Cluster aggregation nucleation

Abruna HD (1991) X-ray Absorption Spectroscopy in the Study of Electrochemical Systems. In Electrochemical Interfaces. Abruna HD (ed) VCH Publishers, New York Albertini G, Carsughi F, Casale C, Fiori F, La Monaca A, Musci M (1993) X-ray and neutron small-angle scattering Investigation of nanophase vanadium-titanium oxide particles. Phil Mag B 68 949-955 Alexandrowicz Z (1993) How to reconcile classical nucleation theory with cluster-cluster aggregation. Physica A 200 250-257... 

Structural control in sol-gel processes is complicated because many and diverse variables affect concurrent reactions differently. Inductive and steric factors contribute to the reaction rates. pH is probably the single most important variable in these reactions. It accounts for differences when DCCAs are used and for the rapid gel times in Si(OAc)4 sols, as well as differences in the two predominant growth mechanisms nucleation and growth and cluster-cluster aggregation. 

Hydrolysis and condensation occur by bimolecular nucleophilic displacement reactions involving OH and Si-0 anions. For w > 4, the hydrolysis of all polymeric species is expected to be complete. Dissolution reactions provide a continual source of monomers. Because condensation occurs preferentially between weakly acidic species that tend to be protonated and strongly acidic species that are deprotonated, growth occurs primarily by reaction-limited monomer-cluster aggregation (equivalent to nucleation and growth), leading to compact, nonfractal structures. 

Under the conditions used by Stober et al. [27], primary particles formed by nucleation, growth, and ripening undergo an ordered aggregation process to form monosized spherical particles. This process represents reaction-limited monomer-cluster aggregation occurring concurrently on two length scales. 

Nucleation of the chalcogenide is much simpler in this process, since a solid phase—the metal hydroxide (or other solid phase)—is already present and the process proceeds by a substitution reaction on that solid phase. In this case, the initial step in the deposition is adhesion of the hydroxide to the substrate. This hydroxide is then converted into, e.g., CdS, forming a primary deposit of CdS clusters. More Cd(OFI)2 and, as the reaction proceeds, CdS and partially converted hydroxide diffuses/convects to the substrate, where it may stick, either to uncovered substrate (in the early stages of deposition) or to already deposited material. This is essentially the same process as aggregation, described in Chap-... 

There is a distinct region of small aggregates or clusters which falls between the atomic (or molecular) domain and that of condensed matter. These small particles and clusters possess unique properties and have several technological applications. The formation of these particles involves a vapour-solid, a liquid-solid, a solid-solid or a vapour-liquid-solid type of phase change governed by nucleation and it is important that the size of the growing nucleus is controlled (Multani, 1981 Hadjipanyas Siegel, 1994). 

The scheme of nucleating and growth of particles at deposition of M/SC from gaseous phase on a substrate is submitted in work [43]. The atoms adsorbed from a gas phase (adatoms) diffuse over a surface. During diffusion adatoms are partly desorbed and in part are stabilized on a surface, forming nuclei of a new phase. Such nuclei arise by collision of diffusing adatoms and their aggregation into stable primary clusters or as a result of... 

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