Patent Publication Number: US-2022219236-A1

Title: Fine particle production apparatus and fine particle production method

Description:
TECHNICAL FIELD 
     The present invention relates to a fine particle production apparatus and a fine particle production method using a thermal plasma flame, particularly to a fine particle production apparatus and a fine particle production method that generate a thermal plasma flame by electromagnetic induction by use of two coils and two independent high frequency power sources that separately supply high frequency currents to the two coils, thereby producing fine particles. 
     BACKGROUND ART 
     At present, fine particles such as silicon fine particles, oxide fine particles, nitride fine particles and carbide fine particles are used in a wide variety of fields. One example of a method of producing such fine particles is a gas-phase process. Exemplary gas-phase processes include chemical processes in which various gases or the like are chemically reacted at high temperature and physical processes in which a substance is irradiated with a beam such as an electron beam or a laser beam so as to be decomposed and evaporated, thereby generating fine particles. 
     Another gas-phase process is a thermal plasma process. The thermal plasma process is a process for producing fine particles by instantly evaporating feedstock in a thermal plasma flame and then quenching and solidifying the resulting evaporated product. The thermal plasma process has many advantages; for instance, the thermal plasma process is clean and highly productive, provides a high temperature and therefore is applicable to high melting point materials, and enables relatively easy complexing as compared to other gas-phase processes. Accordingly, the thermal plasma process is actively utilized as a method of producing fine particles. 
     In a fine particle production method using a conventional thermal plasma process, for instance, a feedstock substance is powdered, the powdered feedstock (powdery feedstock, powder) is, together with carrier gas and the like, dispersed and directly supplied as the feedstock into thermal plasma, whereby fine particles are produced. 
     In addition, for instance, Patent Literature 1 describes a fine particle production method that involves dispersing a material used for producing fine particles (raw material) in a dispersion medium to form a slurry, and as the feedstock, introducing the slurry into a thermal plasma flame while transforming the slurry into droplets, thereby producing fine particles. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2006-247446 A 
       
    
     Non-Patent Literature 
     
         
         Non-patent Literature 1: K. Kuraishi, et al., J. Phys. Conf. Ser., 441, 012016 (2013) 
       
    
     SUMMARY OF INVENTION 
     Technical Problems 
     For a fine particle production method, supplying feedstock together with carrier gas into a thermal plasma flame and, as in Patent Literature 1 stated above, supplying feedstock in the form of a slurry have been conventionally known. Among thermal plasma flames, however, in the case of induction thermal plasma generated by electromagnetic induction, a thermal plasma flame is sometimes destabilized due to disturbance from the outside. 
     To eliminate such destabilization of a thermal plasma flame caused by disturbance from the outside, for example, Non-patent Literature 1 proposes generating a thermal plasma flame by use of two coils and two independent high frequency power sources connected separately to the two coils. 
     Meanwhile, in addition to eliminating destabilization of a thermal plasma flame caused by disturbance from the outside as above, it has been required to control the particle size of obtained fine particles and provide the uniformity of obtained fine particles in particle size. 
     Further, even in the configuration for eliminating destabilization of a thermal plasma flame caused by disturbance from the outside as conventionally proposed, when a large amount of feedstock is supplied to a thermal plasma flame for the purpose of increasing the productivity of fine particles, the thermal plasma flame may be destabilized, e.g., extinguished, and therefore this configuration is not good enough to improve the productivity. 
     An object of the present invention is to provide a fine particle production apparatus and a fine particle production method that are capable of controlling the particle size of fine particles and efficiently producing a large amount of fine particles having excellent uniformity in particle size. 
     Solution to Problems 
     In order to attain the above-described object, the present invention provides a fine particle production apparatus comprising: 
     a feedstock supply section configured to supply feedstock for fine particle production into a thermal plasma flame; 
     a plasma torch configured to allow the thermal plasma flame to be generated therein and, by use of the thermal plasma flame, evaporate the feedstock supplied by the feedstock supply section to transform the feedstock into a mixture in a gas phase state; 
     a plasma generation section configured to generate the thermal plasma flame inside the plasma torch, 
     wherein the plasma generation section includes a first coil surrounding a periphery of the plasma torch, a second coil disposed under the first coil and surrounding the periphery of the plasma torch, a first power source section supplying the first coil with first high frequency current amplified-modulated, and a second power source section supplying the second coil with second high frequency current amplitude-modulated, the first coil and the second coil are arranged side by side in a longitudinal direction of the plasma torch, and a degree of modulation of the first high frequency current is smaller than that of the second high frequency current. 
     It is preferable that the fine particle production apparatus includes a gas supply section configured to supply quenching gas to the thermal plasma flame. 
     In addition, it is preferable that the plasma generation section supplies the first high frequency current supplied to the first coil by the first power source section and the second high frequency current supplied to the second coil by the second power source section at a same timing, and that the feedstock supply section increases an amount of supply of the feedstock in a region where current amplitude of the first high frequency current supplied to the first coil and current amplitude of the second high frequency current supplied to the second coil are high. 
     It is preferable that a current value of the second high frequency current amplitude-modulated and supplied to the second coil is 0 ampere in a region where current amplitude of the second high frequency current is low. 
     In addition, it is preferable that the feedstock supply section supplies the feedstock into the thermal plasma flame with the feedstock being dispersed in a particulate form. 
     Further, it is preferable that the feedstock supply section disperses the feedstock in liquid to obtain a slurry and transforms the slurry into droplets to supply the droplets into the thermal plasma flame. 
     In addition, the present invention provides a fine particle production method using a thermal plasma flame generated inside a plasma torch, 
     there being provided a first coil surrounding a periphery of the plasma torch, a second coil disposed under the first coil and surrounding the periphery of the plasma torch, a first power source section supplying the first coil with first high frequency current amplitude-modulated, and a second power source section supplying the second coil with second high frequency current amplitude-modulated, the first coil and the second coil being arranged side by side in a longitudinal direction of the plasma torch, and the thermal plasma flame being generated by the first power source section and the second power source section, the method comprising: 
     a first step of supplying feedstock for fine particle production to the thermal plasma flame generated inside the plasma torch; and 
     a second step of evaporating the feedstock by use of the thermal plasma flame to transform the feedstock into a mixture in a gas phase state and cooling the mixture, 
     wherein in the first step and the second step, the first power source section supplies the first coil with the first high frequency current amplitude-modulated, the second power source section supplies the second coil with second high frequency current amplitude-modulated, and a degree of modulation of the first high frequency current is smaller than that of the second high frequency current. 
     It is preferable that in the second step, quenching gas is supplied to the thermal plasma flame to cool the mixture in a gas phase state. 
     In addition, it is preferable that in the first step, the first high frequency current supplied to the first coil by the first power source section and the second high frequency current supplied to the second coil by the second power source section are supplied at a same timing, and an amount of supply of the feedstock is increased in a region where current amplitude of the first high frequency current supplied to the first coil and current amplitude of the second high frequency current supplied to the second coil are high. 
     It is preferable that a current value of the second high frequency current amplitude-modulated and supplied to the second coil is 0 ampere in a region where current amplitude of the second high frequency current is low. 
     In addition, it is preferable that in the first step, the feedstock is supplied into the thermal plasma flame with the feedstock being dispersed in a particulate form. 
     Further, it is preferable that in the first step, the feedstock is dispersed in liquid to obtain a slurry, the slurry is transformed into droplets, and the droplets are supplied into the thermal plasma flame. 
     Advantageous Effects of Invention 
     With the fine particle production apparatus and the fine particle production method according to the present invention, it is possible to control the particle size of fine particles and efficiently produce a large amount of fine particles having excellent uniformity in particle size. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an example of a fine particle production apparatus according to an embodiment of the invention. 
         FIG. 2  is a partial cross-sectional view schematically showing an example of a plasma torch of the fine particle production apparatus according to the embodiment of the invention. 
         FIG. 3  is a schematic view showing an example of a waveform of high frequency current of a first power source section. 
         FIG. 4  is a schematic view showing an example of a waveform of high frequency current of a second power source section. 
         FIG. 5A  is a graph showing an example of waveforms of first high frequency current and second high frequency current,  FIG. 5B  is a graph showing opening and closing timing of a valve, and  FIG. 5C  is a graph showing the supply of feedstock. 
         FIG. 6  is a graph showing average particle sizes and standard deviations of particle size distributions of Examples 1 to 3. 
         FIG. 7  is a graph showing analysis results of crystal structures of Examples 1 to 3 as obtained by X-ray diffractometry. 
         FIG. 8  is a schematic view showing an SEM image of fine particles of Example 1. 
         FIG. 9  is a schematic view showing an SEM image of fine particles of Example 2. 
         FIG. 10  is a schematic view showing an SEM image of fine particles of Example 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A fine particle production apparatus and a fine particle production method according to the present invention are described below in detail based on a preferred embodiment shown in the accompanying drawings. 
       FIG. 1  is a schematic view showing an example of the fine particle production apparatus according to the embodiment of the invention, and  FIG. 2  is a partial cross-sectional view schematically showing an example of a plasma torch of the fine particle production apparatus according to the embodiment of the invention. 
     A fine particle production apparatus  10  (hereinafter referred to simply as “production apparatus  10 ”) shown in  FIG. 1  is used for producing nanosized fine particles using feedstock for fine particle production. For instance, powder is used as the feedstock for fine particle production. 
     The production apparatus  10  can produce any fine particles with no limitation in the type, specifically, such fine particles as, in addition to metal fine particles, oxide fine particles, nitride fine particles, carbide fine particles and oxynitride fine particles by changing the composition of the feedstock. 
     The production apparatus  10  includes a feedstock supply section  12 , a plasma torch  14 , a chamber  16 , a collection section  18 , a plasma gas supply section  20 , a plasma generation section  21 , a gas supply section  22 , and a control section  24 . 
     The feedstock supply section  12  is connected to the plasma torch  14  through a hollow supply tube  13 . 
     The supply tube  13  between the feedstock supply section  12  and the plasma torch  14  may be provided with an intermittent supply section  15  as described later. The intermittent supply section  15  is not an essential element in the production apparatus  10 . 
     The chamber  16  is disposed below the plasma torch  14 , and the collection section  18  is disposed downstream from the chamber  16 . The plasma generation section  21  is connected to the plasma torch  14 , and a thermal plasma flame  100  is generated in the plasma torch  14  by means of the plasma generation section  21  as described later. 
     The feedstock supply section  12  is provided to supply feedstock for fine particle production into the thermal plasma flame  100  generated in the plasma torch  14 . 
     The type of the feedstock supply section  12  is not particularly limited as long as it can supply the feedstock into the thermal plasma flame  100 , and the following two types are applicable: one supplying the feedstock into the thermal plasma flame  100  with the feedstock being dispersed in a particulate form, and one slurrying the feedstock and transforming the obtained slurry into droplets to supply the droplets into the thermal plasma flame  100 . 
     For example, in the case where powder is used as the feedstock for fine particle production, the feedstock needs to be dispersed in a particulate form when supplied into the thermal plasma flame  100  in the plasma torch  14 . Therefore, the feedstock is for instance dispersed in carrier gas so that the feedstock in a particulate form is supplied. In this case, the feedstock supply section  12  supplies the feedstock powder in a fixed amount into the thermal plasma flame  100  in the plasma torch  14  whilst maintaining the feedstock powder to be in a dispersed state. For the feedstock supply section  12  having such a function, usable examples include devices disclosed in JP 3217415 B and JP 2007-138287 A. 
     For example, the feedstock supply section  12  includes a storage tank (not shown) storing feedstock powder, a screw feeder (not shown) transporting the feedstock powder in a fixed amount, a dispersion section (not shown) dispersing the feedstock powder transported by the screw feeder into a particulate form before the feedstock powder is finally sprayed, and a carrier gas supply source (not shown). 
     Together with carrier gas to which push-out pressure is applied from the carrier gas supply source, the feedstock powder is supplied into the thermal plasma flame  100  in the plasma torch  14  through the supply tube  13 . 
     The configuration of the feedstock supply section  12  is not particularly limited as long as the feedstock supply section  12  can prevent the feedstock powder from agglomerating and spray the feedstock powder into the plasma torch  14  with the feedstock powder being dispersed in a particulate form and the dispersed state being maintained. Inert gases such as argon gas (Ar gas) and nitrogen gas are usable as the carrier gas, for example. 
     For the feedstock supply section  12  supplying the feedstock powder in the form of a slurry, the device disclosed in JP 2011-213524 A may be used, for example. In this case, the feedstock supply section  12  includes a vessel (not shown) storing a slurry (not shown) having feedstock powder dispersed in liquid such as water, an agitator (not shown) agitating the slurry in the vessel, a pump (not shown) applying high pressure to the slurry to supply the slurry into the plasma torch  14  through the supply tube  13 , and an atomization gas supply source (not shown) supplying atomization gas used to transform the slurry into droplets and supply the droplets into the plasma torch  14 . The atomization gas supply source corresponds to the carrier gas supply source. The atomization gas is also called carrier gas. 
     In the case where the feedstock is supplied in the form of a slurry, the feedstock powder is dispersed in liquid such as water to obtain a slurry. The mixing ratio between the feedstock powder and water in the slurry is not particularly limited and is, for example, 5:5 (50%:50%) in the mass ratio. 
     In the case where used is the feedstock supply section  12  slurrying the feedstock powder and supplying the obtained slurry in the form of droplets, atomization gas to which push-out pressure is applied from the atomization gas supply source is, together with the slurry, supplied into the thermal plasma flame  100  in the plasma torch  14  through the supply tube  13 . The supply tube  13  has a two-fluid nozzle mechanism for spraying the slurry to the thermal plasma flame  100  in the plasma torch and transforming it into droplets, and using this mechanism, the slurry is sprayed to the thermal plasma flame  100  in the plasma torch  14 . That is, this makes it possible to transform the slurry into droplets. Similarly to the carrier gas described above, inert gases such as argon gas (Ar gas) and nitrogen gas are usable as the atomization gas, for example. 
     Thus, the two-fluid nozzle mechanism is capable of applying a high pressure to the slurry and atomizing the slurry with gas, i.e., the atomization gas (carrier gas), and is used as a method for transforming the slurry into droplets. 
     It should be noted that the nozzle mechanism is not limited to the two-fluid nozzle mechanism as above, and a single-fluid nozzle mechanism may also be used. As other methods, examples include a method which involves allowing a slurry to fall onto a rotating disk at a constant rate to transform the slurry into droplets (to form droplets) by the centrifugal force and a method which involves applying high voltage to a surface of a slurry to transform the slurry into droplets (to generate droplets). 
     The plasma torch  14  is configured to allow the thermal plasma flame  100  to be generated therein and, by use of the thermal plasma flame  100 , evaporate the feedstock supplied by the feedstock supply section  12  to transform the feedstock into a mixture  45  in a gas phase state. 
     As shown in  FIG. 2 , the plasma torch  14  includes a quartz tube  14   a  and a high frequency oscillation coil  14   b  provided around the outer surface of the quartz tube  14   a  to surround the periphery of the plasma torch  14 . The center portion of the top of the plasma torch  14  is provided with a supply port  14   c  into which the supply tube  13  is inserted, and a plasma gas supply port  14   d  is formed in the peripheral portion of the supply port  14   c  (on the same circumference). 
     For instance, powdery feedstock and the carrier gas such as argon gas or hydrogen gas are supplied into the plasma torch  14  through the supply tube  13 . 
     The plasma gas supply port  14   d  is connected to the plasma gas supply section  20  via, for example, piping which is not shown. The plasma gas supply section  20  is configured to supply plasma gas into the plasma torch  14  through the plasma gas supply port  14   d . For the plasma gas, gases such as argon gas and hydrogen gas are used alone or in combination as appropriate, for instance. 
     A sheath gas supply section (not shown) supplying sheath gas into the plasma torch  14  may be provided in addition to the plasma gas supply section  20 . For the sheath gas, the same gas as the plasma gas can be used. 
     The sheath gas supply section may be provided in place of the plasma gas supply section  20 . 
     The outside of the quartz tube  14   a  of the plasma torch  14  is surrounded by a concentrically formed quartz tube  14   e , and cooling water  14   f  is circulated between the quartz tubes  14   a  and  14   e  to cool the quartz tube  14   a  with the water, thereby preventing the quartz tube  14   a  from having an excessively high temperature due to the thermal plasma flame  100  generated in the plasma torch  14 . 
     The plasma generation section  21  is provided to generate the thermal plasma flame  100  in the plasma torch  14  as described above. The plasma generation section  21  includes a first coil  60  surrounding the periphery of the plasma torch  14 , a second coil  62  surrounding the periphery of the plasma torch  14 , a first power source section  21   a  supplying amplitude-modulated first high frequency current to the first coil  60 , and a second power source section  21   b  supplying amplitude-modulated second high frequency current to the second coil  62 . A degree of modulation of the first high frequency current is smaller than that of the second high frequency current, i.e., amplitude change of the first high frequency current is smaller than that of the second high frequency current. The first high frequency current and the second high frequency current are supplied at the same timing, for example. That is, the first high frequency current and the second high frequency current have the same phase. 
     The first coil  60  and the second coil  62  are arranged side by side in the longitudinal direction of the plasma torch  14 , and the second coil  62  is disposed under the first coil  60 . 
     The first power source section  21   a  and the second power source section  21   b  are both high frequency power sources and are independent of each other. It is preferable that the frequency of high frequency current of the first power source section  21   a  and the frequency of high frequency current of the second power source section  21   b  be different from each other in order to reduce magnetic coupling between the first coil  60  and the second coil  62 . This configuration can suppress the influence of the power source sections on each other. 
     The first coil  60  and the second coil  62  constitute the high frequency oscillation coil  14   b . The numbers of turns of the first coil  60  and the second coil  62  are not particularly limited and are suitably determined depending on the configuration of the production apparatus  10 . Materials of the first coil  60  and the second coil  62  are also not particularly limited and are suitably determined depending on the configuration of the production apparatus  10 . 
     With the use of the two coils and the two independent power source sections in the plasma generation section  21 , a series structure of induction thermal plasma can be built. The provision of the series structure of induction thermal plasma makes it possible to generate a high-temperature field that is long in the axial direction of the plasma torch  14 . When the long high-temperature field as above is used, it is possible to completely evaporate a high melting point material. A thermal plasma flame that is periodically switched between a high temperature state and a low temperature state having a lower temperature than that in the high temperature state at predetermined time intervals, i.e., that is time-modulated in terms of the temperature state, is called a modulated induction thermal plasma flame. 
     In the plasma generation section  21 , for instance, the first power source section  21   a  supplies the first coil  60  with the amplitude-modulated first high frequency current (see  FIG. 3 ), and the second power source section  21   b  supplies the second coil  62  with the amplitude-modulated second high frequency current (see  FIG. 4 ). 
     The first high frequency current supplied to the first coil  60  is also called first coil current, and the second high frequency current supplied to the second coil  62  is also called second coil current. 
       FIG. 3  is a schematic view showing an example of a waveform of high frequency current of the first power source section, and  FIG. 4  is a schematic view showing an example of a waveform of high frequency current of the second power source section. 
       FIG. 3  shows a waveform of the amplitude-modulated first high frequency current described above, where the amplitude is periodically modulated over time.  FIG. 4  shows a waveform of the amplitude-modulated second high frequency current described above, where the amplitude is periodically modulated over time.  FIG. 4  shows square wave amplitude modulation. The amplitude modulation is not limited to the square wave amplitude modulation shown in  FIGS. 3 and 4 , and needless to say, use may be made of a waveform formed of a repetitive wave including a curved line having a triangle wave, a sawtooth wave, a reverse sawtooth wave, a sine wave or the like. 
     In the amplitude-modulated first and second high frequency currents, the high value of the current amplitude is defined as a higher current level (HCL), the low value of the current amplitude is defined as a lower current level (LCL), and the time with HCL and the time with LCL in one modulation cycle are respectively defined as the ON time and the OFF time. Further, the percentage of the ON time in one cycle: (ON time/(ON time+OFF time)×100(%)) is defined as a duty factor (DF). The ratio (LCL/HCL×100(%)) in the amplitude is defined as a current modulation ratio (SCL). The current modulation ratio (SCL) represents the degree of modulation of current amplitude, where 100% SCL represents an unmodulated state and 0% SCL represents that the current amplitude is most largely modulated. At 0% SCL, the current value of high frequency current is 0 ampere (A) during the OFF time, i.e., in a region where the current amplitude of high frequency current is low, which will be described later. 
     The amplitude modulation of the first high frequency current and the amplitude modulation of the second high frequency modulation are not particularly limited as long as the degree of modulation of the first high frequency current is smaller than that of the second high frequency current as described above, and the amplitude modulation may be not less than 0% SCL and less than 100% SCL. In particular, it is most preferable that the amplitude modulation of the second high frequency current be 0% SCL because a value closer to 0% SCL refers to a higher degree of modulation, i.e., larger amplitude modulation. That is, the current value is preferably 0 amperes in a region where the current amplitude of the second high frequency current is low. 
     The ON time (see  FIGS. 3 and 4 ) corresponds to a region where the current amplitude of high frequency current is high, and the OFF time (see  FIGS. 3 and 4 ) corresponds to a region where the current amplitude of high frequency current is low. The OFF time is also called a modulation time. The ON time, the OFF time and one cycle described above are each preferably on the order of microseconds to several seconds. 
     In the plasma generation section  21 , for example, the first power source section  21   a  supplies the first coil  60  with the amplitude-modulated first high frequency current (see  FIG. 3 ) and the second power source section  21   b  supplies the second coil  62  with the amplitude-modulated second high frequency current (see  FIG. 4 ), whereby the thermal plasma flame  100  is generated in the plasma torch  14 . The temperature of the thermal plasma flame  100  can be varied with larger temperature difference by use of the amplitude-modulated first and second high frequency currents respectively supplied to the first coil  60  and the second coil  62 , thereby controlling the temperature inside the plasma torch  14  with larger temperature difference. With this configuration, a varying temperature field in which the temperature state of the thermal plasma flame  100  is time-modulated and the temperature difference is large is obtained, so that the temperature state of the thermal plasma flame  100  is periodically switched between the high temperature state and the low temperature state having a lower temperature than that in the high temperature state. With the largely-varying temperature field provided by the thermal plasma flame  100 , the particle size of fine particles can be controlled, and fine particles with a smaller particle size can be obtained. 
     The ambient pressure inside the plasma torch  14  is suitably determined depending on production conditions of fine particles and is, for example, not higher than the atmospheric pressure. The atmosphere with a pressure of not higher than the atmospheric pressure is not particularly limited, and for example, the pressure may range from 5 Torr (666.5 Pa) to 750 Torr (99.975 kPa). 
     As to the chamber  16 , as shown in  FIG. 1 , from the side closer to the plasma torch  14 , an upstream chamber  16   a  is attached to the plasma torch  14  to be concentric therewith. A downstream chamber  16   b  is provided perpendicularly to the upstream chamber  16   a , and on a further downstream side, there is provided a collection section  18  including a desired filter  18   a  for collecting fine particles. In the production apparatus  10 , a fine particle collection site is for example the filter  18   a.    
     The chamber  16  is connected with the gas supply section  22 . Quenching gas supplied from the gas supply section  22  is used to generate fine particles (not shown) of a material corresponding to the feedstock in the chamber  16 . The chamber  16  also serves as a cooling tank. 
     The collection section  18  includes a collection chamber having the filter  18   a , and a vacuum pump  18   b  connected through a pipe provided at a lower portion of the collection chamber. The fine particles transported from the chamber  16  are sucked by the vacuum pump  18   b  to be introduced into the collection chamber, and those fine particles remaining on the surface of the filter  18   a  are collected. 
     The gas supply section  22  is configured to supply quenching gas to the thermal plasma flame  100  in the chamber  16 . The quenching gas serves as cooling gas. The gas supply section  22  includes a gas supply source (not shown) storing gas and a pressure application section (not shown) such as a compressor or a blower which applies push-out pressure to the quenching gas to be supplied into the chamber  16 . Further, a regulating valve (not shown) controlling the amount of gas supplied from the gas supply source is provided. The gas supply source for use is determined depending on the composition of the quenching gas. The type of the gas is not limited to a single type, and when the quenching gas is a mixed gas, a plurality of gas supply sources are prepared. 
     The type of the quenching gas is not particularly limited as long as it exercises a cooling function. Inert gases unreactive with the feedstock, such as argon gas, nitrogen gas and helium gas, are usable as the quenching gas, for example. The quenching gas may additionally contain hydrogen gas. The quenching gas may contain reactive gas that reacts with the feedstock. Examples of the reactive gas include hydrocarbon gases such as methane, ethane, propane, butane, acetylene, ethylene, propylene and butene. 
     The gas supply section  22  supplies the quenching gas (cooling gas) at an angle of, for example, 45 degrees toward a tail portion  100   b  of the thermal plasma flame  100  (see  FIG. 2 ), i.e., the end of the thermal plasma flame  100  on the opposite side from the plasma gas supply port  14   d , that is, a terminating portion of the thermal plasma flame  100 , and also supplies the quenching gas (cooling gas) downward along an inner wall of the chamber  16 . However, the configuration is not limited to supplying the quenching gas to the terminating portion of the thermal plasma flame  100 . 
     The quenching gas supplied from the gas supply section  22  into the chamber  16  quenches the mixture having been transformed to a gas phase state with the thermal plasma flame  100 , thereby obtaining fine particles of a material corresponding to the feedstock. Besides, the quenching gas above has additional functions such as contribution to classification of fine particles. 
     Immediately after fine particles of a material corresponding to the feedstock is generated, if the fine particles collide with each other to form agglomerates, this causes nonuniform particle size, resulting in lower quality. However, since the quenching gas is supplied toward the tail portion  100   b  (terminating portion) of the thermal plasma flame, the fine particles are diluted with the quenching gas and thereby prevented from colliding with each other to agglomerate together. 
     In addition, since the quenching gas is supplied along the inner wall surface of the chamber  16 , the fine particles are prevented from adhering to the inner wall of the chamber  16  in the process of collection of the fine particles, whereby the yield of the generated fine particles is improved. 
     The method of supplying the quenching gas to the thermal plasma flame  100  by the gas supply section  22  is not particularly limited, and the quenching gas may be supplied from a single direction. Alternatively, the quenching gas may be supplied from plural directions surrounding the periphery of the thermal plasma flame  100 . In this case, plural supply ports for the quenching gas are provided at the outer peripheral surface of the chamber  16  along the circumferential direction, for example, at regular intervals, although the arrangement at regular intervals is not essential. 
     When the quenching gas is supplied from plural directions, the supply timing is not particularly limited, and the quenching gas is supplied from plural directions in a synchronized manner. Alternatively, the quenching gas may be supplied in a clockwise or counterclockwise order, for instance. In this case, the quenching gas generates a gas flow such as a swirl flow in the chamber  16 . When supplied from plural directions, the quenching gas may be supplied in a random order without determining the order of supply. 
     The gas supply section  22  is not necessarily required if fine particles can be generated without the use of the quenching gas. In the case of the configuration without the gas supply section  22 , this leads to simplification of the apparatus configuration of the production apparatus  10  as well as steps of the fine particle production method. 
     As described above, the feedstock supply section  12  supplies the feedstock to the thermal plasma flame  100 , for instance, supplies the feedstock in a predetermined amount, i.e., in a fixed amount regardless of time. 
     The feedstock supply section  12  is not limited to the one supplying a fixed amount of the feedstock and may be one supplying the feedstock into the thermal plasma flame  100  while time-modulating the amount of supply of the feedstock into the thermal plasma flame  100 . With this configuration, a large amount of the feedstock can be supplied during the ON time shown in  FIGS. 3 and 4 . Consequently, a large amount of smaller fine particles can be manufactured. In this case, the supply tube  13  is provided with the intermittent supply section  15 , for example. The feedstock is supplied into the chamber  16  while the supply is time-modulated by means of the intermittent supply section  15 . The change of the amount of supply of the feedstock is not particularly limited and may assume any of sinusoidal, triangular, rectangular and sawtooth waveforms, while the change preferably conforms to the amplitude modulation of the first high frequency current supplied to the first coil  60  and the amplitude modulation of the second high frequency current supplied to the second coil  62 . That is, it is preferable that the change of the amount of supply of the feedstock synchronize with the time-based change of the amplitude modulation of the first high frequency current and the amplitude modulation of the second high frequency current which are expressed as functions. This configuration makes it easy to adjust the ON time and the timing of supply of the feedstock to each other. 
     For the intermittent supply section  15 , for instance, a solenoid valve (electromagnetic valve) connected to the supply tube  13  is used to time-modulate the amount of supply of the feedstock. The control section  24  controls the opening and closing of the solenoid valve. A ball valve may be used instead of the solenoid valve. Also in this case, the control section  24  controls the opening and closing of the ball valve. The control section  24  time-modulates the amount of supply of the feedstock in such a manner that, for instance, the amount of supply of the feedstock is increased during the ON time and the amount of supply of the feedstock is decreased during the OFF time. Consequently, a large amount of smaller fine particles can be manufactured. Therefore, in the supply of the feedstock, it is preferable that the amount of supply of the feedstock be increased during the ON time and the amount of supply of the feedstock be decreased during the OFF time. Thus, the feedstock is supplied during the ON time, whereby a large amount of the feedstock can be evaporated, and this allows generation of a large amount of fine particles, so that a large amount of fine particles can be efficiently produced with high productivity. 
     Next, a fine particle production method using the production apparatus  10  above is described taking metal fine particles as an example. 
     First, for example, Si powder having a volume mean diameter of not more than 30 μm is put into the feedstock supply section  12  as the feedstock powder for metal fine particles. 
     For instance, argon gas is used as the plasma gas. The first power source section  21   a  supplies the first coil  60  with amplitude-modulated first high frequency current. The second power source section  21   b  supplies the second coil  62  with amplitude-modulated second high frequency current. As a consequence, the thermal plasma flame  100  is generated in the plasma torch  14 . The amplitude modulation of the first high frequency current supplied to the first coil  60  is for example at 90% SCL, the amplitude modulation of the second high frequency current supplied to the second coil  62  is for example at 0% SCL, the modulation cycle is 15 ms, the ON time is 10 ms, and the OFF time is 5 ms. 
     Next, the Si powder is transported with gas, e.g., argon gas used as the carrier gas and supplied into the thermal plasma flame  100  in the plasma torch  14  through the supply tube  13  (first step). The supplied Si powder is evaporated in the thermal plasma flame  100  and becomes the mixture  45  (see  FIG. 2 ) in a gas phase state. The mixture  45  (see  FIG. 2 ) in a gas phase state is cooled (second step). As a result, Si fine particles (metal fine particles) are obtained. 
     Then, the Si fine particles obtained in the chamber  16  are collected on the filter  18   a  of the collection section  18  owing to negative pressure (suction force) applied from the collection section  18  by the vacuum pump  18   b.    
     As described above, the thermal plasma flame  100  can be periodically switched in a stable state between the high temperature state and the low temperature state having a lower temperature than that in the high temperature state; therefore, it is possible to control the particle size of fine particles and obtain fine particles having excellent uniformity in particle size. 
     The cooling of the mixture  45  (see  FIG. 2 ) in a gas phase state (second step) is not particularly limited and may be natural cooling that allows the mixture to cool without using a cooling medium such as quenching gas. In the case of using no quenching gas, the temperature of the thermal plasma flame  100  during the OFF time can be reduced with the temperature of the thermal plasma flame  100  during the ON time being maintained by decreasing the value of SCL, i.e., increasing the degree of modulation of the second high frequency current; therefore, Si fine particles (metal fine particles) with smaller size can be obtained even without cooling using the quenching gas. In this case, steps of the fine particle production method can be simplified. 
     Alternatively, for instance, argon gas may be supplied as the quenching gas from the gas supply section  22  to the tail portion  100   b  of the thermal plasma flame  100  (see  FIG. 2 ), i.e., the terminating portion of the thermal plasma flame  100  to quench the mixture  45  (see  FIG. 2 ). Consequently, the thermal plasma flame  100  is quenched whereby Si fine particles (metal fine particles) are generated. At this time, a low temperature region is formed in the chamber  16 , so that still smaller Si fine particles (metal fine particles) can be obtained. 
     When the Si powder is supplied into the thermal plasma flame  100  in the plasma torch  14 , it is preferable to increase the amount of supply of the Si powder during the ON time and decrease the amount of supply of the Si powder during the OFF time, as described above. Alternatively, the supply of the Si powder may be controlled such that the Si powder is supplied during the ON time and not supplied during the OFF time. In any case, it takes time after the solenoid valve is opened until the feedstock is actually transported and the amount of supply of the feedstock into the thermal plasma flame  100  increases, and therefore, the solenoid valve and the like need to be controlled taking the time required for the transportation into account. 
       FIG. 5A  is a graph showing an example of waveforms of first high frequency current and second high frequency current,  FIG. 5B  is a graph showing opening and closing timing of a valve, and  FIG. 5C  is a graph showing the supply of feedstock. 
     In the embodiment, for example, the opening and closing timing of a valve is determined taking the transportation time into account on the basis of a waveform signal  80  (see  FIG. 5A ) of the first coil  60  which is subjected to square wave amplitude modulation and a waveform signal  82  (see  FIG. 5A ) of the second coil  62  which is subjected to square wave amplitude modulation, a timing signal  84  for opening and closing the valve is obtained as shown in  FIG. 5B , and the valve is opened and closed at predetermined time intervals. Consequently, for instance, the feedstock powder is supplied into the plasma torch  14  during the ON time in a waveform  86  shown in  FIG. 5C , and as a result, the feedstock can be intermittently supplied. 
     It is preferable that amplitude modulation is performed while an average input power to the first coil  60  and that to the second coil  62  are each kept constant. In addition, the pressure inside the plasma torch is set to a constant value during production of fine particles, for example. 
     From the foregoing, a varying temperature field with large temperature difference of the thermal plasma flame  100  can be formed by use of the amplitude-modulated first and second high frequency currents, and supply of feedstock during the ON time allows complete evaporation of the feedstock. Further, since feedstock supplied around the ON time is quenched well compared to that supplied during the unmodulated state, it can be expected that the growth of particles in a growing stage can be further suppressed. Thus, when the first high frequency current and the second high frequency current are amplitude-modulated, an efficient nanoparticle generation process can be carried out. Further, since the temperature of the varying temperature field of the thermal plasma flame can be further reduced during the OFF time by making the value of SCL of the second high frequency current smaller than the value of SCL of the first high frequency current, more reliable evaporation and efficient cooling of particles in a growing stage are possible. This configuration makes it possible to obtain fine particles with a smaller size. 
     The production apparatus  10  of this embodiment is capable of producing, for instance, nanosized Si fine particles using Si powder as the feedstock. However, the invention is not limited thereto, and particles of another element may be used as the feedstock for fine particle production to produce fine particles of an oxide, a metal, a nitride or a carbide of that element. In this case, the fine particle production may involve slurrying. 
     When the feedstock is powder, the average particle size thereof is appropriately set to allow easy evaporation of the feedstock in the thermal plasma flame and is, for example, up to 100 μm, preferably up to 10 μm and more preferably up to 5 μm when converted to the BET diameter. 
     For instance, any type of feedstock may be used as long as it can be evaporated by the thermal plasma flame, and the following substances are preferred. That is, there may be appropriately selected one of a single element oxide, a complex oxide, a multiple oxide, an oxide solid solution, a metal, an alloy, a hydroxide, a carbonic acid compound, a halide, a sulfide, a nitride, a carbide, a hydride, a metal salt and a metal-organic compound each of which contains at least one selected from the group consisting of the elements with the atomic numbers 3 to 6, 11 to 15, 19 to 34, 37 to 52, 55 to 60, 62 to 79 and 81 to 83. 
     The single element oxide refers to an oxide formed from one element in addition to oxygen, the complex oxide refers to an oxide constituted of plural types of oxides, the multiple oxide refers to a higher order oxide formed from two or more types of oxides, and the oxide solid solution refers to a solid in which different oxides are dissolved and uniformly mixed with each other. The metal refers to one consisting of one or more metallic elements alone, and the alloy refers to one constituted of two or more metallic elements. The alloy structure assumes the state of a solid solution, a eutectic mixture, an intermetallic compound, or a mixture thereof in some cases. 
     The hydroxide refers to one constituted of a hydroxyl group and one or more metallic elements, the carbonic acid compound refers to one constituted of a carbonic acid group and one or more metallic elements, the halide refers to one constituted of a halogen and one or more metallic elements, and the sulfide refers to one constituted of sulfur and one or more metallic elements. The nitride refers to one constituted of nitrogen and one or more metallic elements, the carbide refers to one constituted of carbon and one or more metallic elements, and the hydride refers to one constituted of hydrogen and one or more metallic elements. The metal salt refers to an ionic compound containing at least one or more metallic elements, and the metal-organic compound refers to an organic compound having a bond between one or more metallic elements and at least one of elements C, O and N, as exemplified by a metal alkoxide and an organometallic complex. 
     Examples of the single element oxide include a titanium oxide (TiO 2 ), a zirconium oxide (ZrO 2 ), a calcium oxide (CaO), a silicon oxide (SiO 2 ), an aluminum oxide (alumina: Al 2 O 3 ), a silver oxide (Ag 2 ), an iron oxide, a magnesium oxide (MgO), a manganese oxide (Mn 3 O 4 ), an yttrium oxide (Y 2 O 3 ), a cerium oxide, a samarium oxide, a beryllium oxide (BeO), a vanadium oxide (V 2 O 5 ), a chromium oxide (Cr 2 O 3 ), and a barium oxide (BaO). 
     Examples of the complex oxide include a lithium aluminate (LiAlO 2 ), an yttrium vanadate, a calcium phosphate, a calcium zirconate (CaZrO 3 ), a titanium lead zirconate, a titanium iron oxide (FeTiO 3 ) and a titanium cobalt oxide (CoTiO 3 ). Examples of the multiple oxide include a barium stannate (BaSnO 3 ), a barium (meta)titanate (BaTiO 3 ), a lead titanate (PbTiO 3 ), and a solid solution in which a zirconium oxide and a calcium oxide dissolve as solids in a barium titanate. 
     The hydroxide can be exemplified by Zr(OH) 4 , the carbonic acid compound by CaCO 3 , the halide by MgF 2 , the sulfide by ZnS, the nitride by TiN, the carbide by SiC, and the hydride by TiH 2 . 
     Fine particle production using the production apparatus  10  shown in  FIG. 1  described above is described more specifically. 
     The time-averaged input power to the first coil and that to the second coil were each set to 10 kW. The frequency of high frequency current of the first coil was set to 460 kHz, and the frequency of high frequency current of the second coil was set to 320 kHz. 
     The first high frequency current and the second high frequency current were each set to one subjected to square wave amplitude modulation. SCL representing the degree of modulation was set to 90% for the first high frequency current and 0% for the second high frequency current. The modulation cycle was set to 15 ms, the ON time to 10 ms, and the OFF time to 5 ms. The duty factor (DF) was 66%. 
     The pressure inside the plasma torch was set to 300 Torr (about 40 kPa). Ar gas was introduced as the sheath gas at a flow rate of 90 liter/min. Furthermore, Si feedstock powder was supplied to a thermal plasma flame by using Ar gas (carrier gas) at a flow rate of 4 liter/min. No quenching gas was used. 
     As the Si feedstock powder, Si powder (purity: 97%) having a volume mean diameter of 26 μm was used. A solenoid valve was operated in synchronization with the modulation of the first high frequency current (modulated current) and the second high frequency current (modulated current) synchronized with the ON time, and the Si feedstock powder was intermittently supplied in an amount of supply of 3.0 g/min. This is hereinafter called Example 1. 
     For comparison, fine particles were produced with the same conditions as those of Example 1 described above except that high frequency current of the first coil was not amplitude-modulated at 100% SCL, high frequency current of the second coil was amplitude-modulated at 0% SCL, and the amount of supply of the Si feedstock powder was set to 1.5 g/min. This is hereinafter called Example 2. 
     Further, for comparison, fine particles were produced with the same conditions as those of Example 1 described above except that neither high frequency current of the first coil nor that of the second coil was amplitude-modulated, i.e., the SCL was set to 100% and the amount of supply of the Si feedstock powder was set to 2.8 g/min. This is hereinafter called Example 3. 
     A particle size distribution of the fine particles of each of Examples 1 to 3 described above was obtained. For the particle size distribution, an SEM image of a plurality of fine particles thus produced was obtained, and 300 fine particles were randomly selected from the SEM image of the plurality of fine particles. The particle size of each of the selected fine particles was measured from the SEM image, and the particle size distribution was obtained on the basis of the particle size of each of the fine particles. The average particle size and the standard deviation of the fine particles were determined from the obtained particle size distribution of the fine particles. The result is shown in  FIG. 6 . The average particle size of the fine particles is an average value of the diameters of the 300 fine particles that were randomly selected. The standard deviation is a value obtained from the diameters of the 300 fine particles that were randomly selected. 
     In  FIG. 6 , reference sign  90   a  represents an average particle size d of Example 1, reference sign  90   b  represents an average particle size d of Example 2, and reference sign  90   c  represents an average particle size d of Example 3. Reference sign  92   a  represents a standard deviation σ of the particle size distribution of Example 1, reference sign  92   b  represents a standard deviation σ of the particle size distribution of Example 2, and reference sign  92   c  represents a standard deviation σ of the particle size distribution of Example 3. The average particle size d and the standard deviation σ of Example 1 were 63.0 nm and 38.5 nm, respectively, the average particle size d and the standard deviation σ of Example 2 were 72.5 nm and 44.5 nm, respectively, and the average particle size d and the standard deviation σ of Example 3 were 82.7 nm and 68.3 nm, respectively. 
     As shown in  FIG. 6 , the average particle size and the standard deviation of Example 1 in which the first high frequency current and the second high frequency current were amplitude-modulated are small. The average particle size and the standard deviation of Example 2 in which the first high frequency current was not amplitude-modulated and only the second high frequency current was amplitude-modulated are larger than those of Example 1 but smaller than those of Example 3. The average particle size and the standard deviation of Example 3 in which neither the first high frequency current nor the second high frequency current was amplitude-modulated are larger than those of Example 2. 
     It should be noted that Examples 1 and 3 have substantially the same amount of supply of the Si feedstock powder, and the amount of supply of the Si feedstock powder of Example 2 is a half of that of Example 1 and small. In general, when an amount of supply of feedstock powder is small, a particle size tends to be small. However, the average particle size and also the standard deviation of Example 2 are larger than those of Example 1. In Example 1, even though the amount of supply of the Si feedstock powder is larger than that of Example 2, fine particles can be produced, and the productivity is high. 
     The crystal structure of the fine particles of each of Examples 1 to 3 was analyzed by using XRD (X-ray diffractometry). The result is shown in  FIG. 7 . In  FIG. 7 , reference sign  94  represents an XRD spectrum of Example 1, reference sign  96  represents an XRD spectrum of Example 2, and reference sign  98  represents an XRD spectrum of Example 3. 
     The analysis results of the crystal structures as obtained by X-ray diffractometry (XRD) as shown in  FIG. 7  revealed that Examples 1 to 3 were all composed of Si. 
       FIG. 8  shows an SEM image of the fine particles of Example 1,  FIG. 9  shows an SEM image of the fine particles of Example 2, and  FIG. 10  shows an SEM image of the fine particles of Example 3. The magnification of each of  FIGS. 8 and 9  is 100000 times, and the magnification of  FIG. 10  is 95000 times. 
     As shown in  FIGS. 8 and 10 , the particle size of the fine particles of Example 1 was smaller than that of the fine particles of Example 3, and further, the number of coarse particles of Example 1 was smaller than that of coarse particles of Example 3, and the fine particles of Example 1 were more uniform than those of Example 3. 
     As shown in  FIGS. 8 and 9 , the particle size of the fine particles of Example 1 was smaller than that of the fine particles of Example 2, and further, the number of coarse particles of Example 1 was smaller than that of coarse particles of Example 3, and the fine particles of Example 1 were more uniform than those of Example 3. 
     In comparison between the fine particles of Example 2 and the fine particles of Example 3, only the second high frequency current was amplitude-modulated for the fine particles of Example 2, and the particle size of the fine particles of Example 2 was smaller than that of the fine particles of Example 3, and further, the number of the coarse particles of Example 2 was smaller than that of the coarse particles of Example 3. 
     The present invention is basically configured as above. While the fine particle production apparatus and the fine particle production method according to the invention are described above in detail, the invention is by no means limited to the foregoing embodiments and it should be understood that various improvements and modifications are possible without departing from the scope and spirit of the invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  fine particle production apparatus (production apparatus) 
               12  feedstock supply section 
               13  supply tube 
               14  plasma torch 
               14   a  quartz tube 
               14   b  high frequency oscillation coil 
               14   c  supply port 
               14   d  plasma gas supply port 
               14   e  quartz tube 
               14   f  cooling water 
               15  intermittent supply section 
               16  chamber 
               16   a  upstream chamber 
               16   b  downstream chamber 
               18  collection section 
               18   a  filter 
               18   b  vacuum pump 
               20  plasma gas supply section 
               21  plasma generation section 
               21   a  first power source section 
               21   b  second power source section 
               22  gas supply section 
               24  control section 
               45  mixture 
               60  first coil 
               62  second coil 
               80 ,  82  waveform signal 
               84  timing signal 
               86  waveform 
               90   a  average particle size of Example 1 
               90   b  average particle size of Example 2 
               90   c  average particle size of Example 3 
               92   a  standard deviation of particle size distribution of Example 1 
               92   b  standard deviation of particle size distribution of Example 2 
               92   c  standard deviation of particle size distribution of Example 3 
               94  XRD spectrum of Example 1 
               96  XRD spectrum of Example 2 
               98  XRD spectrum of Example 3 
               100  thermal plasma flame 
               100   b  tail portion