New structural features observed in heat-treated vapor-grown carbon fibers (VGCF's), produced by the thermal decomposition of hydrocarbon vapor, are reported using image analysis of the lattice plane structure observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The TEM lattice image of well-ordered graphite fibers (heat-treated VGCF's at 2800 °C) was treated by a two-dimensional fast Fourier transform, showing sharp bright spots associated with the 002 and 100 lattice planes. The heat-treated VGCF's consist of a polygonally shaped shell, and the long and short fringe structures in the TEM lattice image reflect the 002 and 100 lattice planes, respectively. From this analysis, new facts about the lattice structure are obtained visually and quantitatively. The 002 lattice planes remain and are highly parallel to each other along the fiber axis, maintaining a uniform interlayer spacing of 3.36 A. The 100 lattice planes are observed to make several inclined angles with the 002 lattice planes relative to the plane normals, caused by the gliding of adjacent graphene layers. This work visually demonstrates coexistence of the graphitic stacking, as well as the gliding of the adjacent graphene layers, with a gliding angle of about 3-20°. These glide planes are one of the dominant stacking defects in heat-treated VGCF's. On the other hand, turbostratic structural evidence was suggested by AFM observations. The structural model of coexisting graphitic, glide, and turbostratic structures is proposed as a transitional stage to perfect three-dimensional stacking in the graphitization process. These structural features could also occur in common carbons and in carbon nanotubes.
The present work presents a useful comparison of micropore size distributions (MPSDs) obtained from gas
adsorption and image analysis of high-resolution transmission electron micrographs. It is shown that the
MPSD obtained for a chemical activated carbon is concordant with that obtained from CO2 adsorption. In
addition, this technique has allowed us to obtain the MPSD of a carbon molecular sieve (CMS) prepared in
our laboratory by a copyrolysis process, which could only be characterized by CO2 adsorption at 273 K (not
by N2 adsorption at 77 K due to diffusional problems). The MPSD obtained by high-resolution transmission
electron microscopy (HRTEM) for the CMS is wider than that obtained by CO2 adsorption, suggesting that
HRTEM is detecting the closed porosity existing in this sample, which is not accessible to gas adsorption.
The existence of closed porosity in the CMS is explained considering the preparation method used. Thus,
HRTEM combined with image analysis seems to be useful for structural analysis of narrow micropores
including closed porosity.
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