A mixture of the oxides was prepared from metal alkoxides, and was fired at 1650C in order to form mullite plus a SiO2-rich glass phase. By leaching-out the glass phase, crystalline mullite powder was obtained. Transmission electron microscopy revealed that the powder consisted of cylindrical particles of monocrystalline mullite. Diffusivity measurements showed that O diffusion, at temperatures ranging from 1100 to 1300C, could be described by:

D (m2/s) = 1.32 x 10-6exp[-397(kJ)/RT]

These data fell on a line which also went through the O diffusion data for forsterite; which had a similar crystal structure to that of mullite.

Oxygen Self-Diffusion in Cylindrical Single-Crystal Mullite. Y.Ikuma, E.Shimada, S.Sakano, M.Oishi, M.Yokoyama, Z.Nakagawa: Journal of the Electrochemical Society, 1999, 146[12], 4672-5