Structural evolution of sol–gel derived mullite fibers with different solid contents during sintering

Continuous mullite ceramic fibers were fabricated by a sol–gel dry spinning process and sintered at 1100 °C to examine the influence of the solid content and process parameters on solidification and structure evolution. The spinning sol was prepared from an aqueous solution of aluminum nitrate (AN), aluminum isopropoxide (AIP) and tetraethylorthosilicate (TEOS). By adjusting the temperature, the gelling degree could stabilize at a certain value and the sol–gel transition can be transferred to the spinning line. The samples were investigated by thermal gravity analysis/differential scanning calorimetry (TG/DSC), scanning electron microscopy (SEM), X-ray diffraction (XRD) and nuclear magnetic resonance (NMR). The results show that only sintering of the precursor fibers from sols with solid content above 50% at 1100 °C yields crack-free mullite ceramic fibers. The ceramic fibers were good handling and had smooth surfaces. The XRD and NMR analysis indicated that sintering of the precursor fibers at 1100 °C yields complete crystal, which is a tetragonal metric-like mullite.