Microstructure and hydriding properties of ball-milled Mg–10 at.%MmNi 5 (Mm = La, Ce-rich mischmetal) composites

Abstract

Mg–10 at.%MmNi 5 nanocomposites were synthesized by ball milling under different gas atmospheres and as a result different particle size and microstructures were obtained. The samples milled part of the time under hydrogen showed narrower particle size distribution and finer particle size compared to the composite milled under argon only. Conversely, the average size of the nanocrystals (∼20 nm) is smaller for the composite milled under Ar. The nanostructure of the composites was found to be stable during hydriding cycling, especially for the sample milled longer time under Ar. Formation of Mg 2Ni and disordered Mg based phase was detected as a result of the milling. After hydrogen sorption at temperatures in the range 200–300 °C the amount of Mg 2Ni increases at the expense of the amorphous phase. The hydriding/dehydriding kinetics of all composites studied are comparable and very fast, due to the fine grain size and stable nanostructure. The hydrogen absorption rates at 300 °C, are in the range 0.040–0.045 wt.%/s. The composites reveal also a low-temperature hydrogen absorption ability with comparable velocities. It is found that LaNi 5 does not decompose completely during milling and subsequent hydriding and has a positive effect on the low-temperature hydrogenation.