Etude de la dispersion par voie aqueuse ou organique de mélanges de céramiques oxydes ou non-oxydes. Influence de la morphologie des grains.

pas de résumé en anglaisL'objectif de ce travail était d'améliorer les caractéristiques d'un matériau composite AL2O3-AION-SiC, en étudiant les conditions de dispersion des poudres initiale en voie humide. Dans un premier temps nous nous sommes intéresses a la dispersion de l'alumine a dans l'eau puis a l'influence de la morphologie de cette poudre sur la dispersion.Dans un second temps nous avons étudié l'influence de l'ajout d'une deuxième phase minérale (AIN) puis d'une troisième phase (SiC) à la suspension d'alumine, en milieu organique. Dans l'eau en particulier, les impuretés à la surface de la poudre (sodium) influencent la nature basique de la poudre d'alumine : lorsque la teneur en sodium dans la poudre augmente le Point de Charge Nulle se trouve déplacé vers les pH basiques. D'autre part le pH de la suspension détermine la configuration de la molécule du dispersant (P.A.A) et par conséquent la stabilité et l'état de dispersion du système. La morphologie de la poudre d'alumine n'apparait pas comme un facteur déterminant dans la stabilité d'une suspension d'alumine. Mais il ce serait plutôt la nature des phases ou des espèces chimiques ( impuretés) à la surface des plaquettes qui seraient responsable de l’instabilité du système alumine poudre-alumine plaquettes(17%pds).En milieu non aqueux , la nature chimique, et principalement le caractère acido-basique de chacun des constituants du système, joue un grand rôle au niveau des phénomènes d'adsorption des molécules dispersantes à la surface d'une poudre. En effet un dispersant d’acide aura une plus grande affinité avec une surface ayant un caractère basique et vice versa. L'alumine sera mieux défloculée par l'Ester Phosphorique (acide) que par le polyvinylpyrrolidone , alors que dans le cas du carbure de silicium, qui a une surface acide, le polyvinylpyrrolidone sera le dispersant le plus approprie. L'optimisation de la dispersion d'un système unaire (AL2O3) ou binaire (AI2O3-AIN) se traduit par une microstructure homogène du matériau densifié. Cependant lorsqu'on introduit la troisième phase (SiC) dans des conditions rhéologiques optimisées, le résultat en est un matériau présentant une microstructure inhomogène. un traitement mécanique (attrition) de la suspension ternaire entraine alors une meilleure homogénéité des différentes phases dans le matériau fritté

Modélisation et simulation du frittage de matériaux dopés et de multimatériaux à l'échelle de la microstructure

Dans le cadre de la simulation du procédé de consolidation de pièces céramiques ou composites CerMet par frittage, cette thèse se propose de modéliser et de simuler deux aspects particuliers du frittage. Le premier consiste à prendre en compte l'effet d'éléments dopants sur l'évolution microstructurale d'un élément volumique représentatif d'un compact de poudre. Pour cela les chemins de diffusion tels que la diffusion volumique et de surface sont pris en compte. Le second aspect est en relation avec l'élaboration de matériaux composites à dispersoïdes et l'étude de l'évolution de leur microstructure par simulation numérique selon les caractéristiques des inclusions ne participant pas au processus de frittage. Ces simulations reposent sur le développement de la méthode Level-Set qui permet de suivre tous les changements de topologie qui interviennent au cours du frittage à l'échelle des grains.Des résultats concluants ont été obtenus à partir de la simulation de l'effet du dopage, dans le cas particulier de l'alumine dopée magnésie, puis généralisés au cas du codopage. Pour les multimatériaux, la première étape de la modélisation a consisté à considérer des inclusions ayant le même type de loi de comportement que la matrice céramique, à savoir élastique, linéaire et isotrope. Cependant, les propriétés des matériaux seront différentes. Du point de vue numérique, la complexité du problème a consisté à bien gérer les deux phases solides, et en particulier leur interface commune en vue de la résolution de l'équilibre mécanique par éléments finis, en utilisant plusieurs fonctions Level-Set.In the framework of the numerical modelling of ceramic powder consolidation or CerMet composites by sintering, this work aims to model and to simulate two aspects of sintering. The first it is to study the doping effect on microstructural evolution of a powder compact, taking into account the main diffusion routes as volume and surface diffusion. The second aspect aims to simulate the evolution of a continue ceramic matrix phase containing a dispersoïde of inclusions that are inert to the diffusion phenomena. Interesting results are obtained in the simulation of the doping effect, and are generalized to the codoping case thanks to the Level-Set method adoped. It is important to recall that the Level-Set method is able to handle topological changes occuring during the simulation. Concerning the multimaterials, a first approach considers that both inclusions and the ceramic matrix have the same constituve elastic, linear and isotropic law but with different materials properties. Numerically, the complexity appears when managing the solid phases (matrix and inclusions), particulary their interface when the momentum conservation problem is solved by finite elements. The diffusion problem is then solved like in the case of the doping effect simulations. Numerical simulations of granular compacts are held to evaluate the dopant and the inclusions effect.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Zirconia matrix composite dispersed with stainless steel particles: Processing and oxidation behavior

International audienceMaterials with non uniform properties are being developed to optimize several functions of industrial components in severe atmospheres at high temperature. These composites called M(p)-CMC(s): "ceramic matrix composites dispersed with metal particles" are candidates for high-temperature structure materials as functionally graded materials (FGMs) such as intermediate components between electrolyte and interconnecting components in SOFC. Preparation of a model composite M(p)-CMC(s) is described. Powder metallurgy process is used to obtain a dense composite (98% of the theoretical density) based on yttrium stabilized zirconia for the ceramic part and on 304L stainless steel for the metallic part. The characteristics of this material as well as its behavior under oxidation at high temperature are reported

3D simulation of the matter transport by surface diffusion within a level-set context

International audienceWithin the framework of the sintering process simulation, this paper proposes a numerical strategy for the direct simulation of the matter transport by surface diffusion. A level-set method is used to describe the topological changes which arise at the free boundary of the sintering particles. The surface velocity is found to be proportional to the surface Laplacian of the curvature, that is, proportional to the fourth-order derivative of the level-set function. Consequently, both curvature and velocity must be computed carefully and with accuracy. Finally, three-dimensional simulations are shown and investigated

Oxidation behaviour of unirradiated sintered UO2 pellets and powder at different oxygen partial pressures, above 350°C

International audienceThe oxidation of sintered UO2 pellets and powder into U3O8 has been studied by thermogravimetry at 370°C, under controlled oxygen partial pressures (PO2 ranging from 2-40 kPa). Sigmoidal curves of oxidation weight gain were measured for both pellet and powder test samples. The rate of oxidation increased as the oxygen partial pressure increased. It has been shown, by simultaneous TG-DSC, that the reaction proceeds in a pseudo steady state. An experimental methodology based on temperature or PO2 jumps has shown that the assumption of a rate-limiting step is validated, and a mean value of activation energy for the formation of U3O8 of 103 kJ.mol-1 was estimated

The growth of COVID-19 scientific literature: A forecast analysis of different daily time series in specific settings

Paper submitted to the ISSI Conference 2021.We present a forecasting analysis on the growth of scientific literature related to COVID-19 expected for 2021. Considering the paramount scientific and financial efforts made by the research community to find solutions to end the COVID-19 pandemic, an unprecedented volume of scientific outputs is being produced. This questions the capacity of scientists, politicians and citizens to maintain infrastructure, digest content and take scientifically informed decisions. A crucial aspect is to make predictions to prepare for such a large corpus of scientific literature. Here we base our predictions on the ARIMA model and use two different data sources: the Dimensions and World Health Organization COVID-19 databases. These two sources have the particularity of including in the metadata information on the date in which papers were indexed. We present global predictions, plus predictions in three specific settings: by type of access (Open Access), by NLM source (PubMed and PMC), and by domain-specific repository (SSRN and MedRxiv). We conclude by discussing our findings

A parasite outbreak in notothenioid fish in an Antarctic fjord

20 pages, 4 figures, supplemental information https://doi.org/10.1016/j.isci.2022.104588.-- Data and code availability: • All data have been deposited at NCBI GenBank: OL630144 and OL630145, NCBI SRA BioProject: PRJNA789574, at MorphoSource Project: 000405843, and at USAP-DC Project: p0010221, and are publicly available as of the date of publication. Biological materials have been deposited at the Zoological Museum of the University of Copenhagen. Additional accession numbers and DOIs are listed in the key resources. • This paper does not report original code. • Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon requestClimate changes can promote disease outbreaks, but their nature and potential impacts in remote areas have received little attention. In a hot spot of biodiversity on the West Antarctic Peninsula, which faces among the fastest changing climates on Earth, we captured specimens of two notothenioid fish species affected by large skin tumors at an incidence never before observed in the Southern Ocean. Molecular and histopathological analyses revealed that X-cell parasitic alveolates, members of a genus we call Notoxcellia, are the etiological agent of these tumors. Parasite-specific molecular probes showed that xenomas remained within the skin but largely outgrew host cells in the dermis. We further observed that tumors induced neovascularization in underlying tissue and detrimentally affected host growth and condition. Although many knowledge gaps persist about X-cell disease, including its mode of transmission and life cycle, these findings reveal potentially active biotic threats to vulnerable Antarctic ecosystemsThis work was funded by the National Science Foundation grants OPP-1947040 (JHP and ArV), PLR-1444167 (HWD), and OPP-1543383 (JHP, TD, and HWD)With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe

Erratum to: Intercomparison of the Arctic sea ice cover in global ocean–sea ice reanalyses from the ORA-IP project

In the original publication of the article, Fig. 11 was published incorrectly. The correct version of Fig. 11 is provided her

Microstructure refinement of alumina Optimisation by gas pressure sintering process

International audienceThis paper investigates densification and grain growth evolutions during gas pressure sintering of alumina. Isothermal sintering runs are performed under different nitrogen pressures: atmospheric pressure and 2 MPa. Experimental data are fitted thanks to constitutive laws in order to understand nitrogen pressure effect on densification and grain growth mechanisms of a fine-grained alumina. An optimal run of densification is proposed as an application of these results. (C) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Multiphysics modelling approach to microwave heating of cerium oxide particles in diverse packing situations

International audienceIn this work, a single-mode resonant microwave cavity (2.45 GHz) is loaded with spherical particles. A weakly coupled electromagnetic (EM)-thermal solver is used iteratively to determine how the microscopic geometry (local curvatures between the particles, grain size and neck size) modifies the EM field, and in turn the thermal field in the particles and also to quantify the microwave effect for an experimental process. The modelling is performed with the conformal finite element solver COMSOL Multiphysics. Moreover, this study will show that the electric field norm increases for one spherical particle of ceria with different sizes and consequently the temperature increases. In addition, the electric field and heating behaviour have been studied in the case of three particles with different neck sizes. Finally, the effect of the configuration versus the E-field direction has been studied for three particles and has a significant result