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A new, full bottom-up approach for the analysis of X-ray diffraction patterns of graphenic carbons

Solving a 80-year-old problem

par PREVOTS Evelyne - publié le

Graphenic carbons are of an utmost technological importance. Predicting their properties requires measuring the average crystallite dimensions, but the bidimensional nature of the turbostratic part of the often-complex crystal structure makes the analysis of diffraction patterns not straightforward. By using a predictive approach of the crystal structure, CEMES has come up with a new methodology which finally allows mowing away from a 80-year-long unsatisfactory empiricism.

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Graphenic materials are of an utmost technological importance for a broad variety of domains. They are obtained from the carbonisation of organic natural or synthetic precursors which elemental composition determines both the anisotropy degree of the material and its ability to ultimately reach the graphite structure. From the very first steps of carbonisation, polyaromatic molecules develop and pile up as the embryos of graphene layers. Crystallites are then formed all over with similar dimensions (La, Lc) which are characteristic of the material. Meanwhile, the graphene stacking sequence may evolve from rotationally random (turbostratic structure) to partially or fully ordered (graphitic, hexagonal structure).

Many of the properties of a graphenic carbons thus depend on the average crystallite dimensions, more importantly La. Measuring the latter is therefore critical, usually by means of X-ray diffraction (XRD). Unfortunately, the bidimensional nature of the turbostratic part of the crystallite structure makes the analysis of XRD patterns an issue which was addressed over the last 80 years by developing a variety of fitting and correcting methods aiming at obtaining the La values, which all failed in being reliable and univocal.

For addressing the issue, CEMES has considered it the reverse way : instead of empirically fitting experimental XRD patterns by pre-existing functions for estimating the La (top-down approach), parametrised functions are created from atomistic modelling of the average crystallite, and then the computer is left free to calculate the related XRD patterns for a whole range of La (bottom -up approach). The calculated pattern which fits the experimental pattern at best provides the right La value. The key of success was to model the average crystallite as composed from a mixture of three Basic Structural Components (BSC) which proportions may vary : "Turbostratic", "AB pair", and "Bernal" (cf. Fig.). Thus, for a graphitisable carbon, the average crystallite evolves from 100% "Turbostratic" at early carbonisation stage (< 1500°C) to 100% "Bernal" at the end of graphitisation ( 3000°C), while passing by combinations in which the three BSCs may co-exist. The methodology also introduces new structural parameters such as the BSC proportions, and Lc which characterises the height of the average "Bernal" BSC within the average crystallite on the path to graphitisation.

The methodology is valid for any type of graphenic material whatever the precursor and the degree of structural maturation.

 

Reference

New insight on carbonisation and graphitisation mechanisms as obtained from a bottom-up analytical approach of X-ray diffraction patterns.
Puech P., Dabrowska A, Ratel-Ramond N., Vignoles G., Monthioux M.
Carbon 147 (2019) 602-611.

 

Contacts

Pascal Puech - Marc Monthioux