Method for producing composite tungsten oxide particles

Abstract

To provide a production method of a composite tungsten oxide particle having a smaller number of processes and capable of using facilities of lower introduction costs.SOLUTION: A production method is of a composite tungsten oxide particle expressed by a general formula MxWyOz (0.001≤x / y≤1, 2.2≤z / y≤3.0). The production method of a composite tungsten oxide particle includes: an aerosol formation process of forming an aerosol, being a treatment object, containing mixed base powder of W source base powder to be served as a tungsten source object and M element source base powder to be served as an M element source; a heat treatment process of applying a heat treatment at 1000 to 1350°C while conveying the aerosol by a carrier gas; and a cooling process of supplying a coolant gas to a product material obtained at the heat treatment process and cooling the gas. The carrier gas is a mixed gas containing an inert gas and H2 of 0.1 to 0.9 vol.%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing composite tungsten oxide particles. [Background technology]

[0002] Various near-infrared shielding technologies have been proposed to reduce solar radiation transmittance while maintaining good visible light transmittance and transparency. Among them, near-infrared shielding technologies using inorganic conductive fine particles have the advantages of superior near-infrared shielding properties compared to other technologies, low cost, radio wave transmittance, and high weather resistance.

[0003] For example, Patent Document 1 discloses an infrared-shielding material fine particle dispersion in which fine particles of an infrared-shielding material are dispersed in a medium, and the fine particles of the infrared-shielding material are represented by the general formula M x W y O z (wherein 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; and 0.001≦x / y≦1, 2.2≦z / y≦3.0), as well as a technique relating to a method for producing the infrared-shielding microparticles. Patent Document 1 also discloses an example of producing an infrared-shielding film, which is a thin-film dispersion of infrared-shielding material microparticles.

[0004] According to Patent Document 1, it is possible to prepare an infrared-shielding material microparticle dispersion having excellent optical properties, such as more efficient shielding of sunlight, particularly light in the near-infrared region, while at the same time maintaining transmittance in the visible light region, etc. For this reason, application of the infrared-shielding particle dispersion disclosed in Patent Document 1 to various applications such as window glass has been considered.

[0005] Various studies have been conducted on methods for producing composite tungsten oxide particles that are useful as a near-infrared shielding material.

[0006] For example, the inventor of Patent Document 1 described in Non-Patent Document 1 that Cs 0.32 proposed a method for synthesizing WO3 nanoparticles. However, the particle size in the synthesis method disclosed in Non-Patent Document 1 was large, and a pulverization process was required to produce nanoparticles. This could increase the number of process steps.

[0007] Non-Patent Document 2 describes Cs by hydrothermal synthesis. x A method for synthesizing WO3 has been disclosed. However, the hydrothermal synthesis method requires a synthesis time of several tens of hours or more. In addition, the hydrothermal synthesis method has a problem in that it requires a large number of steps, such as post-treatment steps.

[0008] Non-Patent Document 3 discloses a synthesis method based on inductively coupled thermal plasma technology. However, this synthesis method requires the introduction of an inductively coupled thermal plasma device, which increases the cost.

[0009] Patent Document 2 describes the chemical formula K x Cs y WO z A process for preparing potassium-cesium-tungsten bronze solid solution particles of the formula: where x+y≦1 and 2≦z≦3, comprising combining a suitable tungsten source with potassium and cesium salts to form a powder mixture, and exposing the powder mixture to a plasma torch under a reducing atmosphere, preferably where the reducing atmosphere is supplied by a sheath gas consisting of a hydrogen / noble gas mixture, is disclosed.

[0010] However, the method disclosed in Patent Document 2 also requires the use of plasma, which increases the cost of introducing a plasma device. Furthermore, the manufacturing method disclosed in Patent Document 2 also discloses that metallic tungsten is mixed in as an impurity.

[0011] As shown in Non-Patent Document 4, a method of supplying aerosolized raw materials to a flame field was proposed. This method of supplying raw material powder is easy to increase productivity and can eliminate the effects of the solvent contained in the raw materials, so it simultaneously solves the problems of "low droplet formation rate (3 g / Hr or less on a lab scale) due to the use of an ultrasonic atomizer" and "requiring latent heat to vaporize and remove the solvent." However, the method in Non-Patent Document 4 involves a combustion reaction to form a flame field, which results in water and carbon being mixed in as by-products, so there remains the problem that "by-products contained in the solvent and raw materials are mixed into the crystals, resulting in low infrared absorption characteristics." [Prior art documents] [Patent documents]

[0012] [Patent Document 1] International Publication No. 2005 / 037932 [Patent Document 2] Special Publication No. 2012-532822 [Non-patent literature]

[0013] [Non-Patent Document 1] Takeda Hiromitsu, and Kenji Adachi, "Near infrared absorption of tungsten oxide nanoparticle dispersions." Journal of the American Ceramic Society,2007 , Vol.90, Issue 12, P.4059-4061 [Non-Patent Document 2] Guo Chongshen, et al., "Novel synthesis of homogenous CsxWO3nanorods with excellent NIR shielding properties by a water controlled-release solvothermal process." Journal of Materials Chemistry,2010, Vol.20, Issue38, P.8227-8229. [Non-Patent Document 3] Mamak Marc, et al., "Thermal plasma synthesis of tungsten bronze nanoparticles for near infra-red absorption applications." Journal of Materials Chemistry, 2010, Vol.20, Issue44, P.9855-9857. [Non-Patent Document 4] Hidayat, Darmawan, et al. "Preparation of size-controlled tungsten oxide nanoparticles and evaluation of their adsorption performance." Materials Research Bulletin 45.2 (2010): 165-173. [Non-Patent Document 5] Okada, Mika, et al. "Oxygen vacancies and pseudo Jahn-Teller destabilization in cesium-doped hexagonal tungsten bronzes." Journal of the American Ceramic Society 102.9 (2019): 5386-5400. Summary of the Invention [Problem to be solved by the invention]

[0014] As described above, composite tungsten oxide particles are useful as a near-infrared shielding material. There is a demand for a method for producing composite tungsten oxide particles that can be produced at low cost and with fewer steps.

[0015] However, the conventional methods for producing composite tungsten oxide particles have problems such as the need to introduce special, expensive equipment and the need for many steps, as described above.

[0016] In view of the problems of the conventional techniques described above, an object of one aspect of the present invention is to provide a method for producing composite tungsten oxide particles that can use equipment with low introduction costs and that requires a small number of steps. [Means for solving the problem]

[0017] In one aspect of the invention, a compound of general formula M x W y O z (However, the M element is Cs , W is tungsten, O is oxygen, and 0.001≦x / y≦1, 2.2≦z / y≦3.0), an aerosol forming step of forming an aerosol containing a mixed raw material powder of a W source raw material powder as a tungsten source and an M element source raw material powder as an M element source, which are objects to be processed; a heat treatment step of carrying the aerosol by a carrier gas at 1000° C. or more and 1350° C. or less; A cooling step of supplying a cooling gas to the product obtained in the heat treatment step to cool it, The present invention provides a method for producing composite tungsten oxide particles, wherein the carrier gas is a mixed gas containing an inert gas and 0.1 volume % or more and 0.9 volume % or less of H2. Effect of the Invention

[0018] One aspect of the present invention provides a method for producing composite tungsten oxide particles that can use equipment with low introduction costs and that requires a small number of steps. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is an explanatory diagram of a composite material production apparatus that can be suitably used when producing the composite tungsten oxide particles according to one embodiment of the present disclosure. [Diagram 2] FIG. 2 is an explanatory diagram of the reduction treatment device. [Diagram 3] FIG. 3 shows a temperature profile in the pipe of the reaction section in Example 1. [Figure 4] FIG. 4 shows XRD profiles of the composite tungsten oxide particles obtained in Example 1 and Comparative Examples 1 to 4. [Diagram 5] FIG. 5 is a TEM image of the composite tungsten oxide particles obtained in Comparative Example 3. [Figure 6] FIG. 6 is a TEM image of the composite tungsten oxide particles obtained in Comparative Example 1. [Figure 7] FIG. 7 is an SEM image of the composite tungsten oxide particles obtained in Example 1. [Figure 8] FIG. 8 is a TEM image of the composite tungsten oxide particles obtained in Examples 1 to 3. [Figure 9] FIG. 9 is an SEM image of the composite tungsten oxide particles obtained in Example 2. [Figure 10] FIG. 10 is an explanatory diagram of a composite material production apparatus used for producing the composite tungsten oxide particles of Comparative Example 5. [Figure 11] FIG. 11 shows the transmission profiles of the inks obtained in Example 3 and Comparative Example 7. [Figure 12] FIG. 12 is a TEM image of the composite tungsten oxide particles obtained in Comparative Example 5. [Figure 13] FIG. 13 is an SEM image of the composite tungsten oxide particles obtained in Comparative Example 6. [Figure 14]FIG. 14 is a STEM-HAADF image of the composite tungsten oxide particles obtained in Example 1 and Example 2. [Figure 15] FIG. 15 shows XRD profiles of the composite tungsten oxide particles obtained in Examples 1 and 2 and Comparative Examples 5 and 6. [Figure 16] FIG. 16 is a diagram showing the content ratio of each phase in the composite tungsten oxide particles obtained in Examples 1 and 2 and Comparative Examples 5 and 6. [Figure 17] FIG. 17 is a diagram showing changes in the lattice constant of the CWO phase in the composite tungsten oxide particles obtained in Examples 1 and 2 and Comparative Examples 5 and 6. [Figure 18] FIG. 18 is an explanatory diagram of the mechanism of the production process of composite tungsten oxide particles. [Figure 19] FIG. 19 is an explanatory diagram of mass flow rates in the synthesis of particles by liquid spray pyrolysis in Comparative Example 5 and in the synthesis of particles by solid spray pyrolysis in Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] A specific example of the method for producing composite tungsten oxide particles according to one embodiment of the present disclosure (hereinafter referred to as "the present embodiment") will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0021] Hereinafter, one configuration example of the method for producing composite tungsten oxide particles of the present embodiment will be described.

[0022] The method for producing the composite tungsten oxide particles of the present embodiment comprises the steps of: x W y O z The method for producing the composite tungsten oxide particles represented by the formula (I) can include the following aerosol formation step, heat treatment step, and cooling step.

[0023] In addition, the M element in the above general formula can be one or more elements selected from 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. In addition, W represents tungsten, O represents oxygen, and x, y, and z preferably satisfy 0.001≦x / y≦1 and 2.2≦z / y≦3.0, respectively.

[0024] The aerosol forming step can form an aerosol containing a mixed raw material powder of a W source raw material powder serving as a tungsten source and an M element source raw material powder serving as an M element source, which is the object to be processed.

[0025] In the heat treatment step, the aerosol formed in the aerosol formation step can be heat-treated at 1000° C. or more and 1350° C. or less while transporting the aerosol formed in the aerosol formation step with a carrier gas.

[0026] In the cooling step, the product obtained in the heat treatment step can be cooled by supplying a cooling gas thereto.

[0027] The carrier gas used in the heat treatment process may be a mixed gas containing an inert gas and 0.1% by volume or more and 0.9% by volume or less of H2. (1) Composite tungsten oxide particles First, the composite tungsten oxide particles produced by the method for producing composite tungsten oxide particles of the present embodiment will be described. (About the composition) The composite tungsten oxide contained in the composite tungsten oxide particles is represented by the general formula M x W y O z The M element, W, O, and x, y, and z in the formula have already been described, so their explanation will be omitted here.

[0028] The composite tungsten oxide can have a tungsten bronze type crystal structure, for example, any of tetragonal, cubic, and hexagonal. The crystal structure of the composite tungsten oxide contained in the composite tungsten oxide particles obtained by the method for producing composite tungsten oxide particles of the present embodiment is not particularly limited, and can have one or more types of crystal structures selected from tetragonal, cubic, and hexagonal.

[0029] However, when the composite tungsten oxide has a hexagonal crystal structure, the composite tungsten oxide particles are preferably provided with a particularly improved light transmittance in the visible light region and a particularly improved light absorption in the near-infrared region, and therefore the composite tungsten oxide particles preferably contain a composite tungsten oxide having a hexagonal crystal structure.

[0030] Moreover, when one or more elements selected from Cs, Rb, K, Tl, Ba, and In are used as the M element, it becomes easier to form a hexagonal crystal. For this reason, it is preferable that the M element contains one or more elements selected from Cs, Rb, K, Tl, Ba, and In. In particular, it is more preferable that the M element contains Cs, and it is even more preferable that the M element is Cs.

[0031] Here, the manner in which the M element is arranged when the composite tungsten oxide has a hexagonal crystal structure will be described.

[0032] An octahedron formed of a unit of W and six O, i.e., an octahedron with O atoms at the vertices and W atoms at the center, is assembled into six hexagonal voids (tunnels) composed of O atoms. Then, M element is arranged in the void to form one unit, and a large number of these units are assembled to form a hexagonal crystal structure. When the composite tungsten oxide having a hexagonal crystal structure has a uniform crystal structure, the amount of M element added is preferably 0.2 to 0.5 in terms of the value of x / y, more preferably 0.33. When z / y=3, it is considered that the value of x / y is 0.33, so that M element is arranged in all of the hexagonal voids.

[0033] Similarly, when z / y=3, each of the composite tungsten oxides of cubic and tetragonal crystals has an upper limit of the amount of M element added due to the structure, and the maximum amount of M element added to 1 mole of tungsten is 1 mole in the case of cubic crystals, and about 0.5 moles in the case of tetragonal crystals. Note that the maximum amount of M element added to 1 mole of tungsten in the case of tetragonal crystals varies depending on the type of M element, but the amount that is easy to manufacture industrially is about 0.5 moles as described above. However, these structures are difficult to simply define, and the range is an example that shows a particularly basic range, so the present invention is not limited to this.

[0034] In addition, even a very small amount of M element is added to generate free electrons in the composite tungsten oxide, and the desired infrared absorbing effect can be obtained. For this reason, it is preferable that x / y satisfies 0.001≦x / y≦1.

[0035] In addition, the composite tungsten oxide has a composition in which an M element is added to tungsten trioxide (WO3). Since tungsten trioxide does not contain effective free electrons, the ratio of oxygen to 1 mole of tungsten must be less than 3 in order to achieve infrared absorption. However, in the composite tungsten oxide, free electrons are generated by adding an M element, and the infrared absorption effect can be obtained. Therefore, the ratio of oxygen to 1 mole of tungsten can be 3 or less. However, the crystal phase of WO2 may absorb or scatter light in the visible light region, thereby reducing the absorption of light in the near infrared region. Therefore, from the viewpoint of suppressing the generation of WO2, it is preferable that the ratio of oxygen to 1 mole of tungsten is greater than 2.

[0036] Therefore, it is preferable to satisfy 2.2≦z / y≦3.0 as described above. (About particle size) The particle size of the composite tungsten oxide particles produced by the method for producing composite tungsten oxide particles of the present embodiment is not particularly limited, and can be selected depending on the intended use, etc.

[0037] For example, when used in applications that require maintaining transparency, it is preferable that the particle size is 800 nm or less. This is because particles with a particle size of 800 nm or less do not completely block light due to scattering, and can maintain high visibility in the visible light range while efficiently maintaining transparency at the same time. In particular, when transparency in the visible light range is important, it is preferable to further consider scattering by particles.

[0038] When emphasis is placed on reducing scattering due to such particles, the particle size is preferably 200 nm or less, and more preferably 100 nm or less.

[0039] This is because if the particle size is small, scattering of light in the visible light region of wavelength 400nm to 780nm due to geometric scattering or Mie scattering is reduced, and the infrared shielding film becomes like cloudy glass, and clear transparency cannot be obtained. If the particle size is 200nm or less, the geometric scattering or Mie scattering is reduced and the Rayleigh scattering region is reached. In the Rayleigh scattering region, scattered light is reduced in proportion to the sixth power of the particle size, so scattering is reduced with the decrease in particle size, and transparency is improved. Furthermore, if the particle size is 100nm or less, the scattered light is very small, which is preferable. From the viewpoint of avoiding light scattering, a small particle size is preferable.

[0040] Therefore, the particle size of the composite tungsten oxide particles produced by the method for producing composite tungsten oxide particles of this embodiment can be selected according to the purpose of use.For example, as described above, when it is required to maintain high visibility in the visible light region, the particle size is preferably 800nm ​​or less, more preferably 200nm or less, and even more preferably 100nm or less.The lower limit of the particle size of the composite tungsten oxide particles produced by the method for producing composite tungsten oxide particles of this embodiment is not particularly limited, but can be, for example, 1nm or more.

[0041] The particle diameter of the composite tungsten oxide particles obtained by the method for producing composite tungsten oxide particles of the present embodiment can be determined by observing the particle with, for example, an SEM or TEM, and determining the diameter of the smallest circumscribing circle that circumscribes the particle.

[0042] The particle size of the resulting composite tungsten oxide particles can be selected by adjusting, for example, the size of the aerosol formed in the aerosol formation step described below, the heat treatment temperature in the heat treatment step, and the like.

[0043] Since infrared-shielding materials containing the composite tungsten oxide particles obtained by the method for producing composite tungsten oxide particles of this embodiment largely absorb light in the near-infrared region, particularly light with a wavelength of about 1000 nm, the transmitted color tone is often bluish to greenish. (2) Manufacturing method of composite tungsten oxide particles Next, the method for producing the composite tungsten oxide particles of the present embodiment will be specifically described.

[0044] The method for producing composite tungsten oxide particles in this embodiment is a powder supply type pyrolysis method (solid spray pyrolysis method) in which a mixed raw material powder is made into an aerosol, and the powder is heated while being transported by a carrier gas in a reducing atmosphere, causing the powder to sublimate, and composite tungsten oxide particles are obtained while controlling the cooling process, which affects the final particle size, crystal phase, and condensation and precipitation processes, with a cooling gas.

[0045] Therefore, the method for producing composite tungsten oxide particles of this embodiment can include an aerosol forming step of aerosolizing a mixed raw material powder obtained by mixing a W source raw material powder as a tungsten source and an M element source raw material powder as an M element source.Then, the aerosol can be supplied with a carrier gas and heat-treated at 1000°C or more and 1350°C or less.Furthermore, the composite tungsten oxide particles obtained through the heat treatment step can be cooled with a cooling gas and collected. (Aerosol formation process) In the aerosol forming process, the aerosol is formed from a mixed raw material powder obtained by mixing a W source raw material powder as a tungsten source and an M element source raw material powder as an M element source, which are the objects to be processed. Note that being a tungsten source and being an M element source mean that tungsten can be supplied and that the M element can be supplied, respectively.

[0046] The aerosol forming step may be performed by any device capable of forming a dispersion state of the mixed raw material powder and supplying it into the airflow, and the specific means are not particularly limited. For example, the aerosol forming section provided in the composite material manufacturing device that can be used in the manufacturing method of the composite tungsten oxide of this embodiment can be composed of a stirring section such as a rotating brush or stirring blade, and a powder supply section including a piston or a screw feeder that sends the mixed raw material powder to the stirring section. The mixed raw material powder supplied from the powder supply section is dispersed into particles that constitute the powder in the stirring section, and an aerosol can be generated from the mixed raw material powder by sending each particle to the carrier gas. The stirring section can select the rotation speed of the brush or stirring blade so that the mixed raw material powder can be dispersed into particles, and it is preferable to rotate it at a high speed.

[0047] In particular, since an aerosol with little aggregation between particles can be stably formed, a rotating brush can be suitably used in the stirring section, and a configuration in which the mixed raw material powder loaded in a reservoir is supplied to the stirring section by a piston can be suitably used in the powder supply section.

[0048] In such an aerosol forming section, the powder concentration in the aerosol changes depending on the transport speed of the powder loaded in the reservoir, for example, the movement speed of the piston.

[0049] The tungsten source is not particularly limited, and a tungsten salt or the like can be used. For example, H2WO4 or ammonium paratungstate can be preferably used.

[0050] In H2WO4, the elements other than tungsten are H (hydrogen) and O (oxygen), and the elements other than tungsten are discharged outside the system in the heat treatment process described below. Therefore, by using H2WO4 as the W source raw material powder that is the tungsten source, composite tungsten oxide particles with reduced impurity contamination can be obtained, and therefore it is preferably used.

[0051] The M element source raw material powder can be, for example, a powder of a salt containing the M element. The type of salt containing the M element is not particularly limited, but for example, one or more types selected from carbonates, acetates, nitrates, hydroxides, etc. of the M element can be used.

[0052] For example, when the M element is cesium, one or more selected from carbonates, acetates, nitrates, hydroxides, etc. can be used, with carbonates being particularly preferred.

[0053] The ratio of the M element to 1 mole of tungsten in the obtained composite tungsten oxide, i.e., the doping amount, is determined by the mixing ratio of the W source raw material powder and the M element source raw material powder when forming the raw material mixture powder. Therefore, the doping amount can be controlled by adjusting, for example, the amount of the W source raw material powder and the amount of the M element source raw material powder.

[0054] The mixed raw powder may be a mixed raw powder obtained by physically mixing the W source raw powder and the M element source raw powder, or may be a mixed raw powder obtained by dissolving the W source raw powder and the M element source raw powder in a solvent such as water to form a solution, mixing them, and then removing the solvent from the solution by drying, etc. In cases where it is particularly required to suppress the variation in the composition of the obtained composite tungsten oxide particles, a mixed raw powder obtained by dissolving the W source raw powder and the M element source raw powder in a solution, mixing them, and then removing the solvent from the solution by drying, etc. may be preferable.

[0055] The particle size of the mixed raw material powder dispersed in the aerosol forming process is not particularly limited, but the particle diameter is preferably 100 μm or less, more preferably 10 μm or less, and even more preferably 3 μm or less. By making the particle diameter 100 μm or less, it is possible to more reliably sublimate the inside of the particle. In addition, the particle size of the mixed raw material powder dispersed in the aerosol forming process is preferably 100 nm or more. The particle diameter of the mixed raw material powder can be measured in the same manner as the particle diameter of the composite tungsten oxide particles described above.

[0056] The aerosol formed in the aerosol forming step can be transported, for example, by a carrier gas and subjected to a heat treatment step. (Heat treatment process) In the heat treatment step, the workpiece is heat treated at 1000°C or higher and 1350°C or lower.

[0057] The mixed powder of raw material containing the W source raw material powder and the M element source raw material powder, which is the object to be treated, is evaporated in the process of heating to 1000°C or more and 1350°C or less, and is considered to be decomposed into the atomic level. It is considered that the mixed powder then forms clusters, and undergoes a condensation process as a composite tungsten oxide to form composite tungsten oxide particles. The heat treatment temperature is more preferably 1200°C or more and 1350°C or less, and even more preferably 1250°C or more and 1350°C or less.

[0058] It is considered that composite tungsten oxide is formed during the decomposition process of the W source raw material powder or the M element source raw material powder, or at higher temperatures, when tungsten reacts with the M element.

[0059] Therefore, in order to sufficiently advance the decomposition of the W source raw material powder or the M element source raw material powder and to suppress the incorporation of impurities into the composite tungsten oxide, it is preferable to perform the heat treatment at a temperature equal to or higher than the decomposition temperature of the W source raw material powder or the M element source raw material powder in the heat treatment step. It is considered that the decomposition of the W source raw material powder or the M element source raw material powder usually occurs at 500° C. or lower. Therefore, in the heat treatment step, the aerosol, which is the object to be treated, can be heat treated at 500° C. or higher.

[0060] According to the studies of the present inventors, the heat treatment temperature also affects the particle size of the resulting composite tungsten oxide particles. Further studies by the present inventors have shown that the particle size of the resulting composite tungsten oxide particles tends to decrease as the heat treatment temperature increases.

[0061] This is presumably because, when the heat treatment temperature is increased, thermal energy is used to sublimate the generated composite tungsten oxide particles, causing the particles to burst upon sublimation, resulting in particles with a fine particle size.

[0062] For this reason, in order to obtain composite tungsten oxide particles that are particularly fine nanoparticles, the heat treatment temperature is preferably 1000° C. or higher. That is, when the purpose is to obtain particularly fine nanoparticles, it is preferable to heat treat the workpiece at 1000° C. or higher in the heat treatment step.

[0063] In the heat treatment process, the upper limit of the temperature is 1350° C. If the heat treatment temperature exceeds 1350° C., a different phase such as metallic W may be generated.

[0064] The aerosol is transported to an electric furnace or the like by a carrier gas, and the above-mentioned heat treatment process can be carried out. Therefore, for example, by controlling the flow rate of the carrier gas, the time of the heat treatment process can be adjusted.

[0065] The time required for the heat treatment step is not particularly limited and can be selected as desired.

[0066] As described above, the mixed powder material containing the W source powder and the M element source powder, which is the object to be treated, evaporates during the process of heating to the heat treatment temperature and is decomposed into atoms. After that, clusters are formed, and the composite tungsten oxide is formed through a condensation process as the composite tungsten oxide.

[0067] The carrier gas used in the heat treatment process is a mixed gas containing an inert gas and 0.1% by volume or more and 0.9% by volume or less of H2. The inert gas of the carrier gas is not particularly limited, and one or more types selected from rare gas (noble gas) and nitrogen gas can be used. The inert gas is particularly preferably Ar. For this reason, the carrier gas is preferably a mixed gas of Ar and H2.

[0068] By containing 0.1% by volume or more and 0.9% by volume or less of H2 in the carrier gas, composite tungsten oxide particles with few heterophases can be obtained even when heat treatment is performed at the heat treatment temperature described above. If the content of H2 exceeds 0.9% by volume, reduction may proceed and metal W may precipitate. As described above, hexagonal composite tungsten oxide particles can be suitably used from the viewpoint of increasing the transmittance of light in the visible light region and the absorption of light in the near infrared region. And, when heat treatment is performed at the heat treatment temperature described above, by setting the content of H2 in the carrier gas to 0.1% by volume or more, composite tungsten oxide particles having a hexagonal crystal structure can be particularly easily obtained. For this reason, it is preferable to set the content of H2 in the carrier gas to 0.1% by volume or more as described above.

[0069] The flow rate of the carrier gas may be any flow rate that can form and maintain an aerosol in the aerosol forming section of the composite material production apparatus described below, and can be appropriately adjusted depending on the size of the composite material production apparatus used to produce the composite tungsten oxide particles. (cooling process) The composite tungsten oxide particles produced in this embodiment are preferably not recovered immediately after heat treatment, but are recovered through a cooling process in which cooling gas is supplied to cool them. In the method for producing composite tungsten oxide particles of this embodiment, the heat treatment temperature is relatively high, at 1000°C or higher and 1350°C or lower. For this reason, it is preferable to cool the product heated in the heat treatment and the high-temperature gas in the cooling process to stop the reaction and suppress the generation of a different phase. On the other hand, if the product is recovered without going through the cooling process, the reaction of the product proceeds with the high-temperature reducing gas discharged from the heat treatment process, and a different phase may be generated.

[0070] The specific operation of the cooling step is not particularly limited, but the product generated in the heat treatment step may be introduced into a container to which a cooling gas at room temperature is supplied. The cooling gas at room temperature means a cooling gas that is in equilibrium with the ambient temperature without heating or cooling, and may be, for example, a gas at 5°C to 35°C. In particular, in the cooling step, it is preferable to supply the cooling gas from multiple directions, i.e., from two or more directions, to the product obtained in the heat treatment step. This is because supplying the cooling gas from two or more directions to the product obtained in the heat treatment step allows for particularly rapid cooling and suppresses the generation of heterogeneous phases.

[0071] The cooling gas is preferably a mixed gas of an inert gas and O2. As the inert gas, a rare gas such as Ar or N2 can be used. In consideration of cost, N2 can be preferably used as the inert gas. Therefore, as the cooling gas, a mixed gas of N2 and O2 can be preferably used. The cooling gas preferably has an oxygen concentration of 1% by volume or more and 5% by volume or less.

[0072] By including O2 in the cooling gas, the reduction reaction that has progressed during the heat treatment process can be actively stopped, which in turn can particularly suppress the formation of heterogeneous phases. (Reduction treatment process) Particles obtained through a heat treatment process and a cooling process, specifically composite tungsten oxide particles, may not exhibit infrared absorbing properties.The inventors of the present invention have conducted research and found that the composite tungsten oxide particles obtained through a heat treatment process and a cooling process can be further subjected to a reduction treatment process to perform a reduction treatment, thereby enabling the composite tungsten oxide particles to more reliably exhibit infrared absorbing properties.

[0073] For this reason, the method for producing composite tungsten oxide particles of the present embodiment preferably includes, after the cooling step, a reduction treatment step of reducing the obtained particles in an atmosphere containing a reducing gas.

[0074] The conditions for the reduction treatment are not particularly limited, but it is preferable to select the conditions for the reduction treatment so that when the composite tungsten oxide particles after the reduction treatment are analyzed by X-ray diffraction pattern, metallic tungsten and the like do not precipitate.

[0075] In the reduction treatment step, the composite tungsten oxide particles obtained after the cooling step can be reduced by raising and lowering the temperature in a reducing atmosphere containing a reducing gas, that is, by performing a heat treatment.

[0076] During the reduction treatment step, the composite tungsten oxide particles may be stirred or left to stand, and the handling of the composite tungsten oxide particles in the reduction treatment step can be appropriately selected, but it is preferable to select handling conditions such that metallic tungsten does not precipitate.

[0077] The temperature of the reduction treatment (reduction treatment temperature) is preferably higher than 400° C. and lower than 700° C., more preferably 500° C. or higher and lower than 700° C., and even more preferably 600° C. or higher and lower than 700° C. In addition, after the temperature is raised from room temperature to the reduction treatment temperature, it can be lowered again to room temperature. Here, room temperature refers to the temperature before the start of the temperature rise or after the temperature drop, and means the ambient temperature or a temperature close to the ambient temperature.

[0078] By setting the reduction temperature to more than 400° C., the reduction process of the composite tungsten oxide particles can be advanced, and the infrared absorption properties can be more reliably exhibited. Also, by setting the temperature to less than 700° C., the composite tungsten oxide particles can be prevented from being reduced to metallic tungsten.

[0079] The reducing atmosphere is preferably an atmosphere of a mixed gas of an inert gas such as argon and a reducing gas such as H2 gas (hydrogen gas), and the reducing gas is preferably H2 gas.

[0080] When H2 gas is used as the reducing gas, the content of H2 gas in the reducing atmosphere can be appropriately selected, but the content of H2 gas is preferably in the range of 0.1 volume % to 10 volume %, more preferably 2 volume % to 10 volume %. If reduction is performed in an atmosphere containing only reducing gas, the reduction reaction may proceed excessively and metallic tungsten may be precipitated, so care must be taken.

[0081] The time of the reduction process is preferably 30 minutes or more in total from temperature rise to temperature fall. The upper limit of the time of the reduction process is not particularly limited, and it is preferable to select it by performing a preliminary test or the like so that the reduction does not proceed excessively. The total time from temperature rise to temperature fall here means the time from starting temperature rise from room temperature to reaching the reduction treatment temperature and then cooling to room temperature. It is preferable that the composite tungsten oxide particles are placed in the reducing atmosphere described above during this time.

[0082] Although the mechanism of the reduction process is unclear, the composite tungsten oxide particles obtained after the cooling process were confirmed by X-ray diffraction patterns to be, for example, CsW 1.6 O6 phase, Cs2O phase, etc. 0.32 It is also possible to convert the heterogeneous phase, which does not contribute much to the infrared absorbing properties, into the target phase capable of exhibiting infrared absorbing properties by carrying out the reduction treatment process, so that it is possible to obtain composite tungsten oxide particles having particularly excellent infrared absorbing properties.

[0083] In addition, according to the study of the inventor of the present invention, the composite tungsten oxide particles obtained after the cooling process may contain two types of composite tungsten oxide crystal phases. Specifically, the composite tungsten oxide particles may contain two crystal structures: a first crystal structure with a lattice constant of a-axis of 7.400 Å or more and 7.405 Å or less, and a lattice constant of c-axis of 7.680 Å or more and 7.700 Å or less, and a second crystal structure with a lattice constant of a-axis of 7.415 Å or more and 7.420 Å or less, and a lattice constant of c-axis of 7.600 Å or more and 7.620 Å or less. However, by performing a reduction treatment process, the composite tungsten oxide particles can be made to have a crystal structure with excellent infrared absorption properties from the phases of these two types of crystal structures.

[0084] In other words, composite tungsten oxide particles with excellent infrared absorption properties can be prepared via the above two crystal structures. (3)Composite material manufacturing equipment An example of the configuration of a composite material production apparatus that can be suitably used in the method for producing composite tungsten oxide particles of this embodiment will be described below.

[0085] FIG. 1 is a schematic diagram of a composite material manufacturing apparatus 10. As shown in FIG.

[0086] The composite material production apparatus 10 can have an aerosol formation section 11, a transport section 21, a reaction section 31, a cooling section 41, and a recovery section 51.

[0087] (Aerosol forming part) The aerosol forming section 11 includes an aerosol generating tube 12, an agitating section 14, and a powder supplying section 16 that supplies mixed raw material powder to the agitating section 14. The aerosol generating tube is tubular and includes an opening 13 that is connected to the agitating section 14. The agitating section 14 includes a rotating brush 15. The brush 15 is disposed at the opening 13 so as to be exposed to the aerosol generating tube 12. Also, an agitating blade may be used instead of the brush.

[0088] The powder supply unit 16 can have a raw material reservoir 17 for storing the mixed raw material powder, and a piston 18 for sending the mixed raw material powder to the stirring unit. The supply speed of the mixed raw material powder can be controlled by the moving speed of the piston 18. A known screw feeder or the like can be used instead of the piston.

[0089] Furthermore, the aerosol generating pipe 12 can also be provided with a function of supplying carrier gas to the composite material manufacturing apparatus 10 by connecting a regulator 19 that controls the flow rate of carrier gas supplied from the outside.

[0090] (Transportation Department) The transport unit 21 can be provided between the aerosol formation unit 11 and the reaction unit 31. Specifically, the aerosol generation tube 12 can be connected to the transport unit 21 on the downstream side of the carrier gas, and the transport unit 21 can be connected to a pipe 32 of the reaction unit 31. The transport unit 21 can supply the aerosol from the aerosol generation tube 12 to the pipe 32.

[0091] (Reaction section) The above-mentioned heat treatment step can be carried out in the reaction section 31. For this purpose, the reaction section 31 can have, for example, a heat-resistant pipe 32 and a heater 33 for heating the pipe 32, as shown in FIG.

[0092] For example, a ceramic pipe can be used as the pipe 32. From the viewpoint of preventing the inclusion of impurities, it is preferable to use a furnace tube made of Al2O3 with high purity. To reduce the cost of the furnace tube, mullite or the like can also be used.

[0093] The length L32 of the pipe 32 of the reaction section 31 is not particularly limited, and is preferably selected so that heating can be performed up to a predetermined temperature in the heat treatment step and sufficient time for the heat treatment step can be secured.

[0094] The length L32 of the pipe 32 in the reaction section 31 is preferably 1 m or more from the viewpoint of heating to a predetermined temperature and ensuring a sufficient time for the heat treatment process. Although there is no particular upper limit to the length of the pipe 32, an excessively long pipe 32 requires a large amount of carrier gas and increases the size of the apparatus, so that the length is preferably 5 m or less.

[0095] In addition, the diameter D32 of the pipe 32 is not particularly limited, but is preferably 1 cm or more from the viewpoint of productivity. The upper limit of the diameter D32 of the pipe 32 is not particularly limited, but it is preferably selected so that the temperature difference between the center and the wall portion is not excessively large, and the diameter D32 of the pipe 32 is preferably, for example, 20 cm or less.

[0096] The diameter D32 of the pipe 32 means the inside diameter of the pipe 32.

[0097] The pipe 32 of the reaction section 31 usually has a temperature gradient along its longitudinal direction. Therefore, for example, the pipe 32 can be configured so that the temperature is low on the reaction section inlet 32A side and increases toward the reaction section outlet 32B.

[0098] It is preferable to set various conditions so that a temperature region with a temperature of 1000° C. or higher can be formed in the vicinity of the reaction part outlet 32B.

[0099] In particular, when the workpiece is heat-treated at a temperature of 1000° C. or higher in the heat treatment step described above, it is preferable to set various conditions so that a temperature region with a temperature of 1000° C. or higher can be formed near the reaction section outlet 32B. In addition, in the heat treatment step, when the heat treatment time at a temperature of 1000° C. or higher is set to 1 second or more, it is preferable to set various conditions so that the time the workpiece passes through the temperature region with a temperature of 1000° C. or higher in the reaction section 31 is 1 second or more. Specifically, for example, it is preferable to measure the temperature distribution in the pipe 32 in advance and adjust the supply rate of the carrier gas, etc.

[0100] However, without being limited to the above form, various conditions may be set so that a temperature gradient is not formed along the longitudinal direction of the piping 32, but the temperature in the longitudinal direction of the piping 32 is made almost uniform, and a temperature region above a predetermined temperature is formed.

[0101] The cooling section 41 may have a cooling vessel 42, a cooling gas introduction pipe 44 for introducing a cooling gas, and a thermocouple 45. The cooling vessel 42 includes a connection part 43, which is connected to the reaction outlet 32B, and can receive the high-temperature gas discharged from the reaction section 31. In addition, it is preferable to make the distance between the reaction outlet 32B and the cooling vessel 42, i.e., the length L43 of the connection part 43 as short as possible, and the length L43 of the connection part 43 is at most 20 cm or less, preferably 10 cm or less, and more preferably 7 cm or less. In order to prevent the carrier gas from leaking from the joint between the reaction outlet 32B and the connection part 43, it is preferable to seal it with a commercially available heat-resistant ceramic adhesive mainly composed of ceramics such as alumina. If the carrier gas leaks near the reaction outlet 32B, in this embodiment, since the carrier gas contains H2, the hydrogen may come into contact with the air and react and burn, and the heat caused by the combustion may escape from the system, which may affect the characteristics of the composite tungsten oxide particles obtained. A cooling gas supply means (not shown) can be connected to the cooling gas introduction pipe 44. Although FIG. 1 shows an example in which one cooling gas introduction pipe 44 is provided, the present invention is not limited to this form. For example, two or more cooling gas introduction pipes 44 may be provided surrounding the periphery of the connection part 43. For example, the cooling gas introduction pipes can be arranged at intervals of 120° around the periphery of the connection part 43 so that cooling gas can be introduced from three directions toward the high-temperature gas heated in the reaction part 31 and introduced from the connection part 43. By providing multiple cooling gas introduction pipes in this way and supplying cooling gas from multiple directions, the product of the heat treatment process that contains the high-temperature gas (discharge gas) can be rapidly and efficiently cooled. For this reason, the generation of different phases can be particularly suppressed.

[0102] For example, the temperature inside the cooling vessel 42 may be measured by the thermocouple 45 described above, and the supply rate, temperature, etc. of the cooling gas may be controlled.

[0103] In the recovery section 51, the composite tungsten oxide particles cooled in the cooling section 41 can be recovered. The configuration of the recovery section 51 is not particularly limited, and can be selected according to the particle size of the composite tungsten oxide particles to be produced. As the recovery section 51, for example, various filters 52 can be used. Alternatively, an electrostatic collector can be used. A valve 61 and a pump 62 can be provided downstream of the recovery section 51. (Reduction treatment device) In the reduction treatment device, the reduction treatment process described above can be carried out.

[0104] The reduction treatment device is not particularly limited as long as it is configured to perform the reduction treatment step described above. For example, the reduction treatment device may include a container for storing the composite tungsten oxide particles obtained by the composite material production device described above, a gas pipe for supplying a mixed gas that creates a reducing atmosphere in the container into the container, and a heat source for heating the container.

[0105] In addition, a mixed gas that creates a reducing atmosphere in the container can be introduced and exhausted, and the composite tungsten oxide particles that are the object to be treated can be placed under a gas flow of the mixed gas. In this case, a supply pipe for the mixed gas and an exhaust pipe can be provided as gas pipes so that such a gas flow can be formed.

[0106] Furthermore, a stirring blade for stirring the composite tungsten oxide particles in the container may also be used in combination.

[0107] FIG. 2 is a diagram showing a schematic configuration example of a reduction treatment device, and shows a cross-sectional view taken along a plane passing through the central axis of a reaction tube 71 of the reduction treatment device .

[0108] The reduction treatment device 70 is a horizontal tubular furnace, and can be used by attaching a gas inlet pipe (not shown) to one port 71A of the reaction tube 71 and a gas exhaust pipe (not shown) to the other port 71B of the tubular furnace. Then, by supplying a mixed gas that forms a reducing atmosphere from the one port 71A side, the inside of the reaction tube 71 can be made into a reducing atmosphere.

[0109] A heater 72 can be provided around the reaction tube 71, and the composite tungsten oxide particles can be placed in a ceramic container 73 such as a boat and placed at a position corresponding to the heater 72 in the reaction tube 71 of the tubular furnace.

[0110] Using this reduction treatment device 70, a reducing atmosphere is created inside a reaction tube 71, and the inside of the reaction tube 71 is heated to a desired temperature by a heater 72, whereby a reduction treatment can be performed on composite tungsten oxide particles 74 placed in a container 73.

[0111] According to the method for producing composite tungsten oxide particles of the present embodiment described above, it is possible to use equipment with low introduction costs, such as an aerosol forming unit and a heater, and the number of steps can be reduced, thereby making it possible to easily produce composite tungsten oxide particles. EXAMPLES

[0112] The present invention will be described below with reference to specific examples, but is not limited to these examples. (1) Evaluation method (1-1) Powder X-ray Diffraction The composite tungsten oxide particles obtained in the following Examples and Comparative Examples were subjected to measurement of powder X-ray diffraction patterns (XRD patterns) using a powder X-ray diffractometer (Bruker, Model: D2 PHASER). The powder X-ray diffraction patterns were measured using CuKα radiation as the radiation source, with a tube voltage of 40 kV and a tube current of 30 mA. (1-2) TEM Image Observation The obtained composite tungsten oxide particles were observed using a transmission electron microscope (TEM, Model: JEM-2011, manufactured by JEOL Ltd.) with an applied voltage of 200 kV. (1-3)Visible Light Transmittance, Solar Transmittance The optical properties of the ink were measured using a spectrophotometer (U-4100 manufactured by Hitachi, Ltd.), and the visible light transmittance and solar radiation transmittance were calculated according to JIS R 3106 (2019). (2) Examples and Comparative Examples (2-1) Example 1, Comparative Examples 1 to 4 [Example 1] In Example 1, particles are synthesized by solid spray pyrolysis. The synthesis method will be described.

[0113] Using the composite material manufacturing apparatus 10 shown in FIG. 1, composite tungsten oxide particles were prepared by the following method. 0.32 WO3 particles were produced and evaluated. The specific conditions are explained below.

[0114] 1 includes an aerosol formation section 11, a transport section 21, a reaction section 31, a cooling section 41, and a collection section 51. The aerosol formation section 11, the transport section 21, the reaction section 31, the cooling section 41, and the collection section 51 are connected by piping.

[0115] The aerosol forming section 11 includes an aerosol generating tube 12, a stirring section 14, and a powder supplying section 16 that supplies mixed raw material powder to the stirring section 14. The aerosol generating tube 12 is tubular and includes an opening 13 that is connected to the stirring section 14. The stirring section 14 includes a rotating brush 15. The brush 15 is disposed at the opening 13 so as to be exposed to the aerosol generating tube 12.

[0116] The powder supply section 16 is composed of a raw material reservoir 17 for storing the mixed raw material powder, and a piston 18 for delivering the mixed raw material powder to the stirring section.

[0117] Furthermore, the aerosol generating tube 12 is connected to a regulator 19 that controls the flow rate of a carrier gas supplied from a carrier gas tank (not shown), and also has the function of supplying a carrier gas to the composite material manufacturing apparatus 10 .

[0118] First, a tungsten oxide powder represented by H2WO4 was prepared as a W source raw material powder.

[0119] In addition, cesium carbonate (Sigma-Aldrich) was prepared as the M element source raw material powder. The cesium carbonate was dissolved in water and used as an aqueous solution.

[0120] The Cs2CO3 aqueous solution and H2WO4 powder were mixed so that the mass ratio of the substances was Cs / W = 0.33, and then dried at 100°C for 12 hours to obtain a mixed raw material powder. When the mixed raw material powder was examined with an SEM, it was found to be a powder with a particle size of 1 μm or more and 2 μm or less.

[0121] (Aerosol formation process) In the aerosol forming section 11, the mixed raw material powder was extruded from the powder supplying section 16 at a supplying rate of 2 g / Hr toward the brush 15 rotating at a high speed of 1000 rpm or more. The mixed raw material powder dispersed in the carrier gas by the brush 15 becomes an aerosol. The aerosol is transported to the reaction section 31 via the transporting section 21. That is, the aerosol formed in the aerosol forming process is supplied to the heat treatment process.

[0122] The flow rate of the carrier gas was set to 10 L / min in order to sufficiently disperse the mixed raw material powder.

[0123] A mixed gas of argon gas and hydrogen gas was used as the carrier gas. In order to suppress the generation of metallic tungsten while introducing oxygen vacancies into the composite tungsten oxide particles to be produced, 0.5% by volume H2 / Ar gas containing 0.5% by volume hydrogen gas was used.

[0124] The reaction section 31 is equipped with a ceramic pipe 32, and a cylindrical tube having a length L32 of 1 m and a diameter D32 of 13 mm is used as the pipe 32. The diameter D32 means the inner diameter of the pipe 32.

[0125] In the reaction section 31, the temperature of the heater 33 was set to 1300°C. Figure 3 shows the temperature profile in the reaction section 31. The position on the horizontal axis indicates the distance from the reaction section outlet 32B, and the temperature profile is measured along the central axis of the pipe 32. By introducing the carrier gas, the high temperature region heated by the furnace body shifted toward the discharge direction, i.e., toward the reaction section outlet. It is presumed that the precipitation and condensation reaction of the composite tungsten oxide particles progressed in the temperature change region from this high temperature region toward the low temperature region located on the reaction section outlet side.

[0126] (cooling process) The cooling section 41 is composed of a glass cooling vessel 42, a cooling gas introduction pipe 44 for introducing a cooling gas, and a thermocouple 45. The cooling vessel 42 is provided with a connection part 43, which is connected to the reaction part outlet 32B, and is configured to introduce the high-temperature gas discharged from the reaction part 31 into the cooling vessel 42. In order to shorten the distance between the reaction part outlet 32B and the cooling vessel 42, the length L43 of the connection part 43 is set to 5 cm, and the joint part between the reaction part outlet 32B and the connection part 43 is sealed with a commercially available alumina heat-resistant adhesive. Three cooling gas introduction pipes are arranged around the connection part 43 at intervals of 120° so that the cooling gas can be introduced from three directions toward the high-temperature gas (discharge gas) heated in the reaction part 31 and introduced from the connection part 43. A mixed gas of nitrogen and oxygen was used as the cooling gas, and the oxygen concentration was 3% by volume. That is, 3% by volume O2 / N2 gas was used as the cooling gas. The cooling gas was supplied from the three cooling gas inlet pipes at a total supply rate of 10 L / min.

[0127] In the recovery section 51, a bag filter was placed as the filter 52, and it was configured so that the composite tungsten oxide particles formed after the cooling step in the cooling section 41 could be recovered.

[0128] Under the above conditions, composite tungsten oxide particles were produced.

[0129] In Example 1, the furnace temperature in the reaction section 31 was set to 1300° C. as described above. That is, in the heat treatment step, the supplied aerosol was heated to 1300° C. and heat-treated at 1300° C. The heat treatment time at a heat treatment temperature of 1000° C. or higher was 0.8 seconds.

[0130] [Comparative Example 1] In Comparative Example 1, 1% by volume H2 / Ar gas containing 1% by volume of hydrogen gas was used as the carrier gas. Synthesis was performed in the same manner as in Example 1 except for the above points. [Comparative Example 2] In Comparative Example 2, 3 volume % H2 / Ar gas containing 3 volume % hydrogen gas was used as the carrier gas. Synthesis was performed in the same manner as in Example 1 except for the above points. [Comparative Example 3] In Comparative Example 3, the furnace temperature in the heat treatment step was 1100° C., and 3 volume % H2 / Ar gas containing 3 volume % hydrogen gas was used as the carrier gas. Other than the above, synthesis was performed in the same manner as in Example 1. [Comparative Example 4] In Comparative Example 4, the furnace temperature in the heat treatment step was 1200° C., and 3 volume % H2 / Ar gas containing 3 volume % hydrogen gas was used as the carrier gas. Other than the above, synthesis was performed in the same manner as in Example 1.

[0131] The composite tungsten oxide particles recovered in recovery section 51 were evaluated.

[0132] 4(A) and 4(B) show the XRD measurement results for Example 1 and Comparative Examples 1 to 4. In all conditions, the main phase contained Cs 0.33 The WO3 phase was confirmed.

[0133] In Example 1 and Comparative Examples 1 to 4, the mixed raw material powder was heated in a dispersed aerosol state, so that Cs was easily obtained even in a firing time of a few seconds. 0.33 It was confirmed that the thermal decomposition reaction proceeded up to the WO3 phase.

[0134] FIG. 4(A) shows the results of XRD measurement of the composite tungsten oxide particles of Comparative Examples 2 to 4, which were synthesized by using 3 volume % H2 / Ar as a carrier gas and varying the furnace temperature from 1100°C to 1300°C. 0.33 It was confirmed that the ratio of the W phase (ICDD 04-0806) peak at 2θ of around 40 deg to the WO3 phase peak increased. In Comparative Example 2, in which the furnace temperature was 1300°C, the effect of reduction was strong and the ratio of the metallic W phase was large.

[0135] On the other hand, when the shape of the composite tungsten oxide particles of Comparative Example 3, in which the furnace temperature was set to 1100°C and the proportion of the W phase was low, was confirmed by TEM, it was possible to confirm that, as shown in Figure 5(A), amorphous particles were wrapped around each other and had a particle diameter of 500 nm or more. In addition, as shown in Figure 5(B), spherical particles of several tens of nm in size were confirmed. In other words, it was confirmed that composite tungsten oxide particles with a relatively large particle diameter were obtained when the furnace temperature was 1100°C.

[0136] Therefore, when the purpose is to obtain finer particles, it is presumed that the temperature in the heat treatment process is preferably higher. Therefore, in order to obtain finer composite tungsten oxide particles, the heat treatment temperature is set to 1300°C, and the hydrogen concentration in the carrier gas is changed in Example 1, Comparative Example 1, and Comparative Example 2. The XRD measurement results of the composite tungsten oxide particles obtained in these examples are shown in Figure 4 (B).

[0137] From the comparison of Example 1, Comparative Example 1, and Comparative Example 2, it was found that the decrease in hydrogen concentration in the carrier gas resulted in the formation of Cs 0.33 It was confirmed that the magnitude of the peak intensity of the metallic W phase relative to the spectral intensity of the WO3 phase was significantly reduced.

[0138] Figures 6(A) to 6(D) show TEM images of the composite tungsten oxide particles obtained in Comparative Example 1, in which 1 vol% H2 / Ar gas was used as the carrier gas. Figure 6(E) shows the results of EDS analysis of the area surrounded by dotted line A and the area surrounded by dotted line B in Figure 6(D). As shown in Figures 6(A) and 6(B), it was confirmed that fine composite tungsten oxide particles with particle diameters of 100 nm or less were obtained compared to Comparative Example 3.

[0139] However, when the composite tungsten oxide particles obtained in Comparative Example 1 were subjected to EDS analysis, it was confirmed that the core with high contrast inside was metallic W, and the shell was a CWO phase. This is because when the mixed raw material powder was sublimated, the WO6 octahedron was decomposed and reduced to W, and only W atoms were nucleated and surrounded by Cs 0.33 This is presumably because the WO3 phase grew into particles as a shell phase.

[0140] In contrast, in Example 1, in which the hydrogen concentration in the carrier gas was reduced to 0.5 volume %, it was confirmed that the W peak disappeared in the XRD measurement results shown in Figure 4 (B). In addition, Figures 7 (A) and 7 (B) show SEM images of the composite tungsten oxide particles obtained in Example 1. As shown in Figures 7 (A) and 7 (B), it was confirmed that fine composite tungsten oxide particles with a particle diameter of 200 nm or less were obtained compared to Comparative Example 3. (2-2) Example 2 and Example 3 [Example 2] In Example 2, the composite tungsten oxide particles obtained in Example 1 were subjected to reduction treatment at 500° C. for 1 hour in a mixed gas atmosphere of 3 volume % H2 / Ar (reduction treatment step). The reduction treatment was performed using the reduction treatment device 70 shown in FIG.

[0141] The reduction treatment device 70 is a horizontal tubular furnace, and the composite tungsten oxide particles synthesized in Example 1 are placed in a container 73, which is a ceramic boat, and the container 73 is disposed at a position where the temperature inside the reaction tube 71 is the highest, that is, the reduction treatment temperature. Then, the reduction treatment was performed while flowing a mixed gas of hydrogen and argon containing 3 volume % of hydrogen gas from one port 71A to the other port 71B. [Example 3] In Example 3, an ink was prepared as a dispersion liquid containing 2 mass% of the composite tungsten oxide particles obtained in Example 2 and the remainder being methyl isobutyl ketone as a dispersion medium. The ink was prepared by dispersing and pulverizing the composite tungsten oxide particles and the dispersion medium with 0.3 mmφZrO2 beads for 120 minutes using a paint shaker. In addition, the solvent was dried and removed from the ink according to Example 3 to prepare the composite tungsten oxide particles according to Example 3, which were then evaluated.

[0142] Figures 8(A) to 8(I) show TEM images of the composite tungsten oxide powders of Examples 1 to 3. Figures 8(A) to 8(C) are TEM images of the composite tungsten oxide particles of Example 1, Figures 8(D) to 8(F) are TEM images of the composite tungsten oxide particles of Example 2, and Figures 8(G) to 8(I) are TEM images of the composite tungsten oxide particles of Example 3.

[0143] From FIG. 8(A) to FIG. 8(C), it was confirmed that the composite tungsten oxide particles according to Example 1 are composed of angular particles with a particle diameter of up to 200 nm and hexagonal columnar nanoparticles of 80 nm or less. Hexagonal and square type particles were observed as particles of 80 nm or less shown in FIG. 8(B). FIG. 8(C) shows a high-resolution image of a hexagonal columnar particle. According to the high-resolution image shown in FIG. 8(C), lattice fringes of 3.18 Å belonging to the 200th plane of the CWO phase (composite tungsten oxide phase) were confirmed. However, it was confirmed that the lattice fringes were unclear in the circled areas in FIG. 8(C) and that the lattice fringes were interrupted in the middle, as shown by the arrows. It is suggested that these are areas where W atoms or Cs atoms have been removed.

[0144] From the TEM images shown in Figures 8(D) and 8(E), it was observed that the composite tungsten oxide particles according to Example 2 started to slowly aggregate, and the particles grew to a particle size in the range of about 50 nm to 200 nm. Figures 9(A) and 9(B) show SEM images of the composite tungsten oxide particles according to Example 2. From Figures 9(A) and 9(B), the same tendency in particle size was confirmed.

[0145] Figure 8(F) shows the results of high magnification observation of the internal structure of the composite tungsten oxide particle according to Example 2. From Figure 8(F), clear lattice fringes of 3.64 Å belonging to the 002 plane were confirmed.

[0146] 8(G) and 8(H), rounded particles of about 50 nm to 100 nm and scaly particles of 50 nm or less were confirmed for the composite tungsten oxide particles according to Example 3. From FIG. 8(I), which shows the scaly particles observed at a higher magnification, particles of several nm to several tens of nm in size were confirmed. (2-3) Comparative Examples 5 to 7 [Comparative Example 5] Comparative Example 5 is the synthesis of particles by liquid spray pyrolysis.

[0147] In Comparative Example 5, composite tungsten oxide particles were produced using a composite material production apparatus 80 shown in FIG. 10, and evaluation was carried out.

[0148] A composite material production apparatus 80 shown in Fig. 10 has the same configuration as the composite material production apparatus 10 shown in Fig. 1 except that the aerosol formation unit 11 is replaced with a droplet formation unit 81. For this reason, the same members are given the same numbers and their explanations are omitted.

[0149] 1 aerosolizes a mixed raw material powder, whereas the composite material manufacturing apparatus 80 forms droplets from a solution containing a tungsten source and a solution containing an M element source. The composite material manufacturing apparatus 80 will be described.

[0150] The composite material manufacturing apparatus 80 shown in Figure 10 has a droplet formation section 81, a transport section 21, a reaction section 31, a cooling section 41, and a recovery section 51, and the droplet formation section 81, the transport section 21, the reaction section 31, the cooling section 41, and the recovery section 51 are connected by piping.

[0151] The droplet formation unit 81 was connected to a first storage unit 83 for storing a solution containing a tungsten source, which is a raw material solution, a second storage unit 84 for storing a solution containing an M element source, and a carrier gas tank 87 for storing a carrier gas for transporting the droplets formed in the droplet formation unit 81 to the transport unit 21, etc.

[0152] An ultrasonic spray device is used in droplet formation section 81 of composite material production equipment 80, and ultrasonic irradiation section 82 irradiates ultrasonic waves onto the solution in the equipment to form droplets.

[0153] In Comparative Example 5, the solution containing the tungsten source was (NH4) 10 (W 12 O 41 A 10 mmol / L aqueous solution of ammonium paratungstate was prepared using ammonium paratungstate (ATP) (Kanto Chemical, purity: 88-90%) represented by 1H2O·5H2O and ultrapure water. The aqueous solution of ammonium paratungstate was placed in a first storage unit 83, and was configured to be continuously supplied by a pump 85 to a droplet forming unit 81 connected to the first storage unit 83 by a pipe.

[0154] In addition, a 19.2 mmol / L aqueous cesium carbonate solution was prepared using cesium carbonate (Sigma-Aldrich) and ultrapure water as a solution containing an M element source. The aqueous cesium carbonate solution was placed in a second storage unit 84, and was configured to be continuously supplied by a pump 86 to a droplet forming unit 81 connected to the second storage unit 84 by a pipe.

[0155] As described above, each solution is supplied at a constant flow rate from first storage section 83 and second storage section 84 to droplet formation section 81 via piping by pumps 85, 86, and both solutions are mixed at the top of droplet formation section 81 to form a raw material mixed solution. The supply speed and the concentration of each solution were adjusted so that the ratio of the number of moles of cesium to 1 mole of tungsten in the raw material mixed solution formed in droplet formation section 81 was 0.32. The raw material mixed solution contains 5 mmol / L ammonium paratungstate and 9.6 mmol / L cesium carbonate.

[0156] As described above, the carrier gas tank 87 was connected to the droplet forming unit 81, and the carrier gas tank 87 was filled with a mixed gas of 0.5% H2 / Ar as a carrier gas. During the production of the composite tungsten oxide particles, the above carrier gas was supplied to the droplet forming unit 81 at a flow rate of 10 L / min.

[0157] The reaction section 31, the cooling section 41, and the recovery section 51 are configured in the same manner as in the first embodiment, and therefore a description thereof will be omitted here.

[0158] The droplets generated in the droplet forming section 81 by the carrier gas were supplied to the reaction section 31, and the heat treatment step was carried out. The flow rate of the carrier gas was set to 10 L / min.

[0159] The furnace temperature of the reaction section 31 was set to 1300°C, and the inside of the pipe 32 of the reaction section 31 had the temperature distribution shown in Fig. 3. The droplets supplied to the reaction section 31 were heated to 1300°C and heat-treated at a heat treatment temperature of 1000°C or higher for 0.4 seconds (heat treatment step).

[0160] The product obtained after the heat treatment process was supplied to the cooling section 41, cooled, and then recovered in the recovery section 51. The cooling gas used was 3% by volume O2 / N2 gas, and the cooling gas was supplied from three cooling gas inlet pipes at a total supply rate of 10 L / min. [Comparative Example 6] In Comparative Example 6, the composite tungsten oxide particles obtained in Comparative Example 5 were subjected to reduction treatment at 600° C. for 1 hour in a mixed gas atmosphere of 3 volume % H2 / Ar (reduction treatment step). The reduction treatment apparatus 70 shown in FIG. 2 was used for the reduction treatment. [Comparative Example 7] In Comparative Example 7, an ink was prepared as a dispersion liquid by using 2 mass% of the composite tungsten oxide particles obtained in Comparative Example 6 and the remainder as a dispersion medium, methyl isobutyl ketone. The ink was prepared by dispersing and pulverizing the composite tungsten oxide particles and the dispersion medium with 0.3 mmφZrO2 beads for 120 minutes using a paint shaker.

[0161] Fig. 11A shows the transmission profiles of the ink according to Example 3 and the ink according to Comparative Example 7. According to the transmission profiles shown in Fig. 11A, the decrease in transmittance in the wavelength range of 700 nm or more and 900 nm or less is steeper in Example 3 than in Comparative Example 7.

[0162] This is because, as described below, the Cs obtained in Example 1, which is the basis of Example 3, 0.32 The WO3 particles were obtained in Comparative Example 5, which is the basis for Comparative Example 7. 0.32 It is assumed that the effect was slightly larger than that of WO3 particles.

[0163] 12(A) to 12(D) show TEM images of the composite tungsten oxide particles obtained in Comparative Example 5. When comparing the composite tungsten oxide particles of Comparative Example 5 shown in FIG. 12(A) with the composite tungsten oxide particles of Example 1 shown in FIG. 8(B), which are observed at the same magnification, it can be confirmed that the composite tungsten oxide particles of Comparative Example 5 are finer than those of Example 1. In other words, it was confirmed that the composite tungsten oxide particles synthesized by the liquid spray pyrolysis method are finer than the particles synthesized by the solid spray pyrolysis method.

[0164] For this reason, it is predicted that the particles synthesized by the solid spray pyrolysis method contain slightly larger particles than the composite tungsten oxide particles synthesized by the liquid spray pyrolysis method, and that the effect of Rayleigh scattering is greater. TEM images of the composite tungsten oxide particles of Comparative Example 6 are also shown in Figures 13(A) to 13(D).

[0165] Here, in FIG. 11(A), it was confirmed that the absorption characteristics in the infrared region with wavelengths of 780 nm or more of the composite tungsten oxide particles obtained in Example 3 synthesized by solid spray pyrolysis were approximately 10% better than the composite tungsten oxide particles obtained in Comparative Example 7 synthesized by liquid spray pyrolysis.

[0166] FIG. 11B shows the results of determining the relationship between visible light transmittance (visible light transmittance) and solar transmittance (solar transmittance) while diluting the inks of Example 3 and Comparative Example 7 with the solvent methyl isobutyl ketone.

[0167] In FIG. 11(B), when the inks of Example 3 and Comparative Example 7 are compared at the same Visible Light Transmittance, it was confirmed that the ink of Example 3 has a lower Solar Transmittance. That is, it was confirmed that the ink of Example 3, which is an ink of composite tungsten oxide particles synthesized by the solid spray pyrolysis method, has superior infrared absorption properties compared to the ink of Comparative Example 7, which is an ink of composite tungsten oxide particles synthesized by the liquid spray pyrolysis method. It is presumed that the reason why the ink of composite tungsten oxide particles synthesized by the solid spray pyrolysis method has superior infrared absorption properties to the ink of composite tungsten oxide particles synthesized by the liquid spray pyrolysis method is that the influence of the solvent during aerosol formation was eliminated, weakening the influence on the crystal structure.

[0168] 14(A) to 14(D) show STEM-HAADF images observed with 001 incidence by STEM of the composite tungsten oxide particles of Example 1 and the composite tungsten oxide particles obtained in Example 2 in which the composite tungsten oxide particles obtained in Example 1 were further subjected to a reduction treatment step. Figures 14(A) and 14(B) are STEM-HAADF images of the composite tungsten oxide particles of Example 1, in which low-contrast regions were confirmed throughout the 100, 010, and 110 planes, and it was confirmed that the structure was deficient in both Cs and W.

[0169] Figures 14(C) and 14(D) show STEM-HAADF images of the composite tungsten oxide particles of Example 2, which were obtained by performing a reduction treatment on the composite tungsten oxide particles of Example 1. Figures 14(C) and 14(D) confirm that the low-contrast regions confirmed in Figures 14(A) and 14(B) have been reduced. For this reason, it is presumed that the reduction treatment caused the defect structure introduced into the crystal to almost disappear, and rearrangement in the crystal structure proceeded.

[0170] Figure 15(A) shows the XRD profiles of Example 1, Example 2, Comparative Example 5, and Comparative Example 6. Furthermore, Figure 15(B) shows a detailed view of the range of 2θ from 20° to 30° in the XRD profile of Figure 15(A). From Figure 15(B), it was confirmed that in Example 1, the peak near 23° in the XRD profile was separated into two.

[0171] The XRD profiles of the composite tungsten oxide particles obtained in Example 1, Example 2, Comparative Example 5, and Comparative Example 6 were measured using Cs 0.32 WO3 phase, (Cs2O) 0.44 The ratio of each phase was calculated by fitting to the W2O6 phase and the W phase. Figure 16 shows the mass ratio of the crystalline phase in the powders synthesized in Example 1, Example 2, Comparative Example 5, and Comparative Example 6. Note that Cs 0.32 For the WO3 phase, the coordinates of Cs and W, and the lattice constant were set as variable parameters, while the temperature factor and the occupancy rate of each site were fixed. (Cs2O) 0.44For the W2O6 phase and the W phase, fitting was performed with the coordinates and lattice constants of each element fixed.

[0172] The composite tungsten oxide particles obtained in Example 1 were Cs 0.32 WO3 crystal phase (Cs 0.32 The proportion of WO3-A phase) was 30.48 mass%, and Cs 0.32 WO3 crystal phase (Cs 0.32 The proportion of WO3-B phase) is 55.18 mass%, (Cs2O) 0.44 The W2O6 phase was composed of 13.45 mass% and the W phase was composed of 0.89 mass%.

[0173] In the composite tungsten oxide particles of Example 2, which were obtained by subjecting the composite tungsten oxide particles of Example 1 to a reduction treatment, the Cs 0.32 The WO3 crystal phase is almost entirely composed of Cs 0.32 WO3 crystal phase (Cs 0.32 It was confirmed that the WO3-C phase was a single phase. 0.32 WO3 crystal phase (Cs 0.32 The proportion of WO3-C phase reached 88.42 mass%.

[0174] The lattice constant calculated by Rietveld analysis is Cs in Example 1. 0.32 The a-axis of the WO3-A phase is 7.42014 Å, the c-axis is 7.59587 Å, and the Cs 0.32 The lattice constants of the WO3-B phase were 7.40041 Å for the a-axis and 7.68855 Å for the c-axis. 0.32 The lattice constants of the WO3-C phase were 7.40382 Å for the a-axis and 7.61184 Å for the c-axis.

[0175] From the TEM image shown in Fig. 8(A), it was confirmed that the composite tungsten oxide particles obtained in Example 1, as described above, consist of two types of particles: particles of about several hundred nm and particles of 80 nm or less, for example, about 10 nm. For this reason, it is considered that the two peaks appear around 2θ of 23 degrees in the XRD profile of Example 1 in Fig. 15(A) as described above.

[0176] On the other hand, in Comparative Example 5, Cs 0.32 WO3 phase is 92.07 mass%, (Cs2O) 0.44 The W2O6 phase was 6.76 mass% and the W phase was 1.175 mass%. 0.32 WO3 phase is 93.58 mass%, (Cs2O) 0.44 The W2O6 phase was 2.67% by mass, and the W phase was 3.76% by mass, resulting in an increase in the W phase. 0.32 The lattice constants of the WO3 phase are 7.40955 Å for the a-axis and 7.69694 Å for the c-axis. 0.32 The lattice constants of the WO3 phase were 7.41729 Å for the a-axis and 7.61101 Å for the c-axis.

[0177] FIG. 17 shows the relationship between the lattice constants of Example 1, Example 2, Comparative Example 5, and Comparative Example 6 and the lattice constant of the comparative sample ICDD04-009-6455. In FIG. 17, the comparative sample is shown as Reference. 0.32 The WO3-A phase was located to the lower right compared to the comparative sample. This crystal is located to the lower right compared to the comparative sample, which indicates that the WO3-A phase is located to the lower right compared to the comparative sample. x It is interpreted as a WO3 phase.

[0178] On the other hand, Cs in Example 1 0.32 The WO3-B phase was located at the upper left compared to the control sample. 0.32 The WO3-B phase is the Cs of the composite tungsten oxide particles obtained in Comparative Example 5. 0.32 It was confirmed that the same tendency as the WO3 phase was observed. 0.32 The WO3-B phase is composed of Cs precipitated by sublimation during the heat treatment process and nano-sized Cs lacking W. 0.32 It is assumed to be WO3 particles.

[0179] The Cs content of the composite tungsten oxide particles synthesized by the liquid spray pyrolysis method in Comparative Examples 5 and 6 0.32 It was confirmed that the lattice constant of the WO3 phase was extended due to the residual influence of the solvent even after reduction treatment.

[0180] In contrast, the composite tungsten oxide particles of Examples 1 and 2, which were synthesized by the solid spray pyrolysis method, were not affected by the solvent in which the raw materials were dissolved in the aerosol during the heat treatment process, and therefore it was confirmed that the crystal structure was different from that of the above Comparative Examples 5 and 6. Cs 0.32 The WO3-A phase particles precipitate as nanoparticles by sublimation, and during reduction treatment, (Cs2O) 0.44 The Cs vacancies and W vacancies are almost completely reduced as Cs reacts with the W2O6 phase and is filled with Cs. 0.32 It is believed that the WO3 crystals were transformed into Cs 0.32 It is presumed that the WO3-B phase particles underwent a Cs doping reaction and the introduction of oxygen vacancies similar to those in Non-Patent Document 5 during the reduction treatment.

[0181] The mechanism of formation of particles synthesized by the solid spray pyrolysis method in Example 1 and the like is shown in FIG. From the XRD profile shown in Fig. 15, the synthesized Cs 0.32 WO3 particles are thought to precipitate via two routes.

[0182] The first route is based on the sublimation phenomenon, and consists of nanoparticles of Cs, in which both Cs and W are desorbed. 0.32 This route goes through WO3 particles. It is believed that the mixed raw material powder, which is the aerosolized precursor, becomes amorphous and sublimes when it is put into a furnace heated to a high temperature. It is presumed that the vaporized W precipitates while maintaining the state of WO6 clusters. As shown in the TEM images and EDS results of Comparative Example 1 in Figures 6(A) to 6(E), when the effect of reduction is strengthened at the same temperature, W precipitates as a core, but in Example 1, where the hydrogen concentration is 0.5 volume %, no W is confirmed in the core as shown in Figure 4(B).

[0183] During sublimation, Cs atoms and WO6 clusters are mixed. When Cs / W<0.33, the hexagonal structure is deficient in Cs and W, and when Cs / W>0.33, the pyrochlore (Cs2O) 0.44 It is believed that nanoparticles with a W2O6 phase precipitate during the cooling process.

[0184] On the other hand, the second route is thought to be via the insertion of Cs into a W compound, the same as in the conventional solid-phase method. During the heat treatment process, the W source powder contained in the mixed raw material powder becomes amorphous, and its surface sublimes, but the inside does not sublimate and only mixes with Cs atoms, resulting in Cs4W 11 O 35 Phase etc. via Cs 0.32 It is believed that the WO3 phase precipitates. In Example 1, it is believed that about 55 mass % of the particles in the mixed raw material powder were converted into nanoparticles based on the sublimation phenomenon.

[0185] However, for the composite tungsten oxide particles synthesized by either route, the Cs / W composition ratio approaches the feed ratio of 0.33 by performing reduction or pulverization as necessary, and the Cs / W composition ratio shown as a comparative sample in Figure 17 0.32 It was confirmed that it has the same crystal structure as WO3.

[0186] FIG. 19 shows the mass flow rates for particle synthesis by the liquid spray pyrolysis method of Comparative Example 5 and particle synthesis by the solid spray pyrolysis method of Example 1. The mass flow rate for particle synthesis by the liquid spray pyrolysis method is approximately 3 g / Hr at maximum output, but particle synthesis by the solid spray pyrolysis method easily showed 8 g / Hr, more than double the productivity. With the solid spray pyrolysis method, the supply rate can be easily increased to 400 g / Hr simply by further increasing the cutting speed. Therefore, it was confirmed that the solid spray pyrolysis method is a manufacturing method with particularly excellent productivity.

[0187] From the results of this example, it was confirmed that according to the method for producing composite tungsten oxide particles of the present embodiment, composite tungsten oxide particles can be produced using equipment with low introduction costs, such as droplet forming means and heaters, and the number of steps can also be reduced.

Claims

1. General formula M ｘ W ｙ O ｚ (wherein M element is Cs, W is tungsten, O is oxygen, 0.001≦x / y≦1, 2.2≦z / y≦3.0), an aerosol forming step of forming an aerosol containing a mixed raw material powder of a W source raw material powder serving as a tungsten source and an M element source raw material powder serving as an M element source, which are objects to be processed; a heat treatment step of carrying the aerosol by a carrier gas at a temperature of 1000° C. or more and 1350° C. or less; A cooling step of supplying a cooling gas to the product obtained in the heat treatment step to cool it, The carrier gas is an inert gas and 0.1% to 0.9% by volume of H ２ A method for producing composite tungsten oxide particles, the method comprising the steps of:

2. The carrier gas is Ar and H ２ 2. The method for producing composite tungsten oxide particles according to claim 1, wherein the mixed gas is

3. 3. The method for producing composite tungsten oxide particles according to claim 1 or 2, wherein in the cooling step, the cooling gas is supplied to the product from two or more directions.

4. The cooling gas is N ２ and O ２ The method for producing composite tungsten oxide particles according to any one of claims 1 to 3, wherein the mixed gas is a gas having an oxygen concentration of 1 volume % or more and 5 volume % or less.

5. 5. The method for producing composite tungsten oxide particles according to claim 1, wherein the composite tungsten oxide particles are formed via two-phase crystals: a first crystal structure having an a-axis lattice constant of 7.400 Å or more and 7.405 Å or less and a c-axis lattice constant of 7.680 Å or more and 7.700 Å or less; and a second crystal structure having an a-axis lattice constant of 7.415 Å or more and 7.420 Å or less and a c-axis lattice constant of 7.600 Å or more and 7.620 Å or less.

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