Aluminium nitride, AlN, discovered by Mallet, may be prepared by heating finely divided aluminium in nitrogen at 820° to 1000°, powdering the product, and reheating in the gas once or twice. It is produced when aluminium is heated to 700° in ammonia, and is manufactured by heating a mixture of alumina and carbon in a current of nitrogen: -
Al2O3 + 3C + N2 ⇔ 2AlN + 3CO.
In Serpek's process, a mixture of crushed alumina or bauxite and carbon travels down an inclined rotary kiln, drops into a hopper, and is fed into another rotary kiln, a short length of which is maintained at 1500° to 1800°. Producer gas passing up to the kiln supplies the requisite nitrogen. The carbon monoxide produced is burned and the hot gases blown up the first kiln to preheat the initial charge.
As usually prepared, aluminium nitride forms a grey, amorphous solid. It begins to sublime, with partial dissociation into its elements, at c. 1900° C.; the vapour condenses to colourless, hexagonal needles. It is decomposed by water, slowly at 0°, rapidly at 100°, aluminium hydroxide and ammonia being produced; consequently it dissolves readily in alkali hydroxides. |