Abstract:
The purpose/problem of the present invention is to provide a method for producing tungsten complex oxide particles useful as a heat shield material or the like that permits inexpensive production of a stable composition. This method for producing tungsten complex oxide particles includes a step for preparing a dispersion in which a raw material powder has been dispersed, a step for feeding the dispersion into a thermal plasma flame, and a step for supplying gas containing oxygen to the terminal portion of the thermal plasma flame and producing tungsten complex oxide particles. The dispersion preferably includes a carbon element.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method of producing tungsten complex oxide particles having a median particle size ranging from several nm to 1000 nm, particularly to a method of producing tungsten complex oxide particles by a thermal plasma process using a carbon element-containing dispersion as a raw material. 
       BACKGROUND ART 
       [0002]    At present, tungsten complex oxides are used for piezoelectric elements, electrostrictive elements, magnetostrictive elements and heat ray shielding materials. Several methods of producing particles or the like of such tungsten complex oxides have been heretofore proposed (see Patent Literatures 1 and 2). 
         [0003]    Patent Literature 1 describes a method of obtaining an infrared shielding film by adding one or more media selected from UV curable resins, thermoplastic resins, thermosetting resins, cold setting resins, metal alkoxides, and hydrolytic polymerization products of metal alkoxides to a solution containing dispersed infrared shielding material fine particles to prepare a coating solution, applying the coating solution (solution containing dispersed infrared shielding material fine particles) onto a base surface to form a coating film, and evaporating a solvent from the coating film. An infrared shielding optical member is composed of a base and the foregoing infrared shielding film formed on a surface of the base. 
         [0004]    In the solution containing dispersed infrared shielding material fine particles, infrared shielding material fine particles composed of tungsten oxide fine particles represented by a general formula of WyOz (where W is tungsten, O is oxygen, and 2.2≦z/y≦2.999) or/and composite tungsten oxide fine particles represented by a general formula of MxWyOz (where M is one or more elements selected from H, He, alkali metals, alkaline earth metals, rare earth elements, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, Be, Hf, Os, Bi and I, W is tungsten, O is oxygen, 0.001≦x/y≦1, and 2.2≦z/y≦3) are contained in a solvent, and the infrared shielding material fine particles have a particle size distribution, as measured by dynamic light scattering, in which the 50% diameter is 10 nm to 30 nm, the 95% diameter is 20 nm to 50 nm, and the average particle size is 10 nm to 40 nm. 
         [0005]    Patent Literature 1 describes that the tungsten oxide fine particles represented by the general formula WyOz and the composite tungsten oxide fine particles represented by the general formula of MxWyOz can be obtained by subjecting an ammonium tungstate aqueous solution or a tungsten hexachloride solution, used as a starting material, to thermal treatment in an inert gas atmosphere or a reducing gas atmosphere. 
         [0006]    In a method of producing composite tungsten oxide ultrafine particles described by Patent Literature 2, use is made of, as a raw material, powder obtained by mixing an M element compound and a tungsten compound so that the ratio between M and W elements becomes the same as that in a general formula MxWyOz (where M is the M element described below, W is tungsten, O is oxygen, 0.001≦x/y≦1, and 2.0&lt;z/y≦3.0) having a target composition, or a composite tungsten oxide represented by a general formula MxWyOz (where M is the M element, W is tungsten, O is oxygen, 0.001≦x/y≦1, and 2.0&lt;z/y≦3.0) and produced by a conventional method. 
         [0007]    The raw material and a carrier gas are supplied into thermal plasma generated in an atmosphere containing an inert gas alone or a mixed gas of an inert gas and a hydrogen gas to cause the raw material to be subjected to an evaporation process and a condensation process. As a result, composite tungsten oxide ultrafine particles having a single-phase crystal phase, having the target composition and having a particle size of up to 100 nm are generated. The M element refers to one or more elements selected from H, Li, Na, K, Rb, Cs, Cu, Ag, Pb, Ca, Sr, Ba, In, Tl, Sn, Si and Yb. 
       CITATION LIST 
     Patent Literature 
       [0008]    Patent Literature 1: JP 2009-215487 A 
         [0009]    Patent Literature 2: JP 2010-265144 A 
       SUMMARY OF INVENTION 
     Technical Problems 
       [0010]    As described by Patent Literature 1, the tungsten oxide fine particles and the composite tungsten oxide fine particles represented by the general formula MxWyOz are obtained by carrying out thermal treatment in an inert gas atmosphere or a reducing gas atmosphere. In general, composite tungsten oxide fine particles are obtained through thermal treatment in a reducing gas atmosphere. In the case of employing thermal treatment in a reducing gas atmosphere, the cost for a device is increased, which leads to a higher production cost, disadvantageously. 
         [0011]    Aside from that, in the method of producing composite tungsten oxide ultrafine particles by supplying the raw material and the carrier gas into the thermal plasma generated in an atmosphere containing an inert gas alone or a mixed gas of an inert gas and a hydrogen gas as described in Patent Literature 2, powder is used as the raw material to be supplied to the thermal plasma, and the powder is directly supplied to the thermal plasma. Due to the pulsation during supply of the raw material powder and the segregation of the raw material powder, the raw material composition is not stabilized, disadvantageously. The technique of Patent Literature 2 does not make it possible to produce composite tungsten oxide ultrafine particles with a stable composition. 
         [0012]    An object of the present invention is to solve the problems inherent in the prior art and to provide a production method that makes it possible to produce tungsten complex oxide particles with a stable composition at low cost. 
       Solution to Problems 
       [0013]    In order to attain the foregoing object, the present invention provides a method of producing tungsten complex oxide particles, comprising: a step of preparing a dispersion liquid in which raw material powder is dispersed; a step of supplying the dispersion liquid into a thermal plasma flame; and a step of forming tungsten complex oxide particles by supplying an oxygen-containing gas to a terminating portion of the thermal plasma flame. 
         [0014]    Preferably, the dispersion liquid contains carbon element. A solvent used for the dispersion liquid preferably, but not necessarily, contains carbon element. In this case, the solvent is for instance an organic solvent, and examples of a carbon element-containing solvent include alcohols such as ethanol. The raw material powder preferably contains carbon element. The carbon element is for example contained in the form of at least one of a carbide, a carbonate and an organic compound. For instance, the thermal plasma flame is derived from oxygen gas, and the oxygen-containing gas is a mixed gas of air gas and nitrogen gas. 
       Advantageous Effects of Invention 
       [0015]    The present invention makes it possible to produce tungsten complex oxide particles with a stable composition at low cost. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a graph for explaining the evaluation of optical characteristics of tungsten complex oxide particles. 
           [0017]      FIG. 2  is a schematic view showing a fine particle production apparatus that is used in a method of producing tungsten complex oxide particles according to an embodiment of the invention. 
           [0018]      FIG. 3  is a flowchart showing the method of producing tungsten complex oxide particles according to the embodiment of the invention. 
           [0019]      FIG. 4  is a graph showing analysis results for Cs x WO 3  particles obtained by the production method according to the embodiment of the invention, analyzed by X-ray diffractometry. 
           [0020]      FIG. 5  is a graph showing evaluation results for optical characteristics of Cs x WO 3  particles obtained by the production method according to the embodiment of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    On the following pages, the method of producing tungsten complex oxide particles according to the invention is described in detail with reference to a preferred embodiment shown in the accompanying drawings. 
         [0022]    Tungsten complex oxide particles of the invention have the composition represented by, for instance, a general formula MxWyOz, where M is at least one element selected from H, He, alkali metals, alkaline earth metals, rare earth elements, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, V, Mo, Ta, Re, Be, Hf, Os, Bi and I, W is tungsten, and O is oxygen. 
         [0023]    The tungsten complex oxide particles can be used for piezoelectric elements, electrostrictive elements, magnetostrictive elements and heat ray shielding materials. 
         [0024]      FIG. 1  is a graph for explaining the evaluation of optical characteristics of tungsten complex oxide particles. For example, tungsten complex oxide particles represented by Cs 0.33 WO 3  have the optical characteristics shown in  FIG. 1 , where the absorbance in an infrared light region D IR  is higher than that in a visible light region D VL . The tungsten complex oxide particles represented by Cs 0.33 WO 3  have a heat ray shielding effect owing to the optical characteristics above and therefore can be used for a heat ray shielding material. 
         [0025]    The tungsten complex oxide particles represented by Cs 0.33 WO 3  are obtained by subjecting oxide particles represented by Cs 0.33 WO 3+δ  to reduction treatment. The degree of oxidation of the oxide particles represented by Cs 0.33 WO 3+δ  is higher by the amount corresponding to δ than that of the tungsten complex oxide particles represented by Cs 0.33 WO 3 . 
         [0026]    The oxide particles represented by Cs 0.33 WO 3+δ  have a higher absorbance in the visible light region D VL  and a lower absorbance in the infrared light region D IR  than those of the tungsten complex oxide particles represented by Cs 0.33 WO 3  and therefore are not suitable for a heat ray shielding purpose. 
         [0027]    In measurement of the absorbance of the tungsten complex oxide particles represented by Cs 0.33 WO 3  shown in  FIG. 1 , the tungsten complex oxide particles were dispersed in ethanol to measure the absorbance thereof with an infrared visible spectrophotometer. As to the absorbance of the oxide particles represented by Cs 0.33 WO 3+δ , these oxide particles were dispersed in ethanol to measure the absorbance thereof with an infrared visible spectrophotometer. 
         [0028]      FIG. 2  is a schematic view showing a fine particle production apparatus that is used in the method of producing tungsten complex oxide particles according to the embodiment of the invention. 
         [0029]    A fine particle production apparatus  10  (hereinafter referred to simply as “production apparatus  10 ”) shown in  FIG. 2  is used to produce tungsten complex oxide particles. 
         [0030]    The production apparatus  10  includes a plasma torch  12  generating thermal plasma, a material supply device  14  supplying raw material powder for producing tungsten complex oxide particles into the plasma torch  12  in the form of a dispersion liquid, a chamber  16  serving as a cooling tank for producing primary fine particles  15  of the tungsten complex oxide particles, a cyclone  19  removing, from the produced primary fine particles  15 , coarse particles having a particle size equal to or larger than an arbitrarily specified particle size, and a collecting section  20  collecting secondary fine particles  18  of the tungsten complex oxide particles having a desired particle size as obtained by classification in the cyclone  19 . 
         [0031]    Various devices in, for example, JP 2007-138287 A may be used for the material supply device  14 , the chamber  16 , the cyclone  19  and the collecting section  20 . 
         [0032]    In this embodiment, a dispersion liquid prepared by dispersing raw material powder corresponding to the composition of tungsten complex oxide particles in a solvent is used in the production of the tungsten complex oxide particles. The dispersion liquid preferably contains carbon element. The dispersion liquid may also be called “slurry” in the following description. 
         [0033]    The slurry contains carbon element. In obtaining a slurry containing carbon element, there are three patterns including a pattern in which raw material powder contains carbon element, a pattern in which a solvent used for the dispersion liquid contains carbon element, and a pattern in which a carbon element-containing substance is added to a solvent. 
         [0034]    One example of the raw material powder containing carbon element is mixed powder of CsCO 3  powder and WO 3  powder. In addition, carbonates such as Cs 2 CO 3  powder and carbide powders such as WC powder and W 2 C powder may be used. When raw material powder does not contain carbon element, a carbon element-containing substance may be added. Exemplary carbon element-containing substances include carbon-based polymer compounds such as polyethylene glycol and organic substances such as sugar and flour. Thus, carbon element is contained in the form of at least one of a carbide, a carbonate and an organic compound. 
         [0035]    The average particle size of raw material powder is appropriately set to allow the raw material powder to be readily evaporated in a thermal plasma flame, and is, for example, up to 100 μm, preferably up to 10 μm and even more preferably up to 3 μm. The average particle size may be measured using the BET method. 
         [0036]    The carbon element-containing solvent is, for instance, an organic solvent. Specific examples thereof include alcohols, ketones, kerosenes, octanes and gasolines. Usable alcohols include, for instance, ethanol, methanol, propanol and isopropyl alcohol as well as industrial alcohols. Carbon element in the slurry serves to supply carbon that reacts with a part of the raw material powder to reduce the part. Therefore, it is preferable to use an alcohol that is easily decomposed by a thermal plasma flame  24 , and thus lower alcohols are preferred. The solvent preferably contains no inorganic substance. If the raw material powder contains carbon element, the solvent does not necessarily need to contain carbon element and may be, for instance, water. When water is used as the solvent, powder containing carbon as its main ingredient is added to the raw material powder. 
         [0037]    In the slurry, the raw material powder and the solvent are present at a mixing ratio (raw material powder:solvent) of, for example, 4:6 (40%:60%) in terms of weight ratio. 
         [0038]    The plasma torch  12  includes a quartz tube  12   a  and a coil  12   b  for high frequency oscillation surrounding the outside of the quartz tube. On top of the plasma torch  12 , a supply tube  14   a  to be described later which is for supplying raw material powder into the plasma torch  12  in the form of slurry containing the raw material powder as will be described later is provided at the central portion thereof. A plasma gas supply port  12   c  is formed in the peripheral portion of the supply tube  14   a  (on the same circumference). The plasma gas supply port  12   c  is in a ring shape. 
         [0039]    A plasma gas supply source  22  has a first gas supply section  22   a  and a second gas supply section  22   b , which are connected to the plasma gas supply port  12   c  through pipe  22   c . Although not shown, the first and second gas supply sections  22   a  and  22   b  are each provided with a supply amount adjuster such as a valve for adjusting the supply amount. Plasma gas is supplied from the plasma gas supply source  22  into the plasma torch  12  through the plasma gas supply port  12   c.    
         [0040]    For example, two types of plasma gases such as oxygen gas and argon gas are prepared. Oxygen gas is stored in the first gas supply section  22   a , while argon gas is stored in the second gas supply section  22   b . Oxygen gas and argon gas are supplied as plasma gases from the first and second gas supply sections  22   a  and  22   b  of the plasma gas supply source into the plasma torch  12  in a direction indicated by arrow P after having passed through the ring-shaped plasma gas supply port  12   c  via the pipe  22   c . Then, a high frequency voltage is applied to the coil  12   b  for high frequency oscillation to generate the thermal plasma flame  24  in the plasma torch  12 . 
         [0041]    The plasma gases are not limited to oxygen gas and argon gas. As long as oxygen gas is included, for example, helium gas or the like may be used as an inert gas instead of argon gas. Alternatively, a plurality of inert gases such as argon gas and helium gas may be mixed with oxygen gas. 
         [0042]    It is necessary for the thermal plasma flame  24  to have a higher temperature than the boiling point of the raw material powder. On the other hand, the thermal plasma flame  24  preferably has a higher temperature because the raw material powder is more easily converted into a gas phase state. However, there is no particular limitation on the temperature. For instance, the thermal plasma flame  24  may have a temperature of 6,000° C., and in theory, the temperature is deemed to reach around 10,000° C. 
         [0043]    The ambient pressure inside the plasma torch  12  is preferably up to atmospheric pressure. The ambient pressure of up to atmospheric pressure is not particularly limited and is, for example, in a range of 0.5 to 100 kPa. 
         [0044]    The outside of the quartz tube  12   a  is surrounded by a concentrically formed tube (not shown), and cooling water is circulated between this tube and the quartz tube  12   a  to cool the quartz tube  12   a  with the water, thereby preventing the quartz tube  12   a  from having an excessively high temperature due to the thermal plasma flame  24  generated in the plasma torch  12 . 
         [0045]    The material supply device  14  is connected to the upper portion of the plasma torch  12  through the supply tube  14   a . The material supply device  14  is configured to supply a dispersion liquid containing the raw material powder into the thermal plasma flame  24  in the plasma torch  12 . 
         [0046]    For example, the device disclosed in JP 2011-213524 A may be used as the material supply device  14 . In this case, the material supply device  14  includes a vessel (not shown) for introducing a slurry (not shown), an agitator (not shown) agitating the slurry in the vessel, a pump (not shown) for supplying the slurry into the plasma torch  12  through the supply tube  14   a  with a high pressure applied thereto, and an atomization gas supply source (not shown) which supplies atomization gas for supplying the slurry into the plasma torch  12  in the form of droplets. The atomization gas supply source corresponds to the carrier gas supply source. The atomization gas is also called carrier gas. 
         [0047]    The material supply device  14  supplying the raw material powder in the form of slurry supplies atomization gas, to which a push-out pressure is applied, from the atomization gas supply source together with the slurry into the thermal plasma flame  24  in the plasma torch  12  through the supply tube  14   a . The supply tube  14   a  has a two-fluid nozzle mechanism for spraying the slurry into the thermal plasma flame  24  in the plasma torch and converting it into droplets, whereby the slurry can be sprayed into the thermal plasma flame  24  in the plasma torch  12 , in other words, the slurry can be converted into droplets. As with the carrier gas, for example, inert gases such as argon gas and helium gas mentioned as the plasma gases above may be used for the atomization gas. 
         [0048]    As described above, the two-fluid nozzle mechanism is capable of applying a high pressure to the slurry and atomizing the slurry with gas, i.e., atomization gas (carrier gas), and is used as a method for converting the slurry into droplets. 
         [0049]    It should be noted that the nozzle mechanism is not limited to the above-described two-fluid nozzle mechanism but a single-fluid nozzle mechanism may also be used. Other exemplary methods include a method which involves causing a slurry to fall at a constant speed onto a rotating disk so as to convert the slurry into droplets (to form droplets) by the centrifugal force, and a method which involves applying a high voltage to the surface of a slurry to convert the slurry into droplets (to form droplets). 
         [0050]    The chamber  16  is provided below and adjacent to the plasma torch  12 . The chamber  16  is a section in which the primary fine particles  15  of the tungsten complex oxide particles are formed from the dispersion liquid containing the raw material powder as supplied into the thermal plasma flame  24  in the plasma torch  12  and also serves as a cooling tank. 
         [0051]    A gas supply device  28  includes a first gas supply source  28   a , a second gas supply source  28   b  and pipe  28   c , and further includes a compressor which applies push-out pressure to mixed gas supplied into the chamber  16  which will be described later, and a pressure application device such as a blower (not shown). The gas supply device  28  is also provided with a pressure control valve  28   d  which controls the amount of gas supplied from the first gas supply section  28   a  and a pressure control valve  28   e  which controls the amount of gas supplied from the second gas supply section  28   b . For example, the first gas supply source  28   a  stores air gas, while the second gas supply source  28   b  stores oxygen gas. 
         [0052]    The gas supply device  28  supplies oxygen-containing gas, for example, mixed gas of air gas and oxygen gas at a predetermined angle in a direction of arrow Q toward a tail portion of the thermal plasma flame  24 , that is, an end of the thermal plasma flame  24  on the opposite side from the plasma gas supply port  12   c , i.e., the terminating portion of the thermal plasma flame  24 , and also supplies the mixed gas from above to below along a side wall of the chamber  16 , that is, in a direction of arrow R shown in  FIG. 2 . 
         [0053]    In addition to the function as a cooling gas to quench a tungsten complex oxide product produced in the chamber  16  to form the primary fine particles  15  of the tungsten complex oxide particles as will be described later in detail, the mixed gas supplied from the gas supply device  28  has additional effects including contribution to the classification of the primary fine particles  15  in the cyclone  19 . Gas supplied to the terminating portion of the thermal plasma flame  24  is not particularly limited as long as it is gas containing oxygen. 
         [0054]    The slurry introduced from the material supply device  14  is converted into droplets and supplied to the thermal plasma flame  24  in the plasma torch  12  using atomization gas at a predetermined flow rate. As a result, the slurry is converted into a gaseous substance, that is, a gas phase state. Alcohol in the slurry is decomposed to generate carbon. The gaseous substance and carbon react with each other to reduce a part of the raw material powder. Subsequently, due to the mixed gas supplied to the thermal plasma flame  24  in the direction of the arrow Q, the reduced raw material powder is oxidized by oxygen gas present in the mixed gas whereby the tungsten complex oxide product is produced. Then, the tungsten complex oxide product is quenched by the mixed gas in the chamber  16  to thereby produce the primary fine particles  15  of the tungsten complex oxide particles. In this process, the mixed gas supplied in the direction of the arrow R prevents the primary fine particles  15  from adhering to the inner wall of the chamber  16 . 
         [0055]    As shown in  FIG. 2 , the cyclone  19  for classifying the produced primary fine particles  15  based on a desired particle size is provided on a lower lateral side of the chamber  16 . The cyclone  19  includes an inlet tube  19   a  which supplies the primary fine particles  15  from the chamber  16 , a cylindrical outer casing  19   b  connected to the inlet tube  19   a  and positioned in an upper portion of the cyclone  19 , a truncated conical part  19   c  continuing downward from a lower portion of the outer casing  19   b  and having a gradually decreasing diameter, a coarse particle collecting chamber  19   d  connected to a lower side of the truncated conical part  19   c  for collecting coarse particles having a particle size equal to or larger than the above-mentioned desired particle size, and an inner tube  19   e  connected to the collecting section  20  to be described later in detail and projecting from the outer casing  19   b.    
         [0056]    A gas stream containing the primary fine particles  15  produced in the chamber  16  is blown into the cyclone  19  from the inlet tube  19   a  thereof along the inner peripheral wall of the outer casing  19   b , and this gas stream flows in the direction from the inner peripheral wall of the outer casing  19   b  to the truncated conical part  19   c  as indicated by arrow T in  FIG. 2 , thereby forming a downward swirling stream. 
         [0057]    When the above-described downward swirling stream is inverted to form an upward stream, coarse particles cannot follow the upward stream due to the balance between the centrifugal force and drag but come down along the side surface of the truncated conical part  19   c  and are collected in the coarse particle collecting chamber  19   d . Fine particles which were influenced by the drag more than the centrifugal force are discharged to the outside of the system from the inner tube  19   e  along with the upward stream on the inner wall of the truncated conical part  19   c.    
         [0058]    The apparatus is configured such that a negative pressure (suction force) is generated by the collecting section  20  as will be described in detail below and applied through the inner tube  19   e . The apparatus is also configured such that, under the negative pressure (suction force), the tungsten complex oxide particles separated from the above-mentioned swirling gas stream are sucked as indicated by arrow U and sent to the collecting section  20  through the inner tube  19   e.    
         [0059]    On the extension of the inner tube  19   e , which is an outlet for the gas stream in the cyclone  19 , the collecting section  20  for collecting the secondary fine particles (tungsten complex oxide particles)  18  having a desired particle size on the order of nanometers is provided. The collecting section  20  includes a collecting chamber  20   a , a filter  20   b  provided in the collecting chamber  20   a , and a vacuum pump  29  connected through a pipe  20   c  provided below inside the collecting chamber  20   a . The fine particles delivered from the cyclone  19  are sucked by the vacuum pump  29  to be introduced into the collecting chamber  20   a , and remain on the surface of the filter  20   b  and are then collected. 
         [0060]    It should be noted that the number of cyclones used in the method of producing tungsten complex oxide particles according to the invention is not limited to one but may be two or more. 
         [0061]    Fine particles just after the production collide with each other to form agglomerates, thereby causing unevenness in particle size, which may reduce the quality. However, dilution of the primary fine particles  15  with the mixed gas supplied in the direction of the arrow Q toward the tail portion (terminating portion) of the thermal plasma flame prevents the fine particles from colliding with each other to agglomerate together. 
         [0062]    On the other hand, the mixed gas supplied in the direction of the arrow R along the inner wall of the chamber  16  prevents the primary fine particles  15  from adhering to the inner wall of the chamber  16  in the process of collecting the primary fine particles  15 , whereby the yield of the produced primary fine particles  15  is improved. 
         [0063]    Under these circumstances, the mixed gas needs to be supplied in an amount sufficient to quench the resulting tungsten complex oxide particles in the process of producing the primary fine particles  15  of the tungsten complex oxide particles and is preferably supplied in such an amount that the flow rate enabling classification of the primary fine particles  15  at any classification point in the downstream cyclone  19  is obtained and that stabilization of the thermal plasma flame  24  is not hindered. The supply method, supply position and the like of the mixed gas are not particularly limited as long as the stabilization of the thermal plasma flame  24  is not hindered. In the fine particle production apparatus  10  of the embodiment, a circumferential slit is formed in a top plate  17  to supply the mixed gas but any other method or position may be applied as long as the method or position applied enables reliable supply of gas on the path from the thermal plasma flame  24  to the cyclone  19 . 
         [0064]    The method of producing tungsten complex oxide particles using the above-described production apparatus  10  and tungsten complex oxide particles produced by this production method are described below. 
         [0065]      FIG. 3  is a flowchart showing the method of producing tungsten complex oxide particles according to the embodiment of the invention. 
         [0066]    In this embodiment, a dispersion liquid in which raw material powder is dispersed in a solvent is prepared (Step S 10 ), and the dispersion liquid is used to produce tungsten complex oxide particles. As the raw material powder, for instance, mixed powder of CsCO 3  powder and WO 3  powder is used. Alcohol is used for the solvent. In this example, carbon element is contained in the raw material powder and the solvent. Although not limited, the mixing ratio between the raw material powder and the alcohol in the dispersion liquid is 4:6 (40%:60%) in terms of weight ratio. 
         [0067]    For example, argon gas and oxygen gas are used as plasma gases, and a high frequency voltage is applied to the coil  12   b  for high frequency oscillation to generate the thermal plasma flame  24  in the plasma torch  12 . The amount of oxygen gas to be mixed is, for instance, 2.9 vol %. The thermal plasma flame  24  contains oxygen plasma derived from the oxygen gas. 
         [0068]    A mixed gas of air gas and nitrogen gas is supplied in the direction of the arrow Q from the gas supply device to the tail portion of the thermal plasma flame  24 , i.e., the terminating portion of the thermal plasma flame  24 . At that time, the air gas and the nitrogen gas are supplied also in the direction of the arrow R. The amount of air gas mixed in the mixed gas is, for instance, 10 vol %. 
         [0069]    Next, the material supply device  14  supplies the dispersion liquid in the form of droplets into the thermal plasma flame  24  in the plasma torch  12  through the supply tube  14   a  (Step S 12 ). The dispersion liquid is evaporated and converted into a gas phase state by the thermal plasma flame  24 , and the raw material powder and the solvent become gaseous substances. CsWO 3+δ  is produced from the mixed powder of CsCO 3  powder and WO 3  powder. The alcohol and the raw material powder containing carbon as its main ingredient (CsCO 3  powder) in the dispersion liquid are decomposed into C, H 2 O, CO, CO 2  and the like by oxygen plasma of the thermal plasma flame  24 , whereby carbon is generated. 
         [0070]    The raw material powder which is a gaseous substance reacts with C and CO, leading to the reduction. In this example, CsWO 3+δ  and the like react with carbon to produce CsW, CsWO 3−δ  and the like. 
         [0071]    Subsequently, due to the mixed gas supplied to the thermal plasma flame  24  in the direction of the arrow Q, the reduced raw material powder is oxidized by oxygen present in the mixed gas and cooled by the mixed gas (Step S 14 ). More specifically, CsW and O 2  react with each other to produce CsWO 3  as a tungsten complex oxide product, which is in turn quenched by the mixed gas, thereby obtaining CsWO 3  particles as the tungsten complex oxide particles. Primary fine particles  15  of the tungsten complex oxide particles are thus formed (Step S 16 ). 
         [0072]    The primary fine particles  15  produced in the chamber  16  are blown through the inlet tube  19   a  of the cyclone  19  together with a gas stream along the inner peripheral wall of the outer casing  19   b , and this gas stream flows along the inner peripheral wall of the outer casing  19   b  as indicated by the arrow T in  FIG. 2 , thereby forming a swirling stream, which goes downward. When the above-described downward swirling stream is inverted to form an upward stream, coarse particles cannot follow the upward stream due to the balance between the centrifugal force and drag but come down along the side surface of the truncated conical part  19   c  and are collected in the coarse particle collecting chamber  19   d . Fine particles which were influenced by the drag more than the centrifugal force are discharged to the outside of the system from the inner tube  19   e  along with the upward stream on the inner wall of the truncated conical part  19   c.    
         [0073]    Under the negative pressure (suction force) from the collecting section  20 , discharged secondary fine particles  18  of the tungsten complex oxide particles are sucked in the direction indicated by the arrow U in  FIG. 2  and delivered to the collecting section  20  through the inner tube  19   e  to be collected on the filter  20   b  of the collecting section  20 . The internal pressure of the cyclone  19  at that time is preferably up to atmospheric pressure. For the particle size of the secondary fine particles  18  of the tungsten complex oxide particles, an arbitrary particle size on the order of nanometers is selected according to the intended purpose. 
         [0074]    Thus, in this embodiment, the tungsten complex oxide particles having a uniform particle size and having a narrow particle size distribution with a median particle size ranging from several nm to 1000 nm can be thus obtained easily and reliably by merely subjecting raw material powder to plasma treatment. The average particle size of the tungsten complex oxide particles may be measured using the BET method. 
         [0075]    Aside from that, the use of a dispersion liquid can reduce or prevent the segregation of raw material, thereby obtaining the tungsten complex oxide particles with a stable composition. In addition, what is needed is only to supply a slurry to the thermal plasma flame  24 , and therefore, the tungsten complex oxide particles can be obtained at low cost. 
         [0076]    The present inventors confirmed the production of tungsten complex oxide particles by the method of producing tungsten complex oxide particles according to the invention. The results are shown in  FIG. 4 . In the production of tungsten complex oxide particles, cesium carbonate (Cs 2 CO 3 ) powder and tungsten oxide (WO 3 ) powder were used as raw materials, and argon gas and oxygen gas were used as plasma gases. 
         [0077]    Cs x WO 3  particles indicated by E 1  and Cs x WO 3  particles indicated by E 2  in  FIG. 4  were produced under the same production conditions except that quenching gases in those cases contained air with air concentrations different from each other by 10 vol %. In the case associated with reference sign E 1 , the air concentration in the quenching gas was 5 vol %, while in the case associated with reference sign E 2 , the air concentration in the quenching gas was 15 vol %. 
         [0078]    As can be seen in  FIG. 4 , even when CsWO 3  particles were produced with different production conditions, no peak of tungsten appeared, and Cs x WO 3  particles could be produced. In  FIG. 4 , the mark “∘ (circle)” represents a diffraction peak of Cs x WO 3 . 
         [0079]    The optical characteristics of Cs x WO 3  particles indicated by E 1  and Cs x WO 3  particles indicated by E 2  were evaluated. The results are shown in  FIG. 5 . 
         [0080]      FIG. 5  is a graph showing evaluation results for the optical characteristics of the Cs x WO 3  particles. Note that reference signs E 1  and E 2  in  FIG. 5  represent the same as those in  FIG. 4 . 
         [0081]    As shown in  FIG. 5 , with the method of producing tungsten complex oxide particles according to the invention, the absorbance in the visible light region D VL  can be reduced, while the absorbance in the infrared light region D IR  can be increased. Therefore, the tungsten complex oxide particles of the invention can be used for heat ray shielding materials. 
         [0082]    The present invention is basically configured as above. While the method of producing tungsten complex oxide particles according to the invention has been described above in detail, the invention is by no means limited to the foregoing embodiment 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 
       [0000]    
       
         
           
               10  fine particle production apparatus 
               12  plasma torch 
               14  material supply device 
               15  primary fine particle 
               16  chamber 
               18  fine particle (secondary fine particle) 
               19  cyclone 
               20  collecting section 
               22  plasma gas supply source 
               24  thermal plasma flame 
               28  gas supply device