Abstract :
Two types of complex nanotubes produced by the arc-discharge method are investigated in this study: multi-walled
carbon nanotubes filled with metallic nanowires and composite BN–C nanotubes. Their multi-element character yields
specific spatial chemical arrangements — deduced from transmission electron microscopy TEM.studies — which give
precious information about the growth mechanism of nanotubes. Concerning filled nanotubes, if the metal–graphite cathode
is carbon free, no filling is obtained, whereas if it contains sulfur — either as an impurity of graphite or when added under
controlled quantities — complete or very long fillings are achieved, even for metals with very high melting temperatures.
The chemical analyses revealed various types of fillings: pure sulfides, grains of sulfides alternating with grains of pure
metals, or in some specific cases, pure metals. As for the B–C–N tubes, a total phase separation is observed between BN
and C, and these two phases form concentric shells typically of the CrBNrC type. In this paper, we show that an approach
combining the vapor–liquid–solid VLS.scheme and the characteristics of the solidification given by phase diagrams
account very well for the observed structures. The contrast between the absence of filling, when a sulfur-free carbon–metal
rod is used for the cathode, and the successful fillings, when sulfur is added, is explained by metal–sulfur phase diagrams:
adding sulfur to a liquid metal decreases the solidification temperature. The different types of fillings are also explained by
the nature of the sulfur–metal phase diagram. An eutectic solidification, such as in Ni–S, yields two phases — the pure
metal and the first sulfide — within a given tube, whereas the existence of a miscibility gap in the liquid, such as in Cr–S,
leads to two separate liquids and, therefore, to two different fillings. In the same way, the eutectic-like pseudo-binary C–BN
phase diagram explains not only the complete phase separation between BN and C, but also the observed organisation
between layers: we propose that the latter is due to a sequential solidification of the two phases. As a perspective, this phase
diagram approach is also discussed in the context of the formation of ropes of single-walled carbon nanotubes from the
solidification of a metal–carbon liquid particle. q2000 Elsevier Science B.V. All rights reserved
