Alloy composition description

Our view of the catalyst surface is schematically depicted in Figure 4. The indirect and direct characterization data for Pt indicates that it is present in a zero valence state. The Pt will therefore be distributed among Pt atoms, Pt clusters that are larger than one atom and Pt present as a Pt/Sn alloy. Thus, a description of the state of Pt in the Pt-Sn-alumina catalyst involves determining the fraction present in each of the three states. Furthermore, both of the direct methods for determining the Pt/Sn alloy composition, XRD and TEM, indicates that only the PtSn =1 1 alloy is formed. Thus,... 

Examples of atomistic models addressing PEMFC materials s stability and aging, a) DFT modeling of surface oxide formation b) atomistic description of nanoparticle (Ostwald) ripening c) calculation of surface and bulk composition of alloyed catalyst d) change of potential as function of alloy composition. 

The generally accepted theory of electric superconductivity of metals is based upon an assumed interaction between the conduction electrons and phonons in the crystal.1-3 The resonating-valence-bond theory, which is a theoiy of the electronic structure of metals developed about 20 years ago,4-6 provides the basis for a detailed description of the electron-phonon interaction, in relation to the atomic numbers of elements and the composition of alloys, and leads, as described below, to the conclusion that there are two classes of superconductors, crest superconductors and trough superconductors. 

Solid solutions are very common among structurally related compounds. Just as metallic elements of similar structure and atomic properties form alloys, certain chemical compounds can be combined to produce derivative solid solutions, which may permit realization of properties not found in either of the precursors. The combinations of binary compounds with common anion or common cation element, such as the isovalent alloys of IV-VI, III-V, II-VI, or I-VII members, are of considerable scientific and technological interest as their solid-state properties (e.g., electric and optical such as type of conductivity, current carrier density, band gap) modulate regularly over a wide range through variations in composition. A general descriptive scheme for such alloys is as follows [41]. 

Steels and other structural transition-metal alloys are hardened by various extrinsic factors. The compositions and internal micro-structures of these materials are very complex. Therefore, simple descriptions and/or interpretations of their behaviors cannot be given, so they will not be discussed here. 

No single volume could contain the complete description of the composition, properties, and structures of ferrous alloys. Further, the effect of heat treatment and other methods of changing the properties of alloys constitutes an entire science unto itself. Accordingly, the description given of ferrous metallurgy will be only an overview of this enormously important area. 

The situation in the solid state is generally more complex. Several examples of binary systems were seen in which, in the solid state, a number of phases (intermediate and terminal) are formed. See for instance Figs 2.18-2.21. Both stoichiometric phases (compounds) and variable composition phases (solid solutions) may be considered and, as for their structures, both fully ordered or more or less completely disordered phases. This variety of types is characteristic for the solid alloys. After a few comments on liquid alloys, particular attention will therefore be dedicated in the following paragraphs to the description and classification of solid intermetallic phases. 

An extension of the application of these maps to the systematic description of certain groups of ternary alloys has been presented also by Pettifor (1988a, b). Composition averaged Mendeleev numbers can be used, for instance, in the description of pseudo-binary, ternary or quaternary alloys. All these maps show well-defined domains of structural stability for a given stoichiometry, thus making the search easier for new ternary or quaternary alloys with a particular structure type (and which, as a consequence, may have the potential of interesting properties and applications (Pettifor 1988a, b)). 

Statistical thermodynamic descriptions of these transitions in substitutional alloys have been developed for the cases of both binary and ternary alloys , using a simple nearest neighbor bond model of the surface segregation phenomenon (including strain energy effects). Results of the model have been evaluated here using model parameters appropriate for a Pb-5at%Bi-0.04at%Ni alloy for which experimental results will be provided below. However, the model can be applied in principle to the computation of equilibrium surface composition of any ternary solution. 

---