Experimental analysis and FE Modeling of Aluminium alloys Microstructural Evolution in Extrusion

Extrusion is a process used to form long products of constant cross section, from simple billets, with a high variety of shapes. Aluminum alloys are the materials most processed in the extrusion industry due to their deformability and the wide field of applications that range from buildings to aerospace and from design to automotive industries. The diverse applications imply different requirements that can be fulfilled by the wide range of alloys and treatments, that is from critical structural application to high quality surface and aesthetical aspect. Whether one or the other is the critical aspect, they both depend directly from microstructure. The extrusion process is moreover marked by high deformations and complex strain gradients making difficult the control of microstructure evolution that is at present not yet fully achieved. Nevertheless the evolution of Finite Element modeling has reached a maturity and can therefore start to be used as a tool for investigation and prediction of microstructure evolution. This thesis will analyze and model the evolution of microstructure throughout the entire extrusion process for 6XXX series aluminum alloys. Core phase of the work was the development of specific tests to investigate the microstructure evolution and validate the model implemented in a commercial FE code. Along with it two essential activities were carried out for a correct calibration of the model beyond the simple research of contour parameters, thus leading to the understanding and control of both code and process. In this direction activities were also conducted on building critical knowhow on the interpretation of microstructure and extrusion phenomena. It is believed, in fact, that the sole analysis of the microstructure evolution regardless of its relevance in the technological aspects of the process would be of little use for the industry as well as ineffective for the interpretation of the results.

3D-electron microscopy of different conformations of cephalopod hemocyanins

Die Hämocyanine der Cephalopoden Nautilus pompilius und Sepia officinalis sorgen für den Sauerstofftransport zwischen den Kiemen und den Geweben. Sie bestehen aus einem zylindrischen Dekamer mit interner Kragenstruktur. Während eine Untereinheit (also eine Polypeptidkette) bei NpH aus sieben paralogen funktionellen Domänen (FU-a bis FU-g) besteht, führte ein Genduplikationsereignis der FU-d zu acht FUs in SoH (a, b, c, d, d´, e, f, g). In allen Mollusken Hämocyaninen bilden sechs dieser FUs den äußeren Ring und die restlichen die interne Kragenstruktur. rnrnIn dieser Arbeit wurde ein dreidimensionales Modell des Hämocyanins von Sepia officinalis (SoH) erstellt. Die Rekonstruktion, mit einer Auflösung von 8,8Å (FSC=0,5), erlaubt das Einpassen von Homolologiemodellen und somit das Erstellen eines molekularen Modells mit pseudo atomarer Auflösung. Des Weiteren wurden zwei Rekonstruktionen des Hämocyanins von Nautilus pompilius (NpH) in verschiedenen Oxygenierungszuständen erstellt. Die auf 10 und 8,1Å aufgelösten Modelle zeigen zwei verschiedene Konformationen des Proteins. Daraus ließ sich eine Modellvorstellung über die allosterische Funktionsweise ableiten. Die hier erreichte Auflösung von 8Å ist die momentan höchste eines Molluskenhämocyanins. rnAuf Grundlage des molekularen Modells von SoH konnte die Topologie des Proteins aufgeklärt werden. Es wurde gezeigt, dass die zusätzliche FU-d´ in den Kragen integriert ist und somit die prinzipielle Wandarchitektur aller Mollusken Hämocyanine identisch ist. Wie die Analyse des erstellten molekularen Modells zeigt werden sind die beiden Isoformen (SoH1 und SoH2) in den Bereichen der Interfaces nahezu identisch; auch der Vergleich mit NpH zeigt grosse Übereinstimmungen. Des weiteren konnte eine Fülle von Informationen bezüglich der allosterischen Signalübertragung innerhalb des Moleküls gewonnen werden. rnDer Versuch, NpH in verschiedenen Oxygenierungszuständen zu zeigen, war erfolgreich. Die Datensätze, die unter zwei atmosphärischen Bedingungen präpariert wurden, führten reproduzierbar zu zwei unterschiedlichen Rekonstruktionen. Dies zeigt, daß der hier entwickelte experimentelle Ansatz funktioniert. Er kann nun routinemäßig auf andere Proteine angewandt werden. Wie der strukturelle Vergleich zeigte, verändert sich die Orientierung der FUs durch die Oxygenierung leicht. Dies wiederum beeinflusst die Anordnung innerhalb der Interfaces sowie die Abstände zwischen den beteiligten Aminosäuren. Aus dieser Analyse konnte eine Modellvorstellung zum allosterischen Signaltransfer innerhalb des Moleküls abgeleitet werden, die auf einer Umordnung von Salzbrücken basiert.

Numerical modeling of the Alto Tiberina low angle normal fault

The aim of this Thesis is to obtain a better understanding of the mechanical behavior of the active Alto Tiberina normal fault (ATF). Integrating geological, geodetic and seismological data, we perform 2D and 3D quasi-static and dynamic mechanical models to simulate the interseismic phase and rupture dynamic of the ATF. Effects of ATF locking depth, synthetic and antithetic fault activity, lithology and realistic fault geometries are taken in account. The 2D and 3D quasi-static model results suggest that the deformation pattern inferred by GPS data is consistent with a very compliant ATF zone (from 5 to 15 km) and Gubbio fault activity. The presence of the ATF compliant zone is a first order condition to redistribute the stress in the Umbria-Marche region; the stress bipartition between hanging wall (high values) and footwall (low values) inferred by the ATF zone activity could explain the microseismicity rates that are higher in the hanging wall respect to the footwall. The interseismic stress build-up is mainly located along the Gubbio fault zone and near ATF patches with higher dip (30°3D rupture dynamic models demonstrate that the magnitude expected, after that an event is simulated on the ATF, can decrease if we consider the fault plane roughness.

Finite element modeling of the cement-bone interface beneath the tibial tray of a total knee arthroplasty: Study of the stress shielding effect (modellazione agli elementi finiti dell'interfaccia cemento-osso sotto il piatto tibiale di protesi totale di ginocchio: Analisi dello stress shielding)

The goal of this thesis was the study of the cement-bone interface in the tibial component of a cemented total knee prosthesis. One of the things you can see in specimens after in vivo service is that resorption of bone occurs in the interdigitated region between bone and cement. A stress shielding effect was investigated as a cause to explain bone resorption. Stress shielding occurs when bone is loaded less than physiological and therefore it starts remodeling according to the new loading conditions. µCT images were used to obtain 3D models of the bone and cement structure and a Finite Element Analysis was used to simulate different kind of loads. Resorption was also simulated by performing erosion operations in the interdigitated bone region. Finally, 4 models were simulated: bone (trabecular), bone with cement, and two models of bone with cement after progressive erosions of the bone. Compression, tension and shear test were simulated for each model in displacement-control until 2% of strain. The results show how the principal strain and Von Mises stress decrease after adding the cement on the structure and after the erosion operations. These results show that a stress shielding effect does occur and rises after resorption starts.

Imaging and modeling of pore scale processes in porous media using X-ray computed tomography and lattice Boltzmann solver

In der Erdöl– und Gasindustrie sind bildgebende Verfahren und Simulationen auf der Porenskala im Begriff Routineanwendungen zu werden. Ihr weiteres Potential lässt sich im Umweltbereich anwenden, wie z.B. für den Transport und Verbleib von Schadstoffen im Untergrund, die Speicherung von Kohlendioxid und dem natürlichen Abbau von Schadstoffen in Böden. Mit der Röntgen-Computertomografie (XCT) steht ein zerstörungsfreies 3D bildgebendes Verfahren zur Verfügung, das auch häufig für die Untersuchung der internen Struktur geologischer Proben herangezogen wird. Das erste Ziel dieser Dissertation war die Implementierung einer Bildverarbeitungstechnik, die die Strahlenaufhärtung der Röntgen-Computertomografie beseitigt und den Segmentierungsprozess dessen Daten vereinfacht. Das zweite Ziel dieser Arbeit untersuchte die kombinierten Effekte von Porenraumcharakteristika, Porentortuosität, sowie die Strömungssimulation und Transportmodellierung in Porenräumen mit der Gitter-Boltzmann-Methode. In einer zylindrischen geologischen Probe war die Position jeder Phase auf Grundlage der Beobachtung durch das Vorhandensein der Strahlenaufhärtung in den rekonstruierten Bildern, das eine radiale Funktion vom Probenrand zum Zentrum darstellt, extrahierbar und die unterschiedlichen Phasen ließen sich automatisch segmentieren. Weiterhin wurden Strahlungsaufhärtungeffekte von beliebig geformten Objekten durch einen Oberflächenanpassungsalgorithmus korrigiert. Die Methode der „least square support vector machine” (LSSVM) ist durch einen modularen Aufbau charakterisiert und ist sehr gut für die Erkennung und Klassifizierung von Mustern geeignet. Aus diesem Grund wurde die Methode der LSSVM als pixelbasierte Klassifikationsmethode implementiert. Dieser Algorithmus ist in der Lage komplexe geologische Proben korrekt zu klassifizieren, benötigt für den Fall aber längere Rechenzeiten, so dass mehrdimensionale Trainingsdatensätze verwendet werden müssen. Die Dynamik von den unmischbaren Phasen Luft und Wasser wird durch eine Kombination von Porenmorphologie und Gitter Boltzmann Methode für Drainage und Imbibition Prozessen in 3D Datensätzen von Böden, die durch synchrotron-basierte XCT gewonnen wurden, untersucht. Obwohl die Porenmorphologie eine einfache Methode ist Kugeln in den verfügbaren Porenraum einzupassen, kann sie dennoch die komplexe kapillare Hysterese als eine Funktion der Wassersättigung erklären. Eine Hysterese ist für den Kapillardruck und die hydraulische Leitfähigkeit beobachtet worden, welche durch die hauptsächlich verbundenen Porennetzwerke und der verfügbaren Porenraumgrößenverteilung verursacht sind. Die hydraulische Konduktivität ist eine Funktion des Wassersättigungslevels und wird mit einer makroskopischen Berechnung empirischer Modelle verglichen. Die Daten stimmen vor allem für hohe Wassersättigungen gut überein. Um die Gegenwart von Krankheitserregern im Grundwasser und Abwässern vorhersagen zu können, wurde in einem Bodenaggregat der Einfluss von Korngröße, Porengeometrie und Fluidflussgeschwindigkeit z.B. mit dem Mikroorganismus Escherichia coli studiert. Die asymmetrischen und langschweifigen Durchbruchskurven, besonders bei höheren Wassersättigungen, wurden durch dispersiven Transport aufgrund des verbundenen Porennetzwerks und durch die Heterogenität des Strömungsfeldes verursacht. Es wurde beobachtet, dass die biokolloidale Verweilzeit eine Funktion des Druckgradienten als auch der Kolloidgröße ist. Unsere Modellierungsergebnisse stimmen sehr gut mit den bereits veröffentlichten Daten überein.

Finite element modeling of flow/compression-induced deformation of alginate scaffolds for bone tissue engineering

Trauma or degenerative diseases such as osteonecrosis may determine bone loss whose recover is promised by a "tissue engineering“ approach. This strategy involves the use of stem cells, grown onboard of adequate biocompatible/bioreabsorbable hosting templates (usually defined as scaffolds) and cultured in specific dynamic environments afforded by differentiation-inducing actuators (usually defined as bioreactors) to produce implantable tissue constructs. The purpose of this thesis is to evaluate, by finite element modeling of flow/compression-induced deformation, alginate scaffolds intended for bone tissue engineering. This work was conducted at the Biomechanics Laboratory of the Institute of Biomedical and Neural Engineering of the Reykjavik University of Iceland. In this respect, Comsol Multiphysics 5.1 simulations were carried out to approximate the loads over alginate 3D matrices under perfusion, compression and perfusion+compression, when varyingalginate pore size and flow/compression regimen. The results of the simulations show that the shear forces in the matrix of the scaffold increase coherently with the increase in flow and load, and decrease with the increase of the pore size. Flow and load rates suggested for proper osteogenic cell differentiation are reported.

Statistical modeling of the eye for multimodal treatment planning for external beam radiation therapy of intraocular tumors

Ocular anatomy and radiation-associated toxicities provide unique challenges for external beam radiation therapy. For treatment planning, precise modeling of organs at risk and tumor volume are crucial. Development of a precise eye model and automatic adaptation of this model to patients' anatomy remain problematic because of organ shape variability. This work introduces the application of a 3-dimensional (3D) statistical shape model as a novel method for precise eye modeling for external beam radiation therapy of intraocular tumors.

Clinical and biochemical description of a novel CYP21A2 gene mutation 962_963insA using a new 3D model for the P450c21 protein

OBJECTIVE: A severely virilized 46, XX newborn girl was referred to our center for evaluation and treatment of congenital adrenal hyperplasia (CAH) because of highly elevated 17alpha-hydroxyprogesterone levels at newborn screening; biochemical tests confirmed the diagnosis of salt-wasting CAH. Genetic analysis revealed that the girl was compound heterozygote for a previously reported Q318X mutation in exon 8 and a novel insertion of an adenine between nucleotides 962 and 963 in exon 4 of the CYP21A2 gene. This 962_963insA mutation created a frameshift leading to a stop codon at amino acid 161 of the P450c21 protein. AIM AND METHODS: To better understand structure-function relationships of mutant P450c21 proteins, we performed multiple sequence alignments of P450c21 with three mammalian P450s (P450 2C8, 2C9 and 2B4) with known structures as well as with human P450c17. Comparative molecular modeling of human P450c21 was then performed by MODELLER using the X-ray crystal structure of rabbit P450 2B4 as a template. RESULTS: The new three dimensional model of human P450c21 and the sequence alignment were found to be helpful in predicting the role of various amino acids in P450c21, especially those involved in heme binding and interaction with P450 oxidoreductase, the obligate electron donor. CONCLUSION: Our model will help in analyzing the genotype-phenotype relationship of P450c21 mutations which have not been tested for their functional activity in an in vitro assay.

Correspondence establishment in statistical modeling of shapes with arbitrary topology

Correspondence establishment is a key step in statistical shape model building. There are several automated methods for solving this problem in 3D, but they usually can only handle objects with simple topology, like that of a sphere or a disc. We propose an extension to correspondence establishment over a population based on the optimization of the minimal description length function, allowing considering objects with arbitrary topology. Instead of using a fixed structure of kernel placement on a sphere for the systematic manipulation of point landmark positions, we rely on an adaptive, hierarchical organization of surface patches. This hierarchy can be built on surfaces of arbitrary topology and the resulting patches are used as a basis for a consistent, multi-scale modification of the surfaces' parameterization, based on point distribution models. The feasibility of the approach is demonstrated on synthetic models with different topologies.

Resolving diffusion in clay minerals at different time scales: Combination of experimental and modeling approaches

Since no single experimental or modeling technique provides data that allow a description of transport processes in clays and clay minerals at all relevant scales, several complementary approaches have to be combined to understand and explain the interplay between transport relevant phenomena. In this paper molecular dynamics simulations (MD) were used to investigate the mobility of water in the interlayer of montmorillonite (Mt), and to estimate the influence of mineral surfaces and interlayer ions on the water diffusion. Random Walk (RW) simulations based on a simplified representation of pore space in Mt were used to estimate and understand the effect of the arrangement of Mt particles on the meso- to macroscopic diffusivity of water. These theoretical calculations were complemented with quasielastic neutron scattering (QENS) measurements of aqueous diffusion in Mt with two pseudo-layers of water performed at four significantly different energy resolutions (i.e. observation times). The size of the interlayer and the size of Mt particles are two characteristic dimensions which determine the time dependent behavior of water diffusion in Mt. MD simulations show that at very short time scales water dynamics has the characteristic features of an oscillatory motion in the cage formed by neighbors in the first coordination shell. At longer time scales, the interaction of water with the surface determines the water dynamics, and the effect of confinement on the overall water mobility within the interlayer becomes evident. At time scales corresponding to an average water displacement equivalent to the average size of Mt particles, the effects of tortuosity are observed in the meso- to macroscopic pore scale simulations. Consistent with the picture obtained in the simulations, the QENS data can be described using a (local) 3D diffusion at short observation times, whereas at sufficiently long observation times a 2D diffusive motion is clearly observed. The effects of tortuosity measured in macroscopic tracer diffusion experiments are in qualitative agreement with RW simulations. By using experimental data to calibrate molecular and mesoscopic theoretical models, a consistent description of water mobility in clay minerals from the molecular to the macroscopic scale can be achieved. In turn, simulations help in choosing optimal conditions for the experimental measurements and the data interpretation. (C) 2014 Elsevier B.V. All rights reserved.

Automatic multi-resolution shape modeling of multi-organ structures.

Point Distribution Models (PDM) are among the most popular shape description techniques and their usefulness has been demonstrated in a wide variety of medical imaging applications. However, to adequately characterize the underlying modeled population it is essential to have a representative number of training samples, which is not always possible. This problem is especially relevant as the complexity of the modeled structure increases, being the modeling of ensembles of multiple 3D organs one of the most challenging cases. In this paper, we introduce a new GEneralized Multi-resolution PDM (GEM-PDM) in the context of multi-organ analysis able to efficiently characterize the different inter-object relations, as well as the particular locality of each object separately. Importantly, unlike previous approaches, the configuration of the algorithm is automated thanks to a new agglomerative landmark clustering method proposed here, which equally allows us to identify smaller anatomically significant regions within organs. The significant advantage of the GEM-PDM method over two previous approaches (PDM and hierarchical PDM) in terms of shape modeling accuracy and robustness to noise, has been successfully verified for two different databases of sets of multiple organs: six subcortical brain structures, and seven abdominal organs. Finally, we propose the integration of the new shape modeling framework into an active shape-model-based segmentation algorithm. The resulting algorithm, named GEMA, provides a better overall performance than the two classical approaches tested, ASM, and hierarchical ASM, when applied to the segmentation of 3D brain MRI.

Modeling Techniques for the High-Resolution Interpretation of Cryo-Electron Microscopy Reconstructions

Essential biological processes are governed by organized, dynamic interactions between multiple biomolecular systems. Complexes are thus formed to enable the biological function and get dissembled as the process is completed. Examples of such processes include the translation of the messenger RNA into protein by the ribosome, the folding of proteins by chaperonins or the entry of viruses in host cells. Understanding these fundamental processes by characterizing the molecular mechanisms that enable then, would allow the (better) design of therapies and drugs. Such molecular mechanisms may be revealed trough the structural elucidation of the biomolecular assemblies at the core of these processes. Various experimental techniques may be applied to investigate the molecular architecture of biomolecular assemblies. High-resolution techniques, such as X-ray crystallography, may solve the atomic structure of the system, but are typically constrained to biomolecules of reduced flexibility and dimensions. In particular, X-ray crystallography requires the sample to form a three dimensional (3D) crystal lattice which is technically di‑cult, if not impossible, to obtain, especially for large, dynamic systems. Often these techniques solve the structure of the different constituent components within the assembly, but encounter difficulties when investigating the entire system. On the other hand, imaging techniques, such as cryo-electron microscopy (cryo-EM), are able to depict large systems in near-native environment, without requiring the formation of crystals. The structures solved by cryo-EM cover a wide range of resolutions, from very low level of detail where only the overall shape of the system is visible, to high-resolution that approach, but not yet reach, atomic level of detail. In this dissertation, several modeling methods are introduced to either integrate cryo-EM datasets with structural data from X-ray crystallography, or to directly interpret the cryo-EM reconstruction. Such computational techniques were developed with the goal of creating an atomic model for the cryo-EM data. The low-resolution reconstructions lack the level of detail to permit a direct atomic interpretation, i.e. one cannot reliably locate the atoms or amino-acid residues within the structure obtained by cryo-EM. Thereby one needs to consider additional information, for example, structural data from other sources such as X-ray crystallography, in order to enable such a high-resolution interpretation. Modeling techniques are thus developed to integrate the structural data from the different biophysical sources, examples including the work described in the manuscript I and II of this dissertation. At intermediate and high-resolution, cryo-EM reconstructions depict consistent 3D folds such as tubular features which in general correspond to alpha-helices. Such features can be annotated and later on used to build the atomic model of the system, see manuscript III as alternative. Three manuscripts are presented as part of the PhD dissertation, each introducing a computational technique that facilitates the interpretation of cryo-EM reconstructions. The first manuscript is an application paper that describes a heuristics to generate the atomic model for the protein envelope of the Rift Valley fever virus. The second manuscript introduces the evolutionary tabu search strategies to enable the integration of multiple component atomic structures with the cryo-EM map of their assembly. Finally, the third manuscript develops further the latter technique and apply it to annotate consistent 3D patterns in intermediate-resolution cryo-EM reconstructions. The first manuscript, titled An assembly model for Rift Valley fever virus, was submitted for publication in the Journal of Molecular Biology. The cryo-EM structure of the Rift Valley fever virus was previously solved at 27Å-resolution by Dr. Freiberg and collaborators. Such reconstruction shows the overall shape of the virus envelope, yet the reduced level of detail prevents the direct atomic interpretation. High-resolution structures are not yet available for the entire virus nor for the two different component glycoproteins that form its envelope. However, homology models may be generated for these glycoproteins based on similar structures that are available at atomic resolutions. The manuscript presents the steps required to identify an atomic model of the entire virus envelope, based on the low-resolution cryo-EM map of the envelope and the homology models of the two glycoproteins. Starting with the results of the exhaustive search to place the two glycoproteins, the model is built iterative by running multiple multi-body refinements to hierarchically generate models for the different regions of the envelope. The generated atomic model is supported by prior knowledge regarding virus biology and contains valuable information about the molecular architecture of the system. It provides the basis for further investigations seeking to reveal different processes in which the virus is involved such as assembly or fusion. The second manuscript was recently published in the of Journal of Structural Biology (doi:10.1016/j.jsb.2009.12.028) under the title Evolutionary tabu search strategies for the simultaneous registration of multiple atomic structures in cryo-EM reconstructions. This manuscript introduces the evolutionary tabu search strategies applied to enable a multi-body registration. This technique is a hybrid approach that combines a genetic algorithm with a tabu search strategy to promote the proper exploration of the high-dimensional search space. Similar to the Rift Valley fever virus, it is common that the structure of a large multi-component assembly is available at low-resolution from cryo-EM, while high-resolution structures are solved for the different components but lack for the entire system. Evolutionary tabu search strategies enable the building of an atomic model for the entire system by considering simultaneously the different components. Such registration indirectly introduces spatial constrains as all components need to be placed within the assembly, enabling the proper docked in the low-resolution map of the entire assembly. Along with the method description, the manuscript covers the validation, presenting the benefit of the technique in both synthetic and experimental test cases. Such approach successfully docked multiple components up to resolutions of 40Å. The third manuscript is entitled Evolutionary Bidirectional Expansion for the Annotation of Alpha Helices in Electron Cryo-Microscopy Reconstructions and was submitted for publication in the Journal of Structural Biology. The modeling approach described in this manuscript applies the evolutionary tabu search strategies in combination with the bidirectional expansion to annotate secondary structure elements in intermediate resolution cryo-EM reconstructions. In particular, secondary structure elements such as alpha helices show consistent patterns in cryo-EM data, and are visible as rod-like patterns of high density. The evolutionary tabu search strategy is applied to identify the placement of the different alpha helices, while the bidirectional expansion characterizes their length and curvature. The manuscript presents the validation of the approach at resolutions ranging between 6 and 14Å, a level of detail where alpha helices are visible. Up to resolution of 12 Å, the method measures sensitivities between 70-100% as estimated in experimental test cases, i.e. 70-100% of the alpha-helices were correctly predicted in an automatic manner in the experimental data. The three manuscripts presented in this PhD dissertation cover different computation methods for the integration and interpretation of cryo-EM reconstructions. The methods were developed in the molecular modeling software Sculptor (http://sculptor.biomachina.org) and are available for the scientific community interested in the multi-resolution modeling of cryo-EM data. The work spans a wide range of resolution covering multi-body refinement and registration at low-resolution along with annotation of consistent patterns at high-resolution. Such methods are essential for the modeling of cryo-EM data, and may be applied in other fields where similar spatial problems are encountered, such as medical imaging.

A graph-based approach for latency modeling and optimization in multiview video encoding

We present a novel framework for encoding latency analysis of arbitrary multiview video coding prediction structures. This framework avoids the need to consider an specific encoder architecture for encoding latency analysis by assuming an unlimited processing capacity on the multiview encoder. Under this assumption, only the influence of the prediction structure and the processing times have to be considered, and the encoding latency is solved systematically by means of a graph model. The results obtained with this model are valid for a multiview encoder with sufficient processing capacity and serve as a lower bound otherwise. Furthermore, with the objective of low latency encoder design with low penalty on rate-distortion performance, the graph model allows us to identify the prediction relationships that add higher encoding latency to the encoder. Experimental results for JMVM prediction structures illustrate how low latency prediction structures with a low rate-distortion penalty can be derived in a systematic manner using the new model.

Advances in the modeling, characterization and reliability of concentrator multijunction solar cells

Los sistemas de concentración fotovoltaica (CPV) parecen ser una de las vías más prometedoras para generar electricidad a gran escala a precios competitivos. La investigación actual se centra en aumentar la eficiencia y la concentración de los sistemas para abaratar costes. Al mismo tiempo se investiga sobre la fiabilidad de los diferentes componentes que integran un sistema de concentración, ya que para que los sistemas de concentración sean competitivos es necesario que tengan una fiabilidad al menos similar a los sistemas basados en células de silicio. En la presente tesis doctoral se ha llevado a cabo el estudio de aspectos avanzados de células solares multi-unión diseñadas para trabajar a concentraciones ultra-altas. Para ello, se ha desarrollado un modelo circuital tridimensional distribuido con el que simular el comportamiento de las células solares triple-unión bajo distintas condiciones de funcionamiento, así mismo se ha realizado una caracterización avanzada de este tipo de células para comprender mejor su modo de operación y así poder contribuir a mejorar su eficiencia. Finalmente, se han llevado a cabo ensayos de vida acelerados en células multiunión comerciales para conocer la fiabilidad de este tipo de células solares. Para la simulación de células solares triple-unión se ha desarrollado en la presente tesis doctoral un modelo circuital tridimensinal distribuido el cuál integra una descripción completa de la unión túnel. De este modo, con el modelo desarrollado, hemos podido simular perfiles de luz sobre la célula solar que hacen que la densidad de corriente fotogenerada sea mayor a la densidad de corriente pico de la unión túnel. El modelo desarrollado también contempla la distribución lateral de corriente en las capas semiconductoras que componen y rodean la unión túnel. Por tanto, se ha podido simular y analizar el efecto que tiene sobre el funcionamiento de la célula solar que los concentradores ópticos produzcan perfiles de luz desuniformes, tanto en nivel de irradiancia como en el contenido espectral de la luz (aberración cromática). Con el objetivo de determinar cuáles son los mecanismos de recombinación que están limitando el funcionamiento de cada subcélula que integra una triple-unión, y así intentar reducirlos, se ha llevado a cabo la caracterización eléctrica de células solares monouni ón idénticas a las subcelulas de una triple-unión. También se ha determinado la curva corriente-tensión en oscuridad de las subcélulas de GaInP y GaAs de una célula dobleunión mediante la utilización de un teorema de reciprocidad electro-óptico. Finalmente, se ha analizado el impacto de los diferentes mecanismos de recombinación en el funcionamiento de la célula solar triple-unión en concentración. Por último, para determinar la fiabilidad de este tipo de células, se ha llevado a cabo un ensayo de vida acelerada en temperatura en células solares triple-unión comerciales. En la presente tesis doctoral se describe el diseño del ensayo, el progreso del mismo y los datos obtenidos tras el análisis de los resultados preliminares. Abstract Concentrator photovoltaic systems (CPV) seem to be one of the most promising ways to generate electricity at competitive prices. Nowadays, the research is focused on increasing the efficiency and the concentration of the systems in order to reduce costs. At the same time, another important area of research is the study of the reliability of the different components which make up a CPV system. In fact, in order for a CPV to be cost-effective, it should have a warranty at least similar to that of the systems based on Si solar cells. In the present thesis, we will study in depth the behavior of multijunction solar cells under ultra-high concentration. With this purpose in mind, a three-dimensional circuital distributed model which is able to simulate the behavior of triple-junction solar cells under different working conditions has been developed. Also, an advanced characterization of these solar cells has been carried out in order to better understand their behavior and thus contribute to improving efficiency. Finally, accelerated life tests have been carried out on commercial lattice-matched triple-junction solar cells in order to determine their reliability. In order to simulate triple-junction solar cells, a 3D circuital distributed model which integrates a full description of the tunnel junction has been developed. We have analyzed the behavior of the multijunction solar cell under light profiles which cause the current density photo-generated in the solar cell to be higher than the tunnel junction’s peak current density. The advanced model developed also takes into account the lateral current spreading through the semiconductor layers which constitute and surround the tunnel junction. Therefore, the effects of non-uniform light profiles, in both irradiance and the spectral content produced by the concentrators on the solar cell, have been simulated and analyzed. In order to determine which recombination mechanisms are limiting the behavior of each subcell in a triple-junction stack, and to try to reduce them when possible, an electrical characterization of single-junction solar cells that resemble the subcells in a triplejunction stack has been carried out. Also, the dark I-V curves of the GaInP and GaAs subcells in a dual-junction solar cell have been determined by using an electro-optical reciprocity theorem. Finally, the impact of the different recombination mechanisms on the behavior of the triple-junction solar cell under concentration has been analyzed. In order to determine the reliability of these solar cells, a temperature accelerated life test has been carried out on commercial triple-junction solar cells. In the present thesis, the design and the evolution of the test, as well as the data obtained from the analysis of the preliminary results, are presented.