Industrial Carbon and Graphite Materials. Группа авторов
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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 Lc (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 La [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
Numerous hypotheses on the genesis of natural graphite existed, but today most of them are outdated. It is currently supposed that organic matter was the origin of most natural graphites with the probable exception of the Sri Lanka (vein‐type) deposits. The origin of vein graphite deposits is still not certain. In regionally metamorphosed (granulitic, charnockitic) rocks, graphite is thought to form epigenetically from carbon‐rich hydrothermal or pneumatolytic solutions as interlocking aggregates of coarse graphite crystals in veins containing 75–100% carbon. Disseminated flake graphite deposits (see Figure 5.5) develop synergetically from carbonaceous material in sedimentary rocks that have been subjected to garnet grade or higher regional metamorphism. Concurrent large‐scale folding of the metasedimentary sequences is common, and fold limbs often host deposits. Deposits are usually stratabound and consist of individual beds or lenses in gneisses, schists, and marbles that are richer in graphite than associated beds [15]. Single flakes may range from 0.5 to 25 mm. The decomposition of compounds, e.g. carbide, carbonates, carbonyl, and cyano compounds, is less important for the genesis of graphite deposits.
Natural graphite is divided into vein type, flake type, and microcrystalline type, often incorrectly called amorphous graphite. Crystallinity, refractoriness, and other properties of natural graphite from different deposits may vary substantially because of differences in the precursor materials and the conditions of metamorphosis. In general, natural graphite is a ductile soft mineral that easily cleaves parallel to the basic layer. The flaky and vein grades show a typical metallic luster; the microcrystalline “amorphous” grades