Boron nitride powder, and, method for producing boron nitride powder

Abstract

To provide a method for producing boron nitride powder containing aggregated particles having small crushing strength.SOLUTION: One aspect of the present disclosure provides a method for producing boron nitride powder, comprising a step of heat-treating powder containing aggregated particles, which are composed of primary particles of hexagonal boron nitride aggregated together, to form an oxide layer on at least a part of the surface of the primary particles, wherein the orientation index of the powder is 12.0 or less, and the crushing strength of the aggregated particles contained in the powder is 9.0 MPa or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to boron nitride powder and methods for making the same. [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 agglomerated 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 3 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 provides a method for producing a boron nitride powder containing agglomerated particles with low crushing strength. The present disclosure also provides a boron nitride powder useful as a filler for resins. [Means for solving the problem]

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

[0010] [1] A method for producing a powder containing agglomerated particles formed by agglomerating primary particles of hexagonal boron nitride, the powder being subjected to a heat treatment to form an oxide layer on at least a portion of the surface of the primary particles; The orientation index of the powder is 12.0 or less, A method for producing a boron nitride powder, wherein the agglomerated particles contained in the powder have a crushing strength of more than 9.0 MPa.

[0011] The above-mentioned manufacturing method employs a method in which a powder containing agglomerated particles is subjected to a heat treatment to form an oxide layer on at least a portion of the surface of the primary particles constituting the agglomerated particles, which embrittles the bonds between the primary particles and reduces the crushing strength of the agglomerated particles.

[0012] The manufacturing method of the above [1] may be any of the following [2] to [4].

[0013] [2] The manufacturing method according to [1], wherein the step of providing the oxide layer is carried out in an atmosphere having an oxygen partial pressure of 20% or more. [3] The method according to [1] or [2], wherein the heating temperature in the step of providing the oxide layer is 700 to 1100°C. [4] A step of calcining the boron carbide powder at 1900 to 2200 ° C. in a nitrogen pressurized atmosphere of 0.60 MPa or more to obtain boron carbonitride powder; The method according to any one of [1] to [3], further comprising the step of mixing the boron carbonitride with a boron source and heating the mixture in an atmosphere containing nitrogen to generate primary particles of hexagonal boron nitride, thereby obtaining the agglomerated particles.

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

[0015] [5] A powder containing agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride, having an orientation index of 12.0 or less, The crushing strength of the agglomerated particles is 9.0 MPa or less, A boron nitride powder in which, upon cross-sectional observation of the agglomerated particles, the total area of ​​the region where the primary particles are present relative to the area of ​​the region surrounded by the periphery of the agglomerated particles is 60 area % or more.

[0016] The boron nitride powder contains agglomerated particles to the extent that the orientation index is a predetermined value or less. The agglomerated particles have a dense structure such that the area of ​​the primary particle region in the cross section is a certain value or more, while having low crushing strength. This boron nitride powder is useful because it can broaden the range of resin filler options to suit the kneading conditions with resins when preparing resin compositions and resin molded articles.

[0017] The boron nitride powder of [5] above may be the following [6] or [7].

[0018] [6] Specific surface area is 2.0 to 5.0 m 2 / g. [7] The boron nitride powder according to [5] or [6], having an average particle size of 5.0 to 90.0 μm. [Effects of the Invention]

[0019] The present disclosure provides a method for producing boron nitride powder containing agglomerated particles with low crushing strength, and also provides boron nitride powder useful as a filler for resins. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a scanning electron microscope image showing an example of a cross section of an aggregated particle. [Figure 2] FIG. 2 is a diagram for explaining a method for calculating the area ratio of primary particles. DETAILED DESCRIPTION OF THE INVENTION

[0021] 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.

[0022] 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.

[0023] In this specification, the term "primary particle of hexagonal boron nitride" refers to a single particle of hexagonal boron nitride. The term "aggregated particle" refers to a particle formed by aggregating a plurality of 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 a secondary particle.

[0024] One embodiment of a method for producing boron nitride powder includes a step of heat-treating a powder containing agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride (hereinafter also referred to as raw boron nitride powder) to provide an oxide layer on at least a portion of the surface of the primary particles (hereinafter also referred to as an oxidation step). The oxide layer may be formed by oxidizing boron nitride. The oxide layer may contain or consist of boron oxide (BO).

[0025] The upper limit of the orientation index of the raw material boron nitride powder may be, for example, 12.0 or less, 10.0 or less, 9.0 or less, or 8.0 or less. By using a raw material boron nitride powder whose upper limit of the orientation index is within the above range, the resulting boron nitride powder will have a moderately low orientation index and will have suppressed primary particle orientation, making it more suitable as a filler such as a heat-dissipating filler. The lower limit of the orientation index of the raw material boron nitride powder may be, for example, 6.0 or more, 6.5 or more, 6.8 or more, or 7.0 or more.

[0026] The agglomerated particles contained in the raw boron nitride powder may have a relatively high strength. The lower limit of the crushing strength of the agglomerated particles contained in the raw boron nitride powder is not particularly limited, and may be, for example, more than 9.0 MPa, 11.0 MPa or more, 12.0 MPa or more, 13.0 MPa or more, or 15.0 MPa or more. The upper limit of the crushing strength of the agglomerated particles contained in the raw boron nitride powder may be, for example, 50.0 MPa or less, 30.0 MPa or less, or 20.0 MPa or less. By using a raw boron nitride powder whose upper limit of crushing strength falls within the above range, a long oxidation treatment is not required to reduce the crushing strength to the desired strength, which prevents an increase in the production cost of the boron nitride powder and also prevents a decrease in heat dissipation property.

[0027] The raw material boron nitride powder may be a powder obtained by purchase that has an orientation index of 15.0 or less and a crushing strength of the agglomerated particles of 9.0 MPa or more, or a powder prepared separately may be used.

[0028] When the raw material boron nitride powder is separately prepared, it can be prepared, for example, by a method of pressure nitriding and decarburizing boron carbide (BC). The method for producing the boron nitride powder may further include a step of sintering the boron carbide powder at 1900 to 2200°C under a nitrogen pressure atmosphere of 0.60 MPa or more to obtain boron carbonitride powder (BCN4 powder) (hereinafter also referred to as the pressure nitriding step), and a step of mixing the boron carbonitride with a boron source and heating in a nitrogen-containing atmosphere to generate primary particles of hexagonal boron nitride to obtain the aggregated particles (hereinafter also referred to as the crystallization step).

[0029] In the pressure nitriding step, boron carbide powder is sintered in a pressurized nitrogen atmosphere to obtain boron carbonitride powder (B4CN4 powder). The lower limit of the sintering temperature in the pressure nitriding step may be 1900°C or higher, 2000°C or higher, or 2100°C or higher. By setting the lower limit of the sintering temperature within the above range, the crystallinity of the obtained boron carbonitride can be improved and the proportion of hexagonal boron carbonitride can be increased. Increasing the proportion of hexagonal boron carbonitride in the pressure nitriding step can further improve the thermal conductivity of the boron nitride powder obtained later. The upper limit of the sintering temperature in the pressure nitriding step may be 2200°C or lower, 2150°C or lower, or 2100°C or lower. The sintering temperature may be adjusted within the above range, for example, 1900 to 2200°C, or 2000 to 2200°C.

[0030] The lower limit of the pressure (atmospheric 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 in the pressure nitriding step within the above range, the nitriding of boron carbide can be sufficiently promoted. The upper limit of the pressure (atmospheric 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 in the pressure nitriding step within the above range, an increase in the production cost of the boron nitride powder can be suppressed. The pressure may be adjusted within the above range, and may be, for example, 0.6 to 1.0 MPa, or 0.8 to 0.9 MPa.

[0031] The nitrogen gas concentration in the pressurized nitrogen 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. The firing time in the pressure nitriding step is not particularly limited as long as nitriding proceeds sufficiently, and may be, for example, 6 to 30 hours, 8 to 25 hours, or 10 to 20 hours.

[0032] In the crystallization step, a mixture containing the boron carbonitride and a boron source is heated in an atmosphere containing nitrogen to generate primary particles of boron nitride, and the primary particles are aggregated to obtain aggregated particles.

[0033] In the crystallization step, it is preferable to keep the content of the boron source low. The upper limit of the content of the boron source may be, for example, less than 40 mass%, 35 mass% or less, 33 mass% or less, 30 mass% or less, 28 mass% or less, or 25 mass% or less, based on the total amount of the boron carbonitride powder and the boron source. By setting the upper limit of the content of the boron source within the above range, the amount of liquid phase, which is the growth site for boron nitride primary particles, can be reduced. As a result, excessive growth of the primary particles can be suppressed, and boron nitride containing agglomerated particles with superior crushing strength can be produced. The lower limit of the content of the boron source may be, for example, 15 mass% or more, 18 mass% or more, or 20 mass% or more, based on the total amount of the boron carbonitride powder and the boron source. By setting the lower limit of the content of the boron source within the above range, the crystallinity of boron nitride can be further improved, resulting in primary particles of hexagonal boron nitride with higher purity. The content of the boron source may be adjusted within the above range, and may be, for example, 15 to 40 mass%, 15 to 28 mass%, or 18 to 25 mass%, based on the total amount of the boron carbonitride powder and the boron source.

[0034] The boron source may include at least one selected from the group consisting of boric acid and boron oxide.

[0035] In the manufacturing method according to the present disclosure, the oxidation step may be carried out in an atmosphere with an oxygen partial pressure of 20% or more. The atmosphere in the oxidation step with an oxygen partial pressure of 20% or more may be, for example, air. The upper limit of the oxygen partial pressure may be, for example, 70% or less, 60% or less, or 50% or less. Note that the oxygen partial pressure in this specification refers to the partial pressure of oxygen at the standard state of the gas occupying the atmosphere, and is a value measured by an oxygen concentration meter. For example, the "G1690" (product name) manufactured by Sakaki Corporation can be used as the oxygen partial pressure meter.

[0036] The pressure (atmospheric pressure) in the oxidation step may be, for example, 0.1 to 0.5 MPa, 0.1 to 0.3 MPa, or 0.1 to 0.2 MPa, or may be atmospheric pressure (0.1 MPa).

[0037] The lower limit of the heating temperature in the oxidation step may be, for example, 700°C or higher, 750°C or higher, 800°C or higher, or 850°C or higher. By setting the lower limit of the heating temperature within the above range, the formation of an oxide layer is facilitated, and the crushing strength of the agglomerated particles can be further reduced. The upper limit of the heating temperature in the oxidation step may be, for example, 1100°C or lower, 1000°C or lower, 950°C or lower, or 900°C or lower. By setting the upper limit of the heating temperature within the above range, it is possible to more sufficiently suppress a decrease in the thermal conductivity of the agglomerated particles caused by an increased amount of oxide layer formed. The heating temperature in the oxidation step may be adjusted within the above range, and may be, for example, 700 to 1100°C or 700 to 1000°C.

[0038] The heating time in the oxidation step may be adjusted according to the heating temperature, etc. The lower limit of the heating time may be, for example, 0.5 hours or more, 1.0 hours or more, or 1.5 hours or more. When the lower limit of the heating time is within the above range, an oxide layer can be more sufficiently formed on the surface of the primary particles of the raw material boron nitride powder, and the crushing strength can be further reduced. The upper limit of the heating time may be, for example, 30 hours or less, 25 hours or less, 20 hours or less, or 15 hours or less. When the upper limit of the heating time is within the above range, a decrease in heat dissipation property due to excessive oxidation and collapse of the boron nitride powder can be further suppressed.

[0039] The oxidation step may be carried out so that the lower limit of the total oxygen content of the boron nitride powder obtained in the oxidation step is, for example, 0.05% by mass or more, 0.10% by mass or more, 0.15% by mass or more, 0.20% by mass or more, 0.50% by mass or more, or 0.70% by mass or more, based on the total amount of the boron nitride powder. By carrying out the oxidation step so that the lower limit of the total oxygen content is within the above range, the formation of an oxide layer in the oxidation step is more sufficient, and the crushing strength of the resulting particles can be further reduced. The oxidation step may be carried out so that the upper limit of the total oxygen content of the boron nitride powder obtained in the oxidation step is, for example, 5.00% by mass or less, 4.50% by mass or less, 4.00% by mass or less, 3.50% by mass or less, 3.00% by mass or less, 2.50% by mass or less, 2.00% by mass or less, or 1.50% by mass or less, based on the total amount of the boron nitride powder. By carrying out the oxidation step so that the upper limit of the total oxygen content falls within the above range, it is possible to more sufficiently suppress the decrease in thermal conductivity of the agglomerated particles that would otherwise result from an increase in the amount of oxide layer formed.

[0040] The total oxygen content in this specification refers to the value measured for 0.02 g of boron nitride powder by inert gas fusion non-dispersive infrared (NDIR) using an oxygen / nitrogen simultaneous analyzer, such as the EMGA-920 oxygen / nitrogen analyzer manufactured by Horiba, Ltd.

[0041] The method for producing boron nitride powder according to the present disclosure may include other steps in addition to the oxidation step described above. Examples of such other steps include a water washing step.

[0042] The water-washing step is a step in which the powder that has undergone the oxidation treatment is washed with a solution containing water to remove at least a portion of the oxide layer formed on the surface of the primary particles. Since the oxide layer can be a component that reduces thermal conductivity, it is useful to remove the oxide layer by washing with a solution containing water, depending on the required properties, such as when the boron nitride powder is used as a thermally conductive filler. The washing in the water-washing step may be performed by dispersing or immersing the powder that has undergone the oxidation treatment in the solution and stirring it. After washing, the powder may be filtered to obtain a filter cake, which may then be dispersed or immersed in the solution again and stirred repeatedly. After washing, the powder may be filtered to obtain a filter cake, and the content of the solution may be further reduced or removed to obtain a water-washed powder. The water-washing step may be performed at room temperature, in an environment of 20 to 28°C, or in an environment of 25°C.

[0043] The solution used in the water washing step contains water and may contain a low-boiling organic solvent in consideration of subsequent solvent removal. However, water is preferred from the viewpoint of more thoroughly removing boron oxide (BO) from the oxide layer. When using an organic solvent, for example, a lower alcohol such as methanol or ethanol can be used. The water washing step reduces the oxide layer in the boron nitride powder, and may be performed, for example, focusing on the total oxygen content of the boron nitride powder so that this value is 0.10% by mass or less, or 0.07% by mass or less. The lower limit of the total oxygen content is not particularly limited and may be below the detection limit, but may be, for example, 0.01% by mass or more, or 0.02% by mass or more.

[0044] In the water washing step, the upper limit of the amount of the oxidation-treated powder to be added to 1 L of the aqueous solution may be, for example, 1 kg or less, 0.8 kg or less, 0.5 kg or less, or 0.3 kg or less, from the viewpoint of further reducing the oxide layer. The lower limit of the amount of the oxidation-treated powder to be added to 1 L of the aqueous solution is not particularly limited, but may be, for example, 0.05 kg or more, or 0.1 kg or more.

[0045] The lower limit of the treatment time for one water washing treatment in the water washing step may be, for example, 0.1 hours or more, 0.5 hours or more, or 1.0 hour or more. When the lower limit of the treatment time is within the above range, the oxide layer can be more sufficiently removed, and the oxygen content in the boron nitride powder can be more sufficiently reduced. The upper limit of the treatment time is not particularly limited, but may be 20 hours or less, 15 hours or less, 10 hours or less, or 5 hours or less. When the upper limit of the treatment time is within the above range, a decrease in the treatment efficiency of the water washing step can be more sufficiently suppressed.

[0046] When stirring is performed in the water-washing step, a stirrer can be used. Examples of the stirrer that can be used include a mixer. When filtration is performed after water-washing, for example, a centrifuge, a rotary filter, a filter dryer, etc. can be used.

[0047] The above-described method for producing boron nitride powder according to the present disclosure reduces the crushing strength of agglomerated particles, and therefore the present disclosure can also be said to provide a method for reducing the crushing strength of agglomerated particles.

[0048] One embodiment of the boron nitride powder is a powder containing agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride. The boron nitride powder may be mainly composed of agglomerated particles, but may also contain non-agglomerated primary particles.

[0049] The upper limit of the orientation index of the boron nitride powder may be, for example, 12.0 or less, but may also be, for example, 11.0 or less, 10.5 or less, or 10.0 or less. By ensuring that the upper limit of the orientation index is 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 the molded article can be more sufficiently suppressed. The lower limit of the orientation index of the boron nitride powder may 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 may be adjusted within the above range, and may be, for example, 6.8 to 15.0.

[0050] Since the orientation index is measured on boron nitride powder, if the powder contains agglomerated particles (lump particles) in which the primary particles are not substantially oriented, and if 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, if the powder is composed of primary particles that do not contain agglomerated particles, or if the proportion of these primary particles is high, the orientation index value tends to be high.

[0051] 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.

[0052] The upper limit of the specific surface area of ​​the boron nitride powder is, for example, 5.0 m 2 / g or less, 4.6m 2 / g or less, 4.4m 2 / g or less, 4.2m 2 / g or less, 4.0m 2 / g or less, or 3.8m 2 When the upper limit of the specific surface area is within the above range, the primary particle diameter of the boron nitride is sufficiently large, and when the boron nitride powder is made into a resin composition and a molded article, the boron nitride powder can exhibit superior heat dissipation properties. The lower limit of the specific surface area of ​​the boron nitride powder is, for example, 2.0 m 2 / g or more, 2.5m 2 / g or more, 2.8m 2 / g or more, 3.0m 2 / g or more, or 3.1m 2 / g or more. By ensuring that the lower limit of the specific surface area is within the above range, it is possible to suppress a decrease in the density of the primary particles in the aggregated particles, and to suppress a decrease in the heat dissipation and insulating properties of the resin composition and the molded article. The specific surface area of ​​the boron nitride powder may be adjusted within the above range, for example, from 2.0 to 5.0 m 2 / g.

[0053] 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" (product name) manufactured by QUANTACHROME.

[0054] The upper limit of the total oxygen content of the boron nitride powder may be, for example, 1.00 mass% or less, 0.80 mass% or less, 0.70 mass% or less, or 0.60 mass% or less, based on the total amount of the boron nitride powder. When the upper limit of the total oxygen content is within the above range, a decrease in the thermal conductivity of the agglomerated particles due to the remaining oxide layer can be more sufficiently suppressed. The lower limit of the total oxygen content of the boron nitride powder is not particularly limited and may be below the detection limit, but may be, for example, 0.01 mass% or more, or 0.02 mass% or more.

[0055] The upper limit of the average particle size of the boron nitride powder may be, for example, 90.0 μm or less, 85.0 μm or less, 80.0 μm or less, or 70.0 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 a molded body can be more effectively prevented when the boron nitride powder is filled into a resin and molded into a sheet, resulting in a more homogeneous molded body. The lower limit of the average particle size of the boron nitride powder may be, for example, 5.0 μm or more, 7.0 μm or more, 10.0 μm or more, 15.0 μm or more, or 20.0 μm or more. When the lower limit of the average particle size is within the above range, an increase in viscosity can be more effectively prevented when the boron nitride powder is kneaded with a resin to form a resin composition. The average particle size of the boron nitride powder may be adjusted within the above range, for example, 5.0 to 90.0 μm.

[0056] The average particle size of aggregated particles in this specification refers to the 50% cumulative diameter (median diameter) in the volume-based cumulative particle size distribution. More specifically, it refers to the particle size (D50) at which the cumulative value reaches 50% in the volume-based cumulative particle size distribution obtained by laser diffraction scattering for boron nitride 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 in this specification is measured in the presence of aggregated particles, without treatment using a homogenizer.

[0057] The agglomerated particles contained in the boron nitride powder are prepared so as to have a dense structure while exhibiting a relatively small crushing strength. The upper limit of the crushing strength of the agglomerated particles may be, for example, 9.0 MPa or less, 8.0 MPa or less, 7.0 MPa or less, or 6.8 MPa or less. The upper limit of the crushing strength within the above range can further improve the insulating properties of a resin sheet obtained using the boron nitride powder as a filler. The lower limit of the crushing strength of the agglomerated particles may 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. The lower limit of the crushing strength within the above range can further prevent the agglomerated particles from excessively collapsing during kneading with a resin or molding of a resin composition, thereby preventing a decrease in heat dissipation. The crushing strength of the agglomerated particles may be adjusted within the above range, for example, 1.0 to 9.0 MPa or 1.0 to 7.0 MPa.

[0058] The crushing strength in this specification refers to a value measured in accordance with the description in 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 agglomerate particle is calculated from the dimensionless number α (α=2.48), which varies depending on the position within the agglomerate 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.

[0059] The agglomerated particles contained in the boron nitride powder may be composed of densely agglomerated primary particles. That is, when a cross-section of the agglomerated particles contained in the boron nitride powder is observed, the proportion of the area where hexagonal boron nitride primary particles are present may be large, and the area of ​​voids may be kept low. When observing the cross-section of the agglomerated particles, the lower limit of the ratio of the total area of ​​the regions where the primary particles are present to the area of ​​the region surrounded by the periphery of the agglomerated particles (the area proportion of primary particles) may be, for example, 60 area% or more, 65 area% or more, or 70 area% or more. When the lower limit of the area proportion is within the above range, the boron nitride primary particles have a densely agglomerated structure, thereby achieving better heat dissipation properties. When observing the cross-section of the agglomerated particles, the upper limit of the area proportion of the primary particles is not particularly limited, but may be, for example, 90 area% or less, 85 area% or less, or 80 area% or less. When the upper limit of the area ratio is within the above range, an appropriate amount of voids are present within the aggregated particles, which facilitates penetration of the resin into the aggregated particles, thereby further suppressing deterioration in the insulating properties of the resin composition and the molded article. In cross-sectional observation of the aggregated particles, the total area of ​​the region where the primary particles are present relative to the area of ​​the region surrounded by the periphery of the aggregated particles may be adjusted within the above range, and may be, for example, 60 to 90 area % or 60 to 80 area %.

[0060] In the cross-sectional observation of the agglomerated particles, the ratio of the total area of ​​the region where the primary particles exist to the area of ​​the region surrounded by the periphery of the agglomerated particles (the primary particle area ratio) is a value determined by the following method. First, the boron nitride powder to be measured and an epoxy resin are placed in a test specimen mold, lightly mixed with a spatula, vacuum degassed, and then heated to harden the resin. Next, the hardened resin is scraped with SiC paper to expose a cross section, which is then processed by ion milling and the surface is coated with Os (osmium) to prepare a measurement sample. The cross section of the measurement sample thus prepared is observed using a scanning electron microscope. The cross-sectional SEM image obtained as described above was binarized using Otsu's binarization method described in "A Threshold Selection Method from Gray-Level Histograms," N. Otsu, IEEE Trans. Sys., Man, and Cyber., 9(1), pp. 62-66, 1979, to create a binarized image. Focusing on any one agglomerated particle in the above binarized image, the area of ​​the region surrounded by the periphery of the agglomerated particle was determined and used as the "area of ​​the agglomerated particle cross section" in the following formula (1). For the same agglomerated particle for which the "area of ​​the agglomerated particle cross section" was determined, the total area of ​​the primary particles constituting the agglomerated particle was determined and used as the "area (a) of the primary particle" in the following formula (1). The area ratio of the primary particles for the target agglomerated particle is calculated using the following formula (1). The same measurement is performed on 50 agglomerated particles, and the arithmetic average value is used as the "area ratio of the primary particles" for the boron nitride powder being measured. Figure 1 shows a scanning electron microscope image showing an example of the cross section of an agglomerated particle. Figure 1 shows the cross section of agglomerated particle P observed in the center. Figure 2 is a diagram for explaining a method for calculating the area ratio of primary particles, where (a) in Figure 2 is a binary image for determining the area of ​​the region surrounding the periphery of the agglomerated particle P shown in Figure 1, and (b) in Figure 2 is a binary image for determining the total area of ​​the parts where primary particles exist for the same agglomerated particle P. Area ratio of primary particles (%)=[(area of ​​primary particles (a)) / (cross-sectional area of ​​the above aggregated particles)]×100...(1).

[0061] An example of a boron nitride powder according to the present disclosure may be a powder containing agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride, the powder having an orientation index of 15.0 or less, the agglomerated particles having a crushing strength of 7.0 MPa or less, and in which, upon cross-sectional observation of the agglomerated particles, the total area of ​​the region where the primary particles are present relative to the area of ​​the region surrounded by the periphery of the agglomerated particles is 60 area % or more.

[0062] The agglomerated particles contained in the boron nitride powder according to the present disclosure have a relatively dense structure, resulting in a high density of boron nitride per agglomerated particle. On the other hand, the agglomerated particles have low crushing strength, and therefore can be deformed, collapsed, or the like due to molding pressure, etc., when used as a filler to form a resin sheet. This action minimizes the presence of voids, etc., caused by the agglomerated particles in the resin sheet, thereby improving the insulating properties and heat dissipation properties of the resin sheet. Therefore, the boron nitride powder according to the present disclosure is useful as a filler, and is particularly useful as a heat dissipation filler.

[0063] One embodiment of the resin composition contains a resin and the above-mentioned boron nitride powder. The resin composition can be formed into a molded product by molding or the like. The shape of the molded product is not particularly limited, but may be, for example, a sheet. The sheet-shaped molded product can also be called a resin sheet, and is useful as an insulating sheet, a heat dissipation sheet, or the like.

[0064] Examples of resins include liquid crystal polymers, fluororesins, silicone resins, silicone rubber, acrylic resins, polyolefins (such as polyethylene), epoxy resins, phenolic resins, melamine resins, urea resins, unsaturated polyesters, polyimides, polyamideimides, polyetherimides, polybutylene terephthalate, polyethylene terephthalate, polyphenylene ether, polyphenylene sulfide, wholly aromatic polyesters, polysulfones, polyethersulfones, polycarbonates, maleimide-modified resins, ABS (acrylonitrile-butadiene-styrene) resins, AAS (acrylonitrile-acrylic rubber-styrene) resins, and AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resins.

[0065] The lower limit of the resin content may be, for example, 15% by volume or more, 20% by volume or more, or 30% by volume or more, based on the total volume of the resin composition, and the upper limit of the resin content may be, for example, 60% by volume or less, 50% by volume or less, or 40% by volume or less, based on the total volume of the resin composition.

[0066] The lower limit of the content of the boron nitride powder may be, for example, 30% by volume or more, 40% by volume or more, 50% by volume or more, or 60% by volume or more, based on the total volume of the resin composition. When the lower limit of the content of the boron nitride powder is within the above range, the thermal conductivity of the resin composition is improved, and a molded product with excellent heat dissipation properties can be obtained. The upper limit of the content of the boron nitride powder may be, for example, 85% by volume or less, 80% by volume or less, or 70% by volume or less, based on the total volume of the resin composition. When the upper limit of the content of the boron nitride powder is within the above range, a decrease in the moldability of the resin composition can be suppressed.

[0067] In addition to the resin and boron nitride powder, the resin composition may further contain a curing agent that cures the resin. The curing agent can be appropriately selected depending on the type of resin. When the resin is an epoxy resin, examples of the curing agent include phenol novolac compounds, acid anhydrides, amino compounds, and imidazole compounds. The lower limit of the curing agent content may be, for example, 0.5 parts by mass or more, or 1.0 part by mass or more, per 100 parts by mass of the resin. The upper limit of the curing agent content may be, for example, 15.0 parts by mass or less, or 10.0 parts by mass or less, per 100 parts by mass of the resin.

[0068] 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]

[0069] 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.

[0070] (Comparative 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 boron carbide (BC) powder. The synthesized boron carbide powder was pulverized in a ball mill for 1 hour and sieved through a sieve to remove coarse particles, producing a boron carbide powder (BC powder) with an average particle size of 15 μm.

[0071] [Preparation of boron carbonitride powder] The boron carbide powder was filled into a boron nitride crucible, which was then placed in a resistance heating furnace and heated at 2100°C for 25 hours under a nitrogen gas atmosphere of 0.85 MPa to obtain a fired product containing boron carbonitride (BCN) powder (pressure nitriding process).

[0072] The fired product obtained as described above was heated in an air atmosphere at 750° C. for 5 hours to obtain a heat-treated product (carbon reduction step).

[0073] [Preparation of boron nitride powder] Boric acid was added so that the content of the boron source was 33 parts by mass based on the total amount of the heat-treated product and boric acid (boron source), and the mixture was mixed using a Henschel mixer to obtain a mixture. Next, 1.55 kg of the mixture was filled into a boron nitride container while being appropriately compression-molded.

[0074] Next, the container was placed in a resistance heating furnace, and in a nitrogen gas atmosphere at a pressure of 13 kPa, the temperature was raised from room temperature to 2000°C, and the temperature was maintained at 2000°C for 5 hours, thereby carrying out a heat treatment (crystallization step). This decarburized the boron carbonitride particles, and also crystallized the individual boron carbonitride particles in the aggregated boron carbonitride particles into primary particles of hexagonal boron nitride, thereby synthesizing boron nitride powder containing agglomerated particles in which multiple primary particles were aggregated. The synthesized boron nitride powder was crushed in a mortar for 10 minutes, and then classified using a nylon sieve with 75 μm mesh. The obtained boron nitride powder had an orientation index of 6.8 and a specific surface area of ​​3.5 m. 2 / g, and the crushing strength of the agglomerated particles contained in the boron nitride powder was 9.5 MPa.

[0075] Example 1 The boron nitride powder of Comparative Example 1 obtained as described above was used as a raw material powder to produce the boron nitride powder of Example 1. First, the raw material powder was filled into a mullite container, heated from room temperature to 700°C in an electric furnace, and held there for 1 hour to perform a surface oxidation treatment (oxidation step).

[0076] Water was added to the powder after the oxidation treatment so that the volume was 1 L of pure water per 100 parts by mass of the powder, and the mixture was stirred for 1 hour. Thereafter, the same amount of water (i.e., 1 L of pure water per 100 parts by mass of the powder before the first water wash) was further added to the solid matter recovered by suction filtration, and the mixture was stirred. The solid matter recovered by filtration was dried at 140°C for 12 hours. The dried powder was passed through a sieve with 75 μm openings, and the powder that passed through the sieve was designated as the boron nitride powder of Example 1.

[0077] Examples 2 to 8 Boron nitride powders of each example were produced in the same manner as in Example 1, except that the conditions for the oxidation step were changed as shown in Table 1.

[0078] <Evaluation of boron nitride powder> The boron nitride powders prepared in Comparative Example 1 and Examples 1 to 8 were evaluated for specific surface area, orientation index, average particle size, agglomerate particle crushing strength, and the sum of the area of ​​the region where primary particles exist relative to the area of ​​the region surrounded by the periphery of the agglomerate particle in cross-sectional observation of the agglomerate particle (referred to as primary particle area in the tables). The results are shown in Table 1.

[0079] <Evaluation of boron nitride powder as a heat dissipating filler> The resin sheets prepared using the boron nitride powders prepared in Comparative Example 1 and Examples 1 to 8 as a heat-dissipating filler were evaluated for insulation and heat dissipation as follows. The results are shown in Table 1.

[0080] [Preparation of evaluation sheet] A resin composition was obtained by mixing 100 parts by mass of naphthalene-type epoxy resin (HP4032, manufactured by DIC Corporation) and 10 parts by mass of an imidazole compound (2E4MZ-CN, manufactured by Shikoku Kasei Corporation) as a curing agent with boron nitride powder so that the content was 60% by volume. A Thinky Mixer (Awatori Rentaro, manufactured by Thinky Corporation) was used to knead the resin. The kneading conditions were 1600 rpm for 3 minutes. The obtained resin composition was applied to a PET film to a thickness of 0.3 mm. Then, the mixture was kneaded at a temperature of 150°C and 50 kgf / cm. 2Heating and pressure were applied under relatively mild conditions for 60 minutes under the above conditions to prepare a 0.3 mm resin sheet (evaluation sheet).

[0081] [Measurement of breakdown voltage and evaluation of insulation properties] The dielectric breakdown voltage of the evaluation sheet was measured. The dielectric breakdown voltage of the obtained evaluation sheet was measured using a voltage tester (manufactured by Kikusui Electronics Co., Ltd., device name: TOS-8650) in accordance with the description of JIS C 6481-1996 "Test method for copper-clad laminates for printed wiring boards." From the measurement results, the insulation was evaluated according to the following criteria. A: The breakdown voltage is 40 kV / mm or more. B: The breakdown voltage is 30 kV / mm or more and less than 40 kV / mm. C: The breakdown voltage is 20 kV / mm or more and less than 30 kV / mm. D: The breakdown voltage is 10 kV / mm or more and less than 20 kV / mm. E: The breakdown voltage is less than 10 kV / mm.

[0082] [Measurement of thermal conductivity and evaluation of heat dissipation] The thermal conductivity of the above evaluation sheet was measured. The thermal conductivity H (unit: W / (m K)) is calculated by multiplying the thermal diffusivity A (unit: m 2 / sec), density B (unit: kg / m 3 The thermal diffusivity A was calculated from the values ​​of the specific heat capacity C (unit: J / (kg·K)) using the formula H=A×B×C. The evaluation sheet was cut into a length of 10 mm, a width of 10 mm, and a thickness of 0.3 mm, and the thermal diffusivity A was measured using the laser flash method. The measurement device used was a xenon flash analyzer (manufactured by NETZSCH, product name: LFA447NanoFlash). The density B was measured using the Archimedes method. The specific heat capacity C was measured using a DSC (manufactured by Rigaku Corporation, product name: ThermoPlusEvoDSC8230). From the results, the heat dissipation performance was evaluated according to the following criteria. A: Thermal conductivity is 14W / mK or higher. B: Thermal conductivity is 12 W / mK or more and less than 14 W / mK. C: Thermal conductivity is 10 W / mK or more and less than 12 W / mK. D: Thermal conductivity is 8 W / mK or more and less than 10 W / mK. E: Thermal conductivity is 6 W / mK or more and less than 8 W / mK.

[0083] [Table 1]

[0084] (Comparative Example 2) A boron nitride powder was obtained in the same manner as in Comparative Example 1, except that the content of boric acid in the crystallization step was changed to 28 parts by mass. The obtained boron nitride powder had an orientation index of 7.6 and a specific surface area of ​​3.5 m 2 / g, and the crushing strength of the agglomerated particles contained in the boron nitride powder was 11.7 MPa.

[0085] Examples 9 to 13 The boron nitride powder of each example was produced in the same manner as in Example 1, except that the boron nitride powder of Comparative Example 2 was used as the raw material powder and the conditions for the oxidation step were changed as shown in Table 2.

[0086] (Examples 14 and 15) The boron nitride powders of each example were produced in the same manner as in Example 1, except that the boron nitride powder of Comparative Example 2 was used as the raw material powder, the conditions for the oxidation step were changed as shown in Table 2, and the powder was not washed after the oxidation treatment.

[0087] <Evaluation of boron nitride powder> The boron nitride powders prepared in Comparative Example 2 and Examples 9 to 15 were evaluated for specific surface area, orientation index, average particle size, agglomerate particle crushing strength, and the sum of the area of ​​the region where primary particles exist relative to the area of ​​the region surrounded by the periphery of the agglomerate particle in cross-sectional observation of the agglomerate particle (referred to as primary particle area in the tables) in the same manner as in Example 1. The results are shown in Table 2.

[0088] <Evaluation of boron nitride powder as a heat dissipating filler> Resin sheets prepared using the boron nitride powders prepared in Comparative Example 2 and Examples 9 to 15 as a heat-dissipating filler were evaluated for insulation and heat dissipation in the same manner as in Example 1. The results are shown in Table 2.

[0089] [Table 2] [Industrial Applicability]

[0090] The present disclosure provides a method for producing boron nitride powder containing agglomerated particles with low crushing strength, and also provides boron nitride powder useful as a filler for resins.

Claims

1. a step of heat-treating a powder containing agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride to form an oxide layer on at least a portion of the surface of the primary particles, The orientation index of the powder is 12.0 or less, A method for producing a boron nitride powder, wherein the agglomerated particles contained in the powder have a crushing strength of more than 9.0 MPa.

2. The manufacturing method according to claim 1 , wherein the step of providing the oxide layer is carried out in an atmosphere having an oxygen partial pressure of 20% or more.

3. 3. The method according to claim 1, wherein the heating temperature in the step of providing the oxide layer is 700 to 1100°C.

4. a step of calcining the boron carbide powder at 1900 to 2200°C in a nitrogen pressurized atmosphere of 0.60 MPa or more to obtain boron carbonitride powder; 3. The method according to claim 1, further comprising the step of mixing the boron carbonitride with a boron source and heating the mixture in an atmosphere containing nitrogen to generate primary particles of hexagonal boron nitride, thereby obtaining the agglomerated particles.

5. A powder containing agglomerated particles formed by agglomeration of primary particles of hexagonal boron nitride, the powder having an orientation index of 12.0 or less, The crushing strength of the agglomerated particles is 9.0 MPa or less, A boron nitride powder, wherein, upon cross-sectional observation of the agglomerated particles, the total area of ​​the region where the primary particles are present relative to the area of ​​the region surrounded by the periphery of the agglomerated particles is 60 area % or more.

6. Specific surface area is 2.0 to 5.0 m ２ 6. The boron nitride powder according to claim 5, wherein the boron nitride powder has a SiO2 content of 0.1g / g.

7. The boron nitride powder according to claim 5 or 6, having an average particle size of 5.0 to 90.0 μm.

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