Читать книгу Industrial Carbon and Graphite Materials - Группа авторов - Страница 20

In contrast to isotropic diamond, graphite is an extreme anisotropic substance. Strong planar sigma bonds form a planar layer structure that is associated with π‐bonds rectangular to the σ‐bonded layers. These layers are stacked in a regular sequence (ABAB = hexagonal, ABCABC… = rhombohedral). Neither natural graphite nor synthetic graphite does possess a perfect crystal structure. Defects occur within the planes and in the stacking sequence of the planes. Most perfect forms are high crystalline natural vein graphites and hot‐pressed pyrolytic graphite (HOPG). The crystallographic structure of graphite can be determined by X‐ray diffraction. It was determined as hexagonal with four atoms in the unit cell by Hassel and Mark [12] and Bernal [13] with the stacking sequence of ABAB…. The rhombohedral structure of graphite with a stacking sequence of ABCABC… was first suggested by Debye and Scherrer in 1917 [14]. Rhombohedral graphite can be obtained by physical shearing forces like those appear during milling. This graphite can be easily retransformed to hexagonal graphite by annealing. Graphite vaporizes under atmospheric pressure at 3895–4020 K. The graphite–liquid–vapor equilibrium was found by laser heating technique at a pressure of 10.8 ± 0.2 MPa and a temperature of 4600 K. The density of the liquid carbon was calculated with 1.37 g/cm³ [15].

Figure 2.7 Phase diagram of carbon.