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The difference between hexagonal boron nitride and cubic boron nitride

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Author : NANOTRUN
Update time : 2021-10-14 09:08:54
The difference between hexagonal boron nitride and cubic boron nitride
Boron nitride is a crystal composed of nitrogen atoms and boron atoms. The chemical composition is 43.6% boron and 56.4% nitrogen, with four different variants: hexagonal boron nitride (HBN), rhombohedral boron nitride (RBN), cubic boron nitride (CBN), and wurtzite nitrogen Boron (WBN). The cubic crystal variant (the alternate form of wurtzite of boron nitride) is considered the hardest known substance. It is widely used in the manufacture of alloys, high-temperature resistant materials, semiconductors, nuclear reactors, lubricants, etc.
1. Hexagonal Boron Nitride
Hexagonal boron nitride (H-BN), also known as "white graphene", is a two-dimensional material with a monoatomic layer composed of nitrogen atoms and boron atoms alternately. The layered arrangement is AA'AA'……type, and its structure is Graphene is similar, so it also has many properties similar to graphene.
Hexagonal boron nitride (H-BN) is a graphite lattice, which is in the form of a loose, lubricating, moisture-absorbing, light-weight, insoluble, high-temperature-resistant white powder. Hexagonal boron nitride has no obvious melting point and has chemical resistance properties, and does not react with general metals, rare earth metals, precious metals, glass, molten salts, etc.
Hexagonal boron nitride is an excellent insulator with high thermal conductivity, high oxidation resistance, high chemistry, thermal stability, and good mechanical properties. It is used in composite material modification, sensors, field emission devices, ultraviolet laser devices, and anti-oxidation coatings. Layers and many other aspects have important applications.
2. Cubic Boron Nitride
Cubic boron nitride (C-BN) is a synthetic material whose hardness is second only to diamond. It is a superhard material and is synthesized from hexagonal boron nitride and a catalyst under high temperature and high pressure.
Cubic boron nitride is available in black, amber, and metal-plated on the surface. The particle size is usually below 1mm. It has high thermal stability and chemical inertness, so it is generally used as a material for making super-hard tools.
Therefore, the main difference between hexagonal boron nitride and cubic boron nitride lies in the physical properties:
Hexagonal boron nitride is white, while cubic boron nitride is mostly black, amber, etc.;
Hexagonal boron nitride is soft, while cubic boron nitride is very hard;
Hexagonal boron nitride can be used as raw material to synthesize cubic boron nitride;
Hexagonal boron nitride has a wide range of uses, and cubic boron nitride is generally used to make CBN tools.
 

 
The structure and properties of hexagonal boron nitride
From the molecular structure diagram, hexagonal boron nitride belongs to the hexagonal crystal system and has the same hexagonal crystal structure as graphene. It is stacked by a multilayer structure. The BNB between different layers is linked by van der Waals force, and its crystal lattice The constant a=0.2506±0.0002nm, c=0.667±0.0004nm, the density ρ=2.25g/cm3.
Cubic boron nitride is very stable in the air, has a wide bandgap (5.1eV) and high hardness (Mohs hardness 2), can withstand high temperatures of 2270°C and will sublime at around 3270°C. At the same time, hexagonal boron nitride has the advantages of good insulation, thermal conductivity and chemical stability, and low thermal expansion/shrinkage rate, and it does not react with weak acids and strong bases at room temperature.
The properties of hexagonal boron nitride
Because of the molecular structure characteristics of hexagonal boron nitride, it has many excellent properties, such as high thermal conductivity, high heat resistance, good lubricity, low friction coefficient, low thermal expansion coefficient, excellent dielectric properties, and other physical properties. At the same time, it also has chemical properties such as strong oxidation resistance, strong corrosion-resistance, and stable chemical properties.
(1) High heat resistance. Hexagonal boron nitride (h-BN) will be sublimated only when heated to 3000 or above in 0.1Mpa nitrogen. Its strength at 1800 is twice that of room temperature, so it has excellent thermal shock resistance. It is cooled to 1500 in the air. There will be no rupture at room temperature dozens of times.
(2) High thermal conductivity. The thermal conductivity of hexagonal boron nitride products is about 33W/m·k, which has a thermal conductivity similar to that of stainless steel, and the thermal conductivity is larger.

(3) Low expansion coefficient. The linear expansion coefficient of hexagonal boron nitride is (2.0~6.5)*10-6/, second only to quartz glass. In addition, 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, it is still 104~106Ω•cm.
(5) Good corrosion resistance. Hexagonal boron nitride has good chemical stability and is not wetted by most molten metals, glasses, and salts. Therefore, it has high resistance to acid, alkali, molten metals and glass, and has good chemical inertness.
(6) Lower friction coefficient. Hexagonal boron nitride has excellent lubricating properties, with a friction coefficient of 0.16, does not increase at high temperatures, and has higher temperature resistance than molybdenum disulfide and graphite. The oxidizing atmosphere can be used up to 900°C, and it can be used up to 2000°C under vacuum.
(7) Machinability. Hexagonal boron nitride is very easy to use conventional metal-cutting technology to finish the products, and the turning accuracy can reach 0.05mm, so the complicated shape products can be processed from the hexagonal boron nitride blank.
 
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