Boron nitride is a new ceramic material with excellent properties and great development potential. Hexagonal boron nitride (h-bn) is an isomer of boron nitride. The structure of hexagonal boron nitride is similar to that of graphite, with a hexagonal layered structure, soft texture, strong workability, and white color, commonly known as “white stone ink”.
The following to introduce the commonly used method of making powder of hexagonal boron nitride.
1. Chemical vapor deposition (CVD) method. H-bn powder is generally prepared by CVD method in a hot-wall reactor. Gaseous materials containing B and N are introduced into a reaction chamber through carrier gas, and chemical reactions occur between gaseous materials at high temperature to produce BN powder. Boron sources generally use boron compounds including BF3, BCl3, BBr3, B2H6 and B(OCH3)3, while nitrogen sources are generally NH3 or N2. The purity and sphericity of h-bn powder prepared by CVD method are high.
2. Borax — urea method. Boron nitride was prepared by mixing anhydrous borax with urea (ammonium chloride) and adding heat to ammonia gas. Its reaction equation is:
Na2B4O7 + 2 (NH2) 2 co – > 4 bn + Na2O + 4 h2o + 2 co2
Nh4cl Na2B4O7 + 2 + 2 nh3 – > 4 bn + 2 nacl + 7 h2o
Borax – urea (ammonium chloride) method is a traditional method to prepare h-bn powder, which has lower production cost, less investment, simple process and is suitable for industrial production.
3. Water (solvent) thermal synthesis method. Referred to as hydrothermal method, is in the autoclave, using water (or organic solvents) as the reaction medium, by heating the autoclave, to create a high temperature, high pressure reaction environment, so that usually difficult or insoluble substances dissolve and react to form new crystals. Hydrothermal method is usually used to synthesize oxide or metallic superfine powder, but the research on the preparation of non-oxide superfine powder is still at the beginning stage. The hydrothermal process is relatively easy to control, the product size can reach nanometer level, the uniformity and sphericity are good, but the yield is generally low. Therefore, the selection of appropriate solvents, raw materials and additives to reduce the reaction temperature (under 240 ℃ to achieve large-scale production) and improve the high yield will be the focus of future research.
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