Formation mechanism of 21R AlN-polytypoids in aluminothermic reduction and nitridation process

Synthesis of 21R AlN polytypoids was investigated using Al and ultrafine SiO2 powder in a flowing nitrogen atmosphere by means of thermal gravity (TG), differential scanning calorimeter (DSC), X-ray diffractometer (XRD) and scanning electronic microscope (SEM) linked with energy dispersive spectrometer (EDS). The results showed that the formation mechanism of AlN polytypoids was different from that in reaction sintering process using Si3N4, AlN, Al2O3 and other sintering additive as raw materials. It was suggested that firstly Al reduce SiO2 into Si and is also nitrided into AlN, then AlN, Al2O3 and SiO2 dissolve into silicon liquid until the AlN polytypoids precipitate in saturated liquid in a flowing nitrogen atmosphere at lower than 1700 C.