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What is 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.
Properties of Aluminium Nitride
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.
Aluminium nitride 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.
Applications of Aluminium Nitride
Metallization methods are available to allow aluminium nitride to be used in electronics applications similar to those of alumina and beryllium oxide.
Currently there is much research into developing light-emitting diodes to operate in the ultraviolet using the gallium nitride based semiconductors and, using the alloy aluminium gallium nitride, wavelengths as short as 250 nm have been reported. In May 2006 an inefficient LED emission at 210 nm was reported. The bandgap of single crystal AlN has been measured (using vacuum UV reflectivity) at 6.2 eV. This allows a wavelength of around 200 nm to be achieved, in principle. However, there are many difficulties to be overcome if such emitters are to become a commercial reality.
Industrial Use of Aluminium Nitride
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.
Epitaxially grown crystalline aluminium nitride is also used for surface acoustic wave sensors (SAW's) deposited on silicon wafers because of the aluminium nitride piezoelectric properties. Very few places can reliably fabricate these thin films. Agilent after more than a decade of research now has a RF filter used in mobile phone called the FBAR. This technology is closely associated with engineers working in the MEMS field.
This information has been sourced, reviewed and adapted from materials provided by Precision Ceramics.
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