Structural changes of mullite precursors in presence of polyethyleneimine

Thermal properties of two powders derived from the same calcined gel with a stoichiometric mullite composition (atomic ratio Al/Si = 3/1) wet milled in different solutions were studied by simultaneous differential thermal analysis, thermogravimetry and evolved gas analysis (DTA,TG,EGA, respectively), by density measurements and by 29Si MAS NMR spectroscopy and X-ray diffraction (XRD). Calcined gel was milled either in ethanol or in ethanol with the addition of 0.03 g of polyethyleneimine (PEI) per gram of mullite. DTA, TG and mass spectrometry revealed esterification of unhydrated surface of particles formed by milling in ethanol. The difference in the structural evolution of powders and their dependence on annealing temperature seen in 29Si MAS NMR spectra and XRD patterns is attributed to different degrees of segregation of alumina and silica components, and to differing compositions of transitory spinel phase. In both samples the segregation of silica and alumina was observed at temperatures less than that for mullite crystallization. The degree of segregation was much smaller in the sample milled in ethanol. Therefore, in the latter sample spinel with larger silica content incorporated in γ-Al2O3 will crystallize by annealing at 900°C, and Al-rich mullite at 1100°C. Due to greater segregation of silica and alumina component in the presence of PEI, the crystallization of spinel with smaller amounts of silica incorporated into γ-Al2O3 is shifted to 1100°C, and orthorhombic mullite to about 1200°C. The influence of PEI is indicated by a larger sintering density at lower temperatures