The high-pressure behavior of LuFe2O4 is characterized based on synchrotron X-ray diffraction and neutron diffraction, resistivity measurements, X-ray absorption spectroscopy and infrared spectroscopy studies. The results obtained enabled us to propose a P-T phase diagram. In this study, the low pressure charge-ordering melting could be detected by synchrotron XRD in the P-T space. In addition to the ambient pressure monoclinic C2/m and rhombohedral R(3) over bar m phases, the possible P (1) over bar triclinic phase, the monoclinic high pressure form Pm and metastable modulated monoclinic phases were observed; the latter modulated monoclinic phases were not observed in the present neutron diffraction data. Furthermore, the transition to the Pm phase which was already characterized by strong kinetics is found to be favored at high temperature (373 K). Based on X-ray absorption spectroscopy data the Pm phase, which could be recovered at atmospheric pressure, can be explained by a change in the Fe-local environment from a five-fold coordination to a distorted 5 + 1 one.

L'utilisation intensive des combustibles fossiles et l’émission des produits de leur combustion (principalement du CO2) dans l'air ont déjà impacté le climat mondial. Trouver des solutions technologiques permettant la conversion de l'économie mondiale aux carburants propres et renouvelables devient urgent. Une de possibilités consiste en utilisation de l’hydrogène comme un vecteur d’énergie. Aujourd’hui elle est limitée par l’absence d’un matériau permettant son stockage à des températures ambiantes et à des pressions modérées.Dans ce projet, nous explorons la possibilité de préparer un nouveau matériau pour un stockage réversible de l’hydrogène par physisorption : les carbones nanoporeux substitués au bore. Nous montrons que la synthèse en arc électrique peut être optimisée pour produire des structures graphitisées, avec la variété de tailles, de formes et d'interconnexions entre les fragments de graphène. Leur morphologie, structure, composition chimique et homogénéité de la distribution de l’hétéroatome dans la structure carbonée ont été caractérisés par les techniques SEM, HRTEM, EELS, XRD et spectroscopie RMN. La porosité et propriétés adsorptives ont été étudiées en utilisant les mesures d’adsorption de l’azote à T= 77 K.Les deux paramètres essentiels pour un stockage efficace de l’hydrogène dans les conditions ambiantes sont la surface spécifique de l’adsorbant et l’énergie avec laquelle les molécules du gaz sont adsorbées sur cette surface. Nous montrons que la surface spécifique d’adsorption peut être contrôlée et augmentée par une activation thermique ou chimique pour optimiser le stockage, et que la présence du bore dans les structures carbonées permet de doubler l’énergie d'adsorption d'hydrogène du matériau.

Hanang volcano is the southern volcano of, the southern area of the east part of the East African Rift (the North Tanzanian Divergence) and represents volcanic activity of the first stage of continental break-up. In this study, we investigate glassy melt inclusions in nepheline phenocrysts to constrain the late stage of Mg-poor nephelinite evolution and the behaviour of volatiles (CO2, H2O, S, F, Cl) during magma storage and ascent during early stage rifting. The melt inclusions have a green silicate glass, a carbonate phase and a shrinkage bubble free of gas phase indicating that carbonatite:silicate (18:82) liquid immiscibility occurred during nephelinite magmatic evolution. The silicate glasses have trachytic composition (Na + K/Al = 1.6–7.2, SiO2 = 54–65.5 wt%) with high CO2 (0.43 wt% CO2), sulfur (0.21–0.92 wt% S) and halogens (0.28–0.84 wt% Cl; 0.35–2.54 wt% F) contents and very low H2O content (<0.1 wt%). The carbonate phase is an anhydrous Ca-Na-K-S carbonate with 33 wt% CaO, 20 wt% Na2O, 3 wt% K2O, and 3 wt% S. The entrapped melt in nepheline corresponds to evolved interstitial CO2-rich phonolitic composition (Na + K/Al = 6.2–6.9) with 6 ± 1.5 wt% CO2 at pressure of 800 ± 200 MPa after crystallization of cpx (17%), nepheline (40%) garnet (6.5%) and apatite (1.7%) from Mg-rich nephelinitic magma. During ascent, immiscibility in phonolitic melt inclusions leads to Ca-Na carbonate melt with composition within the range of carbonate melt from Oldoinyo Lengai and Kerimasi, in equilibrium with trachytic silicate melt (closed-system, P < 500 MPa). The CO2-rich phonolitic melt inclusions from Hanang volcano may represent an early stage of differentiation before Na‑carbonatitic magmatism observed at Oldoinyo Lengai.

Aerogels are ultraporous materials with a very low permeability. They exhibithigh specific surface area but, due to their huge porous volume, their mechanical properties are quite poor. Aerogels belong to two main families: inorganic materials as silica, silicates, and a few aluminates and organic aerogels which are easily transformed into carbon aerogels by a simple pyrolysis thermal treatment. In this chapter, we mainly report experiments performed on alcohol-dried aerogels. Due to their broad range of porosity, aerogels canfitin very different applications: from analysis of cosmic dust to nuclear waste containment glasses. Advantages of large porosity are sometimes used directly like in thermal and acoustic insulation, or in catalyzers, but a too high pore volume can also be a drawback like in glass precursor and host matrix. Fortunately, aerogel porosity can be tailored using sintering or room isostatic compression or eventually a combination of both methods. Knowledge in this area allows now to synthesize aerogels with a broad range of porosity and also a very broad range of texture.

Une introduction à la spectroscopie Raman de résonance

Introduction to Raman spectroscopy of carbon nanostructures

Double-walled carbon nanotubes (DWNTs), consisting of two coaxial and weakly van der Waals coupled single-walled carbon nanotubes (SWNTs), are one of the most ideal and fundamental systems to study the effects of inter-tube coupling on the physical properties in one-dimensional structures.On this poster, we report on the investigations of individual, spatially isolated and suspended double-walled carbon nanotubes (DWNTs) by combination of high-resolution electron microscopy, electron diffraction, Rayleigh spectroscopy and Raman spectroscopy (Figure 1) [1,2]. We first present an overview of the effect of the diameters, inter-tube distance and nature (metallic or semiconducting) on the Raman signature (Radial Breathing Like [1,3,4] and Tangential Modes [4-6]) of DWNTs. We then examine the features of tangential modes and find a clear frequency shift of the inner-layer G modes as a function of the inter-tube distance with respect to the corresponding G modes in equivalent single-walled carbon nanotubes (SWNT).[4-6] These results are understood by considering the effects of the relaxation of the layers and the interaction between the relaxed structures.[7] Finally, we study the experimental excitation dependence of the G modes intensity of the constituent inner and outer SWNTs. In particular, we discuss the effects of the quantum interference between different electronic transitions on the observed behaviors.[8]References[1] D.I. Levshov et al., Carbon, 114, 141 (2017). [2] D.I. Levshov et al., Phys Rev. B 96 (2017), 195410.[3] D.I. Levshov et al., Nano Lett. 11(11) (2011), 4800. [4] H.-N. Tran et al., Adv. Nat. Sci. Nanosci. Nanotechnol. (2017), 8, 15018.[5] D. Levshov et al., J. Phys. Chem. C 119(40) (2015), 23196.[6] D.I. Levshov et al., Phys. Status Solidi B 254(11) (2017), 1700251.[7] V.N. Popov et al., Phys. Rev. B 97 (2018), 165417.[8] H.-N. Tran et al., Phys. Rev. B 95 (2017), 95 (20), 205411