Industrial Carbon and Graphite Materials. Группа авторов

of carbon are diamond, graphite, and amorphous carbon (Figure 2.2). These three allotropes are of industrial relevance. The most important by volume is amorphous carbon. Fullerenes represent a new allotropic form whose industrial breakthrough is still pending despite their extraordinary physical properties. The structural forms of the different allotropes are shown in Figure 2.3 [5, 6] and Figure 2.4 [6].

Table 2.1 Physical properties of carbon and its neighbor elements. G=Graphite, D=Diamond.

Property	Boron	Carbon	Nitrogen
Atomic number	5	6	7
Classification	Semimetal	Nonmetal	Nonmetal
State	Solid	Solid	Gas
Density, g/cm^3	2.46	G = 2.26, D = 3.53	0.00125
Mohs hardness	9.3	G = 0.5, D = 10	—
Melting point, K	2349	3773 (sublimation)	63.05
Boiling point, K	4203	5115	77.15
Heat of evaporation, kJ/mol	508	715 (sublimation)	5.58
Specific heat, J/kg K	1260	G = 709, D = 427	1040
Electrical conductivity, S/m	10^−6	G = 2–3·10^5, ∥ 3.3 10^2, ⊥ D = ∼ 10^−13	0
Thermal conductivity, W/mK	27.4	G = 119–165 D = 900–2300	25.8·10^−3

Figure 2.1 Relative abundance of elements in the Earth’s upper crust. Source: Haxel et al. 2002 [2]. Reproduced with permission of United States Geological Survey.

      The element carbon achieves in the form of graphite the highest sublimation point among all elements. In the allotropic form of diamond, carbon has the highest hardness. These outstanding properties are due to the formation of covalent bonds between the carbon atoms. In the case of diamond, four covalent bonds (sp³ hybridization) form a face‐centered cubic lattice with an interatomic distance of 0.154 nm. In the case of graphite, three covalent bonds form a hexagonal planar network with a bond length of 0.142 nm in the plane and an interplane distance of 0.335 nm. The fourth bonding p‐orbital overlaps with other adjacent p‐orbitals and creates the energy band for dislocated electrons (sp² hybridization). The weak bonding between the layers is described as being of metallic character on the order of magnitude of van der Waals forces [7]. This results in a high electrical conductivity comparable with those of metals.

Figure 2.2 Allotropic modifications of the element carbon.

      Carbon is the sine qua non condition for life on Earth. The capability to form complex three‐dimensional molecules by single or double bonds between carbon atoms and the incorporation of heteroatoms such as nitrogen, oxygen, phosphorus, sulfur, and last but not least hydrogen open a tremendous biological diversity. Some examples of these complex molecules are proteins, vitamins, and the genetic makeup in DNA, RNA, and adenosine triphosphate (ATP), the most important molecule for energy transfer in living organisms. Others are carbohydrates like starch or sugar. The research on this group of molecules has once initiated the separation into organic and inorganic chemistry. Meanwhile also synthetic macromolecules, such as polymers, are considered to be organic molecules, formed by covalent bonds between carbon atoms and by incorporating heteroatoms. Fossils like crude oil and coals are ranked as organic substances metamorphosed from once‐living matter. By far the majority of carbon compounds are classified as organic molecules. Only a few ones are located under the group of carbon‐containing inorganic compounds. Examples are carbon monoxide (CO) and carbon dioxide (CO₂), cyanides (CN⁻), all carbides, and carbolic acid. The largest sources of inorganic carbon are limestone and dolomite.

The boundary between organic and inorganic substances is not of clear evidence. Diamond, graphite, and fullerenes are widely considered as inorganic substances. Inagaki and Feiyu [8] provide an excellent schematic about the molecular path from organic sp¹‐, sp²‐, and sp³‐bonded molecules to the inorganic allotropes diamond, graphite, fullerenes, and carbines (Figure 2.5). The aggregation of carbon atoms to huge crystalline “supermolecules” in diamond and graphite goes along with the reduction of the hydrogen content to traces that are positioned at the fringes of the “supermolecules” for saturation purpose. Hydrogen has lost here its chemical dominance compared with hydrocarbons. This is as well the case in fullerenes and carbynes. Hence it is justified to classify these substances as inorganic.

Figure 2.3 The four most important allotropic forms of the solid element carbon and their main derivatives [5, 6]. Source: Adapted from Marsh and Rodriguez‐Reinoso 2000 [5] and Borkos et al. 1973 [6].

Figure 2.4 Bonding hybridization and corresponding crystal structure of carbon allotropes. Source: Borkos et al. 1973 [6]. Reproduced with permission of Taylor & Francis.

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