CARBOTHERMAL NITRIDATION SYNTHESIS OF ALPHA-SI3N4 POWDER FROM PYROLYSED RICE HULLS

摘要

The carbothermal nitridation synthesis of alpha-Si3N4 was studied using a high-temperature tube furnace to react a precursor, comprised of pyrolysed rice hulls (C/SiO2) and additive ''seed'' Si3N4, with N-2. The experimental design for synthesis was a three-level factorial surface response design for determining the effect of temperature (1300-1380 degrees C) and reaction time (1-5 h) on kinetics. In addition, all precursors were reacted at 1460, 1480 and 1500 degrees C for 5 h in order to ensure high conversion suitable for product powder evaluation (composition and morphology). Following excess carbon removal, the product Si3N4 was > 95% alpha-phase and had a surface area of 7.7 m(2)g(-1) with an oxygen content of 3.6 wt% O. The powder was comprised of a bimodal size distribution of submicrometre solid alpha-Si3N4 crystallites centred at 0.03 and 0.22 mu m. No whiskers or high aspect ratio elongated crystallites were found in the powder. The addition of carbon black to the seeded pyrolysed rice hull C/SiO2 mixture had no significant impact on the reaction rate or product powder properties. The reaction was modelled using a nuclei-growth rate expression as (kt)(0.58) = -ln(1 - X) k = 1.09 x 10(10) exp(-50502/T) where (1573 K < T < 1653 K), (3600 < t < 18000 s), (0 < X < 1), and k = rate in s(-1). [References: 23]