Structural evolution in gel-derived mullite precursors

The evolution of mullite from organo-methal gel precursors above 700 degrees C is found to be strongly influenced in both gel pieces and powdered samples by the thermal pretreatment at lower temperatures. Under the present conditions, the optimum preheating temperature was found to be 350 degrees C, at which temperature an anomalously high concentration was found of an Al species with a characteristic Al-27 magic-angle spinning NMR resonance at about 30 ppm. Such Al sites are often described as pentaco-ordinated, but an alternative assignment is considered. The optimum temperature for the formation of this Al site is also optimal for the catalytic formation of aromatic molecules from the residual organic fragments and/or solvent present. Mass spectrometry shows that under the present reaction conditions, these aromatics are thermally stable up to at least 900 degrees C in air, and the prolonged presence of their decomposition products (CO and water) could facilitate the transformation of the gel to crystalline mullite. The Si-29 NMR spectra indicate at least three different Si environment, including one which may arise from the formation of silicon oxycarbide glasses in these gels. (C) 1996 ELsevier Science Limited