Structure and Bonding in Carbon Materials

Department of Materials Science Engineering University of Bath Bath, BA2 7AY United Kingdom 

the bonding between carbon atoms is briefly reviewed fuller accounts can be found in many standard chemistry textbooks, e.g., [1]. The carbon atom [ground state electronic configuration (ls )(2s 2px2py)] can form sp sp and sp hybrid bonds as a result of promotion and hybridisation. There are four equivalent 2sp hybrid orbitals that are tetrahedrally oriented about the carbon atom and can form four equivalent tetrahedral a bonds by overlap with orbitals of other atoms. An example is the molecule ethane, CjH, where a Csp -Csp (or C-C) a bond is formed between two C atoms by overlap of sp orbitals, and three Csp -Hls a bonds are formed on each C atom. Fig. 1, Al. 

A second type of hybridisation of the valence electrons in the carbon atom can occur to form three 2sp hybrid orbitals leaving one unhybridised 2p orbital. 

The sp orbitals are equivalent, coplanar and oriented at 120° to each other and form a bonds by overlap with orbitals of neighbouring atoms, as in the molecule ethene, CjH, Fig. 1, A2. The remaining p orbital on each C atom forms a n bond by overlap with the p orbital from the neighbouring C atom the bonds formed between two C atoms in this way are represented as Csp =Csp, or simply as C=C. 

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McEnaney, B. (1999). Structure and bonding in carbon materials. In Carbon Materials for Advanced Technologies (T.D. Burchell, ed.). Pergamon, Chapter 1, pp. 1-33. 

It is beyond the scope of this chapter to review structure and bonding in each class of engineering carbons listed in Table 5. Instead, a generic description of microstructure and bonding in these materials will be attempted. The evolution in understanding of the structure of engineering carbons and graphites has followed the initial application of X-ray diffraction and subsequent application... 

To reach the fantastic heights required by a space elevator, new materials are essential. Skyscrapers of today are usually composed of a steel framework, but steel is too heavy to use for the space elevator—the tower must be so high and needs so much material that it could not possibly support its own weight if it was made of steel. One possibility for a new material is related to the fullerenes discussed in the text. Carbon nanotubes are cylindrical fullerenes composed of sheets of graphites rolled into tiny cylinders with a diameter of roughly 0.00000004 inches (0.0000001 cm)—a nanometer. The structure and bonds of carbon nanotubes could potentially be used to create a material with about 50-100 times the strength of steel. A slender ribbon of carbon nanotube could be the key to bringing the dream of a space elevator into reality. 

Phosphorus The main role of phosphorus in carbon materials is as an oxidation protector and a fire retardant [136-144]. Its source can be in phosphoric acid, which is used in some technologies of carbon activation [143,144] or in the cross-linking precursor. The phosphorus present in the carbon matrix is stable between 773 and 1273 K. It can be fixed as red phosphorus and/or in chemically bonded forms, such as -C-P-bonds or -C-O-P-bonds [143-145]. During the carbonization process at low temperatures, phosphocarbonaceous species are created. Their content decreases by scission of the P-O-C bonds with an increase in the temperature, due to the growth of aromatic structures [143]. Possible phosphorus-containing functionalities are presented in Figure 2.4. 

Also nanotubes formed by AB semiconductors, and structurally similar to those we have discussed above for carbon, have been considered theoretically. In principle, the existence of the carbon nanotubes can be correlated with the existence of planar graphene sheets that subsequently are rolled up. For carbon, the strong interatomic bonds are formed by sp hybrids, whereas weaker bonds are formed by the p orbitals perpendicular to the sheet. If other materials also can form planar sheets, it may be suggested that nanotubes can exist, too. This is the case, e.g., for BN as well as for BC2N for which a and n orbitals from sp hybrids and p functions, respectively, are responsible for the bonding. For heavier elements the atoms tend to form sp hybrids, which ultimately result in three-dimensional structures and not in sheets. 

Computational methods are typically employed to rationalize experimental findings such as molecular structures and spectroscopic data (vide infra), which also serve as benchmarks for assessing the quality of the methods used. At the same time, it is clear that the tme strength of computational chemistry is the prediction of new, hitherto unknown structures and reactions. In an ideal situation, this would greatly reduce experimental efforts and consequently time and monetary investments however, this is not yet done routinely, especially when the predictions cannot readily be probed. The key problem is that although many computational methods are well tested on a limited set of structures, new and unusual bonding arrangements, as often found in carbon-rich materials (bent bonds. 

Diamond is an important commodity as a gemstone and as an industrial material and there are several excellent monographs on the science and technology of this material [3-5]. Diamond is most frequently found in a cubic form in which each carbon atom is linked to fom other carbon atoms by sp ct bonds in a strain-free tetrahedral array. Fig. 2A. The crystal stmcture is zinc blende type and the C-C bond length is 154 pm. Diamond also exists in an hexagonal form (Lonsdaleite) with a wurtzite crystal structure and a C-C bond length of 152 pm. The crystal density of both types of diamond is 3.52 g-cm. ... 

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