Ceramic Materials: Structure and Properties of Hexagonal Boron Nitride

Overview of Hexagonal Boron Nitride

Hexagonal boron nitride (HBN) is a white powder. Its crystal structure and physicochemical properties are very similar to graphite, so hexagonal boron nitride is also called “white graphite”.

Hexagonal boron nitride is a new type of composite material. It not only has the advantages of low density, high melting point, low hardness, thermal vibration resistance and good machinability, but also has excellent properties such as high temperature resistance, small thermal expansion coefficient, high thermal conductivity and low dielectric constant. Therefore, HBN has important applications in thermal conductivity, lubrication, hydrogen storage, battery separator materials, high temperature anti-oxidation coatings, catalysis and other fields.

Hexagonal boron nitride is also called high temperature boron nitride. When it is heated to 1000℃ in air, various physical and chemical properties remain basically unchanged; when used in nitrogen or argon at a temperature of 3000℃, HBN is still stable.

Structure of Hexagonal Boron Nitride

Hexagonal boron nitride belongs to the hexagonal crystal system, which is stacked by a multi-layer structure. B-N-B between different layers is linked by van der Waals force. Its lattice constants a=0.2506±0.0002nm, c=0.667±0.0004nm, density ρ=2.25 g/cm3.

Hexagonal boron nitride is very stable in air, it has a wide band gap (5.1eV) and high hardness (Mohs hardness 2), can withstand high temperature of 2270 °C, and will not sublime until about 3270 °C. At the same time, hexagonal boron nitride has the advantages of good insulation, thermal conductivity and chemical stability, low thermal expansion/contraction rate, etc., and does not react with weak acids and strong bases at room temperature.

Properties of Hexagonal Boron Nitride

Hexagonal boron nitride has high thermal conductivity and high heat resistance. At the same time, it has excellent physical properties such as good lubricity, low coefficient of friction, low coefficient of thermal expansion, excellent dielectric properties, and excellent chemical properties such as strong oxidation resistance, strong corrosion resistance, and stable chemical properties.

(1) High heat resistance. Hexagonal boron nitride (h-BN) will not sublime when heated in 0.1Mpa nitrogen gas until above 3000℃. Its strength at 1800 °C is twice that of room temperature, so it has excellent thermal shock resistance, and it will not crack after dozens of times of air cooling to room temperature at 1500 °C.

(2) High thermal conductivity. The thermal conductivity of hexagonal boron nitride products is about 33W/m·k, which has a high thermal conductivity similar to that of stainless steel.

(3) Low expansion coefficient. The linear expansion coefficient of hexagonal boron nitride is (2.0~6.5)*10-6/℃, second only to quartz glass, and it has high thermal conductivity, so it has excellent thermal shock resistance.

(4) Excellent electrical insulation. Hexagonal boron nitride has good high temperature insulation, and the maximum volume resistivity of high-purity hexagonal boron nitride can reach 1016~1018Ω•cm, even at a high temperature of 1000 ℃, the value can still maintain 104~106Ω•cm.

(5) Good corrosion resistance. Hexagonal boron nitride has good chemical stability and is not wetted by most molten metals, glass and salts, so it has high resistance to acid, alkali, molten metal and glass erosion, and has good chemical inertness.

(6) Lower friction coefficient. Hexagonal boron nitride has excellent lubricating properties, its friction coefficient is only 0.16, and does not increase at high temperatures. It can be used up to 900°C in an oxidizing atmosphere and 2000°C in a vacuum.

(7) Machinability. Hexagonal boron nitride is very easy to use conventional metal cutting technology to finish the product, and the turning accuracy can reach 0.05mm, so the hexagonal boron nitride blank can be processed to obtain complex shape products.