Mullite fibers were prepared by a new sol-gel method from Al(OPr
i)
3-Al(NO
3)
3-TEOS-H
2O system. In this system, the spinnable range varied depending on solution preparation procedures. When TEOS and Al(OPr
i)
3 were added simultaneously to an Al(NO
3)
3 aqueous solution, precursor fibers could be obtained from solutions in a compositional range of H
2Omol/(Al+Si)mol=20/1-200/1 and Al(OPr
i)
3mol/Al(NO
3)
3mol≤6/1. On the other hand, the spinnable range was narrow, when TEOS was added to the solution of Al(OPr
i)
3 in an aqueous Al(NO
3)
3. When Al(OPr
i)
3 was added after TEOS was dissolved in an Al(NO
3)
3 aqueous solution, no spinnable solutions were obtained. Aluminoxane polymer, oligomer of siloxane or that containing Si-O-Al bond, Al-O dimer, Si(OH)
3-O-Si(OH)
3 and [Al(H
2O)
6]
3+ were observed by
27Al-and
29Si-NMR spectroscopy of the spinnable solutions. To the contrary, the unspinnable solutions contained Al
137+ polycation and three-dimensional polymer having Al-O bonds. The existence of aluminoxane-polymer, Al and Si atomic scale component was deemed necessary for the appearance of spinnability. The DTA curves of all precursor fibers showed a unique and sharp exothermic peak at 1000°C, while those of gels obtained from unspinnable solutions showed only small peaks. The X-ray diffraction analysis revealed that the precursor fiber crystallizes directly from amorphous to mullite at temperature as low as 600°C. It was deduced that structure which has a similar composition to mullite exists in precursor fibers.
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