What's graphitization process?

The graphitization process is a heating treatment procedure. At each temperature stage during graphitization, both endothermic and exothermic processes occur, which can be divided into the following three phases:

Stage 1 (1000–1800°C): At temperatures higher than calcination, the product further expels volatiles. All remaining aliphatic chains, C-H bonds, C=O bonds, etc., sequentially break within this temperature range. At this stage, carbon atoms, hydrogen, oxygen, nitrogen, sulfur, and other monomers or simple molecules (CH₂, CO, CO₂, etc.) between disordered layered structures are also expelled. Some randomly dispersed planar molecules aggregate into macromolecules. The heat absorption process in this temperature zone primarily continues chemical reactions. Concurrent physical processes include the disappearance of some microcrystalline boundaries, releasing the original interfacial energy as heat—a driving force for the ordering of the carbon hexagonal lattice. X-ray analysis indicates that within this temperature range, the stacking of carbon atomic layers significantly increases. Their ordered arrangement occurs within a two-dimensional plane, with the plane dimensions not exceeding 8 nm. Macromolecules remain in a disordered layered structure.

Second Stage (1800–2400 K): Two scenarios unfold during this stage. First, as temperature rises, the system gains additional energy. The thermal vibration frequency of carbon atoms increases, and their amplitude grows. Governed by the principle of minimum free energy, the lattice layers transition toward a three-dimensional graphite structure, with interlayer distances decreasing. Simultaneously, the amplitude of carbon atoms vibrating parallel to the planar lattice direction increases. Dislocation lines and grain boundaries on the crystal plane gradually disappear, releasing latent heat. At 2000K, the system's entropy increment reaches its lowest point and persists above 2000K, as shown in the entropy difference curve in Figure (13-6). X-ray diffraction patterns of graphite treated at this temperature gradually reveal sharp (hko), (001), and some (hkl) lines, confirming the emergence of three-dimensional ordered arrangement—an annealing process releasing internal energy. A parallel reaction occurs between 2000–2400K, where certain impurities form carbides (primarily silicon carbide), which subsequently decompose into metallic vapors and graphite at higher temperatures. Additionally, near 2400K, carbon begins to evaporate and thermal defects emerge, both of which consume energy. As these processes intensify between 2000 and 2400K, the system absorbs thermal energy, manifesting as a renewed increase in entropy change.

Stage 3 (Above 2400K): For graphitizable carbons like petroleum coke and asphalt coke, at 2400K the average a-axis grain length reaches 10–150 nm, while the c-axis extends to approximately 60 layers (about 20 nm). The ordering from the previous stage causes grain contraction and expansion of intergranular voids. According to the grain growth mechanism discussed above, even with further temperature increases, grains cannot approach each other to fuse into larger structures. At this point, grain growth requires a new mechanism: recrystallization.

This recrystallization process involves two aspects: first, the movement of carbon atoms within or between carbon plane molecules to refine the crystal lattice and achieve three-dimensional arrangement; second, at temperatures above 2400K, the evaporation rate of carbon materials increases exponentially with rising temperature.

At this stage, the graphitization system becomes saturated with carbon atoms and molecular gases such as C, C₂, C₃ (C₂ + C), C₄ (C₃ + C), and so on. An extremely active material exchange—recrystallization—occurs between the solid and gas phases.

In summary, the various stages of carbon graphitization are intertwined. At temperatures slightly higher than calcination and roasting, decomposition-polymerization reactions occur. Between 1700-2400K, annealing and microcrystal growth predominate, accompanied by carbide formation and decomposition, which promote graphitization. Above 2400K, recrystallization characterized by carbon atom migration becomes the primary process. Throughout the graphitization process, easily graphitizable carbon undergoes both homogeneous and heterogeneous graphitization. While endothermic processes occur, the fundamental nature is exothermic. The system's entropy increases, achieving greater stability.

Non-graphitizable carbonaceous materials can also undergo multiphase crystallization at high temperatures, though they require higher temperatures than easily graphitizable carbon. Above 3200K, cross-links begin to break. Depending on molecular orientation, numerous crystallization centers form. Sublimated carbon atoms rapidly rearrange around these centers, forming fine-grained crystalline graphite.