Segregation effect on nanocrystalline La(Ni,Al)5 surface

Abstract

Nanocrystalline La(Ni,A1)5 alloys were prepared by mechanical alloying (MA) followed by annealing. Results showed that the surface segregation of La atoms in the MA nanocrystalline samples is stronger compared to that of polycrystalline materials from re'c-melted ingots. Small amounts of Fe impurities, which strongly segregate to the surface, could influence on the hydrogenation properties of the MA nanocrystalline alloys. PACS: 71.20.BeLaNih-based materials combine a high reversible energy storage capacity with fast electrochemical activation, excellent long term cycling stability and good chargedischarge kinetics [1]. Tile partial respective replacement of Ni in LaNi5 by small amounts of AI resulted in a increase in the cycle lifetime without causing much decrease in capacity [2]. Photoemission studies conlbined with measurements of tile magnetic susceptibility [3] showed that La segregates to the surface of the LaNi5 alloy and binds the impurities as 02 or H20 keeping tile Ni subsurface layer metallic, which then is able to split the H2 molecule. In this paper we study tile influence of surface chemical composition and microstructure on the hydrogenation properties of nanocrystalline and polycrystalline La(Ni,AI)5 alloys prepared by mechanical alloying (MA) followed by annealing and arc melting, respectively.MA and arc melting were performed under high purity argon (99.999 vol.%) atmosphere. Tile purity of the starting metallic elements La, Ni, and A1 was 99.9 wt.% or better. The bulk chemical composition of the samples was measured by Xray fluorescence (XRF) method. The surface chemical composition of the smnples was measured by Auger photoelectron spectroscopy (AES) combined with 3 keV argon ion beam sputtering [4]. The MA process of the La-Ni-Al mLxture was studied by X-ray diffraction (XRD) and by scmming electron microscopy (SEM). Pressurecomposition isotherms in the dehydriding process of the hydrogenated material were determined by the Sieverts method at room temperature.Alloy of the La(Ni,Al)5-type was produced by mechanical alloying from crystalline powders. Using LaNi4.2'A10.s as a representative alloy example, the MA pro-*) Presented at ll

XPS studies of nanocrystalline La–Ni and LaNi5−Al (x= 0.2, 0.5, 1) alloy thin films

“…In previous papers [15,16] we have reported a strong segregation of the La atoms to the surfaces in mechanically alloyed bulk nanocrystalline LaNi 5 -type alloys under ambient conditions. However, the XPS studies performed immediately after deposition showed no remarkable segregation of La atoms to the surface in nanocrystalline or polycrystalline La-based alloy thin films (see Figs.…”

XPS valence band studies of hydrogen storage Mg‐based nanocomposites

“…Results on XRF measurements revealed the assumed bulk chemical composition of the microcrystalline and nanocrystalline LaNi 5 -type thin films and bulk materials. However, in the previous paper [18] we have reported a strong segregation of the La atoms to the surfaces in MA (bulk) nanocrystalline La(Ni,Al) 5 alloy.…”

Natural Oxidation of thin Fe Films on V Buffer Layer

“…The chemical composition and the cleanness of all layers were checked in situ, immediately after deposition, transferring the samples to an UHV (4 × 10 −11 mbar) analysis chamber equipped with XPS, Auger electron spectroscopy (AES) and ion gun etching system [23][24][25][26][27]. The XPS spectra were measured at room temperature using a SPECS EA 10 PLUS energy spectrometer with Al K α and Mg K α radiation of 1486.6 eV and 1253.6 eV, respectively.…”