Carbon allotrope: soot

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. 

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... 

Carbon exists in a number of allotropic forms. Allotropes are forms of an element with different physical and chemical properties. Two allotropes of carbon have crystalline structures diamond and graphite. In a crystalline material, atoms are arranged in a neat orderly pattern. Graphite is found in pencil lead and ball-bearing lubricants. Among the noncrystalline allotropes of carbon are coal, lampblack, charcoal, carbon black, and coke. Carbon black is similar to soot. Coke is nearly pure carbon formed when coal is heated in the absence of air. Carbon allotropes that lack crystalline structure are amorphous, or without crystalline shape. 

Another class of carbon allotropes was discovered in 1985 by Harold W. Kroto, James R. Heath, Sean O Brien, Robert Curl, and Richard Smalley. Soccer-ball-shaped spheres of 60 carbon atoms with formulas like Cgg and C were found in carbon soot and later recognized to be ubiquitous in interstellar clouds. Cg is recognized as the most perfectly spherical known molecule. Because the arrangements of the carbon atoms resemble the architecture of geodesic domes, which were invented by Richard Buckminster Fuller, this class of carbon allotropes came to be cdXXed fullerenes. Kroto, Curl, and Smalley shared the 1996 Nobel Prize in chemistry for this discovery. 

Many methods have been reported for production of nanodiamonds (NDs) such as laser ablation, " plasma-assisted chemical vapor deposition," autoclave synthesis from supercritical fluids, ion irradiation of graphite, chlorination of carbides, electron irradiation of carbon onions, and ultrasound cavitation. Smaller NDs can be prepared by detonation processes that yield aggregates of NDs with sizes of 4-5 nm embedded in a detonation soot composed of other carbon allotropes and impurities. An explosive mixture having an overall negative oxygen balance provides a source of both carbon and energy for the conversion. Because of their small size (2-10 nm) detonation NDs have also been referred to as ultradispersed, nanocrystalline... 

Even veteran elements can still present surprises. Many elements exist as different allotropes. This means that the atoms are arranged differently. In the case of carbon, the amorphous (soot), the dull gray graphite, and the brilliant diamond forms were known. It was therefore a great surprise when a new form was discovered in 1982 the fullerenes opened up a completely new area of chemistry. Hence it is not too far-fetching to deduce that further secrets lie buried in the elements, not to mention their compounds. 

The thermodynamically stable allotrope of carbon (Soot contains small crystals of graphite.)... 

This threadlike allotrope of carbon is organized into long spirals of bonded carbon atoms. Each spiral contains 300 to 500 carbon atoms. It s produced by using a laser to zap a graphite rod in a glass container filled with argon gas. The allotrope splatters on the glass walls and is then removed. Because they conduct electricity, these carbon filaments may have uses in microelectronics. Some linear acetylenic carbons may eventually form ftillerenes, whereas others form soot. 

The discovery of buckyball generated tremendous interest within the scientific community. Here was a new allotrope of carbon with an intriguing geometry and unknown properties to investigate. Since 1985 chemists have created a whole class of fullerenes, with 70, 76, and even larger numbers of carbon atoms. Moreover, buckyball has been found to be a natural component of soot, and the fullerenes Ceo and C70 turned up in a rock sample from northwestern Russia. 

Not only the chemical constitution, but also allotropic modifications influence the X-ray intensity ratios. For carbon there are three allotropes diamond, graphite, and soot. Experiments on muonic Lyman series intensity ratios (relative to the 2-1 transition) in these substances have been performed by Schnewly et al. The results are shown in Fig. 17. It is clear that for the higher transition, the effect of allotropic modification is more pronounced. 

Carbon is an important element in the periodic table, which was believed to be discovered in prehistoric times, mostly present in the form of soot and charcoal in earliest human civilization. In early 2500 BC, diamond was found in China whereas charcoal was prepared during Roman times [33]. It was Antoine Lavoisier who listed carbon as an element in his 1789 textbook, and subsequently different allotropic forms of it were discovered. 

Soon after the synthesis of diamond Soviet scientists prepared a new substance, carbine, which, as has since been proved, is a new, third allotropic modification of carbon. The carbon atoms in it comprise long chains. This substance resembles soot. 

Kratschmer and Huffman called the soot Fullerite , but they were always aware that to have a new allotrope of carbon it was necessary to isolate the actual crystal form. By subliming the soot and extracting the sublimate with benzene, Fostiropoulos obtained orange-brown crystals, which could be in the form of hexagonal rods, platelets, or starshaped flakes. Lowell Lamb, working for Huffman at Tucson, confirmed the UV spectra. They published their results [76], but the definitive X-ray diffraction of a single crystal had still not been achieved, or alternatively, a single line NMR spectrum would have been sufficient to show that Ceo did, indeed, have a soccer ball structure. 

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