Nanocrystalline LaNi4.2Al0.8 prepared by mechanical alloying and annealing and its hydride formation

The formation of nanocrystalline LaNi4.2Al0.8 material by mechanical alloying (MA) followed by annealing has been studied by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. The amorphous phase forms directly from the starting mixture of elements, without formation of other phases. Heating the MA powders at 750°C for 0.5 h in high purity argon resulted in the creation of hexagonal CaCu5-type structure. The surface chemical composition of the nanocrystalline LaNi4.2Al0.8 alloy was studied by Auger electron spectroscopy and compared with that of a polycrystalline sample. Results showed that the surface segregation of lanthanum atoms in the MA nanocrystalline LaNi4.2Al0.8 alloy is stronger than that of polycrystalline powders from arc-melted ingots. On the other hand, the level of oxygen impurities trapped in the mechanically alloyed powder during the processing is practically the same as in the arc-melted ingots. Small amounts of Fe impurities, which strongly segregate to the surface, could be responsible for the somewhat lower hydrogen storage capacity of the MA nanocrystalline LaNi4.2Al0.8 alloy if compared with that of polycrystalline samples.