Sur des oxydes de cérium contenant du fer nanostructurés et de morphologies contrôlées

Dans le cadre de ce travail, des composés de formulation Ce1 xFexO2 x2 ont été synthétisésà l aide de deux protocoles : co-précipitation et synthèse assistée par chauffage micro-ondes.L utilisation de cette dernière a ainsi conduit à l obtention de nanoparticules de morphologiescubiques ou bâtonnets et ceci pour des temps de synthèse relativement courts. L analysepar diffraction X a montré notamment que le paramètre de maille diminue en fonction de lateneur en fer, x. L environnement local et le degré d oxydation du fer ont été analysés parspectroscopies Mössbauer, RPE et XANES mettant ainsi en évidence la présence d ions Fe3+isolés au sein de sites octaédriques distordus et sous forme de clusters. Une comparaison entreles deux voies de synthèse a montré que des différences apparaissent à l échelle locale. Lessolutions solides obtenues ont ensuite été caractérisées au cours du traitement thermique etsous différentes atmosphères. Indépendamment de l atmosphère de recuit, une démixtion dela solution solide intervient pour des températures proches de 600C.This work deals with the synthesis and characterization of Ce1 xFexO2 x2 nanoparticles.Two different synthesis routes were used : the coprecipitation technique and the microwaveassisted synthesis route. This later allowed the obtention of controled morphologies such asnanocubic or nanorod particles, characterized by HRTEM. Compared to hydrothermal synthesisroute the time of reaction was limited to one hour. X-Ray Diffraction analysis showedthat the lattice parameter decreases versus the iron content. Local environment and oxidationstate of iron were analyzed by Mössbauer, EPR and XANES spectroscopies showingthe presence of two kinds of iron sites : isolated (distorted) octahedral sites and clusters. Acomparison between both synthesis routes shows clearly the differences at a local scale. Thenanoparticles were then further characterized during annealing and under different atmospheres.The solid solution undergoes a demixtion phenomenon around 600C which does notdepend on the atmosphere conditions.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Controlling the {111}/{110} Surface Ratio of Cuboidal Ceria Nanoparticles

The ability to control size and morphology is crucial in optimizing nanoceria catalytic activity as this is governed by the atomistic arrangement of species and structural features at the surfaces. Here, we show that cuboidal cerium oxide nanoparticles can be obtained via microwave-assisted hydrothermal synthesis in highly alkaline media. HRTEM revealed that the cube edges were truncated by CeO2{110} surfaces and the cube corners by CeO2{111} surfaces. When adjusting synthesis conditions by increasing NaOH concentration, the average particle size increased. Although this was accompanied by an increase of the cube faces, CeO2{100}, the cube edges, CeO2{110}, and cube corners, CeO2{111} remained of constant size. Molecular Dynamics (MD) was used to rationalise this behaviour and revealed that energetically, the corners and edges cannot be atomically sharp, rather they are truncated by {111} and {110} surfaces respectively to stabilise the nanocube; both experiment and simulation agreed a minimum size of ~1.6 nm associated with this truncation. Moreover, HRTEM and MD revealed {111}/{110} faceting of the {110} edges, which balances the surface energy associated with the exposed surfaces, which follows {111}>{110}>{100}, although only the {110} surface facets because of the ease of extracting oxygen from its surface, which follows {111}>{100}>{110}. Finally, MD revealed that the {100} surfaces are ‘liquid-like’ with a surface oxygen mobility 5 orders of magnitude higher than that on the {111} surfaces; this arises from the flexibility of the surface species network that can access many different surface arrangements due to very small energy differences. This finding has implications for understanding the surface chemistry of nanoceria and provides avenues to rationalize the design of catalytically active materials at the nanoscale

Etude de la précipitation et des mécanismes de dislocations dans l'alliage Al(CuMg)2650

Le cahier des charges de l'Avion de Transport Supersonique du Futur prévoit une vitesse de croisière proche de celle du Concorde, mais pendant une durée de vie 4 fois supérieure. L'alliage d'aluminium retenu par l'ONERA pour le fuselage, l'alliage 2650 (Al-Cu-Mg), doit être résistant à 100-130ʿC au fluage de longue durée et tolérant aux dommages. Ma thèse présente un double objectif : 1)comprendre les mécanismes dislocationnels contrôlant dans ces conditions la plasticité de l'alliage, 2)améliorer la connaissance de la précipitation durcissante de cet alliage. Pour cela, l'outil privilégié a été la microscopie électronique en transmission, notamment en modes conventionnel et haute résolution. Nous avons notamment montré que l'activation thermique provoque le mouvement dévié des dislocations ce qui favorise leur multiplication et le contournement des précipités. Ceux-ci sont de trois types dont deux qui créent une contrainte dans la matrice.Severe flying conditions have been retained for the future Supersonic Civil Transport aircraft: this aircraft is planned to have a four times longer lifetime than Concorde's one. The selected aluminium alloy for fuselage skin, the 2650(AlCuMg) one, has therefore to exhibit both a high damage tolerance and a strong resistance to long term creep in the 100ʿC-130ʿC temperature range. My thesis aims consequently at : 1)understanding the dislocation mechanisms which control the alloy plasticity under such conditions, 2)improving the knowledge of the hardening precipitation in this alloy. The main part of this investigation has been carried out using the transmission electron microscopy in its conventional and high resolution modes. We show that thermal activation allows cross-slip events on the dislocations and therefore, dislocation multiplication and precipitates by-passing. Three type of precipitates have been identified, two of them introducing strain fields in the matrixTOULOUSE-INSA (315552106) / SudocSudocFranceF

Corrélations compositions chimiques-structures d'oxydes mixtes (Ce / Zr) à base de Pr4+ / Pr3+ et propriétés de réductibilité

Ce travail est relatif à la synthèse et à la caractérisation d oxydes mixtes Pr1-zCezO2-y et Pr1-xZrxO2-y. Différentes compositions chimiques associées à une valence mixte Pr4+/Pr3+ concomitant à un taux précis de lacunes d oxygènes ont été isolées tant pour les composés à l état oxydé que pour ces derniers réduits sous Ar/H2. A l état réduit, différentes surstructures de la maille fluorine ont été caractérisées par diffraction des RX et de neutrons. Sur la base de mesures magnétiques, d analyses par spectroscopie d absorption X (XANES-EXAFS) aux seuils K du Zr et LIII du Ce/Pr et d analyses thermogravimétriques et/ou TPR (Temperature Programmed Reduction), les évolutions des taux de Pr4+/Pr3+ dans cette série ont été déterminées. Enfin, des mesures in-situ sous atmosphère réductrice par diffraction de neutrons et EELS (Electron Energy Loss Spectroscopy) ont été réalisées dans l objectif de corréler compositions chimiques- édifices structuraux et propriétés de réductibilité.This work deals with the synthesis and the characterization of Pr1-zCezO2-y and Pr1-xZrxO2-y oxides. Various chemical compositions associated with mixed valence state Pr4+/Pr3+ concomitant to precise oxygen vacancies rate were isolated for compounds in both oxidized and reduced compounds. In the reduced state, different superstructures of the fluorine network were characterized by X-ray and neutrons diffraction. On the basis of magnetic measurements, X-ray absorption spectroscopy (XANES-EXAFS) at Zr K-edge and Pr and Ce LII/III-edges and the thermogravimetric analyses and\or TPR (Temperature Programmed Reduction), evolutions of Pr4+/Pr3+ rates in this series were estimated. Finally, in-situ measurements under reducing atmosphere by neutrons diffraction and EELS (Electron Energy Loss Spectroscopy) were realized to correlate chemical compositions- structural features and reducibility properties.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Oxygen non-stoichiometry phenomena in Pr1-xZrxO2-y compounds (0.02 < x < 0.5).

New Pr1-xZrxO2-y oxides with x < 0.5 have been prepared by co-precipitation in basic medium and annealed under air at high temperatures (T≤ 1200 °C). Defined compositions with x = 0.02, 0.1, 0.2, 0.35, 0.40 and 0.5 have been characterized by XRD, Zr-K-edge EXAFS for the local structure, magnetic susceptibility measurements, and Pr LIII-edge XANES in order to identify the variation of the cell parameter and Zr local environment versus Zr content and Pr(n+) (4 < n < 3) oxidation states. The higher the Zr content, the lower the Pr valence state. The Zr amount stabilized in the distorted octahedral site is at the origin of the formation of defined compositions as discovered by Leroy Eyring et al. in the PrnO2n-2m series and the generation of oxygen vacancies stabilized in the fluorite-type network. TGA and TPR analyses help to follow the reduction properties under Ar/5% H2 and show high Pr reducible rates at low temperatures (T < 250 °C). The identification of the fluorite-type superstructure (SG: Ia3[combining macron]) of reduced compositions annealed at T = 900 °C under Ar/5% H2 shows the cationic and oxygen vacancy ordering. This feature plays a key role with Zr(4+) cations stabilized in flattened octahedral sites for the generation of oxygen vacancies and the stabilization of Pr(3+) in the reduced states

Two types of S phase precipitates in Al-Cu-Mg alloys

Transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) have been used to study S phase precipitation in an Al-4.2Cu-1.5Mg-0.6Mn-0.5Si (AA2024) and an Al-4.2Cu-1.5Mg-0.6Mn-0.08Si (AA2324) (wt-%) alloy. In DSC experiments on as solution treated samples two distinct exothermic peaks are observed in the range 250 to 350°C, whereas only one peak is observed in solution treated and subsequently stretched or cold worked samples. Samples heated to 270°C and 400°C at a rate of 10°C/min in the DSC have been studied by TEM. The selected area diffraction patterns show that S phase precipitates with the classic orientation relationship form during the lower temperature peak, and for the solution treated samples, the higher temperature peak is caused by the formation of a second type of S phase precipitates which have an orientation relationship that is rotated by ~4 degrees to the classic one. The effects of Si and cold work on the formation of second type of S precipitates have been discussed

Influence de la morphologie d'oxydes à base de cérium sur les relations (micro)structures/propriétés

Les oxydes à base de Cérium, ont fait l objet de nombreuses études ces dernières décennies et se sont révélés des matériaux de choix, dans le domaine de la catalyse hétérogène. L objectif à l heure actuelle, est donc d accroître la réactivité de ces oxydes, tout en élargissant leur gamme de températures optimales d utilisation. Dans ce contexte particulier, il semble possible de moduler les propriétés des oxydes à base de cérium en contrôlant la morphologie des cristallites. Ce travail de thèse a donc été consacré à la détermination, l élaboration et à la caractérisation de matériaux oxydes à base de cérium de morphologies contrôlées. Nous avons tout d abord déterminé cristallographiquement et thermodynamiquement les morphologies accessibles au système étudié puis par traitement hydrothermale assistée par chauffage micro-ondes nous avons synthétisé les dites morphologies. Après caractérisation de la réactivité par ATG et thermographie Infrarouge nous avons optimisé ces matériaux par un dopage extrinsèque tout d abord (dépôt de métaux précieux), puis par un dopage intrinsèque ensuite (Yttrium et Fer). Enfin, l obtention de morphologies non accessibles cristallographiquement nous a amené à approfondir le(s) processus de germination croissance de ces particules et la forte réactivité des matériaux dopés fer nous a poussé à une caractérisation fine de la microstructure de ces matériaux. Au final nous avons pu corréler l influence de la morphologie des cristallites sur la réactivité propre de l ensemble des familles de matériaux étudiés.Recent decades, numerous studies on cerium-based oxides have been realized and have revealed that cerium-based oxides were materials of choice in the field of heterogeneous catalysis. The aim now is therefore to increase the reactivity of these oxides, while expanding their range of optimal temperatures of Use. In this particular context, it seems possible to modulate the properties of cerium-based oxides by controlling the morphology of the crystallites. This thesis has been devoted to the identification, development and characterization of materials based on cerium oxides of controlled morphology. We first determined possible morphologies for the studied system, by hydrothermal synthesis by micro-wave assisted heating we have synthesized these morphologies and we characterized reactivity of these materials by infrared thermography and TGA. We have optimized these materials first, by extrinsic doping (deposition of precious metals) and then by intrinsic doping (Yttrium and Iron). New morphologies have been synthesized so we have studied the processes of nucleation-growth set in. The high reactivity of iron-doped materials has led us to a detailed characterization of the microstructure of these materials. Finally correlation reactivity/morphologies of crystallites have been achieved.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Infrared absorptive properties of Al-doped ZnO divided powder

Al-doped ZnO powders were synthesised by a Pechini process in order to obtain visible non-absorbent and near-Infrared absorbent particles. Firstly, it has been shown that synthesis under argon combined with the lowest synthesis temperatures (700 °C) allows getting the optimal properties for pure ZnO compounds due to creation of n-type defects segregated on oxide grain surface (Zn/O ratio superior to 1). Nevertheless, the near-Infrared absorption properties of the pure ZnO compounds remain low. The Al3+ doping of ZnO compounds was then investigated. The Al solubility limit inside ZnO doped compounds decreases drastically with the grain size, i.e. with the synthesis temperature. Then, the Al cations distribution varies inside ZnO grains, Al3+ segregation at the grain surfaces taking place for high synthesis temperatures. The optimal optical properties (high near-Infrared absorption) are reached combining Al-doping and adequate synthesis conditions: annealing under argon at low temperatures. In these conditions, the highest extrinsic (via Al doping) and intrinsic n-types defects rates are indeed reached

Characterization of helium bubble size and faceting by electron holography

Iron-9% chromium alloy of controlled purity and microstructure was helium implanted and studied using transmission electron microscopy and electron holography (EH). Fresnel imaging is compared with electron holography for a set of identical helium bubbles of size 2–4 nm. Electron holography reveals that the bubbles are, in fact, faceted and the lateral and projected dimensions of the cavities are measured. Current measurements allow the detection of 1 nm diameter bubbles using EH. The merits of conventional and holographic techniques are discussed and future improvements anticipated. Electron holography seems to be the ideal technique for investigating the morphology of helium bubbles and cavities in general