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Commonly Used Synthetic Methods of Boron Nitride

wallpapers Industry 2020-10-29
Since boron nitride was first synthesized in 1842, many synthesis methods have appeared one after another. Boron nitride is generally synthesized from elemental boron, boron anhydride, boron halide, boron salts, and nitrogen-containing salts in a nitrogen or ammonia atmosphere through gas-solid or gas-gas reactions. At present, the borax-ammonium chloride method and boric anhydride method are mainly used in industrial production.
 
1. Borax-ammonium chloride method
This method uses borax and ammonium chloride as primary raw materials to synthesize boron nitride in an ammonia atmosphere. The process flow is shown in Figure 1. The two raw materials should be dehydrated and recrystallized separately before participating in the reaction. Borax is best dehydrated in a vacuum at 200~400℃. Recrystallization of ammonium chloride dissolves it into a saturated solution and then filter to remove impurities and then crystallize out. It can be repeated several times, depending on the purity requirements. The crushed and dried borax and ammonium chloride are mixed in a mass ratio of 7:3, pressed into a compact, and sent to a synthesis reactor. To speed up the reaction and increase the conversion rate, it is necessary to pass in ammonia (NH3) to make up for reactants' insufficiency forming an ammonia atmosphere. At low temperatures, the amount of ammonia introduced is less in the higher temperature stage. The specific amount of ammonia raised should be adjusted according to the number of reactants and the reactor's volume to ensure that the reaction proceeds altogether. The final heating temperature is 900~1000℃, and heat preservation is 6h. The reaction product is immersed in water to remove the remaining impurities, such as boric acid and sodium chloride. It is then dried and crushed to obtain boron nitride powder with a mass fraction of 96%#98%.
 
2. Boron anhydride method
Nitriding and synthesizing boron nitride with boron anhydride (B4O3) is one of the essential methods for boron nitride's industrial production. The process flow of this method is shown in Figure 2. Due to the low melting point of boric anhydride (the glassy state is 294°C, the crystalline form is 450~600°C), it becomes a high-viscosity melt at the nitriding temperature, which hinders the flow of ammonia. The reaction is slow and too incomplete. To overcome this shortcoming, high melting point substances can be used as fillers to reduce the boric anhydride melt's viscosity. This filler does not participate in the reaction and can be easily removed at the end. Commonly used fillers are magnesium oxide (MgO), calcium carbonate (CaCO3), tricalcium phosphate [Ca3(PO4)2], boron nitride (BN), etc. Among them, tricalcium phosphate is the best because tricalcium phosphate and boron A mixture of anhydrides has the highest nitridation reaction rate.


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Tag: Boron Nitride