Nucleation process acceleration

The induction process is normally dominated by the nucleation process. The acceleration step tends to be dominated by growth, while the deceleration is a consequence of the extinction of growth, because of the impingement of diverse growth regions at grain boundaries [5],... 

On comparing samples A and D, it first appears that Al(OH)3 is a more reactive Al source than Na aluminate, as nearly pure ZSM-20 is already formed after 9 days heating. Secondly, as expected from the mechanistic considerations, an increase of the initial amount of Al markedly accelerates the nucleation process of the ZSM-20 "Figure 8 . Neither the crystallinity nor the Al content of the final zeolite are affected by die initial concentration.Analysis of the final liquid phases confirmed that in all cases all the aluminium was consumed during the growth process, which means that the overall yield in crystalline ZSM-20 has increased accordingly. Indeed this increase is confirmed experimentaly "Table IB". 

Positive proof is given that thermal ions or chemi-ions accelerate the nucleation process of carbon formation. On the other hand, metals which easily produce hydroxides can inhibit carbon formation, and of these, barium is the most eflBcient. Transition metals such as manganese do not seem to play a role either in the formation phase of small soot particles or in their oxidation phase. We suggest that in industrial combustion devices, their intervention occurs in the agglomeration phase from involatile oxides formed in poor combustible zones. These oxides produce positively charged solid particles which can transfer their charges to the small soot particles, and consequently they prevent the agglomeration process. 

As it has been shown by Barret [7], the nucleation of C-S-H occurs very quickly. It is provided also by the rapid drop of silica concentration in solution. Also the addition of CH nuclei, though it causes some acceleration of the process, does not eliminate the induction period [16], There are some reports indicating the poisoning of the CH nucleation process by diluted silica solution and a slowed growth of calcium hydroxide crystals as a result of silicate anions adsorption on the 001 planes [17,18]. However, this has not been proved in the case of C3S hydration a low CH crystallization rate is important because it prevent fast calcium ions removing from the solution. 

Nucleating agent (aluminium salt of p-tert-butylbenzoate) was added to depress the nucle-ation energy barrier and accelerate the nucleation process. Consequently, the portion of metastable a-crystal was significantly increased. ... 

If mechanisms one and two do not act, then under stress, a two-dimensional homogeneous nucleation would be needed. Just as with three-dimensional nucleation of phase change (e.g., condensation or crystallization) homogeneous nucleation of bare regions can be expected to be a slow, and hence rate-limiting, process. If the solid surface is rough, then stress concentrations around asperities might accelerate this nucleation process. 

One of the primary goals for the understanding of crystallization of liquids and amorphous materials is to gain a microscopic picture of the nucleation processes. MTD, which accelerates the occurrence of rare events, appears to be well suited for this purpose. 

Figure 11.8a shows the overall crystallization kinetics of PPDX obtained by DSC for neat and self-nucleated samples. When the sample is self-nucleated, the isothermal DSC data contain information of crystal growth only (assuming the self-nucleation process applied was 100% efficient in creating all necessary nuclei previously). In fact, the acceleration of the overall crystallization kinetics caused by the self-nucleation treatment is evident in Figure 11.8a, because the rates are higher for the self-nucleated sample as compared to neat PPDX at identical crystallization temperatures and also crystallization at lower supercoolings can be achieved in the self-nucleated samples. 

The main objectives of this chapter are to clarify the roles of the hydrophobic emulsifier additives added in the oil phase of O/W emulsions how they modify fat crystallization and where they interact within the emulsion droplets. One may ask why the hydrophobic emulsifiers accelerate the nucleation process. The answer may not be straightforward, because their influences on fat crystallization are controlled by their physical and chemical properties and the nature of the interactions with the fat molecules occurring in the oil phase and at the oil/water interfaces. However, the results we have obtained so far indicate that the addition of hydrophobic emulsifiers in the oil phase has remarkable effects on crystallization. Fat crystals typically form a number of polymorphs, whose crystallization properties are influenced by many factors, such as temperature, rate of crystallization, time evolution for transformation, and impurity effects, as is commonly revealed in various examples [27,28], It is reasonable to expect that these polymorphic properties of fats may interfere with the clarification of the essential properties of the interface heterogeneous nucleation that occurs in O/W emulsions. 

The addition of S-170 [61], L-195 [61], or P-170 [62] retarded the rate of crystal growth (Fig. 11). This result proves that crystal growth does not account for the increase in Tc in the O/W emulsion instead, the additives retard the rate of crystal growth. The addition of 0-170, which showed no detectable effects on the Tc of n-hexadecane in the O/W emulsion system, scarcely retarded the rate of crystal growth. The retardation effects of PGEs on the rate of crystal growth of n-hexadecane were also observed [63]. From this, it is obvious that the acceleration effect by the additives of the hydrophobic emulsifiers must be interpreted by the nucleation process. 

The addition of SOEs accelerated the nucleation process in the emulsion system through two stages as shown in the results for n-hexade-cane (Fig. 7) and PMF. The acceleration effects were more enhanced with SOEs that have long saturated fatty acid chains. 

Although the nucleation process was accelerated by the additives, the rate of crystal growth was retarded. Nevertheless, the total extent of crystallization was increased by the addition of the emulsifiers because of the accelerated nucleation. 

The assumption that all FIPs are active naturally rules out the hypothesis of Janeschitz-Kriegl that the nucleation density increases as a result of an activation process accelerated by flow. Roozemond et al. [150] developed a model based on this hypothesis. They made the following assumptions ... 

The influence of foreign bodies, surfaces, crevices and similar entities that accelerate the nucleation process has been discussed earlier in this chapter. These heterogeneities are fortuitous since they have not been deliberately introduced into the system. In many instances strong efforts are made to remove them. However, there are also cases where specific species are deliberately introduced in order to accelerate the crystallization and alter properties. Such substances have been termed nucleation catalysts. They are usually low molecular weight organic and inorganic... 

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