Luc Girard

Improved thermoelectric properties in double-filled Cey∕2Yby∕2Fe4−x(Co∕Ni)xSb12 skutterudites

We have investigated the thermal and electrical properties of filled skutterudites belonging to the series Ce1−zYbzFe4Sb12 and Cey∕2Yby∕2Fe4−x(Co∕Ni)xSb12, as well as their potential for thermoelectric power generation. In the first series, increasing the Yb fraction decreases the resistivity and thermopower. These effects are related to the variations of valence of Yb with the Yb fraction (from 2.2 in Yb0.92Fe4Sb12 to 2.7 in Ce0.85Yb0.05Fe4Sb12). The power factor is increased in Ce0.40Yb0.53Fe4Sb12 while the thermal conductivity is reduced and this leads to an improved figure of merit in this compound when compared to Ce0.85Fe4Sb12. Co or Ni substitution was used to tune the carrier concentration and improve the power factor: we measure a ZT=0.5 at 500K in Ce0.44Yb0.32Fe3.02Co0.98Sb12 and an extrapolation leads to ZT=0.95 at 800K, which is close to state-of-the-art Ce0.28Fe1.52Co2.48Sb12 although the composition is far from being optimized. Double filling happens to be a promising path to provide good th...

Surface Activity and Molecular Organization of Metallacarboranes at the Air–Water Interface Revealed by Nonlinear Optics

Because of their amphiphilic structure, surfactants adsorb at the water-air interface with their hydrophobic tails pointing out of the water and their polar heads plunging into the liquid phase. Unlike classical surfactants, metallabisdicarbollides (MCs) do not have a well-defined amphiphilic structure. They are nanometer-sized inorganic anions with an ellipsoidal shape composed of two carborane semicages sandwiching a metal ion. However, MCs have been shown to share many properties with surfactants, such as self-assembly in water (formation of micelles and vesicles), formation of lamellar lyotropic phases, and surface activity. By combining second harmonic generation and surface tension measurement, we show here that cobaltabis(dicarbollide) anion {[(C2B9H11)2Co](-) also named [COSAN](-)} with H(+) as a counterion, the most representative metallacarborane, adsorbs vertically at the water surface with its long axis normal to the surface. This vertical molecular orientation facilitates the formation of intermolecular and nonconventional dihydrogen bonds such as the B-H(δ-)···(δ+)H-C bond that has recently been proven to be at the origin of the self-assembly of MCs in water. Therefore, it appears here that lateral dihydrogen bonds are also involved in the surface activity of MCs.

Solvent Extraction: Structure of the Liquid–Liquid Interface Containing a Diamide Ligand

Knowledge of the (supra)molecular structure of an interface that contains amphiphilic ligand molecules is necessary for a full understanding of ion transfer during solvent extraction. Even if molecular dynamics already yield some insight in the molecular configurations in solution, hardly any experimental data giving access to distributions of both extractant molecules and ions at the liquid-liquid interface exist. Here, the combined application of X-ray and neutron reflectivity measurements represents a key milestone in the deduction of the interfacial structure and potential with respect to two different lipophilic ligands. Indeed, we show for the first time that hard trivalent cations can be repelled or attracted by the extractant-enriched interface according to the nature of the ligand.

Experimental determination of the phonon density of states in filled skutterudites: evidence for a localized mode of the filling atom

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.

Structural changes induced by Ce filling in partially filled skutterudites

The structural properties of the Ce?Fe4?xNixSb12 series with 0.06???0.72 have been determined by neutron powder diffraction in the temperature range 10?300?K. Large isotropic atomic displacement parameters are observed for the cerium atom throughout the series, however with two distinct origins. The first term, purely dynamic, is well described as an Einstein oscillator and has essentially no dependence with the filling fraction ? and the lattice parameter. A vibrational frequency of 55(2)?cm?1 fitted the experimental data in the entire phase diagram. The second contribution is essentially static and well evidenced by a large residual displacement parameter extrapolated to 0?K. This latter term does depend strongly on ? and is maximum at around ? = 0.5, representing almost 50% of the room temperature value. A number of structural changes of the Sb sublattice are correlated with the disorder of Ce as well as disorder on the Fe/Ni sublattice.

Are Keggin’s POMs charged nano-colloids or multicharged anions?

Owing to their multiple charges and their nanometric size, polyoxometalates (POMs) are at the frontier between ions and charged colloids. We investigated here the effect of POM-POM electrostatics repulsions on their self-diffusion in water by varying POM and supporting salt concentrations. The self-diffusion coefficients of two Keggins POMs [silicotungstate (SiW12O404-) and phosphotungstate (PW12O403-)] were determined by dynamic light scattering (DLS) and 1H/31P DOSY NMR, whereas POM-POM electrostatic repulsions were investigated by the determination of the static structure factors using small-angle X-ray scattering (SAXS). The self-diffusion coefficients for the two POMs and for different POM/background salt concentrations were collected in a master curve by comparing the averaged POM-POM distance in solution to the Debye length. As for classical charged colloids, we show that the POMs counterions should not be considered in the calculation of the ionic strength that governs POM-POM electrostatic repulsions. This result was confirmed by fitting the POM-POM structure factor by considering a pair potential of spherical charged particles using the well-known Hayter mean spherical approximation (MSA). These Keggin POMs also behave as (super)chaotropic anions (i.e., they have a strong propensity to adsorb on (neutral polar) surfaces, which was also investigated) here on the surface of octyl-β-glucoside (C8G1) micelles. The variations of (i) the chemical shift of 1H/31P NMR signals and (ii) the self-diffusion coefficients obtained by DOSY 1H/31P NMR of PW3- and of C8G1 were in good agreement, confirming the strong adsorption of POMs on the micelle polar surface from static and dynamic points of view. We concluded that Keggins POMs behave (i) as anions because they adsorb on surfaces as chaotropic anions and (ii) as colloids because they can be described by a classical colloidal approach by dynamic and static scattering techniques (i.e., by the investigation of their interparticle electrostatic structure factor and self-diffusion without considering the POMs counterions in the ionic strength calculation). This work highlights the dynamic properties of POMs at soft interfaces compared to bulk aqueous solution, which is essential in the understanding of functional properties of POMs, such as (photo)catalysis and the rational design of POM-based hybrid nanomaterials from soft templating routes (i.e., in aqueous solutions at room temperature).

Self-assembly of a bio-based extractant in methyl esters: combination of small angle X-ray scattering experiments and molecular dynamics simulations

The knowledge of the supramolecular structure of a polar oil that contains a bio-based extractant is necessary for a deep understanding of solubilization processes of polar species in oils. This study focuses on a ternary system containing the polyglyceryl-3-diisostearate (PG3DS), a bio-based extractant in cosmetic formulation, methyl esters (ME-Cn) as model diluent and water. The realms of existence of monophasic regions were determined in the ternary phase diagrams varying the chain lengths, Cn, of the diluent between 7 and 14 carbons. We provide for the first time fundamental information on the impact of the water content and of the organic diluent on the miscibility and aggregation properties of PG3DS in MEs. The structure of reverse PG3DS aggregates in the monophasic region were accessed by combining small and wide angle X-ray scattering (SWAXS) experiments and molecular dynamics simulation.

Micellization in vegetable oils: A structural characterisation

The solubilisation of polar and polyphenol antioxidant in vegetable oils was studied. It was shown that the use of a polyglyceryl-3-diisostearate (PG3DS), a bio-sourced emulsifier well known in cosmetics, increases the yield of solubilisation thanks to some aggregation properties analysed using x-ray scattering technique. We show indeed that PG3DS forms reverse aggregates with a critical concentration that depends on the oil polarity. PG3DS reverse aggregates are elongated with a polar core and cannot be really swollen by addition of water. This supramolecular organisation allows however an efficient solubilisation of polar antioxidants in vegetable oils.

Inelastic neutron scattering study of Ni-substituted Ce0.5Fe4Sb12 skutterudite compounds

An inelastic neutron scattering study of the filled and partially-filled skutterudite compounds RFe4Sb12 and R0.5Fe2.75Ni1.25Sb12 (where R = Ce and La) was carried out to understand the nature of the spin dynamics. Strong magnetic scattering was observed in Ce0.5Fe2.75Ni1.25Sb12 at ~ 5 meV. The integrated intensity of this peak does not follow the Ce3+ form factor, but exhibits a maximum at a momentum transfer (|Q|) of 2 A−1. We attribute this feature to a Ce3+ crystal field excitation in the presence of magnetic exchange interactions. This picture is supported by thermodynamic and magnetic properties. Finally, we confirm the presence of a spin gap in CeFe4Sb12 suggested by our previous work.

Dynamics of La and Ce filled xFe4Sb12 skutterudite structures

We have applied high-resolution neutron time-of-flight spectroscopy and ab initio lattice dynamics calculations to study the guest dynamics of La and Ce in the filled skutterudite system xFe4 Sb12. Both the experiments and the calculation show that the guest dynamics is governed by collective vibrations, i.e. phonons. Unlike the case of localized rattlers the guest modes reveal dispersive character and a non-trivial Q-dependence of the signal intensity. The rather low-energy of the guest modes is due to their higher mass and weaker bonding with the host network