X-ray photoelectron spectroscopic studies on nanoquasicrystalline powders of Al 70 Cu 20 Fe 10 obtained by mechanical al
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Surface chemical properties of nanoguasicrystalline powders of Al–Cu–Fe synthesized by the ball-milling technique have been investigated using x-ray photoelectron spectroscopy (XPS). The samples were exposed to ambient conditions at room temperature as well as higher temperatures. Our XPS results reveal that the surfaces of nanopowders of Al70Cu20Fe10 are coated with an Al2O3 layer within which the quasicrystalline phase resides. It also appears that the thickness of this layer does not increase significantly on either heating below 873 K or prolonged exposure to ambient conditions. The unusual electronic properties observed in quasicrystalline (QC) alloys may be attributed to a strong interrelation between the special QC structure and the electronic configuration. Icosahedral Al–Cu–Fe (i-Al– Cu–Fe) alloys are of some commercial interest due to their excellent properties like high hardness, low friction, and corrosion resistance that are indispensable for their subsequent use as surface coatings.1– 4 From a chemical standpoint, oxidation is an important fundamental chemical reaction and warrants careful consideration. A large volume of work has already been reported on cleaned and highly defined single grain/crystal and thin-film surfaces.5,6 Although some oxidation studies on thin films, coarse grain powders, and bulk ingot alloys of AlCuFe quasicrystalline materials have been reported, the mechanism responsible for surface oxidation process has eluded clear-cut understanding so far.7,8 In most industrial applications, however, the important processes occur under atmosphere conditions. Kang and Dubois2,9 have studied the oxidation behavior of bulk and powder (1 m) Al–Cu–Fe samples using a secondary ion mass spectrometer; however, no work has so far been reported on the surface chemical nature of the nanoquasicrystalline powders of Al–Cu–Fe system by XPS. Although in this technique there is no sharp limit of the sampling depth, the normally accepted value is around 6 nm. The sampling depth depends on the inelastic mean free path (IMFP), which in turn depends on the kinetic energy of the photoelectron.10 In this investigation we accept 6 nm as the maximum depth from which a detectable photoelectron current is recorded. The justification for such assumption is that the IMFP for Al 2p (K.E. ∼ 1181 eV) is around 2.4 nm and the escape depth is three times the a)
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http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 8, Aug 2002
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IMFP.10,11 Recently, it has been shown that a single i-phase in Al70Cu20Fe10 composition could be obtained by mechanical alloying and subsequent heat treatment of the elemental powders.12 The QC materials produced in such a way were observed to have particle size in the range of 15–25 nm. This enabled us to examine the chemical behavior of QCs with high surface to volume ratio. The aim of the present letter is to contribute to the understanding of the oxidation behavior of nano QC
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