Alumina granules

Abstract

To provide alumina granules having excellent low-temperature sintering properties.SOLUTION: Alumina granules are composed of primary alumina particles bound by a binder, wherein the pore radius of the maximum logarithmic differential pore volume is 0.055 μm or less in the pores inside the granules.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to alumina granules. [Background technology]

[0002] As a method for producing a sintered ceramic body, there is a method in which a ceramic raw material is press-molded to form a molded body and the molded body is sintered. As a ceramic raw material for press molding, ceramic granules are known (for example, Patent Documents 1 and 2).

[0003] Patent Document 1 discloses ceramic granules containing an organic component and ceramic powder, the ceramic granules having an average granule particle size Dp of 10 to 90 μm and a compressive fracture strength σp of 0.1 to 5 MPa, and the average granule particle size Dp and the compressive fracture strength σp satisfy the following formula: 10≦Dp×σp≦45 [μm MPa]

[0004] Patent Document 2 discloses ceramic granules for press molding that contain an organic component and ceramic powder, the ceramic granules for press molding having an average granule size of 40 to 100 μm and a moisture content of 0.6 to 1.5% by weight. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2006-27914 A [Patent Document 2] JP 2007-197265 A Summary of the Invention [Problem to be solved by the invention]

[0006] From the viewpoint of reducing energy consumption during sintering, it is desirable to use alumina granules that can produce a dense alumina sintered body at a low sintering temperature (i.e., have excellent low-temperature sintering properties). However, Patent Documents 1 and 2 do not discuss improving the low-temperature sintering properties of alumina granules.

[0007] The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide alumina granules that are excellent in low-temperature sintering property. [Means for solving the problem]

[0008] Aspect 1 of the present invention is Alumina granules formed by binding primary alumina particles with a binder, The alumina granules have a pore radius showing the maximum value of the logarithmic differential pore volume in the pores inside the granules of 0.055 μm or less.

[0009] Aspect 2 of the present invention is The alumina granule according to aspect 1, wherein the specific surface area diameter of the primary alumina particles is 90 nm or more and less than 200 nm.

[0010] Aspect 3 of the present invention is 3. The alumina granule according to claim 1 or 2, wherein the primary alumina particles have an average circularity of 0.74 or more.

[0011] A fourth aspect of the present invention is The alumina granule according to any one of aspects 1 to 3, wherein the average aspect ratio of the primary alumina particles is 1.40 or less.

[0012] A fifth aspect of the present invention is The alumina granule according to any one of aspects 1 to 4, further comprising a sintering aid in an amount of 10 to 3,000 ppm in terms of metal element relative to 100% by mass of the alumina contained in the alumina granule. Effect of the Invention

[0013] According to the present disclosure, alumina granules having excellent low-temperature sintering properties can be provided. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a graph plotting the logarithmic (Log) differential pore volume versus pore radius for the pores of an alumina granule according to the embodiment. [Diagram 2] FIG. 2 is a SEM image of the alumina granules of Examples 1 and 2. [Diagram 3] FIG. 3 is an SEM image of the alumina granules of Comparative Examples 1 and 2. [Figure 4] FIG. 4 is a photograph of the sintered body for measuring the translucency. [Diagram 5] FIG. 5 is a photograph of a colored alumina sintered body. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present inventors have conducted extensive research in order to realize alumina granules with excellent low-temperature sintering properties. As a result, they have found that it is sufficient for the pores inside the alumina granules, which are composed of primary alumina particles, to have a pore radius that indicates the maximum value of the logarithmic differential pore volume of a certain value or less. The alumina granules according to this embodiment will be described in detail below.

[0016] [Alumina granules] (The pore radius showing the maximum value of the logarithmic differential pore volume in the pores inside the granules is 0.055 μm or less.) The alumina granules are composed of primary alumina particles bound together by a binder. Inside the alumina granule, there are pores formed between a plurality of primary alumina particles. The alumina granule of this embodiment has a pore radius indicating the maximum value of the logarithmic differential pore volume of the pores therein (hereinafter simply referred to as "pore radius of the alumina granule") of 0.055 μm or less. Such alumina granules are considered to have excellent low-temperature sintering properties for the following reasons.

[0017] The small pore radius of the alumina granule means that the gap between the primary alumina particles in the alumina granule is narrow. It is considered that such alumina granules promote densification during sintering. As a result of intensive research, the present inventors have found that when the pore radius of the alumina granule is 0.055 μm or less, densification proceeds sufficiently and a high-density alumina sintered body can be obtained even at a low sintering temperature (for example, 1250°C to 1350°C). The pore radius of the alumina granule is preferably 0.050 μm or less, more preferably 0.043 μm or less, and further preferably 0.040 μm or less. The pore radius of the alumina granule is preferably small, for example, 0.001 μm or more.

[0018] The pore radius of the alumina granules is determined from a graph plotting the logarithmic (Log) differential pore volume against the pore radius obtained in the pore size analysis measurement. The pore size analysis is performed using a pore size distribution measurement device (e.g., Micromeritics' AutoPore V9600 pore size distribution measurement device) with a measurement absolute pressure range of 1.07 to 59256 psia. A graph is created in which the logarithmic (Log) differential pore volume is plotted against the pore radius obtained by the pore size distribution measurement device (see FIG. 1). The graph in FIG. 1 has two peaks, the peak with the smaller pore radius (first peak) corresponds to the pores inside the alumina granules, and the peak with the larger pore radius (second peak) corresponds to the gaps (pores) between a plurality of alumina granules. The position of the apex of the first peak, that is, the pore radius showing the maximum value of the logarithmic differential pore volume, is obtained and is taken as the pore radius of the alumina granule.

[0019] (The specific surface area diameter of primary alumina particles is 90nm or more and less than 200nm) The specific surface area diameter of the primary alumina particles constituting the alumina granules is preferably 90 nm or more and less than 200 nm, which can further improve the low-temperature sintering properties of the alumina granules. The specific surface area diameter of the primary alumina particles is preferably 90 nm, more preferably 110 nm or more, even more preferably 130 nm or more, particularly preferably 140 nm or more, and is preferably less than 200 nm, more preferably 190 nm or less, even more preferably 170 nm or less, particularly preferably 145 nm or less.

[0020] The specific surface area diameter of the primary alumina particles is calculated from the following formula (1), which is a general formula showing the relationship between the particle diameter and the specific surface area of ​​a spherical particle. A = 6 / (S × ρ) (1) Where: A: specific surface area diameter of primary alumina particles (μm), S: Specific surface area of ​​primary alumina particles (m 2 / g), and ρ: density of primary alumina particles (g / cm 3 ), and herein referred to as 3.99 g / cm 3 Let us assume that.

[0021] The specific surface area (BET specific surface area) of primary alumina particles is determined by the 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." The specific surface area of ​​primary alumina particles can be measured either after calcining the alumina granules at 600°C to remove the binder, or by measuring the primary alumina particles as they are before being made into granules. Regardless of the state in which the measurement is performed, roughly the same measurement results are obtained. As a specific measurement method, for example, a fully automatic specific surface area measuring device Macsorb manufactured by Mountech is used, 0.1 g of a sample (calcined alumina granules, or primary alumina particles before being made into granules) is placed in a cell, pre-treated at 200°C for 20 minutes, and then measured by nitrogen adsorption.

[0022] (The average circularity of primary alumina particles is 0.74 or more) The primary alumina particles preferably have an average circularity of 0.74 or more, which can further improve the low-temperature sintering property of the alumina granules. This is because the higher the average circularity of the primary alumina particles (i.e., the closer the primary alumina particles are to a sphere), the narrower the gaps between the multiple primary alumina particles become, and the better the packing property becomes. The average circularity of the primary alumina particles is preferably 0.74 or more, more preferably 0.79 or more, and the upper limit of the average circularity of the primary alumina particles is 1.00 or less.

[0023] (The average aspect ratio of primary alumina particles is 1.40 or less) The primary alumina particles preferably have an average aspect ratio of 1.40 or less, which can further improve the low-temperature sintering property of the alumina granules. In this specification, the aspect ratio is determined by measuring the maximum diameter of a primary alumina particle and the particle size in a direction perpendicular to the direction in which the maximum diameter is measured, and calculating the ratio of the "maximum diameter" to the "particle size in the perpendicular direction." The minimum value of the average aspect ratio of primary alumina particles defined in this way is 1. An average aspect ratio of 1.40 or less is presumed to mean that the average aspect ratio is close to 1 and that the primary alumina particles have a shape close to a sphere. Primary alumina particles that are close to a sphere have narrower gaps between multiple primary alumina particles, improving packing properties.

[0024] The average aspect ratio of the primary alumina particles is preferably 1.40 or less, more preferably 1.30 or less. The average aspect ratio of the primary alumina particles may be 1.05 or more, 1.10 or more, or 1.20 or more.

[0025] Both the average circularity and the average aspect ratio of the primary alumina particles are determined by image analysis of SEM images of the alumina granules.

[0026] An example of the measurement method will be described below. Using a scanning electron microscope (e.g., a scanning electron microscope S-5500 manufactured by Hitachi High-Technologies Corporation), SEM images of the alumina granules are taken at an acceleration voltage of 1 keV. When taking SEM images of the alumina granules, the magnification is adjusted so that 100 to 300 primary alumina particles constituting the alumina granules are captured per image. For example, the magnification is 10,000 to 50,000. The SEM images of the alumina granules of Examples 1 and 2 shown in FIG. 2 were taken at 40,000 magnification, the SEM images of the alumina granules of Comparative Example 1 shown in FIG. 3 were taken at 25,000 magnification, and the SEM images of the alumina granules of Comparative Example 2 were taken at 20,000 magnification.

[0027] Image analysis of SEM images can be performed using commercially available image analysis software (e.g., Image J (manufactured by the National Institute of Health)) or image analysis services (e.g., DeepCle (Sakai Chemical Industry Co., Ltd.)). In the examples, AI automatic analysis was performed using Sakai Chemical Industry Co., Ltd.'s AI image analysis service DeepCle (AI model: 3.3087_2f_r). The circularity and aspect ratio of 100 to 300 primary alumina particles are obtained by image analysis. The multiple circularities obtained are arithmetically averaged to obtain the average circularity. Similarly, the multiple aspect ratios obtained are arithmetically averaged to obtain the average aspect ratio.

[0028] (Contains 10-3000ppm of sintering aid) The alumina granules may contain 10 to 3000 ppm of a sintering aid, if necessary. When the alumina granules are sintered, the sintering aid contained in the alumina granules acts to promote sintering. The content (ppm) of the sintering aid is determined as the content of the sintering aid converted into a metal element when the alumina content in the alumina granules is taken as 100 mass%. The content of the sintering aid is preferably 10 ppm or more, more preferably 100 ppm or more, even more preferably 200 ppm or more, particularly preferably 300 ppm or more, and is preferably 3000 ppm or less, more preferably 2000 ppm or less, and even more preferably 1500 ppm or less.

[0029] The sintering aid to be used is one that exerts its effect as a sintering aid during sintering in air. Examples of sintering aids suitable for this embodiment include compounds that become oxides at 1200°C or less in air, which can function as sintering aids during low-temperature sintering (for example, 1250°C to 1350°C). Examples of compounds include oxides, nitrates, acetates, hydroxides, chlorides, and the like. Specifically, compounds of magnesium, titanium, scandium, yttrium, zirconium, hafnium, lanthanum, zinc, tin, and the like are suitable. Among these, magnesium compounds, particularly magnesium oxide, are preferred.

[0030] Furthermore, as a result of the inventors' investigations, it was found that alumina granules containing an appropriate amount of one or more sintering aids selected from the group consisting of magnesium compounds, zinc compounds, and tin compounds (for example, a total of 300 ppm or more and 1200 ppm or less of sintering aids) are suitable for producing an alumina sintered body with excellent translucency. In recent years, alumina sintered bodies have been considered as dental materials, and the alumina granules according to the present embodiment, which have excellent low-temperature sintering properties, have the advantage that the alumina sintered bodies for prosthetic use can be sintered in a simple heating furnace. Furthermore, the alumina granules containing the above-mentioned sintering aid can produce alumina sintered bodies with excellent translucency, making them suitable as a raw material for dental prosthetic materials with a natural transparency.

[0031] The inventors further investigated the sintering temperature when magnesium oxide is used as a sintering aid and found that translucency is exhibited at a sintering temperature of at least 1350°C, and that the translucency is further improved at a sintering temperature of 1450°C.

[0032] The alumina granules may contain a coloring additive to control the color of the alumina sintered body. The coloring additive may be, for example, one or a combination of two or more of compounds of chromium, manganese, cobalt, nickel, iron, etc. Examples of the compounds include oxides, nitrates, acetates, hydroxides, chlorides, etc. The color of the alumina sintered body depends on the type of coloring additive. For example, a red alumina sintered body can be obtained by using a compound containing chromium or manganese, a blue alumina sintered body can be obtained by using a compound containing cobalt, a green alumina sintered body can be obtained by using a compound containing nickel, and a yellow alumina sintered body can be obtained by using a compound containing iron. By appropriately selecting the type of coloring additive, an alumina sintered body having a desired color (for example, bright red) can be obtained.

[0033] The amount of the coloring additive added is preferably 10 to 3000 ppm in terms of metal element when the alumina content in the alumina granules is taken as 100 mass %.

[0034] The contents of sintering aids (e.g., magnesium oxide) and color additives (e.g., chromium oxide) can be determined from quantitative elemental analysis (e.g., quantitative analysis of Mg and Cr) by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0035] The alumina granules may have a cumulative 50% particle size D50 from the fine particle side of a volume-based cumulative particle size distribution of 30 μm or more and 150 μm or less. The D50 of alumina granules is measured by the laser diffraction / scattering method. An example of the measurement method is as follows. Alumina granules are added to a 0.2% aqueous solution of sodium hexametaphosphate, and dispersed by stirring with a stirring rod for 30 seconds. The dispersion is placed in a measurement cell and degassed. Then, using a Microtrack particle size distribution analyzer MT-3300 manufactured by Microtrack Bell, the measurement is performed with 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 D50. Furthermore, if the mixture is stirred for a short period of time with a stirring rod, the alumina granules will hardly separate into primary alumina particles.

[0036] (Binder content: 0.1 to 5.0% by mass) The content of the binder in the alumina granules is desirably an amount sufficient to bind the primary alumina particles together, but which does not adversely affect the pore radius within the alumina granules. From such a viewpoint, the alumina granules preferably contain 0.1 to 5.0% by mass of the binder when the entire alumina granules (including the binder) are taken as 100% by mass. The content of the binder contained in the alumina granules is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, even more preferably 0.5% by mass or more, particularly preferably 0.8% by mass or more, and is preferably 5.0% by mass or less, more preferably 4.0% by mass or less, even more preferably 2.0% by mass or less, particularly preferably 1.8% by mass or less, when the entire alumina granules are taken as 100% by mass. The binder content can be determined from the weight loss measured by thermal analysis.

[0037] [Method of manufacturing alumina granules] Although the method for producing the alumina granules of the embodiment is not particularly limited, it is preferable to adopt the following production method since it is possible to produce alumina granules having the above physical properties with good reproducibility. Note that, a person skilled in the art who has come into contact with the disclosure of the present application may arrive at a different method capable of producing the alumina granules of the embodiment based on the disclosure.

[0038] The method for producing alumina granules includes the steps of preparing primary alumina particles, preparing an alumina slurry containing the primary alumina particles, and granulating alumina granules from the alumina slurry.

[0039] (Preparing primary alumina particles) The step of preparing primary alumina particles includes: preparing a seed crystal slurry having seed crystals dispersed therein; mixing the seed crystal slurry with an aluminum alkoxide to obtain an aluminum hydroxide slurry; a step of drying and calcining the aluminum hydroxide slurry to obtain alumina particles (primary alumina particles); and grinding the resulting alumina particles.

[0040] (1. Step of preparing seed crystal slurry) Alumina particles (raw material of seed crystals) are dispersed in water, and then wet-pulverized in a ball mill. Then, the mixture is centrifuged in a cooled centrifuge (e.g., CR7N manufactured by Himac) to remove precipitates. This results in a seed crystal slurry in which the seed crystals are dispersed. The seed crystals are preferably alpha alumina. By using alpha alumina seed crystals, the alumina can be converted to alpha by low-temperature sintering in the sintering step described below. The seed crystals preferably have a small particle size, and typically have a particle size of 0.01 μm to 0.2 μm.

[0041] The BET specific surface area of ​​the seed crystals can affect the physical properties of the primary alumina particles finally obtained. In order to set the distribution RSD of the alumina powder within the range according to the embodiment (0.25 to 0.80) and / or to set the crystallite size within the preferred range (650 to 1400 Å), the BET specific surface area of ​​the seed crystals is preferably 30 m 2 / g or more, more preferably 40m 2 / g or more, more preferably 50m 2 / g or more, particularly preferably 70m 2 / g or higher. The BET specific surface area is measured in the same manner as the method for measuring the BET specific surface area of ​​primary alumina particles.

[0042] (2. Step of mixing the seed crystal slurry with the aluminum alkoxide to obtain an aluminum hydroxide slurry) The seed crystal slurry and aluminum alkoxide are continuously fed to a stirrer and mixed. This mixture is rotated and sheared at high speed by the stirrer, whereby the water in the seed crystal slurry and the aluminum alkoxide undergo a hydrolysis reaction, producing a slurry (aluminum hydroxide slurry) containing aluminum hydroxide particles as a hydrolysis product.

[0043] As used herein, "high-speed rotational shear mixing" refers to mixing performed by mechanical energy such as shear force, pressure fluctuation, cavitation, collision force, and potential core that occurs between the turbine (rotor) and stator (screen) when the clearance between the turbine (rotor) and stator (screen) is small (e.g., 2 mm or less) and the turbine (rotor) rotates at high speed (e.g., peripheral speed of about 1 m / sec to about 40 m / sec). In the high-speed rotary shear stirring, the rotation speed of the turbine (rotor) is set to 3000 rpm to 21500 rpm, preferably 8000 rpm to 15000 rpm, for example 10000 rpm.

[0044] (3. Step of drying and calcining the aluminum hydroxide slurry to obtain alumina particles (primary alumina particles)) The aluminum hydroxide slurry is dried by a known method, and the resulting aluminum hydroxide is calcined in a calciner to obtain alumina particles (primary alumina particles). The calcination is carried out usually at 800° C. or higher, preferably 900° C. or higher, and usually at 1000° C. or lower, preferably 980° C. or lower, and more preferably 960° C. or lower. The firing may be carried out in air or in an inert gas such as nitrogen gas or argon gas, and it is effective to carry out the firing while maintaining a high partial pressure of water vapor in the atmosphere (for example, a dew point of 30° C. or higher).

[0045] (Step of preparing alumina slurry containing primary alumina particles) The obtained primary alumina particles, binder, dispersant, and solvent are mixed, and optionally, appropriate amounts of plasticizer, sintering aid, and color additive are mixed, and mechanical stirring and mixing is performed. For the stirring and mixing, a conventional method 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.

[0046] 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.

[0047] It is preferable to select a suitable solvent depending on the type of binder used and the granulation method of the alumina granules. When the alumina granules are granulated 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.

[0048] A dispersant can be added as desired, and it is preferable to select a suitable dispersant depending on the type of solvent used together. When the solvent is water, polycarboxylate ammonium salts (e.g., trade name: SN-D5468, manufactured by San Nopco) are mainly used as dispersants. When the solvent is organic solvent, ethyl oleate, sorbitan monooleate, sorbitan trioleate, polycarboxylates, etc. are used as dispersants. Polyesters (trade name: Texahol 3012, manufactured by San Nopco) are also suitable. However, the dispersants are not limited to these, and various dispersants can be used.

[0049] Depending on the organic binder used, it may be possible to prepare a slurry with a lower viscosity without using a dispersant, and therefore to increase the concentration of alumina granules in the slurry. In such a case, it is not necessary to add a dispersant.

[0050] The 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. As the plasticizer used together with the organic binder, ethylene glycol, diethylene glycol, polyethylene glycol, glycerin, polyglycerin, various esters, etc. are used. In particular, when an organic solvent is used, dibutyl phthalate, diethylhexyl phthalate, etc. are used, but the present invention is not limited to these.

[0051] The sintering aid can be added as desired. The sintering aid is preferably added so that the amount of sintering aid, calculated as a metal element, is 10 to 3,000 ppm when the alumina content in the alumina granules is 100 mass %.

[0052] A coloring additive can be added as desired. The coloring additive is preferably added so that the coloring additive amount, calculated as a metal element, is 10 to 3000 ppm when the alumina content in the alumina granules is taken as 100 mass %.

[0053] The obtained alumina slurry may be defoamed under reduced pressure. Various defoaming agents may also be used. Depending on the subsequent molding method, various pH adjusters and coagulants may be added to adjust the viscosity to 10 to 500 centipoise. For example, in order to produce spherical granules in granulation using a spray dryer, it is preferable to adjust the viscosity of the alumina slurry to 30 to 300 centipoise by pH adjustment using an aqueous hydrochloric acid solution or aqueous ammonia. Furthermore, the alumina concentration in the slurry can be increased by settling, centrifugal separation, vacuum concentration using a rotary evaporator, or the like.

[0054] (Process for granulating alumina granules from alumina slurry) The resulting alumina slurry is used to granulate alumina granules. The method for granulating alumina granules from the alumina 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 alumina slurry with a spray drying device (spray dryer) or the like.

[0055] (Variation: Method of directly granulating alumina granules from primary alumina particles) In the above-mentioned method for producing alumina granules, the primary alumina particles are converted into an alumina slurry and then granulated into alumina granules. However, the primary alumina particles may be directly granulated into alumina granules without converting them into an alumina slurry. For example, primary alumina particles, a binder, and various additives as desired may be mixed and granulated in an agitation granulator to produce a granulated powder, and this granulated powder may then be repeatedly extruded and granulated in an oscillating granulator and dried to produce alumina granules.

[0056] 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 for obtaining particles of a predetermined particle size or less by performing several steps of crushing particles granulated to a particle size of about several mm on a mesh and dropping the fine particles by successively finer meshes.

[0057] [Method of manufacturing sintered alumina] An example of a suitable method for producing an alumina sintered body using the alumina granules according to the embodiment will be described below. The alumina granules are subjected to uniaxial press molding, cold isostatic press molding, etc. to produce a molded body. In the case of cold isostatic press molding, the alumina granules are uniaxially press molded at a pressure of 20 to 40 MPa, preferably 25 to 35 MPa, and then isotropically pressed at 98 MPa or more, preferably 150 to 200 MPa, in a cold isostatic press molding machine, and the obtained molded body is processed into a predetermined shape.

[0058] Two examples of firing conditions for producing an alumina sintered body are given below. (Condition 1) Sintering process The compact is sintered in air or oxygen at a temperature of 1250 to 1350° C. for 2 hours or more to obtain an alumina sintered body. The average heating rate from room temperature to the sintering temperature is, for example, 200° C. / hr.

[0059] The alumina granules according to the embodiment have excellent low-temperature sintering properties due to the small radius of the internal pores. For example, even when sintered at a low temperature of 1250 to 1350°C as in Condition 1, a dense alumina sintered body can be produced.

[0060] (Condition 2) ·Degreasing process The molded body is degreased by firing for 1 hour or more at a temperature range of 500 to 1200° C., preferably for 2 hours or more at a temperature range of 600 to 800° C. The average heating rate from room temperature to the firing temperature is, for example, 100° C. / hr. Sintering process After the degreasing process, the alumina sintered body is obtained by firing at a high temperature. In Condition 2, the firing atmosphere is not limited. In the case of sintering in an air atmosphere, the alumina sintered body is obtained by sintering for 2 hours or more at a temperature of 1200 to 1700°C, preferably 1250 to 1450°C. The sintering temperature in the sintering step is set to be equal to or higher than the sintering temperature in the degreasing step. The average heating rate from the sintering temperature in the degreasing step to the sintering temperature in the sintering step is set to, for example, 200°C / hr.

[0061] Condition 2 is suitable for obtaining an alumina sintered body by low-temperature sintering, as well as for obtaining a translucent alumina sintered body by using alumina granules containing a sintering aid. When obtaining a translucent alumina sintered body, the sintering step conditions are preferably sintering in the air or in a vacuum atmosphere at a temperature in the range of 1350°C to 1700°C, and in particular, sintering at a temperature in the range of 1400°C to 1450°C can further improve the translucency of the alumina sintered body. EXAMPLES

[0062] Example 1 (Preparation of primary alumina particles) Alumina particles (raw material for seed crystals) were dispersed in water, and then wet-pulverized in a ball mill. The mixture was then centrifuged at 4000 rpm for 30 minutes in a refrigerated centrifuge (manufactured by Himac: CR7N) to remove the precipitate. This produced a seed crystal slurry in which the seed crystals were dispersed.

[0063] Aluminum isopropoxide and the seed crystal slurry were mixed and hydrolyzed using a precision emulsifying and dispersing machine, Clearmix CLM-2.2S (manufactured by M Technique Co., Ltd.), at a rotation speed of 10,000 rpm, to produce aluminum hydroxide slurry. The compounding ratio of aluminum isopropoxide to the seed crystal slurry, (water contained in the seed crystal slurry) / (aluminum isopropoxide), was adjusted so that the aluminum component contained in the seed crystals (hereinafter simply referred to as "aluminum parts by mass of the seed crystals") was 10 parts by mass per 100 parts by mass of the total amount of the aluminum isopropoxide and the aluminum component contained in the seed crystals, calculated as the oxide of the metal component. The content (parts by mass) of each aluminum component was calculated on the assumption that all of the aluminum isopropoxide used was converted to alumina.

[0064] The resulting aluminum hydroxide slurry was dried at 150°C to obtain aluminum hydroxide particles, which were then placed in an alumina crucible and fired in a gas furnace. The firing conditions were as follows: the temperature was raised to 965°C at a heating rate of 200°C / hour, and the firing temperature was maintained for 4 hours to produce primary alumina particles. The specific surface area (BET specific surface area) of the primary alumina particles before they were made into alumina granules was measured. This result can be considered to be equivalent to the specific surface area of ​​the primary alumina particles after they were made into alumina granules.

[0065] (Preparation of alumina slurry) The obtained primary alumina particles (1700 g), pure water (1054 g), a dispersant (7.95 g, San Nopco SND-5468), and alumina beads (3540 g, Nikkato φ2-SSA999W) were loaded into a pot (volume 3 L) with an alumina-lined inner surface, and uniformly dispersed at 65 rpm for 6 hours to prepare an alumina slurry.

[0066] (Preparation of alumina granules) The obtained alumina slurry (705 g), pure water (69.6 g), binder (17.4 g, Chuorika Kogyo Co., Ltd., SA-261P), and plasticizer (4.35 g, Fujifilm Wako Pure Chemical Industries, Ltd., PEG-400) were mixed and stirred for 10 minutes. The obtained slurry was spray-dried using a spray dryer (Okawahara Kakoki Co., Ltd., Li-8 type) under the following conditions: flow rate 79 g / min, atomizer rotation speed 18000 rpm, inlet temperature 180°C, outlet temperature 90°C, and hot air differential pressure 1.1 kPa to produce alumina granules.

[0067] The obtained alumina granules were subjected to pore size analysis and SEM observation. The pore radius of the alumina granules was determined from the pore size analysis. The SEM images were analyzed to determine the average circularity and average aspect ratio of the primary alumina particles. Each measurement was performed using the measuring equipment and under the measuring conditions exemplified in the embodiment. Note that no sintering aid (MgO) was added when the alumina granules were produced.

[0068] (Preparation of sintered alumina) The obtained alumina granules (5 g) were filled into a cylindrical mold of φ20 mm, and uniaxial molding was performed at a pressure of 30 MPa for 30 seconds, followed by cold isostatic pressing (CIP) molding at a pressure of 98 MPa for 3 minutes to obtain an alumina compact. The compact was heated in air at a rate of 200°C / hr to 1250°C or 1350°C, and held at each temperature for 2 hours to obtain an alumina sintered body. The density of the obtained alumina sintered body was measured by Archimedes' method in water at 22°C.

[0069] (Preparation of sintered body for measuring translucency) The obtained alumina granules (1 g) were filled into a cylindrical mold of φ20 mm, and uniaxial molding was performed at a pressure of 30 MPa for 30 seconds, followed by five rounds of isostatic pressing (CIP) molding at a pressure of 200 MPa for 3 minutes to obtain an alumina molded body. The molded body was heated to 600°C in air at a heating rate of 100°C / hr, and calcined at that temperature for 2 hours. The calcined alumina body was heated to 1350°C or 1450°C in air at a heating rate of 200°C / hr, and then kept at each temperature for 2 hours to obtain an alumina sintered body for translucency measurement (referred to as "sintered body for translucency measurement" in this specification). The surface of the sintered body for translucency measurement was polished, and then the translucency was evaluated.

[0070] The translucency evaluation was performed using a haze meter NDH8000 manufactured by Nippon Denshoku Industries Co., Ltd., and the device settings were in accordance with the measurement method for total light transmittance (TT) and parallel light transmittance (PT) of JIS K7361-1:1997. A small diameter attachment was attached, and the measurement diameter was set to a small diameter. Measurements were performed after warming up the device for 30 minutes after starting up. The thickness of the sintered body for translucency measurement after surface polishing was set to 0.9 to 1.1 mm, and the total light transmittance TT and parallel light transmittance PT were measured. FIG. 4 is a photograph of the sintered bodies for measuring translucency produced in the examples and comparative examples.

[0071] Example 2 Primary alumina particles were produced, an alumina slurry was prepared, alumina granules were produced, and an alumina sintered body and a sintered body for translucency measurement were produced in the same manner as in Example 1, except that an aluminum hydroxide slurry was prepared with a compounding ratio such that the aluminum parts by mass of the seed crystals was 5.6 parts by mass.

[0072] Example 3 An alumina slurry was prepared, alumina granules were produced, and an alumina sintered body was produced in the same manner as in Example 1, except that α-alumina powder AKP-50 (manufactured by Sumitomo Chemical) was used as the primary alumina particles.

[0073] (Examples 4, 6, and 8) In the same manner as in Example 1, primary alumina particles were prepared, and an alumina slurry was prepared. In addition, high-purity magnesia powder (150 g, Ube Materials, vapor-phase magnesia 500A) and pure water (2,850 g) were mixed with a disperser for 10 minutes, 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), and then circulated for 30 minutes to uniformly disperse the mixture and produce a magnesia slurry.

[0074] Alumina slurry (1250g), magnesia slurry (12.79g), pure water (111.25g), binder (30.85g, Chuorika Kogyo, SA-261P), plasticizer (7.71g, Fujifilm Wako Pure Chemical Industries, PEG-400) were mixed and stirred for 10 minutes. The obtained slurry was spray-dried using a spray dryer (Okawahara Kakoki, Li-8 type) under the conditions of flow rate 79g / min, atomizer rotation speed 18000rpm, inlet temperature 180℃, outlet temperature 90℃, and hot air differential pressure 1.1kPa to produce alumina granules. The content of MgO contained in the alumina granules is shown in Table 2 as a metal element equivalent value.

[0075] From the obtained alumina granules, a sintered body for measuring translucency was produced in the same manner as in Example 1.

[0076] (Examples 5, 7, and 9) Primary alumina particles were produced in the same manner as in Example 2, and an alumina slurry was prepared. In the same manner as in Example 4, a magnesia slurry was prepared, alumina granules were produced, and in the same manner as in Example 1, a sintered body for measuring translucency was produced.

[0077] Example 10 Primary alumina particles were produced in the same manner as in Example 1, and an alumina slurry was prepared. A magnesia slurry was prepared in the same manner as in Example 4. In addition, chromium oxide powder (15 g, Fujifilm Wako Pure Chemical Industries, Ltd.) and pure water (285 g) were dispersed in a ball mill for 1 hour to prepare a chromium oxide slurry.

[0078] Alumina slurry (1250g), magnesia slurry (12.79g), pure water (111.25g), binder (30.85g, Chuorika Kogyo, SA-261P), plasticizer (7.71g, Fujifilm Wako Pure Chemical Industries, PEG-400), and chromium oxide slurry (22.5g) were mixed and stirred for 10 minutes. The obtained slurry was spray-dried using a spray dryer (Okawahara Kakoki, Li-8 type) under the conditions of flow rate 79g / min, atomizer rotation speed 18000rpm, inlet temperature 180℃, outlet temperature 90℃, and hot air differential pressure 1.1kPa to produce alumina granules. The contents of MgO and Cr2O3 contained in the alumina granules are shown in Table 4 as metal element equivalent values.

[0079] (Preparation of colored alumina sintered body) The alumina granules (1 g) obtained in Example 10 were filled into a cylindrical mold having a diameter of 20 mm, and uniaxial molding was performed at a pressure of 30 MPa for 30 seconds, followed by five rounds of CIP molding at a pressure of 200 MPa for 3 minutes to obtain an alumina molded body. The molded body was heated to 600°C in air at a heating rate of 100°C / hr, and calcined at that temperature for 2 hours. The calcined alumina body was heated to 1450°C in air at a heating rate of 200°C / hr, and then kept at that temperature for 2 hours to obtain a colored alumina sintered body (colored alumina sintered body). The appearance (color) of the colored alumina sintered body was observed with the naked eye. The results of the appearance observation are shown in Table 4. FIG. 5 is a photograph of the colored alumina sintered bodies produced in the examples and comparative examples.

[0080] Example 11 Except for producing primary alumina particles in the same manner as in Example 2, an alumina slurry, a magnesia slurry, a chromium oxide slurry, alumina granules, and a colored alumina sintered body were prepared in the same manner as in Example 10. The contents of MgO and Cr2O3 contained in the alumina granules are shown in Table 4 as metal element equivalent values.

[0081] Comparative Example 1 An alumina slurry was prepared in the same manner as in Example 1, except that α-alumina powder AKP-30 (manufactured by Sumitomo Chemical) was used as the primary alumina particles. Further, alumina granules were prepared in the same manner as in Example 1, and an alumina sintered body was produced.

[0082] Comparative Example 2 An alumina slurry was prepared in the same manner as in Example 1, except that α-alumina powder AKP-20 (manufactured by Sumitomo Chemical) was used as the primary alumina particles. Further, alumina granules were prepared in the same manner as in Example 1, and an alumina sintered body was produced.

[0083] Comparative Example 3 The same primary alumina particles as those in Comparative Example 1 were prepared, and an alumina slurry was prepared in the same manner as in Comparative Example 1. A magnesia slurry was prepared in the same manner as in Example 4. Alumina granules were prepared in the same manner as in Example 6. In the same manner as in Example 1, a sintered body for measuring translucency was produced. The content of MgO in the alumina granules is shown in Table 2 as a metal element equivalent value.

[0084] Comparative Example 4 The same primary alumina particles as those in Comparative Example 2 were prepared, and an alumina slurry was prepared in the same manner as in Comparative Example 2. A magnesia slurry was prepared in the same manner as in Example 4. Alumina granules were prepared in the same manner as in Example 6. In the same manner as in Example 1, a sintered body for measuring translucency was produced. The content of MgO in the alumina granules is shown in Table 2 as a metal element equivalent value.

[0085] Comparative Example 5 Except for preparing the same primary alumina as in Comparative Example 1, an alumina slurry, a magnesia slurry, a chromium oxide slurry, alumina granules, and a colored alumina sintered body were prepared in the same manner as in Example 10. The contents of MgO and Cr2O3 contained in the alumina granules are shown in Table 4 as metal element equivalent values.

[0086] Tables 1 to 4 show various measurement results of the alumina granules and the alumina sintered body according to the examples and comparative examples.

[0087] [Table 1]

[0088] [Table 2]

[0089] [Table 3]

[0090] [Table 4]

[0091] The measurement results in Tables 1 to 4 will be considered. From the results of Table 1, it was found that in the sintering test at low temperatures (1250°C, 1350°C), the alumina granules according to the examples that satisfy the requirements set forth in this embodiment can produce an alumina sintered body with higher density than the alumina granules of the comparative example. This confirmed that the alumina granules according to this embodiment have excellent low-temperature sintering properties.

[0092] From the results of Tables 2 and 3, it was found that the alumina granules according to the examples that satisfy the requirements defined in this embodiment can produce an alumina sintered body with excellent light transmittance, compared to the alumina granules of the comparative examples. Also, when comparing among the groups of examples that differ only in the amount of MgO added (the group consisting of Examples 1, 4, 6, and 8, and the group consisting of Examples 2, 5, 7, and 9), it was found that the use of alumina granules with an added amount of MgO of 500 to 1000 ppm can produce an alumina sintered body with particularly excellent transmittance.

[0093] From the results in Table 4, it was found that the alumina granules according to the examples that satisfy the requirements stipulated in this embodiment can produce an alumina sintered body with a more vivid red color than the alumina granules of the comparative examples (see FIG. 5). This confirmed that the alumina granules according to this embodiment can provide a colored alumina sintered body with vivid color development.

Claims

1. Alumina granules formed by binding primary alumina particles with a binder, An alumina granule, in which the pore radius showing the maximum value of the logarithmic differential pore volume in the granule is 0.055 μm or less.

2. The alumina granule according to claim 1 , wherein the specific surface area diameter of the primary alumina particles is 90 nm or more and less than 200 nm.

3. 3. The alumina granule according to claim 1, wherein the average circularity of the primary alumina particles is 0.74 or more.

4. 3. The alumina granule according to claim 1, wherein the primary alumina particles have an average aspect ratio of 1.40 or less.

5. The alumina granule according to claim 1 or 2, comprising a sintering aid in an amount of 10 to 3,000 ppm in terms of metal element relative to 100 mass% of the alumina content in the alumina granule.

Images