Tungsten oxide powder, electrochromic element using same, and method for producing tungsten oxide powder

Abstract

A tungsten oxide powder according to an embodiment is characterized in that the XRD profile (2θ) of the tungsten oxide powder is such that no peak is detected at 42°±1° and no peak is detected at 14°±1°, or, if detected, the full width at half maximum of the detected peak is 0.45° or more. Furthermore, the number of XRD peaks detected in the range from 10° to 70° is preferably from 0 to 5. Furthermore, the number of XRD peaks detected in the range from 10° to 70° is preferably 0.

Description

Tungsten oxide powder and electrochromic element using the same, method for producing tungsten oxide powder

[0001] The embodiments described later generally relate to tungsten oxide powder, electrochromic elements using the same, and methods for producing tungsten oxide powder.

[0002] Tungsten oxide powder is used in electrochromic devices, battery electrode materials, photocatalysts, and other applications. Tungsten oxide has crystalline structures such as monoclinic, triclinic, and hexagonal. For example, Japanese Patent No. 5546768 (Patent Document 1) discloses tungsten oxide powder having monoclinic and triclinic structures. Japanese Patent No. 6328100 (Patent Document 2) discloses tungsten oxide powder having a hexagonal structure. The tungsten oxide powder in Patent Document 1 has improved photocatalytic properties. The tungsten oxide powder in Patent Document 2 has improved battery capacity. International Publication No. WO2022 / 196454 (Patent Document 3) discloses tungsten oxide powder in which crystalline and amorphous phases are mixed. The tungsten oxide powder in Patent Document 3 improves the response speed of electrochromic devices. The tungsten oxide powders described in Patent Documents 1 to 3 have improved properties by incorporating a crystalline phase. When tungsten oxide powder is used in battery electrode layers or electrochromic layers, it is formed using a paint mixed with resin. For example, Japanese Patent No. 5087782 (Patent Document 4) describes forming an electrochromic layer using a paint mixed with a resin binder and tungsten oxide powder.

[0003] Japanese Patent No. 5546768, Japanese Patent No. 6328100, International Publication No. WO2022 / 196454, Japanese Patent No. 5087782

[0004] Conventional coatings made by mixing tungsten oxide powder with resin have a problem: the resin is decomposed by the photocatalytic activity of tungsten oxide, resulting in poor light resistance under strong light. Furthermore, the coating process does not always easily form a film. Upon investigating the cause, it was found that highly crystalline tungsten oxide powder has an active surface similar to a photocatalyst, and is highly aggressive towards the surrounding resin. This aggressiveness towards the resin creates a space between the resin and the tungsten oxide, reducing conductivity and causing variability and degradation in electrochromic performance. This embodiment addresses these problems.

[0005] The tungsten oxide powder according to this embodiment is characterized in that the XRD profile (2θ) of the tungsten oxide powder shows no peak detected at 42°±1°, and no peak detected at 14°±1°, or the full width at half maximum of the detected peak is 0.45° or more.

[0006] A figure showing an example of the XRD profile of tungsten oxide powder according to the embodiment. A figure showing another example of the XRD profile of tungsten oxide powder according to the embodiment. A figure showing an example of the XRD profile of tungsten oxide powder according to the prior art. A schematic diagram showing an example of tungsten oxide powder according to the embodiment. A schematic diagram showing an example of an electrochromic element according to the embodiment. Embodiment

[0007] The tungsten oxide powder according to the embodiment is characterized in that the XRD profile (2θ) of the tungsten oxide powder shows no peak detected at 42°±1°, and no peak detected at 14°±1°, or the full width at half maximum of the detected peak is 0.45° or more. Figures 1 and 2 show examples of the XRD profile (2θ) of the tungsten oxide powder according to the embodiment. Figure 3 shows an example of the XRD profile (2θ) of tungsten oxide powder according to the prior art. Figure 3 shows an example of the XRD profile (2θ) of tungsten oxide powder in which crystalline and amorphous phases are mixed, according to Patent Document 3. In Figures 1 to 3, the horizontal axis is the diffraction angle (2θ), and the vertical axis is the intensity. The XRD profile (2θ) is sometimes called the diffraction pattern. The XRD profile (2θ) is sometimes simply called the XRD profile.

[0008] XRD profiles are used for qualitative and quantitative analysis of materials. The peak positions of XRD profiles are determined by Bragg's law. Primarily, the peak positions are determined according to the crystal structure. Detecting peaks at specific positions allows for use in qualitative analysis. Figure 1 shows an example of tungsten oxide powder in which XRD peaks are detected within a preferred range. Figure 1 illustrates a profile in which five XRD peaks P are detected in the range of 2θ = 10° to 70°. Figure 2 shows an example of tungsten oxide powder in which no XRD peaks are detected. Figure 3 shows an example of conventional tungsten oxide powder in which numerous XRD peaks are detected. Figure 3 illustrates a profile in which 19 XRD peaks P are detected in the range of 2θ = 10° to 70°. The peak intensity of the XRD profile is primarily determined by the amount of crystals. Higher peak intensity allows for use in quantitative analysis. The peak shape of the XRD profile indicates the crystalline state. Sharper peaks with smaller full width at half maximum indicate higher crystallinity. Conversely, the broader the peak with a large half-width, the lower the crystallinity and the greater the proportion of the amorphous phase. XRD peaks are also sometimes used for qualitative analysis of crystals.

[0009] The XRD profile measurement method is described below. XRD measurements are performed using a Cu target, tube voltage of 40kV, tube current of 40mA, operating axis of 2θ / θ, scanning range (2θ) of 10° to 70°, scanning speed of 0.1° / sec, and step width of 0.02°. The XRD device used should be a Bruker D8-Advance or equivalent. The presence or absence of peaks should be determined using the analysis software (DIFFRAC.SUITE) included with the XRD device. The analysis software will determine the presence or absence of peaks, or whether the peaks are below the background level. The analysis software settings should be left at their default values. The scanning range (2θ) of 10° to 70° is the range in which the main peaks of tungsten oxide crystals such as monoclinic, triclinic, tetragonal, orthorhombic, and hexagonal crystals are detected. While the scanning range (2θ) is specified as 10° to 70°, the scanning range may be widened. Peaks with a full width at half maximum (FWHM) of 0.1° or greater are detected as XRD peaks. In the default settings of the analysis software mentioned above, peaks with a FWHM of less than 0.1° are usually below the detection limit and are not detected. While peak detection may vary depending on the XRD instrument and analysis software, here, peaks with a FWHM of less than 0.1° are not considered peaks. Furthermore, peaks determined to be below the background level are considered undetected.

[0010] Tungsten oxide powder is an oxide of tungsten. Examples of tungsten oxide include WO ３ WO ２ , W ２ O ３ These are some examples. Also, tungsten oxide powder is, for example, WO ３－ｘThe tungsten oxide powder may have oxygen vacancies such that 3 > x > 0 when expressed as . It may also contain metal components other than tungsten. Examples of metal components other than tungsten include one or more selected from potassium (K), sodium (Na), lithium (Li), and magnesium (Mg). In the XRD profile (2θ) of the tungsten oxide powder according to the embodiment, no peak is detected at 42° ± 1°. That is, the tungsten oxide powder mainly consists of an amorphous phase. The crystal structure of tungsten oxide mainly includes monoclinic, triclinic, tetragonal, orthorhombic, and hexagonal crystal structures. For example, tungsten trioxide (WO ３ The crystal structure of ) depends on the temperature during manufacturing. For example, the temperature conditions in liquid-phase synthesis affect the resulting crystal structure. Above 740°C it is tetragonal, between 330°C and 740°C it is orthorhombic, between 17°C and 330°C it is monoclinic, and between -50°C and 17°C it is triclinic. ３ At room temperature, it forms a monoclinic crystal. However, by modifying the manufacturing process as described in Patent Document 2, it can also be formed into a hexagonal crystal. Depending on the manufacturing process, multiple crystal structures may coexist. When the temperature conditions are set to the lower end of the temperature range corresponding to each crystal phase, the rate of change is slow, so extending the processing time makes it easier to reliably obtain the desired crystal structure. Since the amorphous phase is an unstable phase, hexagonal crystals may be formed if the drying time is too long. Similarly, since hexagonal crystals are also an unstable phase, monoclinic crystals may be formed by prolonged drying. Note that the change to monoclinic crystals at room temperature takes several years. Therefore, the tungsten oxide powder according to the embodiment will not undergo a phase change from amorphous to monoclinic unless it is left at room temperature for a long period of time, such as several years.

[0011] The absence of a peak at 42°±1° indicates that the monoclinic phase is below the detection limit (including the absence of monoclinic phase) or below the background level. In XRD measurements, "background" refers to unwanted signals detected in addition to the diffraction peaks from the sample. Background indicates noise and other elements included in the measurement data. Being below the background level is essentially synonymous with having no peak or being below the detection limit. Furthermore, the absence of a peak at 14°±1° indicates that the hexagonal phase is below the detection limit (including the absence of hexagonal phase) or below the background level. Additionally, a full width at half maximum (FWHM) of the peak detected at 14°±1° being 0.45° or greater indicates that the hexagonal phase has low crystallinity and approximates an amorphous phase. While there is no particular upper limit to the FWHM of the peak detected at 14°±1°, 1.00° or less is preferable. If the FWHM of the peak detected at 14°±1° exceeds 1.00°, it may overlap with other peaks, making it impossible to detect the other phase. In other words, a small FWHM indicates the absence of the other phase. Therefore, the full width at half maximum (FWHM) of the peak detected at 14°±1° is preferably within the range of 0.45° to 1.00°. Furthermore, if two or more peaks are detected within the 14°±1° range, the FWHM should be measured using the peak with the larger peak height. In other words, if no peak is detected at 42°±1°, and no peak is detected at 14°±1°, or if the FWHM of the detected peak is 0.45° or greater, it indicates that the crystalline phase is below the detection limit (including the absence of a crystalline phase) or that a hexagonal phase with low crystallinity is present. In other words, it indicates that the tungsten oxide powder is substantially formed from an amorphous phase with low crystallinity. Because it is an amorphous phase with low crystallinity, its aggressiveness towards the resin can be reduced. Furthermore, because its aggressiveness towards the resin can be reduced, the long-term reliability of the electrochromic layer can also be improved.

[0012] Also, it is preferable that the XRD profile has 0 to 5 XRD peaks detected within the range of 10° or more and 70° or less. Further, the XRD peaks indicate the crystal structure. Therefore, the smaller the number of XRD peaks, the more preferable. Most preferably, no peaks are detected, that is, the number of detected XRD peaks is 0. That no peaks are detected indicates that the crystal phase is below the detection limit or the peak intensity is below the background (including the case where no crystal phase exists). The absence of the crystal phase can further suppress the aggressiveness to the resin. It is also preferable that no peaks are detected in the ranges less than 10° and exceeding 70°. The main crystal phase of tungsten oxide has peaks detected at 10° or more and 70° or less. Therefore, by examining the range of 10° or more and 70° or less, the presence or absence of the crystal phase can be grasped. On the other hand, in the ranges less than 10° and exceeding 70°, peaks may be detected if there are heterogeneous phases. That no peaks are detected in this range indicates that no heterogeneous phases exist. It is desirable that no heterogeneous phases are caused by the addition of potassium or the like described above. Even if added, no XRD peaks of the crystal phase that becomes a heterogeneous phase are detected if there are no heterogeneous phases. In other words, it is preferable that no XRD peaks of the crystal phase that becomes a heterogeneous phase are detected even when a metal other than W is added. Further, it is desirable that the XRD profile does not contain peaks with significantly higher intensities compared to the profile in the profile of tungsten oxide (for example, FIGS. 1 and 2). There are no high-intensity peaks in the peaks attributed to tungsten oxide itself. That is, when high-intensity peaks are observed, contamination with a large amount of foreign matter is suspected.

[0013] Also, the tungsten oxide powder preferably has a BET specific surface area of 1 m 2 / g or more. If the BET specific surface area is less than 1 m 2 / g, the surface area cannot be utilized, which may cause a decrease in responsiveness and color unevenness when used in an electrochromic device. The upper limit of the BET specific surface area is not particularly limited, but it is preferably 100 m 2 / g or less. 100 m 2If the BET specific surface area exceeds 1 m², manufacturing costs may increase. 2 / g or more 100m 2 / g or less, and even 10m 2 / g or more 50m 2 A range of less than or equal to / g is preferred. Furthermore, the BET specific surface area measurement method shall be the BET single-point method. The adsorbent gas shall be nitrogen gas, and the sample amount shall be 3g. The BET single-point method shall be performed in accordance with JIS-Z-8830:2013. Note that JIS-Z-8830:2013 corresponds to ISO9277:2010.

[0014] Furthermore, it is preferable that the secondary particles of the tungsten oxide powder include flattened particles. It is also preferable that the secondary particles of the tungsten oxide powder include particles with an aspect ratio of 1.5 or more. Furthermore, it is preferable that the secondary particles of the tungsten oxide powder include flattened particles (aspect ratio of 1.5 or more) in number proportion of 50% to 100%. In particular, it is preferable that the secondary particles before and after the crushing process for size adjustment when forming the electrochromic layer have a flattened shape in number proportion of 50% or more. When there are many flattened secondary particles (aspect ratio of 1.5 or more), it is easier to form a uniform layer and a smooth film when mixed with the resin. This is because, in the process of coating the resin mixed with tungsten oxide powder onto the substrate, particles with a larger aspect ratio tend to orient towards the surface of the substrate. The crushing process may be performed before or after mixing the tungsten oxide powder with the resin. The crushing process has the effect of eliminating coarse secondary particles.

[0015] A primary particle is a single particle. A secondary particle is a particle formed by the aggregation of multiple particles. Figure 4 shows a conceptual diagram of primary and secondary particles. In the figure, reference numeral 1 denotes a primary particle of tungsten oxide powder, and reference numeral 2 denotes a secondary particle of tungsten oxide powder. The number of primary particles 1 that aggregate in the secondary particles 2 of tungsten oxide powder is arbitrary. Furthermore, it is preferable that the primary particles of tungsten oxide powder include particles with an aspect ratio of less than 1.5. It is also preferable that 80% to 100% of the primary particles of tungsten oxide powder have an aspect ratio of less than 1.5. By having a large number of primary particles of tungsten oxide powder with an aspect ratio of less than 1.5, it is possible to suppress a decrease in responsiveness and the occurrence of color unevenness when used in an electrochromic element. Furthermore, the film using the tungsten oxide powder according to the embodiment can have flatness in which no convex portions with a height ratio of 1.8 times or more relative to the average film thickness are formed. The height ratio of the protrusions shall be determined by the maximum thickness of the film / the average thickness of the film. For example, if the average film thickness is 100 μm, this indicates that no protrusions of 180 μm or more will be formed. Because it contains flattened secondary particles (aspect ratio of 1.5 or more), the flatness of the film can be improved. A laser microscope is used to measure the flatness of the film. The laser microscope used is a Keyence VK-X1000 or a similar model. The measurement conditions were to examine the film thickness in an arbitrary 1 mm x 1 mm field of view at a magnification of 10x. The average thickness and maximum height within the measurement field of view were determined. Maximum height = height of the protrusions. Protrusion height ratio = maximum height / average film thickness.

[0016] Here, we will explain the method for measuring the shape of primary and secondary particles. Particle shape measurement will be performed using SEM (Scanning Electron Microscope) observation. Observe the tungsten oxide particles as they appear in the SEM image. Tungsten oxide particles that are aggregated are considered secondary particles. Those that are not aggregated are considered primary particles. The longest diagonal of the secondary particle in the SEM image is defined as the major axis. The length of the vertical line drawn from the center of the major axis is defined as the minor axis. The aspect ratio is calculated as major axis / minor axis. Measure the aspect ratio of 100 arbitrary secondary particles and determine the proportion of particles with an aspect ratio of 1.5 or greater. The same method is used for primary particles. The longest diagonal of the secondary particle in the primary particle in the SEM image is defined as the major axis. The length of the vertical line drawn from the center of the major axis is defined as the minor axis. The aspect ratio is calculated as major axis / minor axis. Measure the aspect ratio of 100 arbitrary primary particles and determine the proportion of particles with an aspect ratio of less than 1.5.

[0017] Furthermore, the tungsten oxide powder may contain one or more of the following elements: potassium, sodium, lithium, and magnesium, in an amount of 0.01 mol% to 50 mol%. By including these elements in the tungsten oxide powder, the conductivity of the tungsten oxide powder can be increased. By increasing the conductivity of the tungsten oxide powder, the response speed can be increased. If the content is less than 0.01 mol%, the effect of including the elements is insufficient. Also, if the content exceeds 50 mol%, the advantages of tungsten oxide cannot be utilized. For this reason, the content is preferably between 0.01 mol% and 50 mol%, and more preferably between 1 mol% and 20 mol%.

[0018] The content of elements such as potassium can be measured by energy dispersive X-ray spectrometry (EDX) analysis. The following is an example of how to determine the potassium content. The sodium, lithium, and magnesium content should be read by substituting potassium. EDX analysis is used to color map the potassium contained in the tungsten oxide powder. The measurement conditions are an acceleration voltage of 15.0 kV, and surface analysis is performed at a magnification of 1,000 times or more. The potassium content is determined from the atomic ratio of K (potassium) and W (tungsten) obtained by the ZAF method. The ZAF method is a correction method that combines three types: atomic number correction (Z), absorption correction (A), and fluorescence correction (F). Atomic number correction (Z) corrects the ratio of electrons irradiated onto the sample that are divided into entering electrons and scattered electrons. Absorption correction (A) corrects the amount of characteristic X-rays generated in the sample that are absorbed before they leave the sample. Fluorescence (excitation) correction (F) corrects the intensity of fluorescent X-rays excited by X-rays generated in the sample. This is a common correction method for EDX.

[0019] The tungsten oxide powder according to this embodiment can be used in various fields. Examples include electrochromic elements and batteries (including rechargeable batteries). The tungsten oxide powder according to this embodiment can be used in electrochromic elements. Figure 5 shows an example of a schematic diagram of the cell structure of an electrochromic element. In the figure, reference numeral 10 denotes the cell (electrochromic element), reference numeral 11 denotes a glass substrate, reference numeral 12 denotes a transparent electrode, reference numeral 13 denotes an electrochromic layer, reference numeral 14 denotes a counter electrode, and reference numeral 15 denotes an electrolyte. The glass substrate 11 has good light transmittance. If light transmittance is not desired, a substrate other than glass may be used. Alternatively, a resin film may be used instead of a glass substrate, or a part of the glass substrate may be replaced with a resin film. In addition, materials such as ITO can be used for the transparent electrode 12.

[0020] The electrochromic layer 13 is made using tungsten oxide powder according to the embodiment. The electrochromic layer 13 is formed by applying and drying a tungsten oxide powder paste onto the transparent electrode 12. The drying process is preferably carried out in a temperature range of 120°C to 270°C. In addition, a resin or solvent can be mixed with the tungsten oxide powder paste. Examples of resins include binders. Examples of solvents include water and alcohol. The tungsten oxide powder paste mixed with a resin or solvent is easier to apply onto the transparent electrode 12. During the drying process, the solvent is removed, but the binder remains, and the electrochromic layer 13 is formed. Note that the electrochromic layer 13 may contain substances other than tungsten oxide powder.

[0021] Platinum is one example of a material used for the counter electrode 14. The counter electrode 14 is provided on a glass substrate (not shown). An electrolyte 15 is filled between the electrochromic layer 13 and the counter electrode 14. The electrolyte 15 is also sealed. When a voltage is applied to the transparent electrode 12 and the counter electrode 14, the electrochromic layer 13 becomes transparent. The electrochromic element can switch between transparency and color by switching the charge on and off. Electrochromic elements are used in displays and dimming systems. Examples of dimming systems include dimmable glass, dimmable glasses, and anti-glare mirrors. Dimming systems are also used in various fields such as vehicles, aircraft, and buildings. For example, when used as dimmable glass in building windows, it is possible to switch the on and off of incident sunlight. It can also suppress the transmission of ultraviolet rays. In other words, it is suitable as an electrochromic element for controlling the on and off of incident sunlight.

[0022] Furthermore, it is preferable that the electrochromic layer 13 contains tungsten oxide powder according to the embodiment in an amount of 50% to 100% by mass. If the content of tungsten oxide powder according to the embodiment is low, the effect may be reduced. In addition, the tungsten oxide powder according to the embodiment has low aggressiveness towards resins. Because it has low aggressiveness towards resin binders and the like, the shape of the electrochromic layer 13 can be maintained. It is possible to prevent the tungsten oxide powder from detaching from the electrochromic layer 13. It is also possible to prevent the formation of pores in the electrochromic layer 13. This makes it possible to suppress a decrease in responsiveness and the occurrence of color unevenness. For example, the electrochromic layer 13 is a thin layer with a thickness of 0.05 μm to 5 μm. If the tungsten oxide powder detaches from the electrochromic layer 13, regions with less tungsten oxide powder will be formed. This may cause a decrease in responsiveness and color unevenness.

[0023] Next, a method for producing tungsten oxide powder according to the embodiment will be described. As long as the tungsten oxide powder according to the embodiment has the above configuration, the method for producing it is not particularly limited, but the method for obtaining it with good yield is as follows.

[0024] The method for producing tungsten oxide powder according to this embodiment comprises an alkali treatment step of dissolving a tungsten oxide precursor in an alkaline solution, a neutralization step of precipitating tungsten oxide powder in an acidic solution, a filtration step of filtering the obtained powder, and a drying step of drying the filtered powder, wherein the drying step is carried out within the range of a heating rate of 0.05°C / min to 20°C / min from room temperature and a temperature of 250°C to 370°C.

[0025] First, an alkali treatment step is performed in which the tungsten oxide precursor is dissolved in an alkaline solution. Examples of tungsten oxide precursors include ammonium tungstate and WO2. ３ WO ２ , or H ２ WO ４It can be used. In addition, when including doping agents such as potassium, a doping agent precursor shall be used. For example, as a potassium precursor, K ２ WO ４ Alternatively, Na2WO4 as a precursor of KOH and Na, or LiWO4 as a precursor of NaOH and Li. ４ MgWO ４ or Mg(OH) ２ This can be used. Furthermore, when adding a doping agent, the doping agent precursor should be mixed in during the step of dissolving the tungsten oxide precursor with alkali. In the alkali treatment step of dissolving the tungsten oxide precursor with alkali, the tungsten oxide precursor is dispersed in water, and the pH is adjusted to within the range of 8-12 using ammonia or KOH to dissolve it. This is the step of preparing the precursor into an alkaline aqueous solution. This method of synthesis in water is sometimes called liquid-phase synthesis.

[0026] Next, a neutralization step is performed to precipitate tungsten oxide powder in an acidic solution. The neutralization step is a step in which the pH of the alkaline aqueous solution subjected to the alkali treatment step is adjusted to the range of pH 5-7 with hydrochloric acid (HCl) solution. By neutralizing the alkaline aqueous solution, WO ３Powder can be precipitated. Instead of hydrochloric acid, sulfuric acid (H2SO4), nitric acid (HNO3), or citric acid may be used. If the pH of the neutralization step is lower than 5, the particle size may become too small, making it difficult to control the specific surface area. Also, if the pH is greater than 7, the formation of the alkali tungstate compound may be insufficient. For this reason, the pH of the neutralization step is preferably between 5 and 7, and more preferably between 6 and 7. It is also preferable to mix the acidic aqueous solution while immersing a pH meter in the alkaline aqueous solution. Furthermore, the acidic aqueous solution is preferably in the range of 5 to 45% by mass in terms of concentration. If the concentration is less than 5% by mass, the amount of acid component will be small. Therefore, the amount of acidic aqueous solution needed for pH adjustment will increase. Also, if the concentration exceeds 45% by mass, the acidity will be too strong, making it difficult to handle safely. Furthermore, it is preferable to adjust the pH of the alkaline mixed aqueous solution or the acid mixed aqueous solution while stirring. By stirring, each aqueous solution can be mixed uniformly. By mixing uniformly, the particle size of the resulting alkali tungstate compound crystals can be made uniform. It is preferable to stir for 30 minutes or more. While there is no particular upper limit to the stirring time, it is preferable to keep it at 10 hours or less. If the stirring time is too long, the reaction may proceed too quickly, potentially leading to the detection of a crystalline phase. Also, the specific surface area of ​​the tungsten oxide powder may decrease (the particle size may increase). Furthermore, if the stirring time is too long, the primary particles will grow isotropically, potentially resulting in the formation of large, spherical secondary particles during the drying process. The powdery precipitate obtained from the neutralization process is filtered and dried to recover the powder. The drying process is preferably carried out within the range of a heating rate of 0.05°C / min to 20°C / min from room temperature and a temperature of 250°C to 370°C. If the drying temperature is below 250°C, the drying time will be longer, reducing manufacturing efficiency. Also, if the temperature is higher than 370°C, particle growth will occur, increasing the specific surface area to 1 m². ２It may be less than / g. Therefore, the drying temperature is preferably in the range of 250°C or higher and 370°C or lower, more preferably 270°C or higher and 350°C or lower. Also, the holding time of the drying temperature is preferably in the range of 30 minutes or longer and 24 hours or shorter. Specifically, it is preferable to hold at the set temperature of the drying device for 30 minutes or longer and 24 hours or shorter at the set temperature.

[0027] Also, in the drying process, the heating rate from room temperature to the drying temperature is preferably 0.05°C / min or higher and 20°C / min or lower. By slowing down the heating rate, the formation of a crystal phase in the tungsten oxide powder can be suppressed. If the heating rate is less than 0.05°C / min, the heating time is long, so the drying process takes time. Also, if it is faster than 20°C / min, the crystal phase may be formed more than necessary. Therefore, in the drying process, the heating rate from room temperature to the drying temperature is preferably 0.05°C / min or higher and 20°C / min or lower, more preferably 0.2°C / min or higher and 15°C / min or lower.

[0028] Also, the cooling rate to 200°C after the drying process is preferably in the range of 0.01°C / min or higher and 2°C / min or lower. After the drying process means after holding at the drying temperature. By slowly reducing the cooling rate from the drying process to 200°C, the formation of a crystal phase can be prevented. The cooling process controls the cooling rate while remaining held in the drying device. When natural cooling is performed without controlling the cooling rate, the tungsten oxide powder may be taken out of the drying device after the drying process.

[0029] By this process, the tungsten oxide powder according to the embodiment can be produced. By using the liquid-phase synthesis method, a tungsten oxide powder with a small crystal phase and mainly an amorphous phase can be used. Also, when using plasma treatment as in the conventional method, since it is produced at a high temperature, there is a high possibility of obtaining a tungsten oxide powder mainly composed of a crystal phase.

[0030] (Examples) (Examples 1 to 5, Comparative Examples 1 to 3) Tungsten oxide powder was prepared using a liquid-phase synthesis method in Examples 1 to 5 and Comparative Example 1. Ammonium tungstate was used as the precursor. An ammonium tungstate aqueous solution containing 50 to 90% by mass of ammonium tungstate was prepared. The alkali treatment step of the ammonium tungstate aqueous solution was adjusted to pH 8 to 12 using an aqueous ammonia solution. Also, in the neutralization step, it was adjusted to pH 5 to 7 using an aqueous hydrochloric acid solution. After the neutralization step, a filtration step of filtering the obtained powder and a drying step of drying the filtered powder were performed. Also, the alkali treatment and neutralization steps were performed while stirring. Also, pH adjustment was performed while immersing a pH meter in the aqueous solution. Tungsten oxide powder was produced by this process. The stirring time in the neutralization step, the heating rate, drying temperature, drying time, and cooling rate in the drying step are as shown in Table 1.

[0031]

[0032] Also, tungsten oxide powder prepared using plasma treatment was prepared as Comparative Example 3. The BET specific surface area and XRD profile were measured for the tungsten oxide powders according to the examples and comparative examples. The measurement conditions are as described above. The results are shown in Table 2.

[0033]

[0034] As can be seen from the table, the BET specific surface area of the tungsten oxide powders according to Examples 1 to 5 was all 1 m ２ / g or more and within the range of 1 to 100 m ２ / g. Also, in Comparative Example 2, since the stirring time was long, the reaction proceeded too far and the BET specific surface area was less than 1 m ２ / g. Also, since Comparative Example 3 was produced by plasma treatment, it was fine powder and the BET specific surface area was 100 m ２The values ​​exceeded / g. Furthermore, no XRD peaks at 42°±1° were detected in the tungsten oxide powders of Examples 1 to 5 (including below background). In Example 1, a peak at 14°±1° was detected, but its full width at half maximum was 0.45° or more. The number of peaks in the 10° to 70° range was between 0 and 5. In Examples 1 to 5, no XRD peaks were detected in the ranges below 10° and above 70°. In contrast, in Comparative Examples 1 and 3, a peak at 42°±1° was detected. In Comparative Examples 1 to 3, the full width at half maximum of the 14°±1° peak was less than 0.45°. In Comparative Examples 1 and 3, the number of peaks in the 10° to 70° range was high at 10. From these results, it can be seen that Examples 1 to 5 are tungsten oxide powders with low crystallinity, mainly consisting of an amorphous phase.

[0035] Next, the aspect ratios of the primary and secondary particles of the tungsten oxide powder in the examples and comparative examples were measured. Furthermore, the percentage of flattened secondary particles (aspect ratio of 1.5 or greater) was determined. The measurement conditions were as described above. The samples were extracted after drying to room temperature. The results are shown in Table 3.

[0036]

[0037] As can be seen from the table, in the tungsten oxide powders of Examples 1 to 5, the secondary particles were flattened (aspect ratio of 1.5 or greater) and accounted for 50% to 100% of the total number of particles. In contrast, although secondary particles were present in Comparative Example 1, they were all spherical with an aspect ratio of less than 1.5. Secondary particles with an aspect ratio of 1.5 or greater were confirmed in Comparative Example 2. Secondary particles were not observed in Comparative Example 3. Therefore, secondary particles with an aspect ratio of 1.5 or greater were not observed. Next, an electrochromic layer was formed using the tungsten oxide powders of the Examples and Comparative Examples. In the formation of the electrochromic layer, the tungsten oxide powder was crushed and then made into a paste. The paste was a mixture of tungsten oxide powder, a resin binder, and a solvent. In Examples 1 to 5, secondary particles of tungsten oxide powder remained even after the crushing process. The obtained tungsten oxide powder paste was applied to form an electrochromic layer.

[0038] First, the flatness of the electrochromic layer was examined. Flatness was determined by checking for the presence or absence of protrusions where the height ratio of the protrusions to the average film thickness was 1.8 times or more. The measurement method was as described above. Next, electrochromic elements were fabricated using the electrochromic layer. The light resistance of the electrochromic layer and the responsiveness of the electrochromic elements were evaluated. The responsiveness was measured using a UV-Vis spectrophotometer, and the time it took for the transmittance of light at a wavelength of 600 nm to change from 20% to 70% when a voltage of 1.2 V was applied was measured. For light resistance, the fabricated electrochromic elements were placed 1 cm below a chemical lamp, and those with a transmittance of 70% or more after 1000 hours of irradiation were deemed acceptable, while those with a transmittance of less than 70% were deemed unsuitable (failed). The results are shown in Table 4.

[0039]

[0040] As can be seen from the table, the examples showed good film flatness. Furthermore, the responsiveness was comparable for Examples 1-5 and Comparative Examples 1 and 3. In addition, it was found that the lightfastness was improved in Examples 1-5. This indicates that color unevenness is reduced and the long-term reliability of the electrochromic layer is improved. On the other hand, Comparative Example 2 showed decreased responsiveness. This is because it has a small specific surface area. Also, in Examples 1-5 and Comparative Example 2, the films coated after mixing with the resin showed good flatness. This is because many of the secondary particles had an aspect ratio of 1.5 or higher, making it easy to obtain a flat film. This point is also thought to contribute to the effect of reducing color unevenness.

[0041] Although several embodiments of the present invention have been illustrated above, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. Modifications of these embodiments are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. Furthermore, the embodiments described above can be implemented in combination with each other. Several embodiments of the invention are described below. [1] Tungsten oxide powder having an XRD profile (2θ) in which no peak is detected or is below the background at 42°±1°, and no peak is detected or the full width at half maximum of the detected peak is 0.45° or more at 14°±1°. [2] The tungsten oxide powder according to [1], wherein there are 0 to 5 XRD peaks detected in the range of 10° to 70°. [3] The tungsten oxide powder according to [1] or [2], wherein there are zero XRD peaks detected in the range of 10° to 70°. [4] The BET specific surface area is 1 m². 2 Tungsten oxide powder as described in [1] or [2], having a BET specific surface area of ​​1 m² or more. [5] 2[3] Tungsten oxide powder according to [3], which is 1 / g or more. [6] Tungsten oxide powder according to [1] or [2], wherein the secondary particles of the tungsten oxide powder are flattened. [7] Tungsten oxide powder according to any one of [3] to [5], wherein the secondary particles of the tungsten oxide powder are flattened. [8] Tungsten oxide powder according to [1] or [2], for use in electrochromic elements. [9] Tungsten oxide powder according to any one of [3] to [7], for use in electrochromic elements.

[10] Electrochromic element using tungsten oxide powder according to [8] or [9].

[11] A method for producing tungsten oxide powder according to any one of [1] to [9], comprising an alkali treatment step of dissolving a tungsten oxide precursor in an alkaline solution, a neutralization step of precipitating tungsten oxide powder in an acidic solution, a filtration step of filtering the obtained powder, and a drying step of drying the filtered powder, wherein the drying step is performed within the range of a heating rate from room temperature of 0.05°C / min or more and 20°C / min or less, and a temperature of 250°C or more and 370°C or less.

[12] A method for producing tungsten oxide powder according to

[11] , wherein the cooling rate to 200°C after the drying step is within the range of 0.01°C / min or more and 2°C / min or less.

[13] A method for producing tungsten oxide powder according to

[11] , wherein no cooling step is performed after the drying step.

[0042] 1... Primary particles of tungsten oxide powder 2... Secondary particles of tungsten oxide powder 10... Cell (electrochromic element) 11... Glass substrate 12... Transparent electrode 13... Electrochromic layer 14... Counter electrode 15... Electrolyte

Claims

1. Tungsten oxide powder whose XRD profile (2θ) shows no peak detected at 42°±1°, and no peak detected at 14°±1°, or the full width at half maximum of the detected peak is 0.45° or greater.

2. The tungsten oxide powder according to claim 1, wherein the number of XRD peaks detected in the range of 10° to 70° is 0 to 5.

3. The tungsten oxide powder according to claim 1, wherein there are zero XRD peaks detected within the range of 10° to 70°.

4. BET specific surface area is 1 m 2 Tungsten oxide powder according to any one of claims 1 to 3, wherein the amount is 1 / g or more.

5. The tungsten oxide powder according to any one of claims 1 to 3, wherein the secondary particles of the tungsten oxide powder include flattened particles.

6. The tungsten oxide powder according to claim 4, wherein the secondary particles of the tungsten oxide powder include flattened particles.

7. Tungsten oxide powder according to any one of claims 1 to 3, for use in electrochromic devices.

8. The tungsten oxide powder according to claim 6, for use in electrochromic devices.

9. An electrochromic element using tungsten oxide powder as described in claim 7.

10. An electrochromic element using tungsten oxide powder as described in claim 8.

11. A method for producing tungsten oxide powder according to any one of claims 1 to 3, comprising: an alkali treatment step of dissolving a tungsten oxide precursor in an alkaline solution; a neutralization step of precipitating tungsten oxide powder in an acidic solution; a filtration step of filtering the obtained powder; and a drying step of drying the filtered powder, wherein the drying step is performed within the range of a heating rate from room temperature of 0.05°C / min to 20°C / min and a temperature of 250°C to 370°C.

12. The method for producing tungsten oxide powder according to claim 11, wherein the rate of cooling down to 200°C after the drying step is in the range of 0.01°C / min or more and 2°C / min or less.

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