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Finding effective cathode materials is currently one of the key barriers to the development of magnesium batteries, which offer enticing prospects of larger capacities alongside improved safety relative to Li-ion batteries. Here, we report the hydrothermal synthesis of several types of WS2 nanostructures and their performance as magnesium battery cathodes. The morphology of WS2 materials was controlled through the use of sodium oxalate as a complexing agent and different templating agents, including polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and hexadecyltrimethyl ammonium bromide (CTAB). A high capacity of 142.7 mAh/g was achieved after 100 cycles at a high current density of 500 mA/g for cathodes based on a nanostructured flower-like WS2. A solution consisting of magnesium (II) bis(trifluoromethanesulfonyl)imide (MgTFSI2) and magnesium (II) chloride (MgCl2) in dimethoxyethane (DME) was used as an effective electrolyte, which contributes to favorable Mg2+ mobility. Weaker ionic bonds and van der Waals forces of WS2 compared with other transition metal oxides/sulfides lay the foundation for fast discharge/charge rate. The enhanced surface area of the nanostructured materials plays a key role in enhancing both the capacity and discharge/charge rate.

Through comprehensive density functional calculations, we predict the stability of a rhenium-based ferrite, ReFe2O4, in a distorted spinel-based structure. In ReFe2O4, all Re and half of the Fe ions occupy the octahedral sites while the remaining Fe ions occupy the tetrahedral sites. All Re ions are predicted to be at a + 4 oxidation state with a low spin configuration (S = 3/2), while all Fe ions are predicted to be at a + 2 oxidation state with a high-spin state configuration (S = 2). Magnetically, ReFe2O4adopts an unconventional ferrimagnetic state in which the magnetic moment of Re opposes the magnetic moments of both tetrahedral and octahedral Fe ions. The spin–orbit coupling is found to cause a slight spin canting of ~ 1.5°. The predicted magnetic ground state is unlike the magnetic alignment usually observed in ferrites, where the tetrahedral cations oppose the spin of the octahedral cations. Given that the density of states analysis predicts a half-metallic character driven by the presence of Ret2gstates at the Fermi level, this compound shows promise towards potential spintronics applications.

Hypothesis Control of capillary flow through porous media has broad practical implications. However, achieving accurate and reliable control of such processes by tuning the pore size or by modification of interface wettability remains challenging. Here we propose that the flow of liquid by capillary penetration can be accurately adjusted by tuning the geometry of porous media and develop numerical method to achieve this. Methodologies On the basis of Darcys law, a general framework is proposed to facilitate the control of capillary flow in porous systems by tailoring the geometric shape of porous structures. A numerical simulation approach based on finite element method is also employed to validate the theoretical prediction. Findings A basic capillary component with a tunable velocity gradient is designed according to the proposed framework. By using the basic component, two functional capillary elements, namely, (i) flow amplifier and (ii) flow resistor, are demonstrated. Then, multi functional fluidic devices with controllable capillary flow are realized by integrating the designed capillary elements. All the theoretical designs are validated by numerical simulations. Finally, it is shown that the proposed model can be extended to three dimensional designs of porous media

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.

International audience; The water-gas shift reaction (WGS) is important and widely applied in the production of H2. Cu-modified perovskites are promising catalysts for WGS reactions in hydrogen generation. However, the structure-dependent stability and reaction pathways of such materials remain unclear. Herein, we report catalytically active Cu-modified SrTiO3 (nominally SrTi1−xCuxO3) prepared by a modified polymeric precursor method. Microstructural analysis revealed a partially segregated CuO phase in the as-prepared materials. Operando X-ray diffraction and absorption spectroscopy showed the reduction of CuO into a stable metallic phase under conditions of WGS reactions for all compositions. Among the characterized materials, the x = 0.20 composition showed the highest turnover frequency, lowest activation energy, and the highest WGS rate at 300 °C. According to density functional calculations, the formation of CuO is energetically less favorable compared with, SrTiO3, explaining why the segregated CuO phase on the SrTiO3 surface is reduced to Cu during the catalytic reaction, while SrTiO3 remains. For x = 0.20, the size of the segregated CuO phase is optimum for facilitating the catalytic reaction. In contrast, a higher Cu content (x = 0.3) results in an aggregation of smaller CuO particles, resulting in fewer surface active sites and a net decrease in catalytic performance.

International audience; Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of well‐controlled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freeze‐casting processes are described and the relationships between processing and structure are examined. Recent progress in freeze‐casting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored micro‐ and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freeze‐cast geometries—beads, fibers, films, complex macrostructures, and nacre‐mimetic composites—are presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freeze‐casting techniques and low‐dimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.; (traduit par un robot)La coulée sous pression, également connue sous le nom de modèle de glace, est une technique particulièrement polyvalente qui a été largement appliquée pour la fabrication de matériaux poreux biomimétiques bien contrôlés à base de céramique, de métaux, de polymères, de biomacromolécules et de nanomatériaux de carbone, leur conférant de nouvelles propriétés et leur élargissement. leur applicabilité. Les principes des différents processus de coulée sous pression directionnelle sont décrits et les relations entre le traitement et la structure sont examinées. Les progrès récents dans l'assemblage assisté par coulée sous pression de blocs de construction de faible dimension, y compris le graphène et les nanotubes de carbone, en micro et macrostructures sur mesure sont ensuite résumés. Les tendances émergentes concernant les nouveaux matériaux comme les blocs de construction et les nouvelles géométries coulées sous pression - perles, fibres, films, macrostructures complexes et composites mimétiques de nacre - sont présentées. Par la suite, les moyens par lesquels les structures poreuses alignées et les matériaux mimétiques de nacre pouvant être obtenus par le biais de techniques de coulée sous pression et de blocs de construction de faible dimension récemment développés peuvent faciliter la fonctionnalité des matériaux dans de multiples domaines d'application, y compris le stockage et la conversion d'énergie, l'assainissement de l'environnement, la gestion thermique et les matériaux intelligents sont discutés.

Hydroxyapatite (HA) synthesized by a wet chemical route was subjected to heavy ion irradiation, using 4 MeV Krypton ion (Kr17+) with ion fluence ranging from 1 × 1013 to 1 × 1015 ions/cm2. Glancing incidence X-ray diffraction (GIXRD) results confirmed the phase purity of irradiated HA with a moderate contraction in lattice parameters, and further indicated the irradiation-induced structural disorder, evidenced by broadening of the diffraction peaks. High-resolution transmission electron microscopy (HRTEM) observations indicated that the applied Kr irradiation induced significant damage in the hydroxyapatite lattice. Specifically, cavities were observed with their diameter and density varying with the irradiation fluences, while a radiation-induced crystalline-to-amorphous transition with increasing ion dose was identified. Raman and X-ray photoelectron spectroscopy (XPS) analysis further indicated the presence of irradiation-induced defects. Ion release from pristine and irradiated materials following immersion in Tris (pH 7.4, 37 ℃) buffer showed that dissolution in vitro was enhanced by irradiation, reaching a peak at 0.1dpa. We examined the effects of irradiation on the early stages of the mouse osteoblast-like cells (MC3T3-E) response. A cell counting kit-8 assay (CCK-8 test) was carried out to investigate the cytotoxicity of samples, and viable cells can be observed on the irradiated materials.

In water treatment applications, the use of ceramic membranes is associated with numerous advantages relative to polymer-based filtration systems. High-temperature stability, fouling resistance, and low maintenance requirements contribute to lower lifecycle costs in such systems. However, the high production costs of most commercially available ceramic membranes, stemming from raw materials and processing, are uneconomical for such systems in most water treatment applications. For this reason, there is a growing demand for new ceramic membranes based on low-cost raw materials and processes. The use of unrefined mineral feedstocks, clays, cement, sands, and ash as the basis for the fabrication of ceramic membranes offers a promising pathway towards the obtainment of effective filtration systems that can be economically implemented in large volumes. The design of effective ceramic filtration membranes based on low-cost raw materials and energy-efficient processes requires a balance of pore structure, mass flow, and robustness, all of which are highly dependent on the composition of materials used, the inclusion of various pore-forming and binding additives, and the thermal treatments to which membranes are subjected. In this review, we present recent developments in materials and processes for the fabrication of low-cost membranes from unrefined raw materials, including clays, zeolites, apatite, waste products, including fly ash and rice husk ash, and cement. We examine multiple aspects of materials design and address the challenges relating to their further development.

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

Structural characteristics are considered to be the dominant factors in determining the effective properties of granular media, particularly in the scope of transport phenomena. Towards improved heat management, thermal transport in granular media requires an improved fundamental understanding. In this study, the effects of packing structure on heat transfer in granular media are evaluated at macro- and grain-scales. At the grain-scale, a gas-solid coupling heat transfer model is adapted into a discrete-element-method to simulate this transport phenomenon. The numerical framework is validated by experimental data obtained using a plane source technique, and the Smoluschowski effect of the gas phase is found to be captured by this extension. By considering packings of spherical SiO2 grains with an interstitial helium phase, vibration induced ordering in granular media is studied, using the simulation methods developed here, to investigate how disorder-to-order transitions of packing structure enhance effective thermal conductivity. Grain-scale thermal transport is shown to be influenced by the local neighbourhood configuration of individual grains. The formation of an ordered packing structure enhances both global and local thermal transport. This study provides a structure approach to explain transport phenomena, which can be applied in properties modification for granular media., 11 figures, 29 pages

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; Granular crystallisation is an important phenomenon whereby ordered packing structures form in granular matter under vibration. However, compared with the well-developed principles of crystallisation at the atomic scale, crystallisation in granular matter remains relatively poorly understood. To investigate this behaviour further and bridge the fields of granular matter and materials science, we simulated mono-disperse spheres confined in cylindrical containers to study their structural dynamics during vibration. By applying adequate vibration, disorder-to-order transitions were induced. Such transitions were characterised at the particle scale through bond orientation order parameters. As a result, emergent crystallisation was indicated by the enhancement of the local order of individual particles and the number of ordered particles. The observed heterogeneous crystallisation was characterised by the evolution of the spatial distributions via coarse-graining the order index. Crystalline regimes epitaxially grew from templates formed near the container walls during vibration, here termed the wall effect. By varying the geometrical dimensions of cylindrical containers, the obtained crystallised structures were found to differ at the cylindrical wall zone and the planar bottom wall zone. The formed packing structures were quantitatively compared to X-ray tomography results using again these order parameters. The findings here provide a microscopic perspective for developing laws governing structural dynamics in granular matter.https://doi.org/10.1007/s10035-019-0876-8; La cristallisation granulaire est un phénomène important dans lequel des structures de garnissage ordonnées se forment dans une matière granulaire sous vibration. Cependant, comparée aux principes bien développés de la cristallisation à l'échelle atomique, la cristallisation dans la matière granulaire reste relativement mal comprise. Pour étudier plus avant ce comportement et relier les domaines de la matière granulaire et de la science des matériaux, nous avons simulé des sphères mono-dispersées confinées dans des conteneurs cylindriques afin d'étudier leur dynamique structurelle lors de la vibration. En appliquant une vibration adéquate, des transitions désordre à ordre ont été induites. Ces transitions ont été caractérisées à l'échelle des particules par le biais de paramètres d'ordre d'orientation de liaison. En conséquence, la cristallisation émergente était indiquée par l'amélioration de l'ordre local des particules individuelles et du nombre de particules ordonnées. La cristallisation hétérogène observée a été caractérisée par l'évolution des distributions spatiales via la granulation grossière de l'indice d'ordre. Les régimes cristallins se développaient par épitaxie à partir de gabarits formés près des parois du conteneur lors de vibrations, appelé ici effet de paroi. En faisant varier les dimensions géométriques des récipients cylindriques, il s'est avéré que les structures cristallisées obtenues différaient au niveau de la zone de paroi cylindrique et de la zone de paroi de fond plane. Les structures de garnissage formées ont été comparées quantitativement aux résultats de tomographie à rayons X en utilisant à nouveau ces paramètres d'ordre. Les résultats présentés ici fournissent une perspective microscopique pour l'élaboration de lois régissant la dynamique structurelle dans la matière granulaire.https://doi.org/10.1007/s10035-019-0876-8

The crumpling of precursor materials to form dense three dimensional geometries offers an attractive route towards the utilisation of minor-value waste materials. Crumple-forming results in a mesostructured system in which mechanical properties of the material are governed by complex cross-scale deformation mechanisms. Here we investigate the physical and mechanical properties of dense compacted structures fabricated by the confined uniaxial compression of a cellulose tissue to yield crumpled mesostructuring. A total of 25 specimens of various densities were tested under compression. Crumple formed specimens exhibited densities in the range 0.8–1.3 g cm−3, and showed high strength to weight characteristics, achieving ultimate compressive strength values of up to 200 MPa under both quasi-static and high strain rate loading conditions and deformation energy that compares well to engineering materials of similar density. The materials fabricated in this work and their mechanical attributes demonstrate the potential of crumple-forming approaches in the fabrication of novel energy-absorbing materials from low-cost precursors such as recycled paper. Stiffness and toughness of the materials exhibit density dependence suggesting this forming technique further allows controllable impact energy dissipation rates in dynamic applications.

In this paper we apply lattice models of finite binary percolation networks to examine the effects of network configuration on macroscopic network responses. We consider both square and rectangular lattice structures in which bonds between nodes are randomly assigned to be either resistors or capacitors. Results show that for given network geometries, the overall normalised frequency-dependent electrical conductivities for different capacitor proportions are found to converge at a characteristic frequency. Networks with sufficiently large size tend to share the same convergence point uninfluenced by the boundary and electrode conditions, can be then regarded as homogeneous media. For these networks, the span of the emergent scaling region is found to be primarily determined by the smaller network dimension (width or length). This study identifies the applicability of power-law scaling in random two phase systems of different topological configurations. This understanding has implications in the design and testing of disordered systems in diverse applications.

The hydro-mechanical behaviour of partially saturated granular materials is greatly influenced by the spatial and temporal distribution of liquid within the media. The aim of this paper is to characterise the distribution of saturated clusters in granular materials using an optical imaging method under different water drainage conditions. A saturated cluster is formed when a liquid phase fully occupies the pore space between solid grains in a localized region. The samples considered here were prepared by vibrating mono-sized glass beads to form closely packed assemblies in a rectangular container. A range of drainage conditions were applied to the specimen by tilting the container and employing different flow rates, and the liquid pressure was recorded at different positions in the experimental cell. The formation of saturated clusters during the liquid withdrawal processes is governed by three competing mechanisms arising from viscous, capillary, and gravitational forces. When the flow rate is sufficiently large and the gravity component is sufficiently small, the viscous force tends to destabilize the liquid front leading to the formation of narrow fingers of saturated material. As the water channels along these liquid fingers break, saturated clusters are formed inside the specimen. Subsequently, a spatial and temporal distribution of saturated clusters can be observed. We investigated the resulting saturated cluster distribution as a function of flow rate and gravity to achieve a fundamental understanding of the formation and evolution of such clusters in partially saturated granular materials. This study serves as a bridge between pore-scale behavior and the overall hydro-mechanical characteristics in partially saturated soils.

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

Due to neutron irradiation, solid breeder blankets are subjected to complex thermo-mechanical conditions. Within one breeder unit, the ceramic breeder bed is composed of spherical-shaped lithium orthosilicate pebbles, and as a type of granular material, it exhibits strong coupling between temperature and stress fields. In this paper, we study these thermo-mechanical problems by developing a thermal discrete element method (Thermal-DEM). This proposed simulation tool models each individual ceramic pebble as one element and considers grain-scale thermo-mechanical interactions between elements. A small section of solid breeder pebble bed in HCPB is modelled using thousands of individual pebbles and subjected to volumetric heating profiles calculated from neutronics under ITER-relevant conditions. We consider heat transfer at the grain-scale between pebbles through both solid-to-solid contacts and the interstitial gas phase, and we calculate stresses arising from thermal expansion of pebbles. The overall effective conductivity of the bed depends on the resulting compressive stress state during the neutronic heating. The thermal-DEM method proposed in this study provides the access to the grain-scale information, which is beneficial for HCPB design and breeder material optimization, and a better understanding of overall thermo-mechanical responses of the breeder units under fusion-relevant conditions., 6 Pages, 3 Tables, 4 Figures, Fusion Science and Technology, 2014

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

We carried out density functional theory calculations to model the electronic structure and the magnetic interactions in copper doped anatase and rutile titanium dioxide in order to shed light on the potential of these systems as magnetic oxides using different density functional schemes. In both polymorphs copper dopant was found to be most stable in substitutional lattice positions. Ferromagnetism is predicted to be stable well above room temperature with long range interactions prevailing in the anatase phase while the rutile phase exhibits only short range superexchange interaction among nearest neighbouring Cu ions. Additionally, energetic evaluation of dopants in scattered and compact configurations reveals a dopant clustering tendency in anatase TiO2., 13 pages, five figures, one table

Ab-initio density functional theory (DFT) calculations of the relative stability of anatase and rutile polymorphs of TiO2 were carried using all-electron atomic orbitals methods with local density approximation (LDA). The rutile phase exhibited a moderate margin of stability of ~ 3 meV relative to the anatase phase in pristine material. From computational analysis of the formation energies of Si, Al, Fe and F dopants of various charge states across different Fermi level energies in anatase and in rutile, it was found that the cationic dopants are most stable in Ti substitutional lattice positions while formation energy is minimised for F- doping in interstitial positions. All dopants were found to considerably stabilise anatase relative to the rutile phase, suggesting the anatase to rutile phase transformation is inhibited in such systems with the dopants ranked F>Si>Fe>Al in order of anatase stabilisation strength. Al and Fe dopants were found to act as shallow acceptors with charge compensation achieved through the formation of mobile carriers rather than the formation of anion vacancies.

The agglomeration, electrokinetic properties and electrophoretic deposition behaviour of aqueous suspensions of ZrO2 with carboxylic acid additives were studied in comparison with conventional pH adjustment. It was found that citric acid imparted negative zeta-potential values and electrosteric stabilisation to particles in suspensions at all pH levels. The examination of additions of carboxylic acids to ZrO2 suspensions revealed that these reagents cause a sharp drop in zeta-potential at distinct addition levels, which correspond to surface saturation of the particles with negatively charged carboxylate groups. Adsorption cross sections of citric acid, EDTA and oxalic acid were evaluated from these results, showing that both citric acid and EDTA coordinate to ZrO2 surfaces by two carboxylate groups while oxalic acid is coordinated by one group. The use of carboxylic acids was shown to facilitate superior electrophoretic deposition in comparison with zeta-potential modification by conventional pH adjustment through improved suspension stability.

International audience; The dispersion of anatase phase TiO 2 powder in aqueous suspensions was investigated by zetapotential and agglomerate size analysis. The iso-electric point (IEP) of anatase was determined to be at pH 2.8 using monoprotic acids for pH adjustment. In comparison, it was found that the use of carboxylic acids, citric and oxalic, caused a decrease in zeta-potential through the adsorption of negatively charged groups to the particle surfaces. The use of these reagents was shown to enable effective anodic electrophoretic deposition (EPD) of TiO 2 onto graphite substrates at low pH levels with a decreased level of bubble damage in comparison with anodic EPD from basic suspensions. The results obtained demonstrate that the IEP of TiO 2 varies with the type of reagent used for pH adjustment. The low pH level of the IEP and the ability to decrease the zeta-potential through the use of carboxylic acids suggest that the anodic EPD of anatase is more readily facilitated than cathodic EPD.

Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.