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

Abstract

Provided are plate-like alumina particles which each contain elemental titanium and at least one element (A) that is selected from the group consisting of elemental iron, elemental nickel, elemental copper, elemental chromium, elemental cobalt, elemental zinc, elemental scandium, and elemental ruthenium.

Description

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

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

[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 the raw material. 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 problems of aggregation, such as multiple overlapping particles and twin crystal formation.

[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 obtained 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 obtained 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.

[0008] Japanese Patent Publication No. 35-6977 Publication No. 9-77512 Publication of Special Publication No. 2008-534417 Publication of Special Publication No. 2010-502539 Publication of Special Publication No. 2017-516734

[0009] However, the plate alumina obtained by these methods requires a firing process at temperatures of 1100°C or higher, which requires a huge amount of energy for production. Recently, there has been a growing movement to reduce 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.

[0011] The present invention encompasses 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) Plate-like alumina particles according to (1) above, containing 0.02 to 2 mass% of the at least one element (A) in terms of oxide. (3) Plate-like alumina particles according to (1) or (2) above, containing 0.1 to 10 mass% of silicon or a silicon compound in terms of silicon dioxide. (4) Plate-like alumina particles according to any one of (1) to (3) above, containing silicon or a silicon compound in a surface layer. (5) 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.

[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.

[0013] 1 is a SEM image of plate-like alumina particles obtained in Example 1. FIG. 2 is a SEM image of plate-like alumina particles obtained in Example 8. FIG. 3 is a SEM image of alumina particles obtained in Comparative Example 1. FIG. 4 is a SEM image of alumina particles obtained in Comparative Example 2. FIG. 5 is a SEM image of alumina particles obtained in Comparative Example 6. FIG. 6 is a SEM image of alumina particles obtained in Comparative Example 8.

[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-like alumina particles> The plate-like alumina particles according to this embodiment contain at least one element (A) (hereinafter also 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 the 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 element selected from the group consisting of iron, nickel, copper, chromium, cobalt, zinc, scandium, and ruthenium, preferably at least one element selected from the group consisting of iron, nickel, copper, chromium, cobalt, and zinc, and more preferably at least one element 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) in terms of 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, making it easier to achieve a uniform particle size. Furthermore, when the content is equal to or less than the above-mentioned upper limit, the plate-like alumina particles are prevented from exhibiting a dark color, making them suitable for use as a luminescent pigment or 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) is present 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, and therefore, it is presumed that the element (A) exists as a divalent element.

[0022] (Titanium element) In the present embodiment, the titanium element is preferably derived from titanium oxide from the viewpoints 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, calculated as oxide, more preferably 0.04 to 1.5 mass %, and even more preferably 0.06 to 1 mass %. When the titanium element content is within the above-mentioned preferred range, the plate-like alumina particles can be produced at a lower firing temperature (e.g., 1000°C or lower) than conventional. Furthermore, when the titanium element content is equal to or lower than the upper limit, the plate-like alumina particles are prevented from turning deep yellow, making them suitable for use as a luminescent pigment or cosmetic.

[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 calculating 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 calculated in terms of oxide (mass %) by comparing the intensity with that of a sample with a known content (calibration curve sample) using 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 presumed that when the divalent element (A) and tetravalent titanium element are added simultaneously during firing, two adjacent aluminum atoms in the crystal structure of the plate-like alumina particles are substituted with one element (A) and one titanium element, respectively, so as to compensate for each other's charges, thereby changing the light absorption and reflection of the resulting plate-like alumina particles and giving the particles a metallic silver color 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 plate-like alumina particles preferably have a thickness of 0.05 μm to 1 μm, more preferably 0.1 μm to 1 μm, and even more preferably 0.1 μm to 0.8 μm, because alumina particles having the above thicknesses have a high aspect ratio and are excellent in brightness and mechanical strength.

[0029] The plate-like alumina particles have an average particle diameter (D ５０ The average particle diameter (D) 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. ５０Alumina particles having an average particle diameter (D) of not more than the above upper limit have a large light reflecting surface area and are therefore particularly excellent in 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 two-dimensional blending characteristics can be obtained, and when the aspect ratio of the plate-like alumina particles is 500 or less, it is preferable because excellent mechanical strength can be obtained. When the aspect ratio is 15 or more, it is preferable because high brightness can be obtained 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 from the viewpoint of ease of handling and production, the particle shape is preferably, for example, polygonal or circular.

[0033] The plate-like alumina particles may be obtained by any production method. However, in terms of a higher aspect ratio, better dispersibility, and higher productivity, they are preferably obtained by calcining an aluminum compound in the presence of a molybdenum compound (particularly preferably molybdenum trioxide) and a shape control agent. 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, as described below. In the above production method, the 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 calcination process, the molybdenum compound reacts with the aluminum compound at high temperature to form aluminum molybdate, and then, when this aluminum molybdate further decomposes into alumina and molybdenum oxide at a higher temperature, the molybdenum compound is incorporated into the plate-like alumina particles. Molybdenum oxide vaporizes and solidifies again upon cooling, which can be recovered and reused. When the plate-like alumina particles contain mullite in their surface layers, it is believed that during this process, silicon or a compound containing silicon atoms, which is blended as a shape control agent, reacts with an aluminum compound via molybdenum, forming mullite in the surface layers of the plate-like alumina particles. More specifically, the mechanism by which mullite is formed is believed to be that, 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 not incorporated into the plate-like alumina particles is preferably recovered by evaporation and reused. This reduces the amount of molybdenum oxide adhering to the surface of the plate-like alumina, and when the alumina is dispersed in a dispersion medium such as an organic binder like a resin or an inorganic binder like glass, the molybdenum oxide does not get mixed into the binder, making it possible to maximize the inherent properties of the plate-like alumina.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-like alumina particles have a density of, for example, 3.70 g / cm ３ 4.10g / cm or more ３ or less, and the density is 3.72 g / cm ３ 4.10g / cm or more ３ It is preferable that the density is 3.80 g / cm or less. ３ 4.10g / cm or more ３ The density can be measured using a dry automatic density meter, Accupyc II1330, manufactured by Micromeritics, at a measurement temperature of 25°C using helium as a carrier gas after pre-treating the plate-like 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, κ, etc., or may contain alumina hydrate in the transition alumina. However, the α-crystal form (α-type) is essentially preferred in terms of superior mechanical strength or thermal conductivity. The α-crystal form is a dense crystalline structure of alumina, which is advantageous for improving the mechanical strength or thermal conductivity of the plate-like alumina. The α-crystallization rate is preferably as close to 100% as possible, as this makes it easier to exhibit the inherent properties of the α-crystal form. The α-crystallization rate 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 of 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 a surface layer. Here, the "surface layer" refers to a region within 10 nm from the surface of the plate-like alumina particles according to the embodiment. This distance corresponds to the detection depth of the XPS used for measurement 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 silicon or a silicon compound such as mullite, Si, SiO ２, SiO, and aluminum silicate formed by reaction with alumina, and the surface layer may contain the above-mentioned substance. 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] obtained by XRF analysis of the plate-like alumina particles is, for example, 0.003 to 0.04 or less, preferably 0.004 to 0.035 or less, and more preferably 0.005 to 0.02 or less. The plate-like alumina particles having a molar ratio [Si] / [Al] within the above range obtained by the XRF analysis satisfy the above average particle size, thickness, and aspect ratio values, and have a more preferable brilliance and 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. The deposits include SiO ２ These particles are thought to be generated due to excess Si when the generation of mullite on the surface layer 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 manufacturing method. The silicon content relative to 100 mass% of the plate-like alumina particles is preferably 10 mass% or less, more preferably 0.1 to 10 mass%, and even more preferably 0.1 to 5 mass%, calculated as silicon dioxide. When the silicon content is within the above range, the above-mentioned values ​​of average particle size, thickness, and aspect ratio are satisfied, the brilliance is more preferable, and the plate-like shape is well formed. In addition, SiO ２The quality is excellent because deposits that appear to be particles are less likely to adhere to the surface of the plate-like alumina particles.

[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, and allows the inorganic coating portion to be efficiently formed on the plate-like alumina particles. The inclusion of mullite in the surface layer of the plate-like alumina particles results in a significant reduction in wear of the equipment. The "mullite" that the plate-like alumina particles may contain in the surface layer is a composite oxide of Al and Si, and Al Ｘ Si Ｙ O ｚ However, there is no particular limitation on the values ​​of x, y, and z. A more preferable range is Al ２ Si １ O ５ ~Al ６ Si ２ O １３ In the examples described later, the XRD peak intensity is confirmed for Al. ２．８５ Si １ O ６．３ , Al ３ Si １ O ６．５ , Al ３．６７ Si １ O ７．５ , Al ４ Si １ O ８ , or Al ６ Si ２ O １３ The plate-like alumina particles contain Al ２．８５ Si １ O ６．３ , Al ３ Si １ O ６．５ , Al ３．６７ Si １ O ７．５ , Al ４ Si １ O ８ , and Al ６ Si ２ O １３The 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 for measurements in the examples. The plate-like alumina particle preferably has mullite unevenly distributed in the surface layer. Here, "unevenly distributed in the surface layer" refers to a state in which 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. Preferably, the plate-like alumina particles contain molybdenum in their surface layers. The molybdenum may be derived from a molybdenum compound used as a fluxing agent in the alumina particle manufacturing method 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 remaining unreacted raw materials and transition alumina. The molybdenum compound may be contained in the plate-like alumina particles in any of its possible polymorphs or in combination, and may be α-MoO ３ , β-MoO ３ , MoO ２ , MoO, molybdenum cluster structure, etc. may be contained in the plate-like alumina particles.

[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 substitutes 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 obtained by XRF analysis is preferably 10% by mass or less. 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. XRF analysis should be performed under the same conditions as those described in the examples below, or under compatible conditions that yield 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 inevitable 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 examples thereof 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 for producing plate-like alumina particles> The method for producing 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 flux method using molybdenum oxide can be 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, optionally, an 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 from Groups 1 to 13. More preferably, it 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 a step of mixing an aluminum compound, a molybdenum compound, a silicon compound, and a metal or transition metal additive to obtain a mixture, and a step of calcining the mixture.

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

[0060] (Aluminum Compound) 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 alumina (γ-alumina, δ-alumina, θ-alumina, etc.), α-alumina, mixed alumina 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 in the range of several nm to several hundred μm, as suitably used in 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 content of the organic compound 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 compound) 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 artificially synthesized silicon compounds such as metallic silicon, organosilanes, silicon resins, silica microparticles, silica gel, mesoporous silica, SiC, and mullite; and natural silicon compounds such as biosilica. Among these, organosilanes, silicon resins, and silica microparticles are preferably used from the viewpoint of forming a more uniform composite or mixture with the aluminum compound. The silicon or silicon-containing silicon compound may be used alone or in combination of two or more. Furthermore, it may be used in combination with other shape control agents as long as the effect of the present invention is 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] Among the molybdenum oxides described above, molybdenum trioxide is preferably used 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 described 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 Compound) 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, but for example, when the total amount of raw materials calculated as oxides is taken as 100 mass %, the following mixture may be fired. ２ O ３ In terms of the total mass of the aluminum compound, 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, MoO ３ molybdenum oxide, in terms of SiO, 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; ２a mixture obtained by mixing, in oxide equivalent, preferably 0.1 mass % or more and 1.5 mass % or less of silicon or silicon compounds, more preferably 0.2 mass % or more and less than 1.3 mass % of silicon or silicon compounds, even more preferably 0.3 mass % or more and 1.0 mass % or less of silicon or silicon compounds; a mixture obtained by mixing, in oxide equivalent, preferably 0.05 mass % or more and 1.5 mass % or less of compound (A), more preferably 0.07 mass % or more and less than 1.0 mass % of compound (A), even more preferably 0.1 mass % or more and 0.5 mass % or less of compound (A); and a mixture obtained by mixing, in oxide equivalent, preferably 0.05 mass % or more and 1.5 mass % or less of titanium compounds, more preferably 0.07 mass % or more and less than 1.0 mass % of titanium compounds, even more preferably 0.1 mass % or more and 0.5 mass % or less of titanium compounds.

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

[0075] The flux method is classified as a solution method. More specifically, it is a crystal growth method that takes advantage of 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 at a temperature lower than 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 a driving force for the crystal growth of the solute (flux evaporation method). Note that the crystal growth of the solute can also be induced by cooling the liquid phase of the solute and flux (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 by which α-alumina particles are produced by a flux method using molybdenum oxide as a flux is not entirely clear, but it is presumed to be, for example, based on the following mechanism. That is, when an aluminum compound is calcined in the presence of molybdenum oxide, aluminum molybdate is first formed. As can be understood 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, evaporation of the flux, etc. 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 commonly used 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, and plate-like alumina particles are formed by the action of silicon or a silicon compound. In addition, in the plate-like alumina particles, when the aluminum molybdate decomposes to form alumina and molybdenum oxide, the molybdenum compound is presumably incorporated into the aluminum oxide particles. Although the reason for this is not entirely clear, the incorporation of titanium element into the alumina during the formation reduces the energy required for the reaction to proceed, and element (A) is also incorporated into the alumina during the formation with a change in valence, thereby maintaining a constant energy of the particles and thereby aligning the reaction time. Furthermore, when the calcination temperature exceeds 900°C, in the presence of molybdenum, the Al on the surface of the plate-like alumina particles grows as the crystals of the plate-like alumina particles grow. ２ O ３ and SiO ２ 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] There is no particular limitation on the firing temperature conditions, and the firing temperature is appropriately determined depending on the above-mentioned average particle size and thickness of the target plate-like alumina particles, aspect ratio, formation of mullite, dispersibility, etc. Usually, the lowest firing temperature is 1000 K. ２(MoO ４ ) ３ The temperature is preferably 900°C or higher, which is the decomposition temperature of ), and more preferably 900 to 1000°C, at which plate-shaped alumina having a high aspect ratio can be formed with high efficiency without leaving any unreacted raw material.

[0082] Generally, in order to control the shape of α-alumina obtained after calcination, it is necessary to perform calcination at a high temperature of 2000°C or higher, which is close to the melting point of α-alumina. However, there are significant challenges to industrial application in terms of the burden on the calcination 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 of 1000°C or less, 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 plate-like shape with a high aspect ratio, 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 firing time, it is preferable that the time required for raising the temperature to the predetermined maximum temperature is in the range of 15 minutes to 10 hours, and that the holding time at the maximum firing temperature is in the range of 5 minutes to 30 hours. To efficiently form plate-like alumina particles, a firing holding time of about 10 minutes to 15 hours is more preferable. By selecting conditions of a maximum temperature of 900 to 1000°C and a firing holding time of 10 minutes to 15 hours, dense polygonal plate-like alumina particles of α crystal form are less likely to aggregate and can be easily obtained.

[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, and then the change in chemical potential during the decomposition of this aluminum molybdate serves as the driving force for crystallization, resulting in the formation of hexagonal bipyramidal polyhedral particles with well-developed idiomorphic (113) planes. 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) planes, resulting in relatively rapid growth of the crystal orientation in the plane direction, leading to the growth of the (001) or (006) planes and the formation of a plate-like morphology. Therefore, by using a molybdenum compound as a fluxing agent, it is possible to more easily form molybdenum-containing plate-like alumina particles with a high α-crystallization rate.

[0089] [Cooling Step] 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 by 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 manufacturing 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-treatment step] 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] [Pulverization step] The calcined product may have agglomerated plate-like alumina particles that do not satisfy the particle size range suitable for the present invention. Therefore, the plate-like alumina particles may be pulverized, if necessary, to satisfy the particle size range suitable for the present invention. The method for pulverizing the calcined product is not particularly limited, and conventionally known pulverization methods such as a ball mill, jaw crusher, jet mill, disk mill, spectromill, grinder, and mixer mill can be used.

[0098] [Classification Process] The plate-like alumina particles are preferably classified to adjust the average particle size, improve the powder fluidity, or suppress an increase in viscosity when blended with a binder to form a matrix. "Classification process" refers to the process of grouping particles according to particle size. Classification can be performed using either wet or dry methods, but dry classification is preferred from the perspective of productivity. Dry classification includes sieving and air classification, which classifies particles based on the difference between centrifugal force and fluid drag. Air classification is preferred from the perspective of classification accuracy, and can be performed using a classifier such as an air classifier utilizing the Coanda effect, a swirling air classifier, a forced vortex centrifugal classifier, or a semi-free vortex centrifugal classifier. The above-mentioned pulverization and classification processes can be performed at any necessary stage, including before or after the organic compound layer formation process described below. The average particle size of the resulting plate-like alumina particles can be adjusted, for example, by selecting whether or not to perform pulverization or classification and the conditions for each.

[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.

[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 by Scanning Electron Microscope] Measurement was carried out using a scanning electron microscope SEM-EDS JEOL7000 (manufactured by JEOL Ltd.) under an acceleration voltage condition of 15 kV.

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

[0103] [Analysis of Crystalline Phase by X-ray Diffraction (XRD)] An appropriate amount of the prepared sample was placed in a recess of a 0.5 mm deep measurement sample holder (manufactured by 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 set in a wide-angle X-ray diffraction (XRD) apparatus Ultima IV (manufactured by Rigaku Corporation) and measured under the conditions of 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 carried out 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] [Evaluation of metallic luster] In each example and comparative example, the completion of the reaction was evaluated according to the following criteria: ∘: When the powder was directly spread and applied to the skin, a metallic luster-like brilliance was visually confirmed. ×: When the powder was directly spread and applied to the skin, a metallic luster-like brilliance was not visually confirmed.

[0106] [Evaluation of Reaction Completion] In each Example and Comparative Example, the completion of the reaction was evaluated according to the following criteria: ∘: Raw materials or transition alumina were not confirmed by SEM observation and XRD; ×: Raw materials or transition alumina were confirmed by SEM observation and XRD.

[0107] [Example 1] 10 g of aluminum hydroxide (manufactured by Nippon Light Metal Co., Ltd., average particle size 1.2 μm), 1 g of molybdenum trioxide (manufactured by Nippon Inorganic Chemical Industry Co., Ltd.), 0.05 g of silicon dioxide (manufactured by Tosoh Silica Corporation), 0.0115 g of iron oxide hydroxide (manufactured by Kanto Chemical), and 0.0103 g of titanium oxide (manufactured by 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, and then held at 1000 ° C. for 10 hours to perform firing. After that, the temperature was lowered to room temperature at 5 ° C. / min, and the crucible was removed to obtain 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, confirming that the 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 compositions shown in Table 1 were used. The results are shown in Tables 2 and 3.

[0109] Comparative Example 1: 10 g of aluminum hydroxide (manufactured by Nippon Light Metal Co., Ltd., average particle size 1.2 μm), 1 g of molybdenum trioxide (manufactured by Nippon Inorganic Chemical Industry Co., Ltd.), and 0.05 g of silicon dioxide (manufactured by 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 a rate of 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 a rate of 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, and then filtered through a 106 μm sieve to remove the aqueous ammonia. The resulting mixture was then washed with water and dried to remove the molybdenum remaining on the particle surface, 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 compositions shown in Table 1 were used. The results are shown in Tables 2 and 3.

[0111]

[0112]

[0113]

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

[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. From the results shown in FIGS. 1 to 4, it can be confirmed that the reaction was completed in Examples 1 and 8, and only plate-like alumina particles were obtained. It was 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 and the like 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-shaped alumina particles containing at least one element (A) selected from the group consisting of iron element, nickel element, copper element, chromium element, cobalt element, zinc element, scandium element, and ruthenium element, and titanium element.

2. The plate-shaped alumina particles according to claim 1, containing 0.02 to 2% by mass of the at least one element (A) in terms of oxide conversion.

3. The plate-shaped alumina particles according to claim 1 or 2, containing 0.1 to 10% by mass of silicon or a silicon compound in terms of silicon dioxide conversion.

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

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

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