Les catalyseurs supportés sur alumine ont un intérêt crucial pour un large domaine de procédés de raffinage. La connaissance des propriétés de texture du support catalytique est un paramètre clé pour l'optimisation des propriétés de transport des réactifs et des produits. Basée sur la grande sensibilité du nuage électronique fortement polarisable de l'atome de xénon à son environnement, la RMN 129Xe s’est avérée être une technique très utile pour étudier la porosité des solides microporeux et mésoporeux comme les zéolithes et les silices. L’objectif de cette thèse est alors de caractériser des matériaux non organisés telles que les alumines-γ grâce à cette technique. Les travaux présentés dans ce manuscrit combinent les résultats des différentes mesures de RMN permettant de caractériser les propriétés de texture et de transport des alumines: - Après avoir déterminé les conditions expérimentales permettant de minimiser l’effet des sites d’adsorption forte sur le déplacement chimique du xénon adsorbé, une corrélation a pu être établie entre le déplacement chimique et la taille de pores déterminée par adsorption d’azote à 77 K (Chapitre 4).- Afin de décrire la diffusion de différents adsorbats (Xe, n-hexane), les décrire les coefficients d’autodiffusion ont été déterminées par RMN à gradients de champ pulsé (129Xe et 1H) permettant ainsi de caractériser les phénomènes de diffusion et d’évaluer la tortuosité (Chapitre 5).- Pour compléter la description de la porosité des alumines–γ, la connectivité des pores, l’échange des atomes de Xe entre divers environnements, a été quantifié par des expériences RMN 2D-Echange réalisées à différentes températures (Chapitre 6). / Alumina-based catalysts have a crucial importance for a wide range of refining processes. The knowledge of the textural properties of catalyst supports is decisive for the optimization of transport properties of reactants and products. Based on the sensitivity of the highly polarizable electron cloud of xenon atom to its environment, 129Xe NMR has proved to be a very useful technique to study the porosity of microporous and mesoporous solids such as zeolites and silicas. The objective of this PhD thesis is to characterize disorganized materials such as γ-aluminas with this technique. The work presented in this manuscript combine the results of different NMR measurements to characterize the texture and transport properties of aluminas:- After identifying the experimental conditions that minimize the effect of strong adsorption sites on the chemical shift of xenon adsorbed, a correlation has been established between the chemical shift and the pore size determined by nitrogen adsorption at 77 K (Chapter 4).- In order to describe the diffusion coefficients of different adsorbates (Xe, n-hexane) were determined by pulsed field gradient NMR (129Xe and 1H) to characterize diffusion phenomena and to assess pore tortuosities (Chapter 5).- For completing the description of γ-aluminas porosity, pore connectivities, exchange of Xe atoms between different environments has been quantified using 2D-Exchange NMR experiments performed at different temperatures (Chapter 6).

Title: Investigation of matter transport by means of PFG NMR Author: Mikuláš Peksa Department of low temperature physics Supervisor: doc. RNDr. Jan Lang, Ph.D. Assistant Supervisor: RNDr. Milan Kočiřík, CSc. (ÚFCH JH AV ČR) Abstract: Estimation of transport-structural parameters such as porosity, tortuosity and surface-to-volume ratio of pores in beds of glass beads is the main goal of this study. These parameters were estimated for 5 samples with different distributions of sizes. The second goal is to probe a possibility to use the same approach to describe the self-diffusion in water solution of LiCl confined in two porous materials based on Al2O3 and glass, respectively. The last goal is the measurement of self-diffusion of water molecules in mesoporous geopolymeric material. Its capability of water transport at long scales have been documented. The measurements of apparent self-diffusion coefficients by means of NMR spectroscopy with pulsed field gradients was major methodology of this work. Keywords: porous material, porosity, tortuosity, self-diffusion, NMR

Next-generation batteries with improved microstructure and performance are on their way to meet the market demands for high-energy and power storage systems. Among different types of batteries, Li-ion batteries remain the best choice for their high energy density and long lifetime. There is a constant but slow improvement in Li-ion batteries by developing new materials and fabrication techniques. However, further improvements are still needed to meet government and industry goals for cost, cycling performance, and cell lifetime. A fundamental understanding of particle-level interactions can shed light on designing new porous electrodes for high-performance batteries. This is a complex problem because electrodes have a multi-component, multi-phase microstructure made through multiple fabrication processes (i.e., mixing, coating, drying, and calendering). Each of these processes can affect the final microstructure (particle and pore locations) differently. This work seeks to understand the porous microstructure evolution of Li-ion electrodes during the drying and calendering fabrication processes by a combination of modeling and experimental approaches. The goal is to understand the mechanisms by which the electrode components and fabrication processes determine the battery microstructure and subsequent cell performance. A multi-phase smoothed particle (MPSP) model has been developed on a publically available simulation platform known as LAMMPS. This model was used to simulate particle-level interactions and predict the mechanical and transport properties of four fabricated electrodes (i.e. a graphite anode and three traditional metal oxide cathodes). One challenge was to include different electrode components and their interactions and relate them to physical properties like density and viscosity that can be measured experimentally. Another challenge was to generate required electrode property data for model validation, which in general was not found in the literature. Therefore, a series of experiments were conducted to provide that information, namely slurry viscosity, electronic conductivity, porosity, tortuosity, elastic modulus, and electrode crosssections. Understanding these properties has value to the battery community independent of their use in this study. The MPSP model helps us explain observed transport heterogeneity after calendering but brings up new questions about the drying process that have not been addressed in previous works. Therefore, the drying fabrication step was studied experimentally in more detail to fill this knowledge gap and explain our simulation results. The MPSP model can also be used as a predictive tool to explore the design space of Li-ion electrodes where conducting the actual experiments is very challenging. For example, the distinct effect of particle size, shape, orientation, and stiffness on electrode transport and mechanical properties are difficult to determine independently, and therefore this model is an ideal tool to understand the effect of these properties. The final model, which is publically available, could be used with adjustments by future workers to test new materials, fabrication processes, or electrode design (e.g., a multi-layered structure).

Electrode microstructure directly affects ion and electron transport and, in turn, has a strong correlation to battery performance. Understanding the separate yet complementary effects of ionic and electronic transport in cell behavior is a challenge. This work provides through a combination of experiments and modeling a better understanding of the relationship between three aspects of the cell: ion transport within the electrode, electrode uniformity, and cell degradation. The first part of this work compares two experimental methods that determine ion transport in terms of tortuosity, a dimensionless geometric factor. The polarization-interrupt and blocking-electrolyte methods measure effective diffusivity and conductivity, respectively. The tortuosity of several commercial-quality electrodes was measured using both methods, producing reasonable agreement between the two methods in most cases. Next, the effect of cell cycling on ionic and electronic transport of electrodes was investigated. Using the blocking electrolyte method, the tortuosity of electrode films at varying extents of cycling was determined. Variations in electronic resistivity were quantified by micro-scale measurements using a previously developed micro-four-line probe. The changes in tortuosity and electronic resistivity were investigated for a graphite anode and several cathode chemistries including LiCoO2, LiNixCoyMnzO2, LiFePO4, and blends of transition metal oxides. Clear evidence of changes in tortuosity and electronic resistivity was observed during cell formation and cycling. The magnitude of the changes strongly depended on the chemistry of electrodes and cycling conditions. The results indicate that, under normal cycling conditions, electronic resistivity increases while tortuosity unexpectedly decreases. However, accelerated cycling conditions (i.e. elevated temperature) can lead to both electronic resistivity and tortuosity increase. Finally, the interplay of electrode tortuosity heterogeneity and Li-plating was investigated. The Li-plating reaction was incorporated into a Newman-type model and validated using the voltage profile and capacity-loss data from experiments. The simulation result shows that a heterogeneous anode can cause non-uniform Li plating while cathode heterogeneity did not have a significant effect. The Li-plating profile across the thickness of the anode with cell cycling showed that Li tends to plate at the high tortuosity region near the separator. Unexpectedly, Li plating tends to shift to the current collector side upon a sufficient increase in porosity close to the separator. Simulated capacity loss vs. cycling data indicates that there is a feedback mechanism with cycling: as cycling continues the rate of Li plating for the high-tortuosity region decreases at the separator side and the other two regions will eventually catch up in terms of plating.

This work presents novel modeling and experimental techniques that provide insight into liquid-phase mass transport and electron transfer processes in lithium-ion batteries. These included liquid-phase ionic mass transport (conduction and diffusion), lithium diffuion in the solid phase and electronic transport in the solid phase. Fundamental understanding of these processes is necessary to efficiently design and optimize lithium-ion batteries for different applications. To understand the effect of electrode structure on the electronic resistance of the cathode, we tested power performance of cathodes with combinations of three different carbon conductivity additives: vapor-grown carbon fibers (CF), carbon black (CB) and graphite (GR). With all other factors held constant, cathodes with a mixture of CF+CB were found to have the best power-performance, followed by cells containing CF only and then by CB+GR. Thus, the use of carbon fibers as conductive additive was found to improve the power performance of cells compared to the baseline (CB+GR). The enhanced electrode performance due to the fibers also allows an increase in energy density while still meeting power goals. About one-third of the available energy was lost to irreversible processes when cells were pulse-charged or discharged at the maximum rate allowed by voltage-cutoff constraints. We developed modeling and experimental techniques to quantify tortuosity in electrolyte-filled porous battery structures (separator and active-material film). Tortuosities of separators were measured by two methods, AC impedance and polarization-interrupt, which produced consistent results. The polarization-interrupt experiment was used similarly to measure effective electrolyte transport in porous films of cathode materials, particularly films containing lithium iron phosphate. An empirical relationship between porosity and the tortuosity of the porous structures was developed. Our results demonstrate that the tortuosity-dependent mass transport resistance in porous separators and electrodes is significantly higher than that predicted by the oft-used Bruggeman relationship. To understand the dominant resistances in a lithium battery, we developed and validated a model for lithium iron phosphate cathode. In doing so we considered unique physical features of this active material. Our model is unusual in terms of the range of experimental conditions for which it is validated. Various submodel and experimental techniques were used to find physically realistic parameters. The model was tested with different discharge rates and thicknesses of cathodes, in all cases showing good agreement, which suggests that the model takes into account physical realities with different thicknesses. The model was then used to find the dominant resistance for the tested cathodes. The model suggests that the inter-particle contact resistance between carbon and the active-material particles was a dominant resistance for the tested cathodes.

A new laboratory permeameter was developed for measuring the hydraulic conductivity of gravels ranging from 0.1 to 2 m/s. The release of pneumatic pressure applied to the test specimen induces an underdamped oscillatory response of the water level above the permeameter, similar to an underdamped in situ slug test response in monitoring wells. A closed form model was derived to calibrate the hydraulic minor losses in the permeameter and the hydraulic conductivity of the specimen by performing tests without and with a specimen. The majority of each test series performed on individual specimens produced hydraulic conductivity values within 10% of the average, which is very small for such a measurement. Tests were performed using the permeameter on a collection of subrounded and angular gravels prepared to measured grain size distributions and porosities. The surface area was determined by evaluating the shape and angularity using a method developed in this research and these parameters were used with the measured tortuosity and hydraulic conductivity, to back calculate the packing factor of the Kozeny-Carman equation. The results show that the packing factor for the gravels and materials tested is proportional to the tortuosity cubed. These results provide a valuable update to the Kozeny-Carman equation for predicting the hydraulic conductivity of gravels. Field slug interference tests were performed in pairs of monitoring wells installed at the same elevation in a floodplain deposit of silty sand in Dedham MA. Slug tests were performed in one of the wells while the response was monitored simultaneously in both wells. The measured responses were both analyzed by modifying the KGS model of Hyder et al. (1994) to consider the wellbore storage and filter packs effects. This modification was found to produce estimates of hydraulic conductivity based on the slugged well response that compared well with that estimated based on the observation well's response. Calibrated hydraulic conductivities for the pairs of wells tested ranged from 4x10-6 to 1.5x10-5 m/s and specific storage ranged from 2x10-5 to 7x10-4 m-1.

No description available.

This thesis concerns the modelling of acoustical and vibrational properties of open cell porous foams in multi-layered structures, especially multi-layered panels. The object is to enable optimisation of the microscopic geometry of the foam with respect to macroscopic quantities such as sound pressure level, surface velocity, total mass or cost. The developed method is based on numerical solutions to Biot's equations were scaling laws has been used to connect the microscopic geometry of the foam to macroscopic properties such as density, flow resistivity and characteristic length. Efforts have also been made to establish a scaling law for tortuosity that allows for adaptation to different strut shapes. / QC 20101117

Coronary tortuosity is the abnormal curving and twisting of the coronary arteries. Although the phenomenon of coronary tortuosity is frequently encountered by cardiologists its clinical significance is unclear. It is known that coronary tortuosity has significant influence on the hemodynamics inside the coronary arteries, but it is difficult to draw definite conclusions due to the lack of patient-specific studies and an absence of a clear definition of tortuosity. In this work, in order to investigate a relation of coronary tortuosity to such diseases as atherosclerosis, ischemia, and angina, a numerical investigation of coronary tortuosity was performed. First, we studied a correlation between a degree of tortuosity and flow parameters in three simplified vessels with curvature and zero torsion. Next, a statistical analysis based on flow calculations of 23 patient-based real tortuous arteries was performed in order to investigate a correlation between tortuosity and flow parameters, such as pressure drop, wall shear stress distribution, and a strength of helical flow, represented by a helicity intensity, and concomitant risks. Results of both idealized and patient-specific studies indicate that a risk of perfusion defects grows with an increased degree of tortuosity due to an increased pressure drop downstream an artery. According to the results of the patient-specific study, a risk of atherosclerosis decreases in more tortuous arteries - a result different from an outcome of the idealized study of arteries with zero torsion. Consequently, a modeling of coronary tortuosity should take into account all aspects of tortuosity including a heart shape that introduces additional torsion to arteries. Moreover, strength of a helical flow was shown to depend strongly on a degree of tortuosity and affect flow alterations and accompanying risks of developing atherosclerosis and perfusion defects. A corresponding quantity, helicity intensity, might have a potential to be implemented in future studies as a universal single parameter to describe tortuosity and assess congruent impact on the health of a patient. / Master of Science

Made available in DSpace on 2017-07-21T19:26:07Z (GMT). No. of bitstreams: 1 Sabrina Passoni.pdf: 6849418 bytes, checksum: 548846a6a7358d6de92338d9345d4f38 (MD5) Previous issue date: 2013-07-31 / The X-ray computed microtomography (CT) represents a non-invasive technique that can be used with success to analyze physical properties by the soil scientists without destroying the structure of the soil. The technique has as advantage over conventional methods the characterization of the soil porous system in three dimensions, which allow morphological property analyses such as connectivity and tortuosity of the pores. However, as the soil is a non-homogeneous and complex system, the CT technique needs specific methodologies for digital image processing, mainly during the segmentation procedure. The objectives of this work were: 1) to develop a methodology for microtomographic digital image processing; 2) to characterize the soil structure by using micromorphology analysis of samples submitted to non-tillage and conventional systems collected in three distinct layers (0-10, 10-20 and 20-30 cm); and 3) to identify possible changes in the porous system of the soil analyzed due to the effect of different management systems. The use of the CT technique and the procedures adopted for microtomographic digital image processing show to be efficient for the micromorphologic characterization of soil porous system. Soil under non-tillage system presented the best results from the agricultural point of view regarding porosity, total number of pores, connectivity and tortuosity in comparison to the conventional tillage. / A microtomografia computadorizada de raios X (μCT) é uma técnica não-invasiva, que pode ser usada com sucesso para analisar as propriedades físicas do solo pelos cientistas, sem destruir a estrutura do solo. A técnica apresenta vantagens sobre os métodos convencionais: a caracterização do sistema poroso do solo em três dimensões, a qual permite análises de propriedades morfológicas, tais como a conectividade e tortuosidade dos poros. No entanto, como o solo é um sistema não-homogêneo e complexo, a técnica de μTC necessita de metodologias específicas para o processamento de imagens digitais, principalmente durante o processo de segmentação. Os objetivos deste trabalho foram: 1) desenvolver uma metodologia de processamento digital de imagens microtomográficas; 2) caracterizar a estrutura do solo por meio de análise micromorfológica das amostras submetidas a sistemas plantio direto e plantio convencional coletadas em três camadas distintas (0-10, 10-20 e 20-30 cm) e 3), identificar possíveis alterações no sistema poroso do solo analisado, devido ao efeito de diferentes sistemas de manejo. O uso da técnica de tomografia computadorizada e os procedimentos adotados para o processamento de imagem digital microtomográfica mostrou-se eficiente para a caracterização micromorfológica do sistema poroso do solo. Os dados do solo sob sistema de plantio direto apresentou os melhores resultados do ponto de vista agrícola sobre a porosidade, o número total de poros, conectividade e tortuosidade em comparação com o plantio convencional.