Porous mullite ceramics through diphasic gels of large boehmite and small silica particles

Mullite formation process has been studied in stoichiometric mullite (3Al₂O₃·2SiO₂) diphasic gel containing large boehmite (∼1 μm) and small silica (∼10 nm) particles. It has been found that initial mullitization did not take place inside the silica phase (cristobalite), but took place in the defect θ-alumina phase. θ-alumina was stabilized by silica when the temperature was below 1350°C. At temperatures above 1350°C, mullite crystallized directly. It was suggested that silica diffused into the pores (<1.8 nm) of θ-alumina and prevented the collapse of θ-alumina pore structure. On the other hand, when silica was not present, the pore structure of θ-alumina collapsed and α-alumina crystallized at ∼1100°C. Porous mullite ceramics were obtained by using special diphasic gels containing large boehmite and small silica particles.