A high-volume spray aerosol generator producing small droplets for low pressure applications

Pergamon

J. Aerosol Sci., Vol. 26, No. 7, pp, 1131-1138, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021 8502/95 $9.50 + 0.00

0021-8502(95)00037-2

A HIGH-VOLUME SPRAY AEROSOL GENERATOR PRODUCING SMALL DROPLETS FOR LOW PRESSURE APPLICATIONS Yun Chan Kang and Seung Bin Park * Department of Chemical Engineering, Korea Advanced Institute of Science & Technology, 373-1 Kusong-dong Yusong-gu, Taejon 305-701, Korea

(First received 26 December 1994; and in final form 7 April 1995)

Abstract--A high-volume spray aerosol generator capable of producing small droplets for lowpressure applications, hereafter named filter expansion aerosol generator (FEAG), is described and characterized. The potential applications of this spray generator are thin film deposition, aerosol etching, and ultrafine particle preparation by spray pyrolysis at reduced pressure. The FEAG is mainly composed of a pneumatic nozzle for dispersing liquid, a porous glass filter with 80 mm diameter, and a vacuum pump (6001min-1). The liquid flow rate through the filter was 3 c m a c m - 2 h -~. The mean droplet size was estimated to be 2.1 #m with a geometric standard deviation 1.76. In principle, this generator can be scaled up by increasing vacuum pump capacity and filter area. INTRODUCTION

Spray aerosol generators can be applied to the formation of thin films (Pike et al., 1993; Lee and Huang, 1994) and preparation of fine particles by spray pyrolysis (Kodas, 1989). In spray pyrolysis for film generation, an aerosol of a solution is formed and then deposited onto a surface where solvent evaporation and chemical reaction take place resulting in a film. The advantage of spray pyrolysis lies in the ability to use a wide variety of precursors with low vapor pressure as long as they have a suitable solvent, while applicability of conventional chemical vapor deposition methods are limited to precursors of high vapor pressure. Additionally, good mixing in a droplet makes the aerosol deposition attractive for the preparation of multicomponent films such as ferroelectric films or superconducting films (Cukauskas et al., 1990). One disadvantage of using an aerosol generator for film formation is the large volume of carrier gas required to produce aerosol and deliver it onto th~ substrate. The large flow of carrier gas results in turbulence which adversely affects the uniformity and conformity of the prepared film (Lee, 1990). One way of achieving laminar flow condition is to reduce the deposition chamber pressure. Most of the currently available liquid aerosol generators are, however, intended to be used under atmospheric pressure. In spray pyrolysis for particles preparation, a metal salt solution is atomized into droplets and sent through a hot-wall tubular reactor. Inside the reactor the solvent evaporates and the metal salts decompose to form the product particles. Morphology and size of the resulting particles are expected to be different from the particles produced from an atmospheric aerosol generator. Droplets of small and uniform size are required for preparation of ultrafine particles and thin films for integrated circuits with submicron features. For these applications, droplet size should be around 1 #m and large production rates are required. However, currently available aerosol generators represent a compromise with regard to production rate and droplet size. Pneumatic nozzles (e.g. Zhang et al., 1991) are capable of producing large aerosol concentrations but droplet size is in the range of 10#m and thus too large. Electro-spray generators (e.g. Meesters et al., 1992) are used to generate droplets below 2 pm but the production rate is on the order of 0.1 g per minute which is too low. Ultrasonic

* Corresponding author. 1131 Z6-7-H

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Y.c. Kang and S. B. Park

spray generators (e.g. Lang, 1962) give reasonably high production rates of several grams per minute and droplet diameters on the order of #m. Thus ultrasonic spray generators have mostly been used for preparing a wide range of ultrafine particles and films. However, scale-up of this generator has proven to be difficult. In this paper, we describe a spray aerosol generator that can be used at reduced pressure and scaled up for large-scale production of droplets smaller than 2 #m. The characteristics of this aerosol generator, mean droplet size and distribution, were measured from the particles prepared by spray pyrolysis and compared with those of an ultrasonic spray generator. The possibility of droplet collisions in the generator is also examined.

D E S C R I P T I O N OF F I L T E R E X P A N S I O N A E R O S O L G E N E R A T O R A schematic diagram of the filter expansion aerosol generator (FEAG) is shown in Fig. 1. The F E A G main elements are a pneumatic nozzle, a porous glass filter, and a vacuum pump. Liquid is sprayed through a pneumatic nozzle using carrier gas on to a glass filter surface where it forms a thin liquid film. This liquid film is pressed through the filter pores by the carrier gas and expanded into a low-pressure chamber. A cross-sectional view of the glass filter is shown in Fig. 2. The liquid and gas flow rates on to a glass filter are controlled at 3 cm 3 c m - 2 h - 1 and 57 1c m - 2 h - 1 respectively, so as to maintain two-phase flow in the pores of the filter. The chamber pressure was maintained at 60 torr with these flow conditions. This aerosol generator, in principle, can be scaled up by increasing vacuum pump capacity and filter area.

M E A S U R E M E N T OF D R O P L E T SIZE AND P A R T I C L E C H A R A C T E R I S T I C S Droplet sizes were estimated by measuring the size distributions of pyrolized particles. For this purpose, the aerosol stream was heated, dried and decomposed in a tubular furnace reactor. It is difficult to measure particle sizes on line due to the low-pressure environment. So direct measurements were made by TEM or SEM. Figure 3 shows a T E M photograph of alumina particles, generated by our F E A G process from 0.2 moll -1 aluminum sulfate solution at 800°C. The particles are spherical and smooth. The particle size distribution calculated from TEM photographs is shown in Fig. 4. The geometric mean diameter and geometric standard deviation are 0.37 #m and 1.76, respectively.

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(h) Fig. 2. SEM photograph and schematic detail of glass filter cross section.

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Y.C. Kang and S. B. Park

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Fig. 3. TEM photograph of alumina particles prepared by FEAG process (0.2 M AI sulfate, 800°C, 60 torr). Fig. 5. SEM photograph of alumina particles prepared by ultrasonic spray pyrolysis (0.2 M AI sulfate, 800°C, 760 torr).

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Assuming one dense particle is derived from one droplet, the mean droplet diameter can be calculated using the following equation:

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(1)

where ddrop and dp,rtiole are the mean diameter of droplets and particles, respectively. Px and Mx are the density and molecular weight of oxide or dried salt particles. C is the molar concentration of salt solution. Here we assume that the particle is not porous. Therefore, the mean droplet size predicted by equation (1) is the maximum possible droplet size. If the particle is hollow or porous, we can estimate the droplet size by the following equation (Zhang and Messing, 1990): darop = dparticlex (Ceq/C) I/3 ,

(2)

where C~q is equilibrium saturation concentration and C is the initial concentration of salt solution. In this equation, we assume that the solute precipitates at the equilibrium concentration and forms a crust shell. Thus, it gives the lower limit of droplet size. The estimated size distributions of liquid droplets along with those of the alumina particles are shown in Fig. 4. The mean droplet size calculated by equation (1) is 2.1/tin and almost all droplets fall below 10 #m. If the alumina particles are assumed to be hollow, equation (2) gives a mean droplet size of 0.7 #m. C O M P A R I S O N W I T H U L T R A S O N I C SPRAY G E N E R A T O R In order to check the validity of the above droplet size measurement and to compare FEAG process characteristics with those of an ultrasonic spray generator, alumina particles were prepared by ultrasonic spray pyrolysis and the droplet sizes estimated by equations (1) and (2) were compared with those of light scattering measurements. The size distributions of

1136

Y.C. Kang and S. B. Park droplet (lower limit) particle

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Fig. 6. Size distributions of alumina particles and droplets prepared by ultrasonic spray pyrolysis

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2O Fig. 7. XRD spectra of particles in droplet collision experiments,

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Table 1. Conditions and results of droplet collisionexperiments Experiment Aerosolsource 1

Aerosol source 2

XRD peaks

1 2 3 4

Nickel nitrate Iron nitrate Mixturesolution Iron nitrate

Nickel oxide Iron oxide Nickelferrite Nickel oxide + iron oxide

Nickel nitrate Iron nitrate Mixture solution Nickel nitrate

droplets geperated by the ultrasonic spray generator from pure water were directly measured by a light scattering device (Malvern 2604C). Figure 5 shows a SEM photograph of alumina particles, generated by ultrasonic spray from 0.2 m o l l - 1 aluminum sulfate solution followed by pyrolysis at 800°C. Particles are also spherical but slightly deformed. The particle size distribution calculated from SEM photographs is shown in Fig. 6. The mean size and standard deviation of prepared alumina particles are 0.99/zm and 0.97, respectively. The size distributions of estimated and measured liquid droplets generated by the ultrasonic spray generator along with alumina particles are shown in Fig. 6. The mean droplets size calculated from equation (1) is 5.8 gm. The lower bound of the mean size calculated by equation (2) is 1.7 #m. Note that the light scattering measurements for droplets from the ultrasonic generator are close to the values from equation (1). This suggests that the alumina particles prepared by ultrasonic spray pyrolysis are indeed dense and so the droplet size estimation by equation (1) is valid. We thus have reason to believe that the alumina particles prepared by our F E A G process are also dense, as indicated in the T E M image of Fig. 3. From these results, we concluded that the mean droplet size generated by the F E A G process is 2.1/~m and thus much smaller than the 5.8 #m of the ultrasonic spray generator. ESTIMATE OF DROPLET COLLISION PROBABILITY The possibility of collision and growth of liquid droplets at the down stream side of the filter was investigated qualitatively. For this purpose, two separate aerosol generator compartments were connected in parallel in addition to the system in Fig. 1 and different solutions were fed at the same liquid flow rate of 2.5 ml m i n - 1. The generated droplets were dried and decomposed at 800°C. The prepared particles were characterized by XRD. The types of solution charged in each compartment and resulting particles were summarized in Table 1. The experiments 1, 2, and 3 show that 800°C is high enough to form nickel oxide, iron oxide, and nickel ferrite particles. In experiment 4, one aerosol source generated droplets of nickel nitrate solution and the other source generated droplets of iron nitrate solution. After decomposing these droplets in the tubular reactor, an X-ray diffraction spectrum was obtained as shown in Fig. 7a. In this spectrum only nickel oxide and iron oxide peaks were observed. If individual droplets of nickel nitrate and iron nitrate solutions would have coalesced to a significant degree and formed a mixture droplet, its XRD spectrum would have appeared more like that of Fig. 7b which shows XRD peaks of nickel ferrite produced from a homogeneous mixture solution of nickel nitrate and iron nitrate. Therefore, we conclude that droplets did not collide excessively with each other to form larger droplets in the F E A G process at the estimated production rate of 1012 droplets per min. REFERENCES Cukauskas, E. J., Allen, L. H., Newman, H. S., Henry, R. L. and Van Damme, P. K. (1990) J. appl. Phys. 67, 6946-6952. Kodas, T. T. (1989)Angew. Chem. Int. Ed. Engl. Adv. Mater. 28, 794-806. Lang, R. T. (1962) J. Acoustical Soc. Amer. 34, 6-9. Lee, C. H. and Huang, C. S. (1994) Mat. Sci. Engn.q B22° 233-240.

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Y.C. Kang and S. B. Park

Lee, H. H. (1990) Fundamentals of Microelectronics Processino, Chap. 6. McGraw-Hill, New York. Meesters, G. M. H., Vercoulen, P. H. W., Marijnissen, J. C. M. and Scarlett, B. (1992) J. Aerosol Sci. 23, 37-49. Pike, R. D., Cui, H., Kershaw, R., Dwight, K. and Wold, A. (1993) Thin Solid Film. 224, 221-226. Zhang, S. C. and Messing, G. L. (1990) In Ceramic Powder Science 111 (Edited by G. L. Messing et al.), p. 49, American Ceramic Society, Columbus, OH. Zhang, S. C., Messing, G. L. and Huebner, W. (1991) J. Aerosol Sci. 22, 585-599.