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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	2.4	1.5	2.9	1.80	3.20
Thermal conductivity [3] (W/cm/K)	20	0.04–0.06	10–15	350	180	150	75	110	55	85.00	45.00
Electrical resistivity (Ω cm)	1020	1	50 × 10−6	3.5 × 10−4	7.0 × 10−4	3.2 × 10−4	6.4 × 10−4	3.3 × 10−4	6.6 × 10−4	4.0 × 10−4	8.0 × 10−4
Magnetic susceptibility [4] (10−6 cm3 g)		−21	−0.3
Elastic modulus [3] (GPa)	1000	36	1060	20	6	22	6.5	24	7.5	26.0	8.0
Mohs hardness [5]	10	9	0.5

Figure 5.2 X‐ray diffraction pattern of non‐graphitic and graphitic carbon materials [1].

The above applied terms follow the IUPAC nomenclature that should be consequently applied [10].

Along with the disorder goes the width of the X‐ray diffraction lines. Whereas the mean crystallite size in c‐direction L_c (stacking height) can be calculated from the width of the (002) interference, the width of the (100) or (110) interference can be taken to calculate the mean crystallite size in a direction L_a [11–13]. It should be noted that graphitic domains are in fact bigger than they appear by X‐ray diffraction methods. Bending of the graphitic sheet structures diminishes the size of coherent scattering areas.

It is evident that the material properties of graphitic and non‐graphitic carbon materials strongly alter with the degree of structural disorder. This broad variety in the crystallographic structure opens manifold areas of application, which are multiplied by the morphological plurality of different forms of carbon.

Closest to the ideal graphite lattice are natural graphites and artificially produced highly oriented pyrolytic graphite (HOPG). The parallel arrangement of graphene layers can be visualized by high‐resolution transmission electron microscopy (HR‐TEM). Figure 5.3a shows an HR‐TEM bright‐field image of a graphitized coke derived from coal‐tar pitch [13]. The high symmetry in the electron diffraction pattern of a highly ordered pyrolytic graphite (HOPG) shows the extreme structural order in this synthetic graphite (Figure 5.3) [13].

Figure 5.3 (a) High‐resolution transmission electron microscopy (HR‐TEM) bright‐field image of graphitized coal tar pitch coke. (b) Electron diffraction pattern of a HOPG.

In 2003 scientists of the University of Augsburg were able to visualize for the first time the completely hexagonal carbon rings in an HOPG material by atomic force microscopy (AFM) (Figure 5.4) [14].

Figure 5.4 AFM image of graphite. The hexagonal carbon rings are visible and the complete lattice surface is imaged [14].