Crystallization Behavior of Nano-Structured Mullite Processed via Mechano-Chemical Route

Mullite can be synthesized from single and diphasic gel obtained by sol-gel method [1, 2]. Thermal analysis investigations have shown that single phase gel shows a sharp exotherm around 1000ºC which is attributed to mullite crystallization from an amorphous precursor, while diphasic gel shows no such exotherm up to 1200ºC [3,4]. In the present study, single phase high purity mullite powder was prepared by the sol-gel route. Effect of intermediate mechanical activation of the dried gel on homogeneity of the system and crystallization of mullite has been investigated.ExperimentalAluminum nitrate and tetraethyl ortho silicate were used as starting materials in preparation of a single phase gel. The prepared gel was severely activated using a planetary ball mill and subjected to further annealing. Thermal behavior of the samples was evaluated by DTA/TGA. Phase compositions were characterized by X-ray diffraction (XRD). The mean crystallite sizes of mullite particles were estimated to be in nano scale using XRD patterns and this was confirmed by TEM investigations.Results and DiscussionFigure 1 shows XRD patterns of the dried gel before and after heat treatment at 500ºC and 850ºC that indicate all of the samples were amorphous. XRD pattern of the sample heat treated at 1100ºC showed formation of mullite together with a spinel type phase and at 1200ºC the spinel phase was fully converted to mullite, Fig.2b. The mean size of crystallites in the resulting mullite particles in the latter sample was calculated to be nearly 90 nm