, with excellent mechanical and superconducting characteristics. The infiltration technique was particularly suitable for pre-forms of crystalline B powders with a grain size of up to about 100μm. However, the resultant products typically exhibited a microstructure that was characterized by a composite morphology; indicating an inhomogeneous distribution of different crystalline phases. The composition and the morphology of typical products were analyzed here using techniques which included X-ray powder diffraction, scanning electron microscopy and X-ray fluorescence microanalysis with the aim of quantifying the spatial distribution of crystalline phases and the presence of minority phases different from MgB2. The minority phases that were commonly present in the MgB2 infiltration preparation were residual elemental Mg and a new B-rich phase (Mg2B25), recently discovered and structurally characterized via unconventional XRPD methods. An analysis was made of the common morphological features of typical MgB2 samples, and cross-checked with the analytical results obtained using various experimental techniques. The results showed that the new Mg2B25 phase amounted to less than 10mol% of the entire MgB2 product, and that it was mainly concentrated, as sub-micron inclusions (within the larger crystalline MgB2 grains) where it could reach concentration values of up to 40mol%.

Analysis of the Minority Crystalline Phases in Bulk Superconducting MgB2 Obtained by Reactive Liquid Mg Infiltration. G.Giunchi, C.Orecchia, L.Malpezzi, N.Masciocchi: Physica C, 2006, 433[3-4], 182-8