Influence d’une texturation déformable sur l’adhésion et la friction

When two objects are in contact, an adhesion experiment is carried out if they are separated and a friction experience if one object slides on the other. A fundamental parameter which controls the adhesion and friction forces is the real area of contact between the surfaces which is largely determined by the surface roughness. To better understand the role of roughness, adhesion and friction experiments were performed with spheres of PDMS elastomer and textured surfaces (hard or soft). The latter’s roughness is modeled by an hexagonal network of cylindrical pillars with micrometrical dimensions and spacing.In adhesion experiments, a JKR set up (for Johnson, Kendall and Roberts) was used to observe the contact between an elastic sphere and a textured surface while controlling the force between the surfaces. At low normal force, the sphere remains at the top of the pillars and the contact is called "top". When the force between the surfaces increases, a full area of contact, where pillars are collapsed ("intimate contact"), appears in the center contact, surrounded by a crown of "top" contact. A model of evolution of this intimate contact which takes into account the adhesion between the pillars and the mechanical properties of surfaces has been achieved. Furthermore, it was possible to measure effective energies of adhesion between the surfaces using a similar analysis to the classical JKR analysis. Studying the evolution of these adhesion energies as a function of the pillars’ surface density below the sphere-plan contact proved to be a challenging task. At last, measurements of the pull off force were realised, corroborating the important role of the nature of contact on the evolution of effectif energies of adhesion.For friction experiments, a tribometer developed in the laboratory was used to measure the dynamic frictional force. During these experiments, the two kinds of contact previously reported were observed. When the contact remains "top", it is natural to introduce a friction stress equal to the friction force divided by the real area of contact. It has been shown that friction stress increases on textured surfaces (relative to the smooth case) and that this increase depends in a complex manner on the geometry of the pillars. Moreover, it has been shown that for small curvature radii of the friction spheres, the friction stress is no longer independent of the real area of contact. Finally, we have shown that the friction stress in the zone of intimate contact is the same as on smooth surfaces.The experimental results obtained in this thesis will serve to validate future numerical models.Lorsque deux objets sont mis en contact, on réalise une expérience d’adhésion si on les sépare et une expérience de friction si on les fait glisser l’un sur l’autre. Lors de ces expériences, on mesure les forces d’adhésion et de friction contrôlées par un paramètre fondamental qui est l’aire réelle de contact. Cette aire réelle dépend fortement de la rugosité des surfaces. Afin de mieux comprendre le rôle de la rugosité, des expériences d’adhésion et de friction ont été réalisées entre des sphères d’élastomère en PDMS et des surfaces texturées (dures ou molles) constituées d’une rugosité modélisée par un réseau hexagonal de plots cylindriques de hauteurs, diamètres et espacements micrométriques.Dans les expériences d’adhésion, un dispositif de type JKR (pour Johnson, Kendall et Roberts) a été utilisé permettant d’observer le contact entre une sphère élastique et un plan texturé tout en contrôlant la force entre les surfaces. À faible force d’appui, la sphère reste au sommet des plots et le contact est dit « posé ». Lorsque la force entre les surfaces augmente, un contact total, où les plots sont écrasés (« contact intime »), apparaît au centre du contact, entourée d’une couronne de contact « posé ». Un modèle d’évolution du contact intime a été réalisé en prenant en compte l’adhésion entre les plots et les caractéristiques mécaniques des surfaces. De plus, en utilisant une analyse similaire à l’analyse classique de type JKR, il a été possible de mesurer les énergies d’adhésion effectives entre les surfaces. L’étude de l’évolution de ces énergies d’adhésion en fonction de la densité surfacique de plots sous le contact sphère-plan s’est révélée complexe. Finalement, des mesures de la force d’arrachement ont été réalisées, confirmant le rôle très important de la nature du contact sur l’évolution des énergies d’adhésion effective.Pour les expériences de friction, un tribomètre développé au laboratoire a été utilisé pour mesurer la force de friction dynamique. Durant ces expériences, les deux types de contact précédemment cités ont également été observés. Dans le cas où le contact reste « posé », il est naturel d’introduire une contrainte de friction égale à la force de friction divisée par l’aire réelle de contact. Il a été montré que cette contrainte de friction augmente sur des surfaces texturées (par rapport au cas lisse) et que cette augmentation dépend de façon complexe de la géométrie des plots utilisés. De plus, il a été montré que pour des petits rayons de courbure des sphères frottantes, la contrainte de friction n’est plus indépendante de l’aire réelle de contact. Finalement, nous avons montré que la contrainte de friction dans la zone de contact intime est la même que pour des surfaces lisses.Ce travail ouvre la voie à des développements théoriques et numériques nouveaux sur l’analyse du champ de contraintes et de déformations pour des contacts texturés modèles

Empowering society by reusing privately held data for official statistics - A European approach

The High-Level Expert Group on facilitating the use of new data sources for official statistics has been created in the context of the data and digital strategy of the European Commission (EC). The task of the Expert Group is to provide recommendations aimed at enhancing data sharing between businesses and government (B2G) for the purpose of producing official statistics (B2G4S). The Expert Group consists of high-level experts with various backgrounds that are particularly relevant to B2G4S. Businesses generate and use data primarily for business-related purposes. The motivation for B2G4S stems from the high societal value that such privately held data can potentially generate when transformed into reliable, relevant and timely official statistics that are made available to everybody, for free. Transforming data into statistical information requires cooperation between private data holders and statistical authorities. On a voluntary basis there have been many collaborative efforts by businesses and statistical authorities to produce statistics based on privately held data, but for various reasons the use of such data for official statistics is still far below the level required to provide society with the high-quality and timely official statistics it needs in the increasingly data-driven world

Fonctionnalisation de matériaux carbonés comme supports de métaux nobles pour la catalyse hétérogène en phase liquide

Carbon materials are used extensively as supports in heterogeneous catalysis, even if they are relatively inert when compared to the inorganic oxides used commonly. These materials are cheap, have a high specific surface area and can withstand high temperature and high moisture degree. However, carbonaceous materials are not well understood due to the difficulty in characterising them spectroscopically. In consequence, the preparation of carbon-based materials still relies on trials and errors. This study aims at showing that well-characterized catalysts, based on a carbon support, can be synthesized in a controlled manner. A step-by-step methodology was developped in order to improve the interaction between the metallic precursors and the support. The first step consists in modifying the carbon surface by adding oxygenated functional groups. These functions are then used as anchoring sites for the grafting of coordination compounds. It is clear that one of the challenges of this work stands in the characterization of the carbonaceous samples at each step of the synthesis. The first part of this work consisted in developping a method to functionalize the carbon supports, in order to obtain a set of supports with different surface properties. A detailled characterization of these supports – before and after the functionalization – was carried out. Two reactants were envisaged : nitric acid and hydrogen peroxide. It was shown that those reactants are capable of oxidazing the support. As the intensity of oxidation depends on the experimental conditions, supports with different or native amount of oxygenated functions were obtained. Some of these functions were removed selectively by a thermal treatment, to give thermally stable supports. In a second part, the grafting of a Pd complex onto these modified carbon supports was undertaken. The grafting mecanism was studied by XPS and it was shown to consist in ligands exchange. A tool was developped to assess the amount of grafted complex: it was suggested that the grafted amount is related to the proportion of PdII in the sample. The functionalization of the support preceeding Pd incorporation was proved to be essential. Indeed a stabilization of the complex on the functionalized supports was observed and it was shown to be linked to the surface acidity. However, this stabilization hinders the reduction during the activation process. Despite this fact, the obtained catalysts give rise to good catalytic performance in the hydrogenation of 2-méthyl-2-nitropropane into t-butylamine. The activity increases with the extend of functionalization. This is due to a decrease in particles sizes with the initial support acidity. However, the use of a support functionalized under harsh conditions is not beneficial, which can be attributed to a negative effect of the oxygenated functions on the catalytic activity. The last part consisted in reproducing the same grafting method for other metallic complexes, in order to synthesize bimetallic catalysts. In this way, the incorporation of Bi and Ru onto carbon supports was studied and seemed to be easily carried out. Bimetallic Pd-Bi/C and Pd-Ru/C were thus prepared using different methodologies (simultaneous, consecutive grafting or a method combining deposition and grafting). The obtained Pd-Bi/C catalysts were tested in the oxidation of glucose into gluconic acid but no improvement in catalytic performance or in terms of Bi losses in solution was obtained. The advantage of using a grafting process compared to the deposition method previously used was an increase of homogeneity in the samples. Small particles well dispersed on the support were, indeed, observed by TEM. The functionalization of the support allows to avoid agglomeration. Pd-Ru/C catalysts showed variable particles sizes in function of the degree of functionalization and the grafting pH. These catalysts were tested in the hydrogenation of MNP into TBA. It was shown that all the catalysts were very active in this reaction and that their activities may be linked to the initial acidity of the support but also, in a more pronounced manner, to the grafting pH.Ce travail s'inscrit dans le cadre de la préparation de catalyseurs supportés sur carbone. L'objectif est de mettre au point une méthodologie permettant un greffage réel de complexes métalliques sur le carbone, et de le prouver. Le greffage se définit au sens strict par la création d'un lien covalent entre les fonctions de surface du support et un complexe métallique. Une fonctionnalisation préalable de la surface du support carboné est cependant indispensable, pour introduire des groupes fonctionnels oxygénés qui serviront de sites pour le greffage de composés de coordination. La première partie du travail a consisté à mettre en oeuvre une méthode de fonctionnalisation des supports carbonés. En jouant sur les conditions opératoires, une série de supports présentant des caractéristiques de surface différentes ont pu être obtenus et les fonctions de surface caractérisées de manière qualitative et quantitative. Dans une seconde partie, la possibilité de greffer un complexe de Pd sur ces supports a été démontrée. L'étude du mécanisme de greffage a montré que celui-ci consiste en un échange de ligands. Après activation, les catalyseurs Pd/C obtenus se sont révélés actifs et présentent de fines particules de Pd bien dispersées. Les paramètres de synthèse de ces catalyseurs ont une influence sur les caractéristiques et l’activité obtenues. La méthode de synthèse a ensuite été reproduite pour d'autres complexes afin d'obtenir des catalyseurs bimétalliques Pd-Bi/C et Pd-Ru/C. Bien que le greffage du Bi et du Ru n'ait pu être clairement démontré, plusieurs indices laissent penser qu'il a bien lieu. De plus, la fonctionnalisation du support carboné préalable au greffage s'est avérée essentielle à l'obtention de catalyseurs présentant une répartition homogène des particules. Dans le cas Pd-Bi, celles-ci sont bimétalliques, tandis que dans le cas Pd-Ru, les deux métaux sont ségrégés à la surface.(CHIM 3) -- UCL, 201

Influence of deformable texturation on the adhesion and the friction properties

Lorsque deux objets sont mis en contact, on réalise une expérience d’adhésion si on les sépare et une expérience de friction si on les fait glisser l’un sur l’autre. Lors de ces expériences, on mesure les forces d’adhésion et de friction contrôlées par un paramètre fondamental qui est l’aire réelle de contact. Cette aire réelle dépend fortement de la rugosité des surfaces. Afin de mieux comprendre le rôle de la rugosité, des expériences d’adhésion et de friction ont été réalisées entre des sphères d’élastomère en PDMS et des surfaces texturées (dures ou molles) constituées d’une rugosité modélisée par un réseau hexagonal de plots cylindriques de hauteurs, diamètres et espacements micrométriques.Dans les expériences d’adhésion, un dispositif de type JKR (pour Johnson, Kendall et Roberts) a été utilisé permettant d’observer le contact entre une sphère élastique et un plan texturé tout en contrôlant la force entre les surfaces. À faible force d’appui, la sphère reste au sommet des plots et le contact est dit « posé ». Lorsque la force entre les surfaces augmente, un contact total, où les plots sont écrasés (« contact intime »), apparaît au centre du contact, entourée d’une couronne de contact « posé ». Un modèle d’évolution du contact intime a été réalisé en prenant en compte l’adhésion entre les plots et les caractéristiques mécaniques des surfaces. De plus, en utilisant une analyse similaire à l’analyse classique de type JKR, il a été possible de mesurer les énergies d’adhésion effectives entre les surfaces. L’étude de l’évolution de ces énergies d’adhésion en fonction de la densité surfacique de plots sous le contact sphère-plan s’est révélée complexe. Finalement, des mesures de la force d’arrachement ont été réalisées, confirmant le rôle très important de la nature du contact sur l’évolution des énergies d’adhésion effective.Pour les expériences de friction, un tribomètre développé au laboratoire a été utilisé pour mesurer la force de friction dynamique. Durant ces expériences, les deux types de contact précédemment cités ont également été observés. Dans le cas où le contact reste « posé », il est naturel d’introduire une contrainte de friction égale à la force de friction divisée par l’aire réelle de contact. Il a été montré que cette contrainte de friction augmente sur des surfaces texturées (par rapport au cas lisse) et que cette augmentation dépend de façon complexe de la géométrie des plots utilisés. De plus, il a été montré que pour des petits rayons de courbure des sphères frottantes, la contrainte de friction n’est plus indépendante de l’aire réelle de contact. Finalement, nous avons montré que la contrainte de friction dans la zone de contact intime est la même que pour des surfaces lisses.Ce travail ouvre la voie à des développements théoriques et numériques nouveaux sur l’analyse du champ de contraintes et de déformations pour des contacts texturés modèles.When two objects are in contact, an adhesion experiment is carried out if they are separated and a friction experience if one object slides on the other. A fundamental parameter which controls the adhesion and friction forces is the real area of contact between the surfaces which is largely determined by the surface roughness. To better understand the role of roughness, adhesion and friction experiments were performed with spheres of PDMS elastomer and textured surfaces (hard or soft). The latter’s roughness is modeled by an hexagonal network of cylindrical pillars with micrometrical dimensions and spacing.In adhesion experiments, a JKR set up (for Johnson, Kendall and Roberts) was used to observe the contact between an elastic sphere and a textured surface while controlling the force between the surfaces. At low normal force, the sphere remains at the top of the pillars and the contact is called "top". When the force between the surfaces increases, a full area of contact, where pillars are collapsed ("intimate contact"), appears in the center contact, surrounded by a crown of "top" contact. A model of evolution of this intimate contact which takes into account the adhesion between the pillars and the mechanical properties of surfaces has been achieved. Furthermore, it was possible to measure effective energies of adhesion between the surfaces using a similar analysis to the classical JKR analysis. Studying the evolution of these adhesion energies as a function of the pillars’ surface density below the sphere-plan contact proved to be a challenging task. At last, measurements of the pull off force were realised, corroborating the important role of the nature of contact on the evolution of effectif energies of adhesion.For friction experiments, a tribometer developed in the laboratory was used to measure the dynamic frictional force. During these experiments, the two kinds of contact previously reported were observed. When the contact remains "top", it is natural to introduce a friction stress equal to the friction force divided by the real area of contact. It has been shown that friction stress increases on textured surfaces (relative to the smooth case) and that this increase depends in a complex manner on the geometry of the pillars. Moreover, it has been shown that for small curvature radii of the friction spheres, the friction stress is no longer independent of the real area of contact. Finally, we have shown that the friction stress in the zone of intimate contact is the same as on smooth surfaces.The experimental results obtained in this thesis will serve to validate future numerical models

Bimetallic Pd-Bi/C Catalysts Prepared by Grafting of Complexes with O-Donor Ligands

Bimetallic carbon-supported PdBi catalysts were prepared by grafting. To do so a soluble complex of bismuth with exchangeable O-donor ligands was selected, and the carbon support was functionalized to increase the number of surface functions. The grafting procedure was carried out by contacting the various supports with solutions of Pd and/or Bi complexes, to allow ligand exchange reactions to take place between the complexes and the surface O-groups. The grafted fragments were then activated into carbon-supported nanoparticles, which were characterized to unravel the impact of grafting experimental variables on the physico-chemical characteristics of the materials obtained. The PdBi/C catalysts were finally tested in the archetypal glucose selective oxidation reaction, to assess the impact of those characteristics on the catalytic performance. It was found that simultaneous grafting gave larger nanoparticles than consecutive grafting, while higher number of stable surface functions allowed to obtain small and nicely distributed bimetallic nanoparticles. However, the surface functions were found to be deleterious for the catalytic activity, and the placement of the Bi promoter with respect to Pd active phase was identified as another key parameter for the activity, with grafting allowing to compare neatly samples where Bi is underneath, above or beside Pd

Pd nanoparticles prepared by grafting of Pd complexes on phenol-functionalized carbon supports for liquid phase catalytic applications

An activated carbon was functionalized by oxidation with HNO3 or H2O2 to increase the number of acidic groups on its surface. A sample of HNO3-oxidized carbon was treated thermally to remove unstable functions. Detailed characterization by a combination of techniques revealed that this stabilization procedure allowed selecting weaker acid sites of phenolic type. H2O2 functionalization gave also mainly surface phenols but in lower amounts. Palladium grafting was carried out in water at room temperature by ligand exchange between a carboxylate complex and the surface oxygenated groups. Activation was carried out thermally and the samples grafted on the most oxidized supports proved more difficult to reduce. The catalytic activity of the obtained Pd/C catalysts was evaluated in the reduction of 2-methyl-2-nitropropane (MNP) into t-butylamine (TBA). The most active materials were those prepared on H2O2-modified carbon and on the stabilized support. In the latter case, the superior activity was explained by the robustness of the grafting anchors which could not decompose during thermal activation together with the active role of weak acidic groups in the catalytic reaction itself

Functionalization of carbon xerogels for the preparation of Pd/C catalysts by grafting of Pd complex

peer reviewe

Facing 802.11 Anomaly and Improving 802.11 WLANs QoS Using a Cross-layer

International audienc

Nanostructured Pd/C catalysts prepared by grafting of model carboxylate complexes onto functionalized carbon

Pd(5 wt%)/C catalysts were prepared by grafting carboxylate precursors onto functionalized carbon. The functionalization of the support was carried out with HNO3 or H2O2, and the number and nature of oxygenated functions introduced were determined by a combination of simplified Boehm's titration, XPS, BET, and DRIFTS. These functions were then used as anchors for model Pd precursors through covalent bonding. The underlying ligand-exchange mechanism was elucidated through detailed XPS studies. The thermal activation of the grafted materials was followed by in situ mass spectrometry, which demonstrated that it consisted of ligand losses. The activated samples were characterized by SEM, XRD, CO chemisorption, and XPS and used in the reduction of 2-methyl-2-nitropropane (MNP) into t-butylamine (TBA). The catalytic activity was shown to be correlated),with the initial carbon acidity and the Pd dispersion. (c) 2006 Elsevier Inc. All fights reserved