Aggregated particles, powder, dispersion, molded body, sintered body, optical element, optical system, optical device, method for producing aggregated particles, method for producing molded body, and method for producing sintered body

Abstract

The present invention provides aggregated particles comprising a compound containing an alkaline earth metal and fluorine, wherein the circularity shown in formula (1) when viewed in plan view, is 0.65 or less.　Formula (1): 4π × area / perimeter2

Description

Associated particles, powders, dispersions, molded bodies, sintered bodies, optical elements, optical systems, optical devices, methods for manufacturing associated particles, methods for manufacturing molded bodies, methods for manufacturing sintered bodies

[0001] The present invention relates to aggregate particles, powders, dispersions, molded articles, sintered articles, optical elements, optical systems, optical devices, methods for manufacturing aggregate particles, methods for manufacturing molded articles, and methods for manufacturing sintered articles.

[0002] As described in Patent Document 1, a technique for obtaining transparent ceramics by molding raw material powders (particles) and then sintering them is known. In addition to the manufacture of such transparent ceramics, powders (particles) are used in polishing and various other applications, and there is a demand for new particles suitable for each application. In particular, there is a demand for new particles of alkaline earth metals.

[0003] Japanese Patent Publication No. 2005-261943

[0004] One aspect of the present invention is an aggregate particle comprising a compound containing an alkaline earth metal and fluorine, wherein the roundness shown in the following formula (1) when viewed from above is 0.65 or less: 4π × area / perimeter ２ ... (1)

[0005] Another aspect of the present invention is a powder comprising aggregate particles made of a compound containing an alkaline earth metal and fluorine, wherein the average value of the roundness shown in the following formula (1) when 20 randomly selected aggregate particles are viewed in a plan view in the direction of observation by the scanning electron microscope is 0.65 or less: 4π × area / perimeter ２ ... (1)

[0006] Another aspect of the present invention is an associated particle in which primary particles comprising a compound containing three or more alkaline earth metals and fluorine are associated, and which have a non-linearly symmetric shape when viewed from above, with an outline that is uneven, zigzag, or indented.

[0007] Another aspect of the present invention is a powder containing the above-mentioned aggregate particles.

[0008] Another aspect of the present invention is a dispersion comprising the above-mentioned powder and a liquid dispersion medium.

[0009] Another aspect of the present invention is a molded article containing the above-mentioned powder.

[0010] Another aspect of the present invention is a sintered body of the molded body described above.

[0011] Another aspect of the present invention is an optical element using the sintered body described above.

[0012] Another aspect of the present invention is an optical system including the optical elements described above.

[0013] Another aspect of the present invention is an optical device including the optical system described above.

[0014] Another aspect of the present invention is a method for producing associated particles, comprising a flow step of flowing primary particles made of a compound of an alkaline earth metal and fluorine in a first solution containing a first cation of an alkaline earth metal and a first fluoride ion at a temperature of 40°C to 99°C.

[0015] Another aspect of the present invention is a method for manufacturing a molded article, which includes a molding step of molding aggregate particles produced by the above-described manufacturing method to obtain a molded article.

[0016] Another aspect of the present invention is a method for manufacturing a sintered body, which includes a sintering step of sintering a molded body manufactured by the above-described manufacturing method to obtain a sintered body.

[0017] This diagram shows the steps of the manufacturing method according to an embodiment of the present invention. This is an SEM (Scanning Electron Microscope) image showing the particle shape of Example 4. This is an SEM image showing the particle shape of the comparative example. This is an SEM image showing the particle shape of the reference example.

[0018] The following describes embodiments of the present invention (hereinafter referred to as "these embodiments"). These embodiments are illustrative examples for explaining the present invention and are not intended to limit the present invention to the following content. The present invention can be implemented by modifying it as appropriate within the scope of its gist.

[0019] In this specification, unless otherwise specified, the content of each component shall be given in mass percent (mass percentage).

[0020] The embodiments of this embodiment will be described in detail below.

[0021] <Associated Particles> One embodiment of the associated particles according to this embodiment consists of a compound containing an alkaline earth metal and fluorine, and has a circularity of 0.65 or less as shown by the following formula (1) when viewed from above: 4π × area / perimeter ２ ... (1)

[0022] The roundness shown in equation (1) is a numerical value that serves as an indicator of circularity, with the closer it is to 1, the closer it is to a perfect circle. A value less than 1 suggests that the outer shape of the particle is distorted. The upper limit of the roundness of the associated particle is preferably 0.6, more preferably 0.5, and even more preferably 0.3. The lower limit of the roundness of the associated particle is, for example, 0.1.

[0023] Another embodiment of the associated particles according to this embodiment consists of a compound containing an alkaline earth metal and fluorine, and has a circularity of 0.65 or less as shown in the following formula (2) when viewed from above: 4 × area / π × (length of the major axis) ２ ... (2) In equation (2), the major axis is the length of the major axis of the ellipse that best fits the shape of the selected particle.

[0024] The circularity shown in equation (2) is a numerical value that serves as an indicator of circularity, with the closer it is to 1, the closer it is to a perfect circle. A value less than 1 suggests that the particles are not spherical, but rather elongated or have protruding parts. The upper limit of the circularity of the associated particles is preferably 0.6, more preferably 0.55, and even more preferably 0.5. The lower limit of the circularity of the associated particles is, for example, 0.1.

[0025] Another embodiment of the associated particles according to this embodiment is one in which primary particles, each composed of a compound containing three or more alkaline earth metals and fluorine, associate to have a non-linearly symmetrical shape when viewed from above, with an outline that is convex, zigzag, or indented.

[0026] Of the three or more primary particles that associate, at least one may associate with only one other primary particle. Compared to the case where all particles associate with many other particles, in this case the overall particle shape can be said to have a protruding portion.

[0027] The associated particles according to this embodiment are, for example, associated primary particles with a major axis of 400 nm to 1500 nm. The lower limit of the major axis of the associated primary particles is preferably 410 nm, more preferably 415 nm, and even more preferably 420 nm. The upper limit of the major axis of the primary particles is preferably 1200 nm, more preferably 1000 nm, and even more preferably 900 nm.

[0028] Furthermore, as shown in Figure 2, the aggregated particles in this embodiment have a shape in which primary particles with a near-spherical shape are branched out and linked together in a bead-like fashion. Therefore, the diameter (BET diameter) when the primary particles are considered as spheres can be determined from the specific surface area obtained by the BET method, and the major axis of the primary particles can be estimated. Specifically, the specific surface area S (m²) obtained by the BET method... ２ The density ρ (g / cm³) of a compound containing alkaline earth metals and fluorine, and the density ρ (g / cm³) of the compound containing alkaline earth metals and fluorine. ３ Using ), the BET diameter (nm) = 6000 / (ρ (g / cm²) ３ ) × S (m ２ It can be calculated using the formula ( / g).

[0029] Here, alkaline earth metals refer to one or more of magnesium, calcium, strontium, and barium, for example. That is, compounds containing alkaline earth metals and fluorine refer to one or more of magnesium fluoride, calcium fluoride, strontium fluoride, and barium fluoride, for example. Furthermore, depending on the purpose, the compound may also contain one or more other elements. For example, europium or terbium may be included to give fluorescence to the associated particles, and gadolinium may be included to give the associated particles a desired refractive index. Other elements that may be included but are not limited to lanthanum, praseodymium, neodymium, samarium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium may also be included. In other words, compounds containing alkaline earth metals and fluorine may also be rare earth-added compounds. When other elements are included, in order to obtain a good shape of associated particles, when the molar ratio of alkaline earth metal to other elements is (1-X):X, X is preferably 0.5 or less, more preferably 0.3 or less, and even more preferably 0.1 or less. Furthermore, the statement that a compound consists of alkaline earth metals and fluorine essentially means that it consists of essentially one type of compound, and does not exclude the possibility of unavoidable contamination.

[0030] The associated particles according to this embodiment can be used, for example, as a raw material for ceramics. They can also be used as an abrasive by mixing them with a dispersion medium.

[0031] <Method for producing associated particles> The method for producing associated particles according to this embodiment includes a flow step (S2) in which primary particles made of a compound of an alkaline earth metal (which is the same element as the first cation) and fluorine are flowed in a first solution containing a first cation of an alkaline earth metal and a first fluoride ion at a temperature of 40°C to 99°C.

[0032] The method for producing associated particles according to this embodiment includes a reaction step (S1) in which a second cation of an alkaline earth metal (which is the same element as the first cation) and a second fluoride ion are reacted in a second solution to produce primary particles.

[0033] In the method for producing aggregated particles according to the present embodiment, the second solution in the reaction step (S1) can be used as it is as the first solution in the fluidization step (S2). In this case, the first cation is the second cation remaining in an ionic state in the solution after the reaction step (S1), and the first fluoride ion is the second fluoride ion remaining in an ionic state in the solution after the reaction step (S1). That is, in such a case, it can be stated that: the first solution is the second solution after the reaction step (S1), the first cation includes at least a part of the second cation, and the first fluoride ion includes at least a part of the second fluoride ion.

[0034] The first solution and the second solution may be different from each other. In that case, after the reaction step (S1) of the second solution, another solution containing the first cation of alkaline earth metal and the first fluoride ion may be prepared as the first solution, and mixed into the second solution after the reaction step (S1). Alternatively, after the reaction step (S1), the primary particles in the second solution may be taken out and mixed into the first solution prepared as a separate solution.

[0035] Alkaline earth metals refer to, for example, magnesium, calcium, strontium, and barium. It is preferable that the alkaline earth metal constituting the primary particles, the alkaline earth metal of the first cation, and the alkaline earth metal of the second cation are the same element. The alkaline earth metal is preferably calcium.

[0036] The first cation and the second cation are respectively added to the first solution and the second solution in one or more forms selected from nitrate, acetate, carbonate, hydroxide, and the like.

[0037] The first cation and the second cation are respectively added to the first solution and the second solution in the form of an inorganic compound. By being added in the form of an inorganic compound, the amount of residual carbon in the resulting particles can be reduced.

[0038] The first fluoride ion and the second fluoride ion are respectively added to the first solution and the second solution in one or more forms selected from, for example, hydrofluoric acid, ammonium fluoride, and the like.

[0039] The concentration of the first cation in the first solution is 0.01% by mass or more and 50% by mass or less. The lower limit of the concentration is preferably 0.1% by mass, more preferably 1% by mass. The upper limit of the concentration is preferably 30% by mass, more preferably 10% by mass.

[0040] The concentration of the first fluoride ion in the first solution is 0.01% by mass or more and 50% by mass or less. The lower limit of the concentration is preferably 0.1% by mass, more preferably 1% by mass. The upper limit of the concentration is preferably 30% by mass, more preferably 10% by mass.

[0041] The concentration of the second cation in the second solution is 0.01% by mass or more and 50% by mass or less. The lower limit of the concentration is preferably 0.1% by mass, more preferably 1% by mass. The upper limit of the concentration is preferably 30% by mass, more preferably 10% by mass.

[0042] The concentration of the second fluoride ion in the second solution is 0.01% by mass or more and 50% by mass or less. The lower limit of the concentration is preferably 0.1% by mass, more preferably 1% by mass. The upper limit of the concentration is preferably 30% by mass, more preferably 10% by mass.

[0043] When the reaction step (S1) is performed at a relatively low temperature, the sizes of the primary particles become uniform. When these primary particles are subjected to the fluidization step (S2), associated particles in which the uniformly sized primary particles are linked can be obtained. When the reaction step (S1) is performed at a relatively high temperature, the sizes of the primary particles tend to be more uneven compared to when the reaction is performed at a low temperature. Therefore, when more homogeneous particles are desired, for example, for use in ceramic production, it is preferable to perform the reaction step (S1) at a relatively low temperature. In this case, the reaction step (S1) is performed at a temperature of 0°C or higher and lower than 40°C. From this viewpoint, the lower limit of the temperature of the reaction step (S1) is preferably 2°C, more preferably 5°C. The upper limit of the temperature of the reaction step (S1) is 30°C, more preferably 25°C, even more preferably 20°C.

[0044] In the reaction step (S1), the second solution may be prepared by adding a solution containing the second cation and a solution containing the second fluoride ion respectively into the solution; alternatively, the second solution may be prepared by adding the solution containing the second fluoride ion to the solution containing the second cation, and the reverse order is also acceptable.

[0045] Depending on the purpose, one or more other ions may be added to the first and second solutions. For example, europium or terbium can be added to impart fluorescence to the associated particles, and gadolinium can be added to give the associated particles a desired refractive index. Other ions that can be added include, but are not limited to, lanthanum, praseodymium, neodymium, samarium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

[0046] In the flow process (S2), the primary particles are flowed at a flow rate of, for example, 1 cm / second or more and 100 cm / second or less.

[0047] In the fluidization step (S2), the primary particles can be made to flow by stirring the first solution with a stirring bar, stirring rod, etc. It is preferable to stir with a stirring bar, stirring rod, etc. at a rotation speed of 10 rpm to 500 rpm. The fluidization step (S2) is not limited to stirring; any method that can move the liquid may be used. For example, the first solution may be made to flow through a capillary tube, or the first solution may be placed in a container and the container may be shaken.

[0048] The fluidization process (S2) is carried out at a temperature of 40°C to 99°C. If the fluidization process (S2) is carried out at a low temperature, primary particles tend not to associate with each other, and if the fluidization process (S2) is carried out at a high temperature, the volatilization of the raw materials increases. From this viewpoint, the lower limit of the temperature for the fluidization process (S2) is preferably 40°C, more preferably 60°C. The upper limit of the temperature for the fluidization process (S2) is 99°C, more preferably 95°C, and even more preferably 90°C.

[0049] The fluidization process (S2) is carried out for 1 hour to 150 hours. By stirring for a sufficient amount of time, the particles can be given sufficient irregular shapes. From this viewpoint, the lower limit of the time for which the fluidization process (S2) is carried out is preferably 6 hours, more preferably 12 hours, and even more preferably 24 hours.

[0050] The pH of the first solution when the flowing step (S2) is performed is not particularly limited, but when the first fluoride ions are added in the form of hydrofluoric acid, the pH will be 6 or lower.

[0051] After the flowing step (S2), a holding step (S3) of holding the first solution at a temperature of 40°C or higher and 99°C or lower for 1 hour or more and 150 hours or less may be included. By including the holding step (S3), particle growth further progresses, and aggregated particles having a larger primary particle diameter can be obtained.

[0052] The lower limit of the temperature at which the holding step (S3) is performed is preferably 40°C, more preferably 50°C, and still more preferably 60°C. The upper limit of the temperature at which the holding step (S3) is performed is 99°C, more preferably 95°C, and still more preferably 90°C. Further, the lower limit of the time for which the holding step (S3) is performed is preferably 1 hour, more preferably 2 hours, and still more preferably 5 hours.

[0053] It should be noted that the aggregated particles according to the present embodiment can be obtained by the above-mentioned method for producing aggregated particles, but this is just an example of the method for producing the aggregated particles according to the present embodiment, and the production method is not limited thereto.

[0054] <Powder> One aspect of the powder according to the present embodiment is a powder composed of a plurality of particles made of a compound containing an alkaline earth metal and fluorine, wherein, among the plurality of particles, when 20 randomly selected particles observed with a scanning electron microscope are viewed in plan in the observation direction of the scanning electron microscope, the average value of the circularity represented by the following formula (1) is 0.65 or less. 4π×area / perimeter ２ ......(1)

[0055] The circularity represented by formula (1) is a numerical value that serves as an index of circularity, where a value closer to 1 means a shape closer to a perfect circle, and it can be inferred that the smaller the value is than 1, the more distorted the outer peripheral shape of the particle is. In the powder according to the present embodiment, the upper limit of the average circularity of 20 randomly selected particles is preferably 0.6, more preferably 0.5, and still more preferably 0.3. The lower limit of the average circularity of 20 randomly selected particles is, for example, 0.1.

[0056] In this embodiment, the powder has a standard deviation of roundness of 20 particles of 0.3 or less. The upper limit of the standard deviation of roundness of 20 particles is preferably 0.3, more preferably 0.25, and even more preferably 0.2. The lower limit of the standard deviation of roundness of 20 particles is not particularly limited, but is, for example, 0.01.

[0057] Another embodiment of the powder according to this embodiment is a powder comprising a plurality of particles made of a compound containing an alkaline earth metal and fluorine, wherein the circularity of 20 particles randomly selected from the plurality of particles in a scanning electron microscope observation, when viewed in a plan view in the direction of the scanning electron microscope observation, is 0.65 or less, as shown by the following formula (2): 4 × area / π × (length of the major axis) ２ ... (2) In equation (2), the major axis is the length of the major axis of the ellipse that best fits the shape of the selected particle.

[0058] The circularity shown in equation (2) is a numerical value that serves as an indicator of circularity, with the closer it is to 1, the closer it is to a perfect circle. A value less than 1 suggests that the particles are not spherical, but rather elongated in shape or have protruding parts. In the powder according to this embodiment, the upper limit of the average value of the circularity of 20 randomly selected particles is preferably 0.6, more preferably 0.55, and even more preferably 0.5. The lower limit of the average value of the circularity of 20 randomly selected particles is, for example, 0.1.

[0059] In this embodiment, the powder has a standard deviation of circularity of 20 particles of 0.3 or less. The upper limit of the standard deviation of circularity of 20 particles is preferably 0.3, more preferably 0.25, and even more preferably 0.2. The lower limit of the standard deviation of circularity of 20 particles is not particularly limited, but is, for example, 0.01.

[0060] In addition, the roundness and circularity of a particle when viewed from above using a scanning electron microscope are calculated using the outer shape of the particle when viewed from above. Therefore, even if a hole appears when viewed from above (e.g., it looks like a donut), the hole is not considered when calculating the roundness and circularity; only the outer shape is used.

[0061] The powder according to this embodiment contains associated particles according to this embodiment. The proportion of associated particles among the particles contained in the powder may be 50% or more, 70% or more, or 90% or more. In other words, when 20 particles contained in the powder according to this embodiment are observed randomly, 10 or more may be associated particles according to this embodiment, 14 or more may be associated particles according to this embodiment, or 18 or more may be associated particles according to this embodiment. Furthermore, the alkaline earth metal constituting the associated particles contained in the powder according to this embodiment is preferably calcium.

[0062] <Method for Producing Powder> The powder according to this embodiment can be obtained in the same manner as the method for producing associated particles described above. Alternatively, it can be obtained by mixing the powder obtained in the same manner as the method for producing associated particles described above with other powders. However, the method is not limited to these methods.

[0063] <Dispersion> The dispersion according to this embodiment includes the powder according to this embodiment and a liquid dispersion medium. The type of liquid dispersion medium can be appropriately selected depending on the application, and examples include water and organic solvents. The dispersion according to this embodiment can be used as an abrasive, for example, and because the powder according to this embodiment has an irregular shape, the number of contact points with the object to be polished increases, allowing for efficient polishing.

[0064] <Molded Article> The molded article according to this embodiment includes the powder according to this embodiment. The method for manufacturing the molded article includes a molding step (S4) in which the aggregate particles produced by the manufacturing method according to this embodiment are molded to obtain a molded article. The molding step (S4) may be wet or dry, and any molding method commonly used in the industry may be employed. For example, the molded article can be obtained by press molding the aggregate particles, or by dispersing them in a liquid and then drying them in a mold.

[0065] <Sintered Body> The sintered body according to this embodiment is a sintered body of the molded body according to this embodiment. The method for manufacturing the sintered body includes a sintering step (S6) in which a molded body manufactured by the manufacturing method according to this embodiment is sintered to obtain a sintered body.

[0066] To remove volatile impurities present in the molded body, a degreasing process (S5) can be performed before the sintering process. In the degreasing process, the molded body is slowly heated to a temperature at which the impurities decompose (approximately 400°C), allowing for the removal of impurities without cracking the molded body.

[0067] The sintering process (S6) removes internal pores by sintering the molded body in a vacuum atmosphere at a high temperature below the melting point of the molded body (for example, around 900 to 1000°C).

[0068] To make the sintered body transparent, a transparency step (S7) can be performed after the sintering process. In the transparency step (S7), the sintered body is heated to a high temperature (for example, about 1000 to 1100°C) under pressure in order to further reduce the pores inside the sintered body.

[0069] Through the above process, a transparent sintered body with a low impurity content can be obtained.

[0070] The sintered body according to this embodiment can be suitably used, for example, as an optical element. Such optical elements include mirrors, lenses, prisms, filters, and the like. Examples of optical systems in which the above optical elements are used include objective lenses, focusing lenses, imaging lenses, interchangeable lenses for cameras, and the like. These optical systems can be suitably used in various optical devices such as imaging devices like interchangeable-lens cameras and non-interchangeable-lens cameras, and microscope devices like fluorescence microscopes and multiphoton microscopes. Such optical devices are not limited to the imaging devices and microscopes mentioned above, but also include, but are not limited to, telescopes, binoculars, laser rangefinders, projectors, and the like.

[0071] Examples, comparative examples, and reference examples of the present invention will be described below. However, the present invention is not limited to the following examples.

[0072] <Method for producing associated particles in Examples and Comparative Examples> In Examples 1 to 11 and the Comparative Example, 3000 mL of a 25 mol / L aqueous solution of calcium nitrate and 1800 mL of a 90 mol / L hydrofluoric acid were mixed and reacted at 10°C for 2 hours to obtain a solution containing primary particles of calcium fluoride (S1: reaction step). After the reaction step, in Examples 1 to 11, the obtained solutions were stirred at 300 rpm using a stirring bar at the temperature and time shown in Table 1 (S2: fluidization step). The solution obtained in the Comparative Example was left to stand at the temperature and time shown in Table 1 without performing the fluidization step (S3: holding step). Each sample was dried at 160°C for 24 hours to obtain the calcium fluoride particle powders of Examples 1 to 11 and the Comparative Example.

[0073] <Method for producing the reference example of aggregated particles> 3000 mL of a 25 mol / L aqueous solution of calcium nitrate and 1800 mL of a 90 mol / L hydrofluoric acid were mixed and reacted at 70°C for several minutes to obtain a solution containing primary particles of calcium fluoride (S1: reaction step). 4800 mL of the obtained solution was stirred at 70°C for 6 hours at 300 rpm using a stirring bar (S2: fluidization step). Then, it was left to stand at 120°C for 18 hours (S3: holding step). The sample was dried at 160°C for 24 hours to obtain the reference example of calcium fluoride particle powder.

[0074]

[0075] The obtained powders from Examples 1 to 11, the Comparative Example, and the Reference Example were each dispersed in an appropriate amount of ethanol and observed using a scanning electron microscope. Figure 2 is an SEM image of the powder from Example 4 (magnification: 7500x), Figure 3 is an SEM image of the powder from the Comparative Example (magnification: 9500x), and Figure 4 is an SEM image of the powder from the Reference Example (magnification: 10000x).

[0076] In Examples 1-11, the primary particles aggregated to form irregularly shaped particles, but this did not occur in the Comparative Example. Furthermore, it was confirmed that there was no significant difference in the major axis of the primary particles between Examples 1-5 and the Comparative Example. Examples 6-11 tended to have relatively larger primary particle diameters compared to Examples 1-5. In the Reference Example, the primary particles varied greatly in size, and particularly large clumps were observed.

[0077] <Evaluation of Roundness and Circularity> For Examples 1 to 11 and the Comparative Example, SEM images were obtained of 20 randomly selected particles, and roundness (displayed as "Circ." in ImageJ) and circularity (displayed as "Round" in ImageJ) were calculated using the image processing software ImageJ (ver. 1.54g). Roundness was calculated using the following formula (1), and circularity was calculated using the following formula (2): 4π × Area / Perimeter ２ ... (1) 4 × area / π × (length of the major axis) ２ ... (2) In equation (2), the major axis is the length of the major axis of the ellipse that best fits the shape of the selected particle. Furthermore, the specific surface area of ​​the powder was measured by gas adsorption using a pore distribution analyzer BELSORP-MINI II (manufactured by MICROTRAC), and the BET diameter was calculated. Pure nitrogen was used as the adsorption gas.

[0078] Both roundness and circularity indicate a degree of circularity; the closer the value is to 1, the closer the object is to a perfect circle. Table 2 shows the average values ​​and standard deviations of roundness and circularity for each example and comparative example.

[0079] As shown in Table 2, the average values ​​of the roundness and circularity of the calcium fluoride particles in the comparative example were close to perfect circles (close to 1), whereas the roundness of the calcium fluoride particles in Examples 1 to 11 was lower than that of the comparative example, confirming that they were irregularly shaped. Furthermore, since there was no significant difference in BET diameter between Examples 1 to 5 and the comparative example, it was confirmed that there was no significant difference in the size of the primary particles between Examples 1 to 5 and the comparative example. Examples 6 to 11 tended to have relatively larger primary particle diameters compared to Examples 1 to 5. In the examples, it was found that a higher temperature during stirring tended to result in a larger BET diameter. In addition, all 20 particles in each of Examples 1 to 11 that were observed with SEM were aggregated particles, while all 20 particles in the comparative example were not aggregated particles.

[0080]

[0081] In summary, comparisons with the reference examples show that it is preferable to react at a low temperature to obtain associated particles with uniform primary particle sizes. Furthermore, comparisons with the comparative examples show that irregularly shaped associated particles can be produced by stirring the primary particles.

Claims

1. An associated particle consisting of a compound containing an alkaline earth metal and fluorine, wherein the circularity shown in the following formula (1) when viewed from above is 0.65 or less: 4π × area / perimeter ２ ... (1) 2. A powder comprising a plurality of particles made of a compound containing an alkaline earth metal and fluorine, wherein the average value of the roundness shown in the following formula (1) when 20 particles randomly selected from the plurality of particles are viewed in a plan view in the direction of observation by the scanning electron microscope is 0.65 or less: 4π × area / perimeter ２ ... (1) 3. The powder according to claim 2, wherein the standard deviation of the roundness of the 20 particles is 0.3 or less.

4. An associated particle formed by the association of primary particles consisting of three or more alkaline earth metals and a compound containing fluorine, having a non-linearly symmetrical shape when viewed from above, with an outline that is convex, zigzag, or indented.

5. The associated particle according to claim 4, wherein at least one of the three or more primary particles is associated with only one other primary particle.

6. The associated particle according to claim 4 or 5, wherein the primary particles having a major axis of 400 nm or more and 1500 nm or less are associated.

7. A powder comprising the aggregated particles described in any one of claims 1, 4 to 6.

8. The powder according to any one of claims 2, 3, or 7, wherein the alkaline earth metal is calcium.

9. A dispersion comprising a powder according to any one of claims 2, 3, 7, or 8, and a liquid dispersion medium.

10. A molded article comprising the powder according to any one of claims 2, 3, 7, or 8.

11. A sintered body of the molded article according to claim 10.

12. An optical element using the sintered body described in claim 11.

13. An optical system including the optical element described in claim 12.

14. An optical apparatus including the optical system described in claim 13.

15. A method for producing associated particles, comprising a flow step of flowing primary particles made of a compound of an alkaline earth metal and fluorine in a first solution containing a primary cation of an alkaline earth metal and a primary fluoride ion at a temperature of 40°C to 99°C.

16. A method for producing associated particles according to claim 15, comprising a reaction step of reacting a secondary cation of the alkaline earth metal with a secondary fluoride ion in a second solution to produce the primary particles.

17. The method for producing associated particles according to claim 16, wherein the first solution is the second solution after the reaction step, the first cation comprises at least a portion of the second cation, and the first fluoride ion comprises at least a portion of the second fluoride ion.

18. The method for producing aggregated particles according to any one of claims 15 to 17, wherein the primary particles are made to flow by stirring the first solution with a stirring bar in the flow step.

19. The method for producing associated particles according to any one of claims 15 to 18, wherein the alkaline earth metal is calcium.

20. The method for producing associated particles according to any one of claims 15 to 19, wherein the first cation is added to the first solution in one or more forms selected from nitrate, acetic acid, carbon, and hydroxide.

21. The method for producing associated particles according to any one of claims 16 to 20, wherein the second cation is added to the second solution in one or more forms selected from nitrate, acetic acid, carbon, and hydroxide.

22. The method for producing associated particles according to any one of claims 15 to 21, wherein the first cation is added to the first solution in the form of an inorganic compound.

23. The method for producing associated particles according to any one of claims 16 to 22, wherein the second cation is added to the second solution in the form of an inorganic compound.

24. The method for producing associated particles according to any one of claims 15 to 23, wherein the first fluoride ion is added to the first solution in the form of hydrofluoric acid or ammonium fluoride.

25. The method for producing associated particles according to any one of claims 16 to 24, wherein the second fluoride ion is added to the second solution in the form of hydrofluoric acid or ammonium fluoride.

26. The method for producing associated particles according to any one of claims 16 to 25, wherein the reaction step is carried out at a temperature of 0°C or higher and less than 40°C.

27. The method for producing associated particles according to any one of claims 15 to 26, wherein the flow step is performed for 1 hour or more and 150 hours or less.

28. The method for producing associated particles according to any one of claims 15 to 27, wherein the fluidization step is performed at a pH of 6 or lower.

29. A method for producing associated particles according to any one of claims 16 to 28, comprising a holding step after the flow step, in which the first solution is maintained at a temperature of 40°C to 99°C for 1 hour to 150 hours.

30. A method for manufacturing a molded article, comprising a molding step of molding aggregate particles produced by the manufacturing method described in any one of claims 15 to 29 to obtain a molded article.

31. The method for manufacturing a molded article according to claim 30, wherein the molding step involves press molding the aggregate particles or dispersing them in a liquid and then placing them in a mold and drying them.

32. A method for manufacturing a sintered body, comprising a sintering step of sintering the molded body manufactured by the manufacturing method described in claim 30 or 31 to obtain a sintered body.

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