Hexagonal boron nitride powder

Abstract

Provided is an h-BN powder capable of further improving the thermal conductivity of a heat dissipation member when used as a filler. This hexagonal boron nitride powder comprises primary particles and secondary particles formed by aggregation of the primary particles. In a particle size distribution of the hexagonal boron nitride powder measured in dry condition by a laser diffraction particle size distribution measurement method after the hexagonal boron nitride powder is sprayed at a pressure of 0.3 MPa, the ratio R1 of particles having a particle size of 20 μm or less is 0-50 vol%, the ratio R2 of particles having a particle size of greater than 20 μm and 100 μm or less is 40-100 vol%, and the ratio R3 of particles having a particle size of greater than 100 μm is 0-10 vol%.

Description

Hexagonal boron nitride powder

[0001] This invention relates to hexagonal boron nitride powder.

[0002] Hexagonal boron nitride powder (hereinafter also referred to as h-BN powder) is used in a variety of applications, including solid lubricants, glass release agents, insulating heat dissipation materials, and cosmetic ingredients.

[0003] h-BN powder has a layered structure similar to graphite and possesses the following excellent properties: (1) High thermal conductivity and excellent heat dissipation. (2) High electrical insulation and excellent dielectric strength. (3) The lowest dielectric constant among ceramics.

[0004] One application of h-BN powder that takes advantage of the above properties is as a filler. That is, by adding h-BN powder as a filler to resin materials such as epoxy resin or silicone rubber, sheets and tapes with excellent thermal conductivity (heat dissipation) and insulation properties can be manufactured. In this specification, resin components containing such fillers are collectively referred to as heat dissipation components or heat conduction components.

[0005] However, the thermal conductivity required for heat dissipation components is increasing year by year, and accordingly, various methods have been proposed to further improve the properties of h-BN powder used as a filler.

[0006] For example, Patent Document 1 proposes boron nitride powder in which the amount of fine powder with a particle size of 10 μm or less is adjusted to 27 vol% or less.

[0007] Furthermore, the inventors have newly developed an h-BN powder in which the ratio of the major axis D to the thickness d of the primary particles D / d and the bulk density are defined within an appropriate range, and disclosed this in Patent Document 2.

[0008] Japanese Patent Publication No. 2020-138903 Japanese Patent Publication No. 2011-098882

[0009] According to the technology proposed in Patent Document 1, the thermal conductivity can be improved by reducing the amount of fine powder.

[0010] Furthermore, according to the technology proposed in Patent Document 2, it is possible to improve the packing rate of h-BN powder in the resin, that is, to increase the amount of h-BN powder that can be added to the resin. As a result, the thermal conductivity can be improved.

[0011] However, the thermal conductivity of the heat dissipation components is still not sufficient, and further improvements are needed.

[0012] The present invention was made to solve the above problems, and aims to provide h-BN powder that can further improve the thermal conductivity of a heat dissipation member when used as a filler.

[0013] The gist of the present invention is as follows:

[0014] 1. A hexagonal boron nitride powder comprising primary particles and secondary particles formed by aggregation of the primary particles, wherein, in the particle size distribution measured dry by laser diffraction particle size distribution analysis after spraying the hexagonal boron nitride powder at a pressure of 0.3 MPa, the proportion R1 of particles with a particle size of 20 μm or less is 0 to 50 volume%, the proportion R2 of particles with a particle size greater than 20 μm and 100 μm or less is 40 to 100 volume%, and the proportion R3 of particles with a particle size greater than 100 μm is 0 to 10 volume%.

[0015] 2. The hexagonal boron nitride powder according to item 1, wherein the ratio R1 is 15 to 50 volume%, the ratio R2 is 40 to 85 volume%, and the ratio R3 is 0 to 10 volume%.

[0016] 3. The hexagonal boron nitride powder according to item 1, wherein the ratio R1 is 27 to 50 volume%, the ratio R2 is 40 to 73 volume%, and the ratio R3 is 0 to 10 volume%.

[0017] 4. Bulk density of 0.5 to 1.0 g / cm³ ３ The hexagonal boron nitride powder according to any one of the above 1 to 3, wherein the average value of the ratio D / d of the major axis D to the thickness d of the primary particles is 5 to 10.

[0018] According to the present invention, it is possible to provide h-BN powder that can further improve the thermal conductivity of a heat dissipation member when used as a filler.

[0019] The embodiments of the present invention will be described in detail below. The following description illustrates preferred embodiments of the present invention, and the invention is not limited thereto. Unless otherwise specified, "%" in this specification refers to "volume %".

[0020] [Hexagonal Boron Nitride Powder] The hexagonal boron nitride powder in one embodiment of the present invention is a hexagonal boron nitride powder consisting of primary particles and secondary particles formed by the aggregation of the primary particles.

[0021] In the present invention, it is important that the particle size distribution of hexagonal boron nitride powder, which is dry-measured by laser diffraction particle size distribution analysis after being sprayed at a pressure of 0.3 MPa, satisfies the following conditions.

[0022] ・R1: 0-50% In the particle size distribution, if the proportion R1 of particles with a particle size of 20 μm or less is higher than 50%, the thermal conductivity decreases. This is thought to be because, when the proportion of fine particles with a particle size of 20 μm or less is high, when heat is transferred from one surface to the other in a heat dissipation material such as a heat dissipation sheet, it is necessary to pass through a large number of hexagonal boron nitride particles. In other words, as mentioned above, although hexagonal boron nitride has excellent thermal conductivity, the thermal conductivity decreases at the interface where particles come into contact due to contact resistance. Therefore, by reducing the proportion of fine particles, the number of times heat passes through the interface between particles when it is transferred is reduced, and as a result, the thermal conductivity can be improved. For this reason, the proportion R1 is set to 50% or less. On the other hand, the lower the proportion of fine particles, the better. For this reason, the lower limit of the proportion R1 is set to 0%. However, in order to set the proportion R1 to 0%, it is necessary to remove a large amount of fine powder contained in the manufactured hexagonal boron nitride powder. As a result, the yield decreases and the manufacturing cost increases. Therefore, from the standpoint of manufacturing costs, the ratio R1 is preferably 15% or more, and more preferably 27% or more.

[0023] - R3: 0-10% On the other hand, if the proportion R3 of particles with a particle size greater than 100 μm in the particle size distribution is higher than 10%, the dielectric strength deteriorates. This is because, since the thickness of a typical heat dissipation sheet is about 200-300 μm, an increase in the proportion of coarse particles with a particle size greater than 100 μm increases the likelihood that the particles will penetrate the sheet. For this reason, the proportion R3 should be set to 10% or less. On the other hand, the lower the proportion of coarse particles, the better. For this reason, the lower limit of the proportion R3 should be set to 0%.

[0024] • R2: 40-100% The percentage R2 of the remainder after removing the fine and coarse particles, i.e., particles with a particle size greater than 20 μm and 100 μm or less, is 40-100%. As can be seen from the upper limit of 100%, the hexagonal boron nitride powder of the present invention may consist only of particles with a particle size greater than 20 μm and 100 μm or less. The percentage R2 is preferably 85% or less, and more preferably 73% or less.

[0025] Bulk density: 0.5–1.0 g / cm³ ３ The bulk density of the hexagonal boron nitride powder is 0.5 g / cm³. ３ If the density is less than 0.5 g / cm³, the filling rate into the resin will decrease. Therefore, the bulk density should be 0.5 g / cm³. ３ Preferably, it is 0.6 g / cm³ or more. ３ It is more preferable that the above conditions are met. On the other hand, the upper limit of bulk density in close-packed containers is 1.0 g / cm³. ３ Therefore, the bulk density is 1.0 g / cm³. ３ The following may be true: 0.9 g / cm³ ３ The following is also acceptable.

[0026] The bulk density of hexagonal boron nitride powder can be determined by dividing its volume by its weight. More specifically, it can be measured by the method described in the examples.

[0027] Average value of D / d: 5 to 10 If the average value of the ratio D / d of the major axis D to the thickness d of the primary particles is excessively small, manufacturing becomes difficult. Therefore, from the viewpoint of ease of manufacturing, the average value is preferably 5 or more, more preferably 6 or more. On the other hand, when the average value is greater than 10, since the primary particles are flat, the density of the secondary particles decreases. As a result, the porosity increases when hexagonal boron nitride powder is blended with a resin, and the thermal conductivity decreases. Therefore, from the viewpoint of further improving the thermal conductivity, it is preferable that the average value of D / d is 10 or less, more preferably 9 or less.

[0028] The major axis D of the primary particles is the length of the long side of the hexagonal boron nitride particles. The thickness d of the primary particles is the length of the short side of one hexagonal boron nitride particle. The major axis D of the primary particles and the thickness d of the primary particles can be measured by image analysis of an SEM image taken using a scanning electron microscope (SEM). That is, as the major axis D of the primary particles and the thickness d of the primary particles, the apparent major axis and the apparent thickness in the microscope field of view are used respectively. More specifically, it can be measured by the method described in the examples.

[0029] The shape of the primary particles of hexagonal boron nitride is not particularly limited, but typically may be a flat plate shape (scaly shape). The secondary particles refer to particles (aggregated particles) in which two or more primary particles are aggregated. The primary particles contained in the secondary particles may be chemically bonded to each other.

[0030] [Manufacturing method] Next, the manufacturing method of the hexagonal boron nitride powder in one embodiment of the present invention will be described. The following description shows an example of the manufacturing method of the hexagonal boron nitride powder, and the present invention is not limited thereto.

[0031] ・ Boron carbide First, hexagonal boron nitride is produced from boron carbide (B ４ C) as a raw material. The boron carbide is not particularly limited, and those produced by any method can be used. For example, boric acid (H ３ BO ３It is also possible to produce boron carbide by heating () in a carbon-containing material in a non-oxidizing atmosphere to cause the reaction of the following formula (1). 4H ３ BO ３ + 7C → B ４ C + 6H ２ O + 6CO…(1)

[0032] The method for producing hexagonal boron nitride from boron carbide is not particularly limited, but typically, hexagonal boron nitride can be obtained by subjecting boron carbide to nitridation treatment and decarburization treatment.

[0033] ・Nitridation treatment The nitridation treatment can be carried out by firing boron carbide in a nitrogen atmosphere. By this nitridation treatment, boron carbide becomes boron nitride (BN) as represented by the following formula (2). (1 / 2)B ４ C + N ２ → 2BN + (1 / 2)C…(2)

[0034] In order to smoothly proceed the reaction of the above formula (2), it is necessary to provide a sufficient nitrogen partial pressure and temperature. If the nitrogen partial pressure is less than 5 kPa, the progress of the nitridation reaction becomes slow and the reaction takes a long time. Therefore, the nitrogen partial pressure during the nitridation treatment is preferably 5 kPa or more. On the other hand, from the viewpoint of high-pressure gas safety, the nitrogen partial pressure is preferably 1000 kPa or less.

[0035] Also, if the temperature (firing temperature) during the nitridation treatment is lower than 1800 °C, the progress of the nitridation reaction becomes slow and the reaction takes a long time. Therefore, the firing temperature is preferably 1800 °C or higher, and more preferably 1900 °C or higher. On the other hand, if the firing temperature exceeds 2200 °C, a reverse reaction occurs, and as a result, the progress of the reaction is hindered. Therefore, the firing temperature is preferably 2200 °C or lower, and more preferably 2100 °C or lower.

[0036] - Decarburization Treatment The boron nitride obtained by the above nitridation treatment contains carbon (C) as a by-product. Therefore, the boron nitride is subjected to a decarburization treatment to remove the C contained in the boron nitride. Specifically, one or both of boron trioxide and its precursor (hereinafter referred to as boron trioxide, etc.) are mixed with the boron nitride and heated in a non-oxidizing atmosphere. Thereby, the C mixed in the boron nitride can be removed as CO (gas). The reaction in this decarburization treatment can be represented by the following formula (3). 2BN + (1 / 2)C + (1 / 2)B ２ O ３ →2BN + (1 / 2)CO↑ + (1 / 2)B ２ O ２ ↑…(3)

[0037] Note that the precursor of boron trioxide is a boron compound that can become boron trioxide by heating, and specifically, ammonium salts of boric acid, orthoboric acid, metaboric acid, tetraboric acid, etc. are mentioned. Among boron trioxide and its precursors, boron trioxide is particularly preferred.

[0038] Also, the non-oxidizing atmosphere is preferably an inert gas atmosphere and preferably a nitrogen atmosphere. The decarburization treatment is preferably carried out while flowing an inert gas into the furnace, and more preferably carried out while flowing nitrogen gas.

[0039] The mixing of boron nitride and boron trioxide, etc. can be carried out by any method. For example, a solvent can be added to a ball mill and mixed wet, but it is preferably carried out using a dry mixer such as a V-blender. Also, the mixing is preferably carried out until the mixture becomes uniform. If the mixture is visually homogeneous and gray, it can be considered that it is uniformly mixed.

[0040] In the decarburization treatment, the amount of C contained in the boron nitride is preferably 0.5 mass% or less, and more preferably 0.2 mass% or less.

[0041] The amounts of boron nitride and diboron trioxide are not particularly limited. However, from the viewpoint of increasing the efficiency of carbon removal and sufficiently reducing the amount of carbon contained in boron nitride, it is preferable to add more diboron trioxide than the amount necessary to remove all the carbon contained in the boron nitride (hereinafter referred to as 1 equivalent).

[0042] Furthermore, in order to sufficiently reduce the amount of carbon contained in the boron nitride, it is preferable to set the temperature during the decarburization treatment (decarburization treatment temperature) to 1500°C or higher, and more preferably to 1800°C or higher. On the other hand, it is preferable that the temperature be 2200°C or lower.

[0043] The time required for the decarburization treatment (decarburization treatment time) is not particularly limited, but a certain amount of treatment time is required to ensure that the decarburization reaction proceeds reliably. In addition, by extending the decarburization treatment time, it is possible to evaporate and remove to some extent any excess diboron trioxide that remains unconsumed by the decarburization reaction. For this reason, the decarburization treatment time is preferably 1 hour or more, more preferably 3 hours or more, and even more preferably 6 hours or more. On the other hand, the decarburization treatment time is preferably 30 hours or less.

[0044] ・Removal of residual diboron trioxide As described above, in the decarburization treatment, carbon contained in boron nitride is removed by reacting it with diboron trioxide, etc. At that time, diboron trioxide remains in the boron nitride. It is possible to remove some of the diboron trioxide by evaporation by extending the decarburization treatment time, but it takes a very long time and is inefficient to try to remove all of the diboron trioxide during the decarburization treatment alone. Therefore, it is preferable to perform an additional treatment to efficiently remove the remaining diboron trioxide (hereinafter referred to as the residual diboron trioxide removal treatment) after the decarburization treatment and before the next crushing. By performing the residual diboron trioxide removal treatment, the average value of D / d and the bulk density of the hexagonal boron nitride powder finally obtained can be set to the preferred range described above. This is thought to be because the strength of the hexagonal boron nitride powder is improved by the uniform and efficient removal of diboron trioxide.

[0045] As the residual diboron trioxide removal treatment, it is preferable to perform at least one of a reduced pressure treatment and an inert gas flow rate increase treatment. By performing a reduced pressure treatment, the remaining diboron trioxide can be efficiently removed. When performing the reduced pressure treatment, it is typically preferable to maintain the furnace pressure at less than 100 kPa.

[0046] Furthermore, when the inert gas flow rate is increased, the partial pressure of the inert gas in the furnace rises, which relatively lowers the partial pressure of diboron trioxide. Therefore, residual diboron trioxide can be efficiently removed. For example, if the decarburization treatment is performed in a nitrogen atmosphere, the nitrogen gas flow rate should be increased.

[0047] - After crushing and decarburization, the boron nitride is not in powder form, but rather in block form where the particles are bonded together. Therefore, the decarburized boron nitride is crushed to obtain hexagonal boron nitride powder.

[0048] The crushing can be carried out by any method, without any particular limitations. For example, a pulverizer can be used for the crushing. As the pulverizer, for example, a pulverizer can be used that crushes boron nitride by impact force and / or shear force by colliding it with blades or hammers that rotate at high speed. Preferably, the pulverizer is equipped with a screen of a predetermined opening diameter and is configured to recover the portion of the crushed powder that passes through the screen. However, if the opening diameter of the screen is excessively small, the yield will decrease. Therefore, when using a screen, the opening diameter of the screen is preferably 0.2 mm or more, and preferably 0.4 mm or more.

[0049] The conditions for crushing are not particularly limited, and the conditions should be adjusted so that the particle size distribution of the hexagonal boron nitride powder obtained in the end satisfies the above-mentioned conditions. For example, when the ratio R1 is increased, the conditions should be adjusted to crush the material more finely, and conversely, when the ratio R1 is decreased, the conditions should be adjusted so that the material is not crushed too finely. In particular, it is important to keep R1 at 50% or less in this invention. To achieve this, the crushing conditions should be adjusted so that the amount of fine powder does not increase too much. R2 and R3 can also be adjusted based on the same idea. For example, when the ratio R3 is decreased, the conditions should be adjusted to crush the material more finely, and conversely, when R3 is increased, the conditions should be adjusted so that the material is not crushed too finely. When using the crusher described above, the crushing conditions can be adjusted by changing the rotation speed and the screen opening diameter.

[0050] Next, the crushed hexagonal boron nitride powder is classified. Specifically, the hexagonal boron nitride powder is sieved into two fractions: sieved powder and sieved powder. Here, sieved powder refers to the fraction that passed through the sieve used in the classification, and sieved powder refers to the fraction that did not pass through the sieve.

[0051] Any apparatus can be used for the aforementioned classification, without any particular limitations, but it is preferable to use an air-powered classifier. Here, an air-powered classifier is an apparatus that separates powder dispersed in an airflow into powder that passes through the screen (sieved powder) and powder that does not pass through the screen (sieved powder).

[0052] The aperture diameter of the sieve (screen) used in the classification is not particularly limited and should be adjusted so that the particle size distribution of the hexagonal boron nitride powder obtained in the end satisfies the above-mentioned conditions. However, from the viewpoint of reducing the proportion of coarse grains (particles with a particle size of 100 μm or more) contained in the final hexagonal boron nitride powder, it is preferable to use a mesh with an aperture of 90 to 110 μm as the sieve. It is more preferable that the aperture is 95 to 105 μm.

[0053] - Mixing: The particle size distribution can be adjusted by mixing the sieved powder with hexagonal boron nitride powder whose particle size has been adjusted.

[0054] The hexagonal boron nitride powder with adjusted particle size is not particularly limited, and any hexagonal boron nitride powder can be used. The hexagonal boron nitride powder with adjusted particle size may, for example, be a hexagonal boron nitride powder that has been separately manufactured and then classified.

[0055] Next, the effects of the present invention will be described based on the examples.

[0056] (Manufacturing of Hexagonal Boron Nitride Powder) Hexagonal boron nitride powder was manufactured using the following procedure. First, commercially available boron carbide powder with a purity of 98% by mass was sieved through a sieve with a mesh size of 44 μm. 101.8 g of the boron carbide powder that passed through the sieve was placed in a carbon crucible with an inner diameter of 90 mm and a height of 100 mm. The boron carbide was then subjected to nitriding treatment by firing in a nitrogen atmosphere while maintaining furnace pressure. The firing conditions were a firing temperature of 2000°C and a firing time of 10 hours. The amount of firing product obtained by the firing treatment was 176.6 g.

[0057] Next, 69.3 g was taken from the calcined product and mixed with 35.2 g of commercially available diboron trioxide to obtain a powdered mixture. The mixing was carried out using a V-blender with an internal volume of 1 L, rotating at 1 Hz for 30 minutes.

[0058] The obtained powdered mixture was charged into a carbon crucible with an inner diameter of 90 mm and a height of 100 mm, and subjected to a decarburization treatment to obtain a second calcined product. The decarburization treatment was carried out in a nitrogen atmosphere at 2000°C for 10 hours. During the decarburization treatment, nitrogen gas was supplied to the furnace at a flow rate of 3 mL / min·g per 1 g of filling material.

[0059] After the decarburization treatment described above, a treatment to remove residual diboron trioxide was performed. Specifically, this involved either a reduced pressure treatment to maintain the furnace pressure at less than 100 kPa, or an increase in the nitrogen flow rate to 4 mL / min·g per gram of packing material. For comparison, in some examples, the treatment to remove residual diboron trioxide was omitted. The obtained second calcination product was a white aggregate, and after grinding, it was subjected to X-ray diffraction, which confirmed that it was almost entirely hexagonal boron nitride.

[0060] The second calcination product was crushed to obtain hexagonal boron nitride powder. A pulverizer that uses impact and shear forces was used for the crushing. Next, the crushed powder was classified to separate it into sieved powder and unsieved powder. A wind classifier equipped with a screen with a mesh size of 106 μm was used for the classification. After that, the unsieved powder obtained from the classification was mixed with hexagonal boron nitride powder with adjusted particle size to obtain the final hexagonal boron nitride powder.

[0061] The resulting hexagonal boron nitride powder was a powder containing a mixture of primary particles and secondary particles formed by the aggregation of the primary particles.

[0062] Furthermore, by changing various conditions such as the crushing conditions, classification conditions, and the mixing ratio of the two powders mentioned above, hexagonal boron nitride powders with different particle size distributions were obtained.

[0063] Next, the particle size distribution, primary particle shape, and bulk density of each of the obtained hexagonal boron nitride powders were measured using the following procedure. The measurement results are shown in Table 1.

[0064] (Particle Size Distribution) The particle size distribution of hexagonal boron nitride powder was measured using a dry method with a laser diffraction particle size distribution analyzer (Malvern Panalytical, Mastersizer 3000). The measurement was performed after spraying the hexagonal boron nitride powder at a dispersion pressure of 0.3 MPa. The measurement conditions were as follows: Feed rate: 40% Sample input section: High-energy type venturi Measurement time: Background measurement 5 seconds, sample measurement 3 seconds Particle size type: Non-spherical Particle size reference: Volume reference Refractive index: 1.74 Absorption rate: 0.01 Analytical model: General purpose Sample pretreatment: None

[0065] The proportions R1, R2, and R3 were calculated from the obtained particle size distribution.

[0066] (Bulk Density) Hexagonal boron nitride powder was dried at 105°C until it reached a constant weight. Then, a sample of exactly 3.0 g was taken from the powder and placed in a graduated test tube with a capacity of 20 ml. The test tube was set in a holder with a lid and dropped 400 times from a height of 45 mm at a rate of once every 2 seconds. The volume of the sample in the test tube was then read and measured. The bulk density of the hexagonal boron nitride powder was determined by dividing the weight (3.0 g) by the volume of the obtained sample.

[0067] (Major axis and thickness of primary particles) Hexagonal boron nitride powder was imaged at a magnification of 4000x using a scanning electron microscope (SEM) to obtain SEM images. For each aggregated grain in the obtained SEM image, the major axis and thickness of 10 primary particles were measured. This was done for 5 aggregated grains, and the average values ​​of the major axis D and thickness d of a total of 50 primary particles were calculated. Table 1 shows the values ​​of D and d, as well as D / d. However, the D / d values ​​shown in Table 1 have been rounded to two decimal places for convenience.

[0068] Next, a heat dissipation sheet was actually fabricated using the hexagonal boron nitride powder, and its thermal conductivity and dielectric strength characteristics were evaluated.

[0069] (Preparation of heat dissipation sheet) Hexagonal boron nitride powder and a curing agent were added to the resin that would serve as the base material for the sheet and mixed until uniformly dispersed. The epoxy resin "Epicote 807" (manufactured by Japan Epoxy Resin Co., Ltd.) was used as the resin, and the modified alicyclic amine grade "Epicure 807" (manufactured by Japan Epoxy Resin Co., Ltd.) was used as the curing agent. The amount of hexagonal boron nitride powder added was set to the maximum amount that could be added to the resin. The resin was then molded into a sheet to form a heat dissipation sheet. A heat dissipation sheet with a thickness of 0.2 mm was prepared for measuring thermal conductivity, and a heat dissipation sheet with a thickness of 0.4 mm was prepared for measuring dielectric breakdown voltage.

[0070] (Thermal Conductivity) A test specimen for measuring thermal conductivity was cut from the 0.2 mm thick heat dissipation sheet that was prepared. The size of the test specimen was 10 mm in diameter and 0.2 mm in thickness. The thermal conductivity was measured using the laser flash method with the test specimen.

[0071] (Dielectric Strength Characteristics) To evaluate the dielectric strength characteristics of the heat dissipation sheet, the dielectric breakdown voltage was measured using a method compliant with JIS C 2110. Dielectric breakdown voltage is a parameter that indicates the ability of a solid electrical insulating material to withstand voltage. The measurement was performed using a heat dissipation sheet with a thickness of 0.4 mm, at a voltage boosting rate of 1 kV / s. Based on the measured dielectric breakdown voltage, the dielectric strength characteristics were evaluated according to the following criteria: - Dielectric breakdown voltage of 55 kV / m or higher: "Excellent" - Dielectric breakdown voltage of 50 kV or higher and less than 55 kV / m: "Good" - Dielectric breakdown voltage less than 50 kV: "Poor"

[0072]

Claims

1. A hexagonal boron nitride powder comprising primary particles and secondary particles formed by aggregation of the primary particles, wherein, in the particle size distribution measured dry by laser diffraction particle size distribution analysis after spraying the hexagonal boron nitride powder at a pressure of 0.3 MPa, the proportion R1 of particles with a particle size of 20 μm or less is 0 to 50 volume%, the proportion R2 of particles with a particle size greater than 20 μm and 100 μm or less is 40 to 100 volume%, and the proportion R3 of particles with a particle size greater than 100 μm is 0 to 10 volume%.

2. The hexagonal boron nitride powder according to claim 1, wherein the ratio R1 is 15 to 50 volume%, the ratio R2 is 40 to 85 volume%, and the ratio R3 is 0 to 10 volume%.

3. The hexagonal boron nitride powder according to claim 1, wherein the ratio R1 is 27 to 50 volume%, the ratio R2 is 40 to 73 volume%, and the ratio R3 is 0 to 10 volume%.

4. Bulk density of 0.5 to 1.0 g / cm³ ３ The hexagonal boron nitride powder according to any one of claims 1 to 3, wherein the average value of the ratio D / d of the major axis D to the thickness d of the primary particles is 5 to 10.

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