Industrial Carbon and Graphite Materials. Группа авторов

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].

5.2 Natural Graphite

      5.2.1 Occurrence and Properties