Boron nitride

Boron nitride is an interesting compound used in ceramics and cosmetics. It's moderately heat resistant and can withstand temperatures up to 2,800 C in the absence of oxygen. Boron nitride occurs in several different forms, which are structurally very similar to the allotropes of carbon. Graphite, diamond and lonsdaleite each have boron nitride analogs.

There are several good ways to make boron nitride. I tried a couple of them out. The reaction of urea with boron trioxide I found to be by far the most reliable, so this is the one I decided to present in this post.

The first step is to convert boric acid to the required boron trioxide.

To a metal can, I added 16g of boric acid. I then strongly heated the can over a camping stove. The boric acid began to melt and give off steam. The mixture became a glassy bubbling syrup as the boric acid was converted to boron trioxide. After about 15 minutes, the bubbling had almost completely stopped and I allowed the mixture to cool. Upon cooling, the boron trioxide solidified into a brittle glass-like substance. I scraped the product out of the can and was left with 8.6g of boron trioxide.

With boron trioxide prepared, I moved on to making the boron nitride.

I ground up the 8.6g of boron trioxide (made above) with 20g of urea. I then poured this mixture into a metal can, which I heated over a camping stove for 30 minutes with gradually increasing heat and fairly constant stirring. The mixture formed a bubbling melt, which diminished after a few minutes to a solid mixture. If the heat is increased too rapidly, a significant amount of boron trioxide can fail to react. I made this mistake, so after the 30 minutes of heating, I added an additional 10g of urea and continued heating for 10 minutes. This probably wouldn't have been necessary had I not started heating so strongly at the beginning.

Anyway after this, I allowed the mixture to cool to room temperature. I then crushed the mixture up and added it to a 150ml beaker containing 75ml of dilute hydrochloric acid. I left this to soak overnight. The next morning most of the white boron nitride had settled to the bottom of the beaker. Above it there was a thin layer of brown material. I stirred the mixture up, waited until the most of the boron nitride had settled, then carefully decanted and discarded as much of the supernatant liquid-brown material suspension as I could. I then washed the residual boron nitride with 100ml of cold water. I filtered off the boron nitride, washed it on the filter with 20ml of ethanol, then dried it. I was left with 2.15g of white boron nitride which is a 35% yield.

The procedure I was following (here) called for boiling the product in water to purify it. However I've found in previous runs that this tends to completely destroy the product. It's well known that hot water reacts with boron nitride, so I'm not sure why they suggest this method of purification.

2 B(OH)3 ==> B2O3 + 3 H2O    /   B2O3 + (NH2)2CO ==> 2 BN + 2 H2O + CO2

B2O3 + 3 C2H5OH ==> (C2H5)3BO3 + B(OH)3