Preparation of a dense alumina fiber with nanograins by a novel two-step calcination

Controlling the density and the nanograin size is challenging but essential to prepare continuous alumina fibers with excellent performance. In this study, the continuous alumina fibers were prepared using the sol-gel method. The fibers calcined under different conditions were characterized to reveal the effect of calcining conditions on the fiber microstructure evolution. The results show that the phase transformation from amorphous Al 2 O 3 to γ-Al 2 O 3 and then to α-Al 2 O 3 takes place during the calcining process. The fiber, prepared using a single-step calcining method, cannot obtain a fully dense and nanocrystal microstructure through optimizing the calcination conditions. Thus, a novel two-step calcining process was proposed, through which an almost fully dense α-Al 2 O 3 fiber with an average grain size of ~150 nm was prepared and the tensile strength of fibers reaches 2.2 GPa. Based on the results, the effect mechanism of residual organics, phase transformation, densifying, and grain growth on the fiber microstructure evolution was discussed in depth. Graphical abstract