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

Abstract

The effects of catalytic metal additives on the hydrogen desorption properties of the submicrocrystalline magnesium hydride (β-MgH 2) 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-MgH 2 hydride co-exists with the small amount of retained unreacted Mg phase and the small amount of MgO in all three composites. The β-MgH 2 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 YH 3 and YH 2 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.