Mechanochemical synthesis of NaBH4 starting from NaH–MgB2 reactive hydride composite system

The present investigation focuses on a new synthesis route of NaBH₄ starting from the 2NaH + MgB₂ system subjected to mechanochemical activation under reactive hydrogen atmosphere. The milling process was carried out under two different hydrogen pressures (1 and 120 bar) with two different rotation speeds (300 and 550 rpm). The reaction products were characterized by ex-situ solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR), ex-situ X-ray powder diffraction (XRPD) and Infrared Spectroscopy (IR). From the results of these analyses, it can be concluded that milling in all the considered conditions led to the formation of NaBH₄ (cubic-Fm-3m). In particular, a reaction yield of 5 and 14 wt% is obtained after 20 h of milling at 120 bar of H₂ for the tests performed at 300 rpm and 550 rpm, respectively. The presence of MgH₂ is also detected among the final products on the as milled powders. The influence of the milling conditions and the evaluation of the parameters related the mechanochemical process are here discussed.     

Effect of NaH/MgB2 ratio on the hydrogen absorption kinetics of the system NaH + MgB2

In this work the effect of the ratio of starting reactants on the hydrogen absorption reaction of the system xNaH + MgB₂ is investigated. At a constant hydrogen pressure of 50 bar, depending on the amount of NaH present in the system NaH + MgB₂, different hydrogen absorption behaviors are observed. For two system compositions: NaH + MgB₂ and 0.5NaH + MgB₂, the formation of NaBH₄ and MgH₂ as only crystalline hydrogenation products is achieved. The relation between the ratio of the starting reactants and the obtained hydrogenation products is discussed in detail.     

Structural study of a new B-rich phase obtained by partial hydrogenation of 2NaH + MgB2

The structure of an unknown crystalline phase observed during the hydrogen absorption reaction of the powder mixtures 2NaH + MgB₂ at high pressure has been studied by ab-initio structure determination from powder diffraction. The sequence of un-overlapped peaks extracted from the X-ray powder diffraction pattern could be indexed with a primitive cubic cell with lattice parameter a = 7.319 Å. The diffraction patterns of the peaks are matched with the Pa-3 space group. The stoichiometry of the hydrogen absorption reaction suggests the presence of a high-boron content phase in the compound under investigation. Assuming this phase to be composed only by boron atoms and therefore having a density similar to that found for boron polymorphs, the solution with a space group of Pa-3 leads to reasonable B–B interatomic distances.