JP7624172B2 - Method for producing composite tungsten oxide particles - Google Patents

Description

The present invention relates to a method for producing composite tungsten oxide particles.

A variety of near-infrared shielding technologies have been proposed to date that have good visible light transmittance and maintain transparency while reducing solar radiation transmittance. Among these, near-infrared shielding technology that uses inorganic conductive fine particles has the advantages of superior near-infrared shielding properties compared to other technologies, low cost, radio wave transmittance, and high weather resistance.

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), and a technique relating to a manufacturing method for the infrared-shielding microparticles are disclosed. Patent Document 1 also discloses an example of producing an infrared shielding film that is a thin-film dispersion of fine particles of an infrared shielding material.

According to Patent Document 1, it is possible to produce an infrared shielding material microparticle dispersion that has 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. For this reason, the application of the infrared shielding particle dispersion disclosed in Patent Document 1 to various applications such as window glass is being considered.

Various studies are being conducted on methods for producing composite tungsten oxide particles that are useful as near-infrared shielding materials.

For example, the inventor of Patent Document 1 has proposed a method for synthesizing _{Cs0.32WO3 nanoparticles} by a solid phase method in Non-Patent Document 1. 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.

Non-Patent Document 2 discloses a method for synthesizing Cs _x WO ₃ by hydrothermal synthesis. However, the hydrothermal synthesis requires a synthesis time of several tens of hours or more. In addition, the hydrothermal synthesis has a problem of having a large number of steps, such as a post-treatment step.

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 costs.

Patent Document 2 discloses a process for preparing potassium-cesium-tungsten bronze solid solution particles represented by the chemical formula _KxCsyWOz , where x+y≦1 and 2≦z≦3, which includes mixing a suitable tungsten source with a _potassium salt _and a cesium salt to form a powder mixture, and exposing the powder mixture to a plasma torch under a reducing atmosphere, preferably the reducing atmosphere being supplied by a sheath gas consisting of a hydrogen/rare gas mixture.

However, the method disclosed in Patent Document 2 also requires the use of plasma, which increases the cost of introducing a plasma device. Patent Document 2 also discloses that the manufacturing method disclosed in Patent Document 2 results in the inclusion of metallic tungsten as an impurity.

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 makes it easy to increase productivity and eliminates the effects of the solvent contained in the raw materials, simultaneously resolving 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 "need for 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, leaving the problem that "by-products contained in the solvent and raw materials are mixed into the crystals, resulting in poor infrared absorption characteristics."

International Publication No. 2005/037932 Special Publication No. 2012-532822

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 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. 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. 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. 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.

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

However, the previously disclosed methods for producing composite tungsten oxide particles have problems such as the need for special, expensive equipment and the need for many steps, as described above.

In view of the problems with the conventional technology described above, one aspect of the present invention aims 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.

One aspect of the present invention is a method for producing composite tungsten _{oxide particles} represented by the general formula _MxWyOz (wherein M 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 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 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 .

One aspect of the present invention provides a method for producing composite tungsten oxide particles that can use equipment with low installation costs and has a small number of steps.

FIG. 1 is an explanatory diagram of a composite material production apparatus that can be suitably used when producing composite tungsten oxide particles according to one embodiment of the present disclosure. FIG. 2 is an explanatory diagram of the reduction treatment device. FIG. 3 shows a temperature profile in the pipe of the reaction section in Example 1. FIG. 4 shows XRD profiles of the composite tungsten oxide particles obtained in Example 1 and Comparative Examples 1 to 4. FIG. 5 is a TEM image of the composite tungsten oxide particles obtained in Comparative Example 3. FIG. 6 is a TEM image of the composite tungsten oxide particles obtained in Comparative Example 1. FIG. 7 is an SEM image of the composite tungsten oxide particles obtained in Example 1. FIG. 8 is a TEM image of the composite tungsten oxide particles obtained in Examples 1 to 3. FIG. 9 is an SEM image of the composite tungsten oxide particles obtained in Example 2. FIG. 10 is an explanatory diagram of a composite material production apparatus used for producing the composite tungsten oxide particles of Comparative Example 5. FIG. 11 shows the transmission profiles of the inks obtained in Example 3 and Comparative Example 7. FIG. 12 is a TEM image of the composite tungsten oxide particles obtained in Comparative Example 5. FIG. 13 is an SEM image of the composite tungsten oxide particles obtained in Comparative Example 6. FIG. 14 is a STEM-HAADF image of the composite tungsten oxide particles obtained in Examples 1 and 2. FIG. 15 shows XRD profiles of the composite tungsten oxide particles obtained in Examples 1 and 2 and Comparative Examples 5 and 6. 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. 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. FIG. 18 is an explanatory diagram of the mechanism of the production process of composite tungsten oxide particles. FIG. 19 is an explanatory diagram of mass flow rates in the synthesis of particles by the liquid spray pyrolysis method of Comparative Example 5 and the synthesis of particles by the solid spray pyrolysis method of Example 1.

Specific examples of a 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 intended to include all modifications within the scope of the claims and meaning equivalent to the claims.

Below, an example of the method for producing composite tungsten oxide particles according to this embodiment is described.

The method for producing _composite tungsten oxide particles of the present embodiment relates to a method for producing composite tungsten oxide particles represented by the general formula _{MxWyOz ,} and can include the following aerosol formation step, heat treatment step, and cooling step.

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.

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

The heat treatment process can be performed at 1000°C or higher and 1350°C or lower while transporting the aerosol formed in the aerosol formation process with a carrier gas.

In the cooling process, the product obtained in the heat treatment process can be cooled by supplying a cooling gas.

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 H ₂ .
(1) Regarding Composite Tungsten Oxide Particles First, the composite tungsten oxide particles produced by the composite tungsten oxide particle production method of the present embodiment will be described.
(About the composition)
The composite tungsten oxide contained in the composite tungsten oxide particles is expressed by the general formula _MxWyOz as described above. Since the M element, W, O, and x, _y , and _z in the formula have already been described, description thereof will be omitted here.

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 this embodiment is not particularly limited, and can have one or more types of crystal structures selected from tetragonal, cubic, and hexagonal.

However, when the composite tungsten oxide has a hexagonal crystal structure, this is preferable because the composite tungsten oxide particles have particularly improved light transmittance in the visible light region and light absorption in the near-infrared region. For this reason, it is preferable that the composite tungsten oxide particles contain composite tungsten oxide with a hexagonal crystal structure.

The use of one or more elements selected from Cs, Rb, K, Tl, Ba, and In as the M element makes it 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.

Here, we explain how the M element is arranged when the composite tungsten oxide has a hexagonal crystal structure.

A hexagonal void (tunnel) made of O atoms is formed by assembling six octahedrons each consisting of W and six O atoms, i.e., an octahedron with O atoms at the vertices and a W atom in the center. Then, M element is placed in the void to form one unit, and a large number of these units are assembled to form a hexagonal crystal structure. When a 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 placed in all of the hexagonal voids.

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

In addition, by adding even a very small amount of M element, free electrons are generated in the composite tungsten oxide, and the desired infrared absorption effect can be obtained. For this reason, it is preferable that x/y satisfies 0.001≦x/y≦1.

In addition, the composite tungsten oxide has a composition in which M element is added to tungsten trioxide (WO ₃ ). Since tungsten trioxide does not contain effective free electrons, the ratio of oxygen to 1 mole of tungsten must be less than 3 to achieve the infrared absorption effect. However, in the composite tungsten oxide, free electrons are generated by adding 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 WO ₂ may cause absorption or scattering of light in the visible light region, and may reduce the absorption of light in the near infrared region. Therefore, from the viewpoint of suppressing the generation of WO ₂ , it is preferable that the ratio of oxygen to 1 mole of tungsten is greater than 2.

Therefore, it is preferable to satisfy 2.2≦z/y≦3.0 as described above.
(Regarding 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.

For example, when used in applications where maintaining transparency is required, it is preferable for the particle diameter to be 800 nm or less. This is because particles with a diameter of 800 nm or less do not completely block light due to scattering, and can maintain high visibility in the visible light range while at the same time efficiently maintaining transparency. When transparency in the visible light range is particularly important, it is preferable to further consider scattering by the particles.

When emphasis is placed on reducing scattering by such particles, the particle size is preferably 200 nm or less, and more preferably 100 nm or less.

This is because a small particle size reduces the scattering of light in the visible light region with wavelengths of 400 nm to 780 nm due to geometric scattering or Mie scattering, which makes the infrared shielding film resemble cloudy glass and prevents clear transparency from being obtained. When the particle size is 200 nm 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 as the particle size decreases and transparency is improved. Furthermore, a particle size of 100 nm or less is preferable because the scattered light is very little. From the viewpoint of avoiding light scattering, a small particle size is preferable.

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 depending on the application. For example, when it is required to maintain high visibility in the visible light region as described above, the particle size is preferably 800 nm or less, more preferably 200 nm or less, and even more preferably 100 nm 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, 1 nm or more.

The particle diameter of the composite tungsten oxide particles obtained by the method for producing composite tungsten oxide particles of this 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.

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 process described below, or the heat treatment temperature in the heat treatment process.

Since infrared-shielding materials containing composite tungsten oxide particles obtained by the method for producing composite tungsten oxide particles of the present embodiment largely absorb light in the near-infrared region, particularly light with a wavelength near 1000 nm, many of the transmitted color tones are bluish to greenish.
(2) Method for Producing Composite Tungsten Oxide Particles Next, a method for producing the composite tungsten oxide particles of the present embodiment will be specifically described.

The method for producing composite tungsten oxide particles in this embodiment is a powder supply type pyrolysis method (solid spray pyrolysis method) in which the mixed raw material powder is turned 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 the composite tungsten oxide particles are obtained while controlling the cooling process, which affects the final particle size, crystal phase, condensation, and precipitation process, with a cooling gas.

Therefore, the method for producing composite tungsten oxide particles of this embodiment can have 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, a heat treatment step of supplying the aerosol with a carrier gas and heat treating it at 1000°C or more and 1350°C or less can be included.Furthermore, the composite tungsten oxide particles obtained through the heat treatment step can be cooled with a cooling gas and collected.
(Aerosol forming process)
In the aerosol forming step, the aerosol is formed from a mixed raw material powder obtained by mixing 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 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.

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 may be composed of a stirring section such as a rotating brush or stirring blade, and a powder supply section including a piston or 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 into the carrier gas. The stirring section is capable of selecting 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.

In particular, since it is possible to stably form an aerosol with little particle aggregation, a rotating brush can be suitably used for 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 for the powder supply section.

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.

The tungsten source is not particularly limited, and a tungsten salt or the like can be used. For example, H ₂ WO ₄ or ammonium paratungstate can be preferably used.

In H ₂ WO ₄ , 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 later. Therefore, by using H ₂ WO ₄ 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 can be preferably used.

As the M element source raw material powder, for example, a powder of a salt containing the M element can be used. 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.

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 suitable.

The ratio of M element to 1 mole of tungsten in the resulting 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 W source raw material powder and the amount of M element source raw material powder.

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

The particle size of the mixed raw material powder dispersed in the aerosol formation 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 formation 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 size of the composite tungsten oxide particles described above.

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 more and 1350° C. or less.

The mixed raw material powder containing the W source raw material powder and the M element source raw material powder, which is the material to be treated, is thought to evaporate during the process of heating to 1000°C or higher and 1350°C or lower, and to be completely decomposed to the atomic level. It is thought that the mixed raw material powder then forms clusters, which undergo a condensation process as composite tungsten oxide to form composite tungsten oxide particles. The heat treatment temperature is more preferably 1200°C or higher and 1350°C or lower, and even more preferably 1250°C or higher and 1350°C or lower.

It is believed 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 by the reaction of tungsten with the M element at higher temperatures.

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 process. It is believed 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 process, the aerosol, which is the object to be treated, can be heat treated at 500°C or higher.

According to the studies of the inventors of the present invention, the heat treatment temperature also affects the particle size of the resulting composite tungsten oxide particles. Furthermore, according to further studies of the inventors of the present invention, there is a tendency for the particle size of the resulting composite tungsten oxide particles to become smaller as the heat treatment temperature increases.

This is presumably because, when the heat treatment temperature increases, thermal energy is used to sublimate the composite tungsten oxide particles that are produced, causing the particles to burst as a result of sublimation, resulting in particles with fine particle sizes.

For this reason, in order to obtain composite tungsten oxide particles, which are particularly fine nanoparticles, it is preferable that the heat treatment temperature be 1000°C or higher. In other words, if the aim is to obtain particularly fine nanoparticles, it is preferable to heat treat the workpiece at 1000°C or higher in the heat treatment process.

The upper limit of the temperature during the heat treatment process is 1350°C or less. If the heat treatment temperature exceeds 1350°C, heterogeneous phases such as metallic W may occur.

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, the time of the heat treatment process can be adjusted, for example, by controlling the flow rate of the carrier gas.

The time required for the heat treatment process is not particularly limited and can be selected as desired.

As mentioned above, the mixed powder material containing the W source raw material powder and the M element source raw material powder, which is the material to be treated, evaporates during the process of being heated to the heat treatment temperature and is completely decomposed to the atomic level. After that, clusters are formed, and a composite tungsten oxide is formed through a condensation process as the composite tungsten oxide.

The carrier gas used in the heat treatment process is a mixed gas containing an inert gas and 0.1 volume % or more and 0.9 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 .

By containing 0.1% by volume or more and 0.9% by volume or less of H ₂ in the carrier gas, composite tungsten oxide particles with few heterophases can be obtained even when heat-treated at the heat treatment temperature described above. If the content of H ₂ 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-treated at the heat treatment temperature described above, by setting the content of H ₂ 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 H ₂ in the carrier gas to 0.1% by volume or more as described above.

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, 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 by cooling 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.

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 cooling gas at room temperature is supplied. Note that cooling gas at room temperature means a cooling gas that is in equilibrium with the surrounding temperature without heating or cooling, and for example, a gas between 5°C and 35°C can be used. In particular, in the cooling step, it is preferable to supply cooling gas to the product obtained in the heat treatment step from multiple directions, i.e., from two or more directions. This is because supplying cooling gas to the product obtained in the heat treatment step from two or more directions allows for particularly rapid cooling and suppresses the generation of heterogeneous phases.

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.

By including _O2 in the cooling gas, the reduction reaction that has progressed in the heat treatment process can be actively stopped, and therefore the generation of heterogeneous phases can be particularly suppressed.
(Reduction treatment step)
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.

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

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

In the reduction treatment process, the composite tungsten oxide particles obtained after the cooling process can be reduced by heating and lowering the temperature in a reducing atmosphere containing a reducing gas, i.e., by performing heat treatment.

During the reduction process, the composite tungsten oxide particles may be stirred or left to stand, and the handling of the composite tungsten oxide particles during the reduction process may be selected as appropriate, but it is preferable to select handling conditions that prevent the precipitation of metallic tungsten.

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. After the temperature is raised from room temperature to the reduction treatment temperature, it can be lowered back 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.

By increasing the temperature of the reduction treatment to above 400°C, the reduction treatment of the composite tungsten oxide particles can be advanced, and the infrared absorption properties can be more reliably exhibited. In addition, by setting the temperature below 700°C, the composite tungsten oxide particles can be prevented from being reduced to metallic tungsten.

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.

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 in the range of 2 volume % to 10 volume %. If reduction is performed in an atmosphere of only reducing gas, the reduction reaction may proceed excessively and metallic tungsten may be precipitated, so care must be taken.

The reduction process time is preferably 30 minutes or more, including the total time from heating to cooling. There is no particular upper limit to the reduction process time, and it is preferable to select the time by conducting a preliminary test or the like so that the reduction does not proceed excessively. Note that the total time from heating to cooling means the time from starting heating from room temperature, reaching the reduction process temperature, and cooling to room temperature. Note that during this time, the composite tungsten oxide particles are preferably placed in the reducing atmosphere described above.

Although the mechanism is not clear, by carrying out the reduction treatment step in this manner, it is also possible to convert heterogeneous phases such as _{CsW1.6O6 phase} and _Cs2O phase confirmed by the X-ray diffraction pattern of the composite tungsten oxide particles obtained after the cooling step into a _{Cs0.32WO3 phase} . Therefore, by carrying out the reduction treatment step, it is possible to convert heterogeneous phases that do not contribute much to infrared absorption properties into target phases that can exhibit infrared absorption properties, making it possible to obtain composite tungsten oxide particles that are particularly excellent in infrared absorption properties.

In addition, according to the study of the inventors 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 having an a-axis lattice constant of 7.400 Å to 7.405 Å and a c-axis lattice constant of 7.680 Å to 7.700 Å, and a second crystal structure having an a-axis lattice constant of 7.415 Å to 7.420 Å and a c-axis lattice constant of 7.600 Å to 7.620 Å. However, by performing a reduction treatment process, the composite tungsten oxide particles can be made from the two types of crystal structure phases to have a crystal structure with excellent infrared absorption properties.

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

Figure 1 is a schematic diagram of a composite material manufacturing device 10.

The composite material manufacturing apparatus 10 can have an aerosol forming section 11, a transport section 21, a reaction section 31, a cooling section 41, and a recovery section 51.

(Aerosol forming part)
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 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. Also, a stirring blade may be used instead of the brush.

The powder supply section 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 section. The supply speed of the mixed raw material powder can be controlled by the movement speed of the piston 18. A known screw feeder or the like can be used instead of the piston.

Furthermore, the aerosol generating tube 12 can be connected to a regulator 19 that controls the flow rate of the carrier gas supplied from the outside, and can also be equipped with the function of supplying the carrier gas to the composite material manufacturing apparatus 10.

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

(Reaction Section)
The above-mentioned heat treatment process 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.

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 Al ₂ O ₃ with high purity. In order to reduce the cost of the furnace tube, mullite or the like can also be used.

The length L32 of the pipe 32 in the reaction section 31 is not particularly limited, and is preferably selected so that it can be heated to the specified temperature of the heat treatment process and so that sufficient time for the heat treatment process can be secured.

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 sufficient time for the heat treatment process. There is no particular upper limit to the length of the pipe 32, but an excessively long pipe 32 requires a large amount of carrier gas and increases the size of the device, so it is preferably 5 m or less.

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.

Note that the diameter D32 of the pipe 32 refers to the inner diameter of the pipe 32.

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

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 particular, when the workpiece is heat-treated at a temperature of 1000°C or higher in the heat treatment process 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 process, 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 in the reaction section 31 with a temperature of 1000°C or higher 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.

However, this is not limited to the above form, and various conditions may be set so that a temperature gradient is not formed along the longitudinal direction of the pipe 32, the temperature in the longitudinal direction of the pipe 32 is made almost uniform, and a temperature region above a predetermined temperature is formed.

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 is provided with a connection part 43, which is connected to the reaction part outlet 32B, and can receive the high-temperature gas discharged from the reaction part 31. In addition, it is preferable to make the distance between the reaction part 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 part between the reaction part 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 part outlet 32B, in this embodiment, since the carrier gas contains H ₂ , the hydrogen may come into contact with the atmosphere and react and burn, and the heat caused by the combustion may escape from the system, which may affect the characteristics of the obtained composite tungsten oxide particles. 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 around the connection part 43. For example, the cooling gas introduction pipes can be arranged at intervals of 120° around 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 containing the high-temperature gas (discharge gas) can be rapidly and efficiently cooled. For this reason, the generation of different phases can be particularly suppressed.

For example, the temperature inside the cooling container 42 may be measured using the thermocouple 45 described above, and the supply rate of the cooling gas, the temperature, etc. may be controlled.

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.

The reduction treatment device is not particularly limited as long as it is configured to perform the reduction treatment process described above. For example, it may include a container for storing composite tungsten oxide particles, which are particles obtained by the composite material manufacturing device described above, a gas pipe for supplying a mixed gas into the container to create a reducing atmosphere inside the container, and a heat source for heating the container.

It is also possible to introduce a mixed gas into the container to create a reducing atmosphere in the container, then exhaust the gas, and place the composite tungsten oxide particles to be treated under the 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 to form such an air flow.

A stirring blade may also be used to stir the composite tungsten oxide particles in the container.

Figure 2 is a schematic diagram of one example of the configuration of a reduction treatment device, showing a cross-sectional view taken along a plane passing through the central axis of the reaction tube 71 of the reduction treatment device 70.

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 creates a reducing atmosphere from the port 71A side, the inside of the reaction tube 71 can be made into a reducing atmosphere.

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 inside the reaction tube 71 of the tubular furnace.

By using the reduction treatment device 70, the inside of the reaction tube 71 is made into a reducing atmosphere, and the reaction tube 71 is heated to a desired temperature by the heater 72, whereby the reduction treatment of the composite tungsten oxide particles 74 placed in the container 73 can be performed.

According to the method for producing composite tungsten oxide particles of this 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, making it possible to easily produce composite tungsten oxide particles.

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 For the composite tungsten oxide particles obtained in the following Examples and Comparative Examples, powder X-ray diffraction patterns (XRD patterns) were measured using a powder X-ray diffractometer (Model: D2 PHASER, manufactured by Bruker Corp.) 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: Transmission Electron Microscope, manufactured by JEOL Ltd., Model: JEM-2011). The observation was performed 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 in accordance with 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 a solid spray pyrolysis method. The synthesis method will be described.

Using the composite material _production apparatus 10 shown in Fig. 1, _Cs0.32WO3 particles were produced as composite tungsten oxide particles and evaluated. Specific conditions will be described below.

The composite material manufacturing apparatus 10 shown in FIG. 1 has an aerosol forming section 11, a transport section 21, a reaction section 31, a cooling section 41, and a collection section 51. The aerosol forming section 11, the transport section 21, the reaction section 31, the cooling section 41, and the collection section 51 are connected by piping.

The aerosol generating 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 12 is tubular and includes an opening 13 that connects to the agitating section 14. The agitating section 14 includes a rotating brush 15. The brush 15 is arranged at the opening 13 so as to be exposed to the aerosol generating tube 12.

The powder supply section 16 is composed of a raw material reservoir 17 that stores the mixed raw material powder, and a piston 18 that delivers the mixed raw material powder to the mixing section.

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

First, a tungsten oxide powder represented by H ₂ WO ₄ was prepared as a W source raw material powder.

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.

Then, the _{Cs2CO3 aqueous} solution and _{the H2WO4} powder were mixed so that the ratio of the amounts of 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 confirmed by SEM, it was a powder with a particle size of 1 μm or more and 2 μm or less.

(Aerosol forming process)
In the aerosol forming section 11, the mixed raw material powder was extruded from the powder supplying section 16 at a supply 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.

The flow rate of the carrier gas was set to 10 L/min to thoroughly disperse the mixed raw material powder.

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 vol% _H2 /Ar gas containing 0.5 vol% hydrogen gas was used.

The reaction section 31 is equipped with a ceramic pipe 32, which is a cylindrical tube with a length L32 of 1 m and a diameter D32 of 13 mm. The diameter D32 refers to the inner diameter of the pipe 32.

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 is the temperature profile measured along the central axis of the pipe 32. The introduction of the carrier gas caused the high-temperature region heated by the furnace body to shift 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.

(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 were 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 O ₂ /N ₂ 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.

A bag filter is placed in the recovery section 51 as the filter 52, and the cooling process is completed in the cooling section 41, and the composite tungsten oxide particles formed can be recovered.

Composite tungsten oxide particles were produced under the above conditions.

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 process, 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.

[Comparative Example 1]
In Comparative Example 1, 1 vol % H ₂ /Ar gas containing 1 vol % 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 vol % H ₂ /Ar gas containing 3 vol % 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 vol % H ₂ /Ar gas containing 3 vol % hydrogen gas was used as the carrier gas.
[Comparative Example 4]
In Comparative Example 4, the furnace temperature in the heat treatment step was 1200° C., and 3 volume % H ₂ /Ar gas containing 3 volume % hydrogen gas was used as the carrier gas.

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

4(A) and 4(B) show the results of XRD measurement for Example 1 and Comparative Examples 1 to 4. Under all conditions, the Cs _0.33 WO ₃ phase was confirmed as the main phase.

In Example 1 and Comparative Examples 1 to 4, since the mixed raw material powder was heated in a dispersed aerosol state, it was confirmed that the thermal decomposition reaction to the Cs _0.33 WO ₃ phase proceeded even in a firing time of several seconds.

4(A) shows the results of XRD measurement of the composite tungsten oxide particles of Comparative Examples 2 to 4, which were synthesized using 3 vol% _H2 /Ar as the carrier gas and varying the furnace temperature from 1100°C to 1300°C. It was confirmed that the ratio of the W phase (ICDD 04-0806) peak at 2θ around 40 deg to the _Cs0.33WO3 phase peak increases with increasing furnace temperature. In Comparative Example ₂ , where the furnace temperature was 1300°C, it was confirmed that the effect of reduction was strong and the ratio of the metallic W phase was large.

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 W phase was low, was examined by TEM, it was confirmed that, as shown in Figure 5(A), rectangular particles with a particle diameter of 500 nm or more consisting of amorphous particles were formed. 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 a furnace temperature of 1100°C produced composite tungsten oxide particles with a relatively large particle diameter.

Therefore, if the aim is to obtain finer particles, it is presumed that a higher temperature in the heat treatment process is preferable. Therefore, in order to obtain finer composite tungsten oxide particles, the heat treatment temperature was set to 1300°C, and the hydrogen concentration in the carrier gas was changed. The results of XRD measurements of the composite tungsten oxide particles obtained in Example 1, Comparative Example 1, and Comparative Example 2 are shown in Figure 4 (B).

From a comparison between Example 1, Comparative Example 1, and Comparative Example 2, it was confirmed that the magnitude of the peak intensity of the metal W phase relative to the spectrum intensity of the Cs _0.33 WO ₃ phase was significantly reduced by reducing the hydrogen concentration in the carrier gas.

Figures 6(A) to 6(D) show TEM images of the composite tungsten oxide particles obtained in Comparative Example 1, in which 1 volume % H ₂ /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.

However, when EDS analysis was performed on the composite tungsten oxide particles obtained in Comparative Example 1, it was confirmed that the core part with high contrast inside was metallic W, and the shell part was a CWO phase. This is presumably because when the mixed raw material powder sublimated, the WO _6- octahedron was decomposed and reduced to W, and only W atoms were nucleated, and the Cs _0.33 WO ₃ phase grew around it as a shell phase.

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 the particle diameter was 200 nm or less, and fine composite tungsten oxide particles were obtained compared to Comparative Example 3.
(2-2) Examples 2 and 3
[Example 2]
In Example 2, the composite tungsten oxide particles obtained in Example 1 were subjected to a reduction treatment for 1 hour in a mixed gas atmosphere of 3 volume % H ₂ /Ar at 500° C. (reduction treatment step). The reduction treatment was performed using the reduction treatment device 70 shown in FIG. 2.

The reduction treatment device 70 was a horizontal tubular furnace, and the composite tungsten oxide particles synthesized in Example 1 were placed in a container 73, which was a ceramic boat, and the container 73 was placed at a position where the temperature inside the reaction tube 71 was the highest, i.e., 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, using 2% by mass of the composite tungsten oxide particles obtained in Example ₂ and the remainder as methyl isobutyl ketone, which is 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, and the evaluation was performed.

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.

8(A) to 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 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 detached.

From the TEM images shown in Figures 8(D) and 8(E), it was observed that the composite tungsten oxide particles of Example 2 began to slowly aggregate and 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 of Example 2. The same tendency in particle size was also confirmed from Figures 9(A) and 9(B).

Figure 8 (F) shows the results of observing the internal structure of the composite tungsten oxide particles of Example 2 at a high magnification. From Figure 8 (F), a clear lattice fringe of 3.64 Å belonging to the 002 plane was confirmed.

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), in which the scaly particles were 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.

In Comparative Example 5, composite tungsten oxide particles were produced and evaluated using the composite material production apparatus 80 shown in FIG. 10.

The composite material manufacturing apparatus 80 shown in FIG. 10 has the same configuration as the composite material manufacturing apparatus 10 shown in FIG. 1, except that the aerosol forming unit 11 is replaced with a droplet forming unit 81. For this reason, the same members are given the same numbers and descriptions are omitted.

The composite material manufacturing apparatus 10 shown in FIG. 1 aerosolizes the 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 now be described.

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

The droplet forming section 81 is connected to a first storage section 83 for storing a solution containing a tungsten source, which is a raw material solution, a second storage section 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 forming section 81 to the transport section 21, etc.

The droplet forming section 81 of the composite material manufacturing device 80 uses an ultrasonic spray device, and the ultrasonic irradiation section 82 irradiates the solution in the device with ultrasonic waves to form droplets.

In Comparative Example 5, a 10 mmol/L aqueous solution of ammonium paratungstate was prepared using ammonium paratungstate (ATP) represented by (NH ₄ ) ₁₀ (W ₁₂ O ₄₁ )·5H ₂ O (manufactured by Kanto Chemical Co., Ltd., purity: 88-90%) and ultrapure water as a solution containing a tungsten source. The aqueous solution of ammonium paratungstate was then placed in a first storage unit 83 and continuously supplied by a pump 85 to a droplet forming unit 81 connected to the first storage unit 83 by a pipe.

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

As described above, each solution is supplied at a constant flow rate from the first storage section 83 and the second storage section 84 to the droplet forming section 81 via piping by the pumps 85 and 86, and the two solutions are mixed at the top of the droplet forming 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 the droplet forming section 81 was 0.32. The raw material mixed solution contains 5 mmol/L ammonium paratungstate and 9.6 mmol/L cesium carbonate.

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% H ₂ /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.

The reaction section 31, cooling section 41, and recovery section 51 are configured in the same manner as in Example 1, so their explanation will be omitted here.

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

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 Figure 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 process).

The product obtained after the heat treatment step was supplied to the cooling section 41, cooled, and then recovered in the recovery section 51. The cooling gas used was 3% by volume O ₂ /N ₂ 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 a reduction treatment at 600° C. for 1 hour in a mixed gas atmosphere of 3 volume % H ₂ /Ar (reduction treatment step). The reduction treatment was performed using the reduction treatment device 70 shown in FIG. 2.
[Comparative Example 7]
In Comparative Example 7, an ink was prepared as a dispersion liquid by using 2% by 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.

The transmission profiles of the ink of Example 3 and the ink of Comparative Example 7 are shown in FIG. 11(A). According to the transmission profiles shown in FIG. 11(A), the decrease in transmittance in Example 3 is steeper than that in Comparative Example 7 in the wavelength range of 700 nm to 900 nm.

This is presumably due to the fact that the _Cs0.32WO3 particles obtained in Example 1, which is the basis for Example ₃ , were slightly larger than the _{Cs0.32WO3 particles} obtained in Comparative Example 5, which is the basis for Comparative Example 7, as described below.

Figures 12 (A) to 12 (D) show TEM images of the composite tungsten oxide particles obtained in Comparative Example 5. Comparing the composite tungsten oxide particles of Comparative Example 5 shown in Figure 12 (A) with the composite tungsten oxide particles of Example 1 shown in Figure 8 (B), which are observed at the same magnification, it can be confirmed that the 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.

For this reason, it is predicted that the particles synthesized by the solid spray pyrolysis method will contain slightly larger particles than the composite tungsten oxide particles synthesized by the liquid spray pyrolysis method, and that the effects of Rayleigh scattering will be greater. Figures 13(A) to 13(D) also show TEM images of the composite tungsten oxide particles of Comparative Example 6.

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

Figure 11 (B) shows the results of determining the relationship between visible light transmittance (Visible Light Transmittance) and solar transmittance (Solar Transmittance) when the inks of Example 3 and Comparative Example 7 are diluted with the solvent methyl isobutyl ketone.

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. In other words, 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 effect of the solvent during aerosol formation was eliminated, weakening the effect on the crystal structure.

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

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 areas seen 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 inside the crystal to almost completely disappear, and rearrangement in the crystal structure progressed.

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 degrees in the XRD profile is separated into two.

The XRD profiles of the composite tungsten oxide particles obtained in Example 1, Example ₂ , Comparative Example 5, and Comparative Example 6 were fitted to the _Cs0.32WO3 phase, ( _Cs2O ) _{0.44W2O6 phase} , and W _phase , and the ratio of each phase was calculated. Figure 16 shows the mass ratio of the crystal phase in the powder synthesized in Example 1, Example ₂ , Comparative Example 5, and Comparative Example 6. For the _Cs0.32WO3 phase, the coordinates and lattice constants of Cs and W were set as variable parameters, and the _temperature factor and the occupancy rate of each site were fixed and fitted. For the ( _Cs2O ) _{0.44W2O6 phase} and the W phase, the coordinates and lattice constants of each element were also fixed and fitted.

The composite tungsten oxide particles obtained in Example 1 were composed of 30.48 mass% _{Cs0.32WO3 crystal} phase ( _{Cs0.32WO3 -} A phase), 55.18 mass% _{Cs0.32WO3 crystal} phase ( _{Cs0.32WO3 -} B phase), 13.45 mass% ( _Cs2O ) _{0.44W2O6 phase} , and 0.89 mass% _W phase.

In the composite tungsten oxide particles of Example 2, which were obtained by carrying out a reduction treatment on the composite tungsten _oxide particles of Example 1, it was confirmed that the two separated _Cs0.32WO3 crystal phases were converted into a single phase of almost _{one Cs0.32WO3 crystal} phase ( _Cs0.32WO3 - _C phase). The proportion of the _Cs0.32WO3 crystal phase ( _Cs0.32WO3 - _C phase) reached 88.42 mass%.

The lattice constants calculated by Rietveld analysis were 7.42014 Å for the a-axis and 7.59587 Å for the c-axis of the Cs _0.32 WO ₃ -A phase in Example 1, 7.40041 Å for the a-axis and 7.68855 Å for the c-axis of the Cs _0.32 WO ₃ -B phase, and 7.40382 Å for the a-axis and 7.61184 Å for the c-axis of the Cs _0.32 WO ₃ -C phase in Example 2.

From the TEM image shown in Figure 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 believed that two peaks appear around 2θ of 23 degrees in the XRD profile of Example 1 in Figure 15 (A) as described above.

On the other hand, in Comparative Example 5, the _{Cs0.32WO3 phase} was 92.07% by mass, the ( _Cs2O ) _{0.44W2O6 phase} was 6.76% by mass, and _the W phase was 1.175% by mass. In Comparative Example 6, which was subjected to reduction treatment, _{the Cs0.32WO3 phase} was 93.58% by mass, the ( _Cs2O ) _{0.44W2O6 phase} was 2.67% by mass, and the W phase was 3.76% by mass, resulting in an increase in the W phase. The lattice constants of the _Cs0.32WO3 phase in Comparative Example 5 were 7.40955 Å for the a-axis and 7.69694 Å for the c-axis, and the lattice constants of _{the Cs0.32WO3} phase in Comparative Example ₆ were 7.41729 Å for the a-axis and 7.61101 Å for the c-axis.

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. The _Cs0.32WO3 - _A phase of Example 1 was located to the lower right compared to the comparative sample. This crystal is interpreted as the _CsxWO3 phase with less Cs reported in Non-Patent Document 5 _, since it tends to be located to the lower right than the comparative sample.

On the other hand, the _Cs0.32WO3 - _B phase of Example 1 was located in the upper left compared to the comparative sample. It was confirmed that the _Cs0.32WO3 - _B phase showed a tendency similar to that of the _Cs0.32WO3 phase of the composite tungsten oxide particles obtained in Comparative Example _5. It is presumed that the _Cs0.32WO3 - _B phase of Example 1 is nano-sized _Cs0.32WO3 particles lacking Cs and W that precipitated based on the sublimation phenomenon in the _heat treatment process.

It was confirmed that the lattice constant of the _Cs0.32WO3 phase of the composite tungsten oxide particles synthesized by the liquid spray pyrolysis method in Comparative Examples 5 and ₆ was extended in the crystal structure due to the residual influence of the solvent even after reduction treatment.

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 that dissolved the raw material in the aerosol during the heat treatment process, and it was confirmed that the crystal structure was different from that of the above-mentioned Comparative Examples 5 and 6. It is considered that the particles of the Cs _0.32 WO ₃ -A phase precipitated as nanoparticles due to the sublimation phenomenon, and during the reduction treatment, they reacted with the (Cs ₂ O) _0.44 W ₂ O ₆ phase to be filled with Cs, while the Cs vacancies and W vacancies were almost reduced, and they changed into Cs _0.32 WO ₃ crystals with oxygen vacancies introduced. On the other hand, it is presumed that the particles of the Cs _0.32 WO ₃ -B phase underwent the same Cs doping reaction and introduction of oxygen vacancies as in Non-Patent Document 5 during the reduction treatment.

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, it is believed that the synthesized _{Cs0.32WO3 particles} precipitate via two routes.

The first route is based on the sublimation phenomenon and passes through _{nanoparticles} of _Cs0.32WO3 particles from which both Cs and W have been desorbed. It is believed that the aerosolized precursor mixed raw material powder becomes amorphous and sublimes when it is put into a furnace heated to a high temperature. It is presumed that the vaporized W precipitated while maintaining the state of _WO6 clusters. This is one of the reasons that, 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 with a hydrogen concentration of 0.5 volume%, as shown in Figure 4(B), no W was confirmed in the core.

During sublimation, Cs atoms and _WO6 clusters are mixed, and it is thought that when Cs/W<0.33, they precipitate as nanoparticles with a hexagonal structure lacking Cs and W, and when Cs/W> _0.33 , they precipitate as nanoparticles with _a pyrochlore ( _Cs2O ) _0.44W2O6 phase during the cooling process.

On the other hand, the second route is considered to be via the Cs insertion route into the W compound, which is the same as the normal solid-phase method. In the heat treatment process, the W source raw material powder contained in the mixed raw material powder becomes amorphous, and its surface sublimes, but the inside does not sublimate and only mixing with Cs atoms proceeds, and it is considered to precipitate as a Cs _0.32 WO ₃ phase via a Cs ₄ W ₁₁ O ₃₅ phase, etc. In Example 1, it is considered that about 55 mass% of the particles of the mixed raw material powder were nanoparticled based on the sublimation phenomenon.

However, for the composite tungsten oxide particles synthesized by either route, by carrying out a reduction treatment or a pulverization treatment as necessary, the Cs/W composition ratio approaches the feed ratio of 0.33, and it was confirmed that the composite tungsten oxide particles have a crystal structure equivalent to _that of _Cs0.32WO3 shown as a comparative sample in FIG. 17.

Figure 19 shows the mass flow rates for particle synthesis using the liquid spray pyrolysis method of Comparative Example 5 and the solid spray pyrolysis method of Example 1. The mass flow rate for particle synthesis using the liquid spray pyrolysis method is approximately 3 g/Hr at maximum output, but particle synthesis using 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 easily be 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.

The results of this example confirmed that the method for producing composite tungsten oxide particles of this embodiment can produce composite tungsten oxide particles using equipment with low introduction costs, such as droplet forming means and heaters, and can also reduce the number of steps.

Claims (5)

The present _invention relates to a method for producing composite tungsten oxide particles represented by a general formula _MxWyOz (wherein M 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 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 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 . The method for producing composite tungsten oxide particles according to claim 1 , wherein the carrier gas is a mixed gas of Ar and _H2 . 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. The method for producing composite tungsten oxide particles according to any one of claims 1 to 3 , wherein the cooling gas is a mixed gas of _N2 and _O2 and has an oxygen concentration of 1 vol% or more and 5 vol% or less. 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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