SYNTHESIS OF AIN POWDER BY CARBOTHERMAL REDUCTION-NITRIDATION METHOD - EFFECT OF ADDITIVES ON REACTION-RATE

The effect of additives on carbothermal reduction-nitridation was inve stigated for the preparation of AlN powders from the A12O3-C-N2 system . According to the role of additives in nitridation, they were divided into three categories: ''promote'', ''have no effect'' and ''disturb' '. In particular, CaF2 was an excellent nitridation promoter providing spherical AlN particles. The apparent activation energy of nitridatio n for CaF2 addition was 190 kJ/mol, while those for Y2O3, Yb2O3 and wi thout additive were around 250 kJ/mol. CaF2, Y2O3, Yb2O3 and transitio n metal compounds strongly affected AlN grain morphologies, supplying spherical and rod-like shapes of AlN. It was estimated that the reduct ion-nitridation proceeds through the formation of intermediates like C aF2.5Al2O3 and Y-aluminates.