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In France, High-Level radioactive Wastes (HLW) coming from nuclear spent fuel treatment are confined at the atomic scale in an homogeneous sodium-borosilicate glass called the R7T7 glass. At different stages of the melting process performed at high temperature, crystallized phases which incorporate the radionuclides are temporarily formed before their dissolution. The formation conditions of these crystals have already been studied but, to date, few studies on their dissolution in nuclear glass melt have been performed. The objective of this thesis is the description of the kinetics and mechanisms of the crystals dissolution in the borosilicate melt.This study focuses on the dissolution of rare earth silicates with an apatite structure (Ca2Nd8(SiO4)6O2 type) in three borosilicate melt compositions. Experiments are conducted in isothermal mode and in closed system. The crystalline fractions are followed according to time thanks to the acquisition of SEM images mosaics coupled with image analysis. In addition, chemical profiles at the crystal/melt interfaces are acquired by microprobe.The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, usually applied to describe the crystallization in melt, is successfully applied to the crystalline fractions measured during the dissolution process. The application of this model for different temperatures of thermal treatment validates its effectiveness to describe the dissolution kinetics of rare earth silicates in borosilicate systems in static mode. Its application is furthermore extended to the crystals dissolution in more complex systems where different types of crystals are simultaneously present. This model allows to determine the mechanism limiting the crystals dissolution in borosilicate melts, i.e. the diffusion in the studied systems, the activation energy of the dissolution as well as the constants of dissolution and the characteristic durations of dissolution for each considered temperature., En France, les déchets de Haute Activité à Vie Longue (HAVL) issus du retraitement du combustible nucléaire usé sont confinés à l’échelle atomique dans un verre borosilicaté appelé le verre R7T7. Lors de son élaboration à haute température, des cristaux incorporant les radionucléides se forment de manière transitoire sous différentes conditions dans le liquide avant d’être dissouts. Jusqu’à présent, de nombreuses études se sont portées sur la cristallisation de ces phases mais leur dissolution n’a pas fait l’objet d’étude approfondie. L’objectif de cette thèse est de décrire les cinétiques et les mécanismes de la dissolution des cristaux dans le liquide borosilicaté.Dans ce but, la dissolution de silicates de terres rares de structure apatite, type Ca2TR8(SiO4)6O2, est étudiée en conditions isothermes dans trois compositions borosilicatées différentes en système fermé. Afin de suivre l’évolution des fractions cristallines au cours du temps, une méthodologie basée sur l’acquisition de mosaïques d’images MEB associée à du traitement d’images a été optimisée. En compléments, des profils chimiques aux interfaces cristaux/verre sont également réalisés par microsonde électronique.Le modèle Johnson-Mehl-Avrami-Kolmogorov (JMAK), habituellement employé pour décrire la cristallisation dans les verres, est appliqué avec succès aux fractions cristallines mesurées au cours de la dissolution. L’application de ce modèle pour différentes températures de traitement thermique valide son efficacité à décrire les cinétiques de dissolution des silicates de terres rares dans des systèmes borosilicatés en régime statique. Son application est également étendue à des systèmes plus complexes présentant simultanément plusieurs phases cristallines de nature autre que les silicates de terres rares. De plus, ce modèle permet d’accéder au mécanisme contrôlant la dissolution des cristaux dans les liquides borosilicatés (dans les systèmes étudiés, la diffusion), à l’énergie d’activation de leur dissolution ainsi qu’aux constantes de dissolution et aux durées caractéristiques de la dissolution pour chaque température.

This work describes a new approach to help understand the chemical durability of partially crystallized nuclear waste conditioning matrices. Among the studies carried out on nuclear waste deep geological disposal, long-term behavior studies have so far been conducted on homogeneous glassy matrices. However, as the crystalline phases may generate modifications in the chemical composition and properties of such matrices, the description and a better understanding of their effects on the chemical durability of waste packages are of primary importance.A protocol to study the durability of heterogeneous model matrices of nuclear interest containing different types of crystalline phases was developed. It is based on a detailed description of the morphology, microstructure and structure of the glassy matrix and crystalline phases, and on the study of various alteration regimes. Three crystal phases that may form when higher concentrations of waste are immobilized in Uranium Oxide type conditioning glasses were studied: alkali and alkaline earth molybdates, rare earth silicates and ruthenium oxide.The results highlight the roles of the composition and the structure of the surrounding glassy matrix as the parameters piloting the alteration kinetics of the partially crystallized glassy matrices. This behavior is identical whatever the nature of the crystalline phases, as long as these phases do not lead to a composition gradient and do not percolate within the glassy matrix. Given these results, a methodology to study partially crystallized matrices with no composition gradient is then suggested. Its key development lies firstly in the evaluation of the behavior of partially crystallized matrices through the experimental study of the residual glassy matrix in various alteration regimes. This methodology may be adapted to the case of new glass formulations with more complex compositions (e.g. highly waste-loaded glass), which may contain crystals formed during cooling., Ces travaux proposent une nouvelle approche pour appréhender la durabilité chimique de matrices de conditionnement de déchets radioactifs partiellement cristallisées. Dans le cadre des études menées sur le stockage en formation géologique profonde, les études de comportement à long terme se sont jusqu’alors focalisées sur des matrices vitreuses homogènes. Cependant, les phases cristallines peuvent être à l’origine de modifications de composition chimique des matrices vitreuses et de leurs propriétés : la description et la compréhension de leur influence sur la durabilité chimique sont alors de première importance. Un protocole d’étude de la durabilité de matrices hétérogènes modèles d’intérêt nucléaire contenant des phases cristallines de différentes natures a donc été développé. Il s’appuie sur une description détaillée de la morphologie, microstructure et structure des matrices vitreuses et des phases cristallines ainsi que sur l’étude de différents régimes d’altération. Trois phases cristallines susceptibles de se former lors d’une augmentation du taux d’incorporation en déchet au sein des verres de conditionnement de type Uranium Oxyde ont été étudiées : les molybdates d’alcalins et d’alcalino-terreux, les silicates de terres rares et l’oxyde de ruthénium.Les résultats acquis ont mis en évidence que la composition et la structure du verre englobant sont les paramètres qui pilotent les cinétiques d’altération des matrices vitreuses partiellement cristallisées. Ce comportement est identique quelle que soit la nature des phases cristallines tant qu’elles n’induisent pas de gradient de composition et qu’elles ne percolent pas au sein de la matrice vitreuse. Fort de ces enseignements, une méthodologie d’étude de matrices partiellement cristallisées sans gradient de composition est proposée. Son développement clé réside dans l’évaluation en première approche du comportement des matrices partiellement cristallisées au travers de l’étude expérimentale de la matrice vitreuse englobante dans les différents régimes d’altération. Cette méthodologie pourrait être adaptée aux cas de nouvelles formulations de verres de compositions plus complexes (p. ex. verre à haut taux d’incorporation) pouvant contenir des cristaux formés au cours du refroidissement.

Environmental Barrier Coatings (EBCs) are used as protective coatings for silicon carbide components in gas turbine engines to inhibit surface reactions with water vapor released by hydrocarbon combustion. In this high temperature environment, oxidizing species such as H2O and O2 react with SiC to form gaseous CO and a solid SiO2 layer on the SiC surface. However, subsequent reaction of the normally protective silica layer with water vapor leads to the formation of gaseous Si(OH)4, and volatilization of the ceramic at a rate that increases with temperature, water vapor pressure and flow rate over the surface. All three of these parameters are high in modern gas turbine engines, leading to severe volatility issues for SiC-based ceramics. The primary objective of an environmental barrier coating (EBC) then is to impede access of oxidizing species to the underlying component while maintaining adherence over long durations (in the range of 5,000 hours). The target use temperature of current EBCs is 1316 °C (2400 °F) with repeated cycling between this and the ambient temperature. While several EBC concepts have been proposed, there has been no fundamental study of their response to thermocyclic loading in water vapor environments, and little is known about the mechanisms that govern their performance and durability. Materials property data is also unavailable for many of the candidate materials in their bulk (fully dense) and coating structure forms, and little is known about the relationships between coating microstructure and the way in which the coating is applied. One promising coating system with very low steam volatility is the ytterbium monosilicate/mullite/silicon (Yb2SiO5/Al6Si2O13/Si) tri-layer EBC. EBCs are generally applied by an air plasma spray (APS) deposition process in which particles of the coating material are melted in an atmospheric pressure, Ar-based plasma jet, and the molten droplets sprayed onto the component surface. In collaboration with the NASA Glenn Research Center, this coating system was deposited on α-SiC substrates using a high power APS approach, and the relation between the coating structure and the process environment characterized. Some of the coatings delaminated from the substrate upon cooling, which was indicative of significant variation in the adherence between the SiC substrate and Si layer. The EBC coated α-SiC substrates that did not delaminate were then tested in a steam-cycling furnace designed to reproduce some aspects of the thermocyclic and environmental conditions found during lean combustion engine operation. The thermal cycles consisted of a 60 min 1316 °C hold time in a flowing 90 % H2O and 10 % O2 environment followed by cooling to 110 °C with a hold for 10 min. All of the coatings delaminated from their substrate with coating lifetimes ranging from less than one hundred to several hundred cycles. The primary factor contributing to rapid steam cycling failure of the coatings was rapid penetration of oxidizing species through channel (mud) cracks in the Yb2SiO5 and Al6Si2O13 layers that terminated at the Si bond coat. An analysis of the residual stresses resulting from differences in the coefficients of thermal expansion (CTE) of the three materials used in the coating and that of the SiC substrate indicated that mud cracks were formed as a result of large in-plane biaxial tensile stress in the ytterbium monosilicate and (to a lesser extent) mullite layers. The mud cracks provided a fast transport path to the silicon layer that resulted in rapid formation of a thermally grown oxide (TGO) identified to be the cristobalite phase of SiO2. The TGO layer was then found to undergo severe microfracture as it underwent a β (high) → α (low) cristobalite phase transformation on cooling through ~220 °C that was accompanied by a 4.5 vol% contraction upon cooling. The repetition of this phase transformation with cracking led to rapid oxidation of the silicon layer and failure of the system by spallation of the Yb2SiO5 and Al6Si2O13 layers above it. An improved APS system was designed and installed at the University to enable the deposition process to be studied carefully, and an optimized coating process to be developed. In this revised process, the Si layer was deposited onto the SiC at high temperature (1200 °C) under a reducing environment to improve its adherence with the substrate and ensure the layer had a high relative density (only a few isolated pores). This eliminated premature delamination issues experienced during cooling for some of the high power deposited coatings. The effect of spray parameter selection upon the microstructure of Yb2SiO5 and its disilicate (Yb2Si2O7) counterpart (which has a CTE more closely matched to the substrate) was investigated. Secondary phases were identified and shown to result from SiO evaporation from the powder particles during plasma heating. Their volume fractions, as well as other microstructural and defect features of the layers were all investigated. Though mud cracking was again observed in Yb2SiO5 layers, no such cracking was identified in Yb2Si2O7 layers. This difference in cracking behavior was consistent with differences in the residual stress developed during cooling of the two ytterbium silicates. The (optimized) low power Yb2SiO5/Al6Si2O13/Si EBCs were again tested in steam-cycling, but no statistically significant improvement in coating life was observed. The improved Si layer adherence and microstructure eliminated the primary delamination mode seen in the high power deposited coatings, but a new failure mechanism was observed in the optimized ytterbium monosilicate protected system. As opposed to mud crack termination at the Si bond coat, mud cracks in the low power tri-layer system bifurcated either within the Al6Si2O13 layer or at one of its interfaces. The bifurcated cracks continued to propagate towards the Si bond coat where the crack ligaments turned and propagated as horizontal delamination cracks through the mid-plane of the Si layer. Rapid oxidizing species access to the interior of the EBC still occurred through these mud cracks, but instead of oxidizing the Al6Si2O13 – Si interface, the faces of the delamination cracks within the Si layer were oxidized to form cristobalite. These cracks advanced sequentially during steam cycling until spallation occurred. In collaborations with researchers at UCSB, the thermomechanical competition between the single channel and bifurcated crack damage modes was investigated using Finite Element Analysis (FEA) combined with J-integral methods to calculate the stored elastic strain energy release rate (ERR) during fracture by the two crack propagation paths. Simpler bi-layer Yb2Si2O7/Si EBCs were also deposited on the same SiC substrates using the optimized, low power APS process. The ytterbium disilicate layer in these coatings contained about 15 vol% Yb2SiO5 but did not mud crack upon cooling. Neither spallation failure of the coatings nor cracking of the ytterbium silicate layer was observed during steam furnace testing for up to 2,000 cycles (2,000 hours at 1316 °C). A cristobalite TGO layer was found to form slowly at the Yb2Si2O7 – Si interface, and the thickness of this layer was measured by sectioning samples extracted from the furnace after 250, 500, 750, 1,000, and 2,000 steam cycles. The maximum TGO thickness reached only ~2.5 μm after 2,000 hours of exposure, even though it had begun to microcrack and therefore had lost its protective properties. The TGO layer thickness exhibited a linear dependence upon high temperature exposure time, consistent with growth being limited by diffusion of the oxidizing species through the ytterbium disilicate layer. The linear oxidation rate constant was used to calculate a monatomic oxygen flux and effective oxygen diffusion coefficient through the Yb2Si2O7 layer. Some volatilization of Si from the surface of the Yb2Si2O7 layer was also observed, and the coating edges suffered from preferential attack. However, the steam-cycling performance of this coating was deemed sufficient to merit further investigations of the system for stressed (rotating) environments. Large stand-alone plates of Yb2Si2O7 and Si were deposited by APS. These plates were precision ground into mechanical test specimens through collaboration with technical staff at the NASA Glenn Research Center. The specimens were used to measure the elastic modulus, fracture toughness, flexure strength at low and high temperatures, and obtain estimates of the materials’ creep resistance. The mechanical properties of the ytterbium disilicate material were significantly lower than values for fully dense materials. Reductions of 2-5x were observed across all quasi-static properties. The creep rates of APS Yb2Si2O7 were found to be very rapid (1 % creep strain accumulated in only 25 h at 900 °C and 16MPa). The activation energy for creep in the 10 % porous APS material was in the 130-150 kJ/mol range. The creep rate of the APS Si samples was also high, but in this case similar to bulk polycrystalline silicon. The mechanical properties of the APS deposited ytterbium disilicate/Si system appear insufficient for future application on rotating components. The implications of the various failure mechanisms observed and the material properties measured and calculated for these EBC systems are discussed. The materials selection, processing, and performance relationships are interpreted in the context of further EBC development, and several suggestions are proposed for future work to address the technical challenges of this evolving field.

Polar crystals provide an interesting avenue for materials research both in the structures they form and the properties they possess. This work describes the hydrothermal synthesis and structural characterization of three novel silicate fluorides. Compound (1), LiY 3(SiO 4) 2F 2 crystallizes in space group C2/c, with a=17.651(4) Å, b=4.8868(10) Å, c=11.625(2) Å and β=131.13(3)°. BaY 2(Si 2O 7)F 2 (2) crystallizes in space group P-1, with a=5.1576(10) Å, b=6.8389(14) Å, c=11.786(2) Å, α=93.02(3)°, β=102.05(3)° and γ=111.55(3) °. Finally, the structure of Ba 2Y 3(SiO 4) 2F 5 (3) was determined in the polar orthorhombic space group Pba2, having unit cell parameters a=8.8864(18) Å, b=12.764(3) Å and c=5.0843(10) Å. The structures are compared based on their building blocks and long range polarities. Aligned silicate tetrahedra segregated into a single layer in (3) impart the observed polarity., The National Science Foundation (DMR-0907395)

The ceramic matrix composites can be used in aeronautic engines due to their high temperature stability and their mechanical properties. However, under a corrosive environment, an oxidation and then a recession of the CMC occured because of the volatilization of the silica scale formed at the surface of the composite. Consequently, in order to increase the lifetime of such materials, a external protection against corrosion is required. This is the aim of my Ph-D thesis. The experimental approach is the following : (i) identification of the coating materials ; (ii) validation of the selected materials by studying their structural stability and their chemical and thermomechanical compatibilities with the substrate ; (iii) determination of the thermal stability of the materials under a corrosive environment and (iv) behaviour of the coatings onto the CMC., Les composites à matrice céramique sont utilisés dans les moteurs aéronautiques en raison de leur stabilité à haute température et de leurs propriétés mécaniques. Cependant, quand ils sont soumis à des environnements sévères (haute température, haute pression, environnement oxydant et humide), ils s'oxydent et se dégradent dû à la volatilisation de la silice protectrice formée en surface par oxydation du CMC. Par conséquent, pour augmenter la durée de vie de ces matériaux, il est nécessaire d'appliquer une protection externe contre la corrosion. Ceci constitue l'objectif de ma thèse. La démarche expérimentale a été la suivante : (i) identification des matériaux de revêtement à étudier ; (ii) validation du choix des matériaux par étude de leur stabilité structurale et de leurs compatibilités chimique et thermomécanique avec le substrat ; (iii) étude de la stabilité des matériaux de revêtement sous atmosphère corrosive et enfin (iv) comportement des revêtements sur composites.