Control of continuous α-Al2O3 fibers by self-seeding and SiO2-Sol doping

The effects of self-seeding and SiO2-sol doping on the sol–gel preparation of α-Al2O3 fibers are investigated. Self-seeding induces heterogeneous nucleation of α-Al2O3 from the precursor gels at a low temperature of 800 °C. Using transmission electron microscopy and X-ray diffraction analysis, the primary crystallite size (dp) of α-Al2O3 in the seeded fibers is determined as ∼30 nm, which is much smaller than the size (∼50 nm) in the unseeded fibers. Moreover, self-seeding increases the γ-to-α transformation kinetics and lowers the transformation temperature by 175 °C. Heating at 1500 °C could sinter the self-seeded α-Al2O3 fibers nearly to full density with a grain size of ∼1 μm. By doping with SiO2 sols, this final grain size can be decreased with suppression of the grain boundary migration, at the expense of increased phase transition temperature. Therefore, dense α-Al2O3 fibers of submicron grains could be achieved by the combination of self-seeding and SiO2-sol doping.