Effect of NH4F additive on purification of AlN ceramics

In order to overcome the purification difficulty of aluminum nitride (AlN) ceramics, the sintering of AlN ceramics with ammonium fluoride (NH4F) as an additive had been studied. The results demonstrate that the addition of NH4F evidently affects the phase compositions, the microstructure of grains and the contents of oxygen and nitrogen in the AlN sintered samples. NH4F not only removes oxygen out of AlN grains but also reduces the total oxygen content in AlN ceramics. It is found that relatively high purity of AlN can be acquired when the molar ratio of NH4F/O (oxygen element in raw AlN powder) increases to 0.8. With adequate amount of NH4F, the Al-O-N phases are removed. SEM and TEM results show the hexagonal structures of AlN grains with clean triple-grain junctions. The oxygen content decreases to 0.55 wt% and nitrogen content increases to 33.7 wt%. Thermodynamic analysis illustrates the oxygen removing effects of NH4F by the reaction of NH3 and Al2O3, which inhibits the formation of Al-O-N. NH4F should be at least 2/3 of the oxygen content.