Structural Changes Induced on Mullite Precursors by Thermal Treatment: A 27Al MAS-NMR Investigation

NMR study of mullite precursors has shown that local arrangement of Al in samples synthesized by spray pyrolysis differs considerably from the one adopted by samples obtained by polymeric or colloidal routes. Aluminum is tetra- and pentahedrally coordinated in the first type of samples but is tetra- and octahedrally coordinated in the second ones. Segregation of SiO₂ and Al₂O₃ is directly produced in colloidal preparation; however, this phenomenon occurs only in polymeric gels when they are heated between 980° and 1100°C. In polymeric samples, thermal treatment at ～980°C produces the formation of γ-Al₂O₃. A similar treatment in spray-pyrolized powders gives directly 3:2 mullite. From these results, exothermic and expansive effects detected at ～980°C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (γ-Al₂O₃ or mullite). Above 1200°C, incorporation of Si in the Al-rich phase induces the formation of 3:2 mullite in polymeric and colloidal samples.