Performance of hexagonal boron nitride
Hexagonal boron nitride (HBN) has excellent electrical insulation, excellent chemical stability and excellent dielectric properties.
① Thermal performance. No obvious melting point, sublimation at 3000 ℃ in 0.1MPA nitrogen, melting point at 3000 ℃ in inert gas, heat resistance to 2000 ℃ in neutral reducing atmosphere, temperature up to 2800 ℃ in nitrogen and argon, poor stability in oxygen atmosphere, and temperature below 900 ℃.
Hexagonal boron nitride is one of the materials with the highest thermal conductivity among ceramic materials, with a high thermal conductivity of 60W/(m · K) in the direction perpendicular to the c-axis; The low coefficient of thermal expansion, equivalent to quartz, is the smallest in ceramics. The coefficient of thermal expansion is 41x10 ^ - 6/℃ in the c-axis direction and 2.3x10 ^ - 6/℃ in the d-axis direction, so the thermal shock resistance is very good.
② Mechanical properties. The friction coefficient is as low as 0.16, does not increase at high temperatures, and has higher temperature resistance than molybdenum disulfide and graphite. The oxidation atmosphere can be used up to 900 ℃, and it can be used up to 2000 ℃ under vacuum. The lubrication performance is poor at room temperature, so it is often mixed with fluorinated graphite, graphite, and molybdenum disulfide as high-temperature lubricants.
Hexagonal boron nitride is a soft material with a Mohs hardness of only 2. Due to the graphite like layered structure of BN crystals, the dense HBN ceramic body formed by hot pressing of sheet shaped crystals has a certain degree of directional arrangement, which makes certain properties of HBN products have obvious anisotropic characteristics. The mechanical properties of hot pressed HBN are greater in strength parallel to the compression direction than in strength perpendicular to the compression direction.
Another characteristic is good machinability, which can be turned, milled, planed, drilled, ground, and cut, and has high processing accuracy. Therefore, general mechanical processing methods can be used to process high-precision component products.
③ Electrical performance: Hexagonal boron nitride is a good conductor of heat and a typical electrical insulator. The room temperature conductivity can reach 10-16 Ω· cm, 10-18 Ω· cm, and even at 1000 ℃, the resistivity still has 10-14 Ω· cm, 10-16 Ω· cm. The dielectric constant of HBN is 3-5., The breakdown strength is twice that of Al2O3, reaching 30-40kV/mm, making it an ideal high-frequency insulation, high-voltage insulation, and high-temperature insulation material.
④ Chemical properties: HBN has excellent chemical stability. It does not react with general metals, rare earth metals, precious metals, semiconductor materials, glass, molten salts, inorganic acids, and alkalis. For most metal melts, such as steel, stainless steel, Al, Fe, Ge, Cu, Ni, Zn, etc., neither wetting nor action occurs. Therefore, it can be used as a protective sleeve for high-temperature thermocouples, crucibles and vessels for melting metals, pipelines for conveying liquid metals, pump parts, molds for casting steel, and high-temperature electrical insulation materials.
