Nucleation behavior

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]. 

The technique of self-nucleation [75] can be very useful to study the nucleation and crystallization of block copolymer components, as already mentioned in previous sections. In block copolymers, factors like the volumetric fraction and the degree of segregation affect the type of confinement and therefore modify the self-nucleation behavior. In the case of semicrystalline block copolymers, several works have reported the self-nucleation of either one or both crystallizable components in PS-fc-PCL, PS-b-PB-b-PCL, PS-b-PE-b-PCL, PB-fr-PIB-fr-PEO, PE-fr-PEP-fr-PEO, PS-fc-PEO, PS-h-PEO-h-PCL, PB-b-PEO, PB/PB-fc-PEO and PPDX-fc-PCL [29,92,98,99,101-103,134] and three different kinds of behavior have been observed. Specific examples of these three cases are given in the following and in Table 5 ... 

Table 5 Self-nucleation behavior for diblock and triblock copolymers ...

Fig. 17 a Classical self-nucleation behavior for polyethylene (PE) within E18EP57EO25133 triblock copolymer [101]. b Self-nucleation behavior for PEO within purified E24EP57EO1964 triblock copolymer [29], c Self-nucleation behavior for PE within S35E15C50219 triblock copolymer [29,98]. (a, c from [98,101] with permission, b Reprinted with permission from [29], Copyright 2002 American Chemical Society)... 

Chen et al. [92] also performed self-nucleation experiments by DSC in PB-fr-PEO diblock copolymers and PB/PB-b-PEO blends. The cooling scans presented in their work showed that a classical self-nucleation behavior was obtained for PEO homopolymer and for the PB/PB-b-PEO blend where the weight fraction of PEO was 0.64 and the morphology was lamellar in the melt. For PB/PB-fr-PEO blends with cylinder or sphere morphology, the crystallization temperature remained nearly constant for several self-seeding temperatures evaluated. This observation indicates that domain II or the self-nucleation domain was not observable for these systems, as expected in view of the general trend outlined earlier. 

As described previously in the crystallization from ethanol (EtOH) solutions A and B polymorphs appeared. However, with kinds of the solvent the polymorphic nucleation behavior may change. In this section the solvent effect in the nucleation behavior of Pr-est is shown. 

In c-Hxn solutions the nucleation behavior is similar to that in EtOH solutions. It is presumed that the concentrations of conformer regarding to the A and B form are competitive and the nucleation process of the polymorph is determined by the supersaturation and the kinetic process. On the other hand in MeCN solutiorrs orrly the stable form nucleates. In MeCN solution it is considered that the thermodynamic stability of the meta-stable form is extremely low due to the large solvent-solute... 

Karty et al. [21] pointed out that the value of the reaction order r and the dependence of k on pressure and temperature in the JMAK (Johnson-Mehl-Avrami-Kolmogorov) equation (Sect. 1.4.1.2), and perhaps on other variables such as particle size, are what define the rate-limiting process. Table 2.3 shows the summary of the dependence of p on growth dimensionality, rate-limiting process, and nucleation behavior as reported by Karty et al. [21]. 

Comparison of Eqs. (10) and (11) yields the radius Rcoh for the transition from coherent rotation to curling. For R < i coh, the exchange energy dominates, and the nucleation is realized by coherent rotation, whereas for R > i coh the nucleation behavior is dominated by flux closure and realized by curling. For spheres and wires (cylinders), one obtains Rcoh = 5.099 /ex and... 

Unlike the mobile liquid state, crystals that nucleate from the viscous amorphous state are less governed by thermodynamics than by kinetic constraints. To describe nucleation behavior in kinetically constrained systems, a modification of Equation... 

In most food systems, a wide variety of ingredients are used to provide the desired textural and sensory characteristics. Thus, crystallization nearly always occurs in complex systems where phase behavior may be difficult to ascertain accurately. Furthermore, the specific interactions among ingredients can lead to significant inhibition of nucleation. Because of these often complex interactions, it is frequently difficult to predict nucleation behavior. 

In Fig. 6, point P represents a solution that is unsaturated at this concentration neither nucleation nor crystal growth will occur. Point S represents a labile solution which will nucleate spontaneously, with concentration falling to point R as nucleation and crystal growth occur. Point Q represents a metastable concentration at which growth will take place if crystal seeds are present or added. Although the supersolubility limit is affected by external factors, discussed later, the Miers concept is useful as an empirical representation of nucleation behavior and in treating crystallization from unseeded solutions. 

Observation (i) above can be understood in terms of droplet nucleation and the lack of competition between nucleation and growth. A mechanistic understanding of observation (ii) above was provided by Samer and Schork [64]. Nomura and Harada [136] quantified the differences in particle nucleation behavior for macroemulsion polymerization between a CSTR and a batch reactor. They started with the rate of particle formation in a CSTR and included an expression for the rate of particle nucleation based on Smith Ewart theory. In macroemulsion, a surfactant balance is used to constrain the micelle concentration, given the surfactant concentration and surface area of existing particles. Therefore, they found a relation between the number of polymer particles and the residence time (reactor volume divided by volumetric flowrate). They compared this relation to a similar equation for particle formation in a batch reactor, and concluded that a CSTR will produce no more than 57% of the number of particles produced in a batch reactor. This is due mainly to the fact that particle formation and growth occur simultaneously in a CSTR, as suggested earlier. 

While the classical theory of nucleation is limited by the implicit assumptions in its derivation, it successfully predicts the nucleation behavior of a system. Inspection of the equation above clearly suggests that the nucleation rate can be experimentally controlled by the following parameters molecular or ionic transport, viscosity, supersaturation, solubility, solid-liquid interfacial tension, and temperature. 

Kim et al.f studied the effect of gas pressure on the nucleation behavior of diamond on a Si(lOO) substrate in HFCVD. The pressure was varied from 2 to 50 torr, while a filament temperature of2200°C, a substrate temperature of 850°C, a total flow rate of 20 seem and a CH4 concentration of 0.8 vol.% were used. The characterization of diamond deposits using micro-Raman spectroscopy, SEM and OM revealed that the maximum nucleation density of diamond parades on the unscratched Si substrate occurred at a pressure of 5 torr. The pressure dependence of the nucleation density was explained by the competition effect between P-SiC formation, which increases the diamond nucleation density, and atomic-hydrogen etching, which decreases the nmnber of nucleation sites. On the basis of this finding, a new fabrication approach for high-quality diamond films without... 

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