Advances in the deposition of ceramics by soft chemistry process : example of rare- earth silicate coatings

The dip-coating process consists in immersing a sample to be coated in the liquid medium and then removing it at a controlled speed in order to obtain a film of regular thickness, as shown in Figure 1a). Dip-coating technique is now used in many industrial fields (biomedical, transportation, optics…). It is a very simple, and easy process to implement for the deposition and shaping of different natures of coatings (ceramic, metallic and polymer). In the case of ceramic coatings, after the dip-coating operation, the layers undergo a sintering post-treatment leading to the consolidation and/or the densification of the deposit. The corresponding mechanisms need a rigorous control of many parameters. The parameters involved in the dip-coating process are related to the medium and to the process. Concerning the medium, the dispersion medium nature, the particles concentration, viscosity, and stability are the main ones. The stability of the suspension is a first-order parameter and a preliminary formulation work has been carried out to cope with it. Moreover, parameters relative to the fabrication process such as the number of layers and the thermal profile (intermediary and final temperatures), will also be key factors to be taken into account in the formation of homogeneous and reproducible coatings by dip-coating.This work highlights the influence of these various parameters in the case of rare earth silicates based coatings. The various experiments were carried out in correlation to the coatings quality and microstructure. Homogeneous and conformal ceramic coatings of few tens of micrometers thick, as shown in Figure 1b), were obtained. A multi-layers deposit in a sol loaded at 40% mass generally allows to reach the desired thickness. With these experiments relationship between dip-coating parameters and coatings microstructure and morphology can be established. Please click Additional Files below to see the full abstract

Advances in the control of electrophoretic process parameters to tune the ytterbium disilicate coatings microstructure

Suspensions of ytterbium disilicate in isopropanol were prepared using iodine dispersant. Their zeta potential, electrical conductivity, and pH dependence with iodine concentration is detailed. Electrophoretic deposition was performed on silicon substrates at various voltages (100‐200 V) and times (until 10 minutes) and the growth dynamic was investigated. It was observed that the deposited mass reaches a maximum value for [I2] = 0.2 g/L, and the coating microstructure becomes porous at higher iodine concentrations. Current density and voltage measurements allowed to correlate this behavior to the increase of free protons concentration in the suspension. In these conditions, it was proved that porosity increases with the increase in applied voltage, and a compaction occurs as the deposition time increases. This has been related to the coating resistance increase and subsequent decrease in effective voltage in the suspension. The denser coatings (20% of porosity) were obtained in the case of suspension without iodine, at the minimum applied voltage and for the longest deposition times

Development of new Environmental Barrier Coatings (EBCs) by ElectroPhoretic Deposition (EPD)

Les composites à matrice céramique (CMC) en raison de leur stabilité à haute température et de leurs propriétés mécaniques sont des matériaux de choix pour remplacer les superalliages à base de nickel au niveau des aubes ou des anneaux de turbines. Ces pièces sont soumises à des sollicitations à très haute température, pouvant aller jusqu’à 1500°C en surface, qui induisent des endommagements significatifs tels que la corrosion du CMC. Pour limiter ces dégradations, des revêtements protégeant le substrat contre la corrosion doivent être envisagés. Dans le cadre de cette thèse, l’objectif est de mettre au point une EBC par électrophorèse à partir d’architectures en couches d’épaisseur contrôlée de disilicate d’ytterbium et de monosilicate d’yttrium, tout en limitant les étapes de traitement thermique et dont le procédé soit transposable sur des pièces de forme. L’électrophorèse (EPD) est une technique basée sur la migration, sous l'effet d'un champ électrique, d’espèces chargées, c'est-à-dire de particules dispersées dans un milieu liquide. Les mécanismes conduisant au dépôt électrophorétique sont complexes et un contrôle rigoureux de nombreux paramètres est nécessaire. Les paramètres impliqués dans l’EPD sont à la fois liés au milieu (conductivité électrique, viscosité, nature des espèces migrantes, charge de surface et stabilité) et au procédé (champ électrique appliqué, durée du dépôt et configuration de la cellule). Le premier axe de travail a été d’étudier l’influence de ces paramètres sur les revêtements et leurs microstructures. Le potentiel zêta, la mobilité électrophorétique et la conductivité électrique des suspensions étudiées se révèlent comme des paramètres de premier ordre, influençant à la fois la formation des revêtements : cinétique et mécanismes de croissance, mais également leurs microstructures : densité et homogénéité. L’utilisation de tensions faibles favorise la formation de revêtements plus compacts, et un effet de compaction est également observé au temps long de dépôt. La suite du travail de thèse s’est focalisée sur l’optimisation de l’architecture du système complet en ajustant à la fois les paramètres électriques, les conditions de frittage, mais aussi la composition des suspensions, en vue de caractériser les revêtements en conditions d’usage. Il a été montré qu’un empilement de 3 couches de disilicate d’ytterbium avec une étape de consolidation intermédiaire à 200°C entre les couches était nécessaire pour atteindre les 30m d’épaisseurs visés suivi d’un traitement thermique à 1350°C pendant 5h avec une rampe de montée en température de 300°C/h et de 100°C/h en descente. Ce type de système a conduit à des résultats encourageants en termes de comportement en corrosion avec seulement 3% de fissuration après 500h à basse température sous 50kPa d’eau. Enfin, l’architecture complète développée, en ajoutant la couche de monosilicate d’yttrium en surface, répond aux critères recherchés : l’EBC est adhérente, dense, homogène et couvrante.Ceramic Matrix Composites (CMCs) due to their high thermal stability and mechanical properties are the promising materials to replace nickel superalloys in turbine blades or rings. These parts are subjected to stresses at very high temperatures, up to 1500 °C on the surface, which induce significant damage, such as corrosion of the CMC. To limit this degradation, coatings protecting the substrate against corrosion have to be developed. In the framework of this thesis, the objective is to develop an EBC by electrophoresis allowing to deposit architectures of controlled thickness layers of yttrium disilicate and yttrium monosilicate, limiting the heat treatment steps and with a process able to be transferred on shaped parts. Electrophoresis (EPD) is a method based on the migration under the effect of an electric field of charged species, such as particles dispersed in a liquid, . The mechanisms leading to electrophoretic deposition are complex and a rigorous control of many parameters is necessary. The parameters involved in EPD are both linked to the medium (electrical conductivity, viscosity, nature of migrating species, surface charge and stability) and to the process (applied electric field, deposition duration and cell configuration). The first part of this work was to study the influence of these parameters on the coatings and their microstructures. The zeta potential, the electrophoretic mobility and the electrical conductivity of the suspensions studied appear to be first-order parameters, influencing both the formation of coatings: kinetics and growth mechanisms, but also their microstructures: density and homogeneity. The use of low voltages promotes the formation of more compact coatings, and a compaction effect is also observed over the long deposition times. The other part of the thesis work focused on optimizing the architecture of the complete system by adjusting both the electrical parameters, the sintering conditions, but also the composition of the suspensions, in order to characterize the coatings in working conditions. It has been shown that a stack of 3 layers of ytterbium disilicate with an intermediate consolidation step at 200 ° C between the layers was necessary to reach the target thickness of 30m, after a heat treatment at 1350°C for 5h with a temperature rise ramp of 300 ° C / h and 100° C/h down. This type of system has led to encouraging results in terms of corrosion behavior with only 3% cracking after 500h at 800 ° C under 50kPa of water. Finally, the complete architecture developed, by adding the layer of yttrium monosilicate on the surface, meets the required criteria: the EBC is adherent, dense, homogeneous and covering

Advances in the Electrophoretic Process to Control Oxide Coatings Microstructure for Sandrine Duluard

Please click Additional Files below to see the full abstract

Numerical microstructural optimization for the hydrogen electrode of solid oxide cells

International audienceA multiscale model has been used to optimize the microstructure of a classical hydrogen electrode made of nickel and yttria-stabilized zirconia (Ni-8YSZ). For this purpose, a 3D reconstruction of a reference electrode has been obtained by X-ray nano-holotomography. Then, a large dataset of synthetic microstructures has been generated around this reference with the truncated Gaussian random field method, varying the ratio Ni/8YSZ and the Ni particle size. All the synthetic microstructures have been introduced in a multiscale modeling approach to analyze the impact of the microstructure on the electrode and cell responses. The local electrode polarization resistance in the hydrogen electrode, as well as the complete cell impedance spectra, have been computed for the different microstructures. A significant performance improvement was found when decreasing the Ni particle size distribution. Moreover, an optimum has been identified in terms of electrode composition allowing the minimization of the cell polarization resistance. The same methodology has been also applied to assess the relevance of graded electrodes. All these results allow a better understanding of the precise role of microstructure on cell performances and provide useful guidance for cell manufacturing

Multiscale modelling of solid oxide cells validated on electrochemical impedance spectra and polarization curves

International audienceSolid oxide cells (SOCs) are high temperature energy-conversion devices, which have attracted a growing interest in the recent years. Indeed, this technology presents a high efficiency and a good reversibility in fuel cell (SOFC) and electrolysis (SOEC) modes. Nevertheless, SOCs durability is still insufficient due to performance degradation during operation. In this context, a physics-based model has been proposed to investigate the impact of operating conditions on the electrodes reaction mechanisms and cell performance. This multiscale model has been developed considering a typical cell composed of a dense electrolyte in Y$_{0.16}$ Zr$_{0.84}$ O$_{1.92}$ (8YSZ) sandwiched between an oxygen electrode in La$_{0.6}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3\delta}$-Ce$_{0.8}$Gd$_{0.2}$O$_{2\delta}$ (LSCF-GDC) and a hydrogen electrode made of Ni-YSZ. The model has been validated on global and local polarizations curves in SOFC and SOEC modes, and electrochemical impedance spectra at the open circuit voltage (OCV). The different contributions arising in the impedance spectra have been identified and discussed

Long-term tests and advanced post-test characterizations of the oxygen electrode in solid oxide electrolysis cells

International audienceSolid oxide electrolysis cells (SOEC) are a promising energy conversion technology for the production of green hydrogen via steam electrolysis. However, performance loss remains a bottleneck for large-scale commercialization. Here, we investigate the different types of degradation occurring in a state-of-the-art cell composed of La0.6Sr0.4Co0.2Fe0.8O3-Ce0.8Gd0.2O2 (LSCF-GDC) composite for the oxygen electrode, GDC for the barrier layer, Y0.16Zr0.84O1.92 (8YSZ) for the electrolyte and Ni-YSZ for the fuel electrode. Electrochemical impedance spectra measured before and after operation at 750 °C for 2000 hours at -1 A cm-2 in SOEC mode revealed no significant evolution for the oxygen electrode contribution. To investigate potential degradation of this electrode, we performed multi-modal chemical imaging based on synchrotron X-ray diffraction (µ-XRD) with a resolution at the micrometer-scale. In addition, we monitored the evolution of the chemical composition and chemical environment before and after ageing using laboratory X-ray photoelectron spectroscopy (XPS). All these analyses revealed a slight evolution of the crystal lattice parameter in the oxygen electrode and the inter-diffusion layer. It was also found that Sr segregation accelerates with ageing. However, more severe ageing conditions and/or longer ageing durations are required in order to observe an effect on the performance