Alumina composition

Abstract

An alumina composition contains alumina particles and oxide particles composed of an oxide other than alumina. The oxide particles are composed of an oxide that contains cations having an ionic radius in hexacoordination of 0.41-1.2Å. The cumulative 50% particle diameter D50 from the fine-particle side of the volume-based cumulative particle size distribution is 5.0 μm or less, and the D50 of the alumina particles is 0.40 μm or less.

Description

Alumina Composition

[0001] The present disclosure relates to an alumina composition, and in particular to an alumina composition suitable for artificial teeth.

[0002] All-ceramic artificial teeth are excellent in biosafety and aesthetics. Translucent zirconia is known as a ceramic material suitable for artificial teeth (Patent Document 1). Translucent zirconia has improved translucency and chemical stability by adding yttria to zirconia powder and sintering it.

[0003] JP 2010-150064 A

[0004] Translucent zirconia has lower translucency than natural teeth, and there is room for improvement in terms of aesthetics. Translucent alumina is being considered as a translucent ceramic to replace translucent zirconia. Because translucent alumina has high translucency, it is expected to be possible to manufacture artificial teeth that are more natural and have excellent aesthetics.

[0005] An alumina composition such as alumina powder or alumina granules is used as the raw material for translucent alumina. The alumina raw material is compression-molded, calcined, and sintered to obtain translucent alumina. The translucency of alumina is improved by sintering in a vacuum / hydrogen atmosphere or by hot isostatic pressing (HIP). However, since sintering of artificial teeth is performed by dental technicians, who are the users, normal pressure sintering in an air atmosphere is usually performed using a general-purpose firing furnace. As a result, there is a risk that the translucency of the translucent alumina may not be sufficiently improved.

[0006] Therefore, an object of one embodiment of the present invention is to provide an alumina composition that can produce an alumina sintered body with high translucency even when sintered under atmospheric pressure in an air atmosphere.

[0007] A first aspect of the present invention is an alumina composition comprising alumina particles and oxide particles made of an oxide other than alumina, wherein the oxide particles are made of an oxide containing a cation having an ionic radius in hexacoordination of 0.41 Å or more and 1.2 Å or less, and have a particle size D50 of 5.0 μm or less at a cumulative 50% particle size from the fine particle side in a volume-based cumulative particle size distribution, and the D50 of the alumina particles is 0.40 μm or less.

[0008] A second aspect of the present invention is the alumina composition according to the first aspect, wherein the cation is a cation of one or more elements selected from the group consisting of Ti, Sn, Zr, Y, and Sr.

[0009] A third aspect of the present invention is the alumina composition according to the first or second aspect, wherein the content of the oxide particles is 50 ppm by mass or more and 5,000 ppm by mass or less in terms of elements constituting the cations.

[0010] A fourth aspect of the present invention is the alumina composition according to any one of the first to third aspects, wherein the oxide particles have a D50 of 0.5 μm or less.

[0011] A fifth aspect of the present invention is the alumina composition according to any one of the first to fourth aspects, wherein the alumina content is 90.0 mass % or more.

[0012] In a sixth aspect of the present invention, the BET specific surface area of ​​the alumina particles is 7 m ２ / g or more 20m ２ / g or less.

[0013] A seventh aspect of the present invention is the alumina composition according to any one of the first to sixth aspects, wherein the alumina particles and the oxide particles are present as primary particles or secondary particles.

[0014] Aspect 8 of the present invention is the alumina composition according to any one of Aspects 1 to 7, wherein the alumina composition is a granule formed by binding the alumina particles and the oxide particles with a binder.

[0015] According to the alumina composition according to one embodiment of the present invention, a highly translucent alumina sintered body can be produced even by atmospheric sintering in an air atmosphere.

[0016] Fig. 1 is an electron backscatter diffraction (EBSD) mapping of a cross section of the alumina sintered body obtained in Example 5 (magnification: 500x). Fig. 2 is an electron backscatter diffraction (EBSD) mapping of a cross section of the alumina sintered body obtained in Example 8 (magnification: 500x). Fig. 3 is a scanning transmission electron microscope (STEM) image and element mapping (Al, Sn) of the alumina sintered body obtained in Example 5 (magnification: 120,000x). Fig. 4 is a scanning transmission electron microscope (STEM) image and element mapping (Al, Sn) of the alumina sintered body obtained in Example 5 (magnification: 1,000,000x).

[0017] The inventors have conducted extensive research to obtain an alumina composition that can produce an alumina sintered body with high translucency even when sintered under atmospheric pressure in an air atmosphere. The inventors have found that in order to improve the translucency of an alumina sintered body, the grain size in the alumina sintered body should be small. It is believed that when a conventional alumina composition is sintered under atmospheric pressure in an air atmosphere, the grain size of the alumina sintered body becomes large, resulting in insufficient translucency.

[0018] As a result of investigations by the inventors, it was discovered for the first time that an alumina sintered body with sufficiently high translucency can be produced even by atmospheric sintering in an air atmosphere by using an alumina composition containing oxide particles and alumina particles, by using oxide particles made of an oxide containing cations having a predetermined ionic radius, and by controlling the particle diameters D50 of the oxide particles and alumina particles within an appropriate range, and this discovery led to the completion of the present invention.

[0019] An alumina composition according to one embodiment of the present invention will be described in detail below.

[0020] (1) Alumina Composition The alumina composition according to an embodiment of the present invention contains alumina particles and oxide particles. In this specification, "oxide particles" refers to particles made of oxides other than alumina. In other words, alumina particles are not included in the oxide particles. The alumina composition may optionally contain a binder, a dispersant, a plasticizer, a coloring additive, and the like.

[0021] The alumina composition is preferably in a powder or granular form. In this specification, a powdered alumina composition may be referred to as "alumina powder," and a granular alumina composition may be referred to as "alumina granules." A translucent alumina sintered body can be produced using either the alumina powder or the alumina granules.

[0022] (Powdered alumina composition: alumina powder) In this specification, the term "powdered" refers to a state in which each oxide particle and alumina particle is not bonded to other particles (oxide particles or alumina particles). Specifically, the powdered alumina composition refers to a state in which the alumina particles and oxide particles exist as individual unit particles (primary particles) or as an aggregate of two or more primary particles (secondary particles). The primary particles and secondary particles may be mixed.

[0023] When present as secondary particles, each secondary particle may be composed of only alumina particles, only oxide particles, or a mixture of alumina particles and oxide particles. However, the alumina powder as a whole must contain both alumina particles and oxide particles. Different types of secondary particles may be present. Secondary particles are simply agglomerations of two or more primary particles, and these particles are not bonded to each other. Therefore, secondary particles can easily separate into primary particles. For example, applying ultrasonic vibrations to secondary particles in a dispersion medium can separate them into primary particles.

[0024] (Granular Alumina Composition: Alumina Granules) A ​​granular alumina composition (alumina granules) is composed of one or more types of oxide particles (primary particles) and alumina particles (primary particles) bound together with a binder. That is, a granular alumina composition necessarily contains oxide particles, alumina particles, and a binder. Each granule may be composed only of alumina particles, or may be composed only of oxide particles, or may be composed of alumina particles and oxide particles. However, the alumina granules as a whole necessarily contain both alumina particles and oxide particles. Different types of granules may be mixed. A granular alumina composition is basically composed only of granules, but may contain a small amount (for example, 10 mass % or less of the entire alumina composition) of one or more types of primary particles and secondary particles.

[0025] The content of the binder contained in the alumina granules is an amount sufficient to bind the primary particles together. From this perspective, the alumina granules preferably contain 0.1 to 5.0 mass% of the binder when the entire alumina granules (including the binder) are taken as 100 mass%. The content of the binder contained in the alumina granules is preferably 0.1 mass% or more, more preferably 0.3 mass% or more, even more preferably 0.5 mass% or more, particularly preferably 0.8 mass% or more, and is preferably 5.0 mass% or less, more preferably 4.0 mass% or less, even more preferably 3.0 mass% or less, and particularly preferably 2.5 mass% or less, when the entire alumina granules are taken as 100 mass%. The binder content can be determined from the weight loss in thermal analysis.

[0026] The BET specific surface area of ​​the alumina composition is, for example, 1 to 20 m ２ The BET specific surface area of ​​the alumina composition is determined by a single-point nitrogen adsorption method in accordance with the method specified in JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption."

[0027] (2) Oxide Particles Next, the oxide particles contained in the alumina composition will be described in detail. As described above, the term "oxide particles" in this specification refers to particles made of an oxide other than alumina.

[0028] (Ionic Radius of Cations in Six-Coordination) The oxide particles contained in the alumina composition are composed of oxides containing cations having an ionic radius in six-coordination of 0.41 Å or more and 1.2 Å or less. As a result of various studies conducted by the inventors, it was found that an alumina sintered body having high translucency cannot be produced when the ionic radius of the cation in six-coordination is less than 0.41 Å or more than 1.2 Å. It is presumed that the reason why the ionic radius of the cation in six-coordination affects the translucency of the alumina sintered body is that alumina, which is the base material of the alumina sintered body, is six-coordinated, and some of the cations in the oxide particles exist by substituting for the alumina base material.

[0029] The ionic radius of the cation in hexacoordination (hereinafter sometimes simply referred to as "cation ionic radius") is preferably 0.60 Å or more and 0.90 Å or less, more preferably 0.60 Å or more and 0.73 Å or less, even more preferably 0.68 Å or more and 0.72 Å or less, and particularly preferably 0.68 Å or more and 0.71 Å or less. By setting the ionic radius of the cation within this range, an alumina sintered body with particularly excellent translucency can be produced.

[0030] The type of cation contained in the oxide particles can be identified from cations other than Al originating in the alumina particles by subjecting the alumina composition to elemental analysis using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The ionic radius of each cation in hexacoordination can be found in R.D. Shannon, Acta Crystallogr., Sect. A 32, 751 (1976).

[0031] Preferred cations include cations of one or more elements selected from the group consisting of Ti, Sn, Zr, Y, and Sr. The ionic radii of these cations in hexacoordination are as follows:

[0032]

[0033] (D50 of oxide particles) The particle diameter D50 of the oxide particles at 50% cumulative size from the fine side of the volume-based cumulative particle size distribution (sometimes referred to as "volume-based D50" or simply "D50") is 5.0 μm or less. If the D50 of the oxide particles exceeds 5.0 μm, the grain size of the alumina sintered body increases, and the translucency decreases. The D50 of the oxide particles is preferably 4.0 μm or less, more preferably 3.5 μm or less, even more preferably 2.0 μm or less, still more preferably 1.0 μm or less, particularly preferably 0.5 μm or less, and preferably 0.05 μm or more, more preferably 0.10 μm or more, and even more preferably 0.15 μm or more. By setting the D50 of the oxide particles within this range, an alumina sintered body with particularly excellent translucency can be produced.

[0034] The D50 of oxide particles is measured by a laser diffraction / scattering method. When measuring the D50 of oxide particles after mixing alumina particles and oxide particles (i.e., in the state of an alumina composition), the oxide particles can be isolated from the mixture and D50 can be measured. The isolation method can be a known method based on the difference in specific gravity between alumina particles and oxide particles or the difference in particle size between them. Whether the isolated particles are oxide particles can be confirmed by elemental analysis such as ICP-AES or SEM-EDX. When the alumina composition is in a granular form (alumina granules), the alumina granules are calcined at 600°C to remove the binder, and the oxide particles are isolated and D50 is measured.

[0035] An example of the conditions for measuring D50 of oxide particles will be described. The oxide particles are added to a 0.2% aqueous solution of sodium hexametaphosphate and dispersed by processing for 7 minutes using an ultrasonic homogenizer. The dispersion is placed in a measurement cell and degassed. Then, using a Microtrac particle size distribution analyzer MT-3300 manufactured by Microtrac-Bell, measurements are performed for a measurement time of 10 seconds, two measurements, a particle refractive index of 1.77, and a solvent refractive index of 1.333 to determine the particle size distribution of the oxide particles, and D50 is calculated from the results.

[0036] Another method for measuring the D50 of oxide particles is to observe an alumina composition with an SEM or TEM. First, oxide particles are identified in an SEM or TEM image. Identification methods include identifying oxide particles from image contrast resulting from the difference between the average atomic numbers of the elements constituting the alumina particles and the average atomic numbers of the elements constituting the oxide particles, or identifying oxide particles through elemental analysis such as EDX analysis. All oxide particles contained in the observation area are identified, and the circle-equivalent diameter of each oxide particle is determined using image processing software. From these data, the cumulative 50% particle diameter D50 (sometimes referred to as "number-based D50") from the fine particle side of the cumulative particle size distribution based on the number of oxide particles is determined, and the volume-based D50 is calculated by converting it to a volume-based value. In SEM and TEM observations, the magnification is 1,000 to 200,000 times, and the observation area has a short side of 1.5 μm or more and a long side of 2.0 μm or more.

[0037] (D90 of oxide particles) The particle size D90 of the oxide particles, which is the cumulative 90% particle size from the fine side in the volume-based cumulative particle size distribution, is preferably 20.0 μm or less, more preferably 10.0 μm or less, even more preferably 3.0 μm or less, still more preferably 1.0 μm or less, particularly preferably 0.5 μm or less, and is preferably 0.05 μm or more, more preferably 0.10 μm or more, and even more preferably 0.15 μm or more. The method for measuring D90 of oxide particles is to determine the particle size distribution of the oxide particles in the same manner as in the method for measuring D50, and then determine D90 from the result.

[0038] (Oxide Particle Content) The content of oxide particles contained in the alumina composition is preferably 50 ppm by mass or more and 5000 ppm by mass or less, calculated as the elements constituting the cations. The content of oxide particles is more preferably 100 ppm by mass or more and 3000 ppm by mass or less, even more preferably 200 ppm by mass or more and 2000 ppm by mass or less, and even more preferably 300 ppm by mass or more and 1000 ppm by mass or less. By setting the content of oxide particles within this range, an alumina sintered body with particularly excellent translucency can be produced.

[0039] Regarding the content of oxide particles, as described above, the alumina composition is subjected to elemental analysis (and quantitative analysis) by inductively coupled plasma atomic emission spectroscopy (ICP-AES) to identify the cations contained in the oxide particles, and then the content of the identified cations is determined.

[0040] (3) Alumina Particles Next, the alumina particles contained in the alumina composition will be described in detail.

[0041] (D50 of Alumina Particles) The particle diameter D50 of the alumina particles at 50% cumulative size from the fine particle side in the cumulative particle size distribution on a volume basis is 0.40 μm or less. If the D50 of the alumina particles exceeds 0.40 μm, the grain size of the alumina sintered body increases, resulting in reduced translucency. The D50 of the alumina particles is preferably 0.35 μm or less, more preferably 0.30 μm or less, and even more preferably 0.25 μm or less, and is preferably 0.08 μm or more, more preferably 0.10 μm or more, and even more preferably 0.12 μm or more. By setting the D50 of the alumina particles within this range, an alumina sintered body with particularly excellent translucency can be produced.

[0042] The method for measuring D50 of alumina particles is the same as the method for measuring D50 of oxide particles.

[0043] (BET specific surface area of ​​alumina particles) The BET specific surface area of ​​alumina particles is 7 m ２ / g or more 20m ２ The BET specific surface area of ​​the alumina particles is preferably 8 m / g or less. ２ / g or more 17m ２ / g or less, more preferably 9m ２ / g or more 13m ２ By setting the BET specific surface area of ​​the alumina particles within this range, an alumina sintered body having particularly excellent translucency can be produced.

[0044] The specific surface area (BET specific surface area) of the alumina particles is determined by the single-point nitrogen adsorption method in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption." When measuring the BET specific surface area of ​​the alumina particles after mixing the alumina particles with oxide particles (i.e., in the state of an alumina composition), the alumina particles can be isolated from the mixture and the BET specific surface area can be measured. The method for isolating the alumina particles is the same as the method described above.

[0045] (Alumina Content) The alumina content in the alumina composition is preferably 90.0 mass% or more. The alumina content is more preferably 95.0 mass% or more, even more preferably 96.0 mass% or more, even more preferably 97.0 mass% or more, particularly preferably 98.0 mass% or more, and preferably less than 100 mass%. By setting the alumina content within this range, an alumina sintered body with particularly excellent translucency can be produced. The alumina content can be calculated from the Al content determined by elemental analysis (quantitative analysis) of the alumina composition by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0046] (4) Manufacturing Method of Alumina Composition Although the method for manufacturing the alumina composition of the embodiment is not particularly limited, it is preferable to adopt the following manufacturing method, since it allows for the reproducibility of manufacturing an alumina composition having the above physical properties. Note that a person skilled in the art who has read the disclosure of the present application may arrive at a different method for manufacturing the alumina granules of the present embodiment based on the disclosure. Below, (i) a manufacturing method of a powdered alumina composition (alumina powder) and (ii) a manufacturing method of a granular alumina composition (alumina granules) are described.

[0047] (i) Method for Producing Alumina Powder The method for producing alumina powder includes the steps of preparing oxide particles and alumina particles, and mixing the oxide particles and alumina particles.

[0048] (Step of Preparing Oxide Particles and Alumina Particles) In order to obtain desired oxide particles, first, a cation having an ionic radius of 0.41 Å or more and 1.2 Å or less in hexacoordination (for example, Ti４＋ , Sn ４＋ , Zr ４＋ , Y ３＋ , Sr ２＋ Oxide raw material particles made of oxides containing, for example, ammonium nitrate ...

[0049] In order to obtain the desired alumina particles, alumina raw material particles produced by a known method are prepared. Commercially available alumina particles may be used as the alumina raw material particles. Alumina particles having a D50 of 0.40 μm or less are prepared from these alumina raw material particles. If the alumina raw material particles have a D50 of more than 0.40 μm, they are pulverized using a ball mill or the like to adjust the D50 to 0.40 μm or less. If the alumina raw material particles have a D50 of 0.40 μm or less, they may be used as alumina particles as is, or may be further pulverized using a ball mill or the like.

[0050] The D50 of the oxide raw material particles and the alumina raw material particles is measured in the same manner as the above-mentioned method for measuring the D50 of the oxide particles. In this manner, oxide particles and alumina particles having desired properties are prepared.

[0051] (Step of mixing oxide particles and alumina particles) The obtained oxide particles and alumina particles are mixed in a desired ratio. The mixing method is not particularly limited, but examples thereof include a mixer, a ball mill, a media agitation mill, etc. In this manner, an alumina powder is obtained.

[0052] (ii) Method for producing alumina granules The method for producing alumina granules includes the steps of preparing oxide particles and alumina particles, preparing a slurry containing the oxide particles and alumina particles, and granulating alumina granules from the slurry.

[0053] (Step of Preparing Oxide Particles and Alumina Particles) As in the above-mentioned "(i) Method for Producing a Powdered Alumina Composition", oxide particles and alumina particles adjusted to a predetermined D50 are prepared.

[0054] (Step of Preparing Slurry Containing Oxide Particles and Alumina Particles) The obtained oxide particles, alumina particles, a binder, and a solvent are blended, and optionally, appropriate amounts of a dispersant, a plasticizer, and a coloring additive are blended, followed by mechanical stirring and mixing to prepare a slurry containing the oxide particles and alumina particles. For stirring and mixing, conventional methods can be used, such as a method of stirring and mixing using a stirring blade or a stirrer while irradiating ultrasonic waves from the outside, a method using various grinding media such as a ball mill or a dyno mill, or a method using various agitators such as an attritor or a pin mill.

[0055] As the binder, organic binders such as polyvinyl alcohol, polyvinyl acetal, various acrylic polymers, methyl cellulose, polyvinyl acetate, polyvinyl butyral, various waxes, and various polysaccharides can be used.

[0056] It is preferable to select a suitable solvent depending on the type of binder used and the granulation method of the alumina granules. When granulating the alumina granules using a spray dryer, an acrylic binder is suitable. In this case, water is mainly used as the solvent. Depending on the type of binder used and the granulation method, various organic solvents (acetone, ethanol, toluene, etc.) can be used.

[0057] A dispersant can be added optionally, and it is preferable to select a suitable dispersant depending on the type of solvent used. When the solvent is water, polycarboxylic acid ammonium salts (for example, trade name: SN-D5468, manufactured by San Nopco) are mainly used as dispersants. When an organic solvent is used, ethyl oleate, sorbitan monooleate, sorbitan trioleate, polycarboxylic acid-based dispersants, etc. are used. Polyester-based dispersants (trade name: Texahol 3012, manufactured by San Nopco) are also suitable. However, various dispersants can be used without being limited to these.

[0058] Depending on the organic binder used, it may be possible to prepare a slurry with lower viscosity without using a dispersant, thereby increasing the concentration of alumina granules in the slurry. In such cases, it is not necessary to add a dispersant.

[0059] A plasticizer can be added as desired, and it is preferable to select a suitable plasticizer depending on the type of binder and organic solvent used together. Plasticizers used together with organic binders include ethylene glycol, diethylene glycol, polyethylene glycol, glycerin, polyglycerin, various esters, etc. When an organic solvent is used, dibutyl phthalate, diethylhexyl phthalate, etc. are particularly used, but the present invention is not limited to these.

[0060] Coloring additives can be added as desired. Examples of coloring additives include compounds of chromium, manganese, cobalt, nickel, iron, and the like, which can be added singly or in combination. Examples of compounds include oxides, nitrates, acetates, hydroxides, and chlorides. The coloring additives are preferably added so that the amount of coloring additive, calculated as metal element, is 10 to 3,000 ppm when the alumina content in the alumina granules is taken as 100 mass %.

[0061] The resulting slurry may be defoamed under reduced pressure. Various antifoaming agents may also be used. Depending on the subsequent molding method, various pH adjusters and flocculants may be added to adjust the viscosity to 10 to 500 centipoise. For example, in granulation using a spray dryer, in order to produce spherical granules, it is preferable to adjust the viscosity of the slurry to 30 to 300 centipoise by adjusting the pH with an aqueous hydrochloric acid solution, aqueous ammonia, or the like. Furthermore, the concentration of oxide particles and alumina particles in the slurry can be increased by static settling, centrifugation, vacuum concentration using a rotary evaporator, or the like.

[0062] Another method for preparing a slurry containing oxide particles and alumina particles is to separately prepare an oxide slurry containing oxide particles and an alumina slurry containing alumina particles, and then mix the oxide slurry and the alumina slurry. Instead of blending both oxide particles and alumina particles in the above-described slurry preparation method, the oxide slurry or the alumina slurry can be prepared by blending either oxide particles or alumina particles. By blending the oxide slurry and the alumina slurry in a predetermined ratio and stirring and mixing them, a slurry containing oxide particles and alumina particles can be obtained.

[0063] (Step of Granulating Alumina Granules from Slurry) The obtained slurry is used to granulate alumina granules containing oxide particles and alumina particles. The method for granulating alumina granules from the slurry can be a known spray drying granulation method or oscillating extrusion granulation method. Specifically, the alumina granules can be granulated by spray drying the obtained slurry using a spray drying device (spray dryer) or the like.

[0064] (Variation: Method of Directly Granulating Alumina Granules from Oxide Particles and Alumina Particles) In the above-described method for producing alumina granules, a slurry containing oxide particles and alumina particles is used to granulate the alumina granules, but the oxide particles and alumina particles may be directly granulated into alumina granules without forming a slurry. For example, the oxide particles, alumina particles, a binder, and various additives as desired may be mixed and granulated in an agitation granulator to produce a granulated powder, and the granulated powder may be repeatedly extruded and granulated in an oscillating granulator and dried to produce alumina granules.

[0065] These granulation methods for alumina granules can be appropriately selected depending on the amount of granules for ceramic molding, the properties of the desired ceramic molded body, etc. The oscillating extrusion granulation method is a method in which particles having a particle size of, for example, about several mm are crushed on a mesh and the finer particles are allowed to fall, and the mesh is successively made finer, thereby obtaining particles having a predetermined particle size or less.

[0066] (4) Manufacturing Method of Alumina Sintered Body An example of a suitable manufacturing method of an alumina sintered body using the alumina composition according to the embodiment is described below. The alumina composition is subjected to uniaxial press molding, cold isostatic pressing (CIP) molding, or the like to produce a molded body. In the case of CIP molding, alumina granules are uniaxially press-molded at a pressure of 20 to 40 MPa, preferably 25 to 35 MPa, and then isostatically pressed at 98 MPa or more, preferably 150 to 200 MPa, in a CIP molding machine, and the resulting molded body is processed into a predetermined shape.

[0067] Two examples of firing conditions for producing an alumina sintered body are given below. Note that the firing conditions shown as examples are set assuming that a general-purpose firing furnace is used for normal pressure sintering in an air atmosphere, but a translucent alumina sintered body can also be produced under other firing conditions.

[0068] (Condition 1) Sintering step: The compact is sintered in air at a temperature of 1250 to 1550°C for at least 2 hours to obtain an alumina sintered body. The average heating rate from room temperature to the sintering temperature is, for example, 200°C / hr.

[0069] In the alumina composition according to the embodiment, by controlling the type of cation contained in the oxide particles and the D50 of the oxide particles and alumina particles, crystal grain growth is unlikely to occur even when sintered in an air atmosphere (under atmospheric pressure) at a temperature range of 1250 to 1550°C, for example, as in Condition 1. Therefore, the grain size of the obtained alumina sintered body is small, and an alumina sintered body with high translucency can be produced.

[0070] (Condition 2) Debinding step: The compact is debound by firing in an air atmosphere at a temperature in the range of 500 to 1200°C for at least 1 hour, preferably in the range of 600 to 800°C for at least 2 hours. The average temperature rise rate from room temperature to the firing temperature is, for example, 100°C / hr. Sintering step: After the debinding step, the compact is fired at a temperature in the range of 1200 to 1700°C, preferably in the range of 1250 to 1450°C, for at least 2 hours to obtain an alumina sintered body. The sintering temperature in the sintering step is set to be equal to or higher than the sintering temperature in the debinding step. The average temperature rise rate from the firing temperature in the debinding step to the sintering temperature in the sintering step is, for example, 200°C / hr.

[0071] Condition 2 is suitable not only for obtaining an alumina sintered body by low-temperature sintering, but also for obtaining a translucent alumina sintered body using alumina granules containing a sintering aid. By setting the temperature condition of the sintering step to 1350 to 1700°C, particularly 1400 to 1450°C, the translucency of the alumina sintered body can be further improved.

[0072] (5) Translucency Evaluation of Alumina Sintered Body By using the alumina composition according to the embodiment, an alumina sintered body with high translucency can be produced by atmospheric sintering in an air atmosphere. As an example, an alumina sintered body having a total transmittance (TT) of 40% or more and a parallel transmittance (PT) of 0.3% or more can be produced. The transmittance measurement for the translucency evaluation conforms to the measurement method for total transmittance (TT) and parallel transmittance (PT) of JIS K7361-1:1997. The thickness of the alumina sintered body after surface polishing is set to 0.9 to 1.1 mm, and the total transmittance (TT) and parallel transmittance (PT) are measured.

[0073] An alumina composition (alumina granules) was prepared by the following procedure, and an alumina sintered body was manufactured using the composition. The types and physical properties of the oxide particles and alumina particles used to prepare the alumina granules are summarized in Table 2.

[0074] Example 1 Preparation of Alumina Slurry Alumina particles (1700 g, manufactured by Sumitomo Chemical Co., Ltd., NXA-100), pure water (1054 g), a dispersant (7.95 g, manufactured by San Nopco Co., Ltd., SND-5468), and alumina beads (3540 g, manufactured by Nikkato Co., Ltd., φ2-SSA999W) were charged into a pot (volume 3 L) with an alumina-lined interior, and uniformly dispersed at 65 rpm for 6 hours to obtain an alumina slurry.

[0075] (Preparation of oxide slurry) Tin (IV) oxide particles (153 g, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and pure water (2847 g) were stirred for 10 minutes using a rotary blade, and then circulated once in a wet disperser (manufactured by Shinmaru Enterprises, Dynomill) under conditions of a peripheral speed of 8 m / s, a flow rate of 500 mL / min, and alumina beads φ0.5 mm (2690 g), and then the mixture was circulated for 30 minutes to uniformly disperse the particles, thereby obtaining an oxide slurry (tin oxide slurry).

[0076] (Preparation of Alumina Granules) The obtained alumina slurry (700 g) was mixed with pure water (63.4 g), the obtained oxide slurry (5.1 g), a binder (17.6 g, solid content: 50 mass%, manufactured by Chuo Rika Kogyo Co., Ltd., SA-261P), and a plasticizer (4.3 g, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., PEG-400), and the mixture was stirred for 10 minutes using a rotating blade. The obtained slurry was spray-dried using a spray dryer (manufactured by Okawara Kakoki Co., Ltd., L-8i) under the following conditions: flow rate 79 g / min, atomizer rotation speed 18,000 rpm, inlet temperature 180°C, outlet temperature 90°C, and hot air differential pressure 1.1 kPa, to obtain alumina granules.

[0077] (Preparation of alumina sintered body) The obtained alumina granules (1 g) were filled into a cylindrical mold having a diameter of 20 mm, and subjected to uniaxial press molding at a pressure of 30 MPa for 30 seconds. Then, cold isostatic pressing (CIP) molding at a pressure of 200 MPa for 3 minutes was repeated five times to obtain an alumina compact. The alumina compact was heated to 1,450°C in air at a heating rate of 200°C / hr and held at that temperature for 2 hours to obtain an alumina sintered body. The surface of the alumina sintered body was mirror-polished, and then its translucency was evaluated.

[0078] Example 2 An alumina slurry was prepared using the same procedure as in Example 1. An oxide slurry (zirconia slurry) was prepared by using zirconia (IV) particles (manufactured by Kojundo Chemical Laboratory Co., Ltd.) instead of the tin (IV) oxide particles used in Example 1. Alumina granules and an alumina sintered body were prepared using the same procedure as in Example 1, except that the amount of oxide slurry (zirconia slurry) added was changed to 5.5 g.

[0079] Example 3 An alumina slurry was prepared using the same procedure as in Example 1. An oxide slurry (titanium oxide slurry) was prepared by using titanium (IV) oxide particles (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) instead of the tin (IV) oxide particles used in Example 1. Alumina granules and an alumina sintered body were prepared using the same procedure as in Example 1, except that the amount of oxide slurry (titanium oxide slurry) added was changed to 6.7 g.

[0080] Example 4 An alumina slurry was prepared using the same procedure as in Example 1. An oxide slurry (yttrium oxide slurry) was prepared by using yttrium oxide particles (manufactured by Shin-Etsu Chemical Co., Ltd.) instead of the tin (IV) oxide particles used in Example 1. Alumina granules and an alumina sintered body were prepared using the same procedure as in Example 1.

[0081] Example 5 An alumina slurry was prepared using the same procedure as in Example 1. Tin (IV) oxide particles (153 g, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and pure water (2,847 g) were stirred for 10 minutes using a rotary blade, and then circulated once in a wet disperser (manufactured by Shinmaru Enterprises, Dynomill) under conditions of a peripheral speed of 8 m / s, a flow rate of 500 mL / min, and alumina beads φ0.5 mm (2,690 g). The mixture was then circulated for 95 minutes to uniformly disperse the mixture, yielding an oxide slurry (tin oxide slurry). Using the same procedure as in Example 1, alumina granules and an alumina sintered body were prepared.

[0082] (Example 6) An alumina slurry was prepared in the same procedure as in Example 1. An oxide slurry (tin oxide slurry) was prepared in the same procedure as in Example 5. Alumina granules and a lumina sintered body were prepared in the same procedure as in Example 1, except that the amount of oxide slurry (tin oxide slurry) added was changed to 10.2 g.

[0083] (Example 7) An alumina slurry was prepared in the same manner as in Example 1, except that the type of alumina particles was changed to NXA-150 (manufactured by Sumitomo Chemical Co., Ltd.). An oxide slurry (tin oxide slurry) was prepared in the same manner as in Example 5. Alumina granules and an alumina sintered body were prepared in the same manner as in Example 1.

[0084] Example 8 An alumina slurry was prepared using the same procedure as in Example 1. Tin (IV) oxide particles (153 g, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and pure water (2,847 g) were stirred for 10 minutes using a rotary blade, and then uniformly dispersed by circulating once in a wet disperser (manufactured by Shinmaru Enterprises, Dynomill) under conditions of a peripheral speed of 8 m / s, a flow rate of 500 mL / min, and alumina beads φ0.5 mm (2,690 g), to obtain an oxide slurry (tin oxide slurry). Alumina granules and an alumina sintered body were prepared using the same procedure as in Example 1.

[0085] Example 9: An alumina slurry was prepared using the same procedure as in Example 1. Strontium carbonate particles (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were calcined in air at 1,300°C for 2 hours to obtain strontium oxide particles. The strontium oxide particles (117 g) and pure water (2,223 g) were mixed using a rotary blade for 10 minutes, then circulated once in a wet disperser (manufactured by Shinmaru Enterprises, Dynomill) at a peripheral speed of 8 m / s, a flow rate of 500 mL / min, and 2,690 g of alumina beads φ0.5 mm. The mixture was then circulated for 30 minutes to obtain an oxide slurry (strontium oxide slurry). Alumina granules and an alumina sintered body were prepared using the same procedure as in Example 1, except that the amount of oxide slurry (strontium oxide slurry) added was changed to 4.9 g.

[0086] Comparative Example 1 An alumina slurry was prepared using the same procedure as in Example 1. Silica particles (150 g, EVONIK, AEROSIL 200) and pure water (4,850 g) were stirred for 10 minutes using a rotary blade, and then circulated once in a wet disperser (Shinmaru Enterprise, Dynomill) at a peripheral speed of 8 m / s, a flow rate of 500 mL / min, and alumina beads φ0.5 mm (2,690 g). The mixture was then circulated for 30 minutes to uniformly disperse the mixture, yielding an oxide slurry (silica slurry). Alumina granules and an alumina sintered body were prepared using the same procedure as in Example 1, except that the amount of oxide slurry (silica slurry) added was changed to 14.9 g.

[0087] Comparative Example 2 An alumina slurry was prepared in the same manner as in Example 1, except that the type of alumina particles was changed to AKP-20 (manufactured by Sumitomo Chemical Co., Ltd.). An oxide slurry (tin oxide slurry) was prepared in the same manner as in Example 5. Alumina granules and an alumina sintered body were prepared in the same manner as in Example 1.

[0088] Table 2 shows the physical property values ​​and various measured values ​​of the oxide particles and alumina particles used in the examples and comparative examples. Table 3 shows the composition ratios of oxide, alumina, and binder contained in the obtained alumina compositions (alumina granules). Note that the composition ratios were calculated from the compounding ratios of oxide particles, alumina particles, and binder during the production of the alumina granules, and do not include optional components such as moisture and plasticizers in the granules. Table 4 shows the measured values ​​and evaluation of the light transmittance of the obtained alumina sintered bodies. Note that the measured values ​​listed in Tables 2 and 4 were measured using the following methods.

[0089] (D50 and D90 of oxide particles, D50 of alumina particles) The D50 and D90 of oxide particles and the D50 of alumina particles were measured as follows. The measurements were carried out before mixing the oxide particles and alumina particles. The particles to be measured were added to a 0.2% aqueous solution of sodium hexametaphosphate and dispersed by treatment for 7 minutes using an ultrasonic homogenizer. The dispersion was placed in a measurement cell and degassed, and then the particle size distribution of the particles was determined using a Microtrac particle size distribution analyzer MT-3300 manufactured by Microtrac Bell under the following conditions: measurement time: 10 s, number of measurements: 2, particle refractive index: 1.77, solvent refractive index: 1.333, and the D50 and D90 were calculated from the results.

[0090] (BET Specific Surface Area of ​​Alumina Particles) The specific surface area (BET specific surface area) of the alumina particles was determined by the single-point nitrogen adsorption method in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption." Using a fully automatic specific surface area measuring device, Macsorb manufactured by Mountech, 0.1 g of alumina particles was placed in a cell, pretreated at 200°C for 20 minutes, and then measured by nitrogen adsorption.

[0091] (Translucency of Alumina Sintered Body) To measure the translucency (transmittance) of the alumina sintered body, a haze meter NDH8000 manufactured by Nippon Denshoku Industries Co., Ltd. was used, and the instrument settings conformed to the measurement method for total transmittance (T.T) and parallel transmittance (P.T) of JIS K7361-1:1997. A small diameter attachment was attached, and the measurement diameter was small. The instrument was started and warmed up for 30 minutes before measurement. The thickness of the sintered body for translucency measurement was set to 0.9 to 1.1 mm after surface polishing, and the total transmittance (T.T) and parallel transmittance (P.T) were measured. In the evaluation of translucency, a total transmittance (T.T) of 40% or more and a parallel transmittance (P.T) of 0.3% or more was evaluated as pass (◯), and any other cases were evaluated as fail (x).

[0092]

[0093]

[0094]

[0095] (Observation of sintered structure of alumina sintered body) The alumina sintered bodies of Examples 5 and 8 were cut, and electron backscatter diffraction (EBSD) images were taken at 500x magnification to perform grain mapping (Figs. 1 and 2). Image analysis was used to determine the particle size distribution, and the cumulative 10% particle size (number-based D10), 30% particle size (number-based D30), 50% particle size (number-based D50), 70% particle size (number-based D70), and 90% particle size (number-based D90) were determined from the fine particle side of the cumulative particle size distribution based on number. The analysis results are shown in Table 5.

[0096]

[0097] The alumina sintered body of Example 5 had a smaller grain size than the alumina sintered body of Example 8. Therefore, it was confirmed that the alumina sintered body of Example 5 had higher translucency than the alumina sintered body of Example 8 (Table 4).

[0098] (Mapping of Alumina Sintered Body) The alumina sintered body of Example 5 was cut and observed with a scanning transmission electron microscope (STEM) at magnifications of 120,000 and 1,000,000, and the presence of Al and Sn was mapped by energy dispersive X-ray spectroscopy (EDX) (FIGS. 3 and 4). From the Sn mapping, it was found that SnO, which is an oxide particle, was present. ２ It was confirmed that it exists at the grain boundaries of the crystal grains.

[0099] This application claims priority from Japanese Patent Application No. 2024-130044, filed on August 6, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. An alumina composition comprising alumina particles and oxide particles made of an oxide other than alumina, wherein the oxide particles are made of an oxide containing a cation having an ionic radius in hexacoordination of 0.41 Å or more and 1.2 Å or less, and the cumulative 50% particle size D50 from the fine particle side of the volume-based cumulative particle size distribution is 5.0 μm or less, and the D50 of the alumina particles is 0.40 μm or less.

2. The alumina composition of claim 1, wherein the cations are cations of one or more elements selected from the group consisting of Ti, Sn, Zr, Y, and Sr.

3. The alumina composition according to claim 1, wherein the content of the oxide particles is 50 ppm by mass or more and 5,000 ppm by mass or less in terms of the elements constituting the cations.

4. The alumina composition according to claim 1, wherein the oxide particles have a D50 of 0.5 μm or less.

5. The alumina composition according to claim 1, wherein the alumina content is 90.0 mass% or more.

6. The BET specific surface area of ​​the alumina particles is 7 m ２ / g or more 20m ２ 2. The alumina composition of claim 1, wherein the alumina composition has a SiO2 content of 0.15 wt % or less.

7. The alumina composition of claim 1, wherein the alumina particles and the oxide particles are present as primary particles or secondary particles.

8. The alumina composition according to claim 1, which is a granule formed by binding the alumina particles and the oxide particles with a binder.

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