Computer simulation of random parckings for self-similar particle size distributions in soil and granular materials: porosity and pore size distribution

A 2D computer simulation method of random packings is applied to sets of particles generated by a self-similar uniparametric model for particle size distributions (PSDs) in granular media. The parameter p which controls the model is the proportion of mass of particles corresponding to the left half of the normalized size interval [0,1]. First the influence on the total porosity of the parameter p is analyzed and interpreted. It is shown that such parameter, and the fractal exponent of the associated power scaling, are efficient packing parameters, but this last one is not in the way predicted in a former published work addressing an analogous research in artificial granular materials. The total porosity reaches the minimum value for p = 0.6. Limited information on the pore size distribution is obtained from the packing simulations and by means of morphological analysis methods. Results show that the range of pore sizes increases for decreasing values of p showing also different shape in the volume pore size distribution. Further research including simulations with a greater number of particles and image resolution are required to obtain finer results on the hierarchical structure of pore space.

Recovery of traditional mortars in the laboratory of materials at the school of architectura of Madrid

The Laboratory of Materials seminar is an optional course offered to students in the School of Architecture in Madrid. It is taught during 35 class hours in the laboratory of materials distributed in two hours and forty minutes weekly. One of the working lines is the preparation of traditional mortars made by students groups, each one of 4 or 5 members. It is basically a practical course, and students have to prepare a group of tests pieces in order to confirm the initial hypothesis, or when needed, make innovations. The test pieces are of 150mmx150mmx10mm and applied to big size hollow bricks, prismatic test pieces of 40mmx40mmx160mm to apply physical and mechanical tests and brick wallings of varied dimensions.

Desarrollo de estructuras nanofotonicas de campo cercano para intensificacion localizada de luz en celulas solares de banda intermedia - Near-field nanophotonic structures for localized light enhancement in intermediate band solar cells

This doctoral thesis explores some of the possibilities that near-field optics can bring to photovoltaics, and in particular to quantum-dot intermediate band solar cells (QD-IBSCs). Our main focus is the analytical optimization of the electric field distribution produced in the vicinity of single scattering particles, in order to produce the highest possible absorption enhancement in the photovoltaic medium in their surroundings. Near-field scattering structures have also been fabricated in laboratory, allowing the application of the previously studied theoretical concepts to real devices. We start by looking into the electrostatic scattering regime, which is only applicable to sub-wavelength sized particles. In this regime it was found that metallic nano-spheroids can produce absorption enhancements of about two orders of magnitude on the material in their vicinity, due to their strong plasmonic resonance. The frequency of such resonance can be tuned with the shape of the particles, allowing us to match it with the optimal transition energies of the intermediate band material. Since these metallic nanoparticles (MNPs) are to be inserted inside the cell photovoltaic medium, they should be coated by a thin insulating layer to prevent electron-hole recombination at their surface. This analysis is then generalized, using an analytical separation-of-variables method implemented in Mathematica7.0, to compute scattering by spheroids of any size and material. This code allowed the study of the scattering properties of wavelengthsized particles (mesoscopic regime), and it was verified that in this regime dielectric spheroids perform better than metallic. The light intensity scattered from such dielectric spheroids can have more than two orders of magnitude than the incident intensity, and the focal region in front of the particle can be shaped in several ways by changing the particle geometry and/or material. Experimental work was also performed in this PhD to implement in practice the concepts studied in the analysis of sub-wavelength MNPs. A wet-coating method was developed to self-assemble regular arrays of colloidal MNPs on the surface of several materials, such as silicon wafers, amorphous silicon films, gallium arsenide and glass. A series of thermal and chemical tests have been performed showing what treatments the nanoparticles can withstand for their embedment in a photovoltaic medium. MNPs arrays are then inserted in an amorphous silicon medium to study the effect of their plasmonic near-field enhancement on the absorption spectrum of the material. The self-assembled arrays of MNPs constructed in these experiments inspired a new strategy for fabricating IBSCs using colloidal quantum dots (CQDs). Such CQDs can be deposited in self-assembled monolayers, using procedures similar to those developed for the patterning of colloidal MNPs. The use of CQDs to form the intermediate band presents several important practical and physical advantages relative to the conventional dots epitaxially grown by the Stranski-Krastanov method. Besides, this provides a fast and inexpensive method for patterning binary arrays of QDs and MNPs, envisioned in the theoretical part of this thesis, in which the MNPs act as antennas focusing the light in the QDs and therefore boosting their absorption

MicroRaman Spectroscopy of Si Nanowires: Influence of Size

Si Nanowires (NWs) were studied by Raman microspectroscopy. The Raman spectrum of the NWs reveals important thermal effects, which broaden and shift the one phonon Raman bands. The low thermal conductivity of the NWs and the low thermal dissipation are responsible for the temperature enhancement in the NW under the excitation with the laser beam. We have modeled, using finite element methods, the interaction between the laser beam and the NWs. The Raman spectrum of Si NWs is interpreted in terms of the temperature induced by the laser beam excitation, in correlation with finite element methods (fem) for studying the interaction between the laser beam and the NWs.

Engineering metal precipitate size distributions to enhance gettering in multicrystalline silicon

The extraction of metal impurities during phosphorus diffusion gettering (PDG) is one of the crucial process steps when fabricating high-efficiency solar cells using low-cost, lower-purity silicon wafers. In this work, we show that for a given metal concentration, the size and density of metal silicide precipitates strongly influences the gettering efficacy. Different precipitate size distributions can be already found in silicon wafers grown by different techniques. In our experiment, however, the as-grown distribution of precipitated metals in multicrystalline Si sister wafers is engineered through different annealing treatments in order to control for the concentration and distribution of other defects. A high density of small precipitates is formed during a homogenization step, and a lower density of larger precipitates is formed during extended annealing at 740º C. After PDG, homogenized samples show a decreased interstitial iron concentration compared to as-grown and ripened samples, in agreement with simulations.

Micropillar compression of LiF [111] single crystals: effect of size, ion irradiation and misorientation

The mechanical response under compression of LiF single crystal micropillars oriented in the [111] direction was studied. Micropillars of different diameter (in the range 1–5 lm) were obtained by etching the matrix in directionally-solidified NaCl–LiF and KCl–LiF eutectic compounds. Selected micropillars were exposed to high-energy Ga+ ions to ascertain the effect of ion irradiation on the mechanical response. Ion irradiation led to an increase of approximately 30% in the yield strength and the maximum compressive strength but no effect of the micropillar diameter on flow stress was found in either the as-grown or the ion irradiated pillars. The dominant deformation micromechanisms were analyzed by means of crystal plasticity finite element simulations of the compression test, which explained the strong effect of micropillar misorientation on the mechanical response. Finally, the lack of size effect on the flow stress was discussed to the light of previous studies in LiF and other materials which show high lattice resistance to dislocation motion.

Latent hardening size effect in small-scale plasticity

We aim at understanding the multislip behaviour of metals subject to irreversible deformations at small-scales. By focusing on the simple shear of a constrained single-crystal strip, we show that discrete Dislocation Dynamics (DD) simulations predict a strong latent hardening size effect, with smaller being stronger in the range [1.5 µm, 6 µm] for the strip height. We attempt to represent the DD pseudo-experimental results by developing a flow theory of Strain Gradient Crystal Plasticity (SGCP), involving both energetic and dissipative higher-order terms and, as a main novelty, a strain gradient extension of the conventional latent hardening. In order to discuss the capability of the SGCP theory proposed, we implement it into a Finite Element (FE) code and set its material parameters on the basis of the DD results. The SGCP FE code is specifically developed for the boundary value problem under study so that we can implement a fully implicit (Backward Euler) consistent algorithm. Special emphasis is placed on the discussion of the role of the material length scales involved in the SGCP model, from both the mechanical and numerical points of view.

Estudio microestructural y de los procesos de hidratación de cementos con adiciones

En los últimos años, las sociedades industrializadas han tomado una mayor conciencia sobre el problema que suponen las emisiones indiscriminadas de gases de efecto invernadero a la atmósfera. El hormigón, cuyo principal componente es el cemento, es probablemente el material más utilizado en construcción. En la actualidad, las emisiones globales de CO2 debidas a la combustión del CaCO3 del cemento Pórtland representan entre el 5% y el 10% respecto del total. Estos valores son de gran interés si se considera que el compromiso aceptado al firmar el Protocolo de Kioto es de una reducción del 5% antes del año 2020, sobre el total de gases producidos. El principal objetivo del presente trabajo es el estudio microestructural y de los procesos de hidratación de los cementos con adiciones. Para ello se propone contribuir a la investigación sobre nuevos productos cementicios basados en micropartículas esféricas vítreas que pueden adicionarse al cemento antes del proceso de amasado. Los resultados obtenidos se han contrastado con las adiciones convencionales de más uso en la actualidad. El nuevo material basa su composición en la química del aluminio y el silicio. Al disminuir la cantidad de CaCO3, se contribuye al desarrollo sostenible y a la reducción de emisiones de CO2. La patente creada por el Grupo Cementos Pórtland Valderrivas (GCPV), describe el proceso de producción de las cemesferas (WO 2009/007470, 2010). Los productos que forman la materia prima para la elaboración de las cemesferas son arcillas, calizas, margas o productos o subproductos industriales, que tras su molienda, son fundidos mediante un fluido gaseoso a elevada temperatura (entre 1250ºC y 1600ºC). Este proceso permite obtener un producto final en forma esférica maciza o microesfera, que tras estabilizarse mediante un enfriamiento rápido, consigue una alta vitrificación idónea para su reactividad química, con una mínima superficie específica en relación a su masa. El producto final obtenido presenta prácticamente la finura requerida y no precisa ser molido, lo que reduce las emisiones de CO2 por el ahorro de combustible durante el proceso de molienda. El proceso descrito permite obtener un amplio abanico de materiales cementantes que, no solo pueden dar respuesta a los problemas generados por las emisiones de CO2, sino también a la disponibilidad de materiales en países donde hasta el momento no se puede fabricar cemento debido a la falta de calizas. Complementariamente se ha optimizado el método de cálculo del grado de hidratación a partir de los resultados del ensayo de ATD-TG en base a los modelos de cálculo de Bhatty y Pane. El método propuesto permite interpretar el comportamiento futuro del material a partir de la interpolación numérica de la cantidad de agua químicamente enlazada. La evolución del grado de hidratación tiene una relación directa con el desarrollo de la resistencia mecánica del material. Con el fin de caracterizar los materiales de base cemento, se ha llevado a cabo una amplia campaña experimental en pasta de cemento, mortero y hormigón. La investigación abarca tres niveles: caracterización microestructural, macroestructural y caracterización del comportamiento a largo plazo, fundamentalmente durabilidad. En total se han evaluado ocho adiciones diferentes: cuatro adiciones convencionales y cuatro tipos de cemesferas con diferente composición química. Los ensayos a escala microscópica comprenden la caracterización química, granulométrica y de la superficie específica BET de los materiales anhidros, análisis térmico diferencial y termogravimétrico en pasta de cemento y mortero, resonancia magnética de silicio en pasta de cemento, difracción de rayos X de los materiales anhidros y de las probetas de pasta, microscopía electrónica de barrido con analizador de energía dispersiva por rayos X en pasta y mortero, y porosimetría por intrusión de mercurio en mortero. La caracterización macroscópica del material comprende ensayos de determinación del agua de consistencia normal y de los tiempos de inicio y fin de fraguado en pasta de cemento, ensayos de resistencia mecánica a flexión y compresión en probetas prismáticas de mortero, y ensayos de resistencia a compresión en probetas de hormigón. Para caracterizar la durabilidad se han desarrollado ensayos de determinación del coeficiente de migración de cloruros y ensayos de resistividad eléctrica en probetas de mortero. Todos los ensayos enumerados permiten clarificar el comportamiento de las cemesferas y compararlo con las distintas adiciones de uso convencional. Los resultados reflejan un buen comportamiento resistente y durable de los materiales con adición de cemesferas. La caracterización microscópica refleja su relación con las propiedades mesoscópicas y permite comprender mejor la evolución en los procesos de hidratación de las cemesferas. In recent years industrialised societies have become increasingly aware of the problem posed by indiscriminate emission of greenhouse gases into the atmosphere. Concrete, with a main component being cement, is arguably the most widely used construction material. At present, global emissions of CO2 due to the combustion of CaCO3 from Portland cement represent between 5% and 10% of the total. If the requirement of the Kyoto Protocol of a reduction of 5% of the total gas produced before 2020 is considered, then such values are of significant interest. The main objective of this work is the assessment of the microstructure and the hydration processes of cements with additions. Such an examination proposes research into new cementitious products based on vitreous spherical microparticles that may be added to the cement before the mixing process. The results are compared with the most commonly used conventional additions. The new material bases its composition on the chemistry of aluminium and silicates. By decreasing the amount of CaCO3, it is possible both to contribute to sustainable development and reduce CO2 emissions. The patent created by Grupo Cementos Portland Valderrivas (GCPV) describes the production process of microspheres (WO 2009/007470, 2010). The products that form the raw material for manufacture are clays, lime-stone, marl and industrial products or by-products that melt after being ground and fed into a gaseous fluid at high temperatures (1250°C and 1600°C). This process allows the obtaining of a product with a solid-spherical or micro-spherical shape and which, after being stabilised in a solid state by rapid cooling, obtains a high vitrification suitable for chemical reactivity, having a minimal surface in relation to its mass. Given that the final product has the fineness required, it prevents grinding that reduces CO2 emissions by saving fuel during this process. The process, which allows a wide range of cementitious materials to be obtained, not only addresses the problems caused by CO2 emissions but also enhances the availability of materials in countries that until the time of writing have not produced cement due to a lack of limestone. In addition, the calculation of the degree of hydration from the test results of DTA-TG is optimised and based on Bhatty and Pane calculation models. The proposed method allows prediction of the performance of the material from numerical interpolation of the amount of chemically bound water. The degree of hydration has a direct relationship with the development of material mechanical strength. In order to characterise the cement-based materials, an extensive experimental campaign in cement paste, concrete and mortar is conducted. The research comprises three levels: micro-structural characterisation, macro-structural and long-term behaviour (mainly durability). In total, eight additions are assessed: four conventional additions and four types of microspheres with different chemical compositions. The micro-scale tests include characterisation of chemical composition, particle size distribution and the BET specific surface area of anhydrous material, differential thermal and thermogravimetric analysis in cement paste and mortar, silicon-29 nuclear magnetic resonance in cement paste, X-ray diffraction of the anhydrous materials and paste specimens, scanning of electron microscopy with energy dispersive X-ray analyser in cement paste and mortar, and mercury intrusion porosimetry in mortar. The macroscopic material characterisation entails determination of water demand for normal consistency, and initial and final setting times of cement paste, flexural and compressive mechanical strength tests in prismatic mortar specimens, and compressive strength tests in concrete specimens. Tests for determining the chloride migration coefficient are performed to characterise durability, together with electrical resistivity tests in mortar specimens. All the tests listed allow clarification of the behaviour of the microspheres and comparison with the various additions of conventional use. The results show good resistance and durable behaviour of materials with a microsphere addition. Microscopic characterisation reflects their relationship with mesoscopic properties and provides insights into the hydration processes of the microspheres.

Influence of proportion and particle size gradation of rubber from end-of-life tires on mechanical, thermal and acoustic properties of plaster-rubber mortars

This work presents the main experimental results obtained from the study of plaster test pieces and boards with addition of various volumetric rubber fractions from mechanical grinding of end-of-life tires (ELTs), in three different particle size gradations. It includes a description of the materials employed, and their proportions. The physical and mechanical properties, as well as the thermal conductivity and acoustic insulation properties are analyzed. Experimental results obtained for specimens with addition of recycled rubber are compared with similar ones, carried out on specimens of plaster of identical features without any addition, evaluating the influence of the particle size and mixture proportions. An improvement in thermal and acoustic performance has been obtained as well as a reduction in density, and as a result, some constructive applications for paving and slabs in rehabilitation works are proposed.

Thermodynamic aproach to particle size distribution in granular media

The study of particulate systems is of great interest in many fields of science and technology. Soil, sediments, powders, granular materials, colloidal and particulate suspensions are examples of systems involving many size particles. For those systems, the statistical description of the particle size distribution (PSD), that is, the mathematical distribution that defines the relative amounts of particles present, sorted according to size, is a crutial issue. The PSD can be important in understanding soil hydraulic properties, the geological origin or sediments or the physical and chemical properties of granular materials and ceramics, among others.

Microstructural and mechanical characterization of tungsten based materials for fusion reactors

El wolframio (W) y sus aleaciones se consideran los mejores candidatos para la construcción del divertor en la nueva generación de reactores de fusión nuclear. Este componente va a recibir las cargas térmicas más elevadas durante el funcionamiento del reactor ya que estará en contacto directo con el plasma. En los últimos años, después de un profundo análisis y siguiendo una estrategia de reducción de costes, la Organización de ITER tomó la decisión de construir el divertor integramente de wolframio desde el principio. Por ello, el wolframio no sólo actuará como material en contacto con el plasma (PFM), sino que también tendría aplicaciones estructurales. El wolframio, debido a sus excelentes propiedades termo-físicas, cumple todos los requerimientos para ser utilizado como PFM, sin embargo, su inherente fragilidad pone en peligro su uso estructural. Por tanto, uno de los principales objetivos de esta tesis es encontrar una aleación de wolframio con menor fragilidad. Durante éste trabajo, se realizó la caracterización microstructural y mecánica de diferentes materiales basados en wolframio. Sin embargo, ésta tarea es un reto debido a la pequeña cantidad de material suministrado, su reducido tamaño de grano y fragilidad. Por ello, para una correcta medida de todas las propiedades físicas y mecánicas se utilizaron diversas técnicas experimentales. Algunas de ellas se emplean habitualmente como la nanoindentación o los ensayos de flexión en tres puntos (TPB). Sin embargo, otras fueron especificamente desarrolladas e implementadas durante el desarrollo de esta tesis como es el caso de la medida real de la tenacidad de fractura en los materiales masivos, o de las medidas in situ de la tenacidad de fractura en las láminas delgadas de wolframio. Diversas composiciones de aleaciones de wolframio masivas (W-1% Y2O3, W-2% V-0.5% Y2O3, W-4% V-0.5% Y2O3, W-2% Ti-1% La2O3 y W-4% Ti-1% La2O3) se han estudiado y comparado con un wolframio puro producido en las mismas condiciones. Estas aleaciones, producidas por ruta pulvimetalúrgica de aleado mecánico (MA) y compactación isostática en caliente (HIP), fueron microstructural y mecánicamente caracterizadas desde 77 hasta 1473 K en aire y en alto vacío. Entre otras propiedades físicas y mecánicas se midieron la dureza, el módulo elástico, la resistencia a flexión y la tenacidad de fractura para todas las aleaciones. Finalmente se analizaron las superficies de fractura después de los ensayos de TPB para relacionar los micromecanismos de fallo con el comportamiento macroscópico a rotura. Los resultados obtenidos mostraron un comportamiento mecánico frágil en casi todo el intervalo de temperaturas y para casi todas las aleaciones sin mejoría de la temperatura de transición dúctil-frágil (DBTT). Con el fin de encontrar un material base wolframio con una DBTT más baja se realizó también un estudio, aún preliminar, de láminas delgadas de wolframio puro y wolframio dopado con 0.005wt.% potasio (K). Éstas láminas fueron fabricadas industrialmente mediante sinterizado y laminación en caliente y en frío y se sometieron posteriormente a un tratamiento térmico de recocido desde 1073 hasta 2673 K. Se ha analizado la evolución de su microestructura y las propiedades mecánicas al aumentar la temperatura de recocido. Los resultados mostraron la estabilización de los granos de wolframio con el incremento de la temperatura de recocido en las láminas delgadas de wolframio dopado con potasio. Sin embargo, es necesario realizar estudios adicionales para entender mejor la microstructura y algunas propiedades mecánicas de estos materiales, como la tenacidad de fractura. Tungsten (W) and tungsten-based alloys are considered to be the best candidate materials for fabricating the divertor in the next-generation nuclear fusion reactors. This component will experience the highest thermal loads during the operation of a reactor since it directly faces the plasma. In recent years, after thorough analysis that followed a strategy of cost reduction, the ITER Organization decided to built a full-tunsgten divertor before the first nuclear campaigns. Therefore, tungsten will be used not only as a plasma-facing material (PFM) but also in structural applications. Tungsten, due to its the excellent thermo-physical properties fulfils the requirements of a PFM, however, its use in structural applications is compromised due to its inherent brittleness. One of the objectives of this phD thesis is therefore, to find a material with improved brittleness behaviour. The microstructural and mechanical characterisation of different tunsgten-based materials was performed. However, this is a challenging task because of the reduced laboratory-scale size of the specimens provided, their _ne microstructure and their brittleness. Consequently, many techniques are required to ensure an accurate measurement of all the mechanical and physical properties. Some of the applied methods have been widely used such as nanoindentation or three-point bending (TPB) tests. However, other methods were specifically developed and implemented during this work such as the measurement of the real fracture toughness of bulk-tunsgten alloys or the in situ fracture toughness measurements of very thin tungsten foils. Bulk-tunsgten materials with different compositions (W-1% Y2O3, W-2% V- 0.5% Y2O3, W-4% V-0.5% Y2O3, W-2% Ti-1% La2O3 and W-4% Ti-1% La2O3) were studied and compared with pure tungsten processed under the same conditions. These alloys, produced by a powder metallurgical route of mechanical alloying (MA) and hot isostatic pressing (HIP), were microstructural and mechanically characterised from 77 to 1473 K in air and under high vacuum conditions. Hardness, elastic modulus, flexural strength and fracture toughness for all of the alloys were measured in addition to other physical and mechanical properties. Finally, the fracture surfaces after the TPB tests were analysed to correlate the micromechanisms of failure with the macroscopic behaviour. The results reveal brittle mechanical behaviour in almost the entire temperature range for the alloys and micromechanisms of failure with no improvement in the ductile-brittle transition temperature (DBTT). To continue the search of a tungsten material with lowered DBTT, a preliminary study of pure tunsgten and 0.005 wt.% potassium (K)-doped tungsten foils was also performed. These foils were industrially produced by sintering and hot and cold rolling. After that, they were annealed from 1073 to 2673 K to analyse the evolution of the microstructural and mechanical properties with increasing annealing temperature. The results revealed the stabilisation of the tungsten grains with increasing annealing temperature in the potassium-doped tungsten foil. However, additional studies need to be performed to gain a better understanding of the microstructure and mechanical properties of these materials such as fracture toughness.