Electrically-assisted flame aerosol synthesis of fumed silica at high production rates

doi:10.1016/S0255-2701(99)00082-3

Chemical Engineering and Processing: Process Intensification

Volume 39, Issue 3, May 2000, Pages 219-227

https://doi.org/10.1016/S0255-2701(99)00082-3 Get rights and content

Abstract

The control of particle size by external electric fields is investigated during flame synthesis of particles at high production rates (up to 87 g h⁻¹). Here needle and plate electrodes are used during synthesis of fumed silica from hexamethyldisiloxane (HMDS) in a coflow double diffusion flame at atmospheric pressure. The average primary particle diameter is reduced by a factor of two, when the applied electric field strength between two needle electrodes was increased from 0 to 1.5 kV cm⁻¹. It was demonstrated that electrode placement is crucial for this process. Furthermore, the powder composition (soot content less than 1 wt%) was controlled by the electric field.

Introduction

First commercial production of fumed silicas, marketed under the name Aerosil, was realized in the early 1940s [1]. Since then these products have found wide applications in different industrial branches, e.g. as fillers in car tires, toothpaste, thickening and thixotropic agents [2], carriers for catalysts [3] and as base material for optical fibers [4].

Through many process variables can affect the characteristics of flame made particles, external electric fields are intriguing as they can be readily implemented and are quite effective for precise particle size control [5]. The effect of electric fields on oxide particle formation in flames has been studied though in rather low particle production rates. Hardesty and Weinberg [6] injected hexamethyldisiloxane (HMDS) vapor into a premixed methane/air flame and found that the primary particle size of SiO₂ powders was reduced by a factor of three with increasing applied electric potential. Katz and Hung [7] used SiCl₄ and SiH₄ as precursor in a counterflow diffusion H₂/O₂ flame and found that the particle diameter increased (as determined by light scattering) with increasing electric potential along the reactant flow.

Vemury and Pratsinis [8] generated 1 g h⁻¹ TiO₂ by oxidation/hydrolysis of TiCl₄ in a methane/air double diffusion flame in the presence of an electric field created by two needle electrodes across the flame. They showed that the average primary particle diameter and rutile content was reduced by a factor of 2 by increasing the applied potential from 1.2 to 2.0 kV cm⁻¹. They found also that positioning the needles closer to the burner face (0.5 cm) is more effective than positioning the electrodes further away (>1.0 cm). Vemury and Pratsinis [9] demonstrated the controlled synthesis of SiO₂-particles in a premixed methane/air flame from SiCl₄ while investigating various field configurations, using needle–needle, needle–plate, and plate–plate electrodes at production rates up to 4 g h⁻¹ SiO₂. They found that the electric field created by needles influenced most significantly the product powder characteristics. Furthermore, Vemury et al. [10] investigated the application of electrical fields as a tool for the precision synthesis of nanophase TiO₂, SiO₂, and SnO₂ at production rates up to 3 g h⁻¹. Increasing the applied field strength resulted in an increase of the specific surface areas up to a factor of 1.8.

In contrast to this, Spicer et al. [11] showed that increasing the electric field intensity between plate electrodes increased the average primary particle size when making composite silica-soot powders in a premixed flame from SiCl₄ and acetylene at production rates up to 15 g h⁻¹. Morrison et al. [12] used fourier transform infrared (FTIR) spectroscopy to determine the flame temperature, gas composition, and particle size of particle laden flames. Specifically, they studied the effect of an external electric field created by plate electrodes on the flame temperature and found that electrical fields modestly increase the flame temperature. Katzer and Schmidt [13] investigated the influence of an external electric field created by needle electrodes producing TiO₂ particles by oxidation of TiCl₄ in a H₂/O₂ diffusion flame. At production rates of less than 1 g h⁻¹ the mobility equivalent diameter of the product particles was reduced up to a factor of 1.7 using a field strength of 5 kV cm⁻¹. Briesen et al. [14] found that axial electric fields created by ring electrodes during flame synthesis of SiO₂ by HMDS oxidation reduced also the average primary particle size by a factor of 2.8 at production rates of 10 g h⁻¹. However, at higher production rates (>10 g h⁻¹) the applied electric field had little effect on the product primary particle size and even increased soot formation at the highest production rates, 30 g h⁻¹, regardless of the field intensity.

Up to now, the influence of an electric field on flame synthesis of particles was only investigated at rather small production rates. Here the effect of external electric fields during flame synthesis of silica by combustion of HMDS is investigated at relatively high production rates up to 87 g h⁻¹. The effect of needle and plate electrodes as well electrode location along the flame axis on the specific surface area, composition and morphology of the product particles are investigated as a function of the field intensity and HMDS concentration.

Section snippets

Experimental

Fig. 1 shows a schematic of the experimental set-up. Clean, dried argon gas (Wright Brothers, 99.9%) is bubbled through a 1 l glass flask, filled up to 10 cm from the bottom with liquid hexamethyldisiloxane (HMDS, Gelest Inc). An Allihn condenser is placed between the burner and the precursor flask to prevent droplet entrainment resulting in 100% saturation of the Ar-HMDS stream [15]. The condenser, the HMDS vapor manifold and the burner are heated with heating tape 20 K higher than the HMDS

Results and discussion

Fig. 3 shows a ring-like double diffusion flame during high rate of powder production (87 g h⁻¹) in the absence of applied electric fields (0 kV cm⁻¹). This is a stable flame with two combustion fronts as the oxygen diffuses from two sides (Fig. 2) to react with fuel/precursor HMDS. The change of the flame shape can be seen when applying an electric field and increasing its intensity from 0 to 1.0, 1.5 and 2.0 kV cm⁻¹, regardless of polarity. Here, the field is generated by two needle

Conclusions

The role of external electric fields on flame synthesis of silica was investigated at high powder production rates. Fumed silica was made in a double diffusion flame by oxidation of hexamethyldisiloxane. The average primary particle size was reduced by a factor of two using needle electrodes, and by a factor of 1.8 by plate electrodes. The monotonic increase of specific surface area with field intensity shows that electric fields can be used to precisely control the specific surface area of the

Acknowledgements

This work was initiated at the University of Cincinnati, USA, and supported in part by the US National Science Foundation and DAAD (CTS-9619392 and INT-9603196) and Swiss National Science Foundation.

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^☆: This article is dedicated to Professor em. Dr.-Ing. E.U. Schlünder on the occasion of his 70th birthday.

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Electrically-assisted flame aerosol synthesis of fumed silica at high production rates☆

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Mater. Sci. Eng.

J. Aerosol Sci.

Prog. Energy Combust. Sci.

J. Aerosol Sci.

J. Aerosol Sci.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Powder Tech.

Vapor phase process is used to produce colloidal silica of high purity, low water content, and large external surface areas

Ind. Eng. Chem.

Flame synthesis of fine particles

Chem. Eng. News

Corona-assisted flame synthesis of ultrafine particles

Appl. Phys. Lett.