Spherical alumina powder

Abstract

Provided is a spherical alumina powder which has a D50 of 0.1 μm to 40 μm, a circularity of 0.90 or more and 1.00 or less, an α-conversion rate of 60% or more and 100% or less, and a particle surface roughness represented by formula (1) of 1.14 or more and 1.35 or less. Formula (1): (Particle surface roughness) = (BET specific surface area A) / (Sphere equivalent specific surface area Sa calculated from particle size distribution)

Description

Spherical Alumina Powder

[0001] The present disclosure relates to a spherical alumina powder and a method for producing the same.

[0002] In recent years, mobile devices such as smartphones and electronic devices installed in vehicles have become significantly smaller, lighter, and thinner. As a result, the density of electronic components such as IC chips and memory mounted on printed wiring boards inside electronic devices has increased, and the heat generation density inside electronic devices has been increasing. As the heat generation density inside electronic devices increases, the temperature rise caused by the heat generated inside the electronic devices becomes significant, leading to a decrease in the operating performance and reliability of the electronic components. Therefore, it is necessary to quickly transfer and dissipate the heat generated from electronic components to the outside. Therefore, there is an increasing need to improve the thermal conductivity of packaging materials and substrate materials for electronic devices.

[0003] The mainstream method for solving the above problems is to fill insulating resin materials constituting packaging materials and substrate materials for electronic devices with a highly thermally conductive inorganic material powder as a filler at a high filling rate. Known highly thermally conductive inorganic material powders include alumina, magnesia, boron nitride, and aluminum nitride, but alumina powder is generally used from the viewpoints of chemical resistance, moisture absorption stability, etc.

[0004] The shape of the inorganic material powder is preferably spherical, which allows for easy kneading with resin at a high filling rate. Spherical alumina powder is generally known to be produced by a so-called thermal spraying method, in which Bayer process alumina, which is a raw material, is sprayed into a flame and rapidly cooled while melting to form spherical particles. The thermal spraying method is described in Patent Document 1.

[0005] However, in conventional thermal spraying methods, when the particle size is large, such as about 50 μm, spherical alumina with a high content of α-alumina (hereinafter also referred to as the α phase ratio) can be obtained, but as the particle size becomes smaller, the content of low-temperature phases such as δ-alumina increases, resulting in a problem of reduced thermal conductivity of the spherical alumina. Therefore, studies have been conducted to increase the α phase ratio by subjecting spherical alumina to heat treatment (Patent Document 2).

[0006] JP 11-147711 A JP 2014-9140 A

[0007] When the degree of gelatinization is increased by heat treatment of spherical alumina, the heat treatment increases the surface roughness of the spherical alumina, reducing its ability to be mixed with resin. This has led to the problem of the thermal conductivity of resin materials filled with spherical alumina not being increased. On the other hand, because conventional spherical alumina has excellent filling and flow properties, it tends to settle in the mixed and filled resin composition, which can lead to segregation between the resin layer with lower thermal conductivity and the layer where the spherical alumina powder is packed. This leads to further reductions in thermal conductivity and performance variations.

[0008] The present disclosure provides a spherical alumina powder that can provide a resin composition with high thermal conductivity even when the spherical alumina powder has a D50 of 0.1 to 40 μm. The present disclosure also provides a resin composition and a prepreg that contain the spherical alumina powder.

[0009] The present disclosure relates to the following: [1] A spherical alumina powder having a D50 of 0.1 to 40 μm, a circularity of 0.90 to 1.00, a gelatinization rate of 60 to 100%, and a particle surface roughness of 1.14 to 1.35 as expressed by the following formula (1): Particle surface roughness = BET specific surface area A / spherical equivalent specific surface area Sa calculated from particle size distribution [2] A specific gravity of 3.80 g / cm ３[3] The spherical alumina powder according to [1], which has a BET specific surface area A of 0.1 or more and 2.0 or less. [4] The spherical alumina powder according to any one of [1] to [3], which has an oil absorption rate of 30% or more and 50% or less. [5] A resin composition comprising the spherical alumina powder according to any one of [1] to [4] and a resin. [6] A prepreg in which a substrate is impregnated with the resin composition according to [5]. [7] A cured product of the prepreg according to [6] or a laminate thereof. [8] A metal-clad laminate comprising the cured product according to [7] and a metal foil arranged on at least one main surface of the cured product. [9] The resin composition according to [5], which is used as a sealant for an electronic component device.

[10] An electronic component device comprising an element and a cured product of the resin composition according to [5] that seals the element.

[11] A method for producing spherical alumina powder, comprising: a high-temperature step of forming a high-temperature region inside a furnace by a burner that forms a flame; a spheroidizing step of producing pre-heat-treated spherical alumina powder by charging raw alumina powder into the furnace and heating and melting it; a cooling and washing step of charging the pre-heat-treated spherical alumina powder into cooling and washing water in a water tank to cool and wash the pre-heat-treated spherical alumina powder; a recovery step of separating the pre-heat-treated spherical alumina powder from the cooling and washing water and recovering the pre-heat-treated spherical alumina powder; and a heat treatment step of heat-treating the recovered pre-heat-treated spherical alumina powder at 1100 to 1300°C in an air atmosphere to obtain spherical alumina powder.

[0010] According to the present disclosure, it is possible to provide a spherical alumina powder that can give a resin composition with high thermal conductivity even when the spherical alumina powder has a D50 of 0.1 to 40 μm. Furthermore, according to the present disclosure, it is possible to provide a resin composition and a prepreg that contain the spherical alumina powder.

[0011] 1 is a diagram showing a process flow of a method for producing a pre-heat-treated spherical alumina powder according to an embodiment of the present invention, and FIG. 2 is a schematic diagram showing the configuration of a production apparatus applicable to the process flow of FIG.

[0012] Hereinafter, embodiments of the present invention will be described. Note that the embodiments described below are representative examples of the present invention, and the present invention is not limited thereto.

[0013] In this specification, when multiple upper or lower limits are listed, numerical ranges can be created from all combinations of the upper and lower limits. Similarly, when multiple numerical ranges are listed, separate numerical ranges can be created by individually selecting and combining the upper and lower limits from those numerical ranges.

[0014] [Spherical Alumina Powder] In one embodiment, the spherical alumina powder has a D50 of 0.1 to 40 μm, a circularity of 0.90 to 1.00, a rate of alpha conversion of 60 to 100%, and a particle surface roughness of 1.14 to 1.35, as expressed by the following formula (1): Formula (1): Particle surface roughness = BET specific surface area A / spherical equivalent specific surface area Sa calculated from particle size distribution. Because the spherical alumina powder has a high rate of alpha conversion and a particle surface roughness within an appropriate range from the viewpoint of packing, it has high thermal conductivity, is less likely to segregate in a resin composition, and easily forms uniform particle contact points. Therefore, the spherical alumina powder can provide a resin composition with high thermal conductivity.

[0015] (D50) The D50 of the spherical alumina powder is 0.1 μm or more, preferably 0.5 μm or more, and more preferably 1.0 μm or more. The D50 of the spherical alumina powder is 40 μm or less, preferably 10 μm or less, and more preferably 5.0 μm or less. When the D50 is 0.1 μm or more, the thermal conductivity can be increased. When the D50 is 40 μm or less, the fillability into resin can be improved.

[0016] D50 is the 50% particle size in the volume-based cumulative particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer, and is measured by the method described in the examples.

[0017] (Glycated ratio) The gelatinized ratio of the spherical alumina powder is 60% or more, preferably 70% or more, and more preferably 80% or more. The higher the gelatinized ratio, the more improved the thermal conductivity can be expected. The gelatinized ratio of the spherical alumina powder may be 100% or less, 99% or less, or 98% or less.

[0018] The alpha conversion rate is calculated by the following formula (2), where X is the maximum peak intensity of the α-alumina crystal phase at a diffraction angle 2θ=35.2°±0.2° in X-ray diffraction measurement, and Y is the maximum peak intensity of a crystal phase other than α-alumina at a diffraction angle 2θ=67.3°±0.2°. Formula (2): alpha conversion rate=(X / (X+Y))×100(%)

[0019] (Particle surface roughness A / Sa) In this specification, the particle surface roughness is a value calculated by the following formula (1). Formula (1): Particle surface roughness = BET specific surface area A / sphere-equivalent specific surface area Sa calculated from particle size distribution The particle surface roughness of the spherical alumina powder is 1.14 or more, preferably 1.16 or more, and more preferably 1.20 or more. The particle surface roughness of the spherical alumina is 1.35 or less, preferably 1.31 or less, and more preferably 1.29 or less. When the particle surface roughness is 1.14 or more, settling of the alumina particles in the resin composition is suppressed. When the particle surface roughness is 1.35 or less, the fillability into the resin can be improved.

[0020] (BET Specific Surface Area A) The BET specific surface area A of the spherical alumina powder is preferably 0.1 or more, more preferably 0.5 or more, and even more preferably 1.0 or more. The BET specific surface area A of the spherical alumina powder is preferably 2.0 or less, more preferably 1.7 or less, and even more preferably 1.5 or less. When the BET specific surface area A is 0.1 or more, the heat dissipation property is excellent. When the BET specific surface area A is 2.0 or less, the fillability into resin can be improved.

[0021] In this specification, the BET specific surface area A is a value measured and calculated in accordance with "6.2 Fluidized Method (3.5) Single Point Method" of JIS R 1626:1996 (Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method). The measurement is performed by pre-treating the sample by heating it to 180°C and passing nitrogen gas through it for 20 minutes, and then using nitrogen gas as the adsorbate.

[0022] (Sphere-equivalent specific surface area Sa calculated from particle size distribution) In this specification, the sphere-equivalent specific surface area Sa calculated from particle size distribution (also referred to as sphere-equivalent specific surface area Sa) is determined by the method described in the examples.

[0023] The spherical alumina powder has a sphere-equivalent specific surface area Sa of preferably 0.1 or more, more preferably 0.5 or more, and even more preferably 1.0 or more. The spherical alumina powder has a sphere-equivalent specific surface area Sa of preferably 2.0 or less, more preferably 1.7 or less, and even more preferably 1.5 or less. When the sphere-equivalent specific surface area Sa is 0.1 or more, excellent heat dissipation properties are achieved. When the sphere-equivalent specific surface area Sa is 2.0 or less, fillability into resin can be improved.

[0024] (Specific Gravity) The specific gravity of the spherical alumina powder is preferably 3.80 g / cm ３ More preferably, 3.85 g / cm ３ The specific gravity of the spherical alumina powder is preferably 4.00 g / cm ３ or less, more preferably 3.97 g / cm ３ The specific gravity is 3.80 g / cm or less. ３ When the specific gravity is equal to or greater than this, the content of α-alumina is high and the thermal conductivity is excellent. The specific gravity is determined by the method described in the Examples.

[0025] (Circularity) The circularity of the spherical alumina powder is 0.90 or more, preferably 0.92 or more, more preferably 0.93 or more, and even more preferably 0.95 or more. The circularity of the spherical alumina powder is 1.00 or less by definition, but may be 0.99 or less from the viewpoint of productivity. The circularity is measured by the method described in the Examples.

[0026] (Oil absorption rate) The oil absorption rate of the spherical alumina powder is preferably 30% or more, more preferably 35% or more, and even more preferably 38% or more. The oil absorption rate of the spherical alumina powder is preferably 50% or less, more preferably 45% or less, and even more preferably 43% or less. When the oil absorption rate is 30% or more, sedimentation of the alumina particles in the resin composition is suppressed. When the oil absorption rate is 50% or less, the fillability into the resin can be improved. The oil absorption rate is measured by the method described in the examples.

[0027] [Method for Producing Spherical Alumina Powder] Spherical alumina powders produced by a typical flame fusion method are produced by a known method in which raw material powders that can serve as aluminum sources, such as aluminum oxide powder, aluminum hydroxide powder, or metallic aluminum powder, are introduced into a high-temperature flame, melted, and spheroidized by surface tension. The raw material powders are then cooled by air to a temperature at which they can be collected, and the powder is then collected in a collector. In this method, the cooling after melting is gradual, which causes the alumina particles to crystallize during the cooling process, resulting in increased particle surface roughness. In addition, the temperature profile from spheroidization to cooling is not controlled, resulting in variations in the degree of crystallization during the cooling process. Therefore, when such conventional spherical alumina powders are heat-treated, it is difficult to control the gelatinization rate and particle surface roughness within the desired ranges.

[0028] On the other hand, the present inventors have discovered that spherical alumina powder with a controlled alpha conversion rate and particle surface roughness within a desired range can be obtained by spheroidizing raw material powder using the above-mentioned known method, immediately cooling the hot powder in a water tank, and then washing out impurities that may induce particle fusion and sintering, such as soda components and calcium components that have seeped out from the interior of the particles and onto the surface, and then heat-treating the resulting spherical alumina powder. Hereinafter, the spherical alumina powder before heat treatment will be referred to as the pre-heat-treated spherical alumina powder, and the spherical alumina powder after heat treatment will be referred to as the spherical alumina powder, and exemplary production methods thereof will be described.

[0029] (Method for producing pre-heat-treated spherical alumina powder) One embodiment of the method for producing pre-heat-treated spherical alumina powder includes a high-temperature step of forming a high-temperature region inside a furnace using a burner that forms a flame, a spheroidizing step of charging raw alumina powder into the furnace and heating and melting it to produce pre-heat-treated spherical alumina powder, a cooling and washing step of charging the pre-heat-treated spherical alumina powder into cooling and washing water in a water tank to cool and wash the pre-heat-treated spherical alumina powder, and a recovery step of separating the pre-heat-treated spherical alumina powder from the cooling and washing water and recovering the pre-heat-treated spherical alumina powder. Figure 1 shows a process flow of one embodiment of the method for producing pre-heat-treated spherical alumina powder. By simultaneously cooling and washing the pre-heat-treated spherical alumina powder, Na, which may induce particle fusion and sintering and is present on the particle surfaces, can be removed at low cost and in a short time. ＋ It is possible to produce a pre-heat-treated spherical alumina powder having reduced amounts of SiO 2 and other ionic impurities.

[0030] <Raw alumina powder> There are no particular restrictions on the raw alumina powder. ＋ It is preferable to use a small amount. ＋ A specific example of alumina powder with a small amount of content is low soda alumina powder produced by the Bayer process.

[0031] The shape of the raw alumina powder is not limited, but a non-spherical shape is preferred for the purpose of obtaining spherical alumina powder before heat treatment. The circularity of the raw alumina powder is, for example, less than 0.90, 0.86 or less, or 0.84 or less.

[0032] The D50 of the raw alumina powder is preferably 0.1 μm or more and 40 μm or less from the viewpoint of obtaining a spherical alumina powder before heat treatment having a size suitable for a thermally conductive filler, more preferably 0.5 μm or more and 10 μm or less, and even more preferably 1.0 μm or more and 5 μm or less from the same viewpoint.

[0033] <Furnace> Examples of the furnace include a vertical furnace and a horizontal furnace. The shape of the furnace is not limited, and examples include a cylindrical shape and a polygonal prism shape such as a hexagonal prism. A cylindrical shape is preferred because it is easy to uniformly control the temperature inside the furnace. The cylindrical shape means that at least a portion of the furnace is cylindrical, and may include portions of other shapes. The polygonal prism shape means that at least a portion of the furnace is polygonal prism, and may include portions of other shapes. From the viewpoint of improving collection efficiency, the downstream portion of the furnace is preferably an inverted cone shape that narrows toward the discharge hole.

[0034] Although the material of the furnace is not limited, the inner wall is preferably made of stainless steel in order to reduce contamination of the spherical alumina powder with impurities before heat treatment. A water-cooling jacket may be provided around the periphery of the furnace to cool it.

[0035] <High-Temperature Step> The high-temperature step is a step of forming a high-temperature region inside the furnace by a burner that forms a flame.

[0036] A burner is a device that mixes a suitable amount of a combustion-sustaining gas with a combustible gas to form a flame in a furnace. The burner is supplied with the combustible gas from a combustible gas supply source, and with the combustion-sustaining gas from a combustion-sustaining gas supply source. Examples of the combustible gas include liquefied natural gas (LNG) and liquefied petroleum gas (LPG). Examples of the combustion-sustaining gas include air, oxygen gas, and oxygen-enriched air.

[0037] The temperature of the high temperature region is preferably 2100°C or higher. The temperature of the high temperature region is preferably 2500°C or lower, more preferably 2300°C or lower. If the temperature is 2100°C or higher, the temperature is above the melting point of alumina, which makes it easy to increase the circularity of the resulting alumina powder. If the temperature is 2500°C or lower, it is within the heat resistance range of a general burner, which is advantageous in terms of cost.

[0038] <Spheroidizing Step> The spheroidizing step is a step of producing a pre-heat-treated spherical alumina powder by introducing a raw alumina powder into a furnace and heating and melting it. In the spheroidizing step, a carrier gas for supplying the raw alumina powder may be used as needed. Examples of the carrier gas include at least one selected from air, nitrogen, oxygen, and carbon dioxide.

[0039] <Pre-cooling Step> The method for producing the pre-heat-treated spherical alumina powder preferably includes a pre-cooling step in which the pre-heat-treated spherical alumina powder is pre-cooled. The pre-cooling step is carried out after the spheroidizing step and before the cooling and washing step. The equipment for carrying out the pre-cooling step is preferably provided outside the high-temperature region of the furnace or between the furnace and the water tank used in the cooling and washing step. This allows the pre-heat-treated spherical alumina powder to be poured into the cooling and washing water in the water tank in the cooling and washing step to be cooled to a desired temperature in a short period of time.

[0040] Pre-cooling is preferably carried out by spraying water onto the spherical alumina powder before heat treatment, using equipment such as a shower.

[0041] <Cooling and Washing Step> The cooling and washing step is a step in which the pre-heat-treated spherical alumina powder is placed into cooling and washing water in a water tank to simultaneously cool and wash the pre-heat-treated spherical alumina powder. By simultaneously performing cooling and washing, the equipment can be made smaller and the overall process time can be shortened compared to when cooling and washing are performed as separate steps, for example, when the pre-heat-treated spherical alumina powder is air-cooled after the spheroidizing step while being collected in a collector such as a cyclone, and the collected pre-heat-treated spherical alumina powder is washed. In addition, variation in crystallinity can be suppressed. The method for producing pre-heat-treated spherical alumina powder can produce pre-heat-treated spherical alumina powder with a low BET specific surface area due to its short thermal history.

[0042] The water tank contains cooling and washing water for cooling and washing the pre-heat-treated spherical alumina powder. To improve the washing efficiency, it is preferable to stir the cooling and washing water with a stirrer. The cooling and washing water is not limited to, but may be, for example, city water.

[0043] The temperature of the cooling and washing water is preferably 50° C. or higher, more preferably 70° C. or higher, from the viewpoint of efficiently reducing ionic impurities present on the surface of the spherical alumina powder before heat treatment. The temperature of the cooling and washing water is preferably 80° C. or lower, from the viewpoint of suppressing damage to the furnace body due to heat load.

[0044] In the cooling and washing step, the pre-heat-treated spherical alumina powder is preferably introduced into the cooling and washing water within 5 seconds after leaving the furnace. Introducing the pre-heat-treated spherical alumina powder within 5 seconds can keep the BET specific surface area of ​​the pre-heat-treated spherical alumina powder low. Introducing the pre-heat-treated spherical alumina powder within 5 seconds can keep the variation in crystallinity of the pre-heat-treated spherical alumina powder low.

[0045] In the cooling and washing step, the temperature of the pre-heat-treated spherical alumina powder introduced into the cooling and washing water is preferably 200° C. or lower, more preferably 100° C. or lower. More preferably, the temperature of the pre-heat-treated spherical alumina powder from the time it leaves the furnace until it is introduced into the cooling and washing water is 200° C. or lower, particularly preferably 100° C. or lower. If the temperature is 200° C. or lower, there is little risk of the cooling and washing water in the water tank being overheated and evaporating, which facilitates stable production.

[0046] The temperature of the spherical alumina powder before the heat treatment can be determined using simulation software Ansys Fluent (Ansys).

[0047] <Recovery Step> The recovery step is a step of separating the pre-heat-treated spherical alumina powder from the cooling and washing water, and recovering the pre-heat-treated spherical alumina powder. The recovery method is not limited, but includes a method of settling the pre-heat-treated spherical alumina powder dispersed in the cooling and washing water, and removing the supernatant water with a pump or the like. This method removes Na adhering to the particle surface. ＋ , Ca ２＋ This is preferable because it is possible to wash away ionic impurities such as ammonium hydroxide and the like.

[0048] <Drying Step> The method for producing the pre-heat-treated spherical alumina powder preferably includes a drying step. The drying step is a step of drying the pre-heat-treated spherical alumina powder from which the cooling and washing water has been separated. In the drying step, moisture remaining after the recovery step is removed. The drying method is not limited, and may be natural drying or heat drying.

[0049] <Apparatus for manufacturing pre-heat-treated spherical alumina powder> A manufacturing apparatus used in one embodiment of the method for manufacturing pre-heat-treated spherical alumina powder and a preferred embodiment of its configuration will be described in detail below with reference to the drawings. Note that the drawings used in the following description focus on the characteristic parts for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of the respective components may not necessarily be the same as those of the actual apparatus.

[0050] The pre-heat-treated spherical alumina powder manufacturing apparatus will be described with reference to Fig. 2. As shown in Fig. 2, the pre-heat-treated spherical alumina powder manufacturing apparatus is generally configured to include a hopper 1 for storing and charging raw materials, a spheroidizing burner 2, a water-cooled jacket-type spheroidizing furnace 3 (also referred to as the spheroidizing furnace 3) for spheroidizing, a stirring and cooling tank 4 for washing and cooling the pre-heat-treated spherical alumina powder, a separation tank 5 for separating the collected pre-heat-treated spherical alumina powder from a large amount of moisture and extracting the pre-heat-treated spherical alumina powder, and a drying device 6 for drying the pre-heat-treated spherical alumina powder that still retains moisture to obtain a dry pre-heat-treated spherical alumina powder.

[0051] The raw material hopper 1 is equipped with a feeder device or the like, which allows the raw material alumina powder to be quantitatively supplied to the spheroidizing burner 2. The spheroidizing burner 2 supplies the raw material alumina powder supplied from the raw material hopper 1 into the spheroidizing furnace 3. It is also possible to supply the raw material alumina powder to the spheroidizing burner 2 using a carrier gas for transporting the powder.

[0052] The spheroidizing burner 2 is provided in the spheroidizing furnace 3. A combustible gas is supplied to the spheroidizing burner 2 from a combustible gas supply source (not shown), and a combustion-sustaining gas is supplied to the spheroidizing burner 2 from a combustion-sustaining gas supply source (not shown). The spheroidizing burner 2 can form a flame in the spheroidizing furnace 3.

[0053] The spheroidizing furnace 3 is a cylindrical vertical furnace, and the lower part of the furnace has an inverted cone shape that narrows toward the discharge hole side. In the spheroidizing furnace 3, raw alumina powder is charged into a high-temperature region formed by a flame and heated to melt, thereby producing pre-heat-treated spherical alumina powder.

[0054] A shower 10 is provided inside the spheroidizing furnace 3. The shower 10 sprays water onto the pre-heat-treated spherical alumina powder that has left the high-temperature region, thereby pre-cooling the pre-heat-treated spherical alumina powder. In Fig. 2, the shower is provided inside the spheroidizing furnace 3, but the shower may be provided at any location on the path that the pre-heat-treated spherical alumina powder takes from when it leaves the high-temperature region until it comes into contact with the cooling wash water.

[0055] The spheroidizing furnace 3 is connected to the stirring and cooling tank 4 via a short pipe. The short pipe is provided with a duct or the like, which is connected to a scrubber-type collector 8 and an exhaust device 9, allowing the combustion exhaust gas to be discharged outside the system. Since the combustion exhaust gas contains fine pre-heat-treatment spherical alumina powder, it is desirable to pass the combustion exhaust gas through the scrubber-type collector 8 to separate and remove the fine powder from the gas, and then discharge the purified combustion exhaust gas via the exhaust device 9. In Figure 2, there are no gaps between the spheroidizing furnace 3, the short pipe, and the stirring and cooling tank 4, but gaps may be present between them.

[0056] Cooling and washing water for cooling and washing the pre-heat-treated spherical alumina powder is stored in the stirring and cooling tank 4. The cooling and washing water in the stirring and cooling tank 4 is stirred by an agitator to improve washing efficiency. A cooling water pump 7 is connected to the stirring and cooling tank 4, and the cooling water pump 7 is connected to a shower 10, and piping is provided so that the cooling and washing water in the tank can be recycled and reused.

[0057] A separation tank 5 is connected to the stirring and cooling tank 4 to separate the collected pre-heat-treated spherical alumina powder from the cooling and washing water, and a slurry containing the pre-heat-treated spherical alumina powder is supplied from the stirring and cooling tank 4 to the separation tank 5. In the separation tank 5, the cooling and washing water is separated from the solid matter, and the water is returned to the system via a cooling water pump 7 and circulated. At this time, Na adhering to the particle surfaces is removed. ＋The solid matter is fed to a drying device 6, where the remaining moisture adhering thereto is dried, thereby obtaining a pre-heat-treated spherical alumina powder with a small amount of ionic impurities.

[0058] This manufacturing apparatus does not require the installation of expensive, large-scale equipment such as an ion-exchange water unit, and allows the production of pre-heat-treated spherical alumina powder with low ionic impurities using a small-scale spheroidizing facility.

[0059] (Method for producing spherical alumina powder) Spherical alumina powder can be obtained by heat-treating unheat-treated spherical alumina powder in an air atmosphere at 1100 to 1300°C. Heat-treating at 1100°C or higher can promote the transition from phases other than the α-crystalline phase, which have low thermal conductivity, such as amorphous phase, γ-crystalline phase, δ-crystalline phase, and θ-crystalline phase, to the α-crystalline phase, which has high thermal conductivity. Heat-treating at 1300°C or lower can suppress surface roughness. Heat-treating at 1300°C or lower can suppress an increase in particle size due to fusion between particles.

[0060] The heat treatment time is preferably 1 hour or more, more preferably 1 hour to 4 hours. If it is 1 hour or more, the transition time to the target α crystal phase is sufficient, and the target α conversion rate is easily achieved. If it is 4 hours or less, sintering between particles is suppressed, making it easier to obtain powder with the target particle size.

[0061] The heat treatment apparatus may be a general apparatus, specific examples of which include a bogie furnace, a tunnel furnace, and a rotary kiln. The heat treatment atmosphere is not particularly limited, but is preferably an air atmosphere.

[0062] The method for producing spherical alumina powder may optionally include a crushing step of crushing agglomerated particles formed by heat treatment. The method for producing spherical alumina powder may optionally include a classification step of performing a classification treatment. The heat treatment may result in agglomerated particles in which some particles are sintered together. In this case, it is preferable to perform at least one treatment selected from a crushing treatment and a classification treatment. The crushing treatment may be wet or dry. Examples of the crushing treatment include methods using a roll mill, a ball mill, a small-diameter ball mill (also called a bead mill), a pot mill, a media stirring mill, an airflow crusher, a mortar, an automatic kneading mortar, a tank crusher, or a jet mill.

[0063] The spherical alumina powder is preferably used in applications requiring high thermal conductivity, such as heat dissipation materials and semiconductor encapsulation materials, as a filler in materials requiring high thermal conductivity.

[0064] [Resin Composition] The resin composition of one embodiment contains spherical alumina powder and a resin. The resin composition can be used as a heat dissipation and insulation resin composition or a sealing material, for example, a sealing material for electronic component devices. The resin composition is suitable for semiconductor packages and printed wiring boards, which require high thermal conductivity.

[0065] Examples of the resin include thermosetting resins and thermoplastic resins, with thermosetting resins being preferred. Examples of the thermosetting resin include epoxy resins, phenolic resins, unsaturated imide resins, amino resins such as melamine resins, unsaturated polyester resins, allyl resins, dicyclopentadiene resins, silicone resins, and triazine resins. Among these, epoxy resins are preferred because of their excellent moldability and electrical insulation properties. The resins may be used alone or in combination of two or more.

[0066] Examples of epoxy resins include bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, biphenyl type epoxy resins, alicyclic epoxy resins, phenol novolac type epoxy resins, cresol novolac type epoxy resins, bisphenol A novolac type epoxy resins, bisphenol F novolac type epoxy resins, dicyclopentadiene type epoxy resins, naphthalene type epoxy resins, and anthracene type epoxy resins.

[0067] Examples of thermoplastic resins include polyethylene, polypropylene, polystyrene, polyphenylene ether resin, phenoxy resin, polycarbonate resin, polyester resin, polyamide resin, polyamideimide resin, polyimide resin, xylene resin, polyphenylene sulfide resin, polyetherimide resin, polyetheretherketone resin, and polyetherimide resin.

[0068] The content of the spherical alumina powder in the resin composition is preferably 50 to 90% by volume, more preferably 60 to 85% by volume. When the content is 50% by volume or more, the effect of improving thermal conductivity due to the spherical alumina powder is sufficient. When the content is 90% by volume or less, the moldability of the resin composition is good.

[0069] The resin composition may contain an inorganic filler other than spherical alumina. Examples of the inorganic filler include aluminum hydroxide, zinc oxide, magnesium oxide, magnesium carbonate, magnesium hydroxide, titanium oxide, silicon oxide, and boron nitride. Among them, at least one selected from aluminum hydroxide, zinc oxide, magnesium oxide, and boron nitride is preferred from the viewpoint of improving thermal conductivity.

[0070] The resin composition may contain optional components other than the above components, such as a curing agent, a curing accelerator, a flame retardant, an ultraviolet absorber, an antioxidant, an organic solvent, and a surface treatment agent.

[0071] When an epoxy resin is used, examples of the curing agent include polyfunctional phenol compounds such as phenol novolac and cresol novolac; amine compounds such as dicyandiamide, diaminodiphenylmethane and diaminodiphenylsulfone; and acid anhydrides such as phthalic anhydride, pyromellitic anhydride, maleic anhydride and maleic anhydride. The curing agents may be used alone or in combination of two or more.

[0072] When an epoxy resin is used, examples of the curing accelerator include imidazole compounds and derivatives thereof; organic phosphorus compounds; secondary amines; tertiary amines; and quaternary ammonium salts.

[0073] Examples of imidazole compounds and derivatives thereof include imidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 2-undecylimidazole, 1-benzyl-2-methylimidazole, 2-heptadecylimidazole, 4,5-diphenylimidazole, 2-methylimidazoline, 2-phenylimidazoline, 2-undecylimidazole, 2-heptadecylimidazole, 2-isopropylimidazole, 2,4-dimethylimidazole, 2-phenyl-4-methylimidazole, 2-ethylimidazoline, 2-isopropylimidazoline, 2,4-dimethylimidazoline, and 2-phenyl-4-methylimidazoline. The imidazole compounds and derivatives thereof may be masked with a masking agent. Examples of masking agents include acrylonitrile, phenylene diisocyanate, toluidine isocyanate, naphthalene diisocyanate, methylene bisphenyl isocyanate, and melamine acrylate.

[0074] Examples of organic phosphorus compounds include ethylene phosphine, propyl phosphine, butyl phosphine, phenyl phosphine, trimethyl phosphine, triethyl phosphine, tributyl phosphine, trioctyl phosphine, triphenyl phosphine, tricyclohexyl phosphine, triphenyl phosphine / triphenyl borane complex, and tetraphenyl phosphonium tetraphenyl borate.

[0075] Secondary amines include, for example, morpholine, piperidine, pyrrolidine, dimethylamine, diethylamine, dicyclohexylamine, N-alkylarylamines, piperazine, diallylamine, thiazoline, and thiomorpholine.

[0076] Tertiary amines include, for example, benzyldimethylamine, 2-(dimethylaminomethyl)phenol, and 2,4,6-tris(dimethylaminomethyl)phenol.

[0077] Examples of quaternary ammonium salts include tetrabutylammonium iodide, tetrabutylammonium bromide, tetrabutylammonium chloride, tetrabutylammonium fluoride, benzalkonium chloride, benzyldi(2-hydroxyethyl)ethylammonium chloride, and decyldi(2-hydroxyethyl)methylammonium bromide.

[0078] The curing accelerators may be used alone or in combination of two or more.

[0079] The organic solvent can be used for the purpose of adjusting the viscosity of the resin composition. For example, when a prepreg is produced by impregnating a substrate with the resin composition, or when the resin composition is applied, it is preferable to use an organic solvent to adjust the viscosity of the resin composition to an appropriate range. The organic solvent may be removed after the impregnation treatment or application treatment.

[0080] Examples of organic solvents include alcohols such as methanol, ethanol, propanol, and butanol; glycol ethers such as methyl cellosolve, butyl cellosolve, and propylene glycol monomethyl ether; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; esters such as butyl acetate and propylene glycol monomethyl ether acetate; ethers such as tetrahydrofuran; aromatic hydrocarbons such as toluene and xylene; nitrogen-containing solvents such as dimethylformamide, dimethylacetamide, and N-methylpyrrolidone; and sulfur-containing solvents such as dimethyl sulfoxide. These organic solvents may be used alone or in combination.

[0081] Among these, methyl isobutyl ketone, methyl ethyl ketone, propylene glycol monomethyl ether, and methyl cellosolve are preferred from the viewpoint of solubility, and methyl isobutyl ketone and propylene glycol monomethyl ether are more preferred from the viewpoint of low toxicity.

[0082] When dispersing the spherical alumina powder in an organic solvent, a dispersing machine such as a bead mill, homogenizer, jet mill, etc. can be used to improve dispersibility. It is also preferable to pretreat the spherical alumina powder with a surface treatment agent described below or to subject it to integral blending treatment.

[0083] Examples of the surface treatment agent include coupling agents such as silane coupling agents and titanate coupling agents, and silicone oligomers. The spherical alumina powder may be pretreated with a surface treatment agent.

[0084] When a prepreg is produced by impregnating a substrate with the resin composition, the total content of components other than the organic solvent (also referred to as solid content) in the resin composition is preferably 40 to 90 mass %, more preferably 50 to 85 mass %, based on the total resin composition.

[0085] [Method for Producing Resin Composition] The method for producing the resin composition is not particularly limited. Examples of methods for producing the resin composition include a method in which predetermined amounts of components are thoroughly mixed using a mixer or the like, kneaded using a mixing roll, extruder, or the like, and then cooled. More specifically, a method in which predetermined amounts of components are stirred and mixed, and the mixture is kneaded using a kneader, roll, extruder, or the like that has been preheated to 70 to 140°C, and then cooled is exemplified.

[0086] [Prepreg] The prepreg of one embodiment is obtained by impregnating a base material with a resin composition.

[0087] Examples of substrate materials include inorganic fibers such as E-glass, D-glass, S-glass, and Q-glass. Examples of substrate forms include woven fabric, nonwoven fabric, roving, chopped strand mat, and surfacing mat. The material and form of the substrate are selected based on the intended use and performance, and a single material or a combination of two or more materials and forms can be used as needed. From the viewpoints of heat resistance, moisture resistance, and processability, the substrate may be surface-treated. Examples of surface treatments include surface treatment with a silane coupling agent or the like, and mechanical fiber-opening treatment. The thickness of the substrate is, for example, 0.01 to 0.2 mm.

[0088] The prepreg can be produced, for example, by impregnating a substrate with a resin composition containing an organic solvent and then removing the organic solvent. The impregnated resin composition may be semi-cured by heating.

[0089] Prepregs are suitable for semiconductor packages and printed wiring boards, which require high thermal conductivity.

[0090] [Cured product of prepreg or laminate thereof] A cured product of a prepreg or laminate thereof can be produced by heating and pressurizing one or more prepreg sheets. The number of prepreg sheets is not particularly limited, but may be, for example, 2 to 20. Examples of devices for performing heating and pressurization include multi-stage presses, multi-stage vacuum presses, continuous molding machines, and autoclave molding machines. The heating and pressurization conditions may be selected depending on the thermosetting resin, curing agent, etc. used. For example, the temperature is 100 to 250°C, the pressure is 0.2 to 10 MPa, and the time is 0.1 to 5 hours.

[0091] [Metal-clad laminate] In one embodiment, the metal-clad laminate includes a cured product of a prepreg or a laminate thereof, and a metal foil disposed on at least one main surface of the cured product. The metal-clad laminate can be produced, for example, by stacking 1 to 20 prepregs and disposing metal foil on one or both surfaces of the stack, followed by heating and pressing. The heating and pressing apparatus and heating and pressing conditions are the same as those described above. The metal foil is not particularly limited as long as it is used for electronic components. Examples of metal foil include copper foil and aluminum foil.

[0092] [Electronic Component Device] An electronic component device according to one embodiment includes an element and a cured product of a resin composition that encapsulates the element. Examples of the element include active elements such as semiconductor chips, transistors, diodes, and thyristors, and passive elements such as capacitors, resistors, and coils.

[0093] Examples of electronic component devices include those in which an element is mounted on a support member such as a lead frame, a pre-wired tape carrier, a wiring board, glass, a silicon wafer, or an organic substrate, and the resulting element portion is sealed with a resin composition. More specifically, DIP (Dual Inline Package), PLCC (Plastic Leaded Chip Carrier), QFP (Quad Flat Package), SOP (Small Outline Package), SOJ (Small Outline J-lead package), TSOP (Thin Small Outline Package), TQFP (Thin Quad Flat Package), etc., have a structure in which an element is fixed on a lead frame, and terminals of the element such as bonding pads are connected to leads by wire bonding, bumps, etc., and then sealed using a resin composition by transfer molding or the like. Examples of such a device include a general resin-sealed IC such as a TCP (Tape Carrier Package); a TCP (Tape Carrier Package) having a structure in which elements connected to a tape carrier by bumps are sealed with a resin composition; a COB (Chip On Board) module, hybrid IC, multi-chip module, etc. having a structure in which elements connected to wiring formed on a support member by wire bonding, flip chip bonding, solder, or the like are sealed with a resin composition; and a BGA (Ball Grid Array), CSP (Chip Size Package), MCP (Multi Chip Package) having a structure in which elements are mounted on the surface of a support member having terminals for connecting a wiring board formed on the back surface, and the elements are connected to the wiring formed on the support member by bumps or wire bonding, and then the elements are sealed with a resin composition.

[0094] Methods for encapsulating electronic component devices using a resin composition include, for example, low-pressure transfer molding, injection molding, and compression molding. Of these, low-pressure transfer molding is the most common.

[0095] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0096] <Evaluation Method> (D10, D50, D90) 40 mg of a sample was added to 50 mL of water containing two drops of a nonionic surfactant (TRITON (trademark)-X; Roche Applied Science), and the mixture was ultrasonically dispersed for 3 minutes. This dispersion was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac Bell, MT3300II). The 10% particle size, 50% particle size, and 90% particle size in the obtained volume-based cumulative particle size distribution were determined as D10, D50, and D90, respectively.

[0097] (Galactosylation rate) The alpha phase ratio was calculated from the relationship: alpha phase ratio (%) = (X / (X + Y)) × 100, where X is the maximum peak intensity of the α-alumina crystal phase at a diffraction angle 2θ = 35.2° ± 0.2° in X-ray diffraction measurement, and Y is the maximum peak intensity of a crystal phase other than α-alumina at a diffraction angle 2θ = 67.3° ± 0.2°. X-ray diffraction measurement of the alumina powder was performed using an X-ray diffractometer PW3040 / 60X'Pert-MRD (Malvern Panalytical) with a copper X-ray tube at a tube voltage of 45 kV and a tube current of 40 mA.

[0098] (BET Specific Surface Area A) The BET specific surface area A was measured and calculated in accordance with "6.2 Fluid Method (3.5) Single-Point Method" of JIS R 1626:1996 (Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method). Specifically, approximately 2 g of a sample was weighed out and subjected to a preliminary degassing treatment at 180°C for 20 minutes. The sample was then placed in an automatic specific surface area measuring device (Mountec Co., Ltd., Macsorb). The nitrogen gas adsorption amount was measured using pure nitrogen and a nitrogen-helium mixed gas (mixing ratio: nitrogen 30 mol%, He 70 mol%), and the BET specific surface area A was calculated by the single-point method.

[0099] (Sphere-equivalent specific surface area Sa calculated from particle size distribution) The sphere-equivalent specific surface area (Sa) calculated from particle size distribution was calculated by the following formula based on particle size distribution data obtained using a laser diffraction / scattering particle size distribution analyzer (Microtrac Bell Co., Ltd., MT3300II). The sample preparation method was the same as the preparation method used in measuring D10, D50, and D90. dｉ is the average diameter in particle size class i, V ｉ represents the relative volume of particle size class i, and ρ represents the specific gravity. ρ is the specific gravity calculated by the following method.

[0100] (Specific Gravity) 5 mg of sample was placed in a measurement sample cell, and the specific gravity was measured by a gas (helium) substitution method using a dry densitometer (Shimadzu Corporation, product name: Accupyc II 1340). Specifically, the measurement sample cell was set in the dry densitometer, and the volume of the sample was determined by substitution with helium gas, and the specific gravity was calculated by dividing the sample mass by the volume.

[0101] (Circularity) The circularity is the average value of values ​​calculated for 2,000 or more particles by the following formula (3), where S is the area of ​​a projection of a particle and L is the perimeter. Formula (3): (4×π×S) / L ２ Measurements of area S and perimeter L were performed using an FPIA-3000 (Malvern Panalytical). As a pretreatment, due to the measurement range of the instrument, approximately 10 g of sample was placed on a 200 mm diameter, 25 μm mesh gold sieve, and particles larger than 25 μm were removed with shower water. The sample that passed through the sieve was transferred to a plastic container and used as the measurement sample. Measurement conditions were LPF / HPF standard (20x lens) and bright field, and Particle Sheath (Malvern Panalytical) was used as the measurement solvent. 2 g of the measurement sample was weighed into a 50 mL beaker, 50 mL of pure water was added, and the sample was dispersed in a 200 W ultrasonic disperser for 3 minutes, after which it was placed in the instrument and measured. As data processing after measurement, any particles with multiple particles on one screen were deleted, and the circularity was calculated.

[0102] (Oil absorption rate) 10 g of sample was placed in a mortar with a diameter of 10 cm, and a small amount of liquid epoxy resin was dropped therein and mixed thoroughly with the sample. Small amounts of resin were repeatedly dropped until the sample and epoxy resin formed a single mass. The volume of the dropped resin was calculated from the weight of the resin at the time when the sample and resin formed a single mass, and the oil absorption rate was calculated using the following formula. The volume of the sample was calculated from the specific gravity measured above. Oil absorption rate (%) = (volume of dropped resin / (volume of sample + volume of dropped resin)) x 100

[0103] (Thermal Conductivity) Alumina powder was mixed with an epoxy molding compound resin containing an epoxy resin, a polyfunctional phenolic compound as a curing agent, and an organic phosphorus compound as a curing accelerator at the volumetric filling rate (%) shown in Table 1, diluted with methyl ethyl ketone, and then stirred using a mixer rotor to produce a varnish with a solids concentration of 75% by mass. The resulting varnish was coated onto a release PET film to a thickness of 200 μm and dried at 110°C for 10 minutes. The dried resin composition was then pulverized. To mold a composite for measurement using the pulverized resin composition, the press pressure was increased to 10 kN in a heated vacuum press at 80°C for 3 minutes. When the pressure reached 10 kN, the temperature was increased to 160°C at a rate of 20°C / 3 minutes. When the temperature reached 100°C, the press pressure was increased from 10 kN to 40 kN. When the heating temperature reached 160°C, it was maintained for 16 minutes to produce a resin composite with a thickness of 1 mm. The resin composite was post-cured at 175°C for 6 hours to obtain a composite for measurement. The thermal conductivity of this composite for measurement was measured.

[0104] Thermal conductivity is calculated as the product of thermal diffusivity, low-pressure specific heat capacity, and density. Thermal diffusivity was measured using a xenon flash analyzer (NETZSCH JAPAN Co., Ltd., LFA647 HyperFlash). Constant-pressure specific heat capacity was calculated from the specific heat capacity of each material and its blending ratio. Density was measured using an electronic hydrometer (Alpha Mirage Co., Ltd., MDS-3000). Thermal conductivity was calculated from this product. Thermal diffusivity and density were measured at 23°C, and the constant-pressure specific heat capacity value was calculated using the specific heat capacity at 25°C.

[0105] Comparative Example 1: A raw alumina powder (D50: 1.8 μm) was introduced into a high-temperature zone of 2200°C formed in a furnace by a flame formed by LPG and oxygen, and subjected to spheroidization treatment. The pre-heat-treated spherical alumina powder was introduced into cooled wash water at 80°C in a water tank within 5 seconds of leaving the furnace, and cooled and washed. The obtained slurry containing the pre-heat-treated spherical alumina powder was separated into a high-concentration slurry and a supernatant liquid in a recovery step, and the high-concentration slurry was dried in a dryer in an air atmosphere at an ambient temperature of 180°C in a drying step to obtain pre-heat-treated spherical alumina powder 1.

[0106] Comparative Example 2: A raw alumina powder (D50: 1.2 μm) was introduced into a high-temperature zone of 2200°C formed in a furnace by a flame formed by LPG and oxygen, and subjected to spheroidization treatment. The pre-heat-treated spherical alumina powder was introduced into cooled wash water at 80°C in a water tank within 5 seconds of leaving the furnace, and cooled and washed. The obtained slurry containing the pre-heat-treated spherical alumina powder was separated into a high-concentration slurry and a supernatant liquid in a recovery step, and the high-concentration slurry was dried in a dryer in an air atmosphere at an ambient temperature of 180°C in a drying step to obtain pre-heat-treated spherical alumina powder 2.

[0107] (Comparative Example 3) Advanced Alumina AA-2, manufactured by Sumitomo Chemical Co., Ltd.

[0108] (Example 1) Pre-heat-treated spherical alumina powder 1 was heat-treated in a muffle furnace in an air atmosphere at a temperature of 1250°C for 4 hours. After cooling, the powder was crushed using a wet shear crusher Nanomizer, and then dispersed and classified using a high-speed rotating thin film classifier Filmix to obtain spherical alumina powder.

[0109] Example 2 Pre-heat-treated spherical alumina powder 2 was heat-treated in a muffle furnace in an air atmosphere at a temperature of 1250° C. for 4 hours. After cooling, the powder was crushed and classified using a swirling jet mill to obtain spherical alumina powder.

[0110] As shown in Table 1, the thermal conductivities of the resin compositions containing the heat-treated spherical alumina powder of Examples 1 and 2 were 2.1 W / m·K and 1.9 W / m·K, respectively. These demonstrate superior thermal conductivity performance to the resin compositions containing the unheat-treated spherical alumina powder of Comparative Examples 1 and 2. On the other hand, the non-spherical alumina of Comparative Example 3, although not heat-treated, had a high degree of gelatinization, and is presumed to have high thermal conductivity. However, the thermal conductivity of the resin composition containing the non-spherical alumina of Comparative Example 3 was lower than that of Examples 1 and 2. This is thought to be due to the non-spherical alumina having too high a particle surface roughness, resulting in poor packing. Specifically, it is presumed that the poor packing prevents the particle contact points that serve as heat conduction paths from being uniformly and sufficiently formed.

[0111] REFERENCE SIGNS LIST 1 Hopper 2 Spheroidizing burner 3 Water-cooled jacket-type spheroidizing furnace 4 Stirring cooling tank 5 Separation tank 6 Drying device 7 Cooling water pump 8 Scrubber-type collection device 9 Exhaust device 10 Shower

Claims

1. A spherical alumina powder having a D50 of 0.1 to 40 μm, a circularity of 0.90 to 1.00, a degree of alpha conversion of 60% to 100%, and a particle surface roughness of 1.14 to 1.35 as expressed by the following formula (1): Particle surface roughness = BET specific surface area A / spherical equivalent specific surface area Sa calculated from particle size distribution 2. Specific gravity is 3.80 g / cm ３ 2. The spherical alumina powder according to claim 1, wherein the spherical alumina powder is a spherical alumina powder having the above structure.

3. The spherical alumina powder according to claim 1 or 2, wherein the BET specific surface area A is 0.1 or more and 2.0 or less.

4. The spherical alumina powder according to claim 1 or 2, which has an oil absorption rate of 30% or more and 50% or less.

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

6. A prepreg in which the resin composition according to claim 5 is impregnated into a substrate.

7. A cured product of the prepreg or laminate thereof according to claim 6.

8. A metal-clad laminate comprising the cured product according to claim 7 and a metal foil disposed on at least one main surface of the cured product.

9. The resin composition according to claim 5, which is used as a sealing material for electronic component devices.

10. An electronic component device comprising an element and a cured product of the resin composition according to claim 5 that encapsulates the element.

11. A method for producing spherical alumina powder, comprising: a high-temperature step of forming a high-temperature region inside a furnace using a burner that forms a flame; a spheroidizing step of producing pre-heat-treated spherical alumina powder by charging raw alumina powder into the furnace and heating and melting it; a cooling and washing step of charging the pre-heat-treated spherical alumina powder into cooling and washing water in a water tank to cool and wash the pre-heat-treated spherical alumina powder; a recovery step of separating the pre-heat-treated spherical alumina powder from the cooling and washing water and recovering the pre-heat-treated spherical alumina powder; and a heat treatment step of heat-treating the recovered pre-heat-treated spherical alumina powder at 1100 to 1300°C in an air atmosphere to obtain spherical alumina powder.

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