Plate-like alumina particles and method for producing plate-like alumina particles

Abstract

Plate-like alumina particles containing titanium and at least one element (A) selected from the group consisting of iron, nickel, copper, chromium, cobalt, zinc, scandium, and ruthenium.

Description

[Technical Field]

[0001] The present invention relates to plate-like alumina particles and a method for producing plate-like alumina particles. This application claims priority based on Japanese Patent Application No. 2023-221902, filed on December 27, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] Alumina particles, which are inorganic fillers, are used in a variety of applications. In particular, plate-like alumina particles are used in a wide range of applications, such as thermally conductive fillers, high-brightness pigments, cosmetics, abrasives, conductive powder substrates, and lubricants for resin films. In particular, plate-like alumina particles with a high aspect ratio, low coagulation tendency, and high dispersibility are in demand.

[0003] Various methods for producing plate-like alumina particles have been known. For example, Patent Document 1 discloses a method in which a mineralizer such as aluminum fluoride is added during the calcination process of raw materials. However, these methods have difficulty in controlling the particle size, particularly in producing plate-like alumina with a high aspect ratio. In addition, the resulting plate-like alumina particles have aggregation problems, such as multiple overlapping particles and twin crystals.

[0004] There have been various reports in recent years on the production of plate-like alumina particles with controlled size, shape, etc. For example, Patent Document 2 reports that titanium oxide-containing flaky aluminum oxide having an average particle size of 5 to 60 μm, a thickness of 1 μm or less, and an aspect ratio of 20 or more is produced by using titanium oxide as a crystallization control agent and firing at a temperature of 1100° C. or higher in the presence of a sulfate, which is a high-temperature flux.

[0005] Patent Document 3 reports that zinc oxide-containing plate-like α-alumina particles having an average particle size of 15 to 25 μm, a thickness of 0.1 to 0.5 μm, and an aspect ratio of 50 to 250 are produced by using zinc oxide as a crystallization control agent and firing at a temperature of 1150°C in the presence of a sulfate, which is a high-temperature flux.

[0006] Patent Document 4 reports that zinc oxide and tin oxide-containing plate-like α-alumina particles having an average particle thickness of 0.5 μm or less, an average particle diameter of 30 μm or more, and an aspect ratio of 100 or more are produced by using zinc oxide and tin oxide as crystallization control agents and firing at a temperature of 1200°C in the presence of a sulfate, which is a high-temperature flux.

[0007] Patent Document 5 reports that by using zirconium oxide as a crystallization control agent and firing at a temperature of 1150°C in the presence of a sulfate, which is a high-temperature flux, plate-like α-alumina particles having an average thickness of 0.1 to 1 μm, an average diameter of 5 to 25 μm, and an aspect ratio of 25 to 250 are obtained. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Special Publication No. 35-6977 [Patent Document 2] Japanese Patent Application Publication No. 9-77512 [Patent Document 3] Special Publication No. 2008-534417 [Patent Document 4] Special Publication No. 2010-502539 [Patent Document 5] Special Publication No. 2017-516734 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the plate-shaped alumina obtained by these methods requires a firing process at temperatures of over 1100°C, which requires a huge amount of energy for production. Recently, there has been a growing movement to reduce the energy used in production in order to achieve a sustainable society, such as carbon neutrality and SDGs, and there is a demand for lower firing temperatures.

[0010] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide plate-like alumina particles that can be produced at a firing temperature lower than conventional temperatures, and a method for producing the plate-like alumina particles. [Means for solving the problem]

[0011] The present invention includes the following aspects. (1) Plate-like alumina particles containing titanium and at least one element (A) selected from the group consisting of iron, nickel, copper, chromium, cobalt, zinc, scandium, and ruthenium. (2) The plate-like alumina particles according to (1) above, which contain 0.02 to 2 mass % of the at least one element (A) calculated as oxide. (3) The plate-like alumina particles according to (1) or (2) above, which contain silicon or a silicon compound in an amount of 0.1 to 10 mass % calculated as silicon dioxide. (4) The plate-like alumina particles according to any one of (1) to (3) above, which contain silicon or a silicon compound in the surface layer. (5) The plate-like alumina particles according to any one of (1) to (4) above, having a thickness of 0.05 to 1 μm, an average particle diameter of 1 to 30 μm, and an aspect ratio of 5 to 300. (6) A method for producing plate-like alumina particles according to any one of (1) to (5) above, comprising the steps of: mixing an aluminum compound, molybdenum oxide, silicon or a silicon compound, a compound (A) containing at least one element (A) selected from the group consisting of iron, nickel, copper, chromium, cobalt, zinc, scandium, and ruthenium, and a titanium compound to obtain a mixture; and calcining the mixture. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide plate-like alumina particles that can be produced at a firing temperature lower than conventional temperatures, and a method for producing the plate-like alumina particles. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is an SEM observation image of the plate-like alumina particles obtained in Example 1. [Figure 2] 1 is an SEM observation image of the plate-like alumina particles obtained in Example 8. [Figure 3] 1 is an SEM observation image of alumina particles obtained in Comparative Example 1. [Figure 4] 1 is an SEM observation image of alumina particles obtained in Comparative Example 2. [Figure 5] 1 is an SEM observation image of alumina particles obtained in Comparative Example 6. [Figure 6] 1 is an SEM observation image of alumina particles obtained in Comparative Example 8. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, a plate-like alumina particle and a method for producing the plate-like alumina particle according to one embodiment of the present invention will be described in detail.

[0015] <Plate-shaped alumina particles> The plate-like alumina particles according to this embodiment contain at least one element (A) (hereinafter simply referred to as "element (A)") selected from the group consisting of iron, nickel, copper, chromium, cobalt, zinc, scandium, and ruthenium, and titanium.

[0016] In this embodiment, "plate-like" refers to an aspect ratio of 2 or more, which is the average particle diameter of alumina particles divided by their thickness. In this specification, the "thickness of alumina particles" refers to the arithmetic mean value of thicknesses measured for at least 50 plate-like alumina particles randomly selected from an image obtained by a scanning electron microscope (SEM). Furthermore, the "average particle diameter of alumina particles" refers to the volume-based median diameter D from a volume-based cumulative particle size distribution measured by a laser diffraction particle size analyzer. 50 The value calculated as follows:

[0017] (Element (A)) In this embodiment, the element (A) is at least one selected from the group consisting of iron, nickel, copper, chromium, cobalt, zinc, scandium, and ruthenium, preferably at least one selected from the group consisting of iron, nickel, copper, chromium, cobalt, and zinc, and more preferably at least one selected from the group consisting of iron, nickel, and chromium.

[0018] In the present embodiment, from the viewpoints of availability and ease of synthesis, the element (A) is preferably derived from at least one compound (A) (hereinafter also simply referred to as "compound (A)") selected from the group consisting of iron hydroxide, iron oxide, iron oxyhydroxide, nickel oxide, copper oxide, chromium oxide, cobalt oxide, zinc oxide, scandium oxide, and ruthenium oxide, more preferably derived from at least one compound (A) selected from the group consisting of iron hydroxide, iron oxide, iron oxyhydroxide, nickel oxide, chromium oxide, cobalt oxide, and zinc oxide, and even more preferably derived from at least one compound (A) selected from the group consisting of iron hydroxide, iron oxide, iron oxyhydroxide, nickel oxide, and chromium oxide.

[0019] The plate-like alumina particles of this embodiment preferably contain 0.02 to 2 mass %, more preferably 0.04 to 1.5 mass %, and even more preferably 0.06 to 1 mass % of the element (A) calculated as oxide. When the content of the element (A) is within the above-mentioned preferred range, the energy required for generating the plate-like alumina particles is stabilized, and the particle size is easily made uniform. When the content is equal to or less than the upper limit, the plate-like alumina particles are prevented from exhibiting a dark color, making them suitable for use as a luminous pigment or a cosmetic.

[0020] In this embodiment, the content of the element (A) is a value obtained by subjecting the plate-like alumina particles to XRF (X-ray fluorescence) analysis and calculating the total content of each element constituting the element (A) in terms of oxide (mass%) from a previously obtained oxide calibration curve. Alternatively, the content of each element contained in the plate-like alumina particles can be calculated and expressed in terms of oxide (mass%) by comparing the intensity with that of a sample (calibration curve sample) with a known content by ICP (inductively coupled plasma) atomic emission spectroscopy.

[0021] The element (A) exists in a so-called doped state, in which the trivalent aluminum element is substituted with the element (A) in the corundum structure of the plate-like alumina particles. The valence of the element (A) is doped by adding the tetravalent titanium element so as to keep the charge of the entire crystal neutral, so it is presumed that the element (A) exists as a divalent element.

[0022] (Titanium element) In this embodiment, the titanium element is preferably derived from titanium oxide from the viewpoint of availability and ease of synthesis.

[0023] The plate-like alumina particles of this embodiment preferably contain 0.02 to 2 mass % of titanium element in terms of oxide, more preferably 0.04 to 1.5 mass %, and even more preferably 0.06 to 1 mass %. When the titanium content is within the above-mentioned preferred range, the plate-like alumina particles can be produced at a lower firing temperature (for example, 1000°C or lower) than conventionally. Furthermore, when the titanium content is equal to or lower than the above-mentioned upper limit, the plate-like alumina particles are prevented from turning deep yellow, making them suitable for use as a luminous pigment or cosmetic material.

[0024] In this embodiment, the titanium element content is a value obtained by subjecting the plate-like alumina particles to XRF (X-ray fluorescence) analysis and determining the total titanium element content in terms of oxide (mass %) from a previously obtained oxide calibration curve. Alternatively, the titanium element content in the plate-like alumina particles can be calculated and expressed in terms of oxide (mass %) by comparing the intensity with that of a sample (calibration curve sample) with a known content by ICP (inductively coupled plasma) atomic emission spectroscopy.

[0025] The titanium element is present in a so-called doped state in which trivalent aluminum elements are substituted with tetravalent titanium elements in the corundum structure of the plate-like alumina particles.

[0026] Although the detailed principle is unknown, it is speculated that when the divalent element (A) and tetravalent titanium element are added simultaneously during firing, one element (A) and one titanium element are substituted for two adjacent aluminum atoms in the crystal structure of the plate-like alumina particles so as to compensate for each other's charges, which changes the light absorption and reflection of the resulting plate-like alumina particles, causing the particles to take on a metallic silver color that is suitable for high-brightness pigments and cosmetics.

[0027] The alumina particles may be formed in any combination of the thickness, particle diameter, and aspect ratio conditions shown below, as long as they are plate-shaped. The upper and lower limits of the numerical ranges exemplified for these conditions may be freely combined.

[0028] The thickness of the plate-like alumina particles is preferably 0.05 μm or more and 1 μm or less, more preferably 0.1 μm or more and 1 μm or less, and even more preferably 0.1 μm or more and 0.8 μm or less. Alumina particles having the above thickness are preferred because they have a high aspect ratio and are excellent in brightness and mechanical strength.

[0029] The plate-shaped alumina particles have an average particle diameter (D 50) is preferably 1 μm or more and 30 μm or less, more preferably 3 μm or more and 30 μm or less, and even more preferably 5 μm or more and 30 μm or less. The average particle size (D 50 Alumina particles having an average particle diameter (D ) of not more than the above upper limit have a large light reflecting surface area, and therefore have particularly excellent brilliance. 50 ) are suitable for use as fillers.

[0030] The aspect ratio of the plate-like alumina particles, which is the ratio of the average particle diameter to the thickness, is preferably 5 to 300, more preferably 10 to 300, more preferably 15 to 300, more preferably 20 to 200, and even more preferably 30 to 150. When the aspect ratio of the plate-like alumina particles is 2 or more, it is preferable because it can have two-dimensional blending characteristics, and when the aspect ratio of the plate-like alumina particles is 500 or less, it is preferable because it has excellent mechanical strength. When the aspect ratio is 15 or more, it is preferable because it has high brightness when used as a pigment.

[0031] In this embodiment, the plate-like alumina particles preferably have a thickness of 0.05 to 1 μm, an average particle diameter of 1 to 30 μm, and an aspect ratio of 5 to 300.

[0032] The plate-like alumina particles may be polygonal, circular, or elliptical, but the particle shape is preferably, for example, polygonal or circular from the viewpoint of ease of handling and production.

[0033] The plate-like alumina particles may be obtained by any production method, but are preferably obtained by firing an aluminum compound in the presence of a molybdenum compound (particularly preferably molybdenum trioxide) and a shape control agent, in terms of a higher aspect ratio, better dispersibility, and better productivity. The shape control agent is preferably silicon and / or a silicon compound. Silicon or a silicon-containing silicon compound is preferred because it serves as a source of Si for mullite, which will be described later. In the above-mentioned production method, a molybdenum compound is used as a fluxing agent. Hereinafter, this production method using a molybdenum compound as a fluxing agent may be simply referred to as the "flux method." The flux method will be described in detail later. It is believed that, during the firing process, the molybdenum compound reacts with the aluminum compound at high temperature to form aluminum molybdate, which then decomposes into alumina and molybdenum oxide at a higher temperature, resulting in the molybdenum compound being incorporated into the plate-like alumina particles. The molybdenum oxide vaporizes and solidifies again upon cooling, allowing it to be recovered and reused. In addition, when the plate-like alumina particles contain mullite in the surface layer, it is thought that during this process, mullite is formed in the surface layer of the plate-like alumina particles by the reaction of silicon or a compound containing silicon atoms, which is blended as a shape control agent, with an aluminum compound via molybdenum. More specifically, the mullite formation mechanism is thought to be as follows: on the alumina plate surface, molybdenum reacts with Si atoms to form Mo-O-Si, and molybdenum reacts with Al atoms to form Mo-O-Al, and high-temperature firing causes Mo to be released and mullite having Si-O-Al bonds is formed. Molybdenum oxide that is not incorporated into the tabular alumina particles is preferably recovered by evaporation and reused. This reduces the amount of molybdenum oxide that adheres to the surface of the tabular alumina, and prevents the molybdenum oxide from being mixed into the binder when the alumina is dispersed in a dispersion medium such as an organic binder like a resin or an inorganic binder like glass, thereby allowing the inherent properties of the tabular alumina to be maximized. In this specification, a substance that can evaporate in the manufacturing method described below is referred to as a fluxing agent, and a substance that cannot evaporate is referred to as a shape control agent.

[0034] In the production of the plate-like alumina particles, by utilizing molybdenum and a shape control agent, the alumina particles have a high α-crystal ratio and are idiomorphic, thereby achieving excellent dispersibility, mechanical strength, and high thermal conductivity.

[0035] When the plate-like alumina particles contain mullite in the surface layer, the amount of mullite formed in the surface layer of the plate-like alumina particles can be controlled by the proportions of the molybdenum compound and the shape control agent used, and in particular by the proportion of silicon or a silicon compound containing silicon used as the shape control agent. The preferred amount of mullite formed in the surface layer of the plate-like alumina particles and the preferred proportions of the raw materials used will be described in detail later.

[0036] The plate-shaped alumina particles have a density of, for example, 3.70 g / cm 3 More than 4.10g / cm 3 and a density of 3.72 g / cm 3 More than 4.10g / cm 3 Preferably, the density is 3.80 g / cm or less. 3 More than 4.10g / cm 3 More preferably, it is: The density can be measured using a Micromeritics dry automatic density meter, Accupyc II1330, at a measurement temperature of 25°C using helium as the carrier gas after pre-treating the plate-shaped alumina particles at 300°C for 3 hours.

[0037] [alumina] The "alumina" contained in the plate-like alumina particles is aluminum oxide, and may be transition alumina of various crystalline forms, such as γ, δ, θ, and κ, or may contain alumina hydrate in the transition alumina. However, the α-crystalline form (α-type) is generally preferred in terms of superior mechanical strength and thermal conductivity. The α-crystalline form is a dense crystalline structure of alumina, and is advantageous for improving the mechanical strength and thermal conductivity of the plate-like alumina. The α-crystallinity ratio is preferably as close to 100% as possible, since the inherent properties of the α-crystal form are more easily exhibited. The α-crystallinity ratio of the plate-like alumina particles is, for example, 90% or more, preferably 95% or more, and more preferably 99% or more.

[0038] 〔silicon〕 The plate-like alumina particles according to the embodiment may contain silicon (Si). The silicon may be derived from silicon or a silicon compound that can be used as a shape control agent. By utilizing these, plate-like alumina particles having excellent brilliance can be produced in the production method described below.

[0039] The plate-like alumina particles according to the embodiment may contain silicon. The plate-like alumina particles according to the embodiment may contain silicon in the surface layer. Here, the "surface layer" refers to a region within 10 nm from the surface of the plate-like alumina particle according to the embodiment. This distance corresponds to the detection depth of the XPS used for measurements in the examples.

[0040] The plate-like alumina particles may have silicon unevenly distributed in the surface layer. Here, "distributed unevenly in the surface layer" refers to a state in which the mass of silicon per unit volume in the surface layer is greater than the mass of silicon per unit volume outside the surface layer. The uneven distribution of silicon in the surface layer can be determined by comparing the results of surface analysis by XPS and overall analysis by XRF.

[0041] The silicon contained in the plate-like alumina particles may be silicon alone or silicon in a silicon compound. The plate-like alumina particles may contain, as silicon or a silicon compound, at least one selected from the group consisting of mullite, Si, SiO2, SiO, and aluminum silicate produced by reaction with alumina, and may contain the above substance in a surface layer. Mullite will be described later.

[0042] When silicon or a silicon compound containing silicon is used as a shape control agent, Si can be detected in the plate-like alumina particles by XRF analysis. The molar ratio of Si to Al [Si] / [Al] obtained by XRF analysis of the plate-like alumina particles is, for example, 0.04 or less, preferably 0.035 or less, and more preferably 0.02 or less. The value of the molar ratio [Si] / [Al] is not particularly limited, but is, for example, 0.003 or more, preferably 0.004 or more, and more preferably 0.005 or more. The molar ratio of Si to Al [Si] / [Al] of the plate-like alumina particles obtained by XRF analysis is, for example, 0.003 to 0.04, preferably 0.004 to 0.035, and more preferably 0.005 to 0.02. Plate-like alumina particles having a molar ratio [Si] / [Al] obtained by XRF analysis within the above ranges satisfy the above-mentioned average particle size, thickness, and aspect ratio values, exhibiting better brilliance and forming a good plate-like shape. Furthermore, deposits are less likely to adhere to the surface of the plate-like alumina particles, resulting in excellent quality. These deposits are thought to be SiO particles, which are believed to be generated due to excess Si when mullite formation on the surface of the plate-like alumina particles reaches saturation.

[0043] The plate-like alumina particles may contain silicon corresponding to the silicon or silicon compound containing elemental silicon used in the production method thereof. The silicon content relative to 100% by mass of the plate-like alumina particles, calculated as silicon dioxide, is preferably 10% by mass or less, more preferably 0.1 to 10% by mass, and even more preferably 0.1 to 5% by mass. When the silicon content is within the above range, the above-mentioned average particle size, thickness, and aspect ratio values ​​are satisfied, the brilliance is more favorable, the plate-like shape is formed well, and deposits that appear to be SiO2 particles are less likely to adhere to the surface of the plate-like alumina particles, resulting in excellent quality.

[0044] (mullite) The plate-like alumina particles of the embodiment may contain mullite. It is presumed that the inclusion of mullite in the surface layer of the plate-like alumina particles improves the selectivity of the inorganic material constituting the inorganic coating portion, thereby enabling the inorganic coating portion to be efficiently formed on the plate-like alumina particles. Mullite, when contained in the surface layer of plate-like alumina particles, significantly reduces the wear of equipment. Mullite, which may be contained in the surface layer of plate-like alumina particles, is a composite oxide of Al and Si. X Si Y O z However, there are no particular limitations on the values ​​of x, y, and z. A more preferred range is Al2SiO5 to Al6SiO 13 In the examples described later, the XRD peak intensity is confirmed for Al. 2.85 SiO 6.3 , Al3SiO 6.5 , Al 3.67 SiO 7.5 , Al4Si1O8, or Al6Si2O 13 The plate-like alumina particles contain Al 2.85 SiO 6.3 , Al3SiO 6.5 , Al 3.67 SiO 7.5 , Al4Si1O8, and Al6Si2O 13 The surface layer may contain at least one compound selected from the group consisting of: wherein the "surface layer" refers to a region within 10 nm from the surface of the plate-like alumina particle. This distance corresponds to the detection depth of the XPS used in the measurements in the examples. The plate-like alumina particles preferably have mullite unevenly distributed in the surface layer, where "distributed unevenly in the surface layer" means that the mass of mullite per unit volume in the surface layer is greater than the mass of mullite per unit volume outside the surface layer.

[0045] The mullite in the surface layer may form a mullite layer, or may be a mixture of mullite and alumina. The interface between the mullite and alumina in the surface layer may be in a state where the mullite and alumina are in physical contact with each other, or the mullite and alumina may form a chemical bond such as Si-O-Al.

[0046] [molybdenum] The plate-like alumina particles of the embodiment may contain molybdenum, and preferably contain molybdenum in the surface layer thereof. The molybdenum may be derived from a molybdenum compound used as a fluxing agent in the method for producing alumina particles described below.

[0047] Molybdenum has catalytic and optical properties, and by utilizing molybdenum, it is possible to produce plate-like alumina particles that are highly crystalline and have excellent luster, despite their plate-like shape, in the production method described below.

[0048] By increasing the amount of molybdenum used, the particle size and the above-mentioned average particle size, thickness, and aspect ratio values ​​are satisfied, and the resulting alumina particles tend to have even better brilliance. Furthermore, the use of molybdenum promotes the formation of mullite, making it possible to produce plate-like alumina particles with a high aspect ratio and excellent dispersibility. Furthermore, the properties of molybdenum contained in the plate-like alumina particles can be utilized to apply them to applications such as oxidation reaction catalysts and optical materials.

[0049] As the molybdenum compound, molybdenum oxide is preferred, and molybdenum trioxide is particularly preferred. Molybdenum oxide has extremely high acidity and reacts in a gaseous state at temperatures above 800°C, so it reacts particularly easily with aluminum oxide as a fluxing agent, making it suitable for suppressing the residue of unreacted raw materials and transition alumina. The molybdenum compound may be contained in the plate-like alumina particles in any of its possible polymorphic forms or in combination thereof, and may be contained in the plate-like alumina particles as α-MoO 3 , β-MoO 3 , MoO 2 , MoO, molybdenum cluster structures, etc.

[0050] The form in which molybdenum is contained is not particularly limited, and it may be contained in a form in which it adheres to the surface of the plate-like alumina particles, or in a form in which it is substituted for part of the aluminum in the crystal structure of the alumina, or a combination thereof.

[0051] The molybdenum content, calculated as molybdenum trioxide, relative to 100% by mass of the plate-like alumina particles as determined by XRF analysis is preferably 10% by mass or less, and by adjusting the firing temperature, firing time, and evaporation rate of the molybdenum compound, the molybdenum content is more preferably 0.001 to 5% by mass, even more preferably 0.01 to 5% by mass, and particularly preferably 0.1 to 3% by mass. A molybdenum content of 10% by mass or less is preferred because it improves the α single crystal quality of the alumina. The molybdenum content can be determined by XRF analysis, which is carried out under the same conditions as those described in the examples below, or under compatible conditions that give the same measurement results.

[0052] The amount of Mo on the surface of the alumina particles can be analyzed using an X-ray photoelectron spectroscopy (XPS) device.

[0053] [Inevitable impurities] The alumina particles may contain unavoidable impurities.

[0054] Inevitable impurities are impurities that originate from metal compounds used in production, are present in raw materials, or are inevitably mixed into alumina particles during the production process. These impurities are essentially unnecessary, but are present in trace amounts and do not affect the properties of the alumina particles.

[0055] The inevitable impurities are not particularly limited, but include magnesium, calcium, strontium, barium, yttrium, lanthanum, cerium, sodium, etc. These inevitable impurities may be contained alone or in combination of two or more.

[0056] The content of unavoidable impurities in the alumina particles is preferably 10,000 ppm or less, more preferably 1,000 ppm or less, and even more preferably 10 to 500 ppm, based on the mass of the alumina particles.

[0057] <Method of manufacturing plate-like alumina particles> The method for producing the plate-like alumina particles is not particularly limited, and known techniques can be applied as appropriate. However, from the viewpoint of being able to suitably control alumina having a high α-crystallization rate at a relatively low temperature, a production method using a flux method utilizing molybdenum oxide is preferably applied.

[0058] A preferred method for producing plate-like alumina particles includes a step of calcining an aluminum compound in the presence of molybdenum oxide, a silicon compound, and, if desired, any additive (e.g., a metal or transition metal additive). The metal or transition metal additive is not particularly limited, and can be, for example, any metal or transition metal in the range of Groups 1 to 13. More preferably, the additive is a compound (A) (hereinafter simply referred to as "compound (A)") containing at least one element (A) selected from the group consisting of iron, nickel, copper, chromium, cobalt, zinc, scandium, and ruthenium, and a titanium compound. More specifically, a preferred method for producing plate-like alumina particles includes the steps of mixing an aluminum compound, a molybdenum compound, a silicon compound, and a metal or transition metal additive to obtain a mixture, and calcining the mixture.

[0059] [Mixing process] The mixing step is a step of mixing a molybdenum oxide, a silicon compound, compound (A), and a titanium compound to obtain a mixture. The contents of the mixture will be explained below.

[0060] (aluminum compounds) The aluminum compound is a raw material for the plate-like alumina particles of this embodiment, and is not particularly limited as long as it becomes alumina upon heat treatment. For example, aluminum chloride, aluminum sulfate, basic aluminum acetate, aluminum hydroxide, boehmite, pseudo-boehmite, transition aluminas (γ-alumina, δ-alumina, θ-alumina, etc.), α-alumina, mixed aluminas having two or more crystal phases, etc. can be used. The physical form of these aluminum compounds as precursors, such as shape, particle size, and specific surface area, is not particularly limited, but aluminum hydroxide is particularly preferred from the viewpoint of reactivity and because the raw material is easily available at low cost.

[0061] According to the flux method described in detail below, the aluminum compound can be suitably used regardless of its shape, for example, spherical, amorphous, high aspect ratio structure (wire, fiber, ribbon, tube, etc.), sheet, etc.

[0062] Similarly, the particle size of the aluminum compound may be preferably from several nm to several hundred μm in the case of the flux method described in detail below.

[0063] The specific surface area of ​​the aluminum compound is not particularly limited. A larger specific surface area is preferable for the molybdenum compound to function more effectively, but by adjusting the firing conditions and the amount of molybdenum compound used, aluminum compounds with any specific surface area can be used as raw materials.

[0064] The aluminum compound may be composed solely of an aluminum compound, or may be a composite of an aluminum compound and an organic compound. For example, an organic / inorganic composite obtained by modifying an aluminum compound with an organosilane, or an aluminum compound composite having a polymer adsorbed thereon, may also be suitably used. When using such a composite, the organic compound content is not particularly limited, but from the viewpoint of efficiently producing plate-like alumina particles, the content is preferably 60% by mass or less, and more preferably 30% by mass or less.

[0065] (Silicon or silicon compounds) The silicon or silicon-containing silicon compound is not particularly limited, and known compounds can be used. Specific examples of silicon or silicon-containing silicon compounds include metallic silicon, organosilanes, silicon resins, silica microparticles, artificially synthesized silicon compounds such as silica gel, mesoporous silica, SiC, and mullite; and natural silicon compounds such as biosilica. Among these, organosilanes, silicon resins, and silica microparticles are preferred from the viewpoint of forming a more uniform composite or mixture with an aluminum compound. Silicon or silicon-containing silicon compounds may be used alone or in combination of two or more. Furthermore, they may be used in combination with other shape control agents, as long as the effects of the present invention are not impaired.

[0066] The shape of silicon or a silicon compound containing elemental silicon is not particularly limited, and for example, spherical, amorphous, high aspect ratio structures (wires, fibers, ribbons, tubes, etc.), sheets, etc. can be suitably used.

[0067] (Molybdenum oxide) As will be described later, molybdenum oxide functions as a fluxing agent in the growth of α-crystals of alumina. Although there are no particular limitations on the molybdenum oxide, molybdenum trioxide is preferred from the viewpoint of reactivity.

[0068] Of the molybdenum oxides mentioned above, molybdenum trioxide is preferred because it is easily vaporized and is cost-effective. Molybdenum trioxide vaporizes at around 800°C, becoming a gas with higher energy than liquid or solid, which promotes plate formation (gelatinization), making it suitable for low-temperature firing. The molybdenum oxides mentioned above may be used alone or in combination of two or more.

[0069] (Compound (A)) From the viewpoints of availability and ease of synthesis, compound (A) is preferably at least one compound selected from the group consisting of iron hydroxide, iron oxide, iron oxyhydroxide, nickel oxide, copper oxide, chromium oxide, cobalt oxide, zinc oxide, scandium oxide, and ruthenium oxide, more preferably at least one compound selected from the group consisting of iron hydroxide, iron oxide, iron oxyhydroxide, nickel oxide, chromium oxide, cobalt oxide, and zinc oxide, and particularly preferably at least one compound selected from the group consisting of iron oxide, iron hydroxide, iron oxyhydroxide, nickel oxide, and chromium oxide.

[0070] (Titanium compounds) As the titanium compound, titanium oxide is preferred from the viewpoints of availability and ease of synthesis.

[0071] The amounts of the aluminum compound, molybdenum oxide, silicon or silicon compound, compound (A), titanium compound, etc. used are not particularly limited. For example, when the total amount of raw materials calculated as oxides is taken as 100 mass %, the following mixture may be fired. 1) An aluminum compound having an Al2O3 content of preferably 80% by mass or more, more preferably 85% by mass or more and 95% by mass or less, and even more preferably 87% by mass or more and 95% by mass or less, Molybdenum oxide is preferably 20% by mass or less, more preferably 2% by mass or more and 18% by mass or less, and even more preferably 5% by mass or more and 15% by mass or less, calculated as MoO3; silicon or silicon compounds in an amount of preferably 0.1 mass % or more and 1.5 mass % or less, more preferably 0.2 mass % or more and less than 1.3 mass %, and even more preferably 0.3 mass % or more and 1.0 mass % or less, calculated as SiO2; Compound (A) is preferably 0.05% by mass or more and 1.5% by mass or less, more preferably 0.07% by mass or more and less than 1.0% by mass, and even more preferably 0.1% by mass or more and 0.5% by mass or less, calculated as an oxide; Preferably, the titanium compound is present in an amount of 0.05% by mass or more and 1.5% by mass or less, more preferably 0.07% by mass or more and less than 1.0% by mass, and even more preferably 0.1% by mass or more and 0.5% by mass or less of a titanium compound; A mixed mixture.

[0072] The above raw material blending (mass %) conditions may be freely combined for each raw material, and the lower and upper limits for each raw material blending (mass %) may also be freely combined.

[0073] By using various compounds within the above ranges, plate-like alumina particles that satisfy the above average particle size, thickness, and aspect ratio values ​​and have excellent brilliance can be easily produced.

[0074] [Firing process] The firing step is a step of firing the mixture obtained in the mixing step. As described above, this manufacturing method is called the flux method.

[0075] The flux method is classified as a solution method. More specifically, it is a crystal growth method that utilizes the eutectic structure of the crystal-flux binary phase diagram. The mechanism of the flux method is presumed to be as follows: When a mixture of solute and flux is heated, the solute and flux become liquid. Because the flux is a flux, or in other words, because the solute-flux binary phase diagram is eutectic, the solute melts below its melting point and forms a liquid phase. When the flux is evaporated in this state, its concentration decreases. In other words, the effect of the flux on lowering the melting point of the solute is reduced, and the evaporation of the flux acts as the driving force for crystal growth of the solute (flux evaporation method). Note that the solute and flux can also be grown by cooling the liquid phase (slow cooling method).

[0076] The flux method has the advantages of allowing crystal growth at temperatures much lower than the melting point, allowing precise control of the crystal structure, and allowing the formation of euhedral plate-shaped crystals.

[0077] The mechanism of α-alumina particle production by the flux method using molybdenum oxide as a flux is not entirely clear, but it is presumed to be, for example, as follows: When an aluminum compound is calcined in the presence of molybdenum oxide, aluminum molybdate is first formed. As can be seen from the above explanation, this aluminum molybdate grows α-alumina crystals at a temperature lower than the melting point of alumina. Then, for example, alumina particles can be obtained by accelerating crystal growth through the decomposition of aluminum molybdate and evaporation of the flux. That is, molybdenum oxide functions as a flux, and α-alumina particles are produced via an intermediate called aluminum molybdate.

[0078] By the flux method, it is possible to produce plate-like alumina particles that satisfy the above-mentioned values ​​of average particle size, thickness, and aspect ratio and have excellent brilliance.

[0079] The calcination method is not particularly limited and can be performed by any known or conventional method. When the calcination temperature exceeds 700°C, the aluminum compound and molybdenum oxide react to form aluminum molybdate. Furthermore, when the calcination temperature exceeds 900°C, the aluminum molybdate decomposes, forming plate-like alumina particles through the action of silicon or a silicon compound. In the plate-like alumina particles, the aluminum molybdate decomposes to form alumina and molybdenum oxide, and the molybdenum compound is presumably incorporated into the aluminum oxide particles. Although the reason for this is not entirely clear, titanium element is incorporated into the alumina during its formation, reducing the energy required for the reaction. Element (A) is also incorporated into the alumina during its formation, accompanied by a change in valence. By maintaining a constant particle energy, the reaction time can be synchronized. Furthermore, when the firing temperature reaches 900°C or higher, in the presence of molybdenum, the crystals of the plate-like alumina particles grow, and Al2O3 and SiO2 on the surface of the plate-like alumina particles react with each other, forming mullite with high efficiency. However, in this embodiment, from the viewpoint of reducing the energy required for production, the firing temperature is preferably 1000° C. or less.

[0080] Furthermore, the state of the aluminum compound, silicon or silicon compound, molybdenum oxide, compound (A), and titanium compound during firing is not particularly limited, as long as they are present in the same space where they can act on the aluminum compound. Specifically, a simple mixer, such as a Henschel mixer, can be used to mix powders of silicon or silicon compound, molybdenum oxide, compound (A), titanium compound, and aluminum compound, and it is desirable that the particle size distribution, specific surface area, bulk density, etc. of the raw materials do not change before and after mixing.

[0081] The firing temperature is not particularly limited and is determined appropriately depending on the above-mentioned average particle size and thickness of the target plate-like alumina particles, aspect ratio, mullite formation, dispersibility, etc. Generally, the minimum firing temperature is preferably 900°C or higher, which is the decomposition temperature of aluminum molybdate (Al2(MoO4)3), and more preferably 900 to 1000°C, at which plate-like alumina with a high aspect ratio can be formed with high efficiency without leaving any unreacted raw materials.

[0082] Generally, to control the shape of α-alumina obtained after firing, it is necessary to perform firing at high temperatures of 2000°C or higher, which is close to the melting point of α-alumina. However, there are significant challenges to industrial use in terms of the burden on the firing furnace and fuel costs.

[0083] The manufacturing method of the embodiment can be carried out even at high temperatures exceeding 2000°C, but even at temperatures below 1000°C, which are significantly lower than the melting point of α-alumina, it is possible to form α-alumina that has a high α-crystallization rate and a high aspect ratio in a plate-like shape regardless of the shape of the precursor.

[0084] According to one embodiment of the present invention, even under conditions where the maximum firing temperature is 900 to 1000°C, plate-like alumina particles having a high aspect ratio and an α-crystallization rate of 90% or more can be formed efficiently at low cost.

[0085] Regarding the calcination time, it is preferable that the time required to raise the temperature to the predetermined maximum temperature is in the range of 15 minutes to 10 hours, and that the time required to maintain the temperature at the maximum calcination temperature is in the range of 5 minutes to 30 hours. In order to efficiently form plate-like alumina particles, it is more preferable that the calcination maintenance time is about 10 minutes to 15 hours. By selecting conditions of a maximum temperature of 900 to 1000°C and a firing holding time of 10 minutes to 15 hours, dense α-crystalline polygonal plate-like alumina particles are easily obtained without aggregation.

[0086] The firing atmosphere is not particularly limited as long as the effects of the present invention can be obtained. For example, an oxygen-containing atmosphere such as air or oxygen, or an inert atmosphere such as nitrogen, argon, or carbon dioxide is preferred, and an air atmosphere is more preferred when cost is taken into consideration.

[0087] The calcination apparatus is not necessarily limited, and a so-called calcination furnace can be used. The calcination furnace is preferably made of a material that does not react with evaporated molybdenum oxide, and it is preferable to use a highly airtight calcination furnace so that molybdenum oxide is efficiently utilized.

[0088] In the flux method using molybdenum oxide, molybdenum oxide reacts with an aluminum compound to form aluminum molybdate. The change in chemical potential during the decomposition of this aluminum molybdate is the driving force for crystallization, resulting in the formation of hexagonal bipyramidal polyhedral particles with well-developed idiomorphic (113) faces. Furthermore, it is presumed that the shape control agent localizes near the particle surface during the α-alumina growth process, significantly inhibiting the growth of the idiomorphic (113) faces. This results in relatively rapid growth of the crystal orientation in the plane direction, leading to the growth of the (001) or (006) face, resulting in the formation of a plate-like morphology. Therefore, the use of a molybdenum compound as a flux agent makes it easier to form molybdenum-containing plate-like alumina particles with a high α-crystallization rate.

[0089] [Cooling process] When a molybdenum compound is used as a fluxing agent, the method for producing alumina particles may include a cooling step. The cooling step is a step of cooling the alumina crystals grown in the firing step. More specifically, the cooling step may be a step of cooling a composition containing the alumina obtained in the firing step and the liquid-phase fluxing agent.

[0090] The cooling rate is not particularly limited, but is preferably 1 to 1000°C / hour, more preferably 5 to 500°C / hour, and even more preferably 50 to 100°C / hour. A cooling rate of 1°C / hour or more is preferable because it can shorten the production time. On the other hand, a cooling rate of 1000°C / hour or less is preferable because the firing container is less likely to crack due to heat shock and can be used for a long time.

[0091] The cooling method is not particularly limited, and may be natural cooling or a cooling device may be used.

[0092] [Post-processing process] The method for producing plate-like alumina particles according to the embodiment may include a post-treatment step. The post-treatment step is a post-treatment step for the plate-like alumina particles, and is a step for removing a fluxing agent. The post-treatment step may be performed after the firing step, after the cooling step, or after the firing step and the cooling step. Furthermore, the post-treatment step may be repeated two or more times as necessary.

[0093] Post-treatment methods include washing and high temperature treatment, which may be performed in combination.

[0094] The washing method is not particularly limited, but the particles can be removed by washing with water, an aqueous ammonia solution, an aqueous sodium hydroxide solution, or an acidic aqueous solution.

[0095] In this case, the molybdenum content can be controlled by appropriately changing the concentrations and amounts of water, aqueous ammonia, aqueous sodium hydroxide, and aqueous acid solution used, as well as the cleaning locations and cleaning times.

[0096] Moreover, examples of the high-temperature treatment method include a method in which the temperature is raised to the sublimation point or boiling point of the flux or higher.

[0097] [Crushing process] In the fired product, the plate-like alumina particles may aggregate and not satisfy the particle size range suitable for the present invention. Therefore, the plate-like alumina particles may be pulverized as necessary to satisfy the particle size range suitable for the present invention. The method for pulverizing the fired product is not particularly limited, and any conventionally known pulverizing method such as a ball mill, jaw crusher, jet mill, disk mill, spectromill, grinder, or mixer mill can be used.

[0098] [Classification process] The plate-like alumina particles are preferably classified to adjust the average particle size, improve the flowability of the powder, or suppress an increase in viscosity when blended with a binder to form a matrix. "Classification" refers to an operation of classifying particles into groups based on their size. The classification may be either wet or dry, but from the viewpoint of productivity, dry classification is preferred. Dry classification includes classification using a sieve and air classification that classifies based on the difference between centrifugal force and fluid drag, but from the viewpoint of classification accuracy, air classification is preferred, and can be performed using a classifier such as an air classifier that utilizes the Coanda effect, a swirling air classifier, a forced vortex centrifugal classifier, or a semi-free vortex centrifugal classifier. The above-mentioned pulverization step and classification step can be carried out at any stage necessary, including before or after the organic compound layer formation step described later. Depending on whether or not pulverization or classification is performed and the conditions for performing them, for example, the average particle size of the obtained plate-like alumina particles can be adjusted.

[0099] The plate-like alumina particles of the embodiment, or the plate-like alumina particles obtained by the manufacturing method of the embodiment, are preferably those with little or no agglomeration, because they are more likely to exhibit their inherent properties, are easier to handle, and have better dispersibility when dispersed in a dispersion medium. In the manufacturing method of plate-like alumina particles, if particles with little or no agglomeration can be obtained without performing the above-mentioned pulverization step or classification step, there is no need to perform the above steps, and plate-like alumina having the desired excellent properties can be manufactured with high productivity, which is preferable. [Example]

[0100] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0101] [Shape analysis of plate-like alumina particles using a scanning electron microscope] Measurements were carried out using a scanning electron microscope SEM-EDS JEOL7000 (manufactured by JEOL Ltd.) under an accelerating voltage condition of 15 kV.

[0102] [Composition analysis of plate-shaped alumina particles using X-ray fluorescence] Using a Primus IV X-ray fluorescence (XRF) analyzer (Rigaku Corporation), approximately 70 mg of the prepared sample was placed on filter paper and covered with PP film for composition analysis. The amounts of various elements obtained from the XRF analysis results were converted into oxide amounts (mass%) to calculate the content of various oxides.

[0103] [Crystalline phase analysis by X-ray diffraction (XRD)] The prepared sample was placed in an appropriate amount in the recess of a 0.5 mm deep measurement sample holder (Rigaku Corporation), and the plate glass was gently pressed and pulled to fill the sample so that the measurement surface was smooth.The sample was then placed in a wide-angle X-ray diffraction (XRD) instrument, Ultima IV (Rigaku Corporation), and measurements were performed using Cu / Kα radiation, 40 kV / 30 mA, a scan speed of 2 degrees / min, and a scan range of 10 to 70 degrees.

[0104] [Particle size distribution measurement] Measurement was performed using a laser diffraction particle size distribution analyzer HELOS (H3355) & RODOS, R3: 0.5 / 0.9-175 μm (manufactured by Nippon Laser Co., Ltd.) under dry conditions of a dispersion pressure of 3 bar and a suction pressure of 90 mbar.

[0105] [Metallic gloss evaluation] In each of the Examples and Comparative Examples, the completion of the reaction was evaluated according to the following criteria. ○: When the powder was directly spread on the skin, a metallic luster-like shine was visually confirmed. ×: When the powder was directly spread on the skin, no metallic luster or shine was visually observed.

[0106] [Evaluation of reaction completion] In each of the Examples and Comparative Examples, the completion of the reaction was evaluated according to the following criteria. ○: Raw material or transition alumina was not confirmed by SEM observation and XRD. ×: Raw material or transition alumina was confirmed by SEM observation and XRD.

[0107] [Example 1] 10 g of aluminum hydroxide (Nippon Light Metal Co., Ltd., average particle size 1.2 μm), 1 g of molybdenum trioxide (Nippon Inorganic Chemical Industry Co., Ltd.), 0.05 g of silicon dioxide (Tosoh Silica Corporation), 0.0115 g of iron hydroxide oxide (Kanto Chemical Co., Ltd.), and 0.0103 g of titanium dioxide (Nippon Aerosil Co., Ltd.) were weighed into a plastic bag and mixed by hand shaking for 5 minutes to obtain a mixture. The resulting mixture was placed in a crucible and heated to 1000 °C at 5 °C / min in a ceramic electric furnace. It was then held at 1000 °C for 10 hours for firing. The temperature was then lowered to room temperature at 5 °C / min, and the crucible was removed, yielding a silvery white powder. The resulting silvery-white powder was then dispersed in 300 mL of 0.25% aqueous ammonia. The dispersion was stirred for 2 hours at room temperature (25-30°C), then filtered through a 106 μm sieve to remove the aqueous ammonia. The remaining molybdenum on the particle surface was removed by washing with water and drying, yielding a silvery-white powder. SEM observation and particle size distribution measurement confirmed that all raw materials had been consumed, and the resulting particles were plate-like alumina particles with an average particle size of 21.6 μm. Furthermore, XRD analysis revealed a sharp scattering peak derived from α-alumina, with no alumina crystalline peaks other than those of the α-crystalline structure. The results are shown in Tables 2 and 3.

[0108] [Examples 2 to 10] Plate-like alumina particles were obtained in the same manner as in Example 1, except that the raw material composition shown in Table 1 was used. The results are shown in Tables 2 and 3.

[0109] [Comparative Example 1] 10 g of aluminum hydroxide (Nippon Light Metal Co., Ltd., average particle size 1.2 μm), 1 g of molybdenum trioxide (Nippon Inorganic Chemical Industry Co., Ltd.), and 0.05 g of silicon dioxide (Tosoh Silica Corporation) were weighed into a plastic bag and mixed by hand shaking for 5 minutes to obtain a mixture. The resulting mixture was placed in a crucible and heated to 1000 °C at 5 °C / min in a ceramic electric furnace. It was then held at 1000 °C for 10 hours for firing. The temperature was then lowered to room temperature at 5 °C / min, and the crucible was removed to obtain a bluish-white powder. The resulting bluish-white powder was then dispersed in 300 mL of 0.25% aqueous ammonia. The dispersion was stirred at room temperature (25–30 °C) for 2 hours, then filtered through a 106 μm sieve to remove the aqueous ammonia. The remaining molybdenum was removed by washing with water and drying, yielding a bluish-white powder. SEM observation and particle size distribution measurement confirmed that the product was a mixture of alumina particles and fine particles of the raw material. Furthermore, XRD measurement revealed sharp scattering peaks due to α-alumina and δ-alumina, confirming that the reaction was not complete. The results are shown in Tables 2 and 3.

[0110] [Comparative Examples 2 to 8] Alumina particles were obtained in the same manner as in Example 1, except that the raw material composition shown in Table 1 was used. The results are shown in Tables 2 and 3.

[0111] [Table 1]

[0112] [Table 2]

[0113] [Table 3]

[0114] From the results shown in Tables 2 and 3, it was confirmed that in Examples 1 to 10, even at a firing temperature of 1000° C., plate-like alumina particles having a lustrous quality similar to that of a metal could be produced.

[0115] Fig. 1 is an SEM image of the plate-like alumina particles obtained in Example 1. Fig. 2 is an SEM image of the plate-like alumina particles obtained in Example 8. Fig. 3 is an SEM image of the alumina particles obtained in Comparative Example 1. Fig. 4 is an SEM image of the alumina particles obtained in Comparative Example 2. 1 to 4, it can be seen that the reaction was completed and only plate-like alumina particles were obtained in Examples 1 and 8. It was also confirmed that when potassium carbonate was used in combination to generate molybdate in the system and use it as a fluxing agent, unreacted raw materials remained.

[0116] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments. Addition, omission, substitution, and other modifications of the configuration are possible within the scope of the spirit of the present invention. The present invention is not limited by the above description, but is limited only by the scope of the appended claims.

Claims

1. Plate-like alumina particles containing titanium and at least one element (A) selected from the group consisting of iron, nickel, copper, chromium, cobalt, and zinc, The element (A) is present in a state in which trivalent aluminum is substituted with the element (A) in the corundum structure of the plate-like alumina particles.

2. The plate-like alumina particles according to claim 1, comprising the at least one element (A) in an amount of 0.02 to 2 mass % in terms of oxide.

3. 3. The plate-like alumina particles according to claim 1, which contain 0.1 to 10 mass % of silicon or a silicon compound in terms of silicon dioxide.

4. The plate-like alumina particles according to claim 3, which contain silicon or a silicon compound in the surface layer.

5. 3. The plate-like alumina particles according to claim 1, having a thickness of 0.05 to 1 μm, an average particle diameter of 1 to 30 μm, and an aspect ratio of 5 to 300.

6. a step of mixing an aluminum compound, a molybdenum oxide, silicon or a silicon compound, a compound (A) containing at least one element (A) selected from the group consisting of iron, nickel, copper, chromium, cobalt, and zinc, and a titanium compound to obtain a mixture; calcining the mixture; The method for producing plate-like alumina particles according to claim 1 or 2, comprising:

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