The Mg_2B_(25) formation and its role in the preparation of bulk MgB_2 superconductors

摘要

The high boron boride phase Mg_2B_(25) was firstly discovered during the preparation of the super conducting MgB_2 material by reacting liquid Mg at 850 °C with crystalline P-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 Mg_2B_(25) and MgB_2 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 MgB_2 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/cm~3). We have used, as boron reactant, P-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 Mg_2B_(25) phase between the MgB_2 crystalline grains, even if they are not superconducting, may lead to an improvement of the superconducting characteristics of the MgB_2 in high magnetic fields by inducing - inside the MgB_2 matrix - nano-defects which would be able to pin the current vortices developed in the super conducting state.