Formation of MgH1.62F0.38 solid solution by ball milling a MgH2-NbF5 mixture

PIGHIN, SANTIAGO; G. URRETAVIZCAYA; F. J. CASTRO

Río de Janeiro

Conferencia; Hy Sea 2014, International Conference on Hydrogen Storage Embrittlement and Applications; 2014

HySea

MgH2 modified with NbF5 as additive is a promising material for hydrogen storage, and as such has been extensively studied [1,2]. In this contribution we study the properties of a MgH2 + 7 mol% NbF5 mixture prepared by mechanical milling under Ar atmosphere. In particular, we characterize the evolution of the mixture during milling and its thermal decomposition under vacuum. We choose a greater than usual proportion of fluoride (2 mol% is the maximum amount typically found in the literature) to be able to identify and characterize the less abundant phases. The material has been studied by in situ pressure monitoring, XRD, SEM, and hydrogen absorption and desorption in a Sievert?s type apparatus. During milling we have observed the formation of NbHx, NbH2 and two solid solutions with the chemical formulae MgH2-xFx and MgF2-yHy (H-rich and F-rich, respectively). These solutions preserve the TiO2 structure that correspond to isostructural MgF2 and -MgH2. MgH2-xFx forms by the replacement of H anions by F anions in MgH2, whereas MgF2-yHy arises from the substitution of fluoride ions by hydride ions in MgF2. The initial MgF2 is formed by the reaction of MgH2 and NbF5. These processes start in the first hours of milling and finish after 40h, when both solutions merge into a unique phase of stoichiometry MgH1.62F0.38. The first stage in the evolution of the material during thermal decomposition, from room temperature up to 450ºC involves the coexistence of two solid solutions MgH2-x´Fx´ and MgF2-y´Hy´. During this period, the solution MgH2-x´Fx´ decomposes into MgF2-y´Hy´, Mg and H2. Simultaneously, the H content in MgF2-y´Hy´ reduces as temperature grows. After 450ºC, MgH2-x´Fx´ is no longer detected, and up to 600ºC only the release of H2 from the F-rich solution is observed. At this temperature, the solid solution is practically decomposed into MgF2 and Mg. The solid solution can not be formed again by hydriding the sample after desorption. If a high enough H2 pressure is applied -MgH2 is formed, accompanied by MgF2.