Graphitization process of graphite electrode

The graphitization process is a heating heat treatment process. In each temperature stage of the graphitization process, there are both endothermic and exothermic processes, which can be divided into the following three stages:

The first stage (1000~1800℃): At a higher temperature than roasting, the product further discharges volatiles, and all the remaining aliphatic chains, C-H, C=0 bonds, etc. are broken one after another within this temperature range; the carbon atoms, hydrogen, oxygen, nitrogen, sulfur and other monomers or simple molecules (CH, CO, CO2, etc.) between the layers of the chaotic layer structure are also discharged at this time. The heat absorption process of a part of the randomly dispersed planar molecules combined into macromolecules in this temperature zone is mainly the continuation of chemical reactions. At the same time, there are also physical processes, which are manifested in the disappearance of part of the microcrystalline boundaries, and the original interface energy is released in the form of heat as a driving force to promote the ordering of the carbon hexagonal grid. From X-ray analysis, it can be seen that within this temperature range, the stacking of carbon atoms has increased significantly, and their orderly arrangement is carried out in a two-dimensional plane. The size of the two-dimensional plane does not exceed 8nm, and the macromolecules are still in a chaotic layer structure.

The second stage (1800~2400K): There are two situations in this stage. First, as the temperature rises, the system gains more energy. The frequency of thermal vibration of carbon atoms increases, the amplitude increases, and the grid layer transitions to a three-dimensionally arranged graphite structure under the rule of minimum free energy, and the interlayer distance decreases. At the same time, the amplitude of carbon atoms parallel to the plane grid increases, and the dislocation lines and grain boundaries on the crystal plane gradually disappear, releasing latent heat. At 2000K, the entropy increment of the system reaches the lowest point, and it will continue to above 2000K as shown in the entropy difference curve in Figure (13-6). At this temperature, the X-ray diffraction spectrum of the graphite treated gradually shows relatively sharp (hko), (001) and some (hkl) lines, proving that a three-dimensional orderly arrangement has been carried out. This is an annealing process that releases internal energy; another parallel reaction is that at 2000-2400K, some impurities generate carbides (mainly silicon carbide), and decompose into metal vapor and graphite at higher temperatures thereafter. In addition, carbon begins to evaporate and thermal defects appear when approaching 2400K, which consumes energy. Since these processes are mostly carried out between 2000 and 2400K, the system absorbs heat energy, which is manifested as a renewed increase in entropy change.

The third stage (above 2400K): For graphitizable carbons such as petroleum coke and asphalt coke, the average a-axis length of the grains grows to 10-150nm at 2400K, and the c-axis length reaches about 60 layers (about 20nm). Due to the ordering in the previous stage, the grains shrink and the intergranular gaps expand. According to the grain growth mechanism discussed above, even if the temperature is further increased, the grains cannot get close to each other and cannot bond into larger grains. At this time, the growth of the grains must be achieved by a new mechanism, namely the recrystallization process.