Structure armchair

In addition, for two coaxial armchair tubules, estimates for the translational and rotational energy barriers (of 0.23 meV/atom and 0.52 meV/atom, respectively) vvere obtained, suggesting significant translational and rotational interlayer mobility of ideal tubules at room temperature[16,17]. Of course, constraints associated with the cap structure and with defects on the tubules would be expected to restrict these motions. The detailed band calculations for various interplanar geometries for the two coaxial armchair tubules basically confirm the tight binding results mentioned above[16,17]. 

The band structure of four concentric armchair tubules with 10, 20, 30, and 40 carbon atoms around their circumferences (external diameter 27.12 A) was calculated, where the tubules were positioned to minimize the energj for all bilayered pairs 17). In this case, the four-layered tubule remains metallic, similar to the behavior of two double-layered armchair nanotubes, except that tiny band splittings form. 

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... 

The tubes (a, a) and (a, 0) are generated from hexagons with 0 = jt/6 and 0, respectively. These tubes become non-helical and are called, respectively, armchair and zigzag structures. Other condition (0 < 0 < Jt/6) generates the tube (a, b) of helical structures (see Fig. 2). 

These values lie between those of the corresponding armchair and zigzag structures. The explicit expressions have been given in ref. 16. 

Armchair structure Zigzag structure Helical structure All other tubes ... 

Electronic structures of SWCNT have been reviewed. It has been shown that armchair-structural tubes (a, a) could probably remain metallic after energetical stabilisation in connection with the metal-insulator transition but that zigzag (3a, 0) and helical-structural tubes (a, b) would change into semiconductive even if the condition 2a + b = 3N s satisfied. There would not be so much difference in the electronic structures between MWCNT and SWCNT and these can be regarded electronically similar at least in the zeroth order approximation. Doping to CNT with either Lewis acid or base would newly cause intriguing electronic properties including superconductivity. 

The basic building block of carbon is a planar sheet of carbon atoms arranged in a honeycomb structure (called graphene or basal plane). These carbon sheets are stacked in an ordered or disordered manner to form crystallites. Each crystallite has two different edge sites (Fig. 2) the armchair and zig-zag sites. In graphite and other ordered carbons, these edge sites are actually the crystallite planes, while in disordered soft and hard carbons these sites, as a result of turbostratic disorder, may not... 

As mentioned in Section IV, BC2N nanotubes have two isomers with distinctly different structure and properties. One of them, with alternating carbon and B-N chains, was predicted to be a semiconductor. In the armchair (n, n) configuration,... 

Another complication in CNT applicability arises from the way the graphene sheet is rolled up to create the cylindrical structures, which is usually called helicity . Depending on the angle of the wrapping, three different structures (different helicities) can result (1) armchair, (2) chiral or (3) zigzag (Fig. 3.3). Such structures exhibit differ-... 

Two basic issues can be distinguished here (i) the physico-chemical nature of zigzag and armchair sites at the edges of fused-benzene-ring structures, including their differences and similarities with respect to polycyclic aromatic hydrocarbon (PAH) molecules, and (ii) the reconstruction of these edges to potentially more stable structures. 

Chairs, backrests, seats, armchairs can be made out of polypropylene, PMMA, ABS, possibly combined with a metal structure. 

There remains the question why activated carbons differ from carbons heat-treated at 1200° with respect to the relative position of the carboxyl groups. Perhaps this difference is based on the structure of the edges of the carbon layers. Hennig (87, 88) found, by observations with single crystals of graphite, that after oxidation with dry oxygen the armchair configuration of the periphery resulted ... 

There are three general types of CNT structure (Figure 12.10). The zigzag nanotubes correspond to ( ,0) or (0,m) and have a chiral angle of 0°. The carbon-carbon position is parallel to the tube axis. Armchair nanotubes have (n,n) with a chiral angle of 30°. The carbon-carbon positions are perpendicular to the tube axis. Chiral nanotubes have general (n,m) values and a chiral angle of between 0° and 30°, and as the name implies, they are chiral. 

This difference in structure also influences the electrical conductivity with the armchair form being conductive or metal-like in its conductivity, and most of the other forms act as semiconductors. 

Figure 2. From the left Bge cluster segments in form of two 48-rings (Fig. 2a), three 32-rings (Fig. 2b), four 24-rings (Fig. 2c) and six 16-rings (Fig. 2d) with the structures of (24, 24), (16,16), (12,12) and (8,8) armchair nanotubes, respectively.

Three examples of particular structures of SWCNTs, depending on the orientation of the hexagons related to the tube axis, (a) armchair-type tube (0 = 30°), (b) zigzag type tube (0 - 0°), and chiral tube (0 < 0 < 30°). Reprint from Carbon, vol. 33, No. 7, Dresselhaus M.S., Dresselhaus G., Saito R., Physics of carbon nanotubes, pages 883-891, Copyright (1995) with permission from Elsevier. 

Let us recall that nanotubes can be considered as graphene sheets rolled up in different ways. If we consider the so-called chiral vectors c = nai + na2, in which a and a2 are the basis vectors of a 2D graphite lattice, depending on the value of the integers n and m, one can define three families of tubes armchair tubes (n = m), zig-zag tubes (n or m = 0), and chiral tubes (n m 0). Band structure calculations have demonstrated that tubes are either metallic compounds, or zero-gap semiconductors, or semiconductors [6,7]. More commonly, they are divided into metallic tubes (when n-m is a multiple of 3) or semiconducting ones. 

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