Surface reactivity of uranium hexafluoride (UF6)

The present article reviews a selection of results obtained in the AREVA/CNRS/UCA joint research laboratory. It focuses on interfaces formed by uranium hexafluoride (UF₆) with chemical filter (purification), carbon (UF₆ storage), and metallic substrate (corrosion). As a matter of fact, along the nuclear fuel cycle, metallic surfaces of the fluorination reactors, cooling systems (for the liquefaction of UF₆), and storage containers are in contact with UF₆, either in the gas or in the liquid phase. For the removal of volatile impurities before the enrichment, surface of chemical filters with a high specific surface area must be enhanced for both selectivity and efficiency. To store depleted UF₆ (²³⁸U), graphite intercalation compounds are proposed and preliminary results are presented.     

Nanostructure and crystallization phenomena in multilayered films of alternating iPP and PA6 semicrystalline polymers

The present work is concerned with the study of the crystalline morphology and the nanostructure of a multilayered system of two alternating immiscible semicrystalline polymers: isotactic polypropylene (iPP) and polyamide 6 (PA6). Films with a volume ratio of 70/30 were prepared by means of layer multiplying coextrusion. Contrary to previous experiments, performed with semicrystalline/amorphous and amorphous/amorphous nanolayered systems, the studied iPP/PA6 film does not exhibit a well defined maximum in the USAXS patterns. This result accounts for an irregular layered structure, as further confirmed by means of TEM images. Nevertheless, such a layered assembly still influences the crystallization behaviour of both constituent polymers. On the one hand, the crystallization of PA6 within the multilayered material is substantially hindered as evidenced by its weak scattering intensity. Real time studies as a function of temperature undoubtedly detect the presence of a WAXS peak and a SAXS maximum associated to PA6 above the melting temperature of iPP. Room temperature AFM studies also confirm the occurrence of crystalline structures within the PA6 layers. On the other hand, SAXS and WAXS measurements at room temperature reveal the occurrence of an oriented lamellar morphology within the iPP layers bearing uniaxial symmetry around an axis perpendicular to the layers surface. Results show that the crystalline molecular chains are placed mainly parallel to the layer surfaces forming edge-on lamellae. Moreover, X-ray scattering results are in agreement with the occurrence of two populations of lamellae, both edge-on and perpendicular to each other, in agreement with the crosshatched morphology observed by AFM.     

On the flexibility of the structural framework of cubic LAMOX compounds, in relationship with their anionic conduction properties

A mathematical analysis of the cubic crystal structure of fast oxide-ion conductor beta-La(2)Mo(2)O(9) (and derived members of the LAMOX family) shows that its cationic sublattice can behave as a semirigid framework. Tilt/rotation of rigid [O1La(3)Mo] anti-tetrahedral units about their 3-fold axis can open up tunnels in the cationic framework, therefore favoring the mobility of O2 and O3 oxide ions located in these tunnels, as confirmed by molecular dynamics simulations. Such a process is likely to assist the anionic transport and explain the postulated transition from Arrhenius-type to VTF (Vogel-Tamman-Fulcher)-type behavior propounded to account for the peculiar conductivity curvature observed at high temperature in all the cubic LAMOX compounds. It also clarifies the correlated extra cell volume expansion observed at the same temperature in all these materials.     

Reactive adsorption of penicillin on activated carbons

A series of activated carbons with varied surface chemistry, obtained by wet oxidation and thermal treatment, was used for the removal of penicillin from low concentration aqueous solution. It was found that the carbon surface chemistry favors the degradation of the antibiotic, giving rise to various intermediates detected both in solution and in the adsorbed phase (deposited with the pore structure of the activated carbons). The confinement of penicillin molecules entrapped in the nanopores of activated carbons of acidic nature accelerates their degradation compared to that one in the bulk solution, which can be linked the strong local pH fall inside the pores. Degradation also takes place in activated carbons of basic pH, although the nature and partition of the intermediates formed differ from those in the acidic carbons. In both cases most of the breakdown products do not present therapeutic activity.     

Biomass waste-derived activated carbon for the removal of arsenic and manganese ions from aqueous solutions

The goal of this study is to investigate the preparation of low-cost activated carbon from bean pods waste and to explore their potential application for the removal of heavy metals from aqueous solutions. Conventional physical (water vapor) activation was used for synthesizing the adsorbent. The obtained carbon was employed for the removal of As (III) and Mn (II) from aqueous solutions at different initial concentrations and pH values. Adsorption for both ions follows Langmuir-type isotherm, the maximum loading capacities for arsenic (III) and Mn (II) ions being 1.01 and 23.4 mg g⁻¹, respectively. According to the experimental data, it can be inferred that the basic character of the surface, i.e. the high content of basic groups, favors adsorption of ions. Arsenic adsorption capacity on the carbon obtained from agricultural waste was found to be similar to this of more expensive commercial carbons showing high adsorption capability. Regarding manganese adsorption, herein obtained carbon presented higher uptake adsorption than that of activated carbons reported in the literature.     

Composantes de dimension maximale d'un analogue du lieu de Noether-Lefschetz (Components of Maximal Dimension of an Analogue of the Noether-Lefschetz Locus)

Let X ⁴ be a smooth hypersurface of degree d 5, and let S X be a smooth hyperplane section. Assume that there exists a non trivial cycle Z Pic(X) of degree 0, whose image in CH₁(X) is in the kernel of the Abel–Jacobi map. The family of couples (X, S) containing such Z is a countable union of analytic varieties. We show that it has a unique component of maximal dimension, which is exaclty the locus of couples (X, S) satisfying the following condition: There exists a line S and a plane P ⁴ such that P X = , and Z = – dh, where h is the class of the hyperplane section in CH₁(S). The image of Z in CH₁(X) is thus 0. This construction provides evidence for a conjecture by Nori which predicts that the Abel–Jacobi map for 1–cycles on X is injective.     

Controlling the molecular weight of poly(2,5-bis[N-methyl-N-hexylamino]phenylene vinylene) using nitrobenzene as an inhibitor

Poly(2,5-bis[N-methyl-N-hexylamino] phenylene vinylene) (BAM-PPV) has been studied for several decades as an anti-corrosion coating. Although the polymer is readily synthesized via base-promoted radical chain polymerization, BAM-PPV exhibits poor solubility at high-molecular weights (M_w > 100 kDa), which limits its applications. In this work, the molecular weight of BAM-PPV was modulated with nitrobenzene. At nitrobenzene loading as low as 0.9 mol%, the polymerization reaction produced low-molecular weight BAM-PPV (M_w = 5–46 kDa). The polymerization of BAM-PPV was systematically studied via size-exclusion chromatography (SEC), liquid chromatography–mass spectrometry (LC-MS), and ¹H NMR spectroscopy, which revealed suppression of the dimer intermediate before chain polymerization. Collectively, this work could expand the practical applications of BAM-PPV for use in optoelectronic devices or corrosion inhibition coatings.     

H2, N2, CO, and CO2 sorption properties of a series of robust sodalite-type microporous coordination polymers

H2, N2, CO, and CO2 are readily incorporated in the porous, 3D sodalitic frameworks of coordination polymers of the [ML2]n type, with M = Pd(II) or Cu(II) and HL = 2-hydroxypyrimidine or 4-hydroxypyrimidine. The metal ion and ligand functionalization modulate their sorption properties, making these materials suitable for gas storage and separation purposes.     

Relationship between structure and reactivity of carbonaceous materials

Knowledge of the amount and distribution of active sites in carbons is of paramount importance for a better understanding of the kinetics involved in heterogeneous gas solid reactions. In this work a commercial active carbon, CM, was treated at several temperatures in order to obtain a series of samples with different textural and structural properties. The results showed that the loss of reactivity of the samples, determined by thermogravimetric analysis, is related not only to the lower surface area but also to the decrease in the amount of active sites due to a higher structural ordering. This revised version was published online in July 2006 with corrections to the Cover Date.