Alloys alloy surfaces experimental structure

This chapter is arranged as follows experimental strategies for direct determination of surface structure are discussed. Experimental findings are then presented in the areas of ionic adsorption, electrodeposition, alloy surface oxidation, and organic molecular adsorption. 

Fig. 25. Surface structures at FeCrNi(lll). (a) (001) plane of the Cr203 structure situated on the alloy substrate as experimentally determined. (For clarity, only a few layers of the oxide are shown.) (b) Square CrO structure (c) Growth phase structure (11 x 11) (d) Alternative growth phase structure (11 x 3). Reprinted from ref. 54.

A similar growth of the ordered NisAl alloy is observed experimentally during deposition of A1 on the (100) surface of Ni [46]. Here the formation of a stable c-(2x2) ordered NiAl alloy was found on the surface while the second layer was an almost entirely pure Ni layer and the third layer was enriched by Al. This type of structure corresponds to the NiAl termination of the NisAKlOO) surface, which also has an alternative truncation. The surface segregation energy of Al on the (100) surface of Ni is only about -0.1 eV, and as has been shown [24], the NiAl termination is more stable than Ni termination by approximately half of this value. 

Figure 15 Comparison between theoretical (left) and experimental (right) results for diffraction patterns obtained at four different kinetic energies 60, 66, 80, and 94 eV. The diffraction patterns are from Mn emitters in a c(2 x 2) MnNi surface alloy. The model structure used for the theoretical results was a substitutional alloy with the Mn atoms buckled out of the surface layer.

Theoretical and experimental studies of model bimetallic catalysts in recent years have distinguished between thermodynamically stable bulk alloys and so-called near surface alloys. Near surface alloys are materials where the top few surface layers are created in a chemically heterogeneous way, for example, by depositing a monolayer of one metal on top of another metal. These structures are often not the thermodynamic equilibrium states of the material. To give one example, Ni and Pt form an fee bulk solid solution under most (but not all) conditions,73 so if a monolayer of Ni is deposited on Pt and the system comes to equilibrium, all of the deposited Ni will dissolve into the bulk. There is, however, a considerable kinetic barrier to this process, so the near surface alloy of a monolayer on Ni on Pt(lll) is quite stable provided a moderate temperature is used.191 If the deposited monolayer in systems of this type has a tendency to segregate away from the surface, a common near surface alloy structure is the formation of a subsurface layer of the deposited metal.85 The deposition of V on Pd(lll) is one example of this behavior.192... 

Amongst the main experimental techniques that have been deployed to determine the composition and structure of binary alloy surfaces. Auger electron spectroscopy (AES), XPS, FEED and ISS feature most prominently. There is now a very large literature describing the results obtained, which, after some early inconsistencies had been resolved, are now generally in line with theoretical... 

The bonding of aluminium alloy components for structural engineering applications has been the subject of extensive research by the Dutch TNO Institute for Building Materials and Structures(19, 20). Apart from the evaluation and testing of a number of adhesive systems, experimental research was carried out on several structural details. Aluminium alloy surface pretreatment was by degreasing only, to represent a practical procedure. 

Figure 15.3 Relationships between the catalytic properties and electronic structure of PtsM alloys. Relationships between experimentally measured specific activity for the ORR on PtsM surfaces in 0.1 M HCIO4 at...

Surface segregation takes place in practically all metal alloys and is controlled by the chemical equilibrium between the near-surface layers and the bulk. Consequently, a successful theoretical description of this phenomenon demands a consideration of both bulk and surface properties in order to understand correlations between segregation profile, atomic structure, SRO, and temperature. For this reason, the basics of the alloy s bulk properties have to be discussed (Section 11.2) before considering the surfaces and their experimental (Section 11.3.1) as well as theoretical characterizations (Sections 11.3.2 and 11.3.3). In Section 11.3, we will introduce the methods that are in general applied to alloy surfaces. Special focus will be on a very new ab initio-based description that allows for a direct prediction of the segregation profile and the mentioned correlated parameters. This concept will then be applied to two different classes of alloy phases an intermetallic compound and a disordered alloy. The last example will demonstrate which possible effects will take place if an adsorbate comes to the surface. Besides changes in the atomic position of the surface atoms (the so-called adsorbate-induced surface reconstruction),... 

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