Dionigi Periegeta, Oikoumhnhs Perihghsis Introduzione, Testo critico, Traduzione e commento.

Dopo l'introduzione, dedicata essenzialmente alla riproposizione delle varie ipotesi di ricostruzione della tradizione manoscritta dell'opera di Dionigi Periegeta, si presenta il testo originale della "Periegesi" con apparato e traduzione. L'ultima e più cospicua parte della tesi è dedicata al commento dell'opera, incentrato sulle problematiche testuali e interpretative. Vengono altresì affrontati problemi stilistici, di poetica, di ricerca delle fonti

Biasing crystallization in fused silica: An assessment of optimal metadynamics parameters

Metadynamics (MetaD) is a useful technique to study rare events such as crystallization. It has been only recently applied to study nucleation and crystallization in glass-forming liquids such as silicates, but the optimal set of parameters to drive crystallization and obtain converged free energy surfaces is still unexplored. In this work, we systematically investigated the effects of the simulation conditions to efficiently study the thermodynamics and mechanism of crystallization in highly viscous systems. As a prototype system, we used fused silica, which easily crystallizes to β-cristobalite through MetaD simulations, owing to its simple microstructure. We investigated the influence of the height, width, and bias factor used to define the biasing Gaussian potential, as well as the effects of the temperature and system size on the results. Among these parameters, the bias factor and temperature seem to be most effective in sampling the free energy landscape of melt to crystal transition and reaching convergence more quickly. We also demonstrate that the temperature rescaling from T > Tm is a reliable approach to recover free energy surfaces below Tm, provided that the temperature gap is below 600 K and the configurational space has been properly sampled. Finally, albeit a complete crystallization is hard to achieve with large simulation boxes, these can be reliably and effectively exploited to study the first stages of nucleation

Disclosing crystal nucleation mechanism in lithium disilicate glass through molecular dynamics simulations and free-energy calculations

Unraveling detailed mechanism of crystal nucleation from amorphous materials is challenging for both experimental and theoretical approaches. In this study, we have examined two methods to understand the initial stage of crystal precipitation from lithium disilicate glasses using molecular dynamics simulations. One of the methods is a modified exploring method to find structurally similar crystalline clusters in the glass models, enabling us to find three different embryos, such as Li2Si2O5 (LS2), Li2SiO3 (LS) and Li3PO4 (LP), in the 33Li2O·66SiO2·1P2O5 glass (LS2P1), in which P2O5 is added as a nucleating agent. Interestingly, LS2 and LP crystals were found inside the LS2P1 glass while LS crystal appeared on the glass surface, which agrees with experimental observations. The other method is free energy calculation using a subnano-scale spherical crystal embedded in the glass model. This method, which we called Free-Energy Seeding Method (FESM), allows us to evaluate free energy change as a function of crystal radius and to identify critical size of the crystal precipitation. The free energy profiles for LS and LS2 crystal nuclei in the LS2 glass models possess maximum energy at a critical radius as expected by classical nucleation theory. Furthermore, the critical radius and the energy barrier height agree well with recent experimental investigation, proving the applicability of this method to design glass–ceramics by atomistic modeling

Structural origins of the Mixed Alkali Effect in Alkali Aluminosilicate Glasses: Molecular Dynamics Study and its Assessment

The comprehension of the nonlinear effects provided by mixed alkali effect (MAE) in oxide glasses is useful to optimize glass compositions to achieve specific properties that depend on the mobility of ions, such as the chemical durability, glass transition temperature, viscosity and ionic conductivity. Although molecular dynamics (MD) simulations have already been applied to investigate the MAE on silicates, less effort has been devoted to study such phenomenon in mixed alkali aluminosilicate glasses where alkali cations can act both as modifiers, forming non-bridging oxygens and percolation channels, and as charge compensator of the AlO4− units present in the network. Moreover, the ionic conductivity has not been computed yet; thus, the accuracy of the atomistic simulations in reproducing the MAE on the property is still open to question. In this work, we have validated five major interatomic potentials for the classical MD simulations by modelling the structure, density, glass transition temperature and ionic conductivity for three aluminosilicate glasses, (25 − x)Na2O − x(K2O) − 10(Al2O3) − 65(SiO2) (x = 0, 12.5, 25). It was observed that only the core-shell (CS) polarizable force field well reproduces the experimentally measured MAE on Tg and the ionic conductivity as well as the higher conductivity of single sodium aluminosilicate glass at low temperature and the higher conductivity of single potassium aluminosilicate glass at high temperature. The MAE is related to the suppression of jump events of the alkaline ions between dissimilar sites in the percolation channels consisting of both sodium and potassium ions as in the case of alkaline silicates. The superior reproducibility of the CS potential is originated from the larger and the flexible ring structures due to the smaller Si-O-Si inter-tetrahedra angle, creating appropriate percolation channels for ion conductivity. We also report detailed assessments for using the potential models including the CS potential for investigating MAE on aluminosilicates

BeeNet and BEST: the Italian 'beekeeping and environment' projects

Tese de doutoramento em Engenharia Civil, no ramo de Geotecnia, apresentada ao Departamento de Engenharia Civil da Faculdade de Ciências e Tecnologia da Universidade de CoimbraO estudo e a caracterização de solos incoerentes, nomeadamente areias saturadas, foram impulsionados pela ocorrência de diversos incidentes, entre os quais se destacam os sismos de 1964 no Alasca (Estados Unidos da América) e em Niigata (Japão). Os severos danos registados durante estes sismos alertaram a comunidade científica para os efeitos associados aos fenómenos de liquefação, impulsionando o desenvolvimento da disciplina de engenharia sísmica geotécnica. No final do século XX, o início do estudo sistemático do comportamento de solos no domínio das pequenas deformações vem complementar o conhecimento da resposta não drenada de areias, fornecendo ferramentas adequadas para o dimensionamento tanto aos estados limites de utilização como aos estados limites últimos. Apesar dos avanços científicos no estudo do comportamento de areias saturadas, abalos sísmicos mais recentes, como os sismos de Kobe (1995, Japão) e Kocaeli (1999, Turquia), demonstraram que a previsão de fenómenos de liquefação e seus efeitos não estão totalmente compreendidos. Atendendo à história sísmica de Portugal bem como ao facto de as principais cidades portuguesas se situarem no litoral ou nas margens de rios, o Departamento de Engenharia Civil da Universidade de Coimbra, com a colaboração de outras duas instituições de ensino superior nacionais, encetou, em 2008, um vasto programa de investigação sobre ferramentas para o dimensionamento baseado no desempenho em maciços suscetíveis à liquefação. Para esta investigação, foi escolhida uma areia local recolhida nas margens do Rio Mondego que, após o seu tratamento, origina uma fração uniforme designada de areia de Coimbra, material que se propõe caracterizar, dando seguimento a vários estudos já elaborados. Os dados da extensa investigação laboratorial concretizada resultam de ensaios realizados sobre amostras cilíndricas de secção cheia mas, essencialmente, de ensaios executados num HCA Mark II sobre provetes cilíndricos ocos cujas dimensões são 60Di:100Do:200H (mm). A conclusão da instalação deste equipamento, o desenvolvimento de procedimentos de calibração dos seus instrumentos de medição bem como o estabelecimento de técnicas de preparação de provetes e de execução de ensaios constituem a primeira parte desta dissertação. Estas informações técnicas, que incluem a descrição detalhada de todos os componentes do HCA da Universidade de Coimbra, é complementada com uma exposição dos pressupostos e conceitos teóricos que permitem a definição dos estados de tensão e deformação em amostras cilíndricas ocas. A caracterização avançada do comportamento mecânico da areia de Coimbra, que sucede estudos microscópicos e de identificação que complementam informações anteriormente publicadas, perfaz a segunda parte desta dissertação. Os ensaios laboratoriais incidem sobre amostras de areia reconstituídas e saturadas, incluindo ensaios monotónicos e cíclicos em condições não drenadas para diferentes modos de carregamento e estados iniciais. A versatilidade do HCA, além da caracterização no domínio das pequenas deformações do solo analisado, permite a imposição de condições de restrição de deformação pouco convencionais no estudo de liquefação de areias. Por fim, descreve-se o comportamento pós-liquefação da areia de Coimbra, com especial enfoque nas deformações volumétricas e na variação do estado de tensão.The characterisation of cohesionless soils, namely saturated sands, was encouraged by several seismic events such as the 1964 earthquakes in Alaska (USA) and Niigata (Japan). The severe damages observed during and after these earthquakes drew the attention of the scientific community to liquefaction-related phenomena. It is also accepted that these two earthquakes are at the origin of the geotechnical earthquake engineering. By the end of the 20th century, extensive research programmes on the small strain behaviour of soils improved considerably the knowledge of the stress- strain response of sands, providing the appropriate tools for the design of both serviceability and ultimate limit states. Despite all the scientific advances in the study of the behaviour of saturated sand, recent earthquakes, such as Kobe’s earthquake in 1995 (Japan) and the Kocaeli’s earthquake in 1999 (Turkey), sowed that the prediction of liquefaction-related phenomena and their effects were not fully understood. Given Portugal’s seismic history, as well as the fact that its major cities are located on the coast or in proximity to rivers, the Department of Civil Engineering of the University of Coimbra, Portugal, began in 2008, in collaboration with two other Portuguese Universities, a comprehensive research programme with the objective of developing performance- based design tools for liquefiable deposits. A local sand, named Coimbra sand, obtained from sand collected on the banks of the Mondego River, has been used for this investigation. The present investigation aims at characterising the mechanical response of this sand, highlighting some features of its behaviour that have not been studied yet. The comprehensive experimental research carried out comprises data from tests performed on both conventional cylindrical samples and hollow cylindrical samples with dimensions 60Di:100Do:200H (mm) tested in a Hollow Cylinder Apparatus Mark II. The set up of this apparatus, the development of the calibration procedures for the measurement instruments, as well as the proposal of sample preparation techniques, compose the first part of this thesis. This technical information, which comprises a fully detailed description of all of the University of Coimbra’s HCA components, is accompanied by a discussion of all the assumptions and theoretical principles needed for the definition of the stress and strain states in a hollow cylindrical sample. Microscopic and fundamentals studies, which complete the data published in previous works, precede the advanced characterisation of the mechanical behaviour of Coimbra sand, constituting the second part of this thesis. All the laboratory tests are carried on remoulded and saturated sand samples, and comprise monotonic and cyclic undrained shear tests for both different loading modes and initial states. The versatility of the HCA enables not only the characterization of the small strain stiffness behaviour of Coimbra sand but also the use of unconventional deformation-restricted conditions in the study of sand liquefaction. Lastly, the post-liquefaction behaviour of Coimbra sand is described, with particular emphasis on the development of volumetric strains and evolution of stress state.L’étude et la caractérisation des sols incohérents, notamment des sables saturés, ont été encoutagées par de tragiques évènements sismiques, parmi lesquels se trouvent les tremblements de terre de 1964 en Alaska (Etats-Unis d’Amérique) et à Niigata (Japon). Les importants dégâts enregistrés au cours de ces violents tremblements de terre ont alerté la communauté scientifique et face aux effets de ces phénomènes liés à la liquéfaction des sables, une discipline du domaine du Génie Civil a été créée: Parasismique et Géotechnique. Vers la fin du XXe siècle, le début des études du comportement des sols sous l’hypothèse des petites perturbations vient compléter la connaissance du comportement non drainé des sables et fournir les outils nécessaires pour le dimensionnement aux états limites ultimes et en service. Malgré tous les progrès scientifiques, des tremblements de terre récents, comme ceux que ont eut lieu à Kobe en 1995 (Japon) et à Kocaeli en 1999 (Turquie), ont démontré que la prédiction bien comme les effets des phénomènes liés à la liquéfaction n’étaient pas intégralement assimilés. Étant donné l’histoire sismologique du Portugal aussi bien comme le fait que les villes portugaises les plus importantes sont établies sur la côte ou sur les berges de fleuves, le Département de Génie Civil de l’Université de Coimbra au Portugal, en collaboration avec deux autres établissements de génie civil nationaux, a entamé, en 2008, un ample programme de recherche sur les outils pour le dimensionnement basé sur la performance des sols susceptibles à la liquéfaction. Les recherches sont menées sur le sable de Coimbra, qui est le résultat du traitement d’un sable locale recueilli sur les rives du fleuve Mondego. La présente étude vise caractériser ce sol en apportant des renseignements encore jamais étudiés auparavant. Les résultats de cette ample recherche réalisée en laboratoire proviennent d’essais triaxiaux sur des échantillons conventionnels de sable et sur des échantillons cylindriques creux. Ces derniers ont été effectués avec une cellule triaxiale à cylindre creux du type HCA Mark II, qui permet, entre autres, d’essayer des éprouvettes cylindriques creuses dont les dimensions sont de 60Di:100De:200H (mm). L’achèvement de l’installation de cet équipement, le développement de procédures d’étalonnage de ses instruments de mesure bien comme de techniques de préparation d’éprouvettes creuses composent la première partie de cette thèse. Cet exposé inclut une description très détaillée du HCA de l’Université de Coimbra ainsi que l’exposition des hypothèses et des notions théoriques qui permettent de définir les états de contrainte et de déformation dans des essaies de cylindres creux. La deuxième partie de cette thèse rassemble les résultats de la caractérisation du comportement mécanique du sable de Coimbra, qui succède à des études microscopiques et d’identification. Les études en laboratoire comprennent uniquement des échantillons de sable remaniés et saturés, lesquels sont soumis à des essais de cisaillement non drainés sous différents modes de chargements monotones et cycliques pour différentes conditions initiales. La polyvalence de la cellule triaxiale à cylindre creux permet aussi d’imposer des restrictions de déformations non conventionnelles dans l’étude de la liquéfaction des sables e ains que leur caractérisation sous l’hypothèse des petites perturbations. En dernier lieu, ce travail présente des données sur le comportement post liquéfaction du sable de Coimbra, en mettant l’accent sur les déformations volumétriques et la variation de l’état de contrainte