These recommendations, “Nomenclature for the C60‐Ih and C70‐D5h(6) Fullerenes” (IUPAC recommendations 2002) (2002). Pure Appl. Chem. 74 (4): 629–695.4 4 Anderson, P.W. (1958). Phys. Rev. 109: 1492.
5 5 Baker, R.T.K. and Harris, P.S. (1978). Chemistry and Physics of Carbon, vol. 14 (eds. P.L. Walker Jr. and P.A. Thrower), 83–165. New York, NY: Marcel Dekker.
6 6 IUPAC Manual of Symbols and Terminology, Appendix 2, Pt. 1, Colloid and Surface Chemistry (1972). Pure Appl. Chem. 31: 518.
Note
1 † Deceased.
5 Graphite*
Otto Vohler1,†, Ferdinand von Sturm1, Erhard Wege1,†, and Wilhelm Frohs2
1 Sigri GmbH, Werner‐von‐Siemens Straße 18, 86405 Meitingen, Germany
2 SGL Carbon GmbH, Werner‐von‐Siemensstr. 18, 86405 Meitingen, Germany
5.1 Graphite Single Crystal
The ideal crystal lattice of hexagonal graphite has an ABAB… stacking sequence of the single sheets, called graphene (Figure 5.1) The flat molecule layers are the consequence of the sp2 hybridization where each carbon atom has three nearest neighbors within the same layer plane. The distance between in‐plane carbon atoms is 0.1421 nm in graphite (sp2 hybridization), compared with 0.1544 nm in diamond (sp3 hybridization) [2]. Due to the extended delocalization of the π electrons between the layers, the bonding length in plane is slightly higher compared with in benzene with 0.139 n. The bonding length in a localized —C=C— is 0.132 nm. The in‐plane bonding energy of graphite is −430 kJ/mol, thus 80 kJ/mol higher than that of diamond, and one of the highest bonding energies known.
The bonding angle between the carbon atoms is 120°. The overlapping p‐orbitals perpendicular to the C atom planes are filled with delocalized electrons forming the weak π‐bonds with a distance of 0.354 nm between the planes. The bonding energy between the basal planes (graphene layers) is in the range of van der Waals forces only.
This layer structure results in an extreme anisotropy of the physical properties of graphite (Table 5.1) [3–7].
Remarkable is the negative coefficient of thermal expansion (CTE) within the plane. Thus, below 670 K graphite contracts in a direction whereas it expands in c‐direction. Parallel to the basic planes graphite is a metallic conductor, whose electrical resistivity increases with temperature. Perpendicular to the planes graphite behaves like a semiconductor. The thermal conductivity decreases with temperature. At room temperature the in‐plane thermal conductivity is higher than that of copper perpendicular to the planes since graphite is a thermal insulator. The high Young’s modulus of 1060 GPa within the plane should allow a theoretical strength of 100 000 MPa. This unique potential in mechanical properties is partially exploited in carbon fibers, which are used to reinforce different matrices (See Carbon Fibers Chapter 11).
Figure 5.1 (a) Lattice of the cubic diamond and the hexagonal graphite crystal. (b) sp3 hybridization in the diamond lattice. (c) sp2 hybridization in the graphite lattice.
Due to the weak forces between the planes, these can easily be shifted against each other. This gives graphite its lubrication properties that are widely used in industrial and private applications (locks). Two modifications of graphite are known. The energetically preferred stacking sequence is ABAB… (hexagonal modification). Formation of the rhombohedral modification ABCABC… can be achieved to a certain degree by strong mechanical impact and shearing during milling. In some cases natural graphite can contain 30% of the rhombohedral modification. The rhombohedral modification can be re‐transferred into the hexagonal modification by annealing. This change in modification has not yet found any industrial application.
Neither natural graphite nor synthetic graphitic carbons are perfect in structure. Lattice defects in planes and defect in the stacking sequence and bending of planes lead to deviations from the ideal crystal lattice. The most frequently applied structure analysis for carbon materials is X‐ray diffraction. Only materials that show in their X‐ray spectra the modulation of the three‐dimensional (112) interference should be named graphitic carbon. It has become common to use the easily measurable (002) interference to calculate with the Scherrer equation the mean interlayer spacing c/2 instead of measuring the weak three‐dimensional (112)‐interference (Figure 5.2) [1]. With the limits for ideal graphite c/2 = 0.3347 (100%) and for non‐graphitic carbon c/2 = 0.344 nm, the degree of graphitization can be calculated [8, 9]. Materials within these limits should be termed graphitic carbons independent of the degree of graphitization. All other carbons are non‐graphitic carbons. In non‐graphitic carbons the stacking of the graphene layers is totally random (turbostratic) [10].
Table 5.1 Properties of diamond and graphite (single crystals) and synthetic graphite at various temperatures.
| Graphite | Synthetic graphite | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Diamond | c Direction room temperature | a Direction room temperature 1000 K | With grain | Across grain | With grain 1500 K | Across grain | With grain | Across grain 2000 K | With grain | Across grain | |
| Density (g/cm3) | 3.515 | 2.266 | 1.80 | 1.80 | 1.79 | 1.79 | |||||
| Coefficient of thermal expansion (15–150 °C), 10−6 K−1 | 0.8 | 28.3 | −1.5 | 0.1 | 1.5 | 1 |
|
