Method for manufacturing boron nitride powder, powder, and method for manufacturing powder

Abstract

To provide a manufacturing method capable of producing boron nitride powder containing agglomerate particles with a relatively low crushing strength.SOLUTION: One aspect of the present disclosure provides a method for manufacturing boron nitride powder, comprising the steps of: heat-treating a boron carbonitride powder containing agglomerates of boron carbonitride at 850°C or higher for 3 to 15 hours in an atmosphere having an oxygen partial pressure of 20% or more to obtain a powder containing agglomerated particles having carbon, nitrogen, boron and oxygen as constituent elements; and obtaining a heat-treated product with reduced oxygen content and carbon content by heat-treating a mixture containing the powder and a boron source at 1800 to 2200°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a boron nitride powder, a powder, and a method for producing a powder. [Background technology]

[0002] In electronic components such as power devices, transistors, thyristors, and CPUs, efficient heat dissipation is a challenge. Therefore, heat dissipation materials with high thermal conductivity are used together with these electronic components. Meanwhile, boron nitride particles, with their high thermal conductivity and high insulating properties, are widely used as a filler in heat dissipation materials.

[0003] Hexagonal boron nitride primary particles have a thermal conductivity of 400 W / (m·K) in the in-plane direction (a-axis direction) but only 2 W / (m·K) in the thickness direction (c-axis direction), resulting in a large anisotropy in thermal conductivity due to their crystalline structure and scale shape. Furthermore, when hexagonal boron nitride powder is filled into resin and molded, the force applied during molding causes multiple primary particles to orient in the same direction within the resin molded body. In this case, for example, the in-plane direction (a-axis direction) of the hexagonal boron nitride primary particles is oriented perpendicular to the thickness direction of the heat dissipation component, resulting in low thermal conductivity in the thickness direction of the heat dissipation component and not being able to fully utilize the high thermal conductivity of the hexagonal boron nitride particles in the in-plane direction (a-axis direction).

[0004] From the viewpoint of reducing the shape-based anisotropy described above, methods have been studied for forming aggregated particles by suppressing the orientation of a plurality of primary particles and aggregating them. Patent Document 1 discloses boron nitride agglomerated particles formed by aggregating boron nitride primary particles. It is described that the agglomerated particles have a strength sufficient to suppress the collapse of the agglomerated particles even when dispersed in a resin and subjected to a predetermined molding pressure, and that the boron nitride primary particles are prevented from being oriented in the same direction in the molded body.

[0005] Furthermore, Patent Document 2 describes a boron nitride powder containing block boron nitride with excellent particle strength, which is obtained by pressurizing and nitriding boron carbide having a specified average particle size, followed by decarburization and crystallization, so that it can be used in processes that involve strong stress, such as kneading with resin or forming into a sheet. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-135731 [Patent Document 2] International Publication No. 2018 / 066277 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, when considering use as a filler for resins, particularly as a heat-dissipating filler, it is desirable for the agglomerated particles to maintain their shape, and it is desirable for the agglomerated particles to have a crushing strength sufficient to maintain their shape during kneading with resin or molding of the resin composition. On the other hand, from the viewpoint of the insulating properties of the resin molded body, it is desirable for the agglomerated particles to change shape due to molding pressure or the like, thereby reducing voids within the agglomerated particles and voids in the resin molded body. In some cases, it is desirable for the agglomerated particles to change shape during kneading with resin or molding of the resin composition. Furthermore, if the crushing strength is too high, it may be necessary to improve the performance of the kneader used during kneading with resin or the molding machine used during molding of the resin composition. It would be useful to have a method for adjusting the crushing strength of agglomerated particles so that they can be adapted to various manufacturing conditions for resin molded bodies containing boron nitride powder as a filler.

[0008] The present disclosure aims to provide a manufacturing method capable of producing boron nitride powder containing agglomerated particles with a relatively low crushing strength. The present disclosure also aims to provide a powder suitable as a raw material for producing the boron nitride powder, and a manufacturing method thereof. [Means for solving the problem]

[0009] One aspect of the present disclosure provides the following [1].

[0010] [1] A step of heat-treating boron carbonitride powder containing aggregates of boron carbonitride at 850°C or higher for 3 to 15 hours in an atmosphere having an oxygen partial pressure of 20% or higher to obtain a powder containing aggregate particles whose constituent elements are carbon, nitrogen, boron, and oxygen; and heat-treating a mixture containing the powder and a boron source at 1800 to 2200°C to obtain a heat-treated product having reduced oxygen and carbon contents.

[0011] In the above-mentioned method for producing boron nitride powder, in the step of obtaining the powder, boron carbonitride powder is intentionally oxidized by heating it in an oxygen-containing atmosphere at a relatively high temperature for a predetermined period of time, and then a high-temperature heat treatment is performed to volatilize and remove at least a portion of the oxidized portion and decarburize it to boron nitride, thereby producing boron nitride powder containing aggregated particles of boron nitride. The aggregated particles of boron nitride contained in the boron nitride powder produced by this procedure have voids formed inside them due to the reduction in the oxidized portions, and it is possible to obtain a boron nitride powder containing aggregated particles with more voids and relatively low crushing strength compared to conventional aggregated particles of hexagonal boron nitride.

[0012] The method for producing the boron nitride powder may be any one of the following methods [2] to [4].

[0013] [2] The method according to [1], wherein the powder has an oxygen content of 5.0 to 30.0% by mass. [3] The method according to [1] or [2], wherein the heating temperature in the step of obtaining the powder is 950°C or less. [4] The method according to any one of [1] to [3], wherein the powder has a carbon content of 2.0 mass % or less.

[0014] One aspect of the present disclosure provides the following [5].

[0015] [5] A method for producing a powder, comprising: a step of heat-treating boron carbonitride powder containing aggregates of boron carbonitride at 850°C or higher for 3 to 15 hours in an atmosphere having an oxygen partial pressure of 20% or higher, to obtain a powder containing aggregate particles whose constituent elements are carbon, nitrogen, boron, and oxygen.

[0016] The method for producing the powder described above involves heating boron carbonitride powder in an oxygen-containing atmosphere at a relatively high temperature for a predetermined period of time to oxidize it, thereby obtaining a powder containing oxygen. The powder contains agglomerated particles from which the oxidized portions can be removed by further heat treatment, and the boron carbonitride can be converted to boron nitride by decarburization, making it suitable as a raw material for producing boron nitride powder containing agglomerated particles with internal voids and relatively low crushing strength.

[0017] The method for producing the powder may be the following [6] or [7].

[0018] [6] The method according to [5], wherein the powder has an oxygen content of 5.0 to 30.0% by mass. [7] The method according to [5] or [6], wherein the heating temperature in the step of obtaining the powder is 950°C or less.

[0019] One aspect of the present disclosure provides the following [8].

[0020] [8] A powder containing agglomerated particles whose constituent elements are carbon, nitrogen, boron, and oxygen, the oxygen content being 5.0 to 30.0 mass%.

[0021] The powder contains oxygen so that the oxygen content falls within the above range. The powder contains agglomerated particles from which the oxidized portions can be removed by further heat treatment, and boron carbonitride can be converted to boron nitride by decarburization, making the powder suitable as a raw material for producing boron nitride powder containing agglomerated particles with internal voids and relatively low crushing strength.

[0022] The powder may be the following [9] or

[10] .

[0023] [9] The powder according to [8], wherein the carbon content is 2.0 mass% or less.

[10] The powder according to [8] or [9], having an average particle size of 10 to 100 μm. [Effects of the Invention]

[0024] The present disclosure provides a method for producing boron nitride powder containing agglomerated particles with a relatively low crushing strength, a powder suitable as a raw material for producing the boron nitride powder, and a method for producing the powder. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a scanning electron microscope image showing the appearance of the aggregated particles containing carbon, nitrogen, boron, and oxygen as constituent elements prepared in Example 1. [Figure 2] FIG. 2 is a scanning electron microscope image showing a cross section of the aggregated particles containing carbon, nitrogen, boron, and oxygen as constituent elements prepared in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, embodiments of the present disclosure will be described, occasionally with reference to the drawings. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following content. In this specification, a numerical range indicated by the symbol "to" includes a lower limit and an upper limit. In other words, a numerical range indicated by "x to y" means greater than or equal to x and less than or equal to y.

[0027] Unless otherwise specified, the materials exemplified in this specification can be used singly or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the multiple substances present in the composition, unless otherwise specified.

[0028] One embodiment of a method for producing boron nitride powder includes a step of heat-treating boron carbonitride powder containing aggregates of boron carbonitride at 850°C or higher for 3 to 15 hours in an atmosphere having an oxygen partial pressure of 20% or higher to obtain a powder containing aggregated particles whose constituent elements are carbon, nitrogen, boron, and oxygen (hereinafter also referred to as an oxidation step), and a step of heat-treating a mixture containing the powder and a boron source at 1800 to 2200°C to obtain a heat-treated product with reduced oxygen and carbon contents (hereinafter also referred to as a crystallization step).

[0029] The boron carbonitride powder used in the oxidation step can be any powder containing agglomerates of boron carbonitride, and the boron carbonitride powder may be purchased or separately prepared. In other words, the method for producing the boron nitride powder may include a step of preparing the boron carbonitride powder. In this specification, the boron carbonitride agglomerates refer to particles formed by agglomerating a plurality of single particles of boron carbonitride. The boron carbonitride may be hexagonal boron carbonitride.

[0030] Boron carbonitride powder (B4CN4) can be prepared, for example, by nitriding boron carbide. More specifically, the boron carbonitride powder can be prepared by a process (pressure nitriding process) in which boron carbide powder (B4C) is fired at 1900 to 2200°C in a pressurized nitrogen atmosphere to obtain boron carbonitride powder.

[0031] The lower limit of the firing temperature in the pressure nitriding step may be 1900°C or higher, or may be 2000°C or higher. By setting the lower limit of the firing temperature within the above range, the nitriding of boron carbide can be more sufficiently promoted. Furthermore, the upper limit of the firing temperature may be 2200°C or lower, or may be 2150°C or lower. The firing temperature may be adjusted within the above range, and may be, for example, 1900 to 2200°C or 1900 to 2150°C.

[0032] The lower limit of the pressure in the pressure nitriding step may be, for example, 0.6 MPa or more, 0.7 MPa or more, or 0.8 MPa or more. By setting the lower limit of the pressure within the above range, the nitriding of boron carbide can be more fully promoted. The upper limit of the pressure in the pressure nitriding step may be, for example, 1.0 MPa or less, or 0.9 MPa or less. By setting the upper limit of the pressure within the above range, a long oxidation treatment is not required to reduce the crushing strength to the desired strength, and an increase in the production cost of the boron nitride powder can be suppressed. The pressure may be adjusted within the above range, for example, 0.6 to 1.0 MPa.

[0033] The nitrogen gas concentration in the nitrogen pressurized atmosphere in the pressure nitriding step may be, for example, 95.0% by volume or more, 98.0% by volume or more, or 99.9% by volume or more. By setting the nitrogen gas concentration within the above range, nitriding of boron carbide can be carried out under milder conditions. The above nitrogen gas concentration is a concentration based on volume under standard conditions. The firing time in the pressure nitriding step is not particularly limited as long as it is within a range in which nitriding proceeds sufficiently, and may be, for example, 6 to 30 hours, or 8 to 20 hours.

[0034] In the oxidation step of the above-described method for producing boron nitride powder, heat treatment is performed in an atmosphere with an oxygen partial pressure of 20% or higher to introduce oxygen into the boron carbonitride powder. Conventional methods for producing boron nitride powder avoid oxidation of the boron carbonitride powder, based on the assumption that oxidation of the boron carbonitride powder reduces the yield of the resulting boron nitride powder and increases the production cost of the boron nitride powder. In contrast, the method for producing boron nitride powder according to the present disclosure intentionally performs an oxidation treatment on the boron carbonitride powder to increase the oxygen content of the powder and form agglomerated particles with oxidized portions in the agglomerates. This reduces the proportion of the oxidized portions in the subsequent crystallization step, turning the regions where the oxidized portions existed into voids. This makes it possible to prepare aggregated particles with a relatively low crushing strength compared to those obtained by crystallization without oxidation treatment. As used herein, agglomerated particles refer to particles having a structure similar to that of agglomerates formed by the aggregation of multiple boron carbonitride particles, with an oxide layer formed on at least a portion of the surface of the boron carbonitride particles. The powder can also be referred to as a boron carbonitride-based powder containing aggregated particles of boron carbonitride having an oxide layer. The oxide layer may include or consist of boron oxide (B2O3).

[0035] The atmosphere having an oxygen partial pressure of 20% or more may be, for example, air, or a mixed gas with an adjusted oxygen partial pressure. From the viewpoint of reducing production costs, the firing atmosphere is air. When a mixed gas is used, the lower limit of the oxygen partial pressure may be, for example, 22% or more, 25% or more, or 30% or more. When the lower limit of the oxygen partial pressure is within the above range, oxygen can be introduced more easily in the oxidation step, and the amount of oxygen in the powder can be further increased. When a mixed gas is used, the upper limit of the oxygen partial pressure may be, for example, 70% or less, 60% or less, or 50% or less. When the upper limit of the oxygen partial pressure is within the above range, excessive oxidation of the powder can be prevented. When a mixed gas is used, the oxygen partial pressure may be adjusted within the above range, for example, 20 to 80% or 25 to 60%.

[0036] The oxygen partial pressure in this specification refers to the partial pressure of oxygen in the mixed gas occupying the firing atmosphere under standard conditions, and refers to the value measured by an oxygen concentration meter. For example, the "G1690" (product name) manufactured by Sakaki Corporation can be used as the oxygen concentration meter.

[0037] In the oxidation step, the boron carbonitride powder is heat-treated at a relatively high temperature. The lower limit of the heating temperature in the oxidation step may be, for example, 860°C or higher, 870°C or higher, 880°C or higher, 890°C or higher, 900°C or higher, 910°C or higher, 920°C or higher, 930°C or higher, or 940°C or higher. By setting the lower limit of the heating temperature within the above range, decarburization and oxidation of the boron carbonitride powder can be more sufficiently promoted, and the crushing strength can be further reduced. The upper limit of the heating temperature in the oxidation step may be, for example, 1000°C or lower, 980°C or lower, 960°C or lower, or 950°C or lower. By setting the upper limit of the heating temperature within the above range, a decrease in productivity due to excessive oxidation of the boron carbonitride powder can be further suppressed.

[0038] The time for the heat treatment in the oxidation step may be adjusted according to the temperature of the heat treatment, and may be 3 to 15 hours, 5 to 13 hours, 5 to 10 hours, or 5 to 8 hours.

[0039] The powder may be oxidized by heat treatment in the presence of oxygen, resulting in an increased oxygen content. The lower limit of the oxygen content of the powder may be, for example, 5.0 mass% or more, 6.0 mass% or more, 8.0 mass% or more, 10.0 mass% or more, 12.0 mass% or more, 14.0 mass% or more, 16.0 mass% or more, 18.0 mass% or more, 20.0 mass% or more, or 22.0 mass% or more, based on the total amount of the powder. Components such as oxide layers corresponding to the oxygen content are removed from the particles by heating in the crystallization step described below. Therefore, when the lower limit of the oxygen content is within the above range, the powder is more suitable as a raw material for producing aggregated particles of boron nitride having low crushing strength. The upper limit of the oxygen content of the powder may be, for example, 30.0 mass% or less, 28.0 mass% or less, or 25.0 mass% or less, based on the total amount of the powder. When the upper limit of the oxygen content is within the above range, it becomes easier to remove the oxide layer when the obtained powder is used as a raw material for producing boron nitride powder, making the boron nitride powder more suitable for use as a heat-dissipating filler. The oxygen content of the powder may be adjusted within the above range, and may be, for example, 5.0 to 30.0 mass% based on the total amount of the powder.

[0040] The upper limit of the carbon content of the powder may be, for example, 2.0% by mass or less, 1.8% by mass or less, or 1.7% by mass or less, based on the total amount of the powder. By ensuring that the upper limit of the carbon content is within the above range, the amount of boron source blended for decarburization in the crystallization step can be reduced, thereby improving productivity. The lower limit of the carbon content of the powder is not particularly limited, but may be, for example, 0.1% by mass or more, 0.2% by mass or more, 0.5% by mass or more, or 0.8% by mass or more, based on the total amount of the powder.

[0041] In the powder, the total amount of carbon, nitrogen, boron, and oxygen may be, for example, 95% by mass or more, or 98% by mass or more, or 100% by mass (i.e., the particles are composed of carbon, nitrogen, boron, and oxygen), based on the total amount of elements constituting the particles. The composition of the above elements in the particles may be, for example, 0.1 to 2.0% by mass of carbon, 30 to 50% by mass of nitrogen, 30 to 50% by mass of boron, and 5 to 30% by mass of oxygen, or 0.1 to 1.5% by mass of carbon, 40 to 50% by mass of nitrogen, 40 to 50% by mass of boron, and 5 to 20% by mass of oxygen.

[0042] In this specification, the carbon, nitrogen, boron, and oxygen contents refer to values ​​measured by the following methods. The carbon content refers to a value measured using a carbon / sulfur simultaneous analyzer. An example of a carbon / sulfur simultaneous analyzer that can be used is the "IR-412" (trade name) manufactured by LECO. The nitrogen content refers to a value determined by titration. Specifically, a sample is first alkaline decomposed with sodium hydroxide, and ammonia is distilled from the decomposition solution using steam distillation and collected in an aqueous boric acid solution. The nitrogen atom content in the sample can be determined by titrating this collected solution with a normal sulfuric acid solution. The boron content refers to a value determined by titration. Specifically, a sample is first heated and melted in a platinum crucible, completely dissolved in hydrochloric acid, and then titrated with an aqueous sodium hydroxide solution to determine the boron atom content in the sample. The oxygen content refers to a value measured using an oxygen / nitrogen simultaneous analyzer. As the oxygen / nitrogen simultaneous analyzer, for example, "EMGA-910 type" (trade name) manufactured by Horiba Ltd. can be used.

[0043] For example, the powder may have a total amount of carbon, nitrogen, boron, and oxygen of 95 mass% or more, based on the total amount of elements constituting the particles, and an oxygen amount of 5.0 mass% or more, based on the total amount of the powder.

[0044] In the crystallization step, the powder prepared in the oxidation step is heat-treated to reduce the oxygen and carbon content, thereby obtaining boron nitride powder as a heat-treated product. The boron nitride powder includes aggregated particles formed by agglomeration of primary particles of hexagonal boron nitride. In this specification, primary particles of hexagonal boron nitride refer to single particles of hexagonal boron nitride. Furthermore, aggregated particles are particles formed by agglomeration of multiple primary particles whose in-plane directions (a-axis directions) are not oriented in one direction but are oriented in multiple directions, and can also be called secondary particles.

[0045] In the crystallization step, the carbon in the powder is combined with oxygen and removed from the system as carbon dioxide gas, thereby performing a decarburization process, converting boron carbonitride to boron nitride. A boron source, which is heated together with the powder, can accelerate the decarburization process. Examples of the boron source include boric acid, boron oxide, or a mixture thereof. The mixture heated in the crystallization step may contain known additives.

[0046] The blending ratio of the powder and the boron source can be appropriately set depending on the molar ratio of the carbon content in the powder to the oxygen content in the boron source. When at least one of boric acid and boron oxide is used as the boron source, for example, the boron source may be blended so that the total amount of boric acid and boron oxide is 20 to 300 parts by mass, or the boron source may be blended so that the total amount of boric acid and boron oxide is 50 to 250 parts by mass, per 100 parts by mass of the powder.

[0047] The temperature for the heat treatment of the mixture containing the powder and the boron source in the crystallization step is 1800 to 2200°C. The lower limit of the heat treatment temperature may be, for example, 1850°C or higher, 1900°C or higher, 1950°C or higher, or 2000°C or higher. By setting the lower limit of the heating temperature within the above range, the oxygen and carbon contents can be reduced more sufficiently, the crystallinity of the hexagonal boron nitride can be improved, and the voids in the resulting aggregated particles can be increased, thereby further reducing the crushing strength. The upper limit of the heating temperature in the crystallization step may be, for example, 2150°C or lower, or 2100°C or lower. By setting the upper limit of the heating temperature within the above range, yellowing of the boron nitride powder can be further suppressed.

[0048] The crystallization step may be performed under normal pressure (atmospheric pressure) or under pressure exceeding atmospheric pressure (e.g., 10 kPa or higher). When pressure is applied, the pressure may be, for example, 0.5 MPa or lower, or 0.3 MPa or lower. The pressure referred to above is a gauge pressure.

[0049] The heating time in the crystallization step may be 0.5 hours or more, 1 hour or more, 2 hours or more, or 3 hours or more. By setting the lower limit of the heating time within the above range, the oxygen and carbon contents can be reduced more sufficiently, the crystallinity of the hexagonal boron nitride can be improved, and the voids in the resulting aggregated particles can be increased, thereby further reducing the crushing strength. The heating time in the crystallization step may be 40 hours or less, or may be 30 hours or less, 20 hours or less, or 10 hours or less. By setting the upper limit of the heating time within the above range, an increase in the production cost of the boron nitride powder can be suppressed. Note that in this specification, the terms "firing time," "heating time," etc. refer to the time (retention time) during which the temperature of the ambient environment of the object is maintained at a predetermined temperature after it has reached that temperature.

[0050] The upper limit of the crushing strength of the aggregated particles contained in the boron nitride powder obtained by the above-mentioned method for producing boron nitride powder can be, for example, 9.0 MPa or less, 8.0 MPa or less, 7.0 MPa or less, or 6.8 MPa or less. When the upper limit of the crushing strength is within the above-mentioned range, the insulating properties of a resin sheet obtained using the boron nitride powder as a filler can be further improved. The lower limit of the crushing strength of the aggregated particles can be, for example, 1.0 MPa or more, 1.5 MPa or more, 2.0 MPa or more, 2.5 MPa or more, or 3.0 MPa or more. When the lower limit of the crushing strength is within the above-mentioned range, excessive collapse of the aggregated particles during kneading with a resin or molding of a resin composition, which can further suppress a decrease in heat dissipation performance. The crushing strength of the aggregated particles can be adjusted within the above-mentioned range, for example, 1.0 to 9.0 MPa or 1.0 to 7.0 MPa.

[0051] The crushing strength in this specification refers to a value measured in accordance with the description of JIS R 1639-5:2007 "Fine ceramics - Measurement methods for granule characteristics - Part 5: Single granule crushing strength." The crushing strength σ (unit: MPa) of a single aggregate particle is calculated from the dimensionless number α (α=2.48), which varies depending on the position within the aggregate particle, the crushing test force P (unit: N), and the particle diameter d (unit: μm), as follows: σ=α×P / (π×d 2 ) is calculated using the formula. Measurements were performed on 20 or more agglomerated particles, and the value at the cumulative destruction rate of 63.2% was calculated. A micro-compression tester can be used for the measurement. For example, the "MCT-W500" (product name) manufactured by Shimadzu Corporation can be used as a micro-compression tester.

[0052] The upper limit of the orientation index of the boron nitride powder obtained by the above-mentioned method for producing boron nitride powder can be, for example, 12.0 or less, 11.0 or less, 10.5 or less, or 10.0 or less. By keeping the upper limit of the orientation index within the above range, even if at least a portion of the aggregated particles collapse during kneading with the resin, increasing the orientation, the occurrence of significant anisotropy in the heat dissipation properties of the resin composition and molded article can be more sufficiently suppressed. The lower limit of the orientation index of the boron nitride powder can be, for example, 6.8 or more, 7.0 or more, 7.1 or more, 7.2 or more, 7.3 or more, 7.4 or more, 7.5 or more, 7.6 or more, or 7.7 or more. The orientation index of the boron nitride powder can be adjusted within the above-mentioned range, for example, to 6.8 to 12.0.

[0053] Since the orientation index is measured on boron nitride powder, when the powder contains aggregated particles in which the primary particles are not substantially oriented and the proportion of these particles is high, the orientation index value tends to approach a value of about 6 to 7. On the other hand, when the powder is composed of primary particles that do not contain aggregated particles, or when the proportion of these primary particles is high, the orientation index value tends to be high.

[0054] The orientation index in this specification refers to a value measured according to the following method. An X-ray diffraction spectrum of the boron nitride powder is obtained by performing X-ray diffraction measurement on the boron nitride powder. Then, the peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained from the X-ray diffraction spectrum. The obtained peak intensities are used to calculate the orientation index [I(002) / I(100)] of the boron nitride powder. An X-ray diffraction device that can be used is, for example, an "ULTIMA-IV" (product name) manufactured by Rigaku Corporation.

[0055] The method for producing the boron nitride powder may include other steps. Examples of such other steps include a pulverization step and a classification step. In the method for producing the boron nitride powder, for example, a pulverization step may be carried out after the crystallization step. In the pulverization step, a general pulverizer or crusher can be used. For example, a ball mill, a vibration mill, a jet mill, or the like can be used. In this specification, "pulverization" also includes "crushing." The average particle size of the hexagonal boron nitride powder may be adjusted to, for example, 10 to 100 μm or 15 to 100 μm by pulverization and classification.

[0056] The oxidation step in the method for producing the boron nitride powder can also be considered as a method for producing the powder. That is, one embodiment of the method for producing the powder includes a step of heat-treating boron carbonitride powder containing aggregates of boron carbonitride at 850°C or higher for 3 to 15 hours in an atmosphere having an oxygen partial pressure of 20% or higher to obtain a powder containing aggregate particles whose constituent elements are carbon, nitrogen, boron, and oxygen.

[0057] A first embodiment of the powder comprises agglomerated particles of which constituent elements are carbon, nitrogen, boron, and oxygen, and the amount of oxygen in the powder is 5 to 30 mass %.

[0058] The powder can also be referred to as boron carbonitride powder that has undergone an oxidation treatment, and the agglomerated particles may have the same crystal structure as turbostratic boron nitride (t-BN). The crystal structure may include, for example, a crystal structure in which some of the elements constituting turbostratic boron nitride are replaced by carbon, oxygen, etc., and a crystal structure in which carbon or oxygen is dissolved in a crystal structure consisting of boron nitride. The agglomerated particles may also include particles of what is called boron carbonitride, or may be particles of what is called boron carbonitride. Whether the agglomerated particles of interest have the same crystal structure as turbostratic boron nitride can be confirmed by obtaining an X-ray diffraction spectrum.

[0059] The lower limit of the average particle size of the powder may be, for example, 10 μm or more, 12 μm or more, 15 μm or more, or 20 μm or more. When the lower limit of the average particle size is within the above range, the thermal conductivity of a resin composition obtained by filling a resin with the boron nitride powder and kneading it, and a molded body of the resin composition, can be further improved. The upper limit of the average particle size of the powder may be, for example, 100 μm or less, 90 μm or less, 85 μm or less, 80 μm or less, or 70 μm or less. When the upper limit of the average particle size is within the above range, uneven distribution of the boron nitride powder within the molded body can be further suppressed when the boron nitride powder is filled into a resin and molded into a sheet, thereby producing a more homogeneous molded body. Furthermore, when the upper limit of the average particle size is within the above range, the boron nitride powder can be filled into a resin and molded into a thinner sheet, allowing for the production of a thinner heat dissipation member. The average particle size of the powder may be adjusted within the above range, for example, 10 to 100 μm.

[0060] The term "average particle size" as used herein refers to the 50% cumulative diameter (median diameter) in a volume-based cumulative particle size distribution. More specifically, it refers to the particle size (D50) at which the cumulative value reaches 50% in a volume-based cumulative particle size distribution obtained by laser diffraction scattering for a powder. Laser diffraction scattering is measured in accordance with the method described in JIS Z 8825:2013, "Particle Size Analysis - Laser Diffraction and Scattering Method." A laser diffraction scattering particle size distribution analyzer, such as the "LS-13 320" (product name) manufactured by Beckman Coulter, Inc., can be used for the measurement. The average particle size as used herein is measured in the presence of aggregated particles, without homogenization.

[0061] Boron nitride powder prepared using the powder according to the present disclosure as a raw material contains agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride, and the agglomerated particles have low crushing strength. Because the agglomerated particles have low crushing strength, they can be deformed, collapsed, etc., by the molding pressure, etc., applied when used as a filler to form a resin sheet. This action minimizes the presence of voids, etc., caused by agglomerated particles in the resin sheet, thereby improving the insulating properties and heat dissipation properties of the resin sheet. Therefore, the powder according to the present disclosure is suitable as a raw material for producing boron nitride powder.

[0062] Although several embodiments have been described above, the present disclosure is not limited to the above embodiments. Furthermore, the descriptions of the above embodiments can be mutually applied. [Example]

[0063] The present disclosure will be described in more detail below using examples and comparative examples, but the present disclosure is not limited to the following examples.

[0064] Example 1 [Preparation of boron carbide powder] 100 parts by mass of orthoboric acid (manufactured by Nippon Denko Corporation, 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 loaded 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 block of boron carbide (BC) powder.

[0065] [Preparation of boron carbonitride powder] The prepared boron carbide powder was heated in a carbon resistance heating furnace under a nitrogen gas atmosphere at a firing temperature of 2000°C and a pressure of 0.90 MPa for 12 hours to obtain boron carbonitride powder containing aggregates of boron carbonitride (B4CN4) (pressure nitriding process).

[0066] The boron carbonitride powder was heated in a muffle furnace under an air atmosphere at 850°C for 5 hours to prepare an oxidized powder (oxidation step). The carbon, nitrogen, boron, and oxygen contents of the particles contained in the powder thus obtained were measured, revealing that the oxygen content was 6.9% by mass and the carbon content was 1.1% by mass. Furthermore, X-ray diffraction measurements of the particles confirmed that the powder had the same crystalline structure as turbostratic boron nitride. For reference, a scanning electron microscope image showing the appearance of the powder is shown in Figure 1, and a scanning electron microscope image of a cross section is shown in Figure 2.

[0067] [Preparation of boron nitride powder] The powder and boric acid (boron source) were blended so that the boric acid content was 23 mass% based on the total amount of the powder and boric acid, and mixed using a Henschel mixer. 2 kg of this mixture was placed in a boron nitride crucible, and heat-treated in a resistance heating furnace at atmospheric pressure in a nitrogen gas atmosphere by increasing the temperature from room temperature to 2000°C and holding at 2000°C for 5 hours (crystallization step). Crystallization was sufficiently promoted by the heat treatment, and a boron nitride powder containing aggregated particles of hexagonal boron nitride was prepared.

[0068] (Examples 2 to 6, Comparative Examples 1 and 2) A powder containing agglomerated particles having carbon, nitrogen, boron, and oxygen as constituent elements, and a boron nitride powder were prepared in the same manner as in Example 1, except that the heating temperature and heating time in the oxidation step and the amount of boron source blended in the crystallization step were changed as shown in Table 1.

[0069] <Evaluation of boron nitride powder> The crushing strength of the aggregated particles and the orientation index of the powder were evaluated according to the methods described below for the boron nitride powders prepared in Examples 1 to 6 and Comparative Examples 1 and 2. The results are shown in Tables 1 and 2.

[0070] [Measurement of crushing strength of agglomerated particles] The crushing strength was measured in accordance with the description in JIS R 1639-5:2007 "Fine Ceramics - Measurement Methods for Granular Properties - Part 5: Single Granule Crushing Strength" using a microcompression testing machine "MCT-W500" (product name) manufactured by Shimadzu Corporation. The crushing strength σ (unit: MPa) of a single agglomerated particle is calculated as follows: σ = α × P / (π × d) from the dimensionless number α (α = 2.48), which changes depending on the position within the agglomerated particle, the crushing test force P (unit: N), and the particle diameter d (unit: μm). 2 Measurements were performed on 20 or more agglomerated particles, and the value at the cumulative destruction rate of 63.2% was calculated.

[0071] [Measurement of the orientation index of boron nitride powder] The orientation index was measured according to the following method. First, an X-ray diffraction spectrum of the boron nitride powder was obtained by performing X-ray diffraction measurement on the boron nitride powder. Then, the peak intensities I(002) and I(100) corresponding to the (002) and (100) planes were obtained from the X-ray diffraction spectrum, and the orientation index [I(002) / I(100)] of the boron nitride powder was calculated using the obtained peak intensities. For the measurement, an X-ray diffractometer "ULTIMA-IV" (product name) manufactured by Rigaku Corporation was used.

[0072] [Table 1]

[0073] [Table 2] [Industrial Applicability]

[0074] The present disclosure provides a method for producing boron nitride powder containing agglomerated particles with a relatively low crushing strength, a powder suitable as a raw material for producing the boron nitride powder, and a method for producing the powder.

Claims

1. a step of heat-treating a boron carbonitride powder containing aggregates of boron carbonitride at 850°C or higher for 3 to 15 hours in an atmosphere having an oxygen partial pressure of 20% or higher to obtain a powder containing aggregate particles whose constituent elements are carbon, nitrogen, boron, and oxygen; and a step of heat-treating a mixture containing the powder and a boron source at 1800 to 2200°C to obtain a heat-treated product having reduced oxygen and carbon contents.

2. The method according to claim 1, wherein the powder has an oxygen content of 5.0 to 30.0 mass%.

3. The method according to claim 1 or 2, wherein the heating temperature in the step of obtaining the powder is 1000°C or less.

4. The method according to claim 1 or 2, wherein the powder has a carbon content of 2.0 mass % or less.

5. A method for producing a powder, comprising: a step of heat-treating boron carbonitride powder containing aggregates of boron carbonitride at 850°C or higher for 3 to 15 hours in an atmosphere having an oxygen partial pressure of 20% or higher, to obtain a powder containing aggregate particles whose constituent elements are carbon, nitrogen, boron, and oxygen.

6. The method according to claim 5, wherein the powder has an oxygen content of 5.0 to 30.0 mass%.

7. The method according to claim 5 or 6, wherein the heating temperature in the step of obtaining the powder is 1000°C or less.

8. A powder comprising agglomerated particles whose constituent elements are carbon, nitrogen, boron, and oxygen, the oxygen content being 5.0 to 30.0 mass %.

9. The powder according to claim 8, wherein the carbon content is 2.0% by mass or less.

10. The powder according to claim 8 or 9, having an average particle size of 10 to 100 μm.

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