Metal inter-metallic compounds

The reactions include hydrogenolysis or hydrocracking (rupture of C—C bonds), isomerization, dehydrocyclization and aromatization of the C U2n + 2 molecules " . They are observed with varying selectivities, which depend on the experimental conditions, e.g. temperature, pressure and the type of catalyst. A multitude of supported (Al203,Si02) or unsupported catalysts (e.g. monometallic transition metals inter-metallic compounds , bimetallic alloys, etc.) are applied to induce skeletal rearrangements of hydrocarbons. The latter, however, are the most important catalysts used in industrial practice, which allow reforming processes at low pressure and at low temperatures. 

Table 1.19 Rate of ammonia synthesis reaction on rare earth metal inter-metallic compounds ...

In the examples given below, the physical effects are described of an order-disorder transformation which does not change the overall composition, the separation of an inter-metallic compound from a solid solution the range of which decreases as the temperature decreases, and die separation of an alloy into two phases by spinodal decomposition. 

The materials problems in the construction of microchips are related to both diffusion and chemical interactions between the component layers, as shown above. There is probably a link between drese two properties, since the formation of inter-metallic compounds of medium or high chemical stability frequently leads to tire formation of a compound ban ier in which tire diffusion coefficients of both components are lower than in the pure metals. 

A guide to tire stabilities of inter-metallic compounds can be obtained from the semi-empirical model of Miedema et al. (loc. cit.), in which the heat of interaction between two elements is determined by a contribution arising from the difference in work functions, A0, of tire elements, which leads to an exothermic contribution, and tire difference in the electron concentration at tire periphery of the atoms, A w, which leads to an endothermic contribution. The latter term is referred to in metal physics as the concentration of electrons at the periphery of the Wigner-Seitz cell which contains the nucleus and elecUonic structure of each metal atom within the atomic volume in the metallic state. This term is also closely related to tire bulk modulus of each element. The work function difference is very similar to the electronegativity difference. The equation which is used in tire Miedema treatment to... 

These data underline the phase diagrams for tire pairs of elements which are in contact in microchips, where Pd-Si and Pt-Si form stable inter-metallic compounds, and An-Si forms a eutectic system. 

Figure A1.27 shows the unusual silver-strontium phase diagram. It has four inter-metallic compounds. Note that it is just five simple phase diagrams, like the Pb-Sn diagram, stuck together. The first is the Ag-SrAgj diagram, the second is the SrAgj-Sr.Agj diagram, and so on. Each has a eutectic. You can always dissect complicated diagrams in this way.

In the following we review very briefly applications of VASP to alloys and inter-metallic compounds illustrating the potential in making ab-initio predictions of materials properties, in characterizing solid surfaces, and in studying the structure and properties of complex melts. For a more extensive list of applications, see Refs. [10, 17]. 

If the major constituents of a solid alloy in contact with a liquid alloy are highly soluble in the latter without formation of compounds, progressive attack by solution is to be expected. If, on the other hand, a stable inter-metallic compound is formed, having a melting point above the temperature of reaction, a layer of this compound will form at the interface and reduce the rate of attack to a level controlled by diffusion processes in the solid state. By far the most serious attack, however, occurs in the presence of stresses, since in this case the liquid alloy, or a product of its reaction with the solid alloy, may penetrate along the grain boundaries, with resultant embrittlement and serious loss of strength. 

An ordered distribution of spheres of different sizes always allows a better filling of space the atoms are closer together, and the attractive bonding forces become more effective. As for the structures of other types of compound, we observe the validity of the principle of the most efficient filling of space. A definite order of atoms requires a definite chemical composition. Therefore, metal atoms having different radii preferentially will combine in the solid state with a definite stoichiometric ratio they will form an inter-metallic compound. 

For a review on site preference of substitutional additions to CsCl-type inter-metallic compounds see Kao et al. (1994). In this work dilute additions to NiAl, FeAl and CoAl are especially discussed. The addition of a third element to ordered Ni3Al (cP4-AuCu3 type) occurring in different ways (Ochiai etal. 1984) is another... 

In this last paragraph, only the essential crystal structure information is given for a couple of stoichiometries that are frequently found in several families of inter-metallic compounds. 

The complex hydride Mg CoH is very similar to Mg FeH. In the binary system of Mg-Co there is no solubility of Co in either solid or liquid Mg and no inter-metallic compound, Mg Co, exists in equilibrium with other phases. However, in contrast to the Mg-Fe system, the intermetallic compound MgCo exists in equili-brium in the Mg-Co binary system (e.g., [14, p. 251]). The theoretical hydrogen capacity of Mg CoH is only 4.5 wt% which is obviously lower than that of Mg FeHg due to the presence of the heavier Co element and one less H atom in the hydride formula. 

Thorium also forms inter-metallic compounds with iron, copper, aluminum, selenium, nickel, cobalt, manganese, bismuth, and many other metals at elevated temperatures. 

Consider the Mg-Ni binary phase diagram shown above. Note that in this system, inter-metallic compound formation is common for example, Mg2Ni, MgNi2 are stoichiometric compounds. 

W.K Wallace, S. G.Sankar, V. U.S.Rao Crystal Field Effects in Rare-Earth Inter-metallic Compounds... 

Recently Franken and Ponec (44) have published photoelectric work functions of Ni-Al alloy films. Since the phase diagram contains many inter-metallic compounds, the surface composition will now be determined by that compound which yields the lower surface energy, in this case the compound having the largest amount of aluminum. Indeed after an initial steep decrease in work function over a limited concentration range, a plateau of constant work function is found. 

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