Formation mechanism of Sr2AlH7

The mixtures of commercial SrH2 and Al powders were ball milled under hydrogen at a rotation speed of 150 rpm for different time. The ball milled products could not be further hydrogenated to Sr2AlH7. In the case of ball milling of Sr2Al alloy under hydrogen at a rotation speed of 150 rpm for 10 h, however, the milled product consisting of SrH2 and Al can be easily hydrogenated to the Sr2AlH7 hydride at 533 K for 2 days. SEM and TEM observations demonstrated that the Al particles in the ball milled 2SrH2 + Al mixtures were agglomerative and the average size of the Al particles was of the order of 100 nm. In the ball milled Sr2Al alloy, however, the Al particles were dispersive and the average particle size was about 10 nm. The different microstructures were considered to be the most important factor in affecting the kinetics of hydrogenation reaction, which is the reason for the distinct hydrogenation characteristics.