Boron nitride powder

Abstract

To provide boron nitride powder useful for obtaining resin molded articles with excellent heat dissipation and insulation properties. [Solution] The present invention provides a boron nitride powder containing aggregated particles formed by the aggregation of primary particles of hexagonal boron nitride, wherein, in the cumulative distribution of particle size based on volume measured by laser diffraction and scattering, when the particle size at which the cumulative value from small particle size reaches 50% and 100% of the total is defined as D50 and D100, respectively, D50 is 20 to 80 μm and D100 is 200 μm or less, the orientation index is 9 or less, and 50 of the above aggregated particles are selected such that at least one coarse particle with a particle size of 100 μm or more is included, and a single-particle compression test is performed on each of the 50 aggregated particles under a loading rate of 0.27 mN / sec, the average crush strength Cs of the coarse particle is 8 MPa or less.

Description

Technical Field

[0001] The present disclosure relates to boron nitride powder.

Background Art

[0002] Boron nitride powder has high thermal conductivity and insulation properties, and is widely used in applications such as thermal conductive fillers and insulating fillers filled in resin moldings. In Patent Document 1, in boron nitride aggregated particles constituting boron nitride powder, by adjusting the particle diameter and arrangement of primary particles on the surface and the maximum particle diameter of the aggregated particles within a predetermined range, boron nitride aggregated particles capable of obtaining a heat dissipation sheet with high thermal conductivity are disclosed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As a factor that reduces the heat dissipation and insulation properties of a resin molding containing boron nitride powder, voids remaining inside the resin molding are considered. Although such voids are reduced to some extent by a manufacturing process such as hot pressing when forming the resin molding, the internal boron nitride particles may inhibit the reduction of voids. Therefore, it is considered that if voids are reduced and boron nitride powder can be filled at a high density inside the resin molding, the heat dissipation and insulation properties of the resin molding can be improved. The present disclosure aims to provide boron nitride powder useful for obtaining a resin molding excellent in heat dissipation and insulation properties.

Means for Solving the Problems

[0005] One aspect of the present disclosure provides the boron nitride powder of the following [1].

[0006] [1] A boron nitride powder containing aggregated particles formed by the aggregation of primary particles of hexagonal boron nitride, In the volume-based cumulative distribution of particle size by laser diffraction and scattering, when the cumulative value from small particle size reaches 50% and 100% of the total, the particle size is defined as D50 and D100, respectively, D50 is 20-80 μm and D100 is 200 μm or less. The orientation index is 9 or less. Boron nitride powder in which 50 aggregated particles are selected such that at least one coarse particle with a particle diameter of 100 μm or more is included, and a single-particle compression test is performed on each of the 50 aggregated particles under a loading rate of 0.27 mN / sec, the average value of the crushing strength Cs of the coarse particle is 8 MPa or less.

[0007] When the boron nitride powder described in [1] above is compounded with a resin to form a resin molded article, the coarse particles are easily deformed or disintegrated by heating and pressing. As a result, the gaps between the aggregated particles are filled with deformed coarse particles or fine particles generated by disintegration. Consequently, voids originating from the gaps between aggregated particles can be reduced in the resin molded article. This allows the boron nitride particles contained in the resin molded article to be densely packed, improving heat dissipation and insulation properties.

[0008] The boron nitride powder in [1] above may also be the following [2] to [4].

[0009] [2] The boron nitride powder according to [1], wherein the 50 aggregated particles include fine particles with a particle diameter of 10 to 40 μm, and the average crush strength Cs of the fine particles is 7 MPa or more.

[0010] The boron nitride powder described in [2] above has a sufficiently high crush strength of fine particles, as the average crush strength Cs of 10-40 μm fine particles is 7 MPa or higher. When such boron nitride powder is compounded with resin to form a resin molded body, the fine particles remain without collapsing due to heating and pressing, and the fine particles fill the gaps between aggregated particles. As a result, voids originating from the gaps between aggregated particles can be further reduced in the resin molded body. This allows the boron nitride particles contained in the resin molded body to be packed more densely, further improving heat dissipation and insulation properties.

[0011] [3] The boron nitride powder according to [1] or [2], wherein the average value of the crush strength Cs of the 50 aggregated particles is 5 MPa or more. [4] Boron nitride powder according to any one of [1] to [3], wherein the graphitization index is 2.3 or less.

[0012] The boron nitride powder described in [3] above has a sufficiently high average value of the crushing strength Cs of the aggregated particles, which can further improve heat dissipation and insulation properties. The boron nitride powder described in [4] above has a sufficiently low graphitization index, and can therefore be suitably used as a filler for filling resin molded bodies. [Effects of the Invention]

[0013] According to this disclosure, boron nitride powder useful for obtaining resin molded articles with excellent heat dissipation and insulation properties can be provided. [Brief explanation of the drawing]

[0014] [Figure 1] This figure shows the distribution of particle size (μm) and crush strength Cs (MPa) of 50 aggregated particles in the boron nitride powder of Example 1. [Modes for carrying out the invention]

[0015] Hereinafter, embodiments of the present disclosure will be described. However, the following embodiments are examples for explaining the present disclosure and are not intended to limit the present disclosure to the following content. In the following description, when described as "X to Y" (X and Y are any numerical values satisfying X < Y), it means "X or more and Y or less" unless otherwise specified. The upper limit value or lower limit value of the numerical range explicitly stated in the present disclosure may be replaced with any value shown in the examples. Further, the individually described upper limit value and lower limit value may be arbitrarily combined. The materials or components exemplified in the present disclosure can be used alone or in combination of two or more unless otherwise specified.

[0016] The boron nitride powder according to one embodiment is a boron nitride powder containing agglomerated particles in which primary particles of hexagonal boron nitride are agglomerated. In the cumulative distribution of the volume-based particle diameter by the laser diffraction / scattering method, when the integrated value from the small particle diameter reaches 50% and 100% of the whole, the particle diameters are defined as D50 and D100 respectively, D50 is 20 to 80 μm, and D100 is 200 μm or less, the orientation index is 9 or less, and when 50 agglomerated particles are selected so that at least one coarse particle having a particle diameter of 100 μm or more is included, and a single-particle compression test is performed on each of the 50 agglomerated particles under the condition of a loading rate of 0.27 mN / sec, the average value of the crushing strength Cs of the coarse particles is 8 MPa or less.

[0017] In the present disclosure, "coarse particle" means a particle having a particle diameter of 100 μm or more in the agglomerated particles of the boron nitride powder. In the present disclosure, "fine particle" means a particle having a particle diameter of 10 to 40 μm in the agglomerated particles of the boron nitride powder.

[0018] The boron nitride powder contains agglomerated particles formed by agglomerating primary particles of hexagonal boron nitride. The shape of the primary particles of hexagonal boron nitride may be, for example, flaky, disk-shaped, or the like.

[0019] An agglomerated particle refers to a particle in which five or more primary particles (single particles) are agglomerated. The thickness directions of the respective primary particles constituting the agglomerated particle may be different from each other. The state in which the primary particles are agglomerated can be confirmed, for example, by SEM (scanning electron microscope).

[0020] The boron nitride powder contains agglomerated particles (coarse particles) having a particle diameter of 100 μm or more. Select 50 agglomerated particles from the boron nitride powder so that at least one such coarse particle is included. For the selection of 50 agglomerated particles, arbitrarily select 50, and if even one coarse particle is included, perform the following measurement using the 50 agglomerated particles. On the other hand, if no coarse particle is included, arbitrarily select another 50 agglomerated particles. The selection may be repeatedly performed until 50 agglomerated particles containing at least one coarse particle are obtained. If at least one coarse particle is included, next, a single-particle compression test is performed on each of the 50 agglomerated particles under the condition of a loading rate of 0.27 mN / sec.

[0021] By the single-particle compression test, the crushing strength Cs can be measured in accordance with the description of JIS R 1639-5:2007 "Fine Ceramics - Measurement Methods for Particle Characteristics - Part 5: Single Particle Crushing Strength". In this measurement method, the crushing strength Cs (unit: MPa) of one agglomerated particle varies depending on the position within the agglomerated particle, from the dimensionless number α (α = 2.48), the crushing test force P (unit: N), and the particle diameter d (unit: μm), using the formula Cs = α × P / (π × d 2 ). For the measurement, a micro compression tester can be used. As the micro compression tester, for example, "MCT-210" (trade name) manufactured by Shimadzu Corporation can be used. The particle diameter d of the agglomerated particles containing coarse particles and fine particles can also be measured using "MCT-210" (trade name) manufactured by Shimadzu Corporation.

[0022] The average crush strength Cs of coarse particles measured in this type of single-particle compression test is 8 MPa or less. The average crush strength Cs of coarse particles can be calculated by determining the arithmetic mean of the crush strength Cs of coarse particles with a particle diameter of 100 μm or more out of 50 aggregated particles. Because such boron nitride powder contains a sufficient amount of coarse particles with low crush strength Cs, the coarse particles are easily deformed or disintegrated by heating and pressing when compounded with resin to form a resin molded product. As a result, the gaps between aggregated particles are filled with deformed coarse particles or fine particles generated by disintegration, and consequently, voids originating from the gaps between aggregated particles are reduced in the resin molded product. This allows the boron nitride particles contained in the resin molded product to be densely packed, improving heat dissipation and insulation properties.

[0023] The average crush strength Cs of the coarse particles may be 6 MPa or less, or 5 MPa or less, from the viewpoint of further improving the heat dissipation and insulation properties when used in a resin molded product. The average crush strength Cs of the coarse particles may be 1 MPa or more. The average range of the crush strength Cs of the coarse particles may be, for example, 1 to 8 MPa, 1 to 6 MPa, or 1 to 5 MPa. The number of coarse particles among the 50 aggregated particles may be, for example, 1 to 20, 2 to 17, or 3 to 15.

[0024] In the case of 50 aggregated particles, the proportion of coarse particles with a crushing strength Cs of 10 MPa or more may be 20% or less, or 10% or less. Such boron nitride powder contains a sufficient amount of coarse particles that are easily deformed or disintegrated, thus further improving the heat dissipation and insulation properties when molded into a resin body. In the case of 50 aggregated particles, the proportion of particles with a crushing strength Cs of 10 MPa or more may be 0%. Such boron nitride powder contains only coarse particles with a crushing strength Cs of less than 10 MPa, so when molded into a resin body, the coarse particles deform sufficiently, further improving the heat dissipation and insulation properties.

[0025] In aggregated boron nitride powder particles, the average crush strength Cs of fine particles with a particle diameter of 10 to 40 μm may be 7 MPa or higher. Such boron nitride powder can be obtained by selecting the above 50 aggregated particles so that, in addition to coarse particles, at least one fine particle with a particle diameter of 10 to 40 μm is included. The average value of the crush strength Cs of the fine particles can be calculated by determining the arithmetic mean of the crush strength Cs of the fine particles with a particle diameter of 10 to 40 μm from the above 50 arbitrarily selected aggregated particles. Because such boron nitride powder has a sufficiently high crush strength Cs of fine particles, when compounded with resin to form a resin molded product, the fine particles remain without collapsing due to heating and pressing, and the fine particles fill the gaps between the aggregated particles. In other words, when forming a resin molded body containing boron nitride powder by heating and pressing, coarse particles with low crush strength Cs deform or collapse, while fine particles with high crush strength Cs maintain their shape. This allows both coarse and fine particles to efficiently fill the gaps between aggregated particles. Since such a resin molded body containing boron nitride powder has a further reduction in voids, its heat dissipation and insulation properties can be further improved.

[0026] From the viewpoint of further improving the heat dissipation and insulation properties of the resin molded article, the average value of the crush strength Cs of the fine particles in the agglomerated boron nitride powder may be 8 MPa or higher, or 9 MPa or higher. The average value of the crush strength Cs of the fine particles may be 20 MPa or lower, or 18 MPa or lower. The range of the average value of the crush strength Cs of the fine particles may be, for example, 8 to 20 MPa, 9 to 20 MPa, or 9 to 18 MPa. The average value of the crush strength Cs of the fine particles may be the average value of the crush strength Cs of the fine particles contained in 50 agglomerated particles selected so that at least one coarse particle is included. The number of fine particles contained in the 50 agglomerated particles may be, for example, 5 to 40, 10 to 35, or 13 to 33.

[0027] From the viewpoint of further improving the heat dissipation and insulation properties of the resin molded article, the average value of the crush strength Cs of the 50 aggregated particles of boron nitride powder may be 5 MPa or higher, or 6 MPa or higher. From the viewpoint of reducing boron nitride powder particles with excessively high crush strength Cs, the average value of the crush strength Cs may be 15 MPa or lower. From the viewpoint of setting the crush strength Cs of the boron nitride powder within an appropriate range and further improving the heat dissipation and insulation properties of the resin molded article, the average value of the crush strength Cs of the aggregated particles of boron nitride powder may be 5 to 15 MPa, or 6 to 15 MPa. Note that the average value of the crush strength Cs of the aggregated particles may be the average value of the crush strength Cs of 50 aggregated particles selected so that at least one coarse particle is included.

[0028] The D50 of the boron nitride powder is 20 to 80 μm from the viewpoint of improving the filling properties inside the resin molded body and improving the heat dissipation and insulation properties of the resin molded body. From the viewpoint of further improving heat dissipation and insulation properties, the D50 of the boron nitride powder may be, for example, 25 to 70 μm or 25 to 60 μm. The D100 of the boron nitride powder is 200 μm or less from the viewpoint of improving the productivity of the resin molded body while improving the heat dissipation and insulation properties of the resin molded body. From the viewpoint of further improving heat dissipation and insulation properties, the D100 of the boron nitride powder may be, for example, 120 to 200 μm or 140 to 200 μm.

[0029] D50 and D100 are the particle diameters at which the cumulative value from the smallest particle size reaches 50% and 100% of the total in the volume-based cumulative particle diameter distribution by laser diffraction and scattering method. D50 may also be called the average particle diameter, and D100 may be called the maximum particle diameter. D50 and D100 can be measured in accordance with the method described in JIS Z 8825:2013 "Particle diameter analysis - Laser diffraction and scattering method". A laser diffraction and scattering particle size distribution analyzer is used for the measurement. For example, the Microtrac "MT-3300EXII" (product name) manufactured by Microtrac-Bell can be used as a laser diffraction and scattering particle size distribution analyzer. When measuring the average particle diameter of boron nitride powder, the measurement should be performed without treating the powder to be measured with a homogenizer or the like.

[0030] The orientation index of boron nitride powder is 9 or less, and may be 8 or less, from the viewpoint of facilitating isotropic heat conduction when molded into a resin body and improving the thermal conductivity of the resin body. The orientation index of boron nitride powder may be 6 or more. The orientation index of boron nitride powder may be adjusted within the above range, for example, 6 to 9 or 6 to 8.

[0031] The orientation index can be measured according to the following method: X-ray diffraction measurement of boron nitride powder is performed to obtain the X-ray diffraction spectrum of the boron nitride powder. From this X-ray diffraction spectrum, peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained. Using the obtained peak intensities, I(002) / I(100) is calculated. This value is the orientation index of the boron nitride powder. As an X-ray diffractometer, for example, Rigaku Corporation's "ULTIMA-IV" (product name) can be used.

[0032] Since the orientation index is measured for boron nitride powder, if aggregated particles are present in the powder and their proportion is large, the orientation index value tends to decrease and approach a value of around 6 to 7. On the other hand, if the boron nitride powder consists only of primary particles without aggregated particles, or if the proportion of the above primary particles is large, the orientation index value tends to increase.

[0033] The graphitization index (GI) of boron nitride powder may be 2.3 or less, or 2.0 or less, from the viewpoint of achieving a suitable shape as a filler for resin molded articles. The graphitization index of boron nitride powder may be 1.3 or more, or 1.6 or more, from the viewpoint of further improving the heat dissipation and insulation properties of resin molded articles. The graphitization index of boron nitride powder may be adjusted within the above range, for example, 1.3 to 2.3, or 1.6 to 2.0.

[0034] The graphitization index can be calculated from the measurement results by powder X-ray diffraction. Specifically, in the X-ray diffraction spectrum of boron nitride powder, the area value (in arbitrary units) enclosed by the integrated intensity of each diffraction peak corresponding to the (100), (101), and (102) planes of the hexagonal boron nitride primary particles (i.e., each diffraction peak) and its baseline is calculated and designated as S100, S101, and S102, respectively. Using the area values ​​thus calculated, the graphitization index (GI) is calculated based on the following equation (1). As an X-ray diffractometer, for example, "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used. GI = (S100 + S101) / S102 ... (1)

[0035] A resin molded article (evaluation sheet) can be obtained using a resin composition prepared by mixing 100 parts by mass of naphthalene-type epoxy resin and 10 parts by mass of an imidazole compound as a curing agent, adding boron nitride powder to a mixture that constitutes 50% by volume, and then adding a solvent to achieve a coating viscosity. After kneading the above resin composition, a temperature of 150°C and a pressure of 100 kgf / cm² can be applied. 2 The average dielectric breakdown voltage of 100 resin molded bodies obtained by heating and pressurizing under these conditions may be 60kV / mm or higher, or 80kV / mm or higher. Resin molded bodies with an average dielectric breakdown voltage within the above range have sufficient insulation properties. The average dielectric breakdown voltage may be 100kV / mm or lower. The average dielectric breakdown voltage range may be 60 to 100kV / mm, or 80 to 100kV / mm.

[0036] The standard deviation of the dielectric breakdown voltage of the above 100 resin molded bodies may be less than 15% or less than 10% of the average dielectric breakdown voltage, from the viewpoint of reducing the variation in dielectric breakdown voltage. The standard deviation of the dielectric breakdown voltage may be 1% or more of the above average value. The range of the standard deviation of the dielectric breakdown voltage may be 1% or more and less than 15%, or 1% or more and less than 10% of the average dielectric breakdown voltage.

[0037] After kneading the above resin composition, apply a temperature of 150°C and a pressure of 100 kgf / cm². 2The average thermal conductivity (at 20°C) of 100 resin molded bodies with a thickness of 0.2 mm obtained by heating and pressurizing under the specified conditions may be 10.5 W / (m·K) or higher, or 12.0 W / (m·K) or higher. Resin molded bodies with an average thermal conductivity within the above range have sufficient heat dissipation. The average thermal conductivity of the resin molded body may be 20.0 W / (m·K) or lower. The average range of the thermal conductivity of the resin molded body may be 10.5 to 20.0 W / (m·K), or 12.0 to 20.0 W / (m·K).

[0038] An example of a method for producing boron nitride powder according to this disclosure is described below. The method for producing boron nitride powder may include, for example, a pressurized nitriding step of firing boron carbide (B4C) powder A under a pressurized atmosphere to obtain boron carbonitride (B4CN4) powder, a carbon reduction step of firing boron carbonitride powder under an air atmosphere to obtain a heat-treated product containing boron carbonitride powder, and a crystallization step of firing boron carbide powder B having a smaller average particle size than boron carbide powder A and the heat-treated product together with a boron source to obtain hexagonal boron nitride powder.

[0039] Boron carbide powder A and boron carbide powder B may be obtained by calcining a mixture of a boron source and carbon black under an argon gas atmosphere. The boron source may contain at least one selected from the group consisting of boric acid and boron oxide.

[0040] The boron source content may be 60% by mass or more, 85% by mass or less, or 60-85% by mass, based on the total mass of the boron source and carbon black. The boric acid content, based on the total mass of boric acid and carbon black, may also be within the above-mentioned ranges.

[0041] The firing temperature when obtaining boron carbide powder A and boron carbide powder B may be 1800°C or higher, 2500°C or lower, or 1800 to 2500°C. The firing time when obtaining boron carbide powder may be 2 hours or higher, 8 hours or lower, or 2 to 8 hours. In this disclosure, firing time and heating time refer to the time (holding time) from when the temperature of the ambient environment surrounding the object to be heated reaches a predetermined temperature.

[0042] The average particle size (D50) of boron carbide powder A may be 13 μm or more, 15 μm or more, or 20 μm or more. The average particle size of boron carbide powder A may be 40 μm or less, or 35 μm or less. The range of the average particle size of boron carbide powder A may be 13 to 40 μm, or 15 to 35 μm. The average particle size of boron carbide powder A is measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction and scattering method".

[0043] The average particle size (D50) of boron carbide powder B is smaller than the average particle size of boron carbide powder A. The average particle size of boron carbide powder B may be 5 μm or more, 7 μm or more, or 10 μm or more. The average particle size of boron carbide powder B may be 20 μm or less, or 15 μm or less. The range of the average particle size of boron carbide powder B may be 5 to 20 μm, or 7 to 15 μm. The average particle size of boron carbide powder B is measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction and scattering method". Boron carbide powder A and boron carbide powder B can be obtained by adjusting the average particle size by crushing and sieving the synthesized lump of boron carbide.

[0044] The pressurized nitriding process is a process for obtaining boron carbonitride powder by nitriding boron carbide powder A. The pressure in the pressurized nitriding process may be 0.4 MPa or higher, or 0.6 MPa or higher, from the viewpoint of allowing the nitriding of boron carbide to proceed more sufficiently. The pressure in the pressurized nitriding process may be 1.2 MPa or lower, or 1.0 MPa or lower, from the viewpoint of suppressing an increase in the manufacturing cost of boron nitride powder. The pressure in the pressurized nitriding process may be adjusted within the above range, for example, 0.4 to 1.2 MPa or 0.6 to 1.0 MPa. The pressurized nitriding process may be carried out under a pressurized nitrogen gas atmosphere. Unless otherwise specified, the pressures in this specification are absolute pressures.

[0045] The firing temperature in the pressurized nitriding process may be 1500°C or higher, or 1800°C or higher, from the viewpoint of allowing the nitriding of boron carbide to proceed more sufficiently. The firing temperature in the pressurized nitriding process may be 2500°C or lower, or 2200°C or lower, from the viewpoint of suppressing an increase in the manufacturing cost of boron nitride powder. The firing temperature in the pressurized nitriding process may be adjusted within the above range, for example, 1500 to 2500°C, or 1800 to 2200°C.

[0046] The firing time in the pressurized nitriding process may be 20 hours or more, or 30 hours or more, from the viewpoint of allowing the nitriding of boron carbide to proceed more sufficiently and promoting grain growth. The firing time in the pressurized nitriding process may be 60 hours or less, or 50 hours or less, from the viewpoint of suppressing an increase in the manufacturing cost of boron nitride powder. The firing time in the pressurized nitriding process may be adjusted within the above range, for example, 20 to 60 hours, or 30 to 50 hours.

[0047] The carbon reduction process involves heating the boron carbonitride powder in an atmospheric environment, thereby reducing the carbon content in the boron carbonitride powder while allowing grain growth to proceed sufficiently.

[0048] The heating temperature in the carbon reduction process may be 400°C or higher, or 600°C or higher, from the viewpoint of sufficiently reducing carbon. The heating temperature in the carbon reduction process may be 1000°C or lower, or 900°C or lower, from the viewpoint of suppressing the oxidation of boron carbonitride after heating. The heating temperature in the carbon reduction process may be adjusted within the above range, for example, 400 to 1000°C, or 600 to 900°C.

[0049] The heating time in the carbon reduction process may be 3 hours or more, or 4 hours or more, from the viewpoint of reducing the carbon content in the boron carbonitride powder and promoting grain growth. The heating time in the carbon reduction process may be 10 hours or less, or 8 hours or less, from the viewpoint of more sufficiently suppressing a decrease in the processing efficiency of the carbon reduction process. The heating time in the carbon reduction process may be adjusted within the above range, for example, 3 to 10 hours, or 4 to 8 hours.

[0050] In the crystallization process, the heat-treated material containing the boron carbonitride powder obtained in the carbon reduction process and the boron carbide powder B, which is substantially free of boron carbonitride, may be calcined together with a boron source. This decarburizes the boron carbonitride particles obtained from the coarse boron carbide powder A, and crystallizes the individual boron carbonitride particles in the lump-like boron carbonitride particles into primary particles of hexagonal boron nitride, thereby obtaining coarse aggregated particles (coarse particles) in which the primary particles have aggregated. Since grain growth is sufficiently promoted in such coarse particles, the number of bonding points of the primary particles is reduced, and the crush strength Cs is reduced. In this way, coarse particles with low crush strength Cs can be obtained.

[0051] Boron carbide powder B is subjected directly to the crystallization process without undergoing the pressurized nitriding process or the carbon reduction process. In other words, in the crystallization process, boron carbide powder B becomes boron nitride particles without going through the boron carbonitride process. By directly crystallizing the fine boron carbide powder B into boron nitride particles, grain growth is suppressed, resulting in the production of fine aggregated particles (microparticles) formed by the aggregation of fine primary particles. Because these microparticles consist of aggregated, delicate primary particles with suppressed grain growth, there are more bonding points for the primary particles, increasing the crush strength Cs. In this way, microparticles with high crush strength Cs can be obtained.

[0052] The crystallization process can be carried out by calcining a mixture A, which is obtained by mixing a heat-treated product containing boron carbonitride powder with a boron source, and a mixture C, which is obtained by mixing a mixture B, which is obtained by mixing boron carbide powder with a boron source. The boron source in the crystallization process may include at least one selected from the group consisting of boric acid and boron oxide.

[0053] The content of the boron source in mixture A may be 20 to 60 parts by mass, or 30 to 50 parts by mass, based on the total amount of the heat-treated material and the boron source. The content of the boron source in mixture B may be 60 to 90 parts by mass, or 65 to 85 parts by mass, based on the total amount of boron carbide powder B and the boron source. The content of mixture A in mixture C may be 20 to 80 parts by mass, or 25 to 75 parts by mass, based on the total amount of mixture A and mixture B.

[0054] The calcination process in the crystallization step may be carried out under normal pressure (atmospheric pressure), or it may be carried out under pressurized pressure exceeding atmospheric pressure (for example, 10 kPa or more in gauge pressure). The pressurized pressure (gauge pressure) may be, for example, 0.5 MPa or less, or 0.3 MPa or less.

[0055] The heating temperature in the crystallization process may be, for example, 1800 to 2300°C or 1900 to 2200°C. By setting the heating temperature within the above range, crystallization can be carried out more sufficiently. The heating time in the crystallization process may be, for example, 5 to 20 hours or 7 to 17 hours.

[0056] Other steps may be performed after the crystallization step. Examples of other steps include an oxidation treatment step to further reduce impurities in the boron nitride powder, a drying step to dry the boron nitride powder, and a classification step to adjust the particle size of the boron nitride powder.

[0057] The oxidation treatment process involves heating the boron nitride powder obtained in the crystallization process under an oxygen-containing atmosphere to convert the carbon content in the boron nitride powder into carbon dioxide and remove it from the system, thereby reducing the amount of residual carbon in the boron nitride powder. Performing an oxidation treatment process after the crystallization process can further reduce the carbon content in the boron nitride powder.

[0058] The heating temperature in the oxidation treatment process may be 400°C or higher, 500°C or higher, or 600°C or higher, from the viewpoint of further reducing the carbon content. From the viewpoint of further reducing the carbon content, it may be 1000°C or lower, 900°C or lower, or 800°C or lower. The heating temperature in the oxidation treatment process may be adjusted within the above range, for example, 400-1000°C, 500-900°C, 600-900°C, or 600-800°C. The heating time in the oxidation treatment process may be, for example, 2-8 hours or 3-7 hours. The pressure in the oxidation treatment process can be adjusted to, for example, atmospheric pressure or reduced pressure.

[0059] The proportion of oxygen in the atmosphere during the oxidation treatment process may be 15% by volume or more, 18% by volume or more, or 20% by volume or more. By setting the proportion of oxygen within the above range, the carbon content can be sufficiently reduced. The proportion of oxygen in the atmosphere during the oxidation treatment process may be 80% by volume or less, 70% by volume or less, or 60% by volume or less. Note that the above proportion of oxygen refers to the value determined by volume under standard conditions. The proportion of oxygen in the atmosphere during the oxidation treatment process may be adjusted within the above range, for example, 15 to 80% by volume.

[0060] In the drying process, the obtained boron nitride powder may be washed with pure water, and the resulting solid may be dried using suction filtration. The drying temperature in the drying process may be 100-180°C or 120-160°C. The drying time in the drying process may be 7-20 hours or 10-15 hours.

[0061] The classification process allows for adjustment of the D50 and D100 values ​​of the boron nitride powder. By performing the classification process, the D50 and D100 values ​​of the boron nitride powder can be reduced. This makes it possible to obtain boron nitride powder that further improves the heat dissipation and insulation properties when molded into resin, while suppressing the occurrence of appearance defects in the resin molded product and improving the productivity of the resin molded product. The classification process can be carried out, for example, by using a sieve with a mesh size of 50 to 90 μm and recovering the boron nitride powder that has passed through the sieve.

[0062] Through the above process, boron nitride powder can be obtained in which D50 is 20-80 μm, D100 is 200 μm or less, orientation index is 9 or less, and the average crush strength Cs of coarse particles with a particle size of 100 μm or more is 8 MPa or less. Such boron nitride powder can improve the heat dissipation and insulation properties when used in resin molded articles.

[0063] Although several embodiments have been described above, this disclosure is not limited in any way to the embodiments described above. [Examples]

[0064] The contents of this disclosure will be described in more detail below with reference to examples and comparative examples. However, this disclosure is not limited to the following examples.

[0065] (Example 1) [Preparation of boron carbide powder] 100 parts by mass of orthoboric acid (manufactured by Nippon Denko Co., Ltd., hereinafter simply referred to as "boric acid") and 35 parts by mass of acetylene black (product name: HS100, manufactured by Denka Co., Ltd.) were mixed using a Henschel mixer and then packed into a graphite crucible. This graphite crucible was placed in an arc furnace and heated at a firing temperature of 2200°C for 5 hours under an argon gas atmosphere to synthesize lump boron carbide (B4C). The synthesized lump boron carbide was pulverized in a ball mill for 1 hour to obtain boron carbide powder. Coarse particles were removed from this boron carbide powder using a sieve with a mesh size of 106 μm. Then, using a sieve with a mesh size of 26.5 μm, the powder was separated into boron carbide powder A (B4C powder) with an average particle size of 22 μm and boron carbide powder B (B4C powder) with an average particle size of 12 μm.

[0066] [Preparation of boron carbonitride powder] The prepared boron carbide powder A was packed into a boron nitride crucible. The crucible was placed in a resistance heating furnace and heated for 40 hours under conditions of a nitrogen gas atmosphere at a pressure of 0.80 MPa and a firing temperature of 2030°C to obtain a calcined product containing boron carbonitride (B4CN4) powder (pressure nitriding process).

[0067] The calcined material obtained as described above was heated at 800°C for 5 hours in an air atmosphere to obtain a heat-treated material (carbon reduction process).

[0068] [Preparation of boron nitride powder] Based on the total amount of the heat-treated material and boric acid (boron source), boric acid was added to the heat-treated material so that the boric acid content was 40 parts by mass, and the mixture was mixed using a Henschel mixer to obtain mixture A. Next, based on the total amount of boron carbide powder B and boric acid (boron source), boric acid was added to boron carbide powder B so that the boric acid content was 70 parts by mass, and the mixture was mixed using a Henschel mixer to obtain mixture B. Based on the total amount of mixtures A and B, mixtures A and B were combined so that the content of mixture A was 70 parts by mass, and the mixture was mixed using a Henschel mixer to obtain mixture C. Subsequently, 1.55 kg of mixture C was filled into a boron nitride container while compressing it appropriately.

[0069] Next, the aforementioned container was placed in a resistance heating furnace and heated from room temperature to 1200°C at a heating rate of 10°C / min under a nitrogen gas atmosphere with a gauge pressure of 10 kPa. Then, the temperature was further increased to 2000°C at a heating rate of 2°C / min and held at 2000°C for 15 hours to perform the heat treatment (crystallization process). This decarburized the heat-treated product obtained from boron carbide powder A and crystallized the individual boron carbonitride particles in the lump boron carbonitride particles into hexagonal boron nitride primary particles, thereby synthesizing boron nitride powder containing aggregated particles formed by the aggregation of multiple primary particles. Furthermore, in the case of boron carbide powder B, unlike the heat-treated product obtained from boron carbide powder A, the BN conversion and decarburization were carried out directly and simultaneously without going through boron carbonitride, and the growth of hexagonal boron nitride primary particles was suppressed, thereby synthesizing high-strength boron nitride powder. The synthesized boron nitride powder was crushed in a mortar for 10 minutes, and then classified using a nylon sieve with a mesh size of 75 μm. The residue below the sieve was the boron nitride powder of Example 1.

[0070] (Example 2) Bulk boron carbide was synthesized using the same procedure as in Example 1's [Preparation of Boron Carbide Powder], and the synthesized bulk boron carbide was ground in a ball mill for 1 hour to obtain boron carbide powder. Coarse particles were removed from this boron carbide powder using a sieve with a mesh size of 106 μm. Then, using a sieve with a mesh size of 19 μm, the powder was separated into boron carbide powder A (B4C powder) with an average particle size of 16 μm and boron carbide powder B (B4C powder) with an average particle size of 12 μm. Using the boron carbide powders A and B prepared in this way, the boron nitride powder of Example 2 was prepared using the same procedure as in Example 1's [Preparation of Boron Carbonitride Powder] and [Preparation of Boron Nitride Powder].

[0071] (Example 3) Mixture A and Mixture B were prepared using the same procedure as in Example 1. Based on the total amount of Mixture A and Mixture B, Mixture A and Mixture B were blended so that the content of Mixture A was 30 parts by mass, and the mixtures were mixed using a Henschel mixer to obtain Mixture C. Using the obtained Mixture C, a crystallization process was carried out using the same procedure as in Example 1 to synthesize boron nitride powder. The synthesized boron nitride powder was crushed in a mortar for 10 minutes, and then classified using a nylon sieve with a mesh size of 63 μm. The material below the sieve was the boron nitride powder of Example 3.

[0072] (Example 4) Bulk boron carbide was synthesized using the same procedure as in Example 1's [Preparation of Boron Carbide Powder], and the synthesized bulk boron carbide was ground in a ball mill for 1 hour to obtain boron carbide powder. The powder was sieved using a sieve with a mesh size of 106 μm to remove coarse particles. Then, using a sieve with a mesh size of 45 μm and a sieve with a mesh size of 19 μm, the powder was separated into boron carbide powder A (B4C powder) with an average particle size of 30 μm and boron carbide powder B (B4C powder) with an average particle size of 12 μm. Using the boron carbide powders A and B prepared in this way, the boron nitride powder of Example 4 was prepared using the same procedure as in Example 1's [Preparation of Boron Carbonitride Powder] and [Preparation of Boron Nitride Powder].

[0073] (Example 5) In preparing mixture B, the boron nitride powder of Example 5 was prepared using the same procedure as in Example 1, except that the boric acid content was set to 80 parts by mass, based on the total amount of boron carbide powder B and boric acid (boron source).

[0074] (Comparative Example 1) Using the same procedure as in Example 1 [Preparation of Boron Carbide Powder], boron carbide powder A (B4C powder) with an average particle size of 22 μm and boron carbide powder B (B4C powder) with an average particle size of 12 μm were prepared.

[0075] Based on the total amount of boron carbide powder A and boric acid (boron source), boric acid was added to boron carbide powder A so that the boric acid content was 70 parts by mass, and the mixture was mixed using a Henschel mixer to obtain mixture D. Next, based on the total amount of boron carbide powder B and boric acid (boron source), boric acid was added to boron carbide powder B so that the boric acid content was 70 parts by mass, and the mixture was mixed using a Henschel mixer to obtain mixture B. Based on the total amount of mixture D and mixture B, mixture D and mixture B were combined so that the content of mixture D was 70 parts by mass, and the mixture was mixed using a Henschel mixer to obtain mixture E. Subsequently, 1.55 kg of mixture E was filled into a boron nitride container while compressing it appropriately.

[0076] The above-mentioned container was placed in a resistance heating furnace and heated from room temperature to 1200°C at a heating rate of 10°C / min under a nitrogen gas atmosphere with a gauge pressure of 10 kPa. Next, the temperature was further increased to 2000°C at a heating rate of 2°C / min and held at 2000°C for 15 hours to perform the heat treatment (crystallization process). The obtained boron nitride powder was crushed in a mortar for 10 minutes and then classified using a nylon sieve with a mesh size of 75 μm. The residue below the sieve was used as the boron nitride powder of Comparative Example 1.

[0077] (Comparative Example 2) Boron carbide powder A with an average particle size of 22 μm was prepared using the same procedure as in Example 1 [Preparation of Boron Carbide Powder]. Using boron carbide powder A with an average particle size of 22 μm, mixture A was prepared using the same procedure as in Example 1 [Preparation of Boron Carbonitride Powder] and [Preparation of Boron Nitride Powder]. 1.55 kg of mixture A was packed into a boron nitride container while being appropriately compressed. The above container was placed in a resistance heating furnace and heated from room temperature to 1200°C at a heating rate of 10°C / min under a nitrogen gas atmosphere with a gauge pressure of 10 kPa. Next, the temperature was further increased to 2000°C at a heating rate of 2°C / min and held at 2000°C for 15 hours to perform heat treatment (crystallization step). The obtained boron nitride powder was crushed in a mortar for 10 minutes and then classified using a nylon sieve with a mesh size of 63 μm. The sieved portion was used to obtain the boron nitride powder from Comparative Example 2.

[0078] (Comparative Example 3) Boron carbide powder A with an average particle size of 30 μm was prepared using the same procedure as in Example 4. Using boron carbide powder A with an average particle size of 30 μm, mixture A was prepared using the same procedure as in Example 1's [Preparation of Boron Carbonitride Powder] and [Preparation of Boron Nitride Powder]. 1.55 kg of mixture A was packed into a boron nitride container while being appropriately compressed. The container was placed in a resistance heating furnace and heated from room temperature to 1200°C at a heating rate of 10°C / min under a nitrogen gas atmosphere with a gauge pressure of 10 kPa. Next, the temperature was further increased to 2000°C at a heating rate of 2°C / min and held at 2000°C for 15 hours to perform the heat treatment (crystallization step). The obtained boron nitride powder was crushed in a mortar for 10 minutes and then classified using a nylon sieve with a mesh size of 125 μm. The sieved portion was used as the boron nitride powder of Comparative Example 3.

[0079] (Comparative Example 4) Following the same procedure as in Example 1 [Preparation of Boron Carbide Powder], boron carbide powder B with an average particle size of 12 μm was prepared. Based on the total amount of boron carbide powder B and boric acid (boron source), boric acid was added to boron carbide powder B so that the boric acid content was 70 parts by mass, and the mixture was mixed using a Henschel mixer to obtain mixture B. 1.55 kg of mixture B was packed into a boron nitride container while being appropriately compressed. The above container was placed in a resistance heating furnace and heated from room temperature to 1200°C at a heating rate of 10°C / min under a nitrogen gas atmosphere with a gauge pressure of 10 kPa. Next, the temperature was further increased to 2000°C at a heating rate of 2°C / min and held at 2000°C for 15 hours to perform heat treatment (crystallization process). The obtained boron nitride powder was crushed in a mortar for 10 minutes and then classified using a nylon sieve with a mesh size of 75 μm. The sieved portion was used to obtain the boron nitride powder of Comparative Example 4.

[0080] <Evaluation of physical properties of boron nitride powder> [Measurement of D50 and D100] The particle size distribution of boron nitride powder in each example and comparative example was measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction and scattering method". A laser diffraction scattering particle size distribution analyzer (Microtrac "MT-3300EXII" (product name) manufactured by Microtrac-Bell) was used for the measurement. The average particle size (D50) and maximum particle size (D100) values ​​were determined in the obtained particle size distribution. The results are shown in Table 1.

[0081] [Orientation Index] X-ray diffraction measurements were performed on boron nitride powder to obtain its X-ray diffraction spectrum. From this spectrum, the peak intensities I(002) and I(100) corresponding to the (002) and (100) planes were obtained, and the orientation index [I(002) / I(100)] of the boron nitride powder was calculated. The results are shown in Table 1. The X-ray diffractometer used was "ULTIMA-IV" (product name) manufactured by Rigaku Corporation.

[0082] [Graphitization Index (GI)] The graphitization index of boron nitride powder was calculated from the results of powder X-ray diffraction measurements. In the obtained X-ray diffraction spectrum, the area value (in arbitrary units) enclosed by the integrated intensity of each diffraction peak (i.e., each diffraction peak) corresponding to the (100), (101), and (102) planes of the hexagonal boron nitride primary particles and their baseline was calculated. The calculated area values ​​were designated S100, S101, and S102. Using these calculated area values, the graphitization index GI was calculated based on the following equation (1). The results are shown in Table 1. GI = (S100 + S101) / S102 ... (1)

[0083] [Measurement of crush strength Cs] From the boron nitride powder of each example and comparative example, 50 agglomerated particles were selected so that at least one agglomerated particle (coarse particle) with a particle size of 100 μm or larger was included. A single-particle compression test was performed on each of the 50 selected agglomerated particles under a loading rate of 0.27 mN / sec, and the particle size and crush strength Cs of each agglomerated particle were measured. The measurements were performed in accordance with the description in JIS R 1639-5:2007 "Fine ceramics - Method for measuring particle properties - Part 5: Single particle crush strength" and using the "MCT-210" (product name) manufactured by Shimadzu Corporation.

[0084] Of the 50 aggregated particles, those with a particle diameter of 100 μm or more were classified as coarse particles, and those with a particle diameter of 10 to 40 μm were classified as fine particles. The number of coarse particles, the average value of the coarse particle's crushing strength Cs, the percentage of coarse particles with a crushing strength Cs of 10 MPa or more, the number of fine particles, and the average value of the fine particle's crushing strength Cs were calculated for each of the 50 aggregated particles. The average value of the crushing strength Cs for all 50 aggregated particles was also determined. The results are shown in Table 1. For Example 1, the measurement results for the particle diameter and crushing strength Cs of each of the 50 aggregated particles are shown in Table 2, arranged in ascending order of particle diameter. Figure 1 shows the distribution of particle diameter and crushing strength Cs shown in Table 2.

[0085] <Performance evaluation of boron nitride powder> [Preparation of evaluation sheet] A mixture of 100 parts by mass of naphthalene-type epoxy resin (DIC Corporation, HP4032) and 10 parts by mass of an imidazole compound (Shikoku Chemicals, Ltd., 2E4MZ-CN), a curing agent, was prepared. Boron nitride powder was added to this mixture to a concentration of 50% by volume, and a solvent was added to achieve a coating viscosity to obtain a resin composition. The resin composition was kneaded at 2000 rpm for 3 minutes using a foaming mixer manufactured by Thinky Co., Ltd. The kneaded resin composition was then coated onto a PET film to a thickness of 0.2 mm using a coater. Subsequently, a temperature of 150°C and a load of 100 kgf / cm² were applied. 2 A 0.2 mm thick resin molded body (evaluation sheet) was prepared by heating and pressurizing (heat pressing) under the specified conditions.

[0086] [Measurement of thermal conductivity and evaluation of heat dissipation] The thermal conductivity (20°C) of the prepared evaluation sheet was measured. Thermal conductivity H (unit: W / (m·K)) and thermal diffusivity a (unit: m 2 ( / second), density b (unit: kg / m³) 3 The thermal conductivity H was calculated from the values ​​of the thermal conductivity and specific heat capacity c (unit: J / (kg·K)) based on the formula H = a × b × c. The thermal diffusivity a was determined by processing an evaluation sheet to a length: 10 mm, width: 10 mm, and thickness: 0.2 mm and using the laser flash method. The measurement device used was a xenon flash analyzer (NETZSCH, product name: LFA447NanoFlash). The density b was determined using the Archimedes method. The specific heat capacity c was determined using a DSC (Rigaku Corporation, product name: ThermoPlusEvoDSC8230). 100 evaluation sheets were prepared, and the thermal conductivity H of each was measured, and the average value of the thermal conductivity H of the 100 evaluation sheets was calculated. The heat dissipation performance was evaluated according to the following criteria. The results are shown in Table 1.

[0087] A: The average value of the thermal conductivity is 12.0 W / (m·K) or higher. B: The average thermal conductivity is between 10.5 W / (m·K) and less than 12.0 W / (m·K). C: The average value of the thermal conductivity is 9.0 W / (m·K) or higher and less than 10.5 W / (m·K). D: The average value of the thermal conductivity is 7.5 W / (m·K) or higher and less than 9.0 W / (m·K). E: The average value of the thermal conductivity is less than 7.5 W / (m·K).

[0088] [Measurement of dielectric breakdown voltage and evaluation of insulation properties] The dielectric breakdown voltage of the prepared evaluation sheets was measured. In accordance with JIS C 6481-1996 "Test Method for Copper-Clad Laminates for Printed Wiring Boards," a dielectric strength tester (manufactured by Kikusui Electronics Co., Ltd., device name: TOS-8650) was used to measure the dielectric breakdown voltage of the obtained evaluation sheets. 100 evaluation sheets were prepared, and the dielectric breakdown voltage of each was measured. The average value and standard deviation were calculated. Based on the measurement results, the insulation performance and variability of insulation performance were evaluated according to the following criteria. The results are shown in Table 1.

[0089] (Evaluation criteria for insulating properties) A: The average dielectric breakdown voltage is 80kV / mm or higher. B: The average dielectric breakdown voltage is between 60kV / mm and 80kV / mm. C: The average dielectric breakdown voltage is between 50kV / mm and 60kV / mm. D: The average dielectric breakdown voltage is between 40kV / mm and 50kV / mm. E: The average dielectric breakdown voltage is less than 40kV / mm.

[0090] (Evaluation criteria for variations in insulating properties) A: The standard deviation of the dielectric breakdown voltage is less than 10% of the mean. B: The standard deviation of the dielectric breakdown voltage is between 10% and 15% of the mean. C: The standard deviation of the dielectric breakdown voltage is 15% or more but less than 20% of the mean. D: The standard deviation of the dielectric breakdown voltage is 20% or more but less than 25% of the mean. E: The standard deviation of the dielectric breakdown voltage is 25% or more of the mean.

[0091] [Evaluation of productivity using evaluation sheets] Productivity was evaluated during the preparation of evaluation sheets. The resin composition was applied to a PET film using a coater to a thickness of 0.2 mm. Visual inspection was performed to check for streaks, lumps, and surface irregularities on the evaluation sheets after heat pressing. Productivity was evaluated according to the following criteria: Productivity was assessed by the percentage of evaluation sheets with abnormal appearance. Surface abnormalities were defined as those where streaks, lumps, or irregularities were observed during visual inspection. A: The incidence rate of external defects is less than 1%. B: The incidence rate of external abnormalities is 1% or more but less than 3%. C: The incidence rate of external abnormalities is 3% or more but less than 5%. D: The incidence rate of external abnormalities is 5% or more but less than 10%. E: The incidence rate of external defects is 10% or more but less than 15%.

[0092] [Table 1]

[0093] [Table 2]

[0094] As shown in Table 1, the boron nitride powders of Examples 1 to 5, in which D50 is 20 to 80 μm, D100 is 200 μm or less, orientation index is 9 or less, and the average crush strength Cs of coarse particles with a particle size of 100 μm or more is 8 MPa or less, were confirmed to have superior heat dissipation and insulation properties compared to the boron nitride powders of Comparative Examples 1 to 4, which do not meet the above range. [Industrial applicability]

[0095] According to this disclosure, it is possible to provide boron nitride powder that can improve the heat dissipation and insulation properties when used in a resin molded article.

Claims

1. A boron nitride powder containing aggregated particles formed by the aggregation of primary particles of hexagonal boron nitride, In the volume-based cumulative distribution of particle size determined by laser diffraction and scattering, when the cumulative value from small particle size reaches 50% and 100% of the total, the particle size is defined as D50 and D100, respectively. In this case, D50 is 25 to 70 μm, and D100 is 120 to 200 μm. The orientation index is 6 to 9. Boron nitride powder in which 50 aggregated particles are selected such that at least one coarse particle with a particle diameter of 100 μm or more is included, and a single-particle compression test is performed on each of the 50 aggregated particles under a loading rate of 0.27 mN / sec, the average value of the crushing strength Cs of the coarse particle is 1 to 6 MPa.

2. The boron nitride powder according to claim 1, wherein the 50 aggregated particles include fine particles with a particle diameter of 10 to 40 μm, and the average value of the crush strength Cs of the fine particles is 7 to 20 MPa.

3. The boron nitride powder according to claim 1 or 2, wherein the average value of the crush strength Cs of the 50 aggregated particles is 5 to 15 MPa.

4. The boron nitride powder according to claim 1 or 2, wherein the graphitization index is 1.3 to 2.3.

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