STRUCTURAL-CHANGES INDUCED ON MULLITE PRECURSORS BY THERMAL-TREATMENT - A-27 AL 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-degrees and 1100-degrees-C. In polymeric samples, thermal treatment at approximately 980-degrees-C produces the formation of gamma-Al2O3. A similar treatment in spray-pyrolized powders gives directly 3:2 mullite. From these results, exothermic and expansive effects detected at approximately 980-degrees-C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (gamma-Al2O3 or mullite). Above 1200-degrees-C, incorporation of Si in the Al-rich phase induces the formation of 3:2 mullite in polymeric and colloidal samples.