Hexagonal boron nitride powder and method for producing the same

Abstract

The present invention provides h-BN powder that, when used as a filler, can further improve the thermal conductivity of a heat dissipation component. [Solution] A hexagonal boron nitride powder comprising primary particles and secondary particles formed by aggregation of the primary particles, wherein the 30% particle diameter D30 in the cumulative particle size distribution on a volume basis is 11.0 to 30.0 μm.

Description

Technical Field

[0001] The present invention relates to hexagonal boron nitride powder and a method for producing the same.

Background Art

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

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

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

[0005] However, the required thermal conductivity of heat dissipation members has been increasing year by year, and accordingly, various methods for further improving the characteristics of h-BN powder for fillers have been proposed.

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

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

Prior Art Documents

[0008] [Patent Document 1] Japanese Patent Publication No. 2020-138903 [Patent Document 2] Japanese Patent Publication No. 2011-098882 [Overview of the project] [Problems that the invention aims to solve]

[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 an h-BN powder that can further improve the thermal conductivity of a heat dissipation member when used as a filler. [Means for solving the problem]

[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, Hexagonal boron nitride powder with a 30% particle size D30 in the volume-based cumulative particle size distribution being 11.0 to 30.0 μm.

[0015] 2. The hexagonal boron nitride powder according to 1 above, wherein the 70% particle size D70 in the cumulative particle size distribution based on volume is 37.0 to 80.0 μm.

[0016] 3. The hexagonal boron nitride powder according to 1 or 2 above, wherein the proportion of particles having a particle size of 100 μm or more is 10 vol% or less.

[0017] 4. 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, the 50% particle size D50 in the cumulative particle size distribution based on volume is 15.0 to 60.0 μm, and the bulk density is 0.5 to 1.0 g / cm3. The hexagonal boron nitride powder according to any one of 1 to 3 above.

[0018] 5. A method for producing the hexagonal boron nitride powder according to any one of 1 to 4 above, producing hexagonal boron nitride from boron carbide, crushing the hexagonal boron nitride to obtain a hexagonal boron nitride powder, screening the hexagonal boron nitride powder to classify it into undersize powder and oversize powder, A method for producing a hexagonal boron nitride powder, wherein the hexagonal boron nitride powder with adjusted particle size is mixed with the undersize powder to adjust the particle size distribution.

[0019] 6. Resizing the oversize powder, screening the oversize powder after resizing to classify it into a second undersize powder and a second oversize powder, mixing the second undersize powder as the hexagonal boron nitride powder with adjusted particle size with the undersize powder to adjust the particle size distribution, The method for producing a hexagonal boron nitride powder according to 5 above.

Advantages of the Invention

[0020] 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.

Embodiments for Carrying Out the Invention

[0021] 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.

[0022] [Hexagonal boron nitride powder] The hexagonal boron nitride powder in one embodiment of the present invention is a hexagonal boron nitride powder comprising primary particles and secondary particles formed by the aggregation of the primary particles.

[0023] D30: 11.0~30.0 μm If the D30 of hexagonal boron nitride powder is less than 11.0 μm, the proportion of fine particles increases, resulting in a decrease in thermal conductivity. This is thought to be because, when the proportion of fine particles is high, heat must pass through a large number of hexagonal boron nitride particles when transferring heat from one surface to the other in a heat dissipation material such as a heat dissipation sheet. In other words, although hexagonal boron nitride has excellent thermal conductivity as described above, the thermal conductivity decreases at the interface where particles come into contact due to contact resistance. Therefore, by adjusting the proportion of fine particles so that the D30 is 11.0 μm or more, the number of times heat must pass through the interface between particles can be reduced, and as a result, thermal conductivity can be improved. For this reason, in the present invention, the D30 of hexagonal boron nitride powder is set to 11.0 μm or more, preferably 12.0 μm or more, more preferably 13.0 μm or more, and even more preferably 14.0 μm or more.

[0024] On the other hand, in order to make D30 greater than 30.0 μm, 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 significantly and the manufacturing cost increases. For this reason, the D30 of the hexagonal boron nitride powder is set to 30.0 μm or less, preferably 29.0 μm or less, more preferably 27.0 μm or less, and even more preferably 24.0 μm or less.

[0025] In this invention, D30 is defined as the 30% particle size in the cumulative particle size distribution based on volume. The cumulative particle size distribution can be measured by laser diffraction scattering. More specifically, it is measured under the conditions described in the examples.

[0026] D70: 37.0~80.0 μm As mentioned above, a high proportion of fine particles reduces thermal conductivity, but adjusting the particle size distribution by removing fine particles reduces the yield. On the other hand, adjusting the particle size distribution by increasing the amount of relatively large h-BN particles can improve thermal conductivity more effectively without wasting fine particles. For this reason, in the present invention, it is preferable that the D70 of the hexagonal boron nitride powder be 37.0 μm or larger, more preferably 39.0 μm or larger, and even more preferably 41.0 μm or larger.

[0027] On the other hand, if D70 is to be greater than 80.0 μm, it is necessary to either remove a large amount of fine powder contained in the manufactured hexagonal boron nitride powder, or to relatively reduce the amount of fine powder by increasing the proportion of particles with a particle size exceeding 80.0 μm. As a result, the yield decreases and the manufacturing cost increases. Furthermore, as will be described later, an increase in the proportion of coarse particles reduces the dielectric strength. For this reason, in the present invention, it is preferable to set the D70 of the hexagonal boron nitride powder to 80.0 μm or less, more preferably to 60.0 μm or less, and even more preferably to 45.0 μm or less.

[0028] In this invention, D70 is defined as the 70% particle size in the cumulative particle size distribution based on volume. The cumulative particle size distribution can be measured by laser diffraction scattering. More specifically, it is measured under the conditions described in the examples.

[0029] Percentage of particles with a diameter of 100 μm or more: 10 vol% or less Materials such as heat dissipation sheets are generally required to have excellent insulation properties in addition to thermal conductivity. However, as the proportion of coarse particles with a particle size of 100 μm or more increases, the dielectric strength decreases. This is because, since the thickness of a typical heat dissipation sheet is about 200 to 300 μm, an increase in the proportion of coarse particles increases the possibility of particles penetrating the sheet. Therefore, from the viewpoint of further improving the dielectric strength, it is preferable to keep the proportion of particles with a particle size of 100 μm or more at 10 vol% or less, more preferably at 8 vol% or less, and even more preferably at 6 vol% or less.

[0030] On the other hand, the lower limit of the proportion of particles with a particle size of 100 μm or more is not particularly reduced and may be 0 vol%. However, since completely removing particles with a particle size of 100 μm or more reduces productivity, it is preferable that the proportion of particles with a particle size of 100 μm or more be 0.5 vol% or more, and more preferably 1 vol% or more.

[0031] In this specification, the proportion of particles with a diameter of 100 μm or more may simply be referred to as the "proportion of coarse particles."

[0032] Average D / d: 5-10 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, the average value is preferably 5 or greater, and more preferably 6 or greater. On the other hand, if the average value is greater than 10, the density of the secondary particles decreases because the primary particles are flattened. As a result, the porosity increases when hexagonal boron nitride powder is blended into the resin, and the thermal conductivity decreases. Therefore, from the viewpoint of further improving thermal conductivity, it is preferable to set the average value of D / d to 10 or less, and more preferably 9 or less.

[0033] The major axis D of the primary particle is the length of the longer side of the hexagonal boron nitride particle. The thickness d of the primary particle is the length of the shorter side of a single hexagonal boron nitride particle. The major axis D and thickness d of the primary particle can be measured by image analysis of SEM images taken using a scanning electron microscope (SEM). Specifically, the apparent major axis D and apparent thickness d of the primary particle are used as the major axis D and apparent thickness under the microscope field of view, respectively. More specifically, they can be measured by the method described in the examples.

[0034] D50: 15.0~60.0 μm If the D50 of the hexagonal boron nitride powder is less than 15.0 μm, it is difficult to adjust the D30 to the range described above. Therefore, it is preferable that the D50 be 15.0 μm or more, and more preferably 20.0 μm or more. On the other hand, if the D50 is greater than 60.0 μm, the proportion of coarse particles increases, making it difficult to keep the proportion of particles with a particle size of 100 μm or more at 10 vol% or less. Therefore, it is preferable that the D50 be 60.0 μm or less, more preferably 55.0 μm or less, and even more preferably 35.0 μm or less.

[0035] In this invention, D50 is defined as the 50% particle diameter (so-called median diameter) in the cumulative particle size distribution based on volume. The cumulative particle size distribution can be measured by laser diffraction scattering. More specifically, it is measured under the conditions described in the examples.

[0036] 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 should be 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 used: 0.9 g / cm³3 The following is also acceptable.

[0037] 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.

[0038] The shape of the primary particles of hexagonal boron nitride is not particularly limited, but typically they may be flattened, plate-like (scaly). Secondary particles refer to particles formed by the aggregation of two or more primary particles (aggregated particles). The primary particles contained in the secondary particles may be chemically bonded to each other.

[0039] [Manufacturing method] Next, a method for producing hexagonal boron nitride powder according to one embodiment of the present invention will be described. Note that the following description is merely an example of a method for producing hexagonal boron nitride powder, and the present invention is not limited thereto.

[0040] Boron carbide First, hexagonal boron nitride is produced from boron carbide (B4C) as a raw material. The boron carbide is not particularly limited and can be produced by any method. For example, boron carbide may be produced by heating boric acid (H3BO3) with a carbon-containing material in a non-oxidizing atmosphere to produce the reaction shown in equation (1) below. 4H3BO3+7C→B4C+6H2O+6CO…(1)

[0041] 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 nitriding and decarburization treatments.

[0042] • Nitriding Nitriding can be performed by calcining boron carbide in a nitrogen atmosphere. This nitriding process converts boron carbide to boron nitride (BN), as shown in equation (2) below. (1 / 2)B4C+N2→2BN+(1 / 2)C…(2)

[0043] To ensure the reaction in equation (2) above proceeds smoothly, sufficient nitrogen partial pressure and temperature must be provided. If the nitrogen partial pressure is less than 5 kPa, the nitriding reaction will proceed slowly and require a long time. Therefore, it is preferable to set the nitrogen partial pressure to 5 kPa or higher when performing the nitriding treatment. On the other hand, from the viewpoint of high-pressure gas safety, it is preferable to set the nitrogen partial pressure to 1000 kPa or lower.

[0044] Furthermore, if the temperature during the nitriding treatment (firing temperature) is lower than 1800°C, the nitriding reaction will proceed slowly and require a long time. For this reason, 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, the reverse reaction will occur, which will ultimately hinder the progress of the reaction. For this reason, the firing temperature is preferably 2200°C or lower, and more preferably 2100°C or lower.

[0045] • Decarburization treatment The boron nitride obtained by the above nitriding treatment contains carbon (C) as a byproduct. Therefore, the boron nitride is subjected to a decarburization treatment to remove the C contained in the boron nitride. Specifically, one or both of diboron trioxide and its precursors (hereinafter referred to as diboron trioxide, etc.) are mixed with the boron nitride and heated in a non-oxidizing atmosphere. This allows the C present in the boron nitride to be removed as CO (gas). The reaction in this decarburization treatment can be represented by the following equation (3). 2BN+(1 / 2)C+(1 / 2)B2O3→2BN+(1 / 2)CO↑+(1 / 2)B2O2↑…(3)

[0046] The precursors of diboron trioxide are boron compounds that can be converted to diboron trioxide upon heating, and specifically include ammonium salts of boric acid, orthoboric acid, metaboric acid, and tetraboric acid. Among diboron trioxide and its precursors, diboron trioxide is particularly preferred.

[0047] Furthermore, the non-oxidizing atmosphere is preferably an inert gas atmosphere, and more preferably a nitrogen atmosphere. The decarburization treatment is preferably carried out while flowing an inert gas through the furnace, and more preferably while flowing a nitrogen gas through it.

[0048] The mixing of boron nitride and diboron trioxide, etc., can be carried out by any method. For example, the mixture can be wet-mixed by adding a solvent to a ball mill, but it is preferable to use a dry mixer such as a V-blender. Furthermore, it is preferable to continue the mixing until the mixture is uniform. The mixture can be considered uniformly mixed if it is a uniform gray color when viewed with the naked eye.

[0049] In the decarburization treatment described above, it is preferable to reduce the amount of carbon contained in the boron nitride to 0.5% by mass or less, and more preferably to 0.2% by mass or less.

[0050] 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).

[0051] 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.

[0052] 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.

[0053] • Removal treatment of residual boron trioxide As described above, in the decarburization treatment, carbon (C) contained in boron nitride is removed by reacting it with diboron trioxide, etc. However, diboron trioxide remains in the boron nitride. While it is possible to remove some of the diboron trioxide by evaporating it by extending the decarburization treatment time, attempting to remove all of the diboron trioxide during the decarburization treatment alone would take a very long time and be inefficient. 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 obtained can be brought within 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.

[0054] 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.

[0055] 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.

[0056] • Decomposition After decarburization, boron nitride is not in powder form, but rather in block-like structures where particles are bonded together. Therefore, the decarburized boron nitride is crushed to obtain hexagonal boron nitride powder.

[0057] 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 larger, and preferably 0.4 mm or larger.

[0058] The conditions for crushing are not particularly limited; the conditions should be adjusted so that the particle size distribution of the final hexagonal boron nitride powder satisfies the above-mentioned conditions. For example, to lower the value of D30, the conditions should be adjusted to crush the material more finely, while to increase the value of D30, the conditions should be adjusted so that the material is not crushed too finely. In particular, in this invention, it is important to set D30 to 11 μm or higher. To achieve this, the crushing conditions should be adjusted so that the amount of fine powder does not increase excessively. D50 and D70 can also be adjusted based on the same considerations. When using the crusher described above, the crushing conditions can be adjusted by changing the rotation speed and the screen opening diameter.

[0059] ·Classification Next, the crushed hexagonal boron nitride powder is classified. Specifically, the hexagonal boron nitride powder is sieved into two fractions: sub-sieved powder and sieved powder. Here, sub-sieved powder refers to the fraction that passed through the sieve used for classification, and sieved powder refers to the fraction that did not pass through the sieve.

[0060] The aforementioned classification can be performed using any device without particular limitations, but it is preferable to use an air-powered classifier. Here, an air-powered classifier is a device 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).

[0061] 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 opening of 90 to 110 μm, preferably 95 to 105 μm, as the sieve.

[0062] ·mixture Next, the particle size distribution can be adjusted by mixing the sieved powder with hexagonal boron nitride powder whose particle size has been adjusted.

[0063] 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.

[0064] In one embodiment of the present invention, the sieved powder obtained by the above classification can be re-crushed and used. The method for doing so will be described below.

[0065] ·Re-crushing When classifying boron nitride powder to adjust its particle size, it is common practice to discard either the powder that passes through the sieve or the powder that passes through the sieve if it does not meet the required specifications for the product. This results in reduced yield and increased manufacturing costs. In the present invention, the powder that does not pass through the sieve (powder that passes through the sieve) cannot be used as is, thus reducing the yield.

[0066] Therefore, in this embodiment, the powder that did not pass through the sieve (sieved powder) is further crushed to adjust the particle size, making it usable in the product. Thus, this method allows for control of the particle size distribution of the product without reducing the yield.

[0067] The re-crushing of the sieved powder is not particularly limited and can be carried out by any method. For example, re-crushing can be performed using the same pulverizing apparatus as described above. Furthermore, the conditions for re-crushing are not particularly limited and should be adjusted so that the particle size distribution of the hexagonal boron nitride powder obtained in the final product satisfies the conditions described above. However, as with the crushing process described above, if the aperture diameter of the screen used in the re-crushing process is excessively small, the yield will decrease. Therefore, when using a screen, it is preferable that the aperture diameter of the screen be 0.2 mm or larger.

[0068] ·Classification Next, the sieved flour after the re-crushing is further sieved to classify it into a second sieved flour and a second sieved flour. Preferably, the conditions for this classification are the same as those for the first classification described above.

[0069] ·mixture Subsequently, the second sieved powder is mixed with the first sieved powder to adjust the particle size distribution. In other words, in this embodiment, the "second sieved powder" obtained in the second classification is used as the "hexagonal boron nitride powder with adjusted particle size" for mixing with the sieved powder. This makes it possible to utilize the sieved powder from the first classification without wasting it, thereby reducing manufacturing costs. [Examples]

[0070] Next, the effects of the present invention will be described based on the examples.

[0071] (Manufacturing of hexagonal boron nitride powder) Hexagonal boron nitride powder was produced 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. Subsequently, the boron carbide was subjected to nitriding treatment by calcining in a nitrogen atmosphere while maintaining furnace pressure. The calcination conditions were a calcination temperature of 2000°C and a calcination time of 10 hours. The amount of calcined product obtained by the calcination treatment was 176.6 g.

[0072] 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.

[0073] 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.

[0074] 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 resulting 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.

[0075] 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. Subsequently, the obtained sieved powder was crushed again. A pulverizer that uses impact and shear forces was used for the re-crushing. Then, the sieved powder after the re-crushing was classified to separate it into a second sieved powder and a second unsieved powder. A wind classifier equipped with a screen with a mesh size of 106 μm was used for the classification.

[0076] The final hexagonal boron nitride powder was obtained by mixing the sieved powder obtained in the first classification with the second sieved powder obtained in the second classification.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] (particle size distribution) The particle size distribution of hexagonal boron nitride powder was measured using a laser diffraction particle size distribution analyzer. In this measurement, the boron nitride powder was dispersed using a dry method. Specifically, the hexagonal boron nitride powder was dispersed in the gas phase by spraying it at a predetermined dispersion pressure, and then subjected to particle size distribution measurement. The apparatus and measurement conditions used for the measurement are as follows. • Measuring device: Mastersizer 3000 (manufactured by Malvern Panalytical) • 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 standard: Volume standard • Refractive index: 1.74 • Absorption rate: 0.01 • Analysis model: General purpose • Dispersion pressure: 0.3 MPa (3.0 bar) • Sample pretreatment: None

[0081] The following values ​​were calculated from the obtained particle size distribution. • 30% particle size D30 (μm) in the cumulative particle size distribution based on volume. • 50% particle size D50 (μm) in the cumulative particle size distribution based on volume. • 70% particle size D70 (μm) in the cumulative particle size distribution based on volume. • Percentage of particles with a diameter of 100 μm or larger (vol%)

[0082] (Major axis and thickness of primary particles) A scanning electron microscope (SEM) was used to image hexagonal boron nitride powder at a magnification of 4000x, and SEM images were obtained. 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.

[0083] (Bulk density) Hexagonal boron nitride powder was dried at 105°C until a constant weight was reached. Then, a sample of exactly 3.0 g was taken from the powder and placed in a 20 ml graduated test tube. The test tube was placed in a lidded holder 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 obtained sample volume.

[0084] Next, a heat dissipation sheet was actually fabricated using the hexagonal boron nitride powder, and its thermal conductivity and dielectric strength characteristics were evaluated.

[0085] (Creating a 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 limited 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.

[0086] (Thermal conductivity) A test specimen for measuring thermal conductivity was cut from the 0.2 mm thick heat dissipation sheet that was created. 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.

[0087] (Withstand voltage 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 fabricated 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 55kV / m or higher: "Excellent" • Dielectric breakdown voltage: 50kV or higher, less than 55kV / m: "Good" • Dielectric breakdown voltage less than 50kV: "Acceptable"

[0088] [Table 1]

Claims

1. A hexagonal boron nitride powder comprising primary particles and secondary particles formed by aggregation of the primary particles, Hexagonal boron nitride powder having a 30% particle size D30 in the cumulative particle size distribution based on volume of 11.0 to 30.0 μm.

2. The hexagonal boron nitride powder according to claim 1, wherein the 70% particle size D70 in the cumulative particle size distribution based on volume is 37.0 to 80.0 μm.

3. The hexagonal boron nitride powder according to claim 1 or 2, wherein the proportion of particles with a particle size of 100 μm or more is 10 vol% or less.

4. The average value of the ratio D / d of the major axis D to the thickness d of the primary particle is 5 to 10. The 50% particle size D50 in the cumulative particle size distribution based on volume is 15.0 to 60.0 μm, and, Bulk density is 0.5 to 1.0 g / cm³ ３ The hexagonal boron nitride powder according to claim 1 or 2.

5. A method for producing hexagonal boron nitride powder according to claim 1 or 2, Hexagonal boron nitride is produced from boron carbide. The hexagonal boron nitride is crushed to obtain hexagonal boron nitride powder. The hexagonal boron nitride powder is sieved and classified into sieved powder and sieved powder. A method for producing hexagonal boron nitride powder, comprising mixing hexagonal boron nitride powder with adjusted particle size into the sieved powder to adjust the particle size distribution.

6. The sieved powder is then crushed again, The sieved flour after the aforementioned re-crushing is sieved to separate it into a second sieved flour and a second sieved flour. The particle size distribution is adjusted by mixing the second sieved powder, which is a hexagonal boron nitride powder with the adjusted particle size, with the first sieved powder. A method for producing hexagonal boron nitride powder according to claim 5.

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