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Other Synthetic Methods of Boron Nitride

wallpapers Industry 2020-10-29
Borax-urea Method
This method uses borax (Na2B4O7) as the primary boron source. It uses urea to form an ammonia atmosphere and form a sponge-like porous body with borax to promote the complete reaction. Figure 3 is a process flow diagram of this method. Borax and urea are evenly mixed in a ratio of 1:1.5:2 and then loaded into the container and sent to the reactor to participate in the reaction. The box can be boats and crucibles made of quartz glass, corundum, graphite, stainless steel, etc. The reaction furnace is generally a horizontal tubular furnace, such as a small rotary kiln that can achieve continuous production. During the reaction, nitrogen or ammonia is introduced to supplement the shortage of ammonia gas formed by urea.
 
Using ammonia is better than using nitrogen because nitrogen is a sub-inert gas, and the dissociation of ammonia can generate monoatomic active nitrogen. But below 300°C, nitrogen should be used first because ammonia will dissolve in the free water or steam generated by the reactant, which will weaken the reaction. Ammonia starts to decompose at 200~300℃ and participates in the response while decaying during heating. The reaction furnace should be heated according to the specified heating curve. After comparison, the four-stage heat preservation effect is better, that is, heat preservation at 100℃ for 0.5h, 140℃ for 2h, 180℃ for 2h, 800℃ for 2h, and the final temperature is 800~ 10000°C. During the nitriding reaction, the shape of the mixture gradually changed from viscous flour to lose honeycomb.
 
Arc Plasma Method
The primary process of synthesizing boron nitride with borax-urea arc plasma is as follows: After borax is dehydrated, it is uniformly mixed with urea at a mass ratio of 1:1.4 to 1.6, pressed into a compact and installed on the graphite plate bed, placed in the reactor High-temperature reaction zone. The arc plasma flame formed by nitrogen coming out of the plasma spray gun is directed to the reaction zone, causing the green body to be heated and react violently to synthesize boron nitride.
 
The reactor's inner wall is lined with graphite tubes to withstand the high temperature of the plasma flame. There are zirconia or alumina hollow balls, alumina foam bricks, refractory fibers, and steel shells from the graphite tube to the outside. The plasma spray gun's cathode material is a cerium-tungsten alloy, and the anode material is copper. When the working voltage is 170V, the operating current is 200A. The N2 gas passes through the spray gun in a precessing manner at a flow rate of about 100L per minute, the temperature of the inner wall of the graphite tube in the reaction zone is above 1800℃, and the reaction zone is 130mm in diameter and 300mm in length. The area contains 150mm×30mm×15mm compacts, and the synthesis reaction can be completed with only 20~30mm, and the recovery rate is 70%~85%. The synthetic crude BN is purified by acid washing and water washing to increase the purity to more than 98%.


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Tag: Boron Nitride