Learn properties and applications of hexagonal boron nitride

Hexagonal boron nitride (H-BN) ceramics is a kind of high temperature structure ceramic material with great development potential. It is a new material with similar graphite layer structure, ivory white, commonly known as "white stone ink". It has no obvious melting point, no softening phenomenon at high temperature, and is resistant to strong acid and alkali corrosion.  

Structure of hexagonal boron nitride  

The structure of H-BN is similar to that of graphite crystal. Each layer is a six-membered ring network composed of B and N atoms arranged in turn. These six-membered ring atomic layers are arranged in ABABAB mode along the C-axis.

Layers are connected by B and N atoms of different layers through van der Waals force, with weak binding force, large distance between layers and easy to slide between layers.  Therefore, the spacing between B and N atoms in h-BN layer is 0.1446nm, and the spacing between atoms is 0.335nm, showing obvious anisotropy.  

Properties of hexagonal boron nitride  

Thermal properties of H-BN ceramics  

H-BN ceramics have excellent heat resistance, in one atmospheric pressure nitrogen atmosphere, will sublimate at 3000℃, in other inert atmosphere of the highest allowable temperature can reach 2800℃;  In neutral and reducing atmosphere, it can also withstand high temperature of about 2000℃;  Although H-BN is easily oxidized in air atmosphere, its oxidation resistance is good at 900℃, so the use temperature in air atmosphere can also reach 900℃.  

H-BN has excellent unidirectional thermal conductivity, and the thermal conductivity of h-BN ceramics in the parallel direction can reach 60W/(nvK) at room temperature.  The thermal conductivity of BeO is higher than that of BeO when the temperature is above 500℃, which is the highest among ceramic materials.  

The thermal expansion coefficient of H-BN ceramics is low, only higher than that of quartz glass, which belongs to a lower class in ceramics. Its thermal shock resistance is good, and its hot-pressed products are air-cooled to room temperature at 1200℃, and the samples will not crack after hundreds of cycles.  

Electrical properties of H-BN ceramics  

H-BN structure has no free electrons, has good electrical insulation, one of the ideal materials for high temperature insulation.  At room temperature, its breakdown voltage is 2 times of alumina ceramics, dielectric loss is very low, can penetrate infrared and microwave.  

Corrosion resistance of H-BN ceramics  

H-BN ceramics chemical properties is very stable, with most of the molten metal, glass and salt neither wetting nor reaction, has a strong acid, alkali, molten metal and glass slag erosion ability.  

Mechanical properties of H-BN ceramics  

H-BN ceramics have soft mohs hardness of 2, and the friction coefficient between cross sections y is very low, only 0.16.  It has obvious anisotropy and can be machined by turning processing equipment like graphite, and its processing accuracy can reach 0.01.

Application of hexagonal boron nitride  

Boron nitride high temperature resistance and electrical insulation performance, widely used in the lining of rocket combustion chamber, high temperature thermocouple protection sleeve, high temperature insulation parts of high frequency and high pressure welding tools, a variety of heater bushing, spacecraft thermal shielding material.

H-BN can be used as crucible for melting rare metals and precious metals in metallurgical industry because of its good chemical stability, high thermal conductivity and thermal shock resistance.  Can be used as a ceramic nozzle for amorphous materials;  Used as refractory material, heat sink, heat exchanger and other components;  In the manufacture of thin film capacitor, h-BN can be used as the evaporation device of molten aluminum, and the effect of high quality and high yield can be obtained.  

In the electronic industry, H-BN products can be used for all kinds of semiconductor packaging heat dissipation plate and phase shifter heat dissipation rod, traveling wave tube collector heat dissipation tube, etc.  H-BN ceramics can be used as window material of radar in national defense industry or composite with absorbing material to make stealth material.  H-BN ceramics can be used as shielding material for atomic reactor in atomic energy industry due to its strong neutron absorption properties.