CARBOTHERMAL SYNTHESIS OF ALUMINUM NITRIDE AT ELEVATED NITROGEN PRESSURES .2. EFFECT OF PROCESS PARAMETERS ON PARTICLE-SIZE AND MORPHOLOGY

The particle size and morphology of AIN product powders from carbothermal synthesis have been studied by scanning electron microscopy, surface-area measurement and microarea elemental analysis. AIN powders with widely variable grain size and morphology could be produced by varying the pressure, gas-flow rate and alumina type. Discrete AIN particles (approximately 0.5-3 mum) were obtained by using fine gamma-Al2O3 as starting material at 1 600-degrees-C over the pressure and gas-flow rate ranges 0.1-5 MPa and 1-10 l min-1. When coarse alpha-Al2O3 was used as raw material, large aggregates of AIN particles normally formed, unless the reaction was run at high nitrogen pressure (5 MPa) and low gas-flow rate (1 l min-1). Occasionally, whiskers formed at relatively low temperatures and pressures, e. g. 1 500-degrees-C and < 0.5 MPa. It was possible to obtain a discrete and fine AIN powder (approximately 0.5 mum) without seeding. The results suggest that reactions including aluminium-containing gas species play an important role in the control of morphology in carbothermal synthesis of AIN.