MECHANICALLY MILLED MG COMPOSITES FOR HYDROGEN STORAGE - THE TRANSITION TO A STEADY-STATE COMPOSITION

Magnesium alloys are potentially the best materials for gaseous hydrog en storage. However, their practical use is limited by poor hydrogen a bsorption and desorption kinetics. This problem can be resolved by mix ing Mg alloys with other materials to form composites. We present an i nvestigation of the initial hydriding characteristics, as well as the compositional transformation of composites made of La2Mg17 + LaNi5 mec hanically milled in a 2:1 weight ratio. Composites produced with varyi ng durations and intensities of milling were tested. Those milled to t he greatest extent proved to have the best initial hydrogen absorption and desorption kinetics. The kinetics of the most heavily milled comp osite were superior to those of La2Mg17. This composite absorbed 90% o f 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 t he 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 m icroscopy, 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 surr ounded by a fractured La2Mg17 matrix. Hydrogen cycling, at temperature s up to 350 degrees C, induced phase changes, segregation, and disinte gration of the composites. The resulting fine powder (less than 1 mu m ) consisted primarily of Mg, Mg2Ni, and La phases.