Classical nucleation theory

Classical nucleation theory (CNT) is the most common theoretical model used to quantitatively study the kinetics of nucleation. Nucleation is the first step in the spontaneous formation of a new thermodynamic phase or a new structure, starting from a state of metastability. The kinetics of formation of the new phase is frequently dominated by nucleation, such that the time to nucleate determines how long it will take for the new phase to appear. The time to nucleate can vary by orders of magnitude, from negligible to exceedingly large, far beyond reach of experimental timescales. One of the key achievements of classical nucleation theory is to explain and quantify this immense variation. Description The central result of classical nucleation theory is a prediction for the rate of nucleation R, in units of (number of events)/(volume·time). For instance, a rate R = 1000 \ \text{m}^{-3}\text{s}^{-1} in a supersaturated vapor would correspond to an ave

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Stress control in GaN/sapphire templates for the fabrication of crack-free thick layers

Nicolas Grandjean, Marc Ilegems, Denis Martin

The reduction of the tensile stress contained in GaN layers grown oil sapphire by metalorganic vapor phase epitaxy (MOVPE) is achieved using a low density of initial GaN crystallites. The template layers exhibit a significant reduction in dislocation density, down to 5 x 10(7) cm(-2) in similar to 6 mu m thick MOVPE GaN templates. The grain size and their density are controlled during the low temperature nucleation step of GaN on sapphire. Subsequently, very thick (up to similar to 200 mu m) crack-free GaN layers were Successfully grown by hydride vapor phase epitaxy (HVPE). A direct correlation between the stress state of the GaN/sapphire template prepared by MOVPE and the critical thickness for crack appearance during HVPE growth thickening is presented. (c) 2005 Elsevier B.V. All rights reserved.

2006

Increasing Polycrystalline Zinc Oxide Grain Size by Control of Film Preferential Orientation

Arthur Brian Aebersold, Duncan Alexander, Christophe Ballif, Lorenzo Fanni, Cécile Hébert, Aïcha Hessler-Wyser, Monica Morales Masis, Sylvain Nicolay

We investigate the structural evolution of polycrystalline zinc oxide films grown by low pressure metal-organic chemical vapor deposition. The goal is to achieve larger grains-leading to higher charge carrier mobility from lower grain boundary density-by controlling the grain orientation during growth. The results are 2-fold. First we describe how the combination of deposition temperature and gas flow influences the nucleation and film thickening stages: low temperature and high gas flow favor a high nucleation density and the development of c-textured films, whereas high temperature and low gas flow lead to a lower nucleation density and a-textured films. Second we demonstrate how a fine control of the film preferential orientation at the different growth stages allows the fabrication of films with grains that are 25% larger, hence improving the carrier mobility with respect to the reference film.

American Chemical Society2015

On the sluggish recrystallization of a cold-rolled Al–Mn–Fe–Si alloy

Ke Huang, Roland Logé

Annealing of supersaturated AA3xxx alloys at low temperatures usually results in sluggish recrystallization kinetics. This is due to the joint effect of the following factors: low nucleation rate, reduced grain boundary mobility, and large amount of fine precipitates (dispersoids). In this paper, samples of Al–Mn–Fe–Si alloy were appropriately homogenized in different conditions to produce different microchemistries before deformation, i.e. solutes and second-phase particles. The sluggish recrystallization behaviour of these cold-rolled Al–Mn–Fe–Si samples annealed in three different conditions was then investigated, the first condition being recrystallization without precipitation, followed by recrystallization and precipitation occurring concurrently, and finally precipitation occurring before recrystallization. The results clearly show that in all these conditions, an incubation time is involved, which decreases with increasing annealing temperature and cold deformation, as well as with decreasing solute amount. Qualitative analysis of the microstructure evolution after a sudden increase of annealing temperature suggests that the effective retarding force from solute and/or particles decreases at higher temperatures. When recrystallization occurs concurrently with precipitation, the growth of successful nuclei can still be suppressed by concurrent precipitation.

Springer Verlag2016

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This course aims at understanding classical and non-classical nucleation theory, at reviewing different techniques for the synthesis of nanomaterials (mainly nanoparticles and thin films) and at learning about some key applications of these nanomaterials in chemical engineering

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This course aims at giving an overview on the synthesis of nanoparticles, with more focus on colloidal chemistry, and their implementation into devices for energy applications (batteries, solar cells, artificial photosynthesis, light emitting diodes, electrochromic windows).

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Ce cours est une introduction aux transformations de phases liquide-solide et solide-solide. Il aborde les aspects thermodynamiques et cristallographiques. Il traite principalement des matériaux métalliques, et établira des ponts vers les céramiques et la minéralogie.