CONSTITUTION OF THE GAMMA-ALUMINA PHASE IN CHEMICALLY PRODUCED MULLITE PRECURSORS

The temperature development of type II mullite precursor powders have been studied in the temperature range of 150-degrees-C (as-received) and 1150-degrees-C X-ray diffraction (XRD) measurements, infrared (IR) and Si-29 and Al-27 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 gamma-alumina spinel being formed as a transient phase in type II mullite precursors. The as-received precursors consist of relatively large sperical particles (less-than-or-equal-to 0.5 mum) of non-crytalline SiO2 and of much finer-grained agglomerates of pseudo-boehmite crystals (gamma-AlO(OH), almost-equal-to 20 nm), which are embedded in a SiO2 matrix. Above almost-equal-to 350-degrees-C, pseudo-boehmite transforms to spinel type alumina (gamma-Al2O3). During this transformation, all Si existing in the SiO2 matrix of the pseudo-boehmite agglomerates is incorporated into gamma-Al2O3 corresponding to a SiO2 content of almost-equal-to 12 mole% at 500-degrees-C Up to 750-degrees-C, the SiO2 content of the gamma-alumina remains constant but above this temperature it gradually rises and reaches a maximum amount of almost-equal-to 18 mole% at 1150-degrees-C A marginal decomposition of the spherical non-crystalline SiO2 particles may be the sources to provide diffusion of Si species into the gamma-alumina during a temperature increase above 750-degrees-C It is most likely that Si species diffuse into the gamma-alumina crystals along the crystallite boundaries. The diffusion process and Si incorporation are facilitated with the temperature increase.