Rare earth phosphate powder, method for producing same, and light-scattering member

Abstract

A rare earth phosphate powder according to the present invention comprises rare earth phosphate particles containing LnPO4. (In the formula, Ln represents at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu.) The rare earth phosphate powder includes rare earth phosphate particles having internal voids. The proportion of rare earth phosphate particles having voids having a maximum length of at least 3.0 nm is at least 10% by number with respect to the total number of rare earth phosphate particles. The present invention addresses the problem of providing a light-scattering powder that is closer to white.

Description

Rare earth phosphate powder, its manufacturing method, and light scattering member

[0001] The present invention relates to a rare earth phosphate powder and a method for producing the same, and also to a light-scattering member containing the rare earth phosphate powder.

[0002] Light-scattering sheets containing light-scattering particles in a transparent resin are used in various optical devices, such as backlight modules for liquid crystal display devices used in televisions and smartphones, screens for image display devices such as projection televisions, transparent screens for projection by head-up displays and projectors, LED elements and μLED elements used as encapsulants, and lighting fixtures used as covers. Such light-scattering sheets are required to have excellent light-scattering properties while maintaining transparency. They are also required to have a wide viewing angle. For this reason, titania, silica, zirconia, barium titanate, zinc oxide, resin particles, and the like are used as light-scattering particles.

[0003] The present applicant has also previously proposed rare earth phosphate particles as particles to be used in a light-scattering sheet in which light-scattering particles are dispersed in a resin (see Patent Document 1).

[0004] US2019 / 0353828A1

[0005] If light-scattering particles have a color, the color of the reflected light will change when light hits them. From this perspective, it is desirable for the light-scattering particles to be as close to white as possible. Therefore, an object of the present invention is to provide a light-scattering powder that is as close to white as possible.

[0006] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, it was found that providing voids inside rare earth phosphate particles increases the whiteness of rare earth phosphate powder containing the particles. ４The above-mentioned problems have been solved by providing a rare earth phosphate powder comprising rare earth phosphate particles containing: (wherein Ln represents at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu), wherein the rare earth phosphate powder comprises rare earth phosphate particles having voids therein, and the proportion of the number of the rare earth phosphate particles having the voids with a maximum length of 3.0 nm or more relative to the number of all rare earth phosphate particles is 10% or more by number.

[0007] The present invention also provides a method for producing a rare earth phosphate powder, which comprises adding an aqueous solution containing phosphoric acid or a salt thereof and a basic substance to an aqueous solution containing a rare earth element to produce a precipitate containing a rare earth element and a phosphorus element, and calcining the precipitate to obtain a rare earth phosphate powder containing rare earth phosphate particles having internal voids.

[0008] Fig. 1 is a transmission electron microscope image of the yttrium phosphate powder obtained in Example 1. Fig. 2 is a transmission electron microscope image of the yttrium phosphate powder obtained in Example 2. Fig. 3 is a transmission electron microscope image of the yttrium phosphate powder obtained in Comparative Example 4.

[0009] The present invention will be described below based on its preferred embodiments. The present invention relates to a rare earth phosphate powder. The rare earth phosphate powder is an aggregate of rare earth phosphate particles. The phosphate referred to in this specification refers to orthophosphate. While normal salts, hydrogen phosphates, and dihydrogen phosphates are known as orthophosphates, the rare earth phosphate used in the present invention is the normal salt of orthophosphate. The rare earth phosphate is LnPO ４ In the formula, Ln represents at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu. These elements can be used alone or in combination of two or more.

[0010] The rare earth phosphate powder of the present invention may be composed solely of the rare earth phosphate particles, or may contain other particles in addition to the rare earth phosphate particles. The other particles may be contained in the rare earth phosphate powder within a range that does not impair the effects of the present invention.

[0011] Rare earth phosphates are materials with a high refractive index. For this reason, when light is irradiated onto the rare earth phosphate powder of the present invention, significant scattering of light occurs. Furthermore, rare earth phosphates generally have a high Abbe number and exhibit a small wavelength dependency of the refractive index. In other words, rare earth phosphates are substances with a small variation in the degree of refraction when light containing various wavelengths is incident. As a result, scattered light with high contrast can be obtained by using the rare earth phosphate powder of the present invention. From these perspectives, the rare earth element Ln in the rare earth phosphate represented by LnPO4 is preferably at least one element selected from Y, La, Gd, Yb, and Lu, and is particularly preferably Y.

[0012] The rare earth phosphate particles may be crystalline or amorphous (non-crystalline). Generally, when rare earth phosphate particles are produced by the method described below, a crystalline rare earth phosphate powder is obtained. When the rare earth phosphate powder is crystalline, it is preferable because the refractive index is high.

[0013] The rare earth phosphate particles contained in the rare earth phosphate powder of the present invention preferably have voids inside the particles. In other words, the rare earth phosphate particles preferably have spaces inside them. These spaces can be referred to as hollow portions. One or more hollow portions may be present in a single rare earth phosphate particle. The inventors' investigations have revealed that the whiteness of the rare earth phosphate powder of the present invention is enhanced when the rare earth phosphate particles have hollow portions.

[0014] The rare earth phosphate powder of the present invention may be composed of rare earth phosphate particles all of which have voids, or may be a mixture of rare earth phosphate particles having voids and rare earth phosphate particles without voids. In the latter case, from the viewpoint of improving whiteness, it is preferable that the ratio of the number of rare earth phosphate particles having voids to the number of all rare earth phosphate particles is as high as possible.

[0015] The shape of the voids is not particularly limited, and may be, for example, a sphere, a spheroid, a rectangular parallelepiped, a rectangular parallelepiped with rounded edges, a rectangular parallelepiped with rounded corners, a prism, a cylinder, an elliptical cylinder, or a combination thereof.

[0016] In rare earth phosphate particles having voids, it is desirable for the voids to have a predetermined size in order to enhance the whiteness of the rare earth phosphate powder. From this perspective, it is preferable that the rare earth phosphate particles constituting the rare earth phosphate powder have voids with a maximum length of 3.0 nm or more inside. The maximum void length is more preferably 10.0 nm or more, and even more preferably 20.0 nm or more. The upper limit of the maximum void length is determined by the particle size of the rare earth phosphate particles, and the closer the maximum void length is to the particle size of the rare earth phosphate particles, the higher the whiteness of the rare earth phosphate powder tends to be.

[0017] The maximum length of the voids is measured by observing the rare earth phosphate particles with a transmission electron microscope (TEM). The maximum length of the voids is defined as the length of the longest line segment among the line segments that cross the voids observed in the rare earth phosphate particles.

[0018] In the rare earth phosphate powder of the present invention, the higher the proportion of rare earth phosphate particles having voids with a maximum length of 3.0 nm or more, the higher the whiteness of the rare earth phosphate powder. From this perspective, in the present invention, the proportion of the number of rare earth phosphate particles having voids with a maximum length of 3.0 nm or more relative to the number of all rare earth phosphate particles is preferably 10% by number or more, more preferably 35% by number or more, even more preferably 40% by number or more, and even more preferably 50% by number or more. In this specification, "all rare earth phosphate particles" refers to rare earth phosphate particles with voids and rare earth phosphate particles without voids present in the TEM image in the observation field. For convenience in the following description, the proportion of the number of rare earth phosphate particles having voids with a maximum length of 3.0 nm or more will also be referred to as the "void particle content." A rare earth phosphate powder having the above-mentioned void particle content can be suitably produced, for example, by the method described below.

[0019] Regarding the maximum length of the voids in the rare earth phosphate particles, the average value of the maximum length of the voids is preferably 1 nm or more, more preferably 8 nm or more, even more preferably 10 nm or more, and even more preferably 20 nm or more, from the viewpoint of increasing the whiteness of the rare earth phosphate powder. Furthermore, from the viewpoint of ensuring the strength of the rare earth phosphate particles, the average value of the maximum length of the voids is preferably 100 nm or less, more preferably 75 nm or less, even more preferably 60 nm or less, even more preferably 50 nm or less, and even more preferably 38 nm or less. In the following description, for convenience, the average value of the maximum length of the voids is also referred to as the "average maximum length of the voids." The average maximum length of the voids refers to the arithmetic average value of the maximum length of the voids measured for 100 or more rare earth phosphate particles having voids. When two or more voids exist in a particle, the maximum length refers to the arithmetic average value of the maximum length of each void. For example, two voids, void A and void B, exist in one rare earth phosphate particle, and the maximum length of void A is L Ａ and the maximum length of the gap B is L ＢWhen the maximum length of the voids in the rare earth phosphate particles is (L Ａ +L Ｂ ) / 2.

[0020] In the present invention, from the viewpoint of further increasing the whiteness of the rare earth phosphate powder, the number of voids per particle, based on the number of all rare earth phosphate particles, is preferably 0.1 / particle or more, more preferably 0.2 / particle or more, even more preferably 0.3 / particle or more, and even more preferably 2.5 / particle or more. Furthermore, from the viewpoint of ensuring the strength of the rare earth phosphate particles, the number of voids per particle, based on the number of all rare earth phosphate particles, is preferably 100 / particle or less, more preferably 50 / particle or less, even more preferably 40 / particle or less, and even more preferably 10 / particle or less. For simplicity in the following explanation, the ratio of the number of voids per particle, based on the number of all rare earth phosphate particles, is also referred to as "porosity". A rare earth phosphate powder having the above-mentioned porosity can be suitably produced, for example, by the method described below.

[0021] The average particle size D of the rare earth phosphate particles is preferably 10 nm or more, more preferably 80 nm or more, and even more preferably 100 nm or more, from the viewpoint of suppressing an increase in viscosity of a coating liquid containing the rare earth phosphate particles. Furthermore, the average particle size D of the rare earth phosphate particles is preferably 250 nm or less, more preferably 200 nm or less, even more preferably 170 nm or less, and even more preferably 150 nm or less, from the viewpoint of improving the dispersibility of the rare earth phosphate particles in a coating liquid containing the rare earth phosphate particles. The average particle size D of the rare earth phosphate particles is measured by TEM observation of the rare earth phosphate particles. The length of the longest line segment among the line segments crossing the rare earth phosphate particles is defined as the particle size of the rare earth phosphate particles. The particle sizes of 100 or more rare earth phosphate particles are measured, and the arithmetic average value thereof is defined as the average particle size D.

[0022] The degree of whiteness of the rare earth phosphate powder of the present invention, expressed as a whiteness index value defined below, is preferably 0.01 particles / particle or more, from the viewpoint of obtaining a rare earth phosphate powder closer to white. From this viewpoint, the whiteness index value is more preferably 0.1 particles / particle or more, and even more preferably 0.2 particles / particle or more. The whiteness index value is defined as porosity (particles / particle) × average maximum void length (nm) / average particle size D (nm) of the rare earth phosphate particles.

[0023] In relation to the above-mentioned whiteness index value, it is preferable that the rare earth phosphate powder of the present invention has a high whiteness L* value and low redness a* and yellowness b* values, since this prevents the resin molded product containing the rare earth phosphate powder and a resin from being discolored. Specifically, the whiteness L* value is preferably 98.50 to 100, more preferably 99.00 to 100, and even more preferably 99.20 to 100. The redness a* value is preferably -0.10 to 0.1, more preferably -0.04 to 0.04, and even more preferably -0.02 to 0.02. Furthermore, the yellowness b* value is preferably 0 to 0.6, more preferably 0 to 0.5, and even more preferably 0.1 to 0.4. Methods for measuring the whiteness L* value, redness a* value, and yellowness b* value will be described in the examples below.

[0024] The rare earth phosphate powder of the present invention can be subjected to a lipophilic treatment on the surface of the rare earth phosphate particles to improve dispersibility in the resin molded product described below, as long as the effects of the present invention are not impaired. Examples of lipophilic treatment include treatment with various coupling agents. Examples of coupling agents include organometallic compounds. Specific examples of coupling agents that can be used include silane coupling agents, zirconium coupling agents, titanium coupling agents, and aluminum coupling agents.

[0025] The above coupling agents can be used alone or in combination of two or more. When a silane coupling agent is used as the coupling agent, the surface of the rare earth phosphate particles is coated with a silane compound. This silane compound preferably has a lipophilic group, such as an alkyl group or a substituted alkyl group. The alkyl group may be linear or branched. In either case, it is preferable that the alkyl group have 1 to 20 carbon atoms in order to achieve good affinity with the resin. When the alkyl group is substituted, the substituent can be an amino group, a vinyl group, an epoxy group, a styryl group, a methacrylic group, an acrylic group, a ureido group, a mercapto group, a sulfide group, an isocyanate group, or the like. The amount of silane compound coating the surface of the rare earth phosphate particles is preferably 0.01 to 200% by mass, particularly 0.1 to 100% by mass, based on the mass of the rare earth phosphate particles, in order to achieve good affinity with the resin.

[0026] Next, a preferred method for producing the rare earth phosphate powder of the present invention will be described. To produce the rare earth phosphate powder of the present invention, an aqueous solution containing one or more rare earth element sources (hereinafter also referred to as the "first solution") is first prepared. At the same time, an aqueous solution containing phosphoric acid or a salt thereof and a basic substance (hereinafter also referred to as the "second solution") is also prepared.

[0027] The first liquid can be prepared, for example, by dissolving an oxide of a rare earth element in an aqueous solution of a mineral acid (e.g., an aqueous nitric acid solution). The concentration of the rare earth element contained in the first liquid can be, for example, 0.3 mol / L to 1.0 mol / L. The second liquid can be prepared, for example, by dissolving orthophosphoric acid or a salt thereof and a basic substance in water. The concentration of phosphate ions contained in the second liquid can be, for example, 0.3 mol / L to 3.0 mol / L. The concentration of the basic substance contained in the second liquid can be, for example, such that the pH of the second liquid is preferably 2 to 6, more preferably 2 to 4. By adding a basic substance to the second liquid and adjusting the pH of the second liquid, it is possible to successfully produce rare earth phosphate particles having voids. When the second liquid does not contain a basic substance and the pH of the second liquid is not adjusted, voids cannot be formed in the rare earth phosphate particles, as shown in Comparative Examples 1 to 3 described below.

[0028] Examples of the basic substance contained in the second liquid include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, ammonia, ammonium hydrogen carbonate, ammonium carbonate, sodium hydrogen carbonate, sodium carbonate, ethylamine, propylamine, etc. In particular, using ammonia as the basic substance is preferable because it can successfully form voids in the rare earth phosphate particles.

[0029] Next, the first liquid and the second liquid are mixed. The mixing of the two liquids is achieved by adding the second liquid to the first liquid. When the first liquid and the second liquid are added simultaneously, voids cannot be formed in the rare earth phosphate particles, as shown in Comparative Example 4 described below. When mixing the two liquids, the first liquid and / or the second liquid may be heated. From the viewpoint of successfully forming voids in the rare earth phosphate particles, it is preferable to add the second liquid sequentially or continuously to the first liquid in a heated state. In this case, the heating temperature of the first liquid is preferably 80°C or higher and 98°C or lower from the viewpoint of forming voids.

[0030] The amounts of the first and second solutions used are determined based on the ratio of the number of moles of phosphate ions to the number of moles of rare earth elements, i.e., PO ４ ３－ / Ln is preferably 1.2 or more and 3.0 or less, and more preferably 1.5 or more and 2.0 or less.

[0031] By mixing the two liquids, a precipitate containing rare earth elements and phosphorus is generated in the liquid. After mixing the two liquids, the mixed liquid is left to stand for a predetermined time to mature, which allows nucleation and growth of the precipitate to proceed. Once the precipitate is obtained in this way, the mixed liquid is subjected to solid-liquid separation and the precipitate is recovered.

[0032] The recovered precipitate is subjected to a calcination process after washing, drying, pulverization, and classification. From the viewpoint of successfully obtaining the desired phosphate particles, calcination of the precipitate is preferably carried out at 600°C to 1500°C, particularly 700°C to 1500°C, further 800°C to 1200°C, and particularly 1000°C to 1200°C. Setting the calcination temperature high tends to cause small voids to coalesce and form larger voids. The calcination time is preferably 1 hour to 4 hours, more preferably 2 hours to 4 hours, provided that the calcination temperature is within the above-mentioned range. The calcination atmosphere can be, for example, an oxygen-containing atmosphere such as air or an inert atmosphere such as nitrogen or argon. Through these processes, rare earth phosphate particles having voids are obtained. The rare earth phosphate powder obtained in this manner has a high whiteness due to the voids in the rare earth phosphate particles that constitute it.

[0033] The rare earth phosphate powder thus obtained can be dispersed in a resin to form a resin composition, which can then be used as a light-scattering member to improve the light-scattering properties of the resin composition. The form of the resin composition is not particularly limited, and examples include a sheet (film), membrane, powder, pellet (masterbatch), dispersion (coating liquid), etc., but a sheet form is advantageous because it can be easily applied to a light-scattering sheet. There are no particular limitations on the type of resin to which the rare earth phosphate powder of the present invention can be added, and moldable thermoplastic resins and thermosetting resins can be used. In particular, it is preferable to use a thermoplastic resin because it is easy to mold into a sheet. Examples of thermoplastic resins include olefin homopolymers such as polyethylene and polypropylene, copolymers of polyethylene or polypropylene with other α-olefins, polyesters such as polyethylene terephthalate and polybutylene terephthalate, homopolymers and copolymers of acrylic acid-based monomers such as acrylic acid, acrylic acid esters, methacrylic acid and methacrylic acid esters, homopolymers and copolymers of vinyl-based monomers such as styrene, homopolymers and copolymers of chlorine-containing monomers such as vinyl chloride and vinylidene chloride, polycarbonates, and cellulose-based resins such as triacetyl cellulose, etc. These thermoplastic resins may be used alone or in combination of two or more.

[0034] When the rare earth phosphate particles of the present invention are dispersed in a resin to form a light-scattering member, the proportion of the rare earth phosphate powder contained in the light-scattering member is preferably 0.05% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 10% by mass or less, and even more preferably 0.1% by mass or more and 5% by mass or less, relative to the total mass of the light-scattering member, taking into consideration the balance between transmittance and light-scattering properties.

[0035] To obtain a light-scattering member composed of a resin composition containing the rare earth phosphate powder of the present invention and a resin, for example, the rare earth phosphate powder of the present invention may be kneaded into a molten resin, and then molded into a sheet by a known sheet molding method such as an inflation method, a T-die method, a solution casting method, or a calendar method.

[0036] Furthermore, the rare earth phosphate powder of the present invention can also improve the scattering properties of a resin molded product by disposing the powder on the surface of the resin molded product. A method for disposing the rare earth phosphate powder of the present invention on the surface of a resin molded product involves, for example, mixing a composition containing the rare earth phosphate powder of the present invention, an organic solvent, and a binder resin to prepare a coating liquid, and then applying or spraying the coating liquid to the surface of the resin molded product using a roller, a spray gun, or the like. In this case, a light-scattering member is obtained in which a coating layer composed of a resin composition containing the rare earth phosphate powder of the present invention and a resin is disposed on the surface of the resin molded product. Another method for disposing the rare earth phosphate powder of the present invention on the surface of a resin molded product involves directly disposing the rare earth phosphate powder of the present invention on the surface of the resin molded product using sputtering or the like, without using a binder such as a resin.

[0037] As described above, when the coating layer is used as a light-scattering member provided on the surface of a resin molded body, the proportion of the rare earth phosphate powder contained in the coating layer is preferably 0.01 mass % or more and 90 mass % or less, and more preferably 0.1 mass % or more and 65 mass % or less, relative to the total mass of the coating layer, taking into consideration the balance between transmittance and light-scattering properties.

[0038] The light-scattering member obtained by such a method can be suitably produced as, for example, a display, a lighting member, a window member, an illumination member, a light guide plate member, a projector screen, a transparent screen used in a head-up display or the like, an agricultural material such as a greenhouse, etc. The light-scattering member can also be incorporated into an optical device for use. Examples of such optical devices include a liquid crystal TV, a personal computer, a tablet, a smartphone, and other mobile devices.

[0039] In relation to the above-described embodiment, the following rare earth phosphate powder, a method for producing the same, and a light scattering member are further disclosed. [1] LnPO ４(wherein Ln represents at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu), wherein the rare earth phosphate powder comprises rare earth phosphate particles having internal voids, and the ratio of the number of the rare earth phosphate particles having the voids with a maximum length of 3.0 nm or more to the number of all rare earth phosphate particles is 10% or more by number. [2] The rare earth phosphate powder according to [1], wherein the number of the voids per particle is 0.1 or more and 100 or less, based on the number of all rare earth phosphate particles. [3] The rare earth phosphate powder according to [1] or [2], wherein the average maximum length of the voids is 1 nm or more and 100 nm or less. [4] The rare earth phosphate powder according to any one of [1] to [3], wherein the average particle size of the rare earth phosphate particles is 10 nm or more and 150 nm or less. [5] The rare earth phosphate powder according to any one of [1] to [4], wherein Ln is Y. [6] A light-scattering member comprising the rare earth phosphate powder according to any one of [1] to [5] dispersed in a resin. [7] A light-scattering member comprising the rare earth phosphate powder according to any one of [1] to [5] disposed on the surface of a resin molded body. [8] A method for producing a rare earth phosphate powder, comprising adding an aqueous solution containing phosphoric acid or a salt thereof and a basic substance to an aqueous solution containing a rare earth element to produce a precipitate containing a rare earth element and phosphorus, and calcining the precipitate to obtain a rare earth phosphate powder containing rare earth phosphate particles having internal voids. [9] The production method according to [8], wherein the basic substance is ammonia.

[0040] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass."

[0041] Example 1 In this example, a powder containing yttrium phosphate particles was produced. The production procedure is as follows. 3,600 g of water was weighed into a glass container, and 880 g of 67.5% nitric acid and 290 g of yttrium oxide were added and heated to 60°C to dissolve the mixture, thereby obtaining a first liquid. The concentration of yttrium in the first liquid was 0.6 mol / L. 3,900 g of water, 600 g of 85% phosphoric acid, and 320 g of 25% aqueous ammonia were added to another glass container to obtain a second liquid. The concentration of phosphate ions in the second liquid was 1.1 mol / L. The pH of the second liquid was 3. The first liquid was heated to 90°C, and the second liquid was successively added dropwise while stirring. The PO at the time of completion of the dropping was 0.6 mol / L. ４ － The molar ratio of yttrium phosphate to yttrium phosphate was 1.8. By mixing the two liquids, a precipitate was formed in the liquid. After aging for 3 hours, the precipitate was washed by decantation. After washing, the precipitate was recovered by solid-liquid separation using vacuum filtration, dried in the air, and then pulverized in a pulverizer. The pulverized product was fired in the air at 1200°C for 3 hours. In this way, an yttrium phosphate powder containing voided yttrium phosphate particles was obtained.

[0042] Examples 2 to 6 Yttrium phosphate powders were obtained in the same manner as in Example 1, except that the firing was carried out at the temperatures shown in Table 1 below.

[0043] Comparative Example 1 600 g of water was weighed into a glass container, and 61.7 g of 60% nitric acid and 18.8 g of yttrium oxide were added and heated to 60°C to dissolve, thereby obtaining a first liquid. The concentration of yttrium in the first liquid was 0.26 mol / L. 600 g of water and 18.8 g of 85% phosphoric acid were added to another glass container to obtain a second liquid. The concentration of phosphate ions in the second liquid was 0.27 mol / L. The pH of the second liquid was 1. The first liquid was heated to 80°C, and the second liquid was successively added dropwise while stirring. The PO at the time of completion of the dropping was ４ －The molar ratio of yttrium phosphate to yttrium phosphate was 1.0. By mixing the two solutions, a precipitate was formed in the solution. After aging for 1 hour, the precipitate was washed by decantation. After washing, the precipitate was recovered by solid-liquid separation using vacuum filtration, dried in the air, and then pulverized in a pulverizer. The pulverized product was fired in the air at 800°C for 3 hours. In this way, yttrium phosphate powder was obtained.

[0044] Comparative Examples 2 and 3 Yttrium phosphate powders were obtained in the same manner as in Comparative Example 1, except that firing was carried out at the temperatures shown in Table 1 below.

[0045] Comparative Example 4 370 g of water was weighed into a glass container, and 14.4 g of 85% nitric acid and 4.2 g of yttrium oxide were added and heated to 60°C for dissolution to obtain a first liquid. The concentration of yttrium in the first liquid was 0.1 mol / L. A second liquid was obtained by adding 390 g of water, 5.3 g of 25% phosphoric acid, and 9.3 g of 25% aqueous ammonia to another glass container. The concentration of phosphate ions in the second liquid was 0.03 mol / L. The pH of the second liquid was 7. The first and second liquids were simultaneously added dropwise to a tank at 25°C and stirred at high speed with a homogenizer. The PO at the time of completion of the addition was 0.1 mol / L. ４ － The molar ratio of yttrium phosphate to yttrium phosphate was 0.3. By mixing the two solutions, a precipitate was formed in the solution. After aging for 1 hour, the precipitate was washed by decantation. After washing, the precipitate was recovered by solid-liquid separation using vacuum filtration, dried in the air, and then pulverized in a pulverizer. The pulverized product was fired in the air at 800°C for 5 hours. In this way, yttrium phosphate powder was obtained.

[0046] [Evaluation] The average particle size, average maximum void length, void particle content, porosity, and whiteness index value of the yttrium phosphate powders obtained in the Examples and Comparative Examples were measured using the methods described above. The L*, a*, and b* values ​​in the L*a*b* color system were also measured using the following methods, and the color difference ΔE* was calculated from the results using the following method. Furthermore, the total light transmittance and diffuse transmittance were measured using the following methods. The results are shown in Table 2 below. Furthermore, TEM images of the yttrium phosphate powders obtained in Examples 1 and 2 and Comparative Example 4 are shown in Figures 1 to 3.

[0047] [L* Value, a* Value, and b* Value in L*a*b* Color System] Measurements were made using a spectrophotometer (CM-2600d, manufactured by Konica Minolta) in accordance with JIS Z8729 "Method for expressing object colors in the U*V*W* system."

[0048] [Calculation of Color Difference ΔE*] The color difference ΔE* between the L* value, a* value, and b* value measured by the above method and white (ΔL*=100, Δa*=0, Δb*=0) was calculated according to the following formula: ΔE*=((ΔL*) ２ +(Δa*) ２ +(Δb*) ２ ) １／２ The smaller the color difference ΔE*, the closer the object to be measured is to white.

[0049] [Measurement of total light transmittance and diffuse transmittance] The yttrium phosphate powder of the examples and comparative examples was added to an acrylic resin (manufactured by Mitsubishi Chemical Corporation, product name: Dianal LR-167), diluted with a toluene solvent so that the volume concentration of the powder was 10%, and mixed with a paint shaker to prepare a coating liquid. Next, this coating liquid was applied to a PET film (manufactured by Toray Industries, Inc., thickness: 0.1 mm) using a bar coater (#3) and dried at 80 ° C. for 5 minutes to obtain a light-scattering member consisting of a light-scattering layer and a PET film substrate layer. The total light transmittance and diffuse transmittance of the light-scattering member were measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name: NDH4000).

[0050]

[0051]

[0052] As is clear from the results shown in Table 2, the powders containing yttrium phosphate particles obtained in each example had small color difference ΔE* values ​​and high whiteness. Furthermore, as is clear from a comparison of Examples 1 to 6, the maximum length of the voids increases as the firing temperature increases.

[0053] According to the present invention, there is provided a rare earth phosphate powder having high whiteness and useful as a light scattering material.

Claims

1. LnPO ４ (wherein Ln represents at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu), wherein the rare earth phosphate powder comprises rare earth phosphate particles having voids therein, and the ratio of the number of the rare earth phosphate particles having the voids, the maximum length of which is 3.0 nm or more, to the number of all rare earth phosphate particles is 10% or more.

2. The rare earth phosphate powder according to claim 1, wherein the number of voids per particle is 0.1 or more and 100 or less per particle, based on the total number of rare earth phosphate particles.

3. The rare earth phosphate powder according to claim 1, wherein the average maximum length of the voids is 1 nm or more and 100 nm or less.

4. The rare earth phosphate powder according to claim 1, wherein the average particle size of the rare earth phosphate particles is 10 nm or more and 150 nm or less.

5. The rare earth phosphate powder according to claim 1, wherein Ln is Y.

6. A light-scattering member comprising the rare earth phosphate powder according to any one of claims 1 to 5 dispersed in a resin.

7. A light scattering member comprising a resin molded body on the surface of which the rare earth phosphate powder according to any one of claims 1 to 5 is disposed.

8. A method for producing a rare earth phosphate powder, comprising adding an aqueous solution containing phosphoric acid or a salt thereof and a basic substance to an aqueous solution containing a rare earth element to form a precipitate containing the rare earth element and phosphorus, and calcining the precipitate to obtain a rare earth phosphate powder containing rare earth phosphate particles having internal voids.

9. The method of claim 8, wherein the basic substance is ammonia.

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