Analysis of the minority crystalline phases in bulk superconducting MgB2 obtained by reactive liquid Mg infiltration

The preparation of MgB2 starting from the elements (in situ process), when performed in conjunction with the Mg liquid infiltration process, gives rise to very high dense manufacts, with outstanding mechanical and superconducting characteristics. The infiltration technique is particularly suitable when realized into preforms of crystalline boron powders of grain size up to about 100 μm; however, the resulting products typically present a microstructure characterized by a composite morphology, indicating a inhomogeneous distribution of different crystalline phases. In this paper the composition and the morphology of typical products has been analyzed with several techniques, including X-ray powder diffraction (XRPD), scanning electron microscopy and X-ray fluorescence microanalysis, aiming to quantify the (spatial) distribution of crystalline phases and the presence of minority phases different from the MgB2. The minority phases that are commonly present in the MgB2 infiltration preparation are residual elemental magnesium and a new boron-rich magnesium boride phase (Mg2B25), recently discovered and structurally characterized (by unconventional XRPD methods) by our group. We have analyzed the common morphological features of typical MgB2 samples, and cross-checked the analytical results obtained from different experimental techniques. Our results show that the new Mg2B25 phase amounts to less than 10 molar % of the MgB2 entire product, and that it is mainly concentrated, as submicronic inclusions (inside the larger crystalline MgB2 grains) where it can reach concentration values up to 40 molar %.