What is Hexagonal boron nitride (h-BN)
Hexagonal boron nitride (h-BN) is a crystalline structure composed of nitrogen and boron atoms arranged in a hexagonal lattice. It is the only phase of boron nitride that exists naturally in th
e environment. It appears as a soft, loose, smooth, and moisture-absorbing white powder, and it shares similar layered structure characteristics with graphene, earning it the nickname "white graphite."
Properties of hexagonal boron nitride:
1. High heat resistance: Hexagonal boron nitride (h-BN) sublimes only at temperatures above 3000°C in a 0.1 MPa nitrogen atmosphere. Its strength at 1800°C is twice that at room temperature, demonstrating excellent thermal shock resistance. It can undergo multiple cycles of cooling from 1500°C to room temperature without cracking.
2. High thermal conductivity: The thermal conductivity of hexagonal boron nitride products is approximately 34 W/m·K, similar to stainless steel, indicating high thermal conductivity.
3. Low coefficient of thermal expansion: The coefficient of thermal expansion of hexagonal boron nitride is (2.0~6.5) * -10^6/°C, second only to quartz glass. Combined with its high thermal conductivity, it exhibits excellent thermal shock resistance.
4. Excellent electrical insulation: Hexagonal boron nitride has good high-temperature insulation properties. The maximum volume resistivity of high-purity hexagonal boron nitride can reach 10^16-10^18 Ωm. Even at 1000°C, it still maintains resistivity in the range of 10^4-10^6 Ωm.
5. Good corrosion resistance: Hexagonal boron nitride exhibits excellent chemical stability and is not wetted by most molten metals, glasses, and salts. It has high resistance to acid, alkali, molten metals, and glass, displaying good chemical inertness.
6. Low coefficient of friction: Hexagonal boron nitride has excellent lubricating properties with a friction coefficient of 0.16. It does not increase at high temperatures and has a higher temperature resistance than molybdenum disulfide and graphite. It can be used in oxidizing atmospheres up to 900°C and in a vacuum up to 2000°C.
7. Machinability: Hexagonal boron nitride can be easily processed using conventional metal cutting techniques, achieving a machining accuracy of up to 0.05 mm. Complex-shaped products can be produced from hexagonal boron nitride blanks.
Applications of boron nitride ceramics made from hexagonal boron nitride:
1. Due to the excellent chemical stability of hexagonal boron nitride, it can be used as crucibles, boats, liquid metal transfer tubes, rocket nozzles, high-power device bases, pipes for molten metal, pump components, and molds for casting steel.
2. Hexagonal boron nitride's heat and corrosion resistance make it suitable for manufacturing high-temperature components, liners for rocket combustion chambers, thermal shielding for spacecraft, and corrosion-resistant parts for magnetic flux generators.
3. The insulation properties of hexagonal boron nitride find wide applications in high-voltage, high-frequency electrical insulation, insulation materials for plasma arcs, insulators for various heaters, sheaths for heating tubes, and high-temperature, high-frequency, high-voltage insulation and heat dissipation components in high-frequency induction furnaces.
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