Preparation method of rare earth composite oxide particles

Abstract

Rare earth composite oxide particles are prepared by a method including the steps of: (A) heating an aqueous solution containing an ion of at least one rare earth element selected from the group consisting of Sc, Y, Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, any one or both of an Al ion and a Ga ion, an organic compound having a carboxyl group, and urea at not less than 80 DEG C and not more than the boiling point of the aqueous solution; and (B) generating a rare earth composite oxide from the rare earth composite compound by reacting the organic compound, a hydrolysis product of urea, ions of a rare earth element, and either or both of an Al ion and a Ga ion.

Description

Technical Field

[0001] The present invention relates to a method for preparing rare earth composite oxide particles containing aluminum or gallium or both which can be used as ceramic raw materials. Background Art

[0002] Composite oxides containing rare earth elements are used in various functional ceramic products to contribute to the improvement of product properties. Raw material powders of rare earth composite oxides containing aluminum or gallium are used to manufacture ceramics, phosphors, films, coatings, etc. by various means. Recently, in many cases, in order to improve the properties of functional ceramic products, the demand for finer raw material powders has increased.

[0003] As a method for obtaining fine particles of rare earth carbonate or rare earth oxide, for example, a method is described in JP-A 2000-239019 (Patent Document 1). In this method, urea is hydrolyzed by heating in water in the presence of hydrogen peroxide, and a basic carbonate of a rare earth element is precipitated as monodisperse particles from rare earth element ions, and then separated by solid-liquid separation, dried, and calcined to obtain a rare earth oxide.

[0004] In addition, as a method for obtaining a composite compound and a composite oxide of a metal element, for example, JP-A2018-095524 (Patent Document 2) describes a method in which a metal compound and a dicarboxylic acid compound containing each metal element component constituting a perovskite-type composite oxide are dissolved in a solvent to obtain a solution, and then the solution is dried and calcined to obtain a composite oxide of a metal element.

[0005] However, when ions of metal elements other than rare earth elements that are easily hydroxylated and gelled, such as aluminum and gallium, and rare earth element ions are precipitated by the method described in JP-A 2000-239019 (Patent Document 1), a composite compound containing rare earth elements having poor collection properties is obtained due to gelling of hydroxylated aluminum and gallium. In addition, when the obtained composite compound is calcined, the rare earth composite oxide containing aluminum or gallium causes strongly aggregated and enlarged particles due to dehydration condensation of hydroxides.

[0006] In addition, in the method described in JP-A 2018-095524A (Patent Document 2), a solution prepared by dissolving a metal compound containing a metal element component and a dicarboxylic acid compound in a solvent is dried using a rotary evaporator to obtain a composite compound of the metal element, and therefore, this method has poor productivity in terms of large-scale production on an industrial scale.

[0007] Reference List

[0008] Patent Document 1: JP-A 2000-239019

[0009] Patent Document 2: JP-A 2018-095524 Summary of the invention

[0010] The present invention has been completed in consideration of the above circumstances, and an object of the present invention is to provide a method for preparing rare earth composite oxide particles containing either or both of aluminum and gallium in a manner in which gelation is suppressed and the particle size is not increased, and to provide a method for preparing rare earth composite oxide particles containing either or both of aluminum and gallium which has high productivity and advancement in large-scale production.

[0011] In the homogeneous precipitation method, particles of rare earth compounds such as basic carbonates of rare earth elements can be precipitated by heating an aqueous solution containing a rare earth mineral acid salt and urea as an ion source of the rare earth element. However, in the case of metal elements such as aluminum and gallium, particles of metal compounds such as hydroxide salts are precipitated from a low pH range, causing the particles to gel, and because the particles tend to bind to each other, they increase in size. When the homogeneous precipitation method is performed in the presence of both metal elements and rare earth elements that are easy to gel, gelation occurs and enlarged particles are obtained.

[0012] The inventors have conducted serious research on a method for preparing rare earth composite oxide particles containing either or both of aluminum and gallium by a homogeneous precipitation method to solve the above problems. As a result, the inventors have found that by heating an aqueous solution containing an organic compound having a carboxyl group and a rare earth element and either or both of aluminum and gallium, the gelation and enlargement of the precipitated particles are improved, and highly dispersed rare earth composite oxide particles containing either or both of aluminum and gallium can be prepared.

[0013] In one aspect, the present invention provides a method for preparing rare earth composite oxide particles, comprising the following steps:

[0014] (A) heating an aqueous solution containing ions of at least one rare earth element selected from the group consisting of Sc, Y, Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, either or both of Al ions and Ga ions, an organic compound having a carboxyl group and urea at a temperature not lower than 80° C. and not higher than the boiling point of the aqueous solution to react the organic compound, the hydrolyzate of the urea, the ions of the rare earth element and either or both of Al ions and Ga ions to produce particles of a rare earth composite compound, and

[0015] (B) generating the rare earth composite oxide from the rare earth composite compound.

[0016] Preferably, the organic compound is an organic compound having at least one carboxyl group, or an organic compound having a carboxyl group from which a condensation of carboxyl groups is easily formed in an aqueous solution.

[0017] Preferably, step (B) comprises the following steps:

[0018] separating the particles of the rare earth composite compound obtained in step (A) by solid-liquid separation, and

[0019] The obtained solid is calcined at not less than 600° C. in an oxygen-containing atmosphere to produce a rare earth composite oxide.

[0020] Preferably, the method is for producing rare earth composite oxide particles having a volume-based median diameter (D50) of not less than 0.1 μm and not more than 10 μm in a particle size distribution measured by a laser diffraction method.

[0021] Preferably, the organic compound having a carboxyl group is at least one selected from the group consisting of maleic acid, maleic anhydride, malic acid, and citric acid.

[0022] Preferably, step (B) comprises the following steps:

[0023] separating the particles of the rare earth composite compound obtained in step (A) by solid-liquid separation, and

[0024] The obtained solid is calcined at not less than 700° C. in an oxygen-containing atmosphere to produce a rare earth composite oxide.

[0025] Beneficial effects of the present invention

[0026] According to the present invention, by adding an organic compound having a carboxyl group to an aqueous solution for precipitating rare earth composite compound particles containing either or both of aluminum and gallium, even when metal elements such as aluminum and gallium that tend to gel and produce enlarged particles are included, gelation and particle enlargement are suppressed compared to conventional methods, and particles with good dispersibility are obtained. Therefore, the method enables mass production of fine particles that exert high performance, which is in line with the recent trend of finer particles of ceramic raw materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 1 and 2 are electron microscope images of the rare earth composite oxide particles obtained in Example 1 and electron microscope images of the yttrium-aluminum composite oxide particles.

[0028] Figure 2 1 and 2 are electron microscope images of the rare earth composite oxide particles obtained in Example 2 and electron microscope images of the yttrium-aluminum composite oxide particles.

[0029] Figure 3 1 and 2 are electron microscope images of the rare earth composite oxide particles obtained in Example 3 and electron microscope images of the yttrium-aluminum composite oxide particles.

[0030] Figure 4 1 and 2 are electron microscope images of the rare earth composite oxide particles obtained in Example 4 and electron microscope images of the yttrium-aluminum-gallium composite oxide particles.

[0031] Figure 5 1 and 2 are electron microscope images of the rare earth composite oxide particles obtained in Example 5 and electron microscope images of the lutetium-aluminum composite oxide particles.

[0032] Figure 6 1 and 2 are electron microscope images of the rare earth composite oxide particles obtained in Example 6 and electron microscope images of the gadolinium-gallium composite oxide particles.

[0033] Figure 7 1 and 2 are electron microscope images of the rare earth composite oxide particles obtained in Comparative Example 1 and electron microscope images of the yttrium-aluminum composite oxide particles.

[0034] Figure 8 The results of particle size distribution measurement of the yttrium-aluminum composite oxide particles obtained in Example 1 by laser diffraction method are shown.

[0035] Fig. 9 The results of particle size distribution measurement of the yttrium-aluminum composite oxide particles obtained in Example 2 by laser diffraction method are shown.

[0036] Fig.10 The results of particle size distribution measurement of the yttrium-aluminum composite oxide particles obtained in Example 3 by laser diffraction are shown.

[0037] Fig.11 The results of particle size distribution measurement of the yttrium-aluminum gallium composite oxide particles obtained in Example 4 by laser diffraction are shown.

[0038] Fig.12 The results of particle size distribution measurement of the lutetium-aluminum composite oxide particles obtained in Example 5 by laser diffraction method are shown.

[0039] Fig.13 The results of particle size distribution measurement of the gadolinium-gallium composite oxide particles obtained in Example 6 by laser diffraction are shown.

[0040] Fig.14 The results of particle size distribution measurement of the yttrium-aluminum composite oxide particles obtained in Comparative Example 1 by laser diffraction are shown. DETAILED DESCRIPTION

[0041] In the present invention, rare earth composite oxide particles are prepared by a method comprising the following steps:

[0042] (A) heating an aqueous solution containing ions of at least one rare earth element selected from the group consisting of Sc, Y, Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, either or both of Al ions and Ga ions, an organic compound having a carboxyl group and urea at a temperature of not less than 80° C. and not more than the boiling point of the aqueous solution to react the organic compound, a hydrolyzate of urea, ions of the rare earth element and either or both of Al ions and Ga ions to produce particles of a rare earth composite compound, and

[0043] (B) generating a rare earth composite oxide from the rare earth composite compound.

[0044] In the present invention, an aqueous solution containing either or both of rare earth elements, aluminum (Al) and gallium (Ga) ions that is easily gelled, an organic compound having a carboxyl group and urea are heated, and a rare earth composite compound containing either or both of aluminum and gallium is generated through the reaction of the organic compound, the hydrolysis product of urea, the ions of the rare earth elements and either or both of the aluminum ions and the gallium ions, so as to prepare fine particles of the rare earth composite compound containing either or both of aluminum and gallium.

[0045] An aqueous solution containing ions of rare earth elements and either or both of aluminum and gallium ions and optionally containing ions of another metal element described below can be provided by preparing an aqueous solution of a water-soluble rare earth inorganic salt and either or both of aluminum and gallium and optionally other metal elements described below. Examples of inorganic salts include nitrates and chlorides. When metals such as iron and stainless steel are used for parts inside a preparation device in contact with an aqueous solution, it is more preferred to use nitrates that have a lower probability of increasing impurities in the product of the metal of the parts in contact with the aqueous solution.

[0046] As the ions of the rare earth element, ions of at least one rare earth element (first rare earth element) selected from the group consisting of Sc, Y, Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu are contained. The preferred concentration of the ions of the first rare earth element in the aqueous solution is not less than 0.01 mol / L, especially not less than 0.05 mol / L, and not more than 0.3 mol / L, especially not more than 0.2 mol / L. In view of the productivity of the particles, a high concentration of the ions of the rare earth element is preferred, however, at a concentration greater than 0.3 mol / L, the particles may be strongly aggregated.

[0047] In addition, as ions of rare earth elements, in addition to ions of the first rare earth element, ions of rare earth elements different from the first rare earth element (second rare earth element) may also be contained. As ions of the second rare earth element, ions of at least one rare earth element selected from the group consisting of La, Ce, Pr, Sm and Eu are exemplified. In the case of containing the second rare earth element, the preferred concentration of the total of ions of the first rare earth element and ions of the second rare earth element in the aqueous solution is not less than 0.01 mol / L, especially not less than 0.05 mol / L, and not more than 0.3 mol / L, especially not more than 0.2 mol / L. Among the ions of the first rare earth element and the ions of the second rare earth element, the preferred content of the ions of the second rare earth element relative to the total of the ions of the first rare earth element and the ions of the second rare earth element is not more than 50 mol%, especially not more than 30 mol%.

[0048] The preferred concentration of the total of aluminum and gallium in the aqueous solution is not less than 0.01 mol / L, particularly not less than 0.05 mol / L, and not more than 0.3 mol / L, particularly not more than 0.2 mol / L. In view of the productivity of the particles, high concentrations of aluminum and gallium are preferred, however, at a concentration greater than 0.3 mol / L, the particles may be strongly aggregated. In the case where the aqueous solution contains both aluminum ions and gallium ions, the ratio of aluminum ions to gallium ions is not particularly limited. The preferred content of gallium ions relative to the total of aluminum ions and gallium ions is not more than 50 mol%, particularly not more than 40 mol%.

[0049] The aqueous solution containing ions of rare earth elements and either or both of ions of aluminum and ions of gallium may contain ions of metals other than the first rare earth element, the second rare earth element, aluminum and gallium (ions of other metals) in addition to either or both of ions of aluminum and ions of gallium. As ions of other metals, ions of iron are exemplified. In the case of containing ions of other metals, the preferred concentration of the total of ions of aluminum, ions of gallium and ions of other metals in the aqueous solution is not less than 0.01 mol / L, in particular not less than 0.05 mol / L, and not more than 0.3 mol / L, in particular not more than 0.2 mol / L. Among ions of aluminum, ions of gallium and ions of other metals, the preferred content of ions of other metals relative to the total of ions of aluminum, ions of gallium and ions of other metals is not more than 50 mol%, in particular not more than 40 mol%.

[0050] The preferred concentration of the total of the ions of rare earth elements, gallium ions and aluminum ions, or in the case where the aqueous solution contains ions of other metals, the preferred concentration of the total of the ions of rare earth elements, aluminum ions, gallium ions and other metal ions is not less than 0.02 mol / L, particularly not less than 0.1 mol / L, and not more than 0.6 mol / L, particularly not more than 0.3 mol / L. In view of the productivity of particles, a high concentration of the total of the ions of rare earth elements, aluminum ions, gallium ions and optionally other metal ions is preferred, however, at a concentration of more than 0.3 mol / L, the particles may be strongly aggregated.

[0051] The aqueous solution contains an organic compound having a carboxyl group. As the organic compound having a carboxyl group, an organic compound having at least one carboxyl group or an organic compound having a carboxyl group that is easily condensed to form a carboxyl group in an aqueous solution is exemplified. The organic compound having a carboxyl group is preferably at least one selected from the group consisting of maleic acid, maleic anhydride, malic acid and citric acid.

[0052] The preferred amount of the organic compound having a carboxyl group is not less than 0.1 times, particularly not less than 0.2 times, and not more than 1 times, particularly not more than 0.5 times, relative to the total amount of the ions of the rare earth element, the ions of aluminum, the ions of gallium, and the ions of the optional other metals in the aqueous solution in terms of molar ratio. When the amount of the organic compound having a carboxyl group is less than the above range, gelation may not be fully suppressed. When the amount of the organic compound having a carboxyl group exceeds the above range, a highly water-soluble compound may be formed together with the organic compound having a carboxyl group by the ions of the rare earth element, the ions of either or both of aluminum and gallium, and the ions of the optional other metals, and the recovery amount (yield) of the rare earth composite compound as a precipitate may be reduced.

[0053] The aqueous solution contains urea. Relative to the total amount of the ions of the rare earth elements, the ions of aluminum, the ions of gallium and the optional ions of other metals in the aqueous solution, the preferred amount of urea is not less than 5 times, particularly not less than 10 times, and not more than 30 times, particularly not more than 20 times in terms of molar ratio. When the amount of urea is less than the above range, the production time of the rare earth composite compound may be too long, and the recovery amount (yield) of the rare earth composite compound may be reduced. When the amount of urea exceeds the above range, it may be disadvantageous in terms of economic benefit.

[0054] The organic compound having a carboxyl group and urea may be mixed with ions of a rare earth element, ions of either or both of aluminum and gallium, and optionally ions of other metals before heating or after heating starts, i.e., during heating (when the temperature rises or after the temperature has reached a predetermined temperature).

[0055] In the preparation of a rare earth composite compound containing either or both of aluminum and gallium ions, urea is hydrolyzed by heating an aqueous solution (mixed aqueous solution) containing each component, and the hydrolysis products such as carbonate ions and ammonium ions and organic compounds having carboxyl groups generated by the hydrolysis react with the ions of the rare earth element to generate a rare earth composite compound containing either or both of aluminum and gallium ions as a precipitate. The preferred heating temperature is not less than 80°C, especially not less than 90°C, and not greater than the boiling point of the mixed aqueous solution, especially less than the boiling point, for example, more preferably not greater than 100°C. When the amount of urea added is large and the heating temperature is high, the heating time can be shortened, and is generally 60 to 300 minutes.

[0056] The precipitate (solid) is produced in the form of a slurry, and can be separated into solid and liquid by filtering or sedimentation such as decantation and centrifugal methods. The precipitate obtained is very small particles, preferably separated into solid and liquid by centrifugal sedimentation, because particles are more easily passed through the filter by common filtration. When removing the unreacted urea and residual anions contained in the solid, it is preferred that the solid obtained after solid-liquid separation is washed with pure water etc. If necessary, the solid can be dried in an oxygen-containing atmosphere such as air or in an inert gas atmosphere. The particles of the rare earth composite compound of any one or both of the ions containing aluminum and gallium obtained in this way contain carbonate, basic carbonate, hydroxide etc. depending on the type of rare earth element.

[0057] The rare earth composite compound particles containing either or both of the ions of aluminum and gallium obtained as a solid can be used as is. In addition, the rare earth composite compound can be calcined to generate a rare earth composite oxide from the rare earth composite compound, and particles of the rare earth composite oxide containing either or both of the ions of aluminum and gallium can be prepared, such as garnet, monoclinic crystal and perovskite. After solid-liquid separation, when the rare earth composite compound containing either or both of the ions of aluminum and gallium collected as a solid is calcined as is, a block solid will be formed due to aggregation and sintering in many cases. Therefore, in order to recover the rare earth composite oxide obtained after calcination as particles with good dispersibility, after solid-liquid separation, the compound is preferably calcined after drying. The preferred temperature for this drying is not more than 150°C, particularly not more than 80°C, and especially not more than 60°C. Depending on the type of rare earth element, when the drying temperature is too high, the rare earth composite compound may recrystallize and may lose the characteristics as a particle. The preferred drying time is not less than 1 day (24 hours), usually not more than 7 days (168 hours), however, it is not particularly limited. The atmosphere used for drying is not particularly limited, and may be an oxygen-containing atmosphere such as air, or an inert gas atmosphere.

[0058] The dried rare earth composite compound containing either or both of the ions of aluminum and gallium can be crushed by a crusher or the like. The dried particles of the rare earth composite compound containing either or both of the ions of aluminum and gallium are easily separated by crushing with a relatively weak force because it is believed that the particles are bound to each other by very weak forces such as hydrogen bonds. The crusher may be a jet mill, a roller mill, a hammer mill, a bead mill, a ball mill, or the like, and may be appropriately selected depending on the state of the particles to be obtained by crushing.

[0059] The calcination for obtaining the rare earth composite oxide containing either or both of aluminum and gallium ions is preferably carried out in an oxygen-containing atmosphere such as air or oxygen at a preferred temperature of not less than 600° C., particularly not less than 700° C., especially not less than 800° C. and not more than 1500° C., particularly not more than 1300° C. The preferred calcination time is not less than 2 hours, and usually not more than 8 hours.

[0060] By the method of the present invention, rare earth composite oxide particles containing either or both of aluminum and gallium ions having a median diameter (D50) of not less than 0.1 μm and not more than 10 μm in particle size distribution measured by laser diffraction can be obtained.

[0061] Example

[0062] Examples of the invention are given below by way of illustration and not limitation.

[0063] Example 1

[0064] An aqueous solution of yttrium nitrate and an aqueous solution of aluminum nitrate were added so that in 100 L of the aqueous solution, the yttrium ion concentration was 0.06 mol / L and the aluminum ion concentration was 0.10 mol / L, respectively. In addition, maleic anhydride was added in an amount equivalent to 0.4 times the total ion concentration of yttrium ions and aluminum ions, and the solution was stirred. Next, the solution was heated to 98°C, and then urea was added in an amount equivalent to 2.4 mol / L, and the solution was heated at 98°C for 150 minutes. As a result, a solid was deposited.

[0065] Next, the sediment was separated into solid and liquid by a centrifuge, and then the collected solid was washed with about 20 L of pure water. The obtained solid (composite compound particles) was confirmed to be amorphous by X-ray diffraction.

[0066] Next, the obtained composite compound particles were calcined at 700°C for 4 hours under air. The obtained calcined product was crushed by a hammer mill and then calcined at 1100°C for another 2 hours. As a result, particles of yttrium-aluminum composite oxide were obtained. Observation under an electron microscope confirmed that, although the particles were connected to each other, the particles had a primary particle size of not more than 0.1 μm, with very little particle enlargement. An electron microscope image of the particles is shown in Figure 1 In addition, the particles were confirmed to be yttrium-aluminum-garnet by X-ray diffraction. In addition, after the obtained particles were dispersed in pure water by a homogenizer (40W, 3 minutes), the particle size distribution was measured by a laser diffraction / scattering particle size distribution analyzer (Microtrack Bell, MT3300). The results are shown in Figure 8 The median diameter (D50) is 2.05 μm.

[0067] Example 2

[0068] An aqueous solution of yttrium nitrate and an aqueous solution of aluminum nitrate were added so that in 100 L of the aqueous solution, the yttrium ion concentration was 0.06 mol / L and the aluminum ion concentration was 0.10 mol / L, respectively. In addition, citric acid was added in an amount equivalent to 0.2 times the total ion concentration of yttrium ions and aluminum ions, and the solution was stirred. Next, the solution was heated to 98°C, and then urea was added in an amount equivalent to 2.4 mol / L, and the solution was heated at 98°C for 150 minutes. As a result, a solid was deposited.

[0069] Next, the sediment was separated into solid and liquid by a centrifuge, and then the collected solid was washed with about 20 L of pure water. The obtained solid (composite compound particles) was confirmed to be amorphous by X-ray diffraction.

[0070] Next, the obtained composite compound particles were calcined at 700°C for 4 hours under air. The obtained calcined product was crushed by a hammer mill and then calcined at 1100°C for another 2 hours. As a result, particles of yttrium-aluminum composite oxide were obtained. Observation under an electron microscope confirmed that, although the particles were connected to each other, the particles had a primary particle size of about 0.3 to 1 μm with very little particle enlargement. An electron microscope image of the particles is shown in Figure 2 In addition, the particles were confirmed to be yttrium-aluminum-garnet by X-ray diffraction. In addition, after the obtained particles were dispersed in pure water by a homogenizer (40W, 3 minutes), the particle size distribution was measured by a laser diffraction / scattering particle size distribution analyzer MT3300 using a laser diffraction method. The results are shown in Fig. 9 The median diameter (D50) is 5.70 μm.

[0071] Example 3

[0072] An aqueous solution of yttrium nitrate and an aqueous solution of aluminum nitrate were added so that in 100 L of the aqueous solution, the yttrium ion concentration was 0.10 mol / L and the aluminum ion concentration was 0.05 mol / L, respectively. In addition, maleic anhydride was added in an amount equivalent to 0.5 times the total ion concentration of yttrium ions and aluminum ions, and the solution was stirred. Next, the solution was heated to 98°C, and then urea was added in an amount equivalent to 2.4 mol / L, and the solution was heated at 98°C for 150 minutes. As a result, a solid was deposited.

[0073] Next, the sediment was separated into solid and liquid by a centrifuge, and then the collected solid was washed with about 20 L of pure water. The obtained solid (composite compound particles) was confirmed to be amorphous by X-ray diffraction.

[0074] Next, the obtained composite compound particles were calcined at 700°C for 4 hours under air. The obtained calcined product was crushed by a hammer mill and then calcined at 1000°C for another 2 hours. As a result, particles of yttrium-aluminum composite oxide were obtained. Observation under an electron microscope confirmed that, although the particles were connected to each other, the particles had a primary particle size of not more than 0.2 μm, with very little particle enlargement. An electron microscope image of the particles is shown in Figure 3 In addition, the particles were confirmed to be yttrium-aluminum-monoclinic crystals by X-ray diffraction. In addition, after the obtained particles were dispersed in pure water by a homogenizer (40W, 3 minutes), the particle size distribution was measured by a laser diffraction / scattering particle size distribution analyzer MT3300 using a laser diffraction method. The results are shown in Fig.10 The median diameter (D50) is 4.54 μm.

[0075] Example 4

[0076] An aqueous solution of yttrium nitrate, an aqueous solution of aluminum nitrate, and an aqueous solution of gallium nitrate were added so that in 100 L of the aqueous solution, the yttrium ion concentration was 0.06 mol / L, the aluminum ion concentration was 0.06 mol / L, and the gallium ion concentration was 0.04 mol / L, respectively. In addition, malic acid was added in an amount equivalent to 0.4 times the total ion concentration of yttrium ions, aluminum ions, and gallium ions, and then the solution was stirred. Next, the solution was heated to 98°C, and then urea was added in an amount equivalent to 2.4 mol / L, and the solution was heated at 98°C for 165 minutes. As a result, a solid was deposited.

[0077] Next, the sediment was separated into solid and liquid by a centrifuge, and then the collected solid was washed with about 20 L of pure water. The obtained solid (composite compound particles) was confirmed to be amorphous by X-ray diffraction.

[0078] Next, the obtained composite compound particles were calcined at 700°C for 4 hours under air. The obtained calcined product was crushed by a hammer mill and then calcined at 1100°C for another 2 hours. As a result, particles of yttrium-aluminum gallium composite oxide were obtained. Observation under an electron microscope confirmed that, although the particles were connected to each other, the particles had a primary particle size of about 0.5 to 1.5 μm with very little particle enlargement. An electron microscope image of the particles is shown in Figure 4 In addition, the particles were confirmed to be yttrium-(aluminum, gallium)-garnet by X-ray diffraction. In addition, after the obtained particles were dispersed in pure water by a homogenizer (40W, 3 minutes), the particle size distribution was measured by the laser diffraction / scattering particle size distribution analyzer MT3300 using the laser diffraction method. The results are shown in Fig.11 The median diameter (D50) is 8.58 μm.

[0079] Example 5

[0080] An aqueous solution of lutetium nitrate and an aqueous solution of aluminum nitrate were added so that in 100 L of the aqueous solution, the lutetium ion concentration was 0.06 mol / L and the aluminum ion concentration was 0.10 mol / L, respectively. In addition, malic acid was added in an amount equivalent to 0.4 times the total ion concentration of lutetium ions and aluminum ions, and the solution was stirred. Next, the solution was heated to 98°C, and then urea was added in an amount equivalent to 2.4 mol / L, and the solution was heated at 98°C for 165 minutes. As a result, a solid was deposited.

[0081] Next, the sediment was separated into solid and liquid by a centrifuge, and then the collected solid was washed with about 20 L of pure water. The obtained solid (composite compound particles) was confirmed to be amorphous by X-ray diffraction.

[0082] Next, the obtained composite compound particles were calcined at 700°C for 4 hours under air. The obtained calcined product was crushed by a hammer mill and then calcined at 1100°C for another 2 hours. As a result, particles of lutetium-aluminum composite oxide were obtained. Observation under an electron microscope confirmed that, although the particles were connected to each other, the particles had a primary particle size of about 0.5 to 1.5 μm with very little particle enlargement. An electron microscope image of the particles is shown in Figure 5 In addition, the particles were confirmed to be lutetium-aluminum-garnet by X-ray diffraction. In addition, after the obtained particles were dispersed in pure water (40W, 3 minutes) by a homogenizer, the particle size distribution was measured by a laser diffraction / scattering particle size distribution analyzer MT3300 using a laser diffraction method. The results are shown in Fig.12 The median diameter (D50) is 7.59 μm.

[0083] Example 6

[0084] An aqueous solution of gadolinium nitrate and an aqueous solution of gallium nitrate were added so that in 100 L of the aqueous solution, the gadolinium ion concentration was 0.06 mol / L and the gallium ion concentration was 0.10 mol / L, respectively. In addition, maleic anhydride was added in an amount equivalent to 0.3 times the total ion concentration of gadolinium ions and gallium ions and malic acid was added in an amount equivalent to 0.1 times the total ion concentration of gadolinium ions and gallium ions, and then the solution was stirred. Next, the solution was heated to 98°C, and then urea was added in an amount equivalent to 2.4 mol / L, and the solution was heated at 98°C for 180 minutes. As a result, a solid was deposited.

[0085] Next, the sediment was separated into solid and liquid by a centrifuge, and then the collected solid was washed with about 20 L of pure water. The obtained solid (composite compound particles) was confirmed to be amorphous by X-ray diffraction.

[0086] Next, the obtained composite compound particles were calcined at 700°C for 4 hours under air. The obtained calcined product was crushed by a hammer mill and then calcined at 1100°C for another 2 hours. As a result, particles of gadolinium-gallium composite oxide were obtained. Observation under an electron microscope confirmed that, although the particles were connected to each other, the particles had a primary particle size of about 0.1 to 0.3 μm with very little particle enlargement. An electron microscope image of the particles is shown in Figure 6 In addition, the particles were confirmed to be gadolinium-gallium-garnet by X-ray diffraction. In addition, after the obtained particles were dispersed in pure water by a homogenizer (40W, 3 minutes), the particle size distribution was measured by the laser diffraction / scattering particle size distribution analyzer MT3300 using the laser diffraction method. The results are shown in Fig.13 The median diameter (D50) is 1.92 μm.

[0087] Comparative Example 1

[0088] An aqueous solution of yttrium nitrate and an aqueous solution of aluminum nitrate were added so that in 100 L of the aqueous solution, the yttrium ion concentration was 0.06 mol / L and the aluminum ion concentration was 0.10 mol / L, respectively, and the solution was stirred. Next, the solution was heated to 98° C., and then urea was added in an amount equivalent to 2.4 mol / L, and the solution was heated at 98° C. for 150 minutes. As a result, a solid was deposited.

[0089] Next, the sediment was separated into solid and liquid by a centrifuge, and then the collected solid was washed with about 20 L of pure water. The obtained solid (composite compound particles) was confirmed to be amorphous by X-ray diffraction.

[0090] Next, the obtained composite compound particles were calcined at 700°C for 4 hours under air. The blocky solid of the obtained calcined product was too hard to be easily crushed by hand, and since crushing by a hammer mill would scratch the hammer and have the risk of contamination, it was crushed in a mortar and then calcined at 1100°C for another 2 hours. As a result, particles of yttrium-aluminum composite oxide were obtained. Observation under an electron microscope confirmed that, although the particles were connected to each other, the particles had a primary particle size of about 10 μm or more due to the progress of particle increase. An electron microscope image of the particles is shown in Figure 7 In addition, the particles were confirmed to be yttrium-aluminum-garnet by X-ray diffraction. In addition, after the obtained particles were dispersed in pure water by a homogenizer (40W, 3 minutes), the particle size distribution was measured by a laser diffraction / scattering particle size distribution analyzer MT3300 using a laser diffraction method. The results are shown in Fig.14 The median diameter (D50) is 99.66 μm.

Claims

1. A method for preparing rare earth composite oxide particles, comprising the following steps: (A) generating particles of a rare earth composite compound by heating an aqueous solution containing ions of at least one rare earth element selected from the group consisting of Sc, Y, Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, either or both of Al ions and Ga ions, an organic compound having a carboxyl group and urea at a temperature not lower than 80° C. and not higher than the boiling point of the aqueous solution to react the organic compound, a hydrolyzate of urea, the ions of the rare earth element and either or both of Al ions and Ga ions, and (B) generating the rare earth composite oxide from the rare earth composite compound. 2 . The method according to claim 1 , wherein the organic compound is an organic compound having at least one carboxyl group, or an organic compound having a condensed carboxyl group from which a carboxyl group is easily formed in an aqueous solution.

3. The method according to claim 1 or 2, wherein step (B) comprises the following steps: separating the particles of the rare earth composite compound obtained in step (A) by solid-liquid separation, and The obtained solid is calcined at not less than 600° C. in an atmosphere containing oxygen to produce the rare earth composite oxide. 4 . The method according to claim 3 , which is used for preparing the rare earth composite oxide particles having a volume-based median diameter (D50) of not less than 0.1 μm and not more than 10 μm in a particle size distribution measured by a laser diffraction method. 5 . The method according to claim 1 , wherein the organic compound having a carboxyl group is at least one selected from the group consisting of maleic acid, maleic anhydride, malic acid, and citric acid.

6. The method according to claim 5, wherein step (B) comprises the following steps: separating the particles of the rare earth composite compound obtained in step (A) by solid-liquid separation, and The obtained solid is calcined at not less than 700° C. in an atmosphere containing oxygen to produce the rare earth composite oxide.

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