Rozprawy doktorskie na temat „Solids with microstructure”

The vehicles industry is facing increasing demands for fuel efficiency and cost reduction due to environmental legislation, sustainability and customer demands. Therefore, there is a great need to develop and produce lightweight components by using materials and processes that offer higher specific strength and/or design optimization. Semi‐solid aluminium casting offers design freedom and castings with lower shrinkage and gas entrapment defects compared to high pressure die castings. The lack of understanding of microstructure and defect formation, and design data, for semi‐solid castings is a barrier for foundries and designers in the vehicles industry to use semi‐solid castings. In this study, the effect of two grain refiners on slurry formation and surface segregation of semi‐solid Al‐7Si‐0.3Mg castings produced by the Rheometal™ process was evaluated. The influence of grain refinement on primary α‐Al grain size, shape factor and solid fraction was analysed in addition to the solute content of the surface segregation layer. The influence of magnesium on the formation of intermetallic phases during solidification and the heat treatment response of Al‐7Si‐Mg semi‐solid castings was investigated. The magnesium content was varied from 0.3 to 0.6wt.% and the semi-solid castings were analysed in the T5 and T6 conditions. Energy dispersive spectroscopy was used to identify the intermetallic phases formed during solidification. Tensile testing was performed and the results were correlated to the magnesium and silicon concentration measured in the interior of the α‐Al globules formed during slurry preparation. The results suggest that the addition of grain refiner decreases the solid fraction obtained in the Rheometal™ process. However, no significant effect was observed on the α‐Al grain size and shape factor. A good correlation was obtained between the magnesium concentration in the interior of the α‐Al globules formed during slurry preparation and the offset yield strength for all alloys. The low magnesium solubility in α‐Al at temperatures in the solidification range of the Al‐7Si‐Mg alloys is suggested to be the reason for the low hardening response for the T5 heat treatment compared to the T6 condition.

Les propriétés mécaniques macroscopiques d’un matériau hétérogène dépendent non seulement des propriétés mécaniques de ses constituants microscopiques mais aussi fortement de sa microstructure. L’objectif de ce travail de thèse consiste d’abord à construire une classe de microstructures nouvelles pour lesquelles certaines propriétés macroscopiques, linéaires ou non linéaires, peuvent être analytiquement et exactement déterminées. Il vise ensuite à démontrer l’existence de champs uniformes dans des matériaux hétérogènes non linéaires et à dériver des relations exactes pour leur homogénéisation. A partir de l’assemblage de sphères composites de Hashin (1962) et Hashin et Shtrikman (1962), un bon nombre de résultats originaux sont obtenus pour les matériaux hétérogènes linéaires. A l’aide du concept des champs uniformes et du théorème des fonctions implicites, des relations générales nouvelles sont déduites pour l’homogénéisation de matériaux hétérogènes non linéaires
The macroscopic mechanical properties of a heterogeneous material depend non only on the mechanical properties of its microscopic constituents but also strongly on its microstructure. The first objective of this work is to construct a class of new microstructures for which certain macroscopic properties, linear or nonlinear, can be analytically and explicitly determined. The second purpose of the work is to show the existence of uniform fields in nonlinear heterogeneous materials and to derive the exact relations for their homogenization. Starting from the composite sphere assemblage of Hashin (1962) and Hashin and Shtrikman (1962), a good number of original results are obtained for linear heterogeneous materials. Using the concept of uniform fields and the implicit function theorem, new general relations are deduced for the homogenization of nonlinear heterogeneous materials

Les évolutions de microstructures du nouvel alliage chinois Ti B19 ont été étudiées au cours de divers traitements thermiques et des relations entre microstructures et propriétés mécaniques ont été établies. Les cinétiques de transformations isothermes ont été établies par des mesures de résistivité électrique. Les structures et microstructures ont été caractérisées par DRX synchroton, Microscopies Optiques et Electroniques (MEB, MET). Les cinétiques de transformation et les caractéristiques microstructurales. Les cinétiques de transformation isothermes ont été obtenues, analysées (loi JMAK), et cet ensemble de résultats a conduit à l'établissement du diagramme TTT de l'alliage TiB19. Enfin il a été montré que la vitesse de chauffage a une très forte influence sur les transformations mises en jeu, et qu'une pré déformation plastique accélère les cinétiques de transformation (introduction de nouveaux sites de germination). Enfin les cinétiques de transformation ont été caractérisées en refroidissement continu depuis le domaine monophasé bêta. Une première approche de la modélisation des cinétiques de transformation a été menée en utilisant le loi JMAK et le principe d'additivité de Scheil. Enfin les relations entre microstructures et propriétés sont discutés
The phase transformations and microstructure evolutions has been characterized for different thermal treatments, and the relationships between final microstrures and properties have been investigated in the new metastable Ti-B19 alloy. The isothermal phase transformation kinetics and the influence of different heat treatment phaths have been establisheb by using in-situ electrical Resistivity. The structures have been determined by synchrotron X-Ray Diffraction and the microstructures were observed by SEM and TEM. The results show that phase transformation kinetics and microstructure characteristics are strongly dependent on the aging temperature (ranging from 300 to 700°C). The global isothermal phase transformation phase transformation kinetics has been got and anallyzed with JMAK equation, and the TTT diagram of Ti-B19 alloy has been established. We have also shown that the heating rate has remarkable influence on the isothermal phase transformation behaviors and the pre-deformation accelerates the transformation kinetics. The microstructure evolutions during cooling are obviously dependent on the cooling rates. A first attempt has been made to calculate the transformation kinetics during cooling using JMAK law and Scheil principle. Finally, the relationship between mechanical properties and microstructure has been discussed

Le gypse (CaSO4·2H2O) est la matière première du plâtre. Il est chauffé entre 100 et 200 °C pour obtenir par déshydratation partielle l'hémihydrate (CaSO4 · 1 2H2O) qui, mélangé à de l'eau, se dissout tandis que des cristaux de gypse précipitent pour former une matrice résistante. Ce cycle qui paraît simple est compliqué par l'existence d'autres phases dans le système, en particulier deux formes de l'anhydrite (CaSO4). Les microstructures, encore mal comprises, affectent de plus la solubilité et la stabilité en température des produits de déshydratation ainsi que la cinétique des transitions. Le suivi in-situ de la déshydratation par microtomographie de rayons X synchrotron sur des monocristaux de gypse a mis en évidence l'anisotropie de la réaction. L'influence de la pression partielle de vapeur d'eau et de la température sur la cinétique a été mesurée, ainsi que leurs effets sur la microstructure. Celle-ci, organisée sur plusieurs ordres d'échelle, montre une rémanence de la structure du gypse malgré la fracturation microcristalline. La diffraction des rayons X et la microscopie électronique à balayage ont montré que la modification des symétries lors de la transition de l'anhydrite III vers l'anhydrite II donne lieu à un maclage très dense résultant de l'existence de deux variantes symétriques dont l'alternance limite les contraintes mécaniques. Enfin, une méthodologie a été développée pour mesurer a posteriori le taux de gâchage et la morphologie initiale d'échantillons de plâtre grâce à la microtomographie de rayons X synchrotron. Appliquée à une étude de cas portant sur 13 bas-reliefs florentins du XVe siècle, elle a donné des informations inédites sur les pratiques d'atelier
Gypsum(CaSO4·2H2O)istherawmaterialusedinplastermanufacture. Heatedbetween100and200 °C, it partially dehydrates into hemihydrate (CaSO4· 1 2H2O) which, when mixed with water, dissolves while gypsum crystals precipitate and form a tough matrix. This simple cycle is complicated by other phases in the system, including two forms of anhydrite (CaSO4). In addition, microstructures, which are poorly understood, affect the reactivity and stability of the dehydration products as well as the kinetics of transitions. The dehydration of gypsum single crystals, observed by Synchrotron X-ray microtomography, revealed the anisotropy of the reaction. Water vapor pressure and temperature affect the kinetics as well as the microstructures that are organised on several scales and reflect the structure of gypsum in spite of the microcrystalline fragmentation. X-ray powder diffraction and scanning electron microscopy showed that the change in symmetry during the transition from γ-anhydrite to β-anhydrite leads to a high-frequency twinning, resulting from the stacking of two symmetrical variants that minimises mechanical stress. Finally, a methodology was developed to measure the water-to-plaster ratio and the reactive powder morphology in set plaster samples using synchrotron X-ray microtomography. It was applied to a case study of 13 florentine low-relief sculptures from the 15th century, bringing new insights on workshop practices

In this work, the development of theoretically-dense, clean grain boundary, high hardness solid-state sintered silicon carbide (SiC) armor was pursued. Boron carbide and graphite (added as phenolic resin to ensure the carbon is finely dispersed throughout the microstructure) were used as sintering aids. SiC batches between 0.25-4.00 wt.% carbon were mixed and spray dried. Cylindrical pellets were pressed at 13.7 MPa, cold-isostatically pressed (CIP) at 344 MPa, sintered under varying sintering soaking temperatures and heating rates, and varying post hot-isostatic pressing (HIP) parameters. Carbon additive amounts between 2.0-2.5 wt.% (based on the resin source), a 0.36 wt.% B4C addition, and a 2050°C sintering soak yielded parts with high sintering densities (~95.5-96.5%) and a fine, equiaxed microstructure (d50 = 2.525 µm). A slow ramp rate (10°C/min) prevented any occurrence of abnormal grain growth. Post-HIPing at 1900°C removed the remaining closed porosity to yield a theoretically-dense part (3.175 g/cm3, according to rule of mixtures). These parts exhibited higher density and finer microstructure than a commercially-available sintered SiC from Saint-Gobain (Hexoloy Enhanced, 3.153 g/cm3 and d50 = 4.837 µm). Due to the optimized microstructure, Verco SiC parts exhibited the highest Vickers (2628.30 ± 44.13 kg/mm2) and Knoop (2098.50 ± 24.8 kg/mm2) hardness values of any SiC ceramic, and values equal to those of the "gold standard" hot-pressed boron carbide (PAD-B4C). While the fracture toughness of hot-pressed SiC materials (~4.5 MPa m1/2) are almost double that of Verco SiC (2.4 MPa m1/2), Verco SiC is a better performing ballistic product, implying that the higher hardness of the theoretically-dense, clean-grain boundary, fine-grained SiC is the defining mechanical property for optimization of ballistic behavior.

Dans le cadre de l’utilisation potentielle d’oxydes mixtes d’actinides au sein des réacteurs nucléaires de 3ème et 4ème générations, des solutions solides de formules générales U1-xThxO2, U1-xCexO2-y, U0,75Nd0,25O1,875, U0,75Gd0,25O1,875 et Th0,75Nd0,25O1,875 ont été préparées par conversion thermique de précurseurs oxalate. Préalablement à l’évaluation de la durabilité chimique des matériaux, une étape de frittage a été entreprise afin d’obtenir des pastilles denses présentant diverses propriétés physico-chimiques et microstructurales d’intérêt (composition, homogénéité, taux de densification, …) L’étude multiparamétrique de la dissolution, conduite en milieux nitrique, sulfurique et chlorhydrique a souligné l’impact important de la composition chimique au sein du matériau sur la durabilité chimique des échantillons. En effet, plusieurs paramètres (ordres partiels par rapport à l’activité en protons, énergie d’activation apparente, …) ont confirmé une modification significative du mécanisme de dissolution prépondérant pour les échantillons enrichis en uranium. Par ailleurs, le rôle important joué par certaines espèces azotées à l’interface solide/solution a également été démontré. L’évolution de l’interface solide/solution (surface réactive, composition) en cours de dissolution a également été suivie à travers une étude operando par Microscopie Electronique à Balayage en mode Environnemental. Cette étude a souligné l’existence de zones préférentielles de dissolution (jonctions triples, joints de grains, porosités inter- et intragranulaires) pour les échantillons les moins riches en uranium ; laquelle s’accompagne d’une forte augmentation de la surface réactive. En raison d’un phénomène prépondérant d’oxydation de l’uranium(IV) à l’interface, la dissolution des échantillons enrichis en uranium apparaît nettement plus homogène
In the field of the use of actinides mixed oxides as potential fuels for the Gen(III) and Gen(IV) nuclear reactors, solid solutions with general formula U1-xThxO2, U1-xCexO2-y, U0.75Nd0.25O1.875, U0.75Gd0.25O1.875 and Th0.75Nd0.25O1.875 were prepared by thermal conversion of oxalate precursors. Dense pellets exhibiting various physico-chemical and microstructural properties (in terms of composition, homogeneity, densification rate, …) were prepared through sintering then submitted to dissolution tests.The multiparametric study of the dissolution, performed in nitric, sulfuric and hydrochloric media clearly underlined the important effect of the chemical composition on the chemical durability of the samples. Indeed, several parameters (including partial order related to proton activity, apparent activation energy) confirmed the significant modification of the preponderant dissolution mechanism for uranium-enriched samples. Moreover, the role of various nitrogen-based species was evidenced at the solid/solution interface.The evolving of solid/solution interfaces (reactive surface area, composition) during dissolution was monitored by the means of operando ESEM experiments. Preferential dissolution zones (triple junctions, grain boundaries, inter- and intra-granular porosities) were clearly observed for uranium-depleted samples. They induce a significant increase of the reactive surface area even for short progress of the reaction. On the contrary, the dissolution appeared more homogenous for uranium-enriched samples due to the existence of a preponderant mechanism associated to the oxidation of the uranium(IV) at the interface

Solid oxide fuel cells (SOFC) in a flat-plate configuration require a hermetic seal between the fuel and air sides of the electrodes, and this seal must withstand a variety of thermally-induced stresses over the lifetime of the cell. In this study, quantitative microstructure-property relationships are developed to predict optimum seal structures for mechanical properties and thermal expansion coefficient criteria. These relationships are used to create an inverse approach to characterizing the processing method from the desired microstructure. The main focus of the work concentrates on providing tools to enable macroscopic property predictions from the constituent properties using homogenization techniques based on the individual phase properties and microstructure morphology. The microstructure is represented by two-point correlation functions. Statistical continuum mechanics models were then employed and developed to predict the mechanical and thermal properties of the material. The models enable the prediction of elastic modulus and coefficient of thermal expansion of the multi-phase material. The inelastic mechanical behavior was also studied, indicating microstructure dependence. These models will aid in predicting the a proper seal microstructure (with desired elastic stiffness, coefficient of thermal expansion, and viscoelastic behaviors) based on a desired level of crystallization glass-ceramic materials.

Semi-solid metal casting produces a low porosity, globular microstructure, which makes it an attractive competitor to wrought parts. This research explores the suitability of utilising heat treatable, Al-Zn-Mg-Cu alloys for the production of high strength parts for the automotive and aerospace industries, using the semi-solid metal rheocasting process. The process results in segregation, arising from non-equilibrium solidification, thus placing high demands on the post-solidification homogenisation and solution heat treatments, in order to derive optimum strength during subsequent ageing treatments. If the semi-solid metal castings can be age hardened to the same extent as the equivalent wrought competition, then this process could replace the machining of parts from conventional wrought stock. The semi-solid cast microstructure was investigated with the aid of light microscopy, scanning electron microscopy, elemental dispersive x-ray spectroscopy and electron backscatter diffraction, in order to establish the extent of segregation associated with the as-cast condition. Evolution of the as-cast structures of AA7075 and A713 composition alloys was investigated during subsequent heat treatments, with the view to achieving a fully homogenised structure prior to solution treatment and age hardening. In situ SEM heating experiments were performed, in order to document the evolution of the microstructure during homogenisation treatments. The results from the heating experiments and thermal analysis were used to evaluate the possibility of effective homogenisation processes for removal of the as-cast segregation. Owing to its negative effect on the homogenisation process, incipient melting of the solute-rich phases, identified using differential scanning calorimetry, had to be avoided during heat treatments. It was determined during in situ heating experiments that the interaction between the eutectic and the porosity in the semi-solid cast structure during overheating caused the agglomeration of and morphological changes to the isolated porosity, resulting in the formation of large pores within the previously interglobular regions. This could potentially have a negative impact on the structural integrity of a part in service. The evaluation of the as-cast condition provided insight into the mechanisms involved in the formation of the globular microstructure that is characteristic of SSM processing. Evidence showed that the mechanism was one of alternating planar and cellular growth, and that dendritic growth did not occur during the semi-solid metal rheocasting. This was highlighted by the absence of inter-dendritic liquid and the presence of internal crystallographic misorientations in many of the grains in the as-cast condition.

L'objectif de ce travail est d'analyser la possibilite d'obtenir, a partir d'elements immiscibles dans le cu, des alliages x#2#0cu#8#0 (x=fe, co) par broyage mecanique et de suivre leur comportement mecanique lors de la mise en forme. Les resultats les plus marquants obtenus lors de cette etude sont: l'evolution de la taille des cristallites lors du broyage se produit en deux stades distincts. Le premier, dont la cinetique est tres rapide, permet de passer d'une taille initiale de quelques centaines de nanometres a quelques dizaines de nanometres ; le second, beaucoup plus lent, conduit a la taille finale d'environ 25 nm qui apparait comme la limite inferieure susceptible d'etre obtenue par ce procede. Les methodes conventionnelles, diffraction des rayons x, m. E. T. Montrent que lors du broyage le fer c. C. (le co hc) disparait et qu'un alliage se forme avec evolution du parametre de maille. Les mesures d'aimantation (squid) mettent en evidence la realite plus complexe des phenomenes. L'alliage cu#8#0fe#2#0 forme une solution solide mais l'homogeneite n'est pas ideale puisqu'il subsiste de petites entites a base de fer. L'alliage cu#8#0co#2#0 pour des temps raisonnables de broyage (24/48h) possede un comportement superparamagnetique avec des amas riches en cobalt (>35%. At). La densification des poudres fait apparaitre plusieurs stades. Lors du premier, aux plus faibles contraintes, on assiste a une mise en place des grains. La deformation plastique se produit lors du deuxieme stade ; dans les poudres broyees la faible taille des cristallites n'autorise pas le developpement des mecanismes classiques de type source de franck-read, empilement et propagation. La forte proportion de joints de grains autorise a penser que des mecanismes specifiques de sources aux joints ou analogues a ceux intervenant dans les materiaux amorphes pourraient egalement participer a la deformation

One of the challenges in tissue engineering is to fabricate scaffolds which can mimic the natural microenvironments of cells. In a cell niche, biophysical and mechanical cues are crucial factors influencing cell functions. Given the complexity of natural extracellular matrix (ECM) engineered ECMs providing controllable biophysical and mechanical cues are appealing both in enhancing the understanding of cell-matrix interaction and in controlling cell fates in vitro. The ultimate goal of our study is to establish a platform as an engineered ECM by fabricating customized solid protein microstructures from solution using two-photon photochemical crosslinking, a novel laser-based freeform fabrication technique. In this study, protein structures varying from submicron lines, 2D micropatterns and microporous matrices, to 3D micropillars were successfully fabricated, demonstrating freeform fabrication capability with two-photon photochemical crosslinking. Two-photon fluorescent imaging and scanning electron microscope (SEM)-based microstructural characterization revealed that power, scan speed, total exposure time and concentrations of protein (bovine serum albumin) and photosensitizer (rose Bengal) in the solution were crucial processing parameters in this fabrication technique. Quantitative imaging analysis showed that porosity of protein matrices was highly dependent on processing parameters including power, scan speed, number of cycles in time series scan and protein concentrations in the solution. An atomic force microscopy (AFM)-based step change nano-compression test was used to measure the reduced elastic modulus of 3D viscoelastic protein micro-pillars fabricated, as a pilot study. Microporous protein matrices and 3D micropillar arrays fabricated with two-photon photochemical crosslinking can be used as engineered ECM for future study in cell-ECM interactions.
published_or_final_version
Mechanical Engineering
Master
Master of Philosophy

Compacted graphite iron (CGI) is rapidly becoming an attractive alternative material for engine components in the automotive industry, replacing lamellar graphite iron (LGI) in applications where high mechanical strength is desired. However, the gain in mechanical strength comes with a cost; thermal conductivity, process control and machining are three areas that are more challenging for CGI. This generates a need for research regarding various aspects concerning CGI. In this thesis the microstructure formation during solidification and solid state transformation will be the focus of interest. The phase transformations relevant for microstructure formation of importance to properties in CGI were studied. Experiments were performed in an industrial foundry giving this research direct relevance to regular production of CGI castings. Solidification of the grey (graphite/austenite) eutectic will be discussed, focusing on some relevant aspects influencing the graphite morphology of CGI. The formation of graphite nodules has been investigated by studying colour-etched microstructures. In a material containing mainly CGI cells it was found that nodules form either early during solidification as a consequence of high undercooling or late in the solidification sequence due to a combination of high undercooling and segregation of nodularising elements. Solidification of the white (cementite/austenite) eutectic was studied using chill wedges and the influence of some alloying elements on the amount of carbides was examined. To further enhance the understanding of carbide formation in CGI a commercial casting simulation software was used to correlate real castings to simulations. It was found that the alloying elements investigated influence the carbide formation in a similar way as in other graphitic cast irons and that high nodularity CGI is more prone to chill formation than low nodularity CGI. The solid state transformation was studied and a deterministic model was developed. The model divides a eutectic cell into layers, in order to take into account segregation of alloying elements, which was observed to be influential for the ferrite growth. Moreover, the effect of alloying elements on mechanical properties (tensile properties and hardness) was evaluated. Properties were correlated to microstructural features originating from both solidification and solid state transformations. The trends found generally confirmed previous results regarding properties in graphitic cast irons.

Dans le cadre du déploiement des énergies renouvelables par nature intermittentes, le couplage des technologies de piles à combustibles et d'électrolyseurs offre une solution innovante pour absorber les pics de production ou de consommation électrique. Dans ce cadre, la technologie basée sur des "cellules à oxydes solides" (SOCs) apparaît comme une solution attractive permettant de remplir les deux fonctions avec le même objet. Ces convertisseurs électrochimiques spécifiques sont constitués d'un électrolyte céramique oxyde fonctionnant à haute température (800 – 1000 °C) permettant d’atteindre des rendements très importants. Néanmoins, la durabilité des SOCs reste à ce jour insuffisante. Dans ce cadre, un des verrous technologiques important est la déstabilisation chimique et mécanique de l'électrode à oxygène ainsi que sa réactivité chimique avec l’électrolyte en fonctionnement. De plus, la température de fonctionnement élevée des SOFC entraîne des contraintes sévères sur l'assemblage des matériaux et sur les processus de fabrication.Les objectifs de cette thèse sont de mieux comprendre les mécanismes réactionels et le rôle de la microstructure des électrodes à oxygène dans les cellules à oxydes solides fonctionnant à température intermédiaire (650 - 750 °C) et d'étudier le vieillissement à travers une approche couplant caractérisations électrochimiques, physico-chimiques et de modélisation. Dans ce travail, l’électrode à oxygène architecturée LaPrNiO4+δ, nommée LPNO, a été déposée sur l'électrolyte Ce0.9Gd0.1O2-δ en vue de tirer parti des propriétés complémentaires des phases extrêmes, à savoir La2NiO4+δ de plus grande stabilité chimique et Pr2NiO4+δ, de plus grande activité électrochimique. Par ailleurs, les performances des SOCs dépendent non seulement des propriétés intrinsèques des matériaux mais aussi de l'association des matériaux fonctionnels et des propriétés de leurs interfaces. Par conséquent, de nouvelles architectures d’électrodes, à base d’une couche fonctionnelle active (AFL) fabriquée par atomisation électrostatique (ESD) et surmontée d'une couche de collecteur de courant optimisée (CCL) déposée par sérigraphie (SP), ont été optimisées pour diminuer la température de fonctionnement afin d'améliorer les performances et la durée de vie des cellules. Une couche fonctionnelle active nanostructurée avec une microstructure unique de type corail présentant une grande porosité et une bonne adhérence sur l'électrolyte s'est avérée d'une grande importance dans les réactions des électrodes à oxygène. Les performances et la durabilité des électrodes dans les SOC symétriques et dans une cellule d'électrolyse complète ont été étudiées en détail en vieillissement thermique et en mode galvanostatique. Ces données ont été discutées en relation avec l'évolution structurale et microstructurale de l'électrode à oxygène LPNO grâce à des caractérisations post-mortem utilisant la microscopie électronique à balayage, la reconstruction 3D par faisceau d'ions focalisés MEB, la diffraction des rayons X en laboratoire et la -diffraction et µ-fluorescence par rayonnement X synchrotron
Coupling fuel cell and electrolysis technologies provide an attractive solution to absorb the fluctuations induced by the deployement of the intermittent renewable energy sources. In this frame, the technology based on the "Solid Oxide Cells" (SOCs) appears as a promising solution as the same device can be alternatively used in both the electrolysis and fuel cell modes. The SOCs are based on an oxide ceramic electrolyte operating at a high temperature (800-1000 °C) allowing to reach very good efficiency. Hovewer, their durability is still insufficient to envisage their economic deployment. An important issue limiting the SOCs lifetime is the chemical and mechanical destabilizations of the oxygen electrode, as well as its chemical reactivity with the electrolyte upon operation. Moreover, the high operating temperature of SOCs leads to severe constraints on materials assembling and on fabrication processes.The objectives of this PhD thesis are to better understand the role of microstructure and the reaction mechanisms of oxygen electrode in SOCs operating in intermediate temperature (650-800 °C) and to investigate the aging in electrolysis and fuel cell modes through an integrated approach coupling electrochemical and physicochemical characterizations with modeling. In this work, architecturally designed LaPrNiO4+δ, referred to as LPNO, oxygen electrode has been prepared on Ce0.9Gd0.1O2-δ electrolyte by taking advantage of the complementary properties of La2NiO4+δ and Pr2NiO4+δ extremes, i.e. larger chemical stability of La2NiO4+δ and larger electrochemical activity of Pr2NiO4+δ. The performances of SOCs are not only due to the intrinsic properties of materials but also to the association of functional structured materials and the properties of their interfaces. In this frame, novel architectures based on an active functional layer (AFL) fabricated by electrostatic spray deposition (ESD) and topped by an optimized current collecting layer (CCL) deposited by screen-printing (SP) have been optimized aiming to decrease the operating temperature in order to improve the performances and the lifetime. A unique coral-like nanostructured AFL with large porosity and good adhesion on the electrolyte has been proved to be of great importance in the oxygen electrode reactions. The electrode performance and durability in symmetrical SOCs and in a complete electrolysis cell have been investigated in detail in thermal aging and in galvanostatic mode. These data have been thoroughly discussed in relationships with structural and microstructural evolution of LPNO oxygen electrode thanks to post-mortem characterizations using scanning electron microscopy, 3D reconstruction by focused ion beam-SEM, laboratory X-ray diffraction and synchrotron X-ray µ-diffraction and µ-fluorescence

As advances in solid rocket technology push rocket motors to more extreme operating speeds and temperatures, it becomes increasingly important to have well-designed material systems capable of surviving these harsh conditions. One common component in these systems is the use of a fiber- and particle-reinforced EPDM insulation layer between the motor casing and the solid fuel to shield the casing from the temperatures of the burning fuel and from the high velocity of gas particles traveling within the motor. This work studies several insulation materials to determine which exhibits the highest shear strength after being charred. Double-notch shear test specimens of three materials, ARI-2718, ARI-2719, and ARI-2750, were charred and tested to measure the failure strength of each charred material. The ARI-2750 showed the highest shear strength when loaded along the material orientation, but the ARI-2719 was strongest when transversely loaded. The strength measurements for ARI-2750 were highly sensitive to loading direction, unlike ARI-2718 and ARI-2719. Extensive scanning electron microscopy to identify correlations between shear strength and microstructure revealed that the amount of fiber orientation and amount of residual matrix material may have significant impacts on charred shear strength in these materials.
Master of Science