Nucleation and phase growth

The example of a metal deposition system that we have already seen was, in most senses, conventional with the microelectrode being used largely to reduce the iRu drop. The small size of microelectrodes does, however, open up the possibilities for new types of experiments. 

As the electrode size is reduced still further, we reach a point where only a single nucleus is able to develop during the period of the experiment and the determination of the nucleation rate is achieved by measuring the induction time for the appearance of the first nucleus and the growth rate of this single nucleus can be determined from the subsequent current-time transient. The nucleation is, of course, a stochastic process and therefore the induction time will vary. The determination of the nucleation rate therefore requires a large number of transients to be studied. 

There have, as yet, been few studies of this type but, in view of their potential, it seems likely that many more will follow in the near future. 

C0°° bulk concentration of species 0 (mol cm-3) F the Faraday (coulombs mol-1) i current (A) i8 current at a sphere I current density (A cm-2) 

Ferris, Introduction to Bioelectrodes, Plenum Press, New York, 1974. 

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Although the mechanisms discussed above are still topics of debate, it is now firmly established that the electrodeposition of conducting polymers proceeds via some kind of nucleation and phase-growth mechanism, akin to the electrodeposition of metals.56,72-74 Both cyclic voltammetry and potential step techniques have been widely used to investigate these processes, and the electrochemical observations have been supported by various types of spectroscopy62,75-78 and microscopy.78-80... 

Nucleation and growth kinetics — Nucleation-and-growth is the principal mechanism of phase transformation in electrochemical systems, widely seen in gas evolution, metal deposition, anodic film formation reactions, and polymer film deposition, etc. It is also seen in solid-state phase transformations (e.g., battery materials). It is characterized by the complex coupling of two processes (nucleation and phase growth of the new phase, typically a crystal), and may also involve a third process (diffusion) at high rates of reaction. In the absence of diffusion, the observed electric current due to the nucleation and growth of a large number of independent crystals is [i]... 

As initially suggested by Genies et al. [14] and then corroborated by the work of Andrieux et al. [15], the mechanism of electrochemical polymerization consists of an oxidation step generating a cation radical followed by coupling and a deprotonation/rearomatization step generating the dimer, then processing via nucleation and phase-growth deposition (Scheme 9.1). 

Electrochemical nucleation and phase growth Surface-enhanced Raman spectroscopy In-situ X-ray techniques Potentiostat design Microelectrodes... 

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

A. R. Nerheim, E. K. Samuelson, and T. M. Svartaas. Investigation of hydrate kinetics in the nucleation and early growth phase by laser light scattering. In Proceedings Volume, volume 1, pages 620-627. 2nd Soc Offshore Polar Eng et al Offshore Polar Eng Int Conf (San Francisco, CA, 6/14-6/19), 1992. 

Polymer crystallization is usually divided into two separate processes primary nucleation and crystal growth [1]. The primary nucleation typically occurs in three-dimensional (3D) homogeneous disordered phases such as the melt or solution. The elementary process involved is a molecular transformation from a random-coil to a compact chain-folded crystallite induced by the changes in ambient temperature, pH, etc. Many uncertainties (the presence of various contaminations) and experimental difficulties have long hindered quantitative investigation of the primary nucleation. However, there are many works in the literature on the early events of crystallization by var-... 

The exponent n is Unked to the munber of steps in the formation of a nucleus (this is a zone in the soUd matrix at which the reaction occurs), ft, and the number of dimensions in which the nuclei grow, X. It can be difficult to distinguish ft and X without independent evidence, and ft can fall to zero following the consumption of external nuclei sites. Hulbert has analysed the possible values of the exponent, n, for a variety of conditions of instantaneous (/3 = 0), constant (ft = 1) and deceleratory (0 < /I < 1) nucleation and for growth in one, two and three dimensions (X = 1 - 3) [ 17]. He also considered the effects of a diffusion contribution to the reaction rate. This reduces the importance of the acceleratory process and reduces the value of n. For diffusion controlled processes, n = ft + Xjl, whereas for a phase boimdary controlled process n = ft + X. Possible values of n are summarised in Table 1. Interpretation of these values can be difficult, and a given value does not unequivocally allow the determination of the reaction mechanism. 

Interest in the crucial processes of nucleation and the growth of solids from fluid phases has a long and multidisciplinary history [50-53]. This research topic involves chemistry, chemical physics, material science, chemical engineering and physics, and, as a consequence, both theoretical and experimental studies were carried out by specialists in these fields. Thus, the following discussion does not pretend to be an exhaustive literature coverage of what is known about nucleation and growth, but rather, through recent articles, tries to review contributions especially relevant to controlled chemical vapour deposition of nanoparticles, always from a multidisciplinary point of view. 

Crystallization occurs in two phases nucleation and particle growth. During nucleation, molecules in solution come together randomly and form small aggregates. Particle growth requires the addition of more molecules to a nucleus to form a crystal. When a solution contains more solute than should be present at equilibrium, the solution is said to be supersaturated. 

Crystallization in general is a two-step process involving (1) nucleation and (2) growth of the nucleus to a macro size. Nucleation involves the activation of small, unstable particles with sufficient excess surface energy to form a new stable phase. This may occur in supersaturated solutions as a result of mechanical shock, the introduction of small crystals of the desired type, or the presence of certain impurities that can act as centers for growth. 

These brief remarks on Ostwald ripening conclude the discussion of nucleation and early growth stages of heterogeneous reactions at this point. Some of the concepts are deepened in Chapter 12 on phase transformations [see also R. Wagner, R. Kampmann (1991)]. 

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