Method for producing composite particles

Abstract

To provide a method for producing a composite particle excellent in coating characteristics.SOLUTION: A method for producing a composite particle of the present invention comprises the steps of: producing alumina particles by firing a mixture including an aluminum compound containing an aluminum element and a molybdenum compound containing a molybdenum element, or a mixture including an aluminum compound containing an aluminum element, a molybdenum compound containing a molybdenum element, and silicon or a silicon compound containing a silicon element; and forming an inorganic coating part on a surface of the alumina particle. A use amount of the molybdenum compound is 1 mass% or more and 10 mass% or less in terms of MoO3 when the total amount of raw materials converted to oxides is 100 mass%, and a use amount of the silicon compound is 0.5 mass% or more and 7 mass% or less in terms of SiO2 when the total amount of raw materials converted to oxides is 100 mass%.

Description

Technical Field

[0001] The present invention relates to a method for producing composite particles, and particularly to a method for producing composite particles in which a coating portion is provided on alumina particles.

Background Art

[0002] Alumina particles, which are inorganic fillers, are used in various applications. Among them, plate-shaped alumina particles with a high aspect ratio are particularly excellent in thermal and optical properties and the like compared to spherical alumina particles, and further improvement in performance is required. Conventionally, various plate-shaped alumina particles having characteristics in shape such as major axis and thickness have been known in order to improve the above-described characteristics and dispersibility inherent in plate-shaped alumina particles (Patent Documents 1 to 2). Further, as a production method for controlling the shape for the purpose of increasing the aspect ratio of plate-shaped alumina particles, a method of adding a phosphate compound as a shape control agent and performing hydrothermal synthesis (Patent Document 3), a method of adding a fluorosilicate and firing (Patent Document 4), etc. are known. Furthermore, a method for producing plate-shaped alumina using a silicon compound containing silicon or a silicon element as a crystal control agent in the production of plate-shaped alumina (Patent Document 5) is also known.

[0003] As coated alumina particles, alumina particles in which zirconia nanoparticles having an average particle diameter of 100 nm or less are uniformly coated on the surface of alumina particles having an average particle diameter of 0.1 μm or more are known (Patent Document 6). Also, as other coated particles, composite powders are known that include a base powder and spherical barium sulfate particles having a number average particle diameter of 0.5 to 5.0 μm protruding on the surface of the base powder, and the coating rate of the spherical barium sulfate particles is 10 to 70% with respect to the surface area of the base powder (Patent Document 7).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, none of Patent Documents 1 to 5 have any findings regarding the coating characteristics between alumina particles and the coating portion. Furthermore, although Patent Document 6 describes that an alumina sintered body with few pores, dense, high toughness, and high flexural strength can be obtained, there are no findings regarding the coating characteristics between alumina particles and the coating portion. Also, although Patent Document 7 describes that when the coating rate of spherical barium sulfate particles is 10 to 70% with respect to the surface area of the base powder, compound particles can correct unevenness and color defects on the skin and give a natural finish when blended into cosmetics, there are no findings regarding the coating characteristics between alumina particles and the coating portion, and there is room for improvement.

[0006] The present invention has been made in view of such circumstances, and an object thereof is to provide a method for producing composite particles having excellent coating characteristics.

Means for Solving the Problems

[0007] As a result of intensive studies to solve the above problems, the inventors of the present invention have found that when at least one of mullite and molybdenum is present on the surface of alumina particles as a substrate constituting composite particles, the coating efficiency when the alumina particles are coated with an inorganic coating portion is improved, and it becomes easier to coat the alumina particles, and thus the present invention has been completed. That is, the present invention provides the following means to solve the above problems.

[0008] [1] Composite particles including alumina particles containing molybdenum (Mo) and an inorganic coating portion provided on the surface of the alumina particles. [2] The composite particles according to [1] above, wherein the inorganic coating portion is composed of an oxide. [3] The composite particles according to [2] above, wherein the oxide is one or more selected from titanium oxide, iron oxide, and silica. [4] The composite particles according to [1] above, wherein the inorganic coating portion is composed of a metal. [5] The composite particles according to [4] above, wherein the metal is one or more selected from silver, nickel, copper, gold, and platinum. [6] The composite particles according to [1] above, wherein the alumina particles further contain silicon and / or germanium. [7] The composite particles according to [6] above, wherein the alumina particles contain mullite in the surface layer. [8] The composite particles according to any one of claims 1 to 7, wherein the composite particles have either a plate shape or a polyhedral shape. [9] The composite particles according to any one of [1] to [8] above, wherein the composite particles have a plate shape, a thickness of 0.01 μm or more and 5 μm or less, a particle size of 0.1 μm or more and 500 μm or less, and an aspect ratio of 2 or more and 500 or less.

[10] A paint containing the composite particles according to any one of [1] to [9] above.

[11] A step of firing a mixture containing an aluminum compound containing an aluminum element and a molybdenum compound containing a molybdenum element, or a mixture containing an aluminum compound containing an aluminum element, a molybdenum compound containing a molybdenum element, and a shape control agent for controlling the shape of alumina particles to produce alumina particles, A step of forming an inorganic coating portion on the surface of the alumina particles; A method for producing composite particles, comprising:

[12] The method for producing composite particles according to

[11] above, wherein the shape control agent is composed of one or more selected from silicon, a silicon compound containing a silicon element, and a germanium compound containing a germanium element.

[13] The method for producing composite particles according to

[11] or

[12] above, wherein the mixture further contains a potassium compound containing a potassium element.

[14] The composite particles according to

[11] above, wherein the inorganic coating portion is composed of an oxide.

[15] The method for producing composite particles according to

[14] above, wherein the oxide is one or more selected from titanium oxide, iron oxide, and silica.

[16] The method for producing composite particles according to

[11] above, wherein the inorganic coating portion is composed of a metal.

[17] The method for producing composite particles according to

[16] above, wherein the metal is one or more selected from silver, nickel, copper, gold, and platinum.

[0009] According to the present invention, composite particles having excellent coating properties can be provided.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0012] <<First Embodiment>> <Composite Particles> The composite particles according to the first embodiment include alumina particles containing molybdenum (Mo) and an inorganic coating portion provided on the surface of the alumina particles. The alumina particles of this embodiment have a plate-like shape, and the composite particles also have a plate-like shape. Hereinafter, in this embodiment, the alumina particles having a plate-like shape are also referred to as "plate-like alumina particles", "plate-like alumina" or simply "alumina particles".

[0013] <Plate-like Alumina Particles> "Plate-like" as used in the present invention means that the aspect ratio obtained by dividing the average particle diameter of the alumina particles by the thickness is 2 or more. In this specification, the "thickness of the alumina particles" is the arithmetic average value of the thicknesses measured for at least 50 randomly selected plate-like alumina particles from an image obtained by a scanning electron microscope (SEM). Also, the "average particle diameter of the alumina particles" is the volume-based median diameter D 50 calculated as.

[0014] In the case of alumina particles, the following conditions for thickness, particle diameter, and aspect ratio can be combined in any way within the range where it is plate-like. Also, the upper and lower limit values of the numerical ranges exemplified by these conditions can be freely combined.

[0015] The plate-like alumina particles preferably have a thickness of 0.01 μm or more and 5 μm or less, preferably 0.03 μm or more and 5 μm or less, preferably 0.1 μm or more and 5 μm or less, more preferably 0.3 μm or more and 3 μm or less, and even more preferably 0.5 μm or more and 1 μm or less. When using plate-shaped alumina particles with a larger particle size, the thickness is preferably 3 μm or more, and more preferably 5 μm or more and 60 μm or less. The alumina particles having the above thickness are preferable because they have a high aspect ratio and excellent mechanical strength.

[0016] The plate-shaped alumina particles preferably have an average particle diameter (D 50 ) of 0.1 μm or more and 500 μm or less, more preferably 0.5 μm or more and 100 μm or less, and even more preferably 1 μm or more and 50 μm or less. When using plate-shaped alumina particles with a larger particle size, the average particle diameter (D 50 ) is preferably 10 μm or more, more preferably 20 μm or more, even more preferably 22 μm or more, further preferably 25 μm or more, and particularly preferably 31 μm or more. The upper limit value of the above average particle diameter is not particularly limited. As an example, the average particle diameter (D 50 ) of the plate-shaped alumina particles in the embodiment is preferably 10 μm or more and 500 μm or less, preferably 20 μm or more and 300 μm or less, more preferably 22 μm or more and 100 μm or less, further preferably 25 μm or more and 100 μm or less, and particularly preferably 31 μm or more and 50 μm or less. The alumina particles having an average particle diameter (D 50 ) of not less than the above lower limit value are particularly excellent in brilliance because the area of the light reflection surface is large. In addition, the alumina particles having an average particle diameter (D 50 ) of not more than the above upper limit value are suitable for use as a filler.

[0017] The plate-shaped alumina particles preferably have an aspect ratio, which is the ratio of the average particle diameter to the thickness, of 2 or more and 500 or less, preferably 5 or more and 500 or less, preferably 15 or more and 500 or less, more preferably 10 or more and 300 or less, more preferably 17 or more and 300 or less, and even more preferably 33 or more and 100 or less. When the aspect ratio of the plate-shaped alumina particles is 2 or more, it is preferable because they can have two-dimensional blending characteristics. When the aspect ratio of the plate-shaped alumina particles is 500 or less, it is preferable because they have excellent mechanical strength. When the aspect ratio is 15 or more, it is preferable because high brightness can be achieved when used as a pigment. When using larger particle size plate-shaped alumina particles, the aspect ratio, which is the ratio of the average particle diameter to the thickness, is preferably 2 or more and 50 or less, and more preferably 3 or more and 30 or less.

[0018] The plate-shaped alumina particles may be circular plate-shaped or elliptical plate-shaped, but the particle shape is preferably polygonal plate-shaped, for example, from the viewpoints of handleability and ease of manufacture.

[0019] The plate-shaped alumina particles may be obtained based on any manufacturing method, but in terms of having a higher aspect ratio, better dispersibility, and better productivity, it is preferable to obtain them by firing an aluminum compound in the presence of a molybdenum compound (preferably further a potassium compound) and a shape control agent. As the shape control agent, it is preferable to use at least one selected from the group consisting of silicon, silicon compounds, and germanium compounds. Since the shape control agent becomes the source of Si for the mullite described later, it is more preferable to use a silicon compound containing silicon or silicon element. In the above manufacturing method, the molybdenum compound is used as a fluxing agent. Hereinafter, this manufacturing 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. By such firing, the molybdenum compound reacts with the aluminum compound at a high temperature to form aluminum molybdate. After that, when this aluminum molybdate further decomposes into alumina and molybdenum oxide at a higher temperature, it is considered that the molybdenum compound is incorporated into the plate-shaped alumina particles. Molybdenum oxide can be sublimated, recovered, and reused. In addition, when the plate-shaped alumina particles contain mullite in the surface layer, in this process, it is considered that silicon or a compound containing a silicon atom blended as a shape control agent reacts with the aluminum compound through molybdenum, and mullite is formed on the surface layer of the plate-shaped alumina particles. Regarding the generation mechanism of mullite, more specifically, formation of Mo-O-Si by the reaction of molybdenum and Si atoms and formation of Mo-O-Al by the reaction of molybdenum and Al atoms occur on the plate surface of alumina, and when fired at a high temperature, Mo is desorbed and mullite having a Si-O-Al bond is formed. It is preferable to recover and reuse the molybdenum oxide that is not incorporated into the plate-shaped alumina particles by sublimation. By doing so, the amount of molybdenum oxide adhering to the surface of the plate-shaped alumina can be reduced, and when dispersing in a dispersion medium such as an organic binder like resin or an inorganic binder like glass, the molybdenum oxide does not mix into the binder, and it becomes possible to impart the original properties of the plate-shaped alumina to the maximum extent. In addition, in this specification, in the manufacturing method described later, those having the property of being able to sublime are referred to as fluxing agents, and those that cannot sublime are referred to as shape control agents.

[0020] In the production of the plate-shaped alumina particles, by utilizing molybdenum and the shape control agent, the alumina particles have a high α-crystallinity and an autogenous shape, so excellent dispersibility, mechanical strength, and high thermal conductivity can be realized.

[0021] When the plate-shaped alumina particles contain mullite in the surface layer, the amount of mullite generated on the surface layer of the plate-shaped alumina particles can be controlled by the use ratios of the molybdenum compound and the shape control agent, and in particular, it can be controlled by the use ratio of silicon or a silicon compound containing a silicon element used as the shape control agent. The preferred values of the amount of mullite generated on the surface layer of the plate-shaped alumina particles and the preferred use ratios of the raw materials will be described in detail later.

[0022] The plate-shaped alumina particles are plate-shaped alumina particles having an aspect ratio of 5 to 500 from the viewpoint of improving the brilliance, and are solid 27 In the 27Al NMR analysis, the longitudinal relaxation time T for the peak of six-coordinate aluminum at 10 to 30 ppm at a static magnetic field strength of 14.1 T 1 is preferably 5 seconds or more.

[0023] The above longitudinal relaxation time T 1 being 5 seconds or more means that the crystallinity of the plate-shaped alumina particles is high. It has been reported that when the longitudinal relaxation time in the solid state is large, the symmetry of the crystal is good and the crystallinity is high (Previous report: Susumu Kitakawa et al., "Selected Books of the Chemical Society of Complexes 4 Solution and Solid NMR of Multinuclear Species", Sankyo Publishing Co., Ltd., p80-82).

[0024] In the plate-shaped alumina particles, the above longitudinal relaxation time T 1 is preferably 5 seconds or more, more preferably 6 seconds or more, and even more preferably 7 seconds or more. In the plate-shaped alumina particles of the embodiment, the upper limit value of the above longitudinal relaxation time T 1 is not particularly limited, but for example, it may be 22 seconds or less, 15 seconds or less, or 12 seconds or less. As an example of the numerical range of the above longitudinal relaxation time T 1 exemplified above, it may be 5 seconds or more and 22 seconds or less, 6 seconds or more and 15 seconds or less, or 7 seconds or more and 12 seconds or less.

[0025] The plate-shaped alumina particles are solid 27It is preferable that no peak of 4-coordinate aluminum is detected at 60 to 90 ppm in Al NMR analysis at a static magnetic field strength of 14.1 T. Such plate-like alumina particles are considered to be less susceptible to breakage or falling off due to distortion of crystal symmetry caused by the inclusion of crystals with different coordination numbers, and tend to have better shape stability.

[0026] Conventionally, the degree of crystallinity of inorganic substances has generally been evaluated by the results of XRD analysis or the like. However, the inventors' research has revealed that the crystallinity of alumina particles can be evaluated by the longitudinal relaxation time T 1 It has been found that by using the longitudinal relaxation time T 1 The plate-like alumina particles according to the embodiment are considered to have high crystallinity, which is probably due to the high crystallinity, suppressing diffuse reflection on the crystal surface and improving light reflection, resulting in excellent brilliance.

[0027] Furthermore, the present inventors have found that the longitudinal relaxation time T 1 It has been found that the value of the longitudinal relaxation time T 1 The correlation between the value of the longitudinal relaxation time T 1 The plate-like alumina particles having a longitudinal relaxation time T of 5 seconds or more have the advantage that they have good processing stability and can be easily processed into a desired shape when blended with a resin to produce a resin composition. 1Since the value of is long, the crystallinity is enhanced. Therefore, due to the high crystallinity of alumina, the strength of the particles is high, and when the resin and the plate-shaped alumina particles are mixed during the production process of the resin composition, it is considered that the plate is less likely to crack. Furthermore, presumably due to the high crystallinity of alumina, there are few irregularities on the particle surface, and it is considered to have excellent adhesion to the resin. Due to these factors, it is considered that the plate-shaped alumina particles as described above have good processing stability of the resin composition. According to the plate-shaped alumina particles as described above, even when blended in a resin composition, the performance of the original plate-shaped alumina particles is exhibited well.

[0028] Conventionally, it has been difficult to obtain alumina particles with high crystallinity for plate-shaped alumina particles compared to spherical alumina particles. This is presumably because, unlike spherical alumina particles, plate-shaped alumina particles need to cause a bias in the crystal growth direction during their production process. On the other hand, the above-mentioned plate-shaped alumina particles satisfying the value of the longitudinal relaxation time T 1 are plate-shaped while having high crystallinity. Therefore, while having the advantages of plate-shaped alumina particles such as exhibiting excellent thermal conductivity, they are extremely useful with further enhanced shape retention rate and processing stability of the resin composition.

[0029] Also, for the plate-shaped alumina particles of the embodiment, the ratio I(006) / I(113) (hereinafter, I(006) / I(113) is abbreviated as the (006 / 113) ratio.) of the peak intensity I(006) at 2θ = 41.6 ± 0.3 degrees corresponding to the (006) plane and the peak intensity I(113) at 2θ = 43.3 ± 0.3 degrees corresponding to the (113) plane, obtained by X-ray diffraction measurement using Cu-Kα rays, is preferably 0.2 or more and 30 or less, more preferably 1 or more and 20 or less, still more preferably 3 or more and 10 or less, and particularly preferably 7.5 or more and 10 or less. In this case, the plate-shaped alumina particles have, for example, an average particle diameter (D 50 ) of 10 μm or more and a thickness of 0.1 μm or more.

[0030] The fact that the value of the above (006 / 113) ratio is large means that the ratio of the (006) plane to the (113) plane is large, and it is understood that it means that the flat plate-shaped alumina particles have a surface corresponding to the crystal in the orientation of the (006) plane significantly developed. Such flat plate-shaped alumina particles have a large area of the upper or lower surface developed on the plate-shaped surface of the plate-shaped alumina, and the visibility of the reflected light reflected thereon is enhanced. Also, since the formation of the surface corresponding to the crystal in the orientation of the (113) plane is suppressed, even if the mass per grain is small, high brilliance is exhibited.

[0031] The pH of the isoelectric point of the plate-shaped alumina particles is, for example, in the range of 2 to 6, preferably in the range of 2.5 to 5, and more preferably in the range of 3 to 4. The plate-shaped alumina particles with the pH of the isoelectric point within the above range have a high electrostatic repulsive force, and can enhance the dispersion stability when compounded into the above-described dispersion medium by themselves. Also, modification by surface treatment such as a coupling treatment agent intended for further performance improvement becomes easier.

[0032] The value of the pH of the isoelectric point is obtained by measuring the zeta potential with a zeta potential measuring device (Zetasizer Nano ZSP manufactured by Malvern), stirring 20 mg of the sample and 10 mL of 10 mM KCl aqueous solution for 3 minutes in the stirring and defoaming mode with a bubble pickling Rintaro (ARE-310 manufactured by Shin-Kee), allowing to stand for 5 minutes, using the supernatant as the measurement sample, adding 0.1 N HCl to the sample with an automatic titrator, measuring the zeta potential in the range up to pH = 2 (applied voltage 100 V, Monomodl mode), and evaluating the pH of the isoelectric point at which the potential becomes zero.

[0033] The plate-shaped alumina particles have, for example, a density of 3.70 g / cm 3 or more and 4.10 g / cm 3 or less, and preferably have a density of 3.72 g / cm 3 or more and 4.10 g / cm 3 or less, and more preferably have a density of 3.80 g / cm 3 or more and 4.10 g / cm 3 or less. The density can be measured using a dry automatic densitometer AccuPyc II 1330 manufactured by Micromeritics at a measurement temperature of 25°C with helium as the carrier gas after pretreatment of the plate-shaped alumina particles under the conditions of 300°C for 3 hours.

[0034] [Alumina] The "alumina" contained in the plate-shaped alumina particles is aluminum oxide. For example, it may be transition alumina of various crystal forms such as γ, δ, θ, κ, etc., or may contain alumina hydrate in the transition alumina. However, in terms of better mechanical strength or thermal conductivity, it is basically preferably in the α crystal form (α-type). The α crystal form is a dense crystal structure of alumina, which is advantageous for improving the mechanical strength or thermal conductivity of the plate-shaped alumina. It is preferable that the α crystallization rate is as close to 100% as possible because it is easier to exhibit the inherent properties of the α crystal form. The α crystallization rate of the plate-shaped alumina particles is, for example, 90% or more, preferably 95% or more, and more preferably 99% or more.

[0035] [Silicon · Germanium] The plate-shaped alumina particles of the embodiment may contain silicon and / or germanium. The silicon or germanium may be derived from silicon, a silicon compound, and / or a germanium compound that can be used as a shape control agent. By utilizing these, plate-shaped alumina particles having excellent brilliance can be produced in the manufacturing method described later.

[0036] (Silicon) The plate-shaped alumina particles of the embodiment may contain silicon. The plate-shaped alumina particles according to the embodiment may contain silicon in the surface layer. Here, the "surface layer" refers to within 10 nm from the surface of the plate-shaped alumina particles according to the embodiment. This distance corresponds to the detection depth of XPS used for measurement in the examples.

[0037] The platy alumina particles may have silicon unevenly distributed on the surface layer. Here, "unevenly distributed on the surface layer" means a state where 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. Whether silicon is unevenly distributed on the surface layer can be determined by comparing the results of surface analysis by XPS and overall analysis by XRF.

[0038] The silicon contained in the platy alumina particles may be elemental silicon or silicon in a silicon compound. The platy alumina particles, as silicon or a silicon compound, Mullite , Si, SiO 2 , SiO, and at least one selected from the group consisting of aluminum silicate formed by reacting with alumina may be included, and the above substances may be included in the surface layer. Mullite will be described later.

[0039] When a silicon compound containing silicon or silicon element is used as a shape control agent for the platy alumina particles, Si can be detected by XRF analysis. For the platy alumina particles, the molar ratio [Si] / [Al] of Si to Al obtained by XRF analysis is, for example, 0.04 or less, preferably 0.035 or less, and more preferably 0.02 or less. Also, 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. For the platy alumina particles, the molar ratio [Si] / [Al] of Si to Al obtained by XRF analysis is, for example, 0.003 or more and 0.04 or less, preferably 0.004 or more and 0.035 or less, and more preferably 0.005 or more and 0.02 or less. The platy alumina particles with the value of the molar ratio [Si] / [Al] obtained by the XRF analysis within the above range satisfy the value of the above (006 / 113) ratio, and have more preferable luster, and the platy shape is well formed. Also, deposits hardly adhere to the surface of the platy alumina particles, and the quality is excellent. This deposit is SiO 2It is considered to be in the form of grains, and the formation of mullite on the surface layer of the plate-shaped alumina particles reaches a saturated state and is generated from the excessive Si. When using plate-shaped alumina particles with a larger particle size, the molar ratio [Si] / [Al] of Si to Al in the plate-shaped alumina particles obtained by XRF analysis is preferably 0.0003 or more and 0.01 or less, more preferably 0.0005 or more and 0.0025 or less, and even more preferably 0.0006 or more and 0.001 or less.

[0040] The plate-shaped alumina particles may contain silicon corresponding to the silicon or silicon compound containing a silicon element used in the manufacturing method. The content of silicon based on 100% by mass of the plate-shaped alumina particles is preferably 10% by mass or less in terms of silicon dioxide conversion, more preferably 0.001 to 5% by mass, even more preferably 0.01 to 4% by mass, still more preferably 0.3 to 2.5% by mass, and particularly preferably 0.6 to 2.5% by mass. When the content of silicon is within the above range, the value of the above (006 / 113) ratio is satisfied, and the brilliance is more preferable, and the plate shape is formed well. Also, SiO 2 The deposits regarded as grains hardly adhere to the surface of the plate-shaped alumina particles, and the quality is excellent. When using plate-shaped alumina particles with a larger particle size, the content of silicon based on 100% by mass of the plate-shaped alumina particles is preferably 10% by mass or less in terms of silicon dioxide conversion, more preferably 0.001 to 3% by mass, even more preferably 0.01 to 1% by mass, and particularly preferably 0.03 to 0.3% by mass.

[0041] (Mullite) The plate-shaped alumina particles of the embodiment may contain mullite. By containing mullite in the surface layer of the plate-shaped alumina particles, it is presumed that the selectivity of the inorganic material constituting the inorganic coating part is improved, and the inorganic coating part can be efficiently formed on the plate-shaped alumina particles. Mullite is contained in the surface layer of tabular alumina particles, resulting in a remarkable reduction in equipment wear. The "mullite" that may be contained in the surface layer of tabular alumina particles is a composite oxide of Al and Si, and is represented as Al X Si Y O z However, the values of x, y, and z are not particularly limited. A more preferable range is Al 2 Si 1 O 5 ~Al 6 Si 2 O 13 It should be noted that the XRD peak intensities confirmed in the examples described later are 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 tabular alumina particles may contain at least one compound selected from the group consisting of 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 13 in the surface layer. Here, the "surface layer" refers to within 10 nm from the surface of the tabular alumina particles. This distance corresponds to the detection depth of XPS used for measurement in the examples. The tabular alumina particles preferably have mullite unevenly distributed on the surface layer. Here, "unevenly distributed on the surface layer" means a state where 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. As shown in the examples described later, the uneven distribution of mullite on the surface layer can be determined by comparing the results of surface analysis by XPS and overall analysis by XRF.

[0042] In addition, the mullite in the surface layer may form a mullite layer or may be in a state where mullite and alumina are mixed. The interface between the mullite and alumina in the surface layer may be a state where mullite and alumina are physically in contact, or mullite and alumina may form a chemical bond such as Si-O-Al.

[0043] (Germanium) The tabular alumina particles of the embodiment may contain germanium. Further, the tabular alumina particles may contain germanium in the surface layer. Although it varies depending on the raw materials used, the tabular alumina particles contain germanium or a germanium compound, for example, Ge, GeO 2 , GeO, GeCl 2 , GeBr 4 , GeI 4 , GeS 2 , AlGe, GeTe, GeTe 3、 As 2 , GeSe, GeS 3 As, SiGe, Li 2 , Ge, FeGe, SrGe, GaGe and other compounds, and at least one selected from the group consisting of oxides thereof may be included, and the above substances may be included in the surface layer. Note that the "germanium or germanium compound" contained in the tabular alumina particles may be the same type of germanium compound as the "raw material germanium compound" used as the raw material shape control agent. For example, GeO 2 may be detected in the tabular alumina particles produced by adding 2 GeO.

[0044] Germanium or a germanium compound is contained in the surface layer of the plate-like alumina particles, thereby achieving remarkable wear reduction of the device. Here, the "surface layer" refers to within 10 nm from the surface of the plate-like alumina particles. It is preferable that the germanium or germanium compound is unevenly distributed in the surface layer of the plate-like alumina particles. Here, "unevenly distributed in the surface layer" means a state where the mass of germanium or germanium compound per unit volume in the surface layer is larger than the mass of germanium or germanium compound per unit volume outside the surface layer. Whether germanium or germanium compound is unevenly distributed in the surface layer can be determined by comparing the results of surface analysis by XPS and overall analysis by XRF.

[0045] The plate-like alumina particles contain germanium corresponding to the raw material germanium compound used in the production method thereof. The content of germanium relative to 100% by mass of the plate-like alumina particles is preferably 10% by mass or less, more preferably 0.001 - 5% by mass, still more preferably 0.01 - 4% by mass, and particularly preferably 0.1 - 3.0% by mass in terms of germanium dioxide conversion. When the content of germanium is within the above range, the amount of germanium or germanium compound is appropriate, the value of the above (006 / 113) ratio is satisfied, and the brilliance is more preferable, so it is preferred. The above content of germanium can be determined by XRF analysis. The XRF analysis shall be carried out under the same conditions as the measurement conditions described in the examples below, or under compatible conditions that can obtain the same measurement results.

[0046] In addition, the germanium or germanium compound in the surface layer may form a layer, or may be in a state where germanium or germanium compound and alumina are mixed. The interface between the germanium or germanium compound and alumina in the surface layer may be in a state where germanium or germanium compound and alumina are physically in contact, or germanium or germanium compound and alumina may form a chemical bond such as Ge - O - Al.

[0047] [Molybdenum] The plate-shaped alumina particles of the embodiment contain molybdenum. Further, it is preferable that the plate-shaped alumina particles contain molybdenum in their surface layer. It is presumed that this improves the selectivity of the inorganic material constituting the inorganic coating portion and enables the inorganic coating portion to be efficiently formed on the plate-shaped alumina particles.

[0048] The molybdenum may be derived from a molybdenum compound used as a fluxing agent in the method for producing alumina particles described later.

[0049] Molybdenum has a catalytic function and an optical function. Further, by utilizing molybdenum, plate-shaped alumina particles having high crystallinity and excellent brilliance while having a plate shape can be produced in the production method described later.

[0050] By increasing the amount of molybdenum used, the particle size and the value of the above (006 / 113) ratio are satisfied, and the brilliance of the obtained alumina particles tends to be further excellent. Furthermore, by utilizing molybdenum, the formation of mullite is promoted, and plate-shaped alumina particles having a high aspect ratio and excellent dispersibility can be produced. Also, by utilizing the properties of molybdenum contained in the plate-shaped alumina particles, it may be possible to apply them to uses such as oxidation reaction catalysts and optical materials.

[0051] The molybdenum is not particularly limited, and includes, in addition to molybdenum metal, molybdenum oxide, a molybdenum compound partially reduced, molybdates, and the like. Any polymorph of molybdenum compounds, or a combination thereof, may be contained in the plate-shaped alumina particles, and α-MoO 3 , β-MoO 3 , MoO 2 , MoO, molybdenum cluster structures, etc. may also be contained in the plate-shaped alumina particles.

[0052] The form of molybdenum content is not particularly limited and may be included in a form attached to the surface of the plate-shaped alumina particles, may be included in a form substituted for a part of aluminum in the crystal structure of alumina, or may be a combination of these.

[0053] In the XRF analysis, the content of molybdenum relative to 100% by mass of the plate-shaped alumina particles is preferably 10% by mass or less in terms of molybdenum trioxide. By adjusting the firing temperature, firing time, and sublimation rate of the molybdenum compound, it is more preferably 0.001 to 5% by mass, still more preferably 0.01 to 5% by mass, and particularly preferably 0.1 to 1.5% by mass. When the molybdenum content is 10% by mass or less, it is preferable because it improves the α single crystal quality of alumina. When using plate-shaped alumina particles with a larger particle diameter, the content of molybdenum relative to 100% by mass of the plate-shaped alumina particles according to the embodiment is preferably 10% by mass or less in terms of molybdenum trioxide. By adjusting the firing temperature, firing time, and sublimation rate of the molybdenum compound, it is more preferably 0.1 to 5% by mass, and still more preferably 0.3 to 1% by mass. The above-mentioned molybdenum content can be determined by XRF analysis. The XRF analysis shall be carried out under the same conditions as the measurement conditions described in the examples below, or under compatible conditions that can obtain the same measurement results.

[0054] In addition, the analysis of the Mo amount on the surface of the alumina particles can be carried out using the above-mentioned X-ray photoelectron spectroscopy (XPS) apparatus.

[0055] [Potassium] The plate-shaped alumina particles may further contain potassium.

[0056] The potassium may be derived from potassium that can be used as a flux in the method for producing alumina particles described below. By utilizing potassium, in the method for producing alumina particles described below, the particle diameter of the alumina particles can be appropriately improved.

[0057] The potassium is not particularly limited, and examples thereof include potassium metal, potassium oxide, and potassium compounds in which a part thereof is reduced, etc.

[0058] The form of potassium content is not particularly limited, and it may be included in a form adhering to the surface of the tabular alumina of the tabular alumina particles, may be included in a form substituted for a part of aluminum in the crystal structure of alumina, or may be a combination thereof.

[0059] In the XRF analysis, the content of potassium with respect to 100% by mass of the alumina particles is preferably 0.01% by mass or more, more preferably 0.01 to 1.0% by mass, still more preferably 0.03 to 0.5% by mass, and particularly preferably 0.05 to 0.3% by mass in terms of potassium oxide (K 2 2O). Alumina particles having a potassium content within the above range are preferable because they have a polyhedral shape and values such as the average particle size are suitable.

[0060] (other atoms) Other atoms mean those intentionally added to the alumina particles for the purpose of imparting mechanical strength or electrical and magnetic functions within a range that does not inhibit the effects of the present invention.

[0061] The other atoms are not particularly limited, and examples thereof include zinc, manganese, calcium, strontium, yttrium, etc. These other atoms may be used alone or in combination of two or more.

[0062] The content of other atoms in the alumina particles is preferably 5% by mass or less, more preferably 2% by mass or less with respect to the mass of the alumina particles.

[0063] [Inevitable impurities] The alumina particles may contain inevitable impurities.

[0064] Inevitable impurities are those that originate from the metal compounds used in the manufacturing process, are present in the raw materials, or inevitably mix into the alumina particles during the manufacturing process. They are inherently unnecessary, but are present in trace amounts and do not affect the properties of the alumina particles.

[0065] Examples of inevitable impurities include, but are not particularly limited to, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, cerium, sodium, etc. These inevitable impurities may be contained individually or in combinations of two or more.

[0066] The content of inevitable impurities in the alumina particles is preferably 10,000 ppm or less, more preferably 1,000 ppm or less, and even more preferably 10 - 500 ppm, based on the mass of the alumina particles.

[0067] <Inorganic coating part> The inorganic coating part covers at least a part of the surface of the alumina particles and is preferably composed of an inorganic coating layer that covers at least a part of the surface of the alumina particles. In other words, at least a part of the surface of the composite particles is covered by the inorganic coating part, and preferably at least a part of the surface of the composite particles is covered by the inorganic coating layer. As described above, the inorganic coating part is provided on the surface of the alumina particles. "On the surface of the alumina particles" means the outside of the surface of the alumina particles. Therefore, the inorganic coating part formed on the outside of the surface of the alumina particles is clearly distinguished from the surface layer containing mullite or germanium formed on the inside of the surface of the alumina particles.

[0068] The inorganic chemical species constituting the inorganic coating portion may be relatively larger than the alumina particles, but it is preferable that they are relatively smaller than the alumina particles in that an inorganic coating portion with an arbitrary coating amount (or coating thickness) can be easily provided according to the purpose. Examples of the combination include alumina particles on the μm order and inorganic chemical species of 150 nm or less. When providing an inorganic coating portion using inorganic chemical species smaller than the alumina particles on the outer side of the surface of the alumina particles, it is possible to use a small amount of inorganic chemical species to provide an inorganic coating portion on a part of the alumina surface so that the base of the alumina particles can be clearly seen, or it is possible to use a large amount of inorganic chemical species to provide an inorganic coating portion on the surface of the alumina particles such that the base of the alumina particles cannot be seen and they are laminated. The shape of the inorganic chemical species constituting the inorganic coating portion is not limited, but for example, it is preferably spherical or polyhedral in that it can be most densely packed with the minimum usage amount and it is easy to conceal the base.

[0069] The composite particles of the present invention are composed of alumina particles containing molybdenum and an inorganic coating portion composed of inorganic chemical species, and have excellent properties that cannot be exhibited by a simple mixture of alumina particles and inorganic chemical species. In the composite particles of the present invention, in the case of a combination of alumina particles containing molybdenum on the μm order and non-aggregated inorganic chemical species of 150 nm or less, for example, due to intermolecular forces and in some cases local chemical reactions, the interaction between the two increases, resulting in particularly outstanding excellent properties such as higher coating characteristics being obtained, a more uniform inorganic coating portion being easily obtained, and the obtained inorganic coating portion being difficult to peel off from the alumina particles. The contribution of molybdenum contained in the alumina particles can also be expected. Independent inorganic chemical species on the nm order can be obtained, for example, by mechanically pulverizing inorganic chemical species on the μm order, but re-aggregation etc. immediately occur, so handling during use is not easy. When alumina particles not containing molybdenum or aggregated inorganic chemical species are used, they only form a simple mixture and do not exhibit the properties of the composite particles of the present invention. According to the method for producing the composite particles of the present invention described later, composite particles with higher coating efficiency can be more easily produced.

[0070] The inorganic coating part can be composed of, for example, an oxide or a metal. Examples of the oxide include, for example, titanium oxide (TiO 2 ), iron oxide (Fe 2 O 3 ), and silica (SiO 2 ). Examples of the metal include one or more selected from, for example, silver (Ag), nickel (Ni), copper (Cu), gold (Au), and platinum (Pt).

[0071] The shape of the oxide or metal constituting the inorganic coating part is not particularly limited, but is, for example, particulate such as spherical, needle-like, polyhedral, disk-like, hollow, or porous. The average particle diameter of the particles composed of the particulate oxide or metal is preferably, for example, 1 nm or more and 500 nm or less, and more preferably 5 nm or more and 200 nm or less. The particles composed of the oxide or metal may be crystalline or amorphous.

[0072] When the inorganic coating part is an inorganic coating layer, the thickness of one layer of the inorganic coating layer formed on the surface of the alumina particles is preferably 20 nm or more and 400 nm or less, preferably 30 nm or more and 300 nm or less, and particularly preferably 30 nm or more and 200 nm or less.

[0073] When the inorganic coating layer is composed of titanium oxide, a desired interference color can be obtained by changing the thickness of the inorganic coating layer. As the thickness of the inorganic coating layer increases, the color density increases. When the inorganic coating layer is composed of iron oxide, the color of the composite particles is red or reddish-brown.

[0074] The inorganic coating part may be composed of one layer or a plurality of layers. Further, when the inorganic coating part is composed of a plurality of layers, the plurality of layers may be composed of different materials from each other.

[0075] [XRF Coating Rate of Composite Particles] The XRF coating rate (%) of the composite particles according to the embodiment can be measured, for example, using the X-ray fluorescence (XRF) analyzer described below. The XRF coating rate (%) is determined based on, for example, the content of the metal oxide constituting the inorganic coating portion with respect to the content of aluminum oxide constituting the alumina particles, and can be determined from, for example, [MO x / [Al 2 O 3 (mass ratio) obtained from the XRF analysis results.

[0076] [Coating efficiency of composite particles] The coating efficiency of the composite particles according to the embodiment can be determined from the ratio of the XRF coating rate to the theoretical coating rate described below. The coating efficiency is preferably 30% or more, more preferably 80% or more, and even more preferably 90% or more.

[0077] [Organic compound layer on the surface of composite particles] In one embodiment, the composite particles may have an organic compound layer on their surface. The organic compound constituting the organic compound layer is present on the surface of the composite particles and has a function of adjusting the surface physical properties of the composite particles. For example, since composite particles having an organic compound on their surface improve the affinity with a resin, the function of alumina particles as a filler can be maximally exhibited.

[0078] The organic compound is not particularly limited, and examples thereof include organic silanes, alkylphosphonic acids, and polymers.

[0079] Examples of the organic silane include alkyltrimethoxysilanes or alkyltrichlorosilanes having 1 to 22 carbon atoms in the alkyl group such as methyltrimethoxysilane, dimethyldimethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, iso-propyltrimethoxysilane, iso-propyltriethoxysilane, pentyltrimethoxysilane, hexyltrimethoxysilane, 3,3,3-trifluoropropyltrimethoxysilane, tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilanes, phenyltrimethoxysilane, phenyltriethoxysilane, p-chloromethylphenyltrimethoxysilane, p-chloromethylphenyltriethoxysilanes, etc.

[0080] Examples of the phosphonic acid include, for example, methylphosphonic acid, ethylphosphonic acid, propylphosphonic acid, butylphosphonic acid, pentylphosphonic acid, hexylphosphonic acid, heptylphosphonic acid, octylphosphonic acid, decylphosphonic acid, dodecylphosphonic acid, octadecylphosphonic acid, 2-ethylhexylphosphonic acid, cyclohexylmethylphosphonic acid, cyclohexylethylphosphonic acid, benzylphosphonic acid, phenylphosphonic acid, dodecylbenzene phosphonic acid.

[0081] As the polymer, for example, poly(meth)acrylates can be preferably used. Specifically, polymethyl(meth)acrylate, polyethyl(meth)acrylate, polybutyl(meth)acrylate, polybenzyl(meth)acrylate, polycyclohexyl(meth)acrylate, poly-t-butyl(meth)acrylate, polyglycidyl(meth)acrylate, polypentafluoropropyl(meth)acrylate, etc., and general-purpose polymers such as polystyrene, polyvinyl chloride, polyvinyl acetate, epoxy resin, polyester, polyimide, polycarbonate can be mentioned.

[0082] Note that the above organic compounds may be contained alone or in combination of two or more.

[0083] The form of the organic compound contained is not particularly limited, and it may be linked to alumina by a covalent bond or may coat alumina or the material of the inorganic coating portion.

[0084] The content rate of the organic compound is preferably 20% by mass or less, more preferably 10% by mass or more and 0.01% by mass or less, based on the mass of the alumina particles. When the content rate of the organic compound is 20% by mass or less, it is preferable because the physical properties derived from the composite particles can be easily exhibited.

[0085] <Method for producing composite particles> Next, the details of the method for producing the composite particles according to the first embodiment will be exemplified. The method for producing the composite particles according to the present embodiment is not limited to the method for producing the composite particles shown below.

[0086] The method for producing the composite particles according to the present embodiment includes a step of firing a mixture containing an aluminum compound containing an aluminum element, a molybdenum compound containing a molybdenum element, and a shape control agent for controlling the shape of the alumina particles to produce alumina particles, and a step of forming an inorganic coating portion on the surface of the alumina particles.

[0087] <Method for producing plate-shaped alumina particles> The method for producing the plate-shaped alumina particles constituting the composite particles is not particularly limited, and known techniques can be appropriately applied. However, from the viewpoint of being able to preferably control alumina having a high α-crystallization rate at a relatively low temperature, a production method by a flux method using a molybdenum compound can be preferably applied.

[0088] More specifically, the preferred method for producing the plate-shaped alumina particles includes a step of firing an aluminum compound (firing step) in the presence of a molybdenum compound and a shape control agent. The firing step may be a step of firing the mixture obtained in the step of obtaining the mixture to be fired (mixing step).

[0089] [Mixing step] The mixing step is a step of mixing an aluminum compound, a molybdenum compound, and a shape control agent to form a mixture. The mixture preferably further contains a potassium compound. Hereinafter, the content of the mixture will be described.

[0090] (Aluminum compound) The aluminum compound is a raw material for the plate-shaped alumina particles of the present embodiment, and is not particularly limited as long as it can be converted into alumina by heat treatment. For example, aluminum chloride, aluminum sulfate, basic aluminum acetate, aluminum hydroxide, boehmite, pseudo-boehmite, transition alumina (such as γ-alumina, δ-alumina, θ-alumina), α-alumina, and mixed alumina having two or more crystal phases can be used. The physical forms such as the shape, particle size, and specific surface area of the aluminum compound as these precursors are not particularly limited.

[0091] According to the flux method described in detail below, the shape of the aluminum compound can be preferably used in any form such as spherical, amorphous, a structure with a high aspect ratio (wire, fiber, ribbon, tube, etc.), and sheet.

[0092] Similarly, according to the flux method described in detail below, aluminum compound solids with a particle size ranging from several nm to several hundred μm can be preferably used as the aluminum compound.

[0093] The specific surface area of the aluminum compound is not particularly limited. Since the molybdenum compound acts effectively, a larger specific surface area is preferred. However, by adjusting the firing conditions and the amount of the molybdenum compound used, any specific surface area can be used as a raw material.

[0094] Further, the aluminum compound may consist only 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 using an organosilane, an aluminum compound composite adsorbed with a polymer, etc. can also be preferably used. When using these composites, the content of the organic compound is not particularly limited, but from the viewpoint of efficiently producing plate-shaped alumina particles, the content is preferably 60% by mass or less, and more preferably 30% by mass or less.

[0095] (Shape control agent) In order to form the plate-shaped alumina particles according to the embodiment, a shape control agent can be used. The shape control agent plays an important role in the growth of plate-shaped crystals of alumina by firing an alumina compound in the presence of a molybdenum compound.

[0096] The state of existence of the shape control agent is not particularly limited. For example, a physical mixture of the shape control agent and the aluminum compound, a composite in which the shape control agent is uniformly or locally present on the surface or inside of the aluminum compound, etc. can be preferably used.

[0097] Also, the shape control agent may be added to the aluminum compound or may be contained as an impurity in the aluminum compound.

[0098] The shape control agent plays an important role in the growth of plate-like crystals. In the molybdenum oxide flux method, 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 self-shaped face (113)-developed hexagonal bipyramidal polyhedral particles. In the manufacturing method of the embodiment, the shape control agent localizes near the particle surface during the α-alumina growth process, significantly inhibiting the growth of the self-shaped face (113). As a result, the growth of the crystal orientation in the plane direction becomes relatively faster, and the (001) plane or the (006) plane grows, enabling the formation of a plate-like morphology. By using a molybdenum compound as a fluxing agent, plate-like alumina particles containing molybdenum with a high α-crystallization rate can be more easily formed.

[0099] It should be noted that the above mechanism is merely speculative, and even if the effects of the present invention are obtained by a mechanism different from the above mechanism, it is included in the technical scope of the present invention.

[0100] Regarding the type of the shape control agent, from the viewpoint of being able to produce plate-like alumina particles with a higher aspect ratio, better dispersibility, and better productivity, it is preferable to use at least one selected from the group consisting of silicon, silicon compounds, and germanium compounds. Silicon or a silicon compound and a germanium compound can be used in combination. From the viewpoint of being able to efficiently produce mullite as the Si supply source of mullite, it is preferable to use a silicon compound containing silicon or silicon element as the shape control agent. Also, from the viewpoint of being able to produce plate-like alumina particles with a higher aspect ratio and a larger particle size than when using silicon or a silicon compound, it is preferable to use a germanium compound as the shape control agent. By using the above flux method with silicon or a silicon compound as the shape control agent, plate-like alumina particles containing mullite on the surface can be easily produced. As a shape control agent, plate-like alumina particles containing germanium or a germanium compound can be easily produced by the above flux method using a raw material germanium compound.

[0101] · Silicon or a silicon compound The silicon compound containing silicon or a silicon element is not particularly limited, and known ones can be used. Specific examples of the silicon compound containing silicon or a silicon element include metallic silicon, organic silane, silicon resin, silica fine particles, silica gel, mesoporous silica, artificial synthetic silicon compounds such as SiC and mullite; natural silicon compounds such as biosilica, etc. Among these, from the viewpoint that the composite and mixing with an aluminum compound can be formed more uniformly, it is preferable to use organic silane, silicon resin, or silica fine particles. Note that the silicon compound containing silicon or a silicon element may be used alone or in combination of two or more. Also, as long as the effects of the present invention are not impaired, it may be used in combination with other shape control agents.

[0102] The shape of the silicon compound containing silicon or a silicon element is not particularly limited, and for example, spherical, amorphous, a structure with a high aspect ratio (wire, fiber, ribbon, tube, etc.), sheet, etc. can be preferably used.

[0103] · Germanium compound The raw material germanium compound used as a shape control agent is not particularly limited, and known ones can be used. Specific examples of the raw material germanium compound include germanium metal, germanium dioxide, germanium monoxide, germanium tetrachloride, organic germanium compounds having a Ge-C bond, etc. Note that the raw material germanium compound may be used alone or in combination of two or more. Also, as long as the effects of the present invention are not impaired, it may be used in combination with other shape control agents.

[0104] The shape of the starting germanium compound is not particularly limited, and for example, spherical, amorphous, structures with a high aspect ratio (such as wires, fibers, ribbons, tubes, etc.), sheets, etc. can be preferably used.

[0105] (molybdenum compound) As described later, the molybdenum compound functions as a flux in the α-crystal growth of alumina. The molybdenum compound is not particularly limited, but examples include molybdenum oxide, compounds containing an acid radical anion (MoO x n- ) formed by the bond of molybdenum metal with oxygen.

[0106] The compounds containing the acid radical anion (MoO x n- ) are not particularly limited, but examples include molybdic acid, sodium molybdate, potassium molybdate, lithium molybdate, H 3 PMo 12 O 40 , H 3 SiMo 12 O 40 , NH 4 Mo 7 O 12 , molybdenum disulfide, etc.

[0107] It is also possible to include silicon in the molybdenum compound. In that case, the molybdenum compound containing silicon serves both as a flux and a shape control agent.

[0108] Among the above-mentioned molybdenum compounds, molybdenum oxide is preferably used because it is easy to sublime and from the perspective of cost. Also, the above-mentioned molybdenum compounds may be used alone or in combination of two or more.

[0109] Also, potassium molybdate (K 2 Mo n O 3n+1, (n = 1 to 3) contains potassium and may also have the function as a potassium compound described below. In the manufacturing method of the embodiment, using potassium molybdate as a fluxing agent is synonymous with using a molybdenum compound and a potassium compound as a fluxing agent.

[0110] (Potassium compound) A potassium compound may be further used in combination with a shape control agent. The potassium compound is not particularly limited, and examples include potassium chloride, potassium chlorite, potassium chlorate, potassium sulfate, potassium hydrogen sulfate, potassium sulfite, potassium hydrogen sulfite, potassium nitrate, potassium carbonate, potassium hydrogen carbonate, potassium acetate, potassium oxide, potassium bromide, potassium bromate, potassium hydroxide, potassium silicate, potassium phosphate, potassium hydrogen phosphate, potassium sulfide, potassium hydrogen sulfide, potassium molybdate, potassium tungstate, etc. At this time, similar to the case of the molybdenum compound, the potassium compound includes isomers. Among these, it is preferable to use potassium carbonate, potassium hydrogen carbonate, potassium oxide, potassium hydroxide, potassium chloride, potassium sulfate, potassium molybdate, and more preferably potassium carbonate, potassium hydrogen carbonate, potassium chloride, potassium sulfate, potassium molybdate.

[0111] The above-mentioned potassium compound may be used alone or in combination of two or more.

[0112] The potassium compound contributes to the efficient formation of mullite on the alumina surface layer. Also, the potassium compound contributes to the efficient formation of a layer containing germanium on the alumina surface layer.

[0113] Also, it is preferable to use the potassium compound as a fluxing agent together with the molybdenum compound.

[0114] Among the above, potassium molybdate contains molybdenum and thus may also have the function as the above-mentioned molybdenum compound. When potassium molybdate is used as a flux agent, it can exhibit the same effects as when a molybdenum compound and a potassium compound are used as flux agents.

[0115] As the potassium compound used at the time of raw material charging or generated by the reaction in the temperature rising process during firing, a water-soluble potassium compound such as potassium molybdate does not vaporize even in the firing temperature range and can be easily recovered by washing after firing. Therefore, the amount of the molybdenum compound released outside the firing furnace is also reduced, and the production cost can be significantly reduced.

[0116] When a molybdenum compound and a potassium compound are used as flux agents, the molar ratio of the molybdenum element of the molybdenum compound to the potassium element of the potassium compound (molybdenum element / potassium element) is preferably 5 or less, more preferably 0.01 to 3, and even more preferably 0.5 to 1.5 because the production cost can be further reduced. When the molar ratio (molybdenum element / potassium element) is within the above range, it is preferable because plate-like alumina particles having a large particle size can be obtained.

[0117] (Metal compound) As described later, the metal compound may have a function of promoting the crystal growth of alumina. The metal compound can be used during firing if desired. Note that the metal compound is not essential for the production of the plate-like alumina particles according to the present invention because it has a function of promoting the crystal growth of α-alumina.

[0118] The metal compound is not particularly limited, but preferably contains at least one selected from the group consisting of Group II metal compounds and Group III metal compounds.

[0119] Examples of the Group II metal compound include magnesium compounds, calcium compounds, strontium compounds, barium compounds, and the like.

[0120] Examples of the Group III metal compound include scandium compounds, yttrium compounds, lanthanum compounds, cerium compounds, and the like.

[0121] The above-mentioned metal compounds refer to oxides, hydroxides, carbonates, and chlorides of metal elements. For example, in the case of yttrium compounds, yttrium oxide (Y 2 O 3 ), yttrium hydroxide, and yttrium carbonate can be mentioned. Among these, the metal compound is preferably an oxide of the metal element. Note that these metal compounds include isomers.

[0122] Among these, metal compounds of elements in the third period, metal compounds of elements in the fourth period, metal compounds of elements in the fifth period, and metal compounds of elements in the sixth period are preferred, metal compounds of elements in the fourth period and metal compounds of elements in the fifth period are more preferred, and metal compounds of elements in the fifth period are even more preferred. Specifically, it is preferable to use magnesium compounds, calcium compounds, yttrium compounds, lanthanum compounds, more preferably magnesium compounds, calcium compounds, yttrium compounds, and particularly preferably yttrium compounds.

[0123] The addition rate of the metal compound is preferably 0.02 to 20% by mass, more preferably 0.1 to 20% by mass, based on the mass conversion value of aluminum atoms in the aluminum compound. When the addition rate of the metal compound is 0.02% by mass or more, the crystal growth of α-alumina containing molybdenum can proceed favorably, which is preferable. On the other hand, when the addition rate of the metal compound is 20% by mass or less, it is preferable because plate-like alumina particles with a low content of impurities derived from the metal compound can be obtained.

[0124] [Yttrium] When an aluminum compound is calcined in the presence of a yttrium compound as a metal compound, crystal growth proceeds more favorably in this calcination step, and α-alumina and a water-soluble yttrium compound are formed. At this time, since the water-soluble yttrium compound tends to localize on the surface of α-alumina, which is plate-shaped alumina particles, if necessary, the yttrium compound can be removed from the plate-shaped alumina particles by washing with water, alkaline water, a heated liquid thereof, etc.

[0125] The usage amounts of the above aluminum compound, molybdenum compound, silicon or silicon compound, germanium compound, potassium compound, etc. are not particularly limited, but for example, when the total amount of raw materials in terms of oxide is 100% by mass, the following mixtures can be calcined. 1) Al 2 O 3 In terms of conversion, preferably 50% by mass or more of an aluminum compound, more preferably 70% by mass or more and 99% by mass or less of an aluminum compound, still more preferably 80% by mass or more and 94.5% by mass or less of an aluminum compound, and MoO 3 In terms of conversion, preferably 40% by mass or less of a molybdenum compound, more preferably 0.5% by mass or more and 20% by mass or less of a molybdenum compound, still more preferably 1% by mass or more and 7% by mass or less of a molybdenum compound, and SiO 2 In terms of conversion or GeO 2 In terms of conversion, preferably 0.1% by mass or more and 10% by mass or less of silicon, a silicon compound or a germanium compound, more preferably 0.5% by mass or more and less than 7% by mass of silicon, a silicon compound or a germanium compound, still more preferably 0.8% by mass or more and 4% by mass or less of silicon, a silicon compound or a germanium compound, and A mixture obtained by mixing.

[0126] From the viewpoint of obtaining plate-shaped alumina particles with a larger particle size, in the above mixture, MoO 3In terms of conversion, it is preferable to use a molybdenum compound of preferably 7% by mass or more and 40% by mass or less, more preferably 9% by mass or more and 30% by mass or less, and even more preferably 10% by mass or more and 17% by mass or less.

[0127] From the viewpoint of obtaining plate-shaped alumina particles with a larger particle size, in the above mixture, SiO 2 in terms of conversion and / or GeO 2 in terms of conversion, it is preferable to use a silicon, silicon compound and / or germanium compound of preferably 0.4% by mass or more and less than 10% by mass, more preferably 0.5% by mass or more and 10% by mass or less, and particularly preferably 1% by mass or more and 3% by mass or less.

[0128] As the silicon, silicon compound and / or germanium compound of the above shape control agent, it may be a silicon or silicon compound, or it may be a germanium compound. As the above shape control agent, only a silicon or silicon compound may be used, only a germanium compound may be used, or only a combination of a silicon or silicon compound and a germanium compound may be used. When using a germanium compound as the shape control agent, when the total amount of the raw materials in terms of oxide is 100% by mass, GeO 2 in terms of conversion, a germanium compound of preferably 0.4% by mass or more and less than 1.5% by mass, more preferably 0.7% by mass or more and 1.2% by mass or less may be blended into the mixture.

[0129] The above conditions for the raw material formulation (mass%) may be freely combined for each raw material, and the lower limit value and the upper limit value in each raw material formulation (mass%) can also be freely combined.

[0130] By using various compounds within the above range, the value of the above (006 / 113) ratio can be satisfied, and plate-shaped alumina particles excellent in brilliance can be easily produced.

[0131] When the mixture further contains the above potassium compound, the amount of the potassium compound used is not particularly limited. However, when the total amount of the raw materials in terms of oxide is 100% by mass, preferably, in terms of K 2 2O, it is 5% by mass or less, more preferably 0.01% by mass or more and 3% by mass or less, and still more preferably 0.05% by mass or more and 1% by mass or less of the potassium compound can be mixed. It is considered that potassium molybdate formed by the reaction with the molybdenum compound due to the use of the potassium compound contributes to promoting the formation of mullite on the surface of the plate-shaped alumina particles by exerting the effect of Si diffusion. Similarly, it is considered that potassium molybdate formed by the reaction with the molybdenum compound due to the use of the potassium compound contributes to promoting the formation of germanium or germanium compound on the surface of the plate-shaped alumina particles by exerting the effect of raw material germanium diffusion. As the potassium compound used at the time of raw material charging or generated by the reaction during the heating process in firing, a water-soluble potassium compound such as potassium molybdate does not vaporize even in the firing temperature range and can be easily recovered by washing after firing. Therefore, the amount of the molybdenum compound released outside the firing furnace is also reduced, and the production cost can be significantly reduced.

[0132] In the flux method, it is also preferable to use the molybdenum compound and the potassium compound as the fluxing agent. In addition, a compound containing molybdenum and potassium as the fluxing agent can be generated, for example, from a more inexpensive and easily available molybdenum compound and potassium compound during the firing process. Here, when the molybdenum compound and the potassium compound are used as the fluxing agent, the case where a compound containing molybdenum and potassium is used as the fluxing agent, and both cases will be described by taking the case where the molybdenum compound and the potassium compound are used as the fluxing agent as an example.

[0133] From the viewpoint of obtaining plate-shaped alumina particles with a larger particle size, the usage amounts of the above aluminum compound, molybdenum compound, potassium compound, and silicon or silicon compound can preferably be as follows when the total amount of raw materials in terms of oxides is 100% by mass. 2) Al 2 O 3 In terms of conversion, an aluminum compound of 10% by mass or more, a molybdenum compound of 20% by mass or more in terms of MoO 3 In terms of conversion, a potassium compound of 1% by mass or more in terms of K 2 O, and a silicon or silicon compound of less than 1% by mass in terms of SiO 2 In terms of conversion, a mixture obtained by mixing them. In terms of the point that the content of hexagonal plate-shaped alumina can be further increased, when the total amount of raw materials in terms of oxides is 100% by mass, it is more preferable to use the following mixture. 3) Al 2 O 3 In terms of conversion, an aluminum compound of 20% by mass or more and 70% by mass or less, a molybdenum compound of 30% by mass or more and 80% by mass or less in terms of MoO 3 In terms of conversion, a potassium compound of 5% by mass or more and 30% by mass or less in terms of K 2 O, and a silicon or silicon compound of 0.001% by mass or more and 0.3% by mass or less in terms of SiO 2 In terms of conversion, a mixture obtained by mixing them. In terms of the point that the content of hexagonal plate-shaped alumina can be further increased, when the total amount of raw materials in terms of oxides is 100% by mass, it is even more preferable to use the following mixture. 4) Al 2 O 3 In terms of conversion, an aluminum compound of 25% by mass or more and 40% by mass or less, a molybdenum compound of 45% by mass or more and 70% by mass or less in terms of MoO 3 In terms of conversion, a potassium compound of 10% by mass or more and 20% by mass or less in terms of K 2 O, and a silicon or silicon compound of 0.01% by mass or more and 0.1% by mass or less in terms of SiO 2 In terms of conversion, a mixture obtained by mixing them. The content of hexagonal plate-shaped alumina can be maximized, and in order to more suitably promote crystal growth, it is particularly preferable to use the following mixture. 5) When the total amount of raw materials in terms of oxides is 100% by mass, Al2 O 3 An aluminum compound of 35% by mass or more and 40% by mass or less in terms of conversion, and MoO 3 A molybdenum compound of 45% by mass or more and 65% by mass or less in terms of conversion, and K 2 A potassium compound of 10% by mass or more and 20% by mass or less in terms of K2O conversion, and SiO 2 A mixture obtained by mixing silicon or a silicon compound of 0.02% by mass or more and 0.08% by mass or less in terms of conversion.

[0134] By blending various compounds within the above ranges, plate-shaped alumina particles that are plate-shaped, have a large particle size, and are more excellent in luster can be produced. In particular, by increasing the amount of molybdenum used and decreasing the amount of silicon used to some extent, the particle size and crystallite size can be further increased, and hexagonal plate-shaped alumina particles are more likely to be obtained. By blending various compounds within the above more preferable ranges, hexagonal plate-shaped alumina particles are more likely to be obtained, and their content can be further increased, and the luster of the obtained alumina particles tends to be further excellent.

[0135] When the mixture further contains the above yttrium compound, the amount of the yttrium compound used is not particularly limited, but when the total amount of the raw materials in terms of oxide conversion is 100% by mass, preferably 5% by mass or less in terms of Y2O3, more preferably, 0.01% by mass or more and 3% by mass or less of the yttrium compound can be mixed. In order to more preferably promote crystal growth, more preferably, when the total amount of the raw materials in terms of oxide conversion is 100% by mass, 0.1% by mass or more and 1% by mass or less of the yttrium compound in terms of Y2O3 can be mixed. 2 O 3 2 O 3

[0136] The numerical ranges of the amounts of use of the above raw materials can be appropriately combined within the range where their total content does not exceed 100% by mass.

[0137] [Firing process] The firing step is a step of firing an aluminum compound in the presence of a molybdenum compound and a shape control agent. The firing step may be a step of firing the mixture obtained in the mixing step.

[0138] The plate-like alumina particles can be obtained, for example, by firing an aluminum compound in the presence of a molybdenum compound and a shape control agent. As described above, this production method is called the flux method.

[0139] The flux method is classified as a solution method. More specifically, the flux method is a crystal growth method that utilizes the fact that the crystal-flux two-component phase diagram shows a eutectic type. The mechanism of the flux method is presumed to be as follows. That is, when a mixture of a solute and a flux is heated, the solute and the flux become a liquid phase. At this time, since the flux is a fluxing agent, in other words, since the solute-flux two-component phase diagram shows a eutectic type, the solute melts at a temperature lower than its melting point and constitutes a liquid phase. In this state, when the flux is evaporated, the concentration of the flux decreases. In other words, the effect of lowering the melting point of the solute by the flux is reduced, and the evaporation of the flux becomes the driving force for the crystal growth of the solute to occur (flux evaporation method). Incidentally, the crystal growth of the solute can also occur by cooling the liquid phase of the solute and the flux (slow cooling method).

[0140] The flux method has advantages such as being able to grow crystals at a temperature much lower than the melting point, being able to precisely control the crystal structure, and being able to form euhedral polyhedral crystals.

[0141] In the production of α-aluminum oxide particles by the flux method using a molybdenum compound as a flux, the mechanism is not necessarily clear, but it is presumed to be due to the following mechanism, for example. That is, when an aluminum compound is fired in the presence of a molybdenum compound, first, aluminum molybdate is formed. At this time, as can be understood from the above description, the aluminum molybdate grows α-aluminum oxide crystals at a temperature lower than the melting point of alumina. Then, for example, through the decomposition of aluminum molybdate, evaporation of the flux, etc., crystal growth is accelerated to obtain alumina particles. That is, the molybdenum compound functions as a flux, and α-aluminum oxide particles are produced via an intermediate of aluminum molybdate.

[0142] In the production of α-aluminum oxide particles by the flux method when a potassium compound is further used as a flux agent, the mechanism is not necessarily clear, but it is presumed to be due to the following mechanism, for example. First, the molybdenum compound and the aluminum compound react to form aluminum molybdate. Then, for example, the aluminum molybdate decomposes into molybdenum oxide and alumina, and at the same time, the molybdenum compound containing molybdenum oxide obtained by the decomposition reacts with the potassium compound to form potassium molybdate. By the crystal growth of alumina in the presence of the molybdenum compound containing potassium molybdate, the plate-like alumina particles according to the embodiment can be obtained.

[0143] By the above flux method, plate-like alumina particles satisfying the value of the above (006 / 113) ratio and having excellent luster can be produced.

[0144] The firing method is not particularly limited and can be performed by known and commonly used methods. When the firing temperature exceeds 700 °C, the aluminum compound and the molybdenum compound react to form aluminum molybdate. Further, when the firing temperature becomes 900 °C or higher, the aluminum molybdate decomposes, and plate-like alumina particles are formed by the action of the shape control agent. Also, in the plate-like alumina particles, it is considered that when the aluminum molybdate decomposes into alumina and molybdenum oxide, the molybdenum compound is incorporated into the alumina particles. Further, when the firing temperature becomes 900 °C or higher, it is considered that the molybdenum compound (for example, molybdenum trioxide) obtained by the decomposition of aluminum molybdate reacts with the potassium compound to form potassium molybdate. Furthermore, when the firing temperature becomes 1000 °C or higher, in the presence of molybdenum, along with the crystal growth of the plate-like alumina particles, the Al 2 O 3 and SiO 2 react to form mullite with high efficiency. Similarly, when the firing temperature becomes 1000 °C or higher, in the presence of molybdenum, along with the crystal growth of the plate-like alumina particles, the Al 2 O 3 on the surface of the plate-like alumina particles reacts with the Ge compound to form germanium dioxide or a compound having Ge—O—Al with high efficiency.

[0145] Also, when firing, the states of the aluminum compound, the shape control agent, and the molybdenum compound are not particularly limited, and it is sufficient that the molybdenum compound and the shape control agent are present in the same space where they can act on the aluminum compound. Specifically, simple mixing of powders of the molybdenum compound, the shape control agent, and the aluminum compound, mechanical mixing using a pulverizer or the like, or mixing using a mortar or the like may be used, and dry-state or wet-state mixing may be used.

[0146] There is no particular limitation on the firing temperature conditions, and the above-mentioned (006 / 113) ratio value, average particle diameter, aspect ratio, formation of mullite, and the above-mentioned longitudinal relaxation time T 1It is appropriately determined according to values such as the value and dispersibility. Usually, for the firing temperature, the maximum temperature is aluminum molybdate (Al 2 (MoO 4 ) 3 ) and is preferably 900°C or higher, which is the decomposition temperature, more preferably 1000°C or higher at which mullite and germanium compounds are formed efficiently. For the above-mentioned longitudinal relaxation time T 1 , it is more preferably 1200°C or higher at which plate-like alumina particles with a high crystallinity of 5 seconds or more can be easily obtained.

[0147] Generally, when attempting 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, from the perspective of the burden on the firing furnace and fuel costs, there are major issues for industrial use.

[0148] The production method of the embodiment can be implemented even at a high temperature exceeding 2000°C, but even at a temperature much lower than the melting point of α-alumina, which is 1600°C or lower, α-alumina with a high α-crystallization rate and a plate-like shape with a high aspect ratio can be formed regardless of the shape of the precursor.

[0149] According to an embodiment of the present invention, even under the condition that the maximum firing temperature is 900 to 1600°C, plate-like alumina particles with a high aspect ratio and an α-crystallization rate of 90% or more can be formed efficiently at low cost. Firing at a maximum temperature of 950 to 1500°C is more preferable, firing in the range of a maximum temperature of 1000 to 1400°C is even more preferable, and firing at a maximum temperature of 1200 to 1400°C is most preferable.

[0150] Regarding the firing time, it is preferable to raise the temperature to the predetermined maximum temperature in the range of 15 minutes to 10 hours and hold the temperature at the maximum firing temperature in the range of 5 minutes to 30 hours. To efficiently form plate-like alumina particles, it is more preferable to have a firing holding time of about 10 minutes to 15 hours. By selecting the conditions of a maximum temperature of 1000 to 1400 °C and a firing holding time of 10 minutes to 15 hours, dense α-crystalline polygonal plate-like alumina particles are difficult to aggregate and can be easily obtained. By selecting the conditions of a maximum temperature of 1200 to 1400 °C and a firing holding time of 10 minutes to 15 hours, the longitudinal relaxation time T 1 of 5 seconds or more (high crystallinity) plate-like alumina particles can be easily obtained.

[0151] 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 preferable, and an air atmosphere is more preferable in consideration of cost.

[0152] The apparatus for firing is not necessarily limited, and a so-called firing furnace can be used. The firing furnace is preferably made of a material that does not react with sublimated molybdenum oxide, and it is more preferable to use a highly airtight firing furnace so as to efficiently utilize molybdenum oxide.

[0153] In obtaining the above alumina particles, it is preferable to obtain them by firing an aluminum compound in the presence of a molybdenum compound and a shape control agent, or in the presence of a molybdenum compound, a shape control agent, a potassium compound, and a metal oxide.

[0154] That is, a preferable method for producing alumina particles includes a step of firing an aluminum compound (firing step) in the presence of a molybdenum compound and a shape control agent, or in the presence of a molybdenum compound, a shape control agent, and a potassium compound. The mixture preferably further contains the above metal compound. As the metal compound, a yttrium compound is preferable.

[0155] In the flux method using a molybdenum compound, aluminum molybdate is formed by the reaction of molybdenum oxide with an aluminum compound. Then, since the change in chemical potential during the decomposition of this aluminum molybdate serves as the driving force for crystallization, hexagonal dipyramidal polyhedral particles with well-developed (113) planes are formed. And it is speculated that the growth of the (113) plane is significantly inhibited when the shape control agent localizes near the particle surface during the α-alumina growth process, resulting in relatively faster growth in the crystal orientation in the plane direction, and the (001) plane or (006) plane grows to form a plate-like morphology. Therefore, by using a molybdenum compound as a fluxing agent, plate-like alumina particles containing molybdenum with a high α-crystallization rate can be more easily formed.

[0156] [Cooling step] When a molybdenum compound and a potassium compound are used as fluxing agents, the method for producing alumina particles may include a cooling step. The cooling step is a step of cooling the alumina that has undergone crystal growth in the firing step. More specifically, it may be a step of cooling a composition containing the alumina obtained by the firing step and the liquid-phase fluxing agent.

[0157] The cooling rate is not particularly limited, but it is preferably 1 to 1000 °C / hour, more preferably 5 to 500 °C / hour, and even more preferably 50 to 100 °C / hour. When the cooling rate is 1 °C / hour or more, the production time can be shortened, which is preferable. On the other hand, when the cooling rate is 1000 °C / hour or less, the firing container is less likely to crack due to heat shock and can be used for a long time, which is preferable.

[0158] The cooling method is not particularly limited, and it may be natural cooling or the use of a cooling device.

[0159] [Post-treatment step] The method for manufacturing plate-shaped alumina particles according to the embodiment may include a post-treatment step. The post-treatment step is a post-treatment step for the plate-shaped alumina particles and is a step of removing the fluxing agent. The post-treatment step may be performed after the above-described firing step, after the above-described cooling step, or after both the firing step and the cooling step. Further, if necessary, it may be repeated two or more times.

[0160] Examples of the post-treatment method include washing and high-temperature treatment. These can be performed in combination.

[0161] The washing 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.

[0162] At this time, the molybdenum content can be controlled by appropriately changing the concentration, usage amount of the water, aqueous ammonia solution, aqueous sodium hydroxide solution, acidic aqueous solution to be used, and the washing site, washing time, etc.

[0163] Examples of the high-temperature treatment method include a method of raising the temperature to a temperature equal to or higher than the sublimation point or boiling point of the flux.

[0164] [Grinding step] The fired product may have the plate-shaped alumina particles aggregated and not satisfy the range of particle diameters suitable for the present invention. Therefore, the plate-shaped alumina particles may be ground as necessary so as to satisfy the range of particle diameters suitable for the present invention. The method for grinding the fired product is not particularly limited, and conventionally known grinding methods such as a ball mill, jaw crusher, jet mill, disk mill, spectromill, grinder, mixer mill, etc. can be applied.

[0165] [Classification step] The plate-shaped alumina particles are preferably classified in order to adjust the average particle size, improve the fluidity of the powder, or suppress an increase in viscosity when blended with a binder for forming a matrix. "Classification" refers to an operation of grouping particles according to their size. The classification may be either wet or dry, but from the viewpoint of productivity, dry classification is preferred. For dry classification, in addition to classification by a sieve, there is also air classification that classifies by the difference between centrifugal force and fluid resistance. From the viewpoint of classification accuracy, air classification is preferred and can be performed using a classifier such as an air classifier utilizing the Coandă effect, a cyclone air classifier, a forced vortex centrifugal classifier, or a semi-free vortex centrifugal classifier. The above-described pulverization step and classification step can be performed at necessary stages, including before and after the organic compound layer formation step described later. By the presence or absence of these pulverization and classification and the selection of their conditions, for example, the average particle size of the obtained plate-shaped alumina particles can be adjusted.

[0166] The plate-shaped alumina particles of the embodiment or the plate-shaped alumina particles obtained by the production method of the embodiment, those with little or no aggregation are likely to exhibit their original properties, are excellent in their own handleability, and are also preferred from the viewpoint of being more excellent in dispersibility when dispersed in a dispersion medium and used. In the production method of plate-shaped alumina particles, if particles with little or no aggregation can be obtained without performing the above-described pulverization step and classification step, there is no need to perform the above steps, and plate-shaped alumina having the desired excellent properties can be produced with high productivity, which is preferable.

[0167] [Inorganic coating part formation step] Next, an inorganic coating part is formed on the surface of the plate-shaped alumina particles obtained above. In this embodiment, an inorganic coating part composed of an oxide can be formed. The method of layer formation is not particularly limited, and examples include a liquid phase method and a gas phase method. For forming the inorganic coating part, any of the inorganic chemical species described above can be used. In a preferred embodiment of the present invention, examples of the inorganic chemical species include metal oxides. When forming a coating part of a metal oxide on alumina particles, a liquid medium dispersion of alumina particles containing molybdenum and the metal oxide itself or its dispersion can be mixed, followed by filtration and drying. Further, when it is desired to enhance the interaction between the alumina particles and the metal oxide and exhibit particularly outstanding properties such as obtaining higher coating characteristics, obtaining a more uniform inorganic coating part, and making the obtained inorganic coating part difficult to peel from the alumina particles, a solution of a metal inorganic salt soluble in a liquid medium corresponding to a precursor of the metal oxide and alumina particles containing molybdenum or its liquid medium dispersion are mixed, and after sufficiently contacting the dissolved molecular metal inorganic salt with the alumina particles containing molybdenum, it is preferable to convert the fine metal inorganic salt with a size of 150 nm or less deposited on the alumina particles into a metal oxide. If necessary, further filtration and drying can also be performed. When converting the metal inorganic salt into a metal oxide, if the conversion is not easy due to low temperature or pH change, firing can be performed if necessary. By doing so, a strong interaction between the alumina particles and the metal oxide, which is not present in a simple mixture, can be exhibited, and the particularly outstanding properties described above can be easily exhibited. The firing conditions in the process of forming the inorganic coating part may be appropriately selected based on the conditions of the above-mentioned alumina particles and then adopted after selecting the optimal conditions.

[0168] As the liquid phase method, for example, a dispersion in which plate-shaped alumina particles are dispersed is prepared. After adjusting the pH and heating the dispersion as necessary, an aqueous solution of a metal chloride such as titanium chloride is dropped into the dispersion. At this time, it is preferable to maintain the pH constant with an alkaline aqueous solution. Then, the dispersion is stirred for a predetermined time, followed by filtration, washing, and drying to obtain a powder. Thereby, an inorganic coating part composed of an oxide such as titanium oxide is formed on the surface of the alumina particles having a plate shape. Further, as other oxides, an inorganic coating part may be formed of iron oxide or silica. Furthermore, an inorganic coating part may be formed of a plurality selected from titanium oxide, iron oxide, and silica.

[0169] In this process, instead of the oxide, an inorganic coating portion made of a metal can also be formed. In this case, the inorganic coating portion can be formed by a liquid phase method, a gas phase method, or the like. For example, an inorganic coating portion is formed on the surface of plate-shaped alumina particles with silver, nickel, copper, gold, or platinum. Also, an inorganic coating portion may be formed with a plurality selected from silver, nickel, copper, gold, and platinum.

[0170] Also, in this process, an inorganic coating layer may be formed so as to cover at least a part of the surface of the plate-shaped alumina particles. In this case, for example, layers are formed in a state where metal oxides or particles made of a metal are aggregated with each other.

[0171] [Organic Compound Layer Formation Process] In one embodiment, the method for producing plate-shaped alumina particles may further include an organic compound layer formation process of forming an organic compound layer on the surface of the inorganic coating portion (also referred to as the composite particle surface) after the inorganic coating portion formation process. The organic compound layer formation process is usually performed after the firing process or after the post-treatment process.

[0172] The method for forming the organic compound layer is not particularly limited, and known methods can be appropriately employed. For example, a method of bringing a liquid containing an organic compound into contact with plate-shaped alumina particles containing molybdenum and drying it can be mentioned.

[0173] Incidentally, examples of the organic compound that can be used for forming the organic compound layer include organic silane compounds.

[0174] (Organic Silane Compound) In the case where the plate-shaped alumina particles contain silicon atoms and / or inorganic silicon compounds, the above-described surface modification effect can be expected as compared with the case where they do not contain them. Furthermore, they can also be used after being made into a reaction product of alumina particles containing silicon atoms and / or inorganic silicon compounds and an organic silane compound. Compared with plate-shaped alumina particles containing silicon atoms and / or inorganic silicon compounds, plate-shaped alumina particles which are a reaction product of the former and an organic silane compound are preferable because the affinity with the matrix can be made better based on the reaction between silicon atoms and / or inorganic silicon compounds localized on the surface of the plate-shaped alumina particles and the organic silane compound.

[0175] Examples of the organic silane compound include alkyltrimethoxysilanes or alkyltrichlorosilanes having 1 to 22 carbon atoms in the alkyl group such as methyltrimethoxysilane, dimethyldimethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, iso-propyltrimethoxysilane, iso-propyltriethoxysilane, pentyltrimethoxysilane, hexyltrimethoxysilane, etc., 3,3,3-trifluoropropyltrimethoxysilane, tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilanes, phenyltrimethoxysilane, phenyltriethoxysilane, p-chloromethylphenyltrimethoxysilane, p-chloromethylphenyltriethoxysilanes, etc., epoxy silanes such as γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, etc., aminosilanes such as γ-aminopropyltriethoxysilane, N-β(aminoethyl)γ-aminopropyltrimethoxysilane, N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane, γ-aminopropyltrimethoxysilane, γ-ureidopropyltriethoxysilane, etc., mercaptosilanes such as 3-mercaptopropyltrimethoxysilane, p-styryltrimethoxysilane, vinyltrichlorosilane, vinyltris(β-methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, vinylsilanes such as γ-methacryloxypropyltrimethoxysilane, and further, epoxy-based, amino-based, and vinyl-based polymer type silanes. The above organic silane compound may be contained alone or in combination of two or more.

[0176] The organic silane compound may be linked to at least a part or all of the silicon atoms on the surface of the plate-like alumina particles and / or the inorganic silicon compound by a covalent bond through a reaction, and not only a part of the alumina but also the whole may be coated with the above reactant. As a method for providing it on the alumina surface, adhesion by immersion or chemical vapor deposition (CVD) can be employed.

[0177] The amount of the organosilane compound used is preferably 20% by mass or less, more preferably 10 - 0.01% by mass, based on the mass of silicon atoms or inorganic silicon compounds contained on the surface of the plate-shaped alumina particles, in terms of silicon atoms. When the amount of the organosilane compound used is 20% by mass or less, it is preferable because the physical properties derived from the alumina particles can be easily exhibited.

[0178] The reaction between the alumina particles containing silicon atoms and / or inorganic silicon compounds and the organosilane compound can be carried out by a publicly known and commonly used method for surface modification of fillers. For example, a spraying method using a fluid nozzle, stirring with shear force, a dry method such as a ball mill or a mixer, or a wet method such as an aqueous system or an organic solvent system can be employed. The treatment using shear force is desirably carried out to such an extent that the alumina particles used in the embodiment are not broken.

[0179] The temperature in the system in the dry method or the drying temperature after treatment in the wet method is appropriately determined in a region where the organosilane compound does not thermally decompose, depending on the type of the organosilane compound. For example, when treating with the organosilane compound as described above, a temperature of 80 - 150°C is desirable.

[0180] <Resin composition> As one embodiment, a resin composition containing a resin and the composite particles of the embodiment is provided. The resin is not particularly limited, and examples thereof include thermosetting resins and thermoplastic resins.

[0181] The resin composition can be cured to obtain a cured product of the resin composition, and can be cured and molded to obtain a molded product of the resin composition. For molding, treatments such as melting and kneading can be appropriately performed on the resin composition. Examples of the molding method include compression molding, injection molding, extrusion molding, and foam molding. Among them, extrusion molding using an extruder is preferable, and extrusion molding using a twin-screw extruder is more preferable. When the resin composition is used as a coating agent, a paint, or the like, the resin composition can be applied to an object to be coated to form a coating film having a cured product of the resin composition.

[0182] <Method for producing resin composition> According to one embodiment of the present invention, a method for producing a resin composition is provided.

[0183] The production method includes a step of mixing the composite particles of the embodiment and a resin. Since the plate-like alumina particles described above can be used, the description thereof is omitted here.

[0184] In addition, surface-treated composite particles can be used.

[0185] Also, only one type of composite particles may be used, or two or more types may be used in combination.

[0186] Furthermore, the composite particles may be used in combination with other fillers (such as alumina, spinel, boron nitride, aluminum nitride, magnesium oxide, magnesium carbonate, etc.).

[0187] The content of the composite particles is preferably 5 to 95% by mass, more preferably 10 to 90% by mass, and even more preferably 30 to 80% by mass with respect to 100% by mass of the mass of the resin composition. When the content of the composite particles is 5% by mass or more, it is preferable because the high thermal conductivity of the composite particles can be efficiently exhibited. On the other hand, when the content of the composite particles is 95% by mass or less, it is preferable because a resin composition excellent in moldability can be obtained. When the resin composition is used as a coating agent, paint, etc., from the viewpoint of exhibiting excellent glossiness and facilitating the formation of a coating film, the content of the composite particles is preferably 0.1 to 95% by mass, more preferably 1 to 50% by mass, and even more preferably 3 to 30% by mass with respect to 100% by mass of the solid content mass of the resin composition.

[0188] (Resin) The resin is not particularly limited, and examples thereof include thermoplastic resins and thermosetting resins.

[0189] The thermoplastic resin is not particularly limited, and known and commonly used resins for molding materials and the like can be used. Specifically, polyethylene resin, polypropylene resin, polymethyl methacrylate resin, polyvinyl acetate resin, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, polyvinyl chloride resin, polystyrene resin, polyacrylonitrile resin, polyamide resin, polycarbonate resin, polyacetal resin, polyethylene terephthalate resin, polyphenylene oxide resin, polyphenylene sulfide resin, polysulfone resin, polyethersulfone resin, polyetheretherketone resin, polyallylsulfone resin, thermoplastic polyimide resin, thermoplastic urethane resin, polyaminobismaleimide resin, polyamideimide resin, polyetherimide resin, bismaleimide triazine resin, polymethylpentene resin, fluorinated resin, liquid crystal polymer, olefin-vinyl alcohol copolymer, ionomer resin, polyarylate resin, acrylonitrile-ethylene-styrene copolymer, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-styrene copolymer, etc. can be mentioned.

[0190] The thermosetting resin is a resin having the property of being able to change to substantially insoluble and infusible when cured by means such as heating, radiation, or a catalyst. Generally, known and commonly used resins for molding materials and the like can be used. Specifically, novolak-type phenolic resins such as phenol novolak resin and cresol novolak resin; resol-type phenolic resins such as unmodified resol phenolic resin, and oil-modified resol phenolic resin modified with tung oil, linseed oil, walnut oil, etc.; phenolic resins; bisphenol-type epoxy resins such as bisphenol A epoxy resin and bisphenol F epoxy resin; novolak-type epoxy resins such as aliphatic chain-modified bisphenol-type epoxy resin, novolak epoxy resin, and cresol novolak epoxy resin; epoxy resins such as biphenyl-type epoxy resin and polyalkylene glycol-type epoxy resin; resins having a triazine ring such as urea (carbamide) resin and melamine resin; vinyl resins such as (meth)acrylic resin and vinyl ester resin; unsaturated polyester resin, bismaleimide resin, polyurethane resin, diallyl phthalate resin, silicone resin, resin having a benzoxazine ring, cyanate ester resin, and the like can be mentioned.

[0191] The above resins may be used alone or in combination of two or more. In this case, two or more thermoplastic resins may be used, two or more thermosetting resins may be used, or one or more thermoplastic resins and one or more thermosetting resins may be used.

[0192] The content of the resin is preferably 5 to 90% by mass, more preferably 10 to 70% by mass, based on 100% by mass of the mass of the resin composition. When the content of the resin is 5% by mass or more, it is preferable because excellent moldability can be imparted to the resin composition. On the other hand, when the content of the resin is 90% by mass or less, it is preferable because high thermal conductivity can be obtained by molding into a compound.

[0193] (Curing agent) A curing agent may be mixed into the resin composition as necessary.

[0194] The hardening agent is not particularly limited, and known ones can be used.

[0195] Specifically, amine compounds, amide compounds, acid anhydride compounds, phenolic compounds, etc. can be mentioned.

[0196] Examples of the amine compounds include diaminodiphenylmethane, diethylenetriamine, triethylenetetramine, diaminodiphenylsulfone, isophoronediamine, imidazole, BF 3 -amine complex, guanidine derivatives, etc.

[0197] Examples of the amide compounds include dicyandiamide, polyamide resin synthesized from a dimer of linolenic acid and ethylenediamine, etc.

[0198] Examples of the acid anhydride compounds include phthalic anhydride, trimellitic anhydride, pyromellitic anhydride, maleic anhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylnadic anhydride, hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, etc.

[0199] Examples of the phenolic compound include polyphenol novolak resins synthesized from polyhydric hydroxy compounds and formaldehyde, such as phenol novolak resin, cresol novolak resin, aromatic hydrocarbon formaldehyde resin-modified phenol resin, dicyclopentadiene phenol addition type resin, phenol aralkyl resin (Zylock resin), resorcinol novolak resin, naphthol aralkyl resin, trimethylolmethane resin, tetraphenylol ethane resin, naphthol novolak resin, naphthol-phenol co-condensed novolak resin, naphthol-cresol co-condensed novolak resin, biphenyl-modified phenol resin (a polyhydric phenol compound in which phenol nuclei are linked by bismethylene groups), biphenyl-modified naphthol resin (a polyhydric naphthol compound in which phenol nuclei are linked by bismethylene groups), aminotriazine-modified phenol resin (a polyhydric phenol compound in which phenol nuclei are linked by melamine, benzoguanamine, etc.), alkoxy group-containing aromatic ring-modified novolak resin (a polyhydric phenol compound in which a phenol nucleus and an alkoxy group-containing aromatic ring are linked by formaldehyde), and the like.

[0200] The above curing agents may be used alone or in combination of two or more.

[0201] (Curing accelerator) A curing accelerator may be mixed into the resin composition as necessary.

[0202] The curing accelerator has a function of accelerating curing when the composition is cured.

[0203] Examples of the curing accelerator include, but are not particularly limited to, phosphorus compounds, tertiary amines, imidazoles, organic acid metal salts, Lewis acids, amine complex salts, and the like.

[0204] The above curing accelerators may be used alone or in combination of two or more.

[0205] (Curing catalyst) A curing catalyst may be mixed into the resin composition as necessary.

[0206] The curing catalyst has a function of promoting the curing reaction of a compound having an epoxy group, instead of the above-mentioned curing agent.

[0207] The curing catalyst is not particularly limited, and known and commonly used thermal polymerization initiators or active energy ray polymerization initiators can be used.

[0208] In addition, the curing catalyst may be used alone or in combination of two or more.

[0209] (Viscosity regulator) A viscosity regulator may be mixed into the resin composition as necessary.

[0210] The viscosity regulator has a function of adjusting the viscosity of the composition.

[0211] The viscosity regulator is not particularly limited, and organic polymers, polymer particles, inorganic particles, etc. can be used.

[0212] In addition, the viscosity regulator may be used alone or in combination of two or more.

[0213] (Plasticizer) A plasticizer may be mixed into the resin composition as necessary.

[0214] The plasticizer has a function of improving the processability, flexibility, weather resistance, etc. of the thermoplastic synthetic resin.

[0215] The plasticizer is not particularly limited, and phthalic acid esters, adipic acid esters, phosphoric acid esters, trimellitic acid esters, polyesters, polyolefins, polysiloxanes, etc. can be used.

[0216] In addition, the above-mentioned plasticizer may be used alone or in combination of two or more.

[0217] [Mixing step] The resin composition according to this embodiment is obtained by mixing composite particles, a resin, and, if necessary, other formulations. There are no particular limitations on the mixing method, and it is mixed by a known and commonly used method.

[0218] When the resin is a thermosetting resin, as a general method for mixing a thermosetting resin with composite particles, etc., after sufficiently mixing a predetermined blending amount of the thermosetting resin, composite particles, and other components as necessary with a mixer or the like, kneading is performed with a three-roll mill or the like to obtain a fluid liquid composition. Also, as a method for mixing a thermosetting resin with composite particles, etc. in another embodiment, after sufficiently mixing a predetermined blending amount of the thermosetting resin, composite particles, and other components as necessary with a mixer or the like, melt-kneading is performed with a mixing roll, an extruder, or the like, and then cooling is performed to obtain a solid composition. Regarding the mixing state, when a curing agent, a catalyst, or the like is blended, it is sufficient that the curable resin and these formulations are sufficiently uniformly mixed, but it is more preferable that the composite particles are also uniformly dispersed and mixed.

[0219] As a general method for mixing a thermoplastic resin with composite particles, etc. when the resin is a thermoplastic resin, the thermoplastic resin, composite particles, and other components as necessary are premixed using various mixers such as a tumbler or a Henschel mixer, and then melt-kneaded with a mixer such as a Banbury mixer, a roll, a Brabender, a single-screw kneading extruder, a twin-screw kneading extruder, a kneader, or a mixing roll. The temperature of the melt-kneading is not particularly limited, but is usually in the range of 100 to 320°C.

[0220] Since the fluidity of the resin composition and the filler filling property of the composite particles, etc. can be further enhanced, a coupling agent may be externally added to the resin composition. By externally adding the coupling agent, the adhesion between the resin and the composite particles can be further enhanced, the interfacial thermal resistance between the resin and the composite particles can be reduced, and the thermal conductivity of the resin composition can be improved.

[0221] The above-mentioned coupling agent may be used alone or in combination of two or more.

[0222] The addition amount of the coupling agent is not particularly limited, but it is preferably 0.01 to 5% by mass, more preferably 0.1 to 3% by mass, based on the mass of the resin.

[0223] According to one embodiment, the resin composition is used as a heat conductive material.

[0224] Since the composite particles contained in the resin composition are excellent in heat conductivity, the resin composition is preferably used as an insulating heat radiating member. Thereby, the heat radiating function of the device can be improved, contributing to the miniaturization and high performance of the device.

[0225] Since the composite particles contained in the resin composition are excellent in luster, the resin composition is suitably used as a coating agent, paint, etc.

[0226] <Method for manufacturing a cured product> According to one embodiment of the present invention, a method for manufacturing a cured product is provided. The manufacturing method includes curing the resin composition manufactured above.

[0227] The curing temperature is not particularly limited, but it is preferably 20 to 300°C, more preferably 50 to 200°C.

[0228] The curing time is not particularly limited, but it is preferably 0.1 to 10 hours, more preferably 0.2 to 3 hours.

[0229] The shape of the cured product varies depending on the desired application and can be appropriately designed by those skilled in the art.

[0230] In the above-described resin composition, method for manufacturing the resin composition, and cured product, composite particles having a plate shape are used, but it is not limited thereto, and composite particles having a polyhedral shape described later may also be used.

[0231] <<Second Embodiment>> <Composite particles> The composite particles according to the second embodiment include alumina particles containing molybdenum (Mo) and an inorganic coating portion provided on the surface of the alumina particles. The alumina particles of this embodiment have a polyhedral shape, and the composite particles also have a polyhedral shape. Hereinafter, in this embodiment, the alumina particles having a polyhedral shape are also referred to as "polyhedral alumina particles", "polyhedral alumina" or simply "alumina particles".

[0232] <Polyhedral alumina particles> The polyhedral alumina particles of this embodiment are particles containing molybdenum with an order of μm or less. The order of μm or less means that the average particle diameter is 1000 μm or less, including the μm region of 1 μm to 1000 μm and the nm region of less than 1000 nm.

[0233] Generally, it is known that aluminum oxide with a higher purity exhibits higher thermal conductivity. The reason is that impurity components are considered to cause phonon scattering and reduce the thermal conductivity. The alumina particles of this embodiment contain molybdenum and, in some cases, also contain impurities derived from raw materials, and exhibit high thermal conductivity despite the low content of the aluminum oxide component.

[0234] Whether the unknown aluminum oxide corresponds to the alumina particles used in the present invention can be determined, for example, by the presence or absence of coloring. The alumina particles of this embodiment are not the white particles of ordinary aluminum oxide, but are light blue to dark blue close to black, and have the characteristic that the color becomes darker as the molybdenum content increases. Also, when other small amounts of metals are mixed in, for example, the mixture of chromium results in red, and the mixture of nickel results in yellow, and the alumina particles of this embodiment are characterized by being colored particles that are not white.

[0235] [Crystal form · α crystallization rate] The alumina particles are aluminum oxide, and the crystal form is not particularly limited. For example, they can be various transition aluminas such as γ, δ, θ, κ, etc., or those containing alumina hydrate in the transition alumina. However, in terms of better mechanical strength or thermal conductivity, it is basically preferable that they are in the α crystal form.

[0236] The α-crystallinity of the alumina particles can be determined by XRD measurement in the same manner as described above. The α-crystallinity varies depending on the firing conditions and the raw materials used. From the perspective of improving the crushing strength and fluidity of the alumina particles, the α-crystallinity is preferably 90% or more, and more preferably 95% or more.

[0237] [Average particle size] The average particle size of the alumina particles is preferably 3 μm or more and 300 μm or less, and more preferably 10 μm or more and 100 μm or less. In this specification, the "average particle size of the alumina particles" refers to the value calculated as the volume-based median diameter D 50 from the volume-based cumulative particle size distribution measured by a laser diffraction dry particle size distribution analyzer.

[0238] [Maximum particle size] Also, the volume-based maximum particle size of the alumina particles (hereinafter, also simply referred to as "maximum particle size") is not particularly limited, but is usually 1000 μm or less, and more preferably 500 μm or less.

[0239] If the maximum particle size of the alumina particles is larger than the above upper limit, when used by blending with a solvent or a binder serving as a matrix, depending on the form of the final application, the alumina particles may protrude from the surface of the binder layer, which may cause poor appearance, so it is not preferable.

[0240] The average particle diameter and the maximum particle diameter of the alumina particles referred to herein can be estimated by a wet method, for example, by dispersing them in a suitable solvent, specifically, by measuring a sample in which the alumina particles are dispersed in an aqueous medium containing sodium hexametaphosphate or the like as a dispersion stabilizer using a laser diffraction / scattering particle size distribution measuring apparatus. Further, the average particle diameter and the maximum particle diameter can also be determined by a dry method in which the alumina particles themselves are measured using a laser diffraction particle size distribution meter.

[0241] [Molybdenum] The alumina particles of the embodiment contain molybdenum. Further, it is preferable that the polyhedral alumina particles contain molybdenum on their surface layer. It is presumed that this improves the selectivity of the inorganic material constituting the inorganic coating portion and enables the inorganic coating portion to be efficiently formed on the polyhedral alumina particles.

[0242] Molybdenum may be derived from a molybdenum compound used as a flux in the method for producing alumina particles described later.

[0243] Molybdenum has a catalytic function and an optical function. Further, by utilizing molybdenum, alumina particles excellent in thermal conductivity can be produced in the production method described later.

[0244] The molybdenum is not particularly limited, and includes, in addition to molybdenum metal, molybdenum oxide, a molybdenum compound partially reduced, molybdates, etc. Any polymorph of the molybdenum compound, or a combination thereof, may be contained in the polyhedral alumina particles, such as α-MoO 3 , β-MoO 3 , MoO 2 , MoO, molybdenum cluster structure, etc., may be contained in the polyhedral alumina particles.

[0245] The form of molybdenum inclusion is not particularly limited and may be included in a form attached to the surface of polyhedral alumina, may be included in a form in which a part of aluminum in the crystal structure of alumina is substituted, or may be a combination of these.

[0246] The content of molybdenum with respect to 100% by mass of polyhedral alumina particles obtained in XRF analysis is preferably 20% by mass or less in terms of molybdenum trioxide (MoO 3 ). By adjusting the firing temperature, firing time, and flux conditions, it is preferably 0.01% by mass or more and 18% by mass or less, more preferably 0.01% by mass or more and 10% by mass or less, and still more preferably 0.05% by mass or more and 5% by mass or less. When the content of molybdenum is 5% by mass or less, it is preferable because it improves the α single crystal quality of alumina.

[0247] In addition, the analysis of the amount of Mo on the surface of alumina particles can be performed using the above X-ray photoelectron spectroscopy (XPS) apparatus.

[0248] [Potassium] The alumina particles may further contain potassium.

[0249] The potassium may be derived from potassium that can be used as a flux in the method for producing alumina particles described below. By utilizing potassium, alumina particles with excellent coating efficiency can be produced with high efficiency in the method for producing alumina particles described below.

[0250] The potassium is not particularly limited, and examples include potassium metal, potassium oxide, and potassium compounds in which a part has been reduced.

[0251] The form of potassium inclusion is not particularly limited and may be included in a form attached to the surface of polyhedral alumina, may be included in a form in which a part of aluminum in the crystal structure of alumina is substituted, or may be a combination of these.

[0252] In XRF analysis, the potassium content in terms of potassium oxide (K 2 O) conversion with respect to 100% by mass of the polyhedral alumina particles is preferably 0.01% by mass or more, more preferably 0.01 to 1% by mass, even more preferably 0.03 to 0.5% by mass, and particularly preferably 0.05 to 0.3% by mass. Alumina particles having a potassium content within the above range are preferable because they have a polyhedral shape and values such as the average particle size are suitable.

[0253] [Other atoms] Other atoms mean those intentionally added to the alumina particles for the purpose of imparting mechanical strength or electrical and magnetic functions, as long as they do not inhibit the effects of the present invention.

[0254] The other atoms are not particularly limited, and examples include zinc, manganese, calcium, strontium, yttrium, etc. These other atoms may be used alone or in combination of two or more.

[0255] The content of other atoms in the alumina particles is preferably 5% by mass or less, more preferably 2% by mass or less, based on the mass of the alumina particles.

[0256] <Inorganic coating part> The inorganic coating part covers at least a part of the surface of the alumina particles, and is preferably composed of an inorganic coating layer covering at least a part of the surface of the alumina particles. In other words, at least a part of the surface of the composite particles is covered by the inorganic coating part, and preferably at least a part of the surface of the composite particles is covered by the inorganic coating layer. As described above, the inorganic coating part is provided on the surface of the alumina particles. "On the surface of the alumina particles" means the outside of the surface of the alumina particles. Therefore, the inorganic coating part formed on the outside of the surface of the alumina particles is clearly distinguished from the surface layer containing mullite or germanium formed on the inside of the surface of the alumina particles.

[0257] The inorganic chemical species constituting the inorganic coating portion may be relatively larger than the alumina particles, but it is preferable that they are relatively smaller than the alumina particles in that an inorganic coating portion with an arbitrary coating amount (or coating thickness) can be easily provided according to the purpose. Examples of the combination include alumina particles on the μm order and inorganic chemical species of 150 nm or less. When providing an inorganic coating portion using inorganic chemical species smaller than the alumina particles on the outer side of the surface of the alumina particles, it is possible to provide an inorganic coating portion on a part of the alumina surface using a small amount of inorganic chemical species so that the base of the alumina particles can be clearly seen, or it is possible to provide an inorganic coating portion on the surface of the alumina particles such that they are laminated using a large amount of inorganic chemical species so that the base of the alumina particles cannot be seen. The shape of the inorganic chemical species constituting the inorganic coating portion is not limited, but for example, from the viewpoint of being able to achieve closest packing with the minimum usage amount and easily concealing the base, it is preferably spherical or polyhedral.

[0258] The composite particles of the present invention are composed of alumina particles containing molybdenum and an inorganic coating portion composed of inorganic chemical species, and have excellent properties that cannot be exhibited by a simple mixture of alumina particles and inorganic chemical species. In the composite particles of the present invention, in the case of a combination of alumina particles containing molybdenum on the μm order and non-aggregated inorganic chemical species of 150 nm or less, for example, due to intermolecular forces and in some cases local chemical reactions, the interaction between the two increases, resulting in higher coating characteristics, it is easier to obtain a more uniform inorganic coating portion, and the obtained inorganic coating portion is less likely to peel off from the alumina particles, etc., and particularly excellent properties are exhibited. The contribution of molybdenum contained in the alumina particles can also be expected. Independent inorganic chemical species on the nm order can be obtained, for example, by mechanically pulverizing inorganic chemical species on the μm order, but immediate re-aggregation etc. occurs, so handling during use is not easy. When alumina particles not containing molybdenum or aggregated inorganic chemical species are used, the two only form a simple mixture and do not exhibit the properties of the composite particles of the present invention. According to the method for producing the composite particles of the present invention described later, composite particles with higher coating efficiency can be more easily produced.

[0259] The inorganic coating portion can be composed of, for example, an oxide or a metal. Examples of the oxide include titanium oxide (TiO 2 ), iron oxide (Fe 2 O 3 ), and silica (SiO 2 ), and one or more selected therefrom. Examples of the metal include one or more selected from silver (Ag), nickel (Ni), copper (Cu), gold (Au), and platinum (Pt).

[0260] The shape of the oxide or metal constituting the inorganic coating portion is not particularly limited, but is, for example, particulate such as spherical, needle-like, polyhedral, disk-like, hollow, or porous. The average particle diameter of the particles composed of the particulate oxide or metal is preferably, for example, 1 nm or more and 500 nm or less, and more preferably 5 nm or more and 200 nm or less. The particles composed of the oxide or metal may be crystalline or amorphous.

[0261] The inorganic coating portion may be composed of one layer or a plurality of layers. Further, when the inorganic coating portion is composed of a plurality of layers, the plurality of layers may be composed of different materials from each other.

[0262] [XRF coating rate of composite particles] The XRF coating rate (%) of the composite particles according to the embodiment can be determined by the same method as the above-described XRF analysis. The XRF coating rate (%) is determined, for example, based on the content of the metal oxide constituting the inorganic coating portion with respect to the content of aluminum oxide constituting the alumina particles, and is determined, for example, from [MO x / [Al 2 O 3 (mass ratio) based on the XRF analysis result.

[0263] [Coating efficiency of composite particles] The coating efficiency of the composite particles according to the embodiment can be determined from the ratio of the XRF coating rate to the theoretical coating rate described later. The coating efficiency is preferably 30% or more, more preferably 80% or more, and still more preferably 90% or more.

[0264] <Method for producing composite particles> Next, the details of the method for producing composite particles according to the second embodiment will be exemplified. The method for producing composite particles according to this embodiment is not limited to the method for producing composite particles shown below.

[0265] The method for producing composite particles according to this embodiment includes a step of firing a mixture containing an aluminum compound containing an aluminum element and a molybdenum compound containing a molybdenum element to produce alumina particles, and a step of forming an inorganic coating portion on the surface of the alumina particles.

[0266] The alumina particles constituting the composite particles according to the embodiment may be obtained based on any production method as long as they contain molybdenum. However, it is also possible to obtain them by pulverizing aluminum oxide containing molybdenum on the order of mm or more to the order of μm. However, a large amount of energy is required to obtain it, and the particle size distribution becomes broad, so it is not preferable.

[0267] Therefore, it is possible to sharpen the particle size distribution without classification, etc., and in terms of better thermal conductivity and better productivity, the alumina particles are preferably alumina obtained in the step of firing an aluminum compound in the presence of a molybdenum compound. Specifically, the alumina particles used in this embodiment preferably have, in the above firing step, a step in which a molybdenum compound reacts with an aluminum compound at a high temperature to form aluminum molybdate, and a step in which this aluminum molybdate further decomposes into alumina and molybdenum oxide at a higher temperature. In such a process, the molybdenum compound is incorporated into the alumina particles, and the alumina becomes a high-purity crystal with controlled particle size and shape. Hereinafter, this production method is referred to as the flux method. The flux method will be described in detail later.

[0268] The shape, size, specific surface area, etc. of the alumina particles can be controlled by selecting the usage ratio of the aluminum compound and the molybdenum compound, the firing temperature, and the firing time.

[0269] The alumina particles may be, for example, various crystal forms such as β, γ, δ, θ, etc., but in terms of better thermal conductivity, it is preferably basically in the α crystal form. The crystal structure of general α-type alumina is a dense hexagonal lattice, and the most thermodynamically stable crystal structure is a plate shape with developed

[0001] plane. However, in the flux method described in detail below, by firing an aluminum compound in the presence of a molybdenum compound, the molybdenum compound acts as a fluxing agent, and it is easier to form alumina particles containing molybdenum with a high α-crystallization rate having a crystal plane other than the

[0001] plane as the main crystal plane, especially with an α-crystallization rate of 90% or more. The fact that a crystal plane other than the

[0001] plane is the main crystal plane means that the area of the

[001] plane is 20% or less of the total area of the fine particles.

[0270] The alumina particles of the embodiment have a polyhedral shape as described above. When the alumina particles are polyhedral particles, it is advantageous in that they are easily filled into the resin composition. For example, by using a molybdenum compound as a flux agent in the flux method detailed below, polyhedral particles that are basically close to spherical can be obtained. These polyhedral particles that are close to spherical are in an advantageous form that is easily filled when filled into the resin composition. Among them, the area of the largest flat surface is preferably 1 / 8 or less of the area of the structure, and particularly preferably 1 / 16 or less of the area of the structure. In addition, when the alumina particles are polyhedral particles, it is considered that when the particles come into contact with each other in the resin composition, surface contact with high thermal conductivity is achieved, and it is considered that high thermal conductivity can be obtained even at the same filling rate compared to spherical particles.

[0271] In addition, alumina obtained by the flux method using a large amount of flux agent generally used has a shape of a hexagonal bipyramid and has sharp angles. Therefore, when manufacturing a resin composition or the like containing the composite particles according to this embodiment, problems such as damage to equipment may occur. However, since the alumina used in this embodiment is basically not in the shape of a hexagonal bipyramid, it is difficult to cause problems such as damage to equipment. Furthermore, the alumina of this embodiment is basically a polyhedron of octahedron or more and has a shape close to spherical, and thus has the characteristic of being difficult to cause problems such as damage to equipment.

[0272] The average particle diameter of the alumina particles is not particularly limited as long as it is 1000 μm or less. However, considering their use in the resin composition, it is preferably 0.1 μm (100 nm) to 100 μm. If the average particle diameter of the alumina particles is 0.1 μm or more, when filled in the resin composition, the viscosity of the resin composition can be kept low, which is preferable for workability and the like. Further, if the average particle diameter of the aluminum oxide used in the present invention is 100 μm or less, for example, in a thermoplastic resin composition, roughness hardly occurs on the surface of the molded article, and a good molded article can be easily obtained. Further, for example, in the case of a thermosetting resin composition, when bonding a substrate to a substrate, etc., the adhesive force at the interface between the cured product and the substrate does not decrease, and it is excellent in crack resistance in a cold-heat cycle or the like and peelability at the bonding interface, which is preferable. For the same reason as described above, a more preferable average particle diameter of the aluminum oxide used in the present invention is 1 μm to 50 μm.

[0273] Further, for example, by the flux method described in detail below, the specific surface area of the alumina particles obtained from the aluminum compound as a raw material is significantly reduced by firing compared to the aluminum compound. Depending on the properties of the aluminum compound and the firing conditions, the specific surface area of the obtained alumina particles is 0.0001 m 2 / g to 50 m 2 / g, and those in the range of 0.001 m 2 / g to 10 m 2 / g can be preferably obtained.

[0274] In the flux method using a molybdenum compound as a fluxing agent, although most of the used molybdenum compound sublimes by the firing treatment at a high temperature, a part of molybdenum remains, and aluminum oxide containing molybdenum is obtained. The content of molybdenum in the alumina particles is preferably 10% by mass or less, and those adjusted to 1% by mass or less by adjusting the firing temperature, firing time, and sublimation rate of the molybdenum compound are more preferably used.

[0275] The form of molybdenum contained in the alumina particles is not particularly limited. For example, it may be contained in the form of molybdenum metal, molybdenum compounds such as molybdenum trioxide or partially reduced molybdenum dioxide, or a form in which a part of aluminum in the structure of aluminum oxide is substituted by molybdenum.

[0276] (aluminum compound) The aluminum compound is a raw material for the alumina particles of the present embodiment and is not particularly limited as long as it becomes aluminum oxide by 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, and the physical forms such as the shape, particle size, and specific surface area of the aluminum compound as these precursors are not particularly limited.

[0277] According to the flux method described in detail below, the shape of the raw material aluminum compound can be suitably used regardless of whether it is, for example, spherical, amorphous, a structure with a high aspect ratio (wire, fiber, ribbon, tube, etc.), sheet, or the like.

[0278] Similarly, according to the flux method described in detail below, a solid of an aluminum compound with a particle size ranging from several nm to several hundred μm can be suitably used as the raw material aluminum compound.

[0279] The specific surface area of the aluminum compound is not particularly limited. Since the molybdenum compound acts effectively, a larger specific surface area is preferable, but any specific surface area can be used as a raw material by adjusting the firing conditions and the amount of the molybdenum compound used.

[0280] In addition, the aluminum compound may consist only 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 alumina using an organosilane, an aluminum compound composite adsorbed with a polymer, etc. can also be preferably used. When using these composites, the content of the organic compound is not particularly limited, but from the viewpoint of efficiently producing α-type alumina fine particles close to spherical shape, the content is preferably 60% by mass or less, more preferably 30% by mass or less.

[0281] (Molybdenum compound) The molybdenum compound may be, for example, molybdenum oxide or a compound containing an acid radical anion (MoO x n- ) formed by bonding of molybdenum metal with oxygen. The compound containing an acid radical anion formed by bonding of the molybdenum metal with oxygen is not particularly limited as long as it can be converted to molybdenum trioxide by high-temperature firing. Examples of such molybdenum compounds include molybdic acid, hexaammonium heptamolybdate, diammonium molybdate, phosphomolybdic acid, molybdenum disulfide, etc., which can be preferably used.

[0282] The amount of the molybdenum compound used is not particularly limited, but it is preferably 0.01 to 1 mol, more preferably 0.015 to 0.8 mol, and still more preferably 0.02 to 0.6 mol as the molybdenum metal of the molybdenum compound per 1 mol of the aluminum metal of the aluminum compound. Also, when the molybdenum compound is used as a fluxing agent when adopting the flux method, since the alumina particles contain molybdenum, it can be used as evidence to specify how the unknown alumina particles were produced.

[0283] (Potassium compound) In the flux method, the mixture may further contain a potassium compound containing potassium. That is, it is also preferable to use a molybdenum compound and a potassium compound as a flux agent. When using a molybdenum compound and a potassium compound as a flux agent, the potassium compound is not particularly limited, and examples thereof include potassium chloride, potassium chlorite, potassium chlorate, potassium sulfate, potassium hydrogen sulfate, potassium sulfite, potassium hydrogen sulfite, potassium nitrate, potassium carbonate, potassium hydrogen carbonate, potassium acetate, potassium oxide, potassium bromide, potassium bromate, potassium hydroxide, potassium silicate, potassium phosphate, potassium hydrogen phosphate, potassium sulfide, potassium hydrogen sulfide, potassium molybdate, potassium tungstate, etc. At this time, similar to the case of the molybdenum compound, the potassium compound contains isomers. Among these, it is preferable to use potassium carbonate, potassium hydrogen carbonate, potassium oxide, potassium hydroxide, potassium chloride, potassium sulfate, or potassium molybdate, and it is more preferable to use potassium carbonate, potassium hydrogen carbonate, potassium chloride, potassium sulfate, or potassium molybdate.

[0284] Note that the above-mentioned potassium compounds may be used alone or in combination of two or more.

[0285] The potassium compound contributes to the efficient formation of mullite on the alumina surface layer. In addition, the potassium compound contributes to the efficient formation of a layer containing germanium on the alumina surface layer.

[0286] Also, it is preferable to use the potassium compound together with the molybdenum compound as a flux agent.

[0287] Among the above, since potassium molybdate contains molybdenum, it may also have the function as the above-mentioned molybdenum compound. When potassium molybdate is used as a flux agent, it can exhibit the same action as when using a molybdenum compound and a potassium compound as a flux agent.

[0288] As a potassium compound used during raw material charging or generated by the reaction during the heating-up process in firing, a water-soluble potassium compound such as potassium molybdate does not vaporize even in the firing temperature range and can be easily recovered by washing after firing. Therefore, the amount of molybdenum compound released outside the firing furnace is also reduced, and the production cost can be significantly reduced.

[0289] When a molybdenum compound and a potassium compound are used as a fluxing agent, the molar ratio of the molybdenum element of the molybdenum compound to the potassium element of the potassium compound (molybdenum element / potassium element) is preferably 5 or less, more preferably 0.01 to 3, and even more preferably 0.5 to 1.5 because the production cost can be further reduced. When the molar ratio (molybdenum element / potassium element) is within the above range, alumina particles with a preferable particle size can be obtained.

[0290] (metal compound) As described later, the metal compound has a function of promoting the crystal growth of alumina, and the metal compound can be used during firing if desired. That is, the mixture may contain a metal compound. Since the metal compound has a function of promoting the crystal growth of α-alumina, it is not essential for the production of composite particles.

[0291] The metal compound is not particularly limited, but preferably contains at least one selected from the group consisting of Group II metal compounds and Group III metal compounds.

[0292] Examples of the Group II metal compound include magnesium compounds, calcium compounds, strontium compounds, barium compounds, and the like.

[0293] Examples of the Group III metal compound include scandium compounds, yttrium compounds, lanthanum compounds, cerium compounds, and the like.

[0294] The above-mentioned metal compound means an oxide, hydroxide, carbonate, or chloride of a metal element. For example, in the case of a yttrium compound, yttrium oxide (Y 2 O 3 ), yttrium hydroxide, and yttrium carbonate can be mentioned. Among these, the metal compound is preferably an oxide of the metal element. Note that these metal compounds include isomers.

[0295] Among these, it is preferably a metal compound of a third-period element, a fourth-period element, a fifth-period element, or a sixth-period element, more preferably a metal compound of a fourth-period element or a fifth-period element, and even more preferably a metal compound of a fifth-period element. Specifically, it is preferable to use a magnesium compound, a calcium compound, a yttrium compound, or a lanthanum compound, more preferably a magnesium compound, a calcium compound, or a yttrium compound, and particularly preferably a yttrium compound.

[0296] The addition rate of the above-mentioned metal compound is preferably 0.02 to 20% by mass, more preferably 0.1 to 20% by mass, based on the mass conversion value of aluminum atoms in the aluminum compound. When the addition rate of the metal compound is 0.02% by mass or more, the crystal growth of α-alumina containing molybdenum can proceed suitably, which is preferable. On the other hand, when the addition rate of the metal compound is 20% by mass or less, it is preferable because alumina particles with a low content of impurities derived from the metal compound can be obtained.

[0297] (yttrium) When the aluminum compound is calcined in the presence of a yttrium compound as the metal compound, in this calcination step, crystal growth proceeds more suitably, and α-alumina and a water-soluble yttrium compound are formed. At this time, since the water-soluble yttrium compound is likely to localize on the surface of α-alumina, which is an alumina particle, if necessary, the yttrium compound can be removed from the alumina particles by washing with water, alkaline water, a heated liquid thereof, or the like.

[0298] When using a molybdenum compound as a fluxing agent, the usage amounts of the above aluminum compound and molybdenum compound are not particularly limited. For example, when the total amount of raw materials in terms of oxide is 100% by mass, firing the following mixture can be mentioned. 1) Al 2 O 3 An aluminum compound containing 70% by mass or more of aluminum element in terms of conversion, and a molybdenum compound of 1% by mass or more in terms of MoO 3 conversion, and a mixture obtained by mixing them.

[0299] In the above 1), when the total amount of raw materials in terms of oxide is 100% by mass, the molybdenum compound is preferably 1% by mass or more in terms of MoO 3 conversion, more preferably 1% by mass or more and 10% by mass or less, and even more preferably 2% by mass or more and 8% by mass or less.

[0300] When using a molybdenum compound and a potassium compound as fluxing agents, the usage amounts of the above aluminum compound, molybdenum compound, potassium compound, and yttrium compound are not particularly limited. For example, when the total amount of raw materials in terms of oxide is 100% by mass, firing the following mixture can be mentioned. 2) Al 2 O 3 An aluminum compound containing 30% by mass or more of aluminum element in terms of conversion, and a molybdenum compound of 30% by mass or more in terms of MoO 3 conversion, and K 2 a potassium compound of 10% by mass or more in terms of K Y 2 O 3 a yttrium compound of 0.05% by mass or more in terms of conversion, and a mixture obtained by mixing them.

[0301] In the above 2), when the total amount of raw materials in terms of oxide is 100% by mass, the molybdenum compound is MoO 3It is preferably 30% by mass or more in terms of conversion, more preferably 30% by mass or more and 60% by mass or less, and still more preferably 40% by mass or more and 50% by mass or less. In the above (2), when the total amount of the raw materials in terms of oxide is 100% by mass, the potassium compound is K 2 It is preferably 10% by mass or more in terms of K O conversion, more preferably 10% by mass or more and 30% by mass or less, and still more preferably 12% by mass or more and 20% by mass or less. 2 O 3 In the above (2), when the total amount of the raw materials in terms of oxide is 100% by mass, the yttrium compound is preferably 0.05% by mass or more in terms of Y

[0302] [Firing step] The firing step is a step of firing an aluminum compound in the presence of a molybdenum compound. As described above, this production method is called the flux method. Since what is contained in the resin composition of the present invention is aluminum oxide containing molybdenum of μm order or less, in a production method using a large amount of molybdenum compound as a flux agent and taking a considerably long time, it is not preferable because aluminum oxide containing molybdenum of mm order or more is generated.

[0303] The firing method is not particularly limited and can be carried out by a known and commonly used method. When the firing temperature exceeds 700 ° C, the aluminum compound and the molybdenum compound react to form aluminum molybdate. Further, when the firing temperature is 900 ° C or higher, aluminum molybdate decomposes, and when it becomes aluminum oxide and molybdenum oxide, it can be obtained by incorporating the molybdenum compound into the aluminum oxide particles.

[0304] Also, when firing, the states of the aluminum compound and the molybdenum compound are not particularly limited, and it is only necessary for the molybdenum compound to be present in the same space where it can act on the aluminum compound. Specifically, even if the two are not mixed, it may be a simple mixing of powders, mechanical mixing using a pulverizer or the like, or mixing using a mortar or the like, and it may be a dry-state or wet-state mixing.

[0305] There is no particular limitation on the firing temperature conditions, and it is appropriately determined according to the particle diameter, morphology, etc. of the target alumina particles. Usually, regarding the firing temperature, it may be 900°C or higher, which is a temperature equal to or higher than the decomposition temperature of aluminum molybdate (Al 2 (MoO 4 ) 3 ). In particular, for efficiently forming the alumina particles used in the present invention, which are polyhedral particles close to spherical and have an α-crystallization rate of 90% or more, firing at 950 to 1100°C is more preferable, and firing in the range of 970 to 1050°C is most preferable.

[0306] Generally, when trying to control the shape of the alumina particles 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, from the viewpoints of the burden on the firing furnace and fuel cost, there are major problems for industrial use.

[0307] Alumina particles can be produced even at a high temperature exceeding 2000°C. However, by using the flux method, even at a temperature much lower than the melting point of α-alumina, which is 1600°C or lower, aluminum oxide with a high α-crystallization rate and a polyhedral shape can be formed regardless of the shape of the precursor. In the above flux method, even under the condition that the maximum firing temperature is 900°C to 1600°C, it is possible to efficiently and at low cost form highly thermally conductive alumina particles that are close to spherical and have an α-crystallization rate of 90% or more. Firing at a maximum temperature of 950 to 1500°C is more preferable, and firing in the range of 1000 to 1400°C of the maximum temperature is most preferable.

[0308] Regarding the firing time, it is preferable to carry out the heating time to the predetermined maximum temperature in the range of 15 minutes to 10 hours, and the holding time at the firing maximum temperature in the range of 5 minutes to 30 hours. To efficiently form alumina particles, it is more preferable that the firing holding time is about 10 minutes to 5 hours.

[0309] 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 or argon is preferable, and an air atmosphere is more preferable in consideration of cost.

[0310] The apparatus for firing is not necessarily limited, and a so-called firing furnace can be used. The firing furnace is preferably composed of a material that does not react with sublimated molybdenum oxide, and it is preferable to use a highly airtight firing furnace so as to efficiently utilize molybdenum oxide. Examples of the firing furnace that can be used at this time include a tunnel furnace, a roller hearth furnace, a rotary kiln, a muffle furnace, etc.

[0311] In addition, in order to adjust the crystal state of the alumina particles or remove impurities on the surface of the particles, etc., after forming aluminum oxide containing molybdenum, firing may be further carried out at a temperature higher than the temperature at which alumina particles are formed.

[0312] In obtaining the above-mentioned alumina particles, it is preferable to obtain them by firing an aluminum compound in the presence of a molybdenum compound and a potassium compound, or in the presence of a molybdenum compound, a potassium compound and a metal oxide.

[0313] That is, a preferable method for producing alumina particles includes a step of firing an aluminum compound (firing step) in the presence of a molybdenum compound and a potassium compound, or in the presence of a molybdenum compound, a potassium compound and a metal oxide. The mixture preferably further contains the above-mentioned metal compound. As the metal compound, a yttrium compound is preferable.

[0314] In the flux method using a molybdenum compound, molybdenum oxide reacts with an aluminum compound to form aluminum molybdate, and then, since the change in chemical potential during the decomposition of this aluminum molybdate serves as the driving force for crystallization, hexagonal dipyramidal polyhedral particles with well-developed (113) faces are formed. Therefore, by using a molybdenum compound as a fluxing agent, it is possible to more easily form alumina particles composed of polyhedral alumina containing molybdenum, which has a high α-crystallization rate, especially an α-crystallization rate of 90% or more.

[0315] Even if the effects of the present invention are obtained by a mechanism different from the above mechanism, they are included in the technical scope of the present invention.

[0316] [Cooling step] When using a molybdenum compound and a potassium compound as fluxing agents, the method for producing alumina particles may include a cooling step. The cooling step is a step of cooling the alumina that has grown crystallographically in the firing step. More specifically, it may be a step of cooling a composition containing the alumina obtained by the firing step and the liquid-phase fluxing agent.

[0317] 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. When the cooling rate is 1 °C / hour or more, the production time can be shortened, which is preferable. On the other hand, when the cooling rate is 1000 °C / hour or less, the firing container is less likely to crack due to heat shock and can be used for a long time, which is preferable.

[0318] The cooling method is not particularly limited, and it may be natural cooling or the use of a cooling device.

[0319] [Post-treatment step] The method for manufacturing the composite particles according to the embodiment may include a post-treatment step. The post-treatment step is a post-treatment step for the polyhedral alumina particles and is a step of removing the fluxing agent. The post-treatment step may be performed after the above-described firing step, after the above-described cooling step, or after the firing step and the cooling step. Further, if necessary, it may be repeated two or more times.

[0320] Examples of the post-treatment method include washing and high-temperature treatment. These can be performed in combination.

[0321] The washing 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.

[0322] At this time, the molybdenum content can be controlled by appropriately changing the concentration, usage amount of the water, aqueous ammonia solution, aqueous sodium hydroxide solution, acidic aqueous solution used, as well as the washing site, washing time, etc.

[0323] Further, examples of the high-temperature treatment method include a method of raising the temperature to a temperature equal to or higher than the sublimation point or boiling point of the flux.

[0324] [Inorganic Coating Formation Step] Next, an inorganic coating portion is formed on the surface of the polyhedral alumina particles obtained above. In the present embodiment, an inorganic coating portion composed of an oxide can be formed. The method for forming the layer is not particularly limited, and examples thereof include a liquid phase method and a gas phase method. For forming the inorganic coating part, any of the above-described inorganic chemical species can be used. In a preferred embodiment of the present invention, examples of such inorganic chemical species include metal oxides. When forming a coating part of a metal oxide on alumina particles, a liquid medium dispersion of alumina particles containing molybdenum and the metal oxide itself or its dispersion can be mixed, followed by filtration and drying. Further, when it is desired to enhance the interaction between the alumina particles and the metal oxide and exhibit particularly outstanding properties such as obtaining the above-described higher coating characteristics, obtaining a more uniform inorganic coating part, and making the obtained inorganic coating part difficult to peel from the alumina particles, a solution of a metal inorganic salt soluble in a liquid medium corresponding to a precursor of the metal oxide and an alumina particle containing molybdenum or its liquid medium dispersion are mixed, and after allowing the dissolved molecular metal inorganic salt to sufficiently contact the alumina particle containing molybdenum, it is preferable to convert the fine metal inorganic salt of 150 nm or less deposited on the alumina particle into a metal oxide. If necessary, further filtration and drying can also be performed. When converting the metal inorganic salt into a metal oxide, if the conversion is not easy due to low temperature or pH change, firing can be performed if necessary. By doing so, a strong interaction between the alumina particle and the metal oxide, which is not present in a simple mixture, can be exhibited, and the above-described particularly outstanding properties can be easily exhibited. The firing conditions in the step of forming the inorganic coating part may be appropriately selected as optimal conditions with reference to the conditions for the above-described alumina particles and then adopted.

[0325] As the liquid phase method, for example, a dispersion in which alumina particles are dispersed is prepared, and after adjusting the pH and heating the dispersion as necessary, an aqueous solution of a metal chloride such as titanium oxide is dropped into the dispersion. At this time, it is preferable to maintain the pH constant with an alkaline aqueous solution. Then, the dispersion is stirred for a predetermined time, followed by filtration, washing, and drying to obtain a powder. Thereby, an inorganic coating part composed of an oxide such as titanium oxide is formed on the surface of the alumina particles having a polyhedral shape. Further, as another oxide, an inorganic coating part may be formed with iron oxide or silica. Furthermore, an inorganic coating part may be formed with a plurality selected from titanium oxide, iron oxide, and silica.

[0326] In this process, instead of the oxide, an inorganic coating portion composed of a metal can also be formed. In this case, the inorganic coating portion can be formed by a liquid phase method, a vapor phase method, or the like. For example, an inorganic coating portion is formed on the surface of alumina particles having a polyhedral shape with silver, nickel, copper, gold, or platinum. Further, an inorganic coating portion may be formed with a plurality selected from silver, nickel, copper, gold, and platinum.

[0327] Further, in this process, an inorganic coating layer may be formed so as to cover at least a part of the surface of the alumina particles. In this case, for example, particles composed of a metal oxide or a metal are formed into a layer in an aggregated state with each other.

Example

[0328] Next, examples will be shown to explain the present invention in more detail, but the present invention is not limited to the following examples.

[0329] <Example 1> First, plate-like alumina serving as a substrate of the composite particles was produced. 146.2 g of aluminum hydroxide (manufactured by Nippon Light Metal Co., Ltd., average particle size 10 μm) (92.2% by mass in terms of oxide of Al 2 O 3 ), 5.0 g of molybdenum trioxide (manufactured by Taiyo Kogyo Co., Ltd.) (4.9% by mass in terms of oxide of MoO 3 ), and 3.0 g of silicon dioxide (manufactured by Fuji Silysia Chemical Ltd., average particle size 3 μm) (2.9% by mass in terms of oxide of SiO 2 ) were mixed in a mortar to obtain a mixture. The obtained mixture was placed in a crucible and heated in a ceramic electric furnace to 1100°C at a rate of 5°C / min, held at 1100°C for 10 hours for firing. Then, after cooling to room temperature at a rate of 5°C / min, the crucible was taken out to obtain 98.0 g of a light blue powder. The obtained powder was crushed in a mortar until it passed through a 2 mm sieve. Subsequently, 95.0 g of the obtained light blue powder was dispersed in 300 mL of 0.25% aqueous ammonia. After stirring the dispersion solution at room temperature (25 - 30 °C) for 3 hours, it was passed through a 106 μm sieve, and the aqueous ammonia was removed by filtration, followed by washing with water and drying to remove the molybdenum remaining on the particle surface, obtaining 90.0 g of a light blue powder. D 50 Platy alumina particles with a 50 value of 5 μm were prepared.

[0330] Next, D 50 5 g of platy alumina with a 50 value of 5 μm was dispersed in 45 mL of water to obtain a dispersion. Using 1 Mol of HCl, the dispersion was adjusted to pH 1.8, and at the same time, the temperature of the dispersion was adjusted to 70 °C. While stirring the dispersion, 23 g of a 1.5% TiCl 4 aqueous solution was added dropwise within 8 hours (theoretical coating rate 11.1%). At the same time, 39 g of a 2% NaOH aqueous solution was used to maintain the dispersion at pH 1.8. After the TiCl 4 aqueous solution was added dropwise, the dispersion was further stirred for 4 hours, and then the dispersion was filtered and washed with water. After drying at 110 °C for 10 hours, 5.32 g of a powder was obtained. The 5.32 g of powder was calcined at 800 °C for 2 hours, obtaining 5.30 g of a sample of platy alumina particles coated with titanium oxide.

[0331] <Example 2> D 50 Platy alumina with a 50 value of 5 μm was prepared by the same production method as in Example 1. 7.5 g of this platy alumina was dispersed in 75 mL of water to obtain a dispersion. Using 1 Mol of HCl, the dispersion was adjusted to pH 2.7, and at the same time, the temperature of the dispersion was adjusted to 75 °C. While stirring the dispersion, an 8.1% FeCl 3 aqueous solution was added dropwise at a rate of 0.17 g / min within 2 hours (theoretical coating rate 11.1%). At the same time, 20 g of a 5% NaOH aqueous solution was used to maintain the dispersion at pH 2.7. After the FeCl 3 aqueous solution was added dropwise, the dispersion was further stirred for 4 hours, and then the dispersion was filtered and washed with water. After drying at 110 °C for 10 hours, 8.25 g of a powder was obtained. The 5g powder was calcined at 800°C for 2 hours. As a result, 4.90g of a sample of plate-like alumina particles coated with iron(III) oxide was obtained. The color of this composite particle was reddish-brown.

[0332] <Example 3> D 50 A 5.0g sample of plate-like alumina particles coated with titanium oxide was obtained in the same manner as in Example 1, except that plate-like alumina with a D value of 13μm was used.

[0333] <Example 4> D 50 A 5.0g sample of plate-like alumina particles coated with iron(III) oxide was obtained in the same manner as in Example 2, except that plate-like alumina with a D value of 13μm was used. The color of this composite particle was reddish-brown.

[0334] <Example 5> FeCl 3 A 5.0g sample of plate-like alumina particles coated with iron(III) oxide was obtained in the same manner as in Example 4, except that the dropping time of the FeCl aqueous solution was changed to within 4 hours. The color of this composite particle was reddish-brown.

[0335] <Example 6> FeCl 3 A 5.0g sample of plate-like alumina particles coated with iron(III) oxide was obtained in the same manner as in Example 4, except that the dropping time of the FeCl aqueous solution was changed to within 7 hours. The color of this composite particle was reddish-brown.

[0336] <Example 7> Plate-like alumina serving as the substrate of the composite particle was produced. 100g of generally commercially available sodium hydroxide (average particle size 1 - 2μm) (90.1% by mass in terms of the oxide of Al 2 O 3 ), 6.5g of molybdenum trioxide (manufactured by Taiyo Kogyo Co., Ltd.) (9.0% by mass in terms of the oxide of MoO 3 ), and 0.65g of silicon dioxide (manufactured by Kanto Chemical Co., Inc., special grade) (SiO 2They were mixed in a mortar with 0.9% by mass in terms of oxide to obtain a mixture. The obtained mixture was put into a crucible and heated in a ceramic electric furnace to 1200 °C at a rate of 5 °C / min, held at 1200 °C for 10 hours for firing. Then, after cooling to room temperature at a rate of 5 °C / min, the crucible was taken out to obtain 67.0 g of a light blue powder. The obtained powder was crushed in a mortar until it passed through a 2 mm sieve.

[0337] Subsequently, 65.0 g of the obtained light blue powder was dispersed in 250 mL of 0.25% aqueous ammonia. After stirring the dispersion solution at room temperature (25 - 30 °C) for 3 hours, it was passed through a 106 μm sieve, and the aqueous ammonia was removed by filtration, followed by washing with water and drying to remove the molybdenum remaining on the particle surface, obtaining 60.0 g of a light blue powder. D 50 Plate-shaped alumina particles with a D value of 28 μm were produced.

[0338] The obtained powder was confirmed to be plate-shaped alumina particles with a polyhedral plate shape by SEM observation, with extremely few aggregates and excellent handleability. Furthermore, when XRD measurement was performed, sharp peak scattering derived from α-alumina appeared, and no alumina crystal system peaks other than the α crystal structure were observed, confirming that it was plate-shaped alumina with a dense crystal structure. Furthermore, from the results of fluorescent X-ray quantitative analysis, it was confirmed that the obtained particles contained 0.61% molybdenum in terms of molybdenum trioxide.

[0339] And D 50 Except for using plate-shaped alumina with a D value of 28 μm, in the same manner as in Example 1, 5.0 g of a sample of plate-shaped alumina particles coated with titanium oxide was obtained.

[0340] <Example 8> D 50 Plate-shaped alumina with a D value of 28 μm was produced by the same production method as in Example 7. D 50 Using plate-shaped alumina with a D value of 28 μm and FeCl 3Except that the dropping time of the aqueous solution was changed within 1 hour, in the same manner as in Example 2, 5.0 g of a sample of plate-shaped alumina particles coated with iron(III) oxide was obtained. The color of this composite particle was reddish brown.

[0341] <Example 9> D 50 Plate-shaped alumina with a D value of 28 μm was prepared by the same production method as in Example 7. D 50 Except that plate-shaped alumina with a D value of 28 μm was used, in the same manner as in Example 2, 5.0 g of a sample of plate-shaped alumina particles coated with iron(III) oxide was obtained. The color of this composite particle was reddish brown.

[0342] <Example 10> FeCl 3 Except that the dropping time of the aqueous solution was changed within 4 hours, in the same manner as in Example 9, 5.0 g of a sample of plate-shaped alumina particles coated with iron(III) oxide was obtained. The color of this composite particle was reddish brown.

[0343] <Example 11> FeCl 3 Except that the dropping time of the aqueous solution was changed within 7 hours, in the same manner as in Example 9, 5.0 g of a sample of plate-shaped alumina particles coated with iron(III) oxide was obtained. The color of this composite particle was reddish brown.

[0344] <Example 12> FeCl 3 Except that the dropping time of the aqueous solution was changed within 11 hours, in the same manner as in Example 9, 5.0 g of a sample of plate-shaped alumina particles coated with iron(III) oxide was obtained. The color of this composite particle was reddish brown.

[0345] <Example 13> Using germanium dioxide instead of silicon dioxide, 100 g of generally commercially available sodium hydroxide (average particle diameter 1 to 2 μm) (90.1% by mass in terms of the oxide of Al 2 O 3 and 6.5 g of molybdenum trioxide (manufactured by Taiyo Kogyo Co., Ltd.) (MoO 3in terms of oxide, 9.0% by mass), and 0.65 g of germanium dioxide (manufactured by Mitsubishi Materials Electronic Chemicals Co., Ltd.) (GeO 2 in terms of oxide, 0.9% by mass), were mixed in a mortar to obtain a mixture. In the same manner as in Example 1, except for this, 5.0 g of a sample of plate-like alumina particles coated with titanium oxide was obtained.

[0346] <Example 14> Plate-like alumina serving as the substrate of the composite particles was produced. 50 g of aluminum oxide (manufactured by CHALCO, Shandong, transition alumina, average particle size 45 μm) (Al 2 O 3 in terms of oxide, 35.9% by mass), 67 g of molybdenum trioxide (manufactured by Taiyo Mining Co., Ltd.) (MoO 3 in terms of oxide, 48.2% by mass), 0.025 g of silicon dioxide (manufactured by Kanto Chemical Co., Inc.) (SiO 2 in terms of oxide, 0.0% by mass), 32 g of potassium carbonate (manufactured by Kanto Chemical Co., Inc.) (K 2 in terms of oxide of O, 15.7% by mass), 0.25 g of yttrium oxide (manufactured by Kanto Chemical Co., Inc.) (Y 2 O 3 in terms of oxide, 0.2% by mass), were mixed in a mortar to obtain a mixture. The obtained mixture was placed in a crucible and heated in a ceramic electric furnace to 1000 °C at a rate of 5 °C / min, and held at 1000 °C for 24 hours for firing. Then, after cooling to room temperature at a rate of 5 °C / min, the crucible was taken out to obtain 136 g of a light blue powder. Subsequently, 136 g of the obtained light blue powder was washed with an approximately 1% aqueous sodium hydroxide solution. Then, while continuing vacuum filtration, it was washed with pure water. It was dried at 110 °C to obtain 47 g of plate-like alumina particles composed of α-alumina of the light blue powder. D 50 Plate-like alumina particles with a D value of 50 μm were produced.

[0347] And, except that plate-like alumina with a D 50 value of 50 μm was used, in the same manner as in Example 1, 5.0 g of a sample of plate-like alumina particles coated with titanium oxide was obtained.

[0348] <Example 15> A polyhedral alumina serving as the matrix of the composite particles was produced. 50 g of aluminum oxide (manufactured by CHALCO, Shandong, transitional alumina, average particle size 45 μm) (35.7 mass% in terms of the oxide of Al 2 O 3 ), 66.75 g of molybdenum trioxide (manufactured by Aladdin Industrial Corporation) (47.7 mass% in terms of the oxide of MoO 3 ), 33.75 g of potassium carbonate (manufactured by Aladdin Industrial Corporation) (16.4 mass% in terms of the oxide of K 2 O), and 0.25 g of yttrium oxide (manufactured by Aladdin Industrial Corporation) (0.2 mass% in terms of the oxide of Y 2 O 3 ) were mixed in a mortar to obtain a mixture. The obtained mixture was placed in a crucible and heated in a ceramic electric furnace to 950 °C at a rate of 5 °C / min, held for 10 hours for firing. Then, after cooling to room temperature at a rate of 5 °C / min, the crucible was taken out and the content was washed with ion-exchanged water. Finally, drying was performed at 150 °C for 2 hours to obtain a blue powder of α-alumina containing molybdenum. Polyhedral alumina particles with a D 50 value of 50 μm were produced.

[0349] And, except that polyhedral alumina with a D 50 value of 50 μm was used, in the same manner as in Example 1, 5.0 g of a sample of polyhedral alumina particles coated with titanium oxide was obtained.

[0350] <Example 16> A polyhedral alumina serving as the matrix of the composite particles was produced. 146.2 g of aluminum hydroxide (manufactured by Nippon Light Metal Co., Ltd., average particle size 12 μm) (95.0 mass% in terms of the oxide of Al 2 O 3 ), 5.0 g of molybdenum trioxide (manufactured by Taiyo Kogyo Co., Ltd.) (MoO 3They were mixed in a mortar with 5.0% by mass in terms of oxide to obtain a mixture. The obtained mixture was placed in a crucible and heated in a ceramic electric furnace to 1000 °C at a rate of 5 °C / min, held at 1000 °C for 10 hours for firing. Then, after cooling to room temperature at a rate of 5 °C / min, the crucible was taken out and the content was washed with ion-exchanged water. Finally, it was dried at 150 °C for 2 hours to obtain 98.0 g of blue α-alumina powder containing molybdenum. D 50 Polyhedral alumina particles with a D value of 5 μm were prepared.

[0351] And D 50 A 5.0 g sample of polyhedral alumina particles coated with titanium oxide was obtained in the same manner as in Example 1, except that polyhedral alumina with a D value of 5 μm was used.

[0352] <Example 17> D 50 Polyhedral alumina with a D value of 5 μm was prepared by the same production method as in Example 16. D 50 A 5.0 g sample of polyhedral alumina particles coated with iron(III) oxide was obtained in the same manner as in Example 2, except that polyhedral alumina with a D value of 5 μm was used. The color of this composite particle was reddish-brown.

[0353] <Comparative Example 1> D 50 A 5.0 g sample of polyhedral alumina particles coated with iron(III) oxide was obtained in the same manner as in Example 2, except that commercially available alumina particles (manufactured by Zhengzhou Research institute of Chalco, product name "A-SF-60") with a D value of 30 μm were used. The color of this composite particle was light red.

[0354]

Table 1

[0355]

Table 2

[0356] ≪Evaluation≫ The powders of Examples 1 to 17 and Comparative Example 1 described above were used as samples, and the following evaluations were performed. The measurement methods are shown below.

[0357] [Measurement of the major axis L of alumina particles] Using a laser diffraction particle size analyzer (manufactured by Shimadzu Corporation, SALD-7000), 1 mg of alumina powder was dispersed in an aqueous solution of 0.2 wt% sodium hexametaphosphate (manufactured by Fujifilm Wako Pure Chemical Corporation) to a total of 18 g, and this was used as a sample for measurement to obtain the average particle size D 50 value (μm), which was taken as the major axis L.

[0358] [Measurement of the thickness D of alumina particles] Using a scanning electron microscope (SEM), the average value of the thicknesses of 50 particles was measured and adopted as the thickness D (μm).

[0359] [Aspect ratio L / D] The aspect ratio was determined using the following formula. (Aspect ratio) = (major axis L of alumina particles / thickness D of alumina particles)

[0360] [Analysis of the presence or absence of mullite on the surface of alumina particles] The inorganic coating layer of the composite particles was dissolved using hot concentrated sulfuric acid in the case of titanium oxide and sulfuric acid in the case of iron(III) oxide to expose the alumina particles. The prepared sample was placed in a holder for measurement samples with a depth of 0.5 mm and filled so as to be flat under a certain load. Then, it was set in a wide-angle X-ray diffractometer (manufactured by Rigaku Corporation, Ultima IV), and measurement was performed under the conditions of Cu / Kα ray, 40 kV / 30 mA, scan speed 2 degrees / minute, and scan range 10 to 70 degrees. The peak height of mullite observed at 2θ = 26.2 ± 0.2 degrees was defined as A, the peak height of α-alumina of the (104) plane observed at 2θ = 35.1 ± 0.2 degrees was defined as B, and the value of the baseline at 2θ = 30 ± 0.2 degrees was defined as C. The presence or absence of mullite was determined from the following formula. When the value of R is 0.02 or more, mullite is determined to be "present" on the surface of alumina particles, and when it is less than 0.02, mullite is determined to be "absent" on the surface of alumina particles. R = (A - C) / (B - C) (R: Ratio of the peak height A of mullite to the peak height B of the (104) plane of α-alumina)

[0361] [Analysis of the amount of Mo on the surface of alumina particles] Using an X-ray photoelectron spectroscopy (XPS) apparatus (manufactured by ULVAC-PHI, Inc., Quantera SXM), the prepared sample was press-fixed on double-sided tape, and compositional analysis was performed under the following conditions. · X-ray source: Monochromatized AlKα, beam diameter 100 μmφ, output 25 W · Measurement: Area measurement (1000 μm square), n = 3 · Charge correction: C1s = 284.8 eV The [Mo] / [Al] obtained from the XPS analysis results was taken as the amount of Mo on the surface of alumina particles. When the amount of Mo was 0.0005 or more, Mo was determined to be "present" on the surface of alumina particles, and when the amount of Mo was less than 0.0005, Mo was determined to be "absent" on the surface of alumina particles.

[0362] [Theoretical coating rate] The theoretical coating rate (%) of the composite particles was calculated from the ratio of the masses of Fe 3 and TiCl 4 charged and the mass of alumina charged, which is the ratio of the mass of Fe 2 O 3 and TiO 2 obtained and the mass of the alumina charged (mass of inorganic coating layer / mass of alumina).

[0363] [XRF coating rate] Using a fluorescent X-ray (XRF) analyzer (manufactured by Rigaku Corporation, Primus IV), approximately 70 mg of the prepared sample was placed on filter paper, covered with a PP film, and compositional analysis was performed. The XRF coating rate (%) of the composite particles was determined from the [TiO 2 / [Al 2 O 3 or [Fe 2 O 3 / [Al2 O 3 (Calculated from the mass ratio).

[0364] [Coating efficiency] The coating efficiency (%) of the composite particles was determined from the ratio of the XRF coating rate to the above theoretical coating rate.

[0365] [Coating evaluation] When the coating efficiency (%) of the composite particles is 90% or more, it is extremely good "◎"; when it is 80% or more and less than 90%, it is good "〇"; when it is 30% or more and less than 80%, it is almost good "△"; when it is less than 30%, it is poor "×". The evaluation results are shown in Tables 1 and 2.

[0366] First, as shown in Table 1, in the powders obtained in Examples 1 to 12 above, the value of the above XRD peak intensity ratio was 0.02 or more, and the presence of mullite was confirmed. On the other hand, in the powders obtained in Examples 13 to 17, the presence of mullite was not confirmed.

[0367] Also, the powders obtained in Examples 1 to 17 and Comparative Example 1 above were confirmed to have the values of particle diameter (D 50 ), thickness, and aspect ratio described in Table 1 above.

[0368] The SEM observation images of the plate-like alumina particles of Example 1 are shown in FIGS. 1 and 2. As shown in FIGS. 1 and 2, it was confirmed that the surface of the plate-like alumina of Example 1 was coated with particulate titanium oxide (TiO 2 ). The SEM observation images of the plate-like alumina particles of Example 2 are shown in FIGS. 3 to 5. As shown in FIGS. 3 to 5, it was confirmed that the surface of the plate-like alumina of Example 2 was coated with particulate iron(III) oxide (Fe 2 O 3 ).

[0369] Also, in the composite particles of Examples 1 to 12, D 50Plate-shaped alumina with a value of 5 μm, 13 μm, or 28 μm is used. The presence of Mo and Si is confirmed on the surface of the plate-shaped alumina, and the presence of mullite is also confirmed on the surface of the plate-shaped alumina. When an inorganic coating layer is formed with titanium oxide or iron(III) oxide, in any case, the coating efficiency is 80% or more, which is good or extremely good. Therefore, it was found that when Mo and mullite are present on the surface of the plate-shaped alumina, it is easy to form both an inorganic coating layer of titanium oxide and iron(III) oxide on the plate-shaped alumina. In particular, in Examples 2, 5, 6, 8 to 12, when an inorganic coating layer is formed with iron(III) oxide, in any case, the coating efficiency is 90% or more, which is extremely good. Therefore, it was found that when Mo and mullite are present on the surface of the plate-shaped alumina, it is very easy to form an inorganic coating layer of iron(III) oxide on the plate-shaped alumina. Also, in Example 7, when an inorganic coating layer is formed with titanium oxide, the coating efficiency is 90% or more, which is extremely good. Therefore, it was found that when Mo and mullite are present on the surface of the plate-shaped alumina, it is very easy to form an inorganic coating layer of titanium oxide on the plate-shaped alumina.

[0370] In the composite particles of Example 13, D 50 Plate-shaped alumina with a value of 22 μm is used, and the presence of Mo is confirmed on the surface of the plate-shaped alumina. When an inorganic coating layer is formed with titanium oxide, the coating efficiency is 87.5% or more, which is good. Therefore, it was found that when Mo is present on the surface of the plate-shaped alumina, it is easy to form an inorganic coating layer of titanium oxide on the plate-shaped alumina.

[0371] In the composite particles of Example 14, D 50 Plate-shaped alumina with a value of 50 μm is used, and the presence of Mo is confirmed on the surface of the plate-shaped alumina. When an inorganic coating layer is formed with titanium oxide, the coating efficiency is approximately 33.3%, which is almost good. Therefore, it was found that when Mo is present on the surface of the plate-shaped alumina, it is easy to form an inorganic coating layer of titanium oxide on the plate-shaped alumina.

[0372] In the composite particles of Example 15, D 50Polyhedral alumina with a value of 50 μm was used, and the presence of Mo was confirmed on the surface of the polyhedral alumina. When an inorganic coating layer was formed with titanium oxide, the coating efficiency was extremely good at 90%. Therefore, it was found that when Mo is present on the surface of the above polyhedral alumina, it is very easy to form an inorganic coating layer of titanium oxide on the polyhedral alumina.

[0373] In the composite particles of Example 16, D 50 Polyhedral alumina with a value of 5 μm was used, and the presence of Mo was confirmed on the surface of the polyhedral alumina. When an inorganic coating layer was formed with titanium oxide, the coating efficiency was extremely good at 91.7%. Therefore, it was found that when Mo is present on the surface of the above polyhedral alumina, it is very easy to form an inorganic coating layer of titanium oxide on the polyhedral alumina.

[0374] In the composite particles of Example 17, D 50 Polyhedral alumina with a value of 5 μm was used, and the presence of Mo was confirmed on the surface of the polyhedral alumina. When an inorganic coating layer was formed with iron(III) oxide, the coating efficiency was almost good at 34.8%. Therefore, it was found that when Mo is present on the surface of the above polyhedral alumina, it is easy to form an inorganic coating layer of titanium oxide on the polyhedral alumina.

[0375] In addition, in Examples 2, 4 to 6, 8 to 12, and 17 coated with iron(III) oxide, it was confirmed that they exhibited red or reddish-brown.

[0376] On the other hand, in the composite particles of Comparative Example 1, D 50 Commercially available plate-shaped alumina with a value of 30 μm was used, and it was confirmed by the above XRD measurement that the plate-shaped alumina had an α crystal structure. Also, the presence of Mo and Si was not confirmed on the surface of the plate-shaped alumina, and the presence of mullite was not confirmed in the plate-shaped alumina either. When an inorganic coating layer was formed with iron(III) oxide, the coating efficiency was poor at only 2.30%.

Industrial Applicability

[0377] Since the composite particles of the present invention have a high coating efficiency of the inorganic coating portion, they can be suitably used for printing inks, paints, automotive coatings, industrial coatings, heat conductive fillers, cosmetic materials, abrasives, highly bright pigments, lubricants, substrates for conductive powders, ceramic materials, and the like.

Claims

1. A method for producing plate-like alumina particles, comprising: firing a mixture containing an aluminum compound containing an aluminum element, a molybdenum compound containing a molybdenum element, and silicon or a silicon compound containing a silicon element; forming an inorganic coating portion on the surface of the plate-like alumina particles; The amount of the molybdenum compound used is MoO ３ In terms of the total mass, the content is 1% by mass or more and 10% by mass or less, The amount of the silicon compound used is SiO when the total amount of the raw materials calculated as oxide is taken as 100 mass %. ２ The method for producing composite particles, wherein the content is 0.5 mass % or more and less than 7 mass % in terms of the total mass of the composite particles.

2. The method for producing composite particles according to claim 1 , wherein the mixture further contains a potassium compound containing potassium element.

3. The method for producing composite particles according to claim 1 , wherein the inorganic coating portion is made of an oxide.

4. The method for producing composite particles according to claim 3 , wherein the oxide is one or more selected from titanium oxide, iron oxide, and silica.

5. The method for producing composite particles according to claim 1 , wherein the inorganic coating portion is made of a metal.

6. The method for producing composite particles according to claim 5 , wherein the metal is one or more selected from the group consisting of silver, nickel, copper, gold and platinum.

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