Mullite Transformation Kinetics in P 2 O 5 ‐, TiO 2 ‐, and B 2 O 3 ‐Doped Aluminosilicate Gels

Mullite transformation kinetics of sol‐gel‐derived diphasic mullite gels doped with P 2 O 5 , TiO 2 , and B 2 O 3 were studied using quantitative X‐ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P 2 O 5 doping because of phase separation and formation of α‐alumina and cristobalite. In TiO 2 ‐doped samples, the mullite transformation temperature decreased with TiO 2 doping, and the transformation rate increased with decreasing TiO 2 particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid‐state mass transport through lattice defects. B 2 O 3 doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.