This work aims at understanding the oxidation mechanism of a TiC-SiC nanocomposite ceramic material. Samples were subjected to heat treatments up to 1400 degrees C under air, varying the relative density of the composite, the heating rate, and the dwell time. The weight variations were followed by thermogravimetric analyses coupled with mass spectrometry. The oxidized samples were then characterized by scanning electron microscopy on cross-sections (SEM-EDX) and their microstructure and composition were studied by transmission electron microscopy (TEM-EDX). The oxidation process was also followed by in situ high-temperature X-ray diffraction and high-temperature environmental scanning electron microscopy. Until 1000 degrees C, the formation of a multilayer alteration scale was observed, with a dense and protective SiO2 layer. Above 1200 degrees C, this layer showed cracks, and the oxidation was increased. Based on the results, a three-step mechanism was proposed for the temperature-dependent conversion of TiC and SiC to the subsequent oxides.

Ion flotation is a separation technology for recovering ions from dilute aqueous solutions. All ion flotation processes known so far make use of ionic foaming agents (surfactants) to selectively extract ions of opposite charge by electrostatic interactions. Recently, it was shown that nanometric-sized ions (nano-ions) with low charge density, such as certain polyoxometalates (POMs), strongly adsorb to neutral hydrated surfaces. In this study, we provide proof-of-concept for a method that we call "superchaotropic ion flotation," which uses nonionic surfactant foams to selectively recover and separate superchaotropic POMs from other ions, including non-superchaotropic ions. Specifically, we investigated the extraction of isopolyoxomolybdates, formed by pH and concentration variation of molybdate aqueous solutions, using foams produced with a commercial polyethoxylated surfactant (BrijO10). The speciation of polyoxomolybdates was investigated by Raman spectroscopy and small angle X-ray scattering (SAXS). SAXS has also confirmed the superchaotropic behaviour of the polyoxomolybdates species via the characterization of their strong adsorption on surfactant micelles. The flotation recovering is high and maximal at pH 1, a pH for which Mo36O1128  , an anion with a low charge density, is the predominant molybdate species. It was found that the surfactant has a strong influence on the molybdate speciation. It was also demonstrated that molybdate may be separated from tungstate at pH 2 by superchaotropic ion flotation. In conclusion, we propose a novel ion separation method via non-ionic surfactant aqueous foams and we show that the superchaotropic effect makes it possible to foresee various applications in separation science.

Second Harmonic Generation (SHG) today represents one of the most powerful techniques to selectively probe all types of interfaces. However, the origin of the SHG signal at a molecular level is still debated since the local dipole contribution, which is strongly correlated to the molecular orientation can be counterbalanced by non-local quadrupole contributions. Here, we propose a method to simulate the SHG signal of a model water/air interface from the molecular response of each contribution. This method includes both local and non-local terms, which are represented, respectively, by the dependency of the polarisability and hyperpolarisability upon the chemical environment of the molecule and by the bulk quadrupole response. The importance of both terms for the sound simulation of the SHG signals and their interpretation is assessed. We demonstrate that the sole dipole term is unable to simulate a SHG signal, even if the dependency of the hyperpolarisability on the local environment is considered. The inclusion of the bulk quadrupole contribution, which largely dominates the dipole contribution, is essential to predict the SHG response, although the accuracy of the prediction is increased when the dependency upon the local environment is considered.

Hypothesis: Weakly hydrated nanometric ions, called superchaotropes, were recently shown to adsorb strongly to non-ionic surfaces affecting drastically the surface's physical-chemical properties due to a charging effect. Superchaotropic ions could serve as stabilizing agents for non-ionic colloidal systems, such as non-ionic surfactant foams. Experiments: We study foams of the non-ionic surfactant BrijO10 (C18:1E10) without and in presence of the superchaotropic Keggin-ion SiW12O404 (SiW). The foams are investigated under free drainage conditions by image analysis and conductimetry to reveal the effect of SiW on the foam stability, liquid drainage, and bubble size. Additionally, small angle neutron scattering on the same foams, but in a dry quasistationary state, provides insight into effects of SiW on the foam films. Findings: SiW strongly stabilizes non-ionic surfactant foams at millimolar concentrations by inducing electrostatic repulsions between foam film interfaces resulting in thicker and monodisperse foam films. A similar effect is observed with the ionic surfactant sodium dodecylsulfate (SDS) but to a lesser extent and with a different mechanism. At the foam films' interface, SiW adsorbs to the polar non-ionic surfactant heads driven by the superchaotropic effect whereas DS anchors between non-ionic surfactant alkyl chains by the hydrophobic effect. The potential of superchaotropic ions as foam stabilizers is herein demonstrated. (C) 2021 Elsevier Inc. All rights reserved.

In concentrating solar power (CSP) technologies, the absorber material should be spectrally selective under severe operating conditions, i.e., similar to 1000 degrees C under air atmosphere. SiC-TiC nanocomposites could be good candidates for this application due to their spectral selectivity at room temperature; however, their behaviour under CSP operating conditions is unknown. Therefore, the spectral selectivity and oxidation resistance of TiC-SiC composites were studied up to 500 degrees C, in air, and at various compositions. During heating, the emittance increased with the temperature. After heating, the TiC grains at the surface were oxidized to TiO2, while SiC grains showed good behaviour. However, despite little oxidation, the spectral selectivity of the nanocomposites remained interesting after heating. Specific oxidation tests of up to 9 days at 1000 degrees C in air showed an increase in the oxidation resistance with the amount of SiC. This result is the consequence of the formation of a protective layer of SiO2 that prevents oxidation of the TiC grains. The composition of 30 at% TiC-70 at% SiC is a good compromise between low oxidation and good selectivity.

Second harmonic generation experiments for probing liquid-liquid interfaces and more specifically the ion transfer through buried interfaces may turn out to be highly relevant. However, it requires an increase in the reflected nonlinear signal and thus an enhancement of the hyperpolarizability of the extractant molecules that boost the ion partitioning. A new amphiphilic ligand-a malonamide azobenzene derivative-was synthesized by combining a diamide functional group with high affinity for trivalent f-block elements and an azobenzene group within its aliphatic part. The ability of this molecule to extract and form hydrophobic complexes with rare-earth cations at the methylcyclohexane/H2O interface was first evaluated. It was also shown that the isomerization of the azobenzene group under UV irradiation induces a slight change in the surface tension between the two liquid phases. As desired, this new extractant molecule has a strong nonlinear response and can be detected at the water/oil interface.

Metalla-bis-dicarbollides, such as the cobalta-bis-dicarbollide (COSAN) anion [Co(C2B9H11)(2)](-), have attracted much attention in biology but a deep understanding of their interactions with cell components is still missing. For this purpose, we studied the interactions of COSAN with the glucose moiety, which is ubiquitous at biological interfaces. Octyl-glucopyranoside surfactant (C8G1) was chosen as a model as it self-assembles in water and creates a hydrated glucose-covered interface. At low COSAN content and below the critical micellar concentration (CMC) of C8G1, COSAN binds to C8G1 monomers through the hydrophobic effect. Above the CMC of C8G1, COSAN adsorbs onto C8G1 micelles through the superchaotropic effect. At high COSAN concentrations, COSAN disrupts C8G1 micelles and the assemblies become similar to COSAN micelles but with a small amount of solubilized C8G1. Therefore, COSAN binds in a versatile way to C8G1 through either the hydrophobic or superchaotropic effect depending on their relative concentrations.

In this study, the potential of SiC-TiC nano-composites as solar absorbers has been studied. For solar thermal applications, materials with high solar absorptance and low emittance are ideally sought for (spectral selectivity). A semi-molecular sol-gel synthesis route leading to nanometric homogenous composites was described. The resulting SiC-TiC nanocomposite powder was sintered at different temperatures to produce samples with various relative densities (from 57% to 96%). The samples morphology and composition were characterized by several techniques including Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDX), X-Ray Diffraction (XRD), carbon and oxygen elemental analyses. The link between the surface roughness and the relative density was precised and the effects on the optical properties (0.25-25 mu m wavelength range) were studied. Comparisons were made with pure SiC and pure TiC samples with various relative densities. Overall, the sample emittance was found to strongly decrease with the increase in the relative density, leading to a great increase in the spectral selectivity, despite a little decrease in the solar absorptance. The TiC-SiC composite has an intermediate reflectance compared to the pure SiC and the pure TiC samples. With an absorptance of 0.76, an emittance of 0.44 and a selectivity of 1.74, the denser SiC-TiC could be a good candidate for bulk solar applications.

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 Keggin's 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 POM's 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 Keggin's 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 POM's 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).

Three different liquid interfaces, water/air, thiophene/air, and water/thiophene, were probed using the second harmonic generation (SHG) technique. Thiophene and water have been chosen because the hyperpolarizability of these molecules has already been measured or calculated and the different values can be found in literature. We have studied the microscopic structure of these interfaces by comparing the components of the second order susceptibility tensor determined from the SHG polarization curve analysis with those determined via a molecular dynamics (MD) simulation of these interfaces. We have indeed computed the structure and orientation of water and thiophene molecules at the liquid/air and liquid/liquid (L/L) interfaces as a function of the distance from the interface. The integrated susceptibility values calculated by MD simulations agree well with SHG results and validate the choice of force fields that should permit to quantify more complex L/L interfaces.

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.

High resolution diffraction measurements of the strained lattice unit of HgCdTe and CdZnTe have been performed at temperatures varying from room temperature to 300°C and for different lattice mismatch between substrate and layer. This investigation makes possible the determination of the coefficients of thermal expansion (CTE) and the evolution of the HgCdTe film stress during the thermal cycles. It is found that the CTE is linear with the zinc fraction for CdZnTe while it can be described by a parabolic variation as a function of the cadmium fraction for HgCdTe. The temperature evolution of the stress is found to be dictated by the CTE difference between substrate and layer up to a temperature of 150°C above which the HgCdTe layer partially relaxes. The evolution of the stress with lattice mismatch enables the determination of the onsets for plastic relaxation for both tensile and compressive stress.

We present high-resolution diffraction measurements of the lattice parameters of HgCdTe and CdZnTe. These measurements were performed at various temperatures ranging from room temperature up to 300°C, enabling the determination of the coefficients of thermal expansion (CTE) and the evolution of the HgCdTe film stress during the thermal cycling. It is found that the CTE is linear with the zinc fraction for CdZnTe, while it can be described by a parabolic variation as a function of the cadmium fraction for HgCdTe. The temperature evolution of the stress is found to be dictated by the CTE difference between the substrate and epilayer up to a temperature of 150°C, for which the stress is partially relaxed. For the sample grown on CdTe/Ge, the HgCdTe lattice is found to be fully relaxed at room temperature and the thermoelastic evolution of the stress of HgCdTe is imposed by the coefficient of thermal expansion of the germanium substrate.

Transmission X-Ray topography (X-ray imaging) was used to characterize the density and type of the native dislocations in ZnO substrates. C-oriented ZnO bulk materials from different origins were compared, either high purity Chemical Vapor Transport (CVT) grown or commercial, hydrothermally grown wafers. Elongated dislocations lying within the substrate were found in hydrothermal substrates. In CVT crystals, the density of dislocations was found to be too high for X-ray determination of their Burgers vector. However, in hydrothermal crystals, the density of dislocations were found to be in the range < 104 / cm2 and extend mainly within the substrate. Whereas complete Burgers vector identification is not achieved, two kinds of dislocations have been evidenced : grown-in, swirled dislocations ("high" temperature) and linear, gliding dislocations, most probably developped during the cooling steps in (0 1–1 0) glide planes.