What exactly does the process of “graphitization” refer to?

“Graphitization”

“Graphitization” refers to a high-temperature heat treatment process (typically conducted at 2000°C to 3000°C or even higher) that transforms the microstructure of carbonaceous materials (such as petroleum coke, coal tar pitch, anthracite coal, etc.) from a disordered or low-ordered state into a layered crystalline structure similar to natural graphite. The core of this process lies in the fundamental rearrangement of carbon atoms, which endows the material with the unique physical and chemical properties characteristic of graphite.

Detailed Process and Mechanism of Graphitization

Heat Treatment Stages

1. Low-Temperature Zone (<1000°C)
   • Volatile components (e.g., moisture, light hydrocarbons) gradually volatilize, and the structure begins to contract slightly. However, carbon atoms remain predominantly disordered or short-range ordered.
2. Medium-Temperature Zone (1000–2000°C)
   • Carbon atoms start to rearrange via thermal motion, forming locally ordered hexagonal network structures (resembling the in-plane structure of graphite). However, interlayer alignment remains disordered.
3. High-Temperature Zone (>2000°C)
   • Under prolonged high-temperature exposure, carbon layers gradually align parallel to each other, forming a three-dimensionally ordered layered crystalline structure (graphitized structure). Interlayer forces weaken (van der Waals interactions), while in-plane covalent bond strength increases.

Key Structural Transformations

• Carbon Atom Rearrangement: Transition from an amorphous “turbostatic” structure to an ordered “layered” structure, with in-plane carbon atoms forming sp² hybridized covalent bonds and interlayer bonding via van der Waals forces.
• Defect Elimination: High temperatures reduce crystalline defects (e.g., vacancies, dislocations), enhancing crystallinity and structural integrity.

Core Objectives of Graphitization

1. Enhanced Electrical Conductivity
   • Ordered carbon atoms create a conductive network, enabling free electron movement within layers and significantly reducing resistivity (e.g., graphitized petroleum coke exhibits resistivity over 10 times lower than non-graphitized materials).
   • Applications: Battery electrodes, carbon brushes, electrical industry components requiring high conductivity.
2. Improved Thermal Stability
   • Ordered structures resist oxidation or decomposition at high temperatures, enhancing heat resistance (e.g., graphitized materials withstand >3000°C in inert atmospheres).
   • Applications: Refractory materials, high-temperature crucibles, spacecraft thermal protection systems.
3. Optimized Mechanical Properties
   • While graphitization may reduce overall strength (e.g., compressive strength decline), the layered structure introduces anisotropy, maintaining high in-plane strength and reducing brittleness.
   • Applications: Graphite electrodes, large-scale cathode blocks requiring thermal shock resistance and wear resistance.
4. Increased Chemical Stability
   • High crystallinity reduces surface active sites, lowering reaction rates with oxygen, acids, or bases, and enhancing corrosion resistance.
   • Applications: Chemical containers, electrolyzer linings in corrosive environments.

Factors Influencing Graphitization

1. Raw Material Properties
   • Higher fixed carbon content facilitates graphitization (e.g., petroleum coke graphitizes more easily than coal tar pitch).
   • Impurities (e.g., sulfur, nitrogen) hinder atomic rearrangement and require pretreatment (e.g., desulfurization).
2. Heat Treatment Conditions
   • Temperature: Higher temperatures enhance graphitization degree but increase equipment costs and energy consumption.
   • Time: Extended holding times improve structural perfection, but excessive duration may cause grain coarsening and performance degradation.
   • Atmosphere: Inert environments (e.g., argon) or vacuums prevent oxidation and promote graphitization reactions.
3. Additives
   • Catalysts (e.g., boron, silicon) lower graphitization temperatures and improve efficiency (e.g., boron doping reduces required temperatures by ~500°C).

Comparison of Graphitized vs. Non-Graphitized Materials

Practical Application Cases

1. Graphite Electrodes
   • Petroleum coke or coal tar pitch is graphitized to produce high-conductivity, high-strength electrodes for electric arc furnace steelmaking, enduring >3000°C and intense currents.
2. Lithium-Ion Battery Anodes
   • Natural or synthetic graphite (graphitized) serves as anode material, leveraging its layered structure for rapid lithium-ion intercalation/deintercalation, improving charge/discharge efficiency.
3. Steelmaking Carburizer
   • Graphitized petroleum coke, with its porous structure and high carbon content, rapidly increases carbon content in molten iron while minimizing sulfur impurity introduction.