The Mg2B25 formation and its role in the preparation of bulk MgB2 superconductors

The high boron boride phase Mg2B25 was firstly discovered during the preparation of the superconducting MgB2 material by reacting liquid Mg at 850 °C with crystalline β-rhombohedral boron powders (Reactive Liquid Mg Infiltration process, Mg-RLI). This nonsuperconducting impurity phase was later on fully characterized from the crystallographic point of view by applying the Rietveld method on X-Ray Powder Diffraction (XRPD) diagrams of a pure powder sample. In this work we monitor the growing characteristics of the crystalline Mg2B25 and MgB2 phases during the Mg-RLI process, by implementing the XRPD analysis and other microscopic characterization techniques. In order to have a clear overview of what happens during the Mg-RLI process, we have varied the MgB2 preparative conditions: samples were prepared at 700, 850 and 900 °C adopting different heating times – 1, 3, 15 and 60 h – and various packing density of the starting boron powders (1.2, 1.4 and 1.6 g/cm3). We have used, as boron reactant, β-rhombohedral crystalline high purity powders, sieved at less than 100 microns. As far as the superconducting applications are concerned, the presence of residues of the Mg2B25 phase between the MgB2 crystalline grains, even if they are not superconducting, may lead to an improvement of the superconducting characteristics of the MgB2 in high magnetic fields by inducing – inside the MgB2 matrix – nano-defects which would be able to pin the current vortices developed in the superconducting state.