Method for producing boron nitride powder, and boron nitride powder

Abstract

To provide a method for producing boron nitride powder that can produce boron nitride powder with excellent insulation in good yield even when low-grade boron carbide is used as a raw material.SOLUTION: One aspect of the present disclosure provides a method for producing boron nitride powder that includes a pressurized nitriding step in which boron carbide powder is fired under a nitrogen pressurized atmosphere to obtain a fired product, an oxidation step in which the fired product is heated under an atmosphere in which the oxygen partial pressure is 20% or more to obtain a heat-treated product, and a crystallization step in which an assembly containing the heat-treated product and a boron source is heated under an atmosphere containing nitrogen to produce primary particles of boron nitride and to obtain aggregated particles composed of the aggregated primary particles. The apparent surface area of the assembly in contact with the nitrogen-containing atmosphere is 250 cm2 / kg or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] One aspect of this disclosure relates to a method for producing boron nitride powder and to boron nitride powder. [Background technology]

[0002] Boron nitride powder possesses lubricating, high thermal conductivity, and insulating properties, and is widely used in applications such as solid lubricants, thermally conductive fillers, and insulating fillers. In recent years, with the increasing performance of electronic devices, there has been a growing demand for boron nitride to have excellent thermal conductivity.

[0003] For example, Patent Document 1 proposes hexagonal boron nitride powder and a method for producing the same, which can increase the thermal conductivity and dielectric strength (dielectric breakdown voltage) of insulating heat dissipation materials such as resins when used as a filler for such materials.

[0004] Hexagonal boron nitride powder is produced by a manufacturing method that includes, for example, a pressurized nitriding step in which boron carbide is nitrided under a pressurized atmosphere containing nitrogen to obtain a nitride, and a decarburization crystallization step in which the nitride is mixed with a boron source and heated to decarburize and promote crystallization to obtain hexagonal boron nitride (for example, Patent Document 2, etc.). The above manufacturing method aims to improve the decarburization performance in subsequent steps by nitriding the boron carbide once. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-116401 [Patent Document 2] International Publication No. 2019 / 073690 [Overview of the project] [Problems that the invention aims to solve]

[0006] When using boron nitride powder as an insulating filler, it is necessary to reduce the conductive impurities contained in the boron nitride powder itself. Therefore, a method using boron carbide with a sufficiently reduced iron content as a raw material has been adopted. However, reducing the iron content in boron carbide is not always easy, and raw material prices tend to rise, leading to an increase in the manufacturing cost of boron nitride powder. In recent years, the demand for insulating fillers has increased, and there is a need for high insulating properties from boron nitride powder. Therefore, it would be beneficial to have a method for manufacturing boron nitride powder that can achieve performance equivalent to or better than conventional methods by using inexpensive, readily available boron carbide in its low grade without reducing the iron content.

[0007] This disclosure aims to provide a method for producing boron nitride powder that can produce boron nitride powder with good yield, even when using low-grade boron carbide as a raw material, and which exhibits excellent insulating properties. This disclosure also aims to provide boron nitride powder that exhibits excellent insulating properties when used as a filler in a resin. [Means for solving the problem]

[0008] One aspect of this disclosure comprises a pressurized nitriding step of firing boron carbide powder under a nitrogen-pressurized atmosphere to obtain a calcined product; an oxidation step of heating the calcined product in an atmosphere with an oxygen partial pressure of 20% or more to obtain a heat-treated product; and a crystallization step of heating an aggregate containing the heat-treated product and a boron source in a nitrogen-containing atmosphere to generate primary boron nitride particles, and obtaining aggregated particles composed of the aggregation of the primary particles, wherein the apparent surface area of ​​the aggregate in contact with the nitrogen-containing atmosphere is 250 cm². 2 The present invention provides a method for producing boron nitride powder in a quantity of 1 kg or more.

[0009] In the method for producing the boron nitride powder described above, boron carbide powder is nitrided and then fired in an atmosphere containing oxygen to reduce the carbon content in the fired product and obtain a heat-treated product. Then, when preparing an aggregate by blending a boron source with the heat-treated product and performing heat treatment, in the production method according to the present disclosure, the area of the aggregate in contact with the nitrogen-containing atmosphere is adjusted to be a predetermined value or more. By adjusting in this way, impurities such as carbon and iron in the aggregate volatilize and can move more easily outside the system where boron nitride crystals grow. Also, at this time, a part of the liquid phase formed by the boron source is appropriately removed. Since this liquid phase is a growth field that promotes the growth of primary particles of boron nitride, by removing a part of the liquid phase, excessive growth of primary particles can be suppressed, and deformation and collapse of agglomerated particles can be reduced. By such an action, the obtained boron nitride powder has a reduced content of impurities such as iron, can be expected to have excellent insulating properties, has sufficient crushing strength for agglomerated particles in the boron nitride powder, and is suppressed from collapsing during kneading with a resin or the like, and can be suitably used as a heat dissipation filler.

[0010] The aggregate may be accommodated in a container, and the container may have a through-hole penetrating the inside and outside of the container. By using a container having a through-hole, the handling of the aggregate becomes easy.

[0011] The Fe content of the boron carbide powder may be 0.2 mass% or more. Boron carbide powder generally contains impurities such as iron. Therefore, in order to obtain boron nitride with excellent insulating properties, boron carbide powder with a reduced content of impurities such as iron is used. As a result, the production cost of boron nitride powder has tended to increase. On the other hand, in the case of the production method of boron nitride powder according to the present disclosure, by performing the crystallization step in an environment where impurities are easily released outside the system, even when using a low-grade material containing a relatively large amount of impurities in the raw material, a boron nitride powder having a quality equal to or better than that of the prior art can be easily produced.

[0012] In the above crystallization step, the content of the boron source may be less than 40% by mass based on the total amount of the heat-treated product and the boron source. By setting the blending amount of the boron source in the crystallization step within the above range, the amount of the liquid phase, which is the growth field of the primary particles of boron nitride, can be reduced, excessive growth of the primary particles can be suppressed, and boron nitride containing agglomerated particles excellent in crushing strength can be produced.

[0013] One aspect of the present disclosure includes agglomerated particles formed by aggregation of primary particles of boron nitride, wherein the crushing strength of the agglomerated particles is 12 MPa or more, the specific surface area is 4.0 m 2 / g or more, the orientation index is 7.5 or less, and the Fe content is 50 ppm or less, and provides boron nitride powder.

[0014] The boron nitride powder has a low content of iron (Fe) suppressed, and the primary particles of boron nitride have grown to an appropriate size and also have a large specific surface area. Further, in the boron nitride powder, the ratio of primary particles that have undergone excessive growth causing disintegration of the agglomerated particles is reduced. For these reasons, the agglomerated particles contained in the boron nitride powder have excellent crushing strength. The boron nitride powder can be suitably used as a heat dissipation filler to be kneaded with a resin and can exhibit excellent insulation properties.

[0015] The boron nitride powder may have a graphitization index of 2.0 or less. By having the graphitization index within the above range, it can be made to have more excellent heat dissipation properties.

Effects of the Invention

[0016] According to the present disclosure, even when using low-grade boron carbide as a raw material, a method for producing boron nitride powder capable of producing boron nitride powder that can exhibit excellent insulation properties with good yield can be provided. According to the present disclosure, boron nitride powder that can exhibit excellent insulation properties when filled and used in a resin can also be provided.

Modes for Carrying Out the Invention

[0017] The embodiments of this disclosure are described below. However, the embodiments described below are illustrative examples for the purpose of illustrating this disclosure and are not intended to limit this disclosure to the following.

[0018] Unless otherwise specified, the materials exemplified herein may be used individually or in combination of two or more. The content of each component in a composition means the total amount of any multiple substances present in the composition, unless otherwise specified, if there are multiple substances corresponding to each component in the composition.

[0019] One embodiment of a method for producing boron nitride powder includes a pressurized nitriding step of firing boron carbide powder under a nitrogen-pressurized atmosphere to obtain a fired product, an oxidation step of heating the fired product in an atmosphere where the oxygen partial pressure is 20% or more to obtain a heat-treated product, and a crystallization step of heating an aggregate containing the heat-treated product and a boron source in a nitrogen-containing atmosphere to generate primary boron nitride particles and obtain aggregated particles composed of the aggregated primary particles. In the crystallization step, the apparent surface area of ​​the aggregate in contact with the nitrogen-containing atmosphere is 250 cm². 2 It is 1 kg or more.

[0020] The boron carbide powder (B4C powder) used in the pressurized nitriding process may have a relatively high iron (Fe) content, and it is not necessary to use powder with reduced iron (Fe) content through pretreatment or other means. The Fe content of commonly available boron carbide powder is approximately 0.2 to 0.5 mass%. The upper limit of the Fe content in the boron carbide powder used in the manufacturing method according to this disclosure may be, for example, 0.5 mass% or less. The lower limit of the Fe content of the boron carbide powder is not particularly limited, but may be, for example, 0.2 mass% or more, or 0.3 mass% or more. When the lower limit of the Fe content of the boron carbide powder is within the above range, the effects of the manufacturing method according to this disclosure are more pronounced.

[0021] The upper limit of the average particle size of boron carbide powder may be, for example, 50 μm or less, 40 μm or less, 30 μm or less, 25 μm or less, or 20 μm or less. By using a raw material powder with a small average particle size, the efficiency of reducing the impurity content in the subsequent crystallization process can be further improved. The lower limit of the average particle size of boron carbide powder may be, for example, 1 μm or more, 2 μm or more, or 5 μm or more. By having the lower limit of the average particle size of boron carbide powder within the above range, it is possible to further reduce the orientation index of boron nitride powder, and the heat dissipation properties can be more fully exhibited.

[0022] In this specification, the average particle size of aggregated particles refers to the 50% cumulative diameter (median diameter) in the volume-based cumulative particle size distribution. More specifically, it refers to the particle size (D50) when the cumulative value in the volume-based cumulative particle size distribution obtained by laser diffraction scattering for boron nitride powder reaches 50%. The laser diffraction scattering method is 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 can be used for the measurement. For example, a laser diffraction scattering particle size distribution analyzer such as the "LS-13 320" (product name) manufactured by Beckman Coulter can be used.

[0023] In the pressurized nitriding process, boron carbide powder is calcined under a nitrogen pressurized atmosphere to obtain a calcined product containing boron carbonitride powder (B4CN4 powder). The lower limit of the calcination temperature in the pressurized nitriding process may be 2000°C or higher, or 2100°C or higher. By setting the lower limit of the calcination temperature within the above range, the crystallinity of the obtained boron carbonitride can be increased, and the proportion of hexagonal boron carbonitride can be increased. By increasing the proportion of hexagonal boron carbonitride in the pressurized nitriding process, the thermal conductivity of the boron nitride powder obtained later can be further improved. The upper limit of the calcination temperature in the pressurized nitriding process may be 2300°C or lower, or 2250°C or lower. The calcination temperature may be adjusted within the above range, for example, 2000 to 2300°C.

[0024] The lower limit of the pressure (atmospheric pressure) in the pressurized nitriding process may be, for example, 0.6 MPa or higher, 0.7 MPa or higher, or 0.8 MPa or higher. By setting the lower limit of the pressure in the pressurized nitriding process within the above range, the nitriding of boron carbide can be sufficiently carried out. The upper limit of the pressure (atmospheric pressure) in the pressurized nitriding process may be, for example, 1.0 MPa or lower, or 0.9 MPa or lower. By setting the upper limit of the pressure in the pressurized nitriding process within the above range, an increase in the manufacturing cost of boron nitride powder can be suppressed. The pressure may be adjusted within the above range, for example, 0.6 to 1.0 MPa or 0.8 to 0.9 MPa.

[0025] The nitrogen gas concentration in the pressurized nitrogen atmosphere during the pressurized nitriding process may be, for example, 95.0% by volume or more, 98.0% by volume or more, or 99.9% by volume or more. The firing time during the pressurized nitriding process is not particularly limited as long as the nitriding is sufficiently advanced, and may be, for example, 6 to 30 hours, 8 to 25 hours, or 10 to 20 hours.

[0026] In the oxidation process, a calcined product containing boron carbonitride powder is calcined in an oxygen-containing atmosphere to decarburize some of the carbon contained in the calcined product. This reduces the amount of boron source used in the subsequent crystallization process, and by suppressing the proportion of the liquid phase in the crystallization process, boron nitride powder with high tap density and excellent crushing strength of aggregated particles can be obtained.

[0027] The oxygen-containing atmosphere in the oxidation process may be an atmosphere with an oxygen partial pressure of 20% or more, such as air. The upper limit of the oxygen partial pressure may be, for example, 70% or less, 60% or less, or 50% or less. In this specification, the oxygen partial pressure refers to the oxygen partial pressure at standard conditions of the gas occupying the atmosphere, and is a value measured by an oxygen concentration meter. As an oxygen partial pressure meter, for example, the "G1690" (product name) manufactured by the following company can be used.

[0028] The pressure (ambient pressure) in the oxidation process may be, for example, 0.1 to 0.5 MPa, 0.1 to 0.3 MPa, or 0.1 to 0.2 MPa, or atmospheric pressure (0.1 MPa).

[0029] The lower limit of the firing temperature in the oxidation process may be, for example, 600°C or higher, 650°C or higher, or 700°C or higher. By keeping the lower limit of the firing temperature within the above range, the carbon content in the boron carbonitride can be further reduced. The upper limit of the firing temperature in the oxidation process may be, for example, 1000°C or lower, or 950°C or lower. By keeping the upper limit of the firing temperature within the above range, oxidation of the boron carbonitride after firing can be suppressed.

[0030] The lower limit of the firing time in the oxidation process may be, for example, 6 hours or more, or 7 hours or more. By keeping the lower limit of the firing time within the above range, it is possible to sufficiently reduce the carbon content in the boron carbonitride. Furthermore, there is no particular limit to the upper limit of the firing time in the oxidation process, but it may be, for example, 20 hours or less, or 15 hours or less. Keeping the upper limit of the firing time within the above range can more effectively suppress the decrease in processing efficiency of the process.

[0031] The crystallization step involves heating an aggregate containing the heat-treated material and a boron source in a nitrogen-containing atmosphere to generate primary boron nitride particles, and obtaining aggregated particles composed of these primary particles. In this crystallization step, the surface area of ​​the aggregate in contact with the atmosphere is adjusted to facilitate the removal of desorbed gases and the like.

[0032] In the crystallization process, the surface area of ​​the aggregate exposed to the nitrogen-containing atmosphere is adjusted to be as large as possible. The apparent surface area of ​​the aggregate exposed to the nitrogen-containing atmosphere is 250 cm². 2 The value is 1 kg or more, but it may be adjusted to be higher from the viewpoint of obtaining boron nitride powder with improved insulating properties. The lower limit of the apparent surface area is, for example, 250 cm². 2 / kg or more, 280cm 2 / kg or more, 300cm 2above / kg or 330 cm 2 It may be above / kg. The upper limit value of the apparent surface area is, for example, 1000 cm 2 / kg or less, 900 cm 2 / kg or less, 800 cm 2 / kg or less, 700 cm 2 / kg or less, 600 cm 2 / kg or less, 500 cm 2 / kg or less, or 400 cm 2 / kg or less. By setting the apparent surface area within the above range, the volatilization of carbon and iron components in the aggregate is promoted, and boron nitride powder with excellent insulation can be easily obtained. The apparent surface area can be controlled, for example, by providing through holes in the wall of the container and adjusting the number and area of the through holes. Also, by blending a binder with the above aggregate or making the molded body obtained by compression molding the above aggregate self-standing, reducing the area in contact with the wall of the container of the molded body, or changing the shape of the above aggregate including the molded body (for example, providing recesses or through holes in the aggregate), etc. can also be controlled.

[0033] The apparent surface area in this specification means the area in contact with the above atmosphere when the fine structure (such as unevenness of 500 μm or less) on the surface of the above aggregate is regarded as a plane without consideration. When the above aggregate is housed in a container and filled so as to contact the inner wall of the container, the apparent surface area shall be calculated by measuring the opening of the container and the outer circumference and inner diameter of the through holes provided in the container. Also, when the above aggregate can hold its shape by itself and is not housed in a container or is installed in the container at a distance from the inner wall of the container, the outer shape of the aggregate is measured and its apparent specific surface area is calculated. Although the manufacturing method of the present disclosure assumes that the mass of the aggregate is a large one of 1 kg or more, the mass of the above aggregate may be less than 1 kg, and in that case, the above apparent surface area is a converted value per 1 kg.

[0034] The above aggregate may be contained in a container. In this case, the container may have through holes that penetrate from the inside to the outside of the container. The number and diameter of the through holes may be adjusted according to the desired properties of the boron nitride powder. To improve insulation, a larger number of through holes and a larger diameter of the through holes are desirable. The container should have excellent heat resistance and low reactivity with the aggregate it contains; for example, it may be a ceramic container.

[0035] In the crystallization process described above, it is preferable to keep the boron source content low. The upper limit of the boron source content may be, for example, less than 40% by mass, 35% by mass or less, 33% by mass or less, 30% by mass or less, 28% by mass or less, or 25% by mass or less, based on the total amount of the heat-treated product and the boron source. By setting the upper limit of the boron source content within the above range, the amount of liquid phase, which is the growth field for primary boron nitride particles, can be reduced. As a result, excessive growth of the primary particles can be suppressed, and boron nitride containing aggregated particles with superior crushing strength can be produced. The lower limit of the boron source content may be, for example, 15% by mass or more, 18% by mass or more, or 20% by mass or more, based on the total amount of the heat-treated product and the boron source. By setting the lower limit of the boron source content within the above range, the crystallinity of boron nitride can be further improved, and primary hexagonal boron nitride particles with superior purity can be obtained. The content of the boron source may be adjusted within the range described above, and may be, for example, 15-40% by mass, 15-28% by mass, or 18-25% by mass, based on the total amount of the heat-treated product and the boron source.

[0036] The above-described manufacturing method may further include other steps in addition to the pressurized nitriding step, oxidation step, and crystallization step. Examples of other steps include a grinding step and a classification step. The grinding step is desirable to be carried out, for example, in the crystallization step, from the viewpoint of breaking down the aggregated particles that have undergone further tertiary aggregation. A general-purpose grinder or crusher can be used in the grinding step. For example, a ball mill, vibratory mill, jet mill, etc. can be used. In this disclosure, "grinding" also includes "breaking down". The average particle size of the boron nitride powder may be adjusted by grinding and classification.

[0037] The boron nitride powder produced by the above-described manufacturing method has a low and suppressed Fe content, even when using low-grade raw materials. Furthermore, by appropriately adjusting the amount of liquid phase derived from the boron source during the crystallization process, it is possible to obtain aggregated particles with excellent crushing strength. One embodiment of the boron nitride powder includes aggregated particles composed of aggregated primary boron nitride particles. In this boron nitride powder, the crushing strength of the aggregated particles is 12 MPa or more, and the specific surface area is 4.0 m². 2 The content is 1 / g or more, and the orientation index is 7.5 or less. The Fe content in the boron nitride powder is 50 ppm or less.

[0038] The lower limit of the crushing strength of the aggregated particles is 10 MPa or higher, but may be, for example, 11 MPa or higher, 12 MPa or higher, or 13 MPa or higher. Having the lower limit of the crushing strength within the above range can further improve the heat dissipation properties of the resin composition and molded article obtained by kneading with the resin. The upper limit of the crushing strength of the aggregated particles may be, for example, 30 MPa or lower, 25 MPa or lower, 23 MPa or lower, or 20 MPa or lower. Having the upper limit of the crushing strength within the above range allows at least a portion of the aggregated particles to collapse appropriately during kneading with the resin, thereby suppressing the generation of voids and further improving the insulation properties of the resin composition and molded article obtained. The crushing strength of the aggregated particles may be adjusted within the above range, for example, 10-30 MPa, 11-25 MPa, or 12-20 MPa.

[0039] In this specification, crushing strength refers to the value measured in accordance with JIS R 1639-5:2007 "Fine ceramics - Method for measuring particle properties - Part 5: Single particle crushing strength". The crushing strength σ (unit: MPa) of a single aggregated particle is given by σ = α × P / (π × d), where α (α = 2.48), a dimensionless number that changes depending on the position within the aggregated particle, P (unit: N), and particle diameter d (unit: μm). 2 The value is calculated using the following formula: ( ). The measurement shall be performed on 20 or more aggregated particles, and the value at the point of cumulative destruction rate of 63.2% shall be calculated. A microcompression tester can be used for the measurement. For example, the "MCT-W500" (product name) manufactured by Shimadzu Corporation can be used as a microcompression tester.

[0040] The lower limit of the specific surface area of ​​boron nitride powder is 4.0 m². 2 It is greater than / g, but for example, 4.2m 2 / g or more, 4.3m 2 / g or more, 4.5m 2 / g or more, or 5.0m 2 It may be 1 / g or more. Having the lower limit of the specific surface area within the above range means that the aggregation of primary boron nitride particles is more dense, and the crushing strength of the aggregated particles can be sufficiently high. This makes it possible to further suppress excessive collapse of aggregated particles when mixing or molding boron nitride powder with resin. The upper limit of the specific surface area of ​​boron nitride powder is, for example, 7.0 m². 2 / g or less, 6.5m 2 / g or less, 6.0m 2 / g or less, or 5.5m 2 It may be less than or equal to / g. When the upper limit of the specific surface area is within the above range, the primary particle size of boron nitride is sufficiently large, and when used in resin compositions and molded articles, it can exhibit better heat dissipation. The specific surface area of ​​the boron nitride powder may be adjusted within the above range, for example, 4.0 to 7.0 m². 2 / g, 4.2~6.5m 2 / g, or 4.3-6.0m 2 / g is acceptable.

[0041] In this specification, specific surface area refers to the value measured using a specific surface area measuring device in accordance with the description in JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption," and is calculated by applying the BET single-point method using nitrogen gas. As a specific surface area measuring device, for example, the "MONOSORB MS-22" (product name) manufactured by QUANTACHROME can be used.

[0042] The boron nitride powder described above has its primary particle orientation sufficiently suppressed. The upper limit of the orientation index of the boron nitride powder is 7.5 or less, but it may be, for example, 7.3 or less, 7.1 or less, or 7.0 or less. By keeping the upper limit of the orientation index within the above range, the resin composition and molded article can exhibit sufficiently sufficient heat dissipation for practical use. The lower limit of the orientation index of the boron nitride powder is not limited, but it is not easy to set it below 6.0, and it may be, for example, 6.0 or more, 6.1 or more, 6.3 or more, or 6.5 or more. The orientation index of the boron nitride powder may be adjusted within the above range, for example, 6.0 to 7.5, 6.3 to 7.3, or 6.5 to 7.1.

[0043] In this specification, the orientation index refers to a value 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 peak intensities I(002) and I(100) corresponding to the (002) plane and (100) plane are obtained from the said X-ray diffraction spectrum. The orientation index [I(002) / I(100)] of the boron nitride powder is calculated using the obtained peak intensities. Since the object of measurement for the orientation index is powder, the value of the orientation index tends to be small when the proportion of aggregated particles (bulbous particles) in the powder, which are substantially not oriented primary particles, is large. On the other hand, the value of the orientation index tends to be large when the proportion of primary particles that do not constitute aggregated particles is large. As an X-ray diffractometer, for example, "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used.

[0044] The Fe content in boron nitride powder is 50 ppm or less, but it can be further reduced depending on the application of the boron nitride powder. The upper limit of the Fe content in boron nitride powder may be, for example, 30 ppm or less, 20 ppm or less, or 10 ppm or less. The lower limit of the Fe content in boron nitride powder is not particularly limited and may be zero (no Fe), but may be, for example, 1 ppm or more, 2 ppm or more, or 3 ppm or more. The Fe content in boron nitride powder may be adjusted within the above range, for example, 1 to 50 ppm or 1 to 10 ppm.

[0045] In this specification, the Fe content refers to the value measured by X-ray fluorescence.

[0046] By preparing boron nitride powder using the manufacturing method described above, impurities such as Fe are reduced, and the powder contains highly crystalline primary particles of boron nitride. The graphitization index (GI) is used as an indicator of the crystallinity of boron nitride powder. The upper limit of the graphitization index of boron nitride powder may be, for example, 2.0 or less, 1.9 or less, 1.8 or less, or 1.7 or less. By having the upper limit of the graphitization index of boron nitride powder within the above range, the primary particles of boron nitride are highly crystalline, and superior heat dissipation can be achieved. The lower limit of the graphitization index of boron nitride powder is not particularly limited, but may be, for example, 0.8 or more, 0.9 or more, or 1.0 or more. By keeping the lower limit of the graphitization index of the boron nitride powder within the above range, the growth of primary boron nitride particles becomes moderate, and the collapse of aggregated particles due to excessive growth of primary particles can be more sufficiently suppressed, thereby suppressing an increase in the orientation index of the boron nitride powder. Due to this effect, resin compositions and molded articles containing boron nitride powder can exhibit more than sufficient heat dissipation for practical purposes. The graphitization index of the boron nitride powder can be adjusted within the above range, for example, 1.0 to 2.0 or 1.0 to 1.7.

[0047] The graphitization index used herein is also known as an index value indicating the degree of crystallinity of graphite (e.g., J. Thomas, et al., J. Am. Chem. Soc. 84, 4619 (1962), etc.). The graphitization index is calculated based on the spectrum measured by powder X-ray diffraction of primary particles of hexagonal boron nitride. First, in the X-ray diffraction spectrum, the area value (in arbitrary units) enclosed by the integrated intensity (i.e., each diffraction peak) of each diffraction peak corresponding to the (100), (101), and (102) planes of the primary particles of hexagonal boron nitride and their baseline is calculated and designated as S100, S101, and S102, respectively. Using the calculated area value, the value of [(S100 + S101) / S102] is calculated to determine the graphitization index. More specifically, it is determined by the method described in the examples herein.

[0048] The upper limit of the average particle size of the boron nitride powder may be adjusted according to the thickness to which it is mixed with the resin and molded, for example, it may be 80 μm or less, 65 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less. The lower limit of the average particle size of the boron nitride powder may be 2 μm or more, 3 μm or more, 5 μm or more, 8 μm or more, or 10 μm or more. By keeping the lower limit of the average particle size within the above range, a boron nitride powder with superior packing properties can be obtained. This further suppresses the increase in viscosity when mixing the boron nitride powder with the resin. The average particle size of the boron nitride powder may be adjusted within the above range, for example, it may be 2 to 80 μm, 3 to 65 μm, or 5 to 50 μm.

[0049] In this specification, the average particle size of boron nitride powder refers to the 50% cumulative diameter (median diameter) in the volume-based cumulative particle size distribution. More specifically, it refers to the particle size (D50) when the cumulative value in the volume-based cumulative particle size distribution obtained by laser diffraction scattering for boron nitride powder reaches 50%. The laser diffraction scattering method is 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 can be used for measurement. For example, the "LS-13 320" (product name) from Beckman Coulter can be used as a laser diffraction scattering particle size distribution analyzer. When measuring, the measurement should be performed in the presence of aggregated particles without homogenizing.

[0050] Although several embodiments have been described above, this disclosure is not limited in any way to the embodiments described above. Furthermore, the descriptions of the embodiments described above are applicable to each other. [Examples]

[0051] The present disclosure will be described in more detail below with reference to examples and comparative examples. However, the present disclosure is not limited to the following examples.

[0052] (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 (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 2200°C for 5 hours in an argon 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 to remove coarse particles, thereby producing boron carbide powder (B4C powder) with an average particle size of 15 μm. The Fe content in the boron carbide powder was 0.3% by mass.

[0053] [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 at 2100°C for 25 hours under a nitrogen gas atmosphere of 0.85 MPa to obtain a calcined product containing boron carbonitride (B4CN4) powder (pressure nitriding process).

[0054] The resulting calcined material was placed in a muffle furnace and heated at 700°C for 5 hours in an atmospheric environment (oxidation process) to obtain a heat-treated product.

[0055] [Preparation of boron nitride powder] Based on the total amount of the above heat-treated material and boric acid (boron source), boric acid was added so that the boron source content reached 30 parts by mass, and the mixture was mixed using a Henschel mixer to obtain a mixture (aggregate). Next, 1.55 kg of the above mixture was filled into a boron nitride container having two through holes (opening diameter: 5.5 cm) on each of its four sides, and a square opening of 15 cm vertically and 15 cm horizontally at the top, while appropriately compressing and molding the mixture, taking care to ensure that the top surface of the mixture was not lower than the upper edge of the through holes on the sides of the container. At this time, the surface area of ​​the above mixture in contact with the atmosphere was 268 cm². 2 / kg(=[π(2.75) 2 The result was (x8 + (15 x 15)] / 1.55).

[0056] Next, the aforementioned container was placed in a resistance heating furnace and subjected to heat treatment by raising the temperature from room temperature to 2000°C and holding it at 2000°C for 5 hours in a nitrogen gas atmosphere at a pressure of 13 kPa (crystallization process). This decarburized the material and synthesized boron nitride powder containing aggregated particles formed by the aggregation of primary particles. The yield was calculated by using the mass ratio after the crystallization process as a reference to the mass of the aggregate before the crystallization process, and it was confirmed to be 66% by mass. 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.

[0057] <Evaluation of boron nitride powder> The boron nitride powder obtained in Example 1 was evaluated for its aggregated particle crushing strength, specific surface area, Fe content, graphitization index, average particle size, and orientation index using the method described later. The results are shown in Table 1.

[0058] [Crushing strength] The crushing strength of the aggregated particles was measured in accordance with JIS R 1639-5:2007 "Fine ceramics - Method for measuring particle properties - Part 5: Crushing strength of single particles". A microcompression tester (manufactured by Shimadzu Corporation, product name "MCT-W500") was used for the measurement. Measurements were performed on 20 or more aggregated particles, and the value was calculated at a cumulative fracture rate of 63.2%.

[0059] [Specific surface area] The specific surface area of ​​the boron nitride powder was calculated using the BET single-point method with nitrogen gas, in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption". The specific surface area measuring device used was the "MONOSORB MS-22" manufactured by QUANTACHROME. The measurement was performed after drying and degassing the boron nitride powder at 300°C for 15 minutes.

[0060] [Fe content] The Fe content in boron nitride powder was measured by X-ray fluorescence spectroscopy. A Rigaku Corporation X-ray fluorescence analyzer (product name: ZSX PrimusII) was used for the measurement.

[0061] [Graphitization Index] 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 corresponding to the (100), (101), and (102) planes of the hexagonal boron nitride primary particles (i.e., each diffraction peak) and its baseline was calculated and designated as S100, S101, and S102, respectively. Using the area values ​​thus calculated, the graphitization index was determined based on the following equation (1). GI = (S100 + S101) / S102 ... (1)

[0062] [Average particle size] The average particle size of the 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 (Beckman Coulter, product name: "LS-13 320") was used for the measurement. The measurement was performed without homogenization, with aggregated particles present.

[0063] [Orientation Index] The orientation index of boron nitride powder was measured according to the following method. An X-ray diffractometer (manufactured by Rigaku Corporation, product name: "ULTIMA-IV") was used for the measurement. First, a measurement sample was prepared by filling a recess with hexagonal boron nitride powder into a recess with a depth of 0.2 mm, which was attached to the X-ray diffractometer, and solidifying it. After irradiating the measurement sample with X-rays and performing baseline correction, the peak intensities of the (002) plane and the (100) plane of the measurement sample were determined, and the ratio [I(002) / I(100)] was taken as the orientation index.

[0064] (Example 2) Boron nitride powder was prepared in the same manner as in Example 1, except that after filling the container with the mixture, five holes with a diameter of 1 cm were made, penetrating from the top surface of the mixture to the bottom surface of the container. At this time, the length from the top surface of the mixture to the bottom surface of the container was 7 cm. In the crystallization process, the surface area of ​​the mixture in contact with the atmosphere was 338 cm². 2 / kg(=[π×7×5+π(2.75) 2 The formula was (×8 + (15 × 15)] / 1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0065] (Example 3) Boron nitride powder was prepared in the same manner as in Example 1, except that the boric acid content was changed to 26 parts by mass during the crystallization process, and that after filling the mixture into a container during the crystallization process, five holes with a diameter of 1 cm were made that penetrated from the top surface of the mixture to the bottom surface of the container. At this time, the length from the top surface of the mixture to the bottom surface of the container was 7 cm. During the crystallization process, the surface area of ​​the above mixture in contact with the atmosphere was 338 cm². 2 / kg(=[π×7×5+π(2.75) 2 The formula was (×8 + (15 × 15)] / 1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0066] (Example 4) Boron nitride powder was prepared in the same manner as in Example 1, except that the boric acid content was changed to 35 parts by mass during the crystallization process. During the crystallization process, the surface area of ​​the above mixture in contact with the atmosphere was 268 cm². 2 / kg(=[π(2.75) 2 The formula was (×8 + (15 × 15)] / 1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0067] (Comparative Example 1) Boron nitride powder was prepared in the same manner as in Example 1, except that, in the crystallization process, a boron nitride container without through holes on the sides and with a square opening of 15 cm by 15 cm on the top was used instead of a boron nitride container with two through holes (opening diameter: 5.5 cm) on each of the four sides and a square opening of 15 cm by 15 cm on the top, and compression molding was not performed when filling the container with the mixture of boron carbonitride and boric acid. In the crystallization process, the surface area of ​​the above mixture in contact with the atmosphere was 145 cm². 2 The yield was / kg (=[15×15] / 1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0068] (Comparative Example 2) Boron nitride powder was prepared in the same manner as in Example 1, except that the boric acid content was changed to 40 parts by mass during the crystallization process, and instead of using a boron nitride container with two through-holes (opening diameter: 5.5 cm) on each of its four sides and a square opening of 15 cm vertically and 15 cm horizontally at the top, a boron nitride container without through-holes on the sides and a square opening of 15 cm vertically and 15 cm horizontally at the top was used, and compression molding was not performed when filling the container with the mixture of boron carbonitride and boric acid. The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0069] (Comparative Example 3) In the crystallization process, a boron nitride container with no through holes on the sides and a square opening of the same size on the top was used instead of a boron nitride container with two through holes (opening diameter: 5.5 cm) on each of its four sides and a square opening of the same size on the top (15 cm vertically and 15 cm horizontally) at the top. Compression molding was not performed when filling the container with the mixture of boron carbonitride and boric acid. After filling the container with the mixture, five holes with a diameter of 1 cm were drilled, penetrating from the top of the mixture to the bottom of the container. In this case, the length from the top of the mixture to the bottom of the container was 7.5 cm. In the crystallization process, the surface area of ​​the mixture in contact with the atmosphere was approximately 221 cm². 2 The result was / kg (=[(π×7·5×5)+(15×15)] / 1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0070] (Reference example 1) For reference, boron nitride powder was produced using high-grade boron carbide with reduced Fe content as a raw material, based on a conventional manufacturing method. Specifically, boron carbide with an Fe content of 0.1 mass% was used, and instead of a boron nitride container with two through holes (opening diameter: 5.5 cm) on each of its four sides and a square opening of 15 cm vertically and 15 cm horizontally at the top, a boron nitride container without through holes on the sides and a square opening of 15 cm vertically and 15 cm horizontally at the top was used, and compression molding was not performed when filling the container with the mixture of boron carbonitride and boric acid. The boron nitride powder was prepared in the same manner as in Example 1. The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0071] [Table 1]

[0072] [Table 2] [Industrial applicability]

[0073] According to this disclosure, a method for producing boron nitride powder that can produce boron nitride powder with excellent insulating properties in high yield, even when using low-grade boron carbide as a raw material, is available. Furthermore, according to this disclosure, boron nitride powder that exhibits excellent insulating properties when used as a filler in resin is also available.

Claims

1. A pressurized nitriding process is performed to obtain a calcined product by calcining boron carbide powder under a nitrogen pressurized atmosphere, The aforementioned calcined product is heated in an atmosphere where the oxygen partial pressure is 20% or more to obtain a heat-treated product in an oxidation step, The process includes a crystallization step in which an aggregate containing the heat-treated material and a boron source is heated in a nitrogen-containing atmosphere to generate primary boron nitride particles, and aggregated particles are obtained by the aggregation of the primary particles. The apparent surface area of ​​the aggregate exposed to the nitrogen-containing atmosphere is 250 cm². ２ / kg or more, A method for producing boron nitride powder, wherein the content of the boron source is 35% by mass or less, based on the total amount of the heat-treated product and the boron source.

2. The manufacturing method according to claim 1, wherein the aggregate is housed in a container, and the container has through holes that penetrate to the inside and outside of the container.

3. The manufacturing method according to claim 1 or 2, wherein the Fe content of the boron carbide powder is 0.2% by mass or more.

4. It contains aggregated particles composed of aggregated primary particles of boron nitride, The crushing strength of the aggregated particles is 12 MPa or more. Specific surface area is 4.0 m² ２ The amount is greater than or equal to / g, and the orientation index is 7.5 or less. The Fe content is 50 ppm or less. Boron nitride powder with a graphitization index of 2.0 or less.

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