Nell'aprile del 1773, Parigi fu scossa dal timore che una cometa provocasse un disastroso cataclisma. All'origine del clamore, il "Mémoire sur les comètes" dell'astronomo Lalande (Bourg-en-Bresse 1732-Paris, 1807), che aveva sostenuto la non impossibilità fisica e matematica di uno scontro/incontro tra Terra e comete.

This work explores the possibility to enhance the performance of photovoltaic devices through Wavelength Shifting and Luminescent Solar Concentration. New matrices for these applications, namely parylene, polysiloxane and fluorinated polyimide have been successfully tested.

Tissue engineering is an interdisciplinary field in which cell biology, biomaterials science, and surgery are combined and its main goal is to repair, replace and reproduce tissues and organs. Following this procedure, cells are seeded on proper scaffolds and induced in sequence to adhere, eventually differentiate, proliferate and finally to produce the wanted extracellular matrix (ECM). During cell culture, the usefulness of applying proper physiological-like stimuli, i.e., biochemical but also mechanical signals to drive and accelerate both cell differentiation and ECM production has been demonstrated. Tissue regeneration can be either conducted entirely in vivo or assisted by a previous in vitro phase. Considering the latter situation, a bioreactor can be defined as any apparatus that attempts to mimic physiological conditions in order to maintain and encourage tissue regeneration in dynamic conditions. Dynamic cell cultures using bioreactors can be considered a good intermediate step between the conventional in vitro static approach and in vivo studies. Therefore it is possible to promote the formation of the specific tissue by simulating physiological conditions via the application of specific mechanical and biochemical stimuli. The proposed work is focused on the design and development of bioreactors for bone and cartilage regeneration, in which optimal cell culture conditions are controlled (temperature, nutrients, carbon dioxide and oxygen levels), and mechanical stimuli are applied on the cell constructs. This study presents a wide investigation concerning these mechanical stimulations in order to understand the best cell culture parameters for the activations of cells, naturally accustomed to similar stresses inside the joint. In particular, direct compression, change in hydrodynamic pressure and perfusion modes are compared and analyzed.

The rapid proliferation of North-South preferential trade agreements (PTAs) during the last quarter century has had broad implications for developing and emerging economies. As a result of North-South power asymmetries and the aggressive trade policy that has been characterized as ‘competitive liberalization,’ it has been argued that these agreements have produced asymmetric results in favor of Northern countries. This thesis advances a novel approach in the assessment of North-South preferential trade relations that goes beyond the simplistic interpretation of North-South trade politics as a phenomenon largely dominated by North-South power asymmetries. By acknowledging that not all North-South PTAs have the same characteristics, this thesis divides North-South PTAs into two sequential categories: first-order, i.e., Southern countries’ first North-South PTAs and second-order, i.e., Southern countries’ subsequent North-South PTAs. The thesis argues that, while first-order North-South PTA negotiations can produce asymmetric outcomes in favor of Northern countries because they have the ability to exert discriminative pressure on Southern countries, second-order North-South PTA negotiations follow a different logic. Having secured preferential access to Northern markets through first-order PTAs, Southern countries become immune to competitive pressures and can themselves exert discriminative pressure on Northern countries during second-order negotiations. The thesis examines the North-South PTA negotiations of Mexico, Chile, Korea, Colombia, and Peru, five countries of the Global South that have been especially active in North-South preferential trade. Based on the author’s personal interviews with EU and US trade officials and primary and secondary sources, this thesis conducts process tracing to account for the process of the five Southern countries’ first-order and second-order North-South PTA negotiations and reveal the impact of first-order North-South PTAs on the bargaining powers of Southern countries in second-order negotiations and hence the outcomes of second-order agreements. The thesis concludes that, albeit to varying extents, first-order agreements improve the bargaining powers of Southern countries in second-order North-South trade negotiations.

This work presents the design, realization and characterization of high efficiency Silicon photovoltaic cells for concentration applications. In order to develop high efficient Si concentrator solar cells two different ways have been followed. The first one aims to optimize the design and the fabrication process of a conventional front-side contacted cell, based on a planar n-p junction. Although this cell structure is rather simple and cheap to produce, we show that a conversion effciency higher than 23% can be reached under concentrated light, if the cell design and the fabrication process are suitably optimized. The second way aims to investigate and propose completely new cell designs which use some 'three-dimensional' structures, such as deep-grooved contacts and through-silicon vertical connections. The new cell designs allow to overcome some intrinsic limits of the conventional front-side contacted cells and could be worthwhile to improve the conversion eciency in future real applications.

The application of powder metallurgy (PM) technologies to the manufacturing of Oil & Gas turbomachineries’ components was investigated in the course of research collaboration with the Material and Processes Engineering Department of General Electric Oil & Gas (Italy). The thesis focused on the study of the pressure-assisted Hot Isostatic Pressing technology for the processing of the corrosion resistant Ni-base alloy N07626. The densification behaviour of the N07626 metal powder in condition of pressure assisted sintering was investigated by experiments conducted on a small scale by uniaxial hot pressing condition using a Spark Plasma Sintering (SPS) machine in the aim of extending the result to the initial stage of densification of HIP. The SPS exepriments demonstrated that the densification rate is strongly affected by the process temperature and it is less sensitive to the variation of applied pressure. The microstructure and mechanical properties of full-dense HIPped N07626 alloy, produced according to a fixed proprietary cycle and several experimental deviations were analyzed. The microstructure was studied by Optical Metallography, Scanning Electron Microscopy, Energy Dispersed X-Ray Spectroscopy and Electron Backscatter Diffraction. The mechanical properties of the alloy were assessed by tensile testing, conventional and instrumented Charpy V-Notch testing, JIC fracture toughness tests and fatigue crack growth rate testing. The tensile and impact toughness properties resulted sensitive to the local accumulation of oxygen in Oxygen Affected Zones (OAZs), that leads to a ductile to brittle transition in the impact toughness of the material. Two models for formation of OAZs were proposed based on the phase transformation and the oxidation/reduction reactions taking place in the HIP. The mechanical properties were discussed on the base of the microstructure of the Prior Particle Boundaries (PPBs) interface, focusing of the phase transformation products, represented by a thin layer of submicrometric oxides and carbides. The fracture mode was explained by the analogy with models of ductile micro-mechanisms of void nucleation and coalescence and with fracture models of particulate reinforced metal-matrix-composite. The Charpy impact toughness and the fracture toughness were correlated to the oxygen concentration and to the density of inclusions. The fatigue crack propagation behavior was discussed focusing on the effect of clustering of inclusions on the crack propagation path. A relation between the Paris slope with the impact toughness was found. Finally the increase of processing temperature (HIP and heat treatment) was found significanty beneficial for the toughness. This effect was investigate by grain-size analysis and was proposed to be related to a reduction of density of PPBs inclusions.

Materials are the heart of engineering, which can be defined as the creative and rational use of materials for practical purposes. Materials have had an essential role in the development of civil engineering: from the beginning of human evolution, man has used many different materials to build houses, bridges, roads and countless other structures to make his life easier. Ancient populations used the raw materials at their disposal, such as stone, clay and timber. Over the centuries, the search for new materials became increasingly important to respond to changing human needs, and men learned how to use clay to form artificial stones, cements and concretes, for instance. While hands-on familiarity rooted in tradition and crafting expertise initially drove these human activities, in modern times the need for a scientific understanding of materials prompted the birth of the material science discipline. Material science studies the composition, behavior and properties of materials to solve problems associated with their use. Engineering and material science work together to create functional, durable and beautiful structures. Among the materials used in civil engineering constructions, wood and cement have had the most important role over the centuries and they are still the main components of our infrastructures. Timber was used as a building material even by primitive man, and a few ancient temples, palaces and bridges built of wood can still be seen today. In the 20th century, although materials such as concrete had become competitive, wood retained its significant role in building. The main problem with the use of wood as a construction material concerns its possibly limited lifetime. Wood is characterized by a limited resistance to moisture and fire, the two elements responsible for the destruction of most wooden buildings in the past. Cement, and consequently concrete, is the most often used material today. The term “cement” is now used to mean a modern binder, the so-called Portland cement, patented in England in 1824. Similar binding materials were already being used from very early on ancient Mesopotamia, Egypt, Greece and Rome. Modern cement formulations have changed a great deal and can be adapted to their intended use and the surrounding environment.

This work was carried out in the frame of an industrial research project in cooperation with the Eurocoating SpA and K4Sint Srl, aiming at developing the commercial pure titanium, the Ti-6Al-4V and the Co-28Cr-6Mo alloys by Spark Plasma Sintering (SPS) for biomedical application. The definition of the process parameters for the production of a highly porous (cp-Ti), full density materials (Ti-6Al-4V and Co-28Cr-6Mo), and their combination in a surface functionalized full density substrate was the central focus. The SPS parameters were optimized to obtain the Co-28Cr-6Mo alloy in full density state for matching the international standards. Tensile and fatigue were the main properties under investigation. In the case of Ti-6Al-4V alloy the best SPS parameters was defined in a previous work by means of densification curve and tensile properties. Therefore, the fatigue resistance was the main property under investigation. The optimization of the sintering parameters was evaluated by the interdependence between the density, microstructure and hardness. Co-sintering of the cp-Ti with the Co alloy and the cp-Ti with the Ti alloy was carried in order to obtain a porous coated full density substrate in one single step. The SPS parameters were optimized in order to achieve a coating like structure containing macropores with specific range of size and highly interconnected. To that, the space holder technique was chosen since it allows a very good control of the pores characteristics. The interactions at the interfaces were characterized and the best SPS strategy was defined. Subsequently, fatigue tests were carried out in order to assess the influence of the porous coating on the fatigue resistance of the full density substrates. As a general conclusion it may be assessed that the process parameters for the production of the investigated biomaterials have been defined and the microstructural characteristics, as well as mechanical, corrosion properties and wear resistance satisfy the requirements on the international standards. These results have been used to produce implants which are under test.

Political inclusion of migrants is at the heart of contested scholarly and political debate. The increasing diversity of European democracies and the exclusion of a large part of the resident population from the political process raise questions about social cohesion and the quality of democracy. In the absence of voting rights for migrant residents, associations are often considered as an alternative for voice, representing migrants and defending their interest in the political process. Yet, little is known about the conditions favoring the political inclusion of migrant associations. Studies on migrants associations tend to consider migrant organizations as discrete units acting independently from each other. Yet, migrant associations do not act in a vacuum and are embedded in webs of relations as well as in a wider political context. Social movement scholars implicitly acknowledge an interaction between institutional and relational context in affecting collective action, but studies analyzing this interaction are scarce. Considering only one of the two structures in which associations are embedded might lead to wrong conclusions as to what fosters political inclusion. Thus, the objective of this study is twofold. First it attempts to link two traditions in the social movement literature: the political opportunity theory and the relational approach. Second, it aims at furthering our knowledge on the mechanisms linking the embeddedness of migrants associations in an institutional and a relational context and their political inclusion. Empirically, this study analyzes network structures in the field of immigration and the political inclusion of migrant organizations in Lyon and in Zurich. To analyze network structures the study draws on a relational approach to social movements that provides useful tools for comparing networks across contexts by defining a typology of modes of coordination of collective action. Modes of coordination are the mechanisms through which resources are allocated within collectivities but also how collective representation are elaborated as well as collective identities. The specific opportunities in the field of immigration draw on the citizenship approaches of nation-states to define the opportunities in the field. Two dimensions of political opportunty structures are defined. An institutional dimension, which refers to the rights and duties offered to immigrants as well as the institutional channels available to them, but also resources to actors acting in the field. The cultural dimension refers to the notions of citizenship and national identity that act as discursive opportunities and provide public recognition of different identities in the field. The main argument of this study is that the specific institutional and discursive opportunities in the field of immigration affect the network structure of migrant organizations and that the political inclusion of migrant organizations is affected by the interaction of both structures. Drawing on the social movement literature, this study analyzes the effect of the specific opportunity structure in the field of immigration on network structure and the political inclusion of migrant organizations. The results show that migrant organizations adapt to the specific opportunities when creating alliances, and the networks that emerge vary between the cities under study. Indeed, associations tend to create alliances on the basis of their access to institutional channels, and the publicly recognized identities in the field. Moreover, it shows that the interaction of the opportunity structure and the network structure affects political inclusion of migrant associations. Indeed, the embeddedness of associations in similar relational structures has different outcomes for political inclusion according to the context. The study concludes by stating that analyzing the interaction of specific political opportunity structures and network structures is necessary to get a better understanding of the conditions favoring or hindering the political inclusion of migrant organizations.

Tribology is known as the science of surfaces in relative motion and involves complex interactions over multiple length and time scales. Therefore, friction, lubrication and wear of materials are intrinsically highly multiphysics and multiscale phenomena. Several modelling and simulation tools have been developed in the last decades, always requiring a trade-off between the available computational power and the accurate replication of the experimental results. Despite nowadays it is possible to model with extreme precision elastic problems at various scales, further eorts are needed for taking into account phenomena like plasticity, adhesion, wear, third-body friction and boundary and solid lubrication. The situation becomes even more challenging if considering non-conventional nano-, as in the case of polymer surfaces and interfaces, or microstructures, as for the hierarchical organisations observed in biological systems. Specically, biological surface structures have been demonstrated to present exceptional tribological properties, for instance in terms of adhesion (e.g., the gecko pad), superhydrophobicity (e.g., the lotus leaf) or fluid-dynamic drag reduction (e.g., the shark skin). This has suggested the study and development of hierarchical and/or bio-inspired structures for applications in tribology. Therefore, by taking inspiration from Nature, we investigate the effect of property gradients on the frictional behaviour of sliding interfaces, considering lateral variations in surface and bulk properties. 3D finite-element simulations are compared with a 2D spring-block model to show how lateral gradients can be used to tune the macroscopic coefficients of friction and control the propagation of detachment fronts. Complex microscale phenomena govern the macroscopic behaviour also of lubricated contacts. An example is represented by solid lubrication or third-body friction, which we study with 3D discreteelement simulations. We show the effects of surface waviness and of the modelling parameters on the macroscopic coefficient of friction. Many other natural systems present complex interfacial interactions and tribological behaviour. Plant roots, for instance, display optimised performance during the frictional penetration of soil, especially thanks to a particular apex morphology. Starting from experimental investigations of different probe geometries, we employ the discrete-element method to compute the expended work during the penetration of a granular packing, conrming the optimal bio-inspired shape. This has allowed to follow also an integrated approach including image acquisition and processing of the actual geometries, 3D printing, experiments and numerical simulations. Finally, another interesting example of advanced biological interface with optimised behaviour is represented by biosensing strucviii tures. We employ fluid-structure interaction numerical simulations for studying the response of spiders' trichobothria, which are among the most sensitive biosensors in Nature. Our results highlight the role of the fluid-dynamic drag on the system performance and allow to determine the optimal hair density observed experimentally. Both the third-body problem and the possibility to tune the frictional properties can be considered as the next grand challenges in tribology, which is going to live a "golden age" in the coming years. We believe the results discussed in this Doctoral Thesis could pave the way towards the design of novel bio-inspired structures with optimal tribological properties, for the future development of smart materials and innovative solutions for sliding interfaces.