Removal of hydroxyl groups and its influence on the microstructures evolution of alumina-mullite fibers fabricated by sol-gel process

Hydroxyl groups are essential groups in the preparation of alumina-mullite fibers by the sol-gel process, which contributes to the formation of spinnable precursor sol. In this work, the removal of hydroxyl groups during pyrolysis and sintering, and its effect on fibers phase transition and microstructural evolution were systematically studied. Five pyrolyzed alumina-mullite fibers samples with different hydroxyl groups content were obtained by adjusting the pyrolysis temperature or introducing water vapor. It was revealed that the removal of hydroxyl groups existed throughout the sintering process, mainly concentrated before 800 degrees C. The mullitization tem-perature of the pyrolyzed fibers with more hydroxyl groups was about 1300 degrees C, while that of pyrolyzed fibers with fewer hydroxyl groups was 700 degrees C. In addition, an appropriate amount of residual hydroxyl groups (13.3 wt %) was beneficial to obtain a dense fine-grained microstructure of the pyrolyzed fibers after sintering, and the tensile strength of the fibers was increased to 1.6 +/- 0.22 GPa. Finally, the influence mechanism of different hydroxyl content on the alumina-mullite fibers was discussed.

Automated summary

The removal of hydroxyl groups and its effect on phase transition and microstructural evolution of alumina-mullite fibers were studied. The removal of hydroxyl groups occurred mainly before 800 degrees C. The mullitization temperature of fibers with high hydroxyl groups content was 1300 degrees C, while that of fibers with low hydroxyl groups content was 700 degrees C. An appropriate amount of residual hydroxyl groups was beneficial for obtaining a dense fine-grained microstructure of the fibers after sintering, with an increased tensile strength of 1.6 +/- 0.22 GPa.