Nucleation sites, preferential

Nucleation sites, preferential (film formation) Positions on a surface that have a high chemical reactivity and will react with mobile adatoms more readily than most of the surface. The site may be due to chemistry or morphology. Examples Steps in the surface providing a high coordination at the base of the step inclusion of tin in one surface of float glass. 

Because of the possibility of focusing laser beams, tlrin films can be produced at precisely defined locations. Using a microscope train of lenses to focus a laser beam makes possible tire production of microregions suitable for application in computer chip production. The photolytic process produces islands of product nuclei, which act as preferential nucleation sites for further deposition, and tlrus to some unevenness in tire product film. This is because the subsuate is relatively cool, and therefore tire surface mobility of the deposited atoms is low. In pyrolytic decomposition, the region over which deposition occurs depends on the drermal conductivity of the substrate, being wider the lower the thermal conductivity. For example, the surface area of a deposit of silicon on silicon is nanower dran the deposition of silicon on silica, or on a surface-oxidized silicon sample, using the same beam geomeU y. 

The nucleation behavior of transition metal particles is determined by the ratio between the thermal energy of the diffusing atoms and the interaction of the metal atoms at the various nucleation sites. To create very small particles or even single atoms, low temperatures and metal exposures have to be used. The metal was deposited as metal atoms impinging on the surface. The metal exposure is given as the thickness (in monolayer ML) of a hypothetical, uniform, close-packed metal layer. The interaction strength of the metals discussed here was found to rise in the series from Pd < Rh < Co ( Ir) < V [17,32]. Whereas Pd and Rh nucleate preferentially at line defects at 300 K and decorate the point defects at 90 K, point defects are the predominant nucleation center for Co and V at 300 K. At 60 K, Rh nucleates at surface sites between point defects [16,33]. 

Despite the fact that the electrodeposition of copper and silver at the water-DCE and the water-dichloromethane interfaces has been generally regarded as the first experimental evidence for heterogeneous ET at externally biased ITIES [171], a very limited amount of work has dealt with this type of process. This reaction has also theoretical interest because the molecular liquid-liquid interface can be seen as an ideal substrate for electrochemical nucleation studies due to the weak interactions between the interface and the newly formed phase and the lack of preferential nucleation sites always present at metallic electrodes. 

Johans et al. derived a model for diffusion-controlled electrodeposition at liquid-liquid interface taking into account the development of diffusion fields in both phases [91]. The current transients exhibited rising portions followed by planar diffusion-controlled decay. These features are very similar to those commonly observed in three-dimensional nucleation of metals onto solid electrodes [173-175]. The authors reduced aqueous ammonium tetrachloropalladate by butylferrocene in DCE. The experimental transients were in good agreement with the theoretical ones. The nucleation rate was considered to depend exponentially on the applied potential and a one-electron step was found to be rate determining. The results were taken to confirm the absence of preferential nucleation sites at the liquid-liquid interface. Other nucleation work at the liquid-liquid interface has described the formation of two-dimensional metallic films with rather interesting fractal shapes [176]. 

Crystallization Crystallization is used to separate the API from its solvent and impurities, or to separate racemic mixtures in solution. Crystallization occurs from a supersaturated solution. Important conditions are the temperature, concentration, stirring rate, and heating and cooling rate. Seeding with the desired API can assist in providing nucleation sites for the preferential crystallization of the API. 

With respect to the rate of nonrandom nucleation, the essence of the rate equation (6.7) is unchanged. However, we may have a spectrum of preferential nucleation sites p, with number density g (p), Therefore, Rn becomes... 

In this paper, the importance of particle and whisker reinforcement to creep and creep rupture behavior of ceramics is discussed. Particle and whisker additions generally increase both the fracture toughness and creep resistance of structural ceramics. These additions also act as nucleation sites for cavities. Cavities form preferentially in tensile specimens. This results in a creep asymmetry, in which composites creep faster in tension than in compression. As a consequence of cavitation, the stress exponent for creep in tension 6-10,... 

Compressive layers may be formed on glass ceramics by controlling the crystallization sequence so that the surface crystallizes and becomes rigid before the interior. As the interior subsequently crystallizes, it shrinks, thereby placing the surface in compression. Preferential surface crystallization may occur if the article is heated in a wet atmosphere or if the surface is abraded to provide nucleation sites. 

Cemented faults and fractures The main porosity reduction mechanism is cementation (Fig. 2a). Cementation is taken here to cover situations where cements have developed from (i) fluids flowing along the fault zone, or (ii) preferential growth in cataclasites because of the high concentration of nucleation sites on the newly created fracture surfaces. 

Cementation in the phreatic zone occurred preferentially in zones of high primary permeability, whereas vadose cementation occurred principally in association with soil development. Pedogenic carbonates may have served as nucleation sites for later phreatic cementation, leading to complex zones of mixed pedogenic and phreatic cements. 

The calculations show that the substitutional Ru should be active for CH3OH dissociation. As mentioned earlier experimentally methanol does not bind at Ru because the sites are preferentially covered by H2O and OH. The Ru atoms in the surface provide nucleation sites only for OHgds formation. Like pure RUio, substituted Ru shows smaller and E for H2O dissociation than does Pt. The calculated D,. hc of H2O on (Pt3)(Pt4Ru3) is 0.31 eV, 0.5 eV less endothermic than on pure Pt (Fig. 6). The activation energy for H2O dissociation on (Pt3)(Pt4Ru3) is also smaller by 0.18 eV than on pure Pt, in agreement with earlier work of Anderson et al. [86]. Assuming that the pre-exponential factors A in the Arrhenius equation, k=A exp(-E /RT), are approximately the same for both pure Pt and mixed Pt-Ru clusters, a decrease of 0.18 eV in E would increase the rate of H2O dissociation reaction by a factor of 1000. 

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