Constitution of the γ-alumina phase in chemically produced mullite precursors

Abstract

Abstract The temperature development of type II mullite precursor powders have been studied in the temperature range of 150°C (as-received) and 1150°C. X-ray diffraction (XRD) measurements, infrared (IR) and 29 Si and 27 Al nuclear magnetic resonance (NMR) spectroscopy and analytical transmission electron microscopy (ATEM) have been performed on the heat-treated precursors. The investigations had the aim of contributing to the frequently discussed question, whether Si is incorporated into the γ-alumina spinel being formed as a transient phase in type II mullite precursors. The as-received precursors consist of relatively large sperical particles (≤ 0·5 μm) of non-crystalline SiO 2 and of much finer-grained agglomerates of pseudo-boehmite crystals (γ-AlO(OH), ≈ 20 nm), which are embedded in a SiO 2 matrix. Above ≈ 350°C, pseudo-boehmite transforms to spinel type alumina (γ-Al 2 O 3 ). During this transformation, all Si existing in the SiO 2 matrix of the pseudo-boehmite agglomerates is incorporated into γ-Al 2 O 3 corresponding to a SiO 2 content of ≈ 12 mole% at 500°C. Up to 750°C, the SiO 2 content of the γ-alumina remains constant but above this temperature it gradually rises and reaches a maximum amount of ≈ 18 mole% at 1150°C. A marginal decomposition of the spherical non-crystalline SiO 2 particles may be the sources to provide diffusion of Si species into the γ-alumina during a temperature increase above 750°C. It is most likely that Si species diffuse into the γ-alumina crystals along the crystallite boundaries. The diffusion process and Si incorporation are facilitated with the temperature increase.