Solubility and microstructural development of TiO_2-containing 3Al_2O_3 centre dot 2SiO_2 and 2Al_2O_3SiO_2 mullites obtained from single-phase gels

摘要

The interdependence of the titanium oxide amount and the anisotropic growth of mullites prepared from single-phase gels were investigated. Gels with stoichiometries 3(Al_(2-x)Ti_xO_3) centre dot 2(SiO_2) and 2(Al_(2-x)Ti_xO_3) centre dot 2(SiO_2), with 0< = x< = 0.15 were prepared by the semialkoxide method. Gels and specimens heated at temperatures between 1200 and 1600 deg C were characterized by using infrared spectroscopy (IR), X-ray diffraction (XRD) and transmission and field emission scanning electron microscopies (TEM and FESEM). Al_2TiO_5 as minor impurity was detected in both series of mullites for gel precursor compositions x = 0.10 and x = 0.15, obtained at temperatures between 1200 and 1600 deg C. Variations of lattice parameters of mullite, processed at temperatures from the range between 1400 and 1600 deg C, with the starting nominal amount of titanium oxide indicated that the solubility limit of titanium oxide was in ranges 3.8-4.1 and 4.1-4.4 wt percent TiO_2 for 3:2 and 2:1 mullites series, respectively. The anisotropic growth of titanium-doped mullite crystalline grains was significant only when the nominal amount of titanium oxide exceeded the limit of solubility into the mullite structure (for both mullite series). Stronger anisotropy occurred for the 3:2 series specimens, i.e. for the SiO_2-richer mullites. In both series of mullites, the anisotropic grain growth was observed for the process temperatures higher than 1400 deg C; the crystalline grains of mullites processed at lower temperatures were equiaxials and of almost the same size.