Hexagonal diamond was found as a by‐product of diamond synthesis and later in meteorites [11]. In the meantime, chemical vapor deposition conditions of methane/hydrogen gas mixtures at low pressure allow the production of synthetic diamonds.
2.3 Graphite
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.
2.4 Non‐graphitic Carbon
Non‐graphitic carbons are all varieties of solids consisting mainly of the element carbon with two‐dimensional long‐range order of the carbon atoms in planar hexagonal networks, but without any measurable crystallographic order in the third direction (c‐direction) apart from more or less parallel stacking [16]. Only non‐graphitic carbons which have passed during thermal degradation (pyrolysis) a liquid or gaseous phase can be transformed by heat treatment (>2500 K) into synthetic graphite. Carbon‐containing substances that remain solid during thermal degradation remain non‐graphitic carbons even after heat treatments beyond 2500 K. They are so‐called non‐graphitizable carbons. One way to transform non‐graphitizable carbons into synthetic graphite is catalytic graphitization. This term is somewhat misleading as in most cases it is a precipitation of carbon dissolved in oversaturated metal melts. The most known catalytic graphite is the so‐called kish graphite, a precipitate from oversaturated iron melts, a well‐known phenomenon in the steel industry. Some more detailed information can be taken from [17].
2.5 Carbyne and Chaoite
Carbyne and chaoite are also considered as allotropic forms of carbon. The chemical structure is an alternating triple bond between the carbon atoms (—C≡C—). The carbon atom is thus in the sp‐hydride state. The estimated bonding lengths in this linear molecule are 0.138 nm for the single bond and 0.121 nm for the triple bond. Young’s modulus of this substance should be 40 times that of diamond. This molecule is not easy to synthesize with chain lengths more than eight acetylenic units. It was claimed that this acetylenic carbon was found in shock‐metamorphosed graphite gneiss in the Nördlinger Ries crater in Germany, later named chaoite [18]. Its existence in various meteorites has been reported also. To summarize, carbynes have no industrial importance and have been subject of academic curiosity.
2.6 Nanoforms of Carbon
Fullerenes are a new allotropic form of carbons belonging to the nanoforms of carbon. Belonging to the group of fullerenes are spherical fullerenes, also called buckyballs, and cylindrical ones. The latter are known as single‐wall nanotubes (SWNT) and multiwall nanotubes (MWNT). The existence of the fullerene C 60 was firstly predicted in 1970 [19] as a spherical molecule built entirely from carbon atoms. To compensate for the molecular tension, fullerenes contain decide hexagonal rings also pentagonal ones. The first fullerene was prepared by Smalley and coworkers [20]. The name of Iijima is today mostly associated with carbon nanotubes (1991) although he had successfully described fullerenes by high resolution electron microscopy already in 1980 [21]. The method for the production of fullerenes in reasonable amounts was developed in 1990 [22].
Due to the extraordinary properties of these new allotropes, a scientific stampede went on accompanied by huge expectations for revolutionary application. Main hopes are in medical application like tumor research and drug delivery. Others are in material reinforcement, superconductivity, hydrogen storage, sensor technology, and electronic circuits. Mostly 30 years after the first preparation of fullerenes, the big expectations have not been realized, and the nanoforms of carbon are still more subject to academic research than industrial application.
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