Aluminum nitride (AlN) is an extremely wide bandgap semiconductor material that can be used for deep ultraviolet optoelectronics. It is nontoxic, highly insulating and thermally conducting and has good corrosion resistance. It has also been used for sealing in electrical devices.
AlN is typically synthesized by combining an aluminum powder with a reducing agent. This process is known as carbothermal reduction. However, this method is not easy to control and does not allow for the production of large-sized crystals.
Moreover, synthesis by this method results in a high loss of purity. This is due to the combustion heat of carbon in the sample. Therefore, a lower temperature should be used.
To overcome this problem, we investigated the use of NH4Cl as an additive in a low-temperature synthesis method for aln without the need of special equipment. We found that the addition of NH4Cl to the starting Al powder allowed the AlN formation to begin at temperatures about 200 degC lower than when only Al powder was heated under a nitrogen stream.
The influence of cyclic oxidation on the AlN coating was investigated by using FeCrAl and AlN-coated APMT as model materials. These models were characterized by fast thermal cycling, which mimics real operating conditions. During cyclic oxidation, the AlN layer is progressively attacked by iron and chromium oxides. Depending on the Al concentration, a pure alumina layer may be formed under these conditions.
Cyclic oxidation of the coated APMT is less severe than cyclic oxidation of FeCrAl. This is likely because the APMT was oxidized at a slightly lower temperature than FeCrAl, which resulted in a thinner oxide layer.