Mullite formation from non-crystalline precursors

Abstract

X-ray diffraction, differential thermal analysis, and29Si and27Al MAS NMR spectroscopy were used to characterize the formation of mullite from non-crystalline precursors. The precursors were produced by the sol-gel route from organic starting compounds (SGM), and by coprecipitation of inorganic starting materials (CM). There is NMR evidence that both types of mullite precursor consist of Si-O and Al-O tetrahedra, and of Al-O octahedra. Furthermore, fivefold-coordinated Al-O polyhedra, or alternatively strongly distorted Al-O tetrahedra, do occur in the precursors. SGM and CM precursors transform to mullite in multi-step reactions with intermediate formation ofγ-Al2O3, and a non-crystalline nearly pure SiO2 phase. Theγ-Al2O3 compound has a highly distorted spinel structure, and may contain some minor amount of Si. The diffusion-controlled decomposition of the precursors toγ-Al2O3 + SiO2 at low temperature (⩽1000°C) suggests that the precursors are diphasic, consisting of nanometre-sized, long-range-disordered Al2O3- and SiO2-rich domains. The reaction ofγ-Al2O3 + SiO2 above about 1000°C initially produces Al2O3-rich mullite. This is explained by a possible nucleation of mullite on the surfaces ofγ-Al2O3 crystallites. The gradual transformation of the unstable, low-temperature (⩾1000 to ⩽1400°C) Al2O3-rich mullites to the stable high-temperature (⩾ 1400°C) 3/2-type mullites (3Al2O3 · 2SiO2) is essentially controlled by the annealing temperatures, whereas annealing times play a minor role.