Boron nitride, BN

Boron unites directly with nitrogen at a red heat to form a Boron nitride, BN, (Wohler and Deville; Moissan). Boron nitride was discovered by Balmain in 1842 by fusing boron sesqui-oxide with potassium cyanide, but its true nature was determined and the compound prepared in a nearly pure state by Wohler in 1850. His method consisted in heating an intimate mixture of anhydrous borax (1 pt.) and ammonium chloride (2 pts.) to bright redness in a platinum crucible, boiling the powdered product with very dilute hydrochloric acid as long as boric acid could be extracted, washing and drying the residue. Another method of preparation consists in heating a mixture of boron sesqui-oxide (4 pts.) and carbon (1 pt.) to whiteness in a current of nitrogen. The nitride is produced when boron is heated to redness in ammonia, and may be more readily prepared by heating either boron sesqui-oxide or anhydrous borax in the same gas. Numerous other methods have been proposed, as, for example, heating boron sesqui-oxide or borax with potassium ferrocyanide, potassium cyanide, mercuric cyanide, or urea.

As a convenient method of preparation, Moeser and Eidmann recommend passing ammonia over a strongly heated and previously fused mixture of boron sesqui-oxide and calcium phosphate. The product, when washed with dilute hydrochloric acid, is nearly pure boron nitride. To obtain the nitride in a state of purity, boron tribromide should be slowly dropped into liquid ammonia, the excess of solvent removed by evaporation, and the solid residue gradually heated to 750°. The imide of boron initially formed becomes converted into nitride: -

B₂(NH)₃ = 2BN + NH₃.

The method given by Wohler and Deville for the preparation of boron nitride has been studied by Stahler and Elbert, with respect to its utilisation for the fixation of atmospheric nitrogen. When a mixture of boron sesqui- oxide and carbon is heated in nitrogen, the yield of boron nitrogen varies with the temperature and the pressure of the nitrogen. At a pressure of one atmosphere, the best yield, 26 to 28 per cent, of nitride, is obtained at 1500° to 1700°; but at a pressure of 70 kilos per square centimetre, a yield of more than 85 per cent, is obtained at 1600°. When, however, boron sesqui-oxide is replaced by borocalcite, CaB₄O₇, a nearly theoretical yield of boron nitride, according to the equation

CaB₄O₇ + 8C + 3N₂ ⇔ 4BN + CaCN₂ + 7CO,

is obtained, even at one atmosphere pressure, by heating first to 1850° and subsequently lowering the temperature to 1400°; increasing the pressure of the nitrogen has practically no effect on the yield.

Boron nitride is a light, white, amorphous solid, soft, like talc, to the touch. It can be compressed, wet or dry, into blocks of considerable rigidity. It is infusible at the melting-point of tungsten, but above 1500° it commences to dissociate in vacuo into boron and nitrogen. At 1220°, however, the dissociation pressure does not exceed 9.4 mm. At high temperatures it is the best insulator known. Heated in a flame it exhibits a greenish-white fluorescence.

Boron nitride is a very stable compound. It is very slowly decomposed by boiling water, aqueous potash, hydrochloric and nitric acids, but decomposes more readily when heated in steam or at 200° in a sealed tube with water, hydrochloric or sulphuric acid: -

BN + 3H₂O = H₃BO₃ + NH₃.

Heated with fused potassium hydroxide, ammonia and potassium borate are produced; with fused potassium carbonate the products are potassium cyanate, cyanide, and borate. It is little affected by heating with oxygen, iodine, hydrogen, carbon dioxide, or carbon disulphide. In an alcohol flame fed with oxygen it burns to boron sesqui-oxide, and at high temperatures it is decomposed by chlorine. It is slowly dissolved by hydrofluoric acid, ammonium borofluoride being produced. The oxides of copper, cadmium, mercury, arsenic, antimony, and bismuth are reduced when heated with boron nitride, the products being metal, metallic borate, and nitrous oxide; sulphates are reduced to sulphides; but the oxides of zinc and iron are not reduced.

Boron nitride prepared by heating boron imide at 125° to 130° for a long time is, according to Stock and Blix, much more reactive than the nitride prepared at high temperatures, and they suggest that the latter is a polymer of the former.