Earlier publications describe the counterflow diffusion flame burner and its unique capability to produce oxide particles having certain structures, such as spheres of one material coated with another, spheres of one composition with attached bulbs of another composition, and uniform multicomponent mixtures. Here we describe the production and properties of bulk quantities of powders produced using this burner. Measurements were made of specific surface area and, for titania, of phase composition. It was found that the controls over powder characteristics used in other forms of flame-synthesis are equally effective in the counterflow diffusion flame burner. We found that the specific surface area of both silica and titania powders decrease with increasing precursor concentrations. Transmission electron microscopy analysis of the titania powders indicates that the mean size of the particles that comprise these powders increases with increasing concentration. These trends are consistent with the collision-coalescence theory of particle growth. In addition, the crystalline phase of titania can be controlled by selecting the appropriate feed stream. For example, over the ranges TiCl4 precursor concentrations tested, feeding it only into the oxidizer stream yields mainly anatase TiO2 powders, while feeding only into the fuel stream yields mainly rutile TiO2 powders. These trends can be explained by the known atmosphere-dependent anatase-rutile transformation. The present data demonstrate that, in addition to its unique capability to produce certain particle shapes and morphologies, the counterflow diffusion flame burner can be manipulated to produce either of the major commercial titania phases, and also silica, with a wide range of specific surface areas.

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.

A counterflow diffusion flame burner was used to produce nanophase vanadium-phosphorus oxide powders in a hydrogen-oxygen flame. Liquid precursors, i.e., VOCl3 and PCl3, were used as source materials in a 1:1 ratio. In situ formation processes were investigated at two temperatures by laser light scattering, by emission and absorption spectroscopy, and by collecting particles directly onto carbon-coated TEM grids. At the higher temperature, the collected powders are spherical particles about 30 to 50 nm in diameter. At the lower temperature, the powders collected are chain-like structures composed of particles 5 to 10 nm in diameter. Particles formed in the burner were collected also from the burner's flanges and from two auxiliary strips. Their crystalline phases and surface area were determined by x-ray diffractometry, FT-IR spectroscopy, and BET analysis by nitrogen desorption. These results indicate a strong influence of temperature on the crystalline phases of the powders. At the higher temperature, the powder collected is a mixture of VOPO4 · 2H2O and δ-VOPO4. This mixture forms Λ-VOPO4 upon subsequent reheating at 750 °C. At the lower temperature, the powders collected are a VOHxPO4 · yH2O phase and VO(H2PO4)2, and form β-VOPO4 and V(PO3)3, respectively, upon subsequent reheating at 750 °C.

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