Boron nitride powder

Abstract

To provide a boron nitride powder that can improve the insulating properties when used in a resin molded product. [Solution] In a boron nitride powder containing aggregated particles composed of aggregated primary particles of hexagonal boron nitride, when the upper surface of a flattened powder layer spread to cover a measurement stage is observed from above, the number of carbon-containing foreign matter particles with a particle size of 50 μm or larger is measured per 1 m of the upper surface. 2 The present invention provides boron nitride powder containing 10 or fewer particles per unit area.

Description

[Technical Field]

[0001] This disclosure relates to boron nitride powder. [Background technology]

[0002] Boron nitride powder possesses high thermal conductivity and insulating properties, and is widely used as a thermally conductive filler and insulating filler for resin molded bodies. It is known that reducing impurities in boron nitride powder can improve its thermal conductivity and insulating properties. For example, Patent Document 1 discloses that boron nitride powder with an impurity element content of 500 ppm or less, such as calcium, can impart high thermal conductivity and insulating properties to resin molded bodies. Patent Document 2 also discloses boron nitride powder with a carbon content of 0.01 mass%, listing carbon as a conductive impurity in boron nitride powder. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-133951 [Patent Document 2] International Publication No. 2014 / 003193 [Overview of the project] [Problems that the invention aims to solve]

[0004] Even when the carbon content of boron nitride powder is low, if coarse carbon-containing impurities are present, these impurities can act as current pathways, potentially leading to insulation failure in the resin molded product. Therefore, this disclosure aims to provide boron nitride powder capable of improving the insulation properties when used in a resin molded product. [Means for solving the problem]

[0005] One aspect of this disclosure provides the boron nitride powder described in [1] below.

[0006] [1] A boron nitride powder comprising aggregated particles composed of aggregated primary particles of hexagonal boron nitride, When the top surface of the powder layer, which has been spread flat to cover the measuring platform, is observed from above, the number of carbon-containing foreign matter particles with a diameter of 50 μm or more is found to be within 1 m of the top surface. 2 Boron nitride powder, with 10 or fewer particles per unit.

[0007] The boron nitride powder described in [1] above has a sufficiently low number of carbon-containing impurities with a particle size of 50 μm or larger. Because such boron nitride powder has a sufficiently reduced amount of conductive and coarse carbon-containing impurities, it can improve the insulation properties when used in a resin molded product.

[0008] The boron nitride powder in [1] above may also be one of the following [2] to [7].

[0009] [2] The boron nitride powder according to [1], wherein the orientation index is 10 or less.

[0010] The boron nitride powder described in [2] above has a low orientation index, which can improve the thermal conductivity when used in a resin molded product.

[0011] [3] Boron nitride powder according to [1] or [2], wherein the average particle size is 5 to 120 μm. [4] Boron nitride powder according to any one of [1] to [3], wherein the carbon content is 0.05% by mass or less. [5] Boron nitride powder according to any one of [1] to [4], wherein the graphitization index is 2.5 or less.

[0012] Since the boron nitride powder of [3] above has a sufficient size when filled into a resin molded body, it can be suitably used as a filler for filling into a resin molded body. The boron nitride powder of [4] above has a sufficiently reduced content of carbon as an impurity, and can further improve the insulation of the resin molded body. The boron nitride powder of [5] above has a sufficiently small graphitization index, and can be suitably used as a filler for filling into a resin molded body.

[0013] [6] The number of carbon-containing foreign substances having a particle diameter of 100 μm or more is 1 or less per 1 m of the upper surface 2 The boron nitride powder according to any one of [1] to [5].

[0014] The boron nitride powder of [6] above has a sufficiently small number of coarser carbon-containing foreign substances. Such boron nitride powder can further improve the insulation when made into a resin molded body.

[0015] [7] The number of colored foreign substances having a particle diameter of 50 μm or more is 10 or less per 1 m of the upper surface 2 The boron nitride powder according to any one of [1] to [6].

[0016] The boron nitride powder of [7] above has a small number of coarser colored foreign substances. Such boron nitride powder can further improve the insulation when made into a resin molded body because the conductivity and the number of coarser colored foreign substances are sufficiently reduced.

Advantages of the Invention

[0017] According to the present disclosure, it is possible to provide a boron nitride powder capable of improving the insulation when made into a resin molded body.

Brief Description of the Drawings

[0018] [Figure 1] It is a diagram for explaining a method of detecting colored foreign substances.

Modes for Carrying Out the Invention

[0019] 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), unless otherwise specified, it means "X or more and Y or less". The upper limit value or lower limit value of the numerical range explicitly shown 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.

[0020] The boron nitride powder according to one embodiment is a boron nitride powder containing agglomerated particles formed by aggregation of primary particles of hexagonal boron nitride. When observing the upper surface of a powder layer laid flat so as to cover a measurement stage from above, the number of carbon-containing foreign matters having a particle diameter of 50 μm or more is 2 10 or less per 1 m² of the upper surface.

[0021] In the present disclosure, the "colored foreign matter" refers to particles obtained by the following procedure. First, in a 256-level grayscale image, the upper surface of a powder layer of boron nitride powder having a uniform hue without foreign matters is photographed, and the obtained image is binarized to obtain the average value of the luminance values of the entire image (average luminance value). Then, when photographing and binarizing the upper surface of a powder layer containing particles different from boron nitride particles under the same conditions, if particles having a luminance value of 60% or less of the average luminance value are included, such particles are referred to as "colored foreign matters". The luminance value can be measured by image analysis. The image analysis in the present disclosure can be performed using, for example, image analysis software "ContamiAnalyzer" (manufactured by Mitani Corporation).

[0022] In the present disclosure, the "carbon-containing foreign matter" refers to particles having a carbon content of 50 mass% or more when the above-mentioned colored foreign matter is measured with an energy dispersive X-ray analyzer (EDS).

[0023] The boron nitride powder contains agglomerated particles formed by the aggregation of 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.

[0024] Agglomerated particles refer to secondary particles in which 5 or more primary particles (single particles) are aggregated, and the thickness directions of each primary particle are random. The state in which the primary particles are aggregated can be confirmed, for example, by SEM (scanning electron microscope).

[0025] When observing from above the upper surface of a powder layer that has been spread flat to cover the measurement table in the boron nitride powder, the number of carbon-containing foreign substances with a particle diameter of 50 μm or more is 2 10 or less per 1 m² of the upper surface. Such boron nitride powder has a small number of coarse carbon-containing foreign substances, so the insulation property when made into a resin molded body can be improved. Also, when a plurality of resin molded bodies are produced using such boron nitride powder, the variation in insulation properties among the plurality of resin molded bodies can be suppressed. From the viewpoint of further improving the insulation property when made into a resin molded body, the number of carbon-containing foreign substances with a particle diameter of 50 μm or more can be 2 7 or less, 5 or less, or 3 or less per 1 m² of the upper surface. The particle diameter of the carbon-containing foreign substance or colored foreign substance is the maximum value of the distance between any two points arbitrarily selected on the outer edge of the particle of the carbon-containing foreign substance or colored foreign substance in the image of the upper surface taken.

[0026] The number of carbon-containing foreign substances with a particle diameter of 50 μm or more per 1 m² of the upper surface of the powder layer 2 can be specifically obtained by the following procedure. First, the boron nitride powder is spread flat to cover the measurement table to form a powder layer with a thickness of 0.2 mm and an upper surface area of 100 cm² 2 or more. The upper surface of the formed powder layer is photographed from above with a Charge Coupled Devices (CCD) camera, and the upper surface is 100 cm² 2An image is obtained. The obtained image is binarized and image analysis is performed to identify and recover colored foreign matter with a particle size of 50 μm or larger. Then, the powder layer is reformed and an image of the top surface is taken, and colored foreign matter with a particle size of 50 μm or larger is identified and recovered from the obtained image. The total area of ​​the top surface where this operation is performed is 1 m². 2 Repeat until the top surface reaches 1 meter. 2 Colored foreign matter is collected. Then, EDS analysis is performed on the collected colored foreign matter to confirm whether it is carbon-containing foreign matter, and the number of collected colored foreign matter and carbon-containing foreign matter is counted. In this way, the top 1 m of the powder layer, which has been spread flat to cover the measurement platform, is examined. 2 The number of carbon-containing foreign particles with a particle size of 50 μm or larger can be determined per unit area.

[0027] The powder layer can be formed, for example, by evenly spreading boron nitride powder on a flat measuring stage to a thickness of 0.2 mm using a leveling tool or the like.

[0028] Referring to Figure 1, a method for detecting colored foreign matter will be explained. The imaging device 100 includes a measuring table 50, a powder layer 10 laid out so as to cover the measuring table 50, and a CCD camera 20. Here, "so as to cover the measuring table" means that the powder layer 10 covers the measuring table 50 to such an extent that the surface of the measuring table 50 does not appear in the image when the powder layer 10 is photographed from above. The CCD camera 20 has a lens 22 and is positioned perpendicularly above the upper surface 12 of the powder layer 10 so as to be able to photograph the upper surface 12 from directly above. With such an imaging device 100, the upper surface 12 is photographed by the CCD camera 20. Then, colored foreign matter 30 on the upper surface 12 can be detected by image analysis, and the particle size of the colored foreign matter 30 can be calculated.

[0029] The imaging device 100 may move the measuring stage 50 or the CCD camera 20 parallel to the upper surface 12 while forming the powder layer 10. Since such an imaging device 100 can continuously image different regions, it can efficiently detect colored foreign matter 30.

[0030] Top 1m of the powder layer 2 The number of carbon-containing foreign matter particles with a diameter of 50 μm or more per unit area can be measured, for example, using the Mimic-AF300 (manufactured by FLOVEL). The Mimic-AF300 (manufactured by FLOVEL) can also be used to measure the number of colored foreign matter particles with a diameter of 50 μm or more, the number of carbon-containing foreign matter particles with a diameter of 100 μm or more, and the number of colored foreign matter particles with a diameter of 100 μm or more, as described later.

[0031] Top 1m of the powder layer 2 The number of colored foreign matter particles with a diameter of 50 μm or more per unit area is calculated as follows: 2 The number of particles may be 10 or less, 7 or less, 5 or less, or 3 or less per layer. Keeping the number of colored foreign particles with a particle size of 50 μm or larger within the above range further improves the insulating properties when the resin molded product is formed. (Top 1 m of the powder layer) 2 The number of colored foreign matter particles with a particle size of 50 μm or larger per unit area may be the same as the number of carbon-containing foreign matter particles with a particle size of 50 μm or larger, or it may be greater than or equal to the number of carbon-containing foreign matter particles with a particle size of 50 μm or larger.

[0032] Top 1m of the powder layer 2 The number of carbon-containing foreign matter particles with a diameter of 100 μm or more per unit area is calculated as follows: 2 The number of carbon-containing foreign matter particles with a particle size of 100 μm or larger may be one or less, or even zero. By keeping the number of carbon-containing foreign matter particles with a particle size of 100 μm or larger within the above range, the amount of coarser carbon-containing foreign matter is further reduced, and the insulating properties of the resin molded product can be further improved.

[0033] Top 1m of the powder layer 2 The number of colored foreign matter particles with a diameter of 100 μm or more per unit area is calculated as follows: 2 The number of colored foreign particles per unit may be one or less, or even zero. By keeping the number of colored foreign particles with a particle size of 100 μm or more within the above range, the amount of coarser colored foreign particles is further reduced, and the insulating properties of the resin molded product can be further improved.

[0034] The orientation index of the boron nitride powder may be 10 or less, or 9 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 the boron nitride powder may be 2 or more, 4 or more, or 6 or more. The orientation index of the boron nitride powder may be adjusted within the above range, for example, 2 to 10, 4 to 10, 6 to 10, 2 to 9, 4 to 9, or 6 to 9.

[0035] The orientation index can be measured according to the following method: An X-ray diffraction spectrum of boron nitride powder is obtained by performing an X-ray diffraction measurement on the boron nitride powder, and the peak intensities I(002) and I(100) corresponding to the (002) plane and (100) plane are obtained from the X-ray diffraction spectrum. The orientation index [I(002) / I(100)] of the boron nitride powder is calculated using the obtained peak intensities. As an X-ray diffractometer, for example, "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used.

[0036] Since the orientation index is measured on 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 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.

[0037] The average particle size (D50) of boron nitride may be 5 μm or more, 10 μm or more, 20 μm or more, or 25 μm or more, from the viewpoint of ensuring the thermal conductivity of the resin molded article. The average particle size of boron nitride may be 120 μm or less, 100 μm or less, 80 μm or less, or 60 μm or less, from the viewpoint of further improving the insulating properties of the resin molded article and ensuring an appropriate size as a filler for the resin molded article. The average particle size of boron nitride powder may be adjusted within the above range, for example, 5 to 120 μm, 10 to 100 μm, 20 to 80 μm, or 25 to 60 μm.

[0038] The average particle size (D50) refers to the value 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 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 scattering particle size analyzer. When measuring the average particle size of boron nitride powder, the measurement should be performed without treating the powder with a homogenizer or the like.

[0039] The carbon content of the boron nitride powder may be 0.05% by mass or less, or 0.01% by mass or less, from the viewpoint of further improving the insulating properties of the resin molded article. The carbon content of the boron nitride powder can be measured using a carbon / sulfur simultaneous analyzer (LECO Corporation, product name: IR-412). By keeping the carbon content within the above range, even at low carbon content levels, coarse carbon-containing foreign matter with a particle size of 50 μm or more is reduced, further improving the insulating properties of the resin molded article.

[0040] The graphitization index (GI) of boron nitride powder may be 2.5 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 1.0 or more, or 1.5 or more, from the viewpoint of further improving the insulating and heat dissipating properties of resin molded articles. The graphitization index of boron nitride powder may be adjusted within the above range, for example, 1.0 to 2.5, or 1.5 to 2.0.

[0041] 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 is determined 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)

[0042] 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, with boron nitride powder added to a total of 55% by volume. After kneading the above resin composition at 1600 rpm for 3 minutes, the mixture is then subjected to a temperature of 150°C and a pressure of 100 kgf / cm². 2 The average dielectric breakdown voltage of 20 resin molded bodies with a thickness of 0.12 mm obtained by heating and pressurizing under the specified conditions may be 65 kV / mm or higher, or 70 kV / 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 100 kV / mm or lower. The average dielectric breakdown voltage may be in the range of 65 to 100 kV / mm, or 70 to 100 kV / mm. The average dielectric breakdown voltage can be measured, for example, by the method described in the examples.

[0043] The standard deviation of the dielectric breakdown voltage of the 20 resin molded bodies described above may be 10.0 kV / mm or less, or 7.0 kV / mm or less, from the viewpoint of reducing variations in dielectric breakdown voltage. The standard deviation of the dielectric breakdown voltage may be 3.0 kV / mm or more. The range of the standard deviation of the dielectric breakdown voltage may be 3.0 to 10.0 kV / mm, or 3.0 to 7.0 kV / mm. The standard deviation of the dielectric breakdown voltage can be measured, for example, by the method described in the examples.

[0044] After kneading the above resin composition at 1600 rpm for 3 minutes, the temperature was set to 150°C and the pressure to 100 kgf / cm². 2 The thermal conductivity of a 0.12 mm thick resin molded body 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. A resin molded body with a thermal conductivity within the above range has sufficient heat dissipation. The thermal conductivity of the resin molded body may be 20.0 W / (m·K) or lower. The 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). The thermal conductivity can be measured, for example, by the method described in the examples.

[0045] 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 under a pressurized atmosphere to obtain boron carbonitride (B4CN4) powder; a carbon reduction step of firing the boron carbonitride powder under an air atmosphere at a temperature of less than 850°C for 15 hours or more to obtain a heat-treated product; a crystallization step of firing the heat-treated product together with a boron source under a nitrogen atmosphere to obtain hexagonal boron nitride powder; and an oxidation treatment step of firing the hexagonal boron nitride powder under an air atmosphere.

[0046] Boron carbide powder 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.

[0047] 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.

[0048] The calcination temperature when obtaining boron carbide powder may be 1800°C or higher, 2500°C or lower, or 1800 to 2500°C. The calcination time when obtaining boron carbide powder may be 2 hours or higher, 8 hours or lower, or 2 to 8 hours. In this disclosure, calcination time and heating time refer to the time (holding time) from when the ambient temperature of the object to be heated reaches a predetermined temperature until that temperature is maintained.

[0049] The average particle size of boron carbide powder may be 5 μm or larger, 10 μm or larger, or 12 μm or larger. The average particle size of boron carbide powder may be 25 μm or smaller, or 22 μm or smaller. The range of the average particle size of boron carbide powder may be 5 to 25 μm, or 12 to 22 μm. The average particle size of boron carbide powder is measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction and scattering method".

[0050] 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.

[0051] 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.

[0052] The firing time in the pressurized nitriding process may be 15 hours or more, or 20 hours or more, from the viewpoint of allowing the nitriding of boron carbide to proceed more sufficiently. The firing time in the pressurized nitriding process may be 40 hours or less, or 30 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, 15 to 40 hours, or 20 to 30 hours.

[0053] In the carbon reduction process, the boron carbonitride powder may be calcined in an atmospheric environment at a temperature below 850°C for 15 hours or more. This low-temperature, long-duration heating suppresses the oxidation of boron carbonitride while heating for an extended period, and allows for the conversion of potentially impurity carbon components into carbon dioxide gas, which can be removed before crystallization. This further reduces the number of carbon-containing impurities with a particle size of 50 μm or larger in the boron nitride powder.

[0054] The heating temperature in the carbon reduction process may be 400°C or higher, or 600°C or higher, from the viewpoint of reducing the carbon content in the boron carbonitride powder while further reducing the number of carbon-containing foreign matter particles with a particle size of 50 μm or larger. The heating temperature in the carbon reduction process may be less than 850°C and 830°C or lower, from the viewpoint of suppressing the oxidation of boron carbonitride after heating and suppressing excess oxidation of the intermediate product to further reduce the number of carbon-containing foreign matter particles. The heating temperature in the carbon reduction process may be adjusted within the above range, for example, 400°C or higher and less than 850°C, or 600 to 830°C.

[0055] The heating time in the carbon reduction process may be 20 hours or less, or 18 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, 15 to 20 hours, or 15 to 18 hours.

[0056] In the crystallization process, the heat-treated boron carbonitride powder obtained in the carbon reduction process may be calcined together with a boron source. This decarburizes the boron carbonitride powder and crystallizes the individual boron carbonitride particles in the lump-like boron carbonitride particles into hexagonal boron nitride primary particles, thereby obtaining boron nitride powder containing aggregated particles formed by the aggregation of primary particles. The crystallization process may be carried out under a nitrogen gas atmosphere from the viewpoint of reducing the number of carbon-containing foreign matter particles with a particle size of 50 μm or more.

[0057] In the crystallization process, the boron source may include at least one selected from the group consisting of boric acid and boron oxide. The content of the boron source may be 20 to 60 parts by mass, or 30 to 50 parts by mass, based on the total amount of the heat-treated product and the boron source.

[0058] 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.

[0059] 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, grain growth can be promoted more sufficiently. The heating time in the crystallization process may be, for example, 0.5 to 10 hours, 1 to 8 hours, or 2 to 7 hours.

[0060] The oxidation treatment process involves heating the boron nitride powder obtained in the crystallization process in the presence of oxygen 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. By performing the oxidation treatment process after the crystallization process, carbon-containing impurities in the boron nitride powder are more easily converted into carbon dioxide, and the number of carbon-containing impurities with a particle size of 50 μm or larger can be sufficiently reduced.

[0061] 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 number of carbon-containing foreign matter. From the viewpoint of further reducing the number of carbon-containing foreign matter, 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.

[0062] The pressure in the oxidation process can be adjusted to, for example, atmospheric pressure or reduced pressure. The pressure in the oxidation process may be, for example, 150 kPa or less, 130 kPa or less, or 120 kPa or less. The pressure in the oxidation process may be 15 kPa or more, 20 kPa or more, or 30 kPa or more. The pressure in the oxidation process may be adjusted within the above range, for example, 15 to 150 kPa.

[0063] 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 oxygen proportion within the above range, the number of carbon-containing foreign matter 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 oxygen proportion 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.

[0064] Through the above process, boron nitride powder can be obtained with a reduced number of carbon-containing foreign matter particles larger than 50 μm in size. Such boron nitride powder can improve the insulating properties when used in resin molded articles.

[0065] Other steps may be performed after the oxidation treatment step. Examples of other steps include a drying step to dry the boron nitride powder. In the drying step, 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 step may be 100 to 180°C or 120 to 160°C. The drying time in the drying step may be 7 to 20 hours or 10 to 15 hours.

[0066] Although several embodiments have been described above, this disclosure is not limited in any way to the embodiments described above. [Examples]

[0067] 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.

[0068] (Comparative 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 a lump of boron carbide (B4C) powder. The synthesized lump of boron carbide powder was pulverized in a ball mill for 1 hour, and then sieved using a sieve with a mesh size of 106 μm to remove coarse particles, thereby preparing boron carbide powder (B4C powder) with an average particle size of 20 μm.

[0069] [Preparation of boron carbonitride powder] The prepared boron carbide powder was packed into a boron nitride crucible. The crucible was placed in a resistance heating furnace and heated for 25 hours under conditions of a nitrogen gas atmosphere at a pressure of 0.85 MPa and a firing temperature of 2100°C to obtain a calcined product containing boron carbonitride (B4CN4) powder (pressure nitriding process).

[0070] The calcined material obtained as described above was heated in an air atmosphere at 800°C for 15 hours to obtain a heat-treated material (carbon reduction process).

[0071] [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 a mixture. Next, 1.55 kg of the above mixture was filled into a boron nitride container while appropriately compressing and molding it.

[0072] 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. Then, the temperature was further increased to 2000°C at a heating rate of 2°C / min and held at 2000°C for 3 hours to perform the heat treatment (crystallization process). This decarburized the material and crystallized the individual boron carbonitride particles in the lump-like 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. 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 boron nitride powder that passed through the sieve was designated as Comparative Example 1.

[0073] (Example 1) The boron nitride powder of Comparative Example 1 was heat-treated in an air atmosphere (oxygen: 21 vol%) by raising the temperature from room temperature to 700°C at a heating rate of 10°C / min and holding it at the heating temperature of 700°C for 4 hours (oxidation treatment step). Then, 1 L of pure water was added to 100 parts by mass of the heat-treated boron nitride powder and stirred for 1 hour. After that, the same amount of water (i.e., 1 L of pure water for 100 parts by mass of boron nitride powder before the first wash) was added to the solid material recovered by suction filtration and stirred. The solid material recovered by filtration was dried at 140°C for 12 hours (drying step). The powder after the drying step was sieved through a nylon sieve with a mesh size of 75 μm, and the powder that passed through the sieve was used as the boron nitride powder of Example 1.

[0074] (Example 2) Boron nitride powder was prepared using the same procedure as in Example 1, except that the heating temperature in the oxidation treatment step was set to 900°C.

[0075] (Example 3) Boron nitride powder was prepared using the same procedure as in Example 1, except that the heating temperature in the oxidation treatment step was set to 500°C.

[0076] (Example 4) Boron nitride powder was prepared using the same procedure as in Example 1, except that the holding time in the oxidation treatment step was set to 6 hours.

[0077] (Example 5) In the preparation of boron carbide powder, boron nitride powder was prepared using the same procedure as in Comparative Example 1, except that the synthesized lump boron carbide powder was ground in a ball mill for 1 hour, and then the average particle size of the boron carbide powder was reduced to 15 μm using a sieve with a mesh size of 75 μm. The obtained boron nitride powder was subjected to an oxidation treatment step and a drying step using the same procedure as in Example 1, and the powder after the drying step was sieved through a nylon sieve with a mesh size of 63 μm, and the powder that passed through the sieve was used as the boron nitride powder of Example 5.

[0078] (Example 6) Boron nitride powder was prepared in the same manner as in Comparative Example 1, except that the holding time in the crystallization process was set to 6 hours.

[0079] (Comparative Example 2) In the preparation of boron carbide powder, boron nitride powder was prepared using the same procedure as in Comparative Example 1, except that the synthesized lump boron carbide powder was ground in a ball mill for 1 hour, the average particle size of the boron carbide powder was reduced to 10 μm using a sieve with a mesh size of 63 μm, and the powder after the crystallization process was classified using a nylon sieve with a mesh size of 63 μm.

[0080] <Evaluation of physical properties of boron nitride powder> [Number of colored foreign objects] The boron nitride powder of each example and comparative example was spread flat to cover a measuring platform having a flat surface, forming a powder layer 0.2 mm thick. The top surface of the powder layer was observed from above using a color CCD camera equipped with LED white illumination, at 100 cm. 2 The field of view was scanned. The captured images were binarized, and colored foreign matter was detected where the maximum distance between two arbitrarily selected points on the outer edge of the particle was 50 μm or more, and the brightness value was 60% or less of the average brightness value. The average brightness value was calculated by capturing the top surface of a boron nitride powder layer with a uniform hue free of foreign matter in a 256-level grayscale image, binarizing the obtained image, and finding the average brightness value of the entire image. Hereinafter, the maximum distance between the above two points will be referred to as the "particle diameter".

[0081] The above 100cm 2 The field of view is captured and colored foreign objects are detected on the upper surface observation area totaling 1 m². 2 This process was repeated until the desired result was achieved. During this time, photography was performed to avoid overlapping fields of view. Top surface 1m 2 The number of colored foreign particles with a particle size of 50 μm or larger and 100 μm or larger was determined. The results are shown in Table 1.

[0082] [Number of carbon-containing foreign objects] The detected colored foreign matter was collected and its elemental analysis was performed using an energy-dispersive X-ray spectrometer (EDS). Particles containing 50% or more carbon were classified as carbon-containing foreign matter, and the number of these carbon-containing foreign matter particles was determined. In this manner, the top surface 1 m 2 The number of carbon-containing foreign particles with a particle size of 50 μm or larger and 100 μm or larger per unit area was determined. The results are shown in Table 1.

[0083] [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.

[0084] [Average particle size] The particle size distribution of boron nitride powder was measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction and scattering method". A laser diffraction and scattering particle size distribution analyzer (Microtrac "MT-3300EXII" (product name) manufactured by Microtrac-Bell) was used for the measurement. The D50 value in the obtained particle size distribution was taken as the average particle size of the boron nitride powder. The results are shown in Table 1.

[0085] [Carbon content] The carbon content of the boron nitride powder was measured using a carbon / sulfur simultaneous analyzer (LECO, product name: IR-412).

[0086] [Graphitization Index (GI)] The graphitization index of boron nitride powder was calculated from the results of measurements by powder X-ray diffraction. 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 as S100, S101, and S102. Using these calculated area values, the graphitization index was determined based on the following equation (1). GI = (S100 + S101) / S102 ... (1)

[0087] <Performance evaluation of boron nitride powder> [Preparation of evaluation sheet] A resin composition was obtained by mixing 100 parts by mass of naphthalene-type epoxy resin (DIC Corporation, HP4032) with 10 parts by mass of an imidazole compound (Shikoku Chemicals, Ltd., 2E4MZ-CN) as a curing agent, and then adding boron nitride powder to a mixture containing 55% by volume. A foam mixer manufactured by Thinky Co., Ltd. was used for mixing the resin. Mixing was performed at 1600 rpm for 3 minutes. The obtained resin composition was applied to a PET film to a cured thickness of 0.12 mm, and degassing was performed under reduced pressure of 500 Pa for 10 minutes. Afterward, degassing was performed at a temperature of 150°C and a pressure of 100 kgf / cm². 2 A resin molded body (evaluation sheet) with a thickness of 0.12 mm was prepared by heating and pressurizing under the specified conditions.

[0088] [Measurement of thermal conductivity and evaluation of heat dissipation] The thermal conductivity of the prepared evaluation sheet was measured. Thermal conductivity H (unit: W / (m·K)) is equal to the thermal diffusivity a (unit: m 2 ( / second), density b (unit: kg / m³) 3 The thermal diffusivity a was calculated from the values ​​of the thermal conductivity H 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: 0.12 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). The heat dissipation performance was evaluated from the measurement results of the thermal conductivity H according to the following criteria. The results are shown in Table 1.

[0089] A: The thermal conductivity is 12.0 W / (m·K) or higher. B: The thermal conductivity is 10.5 W / (m·K) or more and less than 12.0 W / (m·K). C: The thermal conductivity is 9.0 W / (m·K) or higher and less than 10.5 W / (m·K). D: The thermal conductivity is 7.5 W / (m·K) or higher and less than 9.0 W / (m·K). E: The thermal conductivity is 6.0 W / (m·K) or more and less than 7.5 W / (m·K). F: The thermal conductivity is less than 6.0 W / (m·K).

[0090] [Measurement of dielectric breakdown voltage and evaluation of insulation properties] The dielectric breakdown voltage of the prepared evaluation sheets was measured. The obtained evaluation sheets were processed into 5cm x 5cm sheets, and measurements were performed in accordance with JIS C 2110. Specifically, the sheets were clamped from above and below with cylindrical electrodes of 25mm and 75mm in diameter, and a short-time breakdown test was performed in an environment of 25°C, and the dielectric breakdown voltage of 20 evaluation sheets was measured. From the measurement results, the average value of the dielectric breakdown voltage was calculated, and the insulation performance was evaluated according to the following criteria. The results are shown in Table 1.

[0091] A: The average dielectric breakdown voltage is 70kV / mm or higher. B: The average dielectric breakdown voltage is between 65kV / mm and 70kV / mm. C: The average dielectric breakdown voltage is 60kV / mm or more and less than 65kV / mm. D: The average dielectric breakdown voltage is between 55kV / mm and 60kV / mm. E: The average dielectric breakdown voltage is less than 55kV / mm.

[0092] [Evaluation of the variation in dielectric breakdown voltage] The standard deviation (kV / mm) was calculated using the measured dielectric breakdown voltage and average value of the 20 evaluation sheets mentioned above. The results are shown in Table 1.

[0093] [Table 1]

[0094] As shown in Table 1, the number of carbon-containing foreign matter particles with a diameter of 50 μm or more is as follows: 2The boron nitride powders of Examples 1-6, which contained 10 or fewer particles per unit area, were found to have superior insulating properties when molded into resin articles compared to the boron nitride powders of Comparative Examples 1 and 2, which contained more than 10 particles per unit area. On the other hand, the carbon content was 0.01% by mass in both the examples and comparative examples, with no difference. Therefore, it was confirmed that even with the same carbon content, the insulating properties of resin articles can be improved by reducing the number of coarse carbon-containing foreign matter particles with a particle size of 50 μm or larger. [Industrial applicability]

[0095] According to this disclosure, it is possible to provide a boron nitride powder that can improve the insulating properties when used in a resin molded article. [Explanation of Symbols]

[0096] 100... Imaging device, 50... Measuring stand, 20... CCD camera, 22... Lens, 10... Powder layer, 12... Top surface, 30... Colored foreign matter.

Claims

1. A boron nitride powder containing aggregated particles composed of aggregated primary particles of hexagonal boron nitride, When the upper surface of the powder layer, which is spread flat to cover the measuring platform, is observed from above, the number of carbon-containing foreign matter particles with a diameter of 50 μm or more is found to be within 1 m of the upper surface. ２ There are 3 or fewer per person. Boron nitride powder with an orientation index of 10 or less.

2. The boron nitride powder according to claim 1, wherein the average particle size is 5 to 120 μm.

3. Boron nitride powder according to claim 1 or 2, wherein the carbon content is 0.05% by mass or less.

4. Boron nitride powder according to claim 1 or 2, wherein the graphitization index is 2.5 or less.

5. The boron nitride powder according to claim 1 or 2, wherein the graphitization index is 1.8 or less.

6. The number of carbon-containing foreign matter particles with a diameter of 100 μm or more is within 1 m of the upper surface. ２ The boron nitride powder according to claim 1 or 2, wherein there is one or fewer particles per unit.

7. The number of colored foreign matter particles with a diameter of 50 μm or more is the number of particles on the upper surface 1 m ２ The boron nitride powder according to claim 1 or 2, wherein each particle contains three or fewer particles.

8. The boron nitride powder according to claim 1 or 2, wherein the number of colored foreign matter particles with a particle size of 100 μm or more is 0 per 1 m² of the upper surface.

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