Systèmes multifonctionnels à base de dioxydes de cérium nanostructurés (nanocomposites catalytiques CeO2-CuOx et bicouches CeO2-VO2 pour commutation en optoélectronique IR)

Le dioxyde de cérium présente de multiples propriétés physico-chimiques (catalytiques, conductimétriques, optiques) et entre déjà dans plusieurs applications courantes. On peut ainsi le classer dans la catégorie des matériaux multifonctionnels. Nous présentons ici deux études l'une portant sur les propriétés catalytiques de systèmes nanostructurés CeO2-CuOx, l'autre sur des systèmes de bicouches CeO2-VO2 thermochromes, à propriétés optiques modulables dans l'infrarouge (IR). Rappelons qu'une application très récente de la phase semi-conductrice de VO2 (T<68C) porte sur son utilisation dans des micro-bolomètres non refroidis (caméra IR). Le premier objectif a été d'élaborer des nanocomposites à base de dioxyde de cérium associés à des additifs à base d'oxydes de cuivre (CuO-Cu2O), afin d'évaluer leur intérêt en vue de détection de gaz. Ces systèmes ont été élaborés à partir de méthodes de co-précipitation, " Sol-Gel " et pulvérisation à basse température. Les propriétés catalytiques de ces composites ont été étudiées à partir d'un montage spécifique mettant en jeu une analyse infrarouge, à température variable et sous flux gazeux à faibles teneurs en CH4 ou CO. Un dispositif "capteur de gaz", reposant sur un multivibrateur "astable" a été réalisé, en partenariat industriel. Les mesures catalytiques révèlent une influence du taux d'additif en cuivre, couplée à une évolution granulométrique caractéristique. Une première modélisation faisant appel à une approche de type Johnson Mel Avrami est proposée pour ce système. Des durées de vie ont été définies et laissent envisager un fort potentiel pour la détection de gaz toxiques. Le second objectif était de réaliser des bicouches CeO2/VO2 où la couche de cérine servirait de protection vis-à-vis de la couche de dioxyde de vanadium, sans toutefois affecter ses propriétés optiques, notamment sa transition thermochrome à 68C. Les résultats obtenus ont permis de mieux connaître les paramètres clés du dépôt des couches de CeO2 et de VO2 par la technique de pulvérisation cathodique RF sur des substrats de Si(001) et SiO2. Des couches de VO2 ont été élaborées à partir d'une cible " à bas coût " de V2O5. L'étude optique des bicouches a montré que la couche de CeO2 ne perturbe pas les contrastes, en transmission et en réflexion, liés à la transition semi-conducteur / métal de VO2 dans le domaine infrarouge. Néanmoins, une influence de l'épaisseur du dépôt de cérine sur le facteur d'émissivité de ces bicouches a été observée. Ces systèmes présentent donc un double avantage : (a) protection de la couche de VO2 donc augmentation de la durée de vie de dispositifs miniaturisés, et (b) modulation de la signature thermique infrarouge (détection nocturne).Cerium dioxide is known as presenting a large variety of physico-chemical properties: it can be considered as a multifunctional compound. We present two studies linked to two types of applications in which cerium dioxide is involved as a multifunctional phase: catalytic behaviours of nanocomposite systems CeO2- CuOx for gas sensor applications, optical responses and chemical stability of CeO2-VO2 bilayers for infrared applications. This last study is connected with new technologies like bolometric applications (IR camera). The first study consisted in preparing nanostructured systems CeO2-CuOx from low temperature routes (soft chemistry including sol-gel, low temperature processes). This nanocomposites were characterized from X-ray diffraction, electron microscopy techniques, BET analyses. Infrared spectroscopy associated with homemade equipment was then used to follow catalytic reactions in presence of gas mixtures (air-CH4 or air-CO). A modelling approach is presented to interpret the IR analyses and the catalytic site saturation, observed in the case of ceria based systems. The CuOx ceria samples were then compacted and inserted in a homemade electronic system to analyse the influence of CO or CH4 reducing gases, on surface conduction of CuOx-CeO2 pellets. The results obtained from FTIR analyses and electronic measurements are found to be coherent: the copper additions decrease the durability of catalytic responses of these nanocomposite systems, however they increase the sensibility in the case of low Cu additions. This might be interesting for gas sensor applications.The second study deals with CeO2-VO2 bilayers obtained from sputtering techniques and deposited in Si and SiO2 substrates. RF sputtering parameters were optimised to deposit first VO2 then CeO2 thin layers. The role of CeO2 was to insure a protection of the VO2 layer from aggressive environments, without degrading optical responses. Starting from a low cost target of V2O5 , layers of VO2 were obtained. We show that these bilayers present a good thermochromic optical contrast (insulating-metal transition at 68C). Ceria is known as being transparent for IR wavelengths and opaque for UV radiation. However, the emissivity depends on ceria layer thickness. These new bilayers present a double interest: they can present increased chemical stability (improvement of lifetimes), and they can be used to modulate infrared emissivity responses in the case of IR detection.TOULON-BU Centrale (830622101) / SudocSudocFranceF

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International audienceThe mechanically assisted synthesis of organic compounds has recently focused considerable attention as it may be unique in features to selectively direct the reaction pathway. In the continuation of our work on the synthesis of modified cyclodextrins (CDs) via mechanochemical activation, we sought to discriminate the contribution of supramolecular effects and grinding during the course of a reaction in the solid state. As such, we recently investigated the influence of the particle size of β-CD in the synthesis of β-CD mesitylene sulfonate (β-CDMts) in the solid state using a vibrating ball-mill. We were particularly interested in the role of the particle size on the kinetics of the reaction. In this study, we show that grinding β-CD reduces the particles size over time down to a limit of 167 nm. The granulometric composition remains rather invariant for grinding times over 1 h. Each type of β-CD particles reacted with mesitylenesulfonyl chloride (MtsCl) to produce β-CDMts. Contrary to what could be intuitively anticipated, smaller particles did not lead to the highest conversions. The impact of grinding on the conversion was limited. Interestingly, the proportion of β-CDMts mono-substituted on the primary face significantly increased over time when the reaction was carried out in the presence of KOH as a base. The data series were confronted with kinetics models to get insight in the way the reactions proceeded. The diversity of possible models suggests that multiple mechanochemical processes can account for the formation of β-CDMts in the solid state. Throughout the study, we found that the reactivity depended more upon diffusion phenomena in the crystalline parts of the material than on the increase in the surface area of the CD particles resulting from grinding

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International audienceHydrogen is a promising energy carrier, compatible with the sustainable energy concept. In this field, solid-state hydrogen-storage is a key challenge in developing hydrogen economy. The capability of absorption of large quantities of hydrogen makes intermetallic systems of particular interest. Light metals, especially magnesium, possess a high gravimetric hydrogen density but not the capabilities of good kinetic and absorption/desorption process. Improvement of these can be achieved by magnesium based alloys and in this context, theoretical prediction of new structures would be useful to orient prospective experimental synthesis. In this work, efforts have been devoted to the theoretical investigation of binary systems with pressure consideration. An efficient prediction of stable alloys under pressure for magnesium-based system was performed at ab initio level. The effect of pressure change radically the minimal energy compositions and various rich-Mg compounds were found. Results are in agreement with recent exploration and new synthesis methods. For interesting alloys, a careful investigation of potential hydrides has been performed, and electronic properties denote interesting informations on hydrogen atom behaviour in magnesium-based alloys. Results are giving attractive insights on identifying destabilized metal hydrides and encouraging the use of similar work to design hydride systems

Photocatalytic and Photocurrent Responses to Visible Light of the Lone-Pair-Based Oxysulfide Sr 6 Cd 2 Sb 6 S 10 O 7

International audienceWe present a combined experimental and computational study on the recently reported oxysulfide Sr 6 Cd 2 Sb 6 S 10 O 7. Our spectroscopy and photoelectrochemical measurements, and tests for photocatalytic activity, indicate the potential of Sr 6 Cd 2 Sb 6 S 10 O 7 for photocatalytic applications. In particular, the transient photocurrent response shows a reproducible photogenerated current which depends on light intensity and which indicates an efficient electron-hole separation upon visible light illumination. Density functional theory calculations, combined with crystal orbital Hamiltonian population analysis, give insights into the electronic structure of Sr 6 Cd 2 Sb 6 S 10 O 7 and the origin of its physical properties. Our comprehensive investigation into Sr 6 Cd 2 Sb 6 S 10 O 7 reveals the roles of its polar structure, polar Sb 3+ coordination environments and the 5s 2 lone pair in making this compound a potential candidate for solar water splitting photocatalysis

Temperature-Dependent Luminescence of Red-Emitting Ba2Y5B5O17: Eu3+ Phosphors with Efficiencies Close to Unity for Near-UV LEDs

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