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

The oxidation of α-Si₃N₄ and β-sialon (Si_6-z Al_zO_zN_8-z) powders with the different z values (z = 1, 2, 3) in a dry atmosphere of Ar/O₂ (80/20 kPa) at 1000-1300°C was studied by X-ray diffraction, ²⁹Si and ²⁷Al magic-angle spinning-nuclear magnetic resonance (MAS-NMR), transmission electron microscopy (TEM), and thermogravimetric analysis. The oxidation products of the β-sialons consisted of amorphous SiO₂ and aluminosilicate, the latter crystallizing to mullite at T ≥ 1200°C. 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 α-Si₃N₄. The activation energies for the linear kinetic regions were 260, 260, 300, and 270 kJ mol ¹ for α-Si₃N₄ and the β-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, ²⁹Si and ²⁷Al MAS-NMR, and kinetic results.