Mixed oxide powders, e.g., Al2O3−TiO2, SiO2−GeO2, and TiO2−SiO2, are used in industry to produce ceramics, optical fibers, catalysts, and paint opacifiers. The properties of these products depend upon the morphology of the powders. Ceramics and optical fibers are produced using either a uniform mixture of multicomponent particles or a uniform solution. The desired morphology for catalysts is a high surface area and many active sites. TiO2 coated with a layer of SiO2 is the desired structure for use as a paint opacifier. In this paper, TiO2−SiO2 mixed oxide powders were synthesized using a counterflow diffusion flame burner. TiCl4 and SiCl4 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. A thermophoretic sampling method also was used to collect particles directly onto carbon coated grids, and their size, morphology, and crystalline form examined using a transmission electron microscope. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The effect of temperature and of Si to Ti concentration ratio on particle morphology was investigated. Strong temperature dependence was observed. At high temperatures, TiO2 particles were covered with discrete SiO2 particles. At low temperatures, the structure changes to TiO2 particles encapsulated by SiO2. TEM diffraction pattern measurements showed that the TiO2 is rutile and the SiO2 is amorphous silica. At high Si to Ti ratios, SiO2-encapsulated TiO2 particles form. At low Si to Ti ratios, one obtains TiO2 particles covered with discrete SiO2 particles.

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Homogeneous nucleation rates for n-nonane were measured as a function of supersaturation at nine temperatures (233 to 315 K) using an upward thermal diffusion cloud chamber. On each isotherm, the supersaturations were set to values which produced nucleation rates ranging from about 5×10−5 to 100 drop cm−3 s−1. The observed dependences on both temperature and supersaturation were compared to the predictions of several nucleation theories. Closest agreement was obtained with classical theory. Nonetheless, a multiplicative temperature-dependent correction that ranges from 2×10−5 at 233 K to 4×103 at 315 K was required to make classical theory agree with experiment. A comparison of our nucleation rate measurements to measurements made using two different expansion cloud chambers showed consistent deviations from classical theory.

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.

V2O5-TiO2 and V2O5-AI2O3 mixed oxide powders were synthesized in a hydrogen-oxygen flame using VOCI3, TiCl 4 , and A1(CH 3 ) 3 as precursors. The particle formation processes were investigated as a function of VOCI3 concentration by laser light-scattering and by collecting particles directly onto transmission electron microscopy grids. In the V2Os-TiO2 system, the oxides condense as an intimate mixture at all three VOCI3 concentrations. Spherical particles, 40 to 70 nm in diameter, are obtained. In the V2O5-AI2O3 system, chain-like particles composed of an intimate mixture of V 2 O 5 and AI2O3 form at the lowest VOCI3 concentration. At high VOCI3 concentrations, the chain-like particles have a core-mantle structure (a core mainly of AI2O3 and a mantle mainly of V 2 O 5 ). The crystalline form and the surface area of these mixed oxides were determined by x-ray diffractometry, FT-IR spectroscopy, and BET analysis by nitrogen desorption. These measurements indicate that amorphous vanadium oxide forms at low VOCI3 concentrations, and V 2 O 5 is obtained at the higher VOC1 3 concentrations. The structure of the amorphous vanadium oxide matches that published for vanadium oxide "supported" catalysts. : Formation of V 2 O5-based mixed oxides in flames oxides 15 17 in flames showed that the flame technique used here allows one to control the shape, size, morphology, and crystalline structure of the powders produced. The results presented here, although preliminary, strongly suggest that a flame process can be used to produce catalysts and to control their formation.