Diamond from carbon ‘onions

Amorphous carbon is a general term that covers non-crystalline forms of carbon such as coal, coke, charcoal, carbon black (soot), activated carbon, vitreous carbon, glassy carbon, carbon fiber, carbon nanotubes, and carbon onions, which are important materials and widely used in industry. The arrangements of the carbon atoms in amorphous carbon are different from those in diamond, graphite, and fullerenes, but the bond types of carbon atoms are the same as in these three crystalline allotropes. Most forms of amorphous carbon consist of graphite scraps in irregularly packing. 

The generation of carbon onions in space has not yet been fuUy elucidated. However, it seems reasonable to assume that they originate from nanoscopic diamond particles. These may be converted into carbon onions upon heating, electron bombardment, or intensive irradiation (Section 4.3.5.4). The existence of nanodiamonds in extraterrestrial material could be confirmed by analyses on different meteorites. Especially the AUende meteorite contains significant amounts of tiny diamond particles (Section 5.1.2). 

From the detonation of carbon-rich explosives, diamonds with primary particles measuring about 5 nm can be obtained. Electron microscopic examination also reveals graphitic portions of the material that partly exist as onion-hke structures or as multilayered graphitic shells on the diamond particles (Section 5.2.2). Hence, it seems reasonable to assume that a complete graphitization of detonation diamond leads to onion-like particles. 

Altogether the thermal transformation of nanodiamond turned out a suitable method to prepare macroscopic amounts of onion-like carbon. It is true that the products obtained are inhomogeneous to some extent and that the resulting onions show various deficiencies (defects, deviations from spherical shape), but still the heating of diamond in vacuo constitutes the best method to date to generate larger amounts of carbon onions and study in principle their physical and chemical properties. 

There is a major drawback, however, to the preparation of carbon onions by electron bombardment the amounts obtained are extremely small and thus render the examination of bulk properties virtually impossible. Only high-energy electron sources outside an HRTEM would enable the production of macroscopic amounts. S till a further development of this method is of considerable interest as the carbon onions made from diamond are very uniform in quality. 

A thermal treatment of different carbon forms can lead to the formation of onionlike species as well. As for diamond particles, their surface structure plays an important role for the actual outcome of the process. If it is covered with functional groups, the bonding sites are saturated which renders a graphitization more difficult From danghng bonds, on the other hand, graphitized domains will arise that can serve as nucleation center to the formation of carbon onions. In this process, a suitable orientation of lattice planes as well as a small particle size that... 

Owing to their curved and defective structure, carbon onions are quite easily converted into other forms of carbon. The transformation of spherical particles into faceted nanoparticles by heating to at least 1900 °C has already been described in Section 4.3.5.3 on the thermal produchon of nano-onions from diamond particles. 

Apart from the irradiation with high-energy electrons, the conversion of carbon onions into diamond also succeeds by bombardment with ions like Ne. The latter are 36000 times heavier than the rather light-weight electrons. Consequently, they require far less velocity and thus smaller accelerator voltages to bear the same effect Diamond-like structures can further be generated by thermal treatment in air at 500 °C or by irradiation with a C02-laser. 

One other method to generate nanoscale diamond particles has already been presented in Section 4.5.2 In the heart of very large carbon onions, a strong selfcompression reduces the interlayer distance from 0.34nm to less than 0.25 nm... 

Diamond particles have been found in combustion products [9], synthesized from fullerenes by pressurization at room temperature [10] or sputtering at high temperatures [11], obtained by decomposition of polymers with an appropriate structure [12], by heating carbon onions with electron team in a TEM [13], by chlorination of SiC [14], or by deposition from supercritical fluids [15-17]. 

In contrast to the spherical carbon onions observed in the first experiments by Ugarte, OLC particles were subsequently produced with polyhedral facets, more closely matching the polyhedral structures predicted from the consideration of nested fullerene structures described above. These polyhedral onion-like particles were synthesized by vacuum heat treatment of carbon sooF and diamond nanoparticles." Figure 10.5 presents HRTEM images of the polyhedral OLC particles produced in the experiments of Kuznetsov et al. The range of synthesis methods available has led to the production of different types of OLC. In addition to their shape, such carbon onions can be characterized by other parameters, such as the number of concentric shells, the spacing between adjacent shells, the size of the innermost shell, and the presence of different types of defects. 

The transformation of carbon nanoparticles to carbon onions can be stimulated by exposure to an intense beam of electrons. Indeed, Ugarte s experiment " demonstrated for the first time that closed, curved carbon nanostructures can be produced not only by the condensation of carbon vapor but also by the transformation of condensed carbon nanoparticles. The other very important aspect of this work was the realization that electron irradiation not only has a destructive effect on carbon materials but can actually be used to synthesize new carbon nanostructures. This discovery opened a completely new field of carbon materials research related to stability and transformation of carbon nanostructures. It was found that not only carbon soot can be converted into carbon onions but also diamond crystals. Qin and lijima demonstrated that carbon onions can form on the surfaces of 1-3 pm diamond crystals under intense electron beam irradiation (150 A/cm ). Growth of carbon onions (which consisted of between 4 and 10 closed graphitic shells) proceeded from the inner shell because of the high mobility of carbon atoms induced by electron irradiation. However, further irradiation resulted in the destruction of the carbon onions. Transformation of nanometer-sized diamond crystals (3-10 nm in diameter) to carbon onions under electron irradiation (20 0 A/cm ) was later observed by Roddatis et al. ° In this case, complete transformation of nanodiamond crystals occurred, starting from their surfaces and proceeding inward. 

FIGURE 10.6 Spherical carbon onion with a monocrystalline diamond core 10 nm in diameter. The core shows the lattice fringes of diamond with a separation of 2.06 A. (Adapted from Banhart, R, Ajayan, P.M.,... 

During the preparation of this chapter, such a new finding that carbon onion synthesized from diamond nanoparticle at 1600 °C shows a low friction coefficient of 0.005 in ultrahigh vacuum. Therefore, further application of nanolubricants to tribology is anticipated. 

Figure 3.2 Scheme of the transformation of nanodiamond into a carbon onion. The (111) planes of diamond are transformed into (001) graphite planes. Reproduced by permission of Elsevier from Kuznetsov et al. [12], (1994) Elsevier... 

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