Solids, binary systems metals

Figure 2.1. Examples of melting phase diagrams of binary systems showing complete mutual solubility in the solid and in the liquid states (L liquid field, S solid field). The melting behaviour of the Mo-V, Cs-Rb and Ca-Sr alloys is presented. Notice the different ranges of temperature involved. The melting points of the pure metal components are shown on the corresponding vertical axes. The Cs-Rb is an example of a system showing a minimum in the melting temperature. In the Sr-Ca system complete mutual solid solubility is shown in both the allotropic forms a and (3 of the two metals.

Figure 2.22. Compound formation capability in binary systems. The different element combinations are mapped on Mendeleev number coordinates and those systems are indicated in which the formation of intermediate phases has been observed (either from the liquid or in the solid state). Blank boxes indicate systems for which no certain data are available. Notice that the compound-forming alloys are crowded in a region corresponding to a large difference in the Mendeleev numbers of the elements involved (for instance, basic metals with semi-metals).

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. 

Transition Region Considerations. The conductance of a binary system can be approached from the values of conductivity of the pure electrolyte one follows the variation of conductance as one adds water or other second component to the pure electrolyte. The same approach is useful for other electrochemical properties as well the e.m. f. and the anodic behaviour of light, active metals, for instance. The structure of water in this "transition region" (TR), and therefore its reactions, can be expected to be quite different from its structure and reactions, in dilute aqueous solutions. (The same is true in relation to other non-conducting solvents.) The molecular structure of any liquid can be assumed to be close to that of the crystals from which it is derived. The narrower is the temperature gap between the liquid and the solidus curve, the closer are the structures of liquid and solid. In the composition regions between the pure water and a eutectic point the structure of the liquid is basically like that of water between eutectic and the pure salt or its hydrates the structure is basically that of these compounds. At the eutectic point, the conductance-isotherm runs through a maximum and the viscosity-isotherm breaks. Examples are shown in (125). 

A further method of producing amorphous phases is by a strain-driven solid-state reaction (Blatter and von Allmen 1985, 1988, Blatter et al. 1987, Gfeller et al. 1988). It appears that solid solutions of some transition metal-(Ti,Nb) binary systems, which are only stable at high temperatures, can be made amorphous. This is done by first quenching an alloy to retain the high-temperature solid solution. The alloy is then annealed at low temperatures where the amorphous phase appears transiently during the decomposition of the metastable crystalline phase. The effect was explained by the stabilisation of the liquid phase due to the liquid—>glass... 

Note that in contrast to binary systems where only one-phase compound layers can occur at the interface between two elementary substances, in ternary systems, when a two-component alloy or a binary compound reacts with a third metal or non-metal (either solid or liquid), the formation of both compact one-phase layers and two-phase reaction zones is observed. These may have a different morphology, possible types of which were con-sidered, for example, in works by F.J.J. van Loo and J.E. Morrel et al. 

Of the Cr—Mo—S, Cr—W—S, and Mo—W—S ternary systems, only Mo—W—S has been studied from 500—1000 °C, by Moh and Udubasa371. Details of the binary systems Mo—S and W—S are given above. The metallic system Mo—W is characterized by a complete series of solid solutions between the endmembers with continuous liquidus and solidus relations381. Complete solid solution series were also found to occur be-... 

In many cases, there is a substantial degree of solid solubility of one metal or metalloid in another. For example, the binary systems Ag-Au, Ag-Pd, Bi-Sb, Ge-Si, Se-Te,... 

Metal organic chemical vapor deposition (MOCVD) is a well-established, practical technique for forming simple as well as complex solid state films (130). For binary systems the conventional approach is to use mixtures of the most readily available molecules containing the elements of interest. This approach has been employed to prepare borides of several types. For example, iron-boron alloys have been pre-... 

TTie important role of the solvent/electrolyte system is considered finally. In principle, none of the aprotic solvents which are commonly used is thermodynamically stable against lithium metal. The same is true for LiCg, although some improved metastability is found. Binary mixtures of PC (propylene, carbonate)/EC (ethylene carbonate) are widely used [196]. PC alone is not suitable, but it is convenient as a liquefier for the solid EC. Another binary system is EC/DEC (diethyl carbonate). 1 m LiPFe in 30-50 vol% EC has a specific conductivity of k = 7 mS/cm at 20 °C. In DEC alone, k is only 2 mS/cm. In this case, DEC is the stable thirmer. Ternary systems comprising EC/PC. DEC (1 1 2-3 by volume) are also employed. In both cases, the co-insertion of solvent molecules is totally inhibited (see below). A test of... 

We comment elsewhere on the inadequacy of current bonding theory to account for the complexity of some binary systems in which solid phases appear with unexpected formulae and/or properties-for example, the oxides of caesium and the nitrides of calcium. Certain transition metals, notably Ti, V, Nb, Mo, and W, have a suprisingly complex oxide chemistry, and although it may be difficult to appreciate all the features of their structures from diagrams, these compounds are sufficiently important to justify mention here. 

The consequence of the above considerations is that binary systems of alkali metal halides form different types of phase diagrams, starting with the simple eutectic ones through the solid solution eutectic ones, the phase diagrams with the formation of a binary compound up to those with complete solid solubility. Tables 2.4 and 2.5 summarize the main features of individual phase diagrams. 

Table 20. Summary of mass spcctrometric Knudsen effusion studies since about the year 1980 of quasi-binary systems of metal halides. (The gaseous species or solid compounds are underlined if enthalpies of dissociation or formation are given)... 

Table 2.5. Binary semiconductor/metal systems observed to exhibit solid-state amorphization by interdifiiision reactions. Type of experiment, typical reaction temperature (rR), maximum thickness of the amorphous layer (2f and references are listed. (S interdiffusion of polycrystalline metal with semiconductor single crystal, and A interdiffusion of amorphous semiconductor layers with polycrystalline metal)...

A wide open series of solid solution systems, such as ionic alkali halides KCl j jBr binary and pseudobinary metallic or semiconducting alloys Ag—Cu AI—-Cu... 

The sodium-lithium phase system has been studied by thermal analysis in the liquid and solid regions to temperatures in excess of 400°C. Two liquid phases separate at 170.6°C. with compositions of 3.4 and 91.6 atom % sodium. The critical solution temperature is 442° zt 10°C. at a composition of 40.3 atom % sodium. The freezing point of pure lithium is depressed from 180.5°C. to 170.6°C. by the addition of 3.4 atom % sodium, and the freezing point of pure sodium is depressed from 97.8° to 92.2°C. by the addition of 3.8 atom % lithium. From 170.6° to 92.2°C. one liquid phase exists in equilibrium with pure lithium. Regardless of the similarity in the properties of the pure liquid metals, the binary system deviates markedly from simple nonideal behavior even in the very dilute solutions. Correlation of the experimentally observed data with the Scatchard-Hildebrand regular solution model using the Flory-Huggins entropy correction is discussed. 

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