JP7684223B2 - Method for adjusting particle crushing strength of boron nitride powder, boron nitride powder and its manufacturing method - Google Patents

Description

The present invention relates to a method for adjusting the particle crushing strength of boron nitride powder, boron nitride powder, and a method for producing the same.

Efficiently dissipating heat generated during use is an issue for electronic components such as power devices, transistors, thyristors, and CPUs. To address this issue, traditional approaches have been to increase the thermal conductivity of the insulating layer of the printed wiring board on which the electronic components are mounted, or to attach the electronic components or printed wiring board to a heat sink via electrically insulating thermal interface materials. Such insulating layers and thermal interface materials are made of ceramic powders with high thermal conductivity.

As a ceramic powder, boron nitride powder, which has characteristics such as high thermal conductivity, high insulation, and low dielectric constant, has attracted attention. For example, Patent Document 1 discloses a hexagonal boron nitride powder in which the shape of the aggregates is made more spherical to improve packing properties and powder strength, and furthermore, the powder is highly purified to improve the insulation properties and stabilize the voltage resistance of heat transfer sheets and the like filled with the powder, the hexagonal boron nitride powder being characterized in that the ratio of the major axis to the thickness of the primary particles is 5 to 10 on average, the size of the aggregates of the primary particles is 2 μm or more and 200 μm or less in terms of average particle size (D50), and the bulk density is 0.5 to 1.0 g/ ^cm3 .

Hexagonal boron nitride particles have a thermal conductivity of 400 W/(m·K) in the in-plane direction (a-axis direction) and 2 W/(m·K) in the thickness direction (c-axis direction), resulting in a large anisotropy in thermal conductivity due to the crystal structure and scale shape. Furthermore, when hexagonal boron nitride powder is filled into a resin, the particles are aligned in the same direction. Therefore, for example, when manufacturing a thermal interface material, the in-plane direction (a-axis direction) of the hexagonal boron nitride particles and the thickness direction of the thermal interface material become perpendicular, and the high thermal conductivity of the hexagonal boron nitride particles in the in-plane direction (a-axis direction) cannot be fully utilized.

On the other hand, Patent Document 2 describes how, in boron nitride agglomerated particles formed by agglomeration of primary boron nitride particles, the strength is increased to a level that prevents the agglomerated particles from collapsing even at a specified molding pressure, thereby preventing the primary boron nitride particles from orienting in the same direction.

JP 2011-98882 A JP 2016-135731 A

In boron nitride agglomerated particles formed by agglomerating primary particles as disclosed in Patent Document 2, it is considered important to increase the strength of the agglomerated particles in order to prevent the primary particles from orienting in the same direction and decreasing the thermal conductivity. Meanwhile, according to the studies of the present inventors, in order to obtain boron nitride agglomerated particles having the desired properties, it has been found that it is effective to use not only agglomerated particles with high strength (crushing strength), but also a combination of agglomerated particles with appropriate strength.

In other words, while previous research had focused on increasing the strength of boron nitride agglomerated particles, it was found that a technology was needed to obtain agglomerated particles of appropriate strength to be combined with high-strength agglomerated particles. However, because the appropriate level of strength to be combined with the high-strength agglomerated particles varies depending on the strength of the high-strength agglomerated particles, a technology was needed to precisely adjust the strength of the agglomerated particles.

Therefore, one aspect of the present invention aims to provide a novel method for adjusting the particle crushing strength of boron nitride powder.

The inventors have investigated many factors that affect the crushing strength of agglomerated particles and have discovered that when boron nitride powder is obtained by heating boron carbonitride powder in the presence of calcium carbonate, the particle crushing strength of the boron nitride powder produced can be easily adjusted by adjusting the amount of calcium carbonate added, thereby completing the present invention.

One aspect of the present invention provides a method for adjusting the particle crushing strength of the boron nitride powder, comprising a decarburization step of heating boron carbonitride powder in the presence of calcium carbonate to obtain boron nitride powder, by adjusting the amount of calcium carbonate added in the decarburization step.

Another aspect of the present invention provides a method for producing boron nitride powder, comprising the steps of adding calcium carbonate to boron carbonitride powder to adjust the particle crushing strength, and heating the boron carbonitride powder in the presence of calcium carbonate to decarburize it.

In this manufacturing method, in the step of adding calcium carbonate, calcium carbonate may be added in an amount of 0.125 to 1 part by mass per 100 parts by mass of boron carbonitride powder.

Another aspect of the present invention provides boron nitride powder having a standard deviation of particle crush strength of 3 MPa or less.

Another aspect of the present invention provides a set comprising a plurality of assemblies, each of which has boron nitride powder from a different production lot, and in which the standard deviation of particle crush strength of the boron nitride powder in the set is 3 MPa or less.

According to one aspect of the present invention, a novel method for adjusting the particle crush strength of boron nitride powder is provided.

The following describes an embodiment of the present invention. However, the present invention is not limited to the following embodiment.

In this specification, boron nitride powder refers to a collection of agglomerated particles (also called agglomerated particles) formed by the aggregation of primary particles of boron nitride (agglomerated boron nitride powder) obtained in the same production lot (details will be described later). The primary particles of boron nitride may be, for example, scaly hexagonal boron nitride particles.

In this specification, the particle crushing strength refers to the crushing strength of an agglomerated particle. The particle crushing strength (σ: unit MPa) can be measured according to JIS R1639-5, and can be calculated using the formula σ=α×P/(π×d ² ) from the dimensionless number (α=2.48) that changes depending on the position in the agglomerated particle, the crushing test force (P: unit N), and the particle diameter (d: unit μm). The crushing strength of the boron nitride powder (crushing strength of the powder as a whole) can be obtained by measuring the particle crushing strength of 20 agglomerated particles and obtaining the strength at which the cumulative destruction rate is 63.2%. In the above formula for obtaining the particle crushing strength, the crushing test force can be measured using a microcompression tester (for example, MCT-W500 manufactured by Shimadzu Corporation).

The boron nitride powder according to one embodiment is obtained by a manufacturing method including a step of adding calcium carbonate to boron carbonitride (B ₄ CN ₄ ) to adjust the particle crushing strength (addition step), and a step of heating the boron carbonitride powder in the presence of calcium carbonate to decarburize it (decarburization step).

In one embodiment, the adding step includes a step of sintering boron carbide (B ₄ C) powder to obtain boron carbonitride powder (nitriding step).

In the nitriding process, the average particle size of the boron carbide powder may be 3 μm or more, 5 μm or more, or 10 μm or more, and may be 100 μm or less, 60 μm or less, or 40 μm or less. The average particle size of the boron carbide powder was measured using a Beckman Coulter Laser Diffraction Scattering Particle Size Distribution Measurement Device (LS-13 320) without applying a homogenizer to the sample before the measurement process, and is the particle size at 50% of the cumulative value of the cumulative particle size distribution (median diameter, D50).

The carbon content of the boron carbide powder is desirably smaller than that of B ₄ C (21.7% by mass). The carbon content is preferably 18% by mass or more, more preferably 19% by mass or more, and may be 20.5% by mass or less. When the carbon content is 18% by mass or more, the deviation from the theoretical composition is small, and the composition is stable. When the carbon content is 20.5% by mass or less, the amount of carbon volatilized during the decarburization step described below can be reduced, and dense aggregated particles can be obtained. The carbon content of the boron carbide powder can be measured using a carbon analyzer (e.g., IR-412 model manufactured by LECO).

Boron carbide powder can be obtained by known manufacturing methods. For example, boric acid and acetylene black can be mixed and then heated in an inert gas atmosphere at 1800 to 2400°C for 1 to 10 hours to obtain boron carbide blocks. The boron carbide blocks can be crushed, sieved, washed, impurities removed, dried, etc. to obtain boron carbide powder. Commercially available boron carbide powder may be used.

In the nitriding process, the boron carbide powder is heated under pressure in a nitrogen atmosphere. This sinters the boron carbide powder to obtain boron carbonitride powder.

The atmosphere in the nitriding step is an atmosphere that allows the nitriding reaction to proceed, and may be, for example, nitrogen gas, ammonia gas, etc., which may be used alone or in combination of two or more. From the viewpoints of ease of nitriding and cost, the atmosphere is preferably nitrogen gas. The content of nitrogen gas in the atmosphere is preferably 95% by volume or more, more preferably 99.9% by volume or more.

The pressure (atmospheric pressure) in the nitriding step is preferably 0.6 MPa or more, more preferably 0.7 MPa or more, and preferably 1.0 MPa or less, more preferably 0.9 MPa or less. The pressure is further preferably 0.7 to 1.0 MPa. The firing temperature in the nitriding step is preferably 1800°C or more, more preferably 1900°C or more, and preferably 2400°C or less, more preferably 2200°C or less. The firing temperature is further preferably 1800 to 2200°C. The pressure conditions and firing temperature are preferably 1800°C or more and 0.7 to 1.0 MPa, as these conditions further facilitate the nitridation of boron carbide and are industrially appropriate.

The firing time (heating time) in the nitriding process is appropriately selected within a range in which nitriding proceeds sufficiently, and is preferably 6 hours or more, more preferably 8 hours or more, and is preferably 40 hours or less, more preferably 30 hours or less.

The boron carbonitride powder may be a commercially available product. In other words, the addition process does not have to include the nitriding process described above.

In the addition process, calcium carbonate is added to the boron carbonitride powder to adjust the particle crushing strength.

In the addition step, calcium carbonate is added to adjust the particle crushing strength of the boron nitride powder that is finally produced. That is, calcium carbonate is added for a purpose different from the purpose for which it has been used conventionally, such as as a sintering aid. When calcium carbonate is used as a sintering aid, it is sufficient to sufficiently sinter the boron carbonitride powder, and therefore, unlike when calcium carbonate is used for the purpose of adjusting the particle crushing strength, it is not necessary to precisely adjust the amount of calcium carbonate added to the order of 1% or less. In other words, in the addition step in this manufacturing method, the amount of calcium carbonate added is adjusted to the order of 1% or less. As the calcium carbonate, a commercially available product can be used as appropriate.

In the addition process, in order to uniformly act the calcium carbonate on the boron carbonitride powder, the calcium carbonate and the boron carbonitride powder may be mixed using a ball mill, a low-frequency resonant acoustic mixer, or the like. This allows the particle crushing strength to be adjusted more accurately.

The amount of calcium carbonate added is adjusted to obtain the desired particle crushing strength. The amount of calcium carbonate added is not particularly limited, but may be set in the range of 0.125 to 1 part by mass per 100 parts by mass of boron carbonitride. When set in this range, an appropriate amount is set from the range of 0.125 to 1 part by mass according to the desired particle crushing strength. Here, it is important for particle crushing strength adjustment to adjust the set amount of addition so that it does not vary. The set amount of addition does not vary means that it is added within the range of the set amount of addition ±0.05 parts by mass per 100 parts by mass of boron carbonitride. The appropriate amount of addition may be determined by previously determining the relationship between the amount of calcium carbonate added and the particle crushing strength, or may be determined based on past measurement results. In addition, as described below, the smaller the amount of calcium carbonate added, the greater the particle crushing strength, and the greater the amount of calcium carbonate added, the smaller the particle crushing strength. It may also be determined by predicting the tendency.

The amount of calcium carbonate added may be 0.025 parts by mass or more, 0.075 parts by mass or more, or 0.125 parts by mass or more, and may be 0.45 parts by mass or less, 0.375 parts by mass or less, or 0.325 parts by mass or less, per 100 parts by mass of boron carbonitride powder, in order to achieve a particle crushing strength of the boron nitride powder of 6 MPa or more, 6.5 MPa or more, or 7 MPa or more.

The amount of calcium carbonate added may be at least 0.375 parts by mass, at least 0.45 parts by mass, or at least 0.5 parts by mass, and may be at most 1 part by mass, at most 0.95 parts by mass, or at most 0.875 parts by mass, per 100 parts by mass of boron carbonitride powder, in order to set the particle crushing strength of the boron nitride powder to less than 6 MPa, at most 5.5 MPa, or at most 5 MPa.

The amount of calcium carbonate added is preferably 0.125 parts by mass or more, more preferably 0.15 parts by mass or more, and even more preferably 0.2 parts by mass or more, per 100 parts by mass of boron carbonitride powder, from the viewpoint of making it easier to obtain boron nitride powder with a larger particle crushing strength and from the viewpoint of making it easier to obtain boron nitride powder with a smaller variation in particle crushing strength. The amount of calcium carbonate added is preferably 1 part by mass or less, more preferably 0.9 parts by mass or less, and even more preferably 0.88 parts by mass or less, per 100 parts by mass of boron carbonitride powder, from the viewpoint of making it easier to obtain boron nitride powder with a smaller particle crushing strength and from the viewpoint of making it easier to obtain boron nitride powder with a smaller variation in crushing strength. By reducing the amount of calcium carbonate added, the particle crushing strength can be adjusted to a large range, and by increasing the amount added, the particle crushing strength tends to be adjusted to a small range.

In the addition step, in addition to the boron carbonitride powder, a boron source may be further added. The boron source may be boric acid, boron oxide, or a mixture thereof.

The ratio of the boron source to the amount of boron carbonitride powder may be appropriately selected. The ratio of the boron source may be, for example, 100 parts by mass or more, preferably 150 parts by mass or more, and may be, for example, 300 parts by mass or less, preferably 250 parts by mass or less, relative to 100 parts by mass of boron carbonitride powder.

In the addition process, other additives used in the art may be further added to the boron carbonitride powder as necessary.

Next, the above-mentioned boron carbonitride powder (including boron carbonitride powder to which a boron source or the like has been added) is decarbonized by heating in the presence of calcium carbonate to obtain boron nitride powder (decarbonization process). In the decarbonization process, agglomerated particles of boron nitride can be obtained, which are formed by agglomeration of decarbonized and further crystallized primary particles of boron nitride (primary particles of scaly hexagonal boron nitride).

The atmosphere in the decarburization step may be, for example, nitrogen gas, ammonia gas, or the like, and may be one kind alone or a combination of two or more kinds. From the viewpoint of ease of decarburization and cost, the atmosphere is preferably nitrogen gas. The content of nitrogen gas in the atmosphere is preferably 95% by volume or more, more preferably 99.9% by volume or more.

The pressure (ambient pressure) in the decarburization process may be normal pressure (atmospheric pressure) or a pressure higher than normal pressure. If the pressure is higher than normal pressure, the pressure may be, for example, 0.5 MPa or less, or 0.3 MPa or less.

In the decarburization process, for example, the boron carbonitride powder is first heated to a predetermined temperature (the temperature at which decarburization can begin), and then further heated to a holding temperature at the predetermined temperature. The predetermined temperature (the temperature at which decarburization can begin) can be set according to the system, and may be, for example, 1000°C or higher, 1500°C or lower, and preferably 1200°C or lower. The rate of heating from the predetermined temperature (the temperature at which decarburization can begin) to the holding temperature may be, for example, 5°C/min or lower, and may preferably be 4°C/min or lower, 3°C/min or lower, or 2°C/min or lower.

The holding temperature is preferably 1600°C or higher, more preferably 1800°C or higher, from the viewpoint of favorable particle growth and improving the thermal conductivity of the resulting boron nitride powder. The holding temperature is preferably 2200°C or lower, more preferably 2100°C or lower.

The holding time at the holding temperature is appropriately selected within a range in which crystallization proceeds sufficiently, and may be, for example, more than 0.5 hours. From the viewpoint of favorable particle growth, it is preferably 1 hour or more, more preferably 3 hours or more, and even more preferably 5 hours or more. The holding time at the holding temperature may be, for example, less than 40 hours, and from the viewpoint of reducing the decrease in crushing strength due to excessive particle growth and reducing industrial inconveniences, it is preferably 30 hours or less, more preferably 20 hours or less.

In the decarburization process, it is preferable to set the heating rate up to the holding temperature within a range of 5°C/min or less and the holding time within a range of 1 hour to 30 hours so that the heat is applied uniformly to the boron carbonitride particles. Setting a slow heating rate and a long holding time allows heat to be applied more uniformly, suppressing variation in the particle crushing strength of the agglomerated particles.

The boron nitride powder thus obtained is a powder having agglomerated particles formed by agglomeration of primary particles. If necessary, the boron nitride powder may be subjected to a classification step (classification step) using a sieve to obtain boron nitride powder having a desired particle size distribution.

In the boron nitride powder obtained by the manufacturing method described above, the particle crushing strength of the agglomerated particles is adjusted by adjusting the amount of calcium carbonate added. Therefore, according to the manufacturing method described above, agglomerated particles having the desired particle crushing strength can be obtained, and further, boron nitride powder having the desired crushing strength can be obtained. Conventionally, the amount of boron source added has been known to be a factor that affects particle crushing strength, but by adjusting the amount of calcium carbonate added, carbon is less likely to remain in the boron nitride powder than by adjusting the amount of boron source added, and the yield of boron nitride powder can be increased.

The average particle size of the boron nitride powder is not particularly limited, but may be 2 μm or more, 4 μm or more, 6 μm or more, or 8 μm or more, and may be 40 μm or less, 30 μm or less, 20 μm or less, or 15 μm or less. The average particle size may be greater than 40 μm, 45 μm or more, 50 μm or more, or 55 μm or more, and may be 100 μm or less, 80 μm or less, or 75 μm or less. The average particle size of the boron nitride powder was measured using a Beckman Coulter Laser Diffraction Scattering Particle Size Distribution Measuring Device (LS-13 320) without applying a homogenizer to the sample before the measurement process, and is the particle size at 50% of the cumulative value of the cumulative particle size distribution (median diameter, D50).

The particle crushing strength of the boron nitride powder may be 1 MPa or more, 1.5 MPa or more, or 2 MPa or more, and may be 20 MPa or less, 15 MPa or less, or 10 MPa or less. When the average particle size of the boron nitride powder is 40 μm or less, the particle crushing strength may be 1 MPa or more, 1.5 MPa or more, or 2 MPa or more, and may be 10 MPa or less, 7 MPa or less, or 5 MPa or less. When the average particle size of the boron nitride powder is more than 40 μm, the particle crushing strength may be 1 MPa or more, 3 MPa or more, 5 MPa or more, or 7 MPa or more, and may be 20 MPa or less, 15 MPa or less, or 10 MPa or less.

The crushing strength of the boron nitride powder (the strength at which the cumulative fracture rate is 63.2%) may be 1 MPa or more, 1.5 MPa or more, or 2 MPa or more, and may be 20 MPa or less, 15 MPa or less, or 10 MPa or less.

Another embodiment of the present invention includes a decarburization process in which boron carbonitride powder is heated in the presence of calcium carbonate to obtain boron nitride powder, and the amount of calcium carbonate added in the decarburization process is adjusted to adjust the particle crushing strength of the boron nitride powder (a method for adjusting particle crushing strength). The detailed aspects of the decarburization process are as described above. This method makes it easy to adjust the particle crushing strength of the boron nitride powder to a desired range.

Furthermore, according to the above-mentioned manufacturing method, by adjusting the amount of calcium carbonate added in the decarburization process, it is possible to obtain boron nitride powder with reduced variation in particle crushing strength. The variation in particle crushing strength in boron nitride powder means that the standard deviation of particle crushing strength between agglomerated particles contained in the boron nitride powder is small.

In this specification, boron nitride powder refers to powder obtained from the same production lot, so the small variation in particle crushing strength of this boron nitride powder means that the variation in agglomerated particles within the same production lot of boron nitride powder is small. Note that the production lot in this specification refers to boron nitride powder produced at one time through a series of steps in the same facility using the above-mentioned production method.

Another embodiment of the present invention can be said to be a boron nitride powder having a standard deviation of particle crushing strength of 3 MPa or less. In other words, this boron nitride powder is boron nitride powder from the same production lot, and the standard deviation of particle crushing strength between agglomerated particles is 3 MPa or less. The standard deviation of particle crushing strength between agglomerated particles is obtained by taking 20 agglomerated particles from the boron nitride powder obtained by the above-mentioned production method, measuring the particle crushing strength of each particle, and then calculating the standard deviation.

In this boron nitride powder, the standard deviation of the particle crushing strength is 3 MPa or less, and may be 2.9 MPa or less, 2.5 MPa or less, 2.3 MPa or less, or 2 MPa or less. When the average particle size of the boron nitride powder is 40 μm or less, the standard deviation of the particle crushing strength of the boron nitride powder may be 3 MPa or less, 2.5 MPa or less, 2 MPa or less, or 1.7 MPa or less. When the average particle size of the boron nitride powder is more than 40 μm, the standard deviation of the particle crushing strength of the boron nitride powder may be 3 MPa or less, 2.9 MPa or less, or 2.5 MPa or less. Increasing the amount of calcium carbonate added tends to reduce the standard deviation.

Furthermore, according to the above-mentioned manufacturing method, by adjusting the amount of calcium carbonate added in the decarburization process, it is possible to suppress variation in particle crushing strength even between boron nitride powders from different production lots.

Another embodiment of the present invention is a set composed of a plurality of assemblies, each of which has boron nitride powder from a different production lot, and the standard deviation of the particle crushing strength of the boron nitride powder in the set is 3 MPa or less.

The set according to one embodiment is composed of a plurality of assemblies. Each of the plurality of assemblies contains boron nitride powder obtained by the manufacturing method described above, and each of the plurality of assemblies contains boron nitride powder from different manufacturing lots.

In this set, a first aggregate among the multiple aggregates that make up the set contains boron nitride powder from one production lot obtained by the manufacturing method described above.

In one embodiment, the first aggregate may be composed of only boron nitride powder from one production lot (referred to as lot A). In this case, the first aggregate may be composed of boron nitride powder from lot A contained in one packaging bag or the like, or may be composed of a group of boron nitride powder from lot A divided and contained in multiple packaging bags or the like.

In another embodiment, the first assembly may be a mixture (referred to as mixture A) of boron nitride powder from lot A and boron nitride powder from one or more production lots different from lot A. In this case, the first assembly may be composed of the boron nitride powder from mixture A contained in one packaging bag or the like, or may be composed of groups of the boron nitride powder from mixture A divided and contained in multiple packaging bags or the like.

In each embodiment, the second aggregate of the multiple aggregates has boron nitride powder from at least a lot (lot B) different from lot A. "A production lot different from lot A" means a lot that is produced under the same production conditions as lot A and using similar equipment, etc., but is produced at a different time than lot A. The same meaning is also applied to expressions such as "a production lot different from lot B."

The second aggregate may be composed only of boron nitride powder of lot B. In this case, the second aggregate may be composed of boron nitride powder of lot B contained in one packaging bag or the like, or may be composed of a group of boron nitride powder of lot B divided and contained in multiple packaging bags or the like.

Alternatively, the second assembly may be a mixture (mixture B) of boron nitride powder from lot B and boron nitride powder from one or more production lots different from lot B. In this case, the second assembly may be composed of mixture B contained in one packaging bag or the like, or may be composed of a group of mixture B contained in multiple packaging bags or the like. Mixture B may contain boron nitride powder from lot A, so long as the first assembly and the second assembly are not exactly the same.

In this embodiment, the set may further include other assemblies (a third assembly, a fourth assembly, etc.). In this case, the relationship between the assemblies is the same as the relationship between the first assembly and the second assembly described above.

The set described above contains boron nitride powder manufactured by the above-mentioned manufacturing method, so that the variation in particle crushing strength of the boron nitride powder between multiple aggregates can be reduced. In other words, in this set, the standard deviation of particle crushing strength between each aggregate can be made 3 MPa or less.

The particle crushing strength of a set is obtained by taking 20 agglomerated particles of boron nitride powder from each of the aggregates that make up the set, measuring the particle crushing strength of all the particles, and then calculating the standard deviation.

The standard deviation of particle crushing strength in the set is 3 MPa or less, and may be 2.9 MPa or less, 2.5 MPa or less, 2.3 MPa or less, or 2 MPa or less.

The standard deviation of the particle crushing strength in the set may be 3 MPa or less, 2.5 MPa or less, 2 MPa or less, or 1.7 MPa or less when the average particle size of the boron nitride powder is 40 μm or less. The standard deviation of the particle crushing strength in the set may be 3 MPa or less, 2.9 MPa or less, or 2.5 MPa or less when the average particle size of the boron nitride powder is more than 40 μm.

A set is, for example, when the same product of boron nitride powder exists in the form of multiple packages, and multiple packages stored in close proximity in a warehouse or the like constitutes a set. Even if multiple assemblies do not exist simultaneously in time, the same manufacturing entity may manufacture multiple assemblies and immediately transport each assembly after production, which also constitutes the manufacture of a set. Packages that are transported out of a warehouse or manufacturing plant and then stored, for example, as raw materials for the same product, also constitute a set. Even if they do not exist simultaneously in time, the purchase of multiple packages for the manufacture of the same product constitutes the purchase of a set.

A resin composition filled with boron nitride powder can be produced using the above-mentioned boron nitride powder and a set of multiple aggregates containing boron nitride powder. When boron nitride powder with a predetermined particle crushing strength, which has been adjusted in particle crushing strength, is combined with boron nitride powder with a stronger particle crushing strength and filled into a resin to be used as a resin composition, a resin composition with desired properties can be stably produced.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

Example 1
[Preparation of boron carbonitride powder]
100 parts by mass of orthoboric acid (manufactured by Nippon Denko Co., Ltd., hereinafter simply referred to as "boric acid") and 35 parts by mass of acetylene black (HS100, manufactured by Denka Co., Ltd.) were mixed using a Henschel mixer, and then filled into a graphite crucible. The 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 (B ₄ C) powder. The synthesized lump of boron carbide powder was pulverized in a ball mill for 1 hour and sieved into particles of 75 μm or less using a sieve. The sieved boron carbide powder was further washed with an aqueous nitric acid solution to remove impurities such as iron, and filtered and dried to produce a boron carbide powder (B ₄ C powder) with an average particle size of 39.9 μm.

The produced boron carbide powder was filled into a boron nitride crucible and heated in a resistance heating furnace under conditions of 2000° C. and 0.85 MPa in a nitrogen gas atmosphere for 20 hours to obtain boron carbonitride (B ₄ CN ₄ ) powder.

[Preparation of boron nitride powder]
40 parts by mass of boron carbonitride powder and 60 parts by mass of boric acid were added with calcium carbonate in an amount of 0.125% by mass relative to 100 parts by mass of boron carbonitride powder, and mixed by a Henschel mixer. This mixture was filled into a boron nitride crucible and decarbonized by heating using a resistance heating furnace to synthesize boron nitride powder having aggregated particles in which primary particles were aggregated. The heating conditions were a normal pressure nitrogen gas atmosphere, a heating rate from room temperature to 1000°C of 10°C/min, a heating rate from 1000°C of 0.5°C/min, a holding temperature of 2000°C, and a holding time of 5 hours. The synthesized boron nitride powder was crushed into 10 pieces in a mortar, and then classified using a nylon sieve with a sieve mesh of 75 μm. As a result, boron nitride powder (also called "BN powder") with an average particle size of 55.8 μm was obtained.

<Production of a different lot>
Another lot of boron nitride powder was produced by the method of Example 1. A total of three lots of boron nitride powder were obtained.

<Examples 2 to 7>
Boron nitride powders were produced in the same manner as in Example 1, except that the average particle size of the raw material boron carbide powder (B ₄ C powder) and/or the amount of calcium carbonate added were changed as shown in Table 1. In each of Examples 2 to 7, three lots of boron nitride powder were produced.

[Measurement of particle crushing strength]
The particle crushing strength of the boron nitride powder according to the embodiment was measured in accordance with JIS R1639-5:2007. A microcompression tester (MCT-W500, manufactured by Shimadzu Corporation) was used as the measuring device. The particle crushing strength (σ: unit MPa) was measured using the equation σ=α×P/(π×d2) from the dimensionless number (α=2.48) that changes depending on the position in the particle, the crushing test force (P: unit N), and the particle diameter (d: unit μm). For one lot of each embodiment, the particle crushing strength was measured for agglomerated particles of 20 particles each. In addition, from the particle crushing strengths of the measured 20 particles, the value at the time when the cumulative destruction rate was 63.2% was calculated as the crushing strength of the boron nitride powder.

The crushing strength value of the boron nitride powder at a cumulative destruction rate of 63.2% is shown in Table 1. As shown in Table 1, it was found that in the production of boron nitride powder, the particle crushing strength can be adjusted by adjusting the amount of calcium carbonate added, thereby adjusting the crushing strength of the boron nitride powder.

Next, in each example, the standard deviation of particle crushing strength for 20 agglomerated particles of the same lot was calculated. The results are shown in Table 1. In each example, the standard deviation within each of the three lots was equivalent (±0.2), so one representative value is shown. As shown in Table 1, the boron nitride powder according to the example had small variation in particle crushing strength within the same production lot. The boron nitride powders of Examples 1 and 2 had the same crushing strength, but Example 2, in which the amount of calcium carbonate added was increased, had a smaller standard deviation than Example 1, and it was found that the variation in particle crushing strength was even smaller.

In each example, a set of boron nitride powder consisting of three aggregates was constructed, with the first lot of boron nitride powder as the first aggregate, the second lot of boron nitride powder as the second aggregate, and the third lot of boron nitride powder as the third aggregate. 20 particles of agglomerated particles were taken from each aggregate, and the standard deviation of the particle crushing strength (standard deviation of a total of 60 particles) was calculated as the standard deviation in the set (standard deviation between aggregates). As a result, the standard deviation between aggregates in each example was a value that could be said to be equivalent (±0.2) to the standard deviation within the above-mentioned lot.

On the other hand, a set of boron nitride powder consisting of three aggregates was constructed, with one lot of boron nitride powder produced by the method of Example 2 as the first aggregate, one lot of boron nitride powder produced by the method of Example 3 as the second aggregate, and one lot of boron nitride powder produced by the method of Example 4 as the third aggregate. 20 aggregate particles were taken out from each aggregate, and the standard deviation of the particle crushing strength (standard deviation of a total of 60 particles) was calculated as the standard deviation in the set (standard deviation between aggregates), which was 3.3. In other words, it was found that by finely adjusting the amount of calcium carbonate added in the production of boron nitride powder, the variation in particle crushing strength can be reduced in a set consisting of multiple aggregates having boron nitride powder from different production lots.

In Example 1, the resin composition was produced using the boron nitride powder from the above three production lots separately, and a resin composition having the desired properties was obtained under similar production conditions. On the other hand, in Example 5, a resin composition was obtained using the boron nitride powder from the three production lots in the same way, but the production conditions were different from those in Example 1 because the particle crushing strength was different. Since it is practically very difficult to produce a resin composition while minutely changing the production conditions, it can be said that a resin composition having the desired properties can be stably produced by adjusting the particle crushing strength by the amount of calcium carbonate added.

Claims (4)

A decarburization step of heating the boron carbonitride powder in the presence of calcium carbonate to obtain boron nitride powder,
A method for adjusting the particle crushing strength of the boron nitride powder by adjusting the amount of calcium carbonate added in the decarburization step. adding calcium carbonate to the boron carbonitride powder to adjust particle crush strength;
and heating the boron carbonitride powder in the presence of the calcium carbonate to decarburize the boron carbonitride powder. The manufacturing method according to claim 2, wherein in the step of adding calcium carbonate, the calcium carbonate is added in an amount of 0.125 to 1 part by mass per 100 parts by mass of the boron carbonitride powder. Boron nitride powder having a particle crushing strength of 2 MPa or more and a standard deviation of said particle crushing strength of 3 MPa or less.