Additive nucleating

Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubiUties are exceeded in the bulk water. Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface. Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate. 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

Also, it should be noted that solubility of Xe in water is three times higher than Ar, which provides additional nucleation site for cavitation. 

Soot Highly uncertain 0-300% increase Additional nucleation sites for aerosols and surface for catalyzed S02 oxidation to H2S04... 

Solution. A simple model can be constructed based on the idea that the nucleation is heterogeneous and that the heterogeneous nucleation sites (i.e., the particles), are being neutralized as nucleation and growth proceed. Once a nucleation event has occurred at a given particle, the solid will grow and envelop the particle, and it is unlikely that an additional nucleation event will occur there. After the incubation period has passed. Eq. 19.17 may be written for heterogeneous nucleation in the form... 

Calorimetric studies show that pfa retards the reaction of alite in the early stage of reaction (G70,J30,H51,W29), but with one exception (W29) studies on the middle stage show the alite reaction to be accelerated (K45,L47,H51,T44,D12). SEM shows that C-S-H and CH are deposited on the surfaces of pfa grains before these have started to react significantly (D28). The accelerating effect is probably due mainly or entirely to the provision of additional nucleation sites on the pfa, and occurs with other fine powders (K46). A QXDA study showed that the aluminate and ferrite react more rapidly in the presence of pfa, but with belite there was no detectable effect up to 28 days and a marked reduction in rate of consumption thereafter (D12). 

The pumping time is very sensitive to MgO content in the Ceramicrete slurry. A slight increase in MgO accelerates the acid-base reaction, and the slurry sets faster. Figure 15.5 shows the effect of excess MgO on the pumping time at 200 °F (93°C). Excess MgO provides additional surface area for dissolution. It also provides additional nucleation sites... 

Ugelstad et al. [52] have shown how this theory may be used to devise a method to prepare large monodisperse particles of predetermined size. By using the appropriate amount of cosurfactant, polymer particles can be swollen with monomer to the desired size. Polymerization in conditions that prevent additional nucleation results in large monodisperse polymer particles of size 1-100 pm. This method has been criticized by other groups as being in error due to measurement selectivity. 

Alternatively, supersaturation and nucleation can be triggered by a slow ramping of the reaction temperature. Precursors are mixed at low temperature and slowly brought to the temperature at which precursor reaction and decomposition occur sufficiently quickly to result in supersaturation." Supersaturation is again relieved by a nucleation burst , after which temperature is controlled to avoid additional nucleation events, allowing monomer addition to existing nuclei to occur more rapidly than new nuclei formation. Thus, nucleation does not need to be instantaneous, but it should be a single, temporally discreet event to provide for the desired nucleation-controlled narrow size dispersions. ... 

Based on a model on the features of the double-pulse technique, various structures of silver nanoparticles grown onto a thin ITO film covered glass plate were generated and characterized [30]. With this method, the conflict between both optimal conditions for nucleation and growth is partially defused. This is due to the amount of small seed additionally nucleated at the higher polarization and resolved as soon as the potential is switched over to the lower polarization of the growth pulse. The interaction of the pulse parameters was modeled, thus forming the basis for how the electrodeposition process of noble metal clusters can be variably controlled. 

Zeolite Y with Si/Al = 3.5, crystal size < 2pm, was synthesized with 15-crown-5-ether as described in [4] and additional nucleation gel [5,6]. All samples were calcined in air at 873K. 

Small (<1%), to hopefully aid in more controlled nucleation but not adequate to achieve primarily growth on scale-up. It is subject to additional nucleation and bimo-dal distribution of the product. 

Effect of impeller speed on crystallization rate and approach to equilibrium solubility (yield). Failure to achieve equilibrium solubility may indicate accumulation of impurities at the crystallizing surfaces. An increase in impeller speed resulting in further reduction in solution concentration could indicate resumption of growth or additional nucleation (see Example 6-1). 

An increase in impeller speed was effective either because (1) the impurity film was reduced, allowing growth to resume, or (2) the increased shear caused additional nucleation. 

Issues Seeding point, linear versus controlled addition, nucleation, filtration rate... 

If the distributiorr is bimodal, this, indicates (1) that the addition or cooling rate is too high, causing additional nucleation after some growth or (2) that there is breakdown of agglomerates. 

The general recommendation is to keep supersaturation levels in the low to medium range to prevent additional nucleation events. This strategy has an additional beneficial effect on purity owing to slow crystal growth, the purity of the final crystals is usually higher compared to that in the case of fast growth rates. Finally, the addition of seeds efficiently avoids the formation of amorphous material, which can later recrystallize in an uncontrollable fashion. 

In the presence of chlorides an even larger number of transients could be observed, indicating that additional nucleation sites were activated. Measurements in solutions... 

Plaris et al. [1993] investigated also the same blend system and reported that blending had a pronounced effect on the lamellar morphology. Furthermore, the isothermal crystallization experiments indicated that the spherulite growth rate, G, and the nucleation density of the PP phase were enhanced. The authors suggested that these observations could be related to the formation of additional nucleation sites, which arise from the polymer-polymer interfaces created by the blending. 

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