JP7474909B1 - Rare Earth Oxide Powder - Google Patents

Abstract

A powder of an oxide of at least one rare earth element other than Ce, which satisfies (a) or (b). (a) The primary particle size is 10 to 60 nm, and satisfies (I) or (II). (I) D100 is 1 to 10 μm, as measured by a laser diffraction/scattering particle size distribution measurement method after ultrasonic dispersion treatment. (II) When the true density of the rare earth oxide is ρ (g/cm3), the difference (PAD-PTD) between the porosity PAD (%) calculated from the initial bulk density AD and the porosity PTD (%) calculated from the tapped bulk density TD is 2.0 to 5.0%. PAD = (1-AD/ρ) x 100 (%) PTD = (1-TD/ρ) x 100 (%) (b) The primary particle size is 10 nm or more and less than 100 nm, and satisfies (III) and (IV). (III) The value obtained by multiplying the volume (cm3/g) of pores having a diameter of 0.005 to 100 μm by the true density (g/cm3) is 3 to 14. (IV) The value obtained by multiplying the volume (cm3/g) of pores having a diameter of 5 to 50 nm by the true density (g/cm3) is 0 to 2.0.

Description

The present invention relates to rare earth oxide powder.

Rare earth oxides are used in dielectrics or internal electrodes for capacitors, phosphors, refractive index adjusters for optical glass, oxygen sensors, sintering aids for ceramics, catalysts, refractories, etc. They are used in a variety of forms, including coatings (paint films), trace additives, and molded bodies (including sintered bodies).

In the case of coating a slurry containing rare earth oxide powder and a firing process is performed after coating, the thermal diffusivity can be improved by increasing the specific surface area of the rare earth oxide powder or reducing the primary particle size. In addition, when using rare earth oxide as an additive and firing, small primary particles may facilitate thermal diffusion. From these viewpoints, various rare earth oxide fine powders are known. Patent Document 1 describes that ultrafine yttrium oxide particles with an average primary particle size of 80 Å were obtained (Example 1 of Patent Document 1). Patent Document 2 describes that yttrium oxide fine powder was obtained in the form of "scanning electron microscope (SEM) observation (omitted) a group of spherical particles with a uniform particle size of about 100 nm without aggregation" (Example 1 of Patent Document 2). Non-Patent Document 1 also shows TEM images of Dy ₂ O ₃ fine powder crystallized as a result of long-term exposure to the electron beam of the TEM (Fig. 1a b of Non-Patent Document 1).

On the other hand, powders with small primary particle sizes, such as those on the order of several tens of nanometers, easily form agglomerates, and the agglomerated particles become hard agglomerates in the case of fine nanopowder with a small average particle size (see, for example, paragraph [0014] of Japanese Patent No. 6119528). Therefore, in order to coat particles with small primary particles, it is necessary to disintegrate them with high energy using media, etc.

Patent document 3 describes that yttrium oxide powder was dispersed in a slurry using a dispersant and the median particle size D50 was measured to be 5.3 nm (Example 1 of Patent Document 3).

A slurry of fine rare earth oxide powder can be highly dispersed by using an appropriate dispersant (see, for example, paragraphs [0020] and [0029] of JP 2007-126349 A). However, depending on the ingredients, the dispersant may not work, and depending on the application, the dispersant may become an impurity.

Japanese Patent Application Laid-Open No. 4-310516 JP 2014-218384 A US2020/0071180A

J Nanopart Res (2013) 15:1438

The applicant has found that the rare earth oxide powder produced by the method described in Patent Documents 1-2 and Non-Patent Document 1 does not appear to be aggregated when observed with a scanning electron microscope (SEM), but when the aggregate diameter is measured by a macroscopic measurement method (laser scattering method), large coarse particles are measured. If the aggregate diameter is large, it becomes difficult to apply a thin film in the case of a coating liquid. Even if the aggregate diameter is reduced by crushing, the aggregation force becomes strong due to the small size of the primary particles, so that strong dispersion is required, and contamination originating from the grinding media increases. In addition, when the powder is dried after wet crushing, the powder aggregates during drying and the particle size becomes large. In particular, it is difficult to suppress aggregation of fine powders of rare earth oxides other than _CeO2 .

Therefore, the object of the first invention is to provide a rare earth oxide fine powder that can be easily dispersed without intensive crushing treatment and that can form a thin coating film.

After extensive research, the inventors have surprisingly found that the above problems can be solved by having a specific primary particle diameter (SSA equivalent diameter) and setting the agglomeration diameter within a specific range when subjected to a specified ultrasonic treatment, or by having a specific primary particle diameter (SSA equivalent diameter) and setting the porosity difference calculated from the initial bulk density AD and the tapped bulk density TD within a specified range.

The first invention is based on the above findings and provides the following [a1] to [a9].
[a1] A powder of an oxide of at least one rare earth element other than Ce,
A rare earth oxide powder having a primary particle diameter of 10 nm or more and 60 nm or less and satisfying the following (I) or (II):
(I) The volume cumulative particle size D ₁₀₀ at 100% cumulative volume measured by a laser diffraction/scattering particle size distribution measurement method after ultrasonic dispersion treatment at 40 W for 5 minutes is 1 μm or more and 10 μm or less.
(II) When the true density of the rare earth oxide is ρ (g/ ^cm3 ), the difference (P _AD - P _TD ) between the porosity P _AD (%) calculated from the initial bulk density AD using the following formula 1 and the porosity P _TD (%) calculated from the tapped bulk density TD using the following formula 2 is 2.0% or more and 5.0% or less.
Formula 1: P _AD = (1 - AD / ρ) × 100 (%)
Formula 2: P _TD = (1 - TD / ρ) × 100 (%)
[a2] The rare earth oxide powder according to claim 1, which corresponds to (I) above.
[a3] The rare earth oxide powder according to [a1] or [a2], wherein when the true density of the rare earth oxide is ρ (g/cm ³ ), the porosity P _AD (%) calculated from the initial bulk density AD by the following formula 1 is 90.0% or more and 99.0% or less.
Formula 1: P _AD = (1 - AD / ρ) × 100 (%)
[a4] The rare earth oxide powder according to any one of [a1] to [a3], having a primary particle diameter of 35 nm or less.
[a5] The rare earth oxide powder according to any one of [a1] to [a4], wherein the Zr content is 100 ppm by mass or less.
[a6] The rare earth oxide powder according to any one of [a1] to [a5], having a carbon content of 2 mass% or less.
[a7] The rare earth oxide powder according to [a1], which corresponds to the above (II).
[a8] The rare earth oxide powder according to [a7], wherein the porosity P _AD (%) is 90.0% or more and 99.0% or less.
[a9] The rare earth oxide powder according to [a7] or [a8], wherein the volume cumulative particle size _D100 at 100% cumulative volume, as measured after ultrasonic dispersion treatment at 40 W for 5 minutes, is 1 μm or more and ₁₀ μm or less, as determined by a laser diffraction/scattering particle size distribution measurement method, and the ratio _D100 to the volume cumulative particle size _D50 at ₅₀ % cumulative volume, as measured after ultrasonic dispersion treatment, as determined by the above measurement method, is 3.0 or more and 11.0 or less.

Even in rare earth oxide fine powders that appear to have little aggregation when observed with a scanning electron microscope (SEM) as shown in Patent Documents 1 to 3, Non-Patent Document 1, etc., when the primary particles are as small as several tens of nm, the cohesive force is strong and wet dispersing using a dispersant is required to disperse the particles in a slurry.
In view of the above circumstances, it is preferable that the slurry of the rare earth oxide fine powder can be made to be in a highly dispersed state without using a dispersant.
However, it has been difficult to achieve a high dispersion state of the rare earth oxide fine powder in the slurry without using a dispersant, and it has been even more difficult to stably maintain the dispersion state.
In particular, it has been difficult to prepare a highly dispersed slurry of fine powders of oxides of rare earth elements other than Ce without using a dispersant.

Therefore, the object of the second invention is to provide a fine oxide powder of a rare earth element other than Ce that can be made into a highly dispersed slurry without using a dispersant and that can stably maintain the transparency of the slurry.

After extensive research, the inventors have surprisingly found that the above problems can be solved by having a specific primary particle diameter and by setting the product of the pore volume/pore volume and true density within a specific range.

The second invention is based on the above findings and provides the following [b1] to [b7].

[b1] A powder of an oxide of at least one rare earth element other than Ce,
The primary particle size is 10 nm or more and less than 100 nm, and the following (III) and (IV) are satisfied.
(III) The value obtained by multiplying the pore volume (cm ³ /g) of pores having diameters of 0.005 μm or more and 100 μm or less by the true density (g/cm ³ ) is 3 or more and 14 or less.
(IV) The value obtained by multiplying the volume (cm ³ /g) of pores having a diameter of 5 nm or more and 50 nm or less by the true density (g/cm ³ ) is 0 or more and 2.0 or less.
[b2] The rare earth oxide powder according to [b1], having a Na content of 10 ppm by mass or less.
[b3] The rare earth oxide powder according to [b1] or [b2], having a crystallite size of 6 nm or more and 25 nm or less.
[b4] The rare earth oxide powder according to any one of [b1] to [b3], in which when the rare earth oxide powder is mixed with ethanol to prepare an ethanol slurry containing 10 mass% of the rare earth oxide powder, and the following operation (A) is repeatedly performed until the average particle diameter becomes larger than the previous measurement value, the smallest average particle diameter is 10 nm or more and 150 nm or less.
(A): The slurry is subjected to a bead mill treatment for 10 minutes using zirconia beads having a diameter of 0.1 mm, and then the average particle size is measured.
(However, if the average particle diameter becomes larger than the previous measurement, the bead mill treatment is terminated at that point, and even if the average particle diameter does not become larger than the previous measurement in each of the second to twentieth repetitions of (A) 20 times, the treatment is terminated when (A) has been repeated 20 times. The minimum average particle diameter here means the minimum value of the average particle diameter measured by dynamic light scattering method by sampling after each treatment of (A).)
[b5] The rare earth oxide powder according to [b4], wherein the calculated value of the following formula is −15% or more and 25% or less.
[b6] The rare earth oxide powder according to [b4] or [b5], wherein the minimum average particle size obtained by the bead mill treatment is 50 nm or more and 90 nm or less.
[b7] A method for producing a slurry, comprising wet-grinding the rare earth oxide powder according to any one of [b1] to [b6] using a solvent.

Hereinafter, in this specification, the "oxide of a rare earth element" may be referred to as the "rare earth oxide".
The first aspect of the invention will now be described in detail based on its preferred embodiments.
The first invention relates to a powder of an oxide of a rare earth element other than Ce.
_CeO2 , an oxide of Ce, can be obtained unsintered by adding _an oxidizing agent ( _H2O2 ) to cerium hydroxide in water, and can also be easily obtained by sintering the precursor at a low temperature because Ce is easily oxidized. For this reason, _CeO2 is less susceptible to agglomeration caused by sintering during production, and can be easily disintegrated even if it is a powder with primary particles of several tens of nanometers.
On the other hand, rare earth elements other than Ce do not become oxides like Ce when not fired even if an oxidizing agent is added to the hydroxide. In addition, the precursor usually needs to be fired at a relatively high temperature for production, and high-temperature firing is one of the causes of necking. Therefore, in the conventional rare earth element powders other than Ce, it was very difficult to suppress the aggregation of the powder when the primary particles were several tens of nm.

In the first invention, the oxide of a rare earth element may be at least one oxide selected from Sc, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Specifically, the oxide of a rare earth element other than Ce may be _{Sc2O3 , Y2O3 , La2O3 , Pr6O11 , Nd2O3 , Sm2O3 , Eu2O3 , Gd2O3 , Tb4O7 , Dy2O3 , Ho2O3 , Er2O3 , Tm2O3 , Yb2O3 ,} and _{Lu2O3 .} In view of the excellent significance of the effect of the first invention due to the conventional difficulty in suppressing aggregation, the oxide of a rare earth element is preferably at least one oxide selected from Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, more preferably at least one oxide selected from Y, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, even more preferably at least one oxide selected from Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and particularly preferably at least one oxide selected from Y, Dy, Ho, Er and Yb.

The primary particle diameter of the rare earth oxide powder is preferably within a certain range. In rare earth oxide powder, the smaller the primary particle diameter, the better the thermal diffusion, but if it is too small, the stronger the aggregation and the more difficult it is to disintegrate. On the other hand, if the primary particle diameter is too large, necking occurs and the aggregate diameter becomes large, and when the rare earth oxide powder is disintegrated with a bead mill or the like, the particles are crushed, increasing the active surface and resulting in an unstable slurry. From these points, in the first invention, the primary particle diameter of the rare earth oxide powder is preferably 10 nm or more and 60 nm or less, more preferably 15 nm or more and 60 nm or less, and even more preferably 15 nm or more and 35 nm or less. A primary particle diameter of 35 nm or less of the rare earth oxide powder is particularly preferable in terms of improving the coating properties when used as a coating liquid and the large thermal diffusion of the rare earth oxide powder.

In the first invention, the primary particle diameter of the rare earth oxide powder is the primary particle diameter calculated based on the specific surface area, specifically, the particle diameter calculated from the specific surface area s ( ^m2 /g) measured by the BET single point method. For example, the primary particle diameter d (nm) is d = 6000/(ρs) (ρ is the true density (g/ ^cm3 )).

The specific surface area of the rare earth oxide powder of the first invention is, for example, preferably from ¹⁰ to 160 m ² /g, more preferably from ¹⁵ to 110 m ² /g, and particularly preferably from 20 to 80 ^{m 2 /} g.

In the first invention, it is preferable that the volume-based D ₁₀₀ (volume cumulative particle size at 100% cumulative volume) measured by a laser diffraction scattering type particle size distribution measurement method after ultrasonic irradiation is within a specific range in order to enable a thin film to be coated. For example, from the viewpoint of coating properties, D ₁₀₀ after ultrasonic irradiation is particularly preferably 10 μm or less, more preferably 9 μm or less, and even more preferably 7 μm or less. In order to produce a thin film, coating is not possible if there are large coarse particles. In addition, if the dispersibility of the slurry is too good, the stability of the slurry is poor and it may aggregate. This will change the physical properties such as viscosity, and the coating conditions will also change. From the viewpoint of stability, the above D ₁₀₀ is preferably 1 μm or more, and more preferably 2 μm or more.

More specifically, the ultrasonic irradiation is a process of dispersing the sample with 40 W ultrasonic waves at a frequency of 40 kHz for 5 minutes. The irradiation device may be an attachment to a laser diffraction scattering type particle size measuring device, and may be irradiated to a sample in which rare earth oxide powder has been added to a 0.2% by mass aqueous solution of sodium hexametaphosphate. The concentration of the rare earth oxide powder in the dispersion liquid during ultrasonic irradiation is preferably a concentration that the particle size measuring device judges to be an appropriate concentration for particle size measurement, and is usually in the range of 0.002 to 0.2% by mass. The ultrasonic irradiation is specifically performed according to the method described in the examples, but irradiation may be performed using a device other than that attached to the laser diffraction scattering type particle size measuring device used for measurement, as long as it is an equivalent irradiation device. However, when irradiation is performed using a device other than that attached to the laser diffraction scattering type particle size measuring device used for measurement, 0.2 g of the sample is placed in about 100 ml of a 0.2% by mass aqueous solution of sodium hexametaphosphate, and after ultrasonic irradiation, the slurry after ultrasonic irradiation is added to the sample circulation device until the particle size measuring device judges it to be an appropriate concentration for particle size measurement, and then the measurement is performed.

From the standpoint of further improving the coatability and ease of coating, the particles of the first invention preferably have a volume-based _D90 (volume cumulative particle size at 90 volume % cumulative volume) measured by a laser diffraction scattering particle size distribution measurement method after the above-mentioned ultrasonic irradiation of 0.1 μm or more and 2.5 μm or less, even more preferably 0.3 μm or more and 2.3 μm or less, and particularly preferably 0.5 μm or more and 2.0 μm or less.

From the viewpoint of further improving the coatability and ease of coating, the particles of the first invention preferably have a volume-based _D50 (volume cumulative particle size at 50 volume % cumulative volume) measured by the laser diffraction scattering particle size distribution measurement method after the above-mentioned ultrasonic irradiation of 0.3 μm or more and 1.2 μm or less, more preferably 0.5 μm or more and 1.0 μm or less, and even more preferably 0.5 μm or more and 0.7 μm or less.

From the viewpoint of film-forming properties _{, it is particularly preferable that the particle size distribution has a specific range of D100/D50.Specifically, D100 /D50 is preferably} 3.0 or more and 11.0 or less, and more preferably 3.0 or more and 8.5 or less.

The rare earth oxide powder preferably has a small amount of impurities. In particular, by using a grinding media such as beads of a bead mill, D ₁₀₀ can be easily crushed to 1 to 20 μm, more preferably 10 μm, but the media become impurities and cause contamination. In the first invention, since the rare earth oxide powder can be produced without using the above-mentioned media, it is possible to reduce the Zr element, which is generally used as a constituent element of the grinding media, to 100 mass ppm or less. It is also easy to reduce it to 10 mass ppm or less, and it can also be reduced to 2 mass ppm or less. Such rare earth oxide powder is preferable in terms of reducing the risk of contamination, and is also suitable for applications such as corrosion-resistant materials for electronic components and semiconductor manufacturing equipment. The content of the Zr element can be measured by ICP emission spectrometry, and can be measured by the method described in the examples below. The measurement sample can be prepared by a conventional method, for example, by dissolving the rare earth oxide powder in nitric acid or sulfuric acid.

The carbon content of the rare earth oxide powder is preferably low. Rare earth elements and their compounds are frequently used as additives for the purpose of solid solution. If carbon components are present, mass reduction (volume change) occurs during firing, causing film cracks, etc. Therefore, the carbon content is preferably 2 mass% or less, more preferably 1 mass% or less, and particularly preferably 0.7 mass% or less. The carbon content can be measured by the method described in the examples below.
The rare earth oxide powder preferably has a purity of 99% by mass or more. For example, the total content of Zr and carbon is preferably 1% by mass or less.

The present inventors have found that for the ease of dispersion of a rare earth oxide powder having a primary particle size in terms of SSA within a predetermined range, it is particularly preferable that the difference (P _AD - P TD ) (%) between the porosity P _AD (%) obtained from the initial bulk density (AD) using Equation 1 and the porosity P _TD (%) obtained from the tapped bulk density ( _TD ) using Equation 2 be within a predetermined range, where ρ is the true density.
Formula 1: P _AD = (1 - AD / ρ) × 100 (%)
Formula 2: P _TD = (1 - TD / ρ) × 100 (%)

A large value of (P _AD - _{P TD} ) (%) means that the voids are easily clogged by tapping in tap bulk density measurement. Particles with too large a value of (P _AD - _{P TD} ) (%), that is, powders that can be compressed too easily, are powders with strong agglomeration. On the other hand, powders with too small a value of (P _AD - P _TD ) (%) mean that there is a lot of air between the particles, and that the powder is relatively fluffy and difficult to compress in that state. Such powders tend to trap air between the particles, resulting in large agglomeration diameters. Based on these facts, the inventors conducted research based on the assumption that the balance of the degree of compression affects the ease of agglomeration of rare earth oxide fine powders, and as a result, they found that this parameter affects the ease of dispersion of rare earth oxide powders.
Specifically, the difference between (P _AD - P _TD ) (%) is preferably 2.0% or more and 5.0% or less. By having (P _AD - P _TD ) (%) of 2.0% or more and 5.0% or less, the powder has a suitable degree of compressibility, is excellent in dispersibility compared to powders outside this range, and has excellent coatability when made into a coating liquid. From this viewpoint, it is more preferable that (P _AD - _{P TD} ) (%) is 3.0% or more and 5.0% or less, and even more preferable that it is 3.0% or more and 4.5% or less.

Furthermore, in consideration of the balance between inhibition of aggregation and ease of handling, the porosity P _AD (%) calculated from the initial bulk density is preferably 90.0% or more and 99.0% or less, and more preferably 92.0% or more and 98.0% or less. Having the above (P _AD -P _TD ) (%) and having P _AD (%) of 92.0% or more and 98.0% or less is preferable in that the aggregation property is particularly moderate and therefore dispersion is easy.
From the same viewpoint, P _TD calculated from the tapped bulk density is preferably 88.0% or more and 95.5% or less, and more preferably 88.5% or more and 92.0% or less.

In order to obtain the above-mentioned specific surface area ( ^m2 /g), primary particle diameter (nm), particle size distribution, initial bulk density (AD), tapped bulk density (TD), Zr content, carbon content, P _AD , P _TD and P _AD -P _TD , a suitable method for producing rare earth oxide powder described below may be adopted and the mixing, washing or firing conditions may be adjusted.

Next, a preferred method for producing the rare earth oxide powder of the first invention will be described.
This production method includes a reaction/solid-liquid separation step in which an aqueous carbonate solution (hereinafter also referred to as "liquid A") and an aqueous water-soluble salt solution of a rare earth element (hereinafter also referred to as "liquid B") are simultaneously charged into a reaction tank and mixed with high speed stirring so that the pH of the mixed solution becomes 6.5 to 7.0, preferably 6.5 to 6.9, to react the carbonate with the water-soluble salt of the rare earth element, and solid-liquid separation begins within 5 minutes from the start of mixing of liquids A and B; a washing step in which the residue obtained in the reaction/solid-liquid separation step is washed with alcohol; and a calcination step in which the washed residue is calcined.
In order to successfully obtain the rare earth oxide powder of the first invention, the water-soluble salt concentration, calculated as the oxide, in the water-soluble salt aqueous solution of the rare earth element, which is Liquid B, is preferably 10 to 400 g/L, more preferably 50 to 350 g/L, particularly preferably 80 to 300 g/L, and most preferably more than 100 g/L but not more than 300 g/L.

(Reaction and solid-liquid separation process)
In the solution A which is an aqueous carbonate solution, examples of the carbonate include ammonium hydrogen carbonate, ammonium carbonate, sodium carbonate, sodium hydrogen carbonate, etc. In this specification, carbonate includes not only normal salts but also hydrogen carbonate which is an acidic salt. Ammonium hydrogen carbonate is preferred from the viewpoints of easy adjustment of the pH of the mixed solution and reduction of the sodium content.
In the solution B, which is an aqueous solution of a water-soluble salt of a rare earth element, examples of the water-soluble salt of the rare earth element include nitrates, acetates, ammine complexes, and chlorides. Nitrates are preferred from the viewpoints of ease of adjusting the pH of the mixed solution and productivity.

In this manufacturing method, liquid A, which is an aqueous carbonate solution, and liquid B, which is an aqueous solution of a water-soluble salt of a rare earth element, are simultaneously introduced into a reaction vessel so that the pH of the mixture of the two is 6.5 to 7.0, preferably 6.5 to 6.9. The pH here is the pH at the temperature of the mixture of the two. If the pH of the mixture exceeds 7.0, the primary particles will become large. In addition, setting the pH at 6.5 or higher has the advantage that almost all of the rare earth ions in liquid A will precipitate. It is preferable that neither liquid A nor liquid B is heated at the time of introduction. It is preferable that liquid A and liquid B are 5 to 50°C at the time of introduction into the reaction vessel, and the temperature of the mixture is 5 to 40°C. In order to keep the primary particle size within a suitable range, it is preferable to carry out the reaction in an extremely short time and filter it immediately. For this reason, it is preferable to adjust the timing and rate of introduction of both liquids so that the pH of the mixed liquid is within the range of 6.5 to 7.0 from the time when the introduction of liquid A and liquid B into the reaction vessel starts (the time when mixing of liquid A and liquid B starts) to the time when the reaction product is generated (more specifically, the time when solid-liquid separation starts), thereby maintaining the pH constant within the above range. The reaction proceeds smoothly when the pH is within the above range.

The simultaneous addition of liquid A and liquid B means that the time when liquid A is added to the reaction vessel and the time when liquid B is added are at least partially simultaneous. As described above, in order for the pH of the mixed liquid to be within the range of 6.5 to 7.0 from the time when liquid A and liquid B start to be added to the reaction vessel (the time when liquid A and liquid B start to be mixed) to the time when the reaction product is generated (more specifically, the time when solid-liquid separation starts), it is preferable that the start of each addition is almost simultaneous and the addition speed is constant. In addition, the addition speed of liquid A and liquid B is adjusted so that solid-liquid separation can start within 5 minutes, more preferably within 3 minutes, from the start of mixing liquid A and liquid B.

In this manufacturing method, the mixed solution of solution A and solution B is simultaneously introduced into the reaction tank as described above and stirred at high speed under a predetermined pH condition. This embodiment makes it easy to obtain a powder having the above-mentioned primary particle size, agglomeration size D ₁₀₀ after ultrasonic irradiation, and porosity difference within the above-mentioned predetermined range. High-speed stirring can be performed at a rotation speed of 10,000 to 25,000 rpm, and more preferably 18,000 to 21,000 rpm. When the stirrer in the reaction tank of the mixed solution rotates as described above, the capacity of the reaction tank is preferably 50 ml to 1 L, and more preferably 100 ml to 500 ml.
The present inventors have found that, rather than adding liquid B to liquid A that has already been placed in a reaction tank, or pouring liquid A into liquid B that has already been placed in a reaction tank, by simultaneously pouring liquid A and liquid B into the reaction tank and stirring at high speed while maintaining a predetermined pH, the reaction can be completed in a short period of time, and that by subjecting this to a predetermined post-treatment, it is possible to obtain the rare earth oxide powder of the first invention, which has a small primary particle size and can be easily dispersed by low-intensity crushing.

From the viewpoint of making it easier to obtain the above-mentioned primary particle size, agglomeration _size D after ultrasonic irradiation, and porosity difference, the concentration of the carbonate in solution A is preferably 5 to 25 mass %, more preferably 10 to 15 mass %, and the concentration of the water-soluble salt of the rare earth element in solution B is as described above.

(Washing process)
The residue (also referred to as "solid matter") obtained in the solid-liquid separation step is washed with alcohol. The alcohol used in this production method is preferably one having a high purity, for example, an alcohol purity of 99.5 vol% or more. Examples of the alcohol include methanol, ethanol, and isopropanol, and ethanol is preferred from the viewpoint of usability. The amount of alcohol used for washing is preferably 0.1 L to 50 L, more preferably 0.5 L to 20 L, and even more preferably 1 L to 10 L, per 1 g of rare earth oxide used. The amount referred to here is the total amount when the alcohol is passed through the residue several times to wash it.

(Firing process)
The firing temperature is preferably 1000° C. or less in terms of suppressing aggregation and suppressing crystal growth, and more preferably 800° C. or less. The firing temperature is preferably 500° C. or more in terms of reducing the carbon content. From this viewpoint, the firing temperature is more preferably 500° C. or more and 1000° C. or less, and even more preferably 500° C. or more and 800° C. or less. The firing can be performed under an oxygen gas-containing atmosphere such as air atmosphere, and an inert atmosphere such as argon or nitrogen, but it is preferable to perform the firing under an oxygen gas-containing atmosphere, particularly air atmosphere, in terms of reducing the carbon content and costs.

(Crushing process)
It is preferable to crush the coarse particles of the rare earth oxide powder obtained by firing. For crushing, a dry crusher can be used, for example, a crusher (product name: Force Mill, manufactured by Osaka Chemical Co., Ltd.).

The rare earth oxide powder obtained as described above can then be used in a variety of applications, taking advantage of its ease of dispersion. Examples include dielectrics or internal electrodes for capacitors, phosphors, refractive index adjusters for optical glass, oxygen sensors, sintering aids for ceramics, additives to alloys, catalysts, refractories, laser crystal raw materials, and corrosion-resistant materials for semiconductor manufacturing equipment. There are various ways to use the powder, and it can be applied to various forms such as coating (film formation), trace addition, and molded bodies (including sintered bodies). In particular, the rare earth oxide powder of the first invention has good film-forming properties and can be suitably used for coating applications (including film formation).

The second invention will now be described in detail based on preferred embodiments thereof.
The second invention relates to a powder of an oxide of a rare earth element other than Ce.
_CeO2 , an oxide of Ce, can be obtained without _a calcination process by adding an oxidizing agent ( _H2O2 ) to cerium hydroxide in water, and since Ce is easily oxidized, it can be easily obtained by calcining the precursor at a low temperature. For this reason, _CeO2 is less susceptible to agglomeration caused by calcination during production, and can be easily disintegrated even if it is a powder with primary particles of several tens of nanometers.
On the other hand, since rare earth elements other than Ce cannot obtain oxides even if an oxidizing agent is added to their hydroxides, they usually require firing at relatively high temperatures for production, and high-temperature firing is one of the causes of necking. Therefore, in conventional rare earth element powders other than Ce, it has been very difficult to suppress the aggregation of the powder when the primary particles are several tens of nm in size.

In the second invention, the oxide of a rare earth element other than Ce may be at least one oxide selected from Sc, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, _and Lu. Specifically, the oxide of a rare earth element other _than Ce may be _Sc2O3 , _{Y2O3 , La2O3 , Pr6O11 , Nd2O3 , Sm2O3 , Eu2O3 , Gd2O3 , Tb4O7 , Dy2O3 , Ho2O3 , Er2O3 , Tm2O3 , Yb2O3} , and _{Lu2O3 .} In view of the excellent significance of the effect of the second invention due to the difficulty of conventional agglomeration inhibition, the oxide of the rare earth element is preferably at least one oxide selected from Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, more preferably at least one oxide selected from Y, La, Pr, Nd, Eu, Gd, Dy, Ho, Er and Yb, and particularly preferably at least one oxide selected from Y, La, Eu, Gd, Dy, Ho and Yb.

In rare earth oxide powder, the smaller the primary particles, the more difficult they are to pulverize, and the less active surface there is. In the absence of a dispersant, the less active surface there is, the more stable the slurry becomes. However, if the primary particles are too small, the particles will become strongly aggregated and will not be able to be disintegrated. Even if the particles can be disintegrated, when the strongly aggregated parts are disintegrated, the activity of those parts increases, resulting in an unstable slurry. From this perspective, the primary particle size must be within a certain range. Specifically, the primary particles are 10 nm or more and less than 100 nm, preferably 12 nm or more and 60 nm or less, more preferably 15 nm or more and 50 nm or less, and particularly preferably 15 nm or more and 35 nm or less.
In the second invention, the primary particle diameter of the rare earth oxide powder is the primary particle diameter calculated based on the specific surface area, specifically, the particle diameter calculated from the specific surface area s ( ^m2 /g) measured by the BET single point method. For example, the primary particle diameter d (nm) is d = 6000/(ρs) (ρ is the true density (g/ ^cm3 )).

Furthermore, in the second invention, the specific value is a numerical value obtained by multiplying the pore volume (cm ³ /g) of pores having a diameter of 0.005 μm or more and 100 μm or less by the true density (g/cm ³ ).
Here, the reason why the value obtained by multiplying the pore volume by the true density, rather than the pore volume, is specified in the second invention will be explained below.
The pore volume ( ^cm3 /g) depends on the volume per weight of the sample being measured. Therefore, even if the pore volume per volume ( ^cm3 ) is the same, the pore volume per weight ( ^cm3 /g) of a compound with a large true density will have a smaller value.
For this reason, in the second invention, the value of the pore volume independent of the compound type is defined as the value obtained by multiplying the pore volume per gram ( ^cm3 /g) by the true density. The pore volume ( ^cm3 /g) determined by a mercury porosimeter multiplied by the true density (mass per ^cm3 ) gives the pore volume per unit volume.

The value obtained by multiplying the pore volume of pores with diameters of 0.005 μm or more and 100 μm or less by the true density (hereinafter also referred to as the "first pore volume") indicates the total pore volume resulting from the gaps between primary particles and the gaps between aggregated particles, and is a parameter indicating the degree of aggregation. A small first pore volume, which is the pore volume per unit, indicates strong aggregation, while a large first pore volume indicates large or many voids, resulting in a large aggregate diameter, so an appropriate first pore volume leads to ease of disintegration.

Specifically, the first pore volume, which is the value obtained by multiplying the pore volume of pores with a diameter of 0.005 μm or more and 100 μm or less by the true density, is 3 or more and 14 or less. If the value of the first pore volume is low, there will be no gaps between the particles and the particles will be strongly aggregated. Furthermore, if the value exceeds 14, there will be many gaps and the initial aggregate diameter will be large. Furthermore, it is expected that the particles will be more likely to absorb impacts due to the large number of gaps, making it difficult to disintegrate them using a bead mill. From these viewpoints, 4 or more and 12 or less is more preferable, and 5.5 or more and 12 or less is particularly preferable.

In addition, the second invention is characterized in that the numerical value (hereinafter also referred to as the "second pore volume") obtained by multiplying the pore volume ( ^cm3 /g) of pores with a pore diameter of 5 nm or more and 50 nm or less by the true density (g/ ^cm3 ) is also a specific value.
It is known that pores with a diameter of about 1/3 to 1/4 of the particle diameter are formed between particles (see JP-A-7-237982). Pores with a diameter of 5 nm to 50 nm correspond to pores of aggregates with an aggregate diameter of about 15 nm to 200 nm. The aggregate diameter achieved by crushing depends on the bead diameter, but the bead diameter cannot be made too small due to the balance with the crushing energy, as described later. If there are many particles with an aggregate diameter of about 15 nm to 200 nm, they tend to overlap with the particle diameter achieved by bead crushing. Particles with an aggregate diameter relatively close to the particle diameter achieved by bead crushing are difficult to crush even if they are larger than the particle diameter achieved by bead crushing, and even if they are smaller than the particle diameter achieved, the force of aggregation of the aggregates becomes strong, and they tend to form aggregates (tertiary aggregates). For these reasons, powders with a large pore volume with a pore diameter of 5 nm to 50 nm tend to become slurries with large particle diameters of dispersoids. For this reason, in the second invention, the second pore volume, which is the value obtained by multiplying the pore volume of pores with a diameter of 5 nm to 50 nm by the true density, is 2.0 or less, more preferably 1.5 or less, particularly preferably 1.2 or less, even more preferably 1.0 or less, even more preferably 0.8 or less, and particularly preferably 0.7 or less. The lower limit of the second pore volume is not particularly limited as long as it is 0 or more, but from the viewpoint of ease of production, it is preferably 0.01 or more, more preferably 0.02 or more, and even more preferably 0.05 or more.

In addition, the specific surface area of the rare earth element oxide powder of the second invention is preferably, for example, ¹⁰ m2/g or more and 160 ^m2 /g or less in order to easily impart the above-mentioned primary particle diameter, more preferably ¹⁵ m2/g or more and 110 ^m2 /g or less, and even more preferably ²⁰ m2/g or more and 80 ^m2 /g or less.

Furthermore, it is preferable that the amount of impurities in the rare earth oxide powder is small. If Na ions, etc. are present during synthesis, particle growth can be suppressed, but Na is difficult to decompose. Since there are electronic materials that do not like Na, it is preferable that there are few Na ions. From this perspective, the Na content of the rare earth oxide powder is preferably 100 mass ppm or less, and particularly preferably 10 mass ppm or less. The Na content can be measured by the method described in the examples below.

The rare earth oxide powder preferably has a crystallite diameter in a predetermined range. Specifically, the rare earth oxide powder preferably has a crystallite diameter of 6 nm or more and 25 nm or less. By making the crystallite diameter of the rare earth oxide powder greater than a predetermined value, the primary particle diameter is easily made greater than a certain value, and the cohesive force of the primary particles can be reduced. Furthermore, by making the crystallite diameter less than a predetermined value, the primary particle diameter is easily made less than a predetermined value, and necking between particles can be prevented. In consideration of these points, the crystallite diameter of the rare earth oxide powder is more preferably 8 nm or more and 20 nm or less.
The crystallite size can be measured by the method described in the Examples below.

The rare earth oxide powder of the second invention can be made into a slurry by wet crushing it.
The rare earth oxide powder of the second invention preferably has a specific range of aggregate particle size in a specific crushing treatment. Specifically, the rare earth oxide powder of the second invention is preferably mixed with ethanol to prepare an ethanol slurry containing 10% by mass of rare earth oxide powder, and then the operation (A) is repeated until the average particle size becomes larger than the previous measurement value, and the minimum average particle size (Dm) is preferably 10 nm or more and 150 nm or less. As a method for preparing an ethanol slurry containing 10% by mass of rare earth oxide powder, a method of mixing 40.5 g of ethanol having a purity of 99% by mass or more with 4.5 g of rare earth oxide powder to prepare a slurry can be mentioned. No dispersant is used in the slurry for measuring the aggregate particle size. Examples of dispersants include various dispersants described later.
(A): The slurry is subjected to a bead mill treatment for 10 minutes using zirconia beads having a diameter of 0.1 mm, and then the average particle size is measured by a dynamic light scattering method.
The operation (A) is specifically the following (a).
(a): Using zirconia beads having a diameter of 0.1 mm, the slurry:beads mass ratio was set to 45:240, and a bead mill treatment was carried out for 10 minutes in a bead mill having an effective volume of a vessel of 80 cc at a peripheral speed of 4 m/s to 6 m/s, and then the average particle diameter was measured by a dynamic light scattering method.
(However, if the average particle size becomes larger than the previous measurement value, the bead mill treatment is terminated at that point, and even if the average particle size does not become larger than the previous measurement value in each of the second to twentieth repetitions of (A) 20 times, the treatment is terminated when (A) has been repeated 20 times.)
Here, the minimum average particle size (Dm) means the minimum value of the average particle size measured by dynamic light scattering method by sampling every 10 minutes during the bead mill treatment. Note that the average particle size is expressed in nm units, and when there is a decimal value, the value is rounded off to the first decimal place to an integer, and the magnitude is judged based on the value.

The minimum average particle diameter (Dm) obtained by the above measurement indicates the minimum agglomeration diameter by bead crushing treatment, which is a common crushing treatment in the technical field related to oxide fine powder slurries. The minimum average particle diameter (Dm) is preferably 150 nm or less, more preferably 140 nm or less, even more preferably 105 nm or less, and particularly preferably 90 nm or less. This is because the transparency can be improved by setting the minimum average particle diameter (Dm) to a predetermined value or less. Furthermore, the minimum average particle diameter (Dm) is preferably 10 nm or more, more preferably 15 nm or more, even more preferably 20 nm or more, and most preferably 50 nm or more. This is because the dispersion stability of the slurry can be improved by setting this lower limit to a predetermined value or more.

In addition, the polydispersity index (PI) at the time of measuring the minimum average particle size (Dm) is preferably 0.3 or less, since this results in a sharp particle size distribution and makes it easier to maintain high dispersibility and visible light transmittance when made into a slurry, and is more preferably 0.25 or less. The polydispersity index (PI) is a dimensionless index that indicates the spread of the particle size distribution.

The effective volume of the vessel refers to the internal volume of the container (vessel) in which the beads and slurry are contained. The peripheral speed of the bead mill may be any speed between 4 m/s and 6 m/s, but is more preferably between 4 m/s and 5 m/s, and even more preferably 4 m/s.

The measurement of the average particle size and polydispersity index (PI) by dynamic light scattering (photon correlation method) is performed by filling a measurement sample into a dynamic light scattering photometer. The measurement sample is a sample that is not subjected to ultrasonic treatment, with a part of the above-mentioned slurry taken out and the solvent used in wet disintegration as a dispersion medium. The concentration of the measurement sample is a dilution ratio in the range of 1000 to 10000 times by volume that the dynamic light scattering photometer judges to be an appropriate concentration. As the dynamic light scattering photometer, a device that employs a method of determining the average particle size and polydispersity index (PI) by cumulant analysis from the autocorrelation function obtained by the photon correlation method can be used, and for example, the ELSZ-2000ZS manufactured by Otsuka Electronics can be used.
The cumulant method is described in "9.2.1 Cumulant method" of JIS Z 8828:2019 "Particle size analysis - Dynamic light scattering method" and "A.1.2 Cumulant method" of Appendix A of the same JIS.

The rare earth oxide powder of the second invention is crushed as described above to obtain the minimum average particle size (Dm), and the resulting slurry is allowed to stand at room temperature (15 to 25°C), and the calculated value of the following formula is preferably within a predetermined range. The formula below indicates the particle size variation when allowed to stand for 7 days. The value of the formula below is preferably 25% or less, more preferably 20% or less, and even more preferably 15% or less. If it is below this upper limit, it is possible to easily restore the particle size to the same or close to that immediately after crushing by applying ultrasound or the like. Since the average particle size of the slurry is a measurement of fine particles, it is necessary to consider the variation in the measurement. If there is no substantial change in particle size, the particle size may be measured as if it is smaller, so the calculated value is preferably -15% or more, and even more preferably -10% or more.
In the following formula, "average particle size immediately after disintegration (D0)" is the same value as the minimum average particle size (Dm) when the above operation (A) is repeated until the average particle size becomes larger than the previous measurement value, and the average particle size does not become larger than the previous measurement value up to the 20th time. On the other hand, when the average particle size becomes larger than the previous measurement value during the 20th repetition and the bead mill treatment is terminated, the last measurement value becomes "average particle size immediately after disintegration (D0)". In addition, in the following formula, "average particle size 7 days after disintegration (D7)" refers to the average particle size measured again by the above method when the slurry after the bead mill treatment is left to stand for 7 days under the above conditions. Formula: (average particle size 7 days after disintegration (D7) - average particle size immediately after disintegration (D0)) / average particle size immediately after disintegration (D0) x 100 (%)

The rare earth oxide powder of the second invention preferably has an average particle size (D0) immediately after crushing of 210 nm or less, more preferably 150 nm or less, and particularly preferably 120 nm or less. This is because the transmittance after standing can be increased. In addition, the average particle size (D0) immediately after crushing is preferably 25 nm or more, and more preferably 50 nm or more. This is because the dispersibility of the slurry can be increased.

The rare earth oxide powder of the second invention preferably has an average particle size (D7) of 150 nm or less after 7 days of crushing, more preferably 120 nm or less, and particularly preferably 100 nm or less. This is because the transmittance after standing can be increased. In addition, the average particle size (D7) of 25 nm or more after 7 days of crushing is preferably 25 nm or more, and more preferably 50 nm or more. This is because the dispersibility of the slurry can be increased.

The rare earth oxide powder of the second invention has an average particle size (D7S) of preferably 150 nm or less, more preferably 120 nm or less, and particularly preferably 100 nm or less, when 20 ml of the slurry is irradiated with 40 W ultrasound (frequency 40 kHz) for 5 minutes 7 days after crushing. This is because the transmittance after standing can be increased. Moreover, the average particle size (D7S) is preferably 25 nm or more, and more preferably 50 nm or more, because this is because the dispersibility of the slurry can be increased.

In order to obtain the above-mentioned specific surface area ( ^m2 /g), primary particle diameter (nm), first pore volume, second pore volume, Na content, Dm (nm), D0 (nm), D7 (nm), (D7-D0/D0) (%) and D7S (nm), it is sufficient to adopt a suitable manufacturing method for rare earth oxide powder described below and adjust the mixing, washing or firing conditions.

Next, a preferred method for producing the rare earth oxide powder of the second invention will be described.
The present production method includes a reaction/solid-liquid separation step in which an aqueous carbonate solution (hereinafter also referred to as "liquid A") and an aqueous water-soluble salt solution of a rare earth element (hereinafter also referred to as "liquid B") are simultaneously charged into a reaction vessel, and mixed so that the pH of the mixed solution becomes 6.5 to 7.0, preferably 6.5 to 6.9, to react the carbonate with the water-soluble salt of the rare earth element, and solid-liquid separation is initiated within 5 minutes from the start of mixing of liquids A and B;
a washing step of washing the residue obtained in the reaction and solid-liquid separation step with alcohol or aqueous alcohol;
and a calcination step of calcining the washed residue.
In order to successfully obtain the rare earth oxide powder of the second invention, the water-soluble salt concentration in the water-soluble salt aqueous solution of the rare earth element, which is Liquid B, is preferably 10 to 400 g/L, more preferably 20 to 300 g/L, particularly preferably 20 to 200 g/L, and most preferably 20 g/L or more and less than 100 g/L.
This production method differs from the method for producing rare earth oxide powder of the first invention in that the mixed liquid of liquid A and liquid B does not necessarily need to be stirred at high speed, the residue obtained in the reaction and solid-liquid separation step may be washed not only with alcohol but also with hydrous alcohol, and the preferred concentration range of the water-soluble salt solution of the rare earth element in liquid B is different.

(Reaction and solid-liquid separation process)
In the solution A which is an aqueous carbonate solution, examples of the carbonate include ammonium hydrogen carbonate, ammonium carbonate, sodium carbonate, sodium hydrogen carbonate, etc. In this specification, carbonate includes not only normal salts but also hydrogen carbonate which is an acidic salt. Ammonium hydrogen carbonate is preferred from the viewpoints of easy adjustment of the pH of the mixed solution and reduction of the sodium content.
In the solution B, which is an aqueous solution of a water-soluble salt of a rare earth element, examples of the water-soluble salt of the rare earth element include nitrates, acetates, ammine complexes, and chlorides. From the viewpoints of ease of adjusting the pH of the mixed solution and productivity, nitrates are preferred.

When employing this manufacturing method, it is also preferable that the concentrations of Liquid A and Liquid B are within a predetermined range in order to successfully set the primary particle size, the first pore volume, and the second pore volume within the predetermined range. Specifically, the concentration of carbonate in Liquid A is preferably 5 to 25 mass%, and more preferably 10 to 15 mass%. The concentration of the water-soluble salt of the rare earth element in Liquid B is as described above.

In this manufacturing method, liquid A, which is an aqueous carbonate solution, and liquid B, which is an aqueous solution of a water-soluble salt of a rare earth element, are simultaneously introduced into a reaction vessel so that the pH of the mixture of the two is 6.5 to 7.0, preferably 6.5 to 6.9. The pH here is the pH at the temperature of the mixture of the two. If the pH of the mixture exceeds 7.0, the primary particles will become large. In addition, setting the pH at 6.5 or higher has the advantage that almost all of the rare earth ions in liquid A will precipitate. It is preferable that neither liquid A nor liquid B is heated at the time of introduction. It is preferable that liquid A and liquid B are 5 to 50°C at the time of introduction into the reaction vessel, and the temperature of the mixture is 5 to 40°C. In order to keep the primary particle size within a suitable range, it is preferable to carry out the reaction in an extremely short time and filter it immediately. For this reason, it is preferable to adjust the timing and rate of introduction of both liquids so that the pH of the mixed liquid is within a range of 6.5 to 7.0, preferably 6.5 to 6.9, from the time when the introduction of liquid A and liquid B into the reaction vessel starts (the time when mixing of liquid A and liquid B starts) to the time when the reaction product is generated (more specifically, the time when solid-liquid separation starts), thereby maintaining the pH constant within the above range. The reaction proceeds smoothly when the pH is within the above range.

The simultaneous addition of liquid A and liquid B means that the time when liquid A is added to the reaction vessel and the time when liquid B is added are at least partially simultaneous. As described above, in order for the pH of the mixed liquid to remain within the range of 6.5 to 7.0, preferably 6.5 to 6.9, from the time when liquid A and liquid B are added to the reaction vessel (the time when mixing of liquid A and liquid B is started) to the time when the reaction product is generated (more specifically, the time when solid-liquid separation is started), it is preferable that the introduction of liquid A and liquid B having the above concentrations is started almost simultaneously and the introduction speed is constant. The introduction speed of liquid A and liquid B is adjusted so that solid-liquid separation can be started within 5 minutes, more preferably within 3 minutes, from the start of mixing of liquid A and liquid B.
The present inventors have found that, rather than adding liquid B to liquid A that has already been placed in a reaction tank, or pouring liquid A into liquid B that has already been placed in a reaction tank, by simultaneously pouring liquid A and liquid B of predetermined concentrations and mixing them under conditions that maintain a predetermined pH, it is possible to complete the reaction in a short period of time, and by subjecting this to a predetermined post-treatment, it is possible to obtain the rare earth oxide powder of the second invention, which has a small primary particle size, good dispersibility, and provides a slurry with excellent transparency.

It is preferable to stir the mixture of liquids A and B in the reaction vessel in order to successfully obtain the rare earth oxide powder of the second invention. Stirring may be at high, low or medium speed.
The stirring speed is preferably from 100 rpm to 25,000 rpm, and more preferably from 200 rpm to 21,000 rpm.
Low speed stirring includes stirring at 100 rpm or more and less than 1000 rpm, and stirring at 200 rpm or more is more preferable.
Medium speed stirring includes stirring at 1,000 rpm or more and less than 10,000 rpm.
The high speed stirring may be from 10,000 rpm to 25,000 rpm. In the case of high speed stirring, stirring at 18,000 rpm to 21,000 rpm is particularly preferred.
High speed stirring is preferred since it is possible to preferably reduce the primary particle size.
The capacity of the reaction tank may be appropriately determined according to the production amount, etc., so long as the reaction tank is equipped with a stirring device (including one integrated with the reaction tank) capable of stirring at a predetermined rotation speed. However, the capacity of the reaction tank equipped with a high-speed stirring device is limited by the size of the high-speed stirring device, and is preferably 50 mL to 5 L, more preferably 100 mL to 2 L, and even more preferably 100 mL to 1 L.

(Washing process)
It is also important to wash the residue (also referred to as "solid matter") obtained in the solid-liquid separation step with alcohol or hydrous alcohol. Examples of alcohol include methanol, ethanol, isopropanol, etc., but ethanol is preferred from the viewpoint of usability. When hydrous alcohol is used, the alcohol concentration in the hydrous alcohol is preferably 10% by volume or more, more preferably 50% by volume or more. The amount of alcohol used for washing is preferably 0.1 L to 50 L, more preferably 0.5 L to 20 L, and even more preferably 1 L to 10 L per 1 g of oxide equivalent of the solid matter to be washed. The amount referred to here is the total amount when the alcohol is passed through the residue several times to wash it.

(Firing process)
The firing temperature is preferably 1000° C. or less in terms of suppressing aggregation and suppressing crystal growth, and more preferably 800° C. or less. The firing temperature is preferably 500° C. or more in terms of reducing the carbon content. From this viewpoint, the firing temperature is more preferably 500° C. or more and 1000° C. or less, and even more preferably 500° C. or more and 800° C. or less. The firing can be performed under an active gas atmosphere such as air atmosphere, and an inert atmosphere such as argon or nitrogen, but is preferably performed under an active gas atmosphere, particularly air atmosphere.

(Crushing process)
It is preferable to crush the coarse particles of the rare earth oxide powder obtained by firing. For crushing, a dry crusher can be used, for example, a crusher (product name: Force Mill, manufactured by Osaka Chemical Co., Ltd.).

Next, we will explain the method for producing a slurry by wet-grinding the rare earth oxide powder obtained as described above.

Wet disintegration is preferably performed using a bead mill, as this allows for high dispersion. The beads in a bead mill are usually spherical. Bead materials include zirconia, alumina, silicon nitride, silicon carbide, tungsten carbide, wear-resistant steel, stainless steel, etc., with zirconia being preferred. Zirconia here includes stabilized zirconia such as YSZ and PSZ.

The bead diameter during crushing is preferably 0.01 to 0.3 mm. The particle size achieved by crushing is based on the bead diameter, for example, about 1/1000 of the bead diameter, so a smaller bead diameter is better, but if the bead diameter is too small, the crushing energy will be small and agglomerations may not break down. From this perspective, a bead diameter of 0.05 to 0.15 mm during crushing is more preferable.

As a solvent for dispersing rare earth oxide powder, monohydric alcohol is preferable because it has better dispersibility than water, and primary alcohol is especially preferable, and ethanol is especially preferable. Examples of monohydric alcohol solvents include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol. Monohydric alcohols are frequently used as cleaning materials in laboratories and are easy to obtain. In addition, ethanol and 1-propanol are not subject to the Organic Solvent Poisoning Prevention Regulations, so they are easy to handle. It is preferable that the purity of the monohydric alcohol is 99% by volume or more.

In addition, the proportion of rare earth oxide powder in the slurry obtained by the manufacturing method of the second invention is preferably 1 to 50 mass% in order to obtain the desired visible light transmittance, and more preferably 5 to 20 mass%.

In the method for producing a slurry according to the second invention, the average particle size of the resulting slurry is preferably crushed to 200 nm or less, more preferably 10 nm to 150 nm, even more preferably 15 nm to 140 nm, and particularly preferably 20 nm to 105 nm. By being below the upper limit, particularly within the above range, the permeability and stability are excellent.

In the method for producing a slurry according to the second invention, the polydispersity index (PI) of the resulting slurry is preferably 0.3 or less, and more preferably 0.25 or less. The lower the polydispersity index (PI), the better, but from the viewpoint of ease of production, it is preferable that the PI is 0.1 or more.

The method for producing the slurry of the second invention is preferably a method in which the visible light transmittance of the slurry left to stand for 7 days after disintegration is 40 to 95%, and more preferably 50 to 80%. The visible light transmittance can be measured by the method described in the examples below.

The method for producing a slurry of the second invention preferably does not use a dispersant. Examples of dispersants include ionic surfactants, nonionic surfactants, pH adjusters, and soluble salts. For example, ether-type dispersants such as polyoxyethylene alkyl ether, polyoxyethylene secondary alcohol ether, polyoxyethylene alkylphenyl ether, polyoxyethylene, polyoxypropylene block copolymer, and polyoxyethylene polyoxypropylene alkyl ether, and ester-ether-type dispersants such as polyoxyethylene glycerin fatty acid ester, polyoxyethylene castor oil and hardened castor oil, and polyoxyethylene sorbitan fatty acid ester are also included. Polyglycerin fatty acid esters such as diglycerin laurate are also included. In addition, nitric acid, hydrochloric acid, acetic acid, sodium chloride, potassium chloride, magnesium chloride, and calcium chloride are also included. β-diketones described in JP 2007-126349 A can also be mentioned. In addition, dispersants that improve dispersibility by adding additives are also included. By "no dispersant is used," it is preferable to mean that the amount of dispersant in the slurry is 10,000 ppm by mass or less, more preferably 1,000 ppm by mass or less, even more preferably 100 ppm by mass or less, even more preferably 10 ppm by mass or less, and it is particularly preferable that no dispersant is used.

<Examples and Comparative Examples Explaining the First Invention>
The first invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to such examples.

(Example a1)
10 kg of an aqueous ammonium hydrogen carbonate solution (25°C) in which 13.0% by mass of ammonium hydrogen carbonate was dissolved, and 40 L of an aqueous yttrium nitrate solution (25°C) with a concentration of 300 g/L in terms of oxide were prepared. The aqueous ammonium hydrogen carbonate solution was also used in Examples a2 to a9 described later, and the aqueous yttrium nitrate solution was also used in Examples a2 to a4 described later. The aqueous yttrium nitrate solution was simultaneously charged into the same reaction layer at 100 mL/min and the aqueous ammonium hydrogen carbonate solution was simultaneously charged into the same reaction layer at a flow rate such that the pH of the mixture of the two solutions was 6.8, and mixed. When the two solutions were charged, the mixture in the container was stirred at 20,000 rpm. The suspension overflowing from the container was filtered for 20 seconds in sequence, and then the charging and stirring of the two solutions was stopped, and the residue, which was the filtrate, was washed with 10 L of ethanol. This washing operation was repeated five times. The time from the start of mixing the two solutions to the start of filtration was 60 to 120 seconds. The washed residue was calcined at 600° C. in an air atmosphere to obtain about 10 g of Y ₂ O _3. The inner volume of the reaction vessel was 100 ml. The yield of the obtained Y ₂ O ₃ was about 99% based on the 20-second filtration.
The above procedure was repeated 10 times, and _{the Y2O3 samples} obtained in the 10 procedures were mixed. The obtained _Y2O3 was crushed for 30 seconds in a force mill (manufactured by Osaka Chemical Co., Ltd.) to obtain a fine powder _.
For the obtained _{Y2O3 fine} powder, the specific surface area ( ^m2 /g), primary particle size (nm), agglomeration size, initial bulk density (AD), tapped bulk density (TD), Zr content, and carbon content were measured by the following methods, and the values of _PAD , _PTD, and _PAD - _PTD were calculated from the initial bulk density (AD) and tapped bulk density (TD), and the coating properties were also evaluated. The results are shown in Table 1.

(Method of measuring specific surface area)
The measurement was performed by the BET one-point method using a Mountec fully automatic specific surface area meter Macsorb model-1201. The gas used was a nitrogen-helium mixed gas (nitrogen 30 vol%). As a pretreatment for the measurement, the powder was placed in a glass cell and set in the device, and nitrogen gas was passed through the set glass cell to degas it at 300°C for 60 minutes.

(Calculation method of primary particle size)
The particle size was calculated from the specific surface area s ( ^m2 /g) measured by the BET single point method using the following formula: Primary particle diameter d (nm) was d = 6000/(ρs) (ρ is true density (g/ ^cm3 )). The true densities were 5.03 (g/ ^cm3 ) for _Y2O3 , _7.33 (g/ ^cm3 ) _{for Nd2O3 ,} 7.62 (g/ ^cm3 ) for _{Gd2O3 , 7.4} (g/ ^cm3 ) for _{Eu2O3 ,} 7.81 (g/ ^cm3 ) for _Dy2O3 , and 9.22 (g/ ^cm3 ) for _Yb2O3 .

(Agglomeration diameter ( _D100 , _D50 , _D90 ))
Measurements were performed using a Microtrac MT3300EXII manufactured by Nikkiso Co., Ltd. During the measurements, a powdered sample was added to a chamber of a sample circulator filled with an aqueous solution containing 0.2% by mass of sodium hexataphosphate dissolved therein, and dispersed for 5 minutes using an ultrasonic irradiation device equipped in the device at 40 W and a frequency of 40 kHz. After the device determined that the concentration was appropriate, _D100 , _D50 , and _D90 were measured.

(Method of measuring initial bulk density and tapped bulk density)
1. Pretreatment of the sample: The rare earth oxide powder (Y ₂ O ₃ fine powder in Example a1) was crushed for 30 seconds in a force mill manufactured by Osaka Chemical Co., Ltd. immediately before the measurement.
2. Measurement (1) The mass (W _c (g)) of a measurement vessel made of SUS304 having an inner diameter of 10 mm, an outer diameter of 12 mm, a height of 51 mm and an inner surface height of 50 mm (internal volume 3.93 cm ³ ) was measured.
(2) The pretreated sample was passed through a sieve with 2 mm openings and poured into a measurement container until it overflowed.
(3) The powder piled up on the top surface of the measuring container was leveled off using a leveling plate.
(4) The mass (W ₀ (g)) of the measurement container was measured, and the mass (W _c (g)) of the measurement container was subtracted from this to calculate the mass (W _A (g)) of the sample.
(5) An auxiliary cylinder (made of SUS304, inner diameter: lower 12 mm, upper 10 mm, outer diameter 14 mm, height 50 mm (lower 10 mm, upper 40 mm)) was added on top of the measurement container containing the sample, and the sample was further filled in through a sieve with 2 mm openings.
(6) With the auxiliary cylinder still attached, the measurement container containing the sample was tapped 600 times by manually hitting the container against a rubber-covered desk at a tapping height of about 10 mm. During this time, sample was added so that the sample height was always 20 to 30 mm higher than the top surface of the measurement container.
(7) The auxiliary cylinder was removed, and the sample overflowing from the top of the measurement container was scraped off with a scraping plate.
(8) After the sample was leveled off, the mass (W ₁ (g)) of the measurement container was measured, and the mass (W _c (g)) of the measurement container was subtracted to determine the mass of the sample (W _T1 (g)).
(9) The auxiliary cylinder was attached to the measurement container again, and the sample was further filled in through a sieve with 2 mm openings, and tapped 100 times by hand at a tapping height of about 10 mm.
(10) The auxiliary cylinder was removed, and the sample overflowing from the top of the measurement container was scraped off with a scraping plate.
(11) After the sample had been leveled off, the mass ( _W2 (g)) of the measurement container was measured, and the mass ( _Wc (g)) of the measurement container was subtracted to calculate the mass of the sample ( _WT2 (g)). It was confirmed that the difference between this value ( _WT2 (g)) and the previous value ( _WT1 (g)) was within 0.3% of _WT1 . If it exceeded 0.3%, the steps from (9) onwards were repeated until the difference from the previous value was within 0.3%.
(12) The initial bulk density and tapped bulk density were calculated according to the following formula.
Initial bulk density (g/cm ³ )=W _A (g)/3.93 (cm ³ )
Tapped bulk density (g/cm ³ )=W _T2 (g)/3.93 (cm ³ )
(13) The above measurements were carried out three times. The initial bulk density and the tapped bulk density were calculated by averaging the three measurements.

(Porosity)
When the measured initial bulk density and tapped bulk density were taken as ρ ₀ , the porosity P (%) was calculated from P = (1 - ρ ₀ /ρ) x 100 (ρ is the true density (g/cm ³ )).

(Carbon Content)
The measurement was performed by an infrared absorption method using a carbon/sulfur analyzer EMIA-320V manufactured by Horiba Ltd., in which the carbon and sulfur content was measured in an oxygen stream.

(Zr content)
The measurement was carried out using an ICP optical emission spectrometer SPS-3520V-DD manufactured by Hitachi High-Tech Science Corporation.

(Evaluation of coating properties)
Using 3 _{g of} the obtained _Y2O3 fine powder and ethanol, _an ethanol slurry with a _Y2O3 concentration of 30 mass% was prepared, which was dispersed for 5 minutes with 40 W ultrasonic waves (frequency 40 kHz), and then coated on a PET film using an applicator with a gap of 10 μm (manufactured by BEVS Industrial Co., Ltd.). The coating properties were evaluated according to the following criteria.
A: Coating is possible.
B: There are areas where the coating is not completed.
C: Coating is not possible because no dispersed material comes out of the gap.

(Example a2)
The same procedure was followed as in Example a1, except that the firing temperature was 800°C.

(Example a3)
The procedure was the same as in Example a1, except that the firing temperature was 900°C.

(Example a4)
The procedure was the same as in Example a1, except that the firing temperature was 500°C.

(Example a5)
The contents were the same as those of Example a1, except that an aqueous neodymium nitrate solution with an oxide equivalent concentration of 400 g/L was used instead of an aqueous yttrium nitrate solution. The firing temperature was 900° C. Other than these points, the composition was the same as Example a1.

(Example a6)
The contents were the same as those of Example a1, except that an aqueous gadolinium nitrate solution with an oxide equivalent concentration of 250 g/L was used instead of an aqueous yttrium nitrate solution. The firing temperature was 650° C. Other than these points, the same procedure was followed as in Example a1.

(Example a7)
The contents were the same as in Example 1, except that an aqueous europium nitrate solution with an oxide equivalent concentration of 300 g/L was used instead of an aqueous yttrium nitrate solution, and the firing temperature was 650° C. Other than these points, the same procedure was followed as in Example a1.

(Example a8)
The contents were the same as those of Example a1, except that an aqueous solution of dysprosium nitrate having an oxide equivalent concentration of 300 g/L was used instead of an aqueous solution of yttrium nitrate. The firing temperature was set to 600° C. Other than these points, the composition was the same as that of Example a1.

(Example a9)
The content of Example a1 was the same as that of Example a1, except that an aqueous ytterbium nitrate solution with an oxide equivalent concentration of 300 g/L was used instead of an aqueous yttrium nitrate solution. The firing temperature was set to 600° C. Other than these points, the composition was the same as that of Example a1.

(Comparative Example a1)
This comparative example corresponds to Example 1 of JP 2014-218384 A.
5 g of sodium oleate was added to 100 mL of a 1.0 mol/L aqueous solution of yttrium nitrate, and the mixture was stirred for 2 hours. Next, 1000 mL of cyclohexane and 1.0 g of a nonionic surfactant Span 80 (sorbitan monooleate, manufactured by Kanto Chemical Co., Ltd.) were added to the aqueous solution, and the mixture was stirred vigorously (at 10,000 rpm), resulting in a W/O type emulsion solution consisting of minute droplets. Next, 13 g of ammonium hydrogen carbonate was dissolved in 50 mL of pure water to prepare an aqueous solution, and the aqueous solution of ammonium hydrogen carbonate was dropped into the emulsion solution while vigorously stirring it. A white precipitate was generated as the aqueous solution of ammonium hydrogen carbonate was dropped. After the dropwise addition was completed, the mixture was aged for 1 hour at room temperature (25°C) while continuing to stir. The precipitate was then filtered out of the aqueous solution using a Buchner funnel, and the resulting precipitate was dried in an oven at 75°C for 12 hours, then placed in an alumina crucible and fired in an air atmosphere at 800°C. Thus, 8 g of yttrium oxide fine powder was obtained. Thereafter, coating was performed as described in Example a1.
Observation of this powder with a scanning electron microscope (SEM) revealed that it was a group of spherical particles with a uniform particle size of about 100 nm without agglomeration. The primary particle size calculated from the specific surface area was also 100 nm or more.

(Comparative Example a2)
This comparative example corresponds to Example 1 of JP 2014-218384 A, except that the firing temperature was changed.
The powder was prepared in the same manner as in Comparative Example 1, except that the firing temperature was set to 600° C. When the powder was observed with a scanning electron microscope (SEM), particles with SEM diameters of approximately 100 nm and 10 nm were mixed, and particles with uniform SEM diameters could not be obtained by low-temperature firing. The primary particle diameter calculated from the specific surface area was also 100 nm or more.

(Comparative Example a3)
This comparative example corresponds to J Nanopart Res (2013) 15:1438.
2 L of 50 mmol/L _yttrium chloride aqueous solution (pH = 5.0) was instantly added to 2 L of 50 mmol/L _Na2CO3 aqueous solution. The resulting suspension was immediately filtered _, and the residue was washed with 10 L of ethanol and then calcined at 600°C to obtain 5 g of _Y2O3 . Then, a coating was performed by the method described in Example 1a.

(Comparative Example a4)
This comparative example corresponds to the example disclosed in Japanese Patent Application Laid-Open No. 4-310516.
_18.0g of _Y2O3 was dissolved in 900mL of diluted hydrochloric acid, and this solution was dropped into 9L of diluted ammonia water (containing 0.2 moles of ammonia) at 40°C. The solution was dropped over 30 minutes, and after the drop was completed, it was aged for 30 minutes, and then an aqueous solution of 144g of ammonium hydrogen carbonate dissolved in 900mL of water was added to precipitate yttrium carbonate. After aging for 2 hours, it was filtered and washed with water, and 900mL of octanol was added to the obtained yttrium carbonate, and the water was evaporated while stirring at 100°C for 3 hours. Then, the yttrium carbonate was taken out by filtration, and dried under reduced _pressure at 13.3 Pa (0.1 torr) and 160°C. Then, it was calcined at 650°C for 2 hours to obtain _10g of _Y2O3 . The crystallite diameter of this _Y2O3 was 80 Å.

As shown in Table 1 above, the powders of each example, which are oxides of at least one rare earth element other than Ce, have a predetermined primary particle size of 10 nm or more and 60 nm or less, have a _D100 measured after ultrasonic dispersion treatment at 40 W of 1 μm or more and 10 μm or less, and have a porosity difference (P _AD - P _TD ) of 2.0% or more and 5.0% or less, had good film-forming properties.
On the other hand, in Comparative Examples a1 to a4 in which the primary particle size was outside the range of 10 nm or more and 60 nm or (P _AD - P _TD ) was outside the range of 2.0% or more and 5.0% or less, the aggregate size after ultrasonic treatment at 40 W was large, indicating that no coating properties were obtained.
From the above, it is clear that the configuration of the present invention can easily disintegrate rare earth oxides other than Ce even if the primary particle diameter (SSA equivalent diameter) is several tens of nm, and can provide rare earth oxide powder that can be used to form a coating film.

The second 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. In the following examples, the internal volume of the reaction vessel was 2 L with low-speed stirring and 100 mL with high-speed stirring.

(Example b1)
10 kg of an aqueous solution of ammonium hydrogen carbonate (25°C) in which 13.0% by mass of ammonium hydrogen carbonate was dissolved, and 40 L of an aqueous solution of yttrium nitrate (25°C) in which the oxide equivalent was 50 g/L were prepared. The aqueous solution of ammonium hydrogen carbonate was also used in Examples b2 to b6 described later, and the aqueous solution of yttrium nitrate was also used in Examples b2 to b4 and b6 described later. The aqueous solution of yttrium nitrate was simultaneously charged into the same container at 600 mL/min and the aqueous solution of ammonium hydrogen carbonate was simultaneously charged into the same container at a flow rate such that the pH of the mixture of both solutions was 6.8 (temperature of the mixture: 20 to 30°C) and mixed. When the two solutions were charged, the mixture in the container was stirred at high speed at 20,000 rpm. The suspension overflowing from the container was filtered for 20 seconds in sequence, and then the charging and stirring of both solutions was stopped, and the residue, which was the filtrate, was washed with 10 L of ethanol. This washing operation was repeated five times. The washed filtered powder was calcined at 700°C in an air atmosphere to obtain 10 g of Y ₂ O ₃ . The obtained _Y2O3 was crushed for 30 seconds in a force mill (manufactured by Osaka Chemical Co. _, Ltd.) to obtain a fine powder. The obtained _Y2O3 fine _powder was measured for specific surface area ( ^m2 /g), primary particle size (nm), pore volume*true density of pore diameter 0.005μm to 100μm, pore volume*true density of pore diameter 5nm to 50nm, Na content (mass ppm), and crystallite size (nm) by the following method, and also the following crushing evaluation was performed. The results are shown in Table 2.

(Specific surface area ( ^m2 /g))
The measurement was carried out in the same manner as in the first embodiment of the present invention.

(Calculation method of primary particle size)
The calculation was carried out in the same manner as in the first embodiment of the present invention.

(Pore volume*true density for pore diameters of 0.005 μm to 100 μm and pore volume*true density for pore diameters of 5 nm to 50 nm)
The pore volume was measured using an Autopore IV manufactured by Micromeritics.
The cumulative volume in the range of pore diameters of 0.005 μm to 100 μm was defined as the pore volume with a pore diameter of 0.005 μm to 100 μm. The cumulative volume in the range of pore diameters of 5 nm to 50 nm was defined as the pore volume with a pore diameter of 5 nm to 50 nm. The value of pore volume * true density was calculated by multiplying the calculated pore volume by the true density of the rare earth oxide. The value of true density used was the same as that used to calculate the primary particle diameter.

(Na content (ppm by mass))
Atomic absorption spectrometry was performed using a Thermo Fisher Scientific CE3300FL.

(Crystallite size (nm))
The crystallite size was determined by the Halder-Wagner method from a profile obtained under the following X-ray diffraction conditions.
(X-ray diffraction measurement conditions)
・Apparatus: Ultima IV (manufactured by Rigaku Corporation)
Radiation source: CuKα radiation Tube voltage: 40 kV
Tube current: 40mA
Scan speed: 2 degrees/min. Step: 0.02 degrees. Scan range: 2θ = 20° to 90°.

(Disintegration evaluation: measurement of average particle size)
Using _{the Y2O3} fine powder obtained above, 45 g of 10 mass% slurry (dispersion medium: 99.5 vol% ethanol) was prepared, and crushed in a bead mill (Apex LABO/A-LABO, manufactured by Hiroshima Metal & Machinery Co., Ltd., effective volume 80 cc) with 240 g of zirconia beads with a diameter of 0.1 mm and a peripheral speed of 4 m/s. Crushing was stopped at 10-minute intervals, and the average particle size was measured by the following method. However, if the average particle size becomes larger than the previous measurement value, the bead mill treatment was terminated at that time, and even if the average particle size does not become larger than the previous measurement value in each of the 20 times (A) of repeating 20 times, the treatment was terminated at the time of repeating 20 times (A). The particle size that is the smallest in the maximum 20 times of repetition was taken as the minimum average particle size (Dm). In addition, the data immediately after crushing was taken as the particle size when the bead mill treatment was terminated ("D0" in Table 2).

(Method of measuring average particle size and polydispersity index)
The average particle size was measured using Otsuka Electronics' ELSZ-2000ZS after the device determined that the concentration was appropriate. The slurry was diluted at the above dilution ratio using 99.5 vol% ethanol as a solvent. The average particle size and polydispersity index obtained by dynamic light scattering (photon correlation method) measured in accordance with JIS Z 8828:2019 were adopted. The same sample was measured three times, and the average value of the three measurements was taken as the average particle size. However, if the absolute value of the difference between the measured value of each measurement and the average value of the three measurements exceeded 2% for even one of the three average measurements, the measurement result was not adopted, and three new measurements were performed. The measurement was performed at 25°C. The polydispersity index shown in Table 2 is the value when the minimum average particle size (Dm) was measured in the disintegration evaluation.

(Evaluation after 7 days of crushing)
The slurry that had been subjected to the disintegration evaluation as described above was allowed to stand for 7 days at room temperature (20°C). The average particle size (D7) after standing for 7 days was measured by the above method, and the value was calculated by the formula: (average particle size 7 days after disintegration (D7) - average particle size immediately after disintegration (D0)) / average particle size immediately after disintegration (D0) x 100 (%). The results are shown in Table 2 as "(D7 - D0) / D0 x 100 (%)". In addition, the average particle size after irradiating 20 ml of the slurry after 1 week with 40 W ultrasonic waves (frequency 40 kHz) for 5 minutes was also measured ("D7S" in Table 2).

After standing for 7 days, the settling property was evaluated according to the following criteria.
Yes: Particle clumps can be seen with the naked eye at the bottom of the container.
None: No clumps of particles are visible at the bottom of the container.

Furthermore, the visible light transmittance (%) after standing for 7 days was measured by the following method.
(Transmittance)
The transmission spectrum from 350 nm to 800 nm was measured using a visible absorption spectrophotometer (U-3100 manufactured by Hitachi, Ltd.). The transmittance indicates the minimum value between the transmission spectrum from 350 nm to 800 nm. The sample was prepared by dropping 0.2 ml of the slurry obtained by crushing (one week after crushing) into 30 ml of the solvent used during crushing.

(Example b2)
The ammonium bicarbonate aqueous solution and the yttrium nitrate aqueous solution were mixed at a low speed of 400 rpm. In addition, the residue was washed with 10 L of 90 vol% ethanol (10 vol% pure water) instead of 10 L of ethanol. Other than these points, the same procedure as in Example b1 was followed.

(Example b3)
The residue was washed with 10 L of 80 vol % ethanol (20 vol % pure water) instead of 10 L of 90 vol % ethanol. Other than that, the same procedure as in Example b2 was followed.

(Example b4)
The residue was washed with 10 L of 50 vol % ethanol (50 vol % pure water) instead of 10 L of 90 vol % ethanol. Other than that, the same procedure as in Example b2 was followed.

(Example b5)
Instead of the yttrium nitrate aqueous solution (25° C.) having a concentration of 50 g/L calculated as oxide, an ytterbium nitrate aqueous solution (25° C.) having a concentration of 50 g/L calculated as oxide was used. The rest of the experiment was the same as in Example b1.

(Example b6)
In the [Crushing Evaluation], the concentration of the slurry to be subjected to the bead mill crushing was changed from 10% by mass to 4% by mass, and the dispersion medium was changed from ethanol to methanol. Other than these points, the same procedure was followed as in Example b1.

(Example b7)
Instead of the yttrium nitrate aqueous solution (25° C.) having a concentration of 50 g/L calculated as oxide, a dysprosium nitrate aqueous solution (25° C.) having a concentration of 50 g/L calculated as oxide was used. The rest of the experiment was the same as in Example b1.

(Comparative Example b1)
The yttrium oxide fine powder obtained in Comparative Example a1 was used as the yttrium oxide fine powder of Comparative Example b1.
As described above, when this powder was observed with a scanning electron microscope (SEM), it was found to be a group of spherical particles with a uniform particle size of about 100 nm without any agglomeration. Since the primary particle size calculated from the specific surface area was also 100 nm or more, some of the above evaluations were not performed.

(Comparative Example b2)
The yttrium oxide fine powder prepared in Comparative Example a2 was used as the yttrium oxide fine powder of Comparative Example b2. As described above, when this powder was observed with a scanning electron microscope (SEM), particles with SEM diameters of approximately 100 nm and 10 nm were mixed, and particles with uniform SEM diameters could not be obtained by low-temperature firing. Since the primary particle diameter calculated from the specific surface area was also 100 nm or more, some of the above evaluations were not performed.

(Comparative Example b3)
_{The Y2O3} produced in Comparative Example a3 was used as the yttrium oxide fine powder of Comparative Example b3. This powder was subjected to various measurements by the method described in Example b1, and was also evaluated for disintegration. Since the slurry was aggregated and settled one week after disintegration, the aggregate diameter ("7D") and transmittance after one week could not be measured. Furthermore, the aggregate diameter "7DS" after one week of ultrasonic irradiation was not measured.

(Comparative Example b4)
The yttrium oxide fine powder produced in Comparative Example a4 was used as the yttrium oxide fine powder of Comparative Example b4.

(Comparative Example b5)
This comparative example corresponds to US 2020/0071180A.
20 L of an aqueous yttrium nitrate solution was prepared so that the concentration of yttrium ions was 0.05 mol/L. 21.1 g of an acetylene glycol-ethylene oxide adduct (Surfynol 485 manufactured by Nissin Chemical Industry Co., Ltd.) was added to the aqueous solution. Then, urea was added in an amount 15 times the molar ratio of the yttrium ions. In order to advance the hydrolysis reaction, the solution was heated to 95°C and held for 90 minutes, and then cooled to room temperature. During heating, the aqueous solution was slowly stirred with a stirring blade so that the temperature distribution of the aqueous solution in the container was uniform.
Next, the precipitate was separated from the liquid after the reaction using a centrifuge. Furthermore, the collected solid content was washed with water to remove undecomposed urea and residual nitrate ions. Next, the obtained rare earth compound particles were dried at 55°C for 5 days and then crushed in a dry bead mill. The crushed rare earth compound particles were fired at 600°C for 4 hours to obtain yttrium oxide particles. Since the obtained yttrium oxide had a primary particle diameter of 100 nm or more, some of the above evaluations were not performed.

As shown in Table 2 above, a powder of an oxide of at least one rare earth element other than Ce,
the primary particle size is 10 nm or more and less than 100 nm, the value obtained by multiplying the pore volume (cm ³ /g) having a pore diameter of 0.005 μm or more and 100 μm or less by the true density (g/cm ³ ) is 3 or more and 14,
The rare earth oxide powders of each example, in which the value obtained by multiplying the pore volume ( ^cm3 /g) with a pore diameter of 5 nm or more and 50 nm or less by the true density (g/ ^cm3 ) was 0 or more and 2.0 or less, had a small particle size after crushing and a high transmittance after being left standing for 7 days.
In contrast, in Comparative Examples b1, b2, and b5, in which the primary particle diameter was outside the range of 10 nm or more and less than 100 nm, the average particle diameter after crushing was large, and the visible light transmittance after 7 days was low. In Comparative Example b4, in which the value obtained by multiplying the pore volume (cm ³ /g) of a pore diameter of 0.005 μm or more and 100 μm or less by the true density (g/cm ³ ) was more than 14, the average particle diameter after crushing was large, and the transmittance after 7 days was low. Furthermore, in Comparative Example 3, in which the value obtained by multiplying the pore volume (cm ³ /g) of a pore diameter of 5 nm or more and 50 nm or less by the true density (g/cm ³ ) was more than 2.0, the dispersoid settled over time, and the visible light transmittance could not be measured.
From the above, it can be seen that the rare earth oxide powder of the present invention can provide a fine oxide powder of a rare earth element other than Ce that can be made into a highly dispersed slurry without using a dispersant and can stably maintain the transparency of the slurry.

According to the first invention, for rare earth oxides other than Ce, a rare earth oxide powder is provided which can be easily dispersed by a simple dispersion process such as ultrasonic treatment and can form a thin coating film.

According to the second invention, a fine powder of oxides of rare earth elements other than Ce is provided, which can be made into a highly dispersed slurry without using a dispersant and can maintain a dispersed state. By crushing such rare earth oxide fine powder, a highly dispersed slurry can be obtained, and the slurry can have an increased visible light transmittance based on its dispersibility. By increasing the visible light transmittance, it is possible to develop materials that require transparency while taking advantage of the effect of adding rare earth oxides. It is also possible to provide a slurry that absorbs only a narrow wavelength range specific to rare earth ions. In addition, according to the rare earth oxide powder of the present invention, the dispersed state in the slurry can be maintained, so the transparency of the slurry can be stably maintained.

Claims (19)

A powder of an oxide of at least one rare earth element other than Ce,
(a) the primary particle size is 10 nm or more and 60 nm or less, and satisfies the following (I) or (II); or (b) the primary particle size is 10 nm or more and less than 100 nm, and satisfies the following (III) and (IV);
Rare earth oxide powder.
(I) The volume cumulative particle size D ₁₀₀ at 100% cumulative volume measured by a laser diffraction/scattering particle size distribution measurement method after ultrasonic dispersion treatment at 40 W for 5 minutes is 1 μm or more and 10 μm or less.
(II) When the true density of the oxide of the rare earth element is ρ (g/ ^cm3 ), the difference (P _AD - P _TD ) between the porosity P _AD (%) calculated from the initial bulk density AD using the following formula 1 and the porosity P _TD (%) calculated from the tapped bulk density TD using the following formula 2 is 2.0% or more and 5.0% or less.
Formula 1: P _AD = (1 - AD / ρ) × 100 (%)
Formula 2: P _TD = (1 - TD / ρ) × 100 (%)
(III) The value obtained by multiplying the pore volume (cm ³ /g) of pores having diameters of 0.005 μm or more and 100 μm or less by the true density (g/cm ³ ) is 3 or more and 14 or less.
(IV) The value obtained by multiplying the volume (cm ³ /g) of pores having a diameter of 5 nm or more and 50 nm or less by the true density (g/cm ³ ) is 0 or more and 2.0 or less. The rare earth oxide powder according to claim 1, wherein (a) the primary particle diameter is 10 nm or more and 60 nm or less, and satisfies (I) or (II) above. The rare earth oxide powder according to claim 2, which satisfies (I) above. 4. The rare earth oxide powder according to claim 3, wherein the true density of the rare earth oxide is ρ (g/ ^cm3 ), and the porosity P _AD (%) calculated from the initial bulk density AD by the following formula 1 is 90.0% or more and 99.0% or less.
Formula 1: P _AD = (1 - AD / ρ) × 100 (%) The rare earth oxide powder according to claim 3 or 4, having a primary particle size of 35 nm or less. The rare earth oxide powder according to claim 3 or 4, wherein the Zr content is 100 mass ppm or less. The rare earth oxide powder according to claim 3 or 4, having a carbon content of 2 mass% or less. The rare earth oxide powder according to claim 2, which satisfies (II) above. 9. The rare earth oxide powder according to claim 8, wherein the porosity P _AD (%) is 90.0% or more and 99.0% or less. _10. The rare earth oxide powder according to claim 8, wherein the volume cumulative particle size _D100 at 100% cumulative volume measured by a laser diffraction scattering type particle size distribution measurement method after ultrasonic dispersion treatment at 40 W for 5 minutes is 1 μm or more and 10 μm or less, and the ratio _D100 to the volume cumulative particle size D50 at 50% cumulative volume measured by a laser diffraction scattering type particle size distribution measurement method after ultrasonic dispersion treatment, _D100 / _D50 , is 3.0 or more and 11.0 or less. (b) The rare earth oxide powder according to claim 1, having a primary particle diameter of 10 nm or more and less than 100 nm, and satisfying the above (III) and (IV). The rare earth oxide powder according to claim 11, wherein the Na content is 100 ppm by mass or less. The rare earth oxide powder according to claim 11, having a crystallite diameter of 6 nm or more and 25 nm or less. The rare earth oxide powder according to claim 12, having a crystallite diameter of 6 nm or more and 25 nm or less. 15. The rare earth oxide powder according to claim 11, wherein when a rare earth oxide powder is mixed with ethanol to prepare an ethanol slurry containing 10% by mass of the rare earth oxide powder, and the following operation (A) is repeatedly performed until the average particle diameter becomes larger than the previous measurement value, the smallest average particle diameter is 10 nm or more and 150 nm or less.
(A): The slurry is subjected to a bead mill treatment for 10 minutes using zirconia beads having a diameter of 0.1 mm, and then the average particle size is measured by a dynamic light scattering method.
(However, when the average particle size becomes larger than the previous measurement value, the bead mill treatment is terminated without performing the next operation (A) , and if the average particle size does not become larger than the previous measurement value in each of the second to twentieth operations of repeating (A) 20 times, the treatment is terminated when (A) has been repeated 20 times.
The minimum average particle size here means the minimum value of the average particle size measured by a dynamic light scattering method by sampling after each treatment of (A). The rare earth oxide powder according to claim 15, wherein the calculated value of the following formula is -15% or more and 25% or less.
Formula: (average particle size 7 days after crushing−average particle size immediately after crushing)/average particle size immediately after crushing×100(%) The rare earth oxide powder according to claim 15, wherein the smallest average particle size obtained by the bead mill treatment is 50 nm or more and 90 nm or less. The rare earth oxide powder according to claim 16, wherein the smallest average particle size obtained by the bead mill treatment is 50 nm or more and 90 nm or less. A method for producing a slurry in which the rare earth oxide powder according to any one of claims 11 and 12 is wet-disintegrated using a solvent.