Monotectic point

This kind of reaction is called a monotectic reaction and the point m is referred to as die monotectic point. 

Rai and Pandey studied the phase behavior of succinonitrile (1) + pyrene (2) mixture system (Rai and Pandey, 2002), which is a typical monotectic system (Figure 5). The enthalpy of fusion of pyrene, 17.65 kj mole-i (Chickos and Acree, 1999), is much higher than that of succinonitrile, 3.7 kJ mole-i (Rai and Pandey, 2002). The monotectic point is 416.5 K (143.3°C) at Xi=0.025. The eutectic temperature is 328.5 K (55.4°C) at i i=0.744 and the upper consolute temperature, tc (465.2 K, 192.0°C), is 48.7 K above the monotectic point. When is between monotectic and eutectic composition, the two liquids, Li (rich in pyrene) and L2 (rich in succinonitrile) are mutually immiscible. However, if the temperature is above the consolute temperature, there is complete miscibility in liquid state, i.e. only one liquid phase exists. 

For the niobium-copper system different phase diagrams of the simple eutectic type (with the eutectic point very close to Cu) have been proposed, either with an S-shaped near horizontal liquidus line or with a monotectic equilibrium. It was stated that the presence of about 0.3 at.% O can induce the monotectic reaction to occur, whereas if a lesser amount of oxygen is present no immiscibility gap is observed in the liquid. 

Monotectic mixtures arise when the individual components have similar melting points, molecular volumes and polymorphic forms. Figure 17.12(a) represents a possible phase diagram for monotectic mixtures. A typical monotectic solution occurs when SSS is mixed with SOS. Lutton (1955) determined that the a form was present and associated with limited solid solution formation, and contrasted with the a forms of other glyceride mixtures that formed continuous solid solutions (Rossell 1967). It was found, for this system, that tristearin incorporates about 50% of the SOS into a solid solution on the other hand, SOS incorporates very little SSS into a solid solution. 

Rag] postulated a quasibinary monotectic at the place, where [1950Vog] assumed the critical point C2 between L and L of the three-phase equilibrium L + L + Fe3P. The calculations, however, show at least, that there is no reason to assume such a quasibinary monotectic. 

Fig. 8.1 Illustration of typical phase diagrams of polymer solutions for (a) UCST phase separation, (b) crystallization, and (c) monotectic triple point formed by the interception of two phase diagrams...

Polymer solutions display a typical phase diagram as illustrated in Fig. 8.1a, which exhibits a highest critical phase separation temperature, called upper critical solution temperature (UCST). Within the same temperature window, polymer solutions may also crystallize below the solution-crystal coexistence line, as illustrated in Fig. 8.1b. Two kinds of phase transitions will interplay with each other, so that an interception point is observed in the corresponding phase diagrams. The interception point is a three-phase-coexisting point, as illustrated in Fig. 8.1c, called the monotectic triple point. At this point, a dilute solution, a concentrated solution and a crystalline phase can coexist. 

Mad] Thermal analysis, light microscopy, chemical analysis of layers. Armko Fe and electrolitic Cu. The alloys were melted in a vertical molybdenum-wire furnace in an atmosphere of cracked ammonia. Alloys with 50 mass% Cu and Fe and fiom 0.05 to 2.0 mass% C. Stable C-Cu-Fe diagram, miscibiUty gap, the compositions of two Uquid layers at 1150°C (invariant monotectic equiUbrium), temperature and constitution of critical point on the miscibiUty curve. 

Binary organic mixtures with PAHs can form monotectic systems. Table 2 lists the monotectic and eutectic point of a few monotectic forming PAH systems. Monotectic systems are characterized by monotectic, eutectic and upper consolute temperatures, though the upper consolute temperature is often not reported. The monotectic temperature, tM, is the temperature at monotectic composition and the upper consolute temperature is the highest melting temperature of the mixture system, i.e. the critical point where the two liquid phases having identical composition become indistinguishable. 

When the L-S curve intersects with the L-L curve in the overlapping temperature windows, both curves will be terminated at the intersection point, which is referred to the monotectic triple point. The typical phase diagram in polymer solutions is shown in Fig. 4.5. 

By means of thermal analysis, metallography and X-ray diffraction studies, Svechnikov et al. (1975) have investigated the properties of cast and annealed Nd-Yb alloys. Alloys were prepared by melting the metals in an electric arc furnace under an argon atmosphere. The interaction between the two metals was represented by a monotectic-type phase diagram (fig. 64). These two elements were found to have limited solubilities in each other in the solid state and also in the liquid state near the melting points. 

In polymer solutions, liquid-liquid (L-L) demixing is another common phase transition besides crystallization. The thermodynamic boundary conditions for both of them behave as the functions of polymer concentrations and temperatures, demonstrated as phase diagrams. The schematic L-L binodal and liquid-solid (L-S) coexistence curves in polymer solutions and their interception are shown in Figure 13.2. The illustrated L-L binodal contains an upper critical solution temperature. Some other solutions also contain binodals with a lower critical solution temperature. When the L-S curve intersects with the L-L curve in the overlapping temperature windows, both curves are terminated at the intersection point, which is referred to as the monotectic triple point. 

Ca-Na monotectic system. There is a solid phase transition from aCa to pCa at 447°C. The Na-aCa eutectic temperature is only 0.45°C below the melting point of Na. (Reprinted from Massalski, T.B., Handbook of Binary Alloy Phase Diagrams, ASM, 1990. Reprinted with permission of ASM International. All rights reserved.)... 

The phase diagram of two-crystalline polymer blends such as that of PEO/diacrylate (DA) reveals a solid-liquid coexistence gap bound by the solidus and liquidus lines intersecting with the upper critical solution temperature (UCST) (Figure 4.18) [79]. The liquidus closely matches with the melting transition points, showing isotropic (I), the coexistence of crystal + liquid (Cri + I), and crystal + crystal (Cri + 0-2) regions. The two horizontal lines represent the monotectic line (upper) and the... 

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