Single-phase gels with compositions 3Al₂O₃.2SiO₂ and 2Al₂O₃.SiO₂ were prepared by gelling mixtures of aluminium nitrate and tetraethylorthosilicate. Gels were fast heated at different temperatures between 900°C and 1600°C. The phase transformation and microstructural changes of both mullite precursor gels over the temperature range were followed by X-ray powder diffraction (XRD), lattice parameter determination (LP), and scanning and transmission electron microscopies (SEM and TEM). The distribution of crystallite sizes and strains were determined by linewidth refinements of X-ray diffraction patterns using the integral breadth method of Langford and the Warren-Averbach analysis. XRD of both heated gels showed the formation of crystalline mullite single phase. Some amount of glassy phase coexisted with mullites at low temperatures, i. e. below 900°C. The compositional range of mullites formed on heating gels at temperatures between 900°C and 1600°C was dependent on the starting nominal composition of gels. SEM and TEM micrographs of both heated gels below 1200°C showed the formation of small, discrete, prismatic, well-shaped nanocrystals in a very ordered arrangement. The size of these nanocrystals was dependant on the nominal composition of gels and increased on rising the heating temperature of gel precursors. The microstructural features obtained from linewidth refinement results of X-ray diffraction patterns also allowed to suggest the formation of prismatic a little elongated nanocrystals at temperatures below 1200°C. Microstrain values were small and only displayed a relatively significant value for mullites processed at 900°C.

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