Effects of Si3N4 addition on formation of aluminum nitride by self-propagating combustion synthesis

An experimental study on the preparation of aluminum nitride (AlN) by self-propagating high-temperature synthesis (SHS) was conducted with powder compacts under nitrogen pressures of 0.79–4.24 MPa. Reactant compacts were diluted by 30–50 wt% of AlN to avoid the melting of aluminum during combustion. In addition, silicon nitride (Si3N4) was adopted as a solid source of nitrogen in the reactant compacts and its contribution to the improvement of nitride formation was explored. For the AlN-diluted compacts with no Si3N4 addition, a preheating temperature of 200 °C is shown to be required to achieve the self-propagating reaction in nitrogen and a large content (i.e., 50 wt%) of AlN dilution is needed to obtain a high degree of nitridation about 90%. The self-sustaining reaction proceeds with a distinct combustion front traversing the entire sample in a steady mode. The flame-front propagation velocity was found to increase with nitrogen pressure, but to decrease with diluent content. After the passage of the flame front, further nitridation caused by continuous infiltration of nitrogen gas leads to the emergence of afterburning glows. On the other hand, the addition of Si3N4 in the reactant compacts not only activates the self-sustaining reaction under the condition without prior heating, but also brings about a high product yield up to 90% in the compacts with a low content (i.e., 30 wt%) of AlN dilution. The morphology of synthesized products from AlN-diluted samples exhibits formation of AlN particles and AlN whiskers, and the former dominates in the case of diluting with 50 wt% AlN. The addition of Si3N4 was also shown to favor the production of AlN with a particulate structure.