NiAl oxidation

Fig. 9. Example of pock formation of NiAl oxidized at 1100K in a silica ampoule containing a mixture of Cu and Cu20 powders.

Figure 2 is an electron diffraction pattern taken from the NiAl/oxide interface. The main spots are due to the [100] zone axis of NiAl. Extra spots can be indexed as the... 

E. Schumann, C. Sarioglu, J. R. Blachere, F. S. Pettit and G. H. Meier, High-temperature stress measurements during the oxidation of NiAl, Oxid. Metals, 53 (2000), 259-272. 

In summary, the alumina nanolayers formed by the high-temperature oxidation on NiAl alloy surfaces are structurally and chemically very different from the bulk-terminated surfaces of the various A1203 phases, and they thus provide very prototypical examples of oxide phases with novel emergent properties because of interfacial bonding and thickness confinement effects. 

NixAlj x (homogeneous between 42 and 69 at.% Ni) with good mechanical and oxidation resistance properties. By quenching from high temperatures the formation of an ordered martensite is obtained which can be considered for its shape memory behaviour. For a discussion on substitutional additions to CsCl-type alloys (site preference for dilute additions to NiAl, FeAl, CoAl, etc.) see Kao et al. (1994). 

Tunnelling electrons from a STM have also been used to excite photon emission from individual molecules, as has been demonstrated for Zn(II)-etioporphyrin I, adsorbed on an ultrathin alumina film (about 0.5 nm thick) grown on a NiAl(l 10) surface (Qiu et al, 2003). Such experiments have demonstrated the feasibility of fluorescence spectroscopy with submolecular precision, since hght emission is very sensitive to tip position inside the molecule. As mentioned before the oxide spacer serves to reduce the interaction between the molecule and the metal. The weakness of the molecule-substrate interaction is essential for the observation of STM-excited molecular fluorescence. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

D. Nicolas-Chaubet, C. Haut, C. Picard, F. Millot, A M. Huntz. Linear weight gain and parabolic oxide thickness variations vs. oxidation time The signature of diffusion along two dimensions in A1203 scale formed on 13-NiAl // Mater. Sci.Eng. A.- 1989.-V. 120.-P.83-89. 

This experimental assembly is much more complex than the preceding one. The oxide surfaces are ultrathin alumina films grown on NiAl(l 1 0) single crystals, in the preparation chamber following a standard procedure [16]. The alumina films are characterized in situ by AES and LEED. The metal clusters are prepared by vacuum condensation at RT of a metal atoms beam generated by an electron bombardment evaporator calibrated by a quartz microbalance. Metal atoms condense only on the sample through an aperture placed closed to it. After preparation the sample is transferred in the reaction chamber. The characterization of the metal clusters is based on STM observations of deposits performed in the same conditions in another UHV chamber [16]. 

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