JP7736220B1 - Plate-like alumina particles and method for producing plate-like alumina particles - Google Patents

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

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.

Alumina particles, an inorganic filler, are used in a variety of applications. In particular, plate-shaped alumina particles are used in a wide range of applications, including as thermally conductive fillers, high-brightness pigments, cosmetics, abrasives, conductive powder base materials, and lubricants for resin films. There is a particular demand for plate-shaped alumina particles with high aspect ratios that exhibit low agglomeration and high dispersion.

Various methods for producing plate-like alumina particles have been known for some time. 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 materials. However, these methods make it difficult to control particle size, particularly when producing plate-like alumina with a high aspect ratio. Furthermore, the resulting plate-like alumina particles can overlap or form twin crystals, resulting in problems with aggregation.

In recent years, there have been various reports on the production of plate-like alumina particles with controlled size, shape, etc. For example, Patent Document 2 reports the production of flaky aluminum oxide containing titanium oxide with an average particle size of 5 to 60 μm, a thickness of 1 μm or less, and an aspect ratio of 20 or more, by using titanium oxide as a crystallization control agent and firing at a temperature of 1,100°C or higher in the presence of sulfate, a high-temperature flux.

Patent document 3 reports that zinc oxide is used as a crystallization control agent and fired at a temperature of 1,150°C in the presence of a sulfate, a high-temperature flux, to produce plate-like α-alumina particles containing zinc oxide with 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.

Patent document 4 reports that by using zinc oxide and tin oxide as crystallization control agents and firing at a temperature of 1200°C in the presence of a high-temperature flux, sulfate, plate-like α-alumina particles containing zinc oxide and tin oxide with 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.

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 sulfate, a high-temperature flux, plate-shaped α-alumina particles with 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 produced.

Special Publication No. 35-6977 Japanese Patent Application Publication No. 9-77512 Special Publication No. 2008-534417 Special Publication No. 2010-502539 Special Publication No. 2017-516734

However, the plate alumina obtained using these methods requires a firing process at temperatures of over 1,100°C, which requires a huge amount of energy to produce. Recently, there has been a growing movement to reduce energy used in manufacturing in order to achieve a sustainable society, such as carbon neutrality and the SDGs, and there is a demand for lower firing temperatures.

The present invention was made in consideration of the above circumstances, and its objective is to provide plate-like alumina particles that can be produced at a lower firing temperature than conventional methods, and a method for producing such plate-like alumina particles.

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) Plate-like alumina particles according to (1) or (2) above, containing silicon or a silicon compound in an amount of 0.1 to 10 mass % in terms of silicon dioxide.
(4) 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) 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.

The present invention provides plate-like alumina particles that can be produced at a lower firing temperature than conventional methods, as well as a method for producing such plate-like alumina particles.

1 is an SEM observation image of the plate-like alumina particles obtained in Example 1. 1 is an SEM observation image of the plate-like alumina particles obtained in Example 8. 1 is a SEM observation image of alumina particles obtained in Comparative Example 1. 1 is a SEM observation image of alumina particles obtained in Comparative Example 2. 1 is a SEM observation image of alumina particles obtained in Comparative Example 6. 1 is a SEM observation image of alumina particles obtained in Comparative Example 8.

The following provides a detailed description of the plate-like alumina particles and the method for producing plate-like alumina particles according to one embodiment of the present invention.

<Plate-shaped 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.

In this embodiment, the term "plate-like" refers to an aspect ratio, calculated by dividing the average particle diameter of alumina particles by their thickness, of 2 or more. 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). The "average particle diameter of alumina particles" refers to the value calculated as the volume-based median diameter _D50 from the volume-based cumulative particle size distribution measured by a laser diffraction particle size analyzer.

(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.

In this embodiment, from the standpoint of ease of availability and ease of synthesis, 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.

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, and are therefore suitable for use as a luminous pigment or a cosmetic.

In this embodiment, the content of element (A) is determined by subjecting the plate-like alumina particles to XRF (X-ray fluorescence) analysis and calculating the total content of each element constituting element (A) in terms of oxide (mass%) from a previously determined oxide calibration curve. Alternatively, the content of each element contained 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.

The element (A) exists in a so-called doped state, where the trivalent aluminum element is replaced by 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 maintain the charge of the entire crystal neutral, so it is presumed that the element (A) exists as a divalent element.

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

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 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 a cosmetic.

In this embodiment, the titanium element content is determined by subjecting the plate-like alumina particles to XRF (X-ray fluorescence) analysis and calculating the total titanium element content, expressed as oxide (mass %), from a previously determined oxide calibration curve. Alternatively, the titanium element content in the plate-like alumina particles can be calculated and calculated as 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.

Titanium element exists in a so-called doped state, in which trivalent aluminum element is replaced by tetravalent titanium element in the corundum structure of the plate-shaped alumina particles.

Although the exact principle is unknown, it is thought 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, 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.

For alumina particles, the thickness, particle size, and aspect ratio conditions shown below can be combined in any way as long as they are plate-shaped. Furthermore, the upper and lower limits of the numerical ranges exemplified for these conditions can be freely combined.

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.

The average particle diameter (D ₅₀ ) of the plate-like alumina particles 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.
Alumina particles having an average particle diameter ( _D50 ) equal to or greater than the above lower limit have a large light reflecting surface area and therefore have particularly excellent brilliance. Furthermore, alumina particles having an average particle diameter ( _D50 ) equal to or less than the above upper limit are suitable for use as a filler.

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. Plate-like alumina particles with an aspect ratio of 2 or more are preferred because they can have two-dimensional blending properties, and plate-like alumina particles with an aspect ratio of 500 or less are preferred because they have excellent mechanical strength. An aspect ratio of 15 or more is preferred because they will have high brightness when used as a pigment.

In this embodiment, it is preferable that the plate-shaped alumina particles 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.

The plate-shaped alumina particles may be polygonal, circular, or elliptical, but a particle shape such as a polygonal or circular plate is preferred from the standpoint of ease of handling and manufacturing.

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-controlling agent, in terms of a higher aspect ratio, better dispersibility, and better productivity. The shape-controlling 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.
It is believed that when the plate-like alumina particles contain mullite in the surface layer, the silicon or silicon atom-containing compound blended as a shape control agent reacts with the aluminum compound via molybdenum during this process, forming mullite in the surface layer of the plate-like alumina particles. 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.

By utilizing molybdenum and a shape control agent in the production of the plate-shaped alumina particles, the alumina particles have a high alpha crystallinity and are idiomorphic, thereby achieving excellent dispersibility, mechanical strength, and high thermal conductivity.

When 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 shape control agent used, but particularly by the proportion of silicon or a silicon-containing silicon compound 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 below.

The plate-like alumina particles have a density of, for example, 3.70 g/cm ³ or more and 4.10 g/cm ³ or less, preferably 3.72 g/cm ³ or more and 4.10 g/cm ³ or less, and more preferably 3.80 g/cm ³ or more and 4.10 g/cm ³ or less.
The density can be measured using a Micromeritics dry automatic density meter, Accupyc II1330, at a measurement temperature of 25°C using helium as a carrier gas after pretreating the plate-like alumina particles at 300°C for 3 hours.

[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.

〔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.

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 measurement in the examples.

The plate-like alumina particles may have silicon unevenly distributed in the surface layer. "Distributed unevenly in the surface layer" here 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.

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, SiO ₂ , SiO, and aluminum silicate produced by reaction with alumina, and may contain the above substance in a surface layer. Mullite will be described later.

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 _SiO2 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.

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 mass% of the plate-like alumina particles, calculated as silicon dioxide, is preferably 10 mass% or less, more preferably 0.1 to 10 mass%, and even more preferably 0.1 to 5 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 thought to be _SiO2 particles are less likely to adhere to the surface of the plate-like alumina particles, resulting in excellent quality.

(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 the plate-like alumina particles, significantly reduces wear of equipment. The "mullite" that may be contained in the surface layer _of the plate- _{like alumina particles is a composite oxide of Al and Si and is expressed as AlXSiYOz, with no particular limitations on the values of x, y, and z. A more preferred range is Al2Si1O5 to Al6Si2O13 . Note that the XRD peak intensities confirmed in the examples} described below are those containing _{Al2.85Si1O6.3} , _Al3Si1O6.5 , _{Al3.67Si1O7.5 , Al4Si1O8 , or Al6Si2O13 .} The plate- _like alumina particles may contain at least one compound selected from the group consisting _{of Al2.85Si1O6.3} , _{Al3Si1O6.5 , Al3.67Si1O7.5 , Al4Si1O8 ,} and _Al6Si2O13 in _their surface layer. Here, 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 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.

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.

[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.

Molybdenum has catalytic and optical properties. Furthermore, 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, using the manufacturing method described below.

By using a larger amount of molybdenum, the particle size and the above-mentioned average particle size, thickness, and aspect ratio values are met, 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 may enable their application as oxidation reaction catalysts and optical materials.

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, and may be contained in the plate-like alumina particles as α-MoO ₃ , β-MoO ₃ , MoO ₂ , MoO, molybdenum cluster structures, etc.

The form in which molybdenum is contained is not particularly limited, and it may be contained in a form in which it is attached 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 alumina crystal structure, or a combination of these.

The molybdenum content relative to 100% by mass of the plate-like alumina particles, as determined by XRF analysis, is preferably 10% by mass or less, calculated as molybdenum trioxide, 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.

In addition, the amount of Mo on the surface of alumina particles can be analyzed using an X-ray photoelectron spectroscopy (XPS) device.

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

Inevitable impurities are impurities that originate from metal compounds used in manufacturing, are present in raw materials, or are inevitably mixed into alumina particles during the manufacturing process.They are not actually necessary, but are present in trace amounts and do not affect the properties of the alumina particles.

Inevitable impurities include, but are not limited to, magnesium, calcium, strontium, barium, yttrium, lanthanum, cerium, sodium, etc. These inevitable impurities may be present alone or in combination of two or more.

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, relative to the mass of the alumina particles.

<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.

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 may be, for example, any metal or transition metal in the range of Groups 1 to 13, and is more preferably 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.

[Mixing process]
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.

(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 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.

According to the flux method described in detail below, the aluminum compound can be suitably used in any shape, such as spherical, amorphous, high aspect ratio structures (wires, fibers, ribbons, tubes, etc.), or sheets.

Similarly, the particle size of the aluminum compound can be suitably determined using the flux method described in detail below, with solid aluminum compounds ranging from a few nm to several hundred μm.

There are no particular restrictions on the specific surface area of the aluminum compound. 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, any specific surface area can be used as a raw material.

The aluminum compound may consist solely of the aluminum compound, or may be a composite of the aluminum compound and an organic compound. For example, organic/inorganic composites obtained by modifying an aluminum compound with an organosilane, or aluminum compound composites with polymer adsorbed thereon, can also be used suitably. When using these composites, there are no particular restrictions on the organic compound content, but from the perspective of efficiently producing plate-like alumina particles, the content is preferably 60% by mass or less, and more preferably 30% by mass or less.

(Silicon or silicon compounds)
The silicon or silicon compound containing silicon element is not particularly limited, and known compounds can be used. Specific examples of silicon or silicon compound containing silicon element 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 an aluminum compound. Silicon or silicon compounds containing silicon element 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.

The shape of silicon or silicon compounds 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.

(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.

Of the molybdenum oxides listed 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 (alpha conversion), making it suitable for low-temperature firing. The molybdenum oxides listed above may be used alone or in combination of two or more.

(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.

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

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 in an amount 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, calculated as Al 2 O 3;
Molybdenum oxide, calculated as _MoO3 , 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,
In terms of _SiO2 , 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, and even more preferably 0.3 mass% or more and 1.0 mass% or less of silicon or silicon compounds;
The compound (A) is present in an amount of 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.

The above raw material composition (mass%) conditions can be freely combined for each raw material, and the lower and upper limits for each raw material composition (mass%) can also be freely combined.

By using various compounds within the above ranges, it is possible to easily produce plate-shaped alumina particles that meet the above average particle size, thickness, and aspect ratio values and have excellent brilliance.

[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.

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, 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, the flux 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 solute and flux can also be grown by cooling the liquid phase (slow cooling method).

The flux method has advantages such as the ability to grow crystals at temperatures much lower than the melting point, the ability to precisely control the crystal structure, and the ability to form idiomorphic plate-shaped crystals.

The mechanism by which α-alumina particles are produced using the flux method, which uses molybdenum oxide as a flux, is not entirely clear, but it is believed to be based on the following mechanism, for example. Specifically, 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, through, for example, the decomposition of aluminum molybdate and the evaporation of the flux, crystal growth is accelerated, resulting in the production of alumina particles. In other words, molybdenum oxide functions as a flux, and α-alumina particles are produced via an intermediate called aluminum molybdate.

The above flux method makes it possible to produce plate-shaped alumina particles that meet the above average particle size, thickness, and aspect ratio values and have excellent brilliance.

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 and forms plate-like alumina particles due to the action of silicon or a silicon compound. In addition, in the plate-like alumina particles, it is thought that when the aluminum molybdate decomposes to form alumina and molybdenum oxide, the molybdenum compound is incorporated into the aluminum oxide particles. Furthermore, although the reason for this is not entirely clear, the incorporation of titanium element into the alumina during production reduces the energy required for reaction progression, and element (A) is also incorporated into the alumina during production with a change in valence, and by maintaining the energy possessed by the particles constant, the reaction time can be synchronized.
Furthermore, when the firing temperature is 900° C. or higher, in the presence of molybdenum, the crystals of the plate-like alumina particles grow, and Al ₂ O ₃ and SiO ₂ on the surfaces 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.

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. 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.

There are no particular limitations 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, mullite formation, dispersibility, etc. Generally, the minimum firing temperature is preferably 900°C or higher, which is the decomposition temperature of aluminum molybdate (Al ₂ (MoO ₄ ) 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.

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

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 is significantly lower than the melting point of alpha-alumina, it is possible to form alpha-alumina with a high alpha crystallization rate and a plate-like shape with a high aspect ratio, regardless of the shape of the precursor.

According to one embodiment of the present invention, even under conditions where the maximum firing temperature is 900 to 1000°C, it is possible to efficiently form plate-shaped alumina particles with a high aspect ratio and an alpha crystallization rate of 90% or more at low cost.

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 calcination temperature is in the range of 5 minutes to 30 hours. In order to efficiently form plate-like alumina particles, a calcination maintenance 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 α-crystalline polygonal plate-like alumina particles can be easily obtained without aggregation.

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

The calcination equipment is not necessarily limited, and a so-called calcination furnace can be used. It is preferable that the calcination furnace be made of a material that does not react with the evaporated molybdenum oxide. Furthermore, it is preferable to use a highly airtight calcination furnace so that the molybdenum oxide is used efficiently.

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 acts as the driving force for crystallization, resulting in the formation of hexagonal bipyramidal polyhedral particles with well-developed idiomorphic (113) planes. Furthermore, it is believed 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, using a molybdenum compound as a flux agent makes it easier to form molybdenum-containing plate-like alumina particles with a high α-crystallization rate.

[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.

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 preferred because it can shorten the manufacturing time. On the other hand, a cooling rate of 1000°C/hour or less is preferred because the firing container is less likely to crack due to heat shock and can be used for a long time.

The cooling method is not particularly limited, and can be left to cool naturally or a cooling device can be used.

[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.

Post-treatment methods include washing and high-temperature treatment, which can be performed in combination.

The cleaning method is not particularly limited, but it can be removed by washing with water, an aqueous ammonia solution, an aqueous sodium hydroxide solution, or an acidic aqueous solution.

In this case, the molybdenum content can be controlled by appropriately changing the concentration and amount of water, ammonia aqueous solution, sodium hydroxide aqueous solution, and acidic aqueous solution used, as well as the cleaning area and cleaning time.

Another high-temperature treatment method is to raise the temperature above the sublimation point or boiling point of the flux.

[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.

[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 forming 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.

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, as 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 crushing and classification steps, then the above steps are not necessary, and plate-like alumina with the desired excellent properties can be produced with high productivity, which is preferable.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

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

[Composition analysis of plate-like alumina particles using fluorescent X-rays]
Using a Primus IV X-ray fluorescence (XRF) analyzer (manufactured by Rigaku Corporation), approximately 70 mg of the prepared sample was placed on filter paper and covered with a 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 contents of various oxides.

[Crystalline Phase Analysis 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 (a product of Rigaku Corporation), and the plate glass was pulled while gently pressing down to fill the sample so that the measurement surface was smooth. The sample was then set in a wide-angle X-ray diffraction (XRD) device, Ultima IV (manufactured by Rigaku Corporation), and measurement was carried out under the conditions of Cu/Kα radiation, 40 kV/30 mA, a scan speed of 2 degrees/minute, and a scan range of 10 to 70 degrees.

[Particle size distribution measurement]
The 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.

[Evaluation of metallic gloss]
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.

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

[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 hydroxide oxide (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, heated to 1000 ° C. at 5 ° C./min in a ceramic electric furnace, and held at 1000 ° C. for 10 hours for 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.

[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.

[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 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, 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 any remaining molybdenum 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.

[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.

The results shown in Tables 2 and 3 confirm that in Examples 1 to 10, plate-shaped alumina particles with a lustrous metallic luster can be produced even at a firing temperature of 1000°C.

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.

While preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments. Additions, omissions, substitutions, and other modifications to the configuration are possible without departing from 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 (6)

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. The plate-like alumina particles according to claim 1 contain 0.02 to 2 mass% of the at least one element (A) in terms of oxide. The plate-like alumina particles according to claim 1 or 2 contain 0.1 to 10 mass% silicon or a silicon compound, calculated as silicon dioxide. The plate-like alumina particles according to claim 3, which contain silicon or a silicon compound in the surface layer. The plate-like alumina particles according to claim 1 or 2 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. 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: