Preparation of Mullite Fiber by Sol-Gel Method

Mullite fibers were prepared by a new sol-gel method from Al(OPri)3-Al(NO3)3-TEOS-H2O system. In this system, the spinnable range varied depending on solution preparation procedures. When TEOS and Al(OPri)3 were added simultaneously to an Al(NO3)3 aqueous solution, precursor fibers could be obtained from solutions in a compositional range of H2Omol/(Al+Si)mol=20/1-200/1 and Al(OPri)3mol/Al(NO3)3mol≤6/1. On the other hand, the spinnable range was narrow, when TEOS was added to the solution of Al(OPri)3 in an aqueous Al(NO3)3. When Al(OPri)3 was added after TEOS was dissolved in an Al(NO3)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(H2O)6]3+ were observed by 27Al-and 29Si-NMR spectroscopy of the spinnable solutions. To the contrary, the unspinnable solutions contained Al137+ 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.