Preparation of Boron Nitride

The traditional method for the preparation of boron nitride is by the fusion of urea with boric acid in an atmosphere of ammonia at 750 °C.54 The product from these reactions is hexagonal boron nitride with a layer structure like that of graphite. Unlike graphite, it is colorless and is not an electronic conductor. Conversion of the hexagonal form to a cubic modification requires heating at 1,800 °C at 85,000 atmospheres pressure. 

The similarity of the two structures led Croft (15) to attempt the preparation of boron nitride intercalation compounds. He used the metal halides SbCls, SbCU, AsCU, CuCl, CuCh, FeCU, Aids, and also BF3, BCls, Br2, ICl, liquid ammonia, and N2H4. The same procedure was used as in the preparation of metal chloride-graphite compounds. 

Preparation of Boron Nitride 17.3.5.4. One-Dimensional Form (Fibers)... 

The preparation of boron nitride from orthoboric acid and urea is described by T. E. O Connor [81]. Special measures are required to remove the last traces of boric oxide commercial preparations are likely to be significantly contaminated with 820 and water. Deacon and Goodman [11] found a small loss of volatiles at 373-423 K from boron nitride subjected to thermogravimetric analysis in air and a large increase in weight at 1123-1173 K, which is the temperature range of the active oxidation of BN to B2O2. 

Borazine (Figure 14.1), which was originally synthesized in 1926 by Stock et al. [9], finds considerable interest as molecular candidate for the preparation of boron nitride, beause it offers the advantages of a source of boron and nitrogen elements with the correct atomic ratio and geometry to yield polymers (i.e., polyborazylene, then boron nitride in a high ceramic yield) [8]. 

In the 1970s, Tanigushi et al. reported the preparation of boron nitride fibers from a melt-spinnable poly[B-aminoborazine] (Eq. 14.4) [25], However, Uttle information has been provided and the lack of reproducibihty in the melt-spinning process rendered the production of BN fibers difficult. 

Wohler described a simple method for the preparation of boron nitride, which had been discovered by Balmain by melting boric acid with potassium cyanide. Wohler in 1828 just failed to discover vanadium in a Mexican lead mineral (see p. 153), and failed to anticipate H. Rose in discovering niobiiun in Bavarian tantalite and pyrochlor. He prepared a specimen of niobium pentoxide and sent it to Berzelius, who could not say if it was a new substance or not. ... 

Interest in borazine-based polymers arises out of their possible utility as precursors for the preparation of boron nitride, a high value ceramic material. It was discovered that borazine readily undergoes a dehydropolymerization at 70-110 °C to afford a soluble polymer, polyborazylene, in about 80-90% yield (see Eq. 5.15) [30, 31]. 

Other uses of diborane include the preparation of boron nitride by the reaction of diborane with ammonia, as a catalyst for polymerization, and for the conversion of olefins to trialkyl bo-ranes. It is also used in the conversion of amines to amine boranes and as a selective reducing agent with carbonyl compounds such as aldehydes and ketones to form alcohols. 

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