The Formation of Alloys

This section is not primarily concerned with the mechanics of making alloys, but rather with the physical chemistry that determines whether they are formed or not. The term alloy has been used indiscriminately in the literature, but we shall restrict its meaning to a material containing two or more elements in the zero-valent state that are mixed at the atomic level. What happens when two metals are brought together depends on the thermodynamic functions that describe their interaction, and on temperature the former depends on their relative sizes and electronic structures. 

We consider first bimetallic substitutional alloys, where atoms of either kind can occupy the same lattice site. Now an ideal solution is one for which the enthalpy of mixing is zero, and the process of alloy formation is purely entropy-driven  

For alloys that are formed endothermically or only slightly exothermically, there are many indications of a mutual perturbation of the electronic structure. As attractive interaction increases and the process becomes more exothermic, random arrangements are replaced at certain compositions by ordered superlattices 

Interstitial alloys are formed between metals and non-metaUic or semi-metallic elements such as boron, phosphorus, carbon and nitrogen the latter occupy holes in the metal structure, which may however have to expand or re-arrange to accommodate them. Carbides and nitrides of metals of the first Transition Series form spontaneously during catalytic reactions where the reactants contain these atoms, and are themselves catalytically active. They do not exist as stable compounds of the noble metals of Groups 8 to 10. 

Electronic properties such as electrical conductance, magnetic behaviour and band structure typically show dramatic changes with alloy composition, especially where the electronic structures of the pure components differ greatly, as happens for example when the d-shell is filled. Alloys of this type (Ni-Cu, Pd-Ag, Pd-Au) were the subject of intensive research in the period 1945-1970, as it was believed that the presence of an incompletely-filled d-shell was an important feature in determining catalytic activity, and that filling would occur at some composition that could be deduced from electronic properties. The experimental results and the theoretical models that form our present state of understanding of the behaviour of electrons in alloys will be considered in the following section. 

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Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

The addition of up to 0-2% of aluminium to the galvanising bath, on the other hand, depresses the formation of alloys and produces lighter and more ductile coatings, which are more suitable for galvanised sheet since they render it more amenable to bending and forming. 

Bimetallic nanoparticles, either as alloys or as core-shell structures, exhibit unique electronic, optical and catalytic properties compared to pure metallic nanopartides [24]. Cu-Ag alloy nanoparticles were obtained through the simultaneous reduction of copper and silver ions again in aqueous starch matrix. The optical properties of these alloy nanopartides vary with their composition, which is seen from the digital photographs in Fig. 8. The formation of alloy was confirmed by single SP maxima which varied depending on the composition of the alloy. 

Chen YH, Yeh CS (2001) A new approach for the formation of alloy nanoparticles laser synthesis of gold-silver alloy from gold-silver colloidal mixtures. Chem Commun 371-372... 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

Alloys.—Investigation into the formation of alloys of niobium with other elements has hitherto been scanty, and even where alloying is known to take place the conditions for the alloy formation and the properties of the products have received little attention. Niobium appears to alloy with iron in all proportions the alloy containing 90 per cent, of iron and 10 per cent, of niobium is extremely hard.1 Niobium, usually in conjunction with tantalum because of the difficulty of their separation, can be used for incorporation into special steels.2... 

Nickel is among the very few metals with which Li does not form alloys even at moderately high temperatures, ca. 450 K [25,40], and, hence, provides unique opportunities for exploring fundamental aspects of the electrochemistry of Li, including its electrodeposition, without complications derived from the formation of alloys or other phases. 

Metals can pack in myriad ways in the formation of alloys and intermetallic compounds. An interesting structural unit is the Friauf polyhedron (Figure 7), first reported by Friauf for the copper atoms in the 24-atom face-centered compound MgCu2 [73]. It consists of... 

The role of tbe additives, which may be used in bi- or muitnnetaltic combinations (Re, Ir, Ru, Ag, Au, Ge etc.) is poorly umterstood. They help to enhance the properties of platinum hy keeping it in a suitable state of dispersion, and to modulate the acidic character of the support. They appear to oppose the sintering of metallic crystallites by the formation of alloys or polymetallic dusters. 

Galvanic replacement reactions with small silver nanoparticies have been studied in detail by Lu et al. [132], They found that a complete gold shell did not form on the surface of each individual silver nanoparticle template. Instead, the replacement reaction resulted in the formation of alloy nanorings and nanocages from multiply-twinned silver nanoparticies of decahedral or icosahedral shape. 

On the other hand, in a number of cases the question of the formation of alloys between the implant and the host or of the migration of oxygen under the influence of the implanted impurity is of importance. 

DRS measurements support the TPR results. The impregnated catalysts and steam treated (IMPV) did not show the presence of V after the reduction. Probably, the hydrogen consumption in the TPR profile is due to the reduction of cerium. The band in the d-d transition can be attributed to the formation of alloys like cerium vanadate, according to the literature [14]. Baugis et al. [15] reported that the presence of vanadate with rare earth decreases the diffusion of vanadium in the zeolite structure [14]. The existence of these compounds may affect the oxidation state, the dispersion, morphology and location of cerium species in the catalyst. 

It is not easy to find other data which show that Eg plays a role in the reactions of solids. There is one case for metals which can be used as a test the formation of alloys, which represents, at least in a sense, the reaction of two solid metals with each other. The stability of alloys of varying composition is influenced by factors such as the relative atom sizes, and the number of valence electrons per atom (the Hume-Rothery rules). 

The main uses of tellurium are as semiconductors in the formation of alloys with lead (to prevent corrosion), cast iron (to improve machinabil-ity), copper, and stainless steel in ceramic materials tinting glass, see also Chalcogens. 

CSRs between Sn(C2Hs)4 and hydrogen adsorbed on supported platinum (see reaction (1) below) has been first described in 1984. Under properly chosen experimental condition the reaction between Sn(C2Hs)4 and surface OH groups of the support has been completely suppressed. Consequently, reaction (1) provided direct tin-platinum interaction that was maintained upon decomposition of the primary surface complex (PSC) in a hydrogen atmosphere (see reaction (2) below). The net result is the formation of alloy type bimetallic surface entities... 

The decomposition of surface organometallic complexes formed in tin anchoring steps (see reactions (3) - (9)) was accomplished as a gas-solid reaction in the temperature range between 25-300 °C. The decomposition in a hydrogen atmosphere led to the formation of alloy-type bimetallic surface entities. More details on the decomposition of different surface organometallic complexes can be found elsewhere. These Sn-Pt catalysts will be referred as (H) type catalysts. 

In both the case of the model catalysts treated in air at high temperature, and of the commercial aged catalysts, the formation of alloyed phases are shown. 

Because the dynamics observed by means of pulse radiolysis indicated that the displacement process was not instantaneous, it was suggested that very short, intense irradiation, with a dose sufficient to achieve the complete reduction of all the ions, could efficiently prevent the segregation, due to electron transfer between the metals. Therefore, the method could enable the formation of alloyed clusters, of major interest for various applications, particularly catalysis. The positive influence of high dose rates, which quench the atoms in an alloyed cluster, has been demonstrated a bilayered cluster would be obtained from the same system by irradiation at a lower dose rate. " Moreover, as for monometallic clusters (Section 3.13.4.3), the high dose rate favors nucleation rather than growth, and the final sizes of the alloyed clusters are particularly small. " " ... 

The optimum methods sometimes involve metal hydrides rather than pure metals. The procedure is useful mainly in the case of metals that form stable hydrides (alkali and alkaline earth metals, Ti to Th, V to Ta, Pd). The hydrides are readily reduced to powders and the contact of the latter with the other components of the alloy is much better than It would be otherwise. The thermal decomposition of the hydrides proceeds so easily that the formation of alloys is not only not slower than in the case of pure metals, but is faster due to the small particle size of the material. In addition, the hydrogen liberated from the hydride may reduce the oxide impurities. One disadvantage inherent in the use of hydrides is that the commercial materials are usually less pure than the correspondii metals. 

Apart from the traditional applications of XPS (detection of extra-zeolite material, assessment of reduction degrees of alloy components), an attempt has been made to prove the formation of alloy particles by observation of alloy shifts with the signals of the metals involved [47]. For this purpose, however, EXAFS is by far superior. 

Cs+Au type of compound, which we have already met, it is not a suitable basis for explaining the formation of alloys. An early attempt to solve at least part of the problem involved looking at a nickel atom in a copper matrix. The 3d electrons of the nickel atom occupied highly localised energy levels around it, but they were broadened by resonant interaction with the 4s electrons of copper. The width of the d-band should increase with nickel concentration as the d-electrons... 

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