Ce travail porte sur la compréhension de l’impact de la microstructure sur les performances des Cellules à Oxyde Solide (SOC), avec une illustration sur l’électrode à oxygène en LSCF-CGO. Une approche couplant de la modélisation géométrique et électrochimique a été adoptée pour cet effet. Le modèle des champs aléatoires plurigaussiens et un autre basé sur des empilements de sphères ont été développés et adaptés pour les microstructures des SOCs. Ces modèles 3D de géométrie stochastique ont été ensuite validés sur différentes électrodes reconstruites par nano-holotomographie aux rayons X au synchrotron ou par tomographie avec un microscope électronique à balayage couplé à une sonde ionique focalisée. Ensuite, des corrélations semi-analytiques ont été proposées et validées sur une large base de microstructures synthétiques. Ces relations permettent de relier les paramètres ‘primaires’ de l’électrode (la composition, la porosité et les diamètres des phases) aux paramètres qui pilotent les réactions électrochimiques (la densité de points triples, les surfaces spécifiques interphases) et sont particulièrement pertinents pour les équipes de mise-en-forme des électrodes qui ont plus de contrôle sur ce premier ensemble de paramètres. Concernant la partie portant sur l’électrochimie, des tests sur une cellule symétrique en LSCF-CGO ont permis de valider un modèle déjà développé au sein du laboratoire, et qui permet de simuler la réponse électrochimique d’une électrode à oxygène à partir des données thermodynamiques et de microstructure. Finalement, le couplage des deux modèles validés a permis d’étudier l’impact de la composition des électrodes, leur porosité ou encore taille des grains sur leurs performances. Ces résultats pourront guider les équipes de mise-en-forme des électrodes vers des électrodes plus optimisées. / This work aims at better understanding the impact of Solid Oxide Cells (SOC) microstructure on their performance, with an illustration on an LSCF-CGO electrode. A coupled 3D stochastic geometrical and electrochemical modeling approach has been adopted. In this frame, a plurigaussian random field model and an in-house sphere packing algorithm have been adapted to simulate the microstructure of SOCs. The geometrical models have been validated on different electrodes reconstructed by synchrotron X-ray nano-holotomography or focused ion-beam tomography. Afterwards, semi-analytical microstructural correlations have been proposed and validated on a large dataset of representative synthetic microstructures. These relationships allow establishing the link between the electrode ‘basic’ parameters (composition, porosity and grain size), to the ‘key’ electrochemical parameters (Triple Phase Boundary length density and Specific surface areas), and are particularly useful for cell manufacturers who can easily control the first set of parameters. Concerning the electrochemical part, a reference symmetrical cell made of LSCF-CGO has been tested in a three-electrode setup. This enabled the validation of an oxygen electrode model that links the electrode morphological parameters to its polarization resistance, taking into account the thermodynamic data. Finally, the coupling of the validated models has enabled the investigation of the impact of electrode composition, porosity and grain size on the cell electrochemical performance, and thus providing useful insights to cell manufacturers.

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<p dir="ltr">To decrease the use of fossil fuels that generate greenhouse gases, there has been a push to find alternative processes for electricity generation. An attractive renewable alternative is to use solar-thermal energy for grid level electricity production. One method used to generate electricity from the conversion of solar-thermal energy is concentrated solar power (CSP) via the power tower paradigm, which involves an array of mirrors that concentrate sunlight to a spot on a tower. The light heats up a heat transfer fluid which later transfers the thermal energy to a working fluid that expands so as to spin a turbine to generate electricity. Current CSP plants have a peak operation temperature of 550℃, but improvements to the heat exchanger are integral to increasing the peak operation temperature of such plants to a 750℃ target. Ceramic/metal composites (cermets) have been proposed for use as heat exchangers in these CSP plants due to the creep resistance of the ceramic component and toughness of the metal component. One potential material that has an attractive combination of properties for this application is the alumina/chromium (Al2O3/Cr) cermet, given the rigidity and creep resistance of the Al2O3 component and the high-temperature toughness of the Cr phase. Compared to other oxidation-resistant oxide/metal cermets, the Al2O3 and Cr components of this cermet have a relatively close average linear thermal expansion match from 25℃ to 750℃, which is advantageous due to the thermal gradients and thermal cycling of the heat exchanger during operation.</p><p dir="ltr">In this dissertation, the Al2O3/Cr cermet was produced via reaction forming (RF) or reactive melt infiltration (RMI). The RF method involves the reaction of Cr2O3 and Al constituent powder mixtures at high temperature and modest pressures to obtain dense Al2O3/Cr plates. The RMI method involves immersing a shaped porous Cr2O3 preform into an Al or Al-Cr alloy bath to infiltrate and react to form Al2O3/Al-Cr plates. For both methods, the plate microstructure was analyzed for the various reaction conditions. The adiabatic temperature increase for the reaction between Cr2O3 and Al liquid or Al-Cr liquid alloys was calculated. Thermal properties (linear coefficient of thermal expansion, heat capacity, thermal diffusivity, thermal conductivity) and mechanical properties for the RF Al2O3/Cr plates were also measured. Lastly, the reaction kinetics between dense, polycrystalline Cr2O3 and a liquid Al-35at% Cr alloy were experimentally determined at various temperatures and compared to models based on different rate-limiting steps.</p>

Electron beam powder bed fusion (PBF-EB/M) has been attracting great research interest as a promising technology for additive manufacturing of titanium aluminide alloys. However, challenges often arise from the process-induced evaporation of aluminum, which is linked to the PBF-EB/M process parameters. This study applies different volumetric energy densities during PBF-EB/M processing to deliberately adjust the aluminum contents in additively manufactured Ti–43.5Al–4Nb–1Mo–0.1B (TNM-B1) samples. The specimens are subsequently subjected to hot isostatic pressing (HIP) and a two-step heat treatment. The influence of process parameter variation and heat treatments on microstructure and defect distribution are investigated using optical and scanning electron microscopy, as well as X-ray computed tomography (CT). Depending on the aluminum content, shifts in the phase transition temperatures can be identified via differential scanning calorimetry (DSC). It is confirmed that the microstructure after heat treatment is strongly linked to the PBF-EB/M parameters and the associated aluminum evaporation. The feasibility of producing locally adapted microstructures within one component through process parameter variation and subsequent heat treatment can be demonstrated. Thus, fully lamellar and nearly lamellar microstructures in two adjacent component areas can be adjusted, respectively.