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Simple generator of ultrafine particles for tests
NOTES Simple Generator of Ultrafine Particles for Tests
A convenient generator for producing ultrafine particles at atmospheric pressure is described. The generator requires no furnace, only resistance wire, quartz tubes, ampere meter, and AC power supply. Hence, it is simple to assemble the parts. The generation o f silver ultrafine particles was attempted by this method. It was confirmed that the silver ultrafine particles produced were free from chain aggregates, which often occur in other methods, and were relatively uniform. The ultrafine particles with the median diameter desired are simple to produce and stable for long periods by means of operating the AC power supply. © 1990AcademicPress,Inc.
INTRODUCTION A generator to produce stable monodispersed ultrafine particles for long periods for use in aerosols, inhalation, hygiene, and gas filtration studies is widely required. There are few simple and convenient generators mainly due to the difficulty in controlling the temperature distribution in a furnace, which is used in many methods. The present paper describes a generator consisting of heating elements only without a furnace.
GENERATOR Many conventional generators which produce high melting temperature particles contain a furnace (1, 2). They require much power due to indirect heating, and as a result it is difficult to control the temperature in the furnace precisely. On the other hand, there are reports of a method which directly varporizes such resistance wires as nichrome (3) and platinum (4) by an electric current. However, we could not obtain results similar to those reported. In the case of nichrome wire, such metal oxides as NiO and Cr203 formed on the surface due to heating in air would prevent the continuous generation ofultrafine particles. In the case of platinum wire, an overly high current in the heating wire often results in wire breakage. We have developed a simple and handy generator without a furnace which is illustrated in Fig. 1. A Kanthal resistance wire of 0.26 m m 4~ diameter and 17 cm length and a silver wire of 0.2 m m 8 diameter are prepared. The silver wire must be tightly wound around the Kanthal wire at a certain interval (approx. 5 m m in the present case). The interval winding of the silver wire allows the electric current to flow in the Kanthal resistance wire only. The
prepared Kanthal resistance wire is placed at the center of the double quartz tubes (2 cm and 0.6 cm diameter, respectively). Air is supplied to the inner tube at 0.6 liter/ min and to the outer tube at 3.5 liters/rain. GENERATION OF SILVER ULTRAFINE PARTICLES When the Kanthal resistance wire is heated above the melting point of silver, the wound silver wires are melted and they subsequently form many spherical droplets which stick to the surface of the Kanthal wire as shown in Fig. 2. Presumably, the temperature of these droplets may attain that of the surface of the Kanthal wire. The photograph shows that the relationship between the surface of the Kanthal wire and the silver droplets is nonwetting, because a metal oxide film is formed on the Kanthal wire during the initial heating. In addition, as the heat of the resistance wire is directly transferred to the silver droplets, the efficiency of the electric power is much higher than for any generator using a furnace.
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®® gas
FIG. 1. Schematic diagram of the generator to prepare ultrafine particles of silver. (1) Quartz tubes, (2) silver wire, (3) Kanthal resistance wire, (4) ampere meter, and (5) AC power supply.
535 Journal of Colloid and Interface Science, Vol. 140,No. 2, December1990
0021-9797/90 $3.00 Copyright© 1990by AcademicPress,Inc. All fightsof reproductionin any formreserved.
536
NOTES
FIG. 2. Silver droplets formed on Kanthal wire.
During operation, ultrafine particles of the desired median diameter are instantly produced by controlling the AC power supply. The ultrafine particles produced are thermally precipitated on the electron microscope grid at the outlet of a glass tube, and observed by using a TEM microscope. Various ultrafine particles of silver produced under different AC power conditions are shown in Fig. 3. A magnified photograph (×84,000) of Fig. 3A is shown in Fig. 4. It is found that ultrafine particles of less than approximately 0.015 um in size are produced in Fig. 3A. The examples of silver aerosol in Fig. 4 are approximately 0.007 ~zm in median diameter and 0.003 um in standard deviation. This value of standard deviation is by no means inferior to that of aerosols generated by conventional methods. The stability of this generator is an important point for practical use. We confirmed that there was little change in properties of particles after a run of 7 days at 8 h per day (56 h). However, as the silver evaporates gradually, further confirmation would have to be carried out for a longer term of the performance. We were unable to determine the concentration of particles generated due to the lack of a measuring instrument. A few seconds was required for sampling for observation with the TEM microscope. This fact will be useful for estimating the concentration of particles. A particular experiment has not been carried out to explore the range of particle diameters that can be obtained. From the experimental results de-
Journal of Colloid and Interface Science. Vol. 140, No. 2, December 1990
scribed above, it was proved that particles of more than 0.1 um could be produced under the condition of electrical power (75 W); however, most of them were not spherical in shape. Although the precipitation on the wall of ultrafine particles produced is an important practical point in previous reports, gas flowing from the outer tube of the double quartz tubes in the present method prevents ultrafine particles from precipitating on the wall. As far as the generation of ultrafine particles of silver is concerned, Muir et al. (5) reported on a method consisting of a furnace. The ultrafine particles produced with the present generator are similar to those produced by them despite the simplicity of assembly and performance.
CONCLUSION A convenient generator using a resistance wire but no furnace to produce ultrafine particles at atmospheric pressure was presented. The desired median diameter of ultrafine particles is quick and simple to produce by controlling the AC power supply. Assembly of various diameter resistance wires may enable us to produce ultrafine particles with the wide range of size distribution desired. Ultrafine particles with higher melting points may be generated simply by the use of wires such as tungsten.
NOTES
537
FIG. 4. Magnified photograph (?<84,000) of Fig. 3A.
REFERENCES 1. Jacohson, R. T., Kerber, M., and Matizevic, E., J. Phys. Chem. 71, 514 (1967). 2. Kasper, G., and Bernnor, A., Staub 38, 183 (1978). 3. Polydorova, M., Staub 29, 248 (1969). 4. Nolan, P. J., and Kennan, E. L., Proc. R. It. Acad. 52, 171 (1949). 5. Muir, D. C. F., and Cena, K., Aerosol Sei. Technol. 6, 303 (1987). A. GOTOH~ H . IKAZAKI M. KAWAMURA
National Chemical Laboratory for Industry Tsukuba Research Center Ibaraki, Japan Received November 27, 1989; accepted April 18, 1990
1To whom all correspondence should be addressed.
FIG. 3 (A-C). Transmission electron micrographs ofultrafine particles of silver (electric power supplied: A, 37 W; B, 50 W, C, 75 W). Journal of Colloid and Interface Science, Vol. 140, No. 2, December 1990
A convenient generator for producing ultrafine particles at atmospheric pressure is described. The generator requires no furnace, only resistance wire, quartz tubes, ampere meter, and AC power supply. Hence, it is simple to assemble the parts. The generation o f silver ultrafine particles was attempted by this method. It was confirmed that the silver ultrafine particles produced were free from chain aggregates, which often occur in other methods, and were relatively uniform. The ultrafine particles with the median diameter desired are simple to produce and stable for long periods by means of operating the AC power supply. © 1990AcademicPress,Inc.
INTRODUCTION A generator to produce stable monodispersed ultrafine particles for long periods for use in aerosols, inhalation, hygiene, and gas filtration studies is widely required. There are few simple and convenient generators mainly due to the difficulty in controlling the temperature distribution in a furnace, which is used in many methods. The present paper describes a generator consisting of heating elements only without a furnace.
GENERATOR Many conventional generators which produce high melting temperature particles contain a furnace (1, 2). They require much power due to indirect heating, and as a result it is difficult to control the temperature in the furnace precisely. On the other hand, there are reports of a method which directly varporizes such resistance wires as nichrome (3) and platinum (4) by an electric current. However, we could not obtain results similar to those reported. In the case of nichrome wire, such metal oxides as NiO and Cr203 formed on the surface due to heating in air would prevent the continuous generation ofultrafine particles. In the case of platinum wire, an overly high current in the heating wire often results in wire breakage. We have developed a simple and handy generator without a furnace which is illustrated in Fig. 1. A Kanthal resistance wire of 0.26 m m 4~ diameter and 17 cm length and a silver wire of 0.2 m m 8 diameter are prepared. The silver wire must be tightly wound around the Kanthal wire at a certain interval (approx. 5 m m in the present case). The interval winding of the silver wire allows the electric current to flow in the Kanthal resistance wire only. The
prepared Kanthal resistance wire is placed at the center of the double quartz tubes (2 cm and 0.6 cm diameter, respectively). Air is supplied to the inner tube at 0.6 liter/ min and to the outer tube at 3.5 liters/rain. GENERATION OF SILVER ULTRAFINE PARTICLES When the Kanthal resistance wire is heated above the melting point of silver, the wound silver wires are melted and they subsequently form many spherical droplets which stick to the surface of the Kanthal wire as shown in Fig. 2. Presumably, the temperature of these droplets may attain that of the surface of the Kanthal wire. The photograph shows that the relationship between the surface of the Kanthal wire and the silver droplets is nonwetting, because a metal oxide film is formed on the Kanthal wire during the initial heating. In addition, as the heat of the resistance wire is directly transferred to the silver droplets, the efficiency of the electric power is much higher than for any generator using a furnace.
0
®® gas
FIG. 1. Schematic diagram of the generator to prepare ultrafine particles of silver. (1) Quartz tubes, (2) silver wire, (3) Kanthal resistance wire, (4) ampere meter, and (5) AC power supply.
535 Journal of Colloid and Interface Science, Vol. 140,No. 2, December1990
0021-9797/90 $3.00 Copyright© 1990by AcademicPress,Inc. All fightsof reproductionin any formreserved.
536
NOTES
FIG. 2. Silver droplets formed on Kanthal wire.
During operation, ultrafine particles of the desired median diameter are instantly produced by controlling the AC power supply. The ultrafine particles produced are thermally precipitated on the electron microscope grid at the outlet of a glass tube, and observed by using a TEM microscope. Various ultrafine particles of silver produced under different AC power conditions are shown in Fig. 3. A magnified photograph (×84,000) of Fig. 3A is shown in Fig. 4. It is found that ultrafine particles of less than approximately 0.015 um in size are produced in Fig. 3A. The examples of silver aerosol in Fig. 4 are approximately 0.007 ~zm in median diameter and 0.003 um in standard deviation. This value of standard deviation is by no means inferior to that of aerosols generated by conventional methods. The stability of this generator is an important point for practical use. We confirmed that there was little change in properties of particles after a run of 7 days at 8 h per day (56 h). However, as the silver evaporates gradually, further confirmation would have to be carried out for a longer term of the performance. We were unable to determine the concentration of particles generated due to the lack of a measuring instrument. A few seconds was required for sampling for observation with the TEM microscope. This fact will be useful for estimating the concentration of particles. A particular experiment has not been carried out to explore the range of particle diameters that can be obtained. From the experimental results de-
Journal of Colloid and Interface Science. Vol. 140, No. 2, December 1990
scribed above, it was proved that particles of more than 0.1 um could be produced under the condition of electrical power (75 W); however, most of them were not spherical in shape. Although the precipitation on the wall of ultrafine particles produced is an important practical point in previous reports, gas flowing from the outer tube of the double quartz tubes in the present method prevents ultrafine particles from precipitating on the wall. As far as the generation of ultrafine particles of silver is concerned, Muir et al. (5) reported on a method consisting of a furnace. The ultrafine particles produced with the present generator are similar to those produced by them despite the simplicity of assembly and performance.
CONCLUSION A convenient generator using a resistance wire but no furnace to produce ultrafine particles at atmospheric pressure was presented. The desired median diameter of ultrafine particles is quick and simple to produce by controlling the AC power supply. Assembly of various diameter resistance wires may enable us to produce ultrafine particles with the wide range of size distribution desired. Ultrafine particles with higher melting points may be generated simply by the use of wires such as tungsten.
NOTES
537
FIG. 4. Magnified photograph (?<84,000) of Fig. 3A.
REFERENCES 1. Jacohson, R. T., Kerber, M., and Matizevic, E., J. Phys. Chem. 71, 514 (1967). 2. Kasper, G., and Bernnor, A., Staub 38, 183 (1978). 3. Polydorova, M., Staub 29, 248 (1969). 4. Nolan, P. J., and Kennan, E. L., Proc. R. It. Acad. 52, 171 (1949). 5. Muir, D. C. F., and Cena, K., Aerosol Sei. Technol. 6, 303 (1987). A. GOTOH~ H . IKAZAKI M. KAWAMURA
National Chemical Laboratory for Industry Tsukuba Research Center Ibaraki, Japan Received November 27, 1989; accepted April 18, 1990
1To whom all correspondence should be addressed.
FIG. 3 (A-C). Transmission electron micrographs ofultrafine particles of silver (electric power supplied: A, 37 W; B, 50 W, C, 75 W). Journal of Colloid and Interface Science, Vol. 140, No. 2, December 1990