Formation and evolution of quasi-aligned AlN nanowhiskers by combustion synthesis

Quasi-aligned AlN nanowhiskers were formed by combustion synthesis (CS) at a low nitrogen gas pressure of 0.25 MPa. The starting charge consists of Al and AlN diluent (40/60 mol%) with a mixture of CaO and NH₄Cl additives (5 wt.% each). The CS proceeded in a mild way with the combustion temperature of 1530 °C and the combustion speed of 0.24 mm/s. The uniform quasi-aligned nanowhiskers were formed inside the reacting Al particles covered with a thin crust. In addition, the nanowhiskers, which are single-crystalline hexagonal wurtzite structure growing along [0 0 1] direction, have the diameters in the range of 60-150 nm and a length of several to several tens of micrometers. A quenching technique was used to investigate the phase transformation and morphologies evolution during the CS of AlN nanowhiskers. In addition, the possible intermediate reactions during the CS process were confirmed by thermodynamic analysis. The results indicated that the formation of quasi-aligned AlN nanowhiskers involves the dissociation of NH₄Cl, formation of micro-reactors (Al/C₁₂A₇ sintered AlN core-crust spheres), nucleation and growth of one-dimensional nanowhiskers in the micro-reactors mostly by VS mechanism.