Solid-solution range of mullite up to 1800 °C and microstructural development of ceramics

Tetraethoxysilane (TEOS) and Al-sec-butylate (Al-O-Bu) were used for the sol-gel synthesis of mullite ceramics. The starting materials had bulk compositions corresponding to values between 72 and 78 wt% Al₂O₃, and 28 and 22 wt% SiO₂, respectively, and were calcined at 400 °C (A-series) and 1100 °C (B-series). B-series samples, despite their higher green densities, could only be sintered to about 65–70% TD (theoretical density) at 1650 °C, whereas A-series samples achieve values of about 93–98% TD. Ceramics with relatively high amounts of glass phase from large tabular mullite crystals, which are embedded in a finer-grained mullite matrix. As soon as the bulk Al₂O₃ content increases, equiaxed mullite grains appear and the mean grain size becomes smaller, showing a significant difference between the nucleation and crystal growth mechanisms of mullites formed in samples with the lower and higher Al₂O₃-bulk compositions. Depending on the bulk composition of the samples, the temperature-controlled solid-solution of mullite ranges between about 72.7 and 74.3 wt% Al₂O₃ at 1600 °C and 74.1 and 75.4 wt% Al₂O₃ at 1800 °C, indicating that the solid-solution region bends over towards the Al₂O₃-side of the Al₂O₃-SiO₂ phase diagram.