Page 29 - Carbon Nanotubes

et
20 V. IVANOV al.
ments (Fig. 7). Thus, we suppose that the formation
of graphite tubules in these conditions is a very rapid
process and the thermal pyrolysis leading to the
30-
formation of amorphous carbon does not have a
great influence. Hence, carbon nanotubules, quasi-
free from amorphous carbon, are formed.
3.2.3 Reaction time. Two series of experiments
were performed in order to study the influence of the
reaction time on the characteristics of surface carbon
structures. In the first series, the hydrocarbon depos-
ition was periodically stopped, the catalyst was cooled
0-3 3-6 6-9 9-12 12-18 down under flowing nitrogen and it was removed
d, nm from the furnace. After taking a small part of the
Fig. 6. Sizedistribution of metal crystallites on the surface reaction mixture for TEM analysis, the remaining
of Co-silica made by precipitation-ionexchange method. amount of the catalyst was put back into the furnace
and the hydrocarbon deposition was further carried
3.2.2 Reaction temperature. The reaction tem- out under the same conditions. In the second series,
perature was vaned in the range 773-1073K. The different portions of catalyst were treated by hydro-
formation of filament structures was observed at all carbon for different times. The results were similar
studied temperatures. As has already been mentioned, for both series of catalysts. Typical images of carbon
the graphitization of carbon into the tubular struc- surface structures grown during different times are
tures on metal-supported catalysts is generally accom- shown in Fig. 8. In accordance with Ref. [4] we
panied by the formation of amorphous carbon. Both observed the dependence of the rate of filament
processes are temperature dependent. The filaments formation on the size of the catalytic particles. In the
grown at low temperature (773 K) are relatively free first (1 minute) reaction period, mostly very thin
of amorphous carbon. The amount of amorphous carbon filaments were observed as grown on the
carbon increases with increasing temperature and smallest metal particles. These filaments were very
represents about 10% of all carbon condensed on the irregular and the metal particles were generally found
external surface of the catalyst at 973 K. However, at the tips of the fibres. With increasing reaction time
crystallinity of the graphite layers in tubules also the amount of well-graphitized tubules progressively
strongly depends on the reaction temperature being increased. At the same time the average length of the
the lowest at low temperature. nanotubules increased. We need, however, to note
The average length of the tubules is not strongly that a relation exists between the lengths of the
influenced by temperature. However, the amorphous tubules and their diameters. The longest tubules are
carbon on the outer layers of filaments produced also the thickest. For instance, the tubules of
under optimal conditions is often deposited in frag- 3&60 pm length have diameters of 35-40 nm corre-

Fig.7. Graphite nanotubule on Co-SiO, with the fragments of amorphous carbon (arrowed) at the
external surface.