TABLE OF CONTENTS As we have seen in this chapter, amazing progress has been made in the catalytic synthesis of nanotubes since the early 1990s. Processes for the bulk production of multiwalled nanotubes have advanced to the point where several companies claim to be making more than 100 tons per year. Single-walled tubes can also now be produced in bulk using CVD, although production volumes are inevitably rather lower, and techniques for making double-walled nanotubes catalytically have been developed. Remarkable advances have also been made in the growth of nanotubes in a controlled manner on substrates. In the case of MWNTs, this has generally involved the vertical growth of tubes on surfaces, producing large arrays which have applications in areas such as field emission. The directed growth of SWNTs, on the other hand, has mainly involved the formation of defined patterns on substrates, with the aim of constructing nanoscale electronic devices. Another area where important progress has been made, notably by the Windle group in Cambridge, is the continuous spinning of nanotube yarns, which may have important applications as components of composite materials.
Are there any disadvantages to the catalytic synthesis of nanotubes? In the case of multiwalled tubes, the answer is yes, as the quality of CVD-produced multiwalled nanotubes is still inferior to those made by arc-evaporation. Generally speaking, catalytically-made MWNTs contain far more defects than those produced in the arc, and this is reflected in their mechanical properties. It might be thought that the properties of CVD-MWNTs could be...
