FORMATION OF MIXED-OXIDE POWDERS IN FLAMES .2. 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-degrees 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, gamma-Al2O3, and aluminum titanate. Although the particle formation process for SiO2-GeO2 is ver different from that for Al2O3-TiO2, both mixed oxides result in very uniform mixtures.