1.Introduction of Boron nitride ceramics
There are three variants of boron nitride (BN): hexagonal, hexagonal close packed and cubic crystals. Hexagonal crystal BN is stable phase at atmospheric pressure. The hexagonal close packed and cubic crystals BN are stable under high pressure and metastable under atmospheric pressure. Boron nitride hexagonal crystals are converted to cubic boron nitride crystals or boron nitride crystals at high temperature and pressure.
Hexagonal boron nitride
Hexagonal crystal boron nitride has a layered structure similar to graphite. Its appearance is white powder and its properties have many similarities, so it is also called "white stone ink".
Boron nitride Nanopowde
2. Manufacturing technology of boron nitride ceramics
(1) Preparation of boron nitride powder
Synthesis methods of hexagonal crystal BN powder are commonly as follows:
① Boron halide method
This method is also known as the gas phase synthesis method. Boron halide reacts with ammonia to form an intermediate amino complex first, and then the intermediate is processed at high temperature to prepare BN. At 125~130℃, the intermediate product is decomposed into B2(NH)3, and when heating continues to 900~1200℃, B2(NH)3 is decomposed into BN.
②boron anhydride method
BN can be prepared by the reaction of boron anhydride with ammonia at high temperature. BN can also be prepared by the reaction of boron anhydride with sodium cyanide (calcium cyanide. BN can also be prepared by the reduction of boron anhydride with graphite crucible.
Borax-ammonium chloride method:Na2B4O7＋2NH4Cl＋2NH3—4BN＋2NaCl＋7H2O
Borax-urea method: Na2B4O7＋2(NH2)2CO—4BN＋Na2O＋4H2O＋2CO2
(2) Forming of boron nitride ceramics
Boron nitride ceramics can adopt various molding methods according to its shape and size. In order to obtain dense boron nitride products, hot pressing is used.
(3) Sintering of boron nitride ceramics
(1) The properties and uses of hexagonal boron nitride ceramics
Hexagonal crystal boron nitride, white, stable performance, good processability, can be used as high temperature lubricant; It can be used as electrical insulation material with good high temperature resistance and insulation. In the electronic industry, its thermal conductivity and penetrating performance to microwave radiation are used as radar transmission window. In the atomic energy industry, it is used as a structural material for nuclear reactors; with its high melting point, small coefficient of thermal expansion and stable performance to almost molten metals, it can be used as high temperature metal smelting crucible, heat-resistant material, heat dissipation fin and heat conduction material (the operating temperature in neutral or reducing atmosphere can be up to 2800℃). Therefore, it is also the best material for making engine parts.
In addition, it is widely used in national defense industry, space navigation, and in the metallurgical industry for billet horizontal continuous casting technology as a separation ring for the crystallizer.
(2) Properties and applications of cubic boron nitride
Cubic boron nitride ceramics are manufactured by a method similar to graphite in synthetic diamonds, made from hexagonal boron nitride crystals via high altitudes after temperature and high pressure treatment from the conversion.
Cubic boron nitride is a high temperature resistant, super hard material (Mohs hardness close to 10). Cubic boron nitride ceramics are widely used as heat-resistant and wear-resistant materials.
Boron nitride products