Boron Nitride is a high-performance ceramic material with significant potential for development, renowned for its excellent thermal stability, chemical inertness, and electrical insulation properties. It exists in multiple crystalline forms, including five allotropes: hexagonal boron nitride (h-BN), wurtzite boron nitride (w-BN), rhombohedral boron nitride (r-BN), cubic boron nitride (c-BN), and orthorhombic boron nitride (o-BN).
Hexagonal boron nitride is one of the thermally conductive materials in ceramics. Thermal conductivity is an important indicator of a material's ability to conduct heat, which is crucial for thermal management and dissipation. Meanwhile, the melting point reflects the material's stability under extreme temperatures, which is a key factor in determining the material's range of applications. Let's explore the thermal conductivity performance of boron nitride from the aspects of thermal conductivity and melting point.
What are the thermal performance indicators of boron nitride?
Melting Point
- It sublimates at 3000°C in 0.1MPa and nitrogen gas.
- The melting point is 3000°C in inert gas.
- It is heat resistant to 2000°C in a neutral-reducing atmosphere.
- In nitrogen and argon gas, the use temperature is up to 2800 ℃.
- It is less stable in an oxygen atmosphere, and the use of temperature is below 1000 ℃.
Thermal Conductivity
- The thermal conductivity of hexagonal boron nitride is ten times that of quartz, with a notably high thermal conductivity of 60 W/(m·K) in the direction perpendicular to the c-axis.
Thermal Expansion Coefficient
- Hexagonal boron nitride has a very low coefficient of thermal expansion, comparable to that of quartz, which is the smallest among ceramics. The coefficient of thermal expansion along the c-axis is 41×10-6 m/K, while along the d-axis, it is 2.3×10-6 m/K, indicating excellent thermal resistance.
Table 1 Thermal properties of h-BN and other material
Material | The Max Use Temperature (℃) | Thermal Conductivity (W/m·K) | Thermal Expansion Coefficient (10-6 m/K) |
h-BN | 900 (Oxygen) 2800 (Nitrogen) | 25.1 | 0.7 (⟂) 7.5 (∥) |
BeO | 2000 | 255.4 | 7.8 |
1750 | 25.1 | 8.6 | |
ZrO2 | 1100 | 2.09 | 10.0 |
Quartz glass | 130 | 1.67-4.19 | 6.5 |
Fluororesin | 25 | -- | -- |
What are the factors that affect the thermal conductivity of boron nitride?
- Material purity: Impurities and defects scatter heat carriers, reducing thermal conductivity.
- Crystal orientation: Hexagonal boron nitride has different thermal conductivities in different crystal directions, with higher conductivity perpendicular to the c-axis than parallel to it.
- Temperature: An increase in temperature leads to more lattice vibrations, causing more scattering of heat carriers and a decrease in thermal conductivity.
Advanced Applications of Boron Nitride
- Insulator for high-temperature furnaces
- Protective tubes and insulating sleeves for thermocouples
- Casting process in steel production
- Crucible materials for melting high-purity substances in molecular epitaxy units
Hexagonal boron nitride, with its exceptional thermal conductivity and stability, finds extensive applications across various domains. In the future, researchers will delve into low-temperature coating processes, applications in the field of nanophotonics, and scalable synthesis methods to further enhance its performance. Additionally, the potential breakthroughs in quantum technology and corrosion and wear prevention applications of boron nitride herald promising advancements, bringing forth more efficient and reliable material solutions.
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