What is the microstructure (crystal structure) of graphitized petroleum coke like?

I. Characteristics of Crystal Structure

Layered Structure: The crystal structure of graphitized petroleum coke consists of planar networks of hexagonal carbon atoms. These planar networks are stacked layer by layer, forming a typical layered structure. The layers are connected by relatively weak van der Waals forces, which endow graphite with lubricity and anisotropy.
Lattice Constants: After graphitization treatment, the lattice constants (a₀ and c₀) of petroleum coke approach those of natural graphite, indicating a high degree of similarity in their crystal structures. This structural feature enables graphitized petroleum coke to exhibit excellent electrical and thermal conductivity.
Microcrystalline Parameters: Using X-ray diffraction, parameters such as the interlayer spacing (d₀₀₂), average layer diameter (Lₐ), and stacking height (Lc) of the microcrystals in graphitized petroleum coke can be calculated. These parameters reflect the size and arrangement of the microcrystals and serve as important indicators for evaluating the degree of graphitization.

II. Effects of the Graphitization Process

Transition from Amorphous to Crystalline State: Prior to graphitization, the carbon structure of petroleum coke is amorphous, characterized by a “long-range disordered, short-range ordered” material structure. Through graphitization treatment (typically carried out at high temperatures ranging from 2500°C to 3000°C), the amorphous carbon gradually transforms into an ordered three-dimensional graphite crystal structure.
Increase in Microcrystallite Size: During graphitization, the average thickness (Lc) and width (Lₐ) of the carbon lattice flakes increase, while the interlayer spacing (d) decreases. This results in an increase in microcrystallite size and a more perfect crystal structure.
Reduction in Resistivity: As the degree of graphitization increases, the resistivity of graphitized petroleum coke significantly decreases. This is because, during graphitization, the arrangement of carbon atoms becomes more ordered, allowing electrons to move more freely within the layer planes, thereby enhancing electrical conductivity.

III. Relationship Between Microstructure and Properties

Electrical Conductivity: The layered crystal structure of graphitized petroleum coke enables electrons to move freely within the layer planes, resulting in excellent electrical conductivity. This property makes graphitized petroleum coke widely applicable in areas such as electrode materials and conductive additives.
Thermal Conductivity: Due to the van der Waals forces connecting the layers, heat can be rapidly transferred within the layer planes. Consequently, graphitized petroleum coke also exhibits good thermal conductivity, making it suitable for manufacturing heat dissipation materials and other applications.
Mechanical Properties: The crystal structure of graphitized petroleum coke provides it with certain mechanical strength. However, compared to metallic materials, its layered structure results in weaker interlayer bonding, leading to relatively lower flexural and compressive strengths. This performance characteristic gives graphitized petroleum coke an application advantage in scenarios where it needs to withstand certain pressures but does not require high strength.