Densification and Microstructure of Sodium-doped Colloidal Mullite

A series of mullite precursor gels to prepared by hetero-coagulation of boehmite and colloidal silica. Samples were doped with up to 1.5 wt% Na2O. Na2O additions as low as 0.16 wt% led to the formation of α-alumina and a stable glassy phase in addition to mullite. No evidence for the incorporation of sodium into the mullita structure was found by either XRD or 27AlMAS NMR. SEM showed mullite doped with 0.16 wt% and greater Na2O had an inter-granular glassy phase and large crystalline grains. It is thought that the majority of the sodium was present in the glassy phase. Sodium addition did not decrease the mullite crystallization temperature and did not lead to enhanced sintering.