Boron nitride is a new type of ceramic material with excellent performance and great potential for development. Hexagonal boron nitride (h-BN) is an isomer of boron nitride. Its structure is similar to that of graphite. It has a hexagonal layer structure, soft texture and high processability. Its color is white, commonly known as “white graphite.”
The h-BN powder prepared by the CVD method generally adopts a hot wall reactor, and the gaseous raw materials containing B and N are introduced into a reaction chamber through a carrier gas, and a chemical reaction occurs between the gaseous raw materials at a high temperature to form a BN powder. Among them, boron sources generally use boron-containing compounds such as BF3, BCl3, BBr3, B2H6 and B(OCH3)3, and the nitrogen source is generally NH3 or N2. The purity and sphericity of the h-BN powder prepared by the CVD method are high, and various factors need to be precisely controlled in the preparation process.
Borax – Urea Method
The boron hydride is mixed with urea (ammonium chloride) and heated in an ammonia gas stream to obtain boron nitride. The reaction equation is:
The borax-urea (ammonium chloride) method is a traditional method for preparing h-BN powder, which has low production cost, low investment, and simple process. This method is more suitable for industrial production because the boron nitride obtained by the synthesis is not high in purity and has poor particle size uniformity.
The Hydrothermal Method
The water (solvent) thermal synthesis method, referred to as the hydrothermal method, is to use a water (or organic solvent) as a reaction medium in an autoclave to heat a high-pressure autoclave to create a high temperature and high-pressure reaction environment. In such an environment, substances that are generally poorly soluble or insoluble can be dissolved and reacted to form new crystals. The hydrothermal method is usually used to synthesize oxide or metal element ultrafine powder, and research on preparation of non-oxide superfine powder is still in its infancy. The process conditions of the hydrothermal method are relatively easy to control, the product particle size can reach nanometer scale, the uniformity and the sphericity are good, but the yield is generally low. Therefore, the selection of suitable solvents, raw materials and additives to reduce the reaction temperature (large-scale production below 240 °C) and increase the yield will be the focus of future research.
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