10 research outputs found
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 Ë10nm. 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 matĂ©riaux polyromatiques -graphĂ©niques- obtenus par CVD/CVI qui sont souvent utilisĂ©s en tant qu'interfase ou en tant que matrice dans des composites C/C pour l'industrie aĂ©ronautique et la rĂ©alisation de freins, tuyĂšres de fusĂ©es etc. Bien que non limitĂ©es Ă ce domaine d'application, cet exemple montre que leurs propriĂ©tĂ©s mĂ©caniques et leur rĂ©sistance aux tempĂ©ratures extrĂȘmes sont des qualitĂ©s qu'il est primordial de dĂ©terminer. Dans ce but, une Ă©tape d'identification des matĂ©riaux est nĂ©cessaire afin de crĂ©er une base de donnĂ©es reliant la description des matĂ©riaux Ă leurs propriĂ©tĂ©s. Le travail de cette thĂšse s'est donc portĂ© sur la description structurale (caractĂšre cristallographique), nano-texturale (degrĂ© de perfection et d'ordre des graphĂšnes au sein des domaines anisotropes) et structurale (Ă©tendues et disposition relative des domaines anisotropes) de certains de ces matĂ©riaux, les pyrocarbones laminaires (des trois types dits 'rugueux', 'lisses', et 'rĂ©gĂ©nĂ©rĂ©s'). Il est articulĂ© autour de deux techniques majeures qui sont la spectroscopie Raman et la microscopie Ă©lectronique en transmission, complĂ©tĂ© par la diffraction des rayons X. Le volet spectroscopie Raman montre une Ă©tude de l'Ă©volution des diffĂ©rentes bandes caractĂ©ristiques du matĂ©riau (bande G, gĂ©nĂ©rĂ©e par les vibrations du rĂ©seau, et bande D, gĂ©nĂ©rĂ©e par les dĂ©fauts) et de leurs importances respectives (aires A ou intensitĂ©s I), comme le rapport ID/IG, en fonction de la longueur d'onde excitatrice et en fonction de la taille moyenne des cristallites dĂ©terminĂ©es par diffusion de neutrons et diffraction des rayons X. Ce travail montre notamment que pour de petites tailles de cristallite (< 6nm), une nouvelle contribution Ă l'intensitĂ© de la bande D de mĂȘme position spectrale apparaĂźt. Cette approche novatrice par rapport aux travaux antĂ©rieurs permet de rendre compte parfaitement des observations expĂ©rimentales dans le domaine ultraviolet, visible et infrarouge. Afin de complĂ©ter cette Ă©tude, une sĂ©rie de cokes de brai a Ă©tĂ© Ă©tudiĂ©e et montre, en cohĂ©rence avec les pyrocarbones, une relation linĂ©aire entre la largeur de la bande G et les tailles de cristallites lorsqu'elles font moins de Ë 10nm. Dans le volet microscopie Ă©lectronique en transmission, une mĂ©thodologie prĂ©alablement mise au point au laboratoire et validĂ©e pour la caractĂ©risation multi-Ă©chelle quantifiĂ©e de pyrocarbones de type pshĂ©rulitiques isotropes a Ă©tĂ© utilisĂ©e pour la premiĂšre fois pour la caractĂ©risation de pyrocarbones laminaires. Pour le recours Ă plusieurs modes de la microscopie Ă©lectronique (diffraction Ă©lectronique Ă aire sĂ©lectionnĂ©e, imagerie de fond noir, imagerie de franges de rĂ©seau), elle a permis, dans une certaine mesure, d'Ă©valuer qualitativement et/ou quantitativement les caractĂ©ristiques texturales et nanotexturales respectives et discriminantes des diffĂ©rents types de pyrocarbones Ă©tudiĂ©s. Le travail a aussi mis en Ă©vidence les limitations de la mĂ©thodologie Ă son application pour le type particulier des matĂ©riaux de l'Ă©tude
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 DâČ 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 ranges from 2 to 5 nm. This size range is between two different Raman behaviors, one for which the D band broadens (for 5 nm) respectively, with increasing . 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 between 2 and 5 nm while the total D band intensity is nearly constant for excitation wavelengths ranging from 0.532 to 0.638 ÎŒm. In contrast, the integrated intensity ratio D/G follows the well-known 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 , when below â 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 . 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<<10 nm. The energy dependence of / was found to vary with the excitation energy as with 1 <<4 depending on both 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 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 (Îș 2 -PâPâČ) 2 ]
International audienceUsing the unsymmetrical PâPâČ phospholyl(phosphino)methane ligand, complex cis-[RuCl2(Îș2-PâPâČ)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(Îș2-PâPâČ)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 cisâtrans isomerisation process requires a 3 kcal molâ1 energy and involves the decoordination of a phosphole arm
Nanotexture of Pyrolytic Carbon Matrices: a review of characterization and modeling studies
International audienc
Novel phospholyl(diphenylphosphino)methane-ruthenium complexes: unexpected non-assisted cis to trans isomerization of [RuCl 2
International audienceUsing the unsymmetrical PâPâČ phospholyl(phosphino)methane ligand, complex cis-[RuCl2(Îș2-PâPâČ)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(Îș2-PâPâČ)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 cisâtrans isomerisation process requires a 3 kcal molâ1 energy and involves the decoordination of a phosphole arm