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Selective influence of electromagnetic fields on aerosol systems
J Aerosol Sci. Vol. 30, Suppl. 1, pp. S321-S322, 1999 © 1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/99/$ - see front matter
Pergamon
SELECTIVE INFLUENCE OF ELECTROMAGNETIC FIELDS ON AEROSOL SYSTEMS Hee Young Kim 1, Hyung Chun Kim 1, V.V. Levdansky 2, p. Moravec a and J. Smolik 3 ~Korea Research Institute of Chemical Technology, Yusong, Taejon 305-600, Korea 2Heat and Mass Transfer Institute NAS RB, 15 P. Brovka St., 220072 Minsk, Belarus 3 . Institute of Chemical Process Fundamentals AS CR, Rozvojovh 135, 165 02 Prague 6, Czech Republic
KEYWORDS aerosol particles, electromagnetic field, heating INTRODUCTION It is known that the electromagnetic field can selectively affect the substance both in gas and in condensed phases. The influence of electromagnetic radiation is different in different ranges of wave lengths. For example the laser radiation can excite the internal degrees of freedom of molecules and the microwave radiation can selectively and uniformly heat substances that are characterized by high coefficient of the microwave absorption. The paper deals with the selective influence of electromagnetic radiation on aerosol systems both in optical and microwave wave length ranges. RESULTS AND DISCUSSION Some effects related to excitation of gas molecules by resonance (e.g. laser) radiation are discussed in the paper. It is shown that the difference in the interaction of excited and nonexcited molecules with the surface introduces new components of photophoretic force acting on aerosol particles in resonance radiation field. The resonance radiation also allows to affect the composition of aerosol particles growing from a gas mixture (Levdansky, 1991). The selective decomposition of gaseous precursors by means of laser radiation can be used for synthesis ofnanoparticles (Moravec et al., 1997). It is known that selective heating of microwave-absorbing components by microwave radiation can increase the reaction rate in the reactor filled with the catalyst particles that respectively contain much smaller chemically active particles (active sites) (Thomas, 1997). Heating of aerosol particles can also increase the reaction rate of some chemically active molecules of a gas phase on the particle surface. The mathematical model is formulated to analyze the effects that are caused by selective heating of aerosol particles with microwaves, including temperature dependence of absorption coefficient and the heat effect of a chemical reaction. Higher temperature of aerosol particles, as compared to surrounding gas, can produce the repulsive force between aerosol particles and thus decrease their coagulation. The study then discusses the possibility to use microwaves for cleaning of the surface of substrate from deposited aerosol particles. The cleaning is a very important step to be executed, in particular, before the deposition of thin films onto the surface during semiconductor processing. It is known that the capillary-condensed water exists at the volume between a spherical aerosol particle and the substrate surface under a moist atmosphere (Fuchs, 1964). It increases force of the particle adhesion to the surface, preventing the detachment of particles from the substrate (e.g. by gas stream). When microwaves are selectively absorbed by water, the particles and/or the substrate, the temperature of the
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Abstracts of the 1999EuropeanAerosolConference
capillary-condensed water increases. This decreases the surface tension of water and the adhesive force of a particle to substrate surface. Moreover, such heating decreases the radius of the film of the capillary-condensed water due to its evaporation. It then reduces contact heat transfer between the particle and the substrate that can lead to further heating of the particle. The evaporation of liquid film on aerosol particles decreases also cohesive forces between the particles. The microwave radiation can be also used in combination with other processes. Let us consider the problem of cleaning of gas phase from toxic or biological aerosol particles that can be realized by condensation of the supersaturated water vapor on the particle surface. The growing water layer prevents the possible evaporation of molecules of toxic substances from aerosol particles. In addition, molecules of gas impurities, including the molecules of noncondensable components, can be trapped by the growing layer of condensate (Levdansky, 1991). When sufficiently enlarged, the water droplets containing both aerosol particles and impurity molecules fall dawn and can be collected. For accumulation and removal of the collected substances the study proposes to use a porous body composed of a microwave-absorbing skeleton material. When saturated with the substance, the porous collector can be taken away from the system for reactivation. Then a proper chemical or thermal treatment of the collector removes and/or decomposes the collected substances and activates the porous body for reuse. Microwave radiation can be applied for this step. Using the selective heating by microwave radiation, the microwaveabsorbing components - such as water, polar impurity molecules and the skeleton of the porous body - can be quickly heated to a sufficiently high temperature for evaporation of the collected substances and for reactivation of the porous collector. During this step the collected toxic substances can decompose. Thus, it is shown that the selective influence of the electromagnetic radiation can be used to control different processes in aerosol systems. It is interesting to note that small particles produced by laser-assisted chemical deposition can be used for manufacture of catalysts that exhibit high effectiveness under influence of microwave radiation. REFERENCES Levdans~-, V.V. (1991) Transfer processes in aerosol systems with influence of resonance radiation. In Heat Transfer in lnfluence of Radiation Fluxes on Materials. HMTI Press, Minsk, 40-52 (in Russian). Moravec, P., Smolik, J., Levdansky, V.V. (1997) Nano-sized particles from the CO2 laser assisted decomposition oftetraethylorthosilicate vapour. J. Mat. ScL Lett. 16, 648-651. Thomas, J.R., Jr. (1997) Heat transfer model of microwave enhanced catalysis. In Microwaves: Theory and Application in Materials Processing IV, Am. Ceramic Soc., Ceramic Transactions. 80, 349-356. Fuchs, N.A. (1964) The Mechanics of Aerosols Pergamon Press, New York.
Pergamon
SELECTIVE INFLUENCE OF ELECTROMAGNETIC FIELDS ON AEROSOL SYSTEMS Hee Young Kim 1, Hyung Chun Kim 1, V.V. Levdansky 2, p. Moravec a and J. Smolik 3 ~Korea Research Institute of Chemical Technology, Yusong, Taejon 305-600, Korea 2Heat and Mass Transfer Institute NAS RB, 15 P. Brovka St., 220072 Minsk, Belarus 3 . Institute of Chemical Process Fundamentals AS CR, Rozvojovh 135, 165 02 Prague 6, Czech Republic
KEYWORDS aerosol particles, electromagnetic field, heating INTRODUCTION It is known that the electromagnetic field can selectively affect the substance both in gas and in condensed phases. The influence of electromagnetic radiation is different in different ranges of wave lengths. For example the laser radiation can excite the internal degrees of freedom of molecules and the microwave radiation can selectively and uniformly heat substances that are characterized by high coefficient of the microwave absorption. The paper deals with the selective influence of electromagnetic radiation on aerosol systems both in optical and microwave wave length ranges. RESULTS AND DISCUSSION Some effects related to excitation of gas molecules by resonance (e.g. laser) radiation are discussed in the paper. It is shown that the difference in the interaction of excited and nonexcited molecules with the surface introduces new components of photophoretic force acting on aerosol particles in resonance radiation field. The resonance radiation also allows to affect the composition of aerosol particles growing from a gas mixture (Levdansky, 1991). The selective decomposition of gaseous precursors by means of laser radiation can be used for synthesis ofnanoparticles (Moravec et al., 1997). It is known that selective heating of microwave-absorbing components by microwave radiation can increase the reaction rate in the reactor filled with the catalyst particles that respectively contain much smaller chemically active particles (active sites) (Thomas, 1997). Heating of aerosol particles can also increase the reaction rate of some chemically active molecules of a gas phase on the particle surface. The mathematical model is formulated to analyze the effects that are caused by selective heating of aerosol particles with microwaves, including temperature dependence of absorption coefficient and the heat effect of a chemical reaction. Higher temperature of aerosol particles, as compared to surrounding gas, can produce the repulsive force between aerosol particles and thus decrease their coagulation. The study then discusses the possibility to use microwaves for cleaning of the surface of substrate from deposited aerosol particles. The cleaning is a very important step to be executed, in particular, before the deposition of thin films onto the surface during semiconductor processing. It is known that the capillary-condensed water exists at the volume between a spherical aerosol particle and the substrate surface under a moist atmosphere (Fuchs, 1964). It increases force of the particle adhesion to the surface, preventing the detachment of particles from the substrate (e.g. by gas stream). When microwaves are selectively absorbed by water, the particles and/or the substrate, the temperature of the
$321
$322
Abstracts of the 1999EuropeanAerosolConference
capillary-condensed water increases. This decreases the surface tension of water and the adhesive force of a particle to substrate surface. Moreover, such heating decreases the radius of the film of the capillary-condensed water due to its evaporation. It then reduces contact heat transfer between the particle and the substrate that can lead to further heating of the particle. The evaporation of liquid film on aerosol particles decreases also cohesive forces between the particles. The microwave radiation can be also used in combination with other processes. Let us consider the problem of cleaning of gas phase from toxic or biological aerosol particles that can be realized by condensation of the supersaturated water vapor on the particle surface. The growing water layer prevents the possible evaporation of molecules of toxic substances from aerosol particles. In addition, molecules of gas impurities, including the molecules of noncondensable components, can be trapped by the growing layer of condensate (Levdansky, 1991). When sufficiently enlarged, the water droplets containing both aerosol particles and impurity molecules fall dawn and can be collected. For accumulation and removal of the collected substances the study proposes to use a porous body composed of a microwave-absorbing skeleton material. When saturated with the substance, the porous collector can be taken away from the system for reactivation. Then a proper chemical or thermal treatment of the collector removes and/or decomposes the collected substances and activates the porous body for reuse. Microwave radiation can be applied for this step. Using the selective heating by microwave radiation, the microwaveabsorbing components - such as water, polar impurity molecules and the skeleton of the porous body - can be quickly heated to a sufficiently high temperature for evaporation of the collected substances and for reactivation of the porous collector. During this step the collected toxic substances can decompose. Thus, it is shown that the selective influence of the electromagnetic radiation can be used to control different processes in aerosol systems. It is interesting to note that small particles produced by laser-assisted chemical deposition can be used for manufacture of catalysts that exhibit high effectiveness under influence of microwave radiation. REFERENCES Levdans~-, V.V. (1991) Transfer processes in aerosol systems with influence of resonance radiation. In Heat Transfer in lnfluence of Radiation Fluxes on Materials. HMTI Press, Minsk, 40-52 (in Russian). Moravec, P., Smolik, J., Levdansky, V.V. (1997) Nano-sized particles from the CO2 laser assisted decomposition oftetraethylorthosilicate vapour. J. Mat. ScL Lett. 16, 648-651. Thomas, J.R., Jr. (1997) Heat transfer model of microwave enhanced catalysis. In Microwaves: Theory and Application in Materials Processing IV, Am. Ceramic Soc., Ceramic Transactions. 80, 349-356. Fuchs, N.A. (1964) The Mechanics of Aerosols Pergamon Press, New York.