graphitization process

Graphitization process is a heating heat treatment process, in the graphitization process of each temperature stage, there are heat absorption and exothermic process, can be divided into the following three stages:

        The first stage (1000 ~ 1800 ℃ ): in a higher temperature than the roasting, the products further discharge volatile components, all the residual aliphatic chain, C-H, C = 0 bond, etc. in this temperature paradigm within the domestic successive fracture; messy layer structure between layers of carbon atoms, hydrogen, oxygen, nitrogen, sulfur, etc., or simple molecules of monomers (CH , CO, CO2, etc.) is also discharged at this time. A part of the chaotic dispersion of plane molecules combined into a large molecule in this temperature zone heat absorption process is mainly a continuation of the chemical reaction. At the same time, there are also physical processes, manifested in part of the microcrystalline boundary disappears, the original interfacial energy in the form of heat release, as a driving force to promote the carbon hexagonal lattice ordering. From the X-ray analysis, it can be seen that in this temperature range, the stacking of carbon atoms at the level of a significant increase in their orderly arrangement is carried out in the two-dimensional plane, the two-dimensional plane size does not exceed 8nm, the macromolecule is still chaotic layer structure.

        The second stage (1800~2400K): there are two cases in this stage, one is that as the temperature rises, the system obtains more energy. The thermal vibration frequency of carbon atoms increases, the amplitude increases, governed by the law of minimum free energy, the lattice level to the three-dimensional arrangement of the graphite structure is excessive, and the distance between the layers is reduced. At the same time, the amplitude of carbon atoms along the direction parallel to the plane grid increases, and the dislocation lines and grain boundaries on the crystal plane gradually disappear, releasing latent heat. To 2000K, the entropy gain of the system to the lowest point, it will continue to 2000K above the entropy difference curve shown in Figure (13-6). In this temperature treated graphite X-ray diffraction spectrum gradually appeared relatively sharp (hko), (001) and some (hkl) line, proved to have carried out the three-dimensional ordering, which is a kind of release of internal energy of the annealing process; another parallel reaction is, in 2000-2400K, some impurities to generate carbide (mainly silicon carbide), and in the following higher temperature decompose into metal vapor and graphite. In addition to this, near 2400K the carbon starts to evaporate and thermal defects appear, all of which consume energy. Since these processes take place more often between 2000 and 2400 K, the system absorbs thermal energy, which is manifested in a renewed increase in entropy.

        The third stage (above 2400K): general petroleum coke and asphalt coke and other easily graphitized carbon at 2400K temperature, the a-axis direction of the grain grows to an average of 10~150nm, and the c-axis direction reaches about 60 layers (about 20nm). Due to the ordering of the upper stage, the shrinkage of the grains is caused and the interfacial gap of the grains is enlarged. If the grain growth mechanism discussed above, even if you continue to increase the temperature cupboard, the grains can not be close to each other, can not be bonded into a larger grain, the growth of the grain to rely on a new mechanism to achieve, that is, the recrystallization process.

       This recrystallization process, on the one hand, is the carbon plane molecules within or between the molecules of carbon atoms to move, lattice refinement and three-dimensional arrangement, on the other hand, in the high temperature of more than 2400K, the evaporation rate of carbon material with the temperature increases exponentially.

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