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Student Number : 0009815Y - PhD thesis - School of Physics - Faculty of Science / Determining the properties of composites and how these relate to those of the components and the microstructure is extremely useful as it enables the understanding of existing materials and the design of new materials with a variety of applications. However, the link between the ac conductivity data and the microstructure and composition of the composite is not a simple one. Simulations of binary composites are presented which show that from relatively simple component properties arise complex composite properties. Accurate identi¯cation of the components of composites, using characteristic frequencies, is demonstrated for simulated and actual experimental data. The Maxwell-Wagner and Brick Layer Models, which are often applied beyond the range of their original derivation, are found to consistently ¯t the data of yttria-stabilized zirconia ceramics measured at various tempera- tures. The results from ¯tting single crystal and polycrystalline sample data indicate that accurate modeling of the properties of the polycrystalline sam- ples requires more theoretical work on the conduction mechanisms in single crystals and the grains of the ceramics. Data from a polyester-resin/silicon system is found to be best ¯tted us- ing the Single Exponent Phenomenological Percolation Equation (SEPPE) with experimentally measured component properties as input. The percola- tion threshold obtained suggests a system where the insulator tends coat the conductor. The results show that, with the actual component properties as input, the SEPPE can be used to qualitatively and semi-quantitatively model and ¯t composite ac conductivity data. Analysis of the ac conductivity of liquid-phase-sintered silicon carbide ce- ramics showed that for this system, the features often observed in the imped- ance spectra are all due to a multi-component grain bondary/binder phase and not the SiC grains. This multi-component grain boundary phase can be ¯tted accurately to the Brick Layer Model, indicating a microstructure where an insulating component coats a more conducting component.

A la suite d'une demande de divers secteurs en développement, comme !'-enfouissement des déchets (nucléaires, industriels et ménagers), et l'amélioration des ressources géothermiques, pétrolières ou en eau souterraine, la recherche sur les circulations de fluides dans les milieux fracturés a connu un essor croissant ces dernières années. Malheureusement, la modélisation des écoulements dans ces milieux se heurte à la complexité géométrique des champs de failles qui présentent une large gamme de longueurs et d'ouvertures, ainsi qu'une distribution spatiale non-homogène. La superposition de ces paramètres géométriques conduit en particulier à des effets d'échelles sur les coefficients de transport qui restent encore mal définis. Après avoir entrepris une analyse statistique bi-dimensionnelle de la géométrie de plusieurs réseaux de failles naturels, nous avons développé une approche fondée sur la théorie de la percolation, dans le but de déterminer les principaux paramètres géométriques gouvernant les propriétés de transport des milieux fracturés. La description géométrique d'une faille nécessite au moins trois paramètres géométriques, que l'on peut supposer a priori indépendants : sa position dans l'espace, sa longueur et son orientation. Pourtant, en analysant séparément leurs distributions spatiales nous avons montré qu'entre les différents paramètres géométriques, il existe des corrélations nettes qui résultent probablement des interactions mécaniques inteNenant lors de la croissance des champs de failles. En particulier, nos résultats suggèrent l'existence d'une relation entre la dimension fractale des barycentres de failles et l'exposant de la loi d'échelle de la distribution des longueurs de failles. L'adaptation de la théorie de la percolation à la géométrie spécifique des milieux fracturés en général, et à leur très large distribution de longueurs, en particulier, a permis de décrire leurs propriétés de connectivité. Cette analyse théorique a pu être vérifiée en deux et trois dimensions, grâce à un .modèle numérique. Les implications de cette analyse reposent en grande partie sur une quantification du rôle des "grandes" et des "petites" failles, qui permet de définir l'obseNable pertinent. La géométrie des chemins de connection (amas percolant) et des chemins d'écoulement (backbone) au seuil de percolation est décrite sous la forme de lois d'échelles. Il est également montré comment certaines distributions de longueurs, fréquentes dans la nature, impliquent nécessairement l'existence d'une échelle au-dessus de laquelle les réseaux de failles sont bien connectés (longueur de corrélation finie). L'application de notre analyse à plusieurs réseaux de failles naturels a permis de montrer l'adéquation de l'approche théorique développée. Ce travail définit un cadre théorique qui permettra de fournir les bases d'une modélisation originale des propriétés de transport des systèmes fracturés. Cette proposition de modèle s'appuie à la fois sur l'éventuelle dépendance d'échelle des coefficients de transport, qui a par ailleurs été obseNée grâce à une étude expérimentale menée sur le site de Ploemeur (Morbihan), et sur l'utilisation de distributions géométriques appropriées aux systèmes fracturés.

This dissertation aims to propose a new and practical method to obtain equivalent fracture network permeability (EFNP), which represents and replaces all the existing fractures located in each grid block for the reservoir simulation of naturally fractured reservoirs. To achieve this, first the relationship between different geometrical properties of fracture networks and their EFNP was studied. A MATLAB program was written to generate many different realizations of 2-D fracture networks by changing fracture length, density and also orientation. Next, twelve different 2-D fractal-statistical properties of the generated fracture networks were measured to quantify different characteristics. In addition to the 2-D fractal-statistical properties, readily available 1-D and 3-D data were also measured for the models showing variations of fracture properties in the Z-direction. The actual EFNP of each fracture network was then measured using commercial software called FRACA. The relationship between the 1-, 2- and 3-D data and EFNP was analyzed using multivariable regression analysis and based on these analyses, correlations with different number of variables were proposed to estimate EFNP. To improve the accuracy of the predicted EFNP values, an artificial neural network with the back-propagation algorithm was also developed. Then, using the experimental design technique, the impact of each fracture network parameter including fracture length, density, orientation and conductivity on EFNP was investigated. On the basis of the results and the analyses, the conditions to obtain EFNP for practical applications based on the available data (1-D well, 2-D outcrop, and 3-D welltest) were presented. This methodology was repeated for natural fracture patterns obtained mostly from the outcrops of different geothermal reservoirs. The validity of the equations was also tested against the real welltest data obtained from the fields. Finally, the concept of the percolation theory was used to determine whether each fracture network in the domain is percolating (permeable) and to quantify the fracture connectivity, which controls the EFNP. For each randomly generated fracture network, the relationship between the combined fractal-percolation properties and the EFNP values was investigated and correlations for predicting the EFNP were proposed. As before, the results were validated with a new set of fracture networks. / Petroleum Engineering

Sandy soils are among the least productive soils because of their inability to store adequate water for plant growth. Their high percolation rate not only allows water to move quickly beyond the root zone, but also washes nutrients below the reach of plant roots. High evaporation occurs from the soil surface. Many acres of these soils around the world are left out of crop production. This study is a contribution to bring these soils into production by increasing their ability to hold more water in the root zone. Several promising methods of enhancing these soils were simulated, surface mulch, buried barrier layer, and a combination of both. The effects of varying texture and thickness of these layers and varying evaporative demand were investigated. The impact of such modifications on solute distribution in the soil was also simulated. A simulation model of water and solute transport in layered soils was developed for this purpose. The Richards equation for one-dimensional water transport in unsaturated soils was modified to account for the water jump between the layers. The solute transport equation was also modified by implementing the same theory of water infiltration in layered soil to the solute convective transport. The Crank-Nicolson scheme was used to solve the transport equations with the help of the Newton-Raphson iteration method. The results of the simulation show that the proposed methods increase water content in the sandy soil by up to 45%. The combination of barriers, which decreases leaching and evaporation was the most effective in conserving water. Most of the contribution came from the influence of the mulch layer in suppressing water losses by evaporation. The combination method traps solute in the root zone, and this decreased solute leaching from the soil may limit plant growth in saline soils. / Graduation date: 1993

Polymer matrix composites reinforced by metal fibers are observed to present an onset of the insulator-to-conductor transition through previous experimental studies. Analytical studies revealed that the percolation threshold occurs when fiber volume fraction reaches the critical value. The numerical study based on Monte Carlo simulations are performed to investigate such a relation. In this work, the conductive fillers are modeled as a three dimensional (3D) network of identical units randomly distributed in the polymer matrix. For the simplest case, straight fibers are used in the simulation. The effects of the aspect ratio and fiber length on the critical volume fraction are also studied. Linearization is made to the logarithm of simulation results. Next, in order to study the effects of emulsion particles and the emulsion particle sizes on the percolation behavior, cubic particles are aligned in the sample model. The gap width to particle size ratio is fixed at 1/10. The calculated critical volume fraction is used in the power-law function to predict the electrical conductivity of the polymer composites. Due to the insensitivity of the thermal conductivity to the percolation threshold, a combination of two empirical equations is used to predict the range of overall thermal conductivity.

Reproduction de : Thèse de doctorat : Dynamique des fluides : Toulouse, INPT : 2005. / Titre provenant de l'écran-titre. Bibliogr. 40 réf.

Thèse de doctorat : Physique : Tours : 2008. / Titre provenant de l'écran-titre.

published_or_final_version / Civil Engineering / Master / Master of Philosophy

published_or_final_version / Civil Engineering / Master / Master of Philosophy