Investigation of the hydrogen desorption properties of Mg + 10 wt.%X (X = V, Y, Zr) submicrocrystalline composites

The effects of catalytic metal additives on the hydrogen desorption properties of the submicrocrystalline magnesium hydride (β-MgH2) formed after hydrogenation of the Mg + 10 wt.%X (X = V, Y, Zr) mechanically (ball) milled composites were studied. The composites with catalytic metals were processed by controlled mechanical milling (CMM) in the magneto-mill Uni Ball Mill 5 under protective Ar atmosphere. X-ray diffraction of the milled powders revealed the formation of Mg nanograins (50-60 nm range) interdispersed with the nanograined metal additives within the powder particles. Scanning electron microscopy showed particle size reduction after milling. After activation and hydrogenation in a Sieverts-type apparatus under about 2 MPa pressure of hydrogen, the tetragonal β-MgH2 hydride co-exists with the small amount of retained unreacted Mg phase and the small amount of MgO in all three composites. The β-MgH2 phase after hydrogenation is submicrocrystalline with the grain size in the range of 127-151 nm. The XRD pattern for Mg + 10 wt.%Y after hydrogenation also reveals the presence of YH3 and YH2 phases. A single peak of ZrH phase on the Mg + 10 wt.%Zr XRD pattern is detectable. Hydrogen desorption at 300, 325 and 350 °C under atmospheric pressure of hydrogen shows that the Mg + 10 wt.%V composite exhibits the highest rate of hydrogen desorption and the lowest activation energy. For submicrocrystalline composites with the V and Y additives two-step desorption is observed when tested in TGA apparatus. A trend of decreasing DSC desorption peak temperature with decreasing powder particle size is also observed.