Patent Publication Number: US-8118170-B2

Title: Particulate size classification apparatus and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application Nos. 2006-338893, filed on Dec. 15, 2006, and 2006-001166, filed on Jan. 6, 2006, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a particulate size classification apparatus and method classifying sizes of nanoparticles to be used in the fields of luminescent materials, cosmetics, electronics, catalysts, and many others. 
     2. Description of the Related Art 
     Since particulates having sizes of the order of nanometers, so called nanoparticles, generally have a unique property presented due to size effects, or a large specific surface area, studies pursing their applications have been vigorously conducted in many areas in recent years. As examples of such applications, use of silicon nanoparticles as luminescent materials (see, for example, Non-Patent Document 1) or use of titanium oxide nanoparticles is known, and in any application, it is very important to control the sizes of nanoparticles. 
     [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-22886 
     [Non-Patent Document 1] T. Orii et al., Appl. Phys. Lett. 83 (2003)3395 
     [Non-Patent Document 2] Shouheng Sun et al., Science 287, 1989 (2000) 
     [Non-Patent Document 3] P. A. Baron, K. Willeke, Aerosol Measurements Principles, Techniques, and Applications, 2nd ed. Wiley, New York, 2001 
     [Non-Patent Document 4] Suzuki et al., APPLIED PHYSICS LETTER, VOL. 78, page 2043, 2001 
     Methods for producing nanoparticles may be classified mainly into methods with liquid phase systems and methods with gas phase systems, and the method using a reaction in a liquid phase system (see, for example, Non-Patent Document 2) has an advantage that particles have relatively uniform sizes, but raises a concern of existence of impurities regarding electric applications intended by the present inventor because the method uses surfactants, organic solvents and the like. 
     In the method using a reaction in a gas phase system, for example a laser ablation method or a plasma CVD (Chemical Vapor Deposition) method, clean particulates are generally obtained, but it is not easy to obtain particles having uniform sizes. For classifying the sizes of such particles, a differential mobility analyzer (DMA) is often used. 
     The DMA is an apparatus classifying particulates according to size using the electric mobility of particulates in a gas, and it is frequently used in areas of aerosols. The DMA is also often used in the subsidiary research institute of the inventor, since particle sizes can be made uniform satisfactorily when nanoparticles are classified using the DMA (see, for example, Patent Document 1). 
     However, the DMA is not perfect, and has a disadvantage that is considered fetal on some application areas. The disadvantage is that the amount of particulates (throughput) obtained through classification is very small particularly when the DMA is used for classification of nanoparticles. 
     Normally, in size classification by the DMA, it is necessary that particulates be charged, but it is difficult to charge nanoparticles, especially nanoparticles of 10 nm or less, with high efficiency, and therefore the aforementioned problem occurs. In this connection, the probability that particulates having sizes of, for example, 10 nm or less are obtained through size classification by the DMA is at most 2 to 3% of the amount of such particulates that actually exist. This raises a very serious problem when particulates having uniform sizes are put to practical use on a commercial basis. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the problem described above and its object is to provide a particulate size classification apparatus and method allowing reliable and easy size classification of particulates called nanoparticles (principally with diameters of 10 nm or less) with high throughput. 
     The particulate size classification apparatus of the present invention comprises a particulate size classifying chamber provided with an exhaust port for particulates, particulate introducing unit having a nozzle ejecting to the inside of the particulate size classifying chamber a carrier gas containing particulates to be classified, and particulate trapping unit provided in the particulate size classifying chamber and selectively trapping particulates ejected from the nozzle, wherein among the particulates ejected from the nozzle, particulates made uniform in size by not being trapped by the particulate trapping unit are ejected from the exhaust port. 
     In the particulate size classification method of the present invention, a carrier gas containing particulates to be classified is ejected to the inside of a particulate size classifying chamber, particulates are selectively trapped by particulate trapping unit provided in the particulate size classifying chamber, and particulates made uniform in size by not being trapped by the particulate trapping unit are collected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view of essential parts necessary for understanding the principle of an impactor of the present invention; 
         FIG. 2  is an explanatory view of essential parts showing a particulate size classification apparatus according to the embodiment; 
         FIG. 3  is an explanatory view of essential parts showing the particulate size classification apparatus of example 1; 
         FIG. 4  is a slant view of essential parts showing the details of the impactor in example 1; 
         FIG. 5  shows an SEM image of the surface of a trapping plate coated with an anodized aluminum oxide film; 
         FIG. 6  is a perspective view of essential parts schematically showing a situation of the aluminum oxide film formed on the surface of the trapping plate; 
         FIG. 7  is a slant view of essential parts showing one example in which carbon nanotubes are formed on the surface of the trapping plate; 
         FIG. 8  is a slant view of essential parts showing another example in which carbon nanotubes are formed on the surface of the trapping plate; 
         FIG. 9  is a characteristic diagram showing the amount and size distribution of particulates deposited on a substrate; 
         FIGS. 10A and 10B  are distribution charts showing results of classifying nanoparticles by the particulate size classification apparatus based on a comparison with a comparative example; 
         FIG. 11  is a characteristic diagram for explaining a few problems in example 1; and 
         FIG. 12  is a slant view of essential parts showing a situation of the trapping plate in example 1. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention uses a method making use of the inertia of particulates rather than depending on the charge state of particulates for performing size classification of particulates with high throughput. Specifically, so called an impactor is used under a low pressure to perform size classification of particulates. 
       FIG. 1  is an explanatory view of essential parts necessary for understanding the principle of the impactor of the present invention. 
     The impactor comprises a particulate size classifying chamber  1  provided with an exhaust port  4  for particulates, a nozzle  2  for ejecting to the inside of the particulate size classifying chamber  1  a carrier gas containing particulates to be classified, a trapping plate  3  as particulate trapping unit provided in the particulate size classifying chamber  1  and selectively trapping particulates ejected from the nozzle  2 . 
     As shown in the figure, the impactor has the trapping plate (baffle plate)  3  placed at the front of the nozzle  2 , and among particulates carried with a gas, those having a size equal to or a greater than a certain level, hence those having an inertia equal to or greater than a certain level, cannot follow the flow of the gas, collide against the trapping plate  3  placed in the downstream of the nozzle  2 , and are thereby trapped. Here, whether or not particulates are trapped is normally described by a dimensionless parameter (function of the particulate size, the inner diameter of the nozzle  2 , the gas velocity and so on) called the Stokes number (Non-Patent Document 2). In this impactor, particulates (of same size) made uniform in size by not being trapped by the trapping plate  3  are ejected through the exhaust port  4  and collected. 
     The impactor is used for trapping particulates principally of submicron or micron size in the area of aerosols. In the present invention, various modifications are made for applying such an impactor for size classification of nanoparticles. 
       FIG. 2  is an explanatory view of essential parts showing the particulate size classification apparatus according to this embodiment. Here, symbols same as those used in  FIG. 1  denote same parts or have same meanings. 
     This particulate size classification apparatus comprises a particulate size classifying chamber  1 , a particulate introducing section  10  having a nozzle  2 , and a trapping plate  3 . 
     The particulate introducing section  10  comprises in addition to the nozzle  2  a transportation pipe  11  for transporting a carrier gas containing particulates, a gas exhaust port  13  for the carrier gas provided separately from the nozzle  2 , and a conductance adjusting valve  12  provided in the upstream of the gas exhaust port  13  and adjusting the flow rate of the carrier gas. Here, reference numeral  14  denotes a particulate classified and ejected from the exhaust port  4 . 
     The trapping plate  3  is a discoid member having a surface having a porous structure, having a rotational axis perpendicular to the surface and made rotatable at a predetermined speed, and is placed so as to be movable (slidable) vertically and laterally. Further, the trapping plate  3  has a structure such that it can be replaced under a low pressure or in vacuum. This structure will be described in detail later. 
     In this embodiment, as particulates classified according to size, those ejected to the downstream of the impactor are used. Normally, the impactor acts to trap particulates of sizes equal to or greater than a certain size, and therefore particulates in the downstream include all particulates of sizes smaller than the certain size, and generally have a broad distribution of sizes. 
     Since nanoparticles, especially nanoparticles of 10 nm or smaller, cannot stably retain their existence, and the number of nanoparticles of small sizes decreases over time due to coagulation and the like, there is a lower limit on the particle size. As a result, the size classification apparatus sufficiently functions merely by removing nanoparticles of large sizes. 
     In the present invention, the degree of decrease in the number of aforementioned nanoparticles of small sizes is controlled to actively control the lower limit on the size of nanoparticles. 
     For employing the impactor in the particulate size classification apparatus of the present invention, the impactor is subjected to various modifications in performing classification of nanoparticles, and those modifications will be summarized below. 
     (1) Use of Low Pressure 
     For performing classification by making use of the inertia of nanoparticles of 10 nm or less, it is preferable that the inside of the particulate size classifying chamber  1  is kept at a low pressure, for example 2.67×10 3  Pa (20 Torr) or less at the time of size classification of particulates. For this purpose, for example, the inside of the particulate size classifying chamber  1  may be evacuated by a vacuum pump or the like to adjust the pressure to be a predetermined value of 2.67×10 3  Pa or less. 
     (2) Use of Helium Gas 
     For using the impactor with good controllability, it is necessary to use the impactor at a gas velocity equal to or less than a sonic velocity. Generally, classification of small nanoparticles becomes easier as the gas velocity increases, and therefore a helium gas inert and having a high sonic velocity is used. 
     (3) Mechanism for Replacement of Nozzle  2  and Trapping Plate  3   
     For controlling the aforementioned stokes number, it is necessary to change the diameter of the nozzle  2 , but because of the low-pressure circumstance, a mechanism capable of replacing the nozzle  2  without disturbing the set low-pressure (vacuum) state was provided. Since excessive deposition of particulates on the trapping plate  3  deteriorates the classification performance, a mechanism scanning the trapping plate  3  to the nozzle  2  and a mechanism capable of replacing the trapping plate  3  in vacuum were provided. 
     (4) Installation of Gas Exhaust Port  13  Capable of Conductance Adjustment in Upstream of Nozzle  2   
     The exhaust port  13  in the upstream of the nozzle is normally connected directly to a pump or the like through the conductance adjusting valve  12 , and particulates carried with a gas are discarded. The exhaust port  13  may be connected to another deposition chamber rather than the pump. By controlling the amount of exhaust gas from the exhaust port  13  and the flow rate of the gas to the nozzle  2 , particles of small sizes can be removed using diffusion losses/coagulation in the transportation pipe  11 , or conversely, small particles can be introduced in a larger amount to the downstream of the nozzle  2 . Details there of will be described later as an example. 
     (5) Application of New Material to Trapping Plate  3   
     Conventionally, a silicone oil is coated on the surface of the trapping plate for preventing the rebound of particulates colliding against the trapping plate in the impactor (see, for example, Non-Patent Document 3). However, when cleanliness is required, for example when nanoparticles are used in electric applications, a trapping plate coated with a silicone oil, or the like, cannot be used. In the present invention, for solving this problem, the surface of the trapping plate  3  is modified. Specifically, a trapping plate having a surface having a complicated porous structure, such as an aluminum anodized plate, is used. By this complicated structure, the rebound of nanoparticles can be prevented. A trapping plate having nanostructures such as carbon nanotubes grown or coated on a substrate is also effective. 
     EXAMPLES 
     In the present invention, the impactor could be used effectively for size classification of nanoparticles as a result of making various improvements described above. Details of the present invention are described below using various embodiments. 
     Example 1 
       FIG. 3  is an explanatory view of essential parts showing a particulate size classification apparatus of example 1. 
     As shown in the figure, reference numeral  21  denotes a particle generating chamber, reference numeral  22  denotes a target of cobalt, reference numeral  23  denotes a pulse laser made of Nd:YAG, reference numeral  24  denotes a transportation pipe for a carrier gas containing particulates, reference numeral  25  denotes a conductance adjusting valve, reference numeral  26  denotes a conductance adjusting gas exhaust port, reference numeral  27  denotes a particulate size classifying chamber, reference  28  denotes a nozzle, reference numeral  29  denotes a trapping plate, reference numeral  31  denotes an exhaust port, reference numeral  32  denotes a particulate deposition chamber, and reference numeral  33  denotes a substrate. The emission wavelength of the Nd:YAG laser  23  is 532 nm, the output is 4 W, the repeated frequency is 20 Hz, the tip orifice diameter of the nozzle  28  is 3.25 mm, and the pressure of the inside of the particulate deposition chamber  32  is adjusted to be 1.33×10 −3  Pa (10 −5  Torr). 
     In this example, cobalt particulates are generated by laser ablation of the target  22  made of cobalt. Namely, the target  22  made of cobalt, which is placed in the particulate generating chamber  21  adjusted to have a pressure of 6.67×10 2  Pa to 1.33×10 3  (5 Torr to 10 Torr), is irradiated with a laser beam generated by the pulse laser  23  to generate a cobalt vapor. 
     The vapor is quenched by a carrier gas flowing at a flow rate of 0.5 slpm to 1 slpm (standard liters per minute) and consisting of He to generate particulates. The particulates are transported through the transportation pipe  24  having a length of about 1 m to an impactor section having the nozzle  28  and the trapping plate  29  as main elements. 
     In the upstream of the nozzle  28 , the conductance adjusting exhaust port  26  is provided via the valve  25  capable of conductance adjustment, and connected from that point onward to a vacuum pump. In this example, the valve  25  is adjusted, whereby 0 to 0.5 slpm of helium flows to the exhaust port  26 , and remaining helium is guided to the nozzle  28  together with particulates. Here, setting the flow rate of helium from the exhaust port  26  to 0 is equivalent to a configuration in which none of the valve  25  and the exhaust port  26  is provided on a particulate introducing section and only the nozzle  28  is an ejection port for particulate-containing helium introduced into the transportation pipe  24 . In this case, in this example, the valve  25  may be adjusted so that the flow rate of helium from the exhaust port  26  is 0, or none of the valve  25  and the exhaust port  26  may be provided on the particulate introducing section. In this connection, in  FIG. 3 , the transportation pipe and the nozzle are placed perpendicularly to each other and the transportation pipe and the conductance adjusting gas exhaust port are placed horizontally to each other, but the positional relationship is not limited thereto. 
       FIG. 4  is a slant view of essential parts showing the details of the impactor in example 1. Here, symbols same as those used in  FIG. 3  denote same parts or have same meanings. 
     In  FIG. 4 , reference numeral  41  denotes a particulate size classifying chamber kept at a low pressure or in vacuum, reference numeral  42  denotes a holder of a trapping plate  29 , reference numeral  43  denotes a nozzle plate having a plurality of ejection ports  28  of nozzles having different tip orifice diameters, reference numeral  44  denotes a tube provided separately from the ejection port  28  and intended for introducing a carrier gas containing particulates, reference numeral  45  denotes a flange, reference numeral  46  denotes an O-ring, reference numeral  47  denotes a drive mechanism for the nozzle plate  43 , and reference numeral  48  denotes a drive mechanism for the trapping plate  29 . 
     In the nozzle plate  43  shown in the figure, a plurality of ejection ports  28  of nozzles having different tip orifice diameters are provided on, for example, a plate having a thickness of about 10 mm, and by moving the nozzle plate  43  up and down, any of a plurality of ejection ports  28  can be selected. 
     In this example, a plurality of ejection ports  28  having tip orifice diameters of 4 mm, 3.6 mm, 3.25 mm and 3 mm are dug in the nozzle plate  43 , but one having an orifice diameter of 3.25 mm is often used in the example described later. 
     The nozzle plate  43  is driven to select, for example, the ejection port  28  having an orifice diameter of 3.25 mm, and the ejection port  28  is pressed against the flange  45  having the O-ring  46  in the tube  44  to fixedly couple the tube  44  and the nozzle  28  to each other. This work can be carried out in the particulate size classifying chamber  41  kept at a low pressure or in vacuum without breaking the vacuum condition of low pressure. 
     In the downstream of the nozzle  28 , the trapping plate  29  is placed at a distance twice as great as the inner diameter (the tip orifice diameter) of the nozzle, and as shown in  FIG. 4 , the trapping plate  29  can be scanned freely in a direction perpendicular to the nozzle  28  and in a circumferential direction by the drive mechanism  48 . 
     In the example shown in the figure, a disc having a diameter of 10 cm is used for the trapping plate  29 , the trapping plate  29  can be scanned in a longitudinal direction and a circumferential direction, the speed in the longitudinal direction is 1 mm/second and the rotational speed is 2 rpm at this time, and in this case, the surface (surface on which particulates are trapped) of the trapping plate  29  is coated with an anodized aluminum oxide film having a thickness of, for example, about 100 nm. 
       FIG. 5  shows an SEM (Scanning Electron Microscope) image of the surface of the trapping plate coated with an anodized aluminum oxide film. 
     An aluminum oxide film  61  formed on the surface of the trapping plate  29  has a porous structure as shown in, for example,  FIG. 6 , and each pore  62  is formed to have a pitch of about 50 nm, a diameter of about 20 nm to 25 nm and a depth of about 100 nm. In this connection, the pore size is variable, and is not limited to this value. 
     Instead of the aluminum oxide film, carbon nanotubes may be formed on the surface of the trapping plate  29 .  FIG. 7  is a slant view of essential parts showing a situation in which carbon nanotubes are formed on the surface of the trapping plate  29 . 
     Here, for inhibiting the rebound of particulates on the surface of the trapping plate  29 , carbon nanotubes  49  were formed on the surface. On the surface of the trapping plate  29 , carbon nanotubes  49  are orientationally grown upward from the surface of the trapping plate  29 . 
     In this case, it is not necessary to specifically limit the diameter, the length and the number of carbon nanotubes  49 , but it is known that a plate on which, for example, carbon nanotubes having a diameter of about 10 nm and a length of about 5000 nm are grown at a density of 10 10  tubes per cm 2  is extremely useful as a trapping plate. 
       FIG. 7  shows carbon nanotubes  49  oriented upward from the surface of the trapping plate  29 , but the structure of  FIG. 8  in which tubes  49  lie and are entangled is also effective. 
     Instead of forming carbon nanotubes, a configuration in which so called nanowires of silicon, zinc oxide and the like having a high aspect ratio structure are grown or placed on the surface of the trapping plate  29  is similarly effective. In this case, the diameter, the length, the number and the like are not limited, but it has been confirmed that a plate on which nanowires having, for example, a diameter of about 15 nm and a length of about 1000 nm are grown at a density of about 5×10 9  wires per cm 2  is effective. 
     A Silicon wafer may be used as a substrate in the trapping plate. In this case, the trapping plate can be replaced via a load lock or the like under vacuum or a low pressure as in conveyance of a wafer in a normal semiconductor process, and for fixation of the trapping plate on a holder, a hooked fastener may be used or an electrostatic chuck may be used. 
     When the flow rate of a particulate-containing carrier gas flowing to the ejection port  28  is 500 sccm and the pressure is 4.79×10 2  Pa (3.6 Torr), particulates with diameters of 1.5 nm or greater are deposited on the trapping plate  29 . Particulates which have not been trapped are guided through the exhaust port  31  to the deposition chamber  32 . In this example, the deposition chamber  32  is kept at a pressure of about 1.33×10 −3  Pa (10 −5  Torr) by differential pumping, so that particulates can reliably be deposited on the substrate  33  by inertia (see, for example, Patent Document 1). 
     In this example, the presence of the conductance adjusting gas exhaust port  26  is important. Here, for simplification of the situation, the conductance adjusting gas exhaust port  26  is a first exhaust port and the exhaust port  31  is a second exhaust port. How the amount and the size distribution of particulates deposited on the substrate  33  are changed with the amounts of helium gas flowing to the first exhaust port and the second exhaust port in this case will be described below. Here, for the amounts of gas flowing to the first exhaust port and the second exhaust port, the sizes of particulates, and the number of particulates, see the characteristic diagram of  FIG. 9 . 
     (1) First Exhaust Port: 0 slpm and Second Exhaust Port: 0.5 slpm 
     At this time, the pressure of the particulate generating chamber is 6.53×10 2  Pa (4.9 Torr) and the pressure of the impactor section is 5.07×10 2  Pa (3.8 Torr). In this case, on the impactor, nanoparticles with diameters of 1.5 nm or greater are trapped (in this case, referred to as cut size of 1.5 nm). As a result, the size distribution of particulates obtained in the downstream of the impactor becomes same as that seen in  FIG. 9 . Here, since the total flow rate is relatively low, 0.5 slpm, a large number of nanoparticles, particularly small nanoparticles, are deposited on the wall surface and lost in the transportation pipe between the generation chamber and the impactor. Actually, the amount of nanoparticles passing through the transportation pipe exponentially depends on the flow rate. As a result, the amount of nanoparticles obtained is relatively small. 
     (2) First Exhaust Port: 0 slpm and Second Exhaust Port: 1 slpm 
     In this case, the pressure of the particulate generating chamber is 1.09×10 3  Pa (8.2 Torr) and the pressure of the impactor section is 9.2×10 2  Pa (6.9 Torr) due to an increase in flow rate. Here, unless the inner diameter of the nozzle is changed, the cut size increases to 2.5 nm due to an increase in pressure. The distribution of nanoparticles at this time is same as that shown in  FIG. 9 . In this case, the loss of nanoparticles in the transportation pipe should be relatively small because of the high flow rate. Actually, the amount of relatively large nanoparticles was found to increase. However, the amount of smaller nanoparticles, e.g. particles of 1 nm to 2 nm, did not particularly increase. This is because smaller nanoparticles are hard to be obtained if the pressure of the generation chamber is high. 
     This problem would be solved if the pressure of the generation chamber could be reduced, but since the exhaust amount is limited by the nozzle, the problem cannot easily be solved. This result cannot be a good result when the purpose is to obtain small nanoparticles, but conversely, small nanoparticles could be reduced to narrow the size distribution, meaning that an effect in another aspect can be obtained. 
     (3) First Exhaust Port: 0.5 slpm and Second Exhaust Port: 0.5 slpm 
     For obtaining smaller nanoparticles, i.e. nanoparticles of 1 nm to 2 nm in a large amount, the total flow rate is set to 1 slpm, 0.5 slpm of which is discarded through the first exhaust port. In this case, despite the total flow rate of 1 slpm, the pressure of the generation chamber is about 7.46×10 2  Pa (5.6 Torr) and there is not a significant increase in pressure, since 0.5 slpm is discharged in the upstream of the nozzle which is not affected by the conductance of the nozzle. 
     Therefore, relatively small particulates are generated, and further, the rate of flow through the transportation pipe is 1 slpm, and therefore the loss is low as compared to the above case (1), and the passage rate increases by a factor of 10 or greater when the size is 1.5 nm. 
     As a result, even though a half of the total amount of nanoparticles is discarded, a larger number of smaller particles are ultimately obtained. In this case, the pressure of the impactor section is same as that in the above case (1) and the cut size remains unchanged. However, for the size distribution of nanoparticles obtained in the downstream of the impactor, smaller nanoparticles are obtained in a large amount as seen in  FIG. 9 . 
     As described above, it will be understood that by a simple configuration in which a new exhaust port is provided in the upstream of the nozzle of the impactor, the size distribution or the amount of nanoparticles can be controlled satisfactorily as compared to the conventional technique. The amount of nanoparticles after size classification is about 100 times as large as the amount when DMA is used, thus obtaining a very favorable result in pursuing applications of nanoparticles. 
     The results of classifying nanoparticles by the particulate size classification apparatus of example 1 will now be described based on comparison with a comparative example. 
       FIG. 10A  shows a comparative example showing a diameter distribution of particulates when there is only laser ablation and no classification unit, which is introduced in Non-Patent Document 4. Thus, generally, the geometric standard deviation of the particulate size is often about 1.6 to 2.0 when there is no classification unit. Here, the geometric standard deviation of, for example, 1.6 is almost equivalent to the standard deviation of about 60%. 
       FIG. 10B  shows one example showing a size distribution of particulates when using the particulate size classification apparatus (having, for example, an apparatus configuration similar to that of  FIG. 3  in example 1 and having a discoid trapping plate as particulate trapping unit) according to the present invention. The type of particulates was cobalt, the flow rate of He as a carrier gas was 1.9 slpm, the tip orifice diameter of the nozzle was 5.5 mm, and the pressure of the inside of the particulate size classifying chamber was 840 Pa. The rate of flow to the conductance adjusting exhaust port was 0. In this case, the size distribution was very narrow with the geometric average of 3.8 nm and the geometric deviation of 1.21. Thus, by using the particulate size classification apparatus comprising the impactor according to the present invention, nanoparticles relatively uniform in size can be obtained. 
     Example 2 
     In this example, another configuration of the impactor, especially particulate trapping unit, will be described. 
     In example 1, a discoid trapping plate was used as particulate trapping unit, and the trapping plate was scanned in vertical and lateral directions or in a rotation direction to change the position of the trapping of particulates. 
     However, when the configuration of example 1 is employed, the distance in which particulates that are not trapped pass on the trapping plate varies depending on the relative position of the nozzle and the trapping plate. Since some of such particulates are trapped on the trapping plate when passing on the trapping plate, there is concern that the amounts of particulates introduced into the particulate deposition chamber (e.g. particulate deposition chamber  32  of  FIG. 3 ) through the impactor vary by several tens % between positions A and B shown in  FIG. 11 . Further, the sizes of particulates introduced into the particulate deposition chamber also vary. 
     In this example, a belt-like trapping sheet  51  is used as particulate trapping unit in place of the trapping plate  3  ( 29 ) as shown in  FIG. 12  for solving a few problems of the configuration of example 1 of the present invention. 
     The trapping sheet  51  moves only in a longitudinal direction over time, and in the example shown in the figure, it is configured to be wound up in, for example, the direction of the arrow shown in the figure from one end portion  52  toward the other end portion  53 . Owing to such a configuration, the distance in which particulates  50  pass on the trapping sheet  51  is always constant (about half value of the width of the trapping sheet  51 ), and particulates  50  are always oriented to a fresh portion of the surface of the trapping sheet  51  which has no deposited particulates (trapped or passing without being trapped; the example shown in the figure illustrates a case where particulates pass). 
     The trapping sheet  51  has its surface coated with an anodized aluminum oxide film as in the trapping plate  29  (e.g. state of  FIG. 6 ). A configuration in which carbon nanotubes are formed on the surface (e.g. state similar to that of  FIG. 7  or  8 ), or nanowires of silicon, zinc oxide and the like having a high aspect ratio structure are grown or placed on the surface of the trapping sheet  51 , instead of the aluminum oxide film, is also suitable. 
     In this example, the tip orifice diameter of the nozzle  2  is about 3.25 mm, and the length of the shorter side of the trapping plate is about 20 mm. In this example, a configuration in which carbon nanotubes are grown in a thickness of about 5 μm, as in  FIG. 7 , on a stainless sheet having a thickness of about 50 μm is used as the trapping sheet  51 . The feeding speed of the trapping sheet  51  is, for example, about 0.05 mm/s. Since the trapping sheet  51  is in the form of a roll, it can be used for a long time without necessity of replacement in a short time. As a result of using the trapping sheet  51  as particulate trapping unit, time variations in the amount of particulates introduced into the particulate deposition chamber decreases to a low value, i.e. several % or less. 
     The present invention may be carried out with many configurations including the embodiments and examples described above. 
     According to the present invention, particulates called nanoparticles can reliably and easily be classified according to size with high throughput, and particulates uniform in size can be put to practical use in a large amount. The particulate size classification apparatus and method can be used without concern of insufficient supply in the electric area of luminescent materials and the areas of cosmetic materials and the like.