Vapor, supersaturated

The manner in which a film is formed on a surface by CVD is still a matter of controversy and several theories have been advanced to describe the phenomena. ] A thermodynamic theory proposes that a solid nucleus is formed from supersaturated vapor as a result of the difference between the surface free energy and the bulk free energy of the nucleus. Another and newer theory is based on atomistic nucle-ation and combines chemical bonding of solid surfaces and statistical mechanics. These theories are certainly valuable in themselves but considered outside the scope of this book. 

A difiiculty with this mechanism is the small nucleation rate predicted (1). Surfaces of a crystal with low vapor pressure have very few clusters and two-dimensional nucleation is almost impossible. Indeed, dislocation-free crystals can often remain in a metastable equilibrium with a supersaturated vapor for long periods of time. Nucleation can be induced by resorting to a vapor with a very large supersaturation, but this often has undesirable side effects. Instabilities in the interface shape result in a degradation of the quality and uniformity of crystalline material. 

Becker, R., and Doting, W. (1935). The kinetic treatment of nucleus formation in supersaturated vapors. Ann. Phys. Leipzig)[5 24, 719, 752. 

Farkas, L. (1927). The velocity of nucleus formation in supersaturated vapors. Z. Phys. Chem. 125, 236. 

Accumulation of condensed material as aerosols in the atmosphere may take place by two basic processes by condensation of supersaturated vapor or chemical reaction that leads to spontaneous formation of new particles, and by condensation, absorption, or reaction on existing particles. In the latter case, the chemical reactions may actually take place on the surface of or within existing particles. 

In practice then, a liquid having a low vapor pressure such as oleic acid or lubricating oils is carried by an inert gas to a heater where it vaporizes. As it leaves the heated area, it condenses to form the aerosol. Different generator designs based on condensation of a supersaturated vapor are discussed in the LBL report (1979). 

While condensation from a supersaturated vapor can be used to produce liquid particles, it is not as easily applied to the generation of solid particles except those that can be liquefied at modest elevated temperatures. However, these may be generated using different... 

Here, /x, /xjs>, and are, respectively, the chemical potentials of the supersaturated vapor (or the ambient phase such as the solution phase), the saturated phase, and the solid phase. From this, a generalized driving force maybe expressed as... 

Condensation—The condensation method requires the formation of a vapor-air mixture that is supersaturated with respect to the vapor. Condensation from the supersaturated vapor may then... 

Hirschfelder and co-workers (H6) give a detailed discussion of this question. It is proved that a liquid in tension such as that at temperature located on the line BC is unstable to macroscopic fluctuations in density but is stable (thermodynamically) to tiny fluctuations. Portion EF represents supersaturated vapor which also is stable (thermodynamically) to tiny fluctuations. The line CDE is proved to correspond to liquid or vapor which is thermodynamically unstable to density fluctuations of any magnitude whatever. This means that states corresponding to CDE are completely unattainable. ... 

Yet this description is not entirely correct, because even when the system is at a site in Da, and even inside Aai there is still a probability, however small, for a giant fluctuation to occur, which takes it across into Dc. It will then move to the neighborhood of c until a similar giant fluctuation takes it back to Da. Thus a mesostate represented by a probability peak in Aa does not survive forever its probability is slowly depleted in favor of a peak near (f)c. Although a is a stable solution of the macroscopic equation, the related mesostate is not strictly stable but merely long-lived, and may be called metastable. Indeed, a metastable state in thermodynamics, such as supersaturated vapor, also exists because it is stable with respect to small fluctuations, but an improbable giant fluctuation may carry it into a macro-scopically different thermodynamic state. 

Crystals grow from their supersaturated vapor by the addition of vapor atoms at their free surfaces. In this process, the surface is subjected to an effective pressure due to the difference in free energy between the solid and vapor. The interface moves outward toward the vapor as it acts as a sink for the incoming flux of atoms. The mechanism by which atoms leave the vapor phase and eventually become permanently incorporated in the crystal is often relatively complex, and the kinetics of the process depends upon the type of surface involved (i.e., singular, vicinal,... 

SUPERSATURATION (Chemical). The condition existing in a solution when if contains more solute than is needed to cause saturation. Thermodynamically, this type of supersaturatlon is closely allied to supersaturation of a vapor, since the solute cannot crystallize out in solutions free from impurities or seed crystals of the solute. See also Supersaturated Vapor. 

A fluid under a pressure lower than its vapor pressure, and especially under a negative pressure, is unstable thermodynamically with respect to formation of a bubble filled with vapors of the substance and even (in the case of negative pressure) of an empty one. A fluid subjected to negative pressure is completely analogous in this respect to a supersaturated vapor, unstable with respect to formation of a condensate, or to a supercooled liquid, unstable with respect to crystal formation. 

It has been established experimentally that all thermodynamic properties of supersaturated phases (a supersaturated vapor, a liquid heated beyond the boiling point, or a liquid cooled down below the melting point) are in no way remarkable and fail to show any substantial deviations that would point to strong heterophase fluctuations. All the more, d, fortiori, it may hence be concluded that before the transition point and at the point itself the quantity J is negligibly small. 

The nucleation mode (ii ie < 0.1 pm) accounts for the majority of particles by number but because of their small size, these particles rarely account for more than a few percent of the total mass of atmospheric particles. These particles originate from condensation of supersaturated vapors from combustion processes and from the nucleation of atmospheric particles to form fresh particles (Seinfeld and Pandis, 1998 Horvath, 2000). 

There is no reason to expect that as the dimensions of a phase are reduced to the point where it contains a small number of molecules, its surface tension remains constant. This question has obvious relevance to important problems, such as condensation of droplets from supersaturated vapors. Although a number of authors have considered this problem, in these days of supercomputers, it is probably best handled by considering individual intermolecular interactions rather than a bulk property, such as surface tension. 

Nucleation Process of formation of new particles from a supersaturated vapor or a chemical reactive gas. 

If particles (or ions) are already present in a supersaturated vapor, nucleation will take place preferentially on these particles at supersaturations far smaller than for the homogeneous vapor. In this case, nucleation takes place heterogeneously on the existing nuclei at a rate dependent on the free energy of a condensate cap forming on or around the nucleus. Heterogeneous nuclei always occur in the earth s atmosphere. They are crucial to the formation of water clouds and to the formation of ice particles in supercooled clouds. 

Another process involves molecular aggregation by means of direct chemical reactions akin to polymerization. The best known example of this is the process of carbon particles in a premixed acetylene-oxygen flame. Evidently particle formation in this case does not involve condensation from a supersaturated vapor, but proceeds directly through the pyrolysis of the acetylene, forming in the process unstable polyacetylenes as intermediates in the flame. 

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