Formation of mixed oxide powders in flames: Part II. SiO₂−GeO₂ and Al₂O₃−TiO₂

SiO₂−GeO₂ and Al₂O₃−TiO₂ mixed oxide powders were synthesized using a counterflow diffusion flame burner. SiCl₄, GeCl₄, Al(CH₃)₃, and TiCl₄ were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light-scattering. Powders were collected using two different methods, a thermophoretic method (particles are collected onto carbon coated TEM grids) and an electrophoretic method (particles are collected onto stainless steel strips). Their size, morphology, and crystalline form were examined using a transmission electron microscope and an x-ray diffractometer. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The formation of the mixed oxides was investigated using Si to Ge and Al to Ti ratios of 3:5 and 1:1, respectively. Heterogeneous nucleation of the SiO₂ on the surface of the GeO₂ was observed. In Al₂O₃−TiO₂ mixtures, both oxide particles form at the same temperature. X-ray diffraction analysis of particles sampled at temperatures higher than 1553 K showed the presence of rutile, γ–Al₂O₃, and aluminum titanate. Although the particle formation process for SiO₂−GeO₂ is very different from that for Al₂O₃−TiO₂, both mixed oxides result in very uniform mixtures.