JP7832035B2 - Boron nitride powder - Google Patents

Description

This disclosure relates to boron nitride powder.

In electronic components such as power devices, transistors, thyristors, and CPUs, efficiently dissipating the heat generated during use is a challenge. Conventionally, this challenge has been addressed by increasing the thermal conductivity of the insulating layer of the printed circuit board on which the electronic components are mounted, or by attaching the electronic components or printed circuit board to a heatsink via an electrically insulating thermal interface material. Such insulating layers and thermal interface materials utilize ceramic powders with high thermal conductivity.

As a ceramic powder, boron nitride powder, which possesses properties such as high thermal conductivity, high insulation, and low dielectric constant, is attracting attention. Hexagonal boron nitride particles have a thermal conductivity of 400 W/(m·K) in the in-plane direction (a-axis direction), while their thermal conductivity in the thickness direction (c-axis direction) is 2 W/(m·K), indicating significant anisotropy in thermal conductivity due to their crystal structure and flaky appearance. Furthermore, when hexagonal boron nitride powder is filled into resin, the particles align in the same direction. Therefore, for example, during the manufacturing of thermal interface materials, the in-plane direction (a-axis direction) of the hexagonal boron nitride particles and the thickness direction of the thermal interface material become perpendicular, preventing the full utilization of the high thermal conductivity of the hexagonal boron nitride particles in the in-plane direction (a-axis direction).

From the viewpoint of reducing the anisotropy based on the shape described above, methods for forming aggregated particles by aggregating primary particles have been investigated. Patent Document 1 discloses boron nitride aggregated particles formed by the aggregation of boron nitride primary particles. It describes how increasing the strength of the aggregated particles to the extent that the collapse of the aggregated particles can be suppressed even when a predetermined molding pressure is applied suppresses the alignment and orientation of the boron nitride primary particles in the same direction.

Furthermore, Patent Document 2 discloses a method for producing boron nitride powder, comprising the step of mixing boron nitride powder A, which is composed of aggregated primary boron nitride particles and has a most frequent diameter in the region of 5 μm to less than 30 μm in its volume-based particle size distribution, and boron nitride powder B, which is composed of aggregated primary boron nitride particles and has a most frequent diameter in the region of 50 μm to less than 100 μm in its volume-based particle size distribution. While the boron nitride powder obtained by this method has excellent dielectric strength, there is room for improvement in applications requiring higher insulation performance.

Japanese Patent Publication No. 2016-135731 Japanese Patent Publication No. 2020-164365

The aggregated particles mentioned above may partially disintegrate during mixing with the resin, resulting in a mixture of aggregated and primary particles. Depending on the degree of disintegration, this can lead to an imbalance between heat dissipation and insulation properties, potentially preventing the resulting resin sheet from exhibiting the expected performance.

This disclosure aims to provide a boron nitride powder that exhibits excellent fillability into resins and can be used to prepare resin sheets that provide excellent heat dissipation and insulation properties when filled into resins.

One aspect of this disclosure provides boron nitride powder comprising primary boron nitride particles and aggregated particles formed by aggregation of primary boron nitride particles, having a crushing strength of 5 MPa or more, having an orientation index of 15 or less, a tap density of 0.75 g/ ^cm³ or more, having two or more peaks in a volume-based cumulative particle size distribution curve, having a ratio of the minimum frequency value Y between the peaks to the maximum frequency value X of the peaks of 15% or more, having a 10% cumulative diameter of 10 μm or less, a 50% cumulative diameter of 20 to 60 μm, a 90% cumulative diameter of 50 to 100 μm, and a 99% cumulative diameter of 135 μm or less in a volume-based cumulative particle size distribution curve.

The boron nitride powder described above is a mixture containing aggregated particles, which have excellent crushing strength and are formed by the aggregation of primary particles, and primary particles that do not constitute aggregated particles, and has the predetermined particle size distribution described above. Furthermore, the presence of two or more peaks in the cumulative particle size distribution curve, and the ratio of the minimum frequency value Y between the peaks to the maximum frequency value X being within the specified range, can be seen as an indicator that primary particles and aggregated particles of different particle sizes are included over a relatively wide range. In addition to the distribution described above, by adjusting the blending ratio of aggregated particles to primary particles, the orientation index, and the tap density to be within the specified ranges, and by adjusting each cumulative diameter to be within the specified ranges, it is possible to prepare a resin sheet that not only has excellent filling properties for resins but also excellent heat dissipation and insulation properties. The aggregated particles have sufficient crushing strength, and their collapse during kneading with the resin is suppressed.

The boron nitride powder described above may have a specific surface area of 3.5 ^m² /g or less.

The boron nitride powder described above may have an average thickness of 0.5 μm or more for the primary particles.

According to this disclosure, a boron nitride powder can be provided that exhibits excellent fillability into resins and can be used to prepare resin sheets that exhibit excellent heat dissipation and insulation properties when filled into resins.

Figure 1 is a graph showing the particle size distribution of the boron nitride powder obtained in Example 1.

The following describes embodiments of this disclosure. However, these embodiments are illustrative examples for illustrating this disclosure and are not intended to limit this disclosure to the following.

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 refers to the total amount of any multiple substances present in the composition, unless otherwise specified, if multiple substances corresponding to each component exist in the composition.

One embodiment of boron nitride powder comprises primary boron nitride particles and aggregated particles formed by the aggregation of primary boron nitride particles, having a crushing strength of 5 MPa or more. The boron nitride powder has an orientation index of 15 or less and a tap density of 0.75 g/ ^cm³ or more. The boron nitride powder also has two or more peaks in the volume-based cumulative particle size distribution curve, and the ratio of the minimum frequency value Y between the peaks to the maximum frequency value X is 15% or more. Furthermore, the boron nitride powder has a 10% cumulative diameter of 10 μm or less, a 50% cumulative diameter of 20 to 60 μm, a 90% cumulative diameter of 50 to 100 μm, and a 99% cumulative diameter of 135 μm or less in the volume-based cumulative particle size distribution curve.

The lower limit of the crushing strength of the aggregated particles is 5 MPa or higher, but may be, for example, 6 MPa or higher, 7 MPa or higher, 8 MPa or higher, 9 MPa or higher, 10 MPa or higher, 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 further suppresses the collapse of aggregated particles during kneading with the resin, thereby improving the heat dissipation properties of the resulting resin composition and molded article. 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 suppresses the generation of voids during kneading with the resin, thereby improving the insulation properties of the resulting resin composition and molded article. The crushing strength of the aggregated particles may be adjusted within the above range, for example, 5 to 30 MPa, 7 to 30 MPa, 10 to 30 MPa, 11 to 25 MPa, or 12 to 20 MPa.

In this specification, crush strength refers to the value measured in accordance with JIS R 1639-5:2007 "Fine Ceramics - Method for Measuring Granular Properties - Part 5: Single Granular Crushing Strength". The crush strength σ (unit: MPa) of a single agglomerated particle is calculated using the formula σ = α × P / (π × ^d² ) from the values of the dimensionless number α (α = 2.48), which changes depending on the position within the agglomerated particle, the crush test force P (unit: N), and the particle diameter d (unit: μm). The measurement shall be performed on 20 or more agglomerated particles, and the value at the point of cumulative fracture rate of 63.2% shall be calculated. A microcompression tester can be used for the measurement. For example, the "MCT-210" (product name) manufactured by Shimadzu Corporation can be used as a microcompression tester.

The average thickness of the primary boron nitride particles in the boron nitride powder may be adjusted according to the required characteristics when using the boron nitride powder. The lower limit of the average thickness of the primary boron nitride particles may be, for example, 0.5 μm or more, 0.6 μm or more, or 0.7 μm or more. Having the lower limit of the average thickness within this range can result in a boron nitride powder with superior packing properties. The upper limit of the average thickness of the primary boron nitride particles may be, for example, 1.3 μm or less, 1.2 μm or less, or 1.1 μm or less. Having the upper limit of the average thickness within this range can result in a boron nitride powder with a superior balance of orientation, heat dissipation, and packing properties. The average thickness of the primary boron nitride particles may be adjusted within the above range, for example, 0.5 to 1.3 μm, or 0.6 to 1.1 μm.

In this specification, the average thickness refers to the value measured according to the method described later. Using a press molding machine, 3 g of boron nitride powder is molded into a disc shape (diameter: 30 mmφ) at a pressure of 5 MPa to obtain a molded body. The obtained molded body is then embedded in resin (GATAN Corporation, product name: G2 epoxy). Next, a sample is prepared in which the cross-section of the primary boron nitride particles is exposed by performing cross-sectional milling in a direction parallel to the direction in which the pressure was applied. This cross-section is photographed using a scanning electron microscope. The obtained particle image is imported into image analysis software, and the short side of the rectangular particle (corresponding to particle thickness, particle short axis) is measured from the obtained photograph. The measurement shall be performed on 100 arbitrarily selected primary particles. The primary particles to be observed include primary particles that constitute aggregated particles. The arithmetic mean of the measured values is calculated, and this is used to determine the average thickness of the primary particles. As a press molding machine, for example, the "BRE-32" (product name) manufactured by Rigaku Corporation can be used. For scanning electron microscopes, for example, the "JSM-6010LA" (product name) manufactured by JEOL Ltd. can be used. For image analysis software, for example, "Mac-View" (product name) manufactured by Mountec Co., Ltd. can be used.

The average thickness of the primary boron nitride particles contained in the boron nitride powder is relatively large, which can result in a low and suppressed specific surface area of the powder as a whole. The upper limit of the specific surface area of the boron nitride powder may be, for example, 3.5 ^m² /g or less, 3.3 ^m² /g or less, 3.0 ^m² /g or less, or 2.9 ^m² /g or less. By having the upper limit of the specific surface area within the above range, the resulting boron nitride powder may have superior packing properties. The lower limit of the specific surface area of the boron nitride powder may be, for example, 1.0 ^m² /g or more, 1.2 ^m² /g or more, 1.4 ^m² /g or more, or 1.6 ^m² /g or more. By having the lower limit of the specific surface area within the above range, the resulting boron nitride powder may have superior orientation and heat dissipation properties. The specific surface area of the boron nitride powder may be adjusted within the above range, for example, 1.0 to 3.5 ^m² /g, 1.2 to ^{3.3 m²} /g, or 1.4 to 3.0 ^m² /g.

In this specification, the specific surface area refers to the value measured using a specific surface area measuring device in accordance with 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.

The boron nitride powder described above has sufficiently suppressed primary particle orientation. The upper limit of the orientation index of the boron nitride powder is 15 or less, but it may be, for example, 14 or less, 13 or less, 12 or less, 11 or less, or 10 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; for example, it may be 6 or more, 7 or more, or 8 or more. The orientation index of the boron nitride powder may be adjusted within the above range, for example, 6 to 15, 6 to 13, 6 to 10, or 7 to 10.

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 X-ray diffraction measurements on the boron nitride powder. From this X-ray diffraction spectrum, peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained. The orientation index [I(002)/I(100)] of the boron nitride powder is calculated using the obtained peak intensities. Since the orientation index is measured on a powder, the value of the orientation index tends to be small when the proportion of aggregated particles (substantially unoriented primary particles) in the powder is large. Conversely, 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, the "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used.

The boron nitride powder described above has a relatively broad particle size distribution and contains primary particles with a large average thickness, resulting in a relatively high tap density. The lower limit of the tap density of the boron nitride powder is 0.75 g/ ^cm³ or higher, but it may be, for example, 0.80 g/ ^cm³ or higher, 0.81 g/ ^cm³ or higher, or 0.83 g/ ^cm³ or higher. Having the lower limit of the tap density within the above range can further improve the packing properties into the resin. The upper limit of the tap density of the boron nitride powder is not particularly limited, but considering the theoretical density of boron nitride (2.26 g/ ^cm³ ), it may be, for example, around 1.50 g/ ^cm³ , and may be 1.30 g/ ^cm³ or less, or 1.00 g/ ^cm³ or less. The tap density of the boron nitride powder may be adjusted within the above range, for example, 0.75 to 1.50 g/ ^cm³ .

In this specification, tap density refers to the value obtained in accordance with the method described in JIS R 1628:1997 "Method for Measuring the Bulk Density of Fine Ceramic Powders," and specifically, it is determined by the method described in the Examples.

The boron nitride powder described above has a relatively broad particle size distribution, with a moderate amount of primary particles of different sizes present throughout that range. The boron nitride powder exhibits two or more peaks in its volume-based cumulative particle size distribution curve. The number of peaks in the cumulative particle size distribution curve only needs to be two or more; for example, it may be 2 to 5, or 2 to 3, but it may also be 2. Here, a peak in the cumulative particle size distribution curve refers to the position that yields a maximum value when the distribution curve is viewed along the particle size. In some cases, peaks may not be directly visible in the particle size distribution curve. Even in such cases where multiple peaks cannot be observed, if the distribution curve is asymmetrical when the horizontal axis is on a logarithmic scale and shoulder peaks can be observed, the distribution curve is approximated as the sum of log-normal distribution curves corresponding to particle sizes within a standard deviation range on the logarithmic scale. By performing waveform separation, the peaks in the target cumulative distribution curve are detected, and their number is calculated.

In the cumulative particle size distribution curve described above, the ratio of the minimum frequency value Y between the peaks (value of 100 × Y/X) to the maximum frequency value X of the peaks is 15% or more. The lower limit of the ratio of the minimum frequency value Y between the peaks to the maximum frequency value X may be, for example, 20% or more, 25% or more, 28% or more, or 30% or more. By having the lower limit of the ratio within the above range, the resulting boron nitride powder may have superior packing properties. The upper limit of the ratio of the minimum frequency value Y between the peaks to the maximum frequency value X of the peaks is not particularly limited, but may be, for example, 50% or less, 45% or less, 42% or less, or 40% or less. By having the upper limit of the ratio within the above range, the resulting boron nitride powder may have superior heat dissipation properties. The ratio of the minimum frequency value Y between the peaks to the maximum frequency value X of the peaks may be adjusted within the above range, for example, 15-50%, 20-45%, or 25-42%. If there are three or more peaks, there will be multiple points between the peaks where the frequency decreases. In this case, the value of the lowest frequency among these multiple points will be denoted as Y.

The upper limit of the 10% cumulative diameter (D10) in the volume-based cumulative particle size distribution curve of the boron nitride powder described above is 10 μm or less, but may be, for example, 9 μm or less, 8 μm or less, 7 μm or less, or 6 μm or less. Having the upper limit of D10 within this range can result in a boron nitride powder with superior packing and heat dissipation properties. The lower limit of D10 may be, for example, 1 μm or more, 2 μm or more, or 3 μm or more. Having the lower limit of D10 within this range can result in a boron nitride powder with superior packing and insulation properties.

The 50% cumulative particle size (D50, average particle size) in the volume-based cumulative particle size distribution curve of the boron nitride powder described above is 20 to 60 μm. The upper limit of D50 may be, for example, 50 μm or less, 45 μm or less, 40 μm or less, 38 μm or less, or 36 μm or less. Having the upper limit of D50 within this range can result in a boron nitride powder with superior insulating properties. The lower limit of D50 may be, for example, 22 μm or more, 24 μm or more, or 26 μm or more. Having the lower limit of D50 within this range can result in a boron nitride powder with superior heat dissipation properties.

The 90% cumulative diameter (D90) in the volume-based cumulative particle size distribution curve of the boron nitride powder described above is 50 to 100 μm. The upper limit of D90 may be, for example, 95 μm or less, 90 μm or less, 85 μm or less, or 80 μm or less. Having the upper limit of D90 within this range can result in a boron nitride powder with superior insulating properties. The lower limit of D90 may be, for example, 60 μm or more, 63 μm or more, 66 μm or more, or 69 μm or more. Having the lower limit of D90 within this range can result in a boron nitride powder with superior heat dissipation properties.

The 99% cumulative particle size (D99) in the volume-based cumulative particle size distribution curve of the boron nitride powder described above is 135 μm or less. The upper limit of D99 may be, for example, 132 μm or less, 130 μm or less, 128 μm or less, or 126 μm or less. Having the upper limit of D99 within this range can result in a boron nitride powder with superior insulating properties. The lower limit of D99 is not particularly limited, but may be, for example, 80 μm or more, 85 μm or more, 90 μm or more, or 100 μm or more.

In this specification, D10, D50, D90, and D99 for boron nitride powder refer to values measured in accordance with the method described in JIS Z 8825:2013 "Particle Size Analysis - Laser Diffraction and Scattering Method". A laser diffraction scattering particle size analyzer is used for measurement. Furthermore, the measurement should be performed without homogenizing, in the presence of aggregated particles. Examples of laser diffraction scattering particle size analyzers that can be used include the "LS-13 320" (product name) from Beckman Coulter and the "MT-3300EXII" (product name) from Microtrac-Bell.

The boron nitride powder described above is suitable for use as a filler due to its excellent ability to fill resins. One embodiment of the resin composition comprises the boron nitride powder described above and a resin. This resin composition is suitable for use in preparing a resin sheet with excellent heat dissipation and insulation properties.

The lower limit of the boron nitride powder content may be, for example, 30% by volume or more, 40% by volume or more, or 50% by volume or more, based on the total volume of the resin composition. By keeping the lower limit of the boron nitride powder content within the above range, the heat dissipation of the resin sheet obtained by molding the resin composition is improved, and an excellent heat dissipation sheet can be obtained. The upper limit of the boron nitride powder content may be, for example, 85% by volume or less, 80% by volume or less, or 70% by volume or less, based on the total volume of the resin composition. By keeping the upper limit of the boron nitride powder content within the above range, the occurrence of internal voids during molding of the resin composition can be further suppressed, and a decrease in insulation properties and mechanical strength can be suppressed.

Examples of resins include liquid crystal polymers, fluororesins, silicone resins, silicone rubber, acrylic resins, polyolefins (such as polyethylene), epoxy resins, phenolic resins, melamine resins, urea resins, unsaturated polyesters, polyimides, polyamide-imides, polyetherimides, polybutylene terephthalate, polyethylene terephthalate, polyphenylene ether, polyphenylene sulfide, fully aromatic polyesters, polysulfones, polyethersulfones, polycarbonates, maleimide-modified resins, ABS (acrylonitrile-butadiene-styrene) resins, AAS (acrylonitrile-acrylic rubber/styrene) resins, and AES (acrylonitrile-ethylene-propylene-diene rubber/styrene) resins.

The resin content may be, for example, 15% or more by volume, 20% or more by volume, or 30% or more by volume, and may be 70% or less by volume, 60% or less by volume, or 50% or less by volume, based on the total volume of the resin composition.

The resin composition may further contain a curing agent for curing the resin. The curing agent may be appropriately selected depending on the type of resin. For example, if the resin is an epoxy resin, examples of curing agents include phenol novolac compounds, acid anhydrides, amino compounds, and imidazole compounds. The curing agent content may be, for example, 0.5 parts by mass or more, or 1.0 part by mass or more, and 15.0 parts by mass or less, or 10.0 parts by mass or less, per 100 parts by mass of the resin.

The boron nitride powder described above can be manufactured, for example, by the following method. One embodiment of the method for manufacturing boron nitride powder includes a step of mixing a first powder containing aggregates formed by the aggregation of primary boron nitride particles with a second powder containing primary boron nitride particles having an average thickness of 1.0 μm or more. In the above method for manufacturing boron nitride powder, the first powder and the second powder may each be a mixed powder of separately prepared boron nitride powders.

The first powder containing aggregates formed by the aggregation of primary boron nitride particles may be prepared, for example, by a manufacturing method that applies the so-called _B4C method. An example of a manufacturing method that applies the _B4C method includes the steps of: firing boron carbide powder under a nitrogen pressurized atmosphere to obtain a calcined product containing boron carbonitride (nitriding step); removing at least a portion of the coarse particles of the calcined product to adjust the average particle size to 45 μm or less (particle size adjustment step); and heating a mixed powder containing the calcined product and a boron-containing compound containing boric acid to generate flaky primary boron nitride particles, and obtaining a first powder containing lumpy particles formed by the aggregation of primary particles (crystallization step). In the above manufacturing method, the content of the boron-containing compound is 50 parts by mass or more per 100 parts by mass of boron carbonitride powder.

Boron carbide powder can be prepared, for example, by the following procedure. Boric acid and acetylene black are mixed, and then heated in an inert gas atmosphere at 1800-2400°C for 1-10 hours to obtain boron carbide lumps. These boron carbide lumps are then crushed, sieved, washed, impurity removed, dried, etc., as appropriate to prepare boron carbide powder. Here, boron carbide powder can be obtained, for example, by crushing under relatively mild conditions, followed by a combination of classification using a vibrating sieve and airflow classification. Specifically, it may be obtained by removing particles larger than a predetermined size using a vibrating sieve and removing particles smaller than a predetermined size using airflow classification.

In the nitriding process, boron carbide powder is calcined under a nitrogen pressurized atmosphere to obtain a calcined product containing _boron carbonitride ( _B4CN4 ). The calcination temperature in the nitriding process may be, for example, 1800-2400°C, 1900-2400°C, 1800-2200°C, or 1900-2200°C.

The pressure in the nitriding process may be 0.6–1.0 MPa, 0.7–1.0 MPa, 0.6–0.9 MPa, or 0.7–0.9 MPa. By setting the lower limit of the pressure within these ranges, the nitriding of boron carbide can be carried out more sufficiently. On the other hand, if the pressure is too high, manufacturing costs tend to increase.

The nitrogen gas concentration in the nitrogen pressurized atmosphere during the nitriding process may be 95.0% by volume or higher, or 99.9% by volume or higher. The firing time during the nitriding process is not particularly limited as long as sufficient nitriding is achieved; for example, it may be 6 to 30 hours, or 8 to 20 hours.

The particle size adjustment process involves removing at least a portion of the coarse particles from the above-mentioned calcined material to adjust the average particle size to 45 μm or less.

In the crystallization process, a mixture containing the calcined product with boron carbonitride obtained in the nitriding process and a boron-containing compound is heated to generate flaky primary particles of boron nitride. Boron nitride powder containing lumpy particles formed by the aggregation of these primary particles is then obtained. In other words, the crystallization process decarburizes the boron carbonitride while simultaneously generating flaky primary particles of a predetermined size. These particles are then aggregated to obtain boron nitride powder containing lumpy particles.

Examples of boron-containing compounds include boric acid and boron oxide. The mixed powder heated during the crystallization process may contain known additives.

In the mixed powder, the mixing ratio of boron carbonitride and boron-containing compound can be appropriately set according to the molar ratio. The content of the boron-containing compound in the mixed powder may be, for example, 50 to 300 parts by mass, 100 to 300 parts by mass, 100 to 250 parts by mass, or 150 to 250 parts by mass, per 100 parts by mass of boron carbonitride. By including the boron-containing compound in excess of the boron carbonitride and then heat-treating the mixture, the unreacted boron carbide and the boron carbonitride can be sufficiently reacted, further reducing the content of unreacted substances.

The heating temperature for the mixed powder in the crystallization process may be, for example, 1800-2200°C, 2000-2200°C, or 2000-2100°C. By setting the heating temperature within these ranges, grain growth can be more sufficiently advanced. The crystallization process may be carried out under normal pressure (atmospheric pressure), or it may be carried out under pressure exceeding atmospheric pressure. If pressurization is used, the pressure may be, for example, 0.5 MPa or less, or 0.3 MPa or less.

The heating time in the crystallization process may be, for example, 0.5 to 40.0 hours, 0.5 to 35.0 hours, or 1.0 to 30.0 hours. If the heating time is too short, grain growth tends not to proceed sufficiently. On the other hand, if the heating time is too long, it tends to be industrially disadvantageous.

Through the above process, a first powder containing aggregated primary boron nitride particles can be obtained. A grinding step may be performed after the crystallization step. A general grinder or crusher can be used in the grinding step. For example, a ball mill, vibration mill, jet mill, etc., can be used. In this disclosure, "grinding" also includes "crushing." The average particle size of the first powder may be adjusted to 15 to 200 μm by grinding and classification.

The second powder, containing primary boron nitride particles with an average thickness of 1.0 μm or more, may be prepared, for example, by a manufacturing method applying the so-called carbon reduction method. An example of a manufacturing method for the second powder using the carbon reduction method includes: a low-temperature firing step to obtain a first heat-treated product by heating a raw material composition containing a boron-containing compound and a carbon-containing compound at a temperature of 1600°C or higher in a gas atmosphere containing a nitrogen-containing compound and under a pressure of 0.25 MPa or more and less than 5.0 MPa; a firing step to obtain a second heat-treated product by heating the first heat-treated product at a temperature higher than the low-temperature firing step but less than 1850°C; and a high-temperature firing step to obtain a second powder by firing the second heat-treated product at a temperature higher than the firing step. In the above manufacturing method, the content of the boron-containing compound may be 350 parts by mass or more per 100 parts by mass of the carbon-containing compound.

The low-temperature firing process is a process in which boron nitride is produced by pressurizing and heating the raw material composition in the presence of a nitrogen-containing compound. The raw material composition includes a boron-containing compound and a carbon-containing compound.

Boron-containing compounds are compounds that have boron as a constituent element. Boron-containing compounds react with carbon-containing compounds and nitrogen-containing compounds to form boron nitride. High-purity and relatively inexpensive raw materials can be used as boron-containing compounds. Examples of such boron-containing compounds include boric acid and, for example, boron oxide. Boron-containing compounds contain boric acid, which dehydrates upon heating to form boron oxide. This boron oxide forms a liquid phase during the heat treatment of the raw material composition and also acts as an aid to promote grain growth. Furthermore, boric acid can be easily removed from the system by heating under low pressure.

Carbon-containing compounds are compounds that have carbon atoms as constituent elements. Carbon-containing compounds react with boron-containing compounds and nitrogen-containing compounds to form boron nitride. High-purity and relatively inexpensive raw materials can be used as carbon-containing compounds. Examples of such carbon-containing compounds include carbon black and acetylene black.

In the raw material composition, the boron-containing compound is blended in excess of the carbon-containing compound. The content of the boron-containing compound in the raw material composition may be, for example, 350 to 1000 parts by mass, 400 to 800 parts by mass, 450 to 700 parts by mass, or 450 to 600 parts by mass per 100 parts by mass of the carbon-containing compound. By including an excess amount of the boron-containing compound in the raw material composition and then heat-treating it to thoroughly react with the carbon-containing compound, thereby reducing its content, the purity of the resulting second powder can be further improved.

The above-described method for producing the second powder may, for example, include a step for preparing the raw material composition. This raw material composition preparation step may include a step for dehydrating the boron-containing compound. Including a step for dehydrating the boron-containing compound can improve the yield of boron nitride obtained in the low-temperature calcination step. Furthermore, the raw material composition preparation step may involve a pulverizing and mixing process using an impact pulverizer or the like, from the viewpoint of further homogeneously mixing the raw materials and carrying out the heating reaction of the raw material composition in a more homogeneous environment.

The raw material composition may contain other compounds in addition to carbon-containing and boron-containing compounds. Examples of other compounds include boron nitride as a nucleating agent. Including boron nitride as a nucleating agent in the raw material composition makes it easier to control the average particle size of the synthesized second powder. The raw material composition preferably contains a nucleating agent. When the raw material composition contains a nucleating agent, it becomes easier to produce a second powder with a small specific surface area.

When using boron nitride powder as a nucleating agent, the content of the nucleating agent may be, for example, 0.05 to 8.00 parts by mass per 100 parts by mass of the raw material composition. Setting the lower limit of the nucleating agent content to 0.05 parts by mass or more can further enhance the effect of including the nucleating agent. Setting the upper limit of the nucleating agent content to 8.00 parts by mass or less can improve the yield of the second powder.

Nitrogen-containing compounds are compounds that have nitrogen atoms as constituent elements and react with carbon-containing compounds and boron-containing compounds to form boron nitride. Examples of nitrogen-containing compounds include nitrogen and ammonia. Nitrogen-containing compounds may be supplied in gaseous form; in this case, the nitrogen-containing compound is also called a nitrogen-containing gas. From the viewpoint of promoting the formation of boron nitride through the nitriding reaction and reducing costs, the nitrogen-containing gas preferably contains nitrogen gas, and more preferably nitrogen gas. When a mixture of multiple gases is used as the nitrogen-containing gas, the proportion of nitrogen gas in the mixture may preferably be 95% by volume. Note that the above proportion of nitrogen gas refers to the value determined by volume under standard conditions.

The low-temperature firing process is carried out under pressure. The pressure during the low-temperature firing process may be, for example, 0.25 MPa or more but less than 5.00 MPa, 0.25 to 3.00 MPa, 0.25 to 2.00 MPa, 0.25 to 1.00 MPa, 0.25 MPa or more but less than 1.00 MPa, 0.30 to 2.00 MPa, or 0.50 to 2.00 MPa. Increasing the pressure during the low-temperature firing process further suppresses the volatilization of raw materials such as boron-containing compounds, thereby suppressing the formation of boron carbide, a by-product. Furthermore, increasing the pressure during the low-temperature firing process can suppress the increase in the specific surface area of the boron nitride powder. By setting the upper limit of the pressure during the low-temperature firing process within the above range, the growth of primary boron nitride particles can be further promoted.

The low-temperature firing process is carried out under heating. The heating temperature in the low-temperature firing process may be, for example, 1650°C or higher but less than 1800°C, 1650°C to 1750°C, or 1650°C to 1700°C. By setting the lower limit of the heating temperature in the low-temperature firing process within the above range, the reaction can be promoted, and the yield of boron nitride obtained in the low-temperature firing process can be improved. By setting the upper limit of the heating temperature in the low-temperature firing process within the above range, the formation of by-products can be sufficiently suppressed. While there are no particular restrictions on the heating rate in the low-temperature firing process, it may be, for example, 0.5°C/min or higher.

The heating time in the low-temperature firing process may be, for example, 1 to 10 hours, 1 to 5 hours, or 2 to 4 hours. By maintaining a relatively low temperature for a predetermined period during the low-temperature firing process, which is the initial stage of the reaction for synthesizing boron nitride, the reaction system can be made more homogenized, and consequently, the boron nitride formed in the low-temperature firing process can be made more homogenized.

The firing process involves further heating the first heat-treated product obtained in the low-temperature firing process at a higher temperature than that of the low-temperature firing process to obtain a second heat-treated product. This process promotes crystal grain growth and allows for more efficient consumption of the additives in the reaction system.

The heating temperature in the firing process is higher than that of the low-temperature firing process, and is below 1850°C. The firing process may be performed immediately after the low-temperature firing process, and all conditions other than the temperature in the low-temperature firing process may be maintained. That is, the low-temperature firing process may also be a process of heating the first heat-treated material under a pressurized environment containing nitrogen-containing gas, etc.

The heating time in the firing process may be, for example, 3 to 15 hours, 5 to 10 hours, or 6 to 9 hours.

The high-temperature firing process involves further firing the second heat-treated material obtained in the firing process at a higher temperature to obtain a second powder. In the high-temperature firing process, primary boron nitride particles with improved crystallinity are obtained. These primary boron nitride particles have a flaky shape. Furthermore, by setting a higher heating temperature in this process, the amount of residual additives is reduced, and the purity is further improved, making the resulting boron nitride powder more suitable as a raw material for sintered bodies.

The pressure in the high-temperature firing process may be the same as or different from that in the low-temperature firing and firing processes. If the pressure in the high-temperature firing process differs from that in the low-temperature firing and firing processes, the pressure in the high-temperature firing process may be lower than that in the low-temperature firing and firing processes.

The pressure during the high-temperature firing process may be, for example, 0.25 MPa or more and less than 5.00 MPa, 0.25 to 3.00 MPa, 0.25 to 2.00 MPa, 0.25 to 1.00 MPa, 0.25 MPa or more and less than 1.00 MPa, 0.30 to 2.00 MPa, or 0.50 to 2.00 MPa. Increasing the pressure during the high-temperature firing process can further improve the purity of the resulting second powder. By setting the upper limit of the pressure during the high-temperature firing process within the above range, the manufacturing cost of the second powder can be further reduced, which is industrially advantageous.

The firing temperature in the high-temperature firing process is set higher than the heating temperature in the firing process described above. The firing temperature in the high-temperature firing process may be, for example, 1850-2100°C, 1850-2050°C, or 1900-2025°C. By increasing the firing temperature in the high-temperature firing process, the purity of the secondary powder can be further improved, and the growth of primary particles can be promoted, resulting in a smaller specific surface area of the secondary powder. By setting the upper limit of the firing temperature in the high-temperature firing process within the above range, the yellowing of boron nitride can be suppressed.

The firing time (heating time at high temperature) in the high-temperature firing process may be, for example, 0.5 to 30 hours, 1 to 25 hours, or 3 to 10 hours. By setting the firing time in the high-temperature firing process within the above ranges, the purity of the secondary powder can be further improved, and the growth of the primary particles can be more sufficient. By keeping the firing time in the high-temperature firing process within the above ranges, the secondary powder can be manufactured at a lower cost.

The above-described method for producing the second powder may include other steps in addition to the low-temperature calcination step, calcination step, and high-temperature calcination step. Examples of these other steps include the preparation step of the raw material composition, the dehydration step of the raw material composition, the pressure molding step of the raw material composition, the pulverization steps of the first and second heat-treated materials, and the pulverization step of boron nitride. If the above-described method includes a pressure molding step of the raw material composition, calcination can be performed in an environment where the raw material composition is present at high density, thereby further improving the yield of boron nitride obtained in the low-temperature calcination step and the calcination step. Note that the pulverization step in this specification includes not only pulverization but also crushing.

Grinding may be carried out using, for example, a grinding device. As a grinding device, for example, an impact-type grinder (pulperizer) may be used. For impact-type grinders, those that allow for particle size adjustment of the ground material using a screen, such as an impact-type screen-type fine grinder, are preferably used. The screen opening may be, for example, 0.1 to 1.0 mm, or 1.0 to 3.0 mm.

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.

The present disclosure will be described in more detail below using examples and comparative examples. However, this disclosure is not limited to the following examples.

(Example 1)
[Preparation of the first powder containing aggregated particles]
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 vibratory mill for 1 hour and then sieved through a 106 μm sieve to produce boron carbide powder ( _B4C powder) with an average particle size of 20 μm.

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

To 100 parts by mass of the mixture of the above heat-treated material and boric acid (boron source), boric acid was added so that its proportion as the boron source was 36 parts by mass. The mixture was then mixed using a Henschel mixer to obtain a mixture. The mixture was packed into a boron nitride crucible and decarburized by heating in a resistance heating furnace to synthesize boron nitride powder containing aggregated particles formed by the aggregation of primary particles (crystallization step). The crystallization conditions were a nitrogen gas atmosphere at a pressure of 5 kPa, with the temperature raised from room temperature to 2000°C and held at 2000°C for 5 hours. After crushing the synthesized boron nitride powder using a Henschel mixer, classification was performed using a nylon sieve with a mesh size of 75 μm. In this way, powder 1A containing aggregated particles formed by the aggregation of primary particles of boron nitride was obtained.

Next, 100 parts by mass of orthoboric acid (manufactured by Nippon Denko Co., Ltd.) 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 vibratory mill for 30 minutes and then sieved through a 63 μm sieve to produce boron carbide powder ( _B4C powder) with an average particle size of 35 μm.

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

To 100 parts by mass of the mixture of the above heat-treated material and boric acid (boron source), boric acid was added so that its proportion as the boron source was 36 parts by mass. The mixture was then mixed using a Henschel mixer to obtain a mixture. The mixture was packed into a boron nitride crucible and decarburized by heating in a resistance heating furnace to synthesize boron nitride powder containing aggregated particles formed by the aggregation of primary particles (crystallization step). The crystallization conditions were a nitrogen gas atmosphere at a pressure of 5 kPa, with the temperature raised from room temperature to 2000°C and held at 2000°C for 5 hours. After crushing the synthesized boron nitride powder using a Henschel mixer, classification was performed using a nylon sieve with a mesh size of 106 μm. In this way, powder 1B containing aggregated particles formed by the aggregation of primary particles of boron nitride was obtained.

The first powder was prepared by mixing powder 1A and powder 1B, prepared as described above, in a mass ratio of 50:30.

[Preparation of a second powder containing thick primary particles]
A mixed powder was obtained by mixing 100 parts by mass of boric acid (manufactured by Kojun Chemical Laboratory Co., Ltd.), 22 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: FX-35, specific surface area: 130 ^m² /g), and 1 part by mass of sodium carbonate (purity: 99.5% by mass or higher) using a Henschel mixer. The resulting mixed powder was placed in a 250°C dryer and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a 100Φ diameter mold of a press molding machine and molded under the conditions of heating temperature: 200°C and pressing pressure: 30 MPa. The pellets of the mixed powder obtained in this way were subjected to subsequent heat treatments.

First, the pellets were placed in a carbon atmosphere furnace and heated to 1750°C at a rate of 5°C/min in a nitrogen atmosphere pressurized to 0.65 MPa. The temperature was then maintained at 1750°C for 3 hours to heat-treat the pellets and obtain the first heat-treated product (low-temperature firing process). Next, the temperature inside the carbon atmosphere furnace was further increased to 1800°C at a rate of 2°C/min, and the temperature was maintained at 1800°C for 3 hours to heat-treat the first heat-treated product and obtain the second heat-treated product (firing process). Subsequently, the temperature inside the carbon atmosphere furnace was further increased to 2050°C at a rate of 2°C/min, and the temperature was maintained at 2050°C for 7 hours to fire the second heat-treated product at a high temperature (high-temperature firing process). The loosely aggregated boron nitride after calcination was crushed using a Henschel mixer. The resulting powder was dried at 250°C for 5 hours using a vacuum dryer, and then passed through a sieve with a mesh size of 63 μm to obtain the powder that passed through the sieve. In this way, powder 2A containing primary particles of boron nitride powder was prepared. Powder 2A was designated as the second powder.

[Preparation of boron nitride powder]
As described above, the first boron nitride powder (a mixed powder of powder 1A and powder 1B) and the second powder (powder 2A) were mixed in a mass ratio of 80:20 to obtain boron nitride powder. For mixing, a V-type mixer "SVM-10" (product name) manufactured by Seishin Corporation was used, and mixing was carried out at 20 rpm for 30 minutes.

<Evaluation of the properties of boron nitride powder>
The crushing strength of aggregated particles, orientation index of boron nitride powder, tap density, particle size distribution, area, and average thickness of primary particles of the obtained boron nitride powder were measured according to the method described later. The results are shown in Table 1. For reference, Figure 1 shows a graph of the particle size distribution of the boron nitride powder obtained in Example 1.

[Crushing strength]
The crushing strength of the aggregated particles was measured in accordance with JIS R 1639-5:2007 "Fine Ceramics - Method for Measuring Granular Properties - Part 5: Single Granular Crushing Strength". A microcompression tester (manufactured by Shimadzu Corporation, product name "MCT-210") was used for the measurement. The measurement was performed on 20 or more aggregated particles, and the value was calculated at a cumulative fracture rate of 63.2%.

[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 boron nitride powder into a glass cell with a depth of 0.2 mm, which is attached to the X-ray diffractometer, and allowing it to solidify. 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.

[Tap density]
The tap density of boron nitride powder was measured in accordance with the method described in JIS R 1628:1997 "Method for Measuring the Bulk Density of Fine Ceramic Powders". Specifically, boron nitride powder was filled into a 100 ^cm³ dedicated container, and after tapping under the conditions of a tapping time of 180 seconds, 180 tapping cycles, and a tap lift of 18 mm, the bulk density was measured using a "powder tester" manufactured by Hosokawa Micron, and the obtained value was defined as the tap density.

[Particle size distribution (determination of D10, D50, D90, D99)]
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 scattering particle size distribution analyzer (Microtrac-Bell, product name: "Microtrac MT-3300EXII") was used for the measurement. For the measurement, 2 mL of sodium hexametaphosphate aqueous solution and 200 mL of water were placed in a 300 mL beaker, 60 mg of boron nitride powder was added, and the mixture was stirred at 100 rpm for 1 minute using a stirrer without homogenization, and the measurement was performed in the presence of aggregated particles. Water was used as the dispersant with a refractive index of 1.33. The channel divisions were 129 divisions, ranging from 1408 μm to 0.021 μm. In the obtained particle size distribution curve, peaks were detected, the value of the maximum peak frequency X and the value of the minimum peak frequency Y were determined, and the ratio of the value of the minimum peak frequency Y to the value of the maximum peak frequency X (value of 100 × Y/X) was calculated.

[Average thickness of primary particles]
The average thickness of primary particles in boron nitride powder was measured according to the method described below. Using a press molding machine (manufactured by Rigaku Corporation, product name: BRE-32), 3 g of boron nitride powder was molded into a disc shape (diameter: 30 mmφ) at a pressure of 5 MPa. The resulting molded body was embedded in resin (manufactured by GATAN Corporation, product name: G2 epoxy). Next, a sample in which the cross-section of the primary particles of boron nitride was exposed was prepared by performing cross-sectional milling in a direction parallel to the direction in which pressure was applied. This cross-section was photographed using a scanning electron microscope (manufactured by JEOL Ltd., product name: JSM-6010LA). The obtained particle images were imported into image analysis software (manufactured by Mountec Co., Ltd., product name: Mac-View), and the short side (corresponding to particle thickness, particle short axis) of the rectangular particles was measured from the obtained photographs. The measurement was performed on 100 arbitrarily selected primary particles. The primary particles to be observed included primary particles that constitute aggregated particles. The arithmetic mean of the measured values was calculated and used as the average thickness of the primary particles.

<Evaluation of boron nitride powder as a filler>
The performance of the obtained boron nitride powder as a filler for use in resin was evaluated according to the method described later. The results are shown in Table 1.

[Fillability]
The packing performance when boron nitride powder is used as a filler was evaluated. Specifically, boron nitride powder was added to silicone oil (Shin-Etsu Chemical Co., Ltd., product name: KF96-100) at a concentration of 20% by volume relative to the total amount of the resin composition. A slurry was prepared by stirring the mixture at 2000 rpm for 3 minutes using a rotation/revolution mixer (Sinky Co., Ltd., product name: Awatori Rentaro RE-310). The viscosity of the slurry was measured using a rheometer (Signal Hegner Japan Co., Ltd., product name: MCR300, circular plate (diameter: 25 mmφ), gap: 1 m), and the packing performance was evaluated based on the results obtained according to the following criteria. Note that if the viscosity of the resin composition is high when powder is added, handling and moldability will be poor, requiring a reduction in the amount of powder added, and making it difficult to increase the amount of powder that can be packed into the molded body. Therefore, a lower shear viscosity obtained by this evaluation is desirable from the viewpoint of packing performance.
A: The viscosity at a shear rate of 20 rpm is 2000 mPa·s or less.
B: The viscosity at a shear rate of 20 rpm is greater than 2000 mPa·s and less than or equal to 4000 mPa·s.
C: The viscosity at a shear rate of 20 rpm is greater than 4000 mPa·s and less than or equal to 8000 mPa·s.
D: The viscosity at a shear rate of 20 rpm is greater than 8000 mPa·s.

[Heat dissipation]
A resin sheet was prepared from a resin composition using boron nitride powder as a filler, and the heat dissipation properties of the resin sheet were evaluated. First, a resin composition was obtained by mixing 100 parts by mass of naphthalene-type epoxy resin (DIC Corporation, HP4032) and 10 parts by mass of imidazole compound (Shikoku Chemicals Co., Ltd., 2E4MZ-CN) as a curing agent, and then adding boron nitride powder so that the boron nitride powder content was 60% by volume. A product called "Awatori Rentaro" (product name) manufactured by Thinky Co., Ltd. was used for kneading with the resin. The kneading conditions were 1600 rpm for 5 minutes. After applying the obtained mixture onto a polyethylene terephthalate (PET) sheet, degassing was performed for 10 minutes under reduced pressure conditions of 500 Pa. Next, the epoxy resin composition was applied onto a PET film with a thickness of 0.05 mm so that the thickness after curing would be 0.10 mm, and it was heated at 100°C for 15 minutes to dry. The PET sheet coated with the aforementioned mixture and the PET film coated with the epoxy resin composition were laminated facing each other, and the sheets were heated at 180°C for 180 minutes while applying a surface pressure of 160 kgf/ ^cm² using a press machine to cure them, thereby obtaining a heat dissipation sheet with a thickness of 0.1 mm.

The thermal conductivity of the heat dissipation sheet described above was evaluated. The thermal conductivity H (unit: W/(m·K)) was calculated from the values of thermal diffusivity A (unit: ^m² /sec), density B (unit: kg/ ^m³ ), and specific heat capacity C (unit: J/(kg·K)) based on the formula H = A × B × C. The thermal diffusivity A was determined by the laser flash method after fabricating a heat dissipation sheet with dimensions of 10 mm (length) x 10 mm (width) x 0.3 mm (thickness). The measurement device used was a xenon flash analyzer (NETZSCH, product name: LFA447NanoFlash). Density B was determined using the Archimedes method. Specific heat capacity C was determined using a DSC (Rigaku Corporation, product name: ThermoPlusEvoDSC8230). Based on the obtained thermal conductivity values, evaluation was performed according to the following criteria.
A: The thermal conductivity is 15 W/mK or higher.
B: The thermal conductivity is 13 W/mK or more and less than 15 W/mK.
C: The thermal conductivity is 11 W/mK or higher and less than 13 W/mK.
D: The thermal conductivity is less than 11 W/mK.

[Insulating properties]
The insulating properties were evaluated using the same resin sheet prepared for the heat dissipation evaluation described above. Specifically, the dielectric breakdown voltage of the resin sheet was measured in accordance with JIS C 6481:1996 "Test Method for Copper-Clad Laminates for Printed Wiring Boards". A withstand voltage and insulation tester (product name: TOS 8650) manufactured by Kikusui Electronics Co., Ltd. was used for the measurement, and 100 samples were measured, with the arithmetic mean value being taken as the dielectric breakdown voltage. Based on the obtained dielectric breakdown voltage values, the following criteria were used for evaluation.
A: The dielectric breakdown voltage is 85 kV/mm or higher.
B: The dielectric breakdown voltage is 75 kV/mm or more and less than 85 kV/mm.
C: The dielectric breakdown voltage is 65 kV/mm or more and less than 75 kV/mm.
D: The dielectric breakdown voltage is less than 65 kV/mm.

(Example 2)
[Preparation of the first powder containing aggregated particles]
100 parts by mass of orthoboric acid (manufactured by Nippon Denko Co., Ltd.) 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 produce a lump of boron carbide ( _B4C ) powder.

The prepared boron carbide powder was pulverized in a vibratory mill for 200 minutes, and then sieved through a 45 μm sieve to produce boron carbide powder ( _B4C powder) with an average particle size of 10 μm. The prepared boron carbide powder was then 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 step). The obtained calcined product was placed in a muffle furnace and heated at 700°C for 5 hours under an atmospheric atmosphere (oxidation step) to obtain a heat-treated product.

To 100 parts by mass of the mixture of the above heat-treated material and boric acid (boron source), boric acid was added so that its proportion as the boron source was 36 parts by mass. The mixture was then mixed using a Henschel mixer to obtain a mixture. The mixture was packed into a boron nitride crucible and decarburized by heating in a resistance heating furnace to synthesize boron nitride powder containing aggregated particles formed by the aggregation of primary particles (crystallization step). The crystallization conditions were a nitrogen gas atmosphere at a pressure of 5 kPa, with the temperature raised from room temperature to 2000°C and held at 2000°C for 5 hours. 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 45 μm. In this way, powder 1C containing aggregated particles formed by the aggregation of primary particles of boron nitride was obtained.

Powder 1A, powder 1B, and powder 1C, prepared in the same manner as in Example 1, were mixed in a mass ratio of 20:40:20 to prepare the first powder.

[Preparation of boron nitride powder]
The first boron nitride powder (a mixed powder of powders 1A, 1B, and 1C) and the second powder (powder 2A), obtained as described above, were mixed in a mass ratio of 80:20 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 1.

(Example 3)
As the first powder, a mixed powder was used, which consisted of powder 1A and powder 1B mixed in a mass ratio of 70:20. The first powder (mixed powder of powder 1A and powder 1B) and the second powder (powder 2A) were then mixed in a mass ratio of 90:10 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. For the obtained boron nitride powder, the crushing strength of aggregated particles, the orientation index of the boron nitride powder, tap density, particle size distribution, area, and the average thickness of primary particles were measured in the same manner as in Example 1. The results are shown in Table 1.

(Example 4)
[Preparation of a second powder containing thick primary particles]
A mixed powder was obtained by mixing 100 parts by mass of boric acid (manufactured by Kojun Chemical Laboratory Co., Ltd.), 27 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: 50% pressed, specific surface area: 67 ^m² /g), and 1 part by mass of sodium carbonate (purity: 99.5% by mass or higher) using a Henschel mixer. The resulting mixed powder was placed in a 250°C dryer and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a 100Φ diameter mold of a press molding machine and molded under the conditions of heating temperature: 200°C and pressing pressure: 30 MPa. The pellets of the mixed powder obtained in this way were subjected to subsequent heat treatments.

First, the pellets were placed in a carbon atmosphere furnace and heated to 1750°C at a rate of 5°C/min in a nitrogen atmosphere pressurized to 0.65 MPa. The temperature was then maintained at 1750°C for 3 hours to heat-treat the pellets and obtain the first heat-treated product (low-temperature firing process). Next, the temperature inside the carbon atmosphere furnace was further increased to 1800°C at a rate of 2°C/min, and the temperature was maintained at 1800°C for 3 hours to heat-treat the first heat-treated product and obtain the second heat-treated product (firing process). Subsequently, the temperature inside the carbon atmosphere furnace was further increased to 2050°C at a rate of 2°C/min, and the temperature was maintained at 2050°C for 7 hours to fire the second heat-treated product at a high temperature (high-temperature firing process). The loosely aggregated boron nitride after calcination was crushed using a Henschel mixer. The resulting powder was dried at 250°C for 5 hours using a vacuum dryer, and then passed through a sieve with a mesh size of 45 μm to obtain the powder that passed through the sieve. In this way, powder 2B containing primary particles of boron nitride powder was obtained. Powder 2B was designated as the second powder.

Powder 1A and powder 1B, prepared in the same manner as in Example 1, were mixed in a mass ratio of 50:30 to prepare the first powder.

[Preparation of boron nitride powder]
The first boron nitride powder (a mixed powder of powder 1A and powder 1B) and the second powder (powder 2B), obtained as described above, were mixed in a mass ratio of 80:20 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. For the obtained boron nitride powder, the crushing strength of aggregated particles, the orientation index of the boron nitride powder, tap density, particle size distribution, area, and the average thickness of primary particles were measured in the same manner as in Example 1. The results are shown in Table 1.

(Example 5)
As the first powder, a mixed powder was obtained by mixing powder 1A and powder 1B in a ratio of 20:70. The first powder (mixed powder of powder 1A and powder 1B) and the second powder (powder 2A) were mixed in a mass ratio of 90:10 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. For the obtained boron nitride powder, the crushing strength of aggregated particles, the orientation index of the boron nitride powder, tap density, particle size distribution, area, and the average thickness of primary particles were measured in the same manner as in Example 1. The results are shown in Table 1.

(Example 6)
As the first powder, a mixed powder was used, which consisted of powder 1A and powder 1C mixed in a mass ratio of 40:50. The first powder (mixed powder of powder 1A and powder 1C) and the second powder (powder 2A) were mixed in a mass ratio of 90:10 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. For the obtained boron nitride powder, the crushing strength of aggregated particles, the orientation index of the boron nitride powder, tap density, particle size distribution, area, and the average thickness of primary particles were measured in the same manner as in Example 1. The results are shown in Table 1.

(Example 7)
[Preparation of the first powder containing aggregated particles]
100 parts by mass of orthoboric acid (manufactured by Nippon Denko Co., Ltd.) 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 prepare a lump of boron carbide ( _B4C ) powder.

The prepared lump of boron carbide powder was pulverized in a vibratory mill for 20 minutes, and then sieved through a 75 μm sieve to produce boron carbide powder ( _B4C powder) with an average particle size of 45 μm. The prepared boron carbide powder was then 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 step). The obtained calcined product was placed in a muffle furnace and heated at 700°C for 5 hours under an atmospheric atmosphere (oxidation step) to obtain a heat-treated product.

To 100 parts by mass of the mixture of the above heat-treated material and boric acid (boron source), boric acid was added so that its proportion as the boron source was 36 parts by mass. The mixture was then mixed using a Henschel mixer to obtain a mixture. The mixture was packed into a boron nitride crucible and decarburized by heating in a resistance heating furnace to synthesize boron nitride powder containing aggregated particles formed by the aggregation of primary particles (crystallization step). The crystallization conditions were a nitrogen gas atmosphere at a pressure of 5 kPa, with the temperature raised from room temperature to 2000°C and held at 2000°C for 5 hours. 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 125 μm. In this way, powder 1D containing aggregated particles formed by the aggregation of primary particles of boron nitride was obtained.

Powder 1A and powder 1D, prepared in the same manner as in Example 1, were mixed in a mass ratio of 60:20 to prepare the first powder.

[Preparation of boron nitride powder]
The first boron nitride powder (a mixed powder of powder 1A and powder 1D) and the second powder (powder 2A), obtained as described above, were mixed in a mass ratio of 80:20 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 1.

(Example 8)
[Preparation of a second powder containing thick primary particles]
A mixed powder was obtained by mixing 100 parts by mass of boric acid (manufactured by Kojun Chemical Laboratory Co., Ltd.), 22 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: FX-35, specific surface area: 130 ^m² /g), and 1 part by mass of sodium carbonate (purity: 99.5% by mass or higher) using a Henschel mixer. The resulting mixed powder was placed in a 250°C dryer and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a 100Φ diameter mold of a press molding machine and molded under the conditions of heating temperature: 200°C and pressing pressure: 30 MPa. The pellets of the mixed powder obtained in this way were subjected to subsequent heat treatments.

First, the pellets were placed in a carbon atmosphere furnace and heated to 1750°C at a rate of 5°C/min in a nitrogen atmosphere pressurized to 0.65 MPa. The temperature was then maintained at 1750°C for 3 hours to heat-treat the pellets and obtain the first heat-treated product (low-temperature firing process). Next, the temperature inside the carbon atmosphere furnace was further increased to 1800°C at a rate of 2°C/min, and the temperature was maintained at 1800°C for 3 hours to heat-treat the first heat-treated product and obtain the second heat-treated product (firing process). Subsequently, the temperature inside the carbon atmosphere furnace was further increased to 2050°C at a rate of 2°C/min, and the temperature was maintained at 2050°C for 7 hours to fire the second heat-treated product at a high temperature (high-temperature firing process). After calcination, the loosely aggregated boron nitride was pulverized using a pulperizer. The mixture was then stirred in a 0.1 N dilute nitric acid aqueous solution for two hours, thoroughly washed with pure water, and filtered by suction. The resulting powder was then dried at 250°C for five hours using a vacuum dryer. Finally, it was passed through a sieve with a mesh size of 63 μm to obtain the powder that passed through the sieve. In this way, powder 2C, containing primary particles of boron nitride powder, was prepared. Powder 2C was designated as the second powder.

[Preparation of boron nitride powder]
Boron nitride powder was prepared in the same manner as in Example 1, except that second powder (powder 2C) was used instead of second powder (powder 2A). The crushing strength of aggregated particles, orientation index of boron nitride powder, tap density, particle size distribution, area, and average thickness of primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 9)
[Preparation of the first powder containing aggregated particles]
Regarding the preparation of powders 1A and 1B, in the heat treatment process after the preparation of the boron carbide raw material, the material was heated for 12 hours under conditions of a nitrogen gas atmosphere, a calcination temperature of 2150°C, and a pressure of 0.90 MPa. During calcination, nitrogen gas was supplied in excess of the chemically equivalent amount, and 30 equivalents of nitrogen gas were supplied relative to the required amount. Before creating a nitrogen gas atmosphere during the crystallization process, the material was held at 420°C for one hour in a vacuum atmosphere of 100 Pa, thereby preparing powders 1E and 1F.

The first powder was prepared by mixing powder 1E and powder 1F, prepared as described above, in a mass ratio of 50:30.

[Preparation of boron nitride powder]
Boron nitride powder was prepared in the same manner as in Example 1, except that a first powder (a mixed powder of powders 1E and 1F) was used instead of the first powder (a mixed powder of powders 1A and 1B). The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 10)
[Preparation of the first powder containing aggregated particles]
Regarding the preparation of powder 1A and powder 1B, powder 1G and powder 1H were prepared in the same manner as in Example 1, except that the amount of boric acid added in the crystallization process was 39 parts by mass, sodium carbonate was added in an amount of 1% by mass relative to 100% by mass of the above mixture, and the pressure was changed to 50 kPa.

The first powder was prepared by mixing powder 1G and powder 1H, prepared as described above, in a mass ratio of 50:30.

[Preparation of boron nitride powder]
Boron nitride powder was obtained in the same manner as in Example 1, except that a first powder (a mixed powder of powders 1G and 1H) was used instead of the first powder (a mixed powder of powders 1A and 1B). The mixing conditions were the same as in Example 1. The crushing strength of aggregated particles, orientation index of the boron nitride powder, tap density, particle size distribution, area, and average thickness of primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 11)
[Preparation of a second powder containing thick primary particles]
A mixed powder was obtained by mixing 100 parts by mass of boric acid (manufactured by Kojun Chemical Laboratory Co., Ltd.), 22 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: FX-35, specific surface area: 130 ^m² /g), and 1 part by mass of sodium carbonate (purity: 99.5% by mass or higher) using a Henschel mixer. The resulting mixed powder was placed in a 250°C dryer and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a 100Φ diameter mold of a press molding machine and molded under the conditions of heating temperature: 200°C and pressing pressure: 30 MPa. The pellets of the mixed powder obtained in this way were subjected to subsequent heat treatments.

First, the pellets were placed in a carbon atmosphere furnace and heated to 1750°C at a rate of 5°C/min in a nitrogen atmosphere pressurized to 0.65 MPa. The temperature was then maintained at 1750°C for 3 hours to heat-treat the pellets and obtain the first heat-treated product (low-temperature firing process). Next, the temperature inside the carbon atmosphere furnace was further increased to 1800°C at a rate of 2°C/min, and the temperature was maintained at 1800°C for 3 hours to heat-treat the first heat-treated product and obtain the second heat-treated product (firing process). Subsequently, the temperature inside the carbon atmosphere furnace was further increased to 2050°C at a rate of 2°C/min, and the temperature was maintained at 2050°C for 7 hours to fire the second heat-treated product at a high temperature (high-temperature firing process). The loosely aggregated boron nitride after calcination was crushed in a mortar for 10 minutes. The resulting powder was then dried in a vacuum dryer at 250°C for 5 hours. The powder was then passed through a sieve with a mesh size of 63 μm to obtain the remaining powder. In this way, powder 2D, containing primary particles of boron nitride powder, was prepared. This powder 2D was designated as the second powder.

Boron nitride powder was prepared in the same manner as in Example 1, except that second powder (2D) was used instead of second powder (2A). The crushing strength of aggregated particles, orientation index of the boron nitride powder, tap density, particle size distribution, area, and average thickness of primary particles were measured for the obtained boron nitride powder in the same manner as in Example 1. The results are shown in Table 2.

(Example 12)
[Preparation of a second powder containing thick primary particles]
A mixed powder was obtained by mixing 100 parts by mass of boric acid (manufactured by Kojun Chemical Laboratory Co., Ltd.), 22 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: FX-35, specific surface area: 130 ^m² /g), and 1 part by mass of sodium carbonate (purity: 99.5% by mass or higher) using a Henschel mixer. The resulting mixed powder was placed in a 250°C dryer and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a 100Φ diameter mold of a press molding machine and molded under the conditions of heating temperature: 200°C and pressing pressure: 30 MPa. The pellets of the mixed powder obtained in this way were subjected to subsequent heat treatments.

First, the pellets were placed in a carbon atmosphere furnace and heated to 1750°C at a rate of 5°C/min in a nitrogen atmosphere pressurized to 0.65 MPa. The pellets were then heated at 1750°C for 3 hours to obtain the first heat-treated product (low-temperature firing process). Next, the carbon atmosphere furnace was further heated to 1800°C at a rate of 2°C/min and held at 1800°C for 3 hours to heat-treat the first heat-treated product and obtain the second heat-treated product (firing process). Subsequently, the carbon atmosphere furnace was further heated to 2050°C at a rate of 2°C/min and held at 2050°C for 7 hours to fire the second heat-treated product at a high temperature (high-temperature firing process). After firing, the loosely aggregated boron nitride was crushed in a pulperizer and then passed through a sieve with a mesh size of 63 μm to obtain the powder that passed through the sieve. In this way, powder 2E containing primary particles of boron nitride powder was prepared. Powder 2E was designated as the second powder.

[Preparation of boron nitride powder]
Boron nitride powder was prepared in the same manner as in Example 1, except that second powder (2E) was used instead of second powder (2A). The crushing strength of aggregated particles, orientation index of boron nitride powder, tap density, particle size distribution, area, and average thickness of primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 2.

(Example 13)
[Preparation of a second powder containing thick primary particles]
Regarding the preparation of a second powder (powder 1A) containing thick primary particles, powder 2F was prepared in the same manner as in Example 1, except that the pressure of the carbon atmosphere furnace during heat treatment was set to 0.90 MPa and the holding time at 1800°C was changed to 15 hours. Powder 2F was used as the second powder.

[Preparation of boron nitride powder]
Boron nitride powder was obtained by mixing the first powder (a mixture of powder 1A and powder 1B) and the second powder (powder 2F) in a mass ratio of 45:55. The mixing conditions were the same as in Example 1. The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 2.

(Comparative Example 1)
[Preparation of boron nitride powder]
Boron nitride powder was prepared in the same manner as in Example 1, except that the first powder (powder 1A) was used instead of the first powder (a mixed powder of powder 1A and powder 1B), and the first powder (powder 1A) and the second powder (powder 2A) were mixed in a mass ratio of 96:4. The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 3.

(Comparative Example 2)
[Preparation of a third powder containing primary particles with a small thickness]
100 parts by mass of boric acid powder (purity 99.8% by mass or higher, manufactured by Kanto Chemical Co., Ltd.), 80 parts by mass of melamine powder (purity 99.0% by mass or higher, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 1 part by mass of sodium carbonate (purity 99.5% by mass or higher) as an auxiliary agent were added and mixed in a mortar for 10 minutes to obtain a mixed powder. After drying, the mixed powder was placed in a container made of hexagonal boron nitride and placed in an electric furnace. Nitrogen gas was circulated in the electric furnace and the temperature was raised from room temperature to 1000°C at a heating rate of 10°C/min. After holding at 1000°C for 2 hours, heating was stopped and it was allowed to cool naturally. The electric furnace was opened when the temperature fell below 100°C. In this way, a calcined material containing low-crystallinity hexagonal boron nitride was obtained.

100g of the above calcined material was placed in the electric furnace described above. While circulating nitrogen gas through the electric furnace, the temperature was increased from room temperature to 1750°C at a rate of 10°C/min. After maintaining the firing temperature of 1750°C for 4 hours, heating was stopped and natural cooling was allowed. The electric furnace was opened when the temperature fell below 100°C. The resulting calcined material was collected and pulverized in a pulperizer to obtain a powder containing hexagonal boron nitride. Next, to remove impurities from the powder, the coarse powder was added to 0.1N dilute nitric acid and stirred at room temperature for 60 minutes. After stirring, solid-liquid separation was performed by suction filtration, and the filtrate was washed by changing the water until it became neutral. After washing, a dried powder was obtained by drying in a dryer at 120°C for 3 hours. After drying, the powder was sieved through a 200 μm sieve, and the resulting powder 3A was designated as the third powder.

[Preparation of boron nitride powder]
Boron nitride powder was prepared in the same manner as in Example 1, except that the first powder (powder 1A) was used instead of the first powder (a mixture of powders 1A and 1B), and the third powder (powder 3A) was used instead of the second powder (2A), and the first powder (powder 1A) and the third powder (powder 3A) were mixed in a mass ratio of 70:30. The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles were measured for the obtained boron nitride powder in the same manner as in Example 1. The results are shown in Table 3.

(Comparative Example 3)
[Preparation of the first powder containing aggregated particles]
The first powder was prepared by mixing powder 1B, prepared in the same manner as in Example 1, and powder 1D, prepared in the same manner as in Example 7, in a mass ratio of 40:40.

[Preparation of boron nitride powder]
The first boron nitride powder (a mixed powder of powder 1B and powder 1D) and the second powder (powder 2A), obtained as described above, were mixed in a mass ratio of 80:20 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 3.

(Comparative Example 4)
[Preparation of boron nitride powder]
Boron nitride powder was prepared in the same manner as in Example 1, except that the first powder (powder 1C) was used instead of the first powder (a mixed powder of powder 1A and powder 1B), and the first powder (powder 1C) and the second powder (powder 2A) were mixed in a mass ratio of 80:20. The crushing strength of the aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles of the obtained boron nitride powder were measured in the same manner as in Example 1. The results are shown in Table 3.

(Comparative Example 5)
[Preparation of the first powder containing aggregated particles]
100 parts by mass of orthoboric acid (manufactured by Nippon Denko Co., Ltd.) 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 prepare a lump of boron carbide ( _B4C ) powder.

The prepared lump of boron carbide powder was pulverized in a vibratory mill for 15 minutes, and then sieved through a 106 μm sieve to prepare boron carbide powder ( _B4C powder) with an average particle size of 60 μm. 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 30 hours under a nitrogen gas atmosphere of 0.85 _MPa to obtain a calcined product containing boron carbonitride ( _B4CN4 ) powder (pressure nitriding step). The obtained calcined product was placed in a muffle furnace and heated at 700°C for 5 hours under an atmospheric atmosphere (oxidation step) to obtain a heat-treated product.

To 100 parts by mass of the mixture of the above heat-treated material and boric acid (boron source), boric acid was added so that its proportion as the boron source was 36 parts by mass. The mixture was then mixed using a Henschel mixer to obtain a mixture. The mixture was packed into a boron nitride crucible and decarburized by heating in a resistance heating furnace to synthesize boron nitride powder containing aggregated particles formed by the aggregation of primary particles (crystallization step). The crystallization conditions were a nitrogen gas atmosphere at a pressure of 5 kPa, with the temperature raised from room temperature to 2000°C and held at 2000°C for 5 hours. 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 180 μm. In this way, powder 1I containing aggregated particles formed by the aggregation of primary particles of boron nitride was obtained.

Powder 1A, powder 1B, and powder 1I, prepared in the same manner as in Example 1, were mixed in a mass ratio of 55:20:5 to prepare the first powder.

[Preparation of boron nitride powder]
The first boron nitride powder (a mixed powder of powders 1A, 1B, and 1I) and the second powder (powder 2A), obtained as described above, were mixed in a mass ratio of 80:20 to obtain boron nitride powder. The mixing conditions were the same as in Example 1. For the obtained boron nitride powder, the crushing strength of aggregated particles, the orientation index of the boron nitride powder, the tap density, the particle size distribution, the area, and the average thickness of the primary particles were measured in the same manner as in Example 1. The results are shown in Table 3.

According to this disclosure, a boron nitride powder can be provided that exhibits excellent fillability into resins and can be used to prepare resin sheets that exhibit excellent heat dissipation and insulation properties when filled into resins.

Claims (1)

It comprises primary boron nitride particles and aggregated particles formed by the aggregation of primary boron nitride particles, having a crushing strength of 5 MPa or more.
The orientation index is 15 or less.
The tap density is 0.75 g/ ^cm³ or higher.
The volume-based cumulative particle size distribution curve has two or more peaks, and the ratio of the minimum frequency value Y between the peaks to the maximum frequency value X is 15% or more.
In the volume-based cumulative particle size distribution curve, the 10% cumulative diameter is 10 μm or less, the 50% cumulative diameter is 20 to 60 μm, the 90% cumulative diameter is 50 to 100 μm, and the 99% cumulative diameter is 135 μm or less .
The specific surface area is 3.5 ^m² /g or less.
Boron nitride powder having an average thickness of 0.5 μm or more of the primary particles .