Partially crystalline carbon

Key points: Amorphous and partially crystalline carbon in the form of small particles are used on a large scale as adsorbents and as strengthen ing agents for rubber; carbon fibres impart strength to polymeric materials There are many forms of carbon that have a low degree of crys tallinity. These partially crystalline materials have considerable commercial importance; they include carbon black, activated carbon, and carbon fibres. Because single crystals suitable for complete X-ray analyses of these materials are not available, their structures are uncertain. However, what information there is suggests that their structures are similar to that of graphite, but the degree of crystallinity and shapes of the particles differ. ‘Carbon black’ is a very finely divided form of carbon. It is prepared (on a scale that exceeds 8 Mt annually) by the combustion of hydrocarbons under oxygen-deficient conditions. Planar stacks, like those of graphite, and multilayer balls, reminiscent of the fullerenes, have both been proposed for its structure (Fig. 14.7). Carbon black is used on a huge scale as a pigment, in printer’s ink (as on this page), and as a filler for rubber goods, including tyres, where it greatly improves the strength and wear resistance of the rubber and helps to protect it from degradation by sunlight.

Figure 14.7 A proposed structure for a soot particle resulting from imperfect closure of a curved C-atom network. Graphite-like structures have also been proposed.‘Activated carbon’ is prepared from the controlled pyrolysis of organic material, including coconut shells. It has a high surface area (in some cases exceeding 1000 m^-2 g^-1), that arises from the small particle size. It is therefore a very efficient ad sorbent for molecules, including organic pollutants from drinking water, noxious gases from the air, and impurities from reaction mixtures. There is evidence that the parts of the surface defined by the edges of the hexagonal sheets are covered with oxide-tion products, including carboxyl and hydroxyl groups (8).

This structure may account for some of its surface activity. Carbon fibres are made by the controlled pyrolysis of asphalt fibres or synthetic fibres and are incorporated into a variety of high strength plastic products, such as tennis rackets and aircraft com ponents. Their structure bears a resemblance to that of graphite, but in place of the extended sheets the layers consist of ribbons parallel to the axis of the fibre. The strong in-plane bonds (which re semble those in graphite) give the fibre its very high tensile strength.

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مواضيع ذات صلة

Fullerene metal complexes

Carbon clusters

Diamond and graphite

Simple compounds

Occurrence and extraction

Thermodynamic considerations

The atom and its ions

Oxidation states

Occurrence

Enthalpies of atomization

Electronegativity

Ionization energies and electron affinities