Phases Present

Shorthand Notation for Electrochemical Cells Although Figure 11.5 provides a useful picture of an electrochemical cell, it does not provide a convenient representation. A more useful representation is a shorthand, or schematic, notation that uses symbols to indicate the different phases present in the electrochemical cell, as well as the composition of each phase. A vertical slash ( ) indicates a phase boundary where a potential develops, and a comma (,) separates species in the same phase, or two phases where no potential develops. Shorthand cell notations begin with the anode and continue to the cathode. The electrochemical cell in Figure 11.5, for example, is described in shorthand notation as... 

In all appHcations involving zirconia, the thermal instabiHty of the tetragonal phase presents limitations especially for prolonged use at temperatures greater than - 1000° C or uses involving thermal cycling. Additionally, the sensitivity of Y—TZP ceramics to aqueous environments at low temperatures has to be taken into account. High raw material costs have precluded some appHcations particularly in the automotive industry. 

The phases present in products can differ from those predicted from equilibrium diagrams. Nonequilibrium metastable phases form at solidification rates experienced in commercial ingots. Because of the low rate of diffusion of iron in alurninum, equilibrium conditions can only be established by long heat treatments and are very slowly approached at temperatures below about 550 °C. Small additions of other elements, particularly manganese, can also modify the phase relations. 

Low—medium alloy steels contain elements such as Mo and Cr for hardenabiHty, and W and Mo for wear resistance (Table 4) (7,16,17) (see Steel). These alloy steels, however, lose their hardness rapidly when heated above 150—340°C (see Fig. 3). Furthermore, because of the low volume fraction of hard, refractory carbide phase present in these alloys, their abrasion resistance is limited. Hence, low—medium alloy steels are used in relatively inexpensive tools for certain low speed cutting appHcations where the heat generated is not high enough to reduce their hardness significantly. 

Many techniques have been developed to accomplish this, for example, the use of a cooled recirculating system in which the chlorine is dissolved in one part and the allyl chloride is dissolved and suspended in another (61). The streams are brought together in the main reaction zone and thence to a separator to remove water-insoluble products. Another method involves maintaining any organic phase present in the reaction zone in a highly dispersed condition (62). A continuous reactor consists of a recycle system in which make-up water and allyl chloride in a volume ratio of 10—50 1 are added... 

One of the serious defects of the conventional amalgams is the corrosion of the Sn Hg phase, which is normally absent or, if it forms, mostiy disappears in amalgams with the increased copper content hence, the term non-y2 amalgams. The phases present in two conventional amalgams used in clinical testing (56,58,62) are... 

The final factor influencing the stabiHty of these three-phase emulsions is probably the most important one. Small changes in emulsifier concentration lead to drastic changes in the amounts of the three phases. As an example, consider the points A to C in Figure 16. At point A, with 2% emulsifier, 49% water, and 49% aqueous phase, 50% oil and 50% aqueous phase are the only phases present. At point B the emulsifier concentration has been increased to 4%. Now the oil phase constitutes 47% of the total and the aqueous phase is reduced to 29% the remaining 24% is a Hquid crystalline phase. The importance of these numbers is best perceived by a calculation of thickness of the protective layer of the emulsifier (point A) and of the Hquid crystal (point B). The added surfactant, which at 2% would add a protective film of only 0.07 p.m to emulsion droplets of 5 p.m if all of it were adsorbed, has now been transformed to 24% of a viscous phase. This phase would form a very viscous film 0.85 p.m thick. The protective coating is more than 10 times thicker than one from the surfactant alone because the thick viscous film contains only 7% emulsifier the rest is 75% water and 18% oil. At point C, the aqueous phase has now disappeared, and the entire emulsion consists of 42.3% oil and 57.5% Hquid crystalline phase. The stabilizing phase is now the principal part of the emulsion. 

The phase-distribution restrictions reflect the requirement that ff =ff at equilibrium where/is the fugacity. This may be expressed by Eq. (13-1). In vapor-hquid systems, it should always be recognized that all components appear in both phases to some extent and there will be such a restriction for each component in the system. In vapor-liquid-hquid systems, each component will have three such restrictions, but only two are independent. In general, when all components exist in all phases, the uumDer of restricting relationships due to the distribution phenomenon will be C(Np — 1), where Np is the number of phases present. 

For an alloy containing 30% B describe the changes that occur as it is cooled from 1600°C to 0°C. Give the proportions of phases present immediately above and immediately below each temperature at which a reaction occurs. 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

X-ray Diffraction (XRD) is a powerful technique used to uniquely identify the crystalline phases present in materials and to measure the structural properties (strain state, grain size, epitaxy, phase composition, preferred orientation, and defect structure) of these phases. XRD is also used to determine the thickness of thin films and multilayers, and atomic arrangements in amorphous materials (including polymers) and at inter ces. 

As we have seen, the orientation of crystallites in a thin film can vary from epitaxial (or single crystalline), to complete fiber texture, to preferred orientation (incomplete fiber texture), to randomly distributed (or powder). The degree of orientation not only influences the thin-film properties but also has important consequences on the method of measurement and on the difficulty of identifying the phases present in films having multiple phases. 

The density of the polymer will clearly depend on the density of the soft phase (usually low), and the density of the hard phase (generally higher with crystallisable polar blocks) and the ratio of the soft and hard phases present. It will also clearly depend on the additives present and to some extent on the processing conditions, which may affect the crystalline morphology. 

In many cases, both liquid and vapor are present in a vessel. Experiments indicate that the blast wave from expanding vapor is often separate from that generated by flashing liquid. However, it is conservative to assume that the blast waves from each phase present are combined. This method is given in Figure 6.29. 

The ratio of the ionic liquid to the organic phase present in the reactor also plays an important role. A too high level of ionic liquid results in much longer decantation time and causes lower dimer selectivity. To combine efficient decantation and a reasonable size for the settler in the process design, it has been proposed that the separation of the two phases be performed in two distinct settling zones arranged in parallel [38]. 

The term ff denotes the number of independent phase variables that should be specified in order to establish all of the intensive properties of each phase present. The phase variables refer to the intensive properties of the system such as temperature (T), pressure (P), composition of the mixture (e.g., mole fractions, x ), etc. As an example, consider the triple point of water at which all three phases—ice, liquid water, and water vapor—coexist in equilibrium. According to the phase rule,... 

Sample No. Temp. Fe,K Temp. Zn,K AT, K Zn in system. Pa P zn at 793 K, Pa -AGzn kJmol Phases present in final layer... 

Thus the variations of the electrode potential during discharge and charge, as well as the phases present and the charge capacity of the electrode, directly reflect... 

If we look at the mechanistic and crystallographic aspects of the operation of polycomponent electrodes, we see that the incorporation of electroactive species such as lithium into a crystalline electrode can occur in two basic ways. In the examples discussed above, and in which complete equilibrium is assumed, the introduction of the guest species can either involve a simple change in the composition of an existing phase by solid solution, or it can result in the formation of new phases with different crystal structures from that of the initial host material. When the identity and/or amounts of phases present in the electrode change, the process is described as a reconstitution reaction. That is, the microstructure is reconstituted. 

An additional requirement is that the reactant material must have two phases present in the tie-triangle, but the matrix phase only one. This is another way of saying that the stability window of the matrix phase must span the reaction potential, but that the binary titration curve of the reactant material must have a plateau at the tie-triangle potential. It has been shown that one can evaluate the possibility that these conditions are met from knowledge of the binary titration curves, without having to perform a large number of ternary experiments. 

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