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Understanding the carbonisation-graphitisation processes

developing an innovative bottom-up approach for analyzing X-ray diffraction patterns of graphenic carbon materials, and other work : Extracting average crystallite dimension La and Lc in graphenic carbons from X-ray diffraction (XRD) patterns is wanted because they relate to physical properties, but is complex because of the possible and variable presence of the turbostratic periodicity. We developed a new approach which is physically meaningful regarding both the structural behaviour and the fitting mathematical functions from which La and Lc can be reliably obtained. Thanks to atomistic modelling, and in full agreement with the seminal literature, we found that three basic structural components (BSC) have to be considered for the calculated patterns to fit the experimental ones : (i) the single graphene A, (ii) the graphite stacking (ABA, etc.), (iii) the graphene pair AB. Each of the BSCs may co-exist within the same 00l-coherent graphenic crystallite, with proportions depending on the carbonisation/graphitisation state. The accuracy of the approach and the availability of new parameters are discussed. The method is applied to the analysis of very distinct XRD patterns from different carbonisation/graphitisation states and mechanisms and is shown to be applicable to all kinds of graphenic carbons.

The three different Basic Structural Components from the proportions of which any graphenic material can be described, whatever its structural status.

Investigating the effect of pressure : The influence of high pressure in the range of 0–15 GPa on the linewidth and the position of the Raman G band was investigated for graphene‐based materials with controlled crystallite sizes, La, obtained from pitch‐based cokes by increasing annealing temperature. It was observed that the linewidth of the G band increases significantly above a critical pressure of 10 GPa, for all samples, whereas at atmospheric pressure, the linewidth neither depends on variation in the measurement temperature (<400 K) nor on excitation wavelengths. The critical pressure was found not to be influenced by the crystallite size (which increases with the annealing temperatures) but is related to a modification of carbon hybridization. At the same time, the G band linewidth increases significantly for smaller crystallite sizes depending on the annealing temperature at atmospheric pressure due to the merging of the D′ and G phonon bands. This merging of the bands can be explained by a single phonon involved in two physical Raman processes, which cannot be discriminated.

Examples of plots of the G band linewidth with the pressure applied, for ain creasing annealing temperature of o coal-tar pitch coke.

See : Puech et al., Carbon 147 (2019) 602 – Monthioux, Carbon 150 (2019) 142 – Pillet et al., J. Raman Spectrosc. 50 (2019) 1861 – Ouzilleau et al., Carbon 149 (2019) 419 – Puech et al, Ind. J. Eng. Mater. Sci. 27 (2020) 1095.

Carbon fibers made of doped double-wall carbon nanotubes with electrical conductivity approaching that of copper : Why can the electrical conductivity of carbon nanotube fibers, once doped, approach that of copper ? Joint research works of teams originating from Toulouse and US have found out : this is the result of an increase in the number of conduction channels available in the CNTs.
When double-wall carbon nanotubes (DWCNTs) are combined with iodine and chlorosulfonic acid, the charge transfer increases the conductivity by a factor of 7. The doping species move the Fermi level up to 1.1 eV while not changing the average electronic mean free path at 30 nm. Works combining both optical and transport experiments, and simulations reveal the underlying mechanism.
Two strategies are possible to obtain high electrical conductivity values in carbon nanotube (CNT) fibers : (1) increasing the electronic mean free path and (2) increasing the number of electronic conduction channels in each NTC by moving the Fermi level through charge transfer.
Chlorosulfonic acid (CSA) is a true solvent of CNT. If, during CNT fiber synthesis, a part of CSA remains in the material, a charge transfer thus occurs and leads to an up-shift of the Fermi level of 0.7 eV. The consequence is an increase of the conductivity by a factor of 5 compared to a system composed of CNT only. Similar properties can be obtained in pure CNT fiber impregnate with iodine. In that case, the conductivity gain is a slightly larger, approaching factor of 7, and the Fermi level up-shift is 1.1 eV. At high currents, the CNT fibers are heated by Joule effect and the chemical species (iodine or CSA) evaporate. The pure (dedoped) NTC fibers are then obtained allowing the evaluation of the conductivity gain.

Left : Optical and then SEM images of a carbon fiber prepared from doped DWCNTs. Middle : sketch of a typical DWCNT. Right : Principle of the enhanced electrical conduction by adsorbed species.

Raman spectrometry permits to measure the Fermi level displacement in double-wall CNTs. The high expertise of the CEMES group on optical spectroscopy and on C-based materials has allowed designing accurate fitting strategies to extract reliable information. By analyzing the Raman signals from both the inner and outer tubes of the double-wall CNT, they succedded in measuring the charge transfer and, in the case of high doping, to convert it into the Fermi level up-shift. For the first time, it was possible to go further in the analysis in terms of average free path for non-defective CNTs. The value of about 30 nm was obtained independent of the Fermi level position.
The charge transfer process in CSA doped CTNs is relatively obvious while no convincing explanation has been published yet for iodine doped CNTs. Quantum calculations were then carried out to systems typically composed of 500 atoms to state how iodine atoms mutually arranged. Constrained by the material geometry to remain aligned, they form chains which are polarized (6 iodine atoms form 2 tri-iodides in contact for example) and thus, allow the formation of a very large number of ions, the role of counter ion being played by the CNT and explaining the charge transfer experimental result.
These works involving synthesis, optical and electrical characterizations, and numerical simulation were carried out within the framework of three PhD projects and have combined the specific fields of expertise of three French laboratories in Toulouse : CEMES, LPCNO and CIRIMAT and two groupes in the United States : Rensselaer and Rice Universities.
See : Tristant et al., Nanoscale 8 (2016) 19668 - Zubaïr et al., Phys. Rev. Mater. 1 (2017) 064002.

Carbon nanotube/polymer composites
The formation of uniform dispersions from agglomerated carbon nanotubes in polymers is a major challenge for their use in composites. We discovered that agglomerated carbon nanotubes on the surface of a thermoplastic polymer are efficiently dispersed when annealing. Annealing is found to disperse carbon nanotubes surprisingly well. The dispersion of nanotubes depends on whether the tubes are multi-, double- or single-walled. The interface between the composite at the surface and the polymer is well defined. Differences are observed for single-, double- and multi-wall tubes. The electrical conductivity is found de be substantially higher than compared to bulk carbon nanotube composites. The surface nano-composite is suitable for sensor applications.

TEM image of the interface between the diffusion layer and the PEEK matrix. The MWCNT distribution is uniform at scales down to 500 nm.

See : Tishkova, et al., Carbon 53 (2013) 399

Dopage multipliant la conductivité des fibres de nanotubes de carbone d’un facteur 7

Dans les nanotubes de carbone bi-parois associés à la présence d’iode et d’acide chlorosulfonique, le transfert de charge permet d’augmenter jusqu’à un facteur 7 la conductivité. Les espèces dopantes (...)

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Bulle graphène

La nature hautement élastique et flexible du graphène permet la création de grandes bulles stables sur sa surface (plusieurs micromètres de large et un micromètre d’épaisseur) de manière plus ou moins (...)

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Diffraction des rayons X

Analyser de manière fiable les diagrammes de diffraction des rayons X de matériaux carbonés graphéniques nanostructurés est important pour établir une relation avec leurs propriétés. Nous avons utilisé (...)

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