MWCNT

There have been a considerable efforts at synthesis and purification of MWCNT for the measurements of its physical properties. The time is, however, gradually maturing toward its industrial application. As to SWCNT, it could not be efficiently obtained at first and, furthermore, both of its purification and physical-properties measurement were difficult. In 1996, it became that SWCNT could be efficiently synthesized [14,16] and, since then, it has become widely studied mainly from the scientific viewpoints. In what follows, the synthesis and purification of MWCNT and SWCNT are to be summarised itemisingly. 

MWCNT grows only inside the cathode deposit and does not exist in other places in the reactor. Quantity of MWCNT obtained depends on the pressure of He atmosphere in the reactor, which is the most important parameter. The highest quantity of MWCNT is obtained when the pressure of He is ca. 500 Torr. When this value becomes below 100 Torr, almost no MWCNT grow. This contrasts to that the highest quantity of fullerene is obtained when the pressure becomes 100 Torr or less. 

Another important parameter is the electric current for discharge. If the current density is too high, the quantity of the hard shell increases and that of the MWCNT decreases. To keep the arc discharge stable and the electrode cool are effective to increase in the product quantity of MWCNT. A considerable quantity of graphite is produced in the cathode deposit even under the most suitable condition to the synthesis of MWCNT. 

The bundle of MWCNT can be released in ultrasonic cleaner using ethanol as the solvent. The scanning tunnelling microscope (STM) image of thus released MWCNT is shown in Fig. 2. 

Laser-ablation method shown in Fig. 3 was used when Cgo was first discovered in 1985 [15]. This method has also been applied for the synthesis of CNT, but length of MWCNT is much shorter than that by arc-discharge method [17]. Therefore, this method does not seem adequate to the synthesis of MWCNT. However, in the synthesis of SWCNT described later (Sec. 3.1.2), marvelously high yield has been obtained by this method. Hence, laser-ablation method has become another important technology in this respect. 

For extension of the application of MWCNT, the key technology is obviously to develop the method for mass production by which high quality MWCNT can be produced with lower cost. It has been well known for a long time that carbon... 

MWCNT synthesized by catalytic decomposition of hydrocarbon does not contain nanoparticle nor amorphous carbon and hence this method is suitable for mass production. The shape of MWCNT thus produced, however, is not straight more often than that synthesized by arc-discharge method. This differenee could be aseribed to the strueture without pentagons nor heptagons in graphene sheet of the MWCNT synthesized by the catalytic decomposition of hydrocarbon, which would affect its electric conductivity and electron emission. 

Fig. 5. Scanning electron microscope (SEM) images of aligned MWCNT of uniform length (40 pm) and diameters (30-50 nm). Scales bars are 10 pm (top) and 1 pm (bottom) (Courtesy of Drs. M. Terrones and D. R. M. Walton).

Fig. 6. Transmission electron microscope (TEM) image of MWCNT with the open end. The cap of the tube was removed using the purification process,...

As mentioned above, employment of MWCNT for field emitter will be one of the most important applications of MWCNT. For this purpose, MWCNT is prepared by the chemical purification process [30,38], in which graphite debris and nanoparticles are removed by oxidation with the aid of CuCl2 intercalation [38]. Purified MWCNT is obtained in the form of black and thin "mat" (a flake with thickness of ca. a few hundreds of [im). Figure 7 shows a typical transmission electron microscope (TEM) picture of MWCNT with an open end, which reveals that a cap is etched off and the central cavity is exposed. 

Preparation research of SWCNT was also put forth by lijima and his co-worker [3]. The structure of SWCNT consists of an enrolled graphene to form a tube without seam. The length and diameter depend on the kinds of the metal catalyst used in the synthesis. The maximum length is several jim and the diameter varies from 1 to 3 nm. The thinnest diameter is about the same as that of Cgo (i.e., ca. 0.7 nm). The structure and characteristics of SWCNT are apparently different from those of MWCNT and rather near to fullerenes. Hence novel physical properties of SWCNT as the one-dimensional material between molecule and bulk are expected. On the other hand, the physical property of MWCNT is almost similar to that of graphite used as bulk [6c]. 

SWCNT is synthesized by almost the same method as that- for the synthesis of MWCNT. Remarkable difference lies in the fact that metallic catalyst is indispensable to the synthesis of fullerenes. The metal compounds used as the catalyst are listed in Table 2 [8]. 

MWCNT was first discovered by arc-discharge method of pure carbon and successive discovery of SWCNT was also based on the same method in which carbon is co-evaporated with metallic element. Optimisation of such metallic catalyst has recently been performed. Although these electric arc methods can produce gram quantity of MWCNT and SWCNT, the raw product requires rather tedious purification process. 

Among the several known types of carbon fibres the discussion in this chapter is limited to the electric arc grown multi-walled carbon nanotubes (MWCNTs) as well as single-walled ones (SWCNTs). For MWCNT we restrict the discussion to the idealised coaxial cylinder model. For other models and other shapes we refer to the literature [1-6],... 

A diffraction pattern of a single MWCNT (Fig. 1) contains in general two types of reflexions (i) a row of sharp oo.l (/ = even) reflexions perpendicular to the direction of the tube axis, (ii) graphite-like reflexions of the type ho.o (and hh.o) which are situated in most cases on somewhat deformed hexagons inscribed in circles with radii gho.o (or hh.o)-... 

Fig. 1. Typical ED pattern of polychiral MWCNT. The pattern is the superposition of the diffraction patterns produced by several isochiral clusters of tubes with different chiral angles. Note the row of sharp oo.l reflexions and the streaked appearance of 10.0 and 11.0 type reflexions. The direction of beam incidence is approximately normal to the tube axis. The pattern exhibits 2mm planar symmetry [9].

Simulated SWCNT ED patterns will be presented below. Tbe most striking difference with tbe MWCNT ED patterns is tbe absence of tbe row of sharp oo.l reflexions. In tbe diffraction pattern of ropes there is still a row of sharp reflexions perpendicular to the rope axis but which now corresponds to the much larger interplanar distance caused by the lattice of the tubes in the rope. The ho.o type reflexions are moreover not only asymmetrically streaked but also considerably broadened as a consequence of the presence of tubes with different Hamada indices (Fig. 3). 

An image of an MWCNT obtained by using all available reflexions usually exhibits only prominently the oo.l lattice fringes (Fig. 4) with a 0.34 nm spacing, representing the "walls" where they are parallel to the electron beam. The two walls almost invariably exhibit the same number of fringes which is consistent with the coaxial cylinder model. 

Fig. 4. Singularities in MWCNT imaged by means of basal plane lattice fringes, (a) Straight ideal MWCNT. (b) Capped MWCNT. The tube closes progressively by clusters of 2-5 graphene layers. (c)(d) Bamboo-like compartments in straight tubes.

SWCNTs are imaged as two parallel lines with a separation equal to the tube diameter (Fig. 5). By image simulation it can be shown that under usual observation conditions the black lines correspond to graphene sheets seen edge on in MWCNT as well as in SWCNT tubes [7]. 

Fig. 6. High resolution image of straight part of an MWCNT the 0.21 nm spacing is resolved next to the basal 0.34 mm spacing. The 0.21 nm fringes are curved [9].

The diffraction space of MWCNTs can be computed by summing the complex amplitudes due to each of the constituent coaxial tubes. Taking into account possible differences in Hamada indices (Lj, Mj) as well as the relative stacking (described by zQ.j, can formally write... 

The diffraction patterns due to different isochiral clusters are superimposed and well separated in a polychiral MWCNT diffraction pattern, suggesting that interference between waves scattered by tubes with different chiral angles can be neglected. It is therefore meaningful to discuss only isochiral clusters of tubes. Such clusters are only compatible with a constant intercylinder spacing c/2 for pairs of Hamada indices satisfying the condition = L +M +LM - (nc/a). Approximate solutions are for instance (8, 1) and (5, 5) [16,17]. 

Fig. 13. Simulated diffraction space of a 10-layer monochiral MWCNT with Hamada indices (40+8/ , 5+k) with / =0,...,9. In (a), (a ) and (02) the initial stacking at ( q was ABAB. whereas in (b), (b[) and (b2) the initial stacking was random, (a) The normal incidence pattern has a centre of symmetry only. (3 )(a2) The cusps are of two different types. The arc length separating the cusps is c (b) The normal incidence pattern now exhibits 2mm symmetry. (b )(b2) The cusps are distributed at random along the generating circles of the evolutes. These sections represent the diffuse coronae referred to in the "disordered stacking model" [17].

The length and the diameter of MWCNT can be measured directly by TEM. From high-resolution transmission electron microscopy (HRTEM) images exhibiting oo.l fringes follows the number of coaxial tubes and possibly the microstructure of the caps in MWCNT, as viewed along the incident electron beam [24], Also anomalous intercylinder spacings and defects are revealed in this way [1,11]. 

The diffraction patterns of isochiral clusters of tubes with different chiral angles in MWCNTs are superimposed in the composite pattern, the different chiral angles can be measured separately by diffraction contrast imaging [26]. 

On the other hand, TED patterns can assign the fine structure. In general, the pattern includes two kinds of information. One is a series of strong reflexion spots with the indexes of (00/), 002, 004 and 006, and 101 from the side portions of MWCNTs as shown in Eig. 1(b). The indexes follow those of graphite. The TED pattern also includes the information from the top and bottom sheets in tube. The helieity would appear as a pair of arcs of 110 reflexions. In the case of nano-probed TED, several analyses in fine structures have been done for SWCNT to prove the dependence on the locations [11,12]. 

Fig, 6. EEL spectra of bundle of four SWCNTs, MWCNT and graphite in the energy ranges (a) from 0 to 45 eV (plasmon region) and (b) from 280 to 300 eV (carbon K-edge) (modified from ref. 14). 

Figure 9 shows angular distribution of EELS of an MWCNT with a diameter of 100 nm [16j. The core-loss spectra obtained from the 000 and 002 reflexions much resemble those of an MWCNT and graphite (Figs. 6(b) and 7(c)). The n excitation peak is smaller than that of a excitation peak. In contrast, the... 

Right Fig. 9. EEL spectra of an MWCNT obtained from the locations at 000, intermediate and 002 reflexions in the reciprocal space (modified from ref. 16). 

Although CNTs showed similar EELS pattern in plasmon-loss and core-loss regions to graphite, SWCNT and fine MWCNT with a diameter less than 5 nm had different features. Furthermore, it has been found out that the angular-dependent EELS along the direction normal to the longitudinal axis of CNT shows stronger contribution from Jt electrons than [Pg.38]

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