Mullite phase formation and reaction sequences with the presence of pentoxides

Abstract

The effects of pentavalent oxide (M 2O 5, 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 V 2O 5 accelerated the mullite phase formation while Nb 2O 5 and Ta 2O 5 inhibited the mullitization. No reaction was observed between V 2O 5 and Al 2O 3 or SiO 2, while a complicated reaction sequence was found in the mixtures doped with Nb 2O 5 and Ta 2O 5. V 2O 5 doping resulted in anisotropic grain growth, while equiaxed grains were grown from samples doped with Nb 2O 5 and Ta 2O 5. The three groups also demonstrated a different densification behavior. Samples doped with V 2O 5 were of a much lower density than the theoretical density of mullite, due to the anisotropic grain growth. The densification of the Nb 2O 5 and Ta 2O 5 groups was similar to the mixture of Al 2O 3 and SiO 2 without dopants. These differences could be explained in terms of the role that the pentavalent oxides played during the formation of the SiO 2-rich liquid phase in the mixtures.