Effect of residual carbon on the phase transformation and microstructure evolution of alumina-mullite fibers prepared by sol-gel method

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Sol-gel method, as one of the most effective methods, has been widely used in producing high-property ceramics. In this method, the pyrolysis of precursor significantly influences the following microstructure evolution and properties of ceramic materials. In this work, the effect of residual carbon, influenced by the pyrolysis atmosphere, on the phase transition and microstructure evolution of the alumina-mullite fibers was investigated. It was revealed that the phase transition path of the preheated fibers with 0.53 wt% residual carbon was amorphous phase → aluminosilicate → tetragonal mullite → orthorhombic mullite + θ-Al2O3. The phase transition path of the preheated fibers with 10.51 wt% residual carbon became amorphous phase → γ-Al2O3 → orthorhombic mullite + α-Al2O3 + θ-Al2O3. In addition, the residual carbon contributes to improve the densification of alumina-mullite fibers and refine the fiber grain size. The effect of residual carbon on the formation of mullite coarse grains was further discussed.