Sodium-type hexagonal tungsten oxide particles, their manufacturing method, heat-shielding filler, and heat-shielding paint.

Abstract

A method for producing sodium-doped hexagonal tungsten oxide particles, and sodium-type hexagonal tungsten oxide particles are provided. [Solution] The method includes a preparation step of adding hydrochloric acid to an aqueous sodium tungstate solution to adjust the pH to acidic to obtain a prepared solution, a dialysis step of performing electrodialysis on the prepared solution to remove chloride ions and obtain a removed solution, and a hydrothermal synthesis step of heating the removed solution under pressure. The preparation step may also include a primary treatment step of adding hydrochloric acid to adjust the pH to 3 or more and 5 or less to obtain a primary treatment solution, a standing step of letting the primary treatment solution stand for 5 to 20 hours to obtain a standing treatment solution, and a secondary treatment step of adding hydrochloric acid to the standing treatment solution to adjust the pH to 0.5 or more and 1.0 or less to obtain a prepared solution. Furthermore, sodium-type hexagonal tungsten oxide particles have the general formula W 1-X Na X O 3ーY (X = 0.01 to 0.5, Y = 0 to 1.5).

Description

[Technical Field]

[0001] This invention relates to sodium-type hexagonal tungsten oxide particles, a method for producing the same, and a heat-shielding coating. [Background technology]

[0002] Conventionally, the following materials with light absorption properties have been proposed by adding various elements to tungsten oxide. For example, Patent Document 1 (Japanese Patent No. 7280647) describes a UV-C absorber that can absorb UV-C from near-ultraviolet (UV) light while preventing yellowing of wall materials, and also does not cause blue discoloration due to photochromism, as well as a method for producing the main component of the UV-C absorber and a yellowing-preventing coating.

[0003] The method for producing the main component of the UV-C absorber described in Patent Document 1 is characterized by dissolving sodium tungstate in water, adding acid to prepare an aqueous solution of sodium polytungstate with a pH of 3.5 to 7, drying the prepared aqueous solution of sodium polytungstate to produce solid prepared sodium polytungstate, and using this as the main component of the UV-C absorber.

[0004] Patent document 2 (International Publication No. 2023 / 190758) proposes composite tungsten oxide particles containing composite tungsten oxide. The composite tungsten oxide particles described in Patent Document 2 are represented by the general formula MxWyOz (where M is 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, 0.20≦x / y≦0.37, 2.2≦z / y≦3.3), have a hexagonal crystal system, and when the composite tungsten oxide particles are observed from the (010) plane, the occupancy rate of the length of the edges formed by the planes parallel to the c axis among the edges surrounding the (010) plane is 60% or more.

[0005] Patent Document 3 (Japanese Patent Publication No. 2023-155281) proposes a composite tungsten film that maintains transparency in the visible light range while reflecting and shielding infrared light, i.e., a heat shielding function through heat insulation, in addition to having high film smoothness.

[0006] The composite tungsten oxide fine particle dispersion described in Patent Document 3 is a composite tungsten oxide film consisting only of a layer whose main component is a composition represented by the general formula MxWyOz (where M is one or more elements selected from alkali metals, alkaline earth metals, Fe, In, Tl, and Sn, W is tungsten, and O is oxygen), and a sputtered and heat-treated film has been proposed with a transmittance of 50% or more at a wavelength of 550 nm, a transmittance of 30% or less at a wavelength of 1400 nm, and a reflectance of 35% or more at a wavelength of 1400 nm.

[0007] Patent document 4 (Japanese Patent Publication No. 2010-280551) describes tungsten oxide fine particles that allow for control of particle size, are non-aggregating, have excellent dispersibility, and have a uniform particle morphology, as well as a method for producing the same.

[0008] The method for producing tungsten oxide fine particles described in Patent Document 4 is to produce by subjecting a solution or slurry having a pH of less than 7 containing a tungsten compound to a hydrothermal reaction under conditions of 200 °C or higher.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0010] In recent years, the development of light-absorbing materials having the property of absorbing light has been expanding and is widely used in applications such as window materials for buildings and vehicles, and solar radiation shielding materials. Particularly, tungsten oxide has been developed to have absorption performance for visible light or infrared rays by adding elements, but there has been a problem that it is difficult to produce particles because its production method is special. Further, when the crystal structure of tungsten oxide particles is hexagonal, the transmittance of visible light is improved while the light absorption in the near infrared is increased, so it is useful as a heat shielding material. Conventionally, there have been many examples of doping cesium (Cs) and rubidium (Rb) into hexagonal tungsten oxide, but the production method of sodium (Na)-doped tungsten oxide particles has not been sufficiently developed and their physical properties have not been clarified. In recent years, as energy conservation has become a social issue, a heat shielding material with better heat shielding properties is required, and tungsten oxide particles with excellent light absorption performance in the infrared region are needed.

[0011] The UV-C absorber described in Patent Document 1 is produced by dialysis and drying of an aqueous solution of sodium polytungstate, but the resulting substance is a composite material in which various crystalline structures of the sodium polytungstate series are mixed. Therefore, it does not produce tungsten oxide particles having a hexagonal crystalline structure.

[0012] The tungsten oxide particles described in Patent Document 2 disclose a method for producing hexagonal composite tungsten oxide particles. While the specification states that various metals M can be doped, and Na is listed as part of the enumeration, it has not actually been possible to produce sodium-doped hexagonal tungsten oxide particles. Furthermore, Patent Document 2 has the problem of having a complex manufacturing process that requires high levels of control, such as heat treatment of the raw materials in aerosol form at a temperature of 1000°C or higher before reduction treatment.

[0013] The tungsten oxide described in Patent Document 3 is a film created by sputtering and is not suitable for multi-purpose application like particles. Furthermore, according to the examples, when tungsten oxide is doped with Cs (cesium) as the doping element M, it has a crystalline structure that includes hexagonal crystals, but when it is doped with Na (sodium), it has a crystalline structure that does not include hexagonal crystals (Examples 11, 12). Thus, even if Cs and Na are listed equally as dopants in the specification, a method for producing hexagonal tungsten oxide doped with Na using a process that facilitates mass production has not been sufficiently established.

[0014] According to the method for producing tungsten oxide fine particles described in Patent Document 4, sodium hydroxide and the like are added to a tungstic acid slurry to adjust the pH to 3.6-7.0 and a hydrothermal reaction is carried out under conditions of 200°C-300°C. However, in all cases, monoclinic tungsten oxide is obtained, and sodium-doped hexagonal tungsten oxide particles have not been obtained.

[0015] As described above, the methods for producing tungsten oxide particles disclosed in Patent Documents 1 to 5 do not disclose a method for producing sodium-doped hexagonal tungsten oxide particles.

[0016] The main object of the present invention is to provide a method for producing sodium-doped hexagonal tungsten oxide particles, and sodium-type hexagonal tungsten oxide particles. Another object of the present invention is to provide sodium-type hexagonal tungsten oxide particles with excellent infrared absorption properties. [Means for solving the problem]

[0017] (1) A method for producing sodium-type hexagonal tungsten oxide particles that conform to a single plane includes a preparation step of adding hydrochloric acid to an aqueous sodium tungstate solution to adjust the pH to acidic and obtain a prepared solution; a dialysis step of performing electrodialysis on the prepared solution to remove chloride ions and obtain a removed solution; and a hydrothermal synthesis step of heating the removed solution.

[0018] Tungsten oxide particles tend to absorb infrared light in both tetragonal and cubic forms, but the position of light absorption is further in the infrared region for tetragonal particles than for cubic particles, and even further in the infrared region for hexagonal particles than for tetragonal particles. Therefore, by using hexagonal tungsten oxide particles, it is possible to improve light transmittance in the visible region while increasing light absorption in the infrared region (heat ray absorption performance). Conventionally, attempts have been made to form hexagonal tungsten oxide particles by doping them with cesium (Cs), but a method for obtaining hexagonal tungsten oxide particles doped with sodium (Na) had not been clarified.

[0019] According to a method for producing sodium-type hexagonal tungsten oxide particles that follow a single plane, first, hydrochloric acid is added to an aqueous solution of sodium tungstate to make the pH acidic, thereby preparing a solution containing polytungstate ions, chloride ions, and sodium ions <Preparation step>. Next, electrodialysis is performed on this prepared solution to obtain a decontaminated solution from which chloride ions (and some sodium ions) have been removed. By thoroughly performing electrodialysis and desalting, an aqueous solution of sodium polytungstate is obtained <dialysis step>. Furthermore, if water is removed from the decontaminated solution, amorphous structures of the sodium polytungstate series will precipitate. Then, by pressurizing the removal solution at a predetermined pressure and heating it at a predetermined temperature, hexagonal tungsten oxide crystals grow, and these crystals precipitate in a sodium-doped state (hydrothermal synthesis process).

[0020] The hexagonal tungsten oxide particles obtained in this way generate free electron plasmons due to sodium doping, exhibiting excellent light absorption performance in the infrared region. In particular, while most hexagonal tungsten oxides doped with alkali metals (such as Cs) have previously intercalated between the hexagonal layers, the sodium-type hexagonal tungsten oxide particles obtained by the manufacturing method of the present invention have sodium, which has a large ionic radius, dissolved in the hydrothermal synthesis and substituted into the tungsten sites of the hexagonal crystal. This results in a larger lattice constant, making it possible to obtain tungsten oxide particles with excellent infrared absorption. Therefore, it is useful as an energy-saving heat-shielding material. Furthermore, because it has high infrared absorption efficiency, it is also useful as a heat source material that utilizes the heat generated from absorbed infrared radiation.

[0021] (2) The method for producing sodium-type hexagonal tungsten oxide particles according to the second invention is a method for producing sodium-type hexagonal tungsten oxide particles according to one aspect, wherein the pH of the prepared solution may be 0.5 or more and 1 or less in the preparation step.

[0022] The pH of the prepared solution containing polytungstate ions does not change significantly even after desalting during the preparation dialysis process. Therefore, by suitably adjusting the pH of the prepared solution, the pH in the hydrothermal synthesis process becomes suitable, and hexagonal tungsten oxide particles can be grown favorably.

[0023] (3) The method for producing sodium-type hexagonal tungsten oxide particles according to the third invention is one aspect of the method for producing sodium-type hexagonal tungsten oxide particles according to the second invention, and the preparation step may include a primary treatment step of adding hydrochloric acid to raise the pH to 3 or more and 5 or less to obtain a primary treatment solution, a standing step of letting the primary treatment solution stand for 5 hours or more and 20 hours or less to obtain a standing treatment solution, and a secondary treatment step of adding hydrochloric acid to the standing treatment solution to raise the pH to 0.5 or more and 1.0 or less to obtain a prepared solution.

[0024] This makes it possible to grow hexagonal tungsten oxide particles with small particle size and a narrow particle size distribution. Even when hydrochloric acid is added as needed during the preparation process to lower the pH and obtain the prepared solution, hexagonal tungsten oxide particles can be obtained, but the average particle size D 50 The particle size may become larger or the particle size distribution may widen. Through diligent analysis by the inventor, it was found that one of the causes is the generation of secondary particle clusters (particle size approximately 37 μm) during crystal growth, and further, that the generation of clusters is correlated with the method of adding hydrochloric acid in the preparation process. In the preparation process, following the steps of primary treatment, standing, and secondary treatment, hydrogen ions from hydrochloric acid are slowly incorporated into polytungstate ions, preventing the formation of cluster precursors. After desalting, hydrothermal synthesis preferably proceeds, yielding only small-particle hexagonal tungsten oxide particles without clusters. Although the detailed mechanism by which cluster precursors can be prevented is not clear, it is thought that during the standing process, hydrogen ions are incorporated into polytungstate ions, causing the ions to gradually associate with each other. In fact, if the steps of the primary treatment, standing, and secondary treatment are not followed in the preparation process, the prepared solution may take on a yellowish tint, and this yellow component is thought to be the cause (precursor) of cluster formation.

[0025] (4) The method for producing sodium-type hexagonal tungsten oxide particles according to the fourth invention is a method for producing sodium-type hexagonal tungsten oxide particles according to any one of the third inventions from one aspect onward, wherein in the hydrothermal synthesis step, the pressure for pressurizing the removal liquid is 0.1 MPa or more and 1.5 MPa or less, and the heating temperature is 100°C or more and 250°C or less.

[0026] By hydrothermally synthesizing an aqueous solution of sodium polytungstate under predetermined temperature and pressure conditions, hexagonal tungsten oxide can be successfully grown. The hexagonal tungsten oxide particles obtained in this way precipitate as rod-shaped particles and exhibit excellent crystallization properties, resulting in particles with high infrared absorption efficiency.

[0027] (5) The method for producing sodium-type hexagonal tungsten oxide particles according to the fifth invention is a method for producing sodium-type hexagonal tungsten oxide particles according to any of the fourth inventions from one perspective, wherein in the hydrothermal synthesis step, the pressure for pressurizing the removal liquid is 0.1 MPa or more and 1.5 MPa or less, the heating temperature is 130°C or more and 250°C or less, and the heated removal liquid may be left to stand while maintaining its temperature for 30 minutes or more and 2 hours or less.

[0028] This allows for a reduction in the particle size of secondary particles and suppresses the formation of clusters, which are aggregates of secondary particles. Therefore, it is possible to efficiently grow sodium-type hexagonal tungsten oxide particles with a narrow particle size distribution and small particle size.

[0029] (6) The method for producing sodium-type hexagonal tungsten oxide particles according to the sixth invention is a method for producing sodium-type hexagonal tungsten oxide particles according to any of the sixth inventions from one perspective, wherein the average value of the light absorption coefficient at wavelengths of 780 nm to 2500 nm may be 1.2 times or more and 1.5 times or less than that of Cs-doped hexagonal tungsten oxide particles.

[0030] As a result, the Na-doped product has a light absorption coefficient (KM value) in the near-infrared region (780nm to 2500nm) that is 1.2 times to 1.5 times (NIR) higher than that of the Cs-doped product. NaWO3 ÷NIR CsWO3 ) It can be made higher. Therefore, compared to Cs-doped tungsten oxide particles used as heat shielding materials, these particles can have even higher infrared absorption capabilities. Consequently, they can effectively block heat rays, resulting in a more energy-efficient heat shielding material. Furthermore, their high infrared absorption efficiency makes them a superior heat source material.

[0031] (7) Sodium-type hexagonal tungsten oxide particles that follow other orientations have the general formula W 1-X Na X O 3ーY (X = 0.01 to 0.5, Y = 0 to 1.5).

[0032] In this general formula, the value X represents the amount of Na (sodium) doping added to W (tungsten), and the value Y represents the amount of oxygen vacancy in O (oxygen). Conventional hexagonal tungsten oxide doped with alkali metals (such as Cs) mostly intercalates into the free channels of the hexagonal crystal, but the sodium-type hexagonal tungsten oxide particles of the present invention are incorporated by substituting sodium, which has a large ionic radius, into the tungsten sites of the hexagonal crystal. By substituting Na (sodium) into the sites of hexagonal tungsten (W), particles with high infrared absorption capacity can be created. Therefore, heat rays can be effectively blocked, resulting in a more energy-efficient heat shielding material. Furthermore, due to its high infrared absorption efficiency, it can also be used as a superior heat source material.

[0033] (8) The sodium-type hexagonal tungsten oxide particles according to the eighth invention are sodium-type hexagonal tungsten oxide particles that conform to other aspects and may include rod-shaped crystals.

[0034] This results in particles with excellent crystalline properties and high infrared absorption efficiency.

[0035] (9) Furthermore, a heat-shielding filler according to another aspect contains sodium-type hexagonal tungsten oxide particles according to any eighth invention from another aspect.

[0036] This allows for the imparting of high infrared absorption capacity to the substrate when it is mixed in.

[0037] (10) Furthermore, a heat-shielding coating that conforms to other aspects contains sodium-type hexagonal tungsten oxide particles that conform to any of the ninth inventions from other aspects.

[0038] This allows for the creation of a coating with high infrared absorption capacity by applying it to a structure or mixing it with other materials. Therefore, even when heat rays are irradiated onto a coated structure, it is possible to prevent the heat rays from penetrating into the structure, thereby effectively preventing the structure from overheating. Similarly, it is possible to prevent heat rays from leaking out of the structure. Therefore, by applying this heat-shielding paint to a building, for example, the intrusion of heat rays contained in sunlight is effectively prevented, resulting in a building with excellent cooling efficiency. [Brief explanation of the drawing]

[0039] [Figure 1] These are SEM images of sodium-type hexagonal tungsten oxide particles from Examples 1 and 2. [Figure 2] These are SEM images of sodium-type hexagonal tungsten oxide particles from Examples 4 and 7. [Figure 3] This is an SEM image of the sodium-type hexagonal tungsten oxide particles from Example 8. [Figure 4] This is an SEM image of Cs and Ni-doped tungsten oxide particles. [Figure 5] These are the X-ray diffraction measurement results for sodium-type hexagonal tungsten oxide particles of Examples 1 to 7. [Figure 6] These are the results of light transmission measurements of tungsten oxide particles in Example 1 and Comparative Examples 1 and 2. [Figure 7] These are the results of the light absorption measurements of tungsten oxide particles in Example 1 and Comparative Examples 1 and 2. [Figure 8] These are the results of particle size distribution measurements for tungsten oxide particles in Examples 1-5. [Figure 9] These are the results of particle size distribution measurements for tungsten oxide particles in Examples 6-9. [Figure 10] This is a schematic diagram illustrating an electrodialysis machine. [Modes for carrying out the invention]

[0040] The manufacturing method of this embodiment will be described in detail below. Preferred embodiments of the present invention are as follows, but the present invention is not limited thereto. Furthermore, at least a part of the configuration of each embodiment can be appropriately combined with a part of other embodiments without departing from the spirit and scope of the present invention.

[0041] (Method for producing tungsten oxide particles) The method for producing tungsten oxide particles according to an embodiment of the present invention will be described in detail below. The method for producing tungsten oxide particles according to this embodiment includes the following five steps. (1) Dissolution step: Dissolve sodium tungstate in water. This prepares an aqueous solution of sodium tungstate. (2) Preparation step: Acid is added to the sodium tungstate aqueous solution obtained in the dissolution step to adjust the pH to acidic and obtain the prepared solution. (3) Dialysis process: The prepared solution is dialyzed in an electrodialysis machine until the flow rate is 20 mS / cm or less to create the removed solution. (4) Hydrothermal synthesis step: The removal liquid is heated under pressure and allowed to stand for a predetermined time to carry out the hydrothermal reaction. (5) Washing process: The precipitate obtained in the hydrothermal synthesis process is washed with water and then dried to obtain tungsten oxide particles. The following details each step.

[0042] (melting process) A sodium tungstate solution is prepared by dissolving sodium tungstate in water. The concentration of sodium tungstate in the aqueous solution is preferably 0.01 mol / L to 0.9 mol / L, more preferably 0.05 mol / L to 0.7 mol / L, and even more preferably 0.01 mol / L to 0.5 mol / L. A concentration above the lower limit allows for the acquisition of sufficient particles, while a concentration below the upper limit allows for a favorable crystal growth rate and the acquisition of particles with excellent crystallinity. The water temperature of the sodium tungstate aqueous solution during the dissolution process should be 20-25°C. Furthermore, the preferred pH of the sodium tungstate aqueous solution is approximately 8.5-8.8.

[0043] (Preparation process) In the preparation process, the pH of the aqueous solution is adjusted by adding an acid dropwise to the sodium tungstate aqueous solution. The acid used in pH adjustment may be either a strong acid or a weak acid, and may be an inorganic or organic acid. Examples of inorganic acids include hydrochloric acid, sulfuric acid, and nitric acid, while examples of organic acids include citric acid, and among these, hydrochloric acid can be preferably used. By using hydrochloric acid, the precipitation of polymorphic substances of sodium polytungstate can be prevented, and hexagonal tungsten oxide particles can be preferably obtained. Furthermore, by using hydrochloric acid, unwanted ions and other substances are removed in the removal solution after the dialysis process (desalting), and hexagonal tungsten oxide particles can be suitably formed. When adding the acid dropwise, it is preferable to add the sodium tungstate aqueous solution while stirring it with a stirrer or the like, and the stirrer speed is preferably 100 rpm to 800 rpm, and more preferably 200 rpm to 400 rpm.

[0044] The lower limit of the pH of the prepared sodium tungstate aqueous solution is preferably 0.1 or higher, more preferably 0.2 or higher, and even more preferably 0.3 or higher. The upper limit of the pH is preferably 2.5 or lower, more preferably 1.0 or lower, and even more preferably 0.5 or lower. This prevents the generation of polymorphic materials, which are mixtures and complexes of various crystalline structures in the sodium polytungstate series, during hydrothermal synthesis. Therefore, hexagonal crystal growth can be favorably performed, and hexagonal tungsten oxide particles with excellent crystallinity can be obtained.

[0045] In the preparation step, hydrochloric acid may be added dropwise to an aqueous sodium tungstate solution to adjust the pH to a predetermined value. However, it is preferable to first add hydrochloric acid dropwise to an aqueous sodium tungstate solution to obtain a primary treatment solution (primary treatment step), then allow the primary treatment solution to stand at room temperature for a predetermined time (standing step), and then add hydrochloric acid again to obtain the prepared solution (secondary treatment step). This makes it less likely for clusters to form during hydrothermal synthesis, and allows for the production of hexagonal tungsten oxide particles with a small average particle size and a narrow particle size distribution. In the primary treatment step, it is preferable that the pH of the primary treatment solution be between 2 and 5 by dropping hydrochloric acid, and more preferably between 3 and 4. In the settling process, the settling time is preferably 5 hours or more and 24 hours or less, and more preferably 10 hours or more and 17 hours or less. In the secondary treatment step, it is preferable that the pH of the primary treatment solution be adjusted to 0.2 or higher and 1.5 or lower by adding hydrochloric acid dropwise, and more preferably to 0.5 or higher and 1.0 or lower. This prevents the formation of cluster precursors, allowing for the acquisition of only small-particle-sized hexagonal tungsten oxide particles without clusters.

[0046] (dialysis process) By dialyzing the prepared solution obtained in the preparation process using an electrodialysis machine, a desalted solution can be obtained from which excess chloride ions and sodium ions have been removed (desalted). As shown in Fig. 10, an electrodialysis apparatus forms a desalination chamber and a concentration chamber by arranging a cation exchange membrane that permeates Na + and an anion exchange membrane that permeates Cl - between a pair of electrodes. Into this desalination chamber, an aqueous sodium tungstate solution prepared is introduced, and electricity is applied to a pair of electrodes to perform desalination. In this case, it is preferable to perform dialysis until the electrical conductivity of the desalinated solution becomes 100 mS / cm or less, more preferably until it becomes 50 mS / cm or less, and even more preferably until it becomes 20 mS / cm or less.

[0047] When the pH of the aqueous sodium tungstate solution in the adjustment step was changed, the amounts of chloride ions and sodium ions before and after electrodialysis were measured, and the results were as follows. (Measurement method) [Table 1]

[0048] From these results, it was confirmed that chloride ions were sufficiently removed by the dialysis step and hardly remained in the removal solution. In this case, the concentration of the remaining chloride ions is preferably 1,000 ppm or less, more preferably 500 ppm, and even more preferably 200 ppm or less. Also, it is preferable that the removal solution after dialysis contains sodium ions at a predetermined concentration, and the Na + concentration is preferably 5,000 ppm or more and 100,000 ppm or less, more preferably 10,000 ppm or more and 50,000 ppm or less, and even more preferably 14,000 ppm or more and 25,000 ppm or less. Thereby, sodium can be suitably doped into tungsten hexoxide.

[0049] (Hydrothermal synthesis step) In the hydrothermal synthesis process, crystal growth is carried out in high-temperature, high-pressure water to obtain hexagonal tungsten oxide particles. In this embodiment, sodium-doped hexagonal tungsten oxide particles can preferably promote crystal growth, and as shown in Figure 1, the particles contain many rod-shaped hexagonal tungsten oxide crystals. Hydrothermal synthesis reactions, carried out under high temperature and high pressure conditions where the temperature is above the boiling point and the pressure is above the saturated vapor pressure, result in unique reactions that differ from those at room temperature and atmospheric pressure, allowing for crystal growth. In the hydrothermal synthesis step of this embodiment, the temperature of the reaction field is preferably 100°C or higher, more preferably 110°C or higher, and even more preferably 130°C or higher. The upper limit of the heating temperature is 400°C or lower, preferably 300°C or lower, more preferably 250°C or lower, and even more preferably 200°C or lower. Furthermore, the pressure of the reaction field in the hydrothermal synthesis process is preferably at a lower limit of 1 atmosphere or more, and more preferably at 10 atmospheres or more. Furthermore, when the reaction field temperature in hydrothermal synthesis is 110°C to 130°C, the pressure is preferably 0.1 MPa to 0.3 MPa, and when the temperature is 150°C to 170°C, the pressure is preferably 0.5 MPa to 0.7 Pa. This makes it possible to form only tungsten oxide particles having a hexagonal crystal structure from an aqueous solution of sodium polytungstate (an aqueous solution of a complex crystalline structure) obtained in the dialysis process, and to suitably grow the crystals.

[0050] The concentration of the removal solution (aqueous solution of sodium polytungstate) used in hydrothermal synthesis is preferably 0.01 mol / L or more and 0.9 mol / L or less, more preferably 0.05 mol / L or more and 0.7 mol / L or less, and 0.1 mol / L or more and 0.5 m A ratio of ol / L or less is even more preferable. Furthermore, during hydrothermal synthesis, the aqueous solution that serves as the reaction site is preferably left to stand, with a minimum standing time of 30 minutes and a more preferable minimum of 1 hour. The maximum standing time is preferably 24 hours or less, more preferably 8 hours or less, even more preferably 4 hours or less, and most preferably 2 hours or less. This allows for favorable crystal growth of hexagonal tungsten oxide. In this embodiment, hydrothermal synthesis was performed in a static reaction field, but hydrothermal synthesis may also be performed using a batch-type, semi-batch-type, or flow-type reactor equipped with a flow reactor.

[0051] (Washing process) The precipitate obtained in the hydrothermal synthesis process is washed with distilled water, and the distilled water containing the precipitate is subjected to centrifugation. This washing process is repeated multiple times until the electrical conductivity is 0.5 mS / cm or less, thereby removing excess ions and other impurities (washing process). After that, the washed precipitate is dried at 100°C to obtain powdered hexagonal tungsten oxide particles.

[0052] (Na-type hexagonal tungsten oxide particles) The Na-doped hexagonal tungsten oxide particles obtained in this embodiment have a structure in which rod-shaped crystal structures are stacked in parallel, as shown in Figure 1(b) (see Figure 1(b)), and these clusters of rod-shaped crystal structures form secondary particles (see Figure 1(a)). The average particle size of such secondary particles (D 50 The particle size is preferably 1.0 μm to 25 μm, more preferably 1.2 μm to 20.0 μm, and even more preferably 1.4 μm to 8.0 μm.

[0053] Furthermore, depending on the manufacturing method, Na-type hexagonal tungsten oxide particles may further aggregate with other secondary particles to form clusters. The average particle size (D) of the Na-type hexagonal tungsten oxide particles (overall) including clusters. 50The particle size is preferably 1.0 μm or more and 50 μm or less, more preferably 2.0 μm or more and 20.0 μm or less, and even more preferably 2.3 μm or more and 15.0 μm or less.

[0054] The tungsten oxide particles obtained in this embodiment are tungsten oxide (h-WO3) particles having a hexagonal crystal structure. The Na-doped hexagonal tungsten oxide obtained in this embodiment has an increased lattice constant compared to undoped hexagonal tungsten oxide, suggesting that sodium atoms are doped into the tungsten sites. Therefore, the general formula for the Na-doped hexagonal tungsten oxide obtained in this embodiment is W 1-X Na X O 3ーY This is considered to be the case. Here, value X is the amount of Na (sodium) doping relative to W (tungsten), and value Y is the amount of oxygen deficiency. Based on the XRF measurement results, the value X is preferably between 0.01 and 0.50, more preferably between 0.05 and 0.30, and even more preferably between 0.10 and 0.20. The value Y is preferably between 0 and 1.5, more preferably 1.0 or less, and even more preferably 0.5 or less.

[0055] The Na-doped hexagonal tungsten oxide particles obtained in this embodiment have the characteristics of having light absorption ability in the ultraviolet region, light transmittance in the visible region, and strong light absorption ability in the near-infrared region. The average value of the total light transmittance (JIS K 7361-1) in the near-infrared region (780nm to 2500nm) is preferably 10.0% to 50.0%, more preferably 20.0% to 40.0%, and even more preferably 25.0% to 35.0%. Furthermore, the average value of the total light transmittance in the visible light region (380nm to 780nm) is preferably 5.0% to 15.0%, and more preferably 8.0% to 13.0%. Then, the average value of the absorption coefficient of Na-doped hexagonal tungsten oxide particles in the near-infrared region (780 nm to 2500 nm) (NIR NaWO3) and the average value of the light absorption coefficient of Cs-doped hexagonal tungsten oxide particles (NIR CsWO3 ) refers to its magnification (NIR NaWO3 ÷NIR CsWO3 It is preferable that the ratio is 1.2 times or more, and more preferably 1.3 times or more. In addition, the average value of the absorption coefficient (NIR) of Na-doped hexagonal tungsten oxide particles in the near-infrared region (780 nm to 2500 nm) is also specified. NaWO3 ) and the average value of the light absorption coefficient (NIR) of monoclinic hexagonal tungsten oxide particles (undoped). WO3 ) refers to its magnification (NIR NaWO3 ÷NIR WO3 It is preferable that the ratio is 1.5 times or more, and more preferably 1.9 times or more. In this embodiment, the light absorption coefficient K of the Na-doped hexagonal tungsten oxide particles obtained can be calculated from the particle's light scattering coefficient S and light diffuse reflectance R using the KM conversion (Kubelka-Munk) equation. JPEG2026119822000003.jpg51127 The Na-doped hexagonal tungsten oxide particles obtained in this embodiment are thought to exhibit excellent light absorption performance in the infrared region because the doping with Na atoms generates free electron plasmons that are not present in ordinary tungsten oxide.

[0056] (Heat-shielding paint) The Na-doped hexagonal tungsten oxide particles obtained as described above can be mixed with a paint substrate to be applied to various structures or enclosures as a heat-shielding paint, thereby imparting heat-shielding properties to the object being coated. As an example of a base material, a resin base material is preferred, and examples of resin base materials include epoxy resin, urethane resin, polyester resin, fluororesin, silicone resin, acrylic resin, and melamine resin, of which acrylic resin is preferred. Furthermore, among acrylic resins, it is preferable to knead Na-doped hexagonal tungsten oxide particles into an acrylic emulsion resin. The dialysis process in this embodiment removes excess ionic components, preventing aggregation due to reaction with the base material components, thus allowing for desirable dispersion in the base material.

[0057] (Example 1) A 0.5 mol / L sodium tungstate aqueous solution was prepared by dissolving sodium tungstate (manufactured by Nippon Inorganic Chemical Industry Co., Ltd.) in distilled water. <Dissolution process> Next, hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the sodium tungstate aqueous solution obtained in the dissolution step, using a pH meter (LAQUATwin, manufactured by HORIBA Corporation) to adjust the pH to 3.5, thereby obtaining the primary treatment solution. The primary treatment solution was then left to stand at room temperature for 15 hours (standing step), and hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added again to adjust the pH to 0.5, thereby preparing the prepared solution (secondary treatment step). Next, the prepared solution was subjected to electrodialysis using an ion exchange membrane with a molecular weight cutoff of 300 (AC220, SunActis) on an electrodialysis machine (MicroAciLizer® S3, manufactured by Astom Co., Ltd.), with 12V applied until the concentration was 20 mS / cm or less, to obtain the removed solution. <Dialysis process> Next, 10 mL of the removal solution was added to a Teflon® container for hydrothermal synthesis, and the mixture was pressurized to 1.5 MPa and heated to 160°C using a hydrothermal synthesizer (HU-25, manufactured by San-ai Kagaku Co., Ltd.). The removal solution used for hydrothermal synthesis was an aqueous solution of sodium polytungstate with a concentration of 0.5 mol / L. Then, the removal solution was allowed to stand for 1 hour at 1.5 MPa and 160°C to allow crystal growth to occur, and a precipitate was obtained. <Hydrothermal synthesis process> Next, the precipitate obtained in the hydrothermal synthesis process was washed with water, and then subjected to repeated centrifugation (using H-9R manufactured by Kokusan Co., Ltd.) until the electrical conductivity was 0.5 mS / cm or less. Furthermore, the washed precipitate was dried at 100°C to obtain powdered sodium-doped hexagonal tungsten oxide particles. <Washing process>

[0058] (Example 2) Na-doped hexagonal tungsten oxide particles were obtained by following the same procedure as in Example 1, except that the standing time of the removal solution was changed to 4 hours in the hydrothermal synthesis process.

[0059] (Example 3) Na-doped hexagonal tungsten oxide particles were obtained by following the same procedure as in Example 1, except that the temperature was changed to 180°C in the hydrothermal synthesis process.

[0060] (Example 4) The same procedure as in Example 1 was followed to obtain particles, except that the temperature was changed to 200°C during the hydrothermal synthesis process.

[0061] (Example 5) The same procedure as in Example 1 was followed to obtain particles, except that the temperature was changed to 140°C during the hydrothermal synthesis process.

[0062] (Example 6) The same procedure as in Example 1 was followed to obtain particles, except that the temperature was changed to 120°C and the standing time to 24 hours during the hydrothermal synthesis process.

[0063] (Example 7) The same procedure as in Example 1 was followed to obtain particles, except that the temperature was changed to 200°C and the standing time to 24 hours during the hydrothermal synthesis process.

[0064] (Example 8) In the preparation process of Example 2, the primary treatment step, standing step, and secondary treatment step were omitted. Instead, hydrochloric acid was simply added to the sodium tungstate aqueous solution obtained in the dissolution step, using a pH meter, until the pH reached 0.5, thereby preparing the solution. (No standing method) Na-doped hexagonal tungsten oxide particles (160°C for 4 hours) were obtained in the same manner as in Example 2, except that the primary treatment step, standing step, and secondary treatment step were omitted in the preparation process.

[0065] (Example 9) Na-doped hexagonal tungsten oxide particles (120°C for 24 hours) were obtained in the same manner as in Example 6, except that the primary treatment step, standing step, and secondary treatment step were omitted in the preparation process of Example 6, similar to Example 8. (No standing method)

[0066] (Comparative Example 1) As undoped monoclinic tungsten oxide, we prepared tungsten(VI) oxide manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0067] (Comparative Example 2) In the preparation step, 2 parts by weight of cesium chloride were added to the dialysis solution, and in the hydrothermal synthesis step, the heating temperature was set to 200°C. The same procedure as in Example 1 was followed to obtain a precipitate.

[0068] (Comparative Example 3) In the preparation process, 2 parts by weight of cesium chloride were added to the dialysis solution, and a precipitate was obtained by leaving it at room temperature for 15 hours without performing a hydrothermal synthesis step.

[0069] (SEM imaging) Figure 1(a) is an SEM image of tungsten oxide particles obtained in Example 1 (160°C for 1 hour), and Figure 1(b) is a 10x magnified view thereof. Figure 1(c) is an SEM image of tungsten oxide particles obtained in Example 2 (160°C for 4 hours), and Figure 1(d) is a 10x magnified view thereof. As shown in Figures 1(a) and (c), it was confirmed that hexagonal tungsten oxide precipitated in the form of spheres approximately 1 to 10 μm in size. Furthermore, as shown in Figures 1(b) and (d), when the precipitated particles were magnified, it was confirmed that numerous rod-shaped crystalline structures were overlapping and forming bundles. Since the tungsten oxide in Example 1 is hexagonal, it is thought that the hexagonal crystals grew into rod shapes and precipitated. In other words, it was confirmed that hexagonal tungsten oxide, which had grown into a rod shape, aggregated to form spherical secondary particles of about 1 to 10 μm in size.

[0070] As shown in Figures 1(c) and 1(d), the tungsten oxide particles obtained in Example 2 were found to have larger crystals and larger particle sizes compared to the particles in Example 1. In addition, the secondary particles appeared to be generally fluffier and the rod-shaped length also seemed to be longer compared to Example 1. Therefore, it can be inferred that extending the hydrothermal synthesis time allowed the hexagonal tungsten oxide to continue growing, resulting in larger rod shapes and secondary particle sizes.

[0071] Figures 2(e) and (g) show SEM images of tungsten oxide particles obtained in Example 4 (200°C, 1h) and Example 7 (200°C, 24h). Figures 2(f) and (h) are 10x magnified views of Figures 2(e) and (g). In Example 4 (200°C, 1h), the fluffiness of the secondary particles is more pronounced and the particle size is larger compared to Example 1 (160°C, 1h). Therefore, it is thought that the higher hydrothermal synthesis temperature resulted in more favorable crystal growth. On the other hand, in Example 7 (200°C, 24h), although the rod shape is pronounced, the particle properties are gradually being lost compared to Example 4. Therefore, it is thought that a longer hydrothermal synthesis time results in a rod shape that makes it difficult for secondary particles to form.

[0072] Figure 3(i) shows an SEM image of tungsten oxide particles obtained in Example 8 (non-static method, 160°C, 4h). Figure 3(j) is a 10x magnified view of Figure 3(i). In Example 8, compared to Example 1, the secondary particles partially fuse together to form clusters.

[0073] Figure 4(a) is an SEM image of Comparative Example 2 (hexagonal Cs), and Figure 4(b) is an SEM image of Comparative Example 3 (spontaneously precipitated Cs). Furthermore, Figure 4(c) is an SEM image of hexagonal Ni-doped material obtained by hydrothermal synthesis of Ni, and Figure 4(d) is a spontaneous precipitate of tungsten oxide obtained by adding Ni. In this case, Comparative Example 2 (hexagonal Cs) yielded a clear hexagonal crystal structure, and rod-shaped particles like those in the example were not obtained. Furthermore, the hydrothermally synthesized Ni product yielded a monoclinic crystal structure, and rod-shaped particles like those in the example were not obtained.

[0074] (X-ray diffraction test) Figure 5 shows the results of X-ray diffraction measurements of tungsten oxide particles obtained in Examples 1-7. The inorganic materials database PDF card for hexagonal tungsten oxide (h-WO3) is also shown in the figure. The X-ray diffraction measurements were performed using a 9kW X-ray source with Rigaku's SmartLab®.

[0075] As a result, it was confirmed that the tungsten oxide particles obtained in Examples 1 to 7 have a hexagonal crystal structure. In particular, the patterns in Examples 1 to 7 show that the 2θ is shifted to a lower angle than the peak position in the database, suggesting that sodium is doped into the tungsten site. Furthermore, the lattice constants of the obtained tungsten oxide particles were a=7.311 Å and c=3.900 Å, which were larger than those of the hexagonal tungsten oxide database (a=7.285 Å, c=3.885 Å). This also suggests that sodium is doped into the tungsten site. Furthermore, the tungsten oxide particles obtained in Example 7 were confirmed to be high-purity hexagonal tungsten oxide particles, as they lacked a peak around 15-20°.

[0076] (Light absorption measurement) Figure 6 shows the results of total light transmittance measurements of tungsten oxide particles obtained in Example 1 and Comparative Examples 1 and 2. The measurements were performed using a UV-Vis-Near-Infrared Spectrophotometer V770 manufactured by JASCO Corporation, with a powder sample thickness of 0.5 mm placed in a sample packing section of φ5 mm, and measurements were performed according to JIS K 7361-1. As a result, the average total light transmittance in the near-infrared region (780nm to 2500nm) was 10.28% for Example 1 (Na), 19.11% for Comparative Example 1 (undoped), and 14.78% for Comparative Example 2 (Cs). In addition, the average total light transmittance in the visible light region (380nm to 780nm) was 30.34% for Example 1 (Na), 48.13% for Comparative Example 1 (undoped), and 52.73% for Comparative Example 2 (Cs). Therefore, compared to Comparative Examples 1 and 2, Example 1 was semi-transparent in the visible range (approximately 60% transmittance) and transmitted almost no light in the near-infrared range.

[0077] Figure 7 shows the results of optical absorption measurements of tungsten oxide particles in Example 1 and Comparative Examples 1 and 2. The optical absorption measurements were performed using the same method as for total light transmittance measurements, by measuring the light scattering coefficient S and light diffuse reflectance R of the particles and calculating them using KM conversion. As a result, the average value of the near-infrared (780nm~2500nm) light absorption coefficient of Example 1 (NIR NaWO3 ) is the average value of the near-infrared light absorption coefficient of Comparative Example 1 (NIR WO3 The value obtained by dividing by (NIR NaWO3 ÷NIR WO3 ) is 2.05, and the value obtained by dividing the average value of the near-infrared light absorption coefficient of Example 1 by the value of Comparative Example 2 (NIR NaWO3 ÷NIR CsWO3 The value was 1.33. Therefore, it was confirmed that Example 1 has sufficient light absorption capacity in the near-infrared region compared to Comparative Examples 1 and 2.

[0078] The light absorption in the red to near-infrared region (500-2000 nm) is broad in all three examples: Example 1, Comparative Examples 1, and Comparative Examples 2. The absorption rises sharply from 500 nm to around 1400 nm, then slightly increases from 1400 nm to around 2000 nm in Example 1 and Comparative Example 1, while Comparative Example 2 shows a significant increase. This light absorption in the red to near-infrared region is presumed to be due to plasmons and polarons. Example 1 (Na-doped) showed the greatest light absorption in the near-infrared region, and in particular, the slope in the near-infrared region (700 nm to 2000 nm) was the steepest, confirming that it preferably absorbs infrared light.

[0079] (Particle size measurement) Figure 8 shows the particle size distribution diagrams for Examples 1 to 5. Particle size was measured using the laser diffraction scattering method with the Microtrac Bell MT3300II. As a result, the particle size D of Example 1 50 The particle size D of Example 2 was 2.88 μm. 50 The particle size D of Example 3 was 16.26 μm. 50 The particle size D of Example 4 was 4.09 μm. 50 The particle size D of Example 5 was 6.98 μm. 50 The particle size D of Example 6 was 1.55 μm. 50 The particle size D of Example 7 was 12.59 μm. 50 The size was 2.30 μm. In the preparation process, Examples 1 to 5, which included a primary processing step, a standing step, and a secondary processing step, did not produce clusters (peaks around 34 μm) and all yielded a sharp particle size distribution centered on secondary particles. Furthermore, the frequency of particle size distribution was highest in Example 1 (160°C, 1h), and it was confirmed that the particle size increased with increasing hydrothermal synthesis time, and also with increasing hydrothermal synthesis temperature.

[0080] Figure 9 shows the particle size distribution diagrams for Examples 6-9. Compared to Example 2, Example 8 (pre-static method) showed that secondary particles bonded more easily, and clusters were more likely to form around 34 μm. Furthermore, as shown in Examples 6 and 9, when hydrothermal synthesis is performed at low temperatures for a long period of time, clusters tend to form around 34 μm in both cases. However, it was confirmed that the amount of cluster formation can be suppressed by performing a standing process in this case as well (Example 6). On the other hand, it was confirmed that when hydrothermal synthesis is performed at high temperatures for a long time, clusters around 34 μm are less likely to form, but a large number of small particles are more likely to form (Examples 7 and 4). In particular, Example 7 showed the growth of primary particles dispersed in a rod-like shape and secondary particles of small size formed by the aggregation of these primary particles. Therefore, from the viewpoint of obtaining small-particle hexagonal tungsten oxide particles, the hydrothermal synthesis temperature is preferably 120°C to 250°C, more preferably 150°C to 190°C, and the hydrothermal synthesis time is preferably 30 minutes to 4 hours, more preferably 50 minutes to 70 minutes.

[0081] (EDS qualitative analysis measurement) Table 2 shows the EDS measurement results for amorphous tungsten oxide particles obtained in Examples 1 to 7. The analytical measurements were performed using a JAMP-9500F manufactured by JEOL Ltd. [Table 2]

[0082] As a result, the general formula "W 1-X Na X O 3ーY In this case, the average values ​​considering oxygen deficiency (Y value) were X=0.10 and Y=0.21. Furthermore, the results of the XRD measurements mentioned above show a shift to the lower angle side compared to WO3, suggesting that Na was not intercalated, but rather dissolved in the form of substitution at about 10% of the hexagonal W sites, as shown in the general formula above.

[0083] (X-ray fluorescence measurement) Quantitative analysis of tungsten oxide particles obtained in Examples 1 to 7 was performed using X-ray fluorescence. Table 2 shows the analysis results when the oxygen value was fixed at 3. The analysis was performed using a Rigaku ZSPrimusIV wavelength-dispersive X-ray fluorescence analyzer. Fe-Kα was used for iron and W-Lα for tungsten. [Table 3]

[0084] As a result, the general formula "W" does not take oxygen deficiency (Y value) into consideration. 1-X Na X When calculated using "O3", the average value was X = 0.1. From the EDS and XPS results, it was found that in Example 1, tungsten oxide was incorporated into the W site of the tungsten oxide via solid solution during hydrothermal synthesis, resulting in crystal growth, and "W 0.8~0.9 Na 0.05~0.2 O 2.5~3 It is thought that this is what resulted from that.

Claims

1. The preparation step involves adding hydrochloric acid to an aqueous sodium tungstate solution to adjust the pH to an acidic state and obtaining a prepared solution. A dialysis step is performed on the prepared solution to obtain a removed solution from which chloride ions have been removed. A method for producing sodium-type hexagonal tungsten oxide particles, comprising a hydrothermal synthesis step of heating the aforementioned removal liquid under pressure.

2. A method for producing sodium-type hexagonal tungsten oxide particles according to claim 1, wherein in the preparation step, the pH of the prepared solution is 0.5 or more and 1.0 or less.

3. The preparation step includes a primary treatment step in which hydrochloric acid is added to adjust the pH to 3 or more and 5 or less to obtain a primary treatment solution, A standing step is performed in which the primary treatment solution is allowed to stand for 5 hours or more and 20 hours or less to obtain a standing treatment solution. A method for producing sodium-type hexagonal tungsten oxide particles according to claim 1 or 2, comprising a secondary treatment step of adding hydrochloric acid to the static treatment solution to adjust the pH to 0.5 or more and 1.0 or less to obtain the prepared solution.

4. A method for producing sodium-type hexagonal tungsten oxide particles according to claim 1, wherein in the hydrothermal synthesis step, the pressure for pressurizing the removal liquid is 0.1 MPa or more and 1.5 MPa or less, and the heating temperature is 100°C or more and 250°C or less.

5. A method for producing sodium-type hexagonal tungsten oxide particles according to claim 3, wherein in the hydrothermal synthesis step, the pressure for pressurizing the removal liquid is 0.1 MPa or more and 1.5 MPa or less, the heating temperature is 130°C or more and 250°C or less, and the heated removal liquid is allowed to stand while maintaining its temperature for 30 minutes or more and 2 hours or less.

6. A method for producing sodium-type hexagonal tungsten oxide particles according to claim 1, wherein the average value of the light absorption coefficient for wavelengths of 780 nm to 2500 nm is 1.2 times or more and 1.5 times or less than that of Cs-doped hexagonal tungsten oxide particles.

7. The general formula is W １－Ｘ Na Ｘ O ３ーY Sodium-type hexagonal tungsten oxide particles (X = 0.01 to 0.5, Y = 0 to 1.5).

8. Sodium-type hexagonal tungsten oxide particles according to claim 7, comprising secondary particles in which rod-shaped crystals are aggregated.

9. A heat-shielding filler comprising sodium-type hexagonal tungsten oxide particles as described in claim 7.

10. A heat-shielding paint comprising sodium-type hexagonal tungsten oxide particles as described in claim 7.

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