Far infrared absorption by acoustic phonons in titanium dioxide nanopowders

We report spectral features of far infrared electromagnetic radiation absorption in anatase TiO2 nanopowders which we attribute to absorption by acoustic phonon modes of nanoparticles. The frequency of peak excess absorption above the background level corresponds to the predicted frequency of the dipolar acoustic phonon from continuum elastic theory. The intensity of the absorption cannot be accounted for in a continuum elastic dielectric description of the nanoparticle material. Quantum mechanical scale dependent effects must be considered. The absorption cross section is estimated from a simple mechanical phenomenological model. The results are in plausible agreement with the absorption being due to a sparse layer of charge on the nanoparticle surface.Comment: 8 pages, 5 figures, submitted to Journal of Nanoelectronics and Optoelectronic

Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment

The inelastic scattering of neutrons by nanoparticles due to acoustic vibrational modes (energy below 10 meV) confined in nanoparticles is calculated using the Zemach-Glauber formalism. Such vibrational modes are commonly observed by light scattering techniques (Brillouin or low-frequency Raman scattering). We also report high resolution inelastic neutron scattering measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling the observation of such vibrations are discussed. These include a narrow nanoparticle size distribution which minimizes inhomogeneous broadening of the spectrum and the presence of hydrogen atoms oscillating with the nanoparticle surfaces which enhances the number of scattered neutrons.Comment: 3 figures, 1 tabl

Continuous hydrothermal synthesis of nanometric BaZrO3 in supercritical water

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Low-frequency Raman characterization of size-controlled anatase TiO2 nanopowders prepared by continuous hydrothermal syntheses

7 pagesInternational audienceThe advantages of a recently presented continuous hydrothermal elaboration route for size-controlled anatase TiO2 nanoparticles are investigated. Nanopowders prepared by this route and using a soft chemistry route are characterized using X-ray diffraction, surface area measurements, high-resolution electron microscopy and Raman spectroscopy with an emphasis on the determination of the size distribution with low-frequency Raman measurements. The continuous hydrothermal route is shown to be more suitable for producing narrower size distributions

Characteristics of LiFePO4 obtained through a one step continuous hydrothermal synthesis process working in supercritical water

International audienceThe olivine-like material LiFePO4 was prepared via a continuous hydrothermal synthesis process working from subcritical to supercritical water conditions. The influence of some processing parameters–temperature and reaction time–was investigated in terms of material purity, grain size and morphology. Supercritical conditions were found to be attractive to synthesize in one step a well-crystallized material without impurities. The primary particles size was in the nanometric range. They showed a natural tendency to form micron size agglomerates, which were supposed to be the cause of the limited capacity, as demonstrated through a cross study using laser particle size distribution analysis, electrochemical measurements and XRD at different Li contents

A comparative study of methanol carbonation on unsupported SnO2 and ZrO2

International audienceThe aim of this work was to explore the catalytic properties of SnO2 in the coupling of methanol with carbon dioxide to afford dimethyl carbonate. SnO2 nanopowders were produced by hydrolysis of tin tetra-tert-butoxide dissolved in n-butanol. The samples were much more active than a commercial one due to their higher surface areas. In addition, they exhibited excellent recyclability. However, comparison with ZrO2, prepared and tested under the same experimental conditions, showed that zirconia-based catalysts were more selective and are, among the heterogeneous catalysts already reported, still the more selective. SnO2 also catalyzed the formation of dimethyl ether likely due to lower rates in the formation of the key intermediate CH3OC(O)O–Sn and its subsequent alkylation by activated methanol. This lack of selectivity contrasts with that of soluble organotin(IV) which are totally selective to DMC formation. Structural characterization of SnO2 was performed by X-ray powder diffraction, laser Raman spectroscopy, transmission electron microscopy, and nitrogen isotherm. As found from X-ray diffraction line broadening, the crystallite size of all powders was in the nanometric range (cassiterite structure) which was confirmed by transmission electron microscopy. Moreover, the low-frequency Raman scattering allowed to determine an average particle size diameter of 4 nm

Insularités ottomanes

L’Empire ottoman était aussi un empire maritime. Cette facette de sa nature a suscité chez les historiens un intérêt croissant depuis quelques années. Consacré à l’« insularité ottomane », le présent volume est une participation originale aux réflexions en cours. Son objet n’est pas de rassembler des monographies, mais de répondre à la question suivante : en quoi les îles étaient-elles des territoires différents des autres, avec des populations spécifiques, des problèmes spécifiques et des solutions spécifiques ? À partir de sources souvent inédites, les contributeurs s’interrogent ainsi sur la logique présidant à la conquête des îles, sur leur organisation et sur la façon dont la Porte considérait ces territoires un peu particuliers. Abordant également la question du point de vue des îles elles-mêmes, ils se penchent sur la composition ethnique et religieuse des sociétés insulaires, évaluent leur éventuel caractère de conservatoire ethnographique, examinent les modalités des communications des îles entre elles et avec le reste du monde, soulignent les problèmes de sécurité liés notamment au risque pirate. Enfin des éléments de comparaison avec d’autres insularités méditerranéennes permettent de mieux cerner les spécificités ottomanes. Un intérêt particulier est évidemment accordé aux îles méditerranéennes, mais d’autres îles ottomanes, qui n’avaient jamais fait l’objet d’études, sont également traitées : îles de la mer Rouge, îles du Danube

Texte/image — Nouveaux problèmes

Ce volume collectif voudrait témoigner de la variété des questionnements et des approches que suscitent aujourd’hui les études engagées sur le terrain de la confrontation raisonnée des pratiques littéraires (tous genres confondus) et des artefacts artistiques visuels (des réalisations plastiques, figuratives ou non, aux images écrites). Il se propose de rendre compte de la vivacité d’une réflexion qui déplace les frontières notionnelles, redéfinit ses objets et décloisonne ses champs d’investigation et d’application. Le titre « Texte/Image : nouveaux problèmes » avoue cette intention, en même temps qu’il atteste, explicitement, le refus de tout parti pris théorique exclusif. Au dialogue du texte et de l’image correspond ainsi le dialogue ouvert ou le débat contradictoire des chercheurs qui ont participé au colloque de Cerisy-la-Salle et dont les communications recueillies dans le présent volume résument ou développent les propositions