Alumina powder, inorganic powder, and resin composition

Abstract

To provide an alumina powder excellent in lowering viscosity and increasing thermal conductivity when blended with a resin.SOLUTION: Provided is an alumina powder in which a volume frequency particle size distribution measured using the wet laser diffraction scattering technique includes a first peak and a second peak, and when a particle diameter at the first peak is expressed by d1, a particle diameter at the second peak is expressed by d2, and a particle diameter at a lowest-frequency recess portion between the first peak and the second peak is expressed by d3, then d1 is 1 μm or more and less than 50 μm, d2 is 50 μm or more and 150 μm or less, and d32 / d1×d2 satisfies 1.40 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an alumina powder, an inorganic powder, and a resin composition. [Background technology]

[0002] Various developments have been made on alumina powders up to now. One known example of this type of technology is the technology described in Patent Document 1. Patent Document 1 describes an alumina powder having an average particle size (D50) of 50 μm or less. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-193493 Summary of the Invention [Problem to be solved by the invention]

[0004] However, as a result of investigations by the present inventors, it has been found that there is room for improvement in the alumina powder described in Patent Document 1 above in terms of lowering the viscosity and increasing the thermal conductivity when blended with a resin. [Means for solving the problem]

[0005] After further investigation, the inventors found that by making the particle size distribution of alumina coarse powder having a particle size of 50 μm or more and 150 μm or less relatively sharp, it is possible to increase the thermal conductivity while reducing the viscosity of a resin composition containing alumina coarse powder having a sharp particle size distribution, alumina fine powder having a particle size of 1 μm or more and less than 50 μm, and a resin, and thus completed the present invention.

[0006] According to one aspect of the present invention, the following alumina powder, inorganic powder, and resin composition are provided. 1. When the volume frequency particle size distribution measured by a wet laser diffraction scattering method includes a first peak and a second peak, the particle diameter of the first peak is d1, the particle diameter of the second peak is d2, and the particle diameter of the recess with the smallest frequency between the first peak and the second peak is d3, d1 is 1 μm or more and less than 50 μm, d2 is 50 μm or more and 150 μm or less, and d3 2 Alumina powder in which / d1×d2 satisfies 1.40 or more. 2. The alumina powder according to 1., An alumina powder that satisfies at least one of d3 / d1 being 2.90 or more and d2 / d3 being 2.50 or less. 3. The alumina powder according to 1. or 2., When the frequency of the first peak is P1 and the frequency of the second peak is P2, Alumina powder with P2 / P1 of 3.40 or more. 4. The alumina powder according to any one of 1. to 3., The particle diameters at the points where the cumulative volume from the small particle side in the volume frequency particle size distribution is 10%, 50%, and 99% are defined as D 10 , D 50 , D 99 When (D 99 -D 10 ) / D 50 Alumina powder having a viscosity of 2.70 or less. 5. The alumina powder according to any one of 1. to 4., In the volume frequency particle size distribution, the particle diameters at the points where the cumulative volume from the small particle side is 50% and 99% are defined as D 50 , D 99 When D 99 / D 50 Alumina powder having a viscosity of 2.80 or less. 6. The alumina powder according to any one of 1. to 5., An alumina powder having an alumina content of 99.9 mass% or more, calculated as mass. 7. The alumina powder according to any one of 1. to 6., An alumina powder having a sphericity of 0.90 or more as measured using a wet flow image analyzer. 8. An inorganic powder comprising the alumina powder according to any one of 1. to 7. and an inorganic filler other than the alumina powder. 9. A resin composition comprising the alumina powder according to any one of 1. to 7. and a resin. [Effects of the Invention]

[0007] According to the present invention, there are provided an alumina powder that is excellent in reducing viscosity and increasing thermal conductivity when blended with a resin, and an inorganic powder and a resin composition using the same. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view showing the configuration of a thermal spraying device. DETAILED DESCRIPTION OF THE INVENTION

[0009] An outline of the alumina powder of this embodiment will be described.

[0010] The alumina powder of this embodiment is When the volume frequency particle size distribution measured by a wet laser diffraction scattering method includes a first peak and a second peak, the particle diameter of the first peak is d1, the particle diameter of the second peak is d2, and the particle diameter of the recess with the smallest frequency between the first peak and the second peak is d3, d1 is 1 μm or more and less than 50 μm, d2 is 50 μm or more and 150 μm or less, and d3 2 / d1×d2 is 1.40 or more.

[0011] In this embodiment, alumina powder having a particle diameter of 50 μm or more and 150 μm or less is defined as alumina coarse powder, and another alumina powder having a particle diameter of 1 μm or more and less than 50 μm measured in the same manner as alumina coarse powder is defined as alumina fine powder.

[0012] According to the findings of the present inventors, it is possible to sharpen the particle size profile of the volume frequency particle size distribution by removing at least one of the relatively coarse particles and the fine particles contained in the powder. It has also been found that by using an alumina powder that is a mixture of alumina coarse powder and alumina fine powder having a sharp particle size profile, when blended with a resin, the viscosity can be reduced while the thermal conductivity can be increased.

[0013] According to further findings of the inventors, the "d3 2 It was found that by using " / d1×d2" as an index, it is possible to stably evaluate the properties of a resin composition blended with alumina powder and resin, and that by setting this index to a lower limit or higher, it is possible to reduce the viscosity and increase the thermal conductivity of the resin composition.

[0014] Although the detailed mechanism is unclear, it is thought that alumina coarse powder with a sharp particle size distribution suppresses particle size variation and increases packing density, and when combined with alumina fine powder, the fine powder can efficiently fill the gaps between the particles of the coarse powder, making it easier to form heat paths and improving thermal conductivity. Furthermore, it is believed that a mixed powder of coarse powder and fine powder in which the variation in particle size is suppressed has a high particle packing density, which can reduce the viscosity of the resin composition.

[0015] Each component of the alumina powder of this embodiment will be described in detail below.

[0016] The alumina powder may be any powder containing alumina (Al2O3) as the main component. The term "main component" means that the alumina powder contains 99.9 mass % or more of alumina (Al2O3) in terms of mass, based on the total amount of the alumina powder. The alumina powder preferably has a high purity, but the presence of impurities inevitably mixed in from the raw materials or during the manufacturing process is acceptable.

[0017] According to the procedure below, in the volume frequency particle size distribution obtained by the wet laser diffraction scattering method, the particle diameters at the points where the cumulative volume from the small particle side is 10%, 50%, and 99% are determined as D 10 , D 50 , D 99 Let's say.

[0018] The volumetric particle size distribution of the alumina powder, which is related to its particle size, can be measured by a laser diffraction light scattering method using, for example, the "MT-3300EX" manufactured by Nikkiso Co., Ltd. The measurement target was water as the solvent, and as a pretreatment, the alumina powder was dispersed using a homogenizer at 200 W output for 1 minute to obtain a dispersion. The PIDS (Polarization Intensity Differential Scattering) concentration was adjusted to 45-55%. The refractive index of water was set to 1.33, and the refractive index of the powder was determined based on the refractive index of the powder material. For example, the refractive index of amorphous silica was set to 1.50, and the refractive index of alumina was set to 1.76.

[0019] d3 2 The lower limit of / d1×d2 is 1.40 or more, preferably 1.50 or more, and more preferably 1.65 or more, which makes it possible to lower the viscosity and increase the thermal conductivity of the resin composition. d3 2 The upper limit of / d1×d2 is not particularly limited, but may be 3.50 or less, 3.00 or less, or 2.75 or less.

[0020] The alumina powder may also satisfy at least one of the following conditions: d3 / d1 is 2.90 or more, and d2 / d3 is 2.50 or less. The lower limit of d3 / d1 is, for example, 2.90 or more, preferably 3.20 or more, and more preferably 3.30 or more, which can further improve the low viscosity and high thermal conductivity. The upper limit of d3 / d1 is not particularly limited, but may be 5.50 or less, 5.30 or less, or 5.00 or less.

[0021] The upper limit of d2 / d3 is, for example, 2.50 or less, preferably 2.25 or less, and more preferably 2.10 or less, which can further improve the low viscosity and high thermal conductivity. The lower limit of d2 / d3 is not particularly limited, but may be 1.3 or more.

[0022] The frequency of the first peak in the volume frequency particle size distribution is defined as P1, and the frequency of the second peak is defined as P2. The alumina powder may be one in which P2 / P1 satisfies 3.40 or more. The lower limit of P2 / P1 is, for example, 3.40 or more, preferably 3.80 or more, and more preferably 4.00 or more, which can further improve the low viscosity and high thermal conductivity. The upper limit of P2 / P1 is not particularly limited, but may be 5.00 or less, 4.85 or less, or 4.60 or less.

[0023] Alumina powder is (D 99 -D 10 ) / D 50 is also acceptable if the value satisfies 2.70 or less. (D 99 -D 10 ) / D 50 The upper limit of is, for example, 2.70 or less, preferably 2.60 or less, and more preferably 2.55 or less, which can further improve the low viscosity and high thermal conductivity. (D 99 -D 10 ) / D 50 The lower limit of is not particularly limited, but may be, for example, 1.75 or more, 1.90 or more, or 2.00 or more, thereby improving productivity.

[0024] Alumina powder is D 99 / D 50 is also acceptable if the value satisfies 2.80 or less. D 99 / D 50 The upper limit of is, for example, 2.80 or less, preferably 2.70 or less, and more preferably 2.65 or less, which can further improve the low viscosity and high thermal conductivity. D 99 / D50 The lower limit of is not particularly limited, but may be, for example, 1.50 or more, 1.80 or more, or 2.00 or more, thereby improving productivity.

[0025] Alumina powder D 99 The upper limit is, for example, 290 μm or less, preferably 270 μm or less, and more preferably 260 μm or less. D 97 The lower limit of is not particularly limited, but may be, for example, 100 μm or more, 130 μm or more, or 140 μm or more. In this way, D 99 By making the value of the particle diameter equal to or less than the upper limit, the packing density of the particles can be improved.

[0026] Alumina powder D 10 The lower limit is, for example, 1.00 μm or more, preferably 3.00 μm or more, and more preferably 4.50 μm or more. D 10 The upper limit is not particularly limited, but may be 20.0 μm or less, 13.0 μm or less, or 10.0 μm or less. In this way, D 10 By making the value of the particle diameter equal to or larger than the lower limit, the packing density of the particles can be improved.

[0027] The lower limit of the specific surface area of ​​the alumina powder is not particularly limited, but is, for example, 0.1 m 2 / g or more. The upper limit of the specific surface area of ​​alumina powder is, for example, 0.5 m 2 / g or less, preferably 0.35m 2 / g or less.

[0028] The lower limit of the gelatinization rate of the alumina powder is, for example, 30% or more, and preferably 40% or more. The upper limit of the gelatinization rate of the alumina powder is not particularly limited, but may be, for example, 90% or less.

[0029] The alumina powder may have a sphericity of, for example, 0.90 or more, preferably 0.91 or more, more preferably 0.92 or more, as measured using a wet flow image analyzer. However, there is no particular upper limit to the sphericity.

[0030] The alumina powder may be one that has been surface-treated with a silane coupling agent, or may be an untreated product with no silane coupling agent attached to the surface.

[0031] In this embodiment, for example, by appropriately selecting the raw material components of the alumina powder and the manufacturing method of the alumina powder, the above d3 2 / d1×d2, P2 / P1, (D 99 -D 10 ) / D 50 , D 99 / D 50 Among these, for example, with respect to alumina coarse powder and / or alumina fine powder produced by the molten flame method, removing at least one of relatively coarse particles and fine particles contained in the powder, and using alumina coarse powder and alumina fine powder with a sharp particle size distribution in combination, are effective in achieving the above d3. 2 / d1×d2, P2 / P1, (D 99 -D 10 ) / D 50 , D 99 / D 50 are factors for setting the desired numerical range.

[0032] A method for producing the alumina powder of this embodiment will be described.

[0033] Alumina powder is produced, for example, by supplying alumina raw material powder into a high-temperature flame formed by the combustion reaction of a combustible gas and a combustion-supporting gas, and melting and spheroidizing the powder at a temperature above its melting point. The obtained molten spherical particles may be further subjected to classification and sieving treatment, if necessary. The sieving process preferably removes at least one of the relatively coarse particles and the fine particles contained in the powder. Specifically, if the particle size (μm) of the maximum peak is X, for example, coarse particles having a particle size larger than (X + α) are top-cut and / or fine particles having a particle size smaller than (X - β) are under-cut, and the remaining powder can be used as the alumina powder of this embodiment. The remaining ranges (μm) of α and β can be selected appropriately.

[0034] The alumina raw material powder may be, for example, an alumina powder having an average particle size of about 1 to 250 μm. A plurality of raw material powders having different particle sizes may be used as the alumina raw material powder. The aluminum hydroxide powder may be supplied to the high-temperature flame in a dry manner or in a wet manner by forming a slurry with water or the like.

[0035] A resin composition containing the alumina powder of the present invention can be suitably used as a resin molding material.

[0036] The resin composition of the present embodiment contains the alumina powder of the present invention and a resin. The resin composition may contain inorganic powders and known resin additives, as described below, as needed.

[0037] The resin composition may contain alumina powder alone or in combination with other fillers. The resin composition may contain 10 to 99% by mass of alumina powder, or 10 to 99% by mass of inorganic powder containing alumina powder and other inorganic fillers. The content of other inorganic fillers in the inorganic powder may be, for example, 1 to 20% by mass or 3 to 15% by mass relative to 100% by mass of the alumina powder. In this specification, unless otherwise specified, the symbol "to" indicates that the upper and lower limits are included.

[0038] Examples of the other inorganic fillers include alumina powders other than the alumina powder of the present invention, crystalline silica, fused silica, titania, silicon nitride, aluminum nitride, silicon carbide, talc, calcium carbonate, and the like. The other inorganic fillers have an average particle size of, for example, about 0.1 to 100 μm, and there are no particular restrictions on the particle size structure and shape.

[0039] Examples of the resin include epoxy resin, silicone resin, phenol resin, melamine resin, urea resin, unsaturated polyester, fluororesin, polyamide such as polyimide, polyamideimide, polyetherimide, polyester such as polybutylene terephthalate, polyethylene terephthalate, polyphenylene sulfide, wholly aromatic polyester, polysulfone, liquid crystal polymer, polyethersulfone, polycarbonate, maleimide-modified resin, ABS resin, AAS (acrylonitrile-acrylic rubber-styrene) resin, AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resin, etc. These may be used alone or in combination of two or more.

[0040] The resin composition can be produced, for example, by blending raw material components in a predetermined ratio using a blender, a Henschel mixer, or the like, kneading the mixture using a heated roll, a kneader, a single-screw or twin-screw extruder, or the like, cooling the mixture, and then pulverizing it.

[0041] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Example]

[0042] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the descriptions of these examples.

[0043] <Production of alumina powder> A spherical alumina powder was produced using the thermal spraying apparatus 100 shown in FIG. The thermal spraying device 100 shown in FIG. 1 includes a melting furnace 2, a burner 1 installed in the upper part of the melting furnace 2, and a collection system line installed directly connected to the lower part of the melting furnace 2 and consisting of a cyclone 4 and a bag filter 8. Burner 1 has a double-pipe structure that can form an inner flame and an outer flame, and is installed at the top of melting furnace 2, to which combustible gas supply pipe 11, combustion supporting gas supply pipe 12, and raw material supply pipe 13 are connected. In the melting furnace 2, raw material powder is fed into the high-temperature flame through the raw material supply pipe 13 and melted to form molten spherical particles. The molten spherical particles that have passed through the melting furnace 2 are sucked in by the blower 9 together with the combustion exhaust gas, and are transported by the air through the pipes 3 and 5, and are classified and collected in the cyclone 4 or the bag filter 8.

[0044] (coarse alumina powder) Coarse powder 1: Alumina powder (specific surface area: 0.09 m 2 / g, D50:136μm) Using the above-mentioned thermal spraying device 100 and the raw material powder under the following conditions, an alumina powder was produced, which was designated as coarse powder 1. LPG was supplied as a combustible gas from a combustible gas supply pipe 11, and oxygen was supplied as a combustion supporting gas from a combustion supporting gas supply pipe 12. A high-temperature flame was formed by combustion of the LPG and oxygen in the burner 1. Secondary air was supplied to the cyclone 4 by a rotary valve (not shown) installed in the pipe 3. Atmospheric air was used as the secondary air. The degree of opening / closing of the lower valve in the cyclone 4 (lower opening) was set to 100%. As raw material powder, the average particle size (D 50 ) in the range of 60 to 250 μm was used. Coarse powder 2: Alumina powder (specific surface area: 0.11 m 2 / g, D50: 103 μm) As raw material powder, the average particle size (D 50Coarse powder 2 was produced in the same manner as coarse powder 1, except that a plurality of alumina powders having a maximum value of σ in the range of 60 to 120 μm were used. Coarse powder 3: Alumina powder (specific surface area: 0.18 m 2 / g, D50:70μm) As raw material powder, the average particle size (D 50 Coarse powder 3 was produced in the same manner as coarse powder 1, except that a plurality of alumina powders having a maximum value of σ in the range of 30 to 100 μm were used.

[0045] Coarse powder 4: The alumina powder of coarse powder 1 was sieved through a sieve A1 with 150 μm openings, and the part that passed through sieve A1 was sieved through a sieve B1 with 106 μm openings. The alumina powder remaining on sieve B1 was used as coarse powder 4. Coarse powder 5: The alumina powder of coarse powder 2 was sieved through sieve A2 with 150 μm openings, and the part that passed through sieve A2 was sieved through sieve B2 with 75 μm openings. The alumina powder remaining on sieve B2 was used as coarse powder 5. Coarse powder 6: The alumina powder of Coarse powder 3 was sieved through sieve A3 with 100 μm openings, and the part that passed through sieve A3 was sieved through sieve B3 with 53 μm openings. The alumina powder remaining on sieve B3 was used as Coarse powder 5.

[0046] (alumina fine powder) Fine powder 1: Alumina powder (specific surface area: 0.32 m 2 / g, D50:11μm) As raw material powder, the average particle size (D 50 A powder was produced in the same manner as for the coarse powder 1, except that a plurality of alumina powders having a maximum value of ) in the range of 2 to 45 μm were used, and this was designated as the fine powder 1.

[0047] Fine powder 2: Fine powder 1 was sieved through a sieve A4 with 25 μm openings, and the portion that passed through sieve A4 was sieved through a sieve B4 with 3 μm openings, and the alumina powder remaining on sieve B4 was used as the fine powder. Fine powder 3: Fine powder 1 was sieved through a sieve A5 with 25 μm openings, and the fraction that passed through sieve A5 was sieved through a sieve B5 with 1 μm openings, and the fraction remaining on sieve B5 was used as alumina powder.

[0048] (Examples 1 to 4 and Comparative Examples 1 to 3) The coarse powder and fine powder obtained above were mixed according to the blending ratios shown in Table 1 to produce alumina powders of Examples 1 to 4 and Comparative Examples 1 to 3.

[0049] <Particle size distribution> The volume frequency particle size distribution of the powder was determined by a wet laser diffraction scattering method using a particle size distribution analyzer (MT-3300EX, manufactured by Nikkiso Co., Ltd.). Water was used as the solvent, and the powder was dispersed in the solvent for 1 minute using a homogenizer at 200 W output. The dispersion was used as the measurement target. The PIDS (Polarization Intensity Differential Scattering) concentration was adjusted to 45-55% for measurement. Based on the obtained volume frequency particle size distribution, the particle size, frequency, and peak width of the maximum peak, and the particle size at which the cumulative value from the small particle size side becomes X% (D X ) was calculated. In Table 1, the particle size (μm) of the first peak in the volume frequency particle size distribution was designated as d1, the particle size (μm) of the second peak was designated as d2, and the particle size (μm) of the recess with the smallest frequency between the first and second peaks was designated as d3. The frequency (%) of the first peak was designated as P1, and the frequency (%) of the second peak was designated as P2.

[0050] <Sphericity> The sphericity of the powder was determined as follows under the conditions of room temperature of 25°C and humidity of 70%. The sphericity of the obtained alumina powder was measured using a wet flow image analyzer (FPIA-3000, manufactured by Sysmex Corporation). [Measurement procedure] The measurement sample used in the above-mentioned wet flow type image analyzer was prepared as follows. 0.05 g of spherical alumina powder sample was weighed into a 20 ml glass beaker, 10 ml of a 25% by mass propylene glycol aqueous solution was added, and the mixture was dispersed for 3 minutes using an ultrasonic disperser (ASU-10M manufactured by AS ONE Corporation). The entire amount was placed into the FPIA-3000 and measured using the LPF mode / quantitative counting method (total count of 100 particles, repeated measurement once). The perimeter of a single particle projected image and the perimeter of a circle corresponding to the area of ​​the particle projected image were analyzed using the above-mentioned wet flow type image analyzer, and the circularity was calculated using the following formula. Circularity = (perimeter of the projected particle image) / (perimeter of the circle equivalent to the area of ​​the projected particle image) The sphericity and circularity are average values ​​for particles within each particle size class. The sphericity was calculated as the square of the circularity of each particle size class.

[0051] <Specific surface area> The specific surface area of ​​the powder was measured by the BET one-point method using nitrogen gas adsorption. Specifically, a specific surface area measuring device (manufactured by Yuasa Ionics, device name: MONOSORB) was used, and nitrogen gas was used as the adsorption gas and helium gas as the carrier gas. 1 g of sample was dried and degassed at 300°C for 30 minutes before measurement.

[0052] [Table 1]

[0053] [Table 2]

[0054] The alumina powders of each example and comparative example were evaluated for the following items.

[0055] <Method for measuring thermal conductivity> The coarse powder and fine powder shown in Table 1 were mixed at a blending ratio of 60% by mass and 40% by mass to prepare an inorganic powder. 78% by volume of the obtained inorganic powder was mixed with 12% by volume of a liquid silicone resin (DOWSIL SE1885A) and 10% by volume of a liquid silicone resin (DOWSIL SE1885B) using a planetary centrifugal mixer. The resulting mixture was processed into a sheet having a thickness of 3 mm, and heat-treated in a dryer at 120°C for 2 hours to obtain a sample. The obtained samples were cut into a size of 20 mm length x 20 mm width x 3 mm thickness, and the upper heater, sample (cut-out sample), heat flux meter, lower heater, and heat sink were stacked in this order, and the thermal conductivity was measured using the heat flow meter method in accordance with ASTM E 1530. The average thermal conductivity of the 10 samples is shown in Table 2.

[0056] <Viscosity measurement method> The inorganic powder prepared in the above <Method for measuring thermal conductivity> was mixed with bisphenol F-type epoxy (Epikote 807) in a liquid state at 25°C so that the content was 65 mass % to prepare a resin varnish for evaluation. The viscosity of the obtained resin varnish for evaluation was measured using a rheometer at 25°C and a shear rate of 36 [1 / s]. Table 2 shows the relative viscosity values ​​when the viscosity of Comparative Example 1 is normalized to 100.

[0057] The results in Table 2 show that the alumina powders of Examples 1 to 4 can lower the viscosity and increase the thermal conductivity of resin compositions prepared by blending them with resins, compared to the alumina powders of Comparative Examples 1 to 3, respectively. [Explanation of symbols]

[0058] 1 burner 2. Melting furnace 3 Piping 4. Cyclone 5 Piping 8. Bag filter 9 Blower 11 Combustible gas supply pipe 12 Combustion auxiliary gas supply pipe 13 Raw material supply pipe 100 Thermal spraying equipment

Claims

1. When a volume frequency particle size distribution measured by a wet laser diffraction scattering method includes a first peak and a second peak, the particle diameter of the first peak is d1, the particle diameter of the second peak is d2, and the particle diameter of a recess having the smallest frequency between the first peak and the second peak is d3, d1 is 1 μm or more and less than 50 μm, d2 is 50 μm or more and 150 μm or less, and d3 ２ / d1×d2 satisfies 1.40 or more.

2. 2. The alumina powder according to claim 1, An alumina powder that satisfies at least one of d3 / d1 being 2.90 or more and d2 / d3 being 2.50 or less.

3. 3. The alumina powder according to claim 1 or 2, When the frequency of the first peak is P1 and the frequency of the second peak is P2, An alumina powder having a P2 / P1 ratio of 3.40 or more.

4. 3. The alumina powder according to claim 1 or 2, In the volume frequency particle size distribution, the particle diameters at the points where the cumulative volume from the small particle side is 10%, 50%, and 99% are defined as D １０ , D ５０ , D ９９ When (D ９９ -D １０ ) / D ５０ Alumina powder having a viscosity of 2.70 or less.

5. 3. The alumina powder according to claim 1 or 2, In the volume frequency particle size distribution, the particle diameters at the points where the cumulative volume from the small particle side is 50% and 99% are defined as D ５０ , D ９９ When D ９９ / D ５０ Alumina powder having a viscosity of 2.80 or less.

6. 3. The alumina powder according to claim 1 or 2, An alumina powder having an alumina content of 99.9 mass% or more in terms of mass.

7. 3. The alumina powder according to claim 1 or 2, An alumina powder having a sphericity of 0.90 or more as measured using a wet flow image analyzer.

8. An inorganic powder comprising the alumina powder according to claim 1 or 2 and an inorganic filler other than the alumina powder.

9. A resin composition comprising the alumina powder according to claim 1 or 2 and a resin.

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