Thermodiffusion is one of the mechanisms in transport phenomena in which molecules are transported in a multicomponent mixture driven by temperature gradients. Thermodiffusion in associating mixtures presents a larger degree of complexity than non-associating mixtures, since the direction of flow in associating mixtures may change with variations in composition and temperature. In this study a new activation energy model is proposed for predicting the ratio of evaporation energy to activation energy. The new model has been implemented for prediction of thermodiffusion for acetone-water, ethanol-water and isopropanol-water mixtures. In particular, a sign change in the thermodiffusion factor for associating mixtures has been predicted, which is a major step forward in modeling of thermodiffusion for associating mixtures. In addition, a new model for the prediction of thermodiffusion coefficients for linear chain hydrocarbon binary mixtures is proposed using the theory of irreversible thermodynamics and a kinetics approach. The model predicts the net amount of heat transported based on an available volume for each molecule. This model has been found to be the most reliable and represents a significant improvement over the earlier models. Also a new approach to predicting the Soret coefficient in binary mixtures of linear chain and aromatic hydrocarbons using the thermodynamics of irreversible processes is presented. This approach is based on a free volume theory which explains the diffusivity in diffusion-limited systems. The proposed model combined with the Shukla and Firoozabadi model has been applied to predict the Soret coefficient for binary mixtures of toluene and n-hexane, and benzene and n-heptane. Comparisons of theoretical results with experimental data show a good agreement. The proposed model has also been applied to estimate thermodiffusion coefficients of binary mixtures of n-butane & carbon dioxide and n-dodecane & carbon dioxide at different temperature. The results have also been incorporated into CFD software FLUENT for 3-dimensional simulations of thermodiffusion and convection in porous media. The predictions show the thermodiffuison phenomenon is dominant at low permeabilities (0.0001 to 0.01), but as the permeability increases convection plays an important role in establishing a concentration distribution. Finally, the activation energy in Eyring’s viscosity theory is examined for associating mixtures. Several methods are used to estimate the activation energy of pure components and then extended to mixtures of linear hydrocarbon chains. The activation energy model based on alternative forms of Eyring’s viscosity theory is implemented to estimate the thermodiffusion coefficient for hydrocarbon binary mixtures. Comparisons of theoretical results with the available thermodiffusion coefficient data have shown a good performance of the activation energy model.

La thèse porte sur l'étude de la convection capillaire (effet Marangoni) et son couplage avec la convection thermogravitationnelle. Le travail met en oeuvre des mesures de champ de vitesse par vélocimétrie laser, d'une part, et des simulations numériques de ces expériences, d'autre part. Des solutions asymptotiques sont également utilisées, et la transition de la convection stationnaire vers un état d'oscillations spatiales ou spatio-temporelles a été analysée. Le manuscript aborde également des situations impliquant l'effet de thermodiffusion, ainsi que les mouvements dans une bicouche de fluides immiscibles.

Hot microelectrodes are very small electrodes (usually 1 100 µm in diameter), which have a surface temperature much higher than the temperature in the bulk solution. In this work, the heating is achieved by applying an alternating potential of very high frequency (100 MHz 2 GHz) and of high amplitude (up to 2.8 Vrms) to the microelectrode. As a result, very fast (on the order of milliseconds) changes in the temperature of the electrolyte solution surrounding the electrode can be achieved. Due to the size of the heated microelectrodes, the hot zone in solution is small. Therefore, the solution can be easily overheated and temperatures above the boiling point can be reached.<p> The purpose of this research was to investigate and understand the phenomena occurring at ac polarized microelectrodes and to propose new applications of these electrodes. Using both steady-state and fast-scan (10 V/s) cyclic voltammetry measurements, mass transport of redox species has been studied at ac heated microelectrodes. It has been established that the convection at hot-disk microelectrodes is driven primarily by the electrothermal flow of an electrolyte solution. In addition, other effects such as ac dielectrophoresis and Soret (nonisothermal) diffusion are also observed. Numerical simulations have been employed to predict the distribution of temperature in the hot zone, the direction and magnitude of the electrothermal force and the solution flow rate, as well as the voltammetric response of hot-disk microelectrodes. The results of the simulations agree well with the experimental observations. Theoretical findings of this PhD work are very important for the understanding of the fundamentals of high temperature electrochemistry, particularly mass transport. The proposed explanation of the convection mechanism is most likely applicable not only to ac polarized microelectrodes, but also to the microwave heated microelectrodes, since the only difference between these two heating methods is in the way of delivering electrical energy (wired vs. wireless). The results of the studies of Soret diffusion indicate that it contributes significantly to mass transfer of redox species at hot microelectrodes. Taking into account that the magnitude of the Soret effect has been considered negligible by other electrochemists, the results obtained in this work prove the opposite and show that Soret diffusion affects both the faradaic current and the half-wave potential of the redox reaction. Therefore, the Soret effect can not be ignored if working with hot microelectrodes.<p> Hot microelectrodes can have a number of interesting applications. The results of the initial investigations indicate that these electrodes can be successfully used in the arrangement for Scanning Electrochemical Microscopy (such a novel technique is termed Hot-Tip SECM). In addition, the observed dielectrophoretic and electrothermal convection effects can enhance the performance of the electrochemical sensors based on hot microelectrodes. This can lead to the improvement of the detection limits of many biologically important analytes, such as proteins, bacteria and viruses.

Hot microelectrodes are very small electrodes (usually 1 100 µm in diameter), which have a surface temperature much higher than the temperature in the bulk solution. In this work, the heating is achieved by applying an alternating potential of very high frequency (100 MHz 2 GHz) and of high amplitude (up to 2.8 Vrms) to the microelectrode. As a result, very fast (on the order of milliseconds) changes in the temperature of the electrolyte solution surrounding the electrode can be achieved. Due to the size of the heated microelectrodes, the hot zone in solution is small. Therefore, the solution can be easily overheated and temperatures above the boiling point can be reached.<p> The purpose of this research was to investigate and understand the phenomena occurring at ac polarized microelectrodes and to propose new applications of these electrodes. Using both steady-state and fast-scan (10 V/s) cyclic voltammetry measurements, mass transport of redox species has been studied at ac heated microelectrodes. It has been established that the convection at hot-disk microelectrodes is driven primarily by the electrothermal flow of an electrolyte solution. In addition, other effects such as ac dielectrophoresis and Soret (nonisothermal) diffusion are also observed. Numerical simulations have been employed to predict the distribution of temperature in the hot zone, the direction and magnitude of the electrothermal force and the solution flow rate, as well as the voltammetric response of hot-disk microelectrodes. The results of the simulations agree well with the experimental observations. Theoretical findings of this PhD work are very important for the understanding of the fundamentals of high temperature electrochemistry, particularly mass transport. The proposed explanation of the convection mechanism is most likely applicable not only to ac polarized microelectrodes, but also to the microwave heated microelectrodes, since the only difference between these two heating methods is in the way of delivering electrical energy (wired vs. wireless). The results of the studies of Soret diffusion indicate that it contributes significantly to mass transfer of redox species at hot microelectrodes. Taking into account that the magnitude of the Soret effect has been considered negligible by other electrochemists, the results obtained in this work prove the opposite and show that Soret diffusion affects both the faradaic current and the half-wave potential of the redox reaction. Therefore, the Soret effect can not be ignored if working with hot microelectrodes.<p> Hot microelectrodes can have a number of interesting applications. The results of the initial investigations indicate that these electrodes can be successfully used in the arrangement for Scanning Electrochemical Microscopy (such a novel technique is termed Hot-Tip SECM). In addition, the observed dielectrophoretic and electrothermal convection effects can enhance the performance of the electrochemical sensors based on hot microelectrodes. This can lead to the improvement of the detection limits of many biologically important analytes, such as proteins, bacteria and viruses.

Thermodiffusion is one of the mechanisms in transport phenomena in which molecules are transported in a multicomponent mixture driven by temperature gradients. Thermodiffusion in associating mixtures presents a larger degree of complexity than non-associating mixtures, since the direction of flow in associating mixtures may change with variations in composition and temperature. In this study a new activation energy model is proposed for predicting the ratio of evaporation energy to activation energy. The new model has been implemented for prediction of thermodiffusion for acetone-water, ethanol-water and isopropanol-water mixtures. In particular, a sign change in the thermodiffusion factor for associating mixtures has been predicted, which is a major step forward in modeling of thermodiffusion for associating mixtures. In addition, a new model for the prediction of thermodiffusion coefficients for linear chain hydrocarbon binary mixtures is proposed using the theory of irreversible thermodynamics and a kinetics approach. The model predicts the net amount of heat transported based on an available volume for each molecule. This model has been found to be the most reliable and represents a significant improvement over the earlier models. Also a new approach to predicting the Soret coefficient in binary mixtures of linear chain and aromatic hydrocarbons using the thermodynamics of irreversible processes is presented. This approach is based on a free volume theory which explains the diffusivity in diffusion-limited systems. The proposed model combined with the Shukla and Firoozabadi model has been applied to predict the Soret coefficient for binary mixtures of toluene and n-hexane, and benzene and n-heptane. Comparisons of theoretical results with experimental data show a good agreement. The proposed model has also been applied to estimate thermodiffusion coefficients of binary mixtures of n-butane & carbon dioxide and n-dodecane & carbon dioxide at different temperature. The results have also been incorporated into CFD software FLUENT for 3-dimensional simulations of thermodiffusion and convection in porous media. The predictions show the thermodiffuison phenomenon is dominant at low permeabilities (0.0001 to 0.01), but as the permeability increases convection plays an important role in establishing a concentration distribution. Finally, the activation energy in Eyring’s viscosity theory is examined for associating mixtures. Several methods are used to estimate the activation energy of pure components and then extended to mixtures of linear hydrocarbon chains. The activation energy model based on alternative forms of Eyring’s viscosity theory is implemented to estimate the thermodiffusion coefficient for hydrocarbon binary mixtures. Comparisons of theoretical results with the available thermodiffusion coefficient data have shown a good performance of the activation energy model.

Techn. University, Diss., 2007--Darmstadt.

La fraction massique des constituants au sein d'un mélange multi-constituants, cesse d'être homogène lorsque ce dernier est soumis à un gradient thermique. En effet, la présence d'un gradient thermique engendre une diffusion de masse au sein du mélange détruisant cette homogénéité. Cet effet connu depuis le XIXe siècle porte le nom de la thermodiffusion, ou encore l'effet Soret. On définit la diffusion thermogravitationnelle comme étant le couplage entre la thermodiffusion et la convection. La diffusion thermogravitaionnelle peut conduire à des séparations importantes dans les fluides multi-constituants comparée à la thermodiffusion. On s'intéresse dans ce travail de thèse, comportant une partie numérique et théorique, aux différentes structures d'écoulement et à la séparation des espèces d'une solution binaire remplissant une cavité parallélépipédique fluide ou poreuse. La cavité, placée dans le champ de la pesanteur, est soumise au niveau de ses parois horizontales et verticales à des densités de flux uniformes et croisés. L'analyse dimensionnelle de ce problème de convection thermogravitationnelle fait ressortir un certain nombre de paramètres sans dimension à savoir les nombres de Rayleigh thermique / The mass fraction of the components in a multi-component mixture ceases to be homogeneous when it is subjected to a thermal gradient. Indeed, the presence of a thermal gradient causes mass distribution within the mixture which destroys this homogeneity. This effect known since the nineteenth century is called thermodiffusion, or the Soret effect. The thermogravitational diffusion is defined as the coupling between the thermodiffusion and convection. The thermogravitational diffusion may lead to significant separations in fluids multi components compared to thermodiffusion. We focus in this thesis, including both numerical and theoretical parts, on the different flow structures and species separation of a binary solution filling a rectangular fluid or porous cavity. The cavity, placed in the gravity field is subjected to uniform heat fluxes densities at the horizontal and vertical walls. Dimensional analysis of this thermogravitational convection problem reveals a number of dimensionless parameters namely: thermal Rayleigh number

Ce travail vise à étudier la rupture du gainage avec des essais mécaniques plus représentatifs des conditions RIA, en prenant en compte les blisters d'hydrures ainsi que le niveau élevé de biaxialité du chargement mécanique et des vitesses de déformation. Nous avons formé par thermodiffusion en laboratoire des blisters similaires à ceux observés sur des gaines de Zircaloy-4 irradiées en réacteur. Les caractérisations par métallographie, nanodureté, DRX et ERDA ont montré qu'un blister est constitué d'hydrures delta dont la concentration dans la matrice varie entre 80% et 100%, et que la matrice sous-jacente contient des hydrures radiaux. Nous avons modélisé la cinétique de croissance des blisters avec l'hystérésis de la limite de solubilité de l'hydrogène,puis défini le gradient thermique seuil permettant leur formation. Notre étude sur le comportement dilatométrique du zirconium hydruré montre le rôle important de la texture cristallographique du matériau, ce qui peut expliquer des différences de morphologie des blisters. En parallèle, des essais suivis par caméra infrarouge ont montré que des vitesses de déformation supérieures à 0,1/s induisent des échauffements locaux importants qui favorisent la localisation précoce de la déformation. Enfin, nous avons optimisé l'essai d'Expansion Due to Compression pour atteindre un niveau de biaxialité de déformation plane (essais HB-EDC et VHB-EDC), ce qui réduit fortement la déformation à rupture à 25°C et 350°C, mais seulement en l'absence de blisters. Un critère de rupture est proposé pour rendre compte de la baisse de ductilité des gaines en Zircaloy-4 non irradiées en présence de blisters. / Our aim is to study the cladding fracture with mechanical tests more representative of RIA conditions, taking into account the hydrides blisters, representative strain rates and stress states. To obtain hydride blisters, we developed a thermodiffusion setup that reproduces blister growth in reactor conditions. By metallography, nanohardness, XRD and ERDA, we showed that they are constituted by 80% to 100% of delta hydrides in a Zircaloy-4 matrix, and that the zirconium beneath has some radially oriented hydrides. We modeled the blister growth kinetic taking into account the hysteresis of the hydrogen solubility limit and defined the thermal gradient threshold for blister growth. The modeling of the dilatometric behavior of hydrided zirconium indicates the important role of the material crystallographic texture, which could explain differences in the blister shape. Mechanical tests monitored with an infrared camera showed that significant local heating occurred at strain rates higher than 0.1/s. In parallel, the Expansion Due to Compression test was optimized to increase the biaxiality level from uniaxial stress to plane strain (HB-EDC and VHB-EDC tests). This increase in loading biaxiality lowers greatly the fracture strain at 25°C and 350°C only in homogeneous material without blister. Eventually, a fracture criterion of unirradiated Zircaloy-4 cladding tube taking into account the blister depth is proposed.

O presente trabalho investiga o fenômeno termodifusivo em dispersões coloidais de nanopartículas magnéticas de óxidos de ferro em água (ferrofluidos), com a formação de dupla camada elétrica em torno das partículas. A estabilidade da partícula em solução é controlada pela concentração de íons. Ao estabelecer uma diferença de temperatura através da amostra líquida, ocorre o efeito de termodifusão (efeito Soret) das partículas e de íons em solução. Este efeito é o movimento das partículas para o lado frio ou quente do gradiente de temperatura. O acúmulo para um dos lados do gradiente de temperatura depende das características da solução. O efeito Soret de ferrofluidos em soluções ácidas e básicas é descrito a partir da determinação experimental das grandezas físicas envolvidas na difusão das partículas. O coeficiente Soret ST e o coeficiente de difusão são determinados em experimentos ópticos de lente de matéria, utilizando o aparato experimental de Varredura-Z, e de espalhamento Rayleigh forçado para termodifusão. Para investigar a resposta dos íons ao gradiente de temperatura, são realizadas medidas do potencial termoelétrico em uma célula termoelétrica, gerado a partir da difusão das cargas dispersas no líquido. O potencial superficial das partículas também é investigado experimentalmente, para descrever a interação das partículas com o campo termoelétrico. Os experimentos são realizados em função da temperatura da amostra e usados para descrever os resultados ST(T) das partículas, a partir de equações dos principais modelos teóricos. Os resultados mostram as diferenças e semelhanças do efeito Soret das nanopartículas em soluções ácidas e básicas, e que em ambos os casos a termodifusão de nanopartículas reflete o comportamento termodifusivo dos íons dispersos em solução. / This work investigates the thermal diffusion phenomena in colloidal dispersions of iron oxide magnetic nanoparticles dispersed in water (ferrofluid). The particles are stable in water due to electrical double layer around the particles, controlled by the ionic concentration. A temperature gradient throughout the ferrofluid sample causes the thermodiffusion (Soret effect) of dispersed particles and ions. This effect is the movement of particles to the cold or hot side of the temperature gradient. The particles migration for a given side depends on the characteristics of the sample. The Soret effect of ferrofluids in acidic and basic solutions is described by the experimental measurements of the physical parameters associated to particles diffusion. The Soret coefficient ST and the mass diffusion coefficient are measured in the matter lens experiment in the Z-scan experimental setup, and by the use of Thermal Diffusion Forced Rayleigh Scattering experiments. Concerning the ionic response to the temperature gradient the thermoelectric field generated by charges diffusion is measured in a thermoelectric cell. The surface potential of the particles is also measured to describe its interactions with the thermoelectric field. These experiments are made as function of the temperature of the sample and the results are applied to describe the ST(T) of particles by the use of equations from the main theoretical models. The results show differences and resemblances of the Soret effect in acidic and basic nanoparticles solutions. In both kind of solutions the thermodiffusion of nanoparticles is mainly ruled by the thermodiffusion of ions dispersed in solution.

Esta dissertação apresenta um estudo experimental sobre o transporte de massa de nanopartículas magnéticas induzido por um gradiente de temperatura, denominado termodifusão. A técnica de Varredura-Z é utilizada para gerar o aumento de temperatura na região irradiada pelo laser Gaussiano e formar o gradiente de temperatura. A sequente migração de nanopartículas é caracterizada pelo gradiente de concentração gerado no estado estacionário do fluxo de partículas, definindo o coeficiente Soret ST. O objetivo deste trabalho é verificar a variação de ST com o tamanho médio d0 das nanopartículas de ferrofluidos eletrostaticamente carregados em solução ácida. A dependência de ST com d0 surge do coeficiente de difusão de massa, explicando a dependência linear encontrada experimentalmente. Nestes materiais, a migração de nanopartículas ocorre para a região quente da amostra. Mostramos que este comportamento ocorre pela diminuição da carga superficial da nanopartícula na parte mais quente da amostra, diminuindo a repulsão eletrostática. A influência dos íons presentes na solução é obtida através da mudança na amplitude de ST com a diminuição do pH na amostra. Uma previsão teórica, baseada na eletrostática da dupla camada elétrica, concorda com estes dados considerando alta blindagem eletrostática das nanopartículas e a diminuição da carga superficial com o aumento da temperatura. / This dissertation presents an experimental study about the mass transport of magnetic nanoparticles induced by a temperature gradient, called thermodiffusion. The Z-scan technique is used to generate the temperature increasing in the region irradiated by the Gaussian laser beam and create the temperature gradient. The following nanoparticles migration is characterized by the concentration gradient of the stationary particles flux, defining the Soret coefficient ST. The aim of this work is to obtain the variation of ST with the average size d0 of electrostatically charged ferrofluid nanoparticles in acidic solution. The ST dependence with d0 comes from the mass diffusion coefficient, in agreement with the linear dependence found experimentally. In these materials the nanoparticles migration occurs to the hot region of the sample. We show that this behavior is owing to the reduction of the nanoparticle´s surface charge in the hottest region of the sample, decreasing electrostatic repulsion. The influence of the ions from solution is obtained through the change in ST amplitude with reduction of the samples pH. A theoretical prediction, based in the electrostatic of the double layer, agrees with this data considering a high screening of the nanoparticles and decreasing of the surface charge with temperature increasing.