Nuclei formation and growth

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

Although we have covered mechanisms relating to solid state reactions, the formation and growth of nuclei and the rate of their growth in both heterogeneous and homogeneous solids, and the diffusion processes thereby associated, there exist still other processes zifter the particles have formed. These include sequences in particle growth, once the particles have formed. Such sequences include ... 

Knowledge concerning the mechanism of hydrates formation is important in designing inhibitor systems for hydrates. The process of formation is believed to occur in two steps. The first step is a nucleation step and the second step is a growth reaction of the nucleus. Experimental results of nucleation are difficult to reproduce. Therefore, it is assumed that stochastic models would be useful in the mechanism of formation. Hydrate nucleation is an intrinsically stochastic process that involves the formation and growth of gas-water clusters to critical-sized, stable hydrate nuclei. The hydrate growth process involves the growth of stable hydrate nuclei as solid hydrates [129]. 

The overall rate of crystallization is determined by both the rate of nuclei formation and by the crystal growth rate. The maximum crystal growth rate lies at temperatures of between 170 and 190 °C [71, 72], as does the overall crystallization rate [51, 61, 75], The former is measured using hot stage optical microscopy while the latter is quantified by the half-time of crystallization. Both are influenced by the rate of nucleation on the crystal surface and the rate of diffusion of polymer chains to this surface. It has been shown that the spherulite growth rate decreases with increasing molecular weight due to the decrease in the rate of diffusion of molecules to this surface [46, 50, 55, 71, 74],... 

Both the rate of nuclei formation and the crystal growth rate can also be expected to influence the spherulite size. It has been reported (hat, in the temperature range 130-180 °C, the spherulite size increases with increasing temperature [74], This trend can be expected to extend to higher temperatures as the nucleation rate decreases. On the other hand, the presence of nucleating... 

Using the well-defined system of polyoxoanion/Bu4N -stabilized iridium nanoparticles [9, 29] as a model for the studies, Finke and coworkers [30] proposed a method that attempted to explain the formation and growth of transition-metal nanoparticles. This indirect method is based on an autocatalytic mechanism that considers a nudcation step in which a precursor A is converted to a zero-valent nuclei B with a rate constant fej, and a second step that considers the autocatalytic surface growth of the metal nanoparticles where species B catalyzes its own formation with a rate constant tc2 (Scheme 15.5). 

Agl crystals grow in aqueous solution via four steps (I) formation of molecules and complexes, (2) formation and growth of nuclei, (3) aggregation, and (4) ripening. These reaction scheme can be written as... 

Formation and growth of nuclei have been observed microscopically in single crystals in several decomposition and dehydration reactions. 

The conventional theory of the decomposition of solids distinguishes between formation and growth of nuclei in a very real way by ascribing different rate constants kf and kg to these two processes. This is a realistic procedure if a nucleus can be formed as a result of a single reaction step because the first molecular decomposition at a nucleus-forming site is clearly occurring in a different environment from that for subsequent ones. 

Zhdanov (1) describes the mechanism of zeolite crystallization in terms of a quasiequilibrium between the solid and liquid phase in gels and emphasizes that the formation and growth of nuclei occurs in the liquid phase. 

Figure 7. Particle size distributions measured with the SEMS showing the formation and growth of nuclei in the 1-octene photooxidation. The distributions are shown (A) without S02 and (B)with added S02. (Reproduced from reference 49. Copyright 1991 American Chemical Society.)...

We had used the structural information of the packing arrangement of two polymorphs in order to model molecules which selectively inhibit the formation and growth of the nuclei of... 

Oxynitride glasses may be heat treated to form glass-ceramics, effectively multi-phase composites. The process involves heat treatment at two different temperatures, firstly to induce nucleation, then to allow crystal growth of the nuclei. The crystalline phases formed depend on both the composition of the parent glass and the temperatures used for heat treatment. The extent of their formation and growth, the relative amounts and distributions of different phases (including residual glass) and their characteristics will determine the overall properties of the particular composite. The formation of these types of materials and their properties is outlined below. 

Fig. 14 A sequence of AFM phase images of a BA-C8 film obtained at room temperature showing the formation and growth of induced nuclei. The time interval between each consecutive image is approximately 5.8 min [61]...

The microscopic study of the formation and growth of nuclei in solid ammonium perchlorate was studied by a number of authors (Vol. II. p. 481). Raevskii and Manclis [79a found that the decomposition centres of orthorhombic fonn consists of a large number of ellipsoid nuclei of 1-2 /i/m. They are not stationary but moving at the speed of the order 7 -10 m/min at 230 C. Their activation cncrg> is 31 and 33 kcal/mol depending on the direction of tlie movement. 

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