Graphitization treatment typically requires high temperatures ranging from 2300 to 3000℃, with its core principle being the transformation of carbon atoms from a disordered arrangement to an ordered graphite crystal structure through high-temperature heat treatment. Below is a detailed analysis:
I. Temperature Range for Conventional Graphitization Treatment
A. Basic Temperature Requirements
Conventional graphitization necessitates raising the temperature to the range of 2300 to 3000℃, where:
- 2500℃ marks a pivotal turning point, at which the interlayer spacing of carbon atoms significantly decreases, and the degree of graphitization rapidly increases;
- Beyond 3000℃, changes become more gradual, and the graphite crystal approaches perfection, although further temperature increases yield diminishing marginal improvements in performance.
B. Impact of Material Differences on Temperature
- Easy-to-graphitize carbons (e.g., petroleum coke): Enter the graphitization stage at 1700℃, with a notable increase in graphitization degree at 2500℃;
- Difficult-to-graphitize carbons (e.g., anthracite): Require higher temperatures (approaching 3000℃) to achieve a similar transformation.
II. Mechanism by Which High Temperatures Promote Carbon Atom Ordering
A. Phase 1 (1000–1800℃): Volatile Emission and Two-Dimensional Ordering
- Aliphatic chains, C-H, and C=O bonds break down, releasing hydrogen, oxygen, nitrogen, sulfur, and other elements in the form of monomers or simple molecules (e.g., CH₄, CO₂);
- Carbon atom layers expand within the two-dimensional plane, with microcrystalline height increasing from 1 nm to 10 nm, while interlayer stacking remains largely unchanged;
- Both endothermic (chemical reactions) and exothermic (physical processes, such as the release of interfacial energy from microcrystalline boundary disappearance) processes occur simultaneously.
B. Phase 2 (1800–2400℃): Three-Dimensional Ordering and Grain Boundary Repair
- Increased thermal vibration frequencies of carbon atoms drive them to transition into three-dimensional arrangements, governed by the principle of minimum free energy;
- Dislocations and grain boundaries on crystal planes gradually disappear, evidenced by the emergence of sharp (hko) and (001) lines in X-ray diffraction spectra, confirming the formation of three-dimensional ordered arrangements;
- Some impurities form carbides (e.g., silicon carbide), which decompose into metal vapors and graphite at higher temperatures.
C. Phase 3 (Above 2400℃): Grain Growth and Recrystallization
- Grain dimensions increase along the a-axis to an average of 10–150 nm and along the c-axis to approximately 60 layers (about 20 nm);
- Carbon atoms undergo lattice refinement through internal or intermolecular migration, while the evaporation rate of carbon substances increases exponentially with temperature;
- Active material exchange occurs between the solid and gas phases, resulting in the formation of a highly ordered graphite crystal structure.
III. Temperature Optimization through Special Processes
A. Catalytic Graphitization
The addition of catalysts such as iron or ferrosilicon can significantly reduce graphitization temperatures to the range of 1500–2200℃. For example:
- Ferrosilicon catalyst (25% silicon content) can lower the temperature from 2500–3000℃ to 1500℃;
- BN catalyst can reduce the temperature to below 2200℃ while enhancing the orientation of carbon fibers.
B. Ultra-High-Temperature Graphitization
Utilized for high-purity applications such as nuclear-grade and aerospace-grade graphite, this process employs medium-frequency induction heating or plasma arc heating (e.g., argon plasma core temperatures reaching 15,000℃) to achieve surface temperatures exceeding 3200℃ on the products;
- The degree of graphitization exceeds 0.99, with extremely low impurity content (ash content < 0.01%).
IV. Impact of Temperature on Graphitization Effects
A. Resistivity and Thermal Conductivity
For every 0.1 increase in graphitization degree, resistivity decreases by 30%, and thermal conductivity increases by 25%. For instance, after treatment at 3000℃, the resistivity of graphite can drop to 1/4–1/5 of its initial value.
B. Mechanical Properties
High temperatures reduce the interlayer spacing of graphite to near-ideal values (0.3354 nm), significantly enhancing thermal shock resistance and chemical stability (with a linear expansion coefficient reduction of 50%–80%), while also imparting lubricity and wear resistance.
C. Purity Enhancement
At 3000℃, chemical bonds in 99.9% of natural compounds break down, allowing impurities to be released in gaseous form and resulting in a product purity of 99.9% or higher.
Post time: Sep-11-2025