Influence of alumina precursors on microstructure and creep behavior of sol-gel derived mullite ceramics

The effect of alumina precursors on microstructure, sintering and creep behavior of sol-gel derived mullite ceramics was studied by scanning and transmission electron microscopy (SEM, TEM and HRTEM), dilatometry and measuring the creep resistance in 4-point bending mode. When aluminum nitrate nonahydrate was used as a precursor, elongated crystals (with longer axis about 5-7 mum) were embedded into a matrix of polyhedral much smaller grains. This morphology was due to overlapping of mullite crystallization and viscous flow sintering. Equiaxial mullite grains with 1.3 mum mean sizes were obtained using boehmite (gamma-AlOOH) as alumina precursor. Transient alumina formed in situ by the decomposition of boehmite shifted the mullite formation above the sintering temperature, and allowed the formation of equiaxial mullite grains. Four-point bending creep behavior of mullite ceramics was studied in the temperature range of 1320-1400degreesC under the stresses between 40 and 160 MPa. The higher creep rate and the smaller activation energy (Q= 606 +/- 33 kJ/mol) exhibited the sample with bimodally distributed grain sizes, whereas a smaller creep rate and the higher activation energy (Q=743 +/- 18 kJ/mol) were obtained for mullite ceramic with monomodally distributed grain sizes.