Microstructural refinement using ball-milling and spark-plasma sintering of MgH2 based materials for hydrogen storage

In this work, a powder metallurgy procedure for the preparation of bulk hydrogen storage alloys based on MgH2 is presented. To meet the requirements of high specific surface area, high porosity and microstructural stability, all desirable features for this sort of material, a powder mixture with the following composition MgH2-15Nb-2B-1C has been ball-milled to reduce down to a few nanometers the average crystallite size of the main components. Another effect of milling, conducted under a static air atmosphere, was the in situ formation of a limited (5% approx.) amount of MgO, that, as shown by recent literature studies, may beneficially influence both the microstructural stability of the hydride alloy and the kinetics of adsorption and desorption of hydrogen. The powder was subsequently consolidated by spark plasma sintering, a process that, thanks to the limited working temperature and pressure, should not change to a significant extent the nanostructure of the ball-milled powder. The stability against recrystallisation of the consolidated specimens was verified by a number of heating and cooling cycles, up to 100, simulating the temperature conditions involved with the adsorption-desorption cycles. At the end, the crystallite size of the Mg-hydride phase was still in the nanometric range, although an unwanted oxidation, due to air contamination of the furnace atmosphere, reduced significantly the content of the hydrogen storing phase