Mullite phase formation and reaction sequences with the presence of pentoxides

Abstract

The effects of pentavalent oxide (M2O5, M=V, Nb and Ta) on the phase formation of mullite, reaction sequence and microstructure development in the oxide mixtures have been investigated. It was found that V2O5 accelerated the mullite phase formation while Nb2O5 and Ta2O5 inhibited the mullitization. No reaction was observed between V2O5 and Al2O3 or SiO2, while a complicated reaction sequence was found in the mixtures doped with Nb2O5 and Ta2O5. V2O5 doping resulted in anisotropic grain growth, while equiaxed grains were grown from samples doped with Nb2O5 and Ta2O5. The three groups also demonstrated a different densification behavior. Samples doped with V2O5 were of a much lower density than the theoretical density of mullite, due to the anisotropic grain growth. The densification of the Nb2O5 and Ta2O5 groups was similar to the mixture of Al2O3 and SiO2 without dopants. These differences could be explained in terms of the role that the pentavalent oxides played during the formation of the SiO2-rich liquid phase in the mixtures. © 2002 Elsevier Science B.V. All rights reserved.