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

The particle size and morphology of AlN product powders from carbothermal synthesis were studied by scanning electron microscopy, surface-area measurement and microarea elemental analysis. AlN powders with widely variable grain size and morphology could be produced by varying the pressure, gas-flow rate and alumina type. Discrete AlN particles (about 0.5-3 micron) were obtained by using fine gamma-Al2O3 as a starting material at 1600 C over the pressure and gas-flow rate ranges 0.1-5 MPa and 1-10 l/min. When coarse alpha-Al2O3 was used as raw material, large aggregates of AlN particles normally formed, unless the reaction was run at high nitrogen pressure (5 MPa) and low gas flow rate (1 l/min). Occasionally, whiskers formed at relatively low temperatures and pressures, e.g. 1500 C and less than 0.5 MPa. It was possible to obtain a discrete and fine AlN powder (about 0.5 micron) 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 AlN. 3 refs.