OXIDATION BEHAVIOR OF ALUMINUM NITRIDE POWDER

The oxidation kinetics of aluminium nitride powder under isothermal conditions have been studied in the temperature range 1173-1423 K. At all temperatures the results showed that initial oxidation occurred by a linear rate kinetic process followed by a parabolic kinetic process; the activation energies for these processes being 197 and 270 kJ mol(-1), respectively. However, the fraction oxidised during the lineal kinetics stage increased with increasing temperature. Systematic X-ray diffraction studies of typical oxidised powders showed that alpha alumina crystallites of 35 nm size formed, even at 1173 K, via the formation of a poorly crystalline spinel type aluminium oxynitride phase (5 nm size) which could not be detected above 1273 K. Infrared spectroscopic investigations also substantiated this finding. Scanning electron microscopy investigations showed that the morphology of the oxidation product changed fi om a fine grained and coherent structure at 1223 K to a porous fissured structure at 1423 K. Particle size distribution analyses shelved that complete oxidation at 1273 K resulted in no change in size distribution, but at 1423 K resulted, as a result of fissuring, in the formation of 20 wt-% of alpha alumina of size <1 mu m.