Departament de Química

Physico-chemical properties of materials can be strongly modified by confinement because of the quantum effects that appear at such small length scales and also because of the effects of the confinement itself. The aim of this thesis is to show that both the nature of the confining material and the size of the pores and cavities have a strong impact on the confined material. We first show the effect of the pore size of the host mesoporous silica on the temperature of the solid-solid phase transition of silver selenide, a semiconducting material with enhanced magnetoresistive properties under non-stoichiometric conditions. Narrowing the pores from 20 nm to 2 nm raises the phase transition temperature from 139°C to 146°C. This result can be explained by considering the interaction between the confining and confined materials as a driving force. The effects of confinement are also studied in the case of hydrogen and deuterium inside cavities of organized carbon nanostructures. The effects that appear in the adsorption/desorption cycles are much stronger with carbon nanohorns as the host material than with C60 peapods and single-walled carbon nanotubes.

The confinement of silver selenide was investigated using mesoporous silica. Results from x-ray diffraction and electron microscopy show that the confined material still exhibits a β to α transition similar to the one that takes place in the bulk crystalline state but with a transition temperature that depends significantly on the confinement conditions. Decreasing the pore size leads to an increase of the transition temperature, opposite to the behavior of the melting point observed in several metallic and organic materials. In the free particles, on the other hand, no size dependence is observed with particle sizes down to 4 nm.

The effects of confining molecular hydrogen within carbon nanohorns are studied via high-resolution quasielastic and inelastic neutron spectroscopies. Both sets of data are remarkably different from those obtained in bulk samples in the liquid and crystalline states. At temperatures where bulk hydrogen is liquid, the spectra of the confined sample show an elastic component indicating a significant proportion of immobile molecules as well as distinctly narrower quasielastic line widths and a strong distortion of the line shape of the para - ortho rotational transition. The results show that hydrogen interacts far more strongly with such carbonous structures than it does to carbon nanotubes, suggesting that nanohorns and related nanostructures may offer significantly better prospects as lightweight media for hydrogen storage applications.

We present a comparative study of the excitations in bulk and liquid D2 confined within the pores of MCM-41. The material (Mobile Crystalline Material-41) is a silicate obtained by means of a template that yields a partially crystalline structure composed by arrays of nonintersecting hexagonal channels of controlled width having walls made of amorphous SiO2. Its porosity was characterized by means of adsorption isotherms and found to be composed by a regular array of pores having a narrow distribution of sizes with a most probable value of 2.45 nm. The assessment of the precise location of the sample within the pores is carried out by means of pressure isotherms. The study was conducted at two pressures which correspond to pore fillings above the capillary condensation regime. Within the range of wave vectors where collective excitations can be followed up (0.3<Q<3.0 A-1), we found confinement brings forward a large shortening of the excitation lifetimes that shifts the characteristic frequencies to higher energies. In addition, the coherent quasielastic scattering shows signatures of reduced diffusivity.

Cet article analyse la possibilité d’utiliser les nanocornets de carbone (ou nanohorns) comme solution au problème du stockage massif d’hydrogène. Alors qu’une quantité non négligeable d’hydrogène peut être adsorbée par les nanotubes de carbone mono-feuillets (SWNT) sous une pression de 100 bars et à une température de 10 K, il s’avère que pratiquement toute cette quantité s’échappe du matériau lorsque celui-ci est remis dans les conditions ambiantes, et seulement une infime partie est réadsorbée lorsque la température est redescendue à 10 K. Les nanocornets de carbone mono-feuillets (SWNH), structures graphitiques se présentant sous l’aspect de fleurs de dahlia et se formant par agrégation de structures nanotubulaires, apparaissent a priori plus prometteurs puisque le processus d’adsorption semble réversible et la plupart de l’hydrogène libéré à température ambiante est réadsorbé quand la température est à nouveau abaissée. En plus, leurs performances peuvent encore être améliorées en jouant sur la pression, ou en décorant la surface avec des nanoparticules métalliques. Les travaux présentés ici se limitent aux matériaux bruts. Les aspects microscopiques de l’adsorption d’hydrogène dans les nanostructures de carbone sont discutés à partir d’expériences de diffusion de neutrons. Le comportement de l’hydrogène adsorbé dans les SWNT est comparé à celui dans les SWNH en utilisant d’une part l’analyse de la transition rotationnelle para-ortho qui donne accès à des informations sur l’interaction entre les molécules d’hydrogène et la matrice carbonée, et en étudiant d’autre part le pic quasi-élastique dont la largeur dépend de la mobilité translationnelle de l’hydrogène et permet de discriminer les fractions d'hydrogène mobile et immobile.

A spectroscopic study of the amino functionalization of double-walled carbon nanotubes (DWCNT) is performed. Original experimental investigations by near edge X-ray absorption fine structure spectroscopy at the C and O K-edges allow one to follow the efficiency of the chemistry during the different steps of covalent functionalization. Combined with Raman spectroscopy, the characterization gives direct evidence of the grafting of amino-terminated molecules on the structural defects of the DWCNT external wall, whereas the internal wall does not undergo any change. Structural and mechanical investigations of the amino functionalized DWCNT / epoxy composites show coupling between epoxy molecules and the DWCNTs. Functionalization improves the interface between amino-functionalized DWCNT and the epoxy molecules. Electrical transport measurements indicate a percolating network formed only by the inner metallic tubes of the DWCNTs. The activation energy of the barrier between connected metallic tubes is determined around 20 meV.

A radiation curable adhesive comprising (i) (meth)acrylic monomer(s) and/or oligomer(s), wherein said (meth)acrylic monomers or oligomers comprise 5 to 50 wt % of esters of (meth)acrylic acid with polyetherpolyols having the formula HO-((CH2)m-O)n-X , m = 2,3,4 ; n = 2 to 50 ; X = H, linear, branched or aromatic C1 to C12- alkyl, the ester having a molecular weight from 150 to 2000 g/mol; (ii) non-reactive (co)polymer(s) based on unsaturated monomers, wherein said unsaturated monomers are selected from vinyl esters, (meth)
acrylate esters, C2 to C8 unsaturated olefins, (iso)butylene the (co)polymer having a molecular weight from 5000 g/mol to 500000g/mol, such adhesive can form a solid adhesive layer after being cross-linked by radiation, the layer has a non tacky surface but can be adhesively bonded under pressure to a second layer of the adhesive.

Les proprietes physico-chimiques des materiaux confines sont modifiees parfois de maniere drastique, a cause des effets quantiques apparaissant a des tailles aussi petites, mais aussi de par le confinement lui-meme. Le but des travaux effectues durant cette these est de montrer que la nature du materiau confinant et la taille des sites de confinement - autrement dit les pores ou cavites – ont une influence essentielle sur les proprietes des materiaux ou fluides confines. Nous avons ainsi tout d&#39;abord demontre l&#39;effet de la taille des pores de la silice mesoporeuse sur la temperature de transition de phase solide-solide d&#39;un semi-conducteur aux proprietes de magnetoresistance exacerbees dans des conditions non-stœchiometriques, le seleniure d&#39;argent. Il s&#39;avere que plus les pores sont etroits, c&#39;est-a-dire pour des diametres d&#39;ouverture passant de 20 nm a environ 2 nm, cette temperature de transition de phase passe de 139oC a 146 oC, les forces directrices...

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