Kinetic and magic angle spinning-nuclear magnetic resonance studies of dry oxidation of beta-sialon powders

The oxidation of alpha -Si3N4 and beta -sialon (Si6-zAlzOzN8-z) powders with the different z values (z = 1, 2, 3) in a dry atmosphere of Ar/O-2 (80/20 kPa) at 1000-1300 degreesC was studied by X-ray diffraction, Si-29 and Al-27 magic-angle spinning-nuclear magnetic resonance (MAS-NMR), transmission electron microscopy (TEM), and thermogravimetric analysis. The oxidation products of the p-sialons consisted of amorphous SiO2 and aluminosilicate, the latter crystallizing to mullite at T greater than or equal to 1200 degreesC. Very fine acicular mullite grains were formed in the oxidized particles as confirmed by TEM. The initial oxidation kinetics can be described by a two-stage linear law for a degree of reaction less than 10%, followed by parabolic oxidation at 10-60% reaction. In both the linear and parabolic kinetic regions, the oxidation rate constants were very similar for the three sialons but were smaller for alpha -Si3N4. The activation energies for the linear kinetic regions were 260, 260, 300, and 270 kJ mol(-1) for alpha -Si3N4 and the beta -sialons with z = 1, 2, and 3, respectively, whereas the corresponding values for the diffusion process were 410, 430, 410, and 380 kJ mol(-1) The oxidation mechanism is discussed on the basis of the X-ray diffraction, Si-29 and Al-27 MAS-NMR, and kinetic results. (C) 2001 The Electrochemical Society. All rights reserved.