33 research outputs found
Simulation of Metal/Oxide Interface Mobility: Effects of Mechanical Stresses on Geometrical Singularities
During the last decade, an increasing importance has been given to the
feedback of mechanical stresses on the chemical diffusion and, further, on
corrosion. Many works point the active role of stresses on the material ageing
especially on their negative consequences leading to the damaging of
structures. Based on a theoretical study and using numerical tools and
experimental results our previous works [1, on stress/diffusion coupling,
highlight the strong influence of stress field on the diffusion process. The
aim of the present paper is to describe the influence of some particular
morphologies of the metal/oxide interface on both diffusion and oxidation
process. The oxidation is assumed to be driven by a mass conservation law
(Stefan's law) while the diffusion coefficient of oxygen in metal is locally
influenced by the stress field. The stability of a waved-shape interface is
studied in both cases: simple diffusion and coupled stress/diffusion process.
In this purpose we have developed an original numerical model using a virtual
metal/oxide interface of a mono-material with oxygen concentration-dependent
parameters, which allows to operate easily with any shape of interface and to
use simple finite element meshes. Furthermore, in order to underline in a more
obvious way the consequences of mechanical stress on the diffusion process, a
particular geometry is studied
Specific Aspects of Internal Corrosion of Nuclear Clad Made of Zircaloy
In PWR, the Zircaloy based clad is the first safety barrier of the fuel rod,
it must prevent the dispersion of the radioactive elements, which are formed by
fission inside the UO2 pellets filling the clad. We focus here on internal
corrosion that occurs when the clad is in tight contact with the UO2 pellet. In
this situation, with temperature of 400^{\circ}C on the internal surface of the
clad, a layer of oxidised Zircaloy is formed with a thickness ranging from 5 to
15 m. In this paper, we will underline the specific behaviour of this
internal corrosion layer compared to wet corrosion of Zircaloy. Simulations
will underline the differences of stress field and their influences on
corresponding dissolved oxygen profiles. The reasons for these differences will
be discussed as function of the mechanical state at inner surface of the clad
which is highly compressed. Differences between mechanical conditions generated
by an inner or outer corrosion of the clad are studied and their influences on
the diffusion phenomena are highlighted
Iodine Doped Graphene for Enhanced Electrocatalytic Oxygen Reduction Reaction in PEM Fuel Cell Applications
Although doped graphene based materials have been intensively investigated, as electrocatalysts for oxygen reduction reaction (ORR), there is still a number of challenges to be explored in order to design a highly active, durable, thermodynamically stable and low-cost catalyst with full recognized technological importance. The application of iodine-doped graphene in fuel cells (FC) has been recently examined as innovative nanomaterial for cathode fabrication. Up to date microscopic and spectroscopic techniques have been combined with structural and electrochemical investigations for a compendious characterization of developed ORR catalysts. The unique structure of doped graphenes is ascertained by the presence of mesopores, vacancies and high surface area, and favors the ions/electrons transportation at nanometric scale. The chapter discusses (a) how to use the existing knowledge in respect to synthesized doped graphenes and (b) how to improve the FC by taking into account these materials and have an enhanced electrochemical performance as well as long-term durability
Contribution à l'étude des interactions entre contraintes résiduelles et dissolution d'oxygène dans un solide déformable réactif
Cette thèse a pour objet la mise en évidence des interactions entre un champ de contraintes mécaniques et un champ de composition dans un processus de diffusion de matière au sein d un solide déformable réactif. Notre travail a évolué chronologiquement de l étude paramétrique numérique, vers la mise en ouvre expérimentale d une démarche destinée a révéler le rôle moteur des contraintes dans la diffusion de matière.Différentes sources de contraintes mécaniques ont tout d'abord été analysées numériquement a travers leurs impacts sur le processus de diffusion d'oxygène dans un métal (Zr) ou une céramique (UO2) soumis à un environnement oxydant. Cette approche a permis entre autres : de dégager un procédé de traitement de surface (grenaillage) susceptible d'engendrer un champs de contraintes résiduelles particulier comme préalable à la mise en ouvre d'une étude expérimentale destinées à valider les conclusions numériques ; de mettre en évidence le caractère stabilisateur de la contrainte sur la morphologie ondulée d'une interface oxyde/métal (cas du Zr).Dans l'approche expérimentale, différents outils ont été exploités pour caractériser le matériau (SDL, MEB, ATG, MTI, microdureté). Ils ont permis la mise en évidence d'une influence forte de la durée de grenaillage sur le ralentissement de l'oxydation. L analyse comparative des résultats expérimentaux et de simulations est révélatrice d intéractions fortes entre les champs de contraintes et de composition induits par les différents traitements (grenaillage et/ou pré-oxydation)The aim of this PhD work is to highlight the interactions between the mechanical stress and the chemical composition within diffusion of matter process for a reactive solid. The chronological evolution of our work goes from a parametric numerical study to an experimental study and reveals the role of mechanical stresses on the oxygen diffusion process.Different origins of mechanical stress were first numerically analysed from the point of view of their impacts on the process of oxygen diffusion into a metal (Zr) or a ceramic (UO2) subjected to an oxidizing environment. This approach allowed us: to identify a surface treatment (shot-peening) able to generate a residual specific stress field, as a starting point for an experimental study implementation in order to validate the numerical study conclusions; to highlight the ability of the stress field on the stabilisation of the morphology of an undulated metal/oxide interface (case of Zr).In the experimental approach, different technics were used to characterize the material (GDOS, SEM, TGA, hole-drilling method, micro-hardness tests). They permitted the detection of a strong influence of shot-peening on the oxidation rate. The comparison of experimental and numerical simulation results reveals strong interactions between stress and compositions fields induced by the different treatments (shot-peening and/or pre-oxidation)DIJON-BU Doc.électronique (212319901) / SudocSudocFranceF
Influence of catalyst structure on PEM fuel cell performance – A numerical investigation
The effect of the catalyst microstructure on a 5 cm2 PEM fuel cell performance is numerically investigated. The catalyst layer composition and properties (i.e. ionomer volume fraction, platinum loading, particle radius, electrochemical active area and carbon support type), and the mass transport resistance due to the ionomer and liquid water surrounding the catalyst particles, are incorporated into the model. The effects of the above parameters are discussed in terms of the polarization curves and the local distributions of the key parameters. An optimum range of the ionomer volume fraction was found and a gain of 39% in the performance was achieved. As regards the platinum loading and catalyst particle radius, the results showed that a higher loading and a smaller radius leads to an increase in the PEMFC performance. Further, the influence of the electrochemical active area produces an overall increase of 22% in current density and this was due to the use of a new material developed as support for Pt particles, an iodine doped graphene, which has better electrical contacts and additional pathways for water removal. Using this parameter, the numerical model has been validated and good agreement with experimental data was achieved, thus giving confidence in the model as a design tool for future improvements of the catalyst structure
Internal Interface Strains Effects on UO2/U3O7 Oxidation Behaviour
The growth of a U3O7 oxide layer during the anionic oxidation of UO2 pellets
induced very important mechanical stresses due to the crystallographic lattice
parameters differences between UO2 and its oxide. These stresses, combined with
the chemical processes of oxidation, can lead to the cracking of the system,
called chemical fragmentation. We study the crystallographic orientation of the
oxide lattice growing at the surface of UO2, pointing the fact that epitaxy
relations at interface govern the coexistence of UO2 and U3O7. In this work,
several results are given: - Determination of the epitaxy relations between the
substrate and its oxide thanks to the Bollmann's method; epitaxy strains are
deduced. - Study of the coexistence of different domains in the U3O7
(crystallographic compatibility conditions at the interface between two phases:
Hadamard conditions). - FEM simulations of a multi-domain U3O7 connected to a
UO2 substrate explain the existence of a critical thickness of U3O7 layer
PEM fuel cell performance improvement through numerical optimization of the parameters of the porous layers
A numerical model for a PEM fuel cell has been developed and used to investigate the effect of some of the key parameters of the porous layers of the fuel cell (GDL and MPL) on its performance. The model is comprehensive as it is three-dimensional, multiphase and non-isothermal and it has been well-validated with the experimental data of a 5 cm2 active area-fuel cell with/without MPLs. As a result of the reduced mass transport resistance of the gaseous and liquid flow, a better performance was achieved when he GDL thickness was decreased. For the same reason, the fuel cell was shown to be significantly improved with increasing the GDL porosity by a factor of 2 and the consumption of oxygen doubled when increasing the porosity from 0.40 to 0.78. Compared to the conventional constant-porosity GDL, the graded-porosity (gradually decreasing from the flow channel to the catalyst layer) GDL was found to enhance the fuel cell performance and this is due to the better liquid water rejection. The incorporation of a realistic value for the contact resistance between the GDL and the bipolar plate slightly decreases the performance of the fuel cell. Also the results show that the addition of the MPL to the GDL is crucially important as it assists in the humidifying of the electrolyte membrane, thus improving the overall performance of the fuel cell. Finally, realistically increasing the MPL contact angle has led to a positive influence on the fuel cell performance
Influence of the U3O7 domain structure on cracking during the oxidation of UO2
International audienceCracking is observed when a UO2 single crystal is oxidised in air. Previous studies led to the hypothesis that cracking occurs once a critical depth of U3O7 oxidised layer is reached. We present some μ-Laue X-ray diffraction results, which evidence that the U3O7 layer, grown by topotaxy on UO2, is made of domains with different crystalline orientations. This observation was used to perform a modelling of oxidation coupling chemical and mechanical parameters, which showed that the domain patterning induces stress localisation. This result is discussed in comparison with stress localisation observed in thin layer deposited on a substrate and used to propose an interpretation of UO2 oxidation and cracking