Kinetic and Magic Angle Spinning-Nuclear Magnetic Resonance Studies of Wet Oxidation of Beta-Sialon Powders

Wet oxidation of α-Si₃N₄ and β-sialon (Si_{6 z}Al_zO_zN_{8 z}, z = 1, 2, and 3) powders was carried out at 1000-1300°C in atmospheres containing 0-20 kPa water vapor. The oxidation was monitored by XRD, ²⁶Si and ¹⁷O magic-angle spinning-nuclear magnetic resonance (MAS-NMR), transmission electron microscopy, and thermogravimetric analysis. The kinetic results were compared with those of our previous dry oxidation studies in Ar/O₂ (80/20 kPa). The oxide product phase was composed of very tine acicular mullite grains and amorphous SiO₂. The early-stage oxidation is described by a two-stage linear kinetic law, the first stage operating at 0-5% reaction, the second at 5-20% reaction. From 20-80% reaction, the kinetics follow a parabolic law represented by the Ginstling-Brounshtein equation. The diffusion rates for wet oxidation were 2-11 times greater than for dry oxidation. The dependence of the diffusion rate on water vapor pressure over the range 0-20 kPa was approximately parabolic, indicating the diffusion of OH ^̇, When the z = 3 sialon was oxidized in either Ar/H₂¹⁷O or Ar/O₂/H₂¹⁷O atmospheres, ¹⁷O was detected by MAS-NMR in the SiO₂ and mullite oxidation products, indicating that the oxygen atoms from the water vapor contribute in nearly equal amounts to the formation of mullite and SiO₂