K. Smardz

Surface analysis of polycrystalline and nanocrystalline LaNi5-type alloys

Abstract The chemical composition and the cleanness of the surface of polycrystalline and nanocrystalline LaNi 5 , LaNi 4.2 Al 0.8 , and LaNi 3 AlCo alloys were studied by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Results showed that the surface segregation under UHV conditions of lanthanum atoms in the mechanically alloyed nanocrystalline samples is significantly stronger compared to that of polycrystalline powders obtained 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. Furthermore, we have estimated an average native oxide layer thickness of nanocrystalline LaNi 4.2 Al 0.8 as ∼5 nm. A strong segregation of the Fe impurities to the surface could be responsible for the observed slightly lower hydrogen storage capacity of the mechanically alloyed nanocrystalline LaNi 4.2 Al 0.8 compared to that of polycrystalline sample.

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

Abstract The formation of nanocrystalline LaNi 4.2 Al 0.8 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 CaCu 5 -type structure. The surface chemical composition of the nanocrystalline LaNi 4.2 Al 0.8 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 LaNi 4.2 Al 0.8 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 LaNi 4.2 Al 0.8 alloy if compared with that of polycrystalline samples.

Structure and Electronic Properties of La(Ni,Al)5 Alloys

Nanocrystalline and polycrystalline La(Ni,Al) 5 alloys were prepared by mechanical alloying (MA) followed by annealing and arc melting method, respectively. The amorphous phase of MA samples forms directly from the starting mixture of the elements, without other phase formation. Heating the MA powders at 800 °C for 1 h resulted in the creation of hexagonal CaCu 5 -type nanocrystalline compound with mean crystallite size less than 80 nm. XPS studies showed that the shape of the valence band measured for the arc melted (polycrystalline) LaNi 5 is practically the same compared to that reported earlier for the single crystalline sample. The substitution of Ni in LaNi 5 by Al leads to significant modifications of the electronic structure of the polycrystalline sample. On the other hand, the XPS valence band of the MA nanocrystalline LaNi 4.2 Al 0.8 alloy is considerably broader compared to that measured for the polycrystalline sample. The strong modifications of the electronic structure of the nanocrystalline LaNi 4.2 Al 0.8 alloy could significantly influence on its hydrogenation properties.

Growth Properties of Ti/Co Multilayers

Ti/Co multilayers with either wedge-shaped or constant-thickness Co sublayers were prepared using UHV DC/RF magnetron sputtering. The planar growth of the Co and Ti layers was confirmed by X-ray photoelectron spectroscopy and scanning tunnelling microscopy. Results on structural and magnetic studies showed that the cobalt sublayers grow on 2 and 5 nm titanium sublayers in the soft magnetic nanocrystalline phase up to a critical thickness d crit ∼ 3.0 and 3.3 nm, respectively. For a thickness greater than d crit , the Co sublayers undergo a structural transition to the polycrystalline phase with much higher coercivity.