Binary Eutectic Systems

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Figure 12.8 Phase diagram for a binary eutectic system (Gill, 1994).

Figure 3.20. Phase diagram of the simple binary eutectic system.

The equilibrium between the activity of the saturated solution of the solvent A and temperature in a simple binary eutectic system A-B is described by Le Chatelier-Schreder s... 

The use of the differential thermal analysis in the phase diagram determination is illustrated in Figure 3.52. A hypothetical binary eutectic system A-B with the formation of the incongmently melting compound A4B was chosen. There are three thermograms (a) to (c) shown as examples. In thermogram (a), the first heat effect at temperature ai... 

In the process of extractive crystallization, a solvent, say, acetic acid (CAS no. 64-19-7, CH3COOH, clear colorless liquid, m.p. 16.63°, b.p. 118°C), helps in extracting the desired component, say m-cresol from a binary eutectic system. The solvent helps to achieve a higher recovery of the pure component from the eutectic mixture than otherwise what would have been possible. 

Thermodynamics has taught us that, in a simple binary eutectic system, if the pressure dependency of the melting point of component 1 is higher than that of component 2, the eutectic concentration shifts towards component 2. That is,... 

The simple crystallization of a binary eutectic system only produces one of the components in pure form, while the residual mother hquor composition progresses towards that of the eutectic (section 4.3.1). There is often a need, however, to produce both components in pure form, and one way in which this may be achieved is to add a third component to the system which forms a compound with one of the binary components. Phase diagrams for systems with compound formation are discussed in section 4.3.2. 

Table 4.10 Composition of binary eutectic systems of Cg— aromatics...

P8.6 Calculate the SLE diagram for the binary eutectic systems 1,2,3-trichlorobenzene-n-decane and 1,2,4-trichlorobenzene-n-tetra-decane assuming... 

Temperature-time curves through a binary eutectic system. 

Equations (1.76) and (1.77), together with Equations (1.45)and(1.46), indicate that similar information may be expressed in terms of activity. For cases where a two-phase mixture is stable, (Figure 1.6(b)), Equations (1.76) and (1.77) indicate that the activities are constant across the two-phase field. Figure 1.7 illustrates the above points and their relationship to the conventional T versus Xphase diagram for a hypothetical binary eutectic system. 

Figure 1.8 presents the phase equilibria in a hypothetical binary eutectic system similar to that in Figure 1.7, represented on each of the three types of diagrams. This diagram is similar to those for the Ag-Cu and Ni-Cr systems. The plot of T versus ub is a Type 1 diagram and the three-phase equilibrium a-L-(3 is represented by a point. The plot of T versus Ab is a Type 2 diagram and the a-L-(3 equilibrium is represented by three points on a line, the eutectic isotherm. The plot of S versus Xb is a Type 3 diagram and the a-L-(3 equilibrium is represented by an area. Note that the forms of these diagrams correspond to those for the unary system in Figure 1.4. (Numerous examples of the three types of phase diagrams are given for unary, binary and ternary systems in Chapter 13 of Reference [2], Reference [5] and Chapter 2 of Reference [8].

The simplest way to introduce the concepts appropriate to growth of crystals from metal fluxes is with a simple binary eutectic system. Suppose we wish to produce crystals of Si. We know perfectly well that there is an entire industry built... 

Nitrile groups in one of the p-positions increase the dielectric anisotropy. Recently a dielectric of +14 was described for a binary eutectic system with nematic meso-phase at room temperature consisting of 2/3 (c) and 1/3 (d) with a crystalline-nematic temp. —30°C, nematic-isotropic temperature 62°C [81] ... 

Depending on composition, several different types of microstructm-es are possible for the slow cooling of alloys belonging to binary eutectic systems. These possibilities will be considered in terms of the lead-tin phase diagram. Figure 9.8. 

The solubility limit at some temperature corresponds to the maxLmmn concentration of one component that will go into solution in a specific phase. For a binary eutectic system, solubility limits are to be foimd along solidus and solvus phase boundaries. 

W " P + Q Mass fraction of eutectic microconstituent for binary eutectic system (per Figure 9.18) 324... 

Fig. 7.3 Schematic plot of the best glass-forming composition in a simple binary eutectic system...

FIG. 9 Schematic depicting a skewed coupled zone for faceted-nonfaceted binary eutectic system. (From Ref. 53.)... 

A typical example of a phase diagram with coexisting phases is a binary eutectic system such as that of Pb and Sn, commonly used in solders. The Pb-Sn phase diagram is shown in Figure 4.8. There are a number of features to notice. First, both Pb and Sn can dissolve some of the other element in their solid form, but Pb can dissolve much more Sn and this solubility persists to much lower temperatures than for solubility of Pb in Sn. Second, at temperatures below 181°C (the horizontal line on the figure) the system consists of a mixture of solid phases of Pb and Sn or a solid... 

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