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Cerbelaud M.,University of Nantes | Lestriez B.,University of Nantes | Guyomard D.,University of Nantes | Videcoq A.,Limoges National Superior School of Industrial Ceramics | Ferrando R.,CNR Institute of Materials for Electronics and Magnetism
Langmuir | Year: 2012

Dilute aqueous suspensions of silicon nanoparticles and sodium carboxymethylcellulose salt (CMC) are studied experimentally and numerically by Brownian dynamics simulations. The study focuses on the adsorption of CMC on silicon and on the aggregation state as a function of the suspension composition. To perform simulations, a coarse-grained model has first been developed for the CMC molecules. Then, this model has been applied to study numerically the behavior of suspensions of silicon and CMC. Simulation parameters have been fixed on the basis of experimental characterizations. Results of Brownian dynamics simulations performed with our model are found in qualitative good agreement with experiments and allow a good description of the main features of the experimental behavior. © 2012 American Chemical Society.

Ranjbar-Far M.,Limoges National Superior School of Industrial Ceramics | Absi J.,Limoges National Superior School of Industrial Ceramics | Mariaux G.,University of Limoges | Dubois F.,University of Limoges
Materials and Design | Year: 2010

A Finite Element Model (FEM) was developed to evaluate the stresses induced by the thermal cycling in a typical plasma-sprayed thermal barrier coating system (TBCs). The thermo-mechanical model of this multi-layer system takes into account the effects of thermal and mechanical properties, morphology of the top-coat/bond-coat interface and oxidation on the local stresses that are responsible for the micro-crack nucleation during cooling, especially near the metal/ceramic interface. Two top-coat/bond-coat geometries corresponding to different interfacial asperity morphologies (semicircle or sinusoidal) are modeled considering a two dimensional and periodic geometry. The effect of the geometry and the amplitude of asperities on stress distribution are examined to study the cause of the subsequent delamination of the TBCs system. Moreover, the effect of the creep in all layers and plastic deformation in the bond-coat as well as the oxidation in the perpendicular direction of the top-coat/bond-coat interface are examined toward the stress development and critical sites with respect to possible crack paths. In addition, crack initiation and propagation at the system was predicted. © 2009 Elsevier Ltd. All rights reserved.

Ranjbar-far M.,Prisme Institute | Absi J.,Limoges National Superior School of Industrial Ceramics | Mariaux G.,University of Limoges | Smith D.S.,Limoges National Superior School of Industrial Ceramics
Materials and Design | Year: 2011

A finite element model (FEM) is developed to simulate the crack development in a typical plasma sprayed thermal barrier coatings system in consequence of the stresses induced by thermal cycling, the growth of the oxide layer and different interface morphologies. The thermo-mechanical model is designed to takes into account a non-homogenous temperature distribution and the effects of the residual stress generated during coating process. Crack propagation at the top-coat/oxide and oxide/bond-coat interfaces is simulated thanks to the contact tool "Debond" present in the ABAQUS finite element code. Simulations are performed with a geometry corresponding to identical or dissimilar amplitude of asperity and for different thickness of oxide layer. The results show a significant difference between the case with and without presence of crack propagation and an important damage on the interfaces due to the growth of the oxide layer very close to the height of the interface asperities. © 2011 Elsevier Ltd.

Ranjbar-Far M.,Limoges National Superior School of Industrial Ceramics | Absi J.,Limoges National Superior School of Industrial Ceramics | Shahidi S.,University of Pau and Pays de l'Adour | Mariaux G.,University of Limoges
Materials and Design | Year: 2011

The present study deals with a numerical investigation of the residual stresses arising during the plasma-sprayed coatings process and their effects on the final stress state of the thermal barrier coatings system (TBCs) during service. A new thermo-mechanical finite element model (FEM) has been designed to function using a non-homogenous temperature distribution. Several phenomena are taken into account in the model such as: residual stresses generated during the spraying of coatings, morphology of the top-coat/bond-coat interface, oxidation at the top-coat/bond-coat interface, thermal mismatch of the material components, plastic deformation of the bond-coat and creep of all layers during thermal cycling. These phenomena induce local stresses in the TBCs that are responsible of micro-crack propagation during cooling and thermal cycling, specifically near the ceramic/metal interface.The results of the non-homogenous temperature model have been compared with those of the homogenous temperature one. As a result, more stress relaxation in the hotter area due to creep and different oxide growth rate are founded in the non-homogenous temperature model. © 2010 Elsevier Ltd.

Gaalova J.,Czech Institute of Chemical Process Fundamentals | Barbier J.,University of Poitiers | Rossignol S.,Limoges National Superior School of Industrial Ceramics
Journal of Hazardous Materials | Year: 2010

This study was a comparison between Ru-catalysts and similar, previously investigated, Pt-catalysts. In this paper, ruthenium catalysts for catalytic wet air oxidation are prepared, characterized and tested. Both catalysts were supported on commercial CeO2 as well as mixed oxide Zr0.1(Ce0.75Pr0.25)0.9O2. The catalysts were characterized by measuring the oxygen storage capacities (OSC), BET, XRD, FTIR and chemisorption of hydrogen. In addition, the effect of sintering (treatments under H2) was compared with both of the catalysts. The comparison of the results showed that initial intrinsic activity of ruthenium is not significantly influenced by the type of the support, which is contrast to platinum. Furthermore, the particle size of Ru had an important effect on CWAO activity: the higher the particle size, the better the activity. This was different with Pt-catalysts, where the optimal particle size was smaller, having about 15% of metal dispersion. © 2010 Elsevier B.V.

Cerbelaud M.,University of Genoa | Ferrando R.,University of Genoa | Videcoq A.,Limoges National Superior School of Industrial Ceramics
Journal of Chemical Physics | Year: 2010

The influence of dilution on the aggregation process of suspensions composed of two kinds of oxide particles (alumina positively charged particles d1 =400 nm and silica negatively charged particles d2 =250 nm) has been studied by computer simulations. Two kinds of simulations have been performed: Brownian dynamics simulations to study the aggregation process and its kinetics and global minimization searches to find the most stable configurations of aggregates. We show that the rate of dilution has a strong influence on the structure and on the shape of aggregates in Brownian dynamics simulations. By confronting these aggregates with the stable aggregates found by global minimization, we demonstrate that they are metastable and their shape is explained by the competition between the kinetics of aggregate coalescence and the kinetics of aggregate reorganization into more stable configurations. © 2010 American Institute of Physics.

Martias C.,Limoges National Superior School of Industrial Ceramics | Joliff Y.,University of Toulon | Favotto C.,University of Toulon
Composites Part B: Engineering | Year: 2014

This work presents thermomechanical experiments whose results have led to a new formulation of composite panels for building construction. This panel has the advantage to be lightweight and 2 h firebreak. Plaster, under the β-hemihydrate form, is used as a matrix and mineral products (vermiculite, mica, glass fibers) are added as lightweight additives, mechanical reinforcement and thermal insulator. The both effects of the particles size distribution of plaster, and of the amounts of additives, on the mechanical properties are investigated at room temperature. Three approaches are proposed and compared: experimental, analytical and numerical to quantify the impact of additives on the mechanical properties. Thus, the results obtained, including porosity, density and mechanical property, permit to retain a formulation of composite. This formulation is tested under ISO 834 fire conditions to validate its use as passive protection in building construction. © 2014 Elsevier Ltd. All rights reserved.

Boussois K.,Limoges National Superior School of Industrial Ceramics | Tessier-Doyen N.,Limoges National Superior School of Industrial Ceramics | Blanchart P.,Limoges National Superior School of Industrial Ceramics
Journal of the European Ceramic Society | Year: 2013

Multilayer ceramics with a composite and organized microstructure were realized from kaolin and alumina fibers to improve strength and fracture toughness. Dilatometry experiments along 3 directions reveal anisotropic shrinkages, which are in correlation with different activation energy for sintering. Mullite growth is strongly anisotropic, inducing the formation of an organized microstructure, where larger mullite crystals are mainly oriented in plane of layer and perpendicular to alumina fibers. Kinetic data from thermal transformations show that the starting reaction mechanism is mullite nucleation, and it is continued by a strongly anisotropic grain growth. It is explained by topotactic transformations at phyllosilicate faces and along alumina arrangements. Mullite growth kinetics is also favored perpendicularly to fiber main dimension by the anisotropy of alumina diffusion coefficient. It shows the limited importance of mullite crystallization in microstructural transformation, but it also shows that controlled mullite growth is central in microstructural arrangement. © 2012 Elsevier Ltd.

Lebreton F.,CEA Marcoule Nuclear Site | Lebreton F.,Limoges National Superior School of Industrial Ceramics | Belin R.C.,CEA Cadarache Center | Delahaye T.,CEA Marcoule Nuclear Site | Blanchart P.,Limoges National Superior School of Industrial Ceramics
Journal of Solid State Chemistry | Year: 2012

In the frame of minor actinides recycling, americium can be transmuted by adding it in UO 2 or (U, Pu)O 2 fuels. Americium oxides exhibiting a higher oxygen potential than U or Pu oxides, its addition alters the fuel properties. To comprehend its influence, a thorough knowledge of the Am-O phase equilibria diagram and of thermal expansion behavior is of main interest. Due to americium scarcity and high radiotoxicity, few experimental reports on this topic are available. Here we present in-situ high-temperature XRD results on the reduction from AmO 2 to Am 2O 3. We show that fluorite (Fm-3m) AmO 2 is reduced to cubic (Ia-3) C′-type Am 2O 3δ, and then into hexagonal (P6 3/mmc) A-type Am 2O 3, which remains stable up to 1840 K. We also demonstrate the transitional existence of the monoclinic (C2/m) B-type Am 2O 3. At last, we describe, for the first time, the thermal expansion behavior of the hexagonal Am 2O 3 between room temperature and 1840 K. © 2012 Elsevier Inc. All rights reserved.

Lebreton F.,CEA Marcoule Nuclear Site | Lebreton F.,Limoges National Superior School of Industrial Ceramics | Belin R.C.,French Atomic Energy Commission | Prieur D.,CEA Marcoule Nuclear Site | And 2 more authors.
Inorganic Chemistry | Year: 2012

In order to reduce the nuclear waste inventory and radiotoxicity, U 1-xAm xO 2±δ materials are promising fuels for heterogeneous transmutation. In this context, they are generally fabricated from UO 2+δ and AmO 2-δ dioxide powders. In the subsequent solid solution, americium is assumed to be trivalent whereas uranium exhibits a mixed-valence (+IV/+V) state. However, no formation mechanisms were ever evidenced and, more particularly, it was not possible to know whether the reduction of Am(IV) to Am(III) occurs before the solid-solution formation, or only once it is established. In this study, we used high-temperature X-ray diffraction on a UO 2±δ/AmO 2-δ (15 mol%) mixture to observe in situ the formation of the U 1-xAm xO 2±δ solid solution. We show that UO 2+δ is, at relatively low temperature (<700 K), oxidized to U 4O 9-δ, which is likely to be caused by oxygen release from the simultaneous AmO 2-δ reduction to cubic Am 2O 3±δ. Cubic Am 2O 3+δ then transforms to hexagonal Am 2O 3 at 1300 K. Thus, the initial Am(IV) is fully reduced to Am(III) before the solid solution starts forming at 1740 K. The UO 2 fluorite phase vanishes after 4 h at 1970 K, indicating that the formation of the solid solution is completed, which proves that this solid solution is formed after the complete reduction of Am(IV) to Am(III). © 2012 American Chemical Society.

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