Phase formation

The phase behavior of a water-surfactant or an oil-water-surfactant system may be such that when two single-phase mixtures are brought in contact without mixing, and diffusion is allowed to occur, one or more new phases not initially present may form near the surface of contact. We shall call them intermediate 

In this section and the next we develop criteria for intermediate phase formation and for the related phenomenon of spontaneous emulsification. First, we consider quasi-steady-state diffusion processes leading to intermediate phase formation some time after initial contact of the phases. In the next section, intermediate phase formation on initial contact is discussed. 

Accordingly, one may neglect the accumulation term in the diffusion equation for each species. In the absence of convection and for a fluid of uniform density p, the equation for species i simplifies to 

FIGURE 6.20 Video frame showing growth of the intermediate lamellar phase as myelinic figures approximately 21 minutes after contact of 0.05 wt. % CijEj solution with a drop of 5.67 1 n-hexadecane/oleyl alcohol at 50°C. From Miller (1996) with permission. 

Because concentrations must remain finite at r = 0, the solution within the drop can be written 

Dehua and Chuanmei (1999) consider that the formation of the magnesium oxychloride phases are produced in the following reaction sequence the MgO first dissolves in the magnesium chloride solution followed by the formation of the polynuclear complexes [Mgx(0H)J(H20)z]2x y. These complexes then further react with the chloride ion to form a continuous phase or hydrogel, which subsequently converts to the crystalline phases. 

Physical changes of state are observable under suitable conditions as well-defined phenomena. However the very frequent occurrence of superheating and supercooling in liquids, supersaturation of vapors (e.g., in closed chambers), and the persistence of metastable solids (e.g., monoclinic sulfur at 0°C) show that these phase changes can be at times exceedingly 

Jjet us consider as an example the case of a saturated vapor which has been suddenly and adiabatically compressed to a vapor pressure P which is in excess of its equilibrium vapor pressure Po at the final temperature T. In order for liquid to form, it must grow by the growth of small droplets. If, however, we consider a very small droplet of the liquid phase present in the vapor, it will have an excess free energy, compared to bulk liquid, that is due to its extra surface. The magnitude of the excess surface energy is 4irrV, where r is the surface tension and r is the radius of the drop. In order for the drop and vapor to be in equilibrium, the vapor pressure P must exceed the saturation vapor pressure Po by an amount which can be calculated from the Gibbs-Kelvin equation 

For any supcrsatiiration ratio P/Po this equation gives the radius of a critical size drop whose vapor pressure corresponds to P. Drops of smaller radius will have a larger vapor pressure and tend to evaporate, while larger drops will have smaller vapor pressures arid will tend to grow in size indefinitely.  

The model which has been successful in explaining condensation from a supersaturated vapor assumes that in the saturated vapor there is an equilibrium distribution of small droplets whose concentrations can be calculated from the equilibrium constant Kn of the equation 

As a first approximation we can assume that the rate of condensation Rc will be given by the concentration of critical size drops multiplied by the rate Zc at which vapor molecules condense on their surfaces Sc 

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The requirements of thin-film ferroelectrics are stoichiometry, phase formation, crystallization, and microstmctural development for the various device appHcations. As of this writing multimagnetron sputtering (MMS) (56), multiion beam-reactive sputter (MIBERS) deposition (57), uv-excimer laser ablation (58), and electron cyclotron resonance (ECR) plasma-assisted growth (59) are the latest ferroelectric thin-film growth processes to satisfy the requirements. 

For nickel, cobalt, and hon-base alloys the amount of solute, particularly tungsten or molybdenum, intentionally added for strengthening by lattice or modulus misfit is generally limited by the instability of the alloy to unwanted CJ-phase formation. However, the Group 5(VB) bcc metals rely on additions of the Group 6(VIB) metals Mo and W for sohd-solution strengthening. 

The first three of these are solely X T.E-based approaches, involving a series of simple distillation operations and recycles. The final approach also relies on distillation (X T.E), but also exploits another physical phenomena, liqnid-hqnid phase formation (phase splitting), to assist in entrainer recovery. This approach is the most powerful and versatile. Examples of industrial uses of azeotropic distillation grouped by method are given in Table 13-18. 

Thin-film XRD is important in many technological applications, because of its abilities to accurately determine strains and to uniquely identify the presence and composition of phases. In semiconduaor and optical materials applications, XRD is used to measure the strain state, orientation, and defects in epitaxial thin films, which affect the film s electronic and optical properties. For magnetic thin films, it is used to identify phases and to determine preferred orientations, since these can determine magnetic properties. In metallurgical applications, it is used to determine strains in surfiice layers and thin films, which influence their mechanical properties. For packaging materials, XRD can be used to investigate diffusion and phase formation at interfaces... 

A. Khatchaturyan, S.M. Shapiro and S. Semenovskaya, Adaptive phase formation in martensitic... 

The basic guidelines for preventing cracking would seem to be to operate at minimum stress levels, at as low an HjS concentration as possible and to make sure that welding procedures are adequately specified and followed. Furthermore, extensive periods of operation at temperatures that might cause sigma phase formation should be avoided. 

Controlled additions of N have been made to 439 steel weld metal to prevent IGA whilst additions of Y or Ce to an 18Cr-12Ni steel have been found to be beneficial ". In the case of TIG-welded Mo-containing stainless steels, <7-phase formation can be responsibe for IGA in hot oxidising acids ). 

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

Fig. 13. Arrhenius plots of the kinetics of H atom recombination on a Ni77Cu23 alloy film catalyst. Above room temperature—active NiCu film with low activation energy. Below room temperature—film deactivated owing to a 0-hydride phase formation activation energy markedly increased. After Karpinski el al. (65).

Passivation currents, fluctuations in, 293 Passivation potential, and thermodynamic phase formation, 218 Passive film... 

Thermal desorption spectra, 171 Thermodynamic equilibrium, phase transitions at, 219 Thermodynamic phase formation, passivation potential and, 218 Time resolved measurements in the microwave frequency range, 447 photo electrodes and 493 Tin... 

ZeTdovich YaB (1943) On the theory of new phase formation cavitation. Acta Physicochim URSS 18 1-7... 

It was found that the region of formation of the chalcogenide halides depends on the pH, the solvent concentration, and the ratios of the initial components in the charge. Temperature and pressure have practically no influence on the phase formation in these systems (285). The use of bromine (283) and SeBr2 as the solvent leads to a different mechanism, having different kinetics of formation and different growth-forms of the crystals (285). 

In each of the composition diagrams in Fig. 14.2, the numbers represent a series of reactions run at a defined composition and temperature. These are isometric sulfur slices through three-dimensional K/P/RE/S quaternary phase diagrams. As just one example of what we have studied. Table 14.1 identifies the compositions at each point and the resulting phase(s). We have rigorously studied how phase formation is dependent upon the compositions of reactions for the rare-earth elements Y, Eu, and La and we have also discovered key structural relationships between the rare-earth elements, indicating a significant dependence on rare-earth and alkali-metal size for sulfides and selenides. 

In searching to formulate a mechanism of CuInSc2 phase formation by one-step electrodeposition from acid (pH 1-3) aqueous solutions containing millimolar concentrations of selenous acid and indium and copper sulfates, Kois et al. [178] considered a number of consecutive reactions involving the formation of Se, CuSe, and Cu2Se phases as a pre-requisite for the formation of CIS (Table 3.2). Thermodynamic and kinetic analyses on this basis were used to calculate a potential-pH diagram (Fig. 3.10) for the aqueous Cu+In-i-Se system and construct a distribution diagram of the final products in terms of deposition potential and composition ratio of Se(lV)/Cu(ll) in solution. 

Peter LM, Reid ID, Scharifker BR (1981) Electrochemical adsorption and phase formation on mercury in sulphide ion solutions. 1 Electroanal Chem 119 73-91 Da Silva Pereira MI, Peter LM (1982) Photocurrent spectroscopy of semiconducting anodic films on mercury. J Electroanal Chem 131 167-179... 

Totland KM, Harrington DA (1989) Anodic phase formation on lead amalgam electrodes in sodium sulfide solution. J Electroanal Chem 274 61-80... 

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