Page 106 - Carbon Nanotubes

96 A. FONSECA al.
et

Fig. 12. Explanation of the growth mechanism leading to tori (a)-(e) and to regular helices (a)-(h). (a)
Growing nanotubule on an immobilized catalyst particle; (b) the tube reaches obstacle A, (c) elastic
bending of the growing tubule caused by its blockage at the obstacle A; (d) after the formation of the
knee, a second growing stage can occur; (e) second blockage of the growing nanotubule by the obstacle
A, (f) after the formation of the second knee, a new growing stage can occur; (g) the tube reaches obstacle
B (h) formation of the regular helicity in the growing tubules by the obstacle B.

i Perimeter=15ak

Fig. 13. Model of the growth of a nanotubule “bonded” to the catalyst surface. (a) Growth of a straight
(53) nanotubule on a catalyst particle, with perimeter 15ak; (b) growth of a straight (9,O) nanotubule on
a catalyst particle whose perimeter is 18ak (k is a constant and the grey ellipsoids of (a) and (b) represent
catalyst particles, the perimeters of which are equal to 15ak and 18ak, respectively); (c) (5,5)-(9,O) knee,
the two sides should grow optimally on catalyst particles having perimeters differing by ca. 20%.

the catalyst particle perimeter and the nature of the - If the active perimeter of the catalyst particle has a
tubules produced: dimension between 15nak and 18nak, - i.e.
15nak < active perimeter < 18nak - the two differ-
- When the active perimeter of the catalyst particle ent tubules can still be produced, but under stress.
matches perfectly the values 15nak or 18nak (where - The production of heptagon-pentagon pairs
n is the layer order, a is the side of the hexagon in among these hexagonal tubular structures leads to
graphite and k is a constant), the corresponding the formation of regular or tightly wound helices.
straight nanotubules (5n,5n) or (9n,O) will be pro- Each knee provides a switch between the tubule
duced, respectively [ Fig. 13 (a) and (b)] . formation of Fig. 13(a) and (b) [Fig. 13(c)].