Graphite layer

Because they contain many islets of condensed aromatics, the carbon-rich asphaltenes can begin to acquire the spatial organization of graphite layers. 

In general, encapsulated metal particles were observed on all graphite-supported catalysts. According to Ref. [4] it can be the result of a rather weak metal-graphite interaction. We mention the existence of two types of encapsulated metal particles those enclosed in filaments (Fig. 1) and those encapsulated by graphite. It is interesting to note that graphite layers were parallel to the surface of the encapsulated particles. 

As in the case of graphite-supported catalysts, some metal particles were also encapsulated by the deposited carbon (Fig. 4). However, the amount of encapsulated metal was much less. Differences in the nature of encapsulation were observed. Almost all encapsulated metal particles on silica-supported catalysts were found inside the tubules (Fig. 4(a)). The probable mechanism of this encapsulation was precisely described elsewhere[21 ]. We supposed that they were catalytic particles that became inactive after introduction into the tubules during the growth process. On the other hand, the formation of graphite layers around the metal in the case of graphite-supported catalysts can be explained on the basis of... 

The nanotube connections whose diameter differences are different from the 3.8% value characteristic to the (9 ,0)-(5n,5 i) series, will tend to that value with increasing the graphite layer order n. 

The inner (outer) diameter of the observed curved or coiled nanotubules produced by the catalytic method[8] varies from 20 to 100 A (150 to 200 A), which corresponds to the graphite layer order 3< <15 (Table 1). 

As the N knee can have — 1 inner concentric knees, all of them separated by approximately the graphite interplanar distance, is called the graphite layer order . In fact, the number of atoms of the torus is given by 10(24 -l-33 —5n) because 10 atoms are common for adjacent knees at the (5 ,5 )-(5m,5h) connection (see below. Section 2.5). 

As the diameter of the catalyst particle is supposed to be close to that of the single-shell tubule[20], or to that of the inner tubule [8], the number of graphitic layers might depend on the flow rate of acetylene at the catalyst particle. The graphitic layers are supposed to be formed by the Cj units formed on the catalyst particle, exceeding those needed for the growth of the multi-shell tubule inner layer. This generalisation to multi-layer tubules is just a hypothesis, since we do not have any experimental proof yet. 

Wrapped nanocrystals. Metal crystallites covered with well-developed graphitic layers are found in soot-like material deposited on the outer surface of a cathode slag. Figure 6 shows a TEM picture of an a(bcc)-Fe particle grown in the cathode soot. Generally, iron crystallites in the tv-Fe phase are faceted. The outer shell is uniform in thickness, and it usually con-... 

Iron, cobalt, and nickel particles also grow in soot deposited on the chamber walls, but graphitic layers wrapping the metals are not so well-developed as those grown in the cathode soot. Figure 7 shows a TEM picture of iron particles grown in the chamber soot. They... 

Fig. 7. TEM picture of iron nanocrystals collected from the chamber soot nanocrystals are embedded in amorphous carbon globules. On the surface of some core crystals, a few fringes with 0.34-0.35 nm spacing suggesting the presence of graphitic layers are observed, as indicated by arrows.

Bamboo-shaped tubes. A carbon tube with a peculiar shape looking like bamboo, produced by the arc evaporation of nickel-loaded graphite, is shown in Fig. 8. The tube consists of a linear chain of hollow compartments that are spaced at nearly equal separation from 50 to 100 nm. The outer diameter of the bamboo tubes is about 40 nm, and the length typically several /im. One end of the tube is capped with a needle-shaped nickel particle which is in the normal fee phase, and the other end is empty. Walls of each compartment are made up by about 20 graphitic layers[34]. The shape of each compartment is quite similar to the needle-shape of the Ni particle at the tip, suggesting that the Ni particle was once at the cavities. 

The chains of hollow carbon may be initially chains consisting of Ni (or carbide) particles covered with graphitic carbon. The chains lying on the hot surface of the cathode are heated, and Ni atoms evaporate through defects of the outer graphitic carbon because the vapor pressure of Ni is much higher than carbon. Thus, the carbon left forms hollow graphitic layers. 

Fig. I. High-resolution electron micrographs of graphitic particles (a) as obtained from the electric arc-deposit, they display a well-defined faceted structure and a large inner hollow space, (b) the same particles after being subjected to intense electron irradiation (note the remarkable spherical shape and the disappearance of the central empty space) dark lines represent graphitic layers.

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