Formgedächtnislegierungen wie Nickel-Titan (NiTi) können sich nach einer plastischen Verformung und anschließendem Aufheizen an ihre ursprüngliche Form „erinnern“ und diese wieder einnehmen. Als meistverwendete Formgedächtnislegierung kann NiTi als Aktor, zur Dämpfung und zur elastokalorischen Kühlen verwendet werden und kommt von der Medizintechnik bis hin zur Luft- und Raumfahrt zum Einsatz. Der Formgedächtniseffekt basiert auf der martensitischen Phasenumwandlung, einer diffusionslosen Strukturänderung, bei der sich die Kristallsymmetrie ändert. Bei NiTi mit etwa 50 At.-% Ni wandelt die kubische Hochtemperaturphase (Austenit) in die monokline Tieftemperaturphase (Martensit) um. Während dieser Umwandlung entsteht eine Vielzahl an Grenzflächen, wodurch sich ein komplexes martensitisches Gefüge – eine Art dreidimensionales „Puzzle“ bildet. Um NiTi-Formgedächtnislegierungen auf verschiedene Anwendungen zuzuschneiden und deren Eigenschaften zu verbessern, ist es wichtig, das Gefüge zu verstehen. Die häufig eingesetzten polykristallinen NiTi-Schichten haben dabei den Nachteil, dass die enthaltenen Korngrenzen einen zusätzlichen Parameter darstellen, der Gefügeuntersuchungen erschwert. Dagegen werden epitaktische Schichten bereits für andere magnetische Formgedächtnislegierungen als Modellsystem eingesetzt und tragen zu einem besseren Verständnis der martensitischen Umwandlung bei. Epitaktische Schichten sind einkristallin, sodass der Einfluss von Korngrenzen ausgeklammert werden kann. Außerdem dient das Substrat, das die Orientierung der Schicht vorgibt, als festes Referenzsystem. In dieser Arbeit wurden epitaktische NiTi-Schichten mit Magnetron-Sputterdeposition hergestellt, die bei Raumtemperatur martensitisch sind. Dabei wurde der Einfluss von Parametern wie Herstellungstemperatur, chemische Zusammensetzung, Wärmebehandlungsszenarien und Pufferschichten auf das Wachstum und die Eigenschaften der Schichten untersucht. So konnten Schichten in zwei unterschiedlichen Orientierungen, (100) und (111), hergestellt werden. Die so optimierten Schichten wurden anschließend dafür genutzt, das martensitische Gefüge skalenübergreifend zu untersuchen. Mit einer Kombination von Mikroskopie- und Röntgenbeugungsmethoden wurden die auftretenden Zwillingsgrenzen, Habitusebenen und Variantenorientierungen analysiert. So lässt sich feststellen, welche Martensitcluster entstehen, wie sie nukleieren und wachsen und welche Grenzflächen auftreten. Dabei ließ sich ein hierarchischer Aufbau des martensitischen Gefüges feststellen, wobei drei Zwillingsgrenzen auf unterschiedlichen Längenskalen für die Beschreibung des Gefüges nötig sind. Die auftretenden Zwillingsgrenzen sind aus Massivmaterialien bekannt, was zeigt, dass sich die Schichten gut als Modellsystem eignen. Das identifizierte, dreidimensionale Modell des Gefüges wurde mit Röntgenmethoden global bestätigt. Dazu wurden die experimentellen Ergebnisse mit zwei unterschiedlichen Martensittheorien, der phänomenologischen Martensittheorie (PTMC) und der Korrespondenztheorie (CT) verglichen. Der hierarchische Aufbau des Gefüges lässt sich zum Großteil mit den Theorien beschreiben. Die Schichten zeigen aber auch die Limitierungen der bisherigen Theorien und bieten so eine Möglichkeit für deren Weiterentwicklung.

Shape memory alloys (SMAs) can recover large strains (e.g., up to 8%) by undergoing a temperature-induced phase transformation. This strain recovery can occur against large forces, resulting in their use as actuators. The SMA elements in such actuators integrate both sensory and actuation functions. This is possible because SMAs can inherently sense a change in temperature and actuate by undergoing a shape change, associated with the temperature-induced phase transformation. The objective of this work is to develop an SMA based cryogenic thermal conduction switch for operation between dewars of liquid methane and liquid oxygen in a common bulk head arrangement for NASA. The design of the thermal conduction switch is based on a biased, two-way SMA actuator and utilizes a commercially available NiTi alloy as the SMA element to demonstrate the feasibility of this concept. This work describes the design from concept to implementation, addressing methodologies and issues encountered, including: a finite element based thermal analysis, various thermo-mechanical processes carried out on the NiTi SMA elements, and fabrication and testing of a prototype switch. Furthermore, recommendations for improvements and extension to NASA's requirements are presented. Such a switch has potential application in variable thermal sinks to other cryogenic tanks for liquefaction, densification, and zero boil-off systems for advanced spaceport applications. The SMA thermal conduction switch offers the following advantages over the currently used gas gap and liquid gap thermal switches in the cryogenic range: (i) integrates both sensor and actuator elements thereby reducing the overall complexity, (ii) exhibits superior thermal isolation in the open state, and (iii) possesses high heat transfer ratios between the open and closed states. This work was supported by a grant from NASA Kennedy Space Center (NAG10-323) with William U. Notardonato as Technical Officer.

Gas hydrate formation was studied in Gulf of Mexico (GOM) sediments. Sediments studied were from six-meter long cores from Mississippi Canyon Block 118 and a 27-meter core from a cruise in 2002 of the Marion Dufresne. These sediments retained their in situ seawater before testing. Hydrate formation rate and induction times were measured. The hydrate memory effect was studied in GOM sediments with and without in situ seawater. Hydrate induction time was short when in situ seawater was present. Bioproducts adsorbed on particles in the sediments are postulated to shorten the induction times by maintaining seawater structuring around coated particles. Hydrate nucleation was studied by Dynamic Light Scattering and Scanning Electron Microscopy. Particles around 50 to 100 nm nucleated hydrate formation. These small nucleating particles appeared to be clays or surfactant molecules and interactions thereof. Hydrate capillaries were studied and found to be at least 100 nm in diameter because the sediment nucleating particles with bioproducts diffused through the hydrate capillaries. Large complexes of nontronite smectite clay and Emulsan, an anionic biosurfactant, were found to facilitate hydrate formation. It was determined that Emulsan entered the interlayer of nontronite. The clay contents of the GOM sediments were determined. All sediments contained smectite, illite, chlorite, and kaolinite in different proportions. The study gave new insight into the gas hydrate formation mechanism in seafloor sediments.

No description available.

No description available.

Dans ce travail, des modèles thermomécaniques basés sur une approche non-locale sont proposés pour décrire le comportement des Alliages à Mémoire de Forme (AMF) avec la prise en compte des effets de la localisation et de la chaleur latente lors de la transformation de phase. Ces modèles sont obtenus comme des extensions d’un modèle local existant. Pour décrire la localisation de la transformation de phase, l’extension du modèle initial a consisté à le réécrire dans un contexte non-local par l’introduction d’une nouvelle variable, définie comme la contrepartie non-locale de la fraction volumique de martensite déjà présente dans le modèle local. L’exploitation de ce modèle a nécessité le développement d’un élément fini spécial dans ABAQUS avec la fraction volumique non-locale de martensite comme un degré de liberté supplémentaire. Les simulations réalisées montrent la pertinence d’une telle approche dans la description de la transformation de phase dans des structures minces en AMF, soumises à des chargements thermomécaniques. Pour décrire les effets de la chaleur latente, une équation d’équilibre thermique ayant comme terme source des contributions dépendant de la transformation de phase a été adjointe au modèle initial. Là encore, l’exploitation du modèle a nécessité le développement d’un élément fini qui prend en compte le couplage thermomécanique et la formulation proposée pour l’équilibre thermique. Les simulations numériques réalisées ont montré l’effet retardant sur la transformation de phase de la chaleur latente, et le caractère hétérogène possible de la transformation dans ce cas. Ces effets sont d’autant plus importants que la vitesse de déformation est élevée / In this Phd thesis, thermo-mechanical models based on a nonlocal approach are proposed in order to describe the behavior of Shape Memory Alloys (SMA), taking into account localization and latent heat effects during phase transformation. These models are obtained as extensions of an existing local model. In order to describe the localization of phase transformation, the extension of the initial model consisted of rewriting it in a nonlocal context through the introduction of a new variable, defined as the nonlocal counterpart of the martensite volume fraction. The use of this model has required the development of a specific finite element in ABAQUS with the nonlocal martensite volume fraction as an additional degree of freedom. The simulations show the relevance of such an approach in the description of the phase transformation occurring in thin SMA structures subjected to thermo-mechanical loadings. To achieve the description of the latent heat effects, a heat balance equation with a source term depending on contributions of the phase transformation was added to the constitutive equations of the initial model. Even there, the use of the model required the development of a finite element which takes into account the thermo-mechanical coupling and considers the proposed formulation for the thermal balance. Numerical simulations have shown the delaying effect of the latent heat on phase transformation and the possible heterogeneous character of the phase transformation in this case. These effects are even more important as the strain rate is high

A extensa utilização de pesticidas na agricultura moderna tem contribuído para um aumento na contaminação do meio ambiente. Os Hidróxidos Duplos Lamelares (HDLs) ou argilas aniônicas, apresentam uma estrutura lamelar na qual uma variedade de ânions podem ser intercalados entre as lamelas através de interações eletrostáticas. Estes materiais lamelares podem ser utilizados em processos de adsorção/sorção e como suporte para a liberação lenta de compostos químicos. Assim, os principais objetivos deste trabalho foram: o estudo da sorção de ânions orgânicos de herbicidas ácidos 2,4-D, MCPA e Picloram utilizados na agricultura nacional, pela regeneração de HDLs de Mg-Al-CO3 calcinado e por troca aniônica em HDLs de Mg-Al-Cl. Para isso foram investigados a cinética do processo de sorção, além da determinação das isotermas em dois valores diferentes de pH. Assim, estes herbicidas foram intercalados em HDLs de Mg-Al, utilizando dois métodos de síntese indireta: regeneração do material calcinado e troca aniônica em solução, além do método de síntese direta por coprecipitação, sendo os materiais devidamente caracterizados. Todos os materiais obtidos, intercalados com cada um dos herbicidas, foram utilizados para o estudo da cinética de liberação dos mesmos em água. Os materiais obtidos por regeneração contendo cada um dos três herbicidas foram utilizados também no estudo da lixiviação em colunas de solo e no estudo de bio-ensaio com plantas, para testar a eficiência dos HDLs como suportes na liberação lenta dos herbicidas. A partir do estudo da cinética do processo de sorção dos ânions orgânicos presentes em solução utilizando HDLs de Mg-Al contendo cloreto ou carbonato após a calcinação, observou-se que a remoção dos ânions ocorreu através dos processos de troca aniônica no caso do HDL contendo cloreto, ou de regeneração da estrutura lamelar no caso do HDL calcinado. A intercalação dos ânions orgânicos na posição vertical foi verificada no processo de adsorção/sorção, e se mostrou dependente da concentração do herbicida em solução. O HDL calcinado apresentou a maior eficiência na remoção dos ânions da solução. Os herbicidas suportados em HDLs, apresentaram uma liberação em água mais lenta do que os mesmos em sua forma livre, e não se mostrou dependente do método de preparação da matriz utilizada. Os resultados dos estudos de lixiviação utilizando como matriz o HDL obtido por regeneração, demonstraram que os HDLs são bons suportes para os herbicidas pois apresentaram uma liberação lenta do ingrediente ativo, principlamente para o herbicida Picloram. O bio-ensaio mostrou que a forma de liberação dos herbicidas suportados foi eficiente no controle da germinação das sementes de plantas em aplicações pré-emergenciais. / The extensive use of pesticides in modern agriculture has been contributing to an increase in environmental contamination. Layered Double Hydroxides (LDHs), or anionic clays, bear a layered structure, and a variety of anions can be intercalated between these layers through electrostatic interactions. These layered materials can be used in adsorption/sorption processes and in the slow release of chemical compounds. So this work aims at studing the sorption of the acid herbicides bearing organic anions 2,4-D, MCPA and Picloram, used in national agriculture, through regeneration of calcined Mg-Al-CO3 LDH and anion exchange of Mg-Al-Cl ? LDH. For this purpose, the kinetics of the sorption process was investigated, and the determination of isotherms in two different pH values was carried out. The herbicides were intercalated in Mg-Al ? LDH using two indirect synthesis methods: regeneration of the calcined material and anion exchange in solution; besides the direct synthesis method by coprecipitation. The obtained materials were properly characterized. All the obtained materials, intercalated with each of the herbicides, were used in a kinetic release study in water. The materials obtained by regeneration were also used in a leaching and in a bioassay study, to test the efficiency of LDHs as supports for the slow release of herbicides. From the kinetic sorption process of the organic anions in solution using Mg-Al ? LDH containing chloride or carbonate after calcination, it was observed that the removal of anions occurred through an anionic exchange process in the case of the LDH containing chloride, or through regeneration of the layered structure in the case of the calcined LDH. The intercalation of organic anions in a vertical position was observed in the adsorption/sorption process, and it was shown to be dependent on the herbicide concentration in solution. The calcined LDH was the most efficient for removal of anions in solution. The herbicides supported on LDHs, presented a slower release in water than the same compounds in their free form, and the release behavior was not dependent on the methodology used for the preparation of the matrix. Leaching study results using the LDH obtained by regeneration as matrix demonstrated a slow release of the active ingredient, mainly for the Picloram herbicide. The bioassay showed that the release behavior of the supported herbicides was efficient in the control of plant seeds germination at preemergence applications.

No description available.

NiTi based shape memory alloys (SMA) cover most of the commercially produced shape memory devices and components. The reversible martensitic transformation between the phases B2 (austenite) and B19′ (martensite) is responsible for the shape memory effect in these alloys. The amount of strain which can be regained after a permanent deformation through thermal activation, known as the recoverable strain, is a strong function of crystallographic texture and microstructure. Texture influences the activation of a specific martensite variant during stress induced martensitic (SIM) transformation and also the re-orientation of twinned variants during further deformation. The variant selection decides the amount of recoverable strain. Since the NiTi based shape memory alloys inevitably undergo thermo-mechanical processing in the course of component design, the consequent evolution of texture and microstructure regulate the shape memory behavior. The present thesis is aimed to address this issue in some NiTi alloys that are technologically important for different applications, namely a binary Ni-rich NiTi alloy, a copper containing NiTi alloy and a hafnium containing NiTi alloy. The Ni rich NiTi alloy displays pseudoelastic behavior that can be used for couplings, the NiTiCu alloy provides a controlled thermal hysteresis suitable for actuator applications and the NiTiHf alloy can be used for high temperature applications. The first Chapter of the thesis provides a detailed overview of the existing knowledge related to evolution of microstructure and texture during processing, the transformation texture and its role on the shape memory behavior in NiTi alloys. The second chapter includes the experimental procedure followed to generate different textures, namely unidirectional and cross rolling with and without a subsequent annealing and also the details of the techniques used to characterize the structure, microstructure, texture and mechanical properties. The evolution of texture during thermo-mechanical processing of a Ni rich NiTi alloy and its impact on shape memory behavior is addressed in Chapter 3. The two modes of rolling employed at higher temperature led to the formation of different textures. The texture of unidirectionally rolled samples was characterized by a strong <111>||ND fiber, while a strong Goss {100}<110> component along with <111>||ND fiber was observed in the texture of the cross rolled samples. Annealing of the unidirectionally rolled samples generated a strong <100>||ND fiber, and a weak <111>||ND fiber was observed for the cross rolled samples. Microtexture analyses indicated that dynamically recrystallized grains had significantly different texture compared to the statically annealed material. One of the salient features of this study is the analysis of different twin boundaries with coincident site lattice (CSL) relations that has been observed in the hot rolled material. The origin of these twins has been attributed to deformation. The evolution of twin boundaries with CSL relation has strong influence on texture formation. A few of the important texture components have been found to have CSL relation amongst them. The origin of different texture components were found using intra-grain misorientation parameters. In-situ transformation studies in a scanning electron microscope have confirmed the formation of different types of twins at very low amount of strain in the Ni rich NiTi alloy. A Schmid factor based criterion was used to identify the activation of a particular variant. Trace analysis of the surface relief due to SIM transformation was utilized to confirm the theoretically predicted variant. Schmid criterion has been found to be valid in all the cases. Modulus variation with temperature and strain was studied using dynamical mechanical analysis. Microstructural changes during thermal and thermo-mechanical cycling revealed higher orientation gradient along grain boundaries compared to grain interior. The compatibility condition at the grain boundaries were attributed to higher misorientation development. Misorientation development during cycling loading process is also found to be a strong function of texture. Processing condition and texture has a strong influence on the recoverable strain. Particularly, the strength of <111>||ND fiber is influential in deciding the recoverable strain. Study of microstructure and texture evolution in the TiNiCu SMA and subsequent study on its impact on recoverable strain is presented in Chapter 4. Convincing evidences for the mechanisms operating during different dynamic restoration processes have been presented through microstructural investigation. Texture analysis of the austenite phase showed the formation of <111>||ND fiber. Despite the weakening of texture at larger strain, strength of certain deformation texture components like S {123}<634> and Cu {112}<111> increased, which suggested that texture evolution in TiNiCu alloy deviates from the texture of binary NiTi at large strains. Transformation texture analysis was carried out through electron back scattered diffraction technique, using an in-situ heating stage. The analysis of the results showed predominant activation of <011> type II as well as {11 1 } type I twins. A comparison of martensite and austenite pole figures indicated strong variant selection during phase transformation. Like the binary NiTi alloy, cross rolling of TiNiCu alloy also showed ample changes in the texture of martensite phase through the formation of different texture components. Annealing of both unidirectionally and cross rolled samples led to the weakening of texture. The change in volume fraction of Ti2NiCu precipitates, resulting from different processing conditions, influenced the transformation temperature. In this case also, texture and large intra-grain misorientation governed the recoverable strain. Chapter 5 is dedicated to the study of high temperature NiTiHf alloy. X-ray diffraction and differential scanning calorimetric studies confirmed a two step martensitic transformation, a B19` monoclinic and rhombohedral R-phase martensite in the studied alloy (Ni49.4Ti38.6Hf12). Microstructural investigations showed the formation of dendritic (Ti,Hf)2Ni precipitates along the grain boundary. Evolution of R-phase martensite was always observed along with (Ti,Hf)2Ni precipitates, irrespective of the processing condition. Dissolution of (Ti,Hf)2Ni precipitates by solution treatment suppressed the R phase formation. Strong texture of R-phase martensite confirmed variant selection during martensitic transformation. On the contrary, texture of B19` martensite was always weak, suggesting no preference for variant selection. Rolled material with a relatively strong texture exhibited higher recoverable strain compared to annealed material. Finally, all the significant outcomes of the present investigation are summarized in Chapter 6. Based on the conclusions, suggestions for future work have been mentioned.

The objective of this research is to thoroughly investigate the shape memory effect in polymers, characterize, and optimize these polymers for applications in information storage systems. Previous research effort in this field concentrated on shape memory metals for biomedical applications such as stents. Minimal work has been done on shape memory poly- mers; and the available work on shape memory polymers has not characterized the behaviors of this category of polymers fully. Copolymer shape memory materials based on diethylene glycol dimethacrylate (DEGDMA) crosslinker, and tert butyl acrylate (tBA) monomer are designed. The design encompasses a careful control of the backbone chemistry of the materials. Characterization methods such as dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC); and novel nanoscale techniques such as atomic force microscopy (AFM), and nanoindentation are applied to this system of materials. Designed experiments are conducted on the materials to optimize spin coating conditions for thin films. Furthermore, the recovery, a key for the use of these polymeric materials for information storage, is examined in detail with respect to temperature. In sum, the overarching objectives of the proposed research are to: (i) design shape memory polymers based on polyethylene glycol dimethacrylate (PEGDMA) and diethylene glycol dimethacrylate (DEGDMA) crosslinkers, 2-hydroxyethyl methacrylate (HEMA) and tert-butyl acrylate monomer (tBA). (ii) utilize dynamic mechanical analysis (DMA) to comprehend the thermomechanical properties of shape memory polymers based on DEGDMA and tBA. (iii) utilize nanoindentation and atomic force microscopy (AFM) to understand the nanoscale behavior of these SMPs, and explore the strain storage and recovery of the polymers from a deformed state. (iv) study spin coating conditions on thin film quality with designed experiments. (iv) apply neural networks and genetic algorithms to optimize these systems.