Method for producing boron nitride powder

Abstract

To provide a method for producing boron nitride powder having enhanced productivity than conventional methods by using calcium carbonate as an aide.SOLUTION: One aspect of the present disclosure provides a method for producing boron nitride powder, comprising: preparing a molded body by shaping a raw powder containing a carbon material, at least one of boric acid and boron oxide, and calcium carbonate; firing the molded body under a pressurized nitrogen atmosphere to obtain a fired product containing aggregated particles composed of multiple primary particles of hexagonal boron nitride aggregated together; wherein the calcium carbonate has an average particle diameter of 25 μm or more and a bulk density of 0.80 g / cm3 or more.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Boron nitride powder has high thermal conductivity and insulating properties, and is widely used as a thermally conductive filler, an insulating filler, etc. Boron nitride powder is used as a filler in heat dissipation components, which require particularly high thermal conductivity.

[0003] Hexagonal boron nitride primary particles have a relatively thin, scale-like shape. When filled into a resin or other material and molded, the primary particles tend to orient in a certain direction due to factors such as molding pressure. For example, in resin sheets formed by filling hexagonal boron nitride powder and molding it into a sheet using extrusion molding, the major surfaces of the resin sheet generally tend to be oriented parallel to the long axes of the boron nitride primary particles. Furthermore, due to the anisotropy of their shape, the primary particles of hexagonal boron nitride can also exhibit anisotropy in various physical properties. While the thermal conductivity of hexagonal boron nitride primary particles in the in-plane direction (a-axis direction) is high at approximately 400 W / (m·K), the thermal conductivity in the thickness direction (c-axis direction) is only approximately 2 W / (m·K), demonstrating significant anisotropy of physical properties depending on the direction.

[0004] For the reasons mentioned above, methods have been investigated in which hexagonal boron nitride powder is used as a filler for resin, and when preparing a heat-dissipating sheet, the a-axis direction of the primary particles is adjusted to be parallel to the thickness direction of the heat-dissipating sheet, thereby making the most of the high thermal conductivity in the a-axis direction of the primary particles. For example, a technique is known in which the a-axis direction of the primary particles of hexagonal boron nitride is oriented so that it is parallel to the thickness direction of the heat-dissipating sheet (for example, Patent Document 1, etc.).

[0005] Furthermore, from the viewpoint of reducing the anisotropy due to the shape as described above, a method of forming an aggregate composed of a plurality of primary particles, which are aggregated and fused together so that the orientation of the a-axis direction of adjacent primary particles differs, has been investigated. Patent Document 2 discloses boron nitride aggregate particles formed by agglomerating primary particles of boron nitride, and describes that by increasing the strength of the aggregate particles to a degree that the collapse of the aggregate particles can be suppressed even when a predetermined molding pressure is applied, the alignment of the boron nitride primary particles to the same direction can be suppressed. Patent Document 3 discloses a powder containing primary particles of hexagonal boron nitride having a scale shape, an average particle size of 4.0 to 15.0 μm, an orientation index of 25.0 or less, and a tap density of 0.70 g / cm. 3 Thus, a boron nitride powder is disclosed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-154265 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-135731 [Patent Document 3] International Publication No. 2023 / 204139 Summary of the Invention [Problem to be solved by the invention]

[0007] Calcium carbonate is one of the sintering aids that promotes the synthesis of boron nitride and the growth of primary particles. Calcium carbonate has a high boiling point and is difficult to volatilize, which makes it easy to contribute to grain growth. Its remaining between particles allows it to form aggregates composed of boron nitride primary particles. Therefore, calcium carbonate is a useful sintering aid for the synthesis of boron nitride. However, even when calcium carbonate is used, there is still room for improvement in terms of the productivity of the compact itself to be fired.

[0008] An object of the present disclosure is to provide a method for producing boron nitride powder that uses calcium carbonate as an auxiliary agent and is more productive than conventional methods. [Means for solving the problem]

[0009] The present disclosure provides the following [1] to [4].

[0010] [1] a firing step of forming a raw material powder containing a carbon material, at least one of boric acid and boron oxide, and calcium carbonate into a compact, and firing the compact in a pressurized nitrogen atmosphere to obtain a fired product containing agglomerated particles formed by agglomerating a plurality of primary particles of hexagonal boron nitride; The calcium carbonate has an average particle size of 25 μm or more and a bulk density of 0.80 g / cm 3 This is the method for producing boron nitride powder. [2] The compact has a density of 1.30 g / cm 3 or more, and the breaking strength is 20 N or more. [3] The manufacturing method according to [1] or [2], wherein the amount of calcium carbonate blended is 8.0 mass% or less based on the total amount of the raw material powder. [4] The BET specific surface area of ​​the calcium carbonate is 4.0 m 2 The method according to any one of [1] to [3], wherein the concentration is 0.1 to 0.5 μg / g or less. [Effects of the Invention]

[0011] According to the present disclosure, a method for producing boron nitride powder can be provided that uses calcium carbonate as an auxiliary agent and is more productive than conventional methods. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described. 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 equal to or greater than x and equal to or less than y.

[0013] 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.

[0014] The method for producing boron nitride powder according to the present disclosure is a production method that applies the so-called carbon reduction method, and is different from synthesis methods of boron nitride such as the method via boron carbonitride (the B4C method) and the melamine borate method. That is, by reacting carbon with a boron source such as boric acid in a nitrogen-containing atmosphere, oxygen is extracted from the boron source and nitrogen is reacted to synthesize boron nitride.

[0015] One embodiment of a method for producing boron nitride powder includes a firing step of forming a raw material powder containing a carbon material, at least one of boric acid and boron oxide, and calcium carbonate into a molded body, and firing the molded body in a pressurized nitrogen atmosphere to obtain a fired product containing agglomerated particles formed by agglomeration of multiple primary particles of hexagonal boron nitride.

[0016] In the above-mentioned production method, the calcium carbonate has an average particle size of 25 μm or more and a bulk density of 0.80 g / cm 3 By using such calcium carbonate, it is possible to stably form a high-density molded body and perform sintering at a high density, which makes it possible to further improve the productivity of boron nitride powder.

[0017] The carbon material is a carbon source for removing oxygen from a boron source such as boric acid. Any compound capable of supplying carbon can be used as the carbon material, but it is preferable to use a highly pure and relatively inexpensive raw material. Examples of such carbon materials include carbon black and acetylene black.

[0018] The boron source can be a compound containing boron and oxygen. Examples of such a boron source include boric acid and boron oxide. It is preferable to use at least one of boric acid and boron oxide, and it is more preferable to use boric acid. When boric acid is used, it is dehydrated by heating to form boron oxide, which forms a liquid phase during the heat treatment of the raw material powder and also functions as an auxiliary agent for promoting grain growth.

[0019] Calcium carbonate reacts with a boron source, etc., to form a liquid phase, which promotes the growth of primary particles of boron nitride. Calcium oxide, which is produced when calcium carbonate is decarbonized, reacts with boron oxide contained in the boron source to produce calcium borate, and the calcium borate becomes liquid and remains without volatilizing even after firing, causing it to intervene between primary particles of hexagonal boron nitride, causing them to aggregate and form aggregated particles.

[0020] The calcium carbonate may have a large average particle size and a relatively small specific surface area. The upper limit of the BET specific surface area of ​​calcium carbonate is, for example, 4.0 m 2 / g or less, 3.0m 2 / g or less, 2.0m 2 / g or less, or 1.5m 2 / g or less. By using calcium carbonate having an upper limit of the BET specific surface area within the above range, moldability is good and a molded product (e.g., a pellet-shaped molded product) with higher density can be obtained. The lower limit of the BET specific surface area of ​​calcium carbonate is not particularly limited, but may be, for example, 0.2 m 2 / g or more, 0.5m 2 / g or more, or 0.8m 2 / g or more.

[0021] The specific surface area in this specification refers to a value measured using a specific surface area analyzer in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption," and is a value calculated by applying the BET single-point method using nitrogen gas. Examples of specific surface area analyzers that can be used include the "MONOSORB MS-22" (trade name) manufactured by QUANTACHROME.

[0022] The upper limit of the amount of calcium carbonate blended, based on the total amount of raw material powder, may be, for example, 8.0 mass% or less, 7.0 mass% or less, or 6.0 mass% or less. By setting the upper limit of the blended amount within the above range, it is possible to further suppress a decrease in density of the molded body and improve the purity of the obtained boron nitride, thereby achieving a higher level of both heat dissipation and insulation properties of the boron nitride powder. The lower limit of the amount of calcium carbonate blended, based on the total amount of raw material powder, may be, for example, 1.0 mass% or more, 2.0 mass% or more, or 3.0 mass% or more. By setting the lower limit of the blended amount within the above range, it is possible to further promote the growth of primary particles of hexagonal boron nitride and promote the formation of aggregated particles, thereby further improving the yield of boron nitride powder.

[0023] From the viewpoint of further improving productivity, it is desirable to further increase the density of the molded body. The lower limit of the density of the molded body is, for example, 0.90 g / cm 3 More than 1.00g / cm 3 More than 1.10g / cm 3 More than 1.20g / cm 3 or more, or 1.30 g / cm 3 The upper limit of the density of the molded body is not particularly limited, but may be, for example, 1.70 g / cm 3 Below 1.60g / cm 3 Below 1.50g / cm 3 or less, or 1.40 g / cm 3 It may be the following:

[0024] The density of a molded body in this specification refers to a value measured by the method described below. More specifically, the volume is calculated from the external shape (length, thickness, etc.) of the molded body, and the density is determined by taking the ratio to the mass of the molded body. When the molded body is in the form of a pellet, the volume is calculated according to the shape of the tablet. For example, when a pellet has a circular horizontal surface (land portion) on the outer periphery of the central portion in a plan view, the density is determined by taking the value of the molded body as the volume of the molded body using the diameter, thickness, width of the land portion, and radius of curvature of the pellet, and taking the ratio to the mass of the molded body.

[0025] The lower limit of the breaking strength of the compact may be, for example, 20 N or more, 22 N or more, 24 N or more, or 26 N or more. When the lower limit of the breaking strength is within the above range, the compact has good shape retention and is less likely to break, making it possible to produce a compact that is easy to handle in the production of boron nitride powder. The upper limit of the breaking strength of the compact may be, for example, 46 N or less, 44 N or less, 42 N or less, or 40 N or less. When the upper limit of the breaking strength is within the above range, the compact can be stably released during the compacting process, further improving the productivity of the compact.

[0026] The term "breaking density" as used herein refers to a value measured using a digital hardness tester. Measurements are performed using pellets with a pressure surface diameter of 5 mm at a pressure speed of 1 mm / sec. Examples of digital hardness testers that can be used include the "KHT-20N" digital hardness tester manufactured by Fujiwara Seisakusho Co., Ltd.

[0027] The above compact has a density of 1.30 g / cm 3 or more, and the breaking strength may be 20N or more.

[0028] The firing process is carried out under a pressurized nitrogen atmosphere. By performing firing under such conditions, it is possible to provide a sufficient supply of nitrogen as a raw material for boron nitride. In this process, a fired product containing agglomerated particles formed by agglomeration of multiple primary particles of hexagonal boron nitride can be obtained.

[0029] The lower limit of the atmospheric pressure in the firing step may be, for example, 0.5 MPaG or more, 0.6 MPaG or more, 0.7 MPaG or more, or 0.8 MPaG or more. By setting the lower limit of the atmospheric pressure within the above range, volatilization of the boron source is suppressed and the liquid phase of the boron source is maintained, thereby further promoting the growth of primary particles of hexagonal boron nitride and further suppressing the generation of boron carbide as a by-product. The upper limit of the atmospheric pressure in the firing step is not particularly limited, but may be 0.9 MPaG or less in industrial applications. The atmospheric pressure in the firing step may be adjusted within the above range, and may be, for example, 0.5 to 0.9 MPaG. In this specification, pressure refers to gauge pressure.

[0030] The firing temperature in the firing step is, for example, 1800 to 2200°C. The upper limit of the firing temperature may be, for example, 2150°C or lower, or 2100°C or lower. By setting the upper limit of the firing temperature within the above range, it is possible to more sufficiently suppress the generation of by-products. The lower limit of the firing temperature may be, for example, 1850°C or higher, 1900°C or higher, 1950°C or higher, 2000°C or higher, or 2050°C or higher. By setting the lower limit of the firing temperature within the above range, it is possible to further promote the reaction on the carbon material and further improve the yield of boron nitride obtained.

[0031] In the calcination step, it is desirable to set a relatively long time (retention time) for maintaining the temperature after reaching the calcination temperature under the pressure environment. The lower limit of the retention time in the calcination step may be, for example, 7 hours or more, or 8 hours or more. By setting the lower limit of the retention time within the above range, unreacted carbon source in the raw materials can be more thoroughly removed, and boron nitride powder with fewer impurities can be obtained. The upper limit of the retention time in the calcination step is not particularly limited, but from the viewpoint of reducing the production cost of boron nitride powder, it may be, for example, 20 hours or less, 18 hours or less, 16 hours or less, 14 hours or less, or 12 hours or less. The retention time in the calcination step may be adjusted within the above range, and may be, for example, 7 to 20 hours, or 7 to 12 hours.

[0032] The above-described manufacturing method may include other steps in addition to the firing step, such as a crushing step.

[0033] In the crushing step, for example, a crusher such as a Henschel mixer or a grinder mill can be used.

[0034] When a Henschel mixer is used, the rotation speed of the crusher may be as follows: The upper limit of the rotation speed of the crusher may be, for example, 950 rpm or less, 900 rpm or less, or 850 rpm or less. When the upper limit of the rotation speed of the crusher is within the above range, over-crushing of the particles can be prevented. The lower limit of the rotation speed of the crusher may be, for example, 500 rpm or more, or 550 rpm or more. When the lower limit of the rotation speed of the crusher is within the above range, the fired product can be sufficiently crushed, and loose agglomerations of the boron nitride primary particles can be released.

[0035] The lower limit of the disintegration time in the disintegration step may be, for example, 5 minutes or more, 6 minutes or more, 7 minutes or more, or 8 minutes or more. By setting the lower limit of the disintegration time within the above range, the fired product can be sufficiently disintegrated, loose agglomerations of boron nitride primary particles can be broken down, and the sieving yield can be further improved. The upper limit of the disintegration time in the disintegration step may be, for example, 15 minutes or less, 14 minutes or less, 13 minutes or less, or 12 minutes or less. By setting the upper limit of the disintegration time within the above range, excessive crushing of agglomerated particles of hexagonal boron nitride can be more sufficiently suppressed. The disintegration time may be adjusted within the above range, and may be, for example, 5 to 15 minutes, or 8 to 12 minutes.

[0036] 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 applied to each other. [Example]

[0037] 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.

[0038] Example 1 A mixture of 100 parts by mass of boric acid (manufactured by Kojundo Chemical Laboratory Co., Ltd.), 26 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: Li-400), and calcium carbonate (manufactured by Calfine Co., Ltd., average particle size: 27 μm, bulk density: 0.870 g / cm 3 , BET specific surface area: 1.0m 2 The mixture was mixed in a Henschel mixer to obtain a raw powder containing 5.5% by mass of calcium carbonate.

[0039] The obtained mixed powder was placed in a dryer at 250°C and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a 9 mm diameter mold of a rotary press molding machine and molded under pressure to obtain pellets. The maximum pressure required for continuous operation during pellet molding was 33 kN. The density of the pellets was 1.32 g / cm. 3 The breaking strength measured by a digital hardness tester (manufactured by Fujiwara Seisakusho, product name: KHT-20N type) was 28N.

[0040] Next, the pellets were placed in a carbon container (capacity: 3000 cm 3 The pellets were packed into a container, heated to 1900°C at a heating rate of 5°C / min in a nitrogen atmosphere pressurized to 0.5 MPaG, and held at 1900°C for 8 hours to heat-treat the pellets, thereby obtaining a fired product (firing step). At this time, the pellet packing density in the container was 0.831 g / cm. 3 It was.

[0041] The resulting fired product was crushed in a Henschel mixer at a rotation speed of 900 rpm for a crushing time of 10 minutes (crushing step), to prepare a powder containing boron nitride particles.

[0042] Example 2 The raw material powder of calcium carbonate (Takehara Chemical Industry Co., Ltd., average particle size: 6 μm, bulk density: 0.540 g / cm 3 , BET specific surface area: 3.2m2 Except for changing the mass fraction of the boron nitride powder to 1 / g, the boron nitride powder was produced in the same manner as in Example 1. The maximum pressure at which continuous operation was possible during pellet molding was 33 kN.

[0043] (Comparative Example 1) As calcium carbonate, calcium carbonate (manufactured by New Lime Co., Ltd., average particle size: 4 μm, bulk density: 0.378 g / cm 3 , BET specific surface area: 5.5m 2 Except for using 100% ammonium nitrate powder (0.05 wt. / g), boron nitride powder was produced in the same manner as in Example 1. The maximum pressure at which continuous operation was possible during pellet molding was 20 kN.

[0044] <Evaluation of productivity in the manufacturing method of boron nitride powder> In addition to confirming whether stable continuous operation was possible at 30 rpm using a rotary press tableting machine (manufactured by Mori Machinery Corporation, product name: PH345RII), the appearance of 100 pellet-shaped compacts obtained by tableting was also observed. The results were evaluated according to the following criteria to assess the productivity of the boron nitride powder manufacturing method. The results are shown in Table 1. [Evaluation criteria for continuous operation] A: Continuous operation time is more than one hour. B: Tableting cannot be continued and the continuous operation time is less than 1 hour. [Evaluation criteria for capping (presence or absence of defects such as chips)] A: The number of molded bodies in which capping was observed was 10% or less of the molded bodies being observed. B: The number of molded bodies in which capping was observed was more than 10% of the molded bodies to be observed.

[0045] [Table 1] [Industrial Applicability]

[0046] According to the present disclosure, a method for producing boron nitride powder can be provided that uses calcium carbonate as an auxiliary agent and is more productive than conventional methods.

Claims

1. a firing step of forming a raw material powder containing a carbon material, at least one of boric acid and boron oxide, and calcium carbonate into a compact, and firing the compact in a pressurized nitrogen atmosphere to obtain a fired product containing agglomerated particles formed by agglomerating a plurality of primary particles of hexagonal boron nitride; The calcium carbonate has an average particle size of 25 μm or more and a bulk density of 0.80 g / cm ３ This is the method for producing boron nitride powder.

2. The molded body has a density of 1.30 g / cm ３ 2. The method according to claim 1, wherein the thickness of the insulating film is 100 nm or more and the breaking strength is 20 N or more.

3. The method according to claim 1 or 2, wherein the amount of calcium carbonate blended is 8.0 mass % or less based on the total amount of the raw material powder.

4. The BET specific surface area of ​​the calcium carbonate is 4.0 m ２ The method according to claim 1 or 2, wherein the SiO2 content is 1 / g or less.

5. The blending amount of the calcium carbonate is 8.0 mass% or less based on the total amount of the raw material powder, The BET specific surface area of ​​the calcium carbonate is 4.0 m ２ The method according to claim 1 or 2, wherein the SiO2 content is 1 / g or less.

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