Carbon nanotubes bundle structure

Figure 2.4 Carbon nanotubes (a) structure of SWCNT and MWCNT, (b) XRD pattern of Baytube C150P, (c) TEM micrograph showing bundles of CNTs.

Abe, M., Kataura, H., Kira, H., Kodama, T, Suzuki, S., Achiba, Y., Kato, K., Takata, M., Fujiwara, A., Matsuda, K. and Maniwa, Y., Structural transformation from single-wall to double-wall carbon nanotube bundles, Physical Review B, 68,2003,041405. 

Inspired by experimental observations on bundles of carbon nanotubes, calculations of the electronic structure have also been carried out on arrays of (6,6) armchair nanotubes to determine the crystalline structure of the arrays, the relative orientation of adjacent nanotubes, and the optimal spacing between them. Figure 5 shows one tetragonal and two hexagonal arrays that were considered, with space group symmetries P42/mmc P6/mmni Dh,), and P6/mcc... 

An important route to solubilization of carbon nanotubes is to functionalize their surface to form groups that are more soluble in the desired solvent environment. It has been shown that acid treatment of nanotube bundles, particularly with HC1 or HNO3 at elevated temperatures, opens up the aggregate structure, reduces nanotube length, and facilitates dispersion (An et al., 2004 Kordas et al., 2006). Nitric acid treatment oxidizes the nanotubes at the defect sites of the outer graphene sheet, especially at the open ends (Hirsch, 2002 Alvaro et al., 2004), and creates carbonyl, carboxyl, and hydroxyl groups, which aid in their solubility in polar solvents. 

Reich S, Thomsen C, Ordejon P (2002) Electronic band structure of isolated and bundled carbon nanotubes. Phys Rev B 65(15) 155411... 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

Up to now only the electronic properties of single carbon nanotubes have been regarded. In reality, however, frequent use is being made of multiwalled tubes or of bundles of single-walled species. Their electronic properties differ to some extent from those of isolated SWNT. Yet the growing complexity of the systems increasingly complicates the analytical description of the density of states and band structure. Still some recent publications on the matter discuss the phenomena observed upon the assembly of several tubes. 

All known types of carbon nanotubes are potential field emitters (Figure 3.55b). Single and bundled SWNT as well as multiwalled species show this behavior. Individual tubes may be attached to a conductive support, but they might just as well be employed in the shape of ordered or unordered films or as structured arrays (Section 3.3.5). The respective products can be selected depending on the desired application. The choice of single- or multiwalled species influences emission properties, too SWNTs feature a low work function, whereas MWNTs better suit continuous use because they are clearly more stress resistant... 

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