Analyse multiéchellede Carbone pyrolitiques

Pyrocarbons are polyromatic- graphenic- materials obtained by CVD / CVI and are often used as an interphase or as a matrix in C / C composites for the aerospace industry ans the production of brakes, rocket nozzles etc. . . Although not limited to this application domain, this example shows that their mechanical properties and heat resistance are qualities that it is essential to determine and predict. For this purpose, accurate material identification is needed to create a database linking the description of materials to their properties. The work of this thesis is focused on describing the structure (crystallographic point of view), nanotexture (degree of both perfection and order of grapheme layers within anisotropic domains), and texture (extension and relative arrangement of anisotropic domains) of a specific class of these materials, namely laminar pyrocarbons (wich include three kinds of them, known as 'rough', 'smooth' and 'regenerated'). It is mostly based on two major techniques wich are Raman spectroscopy and transmission electron microscopy, completed by X-ray diffraction. The section dedicated to Raman spectroscopy shows a study of the evolution of the various bands characteristic of the material (typically, the G band, generated by the lattice vibrations, and the D band, which is generated by defects) and their respective contributions (areas A or intensities I) as the ID/IG ratio which depends on both the exciting wavelength and the average crystallite size determined by neutron scattering and X-ray diffraction. This work shows that, in particular for small crystallite sizes (< 6 nm), a further contribution to the D band intensity at the same spectral position appears. In comparison to previous works, this innovative approach can account perfectly for the experimental observations recorded for various excitation wave lengths ranging from ultraviolet, to visible, and then to infrared. To complete this study, a series of pitch cokes was studied and shows, consistently with the pyrocarbons, a linear relationship between the width of the G band and the crystallite sizes the latter are less than ˜10nm. The section dedicated to the transmission electron microscopy utilizes a methodology previously developed and validated at CEMES for the quantified multi-scale characterization of another type of pyrocarbons, namely isotropic, spherulitic pyrocarbons has been used for the first time for the characterization of laminar pyrocarbons. By using several modes of the electron microscopy (electron diffraction with selected area diffraction, dark field images, lattice-fringe images), it was possible to evaluate qualitatively and/or quantitatively the structural and nano-textural respectively and to discriminate between the various pyrocarbons studied. This work has also highlighted the limitations of the methodology for its application to the peculiar type of the materials studied.Les Pyrocarbones sont des matériaux polyromatiques -graphéniques- obtenus par CVD/CVI qui sont souvent utilisés en tant qu'interfase ou en tant que matrice dans des composites C/C pour l'industrie aéronautique et la réalisation de freins, tuyères de fusées etc. Bien que non limitées à ce domaine d'application, cet exemple montre que leurs propriétés mécaniques et leur résistance aux températures extrêmes sont des qualités qu'il est primordial de déterminer. Dans ce but, une étape d'identification des matériaux est nécessaire afin de créer une base de données reliant la description des matériaux à leurs propriétés. Le travail de cette thèse s'est donc porté sur la description structurale (caractère cristallographique), nano-texturale (degré de perfection et d'ordre des graphènes au sein des domaines anisotropes) et structurale (étendues et disposition relative des domaines anisotropes) de certains de ces matériaux, les pyrocarbones laminaires (des trois types dits 'rugueux', 'lisses', et 'régénérés'). Il est articulé autour de deux techniques majeures qui sont la spectroscopie Raman et la microscopie électronique en transmission, complété par la diffraction des rayons X. Le volet spectroscopie Raman montre une étude de l'évolution des différentes bandes caractéristiques du matériau (bande G, générée par les vibrations du réseau, et bande D, générée par les défauts) et de leurs importances respectives (aires A ou intensités I), comme le rapport ID/IG, en fonction de la longueur d'onde excitatrice et en fonction de la taille moyenne des cristallites déterminées par diffusion de neutrons et diffraction des rayons X. Ce travail montre notamment que pour de petites tailles de cristallite (< 6nm), une nouvelle contribution à l'intensité de la bande D de même position spectrale apparaît. Cette approche novatrice par rapport aux travaux antérieurs permet de rendre compte parfaitement des observations expérimentales dans le domaine ultraviolet, visible et infrarouge. Afin de compléter cette étude, une série de cokes de brai a été étudiée et montre, en cohérence avec les pyrocarbones, une relation linéaire entre la largeur de la bande G et les tailles de cristallites lorsqu'elles font moins de ˜ 10nm. Dans le volet microscopie électronique en transmission, une méthodologie préalablement mise au point au laboratoire et validée pour la caractérisation multi-échelle quantifiée de pyrocarbones de type pshérulitiques isotropes a été utilisée pour la première fois pour la caractérisation de pyrocarbones laminaires. Pour le recours à plusieurs modes de la microscopie électronique (diffraction électronique à aire sélectionnée, imagerie de fond noir, imagerie de franges de réseau), elle a permis, dans une certaine mesure, d'évaluer qualitativement et/ou quantitativement les caractéristiques texturales et nanotexturales respectives et discriminantes des différents types de pyrocarbones étudiés. Le travail a aussi mis en évidence les limitations de la méthodologie à son application pour le type particulier des matériaux de l'étude

Spatial confinement model applied to phonons in disordered graphene-based carbons

cited By 1International audienceAnalyzing the various bands in Raman spectra, mostly the G (optical carbon-carbon mode) and D (defect-related double resonance process) bands, is powerful at characterizing defects in disordered graphene-based carbons. The crystallite size La could be estimated from the ID/IG (intensity) or AD/AG (area) ratio, but with large uncertainties. Using the spatial confinement model (SCM) of the phonons has fully explained the linewidth variation for the D band, but could not explain the linewidth variation for the G band over the whole La range. Indeed, if the latter for large, Bernal-stacked graphenes is due to the Kohn anomaly and can be explained considering phonon dispersion curves, turbostratically-stacked graphenes with small La generate an additional broadening of the G band not accounted by the phonon-related SCM only. The realistic SCM proposed here explains the way the G band shape is related to La. The model was also validated by successfully duplicating the evolution of the D (mostly related to the electronic band dispersion) and D′ band linewidths with decreasing La down to 5 nm. The work aims to be a guide for future computational works on Raman spectra of graphene-based carbon materials

Behavior of Raman D band for pyrocarbons with crystallite size in the 2-5 nm range

International audienceThe pyrocarbon materials investigated here are examples of disordered graphene-based carbons whose crystallite size $L_a$ ranges from 2 to 5 nm. This $L_a$ size range is between two different Raman behaviors, one for which the D band broadens (for $L_a$ 5 nm) respectively, with increasing $L_a$ . To fully understand the nature of the G band signal, we checked its wavelength behavior from UV to near IR. We demonstrated that the Raman spectrum is well fitted with simply two Lorentzians with various respective contributions centered at the wavenumber of the D band and a Breit-Wigner-Fano shape for the G band. Each intensity contribution for the D band varies linearly with $L_a$ between 2 and 5 nm while the total D band intensity is nearly constant for excitation wavelengths ranging from 0.532 to 0.638 μm. In contrast, the integrated intensity ratio D/G follows the well-known $L_a^{-1}$ law. The two subbands building the D band are related to the lifetime of the electrons involved in the double resonance process which can be scattered twice. Their respective occurrences therefore depend on the crystallite size $L_a$, when below ≈ 5 nm

A Raman study to obtain crystallite size of carbon materials: A better alternative to the Tuinstra-Koenig law

International audienceBy varying excitation energy from ultraviolet to infrared, Raman spectra obtained from both cokes and pyrocarbons revealed the evolution of each band. While the spectra of different samples can be very similar at given wavelength, the wavelength dependence should be considered for correlating spectral features to crystallite size $L_a$. The D' band was found to vanish in the UV range. The D band was fitted with one or two Lorentzians for crystallite sizes larger than ~5 nm or below respectively, both centered on the same wave number. Both the D band intensity and integrated intensity were accurately obtained, and used to question the range of use of the Tuinstra-Koenig law. The G band shape is well fitted and its width increases monotonously with 2<$L_a$<10 nm. The energy dependence of $I_D$/$I_G$ was found to vary with the excitation energy $E_L$ as $E_L^{-b}$ with 1 <$b$<4 depending on both $L_a$ and the sample type. We question the validity of the empirical laws from the literature for having been obtained on a limited sampling and wrongly considered to remain valid over the full ranges of both $L_a$ and wavelengths. Considering the G band width instead is emphasized

Novel phospholyl(diphenylphosphino)methane-ruthenium complexes: unexpected non-assisted cis to trans isomerization of [RuCl 2 (κ 2 -P–P′) 2 ]

International audienceUsing the unsymmetrical P–P′ phospholyl(phosphino)methane ligand, complex cis-[RuCl2(κ2-P–P′)2] is easily prepared from [RuCl2(DMSO)4]. The two phosphole-phosphorus atoms lie in the trans position to the two cis-chloro ligands. This complex slowly isomerizes spontaneously at 20 °C to the trans-[RuCl2(κ2-P–P′)2] diastereoisomer where the two phosphole moieties are mutually trans, as well as the two chloro ligands and the two Ph2P moieties. DFT calculations show that this non-classical cis–trans isomerisation process requires a 3 kcal mol−1 energy and involves the decoordination of a phosphole arm

Measuring domain sizes in graphene-based carbons

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Nanotexture of Pyrolytic Carbon Matrices: a review of characterization and modeling studies

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Novel phospholyl(diphenylphosphino)methane-ruthenium complexes: unexpected non-assisted cis to trans isomerization of [RuCl 2

International audienceUsing the unsymmetrical P–P′ phospholyl(phosphino)methane ligand, complex cis-[RuCl2(κ2-P–P′)2] is easily prepared from [RuCl2(DMSO)4]. The two phosphole-phosphorus atoms lie in the trans position to the two cis-chloro ligands. This complex slowly isomerizes spontaneously at 20 °C to the trans-[RuCl2(κ2-P–P′)2] diastereoisomer where the two phosphole moieties are mutually trans, as well as the two chloro ligands and the two Ph2P moieties. DFT calculations show that this non-classical cis–trans isomerisation process requires a 3 kcal mol−1 energy and involves the decoordination of a phosphole arm