Page 18 - Carbon Nanotubes

8 M. ENDO et al.

of ca. 10 nm (white arrow), observed by field emission
scanning electron microscopy (FE-SEM)[25]. It is,
thus, suggested that at least some of the VGCFs start
as nanotube cores, which act as a substrate for sub-
sequent thickening by deposition of secondary pyro-
lytic carbon material, as in the catalytically primarily
grown hollow fiber. In Fig. 14b is also shown the TEM
image corresponding to the extruded nanotube from
a very thin fiber. It is clearly observed that the exposed
nanotube is continuing into the fiber as a central hol-
low core, as indicated by the white arrow in the figure.
It is interesting that, as indicated before (in Fig. 14a),
the core is more flexible than the pyrolytic part, which
is more fragile.
Fig. 13. HRTEM image of an as-grown thick PCNT. 002
lattice image demonstrates the innermost hollow core (core 8. CONCLUSION
diam. 2.13 nm) presumably corresponding to the “as-formed” Pyrolytic carbon nanotubes (PCNTs), which grow
nanotube. The straight and continuous innermost two fringes
similar to Fig. 5 are seen (arrow). during hydrocarbon pyrolysis, appear to have struc-
tures similar to those obtained by arddischarge tech-
niques using graphite electrodes (ACNTs). The PCNTs
tend to exhibit a characteristic thickening feature due
involved might be separated by pulverizing the VGCF to secondary pyrolytic carbon deposition. Various tip
material. morphologies are observed, but the one most fre-
In Fig. 14a, a ca. 10 wm diameter VGCF that has quently seen has a 20” opening angle, suggesting that,
been broken in liquid nitrogen is depicted, revealing in general, the graphene conical tips possess a cluster
the cylindrical graphitic nanotube core with diameter
of five pentagons that may be actively involved in tube
growth. PCNTs with spindle-like shapes and that have
conical caps at both ends are also observed, for which
a structural model is proposed. The spindle-like struc-
tures observed for the secondary growth thickening
that occurs in PCNTs may be a consequence of the
lower carbon content present in the growth atmo-
sphere than occurs in the case of ACNT growth. Pos-
sible structural models for these spindles have been
discussed. The longitudinal growth of nanotubes ap-
pears to occur at the hemi-spherical active tips and this
process has been discussed on the basis of a closed cap
mechanism[9,11]. The PCNTs are interesting, not only
from the viewpoint of the fundamental perspective
that they are very interesting giant fullerene structures,
but also because they promise to be applications in
novel strategically important materials in the near fu-
ture. PCNT production appears, at this time, more
readily susceptible to process control than is ACNT
production and, thus, their possible value as fillers in
advanced composites is under investigation.

Acknowledgements-Japanese authors are indebted to M. S.
Dresselhaus and G. Dresselhaus of MIT and to A. Oberlin
of Laboratoire Marcel Mathieu (CNRS) for their useful dis-
cussions and suggestions. HWK thanks D. R. M. Walton for
help and the Royal Society and the SERC (UK) for support.
Part of the work by ME is supported by a grant-in-aid for
scientific research in priority area “carbon cluster” from the
Ministry. of Education, Science and Culture, Japan.
Fig. 14. PCNTs (white arrow) appeared after breakage of
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