Low-temperature Ti-containing 3:2 and 2:1 mullite nanocrystals from single-phase gels

摘要

TiO_2-containig single-phase gels with (Al_2O_3 + TiO_2)/(SiO_2) molar ratios 3/2 and 2/1 were prepared by gelling mixtures of aluminium nitrate, tetraethylorthosilicate and titanium isopropoxide. Gels were fast heated at several temperatures up to 1100 deg C. Dried and heated gels were characterized by differential thermal analysis (DTA), magic angle spinning nuclear magnetic resonance (MAS-NMR), X-ray powder diffraction (XRD), and scanning and transmission electron microscopies (SEM and TEM). Coupled DTA and XRD results of gels fast heated at 900 deg C showed the crystallisation of two mullites as well as a small amount of alumina-silica spinel.~(27)Al NMR spectra showed the formation of pentacoor-dinated aluminium before mullite crystallization. The increase of lattice parameters of single-phase mullites heated at 1100 deg C indicated that the amount of TiO_2 incorporated into the mullite structure increased on raising the amount of nominal TiO_2 in both series. SEM and TEM images of heated gels at 1100 deg C displayed the formation of well-shaped parallelepiped of titanium-doped mullite nanocrystals with crystalline anisotropy.