Nucleation site activation

The activated carbon apparently has enough nucleation sites or the correct functionalized surface sites to trap some of these single atoms before they can be incorporated into nanoparticles. 

As stated in Section 2.1, there is a waiting period between the time of release of one bubble and the time of nucleation of the next at a given nucleation site. This is the period when the thermal boundary layer is reestablished and when the surface temperature of the heater is reheated to that required for nucleation of the next bubble. To predict the waiting period, Hsu and Graham (1961) proposed a model using an active nucleus cavity of radius rc which has just produced a bubble that eventually departs from the surface and has trapped some residual vapor or gas that serves as a nucleus for a new bubble. When heating the liquid, the temperature of the gas in the nucleus also increases. Thus the bubble embryo is not activated until the surrounding liquid is hotter than the bubble interior, which is at... 

Anderson, D. L. J., R. L. Judd, and H. Merte, Jr., 1970, Site Activation Phenomena in Saturated Pool Nucleate Boiling, ASME Paper 70-HT-14, Fluids Engineering, Heat Transfer, and Lubrication Conf., Detroit, MI. (2)... 

It has been suggested [21,22] that the presence of Cu and K increases the rates and extent of Fe304 carburization during reaction and the FTS rates, by providing multiple nucleation sites that lead to the ultimate formation of smaller carbide crystallites with higher active surface area. In the present investigation, Cu- and K-promoted iron catalysts performed better than the unpromoted catalysts in terms of (1) a lower CH4 selectivity, (2) higher C5+ and alkene product selectivi-ties, and (3) an enhanced isomerization rate of 1-alkene. 

Cracking tendency, crystallization temperature, in situ atmosphere, active nucleation sites. 

In addition to adsorption, a number of other physical processes can occur in the presence of particles during sonication. For example, particles may serve as nucleation sites for cavitation bubbles. Further experimental work is required in order to adequately interpret these results. Planned Activities... 

Figure 14.15 Mechanism of PET nucleation by sodium ionomers. A particular advantage of such compounds is that they provide active nucleation sites without molecular chain scission...

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

Large concentrations of Fe + develop in the soil solution in the weeks following flooding, often several mM or tens of mM (Figure 4.5). Calculations with chemical equilibrium models show that the ion activity products of pure ferrous hydroxides, carbonates and other minerals are often exceeded 100-fold (Neue and Bloom, 1989). Evidently precipitation of these minerals is inhibited, probably as a result of adsorption of foreign solutes, such as dissolved organic matter and phosphate ions, onto nucleation sites (Section 3.7). However, once a sufficient supersaturation has been reached there is a rapid precipitation of amorphous solid phases, which may later re-order to more crystalline forms. Only a small part of the Fe(II) formed in reduction remains in solution the bulk is sorbed in exchangeable forms or, eventually, precipitated. 

On highly ordered pyrolytic graphite, HOPG(OOOl) electrodes, no UPD has been detected owing to weak carbon-lead interactions [311]. Deposition occurs by three-dimensional island growth according to Volmer-Weber mechanism. Initial steps are controlled by progressive nucleation on active sites and hemispherical diffusion. 

The activation energy of decomposition of both irradiated and unirradiated ammonium perchlorate in the orthorhombic form below 240°C is ca. 18.0 kcal/mole according to Freeman. This seems to indicate that irradiation of ammonium perchlorate does not change the decomposition mechanism but provides many more nucleation sites. 

G denotes the shear modulus and a is the specific interfacial energy. In the sense of Eqn. (6.8), we can use Eqn. (12.5) to calculate the activation energy for the nucleation of martensite. Normally, AGtr >RT, which implies that martensite nucleation is unlikely to be induced by thermal fluctuations. We conclude that the nucleation is heterogeneous and dislocation arrays are the nucleation sites. 

If, however, a thin passivating MeO film covers the surface, stochastic nucleation of active sites for the permeation of H and hydride formation occurs. Such a process can be described phenomenologically as outlined in Section 6.2.3 by... 

At the present time it is not possible to assign a precise function for any alkaline phosphatase. Undoubtedly bone phosphatase is concerned in ossification and two alternative roles have been proposed (1) Precipitation of calcium phosphate is induced by the localized production of high concentrations of Pi owing to phosphatase activity (33) (2) the enzyme permits crystal growth at nucleation sites in the matrix by ensuring the removal and continued absence of PPi which is known to be a crystal poison (60). Other factors must be involved (35) because tissues with high concentrations of alkaline phosphatase (e.g., gut, kidney, and... 

Prior to the aluminum deposition, it was found necessary to "activate" the wafer surface by exposing it to TiCI4 vapor. Presumably, Ti was deposited, and the authors claim that this served to provide numerous nucleation sites for the pyrolysis of the TIBAL. Deposition was carried out at temperatures in the range of 220° to 300°C and pressures were, typically, 200 to 500 mTorr. Film growth rates were about 200 to 800 A/min. Incorporation of Si into the films... 

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