Our project

Our project is devoted to the study of pyrocarbons.

We wish to improve our knowledge of their structure-property relationships.

To do so, many characterization techniques will be used.

We also intend to perform atomistic reconstructions to infer physical & chemical properties by computation.

This will involve several image analysis & synthesis techniques.

We have support from several organisms to develop our approach in a 4-year project.

Nous avons le soutien de différents organismes pour développer une approche sur 4 ans.

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Various kinds of pyrocarbon nanotextures

Pyrocarbons (or pyrolytic carbons) are dense carbons similar to graphite, obtained principally by Chemical Vapor Deposition (CVD) from the pyrolysis of hydrocarbons in presence of a substrate. They constitute the matrices of carbon/carbon composites and the coatings of several devices (e.g. heart valves, electrodes, …)

CVD of pyrocarbons

Hydrocarbons are placed in a hot reactor without air. They undergo pyrolysis, i.e. a thermally induced decomposition. The resulting molecules may recombine together, as in cracking/reforming petrochemical reactors. However, placed upon a soid surface (a substrate), these molecules deposit themselves and contribute to the creation of pyrolytic carbon.

The nanotexture of a pyrocarbon varies depending on the exact composition of the gas phase, which depends on its maturation degree (i.e. advancement of the cracking/reforming process). It is described by : density, coherence lengths along the graphene planes (La) and across them (Lc), defect density, optical anisotropy, …

Three different kinds of Laminar Pyrocarbons

A very important subgroup of pyrocarbons is formed by the Laminar Pyrocarbons, with high anisotropy.

The Rough Laminar (RL) or Columnar Laminar (CL) PyCs are the most dense ones (2.13 g/cm3) and have the largest coherence lengths  La (6-7 nm) and Lc (4-5 nm). They are very anisotropic and display very few defects.

The Regenerative Laminar (ReL) or Highly Anisotropic Laminar (HAL) PyCs are as anisotropic as the RL, though much richer in local defects. Their density (2.09 g/cm3), La (4-5 nm) and Lc (2-4 nm), are somewhat lower than for LR.

The Smooth Laminar (SL) PyCs are less anisotropic. Their density is markedly lower (approx. 1.9 g/cm3) and the coherence lengths La (4-5 nm) and Lc (2-4 nm) too. The defect density is halfway between RL and ReL.

Contrary to RL and ReL, the SL PyCs are not graphitizable, i.e. they are not able to rearrange themselves into polycrystalline graphite upon heat treatment (approx. 2500°C). The thermal and mechanical properties are thus extremely different, though the material is “almost the same” !

Find here more information on pyrocarbon characterization.

 

 

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Project aims

There are  three main objectives.

The first objective is to obtain realistic atomistic representations of several laminar pyrocarbons  (RL,SL,ReL), as processed and after more or less prolonged heat treatments. To do so we will need :

  1. To create an experimental database of materials and properties

    Measured properties

    • Density
    • Hydrogen amount
    • Crystallographic data: d002, La, Lc, etc …
    • pair Distribution Functions (Neutrons or X-ray Scattering)
    • HRTEM micrographs
    • Spectroscopic data (Raman, EELS, etc… )
    • Mechanical (stiffness, toughness, …) and thermal properties
  2. To improve the atomistic reconstruction method utilizing the newly acquired data as reconstruction constraints.

 

The second objective is to evaluate mechanical and thermal properties from the reconstructed atomistic models: it is a key point for the understanding of the structure/property relationships.

The third objective is to achieve a  mesoscopic description of laminar pyrocarbons in terms of coherent or pseudo-coherent domains. To do so direct (HRTEM-DF) and indirect (reconstructions based on HRTEM-LF images) data will be used. The result will be a change of scale : the mesostructure will be understood starting from nanostructure and imaging.

 

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