Polyhedral graphite particles

A cylindrical hard deposit, which is grown on the tip of a cathode (A in Fig. 10.2.2), consists of two regions an inner fibrous black core and an outer gray hard shell (15). The inner core had a columnar structure that was made up of bundles of nanotubes and flocks of polyhedral graphitic particles (32). On the other hand, the hard shell was made of stacked graphitic flakes. 

Thus far, four mechanisms for the formation of concentric shell carbon particles as zero-dimensional carbon allotropes have been proposed. The first mechanism is the formation of a corannulene carbon framework followed by a spiral-shell growth [48], The second mechanism is that the regular concentric arrangements of carbon layers in the onion-like caibon sphere occur tlirough the solidification process of a carbon droplet under ultrafast condensation (49J. The third mechanism is due to a solid—>quasi-liquid—>solid process tliat is, reorganization of soot-containing tubular and polyhedral graphitic particles by... 

Fig. 3. Schematic illustration of the growth process of a graphitic particle (a)-(d) polyhedral particle formed on the electric arc (d)-(h) transformation of a polyhedral particle into a quasi-spherical onion-like particle under the effect of high-energy electron irradiation in (f) the particle collapses and eliminates the inner empty space[25j. In both schemes, the formation of graphite layers begins at the surface and progresses towards the center.

Fig. 4. Onion-like graphitic particles formed by three concentric layers (C o, C240, Cs4o) polyhedral (marked P) and spherical (marked S) structures. For clarity, only a half pan of each shell is shown.

Electron diffraction studies [3] have revealed that hexagons within the sheets are helically wrapped along the axis of the nanotubes. The interlayer spacing between sheets is 0.34 nm which is slightly larger than that of graphite (0.3354 nm). It was dso reported [2] that the helicity aspect may vary from one nanotube to another. Ijima et al. [2] also reported that in addition to nanotubes, polyhedral particles consisting of concentric carbon sheets were also observed. 

The authors (iSl) and Kmetko (ISS) have confirmed definitely, from electron micrographs, that graphitization of carbon blacks of low surface area produces polyhedral particles. 

Usually it is difficult to separate the effect of ciystallite size on carbon reactivity from the effects of crystallite orientation and impurity content. However, Armington (62) attempted to do so by reacting a series of graphi-tized carbon blacks with oxygen and carbon dioxide, as discus.sed earlier in this article. Assuming that upon graphitization all the carbon blacks are converted to polyhedral particles with the surface composed almost completely of basal plane structure, it is possible to eliminate crystallite orientation as a variable. Spectroscopically, the total impurity content of all the graphitized carbon blacks is quite low and to a first approximation, the analyses of the individual constituents are similar. 

The carbon layers of carbon black rearrange to a graphitic order, beginning at the particle surface at temperatures above 1200 °C. At 3000 °C, graphite crystallites are formed and the carbon black particles assume polyhedral shape. 

SEM studies on the cathode deposit reveal multiple features. Curved and compact graphite layers are deposited on the outer shell. Inner core deposits contain bundle-like structures, which contain randomly distributed nanotubes and polyhedral particles (Fig. 9). It is possible that, as a result of carbon vapour... 

Another recent report describes the large scale synthesis of ahgned carbon nanotubes, of uniform length and diameter, by passage of acetylene over iron nanoparticles embedded in mesoporous silica [107]. The latter two methods, based on the pyrolysis of organic precursors over templated/catalysts supports, are by far superior by comparison with plasma arcs, since other graphitic structures such as polyhedral particles, encapsulated particles and amorphous carbon are notably absent (Fig. 16). 

New generadon nano scale fillers are challenging the domination of traditional fillers such a as carbon blacks and silica in the rubbery industry. Nanoscaled fillers such as layered sihcates, carbon nanotubes, carbon nanofibers (CNFs), exfoliated graphite, spherical particles and Polyhedral oligomeric silsesquioxane (POSS), etc., dispersed as a reinforcing phase in a mbber matrix are emerging as a relatively new form of usefiil material. 

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