Microstructure of the pyrocarbon in aligned carbon nanotube/carbon composites

Aligned carbon nanotube reinforced carbon matrix (ACNT/C) composites were fabricated by densifying an ACNT preform with chemical vapor infiltration (CVI) and their microstructure was investigated by XRD, SEM, HRTEM, polarized light microscopy and Raman spectroscopy. Results show that the pyrocarbon in ACNT/C composites was of mainly rough lamella type, while that in traditional carbon/carbon composites obtained using the same process was of typically smooth lamella type with a larger interlayer spacing and a smaller layer stack height. Pyrocarbon around CNTs shows a clear annular graphiticlayered microstructure with good interfacial bonding between CNTs and pyrocarbon. This might be ascribed to the intermolecular interaction between CNTs and aromatic molecules, i.e., CNTs have abundant radial delocalized conjugated π-π electrons, which can attract polynuclear aromatic molecules with a similar structure through van der Waals forces. These polynuclear aromatic molecules are therefore oriented around the CNT outer surface under a "soft epitaxy" mechanism. In contrast, carbon fibers composed of turbostratic carbon have no such effect during CVI.