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International audience; Silicate ceramics are of considerable promise as high frequency dielectrics in emerging millimetre wave applications including high bandwidth wireless communication and sensing. In this review, we show how high quality factors and low, thermally stable permittivities arise in ordered silicate structures. On the basis of a large number of existing studies, the dielectric performance of silicate ceramics is comprehensively summarised and presented, showing how microstructure and SiO4 tetrahedral connectivity affect polarizability and dielectric losses. We critically examine the appropriateness of silicate materials in future applications as effective millimetre wave dielectrics with low losses and tuneable permittivities. The development of new soft chemistry based processing routes for silicate dielectric ceramics is identified as being instrumental towards the reduction of processing temperatures, thus enabling silicate ceramics to be co-fired in the production of components functioning in the mm wave regime.

International audience; Microporous organosilicas assembled from polysilsesquioxane (POSS) building blocks are promising materials that are yet to be explored in-depth. Here, we investigate the processing and molecular structure of bispropylurea bridged POSS (POSS-urea), synthesised through the acidic condensation of 1,3-bis(3-(triethoxysilyl)propyl)urea (BTPU). Experimentally, we show that POSS-urea has excellent functionality for molecular recognition toward acetonitrile with an adsorption level of 74 mmol/g, which compares favourably to MOFs and zeolites, with applications in volatile organic compounds (VOC). The acetonitrile adsorption capacity was 132-fold higher relative to adsorption capacity for toluene, which shows the pores are highly selective towards acetonitrile adsorption due to their size and arrangement. Theoretically, our tight-binding density functional and molecular dynamics calculations demonstrated that this BTPU based POSS is microporous with an irregular placement of the pores. Structural studies confirm maximal pore sizes of ~ 1 nm, with POSS cages possessing an approximate edge length of ~ 3.16 Å.

The hydrophobicity of CeO2 surfaces is examined here. Since wettability measurements are extremely sensitive to experimental conditions, we propose a general approach to obtain contact angles between water and ceria surfaces of specified orientations based on density functional calculations. In particular, we analysed the low index surfaces of this oxide to establish their interactions with water. According to our calculations, the CeO2 (111) surface was the most hydrophobic with a contact angle of {\Theta} = 112.53{\deg} followed by (100) with {\Theta} = 93.91{\deg}. The CeO2 (110) surface was, on the other hand, mildly hydrophilic with {\Theta} = 64.09{\deg}. By combining our calculations with an atomistic thermodynamic approach, we found that the O terminated (100) surface was unstable unless fully covered by molecularly adsorbed water. We also identified a strong attractive interaction between the hydrogen atoms in water molecules and surface oxygen, which gives rise to the hydrophilic behaviour of (110) surfaces. Interestingly, the adsorption of water molecules on the lower-energy (111) surface stabilises oxygen vacancies, which are expected to enhance the catalytic activity of this plane. The findings here shed light on the origin of the intrinsic wettability of rare earth oxides in general and CeO2 surfaces in particular and also explain why CeO2 (100) surface properties are so critically dependant on applied synthesis methods.

International audience; Fluid-filled porous materials are widely encountered in natural and artificial systems. A comprehensive understanding of the elastic behavior of such materials and its dependence on fluid diffusion is therefore of fundamental importance. In this work, a multiscale framework is developed to model the overall elastic response of fluid-filled porous materials. By utilizing a two-dimensional micromechanical model with porosity at two scales, the effects of fluid diffusion and the geometric arrangement of pores on the evolution of effective properties in fluid-filled porous materials are investigated. Initially, for a single-porosity model the effective elastic properties of the dry and fluid-filled porous materials with ordered pores are obtained theoretically by considering a geometrical factor, which is related to the distribution of pores in the matrix. Model predictions are validated by finite element simulations. By employing a double-porosity model, fluid diffusion from macro- to micro-scale pores driven by a pressure gradient is investigated, and the resulting time-dependent effective elastic properties are obtained for both constant pressure and constant injection rate conditions. It is found that the presence and diffusion of pressurized pore fluid significantly affect the elastic response of porous materials, and this must be considered when modeling such materials. It is expected that the proposed theoretical model will advance the understanding of the fluid-governed elastic response of porous materials with implications towards the analysis of geophysical, biological and artificial fluid-filled porous systems.https://doi.org/10.1016/j.ijsolstr.2018.11.028; Les matériaux poreux remplis de fluide sont largement rencontrés dans les systèmes naturels et artificiels. Une compréhension globale du comportement élastique de tels matériaux et de sa dépendance à la diffusion de fluide revêt donc une importance fondamentale. Dans ce travail, un cadre multi-échelles est développé pour modéliser la réponse élastique globale des matériaux poreux remplis de fluide. En utilisant un modèle micromécanique bidimensionnel avec une porosité à deux échelles, on étudie les effets de la diffusion de fluide et la disposition géométrique des pores sur l'évolution des propriétés effectives dans les matériaux poreux remplis de fluide. Initialement, pour un modèle à une seule porosité, les propriétés élastiques effectives des matériaux poreux secs et remplis de fluide à pores ordonnés sont obtenues théoriquement en considérant un facteur géométrique, qui est lié à la distribution des pores dans la matrice. Les prévisions du modèle sont validées par des simulations par éléments finis. En utilisant un modèle à double porosité, on étudie la diffusion de fluide à partir de pores de macro à micro échelle, entraînés par un gradient de pression, et on obtient les propriétés élastiques effectives dépendantes du temps qui en résultent, à la fois pour des conditions de pression constante et de vitesse d'injection constante. On constate que la présence et la diffusion de fluide interstitiel sous pression affectent de manière significative la réponse élastique des matériaux poreux, ce qui doit être pris en compte lors de la modélisation de tels matériaux. Le modèle théorique proposé devrait permettre de mieux comprendre la réponse élastique des matériaux poreux régie par les fluides, avec des implications pour l'analyse de systèmes poreux remplis de fluides géophysiques, biologiques et artificiels.

International audience; In this study, high-resolution transmission electron microscopy (HRTEM) was employed to characterize the nanostructure of strontium-substituted hydroxyapatite (Sr-HA) and its evolution following in vitro immersion in physiological solutions. HRTEM images showed that the substitution of Sr induced local distortions in the hydroxyapatite (HA) lattice: minor levels of edge dislocations were detected at low doping contents of Sr ions (1 at %); when the Sr content exceeded 10 at%, the density of grain boundaries increased notably and triple junctions were clearly observed. The dissolution of undoped HA was initiated at crystallite surfaces, whereas the dissolution of Sr-HA started around grain boundaries. Acicular nanocrystal reprecipitation was observed on grain surfaces immersed in simulated body fluid (SBF), while not in dilute hydrochloric acid (HCl). These findings suggest appropriate levels of Sr incorporation can introduce imperfections in the crystal structure of apatite and thus enhance its dissolution rate towards enhanced physicochemical performance in biomedical applicationshttps://doi.org/10.1016/j.jeurceramsoc.2018.07.056; Dans cette étude, la microscopie électronique à transmission à haute résolution (HRTEM) a été utilisée pour caractériser la nanostructure de l'hydroxyapatite substituée au strontium (Sr-HA) et son évolution après immersion in vitro dans des solutions physiologiques. Les images HRTEM ont montré que la substitution des distorsions locales induites par le Sr dans le réseau d'hydroxyapatite (HA): des niveaux mineurs de dislocations de bords ont été détectés à de faibles teneurs en dopage d'ions Sr (1 at%); lorsque la teneur en Sr dépassait 10% at%, la densité des joints de grains augmentait de manière notable et des triple jonctions étaient clairement observées. La dissolution de l'HA non dopée a été initiée au niveau des surfaces de cristallites, alors que la dissolution de Sr-HA a commencé autour des joints de grains. Une reprécipitation aciculaire de nanocristaux a été observée sur des surfaces de grains immergées dans un fluide corporel simulé (SBF), mais non dans de l'acide chlorhydrique dilué (HCl). Ces découvertes suggèrent que des niveaux appropriés d’incorporation de Sr peuvent introduire des imperfections dans la structure cristalline de l’apatite et ainsi augmenter sa vitesse de dissolution afin d’améliorer les performances physicochimiques dans les applications biomédicales.https://doi.org/10.1016/j.jeurceramsoc.2018.07.056

International audience; Capillary penetration of liquids in porous media is of great importance in many applications and the ability to tune such penetration processes is increasingly sought after. In general, liquid penetration can be retarded or restricted by the evaporation of volatile liquid at the surface of the porous media. Moreover, when capillary penetration occurs in a porous layer with non-uniform cross section, the penetration process can be accelerated or impeded by adjusting the section geometry. In this work, on the basis of Darcy's Law and mass conservation, a theoretical model of capillary penetration combining evaporation effects in two-dimensional homogeneous porous media of varying cross-section is developed and further examined by numerical simulations. The effects of sample geometry and liquid evaporation on capillary penetration are quantitatively analyzed. Results show that the penetration velocity is sensitive to the geometry of the porous layer, and can be tuned by varying the evaporation rate for a given geometry. Under given evaporation conditions, penetration is restricted to a limited region with a predictable boundary. Furthermore, we find that the inhibition of liquid penetration by evaporation can be offset by varying the geometry of the porous layer. In addition, the theoretical model is further extended to model the capillary flow in three-dimensional porous media, and the interplay of geometry and evaporation during the capillary flow process in 3D conditions is also investigated. The results obtained can be used for facilitating the design of porous structures, achieving tunable capillary penetration for practical applications in various fields.; La pénétration capillaire de liquides dans des milieux poreux revêt une grande importance dans de nombreuses applications et la capacité de réglage de tels processus de pénétration est de plus en plus recherchée. En général, la pénétration de liquide peut être retardée ou limitée par l'évaporation de liquide volatil à la surface du support poreux. De plus, lorsque la pénétration capillaire se produit dans une couche poreuse de section transversale non uniforme, le processus de pénétration peut être accéléré ou empêché en ajustant la géométrie de la section. Dans ce travail, sur la base de la loi de Darcy et de la conservation de masse, un modèle théorique de pénétration capillaire combinant les effets d'évaporation dans des milieux poreux homogènes à deux dimensions de sections différentes est développé et examiné plus en détail par des simulations numériques. Les effets de la géométrie de l'échantillon et de l'évaporation du liquide sur la pénétration capillaire sont analysés de manière quantitative. Les résultats montrent que la vitesse de pénétration est sensible à la géométrie de la couche poreuse et peut être ajustée en faisant varier le taux d'évaporation pour une géométrie donnée. Dans des conditions d'évaporation données, la pénétration est limitée à une région limitée avec une limite prévisible. En outre, nous trouvons que l'inhibition de la pénétration du liquide par évaporation peut être compensée en faisant varier la géométrie de la couche poreuse. En outre, le modèle théorique est élargi pour modéliser l'écoulement capillaire dans des milieux poreux tridimensionnels, et les interactions entre la géométrie et l'évaporation pendant le processus d'écoulement capillaire dans des conditions 3D sont également étudiées. Les résultats obtenus peuvent être utilisés pour faciliter la conception de structures poreuses, en réalisant une pénétration capillaire ajustable pour des applications pratiques dans divers domaines.

International audience; The Pechini method was more effective in fabricating Ce/Cr codoped TiO2 nanoparticles of superior photocatalytic properties in comparison with the sol-gel method. The improved efficiency was attributed to: a) formation of mixed-phase anatase + rutile with greater homogeneity of dopant incorporation, b) development of open agglomerated morphologies with significantly higher surface areas, and c) favourable valence changes (increased Ti 3+ concentrations) owing to electron exchange in inter valence and/or multi valence charge transfer reactions. Chen, Wen-Fan, et al. "Enhancement of Ce/Cr Codopant Solubility and Abstract Ce/Cr codoped TiO2 nanoparticles were synthesized using sol-gel and Pechini methods with heat treatment at 400°C for 4 h. A conventional sol-gel process produced well crystallized anatase while Pechini synthesis yielded less ordered mixed-phase anatase + rutile; this suggests that the latter method enhances Ce solubility, increases chemical homogeneity, but destabilizes the TiO2 lattice. Greater structural disruption from the decomposition of the Pechini precursor formed more open agglomerated morphologies while the lower levels of structural disruption from pyrolysis of the dried sol-gel precursor resulted in denser agglomerates of lower surface areas. Codoping and associated destabilization of the lattice reduced the binding energies in both powders. Cr 4+ formation in sol-gel powders and Cr 6+ formation in Pechini powders suggest that these valence changes derive from synergistic electron exchange from intervalence and/or multivalence charge transfer. Since Ce is too large to allow either substitutional or interstitial solid solubility, the concept of integrated solubility is introduced, in which the Ti site and an adjacent interstice are occupied by the large Ce ion. The photocatalytic performance data show that codoping was detrimental owing to the effects of reduced crystallinity from lattice destabilization and surface area. Two regimes of mechanistic behavior are seen, which are attributed to the unsaturated solid solutions at lower codopant levels and supersaturated solid solutions at higher levels. The present work demonstrates that the Pechini method offers a processing technique that is superior to sol-gel because the former facilitates solid solubility and consequent chemical homogeneity.

International audience; The effects of roughness and fractality on the normal contact stiffness of rough surfaces were investigated by considering samples of isotropically roughened aluminium. Surface features of samples were altered by polishing and by five surface mechanical treatments using different sized particles. Surface topology was characterised by interferometry-based profilometry and electron microscopy. Subsequently, the normal contact stiffness was evaluated through flat-tipped diamond nanoindentation tests employing the partial unloading method to isolate elastic deformation. Three indenter tips of various sizes were utilised in order to gain results across a wide range of stress levels. We focus on establishing relationships between interfacial stiffness and roughness descriptors, combined with the effects of the fractal dimension of surfaces over various length scales. The experimental results show that the observed contact stiffness is a power-law function of the normal force with the exponent of this relationship closely correlated to surfaces’ values of fractal dimension, yielding corresponding correlation coefficients above 90 %. A relatively weak correlation coefficient of 60 % was found between the exponent and surfaces’ RMS roughness values. The RMS roughness mainly contributes to the magnitude of the contact stiffness, when surfaces have similar fractal structures at a given loading, with a correlation coefficient of −95 %. These findings from this work can be served as the experimental basis for modelling contact stiffness on various rough surfaces.

International audience; In a range of energy systems, interfacial characteristics at the finest length scales strongly impact overall system performance, including cycle life, electrical power loss, and storage capacity. In this letter, we experimentally investigate the influence of surface topology on interfacial electro-mechanical properties, including contact stiffness and electrical conductance at rough surfaces under varying compressive stresses. We consider different rough surfaces modified through polishing and/or sand blasting. The measured normal contact stiffness, obtained through nanoindentation employing a partial unloading method, is shown to exhibit power law scaling with normal pressure, with the exponent of this relationship closely correlated to the fractal dimension of the surfaces. The electrical contact resistance at interfaces, measured using a controlled current method, revealed that the measured resistance is affected by testing current, mechanical loading, and surface topology. At a constant applied current, the electrical resistance as a function of applied normal stress is found to follow a power law within a certain range, the exponent of which is closely linked to surface topology. The correlation between stress-dependent electrical contact and normal contact stiffness is discussed based on simple scaling arguments. This study provides a first-order investigation connecting interfacial mechanical and electrical behaviour, applicable to studies of multiple components in energy systems.

International audience; In order to investigate the brittle failure of keyhole notched components, the stress distribution at notch tips is studied numerically and theoretically. A semi-analytical formula is developed for the maximum notch-tip-stress, incorporating crack-tip-blunting, stress-concentration and stress-equilibrium. Stress distributions in notched plates are simulated by the finite-element method, showing improved accuracy of the formula relative to established solutions. Application of the developed equation to components containing U-notches and blunt V-notches, is explored, demonstrating its broad applicability. When combined with stress-based failure criteria, the semi-analytical model can be employed to assess brittle failure in notched components with significance toward fracture in heterogeneous materials.; Afin d’étudier la défaillance fragile des composants à encoche en trou de serrure, la répartition des contraintes aux extrémités des entailles est étudiée numériquement et théoriquement. Une formule semi-analytique est élaborée pour la contrainte maximale en pointe, intégrant l’atténuation des fissures, la concentration en contrainte et l’équilibre en contrainte. Les distributions de contraintes dans les plaques à encoches sont simulées par la méthode des éléments finis, ce qui montre une précision améliorée de la formule par rapport aux solutions établies. L’application de l’équation développée aux composants contenant des encoches en U et des encoches en V contondantes est explorée, démontrant ainsi sa large applicabilité. Lorsqu'il est combiné à des critères de rupture fondés sur des contraintes, le modèle semi-analytique peut être utilisé pour évaluer la défaillance fragile de composants entaillés présentant une importance significative pour la rupture dans des matériaux hétérogènes.

We present a newly developed approach for the calculation of interfacial stiffness and contact area evolution between two rough bodies exhibiting self affine surface structures. Using spline assisted discretization to define localised contact normals and surface curvatures we interpret the mechanics of simulated non-adhesive elastic surface-profiles subjected to normal loading by examining discrete contact points as projected Hertzian spheres. The analysis of rough-to-rough contact mechanics for surface profiles exhibiting fractal structures, with fractal dimensions in the regime 1 2, reveals the significant effect of surface fractality on contact mechanics and compliance with surfaces having the same mean roughness but higher fractality showing lower contact stiffness in conditions of initial contact for a given load. The predicted linear development of true contact area with load was found to be consistent with diverse existing numerical and experimental studies. Results from this model demonstrate the applicability of the developed method for the meaningful contact analysis of hierarchical structures with implications for modelling tribological interactions between pairs of rough surfaces

International audience; As candidate tritium breeder materials for use in the ITER helium cooled pebble bed, ceramic multiphasic compounds lying in the region of the quasi-binary lithium metatitanate-lithium orthosilicate system may exhibit mechanical and physical advantages relative to single phase materials. Here we present an organometallic solution-based synthesis procedure for the low-temperature fabrication of compounds in the Li 2 TiO 3-Li 4 SiO 4 region and investigate phase stability and transformations through temperature varied X-ray diffraction and scanning calorimetry. Results demonstrate that the metatitanate and metasilicate phases Li 2 TiO 3 and Li 2 SiO 3 readily crystallise in nanocrystalline form at temperatures below 180°C. Lithium deficiency in the region of 5% results from Li sublimation from Li 4 SiO 4 and/or from excess Li incorporation in the metatitanate phase and brings about a stoichiometry shift and product compounds with mixed lithium orthosilicate/ metasilicate content towards the Si rich region and predominantly Li 2 TiO 3 content towards the Ti rich region. Above 1150°C the transformation of monoclinic to cubic γ-Li 2 TiO 3 disordered solid-solution occurs while the melting of silicate phases indicates a likely monotectic type system with a solidus line in the region 1050°-1100°C. Synthesis procedures involving a lithium chloride precursor are not likely to be a viable option for breeder pebble synthesis as this route was found to yield materials with a more significant Li-deficiency exhibiting the crystallisation of the Li 2 TiSiO 5 phase at intermediate compositions.https://doi.org/10.1016/j.jnucmat.2014.09.028; En tant que matériaux sélectionneurs de tritium candidats à utiliser dans le lit de galets refroidi à l'hélium ITER, les composés céramiques multiphasiques situés dans la région du système métasitanate de lithium-orthosilicate de lithium quasi-binaire peuvent présenter des avantages mécaniques et physiques par rapport aux matériaux monophasés. Nous présentons ici une procédure de synthèse à base de solutions organométalliques pour la fabrication à basse température de composés de la région Li 2 TiO 3-Li 4 SiO 4 et étudions la stabilité de phase et les transformations par diffraction des rayons X à température variée et calorimétrie à balayage. Les résultats démontrent que les phases de métatitanate et de métasilicate, Li 2 TiO 3 et Li 2 SiO 3, cristallisent facilement sous forme nanocristalline à des températures inférieures à 180 ° C. Une déficience en lithium dans la région de 5% résulte de la sublimation de Li de Li 4 SiO 4 et / ou de l'incorporation excessive de Li dans la phase de métatitanate et entraîne un déplacement de la stoechiométrie et des composés du produit avec une teneur mixte en orthosilicate de lithium / métasilicate vers la région riche en Si teneur en Li 2 TiO 3 principalement vers la région riche en Ti. Au-dessus de 1150 ° C, la transformation de la solution solide désordonnée en γ-Li 2 TiO 3 monoclinique en cubique se produit, tandis que la fusion des phases silicatées indique un système de type monotectique probable avec une ligne solide située dans la région allant de 1050 à 1100 ° C. Les procédures de synthèse faisant appel à un précurseur de chlorure de lithium ne constitueront probablement pas une option viable pour la synthèse de galets reproducteurs, car il a été constaté que cette voie conduisait à des matériaux présentant un déficit en Li plus important, présentant la cristallisation de la phase Li 2 TiSiO 5 dans des compositions intermédiaireshttps://doi.org/10.1016/j.jnucmat.2014.09.028