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

The formation of nanocrystalline LaNi4.2Al0.8 material by mechanical alloyi ng (MA) followed by annealing has been studied by X-ray diffraction, scanni ng electron microscopy and differential scanning calorimetry. The amorphous phase forms directly from the starting mixture of elements, without format ion of other phases. Heating the MA powders at 750 degreesC for 0.5 h in hi gh purity argon resulted in the creation of hexagonal CaCu5-type structure. The surface chemical composition of the nanocrystalline LaNi4.2Al0.8, allo y was studied by Auger electron spectroscopy and compared with that of a po lycrystalline sample. Results showed that the surface segregation of lantha num atoms in the MA nanocrystalline LaNi4.2Al0.8 alloy is stronger than tha t of polycrystalline powders from are-melted ingots. On the other hand, the level of oxygen impurities trapped in the mechanically alloyed powder duri ng the processing is practically the same as in the are-melted ingots. Smal l amounts of Fe impurities, which strongly segregate to the surface, could be responsible for the somewhat lower hydrogen storage capacity of the MA n anocrystalline LaNi4.2Al0.8 alloy if compared with that of polycrystalline samples. (C) 2001 Elsevier Science B.V. All rights reserved.