Eutectic phase system

In the case of non—eutectic systems, the solid phase shows nearly ideal mixing, so that the surfactant components distribute themselves between the micelle and the solid in about the same relative proportions (i.e., both the mixed micelle and mixed solid are approximately ideal). However, in the case of the eutectic type system, the crystal is extremely non-ideal (almost a single component), while the micelle has nearly ideal mixing. As seen in earlier calculations for ideal systems, even though the total surfactant monomer concentration is intermediate between that of the pure components, the monomer concentration of an individual component decreases as its total proportion in solution decreases. As the proportion of surfactant A decreases in solution (proportion of surfactant B increases) from pure A, there is a lower monomer concentration of A. Therefore, it requires a lower temperature or a higher added electrolyte level to precipitate it. At some... 

In the eutectic alloy system AB, the compositions of the three conjugate phases of the eutectic are pure A, pure B and liquid of 80% B. Assuming equilibrium solidification of an alloy composed of 40% A and 60% B at a temperature just below the eutectic temperature, calculate the percentage of the primary A. Calculate the percentage of the total A. 

Figure 11.6 shows an example of the phase diagram for a reactive system, in which a compound C is formed from components A and B. An isothermal cut and the polythermal projection are also shown. Such a phase diagram can be obtained via a reaction invariant projection of a higher-dimensional simple eutectic phase diagram. AS and BS are binary nonreactive eutectics, since their presence is not affected by the reaction, while ACSb and BCSa are ternary reactive eutectics. Similar... 

Figure 2.2 Phase diagram for a hypothetical eutectic binary system. There are three single-phase fields (a, fi, and L) and three two-phase fields (a + 3, a + L, and 3 + L).

The polymorphic nature of the multicomponent TAG systems is related to phase behavior that is affected by molecular interactions among the component TAGs. The fat crystals in a miscible phase may exhibit simple polymorphic properties. By contrast, the immiscile eutectic phase may show complicated polymorphic properties as a superposition of the polymorphic forms of the component TAGs. Furthermore, if the molecular compound is formed by specific TAG components, the polymorphic behavior becomes complicated, as shown for the case of POP-OPO (see Section 5.2). Therefore, knowing the phase behavior of the principal TAG components is a prerequisite for precise understanding of the polymorphism of natural fats. 

The phase behavior of the mixed TAG system is influenced by polymorphism. For example, a miscible phase is formed in a and p polymorphs, but it transforms into a eutectic phase in p, as revealed in the SSS-PPP mixture. Then, the polymorphic occurrence is largely affected by cooling rate and temperature fluctuation, and it is therefore necessary to observe the polymorphic properties of the natural fats by varying the rate of coohng or by fluctuating the temperature (so-called tempering). 

In a solid solution, the two components crystallize together in a homogeneous one-phase system. The particle size of the drug in the solid solution is reduced to its molecular size. Thus, a solid solution can achieve a faster dissolution rate than the corresponding eutectic mixture. 

The line Kk corresponds to the three phase system solution + solid A + sohd B this is a divariant system but when p is fixed the representative points for the solution lie on a line. The three lines k K, k K and k K meet at K which is the ternary eutectic point at which the four phases, liquid, solid A, solid B and solid C are in equilibrium. This system is monovariant, but at a given pressure there is only one point representing this state, namely K. 

These two theorems are general and include as particular cases the theorems established in chap. XVIII, 6 and in chap. XXIII. They do not however apply to mono variant or invariant systems. Thus the eutectic point, which is certainly an indifferent point, does not represent, mathematically, an extreme value of or p for it is the point of intersection of two curves each of which refers to a two-phase system e.g. solution + ice or solution-f salt) under constant pressure. Only at the eutectic do three phases (solution + salt + ice) coexist. A mono variant three-phase system does not have an isobaric curve. 

The eutectically solidified ceramics represent a large group of ceramic systems with microstructures that can be tailored during cooling. Since the eutectic solidification is based on the simultaneous crystallization of two solids from one liquid, many complex arrangements of the eutectic phases have been observed [93]. A schematic representation of a eutectic reaction is as follows ... 

Molten Na[AlCl4] catalyses the reaction at 600 C, in a three-phase system with gaseous COCl and solid y-AIjOj, to give very high purity aluminium(III) chloride [962], and this reaction (in the presence of carbon) has formed the basis of a patent for the industrial scale production of aluminium(III) chloride [909]. The chlorination of AljOj by phosgene has been studied in a LiCl-KCl eutectic melt at 470 C as a function of pO it is essentially a two-step process [1842] ... 

LI Eutectic phase relations in water-salt systems These systems usually involve... 

One popular synthetic fluid is a eutectic mixture containing 73.5%i of diphenyl oxide and 26.5%i of biphenyl. This is one of the oldest and most widely used synthetic HTFs. This fluid can be used in both liquid and vapor phase systems. Because of its high freezing point (12°C), it often requires protection against freezing. This fluid has exceptionally high thermal stability. The recommended maximum service temperature is 400°C at a pressure of 16mPa (ISOpsia). However, because of the toxicity and odor problems, this fluid has lost popularity over the years. 

The phase diagram of such a system has four planes. The plane pc(A) is the vertical projection of the plane of primary crystallization of the compound A, plane pc(B) represents the plane of primary crystallization of the component B, and the plane pc(C) refers to the primary crystallization of compound C. Finally, the plane pc(AB) is the projection of the plane of primary crystallization of compound AB. In the case of the congruently melting compound AB its figurative point lies inside the plane of the primary crystallization of this compound. In comparison with the simple eutectic ternary system a new boundary line, Ctj - Ct2, which represents the common crystallization of compounds C and AB, will arise. The joint AB-C divides the ternary system A-B-C into two simple... 

The phase diagram has four planes pc(A), pc(B), pc(C), and pc(ABC), representing the vertical projections of the planes of the primary crystallization of the individual compounds. The figurative point of the ternary compound ABC lies inside the ternary system A-B-C. There are three boundary lines etj -et2, et2 and etj -etj representing the common crystallization of compounds B, C, and A with AB, respectively. The joints of the figurative point ABC with the apexes of the concentration triangle divide the phase diagram into three simple eutectic ternary systems A-ABC-B, B-ABC-C, and A-ABC-C. The points, where the individual joins of ABC with the apexes cross the boundary lines, form the summits of the boundary lines. 

Three possibilities exist when a salt with a polyphosphate x-ray pattern crystallizes from a melt containing an excess of phosphorus pentoxide. 1. The phosphorus pentoxide is incorporated into the polyphosphate chains converting the chains to crystalline ultraphosphates. 2. The excess phosphorus pentoxide does not enter the polyphosphate crystal structure, but forms an amorphous phase between the crystals of polyphosphate. The amorphous phase is not detected by x-ray. 3. The excess phosphorus pentoxide does not enter the crystal structure of the polyphosphate, but forms as an ultraphosphate between the crystalline polyphosphate crystals as a eutectic phase. (This latter case is precisely what happens in the calcium sodium ultraphosphate system from which calcium phosphate fibers are grown (21) and the phase diagram of Hill et. al. is obeyed as it should be.)... 

All the DSC methods of purity determination depend on the applicability of the van t Hoff equation. This restricts the method to systems where the impurity forms a simple eutectic phase diagram with the major component that is, the impurity or impurities are soluble in the melt and the components do not form solid solutions (53). Use of the van t Hoff equation assumes that the solution of impurity in major components above the melting point is an ideal solution in the thermodynamics sense. Also, the method assumes that the solid-liquid system is essentially in true thermodynamic equilibrium during the measurements. Failure to meet any of these conditions will lead to erroneous results. Other possible errors are associated with the instrumentation employed. This involves the use of the smallest possible sample size consistent with homogeneity (50), proper encapsulation to minimize temperature gradients within the sample, and the slowest possible heating rate lo approach equilibrium conditions. It is recommended that the melting... 

Eutectic A system consisting of two or more solid phases and a liquid that coexist at an invariant point (constant temperature in an isobaric system). This temperature is the minimum melting temperature for the assemblage of solids. Any flux of heat into or out of the system will not affect the temperature until one of the phases is exhausted. 

A juxtaposition of the data in Fig. 4.9 with the equilibrium phase diagram of the Pb—Sb system presented in Fig. 4.3 indicates that, at 252 °C and up to 3.5 wt% loading level in the alloy, antimony has the highest solubility in a-Pb dendrites. On cooling, the dendrites get oversaturated with Sb and reorganisation of the structure of the solid phases starts, whereby the content of Sb in the a-Pb dendrites diminishes to 0.1% at room temperature. This improves the mechanical properties of the alloys. At Sb content between 3.0 and 11 wt%, the amount of the eutectic phase in the alloys changes but slightly, which results in minor improvement of their mechanical properties. 

Let us now return to the phase diagram. Beneath the eutectic temperature lies a two-phase system of a very A-rich mixed crystal a and a very B-rich mixed crystal p. Further cooling alters the composition of the mixed crystals. The a mixed crystal... 

Fig. 2.3-4 Phase diagram of a eutectic binary system with total immiscibility in the two sohd phases left), phase diagrams of various types of binary systems with perfect (Type I) or partial (Type II -V) miscibility in the solid phase (right)...

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