Srinivas Vemury

The role of gas mixing in flame synthesis of titania powders

The formation of titania powders by oxidation of TiCl4 was studied in a laminar diffusion flame reactor. The effects of flame configuration (fuel and oxidant flow rate and position) as well as precursor TiCl4 loading on the specific surface area and phase composition of the product titania powder were investigated. Specifically, the specific surface increased from 15 to 120 m2/g and the rutile fraction was reduced from 15 to 0.1 wt.% by exchanging the position of the fuel and oxidant streams in the burner. Increasing the flame temperature increased the rutile fraction and the average primary particle size. Increasing the precursor TiCl4 concentration reduced the specific surface area, especially at low TiCl4 loadings and flame temperatures. However, it had no effect on the phase composition of the titania particles.

Particle formation in gases: A review

A review of recent progress on particle formation in gases for synthesis of materials is given focus in the session Particle Formation in Gases at the First International Forum on Particle Technology held in Denver, CO, in August 1994. Most of the papers dealt with powder synthesis from gases and, to a lesser extent, from suspended droplets. Major advances were reported on particle formation in flames, spray pyrolysis, agglomerate dynamics, reactor design and on formation of solid powders from sprays. This article summarizes the proceedings of the above session and highlights the most important results as they came across at the ensuing discussions.

Self-preserving size distributions of agglomerates

Abstract The attainment of self-preserving distributions by coagulation of agglomerate or aggregate particles has been investigated in the continuum and free-molecular regimes. Self-preserving distributions have been computed for agglomerates of various fractal dimensions. In the free-molecular regime, the self-preserving size distributions broaden as the fractal dimension decreases. In contrast, in the continuum regime, the distributions narrow with decreasing fractal dimension. The aggregate growth rate increases with decreasing fractal dimension of the colliding particles. This effect is more pronounced in the free-molecular than in the continuum regime. In the free-molecular regime, particles of low fractal dimension have larger cross-sectional area than particles of equal mass but with higher fractal dimension. The larger cross-sectional area per unit particle mass results in a direct increase in the coagulation rate. In the continuum regime, the effect of enhanced collision area is reduced by the increased drag of the agglomerates.

Synthesis and evaluation of titania powders for photodestruction of phenol

Abstract The flame synthesis of titania particles with special properties for the photooxidation of organic contaminants in water is investigated. Powders with high specific surface area (> 100 m 2 /g) and high anatase content were produced in a hydrocarbon flame reactor at various conditions. These powders were used for the photodegradation of phenol in non-aerated aqueous solutions under UV-irradiation. The reaction followed first order kinetics and the photodestruction rates of these powders were comparable with or better than those of commercially available titania powders.

Corona‐assisted flame synthesis of ultrafine titania particles

Synthesis of ultrafine titania particles is investigated in a diffusion flame aerosol reactor in the presence of a gaseous electric discharge (corona) created by two needle electrodes. The corona wind flattens the flame and reduces the particle residence time at high temperatures, resulting in smaller primary particle sizes and lower level of crystallinity. Increasing the applied potential from 5 to 8 kV reduces the particle size from 50 to 25 nm and the rutile content from 20 to 8 wt %. Coronas provide a clean and simple technique that facilitates gas phase synthesis of nanosized materials with controlled size and crystallinity.

Charging and coagulation during flame synthesis of silica

The effect of charging during flame synthesis of silica particles is investigated. These particles are made by SiCl4 oxidation/hydrolysis in a premixed, CH4O2N2 flat flame. Unipolar and bipolar DC electric fields are created across the flame axis using various combinations of needle and plate electrodes. Needle electrodes create highly focused electric fieldds resulting in the onset of convection (iOnic wind) across the flame. As a result, these electric fields not only charge the newly formed particles, but they also reduce the flame temperature and the particle residence time at high temperatures. However, the field created with this configuration is not stable. Using a plate as one of the electrodes increases the stability of the electrc field. Using a plate/plate electrode configuration suppresses the onsset of the ionic wind, so the effect of field charging on particle growth may be separated from that of convection (ionic wind). A negative electric field creates the most drasctic effect on the characteristics of the product particles. The primary particles size decreases with increasing field strength regardless of polarity or electrode configuration. Repulsion of the charged particles resulted in electrostatic dispersion, slower coagulation and smaller primary particle sizes. Transmission electron micrographs show that the extent of agglomeration is greatly influenced by electrode polarity and configuration during electrically assisted flame synthesis of silica powders.

Coagulation of symmetric and asymmetric bipolar aerosols

Coagulation of charged aerosols is investigated by developing a sectional numerical model for both particle size and charge distributions and accounting for electrostatic dispersion. Symmetric bipolar charging increases the coagulation rate when particles are highly charged. Asymmetric bipolar charging results in a faster decrease in the particle number concentration, but does not actually result in larger particles as the decrease in number concentration is dominated by the electrostatic dispersion and not by the coagulation of particles. The performance of a simpler model in the literature is evaluated by comparing its predictions against those of the above sectional model. The simple model is most accurate for initially mildly asymmetric bipolar aerosols at short residence times.