Evaluation of T2G band intensity distribution across a surface of an UO2 ceramic

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Highly Nonstoichiometric YAG Ceramics with Modified Luminescence Properties

Y3Al5O12 (YAG) is a widely used phosphor host. Its optical properties are controlled by chemical substitution at its YO8 or AlO6/AlO4 sublattices, with emission wavelengths defined by rare-earth and transition-metal dopants that have been explored extensively. Nonstoichiometric compositions Y3+xAl5-xO12 (x ≠ 0) may offer a route to new emission wavelengths by distributing dopants over two or more sublattices simultaneously, producing new local coordination environments for the activator ions. However, YAG typically behaves as a line phase, and such compositions are therefore challenging to synthesize. Here, a series of highly nonstoichiometric Y3+xAl5-xO12 with 0 ≤ x ≤ 0.40 is reported, corresponding to ≤20% of the AlO6 sublattice substituted by Y3+, synthesized by advanced melt-quenching techniques. This impacts the up-conversion luminescence of Yb3+/Er3+-doped systems, whose yellow-green emission differs from the red-orange emission of their stoichiometric counterparts. In contrast, the YAG:Ce3+ system has a different structural response to nonstoichiometry and its down-conversion emission is only weakly affected. Analogous highly nonstoichiometric systems should be obtainable for a range of garnet materials, demonstrated here by the synthesis of Gd3.2Al4.8O12 and Gd3.2Ga4.8O12. This opens pathways to property tuning by control of host stoichiometry, and the prospect of improved performance or new applications for garnet-type materials.Financial support was provided by the ANR-18-CE08-0012 PERSIST and ANR-20-CE08-0007 CAPRE projects of the French National Research Agency (ANR) and the CNRS, the I+D+I Grants PID2021-122328OB-100 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. PhD studentships for WC and XF were financed by the Chinese Scholarship Council (project numbers 201808450100 and 202008450026). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. EXAFS beamtime was provided by the SOLEIL synchrotron (Gif-sur-Yvette, France) under project 99210047. The project benefitted from the microscopy facilities of the Platform MACLE-CVL which was co-funded by the European Union and Centre-Val de Loire Region (FEDER).Peer reviewe

Tunable SiO2 to SiOxCyH films by ozone assisted chemical vapor deposition from tetraethylorthosilicate and hexamethyldisilazane mixtures

Silica and silica-based materials with tunable functionalities are frequently encountered in low-k material applications, porous membranes, and microelectonic devices. In the present study, an innovative O2/O3 assisted CVD process for the deposition of such films at moderate temperature is presented, based on a dual precursor chemistry from hexamethyldisilazane (HMDS) and tetraethyl orthosilicate (TEOS). Films with tunable carbon content were obtained through variation of the HMDS flow ratio. A comprehensive FT-IR study reveals the transition of the material from a SiOxCyH type film containing -CH3 moieties, to a methyl-free SiOx film with the increase of the temperature. At the same time the water contact angle of 81.0° at 400°C is decreased to 52.8° at 550°C, related to the absence of methyl moieties in the latter. Ion beam analysis (IBA) confirms the lack of carbon in the films when deposition temperatures are equal to or exceed 500°C. The resistance to liquid corrosion is investigated as a function of the deposition temperature; SiOx type films present a low Pliskin etching rate of 15 Å.s-1, with this value increasing to 60 Å.s-1 for the SiOxCy:CH3 films produced at the lower temperatures. It is found that the addition of HMDS to a TEOS chemistry can be utilized to modulate the film composition from SiOx to SiOxCyH and by such, tune the film functional properties, in particular its etching rate, opening the way to the development of new sacrificial films

STEM nanoanalysis of Au/Pt/Ti-Si3N4 interfacial defects and reactions during local stress of SiGe HBTs

A new insight on the behavior of metal contact-insulating interfaces in SiGe heterojunction bipolar transistor is given by high-performance aberration-corrected scanning transmission electron microscopy (STEM) analysis tools equipped with sub-nanometric probe size. It is demonstrated that the presence of initial defects introduced during technological processes play a major role in the acceleration of degradation mechanisms of the structure during stress. A combination of energy-filtered transmission electron microscopy analysis with high angle annular dark field STEM and energy dispersive spectroscopy provides strong evidence that migration of Au-Pt from the metal contacts to Ti/Si3N4 interface is one of the precursors to species interdiffusion and reactions. High current densities and related local heating effects induce the evolution of the pure Ti initial layer into mixture layer composed of Ti, O, and N. Local contamination of Ti layers by fluorine atoms is also pointed out, as well as rupture of TiN thin barrier layer

Network hydration, ordering and composition interplay of chemical vapor deposited amorphous silica films from tetraethyl orthosilicate

The chemical or mechanical performance of amorphous SiO2 films depend on intrinsic physicochemical properties, which are intimately linked to atomic and molecular arrangements in the Si–O–Si network. In this context, the present work focuses on a comprehensive description of SiO2 films deposited from a well-established chemical vapor deposition process involving tetraethyl-orthosilicate, oxygen and ozone, and operating at atmospheric pressure in the range 400–550 °C. The connectivity of the silica network is improved with increasing the deposition temperature (Td) and this is attributed to the decreased content of hydrated species through dehydration-condensation mechanisms. In the same way, the critical load of delamination increases with increasing Td thanks to the silicon substrate oxidation. The utilization of a O2/O3 oxidizing atmosphere involving the oxidation of intermediates species by O2, O3 and O., allows increasing the deposition rate at moderate temperatures, while minimizing carbon, H2O and silanol contents to extremely low values (4.5 at.% of H). The SiOx stoichiometry and Td interplay reveals two distinct behaviors before and above 450 °C. The best corrosion resistance of these films to standard P-etching test is obtained for the minimum silanol content and the best network molecular ordering, with an etching rate of 4.0 ± 0.1 Å/s at pH = 1.5. The elastic modulus and hardness of the films remain stable in the investigated range of deposition temperature, at 64.2 ± 1.7 and 7.4 ± 0.3 GPa respectively, thanks to the low content in silanol groups

La microscopie électronique en transmission appliquée au développement de nouvelles (vitro-)céramiques

Le contrôle de la cristallisation dans les verres d’oxydes est une approche originale développée au laboratoire pour synthétiser de nouvelles (vitro-)céramiques transparentes. Ces matériaux présentent parfois des microstructures assez éloignées des normes couramment admises (taille de grains nanométrique et système à symétrie cubique) comme dans le cas des céramiques biphasiques. La conservation de la transparence du verre parent lors de la cristallisation demande alors la maîtrise fine des compositions mais également du procédé de cristallisation. Pour cela, il apparaît nécessaire de bien comprendre les mécanismes mis en œuvre durant la cristallisation du verre parent. C’est dans ce cadre que mes activités autour de la Microscopie Electronique en Transmission seront présentées au travers de résultats marquants.Lors de ces travaux, nous avons pu mettre en lumière le potentiel de la cristallisation dans les verres d’oxydes pour l’exploration et l’élaboration de nouvelles phases cristallines jusqu’alors inaccessibles par voie solide classique. Pour cela, nous nous sommes notamment appuyés sur des synthèses utilisant un procédé original développé au laboratoire CEMHTI : la lévitation aérodynamique couplée à un chauffage laser. Ce procédé sans contact permet d’éviter les phénomènes de cristallisation hétérogènes de surface, d’atteindre de très hautes températures de synthèse et des vitesses de refroidissement rapides. Il est alors possible d’explorer un large domaine de compositions, conduisant ainsi à la synthèse de nouvelles phases cristallines. La résolution structurale de ces nouvelles phases ou nouveaux polymorphes a été réalisée en utilisant une approche couplant différentes techniques de pointes (DRX, RMN, Raman …). C’est dans ce cadre que je m’emploierai à présenter l’apport que représente la Microscopie Electronique en Transmission via le développement méthodologique de techniques analytiques à l’échelle de la colonne atomique, dans le but de sonder l’organisation structurale mais également la distribution d’un écart à la structure moyenne ainsi qu’à l’existence de défauts

Genèse des microstructures lors du soudage par friction malaxage d'alliages d'aluminium de la série 2000 & 5000 et comportement mécanique résultant

The 2024 alloy (Al-Cu-Mg) is used for minimizing the weight of structural components in the transportation industry. However, this alloy is not easy to weld by traditional techniques. Friction stir welding (FSW) is a recently developed solid state process which removes the solidification defects. In this study, the microstructures of FSW welds of this alloy were finely characterized by SAXS, DSC, TEM, SEM, EBSD and optical microscopy. In order to highlight the interactions between deformation, precipitation and recrystallisation, which all take place during the welding of the 2024 alloy, model experiments were carried out as well as a comparative study between the alloys 5251 and 2024. The combination of the welding characterisation and the model experiments allow to define the metallurgical phenomena controlling the mechanical strength of the welded joints and their microstructure. In addition, a detailed characterisation of the mechanical behaviour of the welded joints was carried out, validated by a finite element model.L'alliage 2024 (Al-Cu-Mg) est utilisé dans le cadre de l'allègement des structures de transport. Cependant, cet alliage est difficilement soudable par voie classique. Le soudage par friction malaxage (FSW) est un nouveau procédé permettant l'assemblage à l'état solide et donc de supprimer les défauts liés à la solidification. A travers cette étude, les microstructures de soudures FSW de cet alliage ont été finement caractérisées par SAXS, DSC, MET, MEB, EBSD et microscopie optique. Afin de mettre en évidence les interactions entre la déformation, la précipitation et la recristallisation qui ont lieu durant le soudage de l'alliage 2024, des expériences modèles ont été menées ainsi qu'une étude comparative entre l'alliage 2024 et l'alliage 5251. L'ensemble de la caractérisation des soudures et des expériences modèles permet de dégager les phénomènes métallurgiques pertinents contrôlant la résistance mécanique des joints soudés et leur microstructure. Par ailleurs, une caractérisation détaillées du comportement mécanique des joints soudés a été menée, validée par une modélisation aux éléments finis

Genèse des microstructures lors du soudage par friction malaxage d'alliages d'aluminium de la série 2000 & 5000 et comportement mécanique résultant

L’alliage 2024 (Al-Cu-Mg) est utilisé dans le cadre de l’allègement des structures de transport. Cependant, cet alliage est difficilement soudable par voie classique. Le soudage par friction malaxage (FSW) est un nouveau procédé permettant l’assemblage à l’état solide et donc de supprimer les défauts liés à la solidification. A travers cette étude, les microstructures de soudures FSW de cet alliage ont été finement caractérisées par SAXS, DSC, MET, MEB, EBSD et microscopie optique. Afin de mettre en évidence les interactions entre la déformation, la précipitation et la recristallisation qui ont lieu durant le soudage de l’alliage 2024, des expériences modèles ont été menées ainsi qu’une étude comparative entre l’alliage 2024 et l’alliage 5251. L’ensemble de la caractérisation des soudures et des expériences modèles permet de dégager les phénomènes métallurgiques pertinents contrôlant la résistance mécanique des joints soudés et leur microstructure. Par ailleurs, une caractérisation détaillées du comportement mécanique des joints soudés a été menée, validée par une modélisation aux éléments finis.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

On the coupling between precipitation and plastic deformation in relation with friction stir welding of AA2024 T3 aluminium alloy

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Atomic scale characterization of deformation induced interfacial mixing in a Cu/V nanocomposite wire

International audienceThe microstructure of a Cu/V nanocomposite wire processed by cold drawing was investigated by high resolution transmission electron microscopy and atom probe tomography. The experimental data clearly reveal some deformation induced interfacial mixing where the vanadium filaments are nanoscaled. The mixed layer is a 2nm wide vanadium gradient in the fcc Cu phase. This mechanical mixing leads to the local fragmentation and dissolution of the filaments and to the formation of vanadium super saturated solid solutions in fcc Cu