Дисертації з теми "532.135:538.4"

My thesis concerns two areas of current research in magnetism, with the common experimental technique of elastic and inelastic neutron scattering on powders and single crystals. The first topic is concerned with rare earth intermetallic compounds with unfilled 4f shells. When a rare earth element with an unfilled shell is placed in an environment with surrounding charges, the crystal electric field acts to lift the J-fold degeneracy. Determination of the Crystal Electric Field (CEF) ground state and excited states are important to understand the complex and interesting low temperature phase diagrams these compounds exhibit. This will be discussed for the rare earth intermetallic compound TmGa3. The other area of my thesis concerns one dimensional Heisenberg antiferromagnetic quantum chains systems. Due to the one-dimensional lattice and low spin, quantum fluctuations play an important role in these systems. From the nature of the lattice, the interactions could give rise to magnetic frustration, resulting in interesting ground states. I will discuss results on the binary halides CuCl2, CuBr2 and on the quaternary oxide LiCuVO4. To measure the thermal properties of the investigated systems, a quasi-adiabatic Nernst type calorimeter was constructed and commissioned. I will describe the design and the properties of the calorimeter.

Магістерська дисертація на тему “Удосконалення ендопротезу колінного суглобу за рахунок використання магнітно-реологічного демпфера”, складається з __3__ розділів, обсяг пояснювальної записки __117__ сторінок основного тексту, включає _75__ рисунків і _31__ таблиця, та __8__ плакатів графічного матеріалу. Метою роботи є підвищення стабільності МРД протезу колінного суглобу в змінних умовах експлуатації. Для досягнення поставленої мети було сформульовано ряд задач: - провести аналіз існуючих конструкцій протезів колінних суглобів; - теоретично обґрунтувати та створити математичні моделі робочих процесів та дослідити вплив температури та напруженості магнітно поля на робочі характеристики МРД; - розробити експериментальну установку для перевірки та уточнення математичних моделей МРД; - провести експериментальні дослідження; - розробити рекомендації щодо створення адаптивних демпферів з урахуванням результатів отриманих при виконанні роботи. Перший розділ даної роботи присвячено аналітичному дослідженню впливу зовнішніх та внутрішніх факторів на демпфери протезу та методи адаптації під них. Другий розділ присвячено розробці математичної моделі яка описує залежність впливу температури на робочу характеристику демпфера. Третій розділ включає в себе створення стенду для проведення відповідних експериментальних досліджень, представлена схема та необхідна апаратура, що дозволили створити безпосередньо стенд для досліджень впливу температури та тиску робочого середовища на робочі характеристики МРД суглобу. Розробити методи експериментальних досліджень. Проведено серії експериментів, для визначення впливу перепаду тиску та температури на робочу характеристику. Об’єктом дослідження в даній роботі є процес функціонування МРД протезу колінного суглобу, а саме його дроселя при різних умовах експлуатації. Предметом дослідження є зв'язок конструктивних параметрів провідності регульованого дроселя та зміни температури робочої рідини. Науковою новизною даного дослідження є запропоновані аналітичні залежності, що відображають зміну витрати від прикладеної сили струму (магнітного поля). Розроблена уточнена математична модель процесу демпфування протезу колінного суглобу. Перелік ключових слів: Демпфер, протезування, моделювання в SolidWorks, адаптивні властивості, дросель. Для дослідження МРД використовувався пакет прикладних програм „MatLab ”, в додатку до нього „Simulink”. Результати дослідження дозволяють вдосконалити конструкцію МР демпфера та зменшити температурний фактор на його характеристики. Публікаціїї. За матеріалами магістерської дисертації було опубліковано 4 праці, а саме 3 тези доповідей на міжнародних науково-технічних конференціях студентів.
The master's thesis on the theme "Improvement of knee joint endoprosthesis due to the use of magnetic-rheological damper" consists of __3__ sections, the volume of the explanatory note __117__ pages of the main text, includes _75__ drawings and _31__ tables, and __8__ posters of graphic material. The aim of the work is to increase the stability of the MRI of the knee joint prosthesis under varying operating conditions. To achieve this goal, a number of tasks were formulated: - to carry out an analysis of existing designs of knee joints; - theoretically substantiate and create mathematical models of work processes and investigate the influence of temperature and intensity of the magnetic field on the operating characteristics of the MRD; - to develop an experimental installation for checking and refinement of mathematical models of MRD; - to conduct experimental research; - to develop recommendations for the creation of adaptive damping taking into account the results obtained during the work. The first section of this paper is devoted to the analytical study of the influence of external and internal factors on the denture dampers and the methods of adaptation under them. The second section is devoted to the development of a mathematical model that describes the dependence of temperature influence on the performance of the damper. The third section includes the creation of a stand for conducting relevant experimental research, a diagram and the necessary equipment are presented, which allowed to create a stand directly for research on the influence of temperature and pressure of the working environment on the working characteristics of the MRD of the joint. Develop experimental research methods. A series of experiments was conducted to determine the effect of pressure drop and temperature on the performance. The object of the research in this paper is the process of functioning of the MRI of the knee joint prosthesis, namely, its chokes under different operating conditions. The subject of the study is the connection of the design parameters of the conductivity of the regulated choke and changes in the temperature of the working fluid. The scientific novelty of this study is the proposed analytical dependence, reflecting the change in flow rate from the applied current strength (magnetic field). The refined mathematical model of the process of damping the knee joint prosthesis is developed. Keyword list: Damper, Prosthesis, Modeling in SolidWorks, Adaptive Properties, Choke. To study MRD, a package of applications "MatLab", in addition to "Simulink" was used. The results of the study allow to improve the design of the MP damper and reduce the temperature factor on its characteristics. Publications On the materials of the master's thesis 4 papers were published, namely 3 theses of reports at international scientific and technical conferences of students.
Магистерская диссертация на тему "Совершенствование эндопротеза коленного сустава за счет использования магнитно-реологического демпфера", состоит из __3__ разделов, объем пояснительной записки __121__ страниц у основного текста, включает _75__ рисунков и _31__ таблиц, и __8__ плакатов графического материала. Целью работы является повышение стабильности МРД протеза коленного сустава в изменяющихся условиях эксплуатации. Для достижения поставленной цели был сформулирован ряд задач: - провести анализ существующих конструкций протезов коленных суставов - теоретически обосновать и создать математические модели рабочих процессов и исследовать влияние температуры и напряженности магнитного поля на рабочие характеристики МРД; - разработать экспериментальную установку для проверки и уточнения математических моделей МРД; - провести экспериментальные исследования; - разработать рекомендации по созданию адаптивных демпферов с учетом результатов полученных при выполнении работы. Первый раздел данной работы посвящен аналитическому исследованию влияния внешних и внутренних факторов на демпферы протеза и методы адаптации в них. Вторая глава посвящена разработке математической модели описывающей зависимость влияния температуры на рабочую характеристику демпфера. Третий раздел включает в себя создание стенда для проведения соответствующих экспериментальных исследований, представлена схема и необходимая аппаратура, позволившие создать непосредственно стенд для исследований влияния температуры и давления рабочей среды на рабочие характеристики М РД сустава. Разработаны методы экспериментальных исследований. Проведено серии экспериментов, для определения влияния перепада давления и температуры на рабочую характеристику. Объектом исследования в данной работе является процесс функционирования МРД протеза коленного сустава, а именно его дросселя при различных условиях эксплуатации. Предметом исследования является связь конструктивных параметров проводимости регулируемого дросселя и изменения температуры рабочей жидкости. Научной новизной данного исследования предложенные аналитические зависимости, отражающие изменение расхода от приложенной силы тока (магнитного поля). Разработана уточненная математическая модель процесса демпфирования протеза коленного сустава. Перечень ключевых слов: Демпфер, протезирование, моделирование в SolidWorks, адаптивные свойства, дроссель. Для исследования МРД использовался пакет прикладных программ "MatLab", в приложении к нему "Simulink». Результаты исследования позволяют усовершенствовать конструкцию МР демпфера и уменьшить температурный фактор на его характеристики. Публикации. По материалам магистерской диссертации было опубликовано 4 работы, а именно 3 тезис а докладов на международных научно - технических конференциях студентов.

Les pastilles supraconductrices peuvent produire des forts champs magnétiques très supérieurs aux aimants permanents. Plusieurs méthodes d’aimantation de ces pastilles existent néanmoins une seule est principalement utilisée pour les applications en génie électrique, l’aimantation par champ magnétique impulsionnel (Pulsed Field Magnetization). Afin de maîtriser l’aimantation de ces pastilles supraconductrices par PFM, nous avons étudié l’influence de la forme de l’inducteur sur le champ magnétique piégé où nous trouvons une influence significative de la forme de l’inducteur sur le champ piégé dans la pastille supraconductrice. Afin de prévoir la mise en oeuvre des pastilles supraconductrices dans des applications en génie électrique, nous avons étudié l’aimantation de ces pastilles dans un circuit magnétique et l’influence de ce circuit sur le champ piégé. Nous remarquons une nette amélioration du champ piégé dans la pastille en utilisant le circuit magnétique. Nous avons étudié, également, l’influence d’un champ démagnétisant impulsionnel et alternatif sur le champ piégé dans une pastille supraconductrice aimantée. Les dégradations observées ne montrent pas de contre-indication à l’utilisation des pastilles supraconductrices aimantées dans les applications en génie électrique
The superconducting bulks can produce very strong magnetic fields greater than those of permanent magnets can. Several methods of magnetization of the superconducting bulks exist, however one is mainly used for the electrical applications, the Pulsed Field Magnetization. In order to control the magnetization of the superconducting bulks by PFM, we studied the influence of the shape of the inductor on the trapped magnetic field where we find a significant influence of the shape of the inductor on the trapped magnetic field in the anticipate superconducting bulk. In order to the implementation of the superconducting bulk in the electrical applications, we studied the magnetization of these bulks in a magnetic core and the influence of this magnetic core on the trapped magnetic field. We notice an important improvement of the trapped magnetic field in the superconducting bulk by using the magnetic core. We studied also the influence of a pulsed and an alternating demagnetizing field on the trapped magnetic field in a superconducting bulk. The observed degradation does not show any contraindication to use the superconducting magnetic bulks in the electrical engineering applications

Il existe actuellement un important besoin en dispositifs électroniques dans le domaine des longueurs d’ondes millimétriques, tels que les circulateurs et les isolateurs fonctionnant dans la gamme de fréquence 30-100 GHz. Les aimants permanents qui garantissent la propagation de l’onde électromagnétique dans ces dispositifs comportent très généralement des éléments terres rares. En raison du coût à l’acquisition de ces éléments, d’une part, ainsi que de leur coût environnemental d’autre part, il existe une demande d’aimants permanents produits sans terres rares. Les ferrites durs peuvent présenter les propriétés demandées pour une intégration dans les dipositifs micoondes non réciproques. Ainsi, les particules de ferrite de baryum (BaM) possèdent un champ d’anisotropie magnétocristallin important, dirigé selon l’axe de facile aimantation. Un matériau autopolarisé, constitué d’un empilement de ces particules, peut donc présenter une aimantation permanente d’intensité suffisante pour les applications visées. De nombreuses méthodes d’élaboration de tels matériaux ont été mises au point. Cependant les mises en oeuvre de ces méthodes sont contraignantes. En revanche, nous avons mis au point dans le présent travail un processus de réalisation de ferrites de BaM massifs autopolarisés, dont l’aspect technologique est simple (basé sur des méthodes de chimie douce et des traitements thermiques adaptés), et très abordable financièrement. Les résultats obtenus sont très compétitifs (aimantation rémanente normalisée MR/MS comprise entre 0.87 et 0.90. Le champ coercitif HC atteint la valeur de 303 kA/m, rendant le matériau peu sensible aux effets démagnétisants), et permettent d’envisager la production de ces matériaux en vue d’applications à des fréquences allant jusqu’à 55 GHz
Presently, there is a critical need for millimeter wave devices, among which are non-reciprocal devices such as isolators and circulators, which operate in the frequency range from about 30 GHz to 100 GHz.Permanent magnets that ensure the propagation of the wave in such devices are for long based on rare-earth elements. Nowadays, the escalation of rare earths cost encourages to look for alternative materials containing much less, or no, rare earth elements selected from the most common and most available. Alternatively, hard ferrites may show the required potential for integration into non-reciprocal microwave devices. Barium ferrite (BaM) is a wellknown, high-performance, permanent magnet material with a large magneto-crystalline anisotropy along the c-axis of its hexagonal structure. A suitable form of barium ferrite for mm-wave applications is a magnetically oriented bulk material.Several constraining processing techniques were therefore set up to perform the alignment and compaction of plate hexaferrite particles. In contrast to these methods, the present study demonstrates the feasibility of simple alternative ways to provide highly oriented bulk compacts made of BaM particles. In the present work efficient and inexpensive chemical processes (co-precipitation, dry mixing and wet mixing) are presented that produces highly oriented bulk compacts made of Barium hexaferrites (BaM) particles. Hysteresis loops that display very competitive squarenesses between 0.87 and 0.90 (normalized remanent magnetization) and coercivity as high as 303 kA/m are obtained. The sensitivity to demagnetizing effects is therfore reduced. These properties make these BaM bulk ferrite materials suitable for selfbiased applications at frequencies up to 55 GHz

Le but de mes recherches est d'étudier les premiers instants de la désaimantation ultrarapide dans divers matériaux ferromagnétiques (Co, Ni et Fe ...) en utilisant différentes méthodes expérimentales, pour obtenir un nouvel aperçu des mécanismes physiques sous-jacents. Plus précisément, les questions particulières qui m'ont motivé étaient: 1. Dans les alliages ferromagnétiques Fe et Ni, les deux éléments sont fortement couplés par l’interaction d'échange. Ce couplage influence-t-il le temps de désaimantation après excitation avec un laser femtoseconde? 2. Le transport de spin superdiffusif pourrait-il être observable entre les couches non-magnétiques (Pt) et magnétiques (Co) d’une multicouche Co/Pt ? 3. La dernière question concerne le passage entre un contrôle cohérent de l’aimantation avec des impulsions de grande longueur d’onde (THz) et une désaimantation incohérente provoquée par une excitation de plus courte longueur d’onde (IR)
The goal of my research is to investigate the early time period of the ultrafast demagnetization in various ferromagnetic materials (Co, Ni, and Fe…) by using different experimental methods, to obtain novel insight on the underlying physical mechanisms. More precisely, the particular questions that motivated me were: 1. In Fe and Ni ferromagnetic alloys, both elements are strongly coupled by exchange interaction. Does this coupling influence the time of demagnetization after pumping with a femtosecond laser? 2. Could the super-diffusive spin-transport be observable to explain the weak spin transport between the layers and the similar demagnetization curve in Co and Pt system? 3. The last question is around the different pump wavelength will truly affect the demagnetization behavior. In this thesis, several different results are clearly presented to try to find the answer of the questions above

Nous avons développé un modèle et un programme d'affinement joint des densités de charge et spin. Lors des premiers tests plusieurs difficultés sont apparues et ont été étudiées puis résolues notamment par la mise en place de contraintes. Après la mise en place d'un programme stable d'affinement joint nous avons testé celui-ci sur le complexe MnCu(pba)...(H2O)3...2H2O, ou pba représente le 1,3-propylenbis(oxamato) en réutilisant les données provenant d'une expérience de diffraction de neutrons polarisés et en effectuant une nouvelle expérience de diffraction des rayons X à 10K, température à laquelle l'expérience de diffraction des neutrons polarisés a été conduite. Cette étude a permis de tester trois schémas de pondération, ainsi que les contraintes. Ces tests ont montré que l'affinement joint permet de retrouver les résultats des différents affinements séparés mais aussi d'aller plus loin en autorisant un affinement de la densité de spin avec plus de paramètres pertinents. Suite à ces premiers tests nous nous sommes intéressés à un complexe azido cuivre (Cu2L2(N3)2 avec L=1,1,1-trifluoro-7-(dimethylamino)-4-méthyle-5-aza-3-heptène-2-onato). L'affinement joint a permis d'avoir accès, pour la première fois, à la densité de valence expérimentale résolue en spin et d'affiner également des paramètres de contraction/dilatation différents pour la valence avec un spin up ou un spin down. Dans le dernier chapitre nous avons étudié un complexe de cobalt qui présentait des propriétés magnétiques intéressantes. Cependant la particularité magnétique du composé venant d'une forte anisotropie magnétique a rendu l'étude par affinement joint délicate dans un premier temps, c'est pourquoi nous avons étudié ce composé uniquement d'un point de vue de la densité de charge. Cette étude a tout de même permis de mettre en évidence expérimentalement à 100K un angle de torsion de 39° entre les axes principaux des atomes de cobalt, prédit par la théorie
We developed a model and a refinement program for charge and spin densities. During the first tests several difficulties have arisen and have been investigated and solved by implementation of constraints. After the establishment of stable joint refinement program, we tested it on the MnCu(pba)...(H2O)3...2H2O, with pba = 1,3-propylenbis(oxamato) complex reusing data from an experiment of polarized neutron diffraction and making a new experience of X-ray diffraction at 10K. This study tested three weighting schemes and constraints. These tests showed that the joint refinement give access to the same results as the separated refinements but also allow us to go further by refining the spin density with more pertinent parameters. Following these initial tests, we were interested in a copper azido complex (Cu2L2(N3)2 with L=1,1,1-trifluoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato). The joint refinement give us access for the first time to the experimental spin-resolved valence density and also to refine the parameters of contraction / expansion for spin up or spin down separately. In the last chapter we studied a cobalt complex which shows interesting magnetic properties. However, the magnetic properties of the compound come from a high magnetic anisotropy which complicates a study by joint refinement. That is why we studied only the charge density of this compound. This study still allowed to show experimentally a torsion angle of 39° between the principal axes of the cobalt atoms, which was predicted by a previous theoretical study

L’utilisation de terres rares dans des dispositifs d’enregistrement de données est très coûteux et polluant. Leur remplacement par de l’oxyde de fer permettrait de s’affranchir de cela. En-dessous d’une taille de 20 nm, les nanoparticules d’oxyde de fer ne peuvent pas être considérées comme des aimants permanents. Une alternative consiste à les combiner à une autre phase magnétique pour permettre d’augmenter leur anisotropie magnétique via un couplage d’échange interfacial au sein de nanoparticules de type coeur@coquille. En revanche la stabilité magnétique de ces dernières reste insuffisante. L’objectif de cette thèse est de concevoir un nouveau type de nanoparticules magnétiques de type coeur@coquille@coquille avec un cœur de Fe3-dO4 et des coquilles de CoFe2O4, CoO ou NiO qui a permis d’augmenter encore les propriétés magnétiques tout en conservant une taille inférieure à 18 nm. L’étude approfondie de leur relation structure-propriété a été réalisée au moyen d’un large éventail de techniques
The use of rare earths in data storage devices is expensive and polluting. Their replacement with iron oxide would make it possible to avoid this. Below a size of 20 nm, iron oxide nanoparticles cannot be considered as permanent magnet. An alternative is to combine them with another magnetic phase to enhance their magnetic anisotropy via interfacial exchange coupling within core@shell nanoparticles. However, the magnetic stability of the latter remains insufficient. The scope of this thesis is to design a new type of magnetic nanoparticles of core@shell@shell structure with a Fe3-dO4 core and CoFe2O4, CoO or NiO as shells which has further enhance the magnetic properties while maintaining a size below 18 nm. The in-depth study of their structure-properties relationship was carried out using a wide set of analytical techniques

Les fluides à seuil ont des propriétés mécaniques étonnantes : en dessous d'une certaine contrainte, appelée contrainte seuil, le fluide se comporte comme un solide. Au-delà de la contrainte seuil, ils s'écoulent comme un gel. Ce mémoire porte sur l'étude des actions des forces magnétiques volumiques sur ces fluides. Nous étudions la possibilité de réaliser la transition solide--gel à partir des forces magnétiques volumiques créées par différents dispositifs magnétiques. Ces fluides n'ont pas de propriétés magnétiques particulières. Il est donc nécessaire de créer des champs magnétiques intenses pour agir sans contact sur ces fluides. Nous étudions différentes configurations magnétiques capables de dépasser la contrainte seuil au sein du fluide. L'influence des différents paramètres des configurations magnétiques est explorée, notamment à l'aide d'études paramétriques. Notre travail mêle des équations de magnétostatique et de mécanique des fluides non-newtoniens. Les calculs de magnétostatique sont menés de façon analytique alors que la partie mécanique et le couplage magnéto-mécanique sont traités par un logiciel de simulation numérique par la méthode des éléments finis. Les conditions magnétiques et mécaniques nécessaires à la transition solide--gel et à la modification d'écoulements de fluides à seuil sont discutées, notamment à l'aide de simulations numériques et de nombres adimensionnels.

Cette thèse concerne l'étude des propriétés structurales et magnétiques de nanoparticules magnétiques creuses (HMNPs), coquille et coquille/coquille. Les effets de surface sont exaltés de par la présence des surfaces interne et externe. L'étude expérimentale de HMNPs basée sur des mesures magnétiques et de spectrométrie Mössbauer du 57Fe a montré une structure magnétique complexe. Les HMNPs ayant une épaisseur ultrafine présentent une structure magnétique décrite par 2 sous-réseaux spero-magnétiques opposés, en plus de la présence d’un champ d'échange bias significatif. L'effet de la taille et de l'épaisseur des HMNPs a été également étudié. Les spectres Mössbauer obtenus sous champ magnétique montrent que la structure magnétique est fortement corrélée au rapport surface/volume. Ces résultats expérimentaux ont été confirmés par simulation Monte Carlo. Après optimisation du modèle, l’approche numérique montre d’abord que l'anisotropie de surface Ks gouverne le comportement magnétique des HMNPs et ensuite que la valeur critique de Ks nécessaire pour obtenir une configuration radiale (spike) diminue lorsque la taille des HMNPs augmente. L'étude numérique menée pour différentes tailles et épaisseurs de coquille, a permis de suivre leurs effets sur la structure magnétique des HMNPs. Par ailleurs, l'étude expéri-mentale menée sur des HMNPs shell/shell, montre que le désordre des spins et le champ d'échange bias deviennent plus importants lorsque les HMNPs sont recouvertes d’une coquille antiferromagnétique (NiO). De ces résultats, on peut déduire l'effet du désordre des spins sur les phénomènes d'échange bias dans un tel système
This thesis concerns the study of structural and magnetic properties of hollow magnetic nanoparticles (HMNPs), shell and shell/shell. These HMNPs present enhanced surface effects resulting from the presence of both inner and outer surface layers. The experimental investigation combining magne-tic measurements and 57Fe Mössbauer spectrometry of such HMNPs has revealed a complex spin magnetic structure. Small HMNPs with ultrathin thickness show highly disordered magnetic structure and the corresponding in-field hyperfine structure can be described by means of 2 speromagnetic antiferromagnetically coupled, in addition to the significant exchange bias phenomenon. The in-field Mössbauer study of the effect of size and thickness of HMNPs shows that the spin disorder is strongly correlated to the surface to volume ratio. Those experimental magnetic behaviors were confirmed using Monte Carlo simulation. Indeed, after improving the numeric model, it is concluded that surface anisotropy Ks has a dominant role in the magnetic behavior of HMNPs and the value of critical Ks necessary to obtain radial (spike) configuration decreases as the size of HMNPs increases, keeping the same thickness. The numeric study for different sizes and shell thicknesses allows the effect of these parameters on the spin structure of HMNPs to be followed. Then, the experi-mental study extended to shell/shell HMNPs indicates that the spin disorder is enhanced in HMNPs with antiferromagnetic shell (NiO) in addition to larger exchange bias field. From those results, one can try to deduce the effect of spin disorder on the exchange bias phenomena in such system

In our pursuit of ever smaller transistors, with greater computational throughput, many questions arise about how material properties change with size, and how these properties may be modelled more accurately. Metallic nanocontacts, especially those for which magnetic properties are important, are of great interest due to their potential spintronic applications. Yet, serious challenges remain from the standpoint of theoretical and computational modelling, particularly with respect to the coupling of the spin and lattice degrees of freedom in ferromagnetic nanocontacts in emerging spintronic technologies. In this thesis, an extended method is developed, and applied for the first time, to model the interplay between magnetism and atomic structure in transition metal nanocontacts. The dynamic evolution of the model contacts emulates the experimental approaches used in scanning tunnelling microscopy and mechanically controllable break junctions, and is realised in this work by classical molecular dynamics and, for the first time, spin-lattice dynamics. The electronic structure of the model contacts is calculated via plane-wave and local-atomic orbital density functional theory, at the scalar- and vector-relativistic level of sophistication. The effects of scalar-relativistic and/or spin-orbit coupling on a number of emergent properties exhibited by transition metal nanocontacts, in experimental measurements of conductance, are elucidated by non-equilibrium Green’s Function quantum transport calculations. The impact of relativistic effects during contact formation in non-magnetic gold is quantified, and it is found that scalar-relativistic effects enhance the force of attraction between gold atoms much more than between between atoms which do not have significant relativistic effects, such as silver atoms. The role of non-collinear magnetism in the electronic transport of iron and nickel nanocontacts is clarified, and it is found that the most-likely conductance values reported for these metals, at first- and lastcontact, are determined by geometrical factors, such as the degree of covalent bonding in iron, and the preference of a certain crystallographic orientation in nickel.
Physics
Ph. D. (Physics)

Dissertação de mestrado em Engineering Rheology
Graphene is one of many nanofillers often incorporated in polymeric matrixes in order to produce polymer nanocomposites. In the time being, melt compunding (e.g., via extrusion) is accepted as the most promising method to produce such materials at a large-scale production scenario. However, since correlations between processing conditions and nanocomposite morphology remains unclear, a general procedure for controlling the final characteristics of these materials to a specific application is still lacking. The main goal of this thesis is to understand how dispersion of polymer/graphene nanocomposites is affected by the processing conditions used on a twin screw extruder. Samples were collected along the axial length of the extruder in order to monitor the evolution of dispersion by means of Small-Amplitude oscillatory shear (SAOS) measurements and bright-field microscopy performed at the lab. Effects of graphene type, feed rate, screw speed, and the evolution of dispersion along the extruder were analyzed. Additionally, the off-line rheological measurements were compared with on-line measurements that were undertaken within the scope of a bigger project done in cooperation with BASF. It is concluded that both graphene types induce a shear thinning behavior over the entire range of frequencies considered and enhanced dispersion is achieved with smaller particle size graphene. Moreover, lower feed rates and lower screw speeds induce better dispersion levels. Furthermore, dispersion is partially improved along the extruder and discrepancies between on-line and off-line results were found.
O grafeno é um dos vários reforços à escala nanométrica que é incorporado em matrizes poliméricas de forma a produzir nanocompósitos. Atualmente, os processos de composição no fundido (p.e., através de extrusão) são aceites como os métodos mais promissores na preparação destes materiais, no cenário de produção em larga escala. Contudo, uma vez que não são claras as correlações entre as condições de processamento e a morfologia dos nanocompositos, falta ainda estabelecer procedimentos para controlar as características finais destes materiais numa aplicação específica. O objetivo principal desta tese é compreender como a dispersão em nanocompósitos de polímero com grafeno são afetadas pelas condições de processamento usadas numa extrusora de duplo-fuso. Foram recolhidas amostras ao longo do comprimento axial da extrusora, de forma a monitorizar a evolução da dispersão através de medições feitas no laboratório de reologia oscilatória (SAOS) e microscopia de campo claro. Foram analizados o efeito do tipo de grafeno na composição, o débito, velocidade dos fusos, e a evolução da dispersão ao longo da extrusora. Para além disso, as medições off-line foram comparadas com medições online realizadas no âmbito de um projeto colaborativo com a BASF. Foi concluído que ambos os tipos de grafeno induzem o comportamento pseudo-plástico ao longo de toda a gama de frequencias usadas e houve uma melhoria da dispersão com o grafeno possuindo um tamanho de partícula inferior. Além disso, a dispersão é parcialmente melhorada ao longo da extrusora and foram encontradas discrepâncias entre os resultados on-line e off-line.

Dissertação de mestrado em Matemática e Aplicações à Mecânica
Na modelação e simulação de sistemas físicos a relação causa – efeito assume grande protagonismo. Assim como é importante quantificar os efeitos provocados por determinadas causas, pequenas causas podem implicar grandes efeitos, também é enriquecedor determinar as causas para obter efeitos desejados. Os problemas inversos são reconhecidos pela intenção de alcançar as causas que proporcionem um determinado efeito conhecido ou desejado. Nos problemas inversos não é possível garantir a existência, unicidade e estabilidade da solução levando a que maior parte destes problemas sejam considerados como mal formulados. Matematicamente, muitos dos problemas inversos são descritos por uma equação integral de Fredholm de 1ª espécie, a qual sustenta a sua resolução em diversas noções de Análise Funcional. O estudo para a determinação do espectro de relaxação de materiais viscoelásticos poder-se-á suportar por vários modelos já anteriormente desenvolvidos, entre eles o modelo de Maxwell, relacionado com a combinação em série de dois mecanismos, uma mola elástica e um amortecedor. Encontradas as funções que possibilitam o cálculo do módulo elástico e do módulo de dissipação, torna-se necessário elaborar uma busca de soluções óptimas para o módulo elástico e tempo de relaxação. Com base numa optimização suportada pelas teorias de John Holand, Algoritmos Genéticos, executou-se uma busca de indivíduos de vários tamanhos estruturados por pares de resultados que se pretendiam optimizar. Os dados recolhidos na fase final do trabalho foram comparados e confrontados entre si de modo a que se vislumbrasse o vector com melhor desempenho segundo as variáveis erro e diferença.
In modeling and simulation of physical systems the relation cause-effect assumes a great importance. So, it is very important to quantify the effects caused by certain causes can lead to great effects; it is also very enriching to determine the causes needed to reach the wanted goals. Inverse problems are recognized by the intension of achieving the causes that can lead to a determined kwon and the wanted effect. In the inverse problems it is not possible to guarantee the existence, the singleness and stability of the solution, in consequence of this major part of these problems are described by an integral equaton of Fredholm of 1st specie which sustains its resolution in various notions of funcional analysis. The study to determine the spectrum of relaxation of viscoelastic materials can be supported by several models previously developed, among them the Maxwell model, related with the combination in series of two mechanisms, one elastic spring and one choker absorder. Once founded the functins that make possible the calculation of the elastic module and of the dissipation module, it becomes necessary to elaborate a research of the best solutions for the elastic module and the relaxation time. Based on the an optimization supported by the theories of John Holand, Genetic Algorithms, it has been performed a research of individuals of several sizes structured by pairs of results which could be optimized. The results obtained in the final step of this task, were compared and confronted between themselves in away that it was possible to see clearly the best vector with the best performance according to the variables of error and diference.

Dissertação de mestrado integrado em Engenharia de Polímeros
A microinjeção pode ser utilizada como um processo de monotorização para a realização da caracterização reológica, no comportamento desenvolvido pelos polímeros fundidos durante o enchimento em microcavidades. O sistema adotado permite obter a informação do comportamento do polímero no interior dos microcanais estudados. A dissertação apresentada é adequada uma vez que enuncia vários fenómenos reológicos tais como, o escorregamento na parede dos microcanais, tensão de superfície gerada, queda de pressão, assim como a frente do fluxo do fundido. Neste estudo desenvolvido o sistema de aquisição de dados para avaliar a queda de pressão foi obtido através do uso de dois sensores incorporados no interior do molde. Os microcanais foram utilizados para a avaliação reológica do polímero fundido, contudo o uso de postiços com diferentes microcanais, permite apenas a informação sobre a queda de pressão ao longo do microcanal, bem como a pressão de injeção desenvolvida e a velocidade da frente do fluxo. Assim através do uso de um molde como ferramenta acoplado á utilização de uma microinjetora foi possível realizar uma quantificação do comportamento reológico do polímero fundido. O uso da microinjeção como sistema de estudo no âmbito da reometria, permitiu a elaboração das curvas de fluxo para dois materiais, polipropileno (Moplen HP 548R) e poliestireno (Edistir N1910), desenvolvidas através da imposição de diferentes velocidades de injeção. Neste trabalho e para as curvas de fluxo obtidas para o microcanal com secção de 100 μm x 400 μm demostra-se claramente o desenvolvimento do escorregamento ao longo das paredes do microcanal, já para o microcanal com secção de 200 μm x 200 μm, não foi possível demostrar-se o efeito de escorregamento. No entanto toda a configuração experimental imposta permitiu obter conclusões importantes sobre o comportamento do fluxo dentro de ambos os microcanais.
Microinjection moulding can be used to process monitoring and rheological characterization, and can help to understand the behaviour of polymer melt flows during the filing of micro cavities. The system used allows a study on characterization of polymer behaviour in microchannels. This approach is appropriate since rheological phenomena such as wall slip, surface tension, melt pressure drop and polymer flow length can be studied. In this system, the data acquisition for the pressure drop has been made by incorporation of two sensors inside the mould. The microchannels were used for rheological evaluation of polymer flow; therefore, the products of the injection tests with these moulding blocks with different microchannels will provide only useful information on pressure drop along the microchannel, injection pressure developed and melts front speed. Thus, through the utilization of a characteristic mould as tool for the quantitative evaluation of the rheological behaviour of polymer melts under typical microinjection moulding conditions was developed. The use of a microinjection moulding tool for rheometry purposes provided the preparation of flow curve for two materials, polypropylene, (Moplen HP 548R) and to polystyrene, (Edistir N1910), developed through the imposition of different injections speeds. In this present work, the flow curves measured by the two sensors into the mould demonstrated that the wallslip effect was clearly observed to microchannel with section of 100 μm x 400 μm and the other microchannel, with section of 200 μm x 200 μm, it was not possible demonstrated the wall slip effect. For the other hand, entire configuration experimental imposed allowed to obtain important conclusions concerning flow behavior within both microchannels.