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

Abstract

The oxidation of and β-sialon powders with the different z values 2, 3) in a dry atmosphere of (80/20 kPa) at 1000-1300°C was studied by X-ray diffraction, and 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 and aluminosilicate, the latter crystallizing to mullite at 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 The activation energies for the linear kinetic regions were 260, 260, 300, and 270 kJ mol−1 for and the β-sialons with 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, and MAS-NMR, and kinetic results. © 2001 The Electrochemical Society. All rights reserved.