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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 the precursor, elongated crystals (with longer axis about 5-7 micro m) 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 micro m 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 1320-1400 oC 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 higher activation energy (Q=743+-18 kJ/mol) were obtained for mullite ceramic with monomodally distributed grain sizes.

Mullite; Grain size; Microstructure; Creep resistance

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