Vincent Sarou-Kanian

High-Temperature Properties of Liquid Boron from Contactless Techniques

The density, surface tension, and spectral and total hemispherical emissivities of liquid boron obtained with contactless diagnostics are reported for temperatures between 2360 and 3100 K. It is shown that, contrary to previous expectations, liquid boron is denser than the solid at its melting point. It is also shown that the high total emissivity of 0.36 is not consistent with that of a liquid metal as recently claimed. Finally, good agreement is found with previously reported surface tensions and spectral emissivities of liquid boron.

Measuring self-diffusion coefficients up to 1500 K: a powerful tool to investigate the dynamics and the local structure of inorganic melts.

Self-diffusion is a fundamental property of liquid dynamics that also provides important structural information. To explore the dynamics in inorganic melts with high liquidus temperature, we propose a new setup based on pulsed field gradient NMR combined with laser heating that makes possible in situ self-diffusion coefficient measurements up to 1500 K. Applied to several corrosive molten fluorides in a wide range of compositions and temperature, we have evidenced the different key parameters of their motion along with their structural characteristics. In alkali fluorides, the self-diffusion coefficient of fluorine depends slightly on the composition compared to the temperature, displaying these systems as an ideal bath of polarizable hard spheres. In contrast, self-diffusion in rare earth and alkali fluorides mixtures presents a complicated balance between temperature and the network-forming process of the ionic long-lived units. These results open wide perspectives in the study of high temperature liquids.

Structure and dynamics in yttrium-based molten rare earth alkali fluorides.

The transport properties of molten LiF-YF3 mixtures have been studied by pulsed field gradient nuclear magnetic resonance spectroscopy, potentiometric experiments, and molecular dynamics simulations. The calculated diffusion coefficients and electric conductivities compare very well with the measurements across a wide composition range. We then extract static (radial distribution functions, coordination numbers distributions) and dynamic (cage correlation functions) quantities from the simulations. Then, we discuss the interplay between the microscopic structure of the molten salts and their dynamic properties. It is often considered that variations in the diffusion coefficient of the anions are mainly driven by the evolution of its coordination with the metallic ion (Y(3+) here). We compare this system with fluorozirconate melts and demonstrate that the coordination number is a poor indicator of the evolution of the diffusion coefficient. Instead, we propose to use the ionic bonds lifetime. We show that the weak Y-F ionic bonds in LiF-YF3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward one of the yttrium cation when the content in YF3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.

On the role of carbon dioxide in the combustion of aluminum droplets

ABSTRACT In this paper, the influence of carbon dioxide on the combustion of aluminum droplets is investigated. Millimeter-sized droplets were heated and ignited by a laser in an aerodynamic levitation system in several CO2 containing atmospheres (H2O/CO2, H2O/CO2/N2) with a large range of compositions (wet – xH2O < 3%, 80/20, 50/50, 12.5/87.5, 50/25/25, 20/40/40). The combustion processes were observed with a high-speed CCD camera, and the droplet radiation was recorded by two optical pyrometers. The ignition occurs with the oxide coating breakdown, which liquefies to form an initial cap moving on the droplet surface. Aluminum vaporizes and burns with the oxidizers as a detached diffusion flame. The droplet regression rate, i.e., the burning rate, strongly depends on the oxidizing atmosphere, from β = 0.58 mm2/s in wet 50% CO2/50% N2 to β = 2.45 mm2/s in 80% H2O/20% CO2. It is shown that CO2 is the worst oxidizer with a smaller “oxidizer efficiency” compared to H2O and O2 . The burning droplet temperature in wet CO2 and in the H2O/CO2 mixtures is around T ≈ 2600 K, and is smaller in H2O/CO2/N2 (T ≈ 2450 K). The non-correlation between the burning rates and the droplet temperatures confirms that the combustion processes are limited by molecular diffusion, and highlights the influence of H2 in the gas-phase transport. An estimation of the exponent n of the “d n law” shows that n decreases with the increase of CO2 from n = 1.7 to n = 0.6. The stagnant burning rates are evaluated as being 7 to 9 times smaller than the measured ones with convective effects. Furthermore, during the droplet regression, the oxide cap dimensions also regress, and it is generally completely removed. Oxide cap regression rates are estimated and are slower than their respective burning rates. Nevertheless, there is a good correlation between the oxide cap regression rate and the droplet temperature that shows that the oxide cap regression results from the chemical decomposition of Al2O3 by the liquid Al droplet producing gaseous AlxOy. For CO2 concentration higher than 40%, a solid phase suddenly appears on the liquid Al surface which entirely covers the droplet leading to the end of the gas-phase combustion. This phenomenon is the consequence of the massive dissolution of carbon in the droplet during burning. Analyses of the unburnt residues showed amounts of dissolved carbon up to 20–23% molar, which is near the saturation concentration limit. Thus, the solid coating corresponds to the ejection of carbon from the droplet because of its continuing regression and is expected to be also present for smaller particles. Therefore, carbon dioxide plays a double role. First, it participates in the gas-phase combustion, but it is the worse oxidizer with smaller burning rates. Second, CO2 causes the carbon dissolution in the Al droplet and finally stops the gas-phase burning. A further implication could be that carbon dioxide may promote the appearance of a combustion regime with surface reactions only.

Metabolite localization in living drosophila using High Resolution Magic Angle Spinning NMR

We have developed new methods enabling in vivo localization and identification of metabolites through their 1H NMR signatures, in a drosophila. Metabolic profiles in localized regions were obtained using HR-MAS Slice Localized Spectroscopy and Chemical Shift Imaging at high magnetic fields. These methods enabled measurement of metabolite contents in anatomic regions of the fly, demonstrated by a decrease in β-alanine signals in the thorax of flies showing muscle degeneration.

First-principles molecular dynamics simulation and conductivity measurements of a molten xLi2O-(1- x)B2O3 system.

The electronic properties and atomic structure of a molten xLi2O-(1 - x)B2O3 system were investigated by measuring conductivity and using first-principles molecular dynamics (MD) simulations. The conductivities obtained were converted to a Li self-diffusion coefficient Dσ, using the Nernst-Einstein equation to assess charge transfer mechanisms. Dσ was compared with a Li self-diffusion coefficient, DNMR, which we measured in a previous study using high-temperature pulsed field gradient NMR. The DNMR/Dσ of xLi2O-(1 - x)B2O3 (0.2 ≤ x ≤ 0.5) at 1250 K ranged from 2.5 to 3.2, following the same trend as room temperature ionic liquids. First-principles MD simulations were performed using our own finite element density functional theory code, FEMTECK (finite element method-based total energy calculation) for molten xLi2O-(1 - x)B2O3 systems at 1250 K. We found that the O-B-O angle distribution functions were characterized by a peak at approximately 120°. Although the electron number from the electronic radial distribution function was arbitrary with regard to the cutoff distance, the net Li charge calculated from the integrated electron number surrounding Li was approximately 0.9 at 0.085 nm. The mean square displacement (MSD) of Li as a function of time was evaluated from the atomic configuration. Li self-diffusion coefficients calculated from the MSD were in better agreement with experimental results than they were using classical MD.

Following lithiation fronts in paramagnetic electrodes with in situ magnetic resonance spectroscopic imaging

Li-ion batteries are invaluable for portable electronics and vehicle electrification. A better knowledge of compositional variations within the electrodes during battery operation is, however, still needed to keep improving their performance. Although essential in the medical field, magnetic resonance imaging of solid paramagnetic battery materials is challenging due to the short lifetime of their signals. Here we develop the scanning image-selected in situ spectroscopy approach, using the strongest commercially available magnetic field gradient. We demonstrate the 7Li magnetic resonance spectroscopic image of a 5 mm-diameter operating battery with a resolution of 100 μm. The time-resolved image-spectra enable the visualization in situ of the displacement of lithiation fronts inside thick paramagnetic electrodes during battery operation. Such observations are critical to identify the key limiting parameters for high-capacity and fast-cycling batteries. This non-invasive technique also offers opportunities to study devices containing paramagnetic materials while operating.

Transport Properties in Cryolitic Melts: NMR Measurements and Molecular Dynamics Calculations of Self-Diffusion Coefficients

Owing to their industrial interest, cryolite-based melts have been extensively studied. It seems however difficult to obtain a structural model consistent with all the experimental data available. The structure of molten NaFAlF3 mixtures have been investigated by Raman spectroscopy, using a captive liquid windowless cell [1] and NMR spectroscopy at high temperature [2] confirming the dissociation scheme proposed by Gilbert et al. for NaF-AlF3 based on the dissociation of AlF6 into AlF5 and AlF. The structural information given by high temperature Nuclear Magnetic Resonance by means of chemical shifts modifications with temperature can be now combined with the measurement of self-diffusion coefficients. This is possible thanks to the development of a new setup based on Pulsed Field Gradient NMR combined with laser heating [3],[4]. This new device makes possible in situ self-diffusion coefficients measurements up to 1500K. In several corrosive molten fluorides in a wide range of compositions and temperature, we have evidenced the different key parameters of their motion along with their structural characteristics. [4],[5]

Investigation of Fluoroacidity in Molten Fluorides by the Combination of High Temperature NMR and Molecular Dynamics

The control of acidity is a well-known requirement of any chemistry experiment performed in an aqueous solvent. In such liquids, acidity is given by a unique quantity, the pH. When dealing with non-aqueous solvents, another definition, which is due to Lewis, has to be used for the acidity concept: it is based on electron pair donors/acceptors. In molten salts, the exchanged species that defines the acid-base systems is the anion, for example O in molten oxides, or F in fluorides. Acidity scales are then built up by classifying the strengths with which different acids react with the different bases for a wide variety of molten salts except in molten oxides and fluorides. This is due to their strong reactivity and high melting temperature, which makes them difficult to handle properly in the laboratory scale. Nevertheless, they are widely used in important industrial applications: oxides in the glass and ceramics industry, fluorides in the electrowinning of aluminium. Our objective is to build quantitative scales of fluoroand oxo-acidities within a combined experimental and theoretical approach. The acidity of a molten salt will act directly on the formation of complexes involving the cations of the melt. The determination of the speciation with varying overall composition therefore provides an indirect measure of the acidity. This opens a route for building up the acidity scale through the use of some specific in situ spectroscopic approaches. To determine the speciation in the liquid state, Nuclear Magnetic Resonance (NMR) appears as the ideal candidate, as it allows estimating the relative concentration of the various complexes formed in the melt. In parallel, theoretical calculations are the unavoidable complement to this experimental technique. As the systems dealt with are liquids at high temperature, the Molecular Dynamics (MD) simulations provide an adequate level of description.

ZSM-5 Zeolite: Complete Al Bond Connectivity and Implications on Structure Formation from Solid-State NMR and Quantum Chemistry Calculations

Al site distribution in the structurally complex and industrially important ZSM-5 zeolite is determined by studying the spectroscopic response of Al(OSi)4 units and using a self-consistent combination of up-to-date solid-state NMR correlations (29Si-27Al and 1H-27Al D-HMQC) and quantum chemistry methods (DFT-D). To unravel the driving forces behind specific Al sitting positions, our approach focuses on ZSM-5 containing its more efficient OSDA, tetrapropylammonium.