PYROLYTIC CARBON NANOTUBES FROM VAPOR-GROWN CARBON-FIBERS

The structure of as-grown and heat-treated pyrolytic carbon nanotubes (PCNTs) produced by hydrocarbon pyrolysis are discussed on the basis o f a possible growth process. The structures are compared with those of nanotubes obtained by the are method (ACNT; are-formed carbon nanotub es). PCNTs, with and without secondary pyrolytic deposition (which res ults in diameter increase) are found to form during pyrolysis of benze ne at temperatures ca. 1060 degrees C under hydrogen. PCNTs after heat treatment at above 2800 degrees C under argon exhibit have improved s tability and can be studied by high-resolution transmission electron m icroscopy (HRTEM). The microstructures of PCNTs closely resemble those of vapor-grown carbon fibers (VOCFs). Some VGCFs that have micro-size d diameters appear to have nanotube inner cross-sections that have dif ferent mechanical properties from those of the outer pyrolytic section s. PCNTs initially appear to grow as ultra-thin graphene tubes with ce ntral hollow cores (diameter ca. 2 nm or more) and catalytic particles are not observed at the tip of these tubes. The secondary pyrolytic d eposition, which results in characteristic thickening by addition of e xtra cylindrical carbon layers, appears to occur simultaneously with n anotube lengthening growth. After heat treatment, HRTEM studies indica te clearly that the hollow cores are closed at the ends of polygonized hemi-spherical carbon caps. The most commonly observed cone angle at the tip is generally ca. 20 degrees, which implies the presence of fiv e pentagonal disclinations clustered near the tip of the hexagonal net work. A structural model is proposed for PCNTs observed to have spindl e-like shape and conical caps at both ends. Evidence is presented for the formation, during heat treatment, of hemi-toroidal rims linking ad jacent concentric walls in PCNTs. A possible growth mechanism for PCNT s, in which the tip of the tube is the active reaction site, is propos ed.