Mechanism of Formation of TiO2-Doped Mullites from Heated Single-Phase Gels Investigated by Structural and Microstructural Parameters

An analysis of the microstructure of TiO 2 -doped mullites prepared from monophasic gels thermally treated in the temperature range 900°-1600°C is reported. Two series of gels with nominal compositions 3(Al 2-x Ti x O 3 ) ·2(SiO 2 ) and 2(Al 2-x Ti x O 3 )·(SiO 2 ), in the range 0≤x≤0.15, were prepared. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) were used to follow the structural and textural evolution from gels to the final doped mullites. The crystallite sizes and microstrains were determined by linewidth refinements of XRD patterns using the integral breadth method of Langford and the Warren-Averbach analysis. XRD patterns showed the formation of Al 2 TiO 5 and/or rutile as minor impurities in both series of mullites for compositions x = 0.10 and 0.15 at temperatures between 1200° and 1600°C. Larger mullite crystallite sizes were produced by increasing either the content of dissolved titanium oxide or the temperature. In general, crystallite sizes are larger in the titanium oxide-doped alumina-rich mullite series while the crystallite aspect ratio values are similar in both series of doped mullites. In mullites produced at lower temperatures, the sizes of nanocrystals estimated by SEM were in agreement with those obtained by line broadening analysis. A mechanism of formation of TiO 2 -doped mullite solid solutions is suggested on the basis of the interpretation of the structural and microstructural evolution from gels to doped mullites.