The effect of ball milling under hydrogen and argon on the desorption properties of MgH2 covered with a layer of Mg(OH)2

It is observed that the activation energy values for hydrogen desorption from the MgH2 covered with a surface layer of Mg(OH)2, which was ball milled in Ar, are somehow erratically scattered as a function of milling time and, on average, are higher than those observed after milling in H2. In contrast, the activation energy values of the samples milled in H2 gradually decrease from ∼217 kJ/mol for the as received MgH2 to about ∼140 kJ/mol after 25 h of milling and then slightly increase with milling time up to 100 h. Apparently, the milling of hydrolyzed MgH2 in H2 is much more beneficial than milling in Ar. Regardless of milling in Ar or H2 the amount of hydrogen desorbed at 375 and 400 °C under atmospheric pressure (0.1 MPa H2) steadily decreases with increasing milling time which is, most likely, due to some sort of stabilization of ball milled hydride. This effect disappears when desorption is carried out under low vacuum.