System, quasibinary

All the points so far considered are concerned with genuine binary systems. A system is binary if both the solute and the solvent are monodisperse. But macromolecular substances generally possess a molar mass distribution consequently they only form quasibinary systems with pure solvents. 

The phase-separation behavior of quasibinary systems is different from that of binary systems. This phenomenon can be most simply described in terms of the cloud-point curves of ternary systems consisting of a solvent and two monodisperse solutes. The cloud-point curve corresponds to the special case of phase separation where the volume of one of the phases tends toward zero. 

The difference between the volume fraction of the solute at the maximum of the cloud-point curve and the critical volume fraction can be used as a measure of the polymolecularity. The same holds for the difference between the maximum cloud-point temperature and the critical demixing temperature. 

Fractionation of polymers according to molar mass represents the most significant analytical application of phase-separation phenomena. The polymers of highest molar mass separate out first on lowering the temperature of a quasibinary endothermic dilute solution system. Of course, this precipitation represents the formation of a highly concentrated gel phase and a dilute sol phase. Successive decreases in temperature lead to further 

Since precipitation temperatures may lie in experimentally unfavorable regions, precipitation fractionation is often carried out by addition of precipitant to the polymer solution at constant temperature. It is advantageous to use a 1 % solution of the polymer in a poor solvent as initial solution and to add a weak nonsolvent as precipitant. To achieve a good fractionation, it is best to heat a precipitated system to redissolution and then cool the well-stirred solution to the original temperature. Further fractions are obtained by adding more precipitant. 

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Figure 6-9. Activity vs. location for a quasibinary system AX-BX during reaction, a) Local equilibrium is established, b) At inter...

If initial phases are chemical compounds, not elementary substances, the growth of the layers of two new chemical compounds in a quasibinary system takes place as a result of counter diffusion of the same-type ions or atoms of smaller size. The common ion usually does not take active part in the layer-growth process. This does not mean, however, that its presence has no effect on the mechanism of formation of the layers. The Rb2AgI3 and RbAg4J5 layers are known to form in the Rbl-AgI system. 5 5 Their formation is due to the following partial chemical reactions ... 

Note that the duplex structure of the layer of a chemical compound, observed in the quasibinary systems formed by the oxides or salts with a common ion, may be of somewhat different nature, frequently being simply a result of different crystallographic orientation of the grains of initial phases. A suitable example is the process of formation of the NiAl204 spinel between the NiO and A1203 oxide phases, studied in detail by D.W. Su-snitzky et al. 

B. Schroder, V. Leute. Solid state reactions and transport properties in the quasibinary system AgJ/RbJ // J.Phys.Chem.Solids - 1980 - V.41, No.8.- P.827-835. 

Ca/P = 2). (b) Phase diagram of the quasibinary system CaO-PjOj-fHjO) at a water partial pressure of 65.5 kPa. (After Riboud, 1973.) Note that incongruent melting of hydroxyapatite (HA) occurs beyond 1570°C under formation of a-C3P (a-TCP) and C4P (TTCP). 

Fee transition metal carbides often form complete solid solutions with each other, particularly at high temperatures and if their lattice parameters do not differ by more than 8-10%. Miscibility gaps have been observed in the quasibinary systems TiC-ZrC, TiC-HfC, VC-NbC and VC-TaC. The solid solubility of MoC and WC in the fee carbides of Groups 4 and 5 is appreciable, while hexagonal WC does not dissolve any of the fee carbides except at very high temperatures. 

Pha diagram studies of the quasibinary systems Te—TeCU Te—TeBt4... 

In 1956, the first tellurium subhalide (characterized as TeJJ was found in X-ray investigations of the Te—Tel system About 10 years later, an additional subiodide (TeJ) was prepared by hydrothermal synthesis d . Systematic reinvestigations of the phase relations in the quasibinary systems Te—TeCl Te—TeBr ... 

THERMODYNAMIC PROPERTIES OF ALLOYS OF THE ZnAsa—CdAsj AND ZnjAsa-CdjAsj QUASIBINARY SYSTEMS ... 

Invariant Equilibria Quasibinary Systems Solid Phases... 

A second quasibinary system exists the FeSi-FeCoSi(T) vertical section. It is presented in Fig. 2 after [1935Vog] with the same minor amendments to take into account the accepted Co-Si and Fe-Si binary systems [Mas2]. 

The 3NbCr2-NbFe2 section seems to be quasibinary because both limiting phases have the same crystal stracture and form congruently in the binary systems. However, the constitution of this quasibinary system is not known. 

Dub] Dubrovskaya, L.B., Geld, P.V., The Quasibinary System a-Leboite-CrSi2 (in Russian),... 

The Uquidus surface in the Fe-Cr-ZrCr2-ZrFc2 region has been reported by [1963Sve]. It looks incomplete because only one mono variant line (from the Fe rich to the Cr rich binary eutectics) is reported. It has been completed here and adapted to the accepted binary and quasibinary systems in order to be shown in Fig. 3. Notice however that it has to be considered poorly reliable. [1978Hao] confirmed that no ternary eutectic is present in the Fe rich comer of the phase diagram. 

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