Synergistic Effects of the Si/Al Stoichiometry and Catalyst Content on the Growth Mechanism of Mullite Whiskers

In this study, mullite single-crystal whiskers were prepared by sintering mullite gel powders using HF as a catalyst via the sol–gel process. The effects of the Al2O3:SiO2 molar ratio on the morphology of mullite whiskers in the Al–Si–F system were comprehensively explored during the catalytic reaction. Furthermore, the synergistic effects of the Si:Al ratio and the catalyst content on the growth mechanism of mullite whiskers were evaluated. The morphological characteristics of the whiskers were determined using transmission electron microscopy and scanning electron microscopy. Moreover, morphological parameters, including the diameter and length of whiskers, were statistically analyzed using the Image J software. Additionally, the compositional variation and phase evolution during the whisker growth process were examined via energy-dispersive spectroscopy and X-ray diffraction, respectively, and the corresponding growth mechanism was elucidated. When HF-mediated catalysis reaches a sufficient level (Al2O3:SiO2:HF = 1:1.5:4.3), the low SiO2 content in the system leads to Al enrichment and the formation of flake-shaped Al2O3 structures, indicating an effect analogous to that of increasing catalyst content. Conversely, the simultaneous reduction in the contents of HF and SiO2 induces different catalytic reactions because of their synergy. Specifically, at relatively low SiO2 and HF contents, F ions enter the Al–Si–O system via SiF4, leading to the generation of fluorine-containing topaz, which subsequently transforms into mullite. At relatively high SiO2 and HF contents, mullite can be directly synthesized via the reaction of AlF3 and SiF4. With a gradual reduction in the SiO2 and HF contents, the mullite whiskers exhibit a varying morphology, predominantly transitioning from rod-shaped to flake-shaped and subsequently to rod-shaped structures. This is due to the synergistic effects of the phase transformation and catalytic reactions within the Al–Si–O system.