Mechanically milled Mg composites for hydrogen storage - The transition to a steady state composition

Magnesium alloys are potentially the best materials for gaseous hydrogen storage. However, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be resolved by mixing Mg alloys with other materials to form composites. We present an investigation of the initial hydriding characteristics, as well as the compositional transformation of composites made of La2Mg17 + LaNi5 mechanically milled in a 2:1 weight ratio. Composites produced with varying durations and intensities of milling were tested. Those milled to the greatest extent proved to have the best initial hydrogen absorption and desorption kinetics. The kinetics of the most heavily milled composite were superior to those of La2Mg17. This composite absorbed 90% of its full hydrogen capacity (3.5 wt.% H-2) in less than 1 min at 250 degrees C and desorbed the same quantity of hydrogen in 6 min. Under the same conditions pure La2Mg17 took 2.5 h to absorb and 3 h to desorb 90% of its full hydrogen capacity (4.9 wt.% H-2). Scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the mechanically milled powders before and after hydriding. The unhydrided powders consisted of LaNi, grains surrounded by a fractured La2Mg17 matrix. Hydrogen cycling, at temperatures up to 350 degrees C, induced phase changes, segregation, and disintegration of the composites. The resulting fine powder (less than 1 mu m) consisted primarily of Mg, Mg2Ni, and La phases.