Nucleation classification

Nucleation, the creation of crystalline bodies within a supersaturated fluid, is a complex event, since nuclei may be generated by many different mechanisms. Most nucleation classification schemes distinguish between primary nucleation - in the absence of crystals and secondary nucleation - in the presence of crystals. Strickland-Constable 27 and Kashchiev 28 have reviewed nucleation, and Garside and Davey(29) have considered secondary nucleation in particular. 

A theoretical analysis of an idealized seeded batch crystallization by McCabe (1929a) lead to what is now known as the AL law . The analysis was based on the following assumptions (a) all crystals have the same shape (b) they grown invariantly, i.e. the growth rate is independent of crystal size (c) supersaturation is constant throughout the crystallizer (d) no nucleation occurs (e) no size classification occurs and (f) the relative velocity between crystals and liquor remains constant. 

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

Fig. 3. Classification of human prion diseases. Sporadic the transformation from PrPc (circle) to PrPSc (square) occurs without apparent cause. Familial a point mutation ( ) is thought to facilitate the transformation. Infectious the transformation arises via PrPSc which acts as a template. The kinetic equations are defined by Eigen (1996). The infectious form includes kuru, iatrogenic CJD (iCJD), variant CJD (vCJD first reported in 1996), bovine spongiform encephalopathy (BSE first reported in 1985), and scrapie. In the nucleation-dependent model, monomeric PrPc and PrPSc are in chemical equilibrium.

Jones et al. (1999) made a classification of the broad range of nucleation likely to be encoimtered in liquids supersaturated with dissolved gas molecules (Jones et al., 1999). Bubble formation from preexisting gas cavities larger than the critical size is referred to as nonclassical heterogeneous bubble nucleation (type TV bubble nucleation, following their... 

Related Calculations. Use this procedure to calculate the crystal size distribution from both class I and class II MSMPR crystallizers. This procedure cannot be used to calculate growth rates and nucleation with crystallizers having either lines destruction or product classification. 

A continuous crystallization process ultimately reaches a steady state, in which the rates of nucleation and growth are constant with time. For a given set of operating conditions, crystal size distribution depends considerably upon the degree to which product classification is practiced. Figures 23 and 24 illustrate schematically the possible extremes between... 

In an attempt to avoid some of the confusion extant in the current literature on the nature of nucleation, Mullin has provided a useful schematic classification for various terms in use ... 

The phenomenological classification of compact 3D Me deposits by Fischer (cf. Section 6.1) can be related to the nucleation and growth parameters discussed above. For example, the field-oriented isolation (FI) and texture (FT) types are caused by electric field-enhanced normal growth, the base-oriented reproduction (B t3 e corresponds to a relatively low nucleation rate and comparable normal and lateral growth rates, and the randomly-oriented dispersion (RD) type to an enhanced nucleation rate. 

The equally critical issues of when to seed and how much seed to use are introduced in each classification. The amount of seed can vary from none to massive and include the familiar classifications of pinch to hopefully avoid complete nucleation, small (<1%) to hopefully achieve some growth, large (5-10%) to improve the probability of growth, and massive (the seed is the product in a continuous or semicontinuous operation) to provide maximum opportunity for all growth. The amount of seed can also be critical in the control of polymorphs and hydration/solvation. 

Special considerations chemical composition of filler surface affects nucleation of filler traces of heavy metals decrease thermal stability and cause discoloration siuface free energy of fillers determines interaction large difference in thermal properties of fillers and polymer may cause stress hydrotalcite is used as acid neutralizer with stabilizing packages anatase titanium dioxide decreases UV stability presence of transition metals (Ni, Zn, Fe, Co) affects thermal and UV stability calcium carbonate and talc were found to immobilize HALS stabilizers in PP with organic masterbatches such as ethylene diamine phosphate V-0 classification can be obtained with 20-25 wt%, at the same time tensile strength and impact strength are substantially reduced... 

One of the most important and fundamental questions in relation to the topochemical classification of Fig.1 is which contributes more to the a-Si nucleation, the gaseous phase photo decomposition or the decomposition of adsorbed molecules. The unambiguous answer to the question can be derived if we could eliminate illumination to the substrate surface without affecting excitation of gaseous phase molecules. Though it is hard to realize such a situation with the diffuse light, a similar... 

FIGURE 7.2 Classification of nucleation. (Adapted from Chow, R.K. et al.. Ultrasonics, 41, 595, 2003. With permission.)... 

Because of the effect of static head, evaporation and cooling occur only in the liquid layer near the magma surface, and concentration and tem wrature gradients near the surface are formed. Also crystals tend to settle to the bottom of the crystallizer, where there may be little or no supersaturation. The crystaUizer will not operate satisfactorily unless the magma is well agitated, to equalize concentration and temperature gradients and suspend the crystals. The simple vacuum crystallizer provides no good method for nucleation control, for classification, or for removal of excess nuclei and very small crystals. 

Fig. 2 The classification of the crystal growth process according to Jackson a-factor (a) rough growth (b) island growth (nucleation and spread) and (c) spiral growth. View this nrt in color at www.dekker.com.)...

---