Nucleation growth processes

A kinetic study of Cu underpotential deposition was carried out to determine if it is best described by adsorption processes or by nucleation processes. The nucleation growth process is classified into two categories instantaneous nucleation growth and progressive nucleation growth. In the case of instantaneous nucleation growth, where nucleation site formation is so fast that no other following nucleation sites are created, the number of nucleation sites N(t) is expressed as... 

Once a continues layer of the film is formed, (often referred to, as a monolayer), further film growth is a reenactment of the nucleation-growth processes occurring under steady state conditions. During this stage, the film evolves into any of, amorphous, polycrystalline or single crystal forms, depending on several parameters such as substrate temperature, nature of the film deposited, and so on. 

Coking of zeolites can be considered as a nucleation-growth process. The key step is the retention of secondary reaction products in the micropores (nucleation). At high reaction temperatures this retention is generally due to a steric blockage (trapping), while at low temperatures it is mainly due to the low volatility of the secondary products (5). 

O. Devos, C. Gabrielli, and B. Tribollet, "Nucleation-Growth Process of Scale Electrodeposition Influence of the Mass Transport," Electrochimica Acta, 52 (2006)285-291. 

The few observations of nucleation in the free troposphere are consistent with binary sulfuric acid-water nucleation. In the boundary layer a third nucleating component or a totally different nucleation mechanism is clearly needed. Gaydos et al. (2005) showed that ternary sulfuric acid-ammonia-water nucleation can explain the new particle formation events in the northeastern United States through the year. These authors were able to reproduce the presence or lack of nucleation in practically all the days both during summer and winter that they examined (Figure 11.16). Ion-induced nucleation is expected to make a small contribution to the major nucleation events in the boundary layer because it is probably limited by the availability of ions (Laakso et al. 2002). Homogeneous nucleation of iodine oxide is the most likely explanation for the rapid formation of particles in coastal areas (Hoffmann et al. 2001). It appears that different nucleation processes are responsible for new particle formation in different parts of the atmosphere. Sulfuric acid is a major component of the nucleation growth process in most cases. 

The existing models describing electrochemical phase formation involving both adsorption and a nucleation/growth process have been recently modified. A generalization of the Avrami s overlap formula for two or more competitive irreversibly growing phases has been proposed [89]. 

The deposition of various other metals on n-Si(lll) surfaces was also studied. In all cases epitaxial growth of the metal was found [57, 60]. However, the processes were not reversible because the corresponding standard potentials occurred much below the conduction band. Accordingly, the metal deposition was possible but not the reoxidation. In addition, the analysis of the first current transient of Cu or Co deposition was disappointing insofar as the nucleation-growth process did not follow the models discussed earher [57, 60]. 

Chen et al [145] have obtained nanocrystalline Co45NiioFc24 films with the mixture of the structure of crystalline and amorphous state from an acidic sulfate bath using cyclic voltammetry technique and electrochemical impedance spectroscopy (EIS) technic was used for investigation of the nucleation/growth process of CoNiFe. 

Fletcher, S., et al.. The response of some nucleation/growth processes to triangular scans of potential. J. Electroanal. Chem. Interfacial Electrochem., 1983. 159 p. 267-85. 

Figure 9.2 Representation of the composition of the crystallites as the devitrification occurs (a) by spinodal decomposition and (b) by a nucleation/growth process. Co corresponds to the initial composition of the glass, C corresponds to the composition of the glassy phase after devitrification and C2 to the composition of the crystallites...

Figure 9.3 Schematic morphology of a glass-ceramic formed (a) by spinodal decomposition and (b) by a nucleation/growth process...

The nucleation/growth process, most commonly encountered, consists of two separate steps. The first one is the nucleation, corresponding to the formation of seeds, also called nuclei, whose chemical composition is different from that of the initial glass, thus forming a new phase. The composition of this new phase remains constant over the whole nucleation/growth process (Figure 9.2). 

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