Compositional surface phase transition

Figure 2d shows the crystal after further cooling to 225°C. In addition to 111, 100 and 227) facets, new facets have appeared at 110 orientations, and the formation ofthose facets is also accompanied by higher Bi and Ni surface concentrations at those regions (about 30 at%Bi and 20 at%Ni). At this temperature, the 100 facets are also found to have undergone the compositional surface phase transition, and display surface compositions similar to those ofthe 110 surfaces. 

While both LRO and SRO effects on segregation are determined largely by the energetic balance as reflected by the magnitude of r, the role of LRO is naturally more prominent, and thus received more attention in this review. Yet, as exemplified for the above mentioned ternary solid solution, SRO associated with appreciable solute-solvent interactions can affect considerably surface compositional phase transitions. Likewise, in ordered fee alloys with both LRO and SRO operative, order-disorder surface phase-transition temperatures are significantly shifted by SRO, thus modifying the individual sub-lattice... 

A renewed interest in the structure and properties of amphiphilic molecules which reside at the air-water interface has occurred in the last decade due to the fact that the monolayers comprise an idealized two dimensional system which can be probed in terms of structure, composition and phase transitions, and due to the fact that the monolayers are used to form highly ordered coatings in Langmuir-Blodgett applications. Monolayers form at an air-water interface upon dissolution of amphiphiles in a water immiscible solvent, spreading of Ae solution on a water surface, and lastly, solvent removal. Such monolayers are insoluble in the aqueous phase. Alternately, monolayers which are soluble in the aqueous phase are formed by adsorption of amphiphiles from the bulk aqueous solution to the air-water interface. A primary difference between insoluble and soluble monolayers... 

Besides the chemical composition, porosity is another property of stone which has great influence on its preservation. An increased porosity increases the exposed surface and pores allow movement of materials such as water and its solutes through the stones. If the pores are blocked or reduced in diameter such substances may be trapped within resulting in increased local interior damage. Exposure to the climatic elements is one important source of decay. Freeze-thaw cycles, in particular, result in pressures on the pore walls of the stone s interior from changes in volume during the phase transition... 

The difference between the static or equilibrium and dynamic surface tension is often observed in the compression/expansion hysteresis present in most monolayer Yl/A isotherms (Fig. 8). In such cases, the compression isotherm is not coincident with the expansion one. For an insoluble monolayer, hysteresis may result from very rapid compression, collapse of the film to a surfactant bulk phase during compression, or compression of the film through a first or second order monolayer phase transition. In addition, any combination of these effects may be responsible for the observed hysteresis. Perhaps understandably, there has been no firm quantitative model for time-dependent relaxation effects in monolayers. However, if the basic monolayer properties such as ESP, stability limit, and composition are known, a qualitative description of the dynamic surface tension, or hysteresis, may be obtained. 

If components 1 and 2 are miscible, then the number of surface phases in equilibrium at the transition pressure is two, and / = 2. In this case, the surface pressure varies continuously with film composition. If the components are immiscible, the number of surface phases in equilibrium at the transition pressure will be 3, and/= 1. Variation of film composition will not alter the transition pressure. 

Where applications to industrial combustion systems involve a relatively limited set of fuels, fire seeks anything that can bum. With the exception of industrial incineration, the fuels for fire are nearly boundless. Let us first consider fire as combustion in the gas phase, excluding surface oxidation in the following. For liquids, we must first require evaporation to the gas phase and for solids we must have a similar phase transition. In the former, pure evaporation is the change of phase of the substance without changing its composition. Evaporation follows local thermodynamics equilibrium between the gas... 

Application of the ignition stimulus (such as a spark or flame) initiates a complex sequence of events in the composition. The solid components may undergo crystalline phase transitions, melting, boiling, and decomposition. Liquid and vapor phases may be formed, and a chemical reaction will eventually occur at the surface... 

Figure 9.2 shows the correlation functions for the reactive state just below the phase transition point (Yco = 0.56 < yz). The steady state is reached in a very short time. The particle composition on the surface is a mixture of A and B particles with many empty sites in between. 

The temperature at which the reactivity of n-hexane has been studied is very near the temperature of demixing of the Ni-Cu alloys. Therefore, the surface phase will be reminiscent of the phase expected for lower temperatures, except that the transition to the solid solution has already started, which can explain the slight dependence on bulk composition. 

Knowledge of the mode of crystn of TNT is essential because it underlies the widespread practice of melt-pouring employed in the preparation of the commonly-used composite expls, such as Composition B. Samples of TNT obtained by sublimation onto a condensing surface held at a temp (78°) close to the mp, or by freezing melts at temps close to the mp, consist solely of the simple monoclinic form (Ref 26). Crystn from solvents at room temp, or from strongly supercooled melts, yields primarily monoclinic variant forms. Orthorhombic TNT is formed by crystn from solvents at low temps. At least seven morphological types of TNT have been identified (Ref 48). Two types have been identified by nuclear quadripole resonance (NQR) (Ref 66) a phase transition was noted at... 

It is possible to control the pressures at which the phase transitions occur by fine tuning the strength of intermolecular interactions between the amphiphilic molecules. The interactions between the hydrophobic tails depend on temperature [37], while the interactions between the hydrophilic heads depend on the chemical composition of the subphase, namely its pH and ionic strength [4], For example, the fatty acid molecules in films prepared on subphase with high pH and high concentration of divalent salt, such as CaCl2 or CdCl2, are normal to the surface, i.e. are in solid state, even at low pressures. Pressure-area isotherms of such films are featureless compressed films are stable and easy to transfer [38]. 

The majority of publications on cuticular lipids involve analyses of lipid composition. Which compounds are present, and what is their function Correlations between lipid composition and water-loss have provided indirect tests of the phase transition hypothesis, under the assumption that changes in lipid composition predictably affect lipid properties. In this section, we summarize available information on how specific structural changes affect the physical properties of pure surface lipids, as well as how different lipids interact with each other. 

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