Nucleation phase

A molten metal alloy would normally be expected to crystallize into one or several phases. To form an amorphous, ie, glassy metal alloy from the Hquid state means that the crystallization step must be avoided during solidification. This can be understood by considering a time—temperature—transformation (TTT) diagram (Eig. 2). Nucleating phases require an iacubation time to assemble atoms through a statistical process iato the correct crystal stmcture... 

The decomposition of a solution with composition outside the spinodal region but within the metastable region can be analyzed in a similar way. Let us assume that a sample with composition in this region is cooled to low temperatures. Small fluctuations in composition now initially lead to an increase in the Gibbs energy and the separation of the original homogeneous solution must occur by nucleation of a new phase. The formation of this phase is thermally activated. Two solutions with different composition appear, but in this case the composition of the nucleated phase is well defined at all times and only the relative amount of the two phases varies with time. 

The mechanism of pitting is usually considered as involving two phases (1) the nucleation phase (initiation phase) when the passive layer... 

This balance is shown in Fig. 1.1. The typical size of Rc is about 100 molecules— between 1 and 2 nm in diameter. Solvent molecules can adsorb on the embryos and change their surface energy the critical radius will therefore depend not only on the material of the nucleating phase but also on the solution phase. 

Breck (1) was the first to investigate the reaction in the hydrothermal formation of zeolites. He found that there is always some delay before crystallization starts. This so-called induction period can be reduced by raising the temperature or alkalinity of the reaction batch (2). As Sand (8) reported in 1968 in connection with the formation of mordenite, the nature of the Si02 material also has a decisive influence on the reaction and the nature of the zeolite crystals. The induction period as a nucleation phase is discussed by Domine and Quobex (4) in connection with kinetic investigations relating to mordenite formation. 

Therefore, Vanderhoff (11) obtained uniform growing with styrene with values of N and S with which VC would have exhibited a new nucleation phase. In the above mentioned work by Vandegaer (10), in his growth Experiments B and D with poly (vinyl toluene) seed (initial diameters 1420 and 1470 A.) he obtained uniform growing. Unfortunately we cannot compare our results because we do not know the S and N limiting values for seeds of PVC having such small diameters. 

Wettability and Surface Morphology. Surface chemical studies on crystallizable polymers have ignored, in general, the nature of the nucleating phase—i.e., vapor, solid, or liquid—and the details of formation of the polymer melt-nucleating phase interface which on solidification by cooling results in a polymer solid-nucleating phase interface (22). 

None of these types of nonstoichiometry establishes a specific phase boundary. The phase limits are determined by a competition between possible (and nucleated) phases for the available atoms,... 

After the initial nucleation phase, the physical and chemical evolution of the aerosol cloud, leading eventually to its disappearance, is dictated, on the long term, by the mechanisms of coagulation and sedimentation and, on the short term, by processes related to its radiative and thermal balance which may determine a variable H2O/H2SO4 ratio. 

The self-assembly of tubulin to form microtubules was described initially in a classic polymerization model of nucleated helical polymerization by Maruyama and Oosawa (4). Assembly involves two phases a nucleation phase followed by an elongation phase. With purified systems in vitro, nucleation can... 

Kinetic transitions between well-defined micromorphologies are usually dominated not by second order but by first order thermodynamics. Recent ideas have shown how propagating fronts of the new nucleating phase may be responsible for the limiting rate. ... 

FIGURE 8.38. Histograms for the pore sizes and densities in (a) the nucleation phase and (b) the stable growth phase. After AI Rifai et... 

David W. Oxtoby is a physical chemist who studies the statistical mechanics of liquids, including nucleation, phase transitions, and liquid-state reaction and relaxation. He received his B.A. (Chemistry and Physics) from Harvard University and his Ph.D. (Chemistry) from the University of California at Berkeley. After a postdoctoral position at the University of Paris, he joined the faculty at The University of Chicago, where he taught general chemistry, thermodynamics, and statistical mechanics and served as Dean of Physical Sciences. Since 2003 he has been President and Professor of Chemistry at Pomona College in Claremont, California. 

Equilibrium properties of the C02/sea-water system have been well researched from an experimental standpoint. In particular, the clathrate hydrate forming conditions T < 285K andP>4MPa) are well established. Several experiments have been performed under conditions mimicking the direct injection process and have attempted to study the dissolution rate of CO2 in seawater. Under direct injection conditions, the injected CO2 is in the form of a liquid droplet and a thin spherical shell of CO2 clathrate hydrate of structure I is observed to form around the CO2 drop, separating it from the sea water. The process of hydrate formation has many similarities with that of crystallization, i.e., it can be divided into a nucleation phase and a growth phase. For CO2 clathrates, the nucleation phase involves the formation of a... 

The in vitro polymerization of G-actin proceeds in three sequential phases (Figure 19-6a). The first nucleation phase is marked by a lag period in which G-actln aggregates into short, unstable oligomers. When the oligomer reaches a cer-... 

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