Method for producing composite tungsten oxide particles

Abstract

To provide a production method of composite tungsten oxide particles capable of producing composite tungsten oxide particles having a target composition.SOLUTION: A production method of composite tungsten oxide particles represented by general formula: MxWyOz, having a particle size of 300 nm or less includes a raw material preparation step for preparing a raw material containing the M element and a tungsten element and a heat treatment step for heat-treating the raw material at a temperature of 500°C or higher. In the raw material preparation step, the raw material is prepared so that the ratio a / b of an amount "a" of the M element and an amount b of the tungsten element contained in the raw material is larger than the ratio x / y of an amount x of the M element and an amount y of the tungsten element in a target composition of the composite tungsten oxide particles by 2.5% or more and 15% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Various technologies have been proposed to reduce solar radiation transmittance while maintaining transparency and having good visible light transmittance as near-infrared shielding technology. Among these, near-infrared shielding technology using inorganic conductive nanoparticles has advantages such as superior near-infrared shielding characteristics, low cost, radio wave transmittance, and high weather resistance compared to other technologies.

[0003] For example, in Patent Document 1, General formula M x W y O z The invention discloses infrared shielding material nanoparticle dispersions in which composite tungsten oxide nanoparticles, expressed as (where M is one or more elements selected from H, He, alkali metals, alkaline earth metals, rare earth elements, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, Be, Hf, Os, Bi, I, W is tungsten, O is oxygen, 0.001≦x / y≦1, 2.2≦z / y≦3.0) are dispersed in a medium such as a resin, as well as technologies related to methods for manufacturing said infrared shielding particles. Patent Document 1 also discloses examples of manufacturing infrared shielding films, which are thin-film infrared shielding material nanoparticle dispersions.

[0004] According to Patent Document 1, it is possible to produce an infrared shielding material particle dispersion that has excellent optical properties, such as more efficiently shielding sunlight, especially light in the near-infrared region, while simultaneously 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.

[0005] By the way, according to Non-Patent Document 2, Cs 0.32 WO3 particles, known as one of the photochromic materials, have the property that the bluish color intensifies under strong UV (ultraviolet) irradiation (hereinafter referred to as the UV coloring phenomenon). According to Non-Patent Document 2, it has also been reported that after the UV coloring phenomenon, when stored in a dark place, it gradually returns to the original light blue color. The above UV coloring phenomenon has emerged as an issue for the popularization of composite tungsten oxide particles. Research has been conducted to reduce the UV coloring phenomenon under such circumstances.

[0006] Non-Patent Document 3 discloses that a composite material of SiO2, UVA, and CWO was synthesized by adding tetraethyl orthosilicate and a UV absorber (ultraviolet-absorbing agent; UVA) to Cs 0.32 WO3 particles.

[0007] Also, Non-Patent Document 4 discloses that by using the melt blending process, Cs 0.32 WO3 particles are kneaded into an inert polymer to suppress the generation of protons present near the surface of Cs 0.32 WO3 particles.

[0008] The methods disclosed in Non-Patent Documents 3, 4, etc. are intended to reduce the UV coloring phenomenon by compositing composite tungsten oxide particles with a UV absorber or the like, and the properties of Cs 0.32 WO3 particles themselves have not been improved.

[0009] On the other hand, various studies have been conducted on the manufacturing method of composite tungsten oxide particles useful as a near-infrared shielding material.

[0010] For example, the inventors of Patent Document 1, in Non-Patent Document 1, prepared Cs by the solid-phase method 0.32A method for synthesizing WO3 nanoparticles is proposed. However, in the synthesis method disclosed in Non-Patent Document 1, the particle size was large, and a grinding process was required to form nanoparticles. Therefore, the number of process steps might increase.

[0011] In Patent Document 2, it is proposed to synthesize potassium cesium tungsten bronze solid solution particles using a plasma torch in a reducing atmosphere.

[0012] In Non-Patent Document 5, a synthesis method of Cs x WO3 by a hydrothermal synthesis method is disclosed. However, the hydrothermal synthesis method requires a synthesis time of tens of hours or more. In addition, the hydrothermal synthesis method also has problems such as a large number of process steps in post-treatment processes.

[0013] In Non-Patent Document 6, a synthesis method based on inductively coupled thermal plasma technology is disclosed. However, such a synthesis method requires the introduction of an inductively coupled thermal plasma device, resulting in high costs.

[0014] In Non-Patent Document 7, a synthesis method of composite tungsten oxides by an aqueous solvent flame spray pyrolysis method is disclosed. However, due to the low Cs content, the infrared absorption characteristics were low.

[0015] In Non-Patent Document 8, a synthesis method of composite tungsten oxides by an aqueous solvent spray pyrolysis method is disclosed, and it is disclosed that a synthesis method of composite tungsten oxides with less Cs desorption on the particle surface and improved lightfastness to coloring can be obtained. However, the infrared absorption characteristics were low.

[0016] In Patent Document 3 and Patent Document 4, a method for manufacturing composite tungsten fine particles by a flame spray method is disclosed.

Prior Art Documents

Patent Documents

[0017]

Patent Document 1

[0018] [Non-licensed 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-licensed Document 2] Adachi, K., Ota, Y., Tanaka, H., Okada, M., Oshimura, N., & Tofuku, A. (2013). Chromatic instabilities in cesium-doped tungsten bronze nanoparticles. Journal of Applied Physics, 114(19), 194304. [Non-licensed Document 3] Zeng, Xianzhe, et al. "The preparation of a high performancenear-infrared shielding CsxWO3 / SiO2 composite resin coating and research on its optical stability under ultraviolet illumination." Journal of Materials Chemistry C 3.31 (2015): 8050-8060. [Non-licensed Document 4] Zhou, Yijie, et al. "CsxWO3 nanoparticle-based organic polymer transparent foils: low haze, high near infrared-shielding ability and excellent photochromic stability." Journal of Materials Chemistry C 5.25 (2017): 6251-6258. [Non-Patent Document 5] Guo Chongshen, et al., "Novel synthesis of homogenous CsxWO3 nanorods with excellent NIR shielding properties by a water controlled-release solvothermal process." Journal of Materials Chemistry,2010, Vol.20, Issue38, P.8227 - P.8229. [Non-Patent Document 6] 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 - P.9857. [Non-Patent Document 7] Hirano, Tomoyuki, et al. "Synthesis of highly crystalline hexagonal cesium tungsten bronze nanoparticles by flame - assisted spray pyrolysis." Advanced Powder Technology 29.10 (2018): 2512 - 2520. [Non-Patent Document 8] Nakakura, Shuhei, et al. "Improved photochromic stability in less deficient cesium tungsten bronze nanoparticles." Advanced Powder Technology 31.2 (2020): 702-707. [Non-Patent Document 9] Machida, K.; Okada, M; Adachi, K. Excitations of free and localized electrons at nearby energies in reduced cesium tungsten bronze nanocrystals. J. Appl. Phys, 2019, 125(10), 103103. [Overview of the project] [Problems that the invention aims to solve]

[0019] As previously mentioned, composite tungsten oxide particles are useful as near-infrared shielding materials. Therefore, there is a need for composite tungsten oxide particles that can be manufactured at low cost and with fewer processes.

[0020] Conventional methods for producing composite tungsten oxide particles have problems such as requiring the introduction of special, high-cost equipment and involving many steps, as described above.

[0021] On the other hand, manufacturing methods that involve spraying raw materials, such as the flame decomposition method, which can directly synthesize composite tungsten oxide particles, may reduce grinding costs.

[0022] However, according to the inventors' research, conventional methods for producing composite tungsten oxide particles using methods such as flame decomposition sometimes resulted in composite tungsten oxide particles that did not have the desired composition. Furthermore, if the composite tungsten oxide particles deviate from the desired composition, they may not be able to exhibit the desired optical properties.

[0023] Therefore, one aspect of the present invention aims to provide a method for producing composite tungsten oxide particles that have a desired composition. [Means for solving the problem]

[0024] One aspect of the present invention is a method for producing composite tungsten oxide particles, The composite tungsten oxide particles are General formula M x W y O z (However, element M is represented as 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, I, W is tungsten, O is oxygen, 0.20≦x / y≦0.37, 2.2≦z / y≦3.3) The particle size is 300 nm or less. A raw material preparation step for preparing a raw material containing the aforementioned M element and a tungsten element, The aforementioned raw materials Using flames, It includes a heat treatment process in which heat treatment is performed at a temperature of 500°C or higher, The present invention provides a method for producing composite tungsten oxide particles, wherein the raw material preparation step involves preparing the raw material such that the ratio a / b, which is the ratio of the amount of substance a of element M to the amount of substance b of tungsten contained in the raw material, is 2.5% to 15% greater than the ratio x / y, which is the ratio of the amount of substance x of element M to the amount of substance y of tungsten in the target composition of the composite tungsten oxide particles. [Effects of the Invention]

[0025] One aspect of the present invention is the ability to provide a method for producing composite tungsten oxide particles having a desired composition. [Brief explanation of the drawing]

[0026] [Figure 1] Figure 1 is a schematic diagram of a composite material manufacturing apparatus that can be suitably used in the method for producing composite tungsten oxide particles according to this embodiment. [Figure 2] Figure 2 is an explanatory diagram of the reduction apparatus used in the reduction process. [Figure 3] Figure 3 shows the XRD diffraction patterns of the powders obtained in Examples 1 to 6. [Figure 4] Figure 4 shows TEM and HAADF-STEM images of the composite tungsten oxide particles obtained in Example 1. [Figure 5] Figure 5 shows TEM and HAADF-STEM images of the composite tungsten oxide particles obtained in Example 2. [Figure 6] Figure 6 shows TEM and HAADF-STEM images of the composite tungsten oxide particles obtained in Example 3. [Figure 7] Figure 7 shows TEM and HAADF-STEM images of the composite tungsten oxide particles obtained in Example 4. [Figure 8] Figure 8 shows TEM and HAADF-STEM images of the composite tungsten oxide particles obtained in Example 5. [Figure 9] Figure 9 shows TEM and HAADF-STEM images of the composite tungsten oxide particles obtained in Example 6. [Figure 10] Figure 10 shows the evaluation results of the Cs / W ratio of the composite tungsten oxide particles obtained in Examples 1 to 6 using ICP, etc. [Figure 11] Figure 11 shows the transmission profiles and molar absorption coefficients of the inks obtained in Examples 7 to 9. [Figure 12]Figure 12 shows the changes in the permeation profile due to grinding and dispersion processing time in the ink obtained in Example 8, and the evaluation results of the particle size distribution of the composite tungsten oxide particles contained in the ink. [Figure 13] Figure 13 shows the changes in the permeation profile due to grinding and dispersion processing time in the ink obtained in Comparative Example 2, and the evaluation results of the particle size distribution of composite tungsten oxide particles contained in the ink. [Figure 14] Figure 14 shows the measurement results of the molar absorption coefficient of composite tungsten oxide particles in the ink obtained in Example 8 and Comparative Example 2. [Modes for carrying out the invention]

[0027] A specific example of a method for producing composite tungsten oxide particles according to one embodiment of this disclosure (hereinafter referred to as "this embodiment") will be described below with reference to the drawings. However, the present invention is not limited to these examples and is intended to be shown in the claims, with all modifications within the meaning and scope equivalent to the claims being included. [Method for producing composite tungsten oxide particles] The composite tungsten oxide particles produced in the method for producing composite tungsten oxide particles of this embodiment have the general formula M x W y O z It is represented as follows.

[0028] In the above general formula, element M 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. Furthermore, W represents tungsten and O represents oxygen, and it is preferable that x, y, and z satisfy 0.20 ≤ x / y ≤ 0.37 and 2.2 ≤ z / y ≤ 3.3, respectively.

[0029] The method for producing composite tungsten oxide particles according to this embodiment comprises the following raw material preparation steps and heat treatment steps.

[0030] In the raw material preparation process, raw materials containing element M and element tungsten can be prepared.

[0031] In the heat treatment process, the raw materials prepared in the raw material preparation process can be heat-treated at temperatures of 500°C or higher.

[0032] In the raw material preparation process, the raw materials can be prepared such that the ratio a / b, which is the ratio of the amount of substance a of element M to the amount of substance b of tungsten in the raw material, is 2.5% to 15% greater than the ratio x / y, which is the ratio of the amount of substance x of element M to the amount of substance y of tungsten in the target composition of the composite tungsten oxide particles. (1) Regarding the composite tungsten oxide particles to be manufactured First, we will describe the composite tungsten oxide particles produced by the method for producing composite tungsten oxide particles of this embodiment. (1-1) About the composition The composite tungsten oxide contained in the composite tungsten oxide particles is, as mentioned above, formulated with the general formula M x W y O z It is expressed as follows. The elements M, W, O, and x, y, and z in the formula have already been explained, so we will omit their explanation here.

[0033] Composite tungsten oxides can adopt one or more tungsten bronze-type crystal structures selected from, for example, tetragonal, cubic, and hexagonal crystal structures.

[0034] When composite tungsten oxide particles have a hexagonal crystal structure, the transmission of visible light and absorption of near-infrared light are improved. However, tetragonal and cubic tungsten bronze structures also function as excellent infrared shielding materials. The absorption position of near-infrared light tends to change depending on the crystal structure of the composite tungsten oxide particles. This absorption position shifts to longer wavelengths when the particle is tetragonal compared to cubic, and further to longer wavelengths when the particle is hexagonal. In addition, along with this variation in absorption position, hexagonal particles absorb the least visible light, followed by tetragonal particles, and cubic particles absorb the most visible light among these. Therefore, the crystal structure of composite tungsten oxide can be selected according to the application, for example, hexagonal tungsten bronze is preferable for applications that require greater transmission of visible light and greater shielding of infrared light.

[0035] As described above, when the composite tungsten oxide has a hexagonal crystal structure, the transmittance of light in the visible light region and the absorption of light in the near-infrared region of the composite tungsten oxide particles are particularly improved. For this reason, in applications where absorption of light in the near-infrared region is particularly required, it is preferable that the composite tungsten oxide particles contain a composite tungsten oxide with a hexagonal crystal structure. Furthermore, using 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, and it is more preferable that the M element contains one or more elements selected from Rb and Cs.

[0036] Here, we will explain the arrangement of element M when the composite tungsten oxide has a hexagonal crystal structure.

[0037] An octahedron is formed using a tungsten (W) atom and six oxygen (O) atoms as units. This octahedron, with an O atom at its vertex and a W atom in the center, is formed by the aggregation of six such octahedra, creating a hexagonal void (tunnel) composed of O atoms. An M element is then placed within this void to form a single unit, and numerous such units aggregate to form a hexagonal crystal structure.

[0038] When a composite tungsten oxide having a hexagonal crystal structure has a uniform crystal structure, the molar ratio of element M to W is 0.20 ≤ x / y ≤ 0.37, and preferably 0.30 ≤ x / y ≤ 0.36. Theoretically, when z / y = 3, the value of x / y becomes 0.33, which suggests that element M is distributed in all hexagonal voids. Note that the above x, y, and z are as described in the general formula M x W y O z These represent x, y, and z in the given context, and so on.

[0039] Similarly, when z / y=3, both cubic and tetragonal composite tungsten oxides have a structure-dependent upper limit on the amount of M element in them. The maximum amount of M element atoms per mole of tungsten is 1 mole for cubic tungsten and approximately 0.5 moles for tetragonal tungsten. Note that the maximum amount of M element atoms per mole of tungsten in the tetragonal case varies depending on the type of M element, but as mentioned above, approximately 0.5 moles is the amount that is easily manufactured industrially.

[0040] Composite tungsten oxides have a composition in which element M is added to tungsten trioxide (WO3). Since tungsten trioxide does not contain effective free electrons, it cannot exhibit an infrared absorption effect unless the ratio of oxygen to 1 mole of tungsten is less than 3. However, in composite tungsten oxides, free electrons are generated by adding element M, and an infrared absorption effect can be obtained. For this reason, the ratio of oxygen to 1 mole of tungsten can be 3 or less. Also, the ratio of oxygen to 1 mole of tungsten may exceed 3. However, the crystalline phase of WO2 may cause absorption and scattering of light in the visible light region, potentially reducing the absorption of light in the near-infrared region. For this reason, from the viewpoint of suppressing the formation of WO2, it is preferable to have a ratio of oxygen to 1 mole of tungsten greater than 2.

[0041] Therefore, it is preferable that the ratio of oxygen to 1 mole of tungsten, z / y, satisfies 2.2 ≤ z / y ≤ 3.3, as described above. (1-2) Regarding particle size When the composite tungsten oxide particles produced by the manufacturing method of this embodiment are used in applications where transparency is required, for example, the composite tungsten oxide particles must have a particle diameter of 800 nm or less. Furthermore, to ensure practical transparency, a particle diameter of 300 nm or less is preferable. This is because particles with a particle diameter of 300 nm or less do not completely block light due to scattering, thus maintaining high visibility in the visible light region and efficiently maintaining transparency at the same time. When transparency in the visible light region is particularly important, it is preferable to further consider scattering by the particles.

[0042] When prioritizing the reduction of scattering by such particles, the particle size is more preferably 200 nm or less, and even more preferably 100 nm or less.

[0043] This is because, with smaller particle sizes, the scattering of light in the visible light region (wavelengths 400nm to 780nm) due to geometric scattering or Mie scattering is reduced, preventing the infrared shielding film from becoming cloudy and losing its clarity. Furthermore, when the particle size is 200nm or less, the above-mentioned geometric scattering or Mie scattering is reduced, and the region shifts to Rayleigh scattering. In the Rayleigh scattering region, scattered light is reduced in proportion to the sixth power of the particle size, so as the particle size decreases, scattering is reduced and transparency improves. Moreover, when the particle size is 100nm or less, scattered light becomes very small, which is desirable. From the viewpoint of avoiding light scattering, smaller particle sizes are preferable.

[0044] Therefore, when it is required to maintain high visibility in the visible light region, for example, as described above, the composite tungsten oxide particles produced by the method for producing composite tungsten oxide particles of this embodiment preferably have a particle diameter of 300 nm or less, more preferably 200 nm or less, and even more preferably 100 nm or less. The lower limit of the particle diameter 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.

[0045] The particle size 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 particles, for example, with a SEM or TEM, and drawing the smallest circumscribed circle that circumscribes the particles. (2) Method for producing composite tungsten oxide particles The outline of the method for producing the composite tungsten oxide particles of this embodiment will be described.

[0046] As described above, the method for producing composite tungsten oxide particles according to this embodiment may include a raw material preparation step and a heat treatment step. Each step will be described below. (2-1) Raw material preparation process In the raw material preparation process, raw materials containing element M and element tungsten (hereinafter also referred to as "element W") can be prepared.

[0047] The inventors of this invention have found that composite tungsten oxide particles obtained by conventional methods for producing composite tungsten oxide particles sometimes deviate from the target composition. Specifically, the molar ratio of element M to element W in the raw materials differs from the molar ratio of element M to element W in the obtained composite tungsten oxide particles, and it has been confirmed that the amount of element M is reduced. This reduction in element M in the obtained composite tungsten oxide particles is thought to be due to sublimation or volatilization of element M during heat treatment or other processes.

[0048] Therefore, in the raw material preparation process, the raw materials can be prepared such that the ratio a / b, which is the ratio of the amount of substance a of element M to the amount of substance b of element W contained in the raw materials, is 2.5% to 15% greater than the ratio x / y, which is the ratio of the amount of substance x of element M to the amount of substance y of element W in the target composition of the composite tungsten oxide particles. In other words, in the raw material preparation process, the raw materials can be prepared so that they contain a larger amount of element M than the amount of element M to be obtained in the composite tungsten oxide particles.

[0049] In the method for producing composite tungsten oxide particles of this embodiment, as described later, the raw materials can be heat-treated at a temperature of 500°C or higher in the heat treatment step so that composite tungsten oxide particles with a particle diameter of 300 nm or less can be obtained.

[0050] However, in the heat treatment process, for example, when the raw materials are heat-treated, the M element is more easily volatile than the W element from aerosols such as fine particles of the raw materials dispersed in the reaction field of the heat treatment. Therefore, in the raw material preparation step of the method for producing composite tungsten oxide particles of this embodiment, the raw materials can be prepared so that the amount of M element is in excess of the amount of M element in the target composite tungsten oxide particles, in order to compensate for the decrease due to the volatilization of the M element.

[0051] In the raw material preparation process, the raw materials can be prepared such that the above a / b ratio in the raw materials is 2.5% to 15% greater than the above x / y ratio in the target composition of the composite tungsten oxide particles. This is because setting it to 2.5% or more prevents the M element from being insufficient in the resulting composite tungsten oxide particles relative to the target composition. Also, setting it to 15% or less prevents the M element from being excessively in excess relative to the target composition.

[0052] Here, for a / b to be 2.5% to 15% greater than x / y, it means that the percentage obtained by subtracting x / y from a / b and dividing that difference by x / y is 2.5% to 15%.

[0053] The state of the raw materials prepared in the raw material preparation process is not particularly limited; they may be liquids or powders, and it is preferable that they can form aerosols by spraying or other means. An aerosol refers to a mixture of fine liquid or solid particles suspended in a gas and the surrounding gas.

[0054] If the raw material is a liquid, it can be prepared by, for example, preparing a solution containing an M element source and a W element source.

[0055] Alternatively, a solution containing the M element source and a solution containing the W element source can be prepared separately in advance, and both solutions can be mixed during the raw material preparation process to create the raw material mixture solution.

[0056] For the heat treatment process described later, the raw materials can also be supplied in the form of droplets, for example. In this case, the raw material preparation process can be carried out by mixing the solution containing the M element source and the solution containing the W element source immediately before supplying them to the droplet forming unit (droplet forming means) or within the droplet forming unit. Then, the aerosol formation process described later can be carried out in the droplet forming unit.

[0057] For example, if mixing a solution containing the M element source and a solution containing the W element source beforehand causes problems such as gelation, it is preferable to prepare both solutions in advance as described above and mix them immediately before the aerosol formation process. When the raw material preparation process is carried out immediately before the aerosol formation process, the molar ratio of the M element to the W element in the raw material can be adjusted to the range described above by adjusting the concentrations of both solutions and the rate at which both solutions are supplied to the droplet formation section.

[0058] As mentioned above, when the raw material preparation process is carried out immediately before the aerosol formation process, the aerosol formation process and the raw material preparation process do not need to be clearly distinguished, and both processes can be carried out continuously.

[0059] As described above, when mixing a solution containing the M element source and a solution containing the W element source in the raw material preparation process, the specific method of mixing is not particularly limited, and any method can be used.

[0060] The W element source is not particularly limited, and tungsten salts can be used, for example, hexacarbonyltungsten is preferably used. Hexacarbonyltungsten can be represented as, for example, W(CO)6. Furthermore, as the solution containing the W element source, an organic solution containing the W element source can be preferably used due to its ease of handling, etc.

[0061] As a solution containing the M element source, for example, a solution of a salt containing the M element can be used. The type of salt of the M element that serves as the M element source 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.

[0062] As a solution containing the M element source, an ethanol solution containing the M element source can be suitably used due to its ease of handling and other factors.

[0063] For example, even when element M is cesium, one or more salts selected from carbonates, acetates, nitrates, hydroxides, etc., can be used as the salt of the element M source, but acetates are particularly preferable. This is because cesium acetate dissolves particularly easily in ethanol.

[0064] Furthermore, the ratio of element M to 1 mole of tungsten in the resulting composite tungsten oxide, i.e., the doping amount, is determined by the ratio of element W source to element M source when forming the raw material mixed solution. Therefore, it can be controlled, for example, by adjusting the concentration of the solution containing element W source or the concentration of the solution containing element M source.

[0065] The concentration of the W element source in the solution containing the W element source, i.e., the concentration of the W element salt, is not particularly limited. For example, it is preferable that the tungsten concentration of the solution containing the W element source be between 0.001 mol / L and 10 mol / L, more preferably between 0.01 mol / L and 10 mol / L, and even more preferably between 0.01 mol / L and 1 mol / L. This is because setting the tungsten concentration of the solution containing the W element source to 0.001 mol / L or higher ensures a sufficient production amount of composite tungsten oxide particles per unit time, allowing for the recovery of a sufficient amount by means of a filter, for example, thereby increasing productivity. Furthermore, setting the tungsten concentration of the solution containing the W element source to 10 mol / L or lower prevents the reprecipitation of the dissolved W element source, suppresses the aggregation of the generated particles, and prevents the inclusion of coarse composite tungsten oxide particles of 1 μm or larger, for example. In addition, additives such as pH adjusting agents and surfactants can be added to the solution containing the W element source.

[0066] Furthermore, the concentration of the M element source in the solution containing the M element source is not particularly limited and can be selected according to the desired composition of the composite tungsten oxide particles to be manufactured, the concentration of the W element source in the solution containing the W element source, etc.

[0067] In addition to solutions containing a tungsten source or an M element source, any other components can be added to the raw material mixture solution.

[0068] Up to this point, the explanation has been based on the example of a liquid raw material, but the raw material can also be a solid, for example, a powder. When the raw material is a powder, it can be prepared by mixing, for example, a powder of an M element compound and a powder of a tungsten compound. Alternatively, for example, a precursor powder can be used as a raw material by adding a tungsten compound powder to a solution containing an M element source, stirring, and removing the solvent by drying or other means.

[0069] When the raw material is solid, the W element source is not particularly limited, and tungsten salts can be used, for example, H2WO4 or ammonium paratungstate can be preferably used.

[0070] In H2WO4, the elements other than tungsten are H (hydrogen) and O (oxygen), and these elements are discharged from the system during the heat treatment process described later. Therefore, using H2WO4 as the W element source is preferable because it allows for the production of composite tungsten oxide particles with suppressed impurity contamination.

[0071] As a source of element M, for example, a powder of a salt containing element M can be used. The type of salt containing element M is not particularly limited, but for example, one or more types selected from carbonates, acetates, nitrates, hydroxides, etc. of element M can be used.

[0072] For example, even when element M is cesium, one or more of the following can be used: carbonate, acetate, nitrate, hydroxide, etc., but carbonate can be used particularly preferably. (2-2) Heat treatment process In the heat treatment process, the raw material can be processed into composite tungsten oxide particles by heat treatment. The heat treatment only needs to be performed at a temperature of 500°C or higher, and the configuration of the heat source is not particularly limited. Therefore, the heat treatment process can be carried out by introducing the raw material into a flame using a carrier gas, or by introducing it into a tubular electric furnace. In both cases, the heat treatment temperature can be set to 500°C or higher. By performing heat treatment at 500°C or higher, compounds contained in the raw material decompose, and tungsten reacts with M element to form composite tungsten oxide.

[0073] The heat treatment temperature should be at least 500°C, as long as it allows the reaction between tungsten and element M to proceed. However, it is preferable to be at least 550°C, and more preferably at least 1000°C. While there is no particular upper limit to the heat treatment temperature, it is preferable to be at least 1500°C from the viewpoint of suppressing energy consumption.

[0074] As mentioned above, a flame can be used in the heat treatment process, allowing the raw materials to be heat-treated using a flame. By using a flame in the heat treatment process, the particle size of the resulting composite tungsten oxide particles can be selected by adjusting the temperature of the flame reaction field.

[0075] When a flame is used in the heat treatment process, the conditions for flame formation are not particularly limited, but the flame can be formed using, for example, a mixed gas containing oxygen and hydrocarbons. By forming a flame with a mixed gas containing oxygen and hydrocarbons, a flame with a stable temperature can be formed, and composite tungsten oxide particles with suppressed variations in particle size can be produced.

[0076] While there are no particular limitations on the method of adjusting the flame size or flame temperature, it is preferable to adjust the flow rates of both gases, for example, by adjusting the flow rate ratio of oxygen in the gas mixture supplied to the flame to that of a combustible gas such as hydrocarbons, while maintaining a flow rate ratio that allows for the combustion of the combustible gas. This is because it allows for adjustment of the flame intensity while ensuring the amount of oxygen necessary for combustion of the combustible gas.

[0077] For example, when a flame is formed using a gas mixture containing oxygen and propane, it is preferable that the ratio of propane to oxygen (burner) flow rates in the gas mixture be 5 to 8 parts oxygen to 1 part propane, and that the propane flow rate be in the range of 0.5 L / min to 2 L / min or more. This is because, when the propane flow rate is 1 part, the oxygen flow rate is 5 or more parts, which sufficiently promotes the combustion of propane, a flammable gas. However, to prevent an excessive supply of oxygen, it is preferable to supply 8 or less parts oxygen to 1 part methane.

[0078] The heat treatment temperature, such as that of the flame reaction field, also affects the particle size of the resulting composite tungsten oxide particles.

[0079] This is presumed to be because the thermal energy in the reaction field, such as a flame, is used for the sublimation of the generated composite tungsten oxide particles, causing the particles to burst during sublimation and yield particles with a very small particle size.

[0080] The composite tungsten oxide particles obtained by heat treatment can be recovered, for example, using a filter. (2-3) Aerosol formation process In the method for producing composite tungsten oxide particles according to this embodiment, it is preferable to supply the raw materials prepared in the raw material preparation step to the heat treatment step in the form of an aerosol. Specifically, it is preferable to transport the aerosol using a carrier gas such as oxygen and subject it to the heat treatment step.

[0081] Therefore, the method for producing composite tungsten oxide particles of this embodiment may also include an aerosol formation step in which the raw materials are converted into an aerosol containing droplets or particles of the raw materials.

[0082] The means and methods for forming the aerosol in the aerosol formation process are not particularly limited and can be selected according to the state of the raw materials, etc.

[0083] When the raw material is a liquid, an aerosol can be formed by spraying the liquid onto a carrier gas using various atomizers such as centrifugal atomizers or two-fluid nozzles. Alternatively, droplets can be formed by applying ultrasonic irradiation to the liquid.

[0084] When the raw material is a powder, an aerosol can be formed by a device that disperses the powder and supplies the powder into an airflow. For example, an aerosol can be formed by an aerosol forming device that includes a stirring section such as a rotating brush or agitator blade, and a powder supply section including a piston or screw feeder that sends the mixed powder raw material to the stirring section. The raw material powder supplied from the powder supply section is dispersed into particles in the stirring section, and an aerosol can be generated from the raw material powder by sending each particle into a carrier gas. The stirring section can select the rotation speed of the brush or agitator blade so that the raw material powder can be dispersed into particles, and it is preferable to rotate it at high speed.

[0085] When forming droplets dispersed in a gas during the aerosol formation process, the size of the droplets to be formed is not particularly limited, but the diameter of the droplets is preferably 100 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. By setting the diameter of the droplets to 100 μm or less, it is possible to prevent the resulting composite tungsten oxide particles from becoming coarse, and to obtain composite tungsten oxide particles on the order of nanometers. The lower limit of the size of the droplets formed in the aerosol formation process is not particularly limited. However, it is difficult to form excessively small droplets, and productivity may decrease, so it is preferable that the size is, for example, 1 μm or more.

[0086] When forming solid particles dispersed in a gas during the aerosol formation process, the particle size 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 setting the particle diameter to 100 μm or less, it becomes possible to more reliably perform heat treatment to the interior of the particles. The particle diameter can be measured in the same way as the particle diameter of the composite tungsten oxide particles described above. (2-4) Reduction process Particles obtained through a heat treatment process, specifically composite tungsten oxide particles, may not exhibit infrared absorption properties. Therefore, the inventors of this invention conducted research and found that by further performing a reduction treatment process on the composite tungsten oxide particles obtained through the heat treatment process, the composite tungsten oxide particles can exhibit infrared absorption properties.

[0087] Therefore, the method for producing composite tungsten oxide particles according to this embodiment may also include a reduction treatment step in which the particles obtained in the heat treatment step are subjected to reduction treatment in an atmosphere containing a reducing gas. Specifically, for example, the method for producing composite tungsten oxide particles according to this embodiment may include a reduction treatment step after the heat treatment step in which the particles are subjected to reduction treatment in an atmosphere containing a reducing gas at a temperature in the range of higher than 400°C and lower than 700°C.

[0088] The conditions for the reduction treatment are not particularly limited, but it is preferable to select the reduction treatment conditions such that, when the composite tungsten oxide particles after the reduction treatment are analyzed by X-ray diffraction, the crystal structure does not change before and after the reduction treatment process, and metallic tungsten or the like does not precipitate.

[0089] In the reduction process, the composite tungsten oxide obtained in the heat treatment process can be reduced by heating and cooling it in a reducing atmosphere containing a reducing gas, i.e., by heat treatment.

[0090] During the reduction process, the composite tungsten oxide particles may be stirred or left to stand. The handling of the composite tungsten oxide particles during the reduction process can be selected as appropriate, but it is preferable to select handling conditions that prevent the deposition of metallic tungsten.

[0091] The reduction treatment temperature is preferably higher than 400°C, more preferably 500°C or higher, and even more preferably 550°C or higher.

[0092] The upper limit of the temperature for the reduction treatment is not particularly limited, but it is preferably less than 700°C, more preferably 650°C or less, and even more preferably less than 650°C.

[0093] Furthermore, when the raw material is liquid, reducing the raw material concentration during the aerosol formation process results in smaller particle sizes. Since smaller particles are easier to reduce, the temperature during the reduction process can be lowered compared to conventional methods. In the reduction process, the temperature can be raised from room temperature to the reduction temperature, and then lowered back down to room temperature.

[0094] The reduction conditions can be determined from the optical properties of the resulting composite tungsten oxide particles.

[0095] By raising the reduction temperature above 400°C, the reduction treatment of the composite tungsten oxide particles can be advanced, allowing them to exhibit infrared absorption properties more reliably. Furthermore, by keeping the temperature below 700°C, the reduction of the composite tungsten oxide particles to metallic tungsten can be suppressed.

[0096] The reducing atmosphere is preferably a mixture of an inert gas such as argon and a reducing gas such as H2 gas (hydrogen gas), with H2 gas being the preferred reducing gas.

[0097] When using H2 gas as the reducing gas, the H2 gas content in the reducing atmosphere can be appropriately selected, but the H2 gas content is preferably in the range of 0.1% to 10% by volume, and more preferably in the range of 2% to 10%. Caution is required because reduction in an atmosphere consisting only of a reducing gas may cause the reduction reaction to proceed excessively, leading to the precipitation of metallic tungsten.

[0098] The reduction treatment process should preferably last at least 30 minutes, including the total time from heating to cooling. There is no particular upper limit to the reduction treatment process time; however, it is preferable to select a suitable time by conducting preliminary tests, for example, to prevent excessive reduction. The total time from heating to cooling refers to the time from when heating starts at room temperature, when the reduction treatment temperature is reached, and when it cools back down to room temperature. During this time, it is preferable that the composite tungsten oxide particles are placed in the reducing atmosphere described above.

[0099] By performing this reduction process, it is possible to convert the unintended phase of the composite tungsten oxide particles obtained after the heat treatment process into the desired composite tungsten oxide phase. [Composite material manufacturing equipment] An example of the configuration of a composite material manufacturing apparatus that can be suitably used in the method for producing composite tungsten oxide particles of this embodiment is described below.

[0100] Figure 1 is a schematic diagram of the composite material manufacturing apparatus 10 of this embodiment.

[0101] The composite material manufacturing apparatus 10 includes a first storage section 11 containing a raw material solution containing M element and tungsten element, a two-fluid nozzle 12 that forms droplets of the raw material and simultaneously forms a flame, and a reaction tube 13 connected to a filter 14 for collecting the formed composite tungsten oxide particles.

[0102] A raw material solution and a carrier gas can be supplied to the two-fluid nozzle 12 to form an aerosol (aerosol formation step). For example, oxygen and hydrocarbons are supplied to the two-fluid nozzle 12, and together a flame reaction field is formed. The formed aerosol can be supplied into the flame and heat treated (heat treatment step). Cooling water pipes 131 are arranged around the reaction tube 13, and the cooling water circulates. The composite tungsten oxide particles introduced into the reaction tube 13 are recovered by a filter 14 such as a bag filter.

[0103] Additionally, an ejector 15 may be provided at the downstream end to adjust the amount of carrier gas supplied.

[0104] Here, we have shown an example of a composite material apparatus that forms droplets of raw materials and heat-treats them using a flame to form composite tungsten oxide particles. However, the apparatus is not limited to this configuration, and the raw materials may be powders, and the heat source used for heat treatment may be an electric furnace, etc. [Reduction treatment device] The reduction treatment apparatus can carry out the reduction treatment process described above.

[0105] The reduction treatment apparatus is not particularly limited, as long as it is configured to carry out the reduction treatment process described above. For example, it may include a container for storing composite tungsten oxide particles, which are particles obtained in the composite material manufacturing apparatus described above, a gas pipe for supplying a mixed gas to form a reducing atmosphere into the container, and a heat source for heating the container.

[0106] Alternatively, a mixed gas that creates a reducing atmosphere can be introduced into the container and exhausted, and the composite tungsten oxide particles to be treated can be placed under the airflow of the mixed gas. In this case, gas piping can be provided, including a supply pipe for the mixed gas and an exhaust pipe, to form such an airflow.

[0107] Additionally, a stirring blade or similar device may be used to agitate the composite tungsten oxide particles inside the container.

[0108] Figure 2 is a schematic diagram showing one example of the configuration of a reduction apparatus, and shows a cross-sectional view of the reaction tube 21 of the reduction apparatus 20 through the central axis.

[0109] The reduction treatment apparatus 20 is a horizontal tubular furnace, and can be used by attaching a gas introduction pipe (not shown) to one opening 21A of the reaction tube 21 and a gas exhaust pipe (not shown) to the other opening 21B of the tubular furnace. By supplying a mixed gas that creates a reducing atmosphere from the opening 21A, a reducing atmosphere can be created inside the reaction tube 21.

[0110] A heater 22 can be provided around the reaction tube 21, and the composite tungsten oxide particles can be placed in a ceramic container 23, such as a boat, and positioned in the reaction tube 21 of the tubular furnace at a location corresponding to the heater 22.

[0111] By using the reduction treatment apparatus 20, the inside of the reaction tube 21 is made into a reducing atmosphere, and the heater 22 is used to heat it to a desired temperature, thereby performing a reduction treatment on the composite tungsten oxide particles 24 placed in the container 23.

[0112] According to the method for producing composite tungsten oxide particles of this embodiment described above, composite tungsten oxide particles with a desired composition ratio can be produced by using raw materials containing M element and W element in predetermined proportions. [Examples]

[0113] The present invention will be described with specific examples below, but it is not limited to these examples. (1) Evaluation method (1-1) Powder X-ray diffraction Powder X-ray diffraction patterns (XRD patterns) were measured for composite tungsten oxide particles using a powder X-ray diffractometer (X'Pert PRO MPD (Malvern Panalytical)). CuKα radiation was used as the radiation source, and the measurements were performed with a tube voltage of 45kV and a tube current of 40mA. (1-2) Observation of TEM images and HAADF images The obtained composite tungsten oxide particles were observed using a transmission electron microscope. High-angle angular dark-field (HAADF) images were also observed using a JEOL STEM (model: JEM-ARM200F). The particle contrast in HAADF images is proportional to the number of atoms in the particle and the number of atoms. Since the contrast is brighter as more heavier atoms are stacked, the brightest and second brightest contrasts are thought to be tungsten oxide (W), the third brightest contrast is thought to be cinnabar (Cs), and oxygen (O) cannot be observed. (1-3) Composition analysis The proportion of each component contained in the composite tungsten oxide particles was evaluated using the following method.

[0114] The mass percentage of Cs was calculated as the average of three measurements taken for each sample using a polarized Zeeman atomic absorption spectroscopy (AAS, model: ZA3300, manufactured by Hitachi High-Tech Corporation).

[0115] The mass percentage of W was calculated as the average value obtained by analyzing each sample three times using inductively-coupled plasma optical emission spectroscopy (ICP-OES, model: ICPE-9800, manufactured by Shimadzu Corporation).

[0116] The mass percentage of oxygen was calculated using an oxygen, nitrogen, and hydrogen analyzer (ON-836, LECO Japan Corp.) with an infrared absorption spectroscopy (IRS) detector for oxygen detection, and the average value was obtained from three analyses performed on each sample. [Example 1] Composite tungsten oxide particles were manufactured and evaluated using the composite material manufacturing apparatus 10 shown in Figure 1. The specific conditions are described below.

[0117] The composite material manufacturing apparatus 10 includes a first storage section 11 containing a solution containing a W element source and an M element source, which are the raw material solutions; a two-fluid nozzle 12 that forms droplets of the raw material and simultaneously forms a flame; and a reaction tube 13 connected to a filter 14 for collecting the formed composite tungsten oxide particles.

[0118] The supply rate of the raw material solution to the two-fluid nozzle 12 was set to 3 g / min. The flow rate of the carrier gas, oxygen, was controlled by the ejector 15 to 9 L / min. The air flow rate in the ejector 15 was controlled to be within the range of 160 L / min to 180 L / min.

[0119] For flame formation using the two-fluid nozzle 12, propane gas and oxygen gas were used, with a propane gas flow rate of 1.0 L / min and an oxygen gas flow rate of 5.0 L / min.

[0120] As raw materials, a mixed solution was used, consisting of a solution in which W(CO)6, a W element source, was dissolved in THF (tetrahydroflon) (solution containing the W element source), and a solution in which cesium acetate, a M element source, was dissolved in ethanol (solution containing the M element source). This mixed solution was then stored in the first storage section 11.

[0121] The raw material mixed solution was prepared so that the ratio a / b, which is the ratio of the amount of substance a of the M element Cs to the amount of substance b of the tungsten element, was 0.33. Furthermore, the above a / b ratio was adjusted to be 6.5% greater than the ratio x / y, which is the ratio of the amount of substance x of the M element to the amount of substance y of the tungsten element in the target composition of the composite tungsten oxide particles. In relation to the target composition in Example 4, described later, the above a / b ratio was adjusted to be 10% greater than the ratio x / y, which is the ratio of the amount of substance x of the M element to the amount of substance y of the tungsten element in the target composition of the composite tungsten oxide particles.

[0122] From the first storage unit 11, the above-mentioned raw material solution and oxygen, which is a carrier gas, were supplied to the two-fluid nozzle 12 to form an aerosol (aerosol formation step).

[0123] As described above, propane gas and oxygen gas were supplied to the two-fluid nozzle 12 to form a flame reaction field, and the formed aerosol was supplied into the flame for heat treatment (heat treatment step).

[0124] The composite tungsten oxide particles obtained in the heat treatment process were introduced into the reaction tube 13. Cooling water pipes 131 were arranged around the reaction tube 13, and the cooling water circulated through them. The composite tungsten oxide particles introduced into the reaction tube 13 were recovered by a bag filter, which is a filter 14.

[0125] The obtained composite tungsten oxide particles were evaluated as described above.

[0126] The XRD patterns of the obtained composite tungsten oxide particles are shown in Figure 3(A). The obtained XRD patterns are Cs 0.33 We confirmed that it contained only the diffraction peak of WO3.

[0127] Furthermore, TEM and HAADF images of the obtained composite tungsten oxide particles are shown in Figures 4(A) to 4(C). From the TEM image shown in Figure 4(A), it was confirmed that the particle size was approximately 50 nm, and less than 300 nm. From the HAADF-STEM images shown in Figures 4(B) and 4(C), W-defect rows were observed throughout the W / Cs row, so these particles are classified as cesium polytungstate.

[0128] Analysis of the Cs / W ratio of these particles using ICP and other methods revealed that the Cs / W ratio decreased from 0.33 (the initial composition) to 0.31, and the O / W ratio was approximately the same as the theoretical composition of 3.00, at 3.06. Figure 10 summarizes the changes in the Cs / W ratio in Examples 1 to 6. It is presumed that when passing through a high-temperature field of about 3000K, Cs and W are decomposed to the atomic scale, but W readily combines with oxygen to form WO6 octahedra, while Cs, which sublimes more easily than W, did not combine with oxygen. [Example 2] The mixed solution used as the raw material was prepared so that the ratio a / b, which is the ratio of the amount of substance a of the M element Cs to the amount of substance b of the tungsten element, was 0.38. Furthermore, the above a / b was prepared to be 2.7% greater than the x / y ratio, which is the ratio of the amount of substance x of the M element to the amount of substance y of the tungsten element in the target composition of the composite tungsten oxide particles. In relation to the target composition in Example 5, described later, the above a / b was prepared to be 11.8% greater than the x / y ratio, which is the ratio of the amount of substance x of the M element to the amount of substance y of the tungsten element in the target composition of the composite tungsten oxide particles.

[0129] Except for the points mentioned above, composite tungsten oxide particles were prepared in the same manner as in Example 1.

[0130] The XRD patterns of the obtained composite tungsten oxide particles are shown in Figure 3(A). In the obtained XRD patterns, Cs 0.33 Diffraction peaks of WO3 and (Cs2O) 0.44 Some diffraction peaks of W2O6ICDD:00-047-0566 were observed.

[0131] Furthermore, TEM and HAADF images of the obtained composite tungsten oxide particles are shown in Figures 5(A) to 5(C). From the TEM image shown in Figure 5(A), it was confirmed that the particle size was approximately 50 nm to 200 nm, and less than 300 nm. It was also confirmed that the particles had facets (flat surfaces). From the HAADF-STEM images shown in Figures 5(B) and 5(C), W deficiency rows were observed throughout the W / Cs row, so these particles are classified as cesium polytungstate.

[0132] Analysis of the Cs / W ratio of these particles by ICP revealed that the Cs / W ratio had decreased from 0.38 (the initial composition) to 0.37. The O / W ratio was 3.13. [Example 3] The mixed solution used as the raw material was prepared so that the ratio a / b, which is the ratio of the amount of substance a of the M element Cs to the amount of substance b of the tungsten element, was 0.42. Furthermore, the above a / b ratio was adjusted to be 7.7% greater than the ratio x / y, which is the ratio of the amount of substance x of the M element to the amount of substance y of the tungsten element in the target composition of the composite tungsten oxide particles. Similarly, in relation to the target composition in Example 6, described later, the above a / b ratio was also adjusted to be 7.7% greater than the ratio x / y, which is the ratio of the amount of substance x of the M element to the amount of substance y of the tungsten element in the target composition of the composite tungsten oxide particles.

[0133] Except for the points mentioned above, composite tungsten oxide particles were prepared in the same manner as in Example 1.

[0134] The XRD patterns of the obtained composite tungsten oxide particles are shown in Figure 3(A). In the obtained XRD patterns, Cs 0.33 Diffraction peaks of WO3 and (Cs2O)0.44 Some diffraction peaks of W2O6ICDD:00-047-0566 were observed.

[0135] Furthermore, TEM and HAADF images of the obtained composite tungsten oxide particles are shown in Figures 6(A) to 6(C). From the TEM image shown in Figure 6(A), it was confirmed that the particle size was approximately 50 nm to 200 nm, and less than 300 nm. From the HAADF-STEM images shown in Figures 6(B) and 6(C), the edges of the particles were rounded, and an amorphous region with low contrast was formed at the interface.

[0136] Analysis of the Cs / W ratio of these particles using ICP and other methods revealed that the Cs / W ratio decreased from 0.42 (the initial composition) to 0.39. The O / W ratio was 2.99. [Example 4] The composite tungsten oxide particles obtained in Example 1 were subjected to reduction treatment using the reduction treatment apparatus 20 shown in Figure 2.

[0137] As shown in Figure 2, under an airflow of 3 vol.% H2 / 97 vol.% Ar, heaters 22 were placed around the reaction tube 21, and the temperature was raised from room temperature to 500°C. After the part where the container 23 was placed reached the reduction treatment temperature, it was held for 2 hours, and then cooled to room temperature to perform the reduction treatment (reduction treatment step).

[0138] The XRD patterns of the obtained composite tungsten oxide particles are shown in Figure 3(B). The obtained XRD patterns are Cs 0.33 We confirmed that it contained only the diffraction peak of WO3.

[0139] Furthermore, TEM and HAADF images of the obtained composite tungsten oxide particles are shown in Figures 7(A) to 7(C). From the TEM image shown in Figure 7(A), it was confirmed that the particle size was approximately 50 nm, and less than 300 nm. From the HAADF-STEM images shown in Figures 7(B) and 7(C), it was confirmed that one entire row of W / Cs was missing, and a region where W / W rows were joined was partially observed.

[0140] Analysis of the Cs / W ratio of these particles using ICP and other methods revealed that the Cs / W ratio decreased further from 0.31 to 0.30 compared to Example 1. The O / W ratio was 2.71.

[0141] Upon examining the lattice constants shown in Figure 3(C), we find that Cs 0.33 It is located approximately 0.015 Å to the lower right of WO3, and is thought to have a hexagonal crystal structure with Cs elimination.

[0142] In other words, although the W deficiencies were almost completely eliminated by the reduction treatment, the Cs / W ratio deviated by about 0.03 from the ideal 0.33 to 0.30, suggesting that the Cs deficiencies remained as a series of Cs defects within the particles. [Example 5] The composite tungsten oxide particles obtained in Example 2 were subjected to reduction treatment using the reduction treatment apparatus 20 shown in Figure 2.

[0143] As shown in Figure 2, under an airflow of 3 vol.% H2 / 97 vol.% Ar, heaters 22 were placed around the reaction tube 21, and the temperature was raised from room temperature to 500°C. After the part where the container 23 was placed reached the reduction treatment temperature, it was held for 2 hours, and then cooled to room temperature to perform the reduction treatment (reduction treatment step).

[0144] The XRD patterns of the obtained composite tungsten oxide particles are shown in Figure 3(B). The obtained XRD patterns are Cs 0.33 We confirmed that it contains only WO3.

[0145] Furthermore, TEM and HAADF images of the obtained composite tungsten oxide particles are shown in Figures 8(A) to 8(C). From the TEM image shown in Figure 8(A), it was confirmed that the particle size was approximately 50 nm to 200 nm, and less than 300 nm. The particles also had facets. From the HAADF-STEM images shown in Figures 8(B) and 8(C), it was confirmed that the W and Cs deficiencies had completely disappeared.

[0146] Analysis of the Cs / W ratio of these particles using ICP and other methods showed that the Cs / W ratio decreased further from 0.37 to 0.34 compared to Example 2. However, the value of 0.34 is 0.01 higher than the theoretical composition ratio of 0.33, and since it is single-phase on the XRD pattern, it is considered that there are virtually no Cs defects. The O / W ratio was 2.83.

[0147] Upon examining the lattice constants shown in Figure 3(C), we find that Cs 0.33 It is located approximately 0.003 Å to the upper left and above WO3 in terms of both the a-axis and c-axis lengths, suggesting that a composite tungsten oxide doped with Cs up to a theoretical ratio of 0.33 was successfully synthesized in nanoparticle form.

[0148] Table 1 shows the semi-quantitative results obtained by XPS measurement of the composite tungsten oxide particles. A VersaProbe II from ULVAC-FI was used for XPS. A monochromatic Al-Kα X-ray source was used, with a beam diameter of 100 μmφ, an X-ray output of 25 W, and a target vacuum of 5.7 × 10⁻⁶. -7 The implementation was carried out at a level below Pa.

[0149] It is known that alkali elements can dopage tungsten to a maximum of 1 / 3 of its molar mass in a hexagonal crystal structure. However, in this example, ratios greater than 0.372 and 0.33 were observed. This suggests that alkali elements may be present in greater quantities on the surface of the particles.

[0150] [Table 1] [Example 6] The composite tungsten oxide particles obtained in Example 3 were subjected to reduction treatment using the reduction treatment apparatus 20 shown in Figure 2.

[0151] As shown in Figure 2, under an airflow of 3 vol.% H2 / 97 vol.% Ar, heaters 22 were placed around the reaction tube 21, and the temperature was raised from room temperature to 500°C. After the part where the container 23 was placed reached the reduction treatment temperature, it was held for 2 hours, and then cooled to room temperature to perform the reduction treatment (reduction treatment step).

[0152] The XRD patterns of the obtained composite tungsten oxide particles are shown in Figure 3(B). In the obtained XRD patterns, Cs 0.33 Diffraction peaks of WO3 and (Cs2O) 0.44 Some diffraction peaks of W2O6ICDD:00-047-0566 were observed.

[0153] Furthermore, TEM and HAADF images of the obtained composite tungsten oxide particles are shown in Figures 9(A) to 9(C). From the TEM image shown in Figure 9(A), it was confirmed that the particle size was approximately 50 nm to 200 nm, and less than 300 nm. From the HAADF-STEM images shown in Figures 9(B) and 9(C), it was confirmed that there were many faceted particles. In addition, the W-defect rows were almost completely gone, but some Cs defects were confirmed in the Cs / W rows.

[0154] Further reduction treatment was performed, but the Cs / W ratio of the powder obtained in this example remained at 0.39. The O / W ratio was 2.77.

[0155] Upon examining the lattice constants shown in Figure 3(C), we find that Cs 0.33 Compared to the lattice constants of WO3, it was confirmed that the a-axis and c-axis lengths were located approximately 0.006 Å to the upper left. Therefore, it is thought that a composite tungsten oxide with Cs doped to a theoretical ratio of 0.33, despite being nanoscale, was synthesized in nanoparticle form. [Example 7] An ink dispersion was prepared using 1% by mass of the composite tungsten oxide particles obtained in Example 4, 1% by mass of an amine-based acrylic copolymer dispersant, and the remainder as methyl isobutyl ketone (hereinafter also referred to as MIBK) as a dispersion medium. The ink was prepared by dispersing the composite tungsten oxide particles and the dispersion medium using 0.3 mmφ ZrO2 beads in a paint shaker for 5 hours and then grinding the mixture.

[0156] The obtained dispersion, which is the ink, was diluted with MIBK to a concentration of 0.02% by mass of composite tungsten oxide particles, and the molar absorption coefficient and transmission profile were measured using a quartz cell with a UV-Vis-NIR spectrophotometer (model: U-4100, manufactured by Hitachi High-Tech Corporation). In the following examples and comparative examples, the spectral characteristics of the dispersion, which is the ink, were measured using the same procedure.

[0157] The transmission profile and molar absorption coefficient of the obtained inks are shown in Figure 11(A) and Figure 11(B), respectively.

[0158] According to the transmission profile with VLT = 80% shown in Figure 11(A), the transmittance at 1355 nm was 13.35%. Furthermore, according to the molar absorption coefficient shown in Figure 11(B), At around 0.93 eV, it showed an absorption of up to approximately 2300 L / mol·cm.

[0159] As is clear from the results in Figure 11(A), it was confirmed that the infrared absorption characteristics were lower compared to the ink of Example 8, which will be described later. The reason for this is thought to be that although the W deficiencies in the composite tungsten oxide particles contained in the ink of this example were almost completely eliminated, the Cs / W ratio was 0.30, which deviates by about 0.03 from the ideal 0.33, and remained as a series of Cs deficiencies inside the particles. [Example 8] An ink dispersion was prepared using 1% by mass of the composite tungsten oxide particles obtained in Example 5, 1% by mass of an amine-based acrylic copolymer dispersant, and the remainder as methyl isobutyl ketone as a dispersion medium. The ink was prepared by dispersing and grinding the composite tungsten oxide particles and dispersion medium using 0.3 mmφ ZrO2 beads with a paint shaker.

[0160] In Example 8, the optical properties of the dispersion were measured by varying the dispersion and grinding time to 0.5 hours, 1 hour, 2 hours, 4 hours, and 5 hours. This allows us to understand the changes that occur during the dispersion and grinding process of the composite tungsten oxide particles from Example 5.

[0161] As Example 8, the permeation profile and molar absorption coefficient of the ink obtained by dispersion and grinding treatment for 5 hours are shown in Figure 11(A) and Figure 11(B), respectively.

[0162] According to the transmission profile with VLT = 80% shown in Figure 11(A), the transmittance at 1355 nm was 8.47%. Furthermore, according to the molar absorption coefficient shown in Figure 11(B), At around 0.93 eV, absorption was observed up to approximately 2500 L / mol·cm.

[0163] Figure 12(A) shows the changes in the measured transmission profiles of inks prepared by varying the grinding time of composite tungsten oxide particles. It was confirmed that the absorption characteristics in the infrared region did not change even when the grinding time was about 2 hours, and that the scattering of light in the visible light region decreased after 2 hours of grinding.

[0164] The particle size distribution measured from TEM images of the composite tungsten oxide powder in the ink obtained after a 5-hour dispersion and grinding treatment is shown in Figures 12(B) and 12(C), respectively. The average particle size was 14.3 nm, and the standard deviation was 2.7 nm. No coarse particles larger than 70 nm were observed.

[0165] Table 1 shows the semi-quantitative results of XPS measurement of the composite tungsten oxide powder in the ink obtained after a 5-hour dispersion and grinding treatment. Although the grinding treatment reduced the Cs / W ratio to 0.307 compared to Example 5, it was confirmed that it was still higher than the Cs / W ratio shown in Comparative Example 2. [Example 9] An ink, which is a dispersion according to Example 9, was obtained in the same manner as in Example 7, except that the composite tungsten oxide particles obtained in Example 6 were used.

[0166] The transmission profile and molar absorption coefficient of the obtained inks are shown in Figure 11(A) and Figure 11(B), respectively.

[0167] According to the transmission profile with VLT = 80% shown in Figure 11(A), the transmittance at 1355 nm was 8.2%. Furthermore, according to the molar absorption coefficient shown in Figure 11(B), At around 0.93 eV, absorption was observed up to approximately 2500 L / mol·cm. [Comparative Example 1] As comparative example 1, Cs 0.33 WO3 powder was synthesized.

[0168] Tungstic acid was added to an aqueous solution of cesium carbonate, stirred, and then the water was dried off by holding the mixture at 100°C for 12 hours to prepare the precursor.

[0169] The precursor has a molar ratio of cesium atoms (Cs) to tungsten atoms (W) of Cs / W = 0.33.

[0170] The obtained precursor was packed into a calcination vessel and calcined at 800°C for 1 hour in a reducing atmosphere with a volume ratio of H2 / N2 = 3 / 97. 0.32 WO3 consisted of coarse particles, even the finest ones being over 100 μm in size. The obtained coarse Cs 0.33 XRD results for WO3 powder: Cs 0.33 The diffraction peak was confirmed to be solely that of WO3. Furthermore, the Cs / W ratio, determined by analysis using ICP, was 0.33.

[0171] In other words, it was not possible to reduce the particle size to 300 nm or less using the solid-phase synthesis method.

[0172] Table 1 shows the semi-quantitative results obtained by XPS measurement of the composite tungsten oxide powder. The Cs / W ratio was 0.400. [Comparative Example 2] An ink dispersion was prepared using 1% by mass of the composite tungsten oxide particles obtained in Comparative Example 1, 1% by mass of an amine-based acrylic copolymer dispersant, and the remainder as methyl isobutyl ketone as a dispersion medium. The ink was prepared by dispersing and grinding the composite tungsten oxide particles, dispersant, and dispersion medium using 0.3 mmφ ZrO2 beads with a paint shaker.

[0173] In Comparative Example 2, the optical properties of the dispersion were measured by varying the dispersion and grinding time to 0.5 hours, 1 hour, 2 hours, 4 hours, 5 hours, and 7 hours. This allows us to understand the changes in the process as the composite tungsten oxide particles of Comparative Example 1 are dispersed and ground.

[0174] Figure 13(A) shows the changes in the measured transmission profile for inks prepared by varying the grinding time of composite tungsten oxide particles. At a grinding time of 0.5 hours, the transmittance in the infrared region was low, around 20%, but by increasing the grinding time to 2 hours or more, the transmittance in the infrared region decreased to around 5%. Further extension of the grinding time reduced the effect of Rayleigh scattering, improving visible transparency. Finally, a practical dispersion was obtained with a grinding time of 7 hours.

[0175] The particle size distribution measured from TEM images of the composite tungsten oxide powder in the ink obtained after a 7-hour dispersion and grinding treatment is shown in Figures 13(B) and 13(C), respectively. The average particle size was 24.6 nm, and the standard deviation was 30 nm.

[0176] Table 1 shows the semi-quantitative results of XPS measurement of the composite tungsten oxide powder in the ink obtained after a 7-hour dispersion and grinding treatment. The Cs / W ratio was 0.283, which is lower than in Example 8, and is presumed to be due to the desorption of Cs from the surface during the long grinding treatment.

[0177] Figure 14 shows the measurement results of the molar absorption coefficient of composite tungsten oxide particles in the dispersion of ink obtained as Comparative Example 2. When the molar absorption coefficient after dispersion was compared with that of the ink from Example 8 in Figure 14(A), Comparative Example 2 showed absorption peaks at 0.8 eV and 1.4 eV, while Example 8 showed an absorption peak at 0.95 eV.

[0178] According to the Drude-Lorentz analysis by Machida et al. in Non-Patent Document 9, Cs prepared by the solid-phase method 0.32 WO 3-yIn the powder, absorption originating from Cs free electrons contributes to the free electron plasmon ⊥ at 0.8 eV and the free electron plasmon / / at 1.0 eV. On the other hand, oxygen vacancies contribute to the free electron ⊥ (0.8 eV) in the xy direction and polarons around 1.4 eV. Therefore, it is thought that the composite tungsten oxide particles prepared in Example 8 showed a large amount of absorption at 0.8 eV and 1.0 eV originating from Cs, and a small amount of absorption originating from oxygen vacancies. [Explanation of Symbols]

[0179] 10 Composite material manufacturing equipment 11. First storage unit 12 Two-fluid nozzle 13 reaction tube 14 filters 15 Ejectors 20 Reduction treatment device 21 reaction tube 21A One side 21B The other mouth 22 Heater 23 Container 24. Composite tungsten oxide particles

Claims

1. A method for producing composite tungsten oxide particles, The composite tungsten oxide particles are General formula M ｘ W ｙ O ｚ (However, element M is represented as 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, I, W is tungsten, O is oxygen, 0.20 ≤ x / y ≤ 0.37, 2.2 ≤ z / y ≤ 3.3) The particle size is 300 nm or less. A raw material preparation step for preparing a raw material containing the aforementioned M element and a tungsten element, The process includes a heat treatment step in which the raw material is heat-treated at a temperature of 500°C or higher using a flame. A method for producing composite tungsten oxide particles, wherein in the raw material preparation step, the raw material is prepared such that the ratio a / b, which is the ratio of the amount of substance a of element M to the amount of substance b of tungsten contained in the raw material, is 2.5% to 15% greater than the ratio x / y, which is the ratio of the amount of substance x of element M to the amount of substance y of tungsten in the target composition of the composite tungsten oxide particles.

2. The method for producing composite tungsten oxide particles according to claim 1, wherein the flame is formed using a mixed gas containing oxygen and hydrocarbons.

3. A method for producing composite tungsten oxide particles according to claim 1 or claim 2, further comprising a reduction treatment step after the heat treatment step, in which the particle is reduced in an atmosphere containing a reducing gas at a temperature in the range of 400°C to less than 700°C.

4. A method for producing composite tungsten oxide particles according to any one of claims 1 to 3, wherein the M element comprises one or more elements selected from Rb and Cs.

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