Combustion synthesis of hexagonal aluminum nitride crystal by aluminum carbides

AlN hexagonal crystals were synthesized by self-propagating combustion of Al4C3 powders under low nitrogen pressure (0.3 MPa) via the reaction: Al4C3 + 2N2 → 4AlN + 3C. The combustion temperature measured for various supplied powers was about 2373–2473 K, which agreed well with the calculated adiabatic temperature (Tad). The effects of supplied power on the combustion were investigated. When the supplied power was above 650 W, the combustion wave propagated successfully. The decrease in ignition time with increasing supplied power was observed. Also, increasing the supplied power raised the combustion temperature slightly. In addition, preheating the reactant produced a nearly pure AlN product and resulted in more amounts of hexagonal AlN crystals. On another experiment, a thin layer of Al4C3 powders was combusted and the results showed AlN crystals were more dispersed with well-faceted hexagonal morphology.A quenched experiment was carried out to study the mechanism of the combustion synthesis. At the transition zones near combustion wavefront, no other intermediates were observed, suggesting that Al4C3 did not decompose in the course of combustion. Hence, it is proposed that carbon atoms of Al4C3 are substituted directly by nitrogen atoms to form a product composed of a mixture of AlN and carbon.