Analysis of Factors Affecting Thermal Conductivity of Carbon Graphite Materials: From Microstructure to Impregnation Process

Thermal conductivity is a key indicator of the heat dissipation capability of carbon graphite seals, directly affecting seal face temperature, friction and wear, and service life. In our long-term research and development of carbon graphite materials at Huahao Sealing Co., Ltd. (霍邱县华豪密封件有限公司), we have systematically studied the influence of various factors on thermal conductivity. Based on experimental data, this article deeply analyzes the microscopic mechanisms and process effects of carbon graphite thermal conductivity, providing theoretical basis for seal selection and thermal design.

1. Microscopic Mechanism of Carbon Graphite Thermal Conductivity

1.1 Crystal Structure and Phonon Heat Transfer

Carbon graphite has a layered hexagonal crystal structure, where carbon atoms form strong covalent bonds through sp² hybridization within layers, with van der Waals bonding between layers. Heat conduction is mainly achieved through phonon propagation within layers. The theoretical in-layer thermal conductivity of graphite can reach 2000 W/(m·K), while the inter-layer thermal conductivity is only 10-20 W/(m·K). Polycrystalline graphite materials used in engineering have macroscopic thermal conductivity between these two values due to varying grain orientations.

1.2 Decisive Role of Graphitization Degree

Graphitization degree is an indicator of the perfection of the graphite crystal structure, characterized by measuring the (002) plane interlayer spacing d002 by X-ray diffraction. Graphitization degree G = (3.440 - d002)/(3.440 - 3.354) × 100%. Experimental data shows:

• Graphitization 70% (d002=3.376 nm): thermal conductivity 100-120 W/(m·K)
• Graphitization 80% (d002=3.367 nm): thermal conductivity 130-150 W/(m·K)
• Graphitization 90% (d002=3.358 nm): thermal conductivity 170-190 W/(m·K)

For every 10% increase in graphitization degree, thermal conductivity rises by about 25-35 W/(m·K). This is why Huahao Sealing Co., Ltd.'s high thermal conductivity graphite products must undergo high-temperature graphitization treatment above 2500°C.

2. Influence of Porosity and Bulk Density

2.1 Inverse Relationship Between Porosity and Thermal Conductivity

Carbon graphite materials contain a large number of pores (open and closed), filled with air (thermal conductivity 0.026 W/m·K), which is the main resistance to heat conduction. Experimental data shows:

• Porosity 10% (density 1.85 g/cm³): thermal conductivity 150 W/(m·K)
• Porosity 15% (density 1.75 g/cm³): thermal conductivity 120 W/(m·K)
• Porosity 20% (density 1.65 g/cm³): thermal conductivity 90 W/(m·K)
• Porosity 25% (density 1.55 g/cm³): thermal conductivity 70 W/(m·K)

2.2 Improvement of Thermal Conductivity by Impregnation

Impregnation processes improve thermal conductivity by filling pores. Different impregnants have significantly different effects:

• Antimony impregnation (metal): thermal conductivity increases from 80 W/(m·K) to 110 W/(m·K), up about 38%
• Copper impregnation (metal): thermal conductivity increases from 80 W/(m·K) to 140 W/(m·K), up about 75%
• Phenolic resin impregnation: thermal conductivity decreases from 80 W/(m·K) to 65 W/(m·K), down about 19% (resin thermal conductivity is only 0.2 W/m·K)
• PTFE impregnation: thermal conductivity decreases from 80 W/(m·K) to 55 W/(m·K), down about 31%

3. Effect of Grain Size and Temperature

3.1 Grain Size Effect

Larger graphite grain sizes result in less grain boundary scattering and higher thermal conductivity. Graphite prepared from needle coke has larger grains (Lc=50-100 nm) with thermal conductivity of 150-200 W/(m·K), while graphite from ordinary petroleum coke has smaller grains (Lc=20-40 nm) with thermal conductivity of only 70-100 W/(m·K).

3.2 Temperature Dependence

Carbon graphite thermal conductivity varies non-monotonically with temperature:

• Room temperature (25°C): 120 W/(m·K)
• 200°C: 100 W/(m·K)
• 400°C: 80 W/(m·K)
• 600°C: 60 W/(m·K)
• 800°C: 50 W/(m·K)
• 1000°C: 45 W/(m·K)

As temperature rises, phonon scattering intensifies and thermal conductivity decreases. However, in sealing applications, since frictional heat concentrates at the seal face and working temperature typically does not exceed 400°C, thermal conductivity can still be maintained above 80 W/(m·K), with sufficient heat dissipation capability.

4. Anisotropic Characteristics

4.1 Difference Between Parallel and Perpendicular Directions

Carbon graphite materials exhibit significant differences in thermal conductivity between the pressing direction (parallel to molding pressure direction) and the perpendicular direction. Molded graphite has an anisotropy ratio of about 1.5:1, while isostatically pressed graphite has an anisotropy ratio of only 1.1:1, closer to isotropic.

4.2 Engineering Application Recommendations

For components requiring radial heat dissipation such as seal rings, isostatically pressed graphite (isotropic) should be selected; for components requiring axial electrical and thermal conductivity such as brushes, molded graphite (anisotropic) can be used.

Conclusion

The thermal conductivity of carbon graphite materials is comprehensively influenced by multiple factors including graphitization degree, porosity, impregnation process, grain size, temperature and anisotropy. Huahao Sealing Co., Ltd.'s high thermal conductivity copper-impregnated graphite products (HCu series) achieve thermal conductivity above 140 W/(m·K), particularly suitable for high-speed heavy-load sealing applications. For technical selection support, please contact our engineering team — we will provide material recommendations based on measured data.