PARAMETRIC STUDY OF THE PLASMA SYNTHESIS OF ULTRAFINE SILICON-NITRIDEPOWDERS

The high-temperature plasma synthesis of ultrafine silicon nitride (Si 3N4) powders through the vapour-phase reaction between SiCl4 and NH3 i n an Ar/H-2 radio frequency (r.f.) inductively coupled plasma was inve stigated. The experiments were carried out at a 25-39 kW plate power l evel and at atmospheric pressure. Special attention was paid to the in fluence of the reactor wall temperature and plasma operating condition s on the quality of the powder. With a cold-wall reactor, the powders obtained were white to light brown in colour and were com posed of cry stalline, amorphous and Si3N4 whisker phases. Both alpha and beta-Si3N 4 were present in these products. The NH4Cl, formed as a by-product of the reaction, could be eliminated from the Si3N4 by thermal treatment . The BET specific surface area of the powder after thermal treatment was about 60 m(2) g(-1). The use of the hot-wall reactor resulted in a considerable reduction in the amount of NH4Cl remaining in the powder (less than 1 wt %) and a considerable increase in the fraction of the powder obtained in crystalline form. These powders were composed of a mixture of amorphous phase and 30 wt % or more of the alpha and beta- Si3N4 crystalline phases. The BET specific surface area of the powder after thermal treatment was found to be 40 m(2) g(-1). The experimenta l results are discussed in relation to their use for optimizing reacto r design for the vapour-phase synthesis of ultrafine ceramic powders.