Nanoscale structure and texture of highly anisotropic pyrocarbons revisited with transmission electron microscopy, image processing, neutron diffraction and atomistic modeling

摘要

We present a comparative study of the structure and texture at the nanoscale of two well-known high-textured laminar pyrocarbons (PyCs), the rough laminar (RL) and regenerative laminar (ReL) PyCs. Structure is assessed with diffraction data in the reciprocal space (coherence lengths L_α and L_c), and in the real space, using a pair distribution function analysis, to finely describe the in-plane features. Texture is characterized from high resolution transmission electron microscopy (HRTEM) images, by analyses of the 002 fringe properties (length and tortuosity) and of new descriptors based on the spatial variations of local orientations, allowing the measurement of the average misorientation angle between crystallites. However, the structure and texture of the RL PyC are recovered when the ReL PyC is heat-treated at 1500 °C. In addition, atomistic models are generated from HRTEM images and thoroughly validated against structural and textural indicators. The latter, essentially containing three-coordinated atoms arranged in hexagonal rings, show that the materials differ from the way these rings are clustered together in large hexagonal graphene domains and connected by grain boundaries (pentagon/heptagon pairs), interlayer crosslinks (screw dislocations) and hydrogen-saturated edges.