Aluminium Nitride AlN
Aluminium nitride (AlN) is an interesting material and is one of the best materials to use if high thermal conductivity is required. When combined with its excellent electrical insulation properties, aluminium nitride is an ideal heat sink material for many electrical and electronic applications.
Aluminium nitride is a (mostly) covalently bonded material, and has a hexagonal crystal structure which is isomorphic with one of the polytypes of zinc sulfide known as wurtzite. The space group for this structure is P63mc.
The material is stable at very high temperatures in inert atmospheres. In air, surface oxidation occurs above 700 °C, and even at room temperature, surface oxide layers of 5-10 nm have been detected. This oxide layer protects the material up to 1370 °C. Above this temperature bulk oxidation occurs. Aluminium nitride is stable in hydrogen and carbon dioxide atmospheres up to 980 °C.
The material dissolves slowly in mineral acids through grain boundary attack, and in strong alkalis through attack on the aluminium nitride grains. The material hydrolyzes slowly in water. Aluminium nitride is resistant to attack from most molten salts including chlorides and cryolite.
AlN is synthesised by carbothermal reduction of alumina or by direct nitridation of aluminium. The use of sintering aids and hot pressing is required to produce a dense technical grade material.
Metallization methods are available to allow aluminium nitride to be used in electronics applications similar to those of alumina and beryllium oxide.
Among the applications of aluminium nitride are:
- Dielectric layers in optical storage media
- Electronic substrates, chip carriers where high thermal conductivity is essential
- Military applications