Mechanism of nucleation

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

Evaporation-of-microlayer theory. A later hypothesis for the mechanism of nucleate boiling considers the vaporization of a micro layer of water underneath the bubble. This was first suggested by Moore and Mesler (1961), who measured... 

Shai, I., 1967, The Mechanism of Nucleate Pool Boiling Heat Transfer to Sodium and the Criterion for Stable Boiling, Ph.D. thesis, Massachusetts Institute of Technology, Cambridge, MA. (2) Shapiro, A. H., 1953, The Dynamics and Thermodynamics of Compressible Fluid Flow, Vol. I, Ronald Press, New York. (3)... 

Time resolved ellipsometry is an ideal technique to investigate the mechanism of nucleation and film growth of electrochemically deposited films. 

In the optical resolution of DL-threonine mixtures by batch preferential crystallization, changes of solution concentration and crystal purity were measured. The mechanism of nucleation of the un-seeded enantiomer was discussed to explain the purity decrease of the resolved crystals. From the observation of crystallization behavior of the seed crystals of L-threonine, it was concluded that the existence of the D-enantiomer on the surface of the seed caused the sudden nucleation when they grew to attain sufficient amounts. 

Mechanism of Nucleation and Crystallization of Zeolites from Gels... 

In zeolite systems chosen for study diffusion in the liquid phase and crystal growth on the crystal-liquid interface were the two major steps in converting gels to mordenite, zeolites A and X, the former being the rate-determining step for mordenite and the latter for zeolite X crystallization. In the mordenite system the effect of seed crystals, with surface areas per unit mass different by an order of magnitude, demonstrated the mechanism of nucleation on the seed crystal surfaces. The data support the hypothesis that crystal growth of the zeolite occurs from the solution phase rather than in the gel phase. 

It was possible for two of the systems chosen that the nucleation and crystallization activation energies could be determined separately by distinguishing the induction period and crystal growth period in the overall crystallization process. Of the two hypotheses proposed for zeolite crystallization, in the gel phase or from the solution phase, the data support the latter hypothesis for crystal growth with the crystal-liquid surface enhancing the nucleation process in seeded systems. The precise mechanism of nucleation in unseeded systems remains to be determined. 

Electrolytes The above issue of double layer structure is important to the mechanism of nucleation and growth in ionic liquids, it may therefore be possible to control the structure at the electrode/solution interface by addition of an inert electrolyte. In this respect most Group 1 metals are soluble in most ionic liquids, although it is only generally lithium salts that exhibit high solubility. In ionic liquids with discrete anions the presence of Group 1 metal ions can be detrimental to the deposition of reactive metals such as A1 and Ta where they have been shown to be co-deposited despite their presence in trace concentrations. 

The constant K contains nucleation and growth parameters n is an integer whose value depends on the mechanism of nucleation and on the form of crystal growth. The... 

Related Calculations. The mechanism of nucleate boiling has not been clearly established, but several expressions are available from which reasonable values of heat-transfer coefficients may be obtained. These do not yield exactly the same numerical results even though based on much of the same data. There is thus neither a prominent nor a unique equation for nucleate-boiling heat transfer. Either convenience or familiarity usually governs the user s selection. 

There can be no doubt that one of the most interesting applications of the results from studies of bilayers is to biomembranes and other biostructures, Investigations of the stability and permeability of biomembranes are particularly of great interest. For instance, it is known [425-427] that a possible mechanism of the transfer of permeant (e.g. water dissolved ions, etc.) across a bilayer biomembrane is the passage of the permeant through microholes in the membrane. The statistically distributed holes in the bilayer biomembrane, formed by the mechanism of nucleation described, may thus turn out to be very important for the permeability of such biomembranes. 

Duff et al. [27] reported a study made by means of DSC and WAXD on SPS/ PPE blends of various compositions, precipitated from ethylbenzene solutions, compression molded at 330 °C for 2 min and then slowly cooled to room temperature. In particular, the WAXD patterns show that in sPS-rich blends (>50 50 wt%) sPS is in a 0 or (3 form, while small amounts of a are present in the 50 50 wt% blend. The kinetics of crystallization and the mechanism of nucleation of sPS were investigated under isothermal and nonisothermal conditions as a function of blend composition and molecular weights of the components. The experimental curves show that the half-time to crystallization, t j2, increases with increasing content and molecular weight of PPE, but is not influenced by the molecular weight of sPS. The crystallization kinetics were... 

In essence, Eq. (16) describes the formation of a two-phase structure in reactive systems, which takes place according to the mechanism of nucleation and growth under the condition that an increase in concentration of the second phase is determined by the chemical reactions. Crystallization may serve as a physical analogue for such a process. Indeed, in Refs. [124,125] a new model of crystallization kinetics was developed, which is reduced to a self-acceleration equation similar to Eq. (16). 

Crystallization data have typically been treated theoretically using either the Fisher-Tumbull model or the Avrami equation. These analyses not only allow lipid crystallization to be modeled but may also shed some hght on the mechanisms of nucleation and growth. However, there is some recent debate about the validity of such models, especially the application of the Avrami equation (42) to accurately depict crystallization of lipids. 

MC) and HPMC could significantly inhibit the crystallization of supersaturated hydrocortisone acetate (HA). The mechanism of nucleation retardation was believed to be due to the hydrogen bonding interactions between HA and the polymers. As to the... 

Cu, Pb, Cd and Ni cathodie deposition on silicon proceeds via formation of 3D nuclei. Mechanisms of nucleation were evaluated from DC current dependences on time (chronoamperograms). Examples of chronoamperograms of metal nueleation on Si are shown in Fig. la. The nucleation is typically progressive, so the deposit properties depend on the nucleation stage. 

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