JP2023120878A - Method for producing boron nitride powder, and boron nitride powder - Google Patents

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

One aspect of the present disclosure relates to a method of making boron nitride powder and a boron nitride powder.

Boron nitride powder has lubricating properties, high thermal conductivity, insulating properties, and the like, and is widely used for applications such as solid lubricants, thermally conductive fillers, and insulating fillers. In recent years, the above-described boron nitride is required to have excellent thermal conductivity due to high performance of electronic equipment and the like.

For example, in Patent Document 1, when used as a filler for an insulating heat dissipating material such as resin, hexagonal boron nitride powder and its A manufacturing method has been proposed.

Hexagonal boron nitride powder is decarburized by, for example, a pressurized nitriding step of nitriding boron carbide in a pressurized atmosphere containing nitrogen to obtain a nitride, and mixing the nitride with a boron source or the like and heat-treating it. It is manufactured by a manufacturing method having a decarburization and crystallization step of obtaining hexagonal boron nitride by proceeding with crystallization (for example, Patent Document 2, etc.). The above-described manufacturing method is intended to improve decarburization performance in subsequent steps by nitriding boron carbide once.

JP 2019-116401 A WO2019/073690

When boron nitride powder is used as an insulating filler or the like, it is required to reduce conductive impurities contained in the boron nitride powder itself. Therefore, a method of using boron carbide with a sufficiently reduced iron content as a raw material is adopted. However, it is not necessarily easy to reduce the iron content in boron carbide, which tends to increase raw material prices and increase the production cost of boron nitride powder. In recent years, the demand for insulating fillers has increased, and against the background of the demand for higher insulating properties from boron nitride powder, low-grade It would be beneficial to have a method for producing boron nitride powder that can be used as-is and can exhibit performance equal to or better than conventional powders.

An object of the present disclosure is to provide a method for producing boron nitride powder that can produce boron nitride powder that can exhibit excellent insulating properties with high yield even when low-grade boron carbide is used as a raw material. and Another object of the present disclosure is to provide a boron nitride powder that can exhibit excellent insulating properties when used by being filled in a resin.

One aspect of the present disclosure is a pressurized nitriding step of firing boron carbide powder in a nitrogen pressurized atmosphere to obtain a fired product, and heating the fired product in an atmosphere with an oxygen partial pressure of 20% or more. and an oxidation step for obtaining a heat-treated product, and heating an assembly containing the heat-treated product and the boron source in an atmosphere containing nitrogen to generate primary particles of boron nitride, and the primary particles are aggregated. and a crystallization step of obtaining agglomerated particles composed of the above, wherein the aggregate has an apparent surface area of 250 cm ² /kg or more in contact with an atmosphere containing nitrogen.

In the method for producing the boron nitride powder described above, the boron carbide powder is nitrided and then fired in an oxygen-containing atmosphere to reduce the carbon content in the fired product and obtain the heat-treated product. Thereafter, when the aggregate is prepared by blending the boron source with the heat-treated product and heat-treated, in the production method according to the present disclosure, the area of the aggregate in contact with the atmosphere containing nitrogen is a predetermined or more. is adjusted to be By adjusting in this way, impurities such as carbon and iron volatilize from the aggregates and can be more easily moved out of the system in which boron nitride crystals grow. At this time, part of the liquid phase formed by the boron source is also moderately removed. Since this liquid phase is a growth field that promotes the growth of primary particles of boron nitride, by removing part of the liquid phase, excessive growth of primary particles is suppressed, and deformation and collapse of aggregated particles are reduced. can be Due to this action, the boron nitride powder obtained has a reduced content of impurities such as iron, and can be expected to have excellent insulating properties. , collapse during kneading with a resin or the like is suppressed, and can be suitably used as a heat-dissipating filler.

The assembly may be housed in a container, and the container may have a through hole penetrating to the inside and outside of the container. By using a container having through holes, handling of the aggregate is facilitated.

The Fe content of the boron carbide powder may be 0.2% by mass or more. Boron carbide powder generally contains impurities such as iron. Therefore, in order to obtain boron nitride having excellent insulating properties, a boron carbide powder in which the content of impurities such as iron is reduced is used. Therefore, the production cost of boron nitride powder tends to increase. On the other hand, in the case of the method for producing boron nitride powder according to the present disclosure, the crystallization process is performed in an environment where impurities are easily released to the outside of the system, so that low-grade products containing relatively many impurities in the raw material are produced. Even if it is used, it is possible to easily produce a boron nitride powder having a quality equal to or higher than that of the conventional one.

In the 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 process within the above range, the amount of the liquid phase, which is the growth field of the primary particles of boron nitride, is reduced, the excessive growth of the primary particles is suppressed, and the crushing strength is improved. Boron nitride with better agglomerated particles can be produced.

One aspect of the present disclosure includes aggregated particles formed by aggregating primary particles of boron nitride, the aggregated particles have a crushing strength of 12 MPa or more and a specific surface area of 4.0 m ² /g or more, Provided is a boron nitride powder having an orientation index of 7.5 or less and an Fe content of 50 ppm or less.

The boron nitride powder has a low iron (Fe) content, and the primary particles of boron nitride have grown to an appropriate size, and the specific surface area is also large. Also, the boron nitride powder has a reduced proportion of primary particles that have undergone excessive growth that can lead to collapse of the agglomerated particles. For this reason, the agglomerated particles contained in the boron nitride powder have excellent crushing strength. The boron nitride powder can be suitably used as a heat dissipating filler that is kneaded with a resin, and can exhibit excellent insulating properties.

The boron nitride powder may have a graphitization index of 2.0 or less. When the graphitization index is within the above range, the heat dissipation property can be further improved.

According to the present disclosure, even when using low-grade boron carbide as a raw material, it is possible to provide a method for producing boron nitride powder that can produce boron nitride powder that can exhibit excellent insulating properties with high yield. According to the present disclosure, it is also possible to provide a boron nitride powder that can exhibit excellent insulating properties when used by being filled in a resin.

Embodiments of the present disclosure will be described below. However, the following embodiments are examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents.

The materials exemplified in this specification can be used singly or in combination of two or more unless otherwise specified. The content of each component in the composition means the total amount of the multiple substances present in the composition unless otherwise specified when there are multiple substances corresponding to each component in the composition. .

One embodiment of the method for producing boron nitride powder includes a pressurized nitriding step of firing boron carbide powder in a nitrogen pressurized atmosphere to obtain a fired product, and a pressure nitriding step in which the fired product has an oxygen partial pressure of 20% or more. An oxidation step for obtaining a heat-treated product by heating in an atmosphere, and heating an assembly containing the heat-treated product and a boron source in an atmosphere containing nitrogen to generate primary particles of boron nitride, and a crystallization step of obtaining aggregated particles composed of aggregated primary particles. In the crystallization step, the aggregate has an apparent surface area of 250 cm ² /kg or more in contact with the nitrogen-containing atmosphere.

The boron carbide powder (B ₄ C powder) used in the pressure nitriding step may have a relatively large iron (Fe) content, and the iron (Fe) content may not necessarily increase due to pretreatment or the like. It is not necessary to use the reduced one. Generally available boron carbide powder has an Fe content of about 0.2 to 0.5% by mass. The upper limit of the Fe content in the boron carbide powder used in the production method according to the present disclosure may be, for example, 0.5% by 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% by mass or more, or 0.3% by mass or more. When the lower limit of the Fe content of the boron carbide powder is within the above range, the effect of the production method according to the present disclosure is more pronounced.

The upper limit of the average particle size of the 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 powder having a small average particle size as the raw material powder, the efficiency of reducing the impurity content in the subsequent crystallization step can be further improved. The lower limit of the average particle size of the boron carbide powder may be, for example, 1 μm or more, 2 μm or more, or 5 μm or more. When the lower limit of the average particle size of the boron carbide powder is within the above range, the orientation index of the boron nitride powder can be further reduced, and the heat dissipation can be exhibited more fully.

The average particle size of aggregated particles in this specification means the 50% cumulative size (median size) in the volume-based cumulative particle size distribution. More specifically, it means the particle diameter (D50) when the cumulative value in the volume-based cumulative particle size distribution obtained by the laser diffraction scattering method for boron nitride powder reaches 50%. The laser diffraction scattering method is measured according to the method described in JIS Z 8825:2013 "Particle size analysis-laser diffraction/scattering method". For the measurement, a laser diffraction scattering particle size distribution analyzer or the like can be used. As a laser diffraction scattering method particle size distribution analyzer, for example, "LS-13 320" (product name) manufactured by Beckman Coulter can be used.

In the pressure nitriding step, the boron carbide powder is fired in a pressurized nitrogen atmosphere to obtain a fired product containing the boron carbonitride powder (B ₄ CN ₄ powder). The lower limit of the firing temperature in the pressurized nitriding step may be 2000° C. or higher, or 2100° C. or higher. By setting the lower limit of the firing temperature within the above range, the crystallinity of the obtained boron carbonitride can be enhanced, and the proportion of hexagonal boron carbonitride can be increased. By increasing the proportion of hexagonal boron carbonitride in the pressure nitriding step, the thermal conductivity of the boron nitride powder obtained later can be further improved. The upper limit of the firing temperature in the pressurized nitriding step may be 2300° C. or lower, or 2250° C. or lower. The firing temperature may be adjusted within the above range, and may be, for example, 2000-2300.degree.

The lower limit of the pressure (atmospheric pressure) in the pressurized nitriding step may be, for example, 0.6 MPa or more, 0.7 MPa or more, or 0.8 MPa or more. By setting the lower limit of the pressure in the pressurized nitriding step within the above range, the nitriding of boron carbide can be sufficiently advanced. The upper limit of the pressure (atmospheric pressure) in the pressurized nitriding step may be, for example, 1.0 MPa or less, or 0.9 MPa or less. By setting the upper limit of the pressure in the pressurized nitriding step within the above range, it is possible to suppress an increase in the manufacturing cost of the boron nitride powder. The pressure may be adjusted within the ranges mentioned above, and may be, for example, 0.6-1.0 MPa, or 0.8-0.9 MPa.

The nitrogen gas concentration of the nitrogen pressurized atmosphere in the pressurized nitriding step 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 in the pressurized nitriding step is not particularly limited as long as the nitridation progresses sufficiently, and may be, for example, 6 to 30 hours, 8 to 25 hours, or 10 to 20 hours.

The oxidation step is a step of decarburizing part of the carbon contained in the fired product by firing the fired product containing the boron carbonitride powder in an oxygen-containing atmosphere. By reducing the amount used and suppressing the ratio of the liquid phase in the crystallization step, it is possible to obtain a boron nitride powder having a high tap density and excellent crushing strength of aggregated particles.

The atmosphere containing oxygen in the oxidation step may be an atmosphere having an oxygen partial pressure of 20% or more, such as the air. The upper limit of the oxygen partial pressure may be, for example, 70% or less, 60% or less, or 50% or less. In addition, the oxygen partial pressure in this specification means the partial pressure of oxygen in the standard state of the gas occupying the atmosphere, and is a value measured by an oxygen concentration meter. As the oxygen partial pressure meter, for example, "G1690" (product name) manufactured by Sashita Corporation can be used.

The pressure (atmospheric pressure) in the oxidation step may be, for example, 0.1 to 0.5 MPa, 0.1 to 0.3 MPa, or 0.1 to 0.2 MPa, and is atmospheric pressure (0.1 MPa). you can

The lower limit of the firing temperature in the oxidation step may be, for example, 600° C. or higher, 650° C. or higher, or 700° C. or higher. When the lower limit of the firing temperature is 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 step may be, for example, 1000° C. or lower, or 950° C. or lower. When the upper limit of the firing temperature is within the above range, oxidation of boron carbonitride after firing can be suppressed.

The lower limit of the baking time in the oxidation step may be, for example, 6 hours or longer, or 7 hours or longer. When the lower limit of the firing time is within the above range, it is possible to sufficiently reduce the carbon content in the boron carbonitride. Also, the upper limit of the baking time in the oxidation step is not particularly limited, but may be, for example, 20 hours or less, or 15 hours or less. When the upper limit of the baking time is within the above range, it is possible to more sufficiently suppress the decrease in the treatment efficiency of the step.

In the crystallization step, by heating an aggregate containing the heat-treated product and the boron source in an atmosphere containing nitrogen, primary particles of boron nitride are generated, and the primary particles are aggregated to form aggregates. get the particles. Here, the crystallization step is carried out by adjusting the area of the aggregates in contact with the atmosphere so as to facilitate the removal of desorbed gas and the like.

In the crystallization step, adjustment is made so that the area of the aggregate that is in contact with the atmosphere containing nitrogen is increased. The apparent surface area of the above aggregates in contact with the nitrogen-containing atmosphere is 250 cm ² /kg or more, but may be adjusted to be larger from the viewpoint of obtaining a boron nitride powder with further improved insulating properties. The lower limit of the apparent surface area may be, for example, 250 cm ² /kg or more, 280 cm ² /kg or more, 300 cm ² /kg or more, or 330 cm ² /kg or more. The upper limit 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} . kg or less. By setting the apparent surface area within the above range, volatilization of carbon and iron in the aggregate is promoted, and boron nitride powder with superior 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. In addition, a molded article obtained by blending a binder in the aggregate or compression molding the aggregate is allowed to stand on its own to reduce the area in contact with the wall of the container of the molded article, or the aggregate including the molded article It can also be controlled by changing the shape of the object (for example, providing a recess or a through hole in the assembly).

The apparent surface area in this specification means the area in contact with the atmosphere when the surface of the aggregate is regarded as a flat surface without considering the fine structure (unevenness of 500 μm or less, etc.) on the surface. When the aggregate is contained in a container and filled so as to contact the inner wall of the container, the apparent surface area is calculated by measuring the opening of the container and the outer circumference and inner diameter of the through hole provided in the container. It shall be. In addition, if the aggregate can maintain its shape by itself, and if it is not housed in a container or is placed within the container at a distance from the inner wall of the container, the outline of the aggregate is measured. and calculate its apparent specific surface area. Although the production method of the present disclosure assumes that the mass of the aggregate is as large as 1 kg or more, the mass of the aggregate may be less than 1 kg, and in that case the apparent surface area is per 1 kg is the conversion value of

The aggregate may be housed in a container. In this case, the container may have a through hole penetrating between the inside and the outside of the container. The number of through-holes and the diameter of the through-holes may be adjusted according to the desired properties of the boron nitride powder. In order to improve the insulation properties, it is desirable that the number of through-holes is large and the diameter of the through-holes is large. desirable. The container should be excellent in heat resistance and have low reactivity with the aggregate to be accommodated, and may be, for example, a ceramic container.

In the crystallization step, it is preferable to keep the content of the boron source low. The upper limit of the content of the boron source is, 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, based on the total amount of the heat-treated product and the boron source. or less, or 25% by mass or less. By setting the upper limit of the content of the boron source 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. As a result, excessive growth of the primary particles can be suppressed, and boron nitride containing agglomerated particles with superior crushing strength can be produced. The lower limit of the content of the boron source 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 content of the boron source within the above range, the crystallinity of boron nitride can be further improved, and the primary particles of hexagonal boron nitride having higher purity can be obtained. The content of the boron source may be adjusted within the above-described range, for example, 15 to 40% by mass, 15 to 28% by mass, or 18 to 25% by mass, based on the total amount of the heat-treated product and the boron source %.

The manufacturing method described above may further include other steps in addition to the pressure nitriding step, the oxidation step, and the crystallization step. Other steps include, for example, a pulverization step and a classification step. The pulverization step is desirably carried out, for example, from the standpoint of crushing tertiary agglomeration of agglomerated particles in the crystallization step. In the pulverization step, a general pulverizer or pulverizer can be used. For example, a ball mill, vibrating mill, jet mill or the like can be used. In the present disclosure, "pulverization" also includes "crushing". The average particle size of the boron nitride powder may be adjusted by grinding and classification.

The boron nitride powder produced by the above-described production method has a suppressed Fe content even when a low-grade raw material is used. In addition, by appropriately adjusting the amount of the liquid phase derived from the boron source in the crystallization step, aggregated particles having excellent crushing strength can be included. One embodiment of the boron nitride powder comprises agglomerated particles composed of agglomerated primary particles of boron nitride. The boron nitride powder has a crushing strength of the aggregated particles of 12 MPa or more, a specific surface area of 4.0 m ² /g or more, and an orientation index of 7.5 or less. The content of Fe in the boron nitride powder is 50 ppm or less.

The lower limit of the crushing strength of the aggregated particles is 10 MPa or more, and may be, for example, 11 MPa or more, 12 MPa or more, or 13 MPa or more. When the lower limit of the crushing strength is within the above range, the heat dissipation properties of the resin composition and the molded article obtained by kneading with the resin can be further improved. The upper limit of crushing strength of aggregated particles may be, for example, 30 MPa or less, 25 MPa or less, 23 MPa or less, or 20 MPa or less. When the upper limit of the crushing strength is within the above range, at least a part of the aggregated particles is appropriately collapsed during kneading with the resin, and the resin composition and the molded product obtained by suppressing the generation of voids. can further improve the insulation. The crush strength of the agglomerated particles may be adjusted within the ranges described above, and may be, for example, 10-30 MPa, 11-25 MPa, or 12-20 MPa.

The crushing strength herein is measured in accordance with the description of JIS R 1639-5:2007 "Fine ceramics-Method for measuring (granular) grain properties-Part 5: Single grain crushing strength". value. The crushing strength σ (unit: MPa) of one aggregated particle is the dimensionless number α (α = 2.48) that changes depending on the position in the aggregated particle, the crushing test force P (unit: N), and the particle diameter d ( unit: μm), it is calculated using the formula σ=α×P/(π×d ² ). The measurement shall be performed on 20 or more aggregated particles, and the value at the time when the cumulative destruction rate is 63.2% shall be calculated. A microcompression tester can be used for the measurement. As the microcompression tester, for example, "MCT-W500" (product name) manufactured by Shimadzu Corporation can be used.

The lower limit of the specific ^surface area of the boron ^nitride powder is 4.0 ^{m 2} /g or more. It may be 0 m ² /g or more. When the lower limit of the specific surface area is within the above range, the aggregation of the primary particles of boron nitride 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 the boron nitride powder is kneaded with resin or molded. The upper limit of the specific surface area of the boron nitride powder may be, for example, 7.0 m ² /g or less, 6.5 m ² /g or less, 6.0 m ² /g or less, or 5.5 m ² /g or less. When the upper limit of the specific surface area is within the above range, the primary particle size of the boron nitride is sufficiently large, and when the resin composition and molded article are formed, more excellent heat dissipation can be exhibited. The specific surface area of the boron nitride powder may be adjusted within the above range, for example, 4.0-7.0 m ² /g, 4.2-6.5 ^{m 2} /g, or 4.3-6.0 m ² /g.

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

The boron nitride powder has sufficiently suppressed orientation of the primary particles. The upper limit of the orientation index of the boron nitride powder is 7.5 or less, and may be, for example, 7.3 or less, 7.1 or less, or 7.0 or less. When the upper limit of the orientation index is within the above range, the resin composition and molded article can exhibit practically sufficient heat dissipation. The lower limit of the orientation index of the boron nitride powder is not limited, but it is not easy to make it less than 6.0, for example, 6.0 or more, 6.1 or more, 6.3 or more, or 6 0.5 or more. The orientation index of the boron nitride powder may be adjusted within the ranges described above, for example, 6.0 to 7.5, 6.3 to 7.3, or 6.5 to 7.1.

The orientation index in this specification means a value measured according to the following method. By performing X-ray diffraction measurement on the boron nitride powder, the X-ray diffraction spectrum of the boron nitride powder is obtained, and from the X-ray diffraction spectrum, the peak intensity I (002) corresponding to the (002) plane and the (100) plane and I(100). The obtained peak intensity is used to calculate the orientation index [I(002)/I(100)] of the boron nitride powder. Since the object of measurement of the orientation index is powder, the value of the orientation index tends to be small when the existence ratio of aggregated particles (aggregated particles) in which the primary particles are not substantially oriented is large in the powder. . On the other hand, when the proportion of primary particles that do not constitute aggregated particles increases, the value of the orientation index tends to increase. As the X-ray diffractometer, for example, "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used.

The content of Fe in the boron nitride powder is 50 ppm or less, but it can be further reduced depending on the use of the boron nitride powder. The upper limit of the Fe content in the 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 the boron nitride powder is not particularly limited and may be zero (not including Fe), but may be, for example, 1 ppm or more, 2 ppm or more, or 3 ppm or more. The Fe content in the boron nitride powder may be adjusted within the above range, and may be, for example, 1-50 ppm, or 1-10 ppm.

Content of Fe in this specification means the value measured by the fluorescent X-ray method.

The boron nitride powder is prepared by the above-described production method, so that impurities such as Fe are reduced and contains primary particles of boron nitride with high crystallinity. A graphitization index (G.I.) is used as an index of the crystallinity of the boron nitride powder. The upper limit of the graphitization index of the boron nitride powder may be, for example, 2.0 or less, 1.9 or less, 1.8 or less, or 1.7 or less. When the upper limit of the graphitization index of the boron nitride powder is within the above range, the primary particles of boron nitride have high crystallinity, and excellent heat dissipation can be exhibited. Although the lower limit of the graphitization index of the boron nitride powder is not particularly limited, it may be, for example, 0.8 or more, 0.9 or more, or 1.0 or more. When the lower limit of the graphitization index of the boron nitride powder is within the above range, the growth of the primary particles of boron nitride becomes moderate, and the collapse of aggregated particles due to excessive growth of the primary particles can be more sufficiently suppressed. Therefore, an increase in the orientation index of the boron nitride powder can be suppressed. Due to such an action, the resin composition and the molded article containing the boron nitride powder can exhibit sufficient heat dissipation from a practical point of view. The graphitization index of the boron nitride powder can be adjusted within the ranges described above, and can be, for example, 1.0 to 2.0, or 1.0 to 1.7.

The graphitization index herein is an index also known as an index value indicating the degree of crystallinity of graphite (for example, J. Thomas, et. al, J. Am. Chem. Soc. 84, 4619 (1962) etc.). The graphitization index is calculated based on the spectrum of primary particles of hexagonal boron nitride measured by powder X-ray diffraction. First, in the X-ray diffraction spectrum, the integrated intensity of each diffraction peak corresponding to the (100) plane, (101) plane and (102) plane of the hexagonal boron nitride primary particles (that is, each diffraction peak) and its baseline and 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 of this specification.

The upper limit of the average particle size of the boron nitride powder may be adjusted according to the thickness to be mixed with the resin and molded. good. The lower limit of the average particle size of the boron nitride powder may be, for example, 2 μm or more, 3 μm or more, 5 μm or more, 8 μm or more, or 10 μm or more. When the lower limit of the average particle size is within the above range, the boron nitride powder can have excellent filling properties. This makes it possible to further suppress an increase in viscosity when the boron nitride powder and the resin are mixed. The average particle size of the boron nitride powder may be adjusted within the ranges described above, and may be, for example, 2-80 μm, 3-65 μm, or 5-50 μm.

The average particle size of the boron nitride powder in this specification means the 50% cumulative size (median size) in the volume-based cumulative particle size distribution. More specifically, it means the particle diameter (D50) when the cumulative value in the volume-based cumulative particle size distribution obtained by the laser diffraction scattering method for boron nitride powder reaches 50%. The laser diffraction scattering method is measured according to the method described in JIS Z 8825:2013 "Particle size analysis-laser diffraction/scattering method". For the measurement, a laser diffraction scattering particle size distribution analyzer or the like can be used. As a laser diffraction scattering method particle size distribution analyzer, for example, "LS-13 320" (product name) manufactured by Beckman Coulter can be used. In addition, the measurement shall be performed in the presence of agglomerated particles without treatment by a homogenizer.

Although several embodiments have been described above, the present disclosure is not limited to the above embodiments. Also, the descriptions of the above-described embodiments can be applied to each other.

Hereinafter, the present disclosure will be described in more detail using examples and comparative examples. It should be noted that the present disclosure is not limited to the following examples.

(Example 1)
[Production of boron carbide powder]
After mixing 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.) using a Henschel mixer, a graphite crucible was added. filled inside. This graphite crucible was placed in an arc furnace and heated at 2200° C. for 5 hours in an argon atmosphere to synthesize massive boron carbide (B ₄ C) powder. The synthesized massive boron carbide powder was pulverized in a ball mill for 1 hour and sieved using a sieve to remove coarse particles, thereby producing a boron carbide powder (B ₄ C powder) having an average particle size of 15 μm. . The content of Fe in the boron carbide powder was 0.3% by mass.

[Preparation of boron carbonitride powder]
A boron nitride crucible was filled with the produced boron carbide powder. The crucible was placed in a resistance heating furnace and heated at 2100° C. for 25 hours in a nitrogen gas atmosphere of 0.85 MPa to obtain a fired product containing boron carbonitride (B ₄ CN ₄ ) powder. (pressurized nitriding process).

The obtained fired product was placed in a muffle furnace and heated at 700° C. for 5 hours in an air atmosphere (oxidation step) to obtain a heat-treated product.

[Production of boron nitride powder]
Based on the total amount of the heat-treated product and boric acid (boron source), boric acid is added so that the content of boric acid as a boron source is 30 parts by mass, mixed with a Henschel mixer, and the mixture ( aggregates) were obtained. Next, for a boron nitride container having two through holes (opening diameter: 5.5 cm) on each of the four sides and a square opening with a length of 15 cm and a width of 15 cm at the top 1.55 kg of the above mixture was filled while moderately compressing the mixture while taking care that the upper surface of the mixture was not positioned below the upper end of the through-hole on the side of the container. At this time, the surface area of the mixture in contact with the atmosphere was 268 cm ² /kg (=[π(2.75) ² ×8+(15×15)]/1.55).

Next, the above container was placed in a resistance heating furnace, heated from room temperature to 2000° C. in a nitrogen gas atmosphere at a pressure of 13 kPa, and held at 2000° C. for 5 hours for heat treatment (crystal conversion process). By this decarburization, a boron nitride powder having agglomerated particles in which primary particles were agglomerated was synthesized. The mass ratio after the crystallization process based on the mass of the aggregate before the crystallization process was calculated as the yield, and it was confirmed to be 66% by mass. The synthesized boron nitride powder was pulverized with a mortar for 10 minutes, and then classified with a nylon sieve having a sieve mesh of 75 μm.

<Evaluation of boron nitride powder>
The boron nitride powder obtained in Example 1 was evaluated for aggregated particle crushing strength, specific surface area, Fe content, graphitization index, average particle size, and orientation index by the methods described later. Table 1 shows the results.

[Crushing strength]
The crushing strength of agglomerated particles was measured according to the description of JIS R 1639-5:2007 "Fine ceramics-Method for measuring (granule) properties-Part 5: Single granule crushing strength". For the measurement, a microcompression tester (manufactured by Shimadzu Corporation, product name "MCT-W500") was used. In addition, the measurement was performed for 20 or more aggregated particles, and the value at the time of cumulative destruction rate of 63.2% was calculated.

[Specific surface area]
The specific surface area of the boron nitride powder was calculated according to the description of JIS Z 8830:2013 "Method for measuring specific surface area of powder (solid) by gas adsorption", applying the BET single-point method using nitrogen gas. As a specific surface area measuring device, "MONOSORB MS-22 type" (product name) manufactured by QUANTACHROME was used. The measurement was performed after the boron nitride powder was dried and degassed at 300° C. for 15 minutes.

[Content of Fe]
The Fe content in the boron nitride powder was measured by a fluorescent X-ray method. For the measurement, a fluorescent X-ray spectrometer manufactured by Rigaku Corporation (product name: ZSX Primus II) was used.

[Graphitization index]
The graphitization index of the boron nitride powder was calculated from the measurement results by the powder X-ray diffraction method. In the obtained X-ray diffraction spectrum, the integrated intensity of each diffraction peak corresponding to the (100) plane, (101) plane and (102) plane of the hexagonal boron nitride primary particles (that is, each diffraction peak) and its base The area values (in arbitrary units) surrounded by the lines were calculated and designated as S100, S101, and S102, respectively. Using the area value thus calculated, the graphitization index was determined based on the following formula (1).
GI=(S100+S101)/S102 (1)

[Average particle size]
The average particle size of the boron nitride powder was measured according to the method described in JIS Z 8825:2013 "Particle size analysis-laser diffraction/scattering method". For the measurement, a laser diffraction scattering method particle size distribution analyzer (manufactured by Beckman Coulter, product name: "LS-13 320") was used. The measurement was performed in the presence of agglomerated particles without treatment with a homogenizer.

[Orientation index]
The orientation index of the boron nitride powder was measured according to the following method. An X-ray diffractometer (manufactured by Rigaku Co., Ltd., trade name: "ULTIMA-IV") was used for the measurement. First, hexagonal boron nitride powder was filled into recesses of a glass cell having recesses of 0.2 mm in depth attached to an X-ray diffractometer, and solidified to prepare a measurement sample. 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 are determined, and this ratio [I (002) / I (100 )] was taken as the orientation index.

(Example 2)
Boron nitride powder was prepared in the same manner as in Example 1, except that in the crystallization step, after the mixture was filled in the container, five holes with a diameter of 1 cm penetrating from the top surface of the mixture to the bottom surface of the container were made. . At this time, the length from the upper 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 cm ² /kg (=[π×7×5+π(2.75) ² ×8+(15×15)]/1.55). . The obtained boron nitride powder was evaluated in the same manner as in Example 1. Table 1 shows the results.

(Example 3)
In the crystallization process, the content of boric acid was changed to 26 parts by mass. In the crystallization process, after the mixture was filled in the container, five holes with a diameter of 1 cm penetrating from the top surface of the mixture to the bottom surface of the container were made. A boron nitride powder was prepared in the same manner as in Example 1, except for the above. At this time, the length from the upper 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 cm ² /kg (=[π×7×5+π(2.75) ² ×8+(15×15)]/1.55). . The obtained boron nitride powder was evaluated in the same manner as in Example 1. Table 1 shows the results.

(Example 4)
A boron nitride powder was prepared in the same manner as in Example 1, except that the content of boric acid was changed to 35 parts by mass in the crystallization step. In the crystallization process, the surface area of the mixture in contact with the atmosphere was 268 cm ² /kg (=[π(2.75) ² ×8+(15×15)]/1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. Table 1 shows the results.

(Comparative example 1)
In the crystallization process, a boron nitride container having two through-holes (opening diameter: 5.5 cm) on each of four sides and a square opening of 15 cm long and 15 cm wide at the top. Instead of using a boron nitride container having a square opening with a length of 15 cm and a width of 15 cm at the top without through holes on the side, and a mixture of boron carbonitride and boric acid A boron nitride powder was prepared in the same manner as in Example 1, except that compression molding was not performed when filling the container. In the crystallization process, the surface area of the mixture in contact with the atmosphere was 145 cm ² /kg (=[15×15]/1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. Table 2 shows the results.

(Comparative example 2)
In the crystallization process, the content of boric acid was changed to 40 parts by mass, and two through holes (opening diameter: 5.5 cm) were provided on each of the four sides, and the top had a length of 15 cm. , A boron nitride container having a square opening of 15 cm in width and a square opening of 15 cm in length and 15 cm in width at the top without through-holes on the side. Boron nitride powder was prepared in the same manner as in Example 1, except that the mixture of boron carbonitride and boric acid was not compression molded when filling the container. The obtained boron nitride powder was evaluated in the same manner as in Example 1. Table 2 shows the results.

(Comparative Example 3)
In the crystallization process, a boron nitride container having two through-holes (opening diameter: 5.5 cm) on each of four sides and a square opening of 15 cm long and 15 cm wide at the top. Instead of using a boron nitride container having a square opening with a length of 15 cm and a width of 15 cm at the top without through holes on the side, a mixture of boron carbonitride and boric acid was used in the container In the same manner as in Example 1, except that compression molding was not performed when filling the container, and five holes with a diameter of 1 cm penetrating from the top surface of the mixture to the bottom surface of the container were made after filling the mixture into the container. to prepare a boron nitride powder. At this time, the length from the upper surface of the mixture to the bottom surface of the container was 7.5 cm. In the crystallization process, the surface area of the mixture in contact with the atmosphere was about 221 cm ² /kg (=[(π×7·5×5)+(15×15)]/1.55). The obtained boron nitride powder was evaluated in the same manner as in Example 1. Table 2 shows the results.

(Reference example 1)
For reference, boron nitride powder was produced according to a conventional production method using high-grade boron carbide with a reduced Fe content as a raw material. Specifically, it uses boron carbide with an Fe content of 0.1% by mass, and has two through holes (opening diameter: 5.5 cm) on each of the four side surfaces, and Change to a boron nitride container having a square opening of 15 cm long and 15 cm wide at the top, without through holes on the side, and having a square opening of 15 cm long and 15 cm wide at the top. Boron nitride powder was prepared in the same manner as in Example 1, except that a container made of boron nitride was used and the mixture of boron carbonitride and boric acid was not compression molded when filling the container. . The obtained boron nitride powder was evaluated in the same manner as in Example 1. Table 2 shows the results.

According to the present disclosure, even when using low-grade boron carbide as a raw material, it is possible to provide a method for producing boron nitride powder that can produce boron nitride powder that can exhibit excellent insulating properties with high yield. According to the present disclosure, it is also possible to provide a boron nitride powder that can exhibit excellent insulating properties when used by being filled in a resin.

Claims (6)

a pressurized nitriding step of firing the boron carbide powder in a pressurized nitrogen atmosphere to obtain a fired product;
an oxidation step of heating the fired product in an atmosphere having an oxygen partial pressure of 20% or higher to obtain a heat-treated product;
Crystallization to obtain aggregated particles formed by aggregating the primary particles by heating the aggregate containing the heat-treated material and the boron source in an atmosphere containing nitrogen to produce primary particles of boron nitride. and
A method for producing boron nitride powder, wherein the aggregate has an apparent surface area in contact with an atmosphere containing nitrogen of 250 cm ² /kg or more. 2. The manufacturing method according to claim 1, wherein the assembly is housed in a container, and the container has a through hole penetrating to the inside and outside of the container. 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. The production method according to any one of claims 1 to 3, wherein the content of the boron source is less than 40% by mass based on the total amount of the heat-treated product and the boron source. Containing agglomerated particles composed of agglomerated primary particles of boron nitride,
The crushing strength of the aggregated particles is 12 MPa or more,
a specific surface area of 4.0 m ² /g or more and an orientation index of 7.5 or less;
A boron nitride powder having an Fe content of 50 ppm or less. 6. Boron nitride powder according to claim 5, having a graphitization index of 2.0 or less.