Mullite Whiskers from Precursor Gel Powders

A mullite precursor sol was prepared by mixing boehmite and silica sols. On addition of 3.5 wt% F− ion as a 47% solution of HF, the mullite sol was gelled. The dried gel was ball-milled and calcined at 1400°C for 1 h in an airtight container. Mullite whiskers grew on the (111) plane along the 〈001〉 direction.