Method for coating the surface of composite tungsten oxide nanoparticles with a zirconium compound film.

Abstract

This invention provides a method for coating the surface of composite tungsten oxide nanoparticles with a zirconium compound, thereby forming a zirconium compound film of uniform thickness on the surface of composite tungsten oxide nanoparticles. [Solution] A method for forming a film on the surface of composite tungsten oxide fine particles, wherein the film is a zirconium compound, and the method comprises a chemical modification step, a composite tungsten oxide fine particle dispersion step, a zirconium compound film formation step, and a cleaning step, thereby providing a method for forming a zirconium compound film on the surface of composite tungsten oxide fine particles.

Description

Technical Field

[0001] The present invention relates to a method for coating a zirconium compound film on the surface of composite tungsten oxide fine particles.

Background Art

[0002] Hexaboride fine particles, tungsten oxide fine particles, and composite tungsten oxide fine particles are known as near-infrared absorption material fine particles because they have a function of absorbing near-infrared rays while having transparency to visible light. A near-infrared absorption material fine particle dispersion in which such near-infrared absorption material fine particles are dispersed in a medium such as a resin that transmits visible light has been widely applied in fields such as near-infrared absorption (heat ray shielding) window films, near-infrared absorption sheet materials, intermediate films for near-infrared absorption laminated glass, infrared cut filters for electronic materials, and solar heat shielding materials for agriculture.

[0003] These materials such as near-infrared absorption window films, near-infrared absorption sheet materials, and solar heat shielding materials for agriculture are used outdoors due to their applications and are thus exposed to sunlight, moisture, etc. The near-infrared absorption material fine particles contained in these materials may have a reduced near-infrared absorption function when exposed for a long time in a high-humidity and high-temperature environment. The cause is considered to be that water and oxygen contained in the environment come into contact with the near-infrared absorption material fine particles, resulting in oxidation of the compound and desorption of the doping element.

[0004] Therefore, Patent Document 1 discloses that zirconium tetraalkoxide was added to lanthanum hexaboride powder to form zirconium oxide-coated lanthanum hexaboride fine particles. [[ID=Q22]]In addition, Patent Document 2 discloses fine particles in which the surface of composite tungsten oxide fine particles is coated with SiO2, and fine particles in which zirconium tributoxyacetylacetonate becomes a film component (ZrO2 / SiO2).

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2004-204173 [Patent Document 2] Japanese Patent Publication No. 2008-291109 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Conventional techniques for coating near-infrared absorbing material nanoparticles have had problems with the uniformity of the coating film thickness. Specifically, if the coating film thickness is uneven, resulting in areas with thicker film thicknesses, the particle size increases significantly due to the coating process, and the magnitude of light scattering per particle (scattering cross-section) increases. The object of the present invention is to provide a method for coating the surface of composite tungsten oxide fine particles with a zirconium compound, which forms a zirconium compound film of uniform thickness on the surface of the composite tungsten oxide fine particles. [Means for solving the problem]

[0007] In this invention, as a means to solve the above-mentioned problems, A method for forming a coating on the surface of composite tungsten oxide fine particles, The aforementioned coating is a zirconium compound, A chemical modification step involves mixing a zirconium compound precursor that forms the zirconium compound with a chemically modified compound in a chemical modification solvent to obtain a chemically modified mixture. A composite tungsten oxide particle dispersion step is performed to obtain a composite tungsten oxide particle dispersion by dispersing composite tungsten oxide particles in a solvent for dispersion, The present invention provides a method for forming a zirconium compound coating on the surface of composite tungsten oxide microparticles, comprising a zirconium compound coating formation step in which the chemically modified admixture liquid is added to the composite tungsten oxide microparticle dispersion to form a zirconium compound coating on the surface of the composite tungsten oxide microparticles. [Effects of the Invention]

[0008] According to the method for coating the surface of composite tungsten oxide fine particles with a zirconium compound according to the present invention, a uniform film of zirconium compound can be formed on the surface of the composite tungsten oxide fine particles. [Brief explanation of the drawing]

[0009] [Figure 1] This flowchart shows the chemical modification step in the method for forming a zirconium compound film on the surface of composite tungsten oxide nanoparticles according to Example 1. [Figure 2] This flowchart shows the composite tungsten oxide nanoparticle dispersion step and the zirconium compound film formation step of the method for forming a zirconium compound film on the surface of composite tungsten oxide nanoparticles according to Example 1. [Figure 3] This flowchart shows the cleaning step in the method for forming a zirconium compound film on the surface of composite tungsten oxide fine particles according to Example 1. [Figure 4] This is a table summarizing the evaluation results of coated composite tungsten oxide fine particles and coated composite tungsten oxide fine particle aqueous dispersions related to Example 1 and Comparative Example 1. [Modes for carrying out the invention]

[0010] The method for coating the surface of composite tungsten oxide fine particles with a zirconium compound film according to this embodiment will be described in the following order: 1. Composite tungsten oxide fine particles, 2. Method for producing composite tungsten oxide fine particles, 3. Formation of a zirconium compound film on the surface of composite tungsten oxide fine particles, and 4. Method for using the aqueous dispersion of coated composite tungsten oxide fine particles with a zirconium compound film formed on its surface.

[0011] 1. Composite tungsten oxide nanoparticles The composite tungsten oxide fine particles according to this embodiment exhibit a transmittance such as a maximum value in the wavelength range of 350 nm to 600 nm and a minimum value in the wavelength range of 800 nm to 2100 nm when dispersed in a near-infrared absorbing material fine particle dispersion in a medium such as a resin that transmits visible light. More specifically, the wavelength range in which the maximum and minimum values ​​of transmittance occur is described as follows: the maximum value occurs in the wavelength range of 440 nm to 600 nm, and the minimum value occurs in the wavelength range of 1150 nm to 2100 nm. In other words, the maximum value of transmittance occurs in the visible light region, and the minimum value of transmittance occurs in the near-infrared region.

[0012] Generally, tungsten trioxide (WO3) lacks effective free electrons, resulting in poor absorption and reflection properties in the near-infrared region, making it ineffective as an infrared shielding material. While it is known that reducing the oxygen-to-tungsten ratio in tungsten trioxide from 3 generates free electrons, the inventors have discovered that there is a specific range within the tungsten-to-oxygen composition of the tungsten oxide that is particularly effective as a near-infrared absorbing material.

[0013] The composition range of tungsten and oxygen is preferably such that the oxygen-to-tungsten ratio is 3 or less, and furthermore, when the tungsten oxide is denoted as WyOz, the z / y ratio is 2.2 ≤ z / y ≤ 2.999. If the z / y value is 2.2 or greater, it is possible to avoid the appearance of the unwanted WO2 crystalline phase in the tungsten oxide, and chemical stability as a material can be obtained, so it can be applied as an effective near-infrared absorbing material. On the other hand, if the z / y value is 2.999 or less, the required amount of free electrons is generated in the tungsten oxide, resulting in an efficient near-infrared absorbing material.

[0014] In addition, in the tungsten oxide fine particles obtained by micronizing the tungsten oxide, when expressed by the general formula WyOz, the so-called "Magnéli phase" having a composition ratio represented by 2.45 ≦ z / y ≦ 2.999 is chemically stable and has good absorption characteristics in the near-infrared region, so it is preferable as a near-infrared absorption material.

[0015] Furthermore, it is also preferable to add an M element to the tungsten oxide to form a composite tungsten oxide. By adopting this configuration, free electrons are generated in the composite tungsten oxide, and absorption characteristics derived from free electrons are exhibited in the near-infrared region, making it effective as a near-infrared absorption material around a wavelength of 1000 nm.

[0016] Here, from the perspective of stability in the composite tungsten oxide to which the M element is added, the M element is more preferably 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.

[0017] By using the above-described control of the oxygen amount and the addition of an element that generates free electrons in combination for the composite tungsten oxide, a more efficient near-infrared absorption material can be obtained. When the general formula of the near-infrared absorption material obtained by combining the control of the oxygen amount and the addition of an element that generates free electrons is expressed as MxWyOz (where M is the M element, W is tungsten, and O is oxygen), it satisfies the relationship of 0.001 ≦ x / y ≦ 1, preferably 0.20 ≦ x / y ≦ 0.37.

[0018] Here, from the perspective of the stability of the MxWyOz to which the M element is added, the M element is more preferably 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. From the perspective of improving the optical properties and weather resistance as a near-infrared absorbing material, those belonging to alkali metals, alkaline earth metals, transition metal elements, Group 4B elements, and Group 5B elements among the M elements are more preferable.

[0019] Next, the value of z / y indicating the control of the oxygen amount will be described. Regarding the value of z / y, in the infrared absorbing material represented by MxWyOz, in addition to the same mechanism as that of the near-infrared absorbing material represented by WyOz described above, since there is also the supply of free electrons due to the addition amount of the M element described above at z / y = 3.0, 2.2 ≤ z / y ≤ 3.0 is preferable.

[0020] Furthermore, when the above-mentioned composite tungsten oxide fine particles have a hexagonal crystal structure, the transmittance of the fine particles in the visible light region is improved, and the absorption in the near-infrared region is improved. In this hexagonal crystal structure, six octahedrons formed by WO6 units gather to form a hexagonal void (tunnel), and the M element is arranged in the void to form one unit, and a large number of these units gather to form a hexagonal crystal structure.

[0021] In order to obtain the effect of improving the transmittance in the visible light region and the absorption in the near-infrared region according to this embodiment, it is sufficient that the composite tungsten oxide fine particles contain a unit structure (a structure in which six octahedrons formed by WO6 units gather to form a hexagonal void, and the M element is arranged in the void).

[0022] When cations of element M are added to these hexagonal voids, absorption in the near-infrared region is improved. Generally, when element M with a large ionic radius is added, the hexagonal crystal is formed. Specifically, it is preferable to add one or more elements selected from Cs, Rb, K, Tl, In, Ba, Li, Ca, Sr, Fe, and Sn, as this facilitates the formation of the hexagonal crystal. Furthermore, in composite tungsten oxide nanoparticles to which one or more elements selected from Cs and Rb among these M elements with large ionic radii are added, it is possible to achieve both absorption in the near-infrared region and transmission in the visible light region.

[0023] When composite tungsten oxide nanoparticles having a hexagonal crystal structure have a uniform crystal structure, the amount of added element M is 0.001 ≤ x / y ≤ 1, preferably 0.2 ≤ x / y ≤ 0.5, more preferably 0.20 ≤ x / y ≤ 0.37, and most preferably x / y = 0.33. This is because, theoretically, when z / y = 3, x / y = 0.33, so it is thought that the added element M is distributed in all of the hexagonal voids.

[0024] Furthermore, the near-infrared absorbing material fine particles according to this embodiment have a particle diameter of 200 nm or less. From the viewpoint of exhibiting even better infrared shielding characteristics, the particle diameter is preferably 10 nm to 100 nm, more preferably 10 nm to 80 nm, even more preferably 10 nm to 60 nm, and most preferably 10 nm to 40 nm. The best infrared shielding characteristics are exhibited when the particle diameter is in the range of 10 nm to 40 nm. Here, particle size refers to the average diameter of individual near-infrared absorbing material microparticles that are not aggregated, and is the average particle size of near-infrared absorbing material microparticles contained in the near-infrared absorbing material microparticle dispersion described later. On the other hand, the particle size does not include the diameter of aggregates of composite tungsten oxide fine particles and is different from the dispersed particle size.

[0025] The average particle size is calculated from electron microscope images of near-infrared absorbing material microparticles. The average particle size of composite tungsten oxide microparticles contained in a near-infrared absorbing material microparticle dispersion can be determined by measuring the particle size of 100 composite tungsten oxide microparticles from a transmission electron microscope image of a thin section of the dispersion obtained by cross-sectional processing, using an image processing device, and calculating the average value. For cross-sectional processing to obtain the thin section, a microtome, cross-section polisher, focused ion beam (FIB) device, etc., can be used. Note that the average particle size of composite tungsten oxide microparticles contained in a near-infrared absorbing material microparticle dispersion is the average value of the particle sizes of composite tungsten oxide microparticles dispersed in the solid medium that serves as the matrix.

[0026] Furthermore, the near-infrared absorbing material nanoparticle dispersion containing composite tungsten oxide nanoparticles according to this embodiment absorbs light in the near-infrared region, particularly around 1000 nm wavelength, and therefore its transmitted color tone is often blue to green. The particle size of the dispersed near-infrared absorbing material nanoparticles can be selected according to the intended use. First, when used in applications where transparency must be maintained, it is even more preferable that the dispersed particle size be 800 nm or less. This is because particles with a dispersed particle size smaller than 800 nm do not completely block light due to scattering, thus maintaining 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 even more preferable to consider scattering by the particles. Furthermore, the dispersed particle diameter of the near-infrared absorbing material fine particles described above is a concept that includes the diameter of the aggregates of the composite tungsten oxide fine particles, and is a different concept from the particle diameter of the near-infrared absorbing material fine particles according to the embodiment described above.

[0027] When prioritizing the reduction of scattering by these particles, the dispersed particle diameter is preferably 200 nm or less, more preferably 10 nm to 200 nm, and even more preferably 10 nm to 100 nm. The reason for this is that if the dispersed particle diameter is small, the scattering of light in the visible light region with wavelengths of 400 nm to 780 nm due to geometric scattering or Mie scattering is reduced, which prevents the infrared shielding film from becoming cloudy and losing its clear transparency. In other words, when the dispersed particle diameter is 200 nm or less, the above-mentioned geometric scattering or Mie scattering is reduced, and the Rayleigh scattering region is reached. In the Rayleigh scattering region, scattered light is proportional to the sixth power of the dispersed particle diameter, so as the dispersed particle diameter decreases, scattering is reduced and transparency improves. Furthermore, when the dispersed particle diameter is 100 nm or less, the scattered light becomes very small, which is preferable. From the viewpoint of avoiding light scattering, a smaller dispersed particle diameter is preferable, and industrial manufacturing is easy if the dispersed particle diameter is 10 nm or more.

[0028] By setting the dispersed particle size to 800 nm or less, the haze value of the near-infrared absorbing material nanoparticle dispersion, in which near-infrared absorbing material nanoparticles are dispersed in a medium, can be reduced to 10% or less when the visible light transmittance is 85% or less. In particular, by setting the dispersed particle size to 100 nm or less, the haze can be reduced to 1% or less. Furthermore, when considering the scattering of light from a dispersion of near-infrared absorbing material microparticles, it is necessary to take into account the aggregation of the near-infrared absorbing material microparticles and therefore the dispersion particle size should be considered.

[0029] 2. Method for producing composite tungsten oxide fine particles The composite tungsten oxide fine particles represented by the general formula MxWyOz according to this embodiment can be produced by a solid-phase reaction method in which a tungsten compound, which is the starting material for tungsten oxide fine particles, is heat-treated in a reducing gas atmosphere or a mixed gas atmosphere of a reducing gas and an inert gas, or in an inert gas atmosphere. The composite tungsten oxide fine particles obtained after this heat treatment and further micronization by grinding or other processes to a predetermined particle size have sufficient near-infrared absorption capacity and possess desirable properties as near-infrared absorbing fine particles.

[0030] To obtain the composite tungsten oxide fine particles represented by the general formula MxWyOz according to this embodiment, the starting material can be a powder obtained by mixing one or more powders selected from tungsten trioxide powder, tungsten dioxide powder, or tungsten oxide hydrate, or tungsten hexachloride powder, or ammonium tungstate powder, or tungsten oxide hydrate powder obtained by dissolving tungsten hexachloride in alcohol and then drying it, or tungsten oxide hydrate powder obtained by dissolving tungsten hexachloride in alcohol, then adding water to precipitate and drying it, or tungsten compound powder obtained by drying an aqueous solution of ammonium tungstate, or metallic tungsten powder, with a powder of the element M or a compound thereof, in a ratio of 0.20 ≤ x / y ≤ 0.37.

[0031] Furthermore, if the tungsten compound used as the starting material for obtaining the composite tungsten oxide nanoparticles is in the form of a solution or dispersion, each element can be easily and uniformly mixed. From this viewpoint, it is even more preferable that the starting material for the composite tungsten oxide fine particles is a powder obtained by mixing an alcoholic solution of tungsten hexachloride or an aqueous solution of ammonium tungstate with a solution of a compound containing the aforementioned M element, and then drying the mixture. From a similar viewpoint, it is also preferable that the starting material for the composite tungsten oxide fine particles is a powder obtained by mixing a dispersion obtained by dissolving tungsten hexachloride in alcohol, then adding water to form a precipitate, with a powder of the element M or a compound containing the element M, or a solution of a compound containing the element M, and then drying the mixture.

[0032] Examples of compounds containing the aforementioned element M include, but are not limited to, tungstates, chlorides, nitrates, sulfates, oxalates, oxides, carbonates, and hydroxides of element M; any compound that forms a solution is acceptable. Furthermore, when industrially producing the composite tungsten oxide fine particles, using tungsten oxide hydrate powder or tungsten trioxide along with carbonates or hydroxides of element M is a preferred manufacturing method because it does not generate harmful gases during heat treatment or other stages.

[0033] Here, we will describe the heat treatment conditions for composite tungsten oxide fine particles in a reducing atmosphere or in a mixed gas atmosphere of a reducing gas and an inert gas. First, the starting material is heat-treated in a reducing gas atmosphere or in a mixed gas atmosphere of a reducing gas and an inert gas. The heat treatment temperature is preferably higher than the temperature at which the composite tungsten oxide fine particles crystallize. Specifically, a temperature of 500°C to 1000°C is preferred, and 500°C to 800°C is more preferred. If desired, the material may be further heat-treated in an inert gas atmosphere at a temperature of 500°C to 1200°C.

[0034] Furthermore, while the reducing gas is not particularly limited, H2 is preferred. When H2 is used as the reducing gas, its concentration is not particularly limited and can be appropriately selected according to the calcination temperature and the amount of starting material. For example, it is 20 vol% or less, preferably 10 vol% or less, and more preferably 7 vol% or less. This is because if the concentration of the reducing gas is 20 vol% or less, the generation of WO2, which does not have a solar radiation shielding function due to rapid reduction, can be avoided. This heat treatment results in a z / y ratio of 2.2 ≤ ≤ 3.0 in the composite tungsten oxide.

[0035] On the other hand, the method for producing composite tungsten oxides is not limited to solid-phase reaction methods. By setting appropriate production conditions, they can also be produced by thermal plasma methods. Examples of such appropriately set production conditions include the supply rate when supplying raw materials into the thermal plasma, the flow rate of the carrier gas used for supplying raw materials, the flow rate of the plasma gas that maintains the plasma region, and the flow rate of the sheath gas that flows just outside the plasma region.

[0036] The bulk material and particles of the composite tungsten oxide may be micronized via a near-infrared absorbing material microparticle dispersion, as described later. To obtain composite tungsten oxide microparticles from this near-infrared absorbing material microparticle dispersion, the solvent can be removed by a known method. Furthermore, the bulk and particle forms of composite tungsten oxide can also be micronized using dry methods such as jet mills. However, even with dry micronization, it is essential to determine grinding conditions (micronization conditions) that allow for the desired particle size, crystallite size, and a-axis and c-axis lengths of the lattice constants of the resulting composite tungsten oxide. For example, if using a jet mill, one should select a jet mill with appropriate airflow and processing time to achieve the desired grinding conditions.

[0037] 3. Formation of a zirconium compound film on the surface of composite tungsten oxide nanoparticles The inventors of this invention have realized that the wavelength range of light transmitted can be varied by changing the refractive index of the region where the surface of the composite tungsten oxide nanoparticles are in contact. Here, Patent Document 1 describes coating the surface of lanthanum hexaboride nanoparticles with a zirconium compound. However, Patent Document 1 does not describe the thickness of the coating film. On the other hand, Patent Document 2 discloses composite tungsten oxide nanoparticles with a ZrO2 / SiO2 composite coating, in which the surface of composite tungsten oxide nanoparticles that have been pre-coated with SiO2 is coated with ZrO2. However, composite tungsten oxide nanoparticles coated with ZrO2 alone are not disclosed.

[0038] The method for coating the surface of a composite tungsten oxide with a zirconium compound film according to this embodiment allows for the control of uniform film thickness of the zirconium compound film by chemically modifying the zirconium atoms during zirconium compound film formation, thereby providing coated composite tungsten oxide fine particles and a coated composite tungsten oxide fine particle dispersion with excellent optical properties.

[0039] The method for coating the surface of composite tungsten oxide fine particles with a zirconium compound film according to this embodiment will be described in the following order: <1. Chemical modification step>, <2. Composite tungsten oxide fine particle dispersion step>, <3. Zirconium compound film formation step>, and <4. Washing step>.

[0040] <1. Chemical modification process> The chemical modification step involves preparing a chemical modification mixture containing a zirconium compound precursor, a chemical modification compound, and a chemical modification solvent, which forms a zirconium compound film on the surface of composite tungsten oxide nanoparticles. As described later, the zirconium compound precursor is chemically modified by reacting the chemically modified mixture with the zirconium compound precursor.

[0041] As zirconium compound precursors, organic compounds containing zirconium atoms are preferred, and zirconium tetraalkoxides are even more preferred. Examples of zirconium tetraalkoxides include zirconium(IV) tetrabutoxide, tetra-t-butoxyzirconium, tetra-n-ptoxyzirconium, octoxy-tridecoxyzirconium, and tributoxy-monoacetonatozirconium.

[0042] The chemically modified mixture is added to the composite tungsten oxide microparticle dispersion, which will be described later. During this process, the water contained in the composite tungsten oxide microparticle dispersion hydrolyzes the zirconium compound precursor contained in the chemically modified mixture, and a zirconium compound film is formed on the surface of the composite tungsten oxide microparticles. The chemically modified compound plays an important role in this hydrolysis process.

[0043] In other words, chemically modified compounds are compounds that chemically modify zirconium compound precursors and control the reactivity of the zirconium compound. The chemically modified compound acts as a ligand, coordinating to the zirconia atoms of the zirconium compound precursor to perform the chemical modification. Because the coordinate sites of the chemically modified zirconium compound precursor are filled by the ligand, it becomes less susceptible to attack by water. As a result, the hydrolysis of the zirconium compound precursor is controlled by the steric hindrance of the chemically modified compound coordinating to the zirconium atoms, and coated composite tungsten oxide nanoparticles can be obtained in which a zirconium compound coating with a uniform film thickness of 3 nm to 15 nm is formed on the surface of the composite tungsten oxide nanoparticles.

[0044] In contrast, if no chemically modified compound is used in the zirconium compound precursor, that is, if the chemically modified compound is not coordinated to the zirconia atoms of the zirconium compound precursor, a zirconium compound film with a thickness exceeding 100 nm will form on a portion of the surface of the composite tungsten oxide nanoparticles. As a result, the optical properties of the composite tungsten oxide nanoparticles may deteriorate.

[0045] Generally, zirconium tetraalkoxide, a precursor of zirconium compounds, undergoes rapid hydrolysis, making it difficult to form a uniform film thickness on surfaces such as fine particles. The configuration of this embodiment, which uses both a zirconium compound precursor and a chemically modified compound, allows for control of the hydrolysis reaction rate, enabling the formation of a uniform zirconium compound film thickness.

[0046] Examples of chemically modified compounds include chelating agents and amino alcohols such as aminoethanol. Examples of chelating agents include compounds containing two or more nitrogen atoms, such as ethylenediamine, and compounds having two or more carbonyl groups, such as β-diketones. Among these, chelating agent compounds having two or more carbonyl groups in the molecule are preferred, and β-diketone compounds and β-ketoester compounds are even more preferred. Examples of compounds having two or more carbonyl groups include acetylacetone, acetoacetic acid, and acetoacetic acid compounds such as ethyl acetoacetate. The lone pairs of electrons of the nitrogen and oxygen atoms contained in these chemically modified compounds coordinate to the zirconium atoms of the zirconium compound precursor.

[0047] Since the solvent for chemical modification needs to be miscible with the complex tungsten oxide fine particle dispersion described later, alcohols with 4 or fewer carbon atoms, such as methanol, ethanol, propanol, and butanol, are preferred. The solvent for chemical modification may be the same alcohol contained in the solvent of the complex tungsten oxide fine particle dispersion.

[0048] The preferred ratio of zirconium compound precursor to chemically modified compound in the chemically modified mixture is 50 to 350 moles of chemically modified compound per 100 moles of zirconium compound precursor. If the chemically modified compound is present in 50 moles or more, the hydrolysis reaction of the zirconium compound precursor can be controlled. On the other hand, if the chemically modified compound is added in amounts exceeding 350 moles, it may excessively delay the hydrolysis reaction of the zirconium compound precursor.

[0049] To prepare a chemically modified mixture, a zirconium compound precursor, a chemically modified compound, and a chemical modification solvent are mixed together. This mixture can be prepared using known stirring methods such as ultrasound or a stirrer.

[0050] <2. Composite Tungsten Oxide Microparticle Dispersion Process> The composite tungsten oxide microparticle dispersion process is a process for preparing a composite tungsten oxide microparticle dispersion containing composite tungsten oxide microparticles and a solvent for the dispersion.

[0051] The composite tungsten oxide fine particle dispersion contains composite tungsten oxide fine particles in an amount of 0.01% to 20% by mass. The content of composite tungsten oxide fine particles in the composite tungsten oxide fine particle dispersion is acceptable as long as it is a content that allows for dispersion of the composite tungsten oxide fine particles, and industrial dispersion is possible if the content is between 0.01% and 20% by mass. In a dispersion of composite tungsten oxide microparticles, the particle size of the composite tungsten oxide microparticles is preferably 200 nm or less.

[0052] The solvent in the dispersion of composite tungsten oxide fine particles can be any solvent that is miscible with the zirconium compound precursor contained in the chemically modified mixing solution added thereto, or with water used to hydrolyze the zirconium compound precursor. For this reason, the dispersion solvent is preferably an alcohol with 4 or fewer carbon atoms, such as methanol, ethanol, propanol, or butanol.

[0053] Furthermore, aprotic polar solvents such as acetonitrile, N,N-dimethylformamide, and dimethyl sulfoxide can be added to the composite tungsten oxide microparticle dispersion. By adding such aprotic polar solvents, the charge of the composite tungsten oxide microparticles in the dispersion can be stabilized, and the dispersion state can be maintained. From this viewpoint, the content of the aprotic polar solvent in the composite tungsten oxide microparticle dispersion is preferably 22% by mass or more and 63% by mass or less.

[0054] As a method for producing a composite tungsten oxide microparticle dispersion, composite tungsten oxide microparticles with a particle size of 200 nm or less may be mixed with a dispersion solvent, and the composite tungsten oxide microparticles may be dispersed in the liquid by irradiating them with ultrasound using an ultrasonic homogenizer or the like. Alternatively, a composite tungsten oxide microparticle dispersion may be obtained by placing coarse particles of composite tungsten oxide, a dispersion solvent, and beads in a paint shaker, crushing the coarse particles with the paint shaker, and dispersing them in the liquid.

[0055] Alternatively, a water dispersion containing composite tungsten oxide microparticles and water can be prepared in advance, and this water dispersion can be added to the aforementioned dispersion solvent to obtain a composite tungsten oxide microparticle dispersion. The amount of this water dispersion added to the dispersion solvent is determined according to the content of composite tungsten oxide microparticles contained in the composite tungsten oxide microparticle dispersion. When this water dispersion is added to the dispersion solvent to prepare a composite tungsten oxide microparticle dispersion, water will be included in the dispersion. If a large amount of water is included in the composite tungsten oxide microparticle dispersion, the water may separate from the dispersion. If water separates from the composite tungsten oxide microparticle dispersion, it may become impossible to control the hydrolysis of the zirconium compound precursor introduced in the zirconium compound film formation process, and a uniform film of zirconium compound may not be formed on the surface of the composite tungsten oxide microparticles. From this perspective, it is preferable that the amount of water that can be contained in the composite tungsten oxide fine particle dispersion is 1.3% by mass or less.

[0056] <3. Zirconium compound coating process> The zirconium compound coating process involves adding a chemically modified mixing solution to a composite tungsten oxide microparticle dispersion to form a zirconium compound coating on the surface of the composite tungsten oxide microparticles. Before adding the chemical modification mixture to the composite tungsten oxide microparticle dispersion, an alkaline compound or water is added. This addition of alkaline compounds or water is to adjust the pH within the coating reaction system when a zirconium compound film is formed from the zirconium compound precursor in the chemical modification mixture added to the composite tungsten oxide microparticle dispersion. The zirconium compound produced from the hydrolysis of the zirconium compound precursor contains zirconia. The isoelectric point of zirconia is around pH 6, and by ensuring an alkaline pH within the coating reaction system, the aggregation of zirconia produced from the hydrolysis of the zirconium compound precursor can be suppressed.

[0057] Alkaline compounds that can be added to a dispersion of composite tungsten oxide fine particles include weakly basic ammonia, magnesium hydroxide, copper hydroxide, and aluminum hydroxide. Of these, ammonia is preferred from the viewpoint of solubility in water. When using magnesium hydroxide, copper hydroxide, or aluminum hydroxide, it is preferable to add the supernatant liquid obtained by dissolving these alkaline compounds in water as the alkaline compound. On the other hand, amine compounds such as water-soluble ethanolamine and triethanolamine can also be cited as weakly alkaline compounds, but since amine compounds chemically modify the zirconium compound precursor, it is not desirable to add them as weak alkalis.

[0058] The alkali compound can be added to the complex tungsten oxide fine particle dispersion by adding it as an aqueous solution. Furthermore, stirring is preferable when adding the alkali compound to the complex tungsten oxide fine particle dispersion. The concentration of the alkali compound in the composite tungsten oxide microparticle dispersion is preferably 0.028 mol / L or more and 0.056 mol / L or less after the addition of the composite tungsten oxide microparticle dispersion. The addition of the alkali compound to the composite tungsten oxide microparticle dispersion can control the aggregation of the zirconia compound.

[0059] Water can be added after the addition of an alkaline compound to the complex tungsten oxide microparticle dispersion. Adding water to the complex tungsten oxide microparticle dispersion can accelerate the hydrolysis of the zirconium compound precursor. The amount of water added should be sufficient to mix the complex tungsten oxide microparticle dispersion with the water. That is, it can be determined appropriately considering the solubility of the dispersion solvent in water contained in the complex tungsten oxide microparticle dispersion. However, if an aqueous solution of the alkaline compound was used when adding the alkaline compound, the amount of water added should be determined considering the amount of water already contained in the aqueous solution of the alkaline compound.

[0060] Once the alkaline compound and water have been added to the composite tungsten oxide fine particle dispersion, the mixture is ready for the addition of the chemically modified mixture. The chemical modification solution is added dropwise to a dispersion of composite tungsten oxide microparticles to which an alkaline compound and water have been added. The addition of the chemical modification solution initiates the coating reaction of the composite tungsten oxide microparticles. It is preferable to stir the dispersion of composite tungsten oxide microparticles during or immediately after the addition of the chemical modification solution. Furthermore, it is desirable to maintain a constant temperature of the dispersion of composite tungsten oxide microparticles during stirring. Specifically, during stirring, the temperature should be kept constant, selected from 20°C to 80°C, and stirring should be continued for 12 hours or more, more preferably 18 hours or more, and even more preferably 24 hours or more. This is because, even during stirring, unreacted zirconium compound precursors exist in the composite tungsten oxide dispersion to which the chemical modification solution has been added, i.e., in the coating reaction system, and these unreacted zirconium compound precursors undergo a hydrolysis reaction, causing a zirconium compound film to grow on the surface of the composite tungsten oxide microparticles. Here, "maintaining a constant temperature" means maintaining it within ±0.1°C of the selected temperature.

[0061] The amount of chemical modification admixture added dropwise to a composite tungsten oxide dispersion containing an alkaline compound and water can be determined by the amount of composite tungsten oxide microparticles to coat the surface. When forming a zirconium compound film on the surface of composite tungsten oxide microparticles with a particle size of 10 nm to 200 nm, it is preferable to add an excess of 5 parts by mass or more, and more preferably 10 parts by mass or more, per 1 part by mass of composite tungsten oxide microparticles. Adding the zirconium compound precursor in excess of the composite tungsten oxide microparticles is necessary to promote the hydrolysis of the unreacted zirconium compound precursor in the coating reaction system during the stirring described above, and to grow the zirconium compound film on the surface of the composite tungsten oxide microparticles. It is also necessary to form a zirconium compound film on the surface of all the composite tungsten oxide microparticles contained.

[0062] On the other hand, the content of composite tungsten oxide microparticles in the coating reaction system during stirring, to which an alkaline compound, water, and a chemically modified mixture are added to the composite tungsten oxide microparticle dispersion, is preferably 5% by mass or less, from the viewpoint of forming a uniform zirconium compound film of uniform thickness on the surface of the composite tungsten oxide microparticles through the reaction of the zirconium compound precursor. If the content of composite tungsten oxide microparticles in the raw material mixture solution is 5% by mass or less, there will be no variation in the thickness of the zirconium compound film from particle to particle. Furthermore, the content of composite tungsten oxide microparticles in the reaction system can be 0.01% by mass or more.

[0063] In the liquid within the coating reaction system after the coating reaction is complete, there are composite tungsten oxide particles with a zirconium compound coating formed on their surface, and unreacted zirconium compound precursors remaining in the liquid.

[0064] Through these operations, the composite tungsten oxide nanoparticles become coated composite tungsten oxide nanoparticles with a zirconium compound coating of 3 nm to 15 nm thickness formed on their surface. Compared to a composite tungsten oxide particle dispersion without such a coating, a dispersion containing coated composite tungsten oxide nanoparticles achieves a qualitative effect of shifting the wavelength of the transmission peak to longer wavelengths while maintaining almost the same peak transmittance in the visible light region. Similar optical properties are obtained in a dispersion obtained by processing the coated composite tungsten oxide nanoparticle dispersion and dispersing the coated composite tungsten oxide nanoparticles in a medium such as resin.

[0065] On the other hand, if the coating reaction with a zirconium compound precursor is carried out on the surface of the composite tungsten oxide nanoparticles without implementing this embodiment (without using a chemically modified compound), the thickness of the zirconium compound coating may partially exceed 100 nm, making it impossible to form a uniform coating. As a result, the particle size of the composite tungsten oxide nanoparticles increases, and the transmittance in the visible light region of the dispersion of the composite tungsten oxide nanoparticles may decrease significantly.

[0066] <4. Washing Process> The washing process involves centrifugal dehydration washing with alcohol or water onto a dispersion of coated composite tungsten oxide microparticles in which the zirconium compound film formation process has been completed and the zirconium compound film has formed on the surface. This process removes unreacted zirconium compound precursors, oligomeric compounds formed by the bonding of these precursors, and alkali.

[0067] First, the coated composite tungsten oxide microparticle dispersion in the completed coating reaction system is washed with an alcohol such as methanol, ethanol, propanol, or butanol, and then solid-liquid separation is performed by centrifugal dehydration. This can be done by repeating the washing with water multiple times and solid-liquid separation by centrifugal dehydration. As a result, an aqueous dispersion of coated composite tungsten oxide microparticles is obtained, in which unreacted zirconium compound precursors and alkalis are removed, and the final form of the zirconium compound film is uniformly formed on the surface of the coated composite tungsten oxide microparticles dispersed in water.

[0068] 4. Method of using a coated composite tungsten oxide fine particle aqueous dispersion with a zirconium compound film formed on its surface. The aqueous dispersion of coated composite tungsten oxide microparticles obtained in the washing process can be used as is as an aqueous dispersion of coated composite tungsten oxide microparticles with a zirconium compound film formed on its surface. Alternatively, if desired, the aqueous dispersion of coated composite tungsten oxide microparticles can be used after solvent replacement with an organic solvent that is immiscible with water, such as toluene.

[0069] By adding a water-soluble resin (e.g., acrylic resin) to an aqueous dispersion of coated composite tungsten oxide fine particles to create a coating solution, and then applying this coating solution to a transparent substrate such as glass and allowing the resin to harden, a near-infrared absorbing material fine particle dispersion can be obtained in which coated composite tungsten oxide fine particles are dispersed within the resin. [Examples]

[0070] The production of the coated composite tungsten oxide microparticle dispersion according to Example 1 will be described with reference to the flowcharts in Figures 1 to 3, which show the method for forming a zirconium compound film on the surface of composite tungsten oxide microparticles according to the present invention. Figures 1-3 show the steps for forming a zirconium compound film on the surface of composite tungsten oxide fine particles according to Example 1. For convenience, the figures are divided into three parts: Figure 1 shows the chemical modification step, Figure 2 shows the composite tungsten oxide fine particle dispersion step and the zirconium compound film formation step, and Figure 3 shows the washing step. Therefore, the chemical modification mixing solution 1 in Figure 1 and the chemical modification mixing solution 1 in Figure 2 are the same substance, and the coated composite tungsten oxide fine particle dispersion in Figure 2 and the coated composite tungsten oxide fine particle dispersion in Figure 3 are the same substance. Furthermore, the present invention is not limited to the embodiments described herein.

[0071] [Example 1] First, a composite tungsten oxide fine particle aqueous dispersion was prepared using the following procedure. Cesium-composite tungsten oxide with an average particle size of 35 nm was dispersed in water using a paint shaker to prepare aqueous dispersion A of composite tungsten oxide fine particles with a concentration of 19.1% by mass.

[0072] <Chemical modification process> As shown in Figure 1, 7.984 μL of 1-butanol was mixed with 14 μL of zirconium(IV) tetrabutoxide and 2 μL of ethyl acetoethyl acetate. The mixture was then stirred by irradiating it with ultrasound for 1 minute to obtain chemically modified mixture 1 according to Example 1.

[0073] <Composite Tungsten Oxide Microparticle Dispersion Process> As shown in the first half of Figure 2, 6.09 mL of 1-butanol was mixed with 24.451 mL of acetonitrile and 16.4 μm of composite tungsten oxide microparticle aqueous dispersion A, and sonicated for 3 minutes to prepare composite tungsten oxide microparticle dispersion 1.

[0074] <Zirconium compound coating formation process> As shown in the latter half of Figure 2, a screw tube containing the composite tungsten oxide microparticle dispersion 1 was placed in a 25°C constant temperature bath, and stirring was started at 300 rpm using a stirring bar. Next, 76 μL of 28% by mass aqueous ammonia was added to the composite tungsten oxide microparticle dispersion 1, and after 10 minutes, 360 μL of water was added. After 10 minutes, 8 mL of the chemically modified mixture from Example 1 was added dropwise over 480 seconds to the composite tungsten oxide microparticle dispersion 1, which had been mixed with ammonia water and water. The mixture was then stirred for 24 hours while maintaining a temperature of 25°C to obtain a coated composite tungsten oxide microparticle dispersion.

[0075] <Washing process> As shown in Figure 3, the coated composite tungsten oxide fine particle dispersion, after 24 hours of continuous stirring, was centrifuged once with ethanol and twice with distilled water (10,000 rpm, 30 minutes). This was then redispersed with distilled water to prepare the coated composite tungsten oxide fine particle aqueous dispersion according to Example 1.

[0076] [Comparative Example 1] 7.984 μL of 1-butanol was mixed with 14 μL of zirconium(IV) tetrabutoxide, and the mixture was stirred by irradiating with ultrasound for 1 minute to prepare coating mixture 1 according to Comparative Example 1. A coated composite tungsten oxide fine particle aqueous dispersion according to Comparative Example 1 was prepared by the same procedure as in Example 1, except that the coated mixing solution 1 according to Comparative Example 1 was used in place of the chemically modified mixing solution 1 according to Example 1.

[0077] [Evaluation and Discussion] The solvent was removed from the aqueous dispersion of coated composite tungsten oxide microparticles according to Example 1 to obtain coated composite tungsten oxide microparticles according to Example 1. Similarly, the solvent was removed from the aqueous dispersion of coated composite tungsten oxide microparticles according to Comparative Example 1 to obtain coated composite tungsten oxide microparticles according to Comparative Example 1.

[0078] Figure 4 shows the TEM images of the coated composite tungsten oxide microparticles obtained in Example 1 and Comparative Example 1, the particle size distribution and average particle diameter of the coated composite tungsten oxide microparticles in the aqueous dispersion of coated composite tungsten oxide microparticles in Example 1 and Comparative Example 1, and the measurement results of the light transmission profile of the aqueous dispersion of coated composite tungsten oxide microparticles in Example 1 and Comparative Example 1. For TEM, a JEOL JEM-2100 was used. Particle size distribution and average particle size were measured using a zeta potential meter (Malvern Zetasizer nano ZS90) capable of measuring zeta potential and particle size. The light transmission profile was measured using a JASCO V-670 UV-Vis-Near-Infrared Spectrophotometer. Note that the transmission profile of uncoated CsWO in the transmission profile column of Figure 4 is the transmission profile of a dispersion obtained by diluting composite tungsten oxide fine particle aqueous dispersion A with water and adjusting it so that the transmittance peak in the visible light region is 90% or higher.

[0079] From the TEM image in Figure 4, it was found that the coated composite tungsten oxide nanoparticles according to Example 1, in which the zirconium atoms were chemically modified during the formation of the zirconium compound film, were uniformly coated with a zirconium compound film thickness of 5.5 nm to 9.4 nm. In contrast, it was found that the coated composite tungsten oxide nanoparticles according to Comparative Example 1, in which the zirconium atoms were not chemically modified during the formation of the zirconium compound film, were thicker, with a zirconium compound film thickness of 8.0 nm to 21.0 nm or more, and were non-uniformly coated. Furthermore, regarding particle size distribution and average particle diameter, the coated composite tungsten oxide microparticles in the aqueous dispersion of coated composite tungsten oxide microparticles according to Example 1 had a particle size peak at approximately 100 nm and an average particle diameter of 105 nm, while the coated composite tungsten oxide microparticles in Comparative Example 1 had a particle size peak at approximately 200 nm and an average particle diameter of 209.6 nm.

[0080] Furthermore, the light transmission profile of the coated composite tungsten oxide microparticle aqueous dispersion according to Example 1 showed almost the same transmittance in the visible light region and a lower transmittance in the near-infrared region compared to the uncoated composite tungsten oxide microparticle aqueous dispersion. In contrast, the light transmission profile of the coated composite tungsten oxide microparticle aqueous dispersion according to Comparative Example 1 showed a significantly lower transmittance in the visible light region and almost the same transmittance in the near-infrared region compared to the uncoated coated composite tungsten oxide microparticle aqueous dispersion. Furthermore, it was confirmed that in the coated composite tungsten oxide fine particle aqueous dispersion according to Example 1, the wavelength of the transmission profile peak was shifted to the longer wavelength side by more than 30 nm compared to the uncoated dispersion. The same was true for Comparative Example 1.

[0081] From the above results, it was found that the method for forming a zirconium compound coating on the surface of composite tungsten oxide fine particles according to the present invention can produce coated composite tungsten oxide fine particles in which a uniform zirconium compound coating with a thickness of 3 nm to 15 nm is formed on the surface of the composite tungsten oxide fine particles. Furthermore, it was found that the manufactured coated composite tungsten oxide microparticle dispersion exhibited superior optical properties, showing nearly the same transmittance in the visible light region and lower transmittance in the near-infrared region compared to an uncoated composite tungsten oxide microparticle aqueous dispersion.

Claims

1. A method for forming a coating on the surface of composite tungsten oxide fine particles, The aforementioned coating is a zirconium compound, A chemical modification step involves mixing a zirconium compound precursor that forms the zirconium compound with a chemically modified compound in a chemical modification solvent to obtain a chemically modified mixture. A composite tungsten oxide particle dispersion step is performed to obtain a composite tungsten oxide particle dispersion by dispersing composite tungsten oxide particles in a solvent for dispersion, A method for forming a zirconium compound coating on the surface of composite tungsten oxide microparticles, comprising a zirconium compound coating formation step of adding the chemically modified admixture liquid to the composite tungsten oxide microparticle dispersion to form a zirconium compound coating on the surface of the composite tungsten oxide microparticles.

2. A method for forming a zirconium compound coating on the surface of composite tungsten oxide fine particles according to claim 1, wherein the chemically modified compound is a chelating agent.

3. A method for forming a zirconium compound film on the surface of composite tungsten oxide fine particles according to claim 1 or 2, wherein the zirconium compound film on the composite tungsten oxide fine particles has a thickness of 3 nm or more and 15 nm or less.

4. A method for forming a zirconium compound film on the surface of composite tungsten oxide fine particles according to claim 1 or 2, wherein the zirconium compound precursor is zirconium tetraalkoxide.

5. The method for forming a zirconium compound film on the surface of composite tungsten oxide fine particles according to claim 1 or 2, wherein the chemically modified compound is a chelating agent having two or more carbonyl groups in its molecule.

6. The method for forming a zirconium compound film on the surface of composite tungsten oxide fine particles according to claim 5, wherein the chelating agent is a chemically modified compound selected from acetylacetone, acetoacetic acid, and acetate acetate.

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