Boron nitride is a new type of ceramic material with excellent performance and great development potential. Hexagonal boron nitride (h-BN) is an isomer of boron nitride. The structure of hexagonal boron nitride is similar to graphite, with a hexagonal layered structure, a soft texture, strong processability, and a white color, commonly known as "white graphite".
Below, we will introduce the commonly used methods for producing hexagonal boron nitride powder.
1. Chemical vapor deposition (CVD) method. The CVD method for preparing h-BN powder generally uses a hot wall reactor, where gaseous raw materials containing B and N are introduced into a reaction chamber through carrier gas. At high temperatures, chemical reactions occur between the gaseous raw materials to generate BN powder. Boron sources commonly use boron containing compounds such as 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, and multiple factors need to be controlled during the preparation process.
2. Borax urea method. Boron nitride is produced by heating and reacting anhydrous borax and urea (ammonium chloride) in an ammonia stream. The reaction equation is:
Na2B4O7+2 (NH2) 2CO → 4BN+Na2O+4H2O+2CO2
Na2B4O7+2NH4Cl+2NH3 → 4BN+2NaCl+7H2O
The borax urea (ammonium chloride) method is a traditional method for preparing h-BN powder, which has low production cost, low investment, simple process, and is suitable for industrial production. However, the purity of the synthesized boron nitride is not high, and the particle size uniformity is poor.
3. Water (solvent) thermal synthesis method. Hydrothermal method, abbreviated as hydrothermal method, is a process in which water (or organic solvents) is used as the reaction medium in a high-pressure reactor. By heating the reactor, a high-temperature and high-pressure reaction environment is created, which dissolves commonly insoluble or insoluble substances and reacts to form new crystals. The hydrothermal method is usually used to synthesize oxide or metal elemental ultrafine powders, and research on the preparation of non oxide ultrafine powders is still in its early stages. The process conditions of hydrothermal method are relatively easy to control, and the product particle size can reach the nanometer level, with good uniformity and sphericity, but the yield is generally low. Therefore, selecting appropriate solvents, raw materials, and additives to reduce reaction temperature (large-scale production can be achieved below 240 ℃) and improve yield will be the focus of future research. 
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