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Product Engineering of Nanoscaled Materials
Published in Wolfgang Sigmund, Hassan El-Shall, Dinesh O. Shah, Brij M. Moudgil, Particulate Systems in Nano- and Biotechnologies, 2008
Surface structures can also be influenced by the coating processes, both from the gas and liquid phases. The acting supersaturation, which is the driving force for the surface growth, determines the mechanisms of growth and the growth kinetics. In crystallization, the following growth mechanisms in integration controlled growth are distinguished: mononuclear and polynuclear as the two limiting cases for low and high supersaturations, respectively, and the birth and spread model as an intermediate case. These mechanisms determine the resulting surface roughness. For instance, under conditions where the lateral diffusion of preformed species on the surface is very fast in comparison with the integration step, fairly smooth layers are obtained (mononuclear case). Integration occurs in this case at a few energetically favored sites such as edges and kinks. In contrast, when the adsorbed species on the surface are immobilized fast and thus integrated into the surface at arbitrary locations so that they are not diffusing over the surface, then a much rougher surface will be the result (polynuclear case) [16]. Of course, these effects depend also on each specifically indexed surface in the case of crystals.
A numerical aerosol model Fractal Aggregate Moment Model (FAMM) to simulate simultaneous nucleation, coagulation, surface growth, and sintering of fractal aggregates
Published in Aerosol Science and Technology, 2019
A new numerical model capable of tracing the formation and growth of fractal aggregates in the course of simultaneous nucleation, surface growth, coagulation, sintering, and condensational obliteration in the framework of LNMM was developed. The new model can predict the changes in the bimodal size distribution and morphology of fractal aggregate particles by tracing five time-dependent variables (and an additional variable for the amount of gas precursor). A bimodal size distribution composed of a monodisperse nuclei mode for spherical nucleus particles with a fixed particle size and an LN accumulation mode for fractal aggregates was assumed. The new model showed good agreement with a more accurate sectional model and with available measured data, except for an imaginary case in which ceaseless nucleation and no coagulation were assumed.
Universal relations between soot effective density and primary particle size for common combustion sources
Published in Aerosol Science and Technology, 2019
The chemical mechanisms of soot formation involve overlapping processes of nucleation, surface growth, carbonization, and aggregation (Frenklach 2002), leading to fractal aggregates of “primary particles” (Figure 1, typical soot aggregates with 10–30 nm primary particles). The primary particle size and degree of graphitization varies substantially, and is not easily predicted from current models. However, the gross structure of a soot aggregate is well-modelled by Brownian coagulation of perfectly-sticking clusters (i.e., Diffusion Limited Cluster Aggregation model) (Meakin 1988). In typical DLCA simulations, a control volume is populated with uniformly-sized primary particles (diameter dp) that are allowed to aggregate until only a single large cluster remains. This model predicts a fractal dimension of Df ∼ 1.8, matching micrographs of real soot.