Combustion synthesis of hexagonal aluminum nitride powders under low nitrogen pressure

The synthesis of aluminum nitride (AlN) powders was carried out by combustion of aluminum (Al) powder under low nitrogen pressure (less than or (double) equal 0.5 MPa) with carbon black (CB) as the dispersion agent to prevent the coalescence of the aluminum. The combustion was successful when the weight ratio between CB and Al, CB/Al, ranged from 0.02 to 1.5, while the combustion failed at CB/Al=0 (pure Al) or 1.75. As CB/Al varied, different combustion behaviors were observed: steady, unsteady and spin combustion. The combustion temperature and velocity reached the respective maximum at an intermediate CB/Al value (0.2). The measured combustion temperature quantitatively agrees with the adiabatic temperature calculated from a thermodynamic analysis for various CB/Al. The as-synthesized AlN product had various morphologies: aggregated fine particles, flakes, aggregated faceted particles, honeycomb-like microstructure, as well as hexagonal crystals. It was found that the hexagonal crystal structure was favored when CB/Al=0.2 and 0.5. The combustion temperature at these ratios was larger than those synthesized at different CB/Al ratios. The reaction yield monotonically increased with the increasing CB/Al up to a ratio of 1.25. The maximum reaction yield at CB/Al=1.25 did not correspond to the largest combustion temperature. An increase of nitrogen pressure greatly increased both the combustion temperature and velocity, and the reaction yield was moderately enhanced. The experimental reaction yield agreed well with the theoretical reaction yield based on a diffusion-limited core-shell model