treatment in air. The once stabilized fibers do not melt during subsequent pyrolysis. Further treatment at graphitization temperatures (high‐modulus carbon fibers) does not result in a graphitic carbon fiber.
Figure 5.7 Gaseous, liquid, and solid pyrolysis and their products.
In the case of the liquid‐phase pyrolysis, the formation of optically anisotropic spherulites (carbonaceous mesophase) is essential for the later structural order of the final graphitic carbon. Figure 5.8 shows an optical micrograph of mesophase spherulites [18]. The changing color under polarized light shows the anisotropy of the spherulites. Brooks and Taylor were the first to discover the formation of these spherules during the pyrolysis of petroleum pitch and coal‐tar pitch as well as during the softening of coal [19]. During heat treatment, pitch gradually forms an isotropic melt and develops small (few microns in size) spherulites with increasing temperature. The diameter of the spherulites increases with temperature, to a lesser extent with the residence time. The spherules are nematic liquid crystals composed of large parallel stacked aromatic molecules.
Figure 5.8 Optical micrograph of carbonaceous mesophase from heated anthracene oil [18].
Figure 5.9 Carbonaceous mesophase structure (a) and mechanism of growth by coalescence of spherulites [22–24] (b).
After the formation of a sufficient number of spherulites, so‐called secondary quinoline insoluble (QIs), they touch each other and coalesce under the formation of bigger spherules. With ongoing growth, the spherulitic mesophase is transferred into the bulk mesophase, viscosity increases, and the carbonaceous material becomes solid (Figure 5.9). Formation and structure of the mesophase are essentially governed by three limiting conditions:
1 1. The shape of the polyaromatic molecules must favor the formation of liquid crystals (i.e. highly aromatic, few heteroatoms, and few aliphatic side chains).
2 2. The fluidity of the system.
3 3. The reaction or condensation rate must be smaller than the ordering rate.
Other factors influencing the structure of the mesophase – which eminently affect structure and properties of the coke or of the synthetic graphite after graphitization, respectively – are the amount of nonmelting constituents and/or the amount and size of impurities [20, 21]. Impurities are the so‐called primary QIs, which are solid particles like ash, soot, refractory particles, cenospheres (porous to hollow carbonaceous sphere‐like particles), and others, which are not soluble in quinoline. These particles tend to settle on the surface of the mesophase spherulites and hinder the coalescence of the mesophase (Figure 5.9) [25, 26].
The chemistry of the pyrolytic conversion of hydrocarbons to solid carbon involves numerous chemical reactions. Generalized schemes have been developed by studying simpler models [25, 26]. The major reactions involved are:
Bond cleavage and formation of free radicals.
Molecular rearrangement.
Thermal polymerization/polycondensation.
Aromatization.
Elimination of aliphatic side chains and dehydrogenation.
A generalized scheme is shown in Figure 5.10 with structural models of the chemical reaction products. The pyrolysis is accompanied by the release of volatile reaction products. This release of volatiles induces the development of pores. Shrinkage reactions during the subsequent solid‐state degradation contribute to the porosity with cracks and slit pores. The pore sizes reach from micropores (<2 nm) over mesopores (2–50 nm) to macropores (>50 nm). The highest porosity is achieved at heat treatment temperatures of around 870 K. Micropores were postulated in the 1950s to buffer the thermal expansion and thus are responsible for the low thermal expansion behavior of polygranular carbon materials [27]. This postulate was experimentally proven later [28].
After pyrolysis further heat treatment to above 2500 K is necessary to obtain synthetic graphite. Industrially most important is the heat treatment of graphitizable carbon compounds above 2500 K by means of electrically heated furnaces known as Acheson [29] and Castner furnaces [30] (see → Carbon, 4. Industrial Carbons). Today the commonly used technical terminus is lengthwise graphitization (LWG). For granular forms of carbon or powders, electrically heated shaft furnaces or fluidized‐bed reactors are used [27, 28, 31].
Figure 5.10 Reaction scheme for carbonization and graphitization.
Other methods for the production of synthetic graphite are vapor deposition of volatile carbon compounds or the condensation of carbon vapor (pyrolytic graphite) [32]. The crystallization of carbon melts at pressures above 15 MPa [33], the precipitation from oversaturated metal melts (Kish graphite) [34], the decomposition of carbide crystals and nitrogenation of calcium carbide [35], and the electrolytic decomposition of carbonate melts [36] have been applied. The precipitation from oversaturated metal melts or the decomposition of carbides is often named catalytic graphitization. Catalytic graphitization is not applied to formed products like graphite electrodes or other polygranular graphitic carbons but in the production for some granular specialties.
Crucial during the heat treatment to graphitization temperatures is the release of the incorporated heteroatoms sulfur and nitrogen. The release of nitrogen starts at around 1670 K followed by the release of sulfur at around 1870 K [37, 38]. Both volatiles lead to an irreversible expansion with the potential to destroy the polygranular artifact. This behavior is named as puffing [39, 40].
During further heat treatment to graphitization temperatures above 2500 K, lattice defects are eliminated, combined with the growth of the graphene layers in a‐ and c‐direction (La = apparent crystallite size, Lc = mean stacking height) and the narrowing of the interlayer distance toward the value of the ideal graphite crystal. The development of the graphite crystal under graphitization temperatures was investigated in situ by X‐ray diffraction up to 2870 K [41]. This investigation showed that prior to the interlayer distance shrinkage, defects in the layers must be healed enabling the necessary parallel arrangement.
