The graphitization process is a heat treatment procedure where both endothermic and exothermic reactions occur at different temperature stages, which can be divided into three phases:
Phase 1 (1000–1800°C): At temperatures higher than calcination, the material further releases volatiles. All remaining aliphatic chains, C-H bonds, C=O bonds, etc., break within this temperature range. Monoatomic or simple molecules (CH, CO, CO₂, etc.) of carbon, hydrogen, oxygen, nitrogen, and sulfur between disordered layer structures are also expelled. Some randomly dispersed planar molecules combine to form larger molecules. The endothermic reactions in this phase primarily involve continued chemical reactions, alongside physical processes where some microcrystalline boundaries disappear, releasing interfacial energy as heat, which drives the ordering of carbon hexagonal networks. X-ray analysis shows that within this temperature range, the stacking of carbon atomic layers does not significantly increase-their ordered arrangement occurs in two-dimensional planes with sizes not exceeding 8 nm, maintaining a turbostratic structure.
Phase 2 (1800–2400K): Two phenomena occur in this stage: As the temperature rises, the system gains more energy. Carbon atoms exhibit increased thermal vibration frequency and amplitude, governed by the principle of minimum free energy. The lattice layers transition toward a three-dimensional graphite structure, with interlayer distances shrinking. Simultaneously, dislocations and grain boundaries on crystal planes gradually disappear, releasing latent heat. By 2000K, the system's entropy increment reaches its lowest point. X-ray diffraction patterns of graphite treated at this temperature show sharper (hk0), (001), and some (hkl) lines, indicating three-dimensional ordering-an exothermic annealing process.
Concurrently, between 2000–2400K, some impurities form carbides (mainly silicon carbide), which decompose into metal vapors and graphite at higher temperatures. Additionally, near 2400K, carbon begins to evaporate, creating thermal defects that consume energy. These processes dominate between 2000–2400K, causing the system to absorb heat and exhibit a renewed entropy increase.
Phase 3 (Above 2400K): For easily graphitizable carbons like petroleum coke and pitch coke at 2400K, crystallites grow to an average size of 10–150 nm along the a-axis and ~60 layers (~20 nm) along the c-axis. Ordered restructuring from the previous phase causes crystallite contraction, expanding inter-grain gaps. Under conventional growth mechanisms, further temperature increases cannot bridge these gaps to form larger crystallites. Instead, growth proceeds via a new mechanism: recrystallization.
Here, the graphitization system becomes saturated with carbon atoms/molecules (C, C₂, C₃, C₄, etc.), enabling vigorous matter exchange between solid and gas phases-recrystallization.
The graphitization stages overlap. At temperatures slightly above calcination/carbonization, decomposition-polymerization reactions occur. Between 1700–2400K, annealing and microcrystalline growth dominate, aided by carbide formation/decomposition. Above 2400K, recrystallization via carbon migration becomes primary. Easily graphitizable carbons undergo homogeneous/heterogeneous graphitization-net exothermic despite local endothermic steps-increasing system entropy and stability.
Hard-to-graphitize carbons require higher temperatures (>3200K) for heterogeneous crystallization. Cross-links break, forming multiple nucleation sites where sublimated carbon rapidly rearranges into fine graphite crystallites.
