Mullite Transformation Kinetics in P2O5‐, TiO2‐, and B2O3‐Doped Aluminosilicate Gels

Abstract

Mullite transformation kinetics of sol‐gel‐derived diphasic mullite gels doped with P2O5, TiO2, and B2O3 were studied using quantitative X‐ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P2O5 doping because of phase separation and formation of α‐alumina and cristobalite. In TiO2‐doped samples, the mullite transformation temperature decreased with TiO2 doping, and the transformation rate increased with decreasing TiO2 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. B2O3 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.