L'objectif de ce travail était de comprendre les mécanismes menant à la formation de SiC-3C sur substrats de SiC hexagonaux lors de la croissance par mécanisme Vapeur-Liquide-Solide (VLS) à partir d'une phase liquide Si-Ge. Notre étude s'est concentrée sur les premiers instants de l'interaction liquide/SiC, c'est-a-dire sans ajout de phase gazeuse réactive (propane). Nous avons montré qu'il se formait très rapidement des îlots de SiC-3C à la surface des germes. L'étude en fonction de divers paramètres (température et durée de plateau, vitesse de chauffage, nature du germe) a permis de mettre en évidence l'influence prononcée du creuset en graphite dans lequel est confinée la réaction. Les observations expérimentales associées à des calculs thermodynamiques ont permis de montrer que l'étape prépondérante, pour la formation du 3C, est sans doute la réaction transitoire entre un liquide très riche en germanium et le germe de SiC. Des considérations d'ordre cinétiques sont cependant à prendre en compte pour expliquer le caractère hors équilibre de la réaction / The aim of this work was to understand the mechanisms that lead to the 3C-SiC formation on hexagonal SiC substrates during the Vapor-Liquid-Solid (VLS) growth from a Si-Ge liquid phase. Our study focused on the early stages of the liquid/SiC interaction, i.e. without reactive gaseous phase (propane) addition. We have shown that 3C-SiC islands were very rapidly formed upon seeds surface. The study of several parameters (such as temperature and duration of the plateau, heating rate, nature of the seed) evidenced the huge influence of the graphite crucible that contains the reaction. Experimental observations combined with thermodynamic calculations show that the most important step for the 3C formation, is the transient reaction between a germanium very rich liquid and the SiC seed. Kinetic effects have to be taken into account to explain the out-of-equilibrium nature of the reaction.

© Springer Science+Business Media Dordrecht 2016 / Reducing AgNO3 by glucose at basic pH coated the surface of silica spheres with a high density of hemispherical silver nanoparticles (average diameter 3.2±1 nm). A much lower silver concentration than is standard favored heterogeneous nucleation of silver on the silica surface at the expense of homogeneous nucleation in solution. The slow growth rate of the nuclei promoted the formation of discrete silver particles rather than a continuous shell. Based on scanning electron microscopy and transmission electron microscopy, the surface coverage of silver seed particles was as high as 25% at 10 °C without prior functionalization of the silica. The particles were composed of metallic silver based on x-ray photoelectron spectroscopy. There was a sharp increase in the silver surface coverage and decrease in the particle size when the temperature was raised from 5 °C to 10 °C and the amount of silica was decreased from 0.2 to 0.025 V/V. The size was controlled by the diffusion barrier through the ion shell surrounding the silica spheres and by maintaining reaction conditions where the particles on the surface compete for silver.

In the atmosphere, cloud droplets can remain in a supercooled liquid phase at temperatures as low as -40 °C. Above this temperature, cloud droplets freeze via heterogeneous ice nucleation whereby a rare and poorly understood subset of atmospheric particles catalyze the ice phase transition. As the phase state of clouds is critical in determining their radiative properties and lifetime, deficiencies in our understanding of heterogeneous ice nucleation poses a large uncertainty on our efforts to predict human induced global climate change. Experimental challenges in properly simulating particle-induced freezing processes under atmospherically relevant conditions have largely contributed to the absence of a well-established model and parameterizations that accurately predict heterogeneous ice nucleation. Conversely, the sparsity of reliable measurement techniques available struggle to be interpreted by a single consistent theoretical or empirical framework, which results in layers of uncertainty when attempting to extrapolate useful information regarding ice nucleation for use in atmospheric cloud models. In this dissertation a new framework for describing heterogeneous ice nucleation is developed. Starting from classical nucleation theory, the surface of an ice nucleating particle is treated as a continuum of heterogeneous ice nucleating activity and a particle specific distribution of this activity g is derived. It is hypothesized that an individual particle species exhibits a critical surface area. Above this critical area the ice nucleating activity of a particle species can be described by one g distribution, 𝑔, while below it 𝑔 expresses itself expresses externally resulting in particle to particle variability in ice nucleating activity. The framework is supported by cold plate droplet freezing measurements for dust and biological particles in which the total surface area of particle material available is varied. Freezing spectra above a certain surface area are shown to be successfully fitted with 𝑔 while a process of random sampling from 𝑔 can predict the freezing behavior below the identified critical surface area threshold. The framework is then extended to account for droplets composed of multiple particle species and successfully applied to predict the freezing spectra of a mixed proxy for an atmospheric dust-biological particle system. The contact freezing mode of ice nucleation, whereby a particle induces freezing upon collision with a droplet, is thought to be more efficient than particle initiated immersion freezing from within the droplet bulk. However, it has been a decades’ long challenge to accurately measure this ice nucleation mode, since it necessitates reliably measuring the rate at which particles hit a droplet surface combined with direct determination of freezing onset. In an effort to remedy this longstanding deficiency a temperature controlled chilled aerosol optical tweezers capable of stably isolating water droplets in air at subzero temperatures has been designed and implemented. The new temperature controlled system retains the powerful capabilities of traditional aerosol optical tweezers: retrieval of a cavity enhanced Raman spectrum which could be used to accurately determine the size and refractive index of a trapped droplet. With these capabilities, it is estimated that the design can achieve ice supersaturation conditions at the droplet surface. It was also found that a KCl aqueous droplet simultaneously cooling and evaporating exhibited a significantly higher measured refractive index at its surface than when it was held at a steady state temperature. This implies the potential of a “salting out” process. Sensitivity of the cavity enhanced Raman spectrum as well as the visual image of a trapped droplet to dust particle collisions is shown, an important step in measuring collision frequencies of dust particles with a trapped droplet. These results may pave the way for future experiments of the exceptionally poorly understood contact freezing mode of ice nucleation.

The industrial practice of grain refinement of aluminium alloys involves the addition of inoculant particles to initiate alpha-aluminium grains at small undercoolings. This results in a uniformly fine, equiaxed as-cast microstructure and is commonly achieved using Al-Ti-B additions. The phase responsible for initiation of grains in aluminium melts inoculated with Al-Ti-B was determined during the 1990s; since that time, scientific understanding of grain refinement has advanced rapidly. However, one of the main problems of addition inoculants is impurities which is added to the melt and may affect the desired characteristics of the product. With regards to this problem other methods of refinement and the mechanisms of refining have not been fully understood and prediction of as-cast Microstructures in aluminium alloys has much scope for improvement. In this thesis: 1-Factors in establishing equiaxed microstructure were analysed and the origin of equiaxed grains were explored. Then the nucleation process and the involved mechanisms were investigated in depth and control of nucleation process to achieve a fine and uniform structure was set as target. 2-Refinement of microstructure with introduction of shearing was evaluated and the process of refinement in the mushy zone (semisolid state) as a base line was established. Then introduction of shearing above liquidus as a development was analysed and outstanding refinement was seen with shearing above liquidus which have not been investigated properly elsewhere. 3- The mechanisms of refinement by introducing shearing were investigated and the refining mechanisms below and specifically above liquidus were investigated systematically. As results an appropriate understanding about the mechanisms of nucleation and refinement above liquidus was established. 4- Finally, with simulation the most dominant factor in approaching fine grain size by applying shear was identified and the results of experimental examination was verified by simulation.

Non disponible. / Aerosol particles are ubiquitous in the Earth’s atmosphere. Although a minor constituent of the atmosphere, the aerosol particles are linked to visibility reduction, adverse health effects and heat balance of the Earth. The secondary aerosols which are formed in the atmosphere from the gaseous phase : precursor gases become particles by nucleation and condensation (Seinfeld and Pandis, 1998) represents the largest source in a number concentration of atmospheric particles. The chemical reactions can play an important role by turning high volatility gases into species with low vapor pressure and thus high saturation ratio, i.e. creating favorable conditions for particulate matter formation. In this work the CHIMERE chemical transport model is used to ameliorate our understanding of the governing processes for aerosol formation and to investigate its capability to reproduce the mass and number concentrations and temporal evolution of the aerosols particles at high altitudes (as for example Puy de Dome research station), and in particular, evaluate its capacity to simulate the formation of new particles due to nucleation. For the studied cases it was investigated the impact of : a fine resolution topographical database on the accuracy of simulation of dynamical parameters at high altitude, of the use of different emissions databases in the accuracy of gas-phase and aerosol concentration predictions, what is the most adequate nucleation parameterization scheme for simulating new particle formation at high altitude and what is the influence of the choice of the primary particle size distribution on the prediction of new particle formation. Also the ability of the different theories to reproduce the occurrence or lack of a nucleation event is evaluated.

Title: Molecular simulations of ice nucleation Author: Eva Pluhařová Department: Department of Physical and Macromolecular Chemistry Faculty of Science UK Advisor: doc. Mgr. Pavel Jungwirth, DSc., IOCB AS CR, v.v.i. Advisor's e-mail address: pavel.jungwirth@uochb.cas.cz Abstract: By means of molecular dynamics simulations we have systematically investigated homogeneous ice nucleation in neat and surface contaminated water. As models of the adsorbates we have assumed pentanol and pentanoic acid. In neat water nucleation preferentially starts in the subsurface region, which accommodates better than the bulk the volume increase associated with freezing. Homogeneous ice nucleation is affected more by alcohol than by acid. Water slabs covered by a disordered layer of pentanol exhibit negligible preference for subsurface nucleation and longer nucleation times in comparison with neat water, while nucleation times are almost unaffected by the presence of pentanoic acid and the subsurface preference is only slightly decreased. We tried to rationalize the differences between the effects of different compounds by their ability to orient water molecules and to change their mobility. The fact that adsorbates differ in the influence on homogeneous ice nucleation has important implications for the microphysics of...

Texture of igneous rocks, which includes size, shape and spatial distribution of grains, represents the final record of kinetic and mechanical processes operating during ascent and final emplacement of a magma. However, traditional geochemical approaches cannot assess and verify the physical processes of magma solidification, in particular, crystal nucleation and growth, textural coarsening, or mechanical crystal-melt interactions. In this work, I apply stereological methods to quantitatively characterize the textures and to interpret the crystallization history of granitic rocks in the Western Krušné hory/Erzgebirge and Vogtland. The Western Krušné hory/Erzgebirge granites consist of three suites: biotite granites (Kirchberg), muscovite-biotite microgranites (Walfischkopf), and topaz-zinnwaldite alkali- feldspar granites (Eibenstock), which consist of eight intrusive units and two aplite dyke sets. The entire granite sequence exhibits an extreme and nearly continuous differentiation range, but in detail the evolutionary trends of each suite are independent, and individual intrusive units are also clearly compositionally separated. The granites consist of 29-43 vol.% quartz, 20-30 vol. % plagioclase, 22-31 vol. % K-feldspar, 2-9 vol. % biotite, <2 vol. % muscovite, and minor topaz and apatite. All...

Le but de cette thèse est de développer un outil microfluidique d'étude de la cristallisation (et plus particulièrement de la nucléation) le plus adapté aux contraintes de l'industrie. C'est-à-dire un outil permettant de réaliser un grand nombre d'expériences de cristallisation tout en utilisant le moins de produits possible et en restant simple à mettre en place expérimentalement. Seule la microfluidique permet, en utilisant des volumes de l'ordre du nL, de répondre simultanément à ces deux contraintes. Les systèmes microfluidiques permettent en effet de générer des gouttes de quelques nanolitres qui sont autant de microcristallisoires permettant l'étude de la nucléation. Ce travail présente la mise au point de systèmes microfluidiques et des méthodes analytiques associées dédiés à l'étude de la nucléation de principes actifs pharmaceutiques. Un système microfluidique existant a été adapté afin de répondre dans un premier temps aux problèmes posés par la cristallisation de protéines dans l'eau. Ce système a permis de mesurer la limite de zone métastable ainsi que la fréquence de nucléation d'une protéine modèle, le lysozyme, également utilisée comme principe actif. Puis ce système a été à nouveau adapté afin de permettre l'étude de la nucléation dans des solvants organiques variés et donc l'étude d'un grand nombre de principes actifs pharmaceutiques. À l'occasion de cette nouvelle adaptation, des méthodes plus polyvalentes d'études de la nucléation ont dû être mises au point afin de résoudre les nouveaux problèmes soulevés. / The aim of this work is to develop a microfluidic tool to study crystallization (and specifically nucleation) adapted to industrial issue, that's mean doing a lot of experiment with only few materials. Microfluidic, thanks to using nanoliters volume, are able to solve simultaneously both issues. Microfluidic system allows us to generate plenty of nanoliters droplets and each droplet is a microcristallizer to study nucleation. Here I present the development of a microfluidic system and the related analytical method dedicated to nucleation study of active pharmaceutical ingredient. As a first step we adapt an existing microfluidic system to study the nucleation of protein in water. Thanks to this system we are able to measure the metastable zone width and nucleation frequency of model protein used as an active pharmaceutical ingredient, the lysozyme. In a second step we modify this system in order to allow nucleation study in organic solvent. Thanks to this new system we can study the nucleations of different APIs using polyvalent methods develop to avoid nucleation problems due to the crystallization of API. This microfluidic system and the method develop to study nucleation of API are really polyvalent and let us imagine to extend their applicative field to all industrial field where using nanoliter volume can reduce the cost (protein crystallization) and/or risk (explosives, radioactive hazard).

Lors de la solidification d'un alliage métallique, des hétérogénéités de composition en éléments d'alliage et des hétérogénéités de taille des structures apparaissent. La modélisation est un outil précieux dans la maitrise de la formation de ces hétérogénéités, qui sont incompatibles avec les exigences industrielles. Au niveau international, le code de calcul SOLID est l'unique code de solidification permettant la prise en compte simultanée de la germination, de la croissance et du transport des grains, de la convection naturelle et du retrait à la solidification à l'échelle industrielle. Le travail de thèse a pour but d'améliorer la description du couplage de la germination et de la croissance des grains dans ce modèle. La diffusion solutale, qui contrôle la croissance des grains, est ainsi modélisée par une longueur de diffusion prenant en compte à la fois l'avancée de l'interface grain-liquide et la convection de la phase liquide. A l'échelle microscopique, une troisième phase est également ajoutée afin d'améliorer la prédiction de la croissance des grains, notamment du développement de leur morphologie. Le modèle permet de prédire quantitativement la compétition entre germination et croissance à l'échelle microscopique. Le modèle est ensuite appliqué à des cas de coulée en lingot fixe et de coulée semi-continue afin d'étudier le couplage entre germination et croissance à l'échelle macroscopique, en présence de transport à la fois des grains, des germes et du soluté. Les prédictions du modèle sont comparées à des mesures expérimentales et permettent de mettre en évidence l'importance du transport, non seulement des grains mais également des germes, dans la formation des hétérogénéités. La prise en compte de la dendritisation des grains au cours de la solidification modifie également de façon importante les prédictions, même lorsque leur morphologie finale est globulaire / During solidification of metallic alloy, heterogeneities of solute composition and microstructure size appear. Mathematical models are valuable tools in the understanding and the control of the formation of these heterogeneities, which are incompatible with industrial requirements. Worldwide, SOLID is the only solidification model that simultaneously accounts for inoculation, the growth and motion of grains, natural convection and solidification shrinkage. The aim of this thesis is to improve the modelling of the coupling of nucleation and grain growth in this model. The solute diffusion, which controls the grain growth, is modelled by a diffusion length that takes into account both the motion of the solid-liquid interface and the convection of the liquid. At the microscopic scale, a third phase is added in order to improve the prediction of the growth of grains, particularly of the development of their morphology. The model can quantitatively predict the competition between the nucleation on inoculant particles and the grain growth at the microscopic scale. The model is applied to solidification of a static ingot and to direct chill casting in order to study the inoculation-growth coupling at the macroscopic scale. At this scale, the transport of the grains, of inoculant particles and of solute are all considered and are fully coupled. Comparisons of the model predictions to experimental measurements show the fundamental role of the transport of inoculants and of grains in the formation of the heterogeneities. The consideration of the development of dendritic grain morphologies considerably influences the predictions, even when their final morphology is globular

Variability of magmatic textures records a wide array of physicochemical and mechanical processes that have operated in a magma chamber during its crystallization. Here I investigate how the final textural record can quantitatively be used to decipher the magma crystallization history and internal dynamics of magma chambers. The thesis is based on a formulation of numerical models of texture formation under the activity of various crystallization processes. Numerical results are then compared to the new quantitative textural datasets derived from four distinct magmatic systems in the Bohemian Massif: (i) Fichtelgebirge-Smrčiny granite batholith; (ii) Krkonoše-Jizera plutonic complex; (iii) Kdyně mafic intrusion; (iv) České středohoří volcanic complex. Combination of the field textural studies with their interpretation via numerical crystallization models provides new implications regarding magmatic crystallization and internal dynamics of magma chamber. The most important results of this Ph.D. thesis are as follows: (i) a new method has been developed that allows the rates of nucleation and growth of crystals to be derived from quantitative textural data. The method requires using the crystallinity evolution in time as an independent constraint in order to provide unique solution. In case of the...