Hexagonal boron nitride aggregate particles, hexagonal boron nitride powder, resin composition, and method for producing hexagonal boron nitride aggregate particles

Abstract

The purpose of one embodiment of the present disclosure is to provide hexagonal boron nitride aggregate particles and the like for obtaining a resin composition having both excellent thermal conductivity and dielectric strength. A hexagonal boron nitride aggregate particle (10) according to one embodiment of the present disclosure has La of 70-250 μm and ratios La / Lc and Lb / Lc of 2.8 or more, respectively, wherein La, Lb and Lc are the lengths of three sides orthogonal to each other of a rectangular parallelepiped that has at least one surface parallel to one surface having the largest projected area among the surfaces of the aggregated particle, circumscribes the aggregate particle and has the smallest volume, provided that La≥Lb≥Lc.

Description

Hexagonal boron nitride aggregate particles, hexagonal boron nitride powder, resin composition, and method for producing hexagonal boron nitride aggregate particles

[0001] The present invention relates to hexagonal boron nitride aggregate particles, hexagonal boron nitride powder, resin compositions, and methods for producing hexagonal boron nitride aggregate particles.

[0002] In recent years, the miniaturization and increased power of electronic components have led to a problem of increased heat generation. Therefore, research is underway to develop materials with excellent thermal conductivity to efficiently dissipate heat from electronic components.

[0003] Resin compositions obtained by blending hexagonal boron nitride powder with a resin exhibit improved thermal conductivity compared to the resin itself, making such resin compositions suitable for use as materials for electronic components. It is known that hexagonal boron nitride powder blended with a resin further improves the thermal conductivity of the resin composition by containing aggregated particles formed by the aggregation of primary particles of hexagonal boron nitride. Patent Document 1 discloses a technique for achieving excellent thermal conductivity by combining spherical aggregated particles of different particle sizes to increase the contact points between particles. Patent Document 2 discloses a technique for oriented and agglomerating primary boron nitride particles in a specific direction by nitriding boron carbide with a large aspect ratio to produce boron nitride.

[0004] Japanese Patent Publication No. 2022-133951, International Publication No. 2020 / 196679

[0005] However, in the powder described in Patent Document 1, the contact between spherical particles is point contact, and the contact area for heat conduction is small, so there is room for further improvement in thermal conductivity.

[0006] The technology described in Patent Document 2 utilizes the property of boron carbide that when boron carbide is nitrided, boron nitride aggregate particles that retain the shape of boron carbide are obtained. Therefore, in order to produce large-particle aggregate particles with high thermal conductivity using the technology of Patent Document 2, it is necessary to use boron carbide of the same large particle size as the raw material. However, since boron carbide is chemically stable and relatively difficult to nitride, when large-particle boron carbide is used, carbon tends to remain inside the produced aggregate particles. Therefore, a problem with the technology of Patent Document 2 is that the dielectric strength of the resin composition containing the produced aggregate particles tends to decrease.

[0007] One aspect of the present invention aims to provide hexagonal boron nitride aggregate particles, etc., for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength.

[0008] To solve the above problems, a hexagonal boron nitride aggregate particle according to one aspect of the present invention is a hexagonal boron nitride aggregate particle formed by the aggregation of hexagonal boron nitride primary particles, wherein the aggregate particle has at least one face parallel to the face having the largest projected area among the faces of the aggregate particle, and in a rectangular parallelepiped that is circumscribed around the aggregate particle and has the smallest volume, when the lengths of the three mutually orthogonal sides are La, Lb, and Lc (where La ≥ Lb ≥ Lc), La is 70 μm or more and 250 μm or less, the ratio La / Lc is 2.8 or more, and the ratio Lb / Lc is 2.8 or more.

[0009] To solve the above problems, a method for producing hexagonal boron nitride aggregate particles according to one aspect of the present invention is a method for producing hexagonal boron nitride aggregate particles formed by the aggregation of hexagonal boron nitride primary particles, comprising a mixing step of mixing boron oxide, a carbon source, an oxygen-containing calcium compound, and boron carbide to obtain a raw material mixture, wherein the boron oxide contains 20% by volume or more of particles with a particle size of 75 μm or less, and 30% by volume or more of particles with a particle size of 425 μm or more and 725 μm or less, and the ratio of the mass of the boron oxide converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) of the raw material mixture is 0.68 or more and 0.95 or less, and the B of the boron oxide is based on B ２ O ３For every 100 parts by mass of the total of the mass converted to and the mass converted to C of the carbon source, the oxygenated calcium compound is contained in an amount of 8 to 25 parts by mass, converted to CaO on a Ca basis, and B on a B basis of the boron oxide. ２ O ３ The process includes a mixing step of adding 10 to 35 parts by mass of boron carbide to 100 parts by mass of the total of the mass converted to C of the carbon source, the mass converted to CaO on a Ca basis of the oxygenated calcium compound, and the mass converted to CaO on a Ca basis of the boron carbide; and a heating step of heating the raw material mixture at a temperature of 1700°C to 1900°C under a nitrogen atmosphere.

[0010] According to one aspect of the present invention, it is possible to provide hexagonal boron nitride aggregate particles, etc., for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength.

[0011] This is a microscopic image illustrating a method for setting up a rectangular parallelepiped for hexagonal boron nitride aggregated particles according to one aspect of the present invention, where (a) is an example of a microscopic image of aggregated particles from the direction in which the projected area of ​​the aggregated particles is maximized, and (b) is an example of a microscopic image of aggregated particles from a direction perpendicular to the direction in (a). This is a schematic diagram illustrating the scheme of a method for producing hexagonal boron nitride aggregated particles according to one aspect of the present invention. This is an SEM image of hexagonal boron nitride powder from Example 1.

[0012] <Hexagonal Boron Nitride Aggregated Particles> In order to solve the problems of the conventional technology described above, the inventors conducted extensive research. As a result, the inventors discovered that plate-shaped hexagonal boron nitride aggregated particles, which have a longitudinal dimension greater than a certain value and a large aspect ratio, are useful as a filler for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength, and thus completed the present invention. When such aggregated particles are filled into a resin, multiple aggregated particles tend to come into contact with each other such that the surfaces with the largest projected area of ​​each aggregated particle face each other. Therefore, the contact area between plate-shaped aggregated particles is larger compared to spherical aggregated particles. For this reason, it is believed that the thermal conductivity of the resin composition filled with plate-shaped aggregated particles is improved.

[0013] In this specification, unless otherwise specified, "aggregated particle" refers to a single independent secondary particle and is distinguished from "powder," which refers to an aggregate of multiple particles. Below, a hexagonal boron nitride aggregated particle according to one aspect of the present invention will be described in detail. In this description, the hexagonal boron nitride aggregated particle according to one aspect of the present invention may be abbreviated as "the aggregated particle."

[0014] These aggregated particles are secondary particles composed of multiple primary particles (single particles) of hexagonal boron nitride that have aggregated. The fact that these aggregated particles are composed of aggregated primary particles can be observed using known microscopy methods such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

[0015] These aggregated particles are plate-like. For example, these aggregated particles are flattened and thin. Typically, these aggregated particles have an aggregated structure composed of multiple randomly oriented primary particles throughout the entire particle. Furthermore, these aggregated particles may not have significantly large hollow spaces in the aggregated structure compared to the gaps between primary particles that are usually observed. An example of a significantly large hollow space is a hollow space that penetrates the aggregated particle in the direction along the side of length La, which will be described later.

[0016] (Method for setting the rectangular parallelepiped) In this specification, the degree to which the aggregated particles are plate-like is evaluated based on the dimensions of a rectangular parallelepiped virtually set based on the microscopic image of the aggregated particles. The method for setting the rectangular parallelepiped for the aggregated particles will be explained with reference to Figure 1. Figure 1 is a microscopic image for illustrating the method for setting the rectangular parallelepiped for the aggregated particles. Figure 1(a) is an example of a microscopic image of the aggregated particles from the direction in which the projected area of ​​the aggregated particles is maximized. Figure 1(b) is an example of a microscopic image of the aggregated particles from a direction perpendicular to the direction in Figure 1(a).

[0017] First, the aggregated particles 10 are observed under a microscope using a rotatable sample stage. As shown in Figure 1(a), each face of the aggregated particles 10 is observed from the field of view direction that maximizes the projected area of ​​that face. The field of view direction corresponding to the face with the largest projected area from that field of view direction (i.e., the principal face) is defined as the Z-axis direction, and a microscopic image of the aggregated particles 10 is obtained from the Z-axis direction. The boundaries between adjacent faces are determined visually using the microscopic image. A rectangular face Sxy circumscribing the aggregated particles 10 is set in the microscopic image of Figure 1(a) such that the area of ​​face Sxy is minimized. Face Sxy is formed by two parallel edges Ex and two parallel edges Ey, with edges Ex and Ey being orthogonal to each other. The direction parallel to edge Ex is defined as the X-axis direction, and the direction parallel to edge Ey is defined as the Y-axis direction. Next, as shown in Figure 1(b), a microscopic image of the aggregated particle 10 is obtained from a direction perpendicular to the edge Ex and parallel to the XY plane and the plane Sxy. An edge Ez parallel to the Z-axis direction, having the length of the aggregated particle 10 along the Z-axis direction, is set in the microscopic image of Figure 1(b). By arranging four of each of the three types of edges Ex, Ey, and Ez in parallel, a rectangular parallelepiped is formed. The formed rectangular parallelepiped has at least one plane Sxy parallel to the plane of the aggregated particle that has the largest projected area, is circumscribing the aggregated particle 10, and has the smallest volume.

[0018] Next, the lengths of the three mutually orthogonal sides Ex, Ey, and Ez are measured, and these measured lengths are denoted as La, Lb, and Lc in descending order of length. That is, the relationship La ≥ Lb ≥ Lc holds. In the example shown in Figure 1, the length of Ex is 94 μm, the length of Ey is 74 μm, and the length of Ez is 17 μm. Therefore, La is 94 μm, Lb is 74 μm, and Lc is 17 μm. Note that the specific numerical values ​​shown in Figure 1 are merely examples for a better understanding of how to set up the rectangular parallelepiped and do not limit the aggregated particles.

[0019] (Length La in the Aggregated Particles) In the aggregated particles, the length La of the longest side of a hypothetically defined rectangular parallelepiped is an indicator of how plate-like the aggregated particles are, and it is preferable to control it within a specific range. The length La is 70 μm or more, preferably 80 μm or more. The longer the length La is within this range, the larger the contact area between the aggregated particles and other particles in the resin composition containing the powder of the aggregated particles, and the greater the thermal conductivity of the resin composition. Also, the length La is 250 μm or less, preferably 140 μm or less. The shorter the length La is within this range, the less likely voids are to form inside the aggregated particles, and when a varnish containing the aggregated particles, resin, and solvent is applied, the number of voids in the coated material is reduced, thus improving the dielectric strength of the resin composition obtained by drying the coated material.

[0020] (Ratios La / Lc and Lb / Lc in the Aggregated Particles) In the aggregated particles, the aspect ratio of a hypothetically set rectangular parallelepiped can be considered as the aspect ratio of the aggregated particles. The aspect ratio of the aggregated particles is an indicator of how plate-like the aggregated particles are, in particular how thin they are, and it is desirable to control it within a specific range. The larger the aspect ratio, the more plate-like the aggregated particles are. The aspect ratio is expressed by the ratios La / Lc and Lb / Lc, respectively. The ratio La / Lc is 2.8 or higher, preferably 4.0 or higher. The larger the ratio La / Lc is within these ranges, the closer the shape of the aggregated particles becomes from spherical to plate-like, resulting in more contact points between the aggregated particles and other particles in the resin composition containing the powder of the aggregated particles, and thus improving the thermal conductivity of the resin composition. Furthermore, the ratio La / Lc is preferably 9 or less, more preferably 6 or less. The smaller the ratio La / Lc is within these ranges, the less likely the aggregated particles are to break, improving the yield during production and handling during use. From a similar viewpoint to that of the ratio La / Lc, the ratio Lb / Lc is 2.8 or higher, preferably 4.0 or higher. Also from a similar viewpoint to that of the ratio La / Lc, the ratio Lb / Lc is preferably 9 or lower, more preferably 6 or lower.

[0021] (Composition of aggregated particles) These aggregated particles contain boron nitride as the main component, and preferably consist substantially of boron nitride. The composition of these aggregated particles is measured by energy-dispersive X-ray analysis, and can be confirmed using, for example, the JCM-7000NeoScope (product name) manufactured by JEOL Ltd. as an energy-dispersive X-ray analyzer.

[0022] These aggregated particles may contain elements other than B and N as impurity elements. Examples of impurity elements include C, O, Ca, Si, Fe, and Al.

[0023] <Hexagonal Boron Nitride Powder> The hexagonal boron nitride powder containing the aggregated particles described above can impart excellent thermal conductivity and dielectric strength to a resin composition containing the powder and resin, due to the aggregated particles. In other words, the hexagonal boron nitride powder containing the aggregated particles is also within the scope of the present invention. Hereinafter, the hexagonal boron nitride powder according to one aspect of the present invention may be abbreviated as "this powder".

[0024] (Proportion of aggregated particles) In this specification, "powder" refers to an aggregate of multiple particles. In this powder, some or all of the multiple particles constituting the aggregate are the aggregated particles. In this powder, it is preferable that the proportion of the aggregated particles among the multiple particles constituting the aggregate is controlled within a specific range. The proportion of the aggregated particles can be evaluated based on the product M・R. Here, M represents the mass (in parts by mass) of particles remaining on a sieve with a mesh size of 70 μm out of 100 parts by mass of this powder. R represents the number proportion (unitless) of the aggregated particles among the particles remaining on a sieve with a mesh size of 70 μm.

[0025] The number ratio R of these aggregated particles is measured using microscopic observation of particles remaining on a sieve with a mesh size of 70 μm. Specifically, multiple particles remaining on a sieve with a mesh size of 70 μm are observed under a microscope, and a microscopic image of each particle is obtained from the direction that maximizes its projected area. From the observed multiple particles, 100 particles are randomly selected. For each selected particle, the aforementioned hypothetical rectangular parallelepiped is set up, and it is determined whether each particle corresponds to these aggregated particles. R is calculated as the ratio of the number of particles determined to be these aggregated particles to the number of selected particles.

[0026] The product M·R is preferably 10 or more, and more preferably 20 or more. The larger the product M·R within this range, the greater the proportion of the aggregated particles in the powder, resulting in more contact points between the aggregated particles when the powder and resin are kneaded, and thus improving the thermal conductivity of the resin composition. Furthermore, the higher the product M·R, the better, but as an example, it is 40 or less, and as another example, 30 or less.

[0027] (Length La in this powder) The length La in this powder is an indicator of the degree to which the particles constituting this powder are plate-like, and it is preferable to control it within a specific range. In this specification, the length La in this powder refers to the numerical average of the lengths La measured for 100 particles randomly selected as described above in order to measure the number ratio R of aggregated particles. The length La is preferably 70 μm or more, and more preferably 80 μm or more. The longer the length La is within this range, the larger the contact area between the particles constituting this powder in the resin composition containing this powder, and the greater the thermal conductivity of the resin composition. The length La is preferably 250 μm or less, and more preferably 140 μm or less. The shorter the length La is within this range, the less likely voids are to form inside the particles constituting this powder, and when a varnish containing this powder, resin, and solvent is applied, the number of voids that form in the coated material is reduced, and the dielectric strength of the resin composition obtained by drying the coated material is improved.

[0028] (Ratio La / Lc and Lb / Lc in this powder) The aspect ratio in this powder is an index indicating how much the particles constituting this powder are plate-shaped, particularly how thin the particles constituting this powder are, and it is preferably controlled within a specific range. The larger the aspect ratio, the more plate-shaped the particles constituting this powder are. In this specification, the aspect ratio in this powder refers to the number average value of the aspect ratios measured for 100 particles randomly selected as described above to measure the number ratio R of the present agglomerated particles. The aspect ratio is represented by each of the ratio La / Lc and the ratio Lb / Lc. The ratio La / Lc is preferably 2.8 or more, more preferably 4.0 or more. The larger the ratio La / Lc within these ranges, the closer the shape of the particles constituting this powder approaches from a spherical shape to a plate shape. Therefore, in the resin composition containing this powder, the contacts between the particles increase, and the thermal conductivity of the resin composition is further improved. Also, the ratio La / Lc is preferably 9 or less, more preferably 6 or less. The smaller the ratio La / Lc within these ranges, the tendency is that it is easier to reduce the degree of thickening when filling this powder into a resin, and the handling property during the use of this powder is improved. From the same viewpoint as the ratio La / Lc, the ratio Lb / Lc is preferably 2.8 or more, more preferably 4.0 or more. Also, from the same viewpoint as the ratio La / Lc, the ratio Lb / Lc is preferably 9 or less, more preferably 6 or less.

[0029] (Composition of this powder) Among the plurality of particles constituting the aggregate in this powder, the particles other than the present agglomerated particles typically contain boron nitride as the main component and preferably consist essentially of boron nitride. Therefore, this powder as a whole typically contains boron nitride as the main component and preferably consists essentially of boron nitride.

[0030] The content of boron nitride in this powder is, for example, 90, 95 or 99% by mass or more, preferably 99.95% by mass or more, more preferably 99.97% by mass or more. The higher the content of boron nitride within these ranges, the more the curing inhibition of the resin composition, which is also caused by impurity elements and leads to a decrease in thermal conductivity and dielectric breakdown voltage, can be reduced.

[0031] The content of boron nitride in this powder is the value obtained by subtracting the content mass ratio (unit: mass %) of elements other than B and N in this powder from 100. The content of boron nitride is measured by X-ray fluorescence analysis. For example, as an X-ray fluorescence analyzer, it can be confirmed by Rigaku ZSX Primus2 (trade name).

[0032] This powder may contain elements other than B and N as impurity elements. Examples of impurity elements include C, O, Ca, Si, Fe, Al, etc.

[0033] The carbon content of this powder is preferably 300 ppm or less, more preferably 200 ppm or less, and still more preferably 100 ppm or less. The lower the carbon content within these ranges, the lower the curing inhibition of the resin composition caused by carbon impurities and leading to a decrease in dielectric breakdown strength can be reduced. There is no particular lower limit for the carbon content of this powder, but as an example, the carbon content may be 10 ppm or more. The carbon content is measured by the oxygen flow combustion-infrared absorption method. For example, as a carbon analyzer, it is measured by Horiba EMIA-110 (trade name).

[0034] (Specific surface area) The specific surface area of this powder is preferably 2.2 m ２ / g or more, more preferably 2.5 m ２ / g or more. The larger the specific surface area within these ranges, the smaller and denser the primary particles constituting the agglomerated particles contained in this powder, and the strength of the agglomeration is improved. Therefore, the effect of improving the thermal conductivity of the resin composition by this powder becomes greater. There is no particular upper limit for the specific surface area of this powder, but as an example, the specific surface area may be 5.0 m ２ / g or less.

[0035] In addition, in this specification, the specific surface area of this powder is measured by the BET single point method. The specific surface area of this powder can be measured, for example, using Micromeritics: FlowSorb III 2310 (trade name).

[0036] <Resin Composition> One application of this powder is as a filler for resins, aimed at improving dielectric strength and thermal conductivity. That is, a resin composition containing this powder and a resin is also within the scope of the present invention. A resin composition according to one aspect of the present invention has excellent thermal conductivity and dielectric strength due to this powder. Hereinafter, a resin composition according to one aspect of the present invention may be abbreviated as "this resin composition". This resin composition can be used for various purposes, for example, as a thermally conductive resin composition or as a material for thermally conductive molded articles.

[0037] In this resin composition, the content of this powder is preferably 30 to 90% by volume, and more preferably 40 to 80% by volume, relative to the resin composition. A powder content of 30% by volume or more facilitates the formation of thermal pathways, making it easier to improve the thermal conductivity of the resin composition. A powder content of 90% by volume or less facilitates the uniform mixing of the resin and the powder, preventing the formation of gaps due to insufficient resin and making it easier to improve the dielectric strength of the resin composition.

[0038] Examples of resins included in this resin composition include: thermoplastic resins such as polyolefins, vinyl chloride resins, methyl methacrylate resins, nylon and fluororesins; thermosetting resins such as epoxy resins, phenolic resins, urea resins, melamine resins, unsaturated polyester resins, silicon resins and bismaleimidotriazine resins; synthetic rubbers; and the like.

[0039] This resin composition may further contain other components in addition to the powder and resin. The other components are appropriately selected from known components depending on the application of the resin composition. Examples of other components include thermally conductive fillers such as aluminum nitride and aluminum oxide; known additives such as polymerization initiators, curing agents, polymerization inhibitors, polymerization retarders, coupling agents, plasticizers, ultraviolet absorbers, pigments, dyes, antibacterial agents, organic fillers, and organic-inorganic composite fillers.

[0040] This resin composition can be used as a material for heat-dissipating molded products such as heat dissipation sheets, phase change sheets, heat dissipation tapes, heat dissipation resin substrates in printed circuit board (PWB) base resin substrates or copper-clad laminate (CCL) base resin substrates, and insulating layers in metal base substrates such as aluminum base substrates or copper base substrates; and as a heat dissipation material such as heat dissipation grease, heat dissipation adhesive, gap filler, heat dissipation paint, heat dissipation coating, and encapsulant for power devices.

[0041] Furthermore, the uses of this powder are not limited to fillers for resins. Examples of other uses include boron nitride processed products such as boron nitride molded products or raw materials for cubic boron nitride, nucleating agents for engineering plastics, phase change materials, solid or liquid thermal interface materials, release agents for molds of molten metal or molten glass, cosmetics, and raw materials for composite ceramics.

[0042] <Method for producing hexagonal boron nitride aggregated particles> In order to solve the problems of the conventional technology described above, the inventors conducted extensive research. As a result, the inventors discovered that by using boron oxide with large and small particle sizes as raw materials in a specific ratio, it is possible to produce plate-shaped hexagonal boron nitride aggregated particles that are useful as a filler for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength, and thus completed the present invention.

[0043] The inventors surmise that these effects are due to the scheme shown in Figure 2. As shown in Figure 2(a), a mixed raw material containing large-particle boron oxide 100a, small-particle boron oxide 100b, a carbon source 101, an oxygen-containing calcium compound 102, and boron carbide 103 is used. When the mixed raw material is heated, as shown in Figure 2(b), the reductive nitridation reaction of the large-particle boron oxide 100a proceeds on the surface of the large-particle boron oxide 100a, and a crude boron nitride product layer 10a is formed. The small-particle boron oxide 100b also undergoes the reductive nitridation reaction, but since it is more volatile than the large-particle boron oxide 100a, when heated further, as shown in Figure 2(c), it also plays a role in concentrating the other raw materials on the surface of the boron oxide 100a by volatilization. As a result, a boron nitride product layer 10b with higher purity is formed. Furthermore, since the boron oxide 100a inside the nucleus of the reductitrative nitridation reaction also volatilizes, the nucleus gradually shrinks. In this way, core-shell particles of boron nitride layer-boron oxide nucleus are formed. When the core-shell particles are decomposed by means of pulverization or other means, as shown in Figure 1(d), the boron nitride layer is easily separated from the shrunken nucleus, and the separated layer is further decomposed into plate-like hexagonal boron nitride aggregate particles 10.

[0044] The following describes in detail a method for producing hexagonal boron nitride aggregate particles according to one aspect of the present invention. In this description, the method for producing hexagonal boron nitride aggregate particles according to one aspect of the present invention may be abbreviated as "this production method." This production method may be the method for producing the aggregate particles described above, although this is not limited to this method. Furthermore, this production method may also be the method for producing hexagonal boron nitride powder containing these aggregate particles, i.e., the powder described above. In addition, the hexagonal boron nitride aggregate particles and hexagonal boron nitride powder produced by this production method are also within the scope of the present invention.

[0045] This manufacturing method includes a mixing step and a heating step in that order. Furthermore, this manufacturing method may further include a grinding step after the heating step. Furthermore, this manufacturing method may further include at least one of an acid washing step and a classification step after the grinding step.

[0046] (Mixing Process) The mixing process is a process of mixing boron oxide, a carbon source, an oxygen-containing calcium compound, and boron carbide as raw materials for hexagonal boron nitride agglomerated particles to obtain a raw material mixture. The amount of each raw material may be selected so that the ratio of the components described below for the raw material mixture is satisfied. The mixing of the raw materials can be carried out using known methods. For example, it can be carried out using mixers such as a vibration mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibration stirrer, a V-type mixer, etc. Even when using three or more raw materials, the order of mixing the raw materials is not particularly limited, and all the raw materials may be mixed simultaneously, or they may be mixed in any order in sequence. Also, in order to mix the raw materials more uniformly, the raw materials may be mixed while being pulverized.

[0047] (Boron Oxide) Boron oxide is a compound represented by the chemical formula B ２ O ３ . In this production method, granular boron oxide is used. In this production method, boron nitride is mainly produced by the reduction nitridation reaction of boron oxide. Therefore, compared with the technology of Patent Document 2 that mainly uses the nitridation reaction of boron carbide, the boron carbide remaining in the produced hexagonal boron nitride agglomerated particles is reduced.

[0048] The volume average particle size of boron oxide is preferably 150 μm or more, more preferably 180 μm or more. Within these ranges, the larger the volume average particle size, the easier it is to control the reaction to obtain hexagonal boron nitride agglomerated particles and to suppress the moisture absorption of boron oxide, making the handling of boron oxide easier. The volume average particle size of boron oxide is preferably 300 μm or less, more preferably 250 μm or less. Within these ranges, the smaller the volume average particle size, the more likely the reduction nitridation reaction of boron oxide is to proceed.

[0049] The boron oxide mixture contains at least 20% by volume of small-particle boron oxide with a particle size of 75 μm or less, preferably at least 30% by volume, per 100% by volume of boron oxide. Furthermore, the boron oxide mixture contains at least 30% by volume of large-particle boron oxide with a particle size of 425 μm to 725 μm, preferably at least 35% by volume. The higher the respective content of large and small-particle boron oxide within the above ranges, the more favorably the mixture can form both a portion where large-particle boron oxide is not mixed with other raw materials and functions as a nucleus for the reduction-nitridation reaction, and a portion where small-particle boron oxide is well mixed with other raw materials and functions as a substrate for the reduction-nitridation reaction. This allows for the more favorable formation of core-shell particles with a boron nitride layer and boron oxide nucleus. There is no particular upper limit to the content of particles with a particle size of 75 μm or less, but as an example, the content of particles with a particle size of 75 μm or less may be 50% by volume or less. Furthermore, there is no particular upper limit on the content of particles with a particle size of 425 μm or more and 725 μm or less; however, as an example, the content of particles with a particle size of 425 μm or more and 725 μm or less may be 60 volume% or less.

[0050] In this specification, the particle size of boron oxide is evaluated based on a volume-based particle size distribution measured by the dry laser diffraction particle size distribution method.

[0051] (Carbon Source) The carbon source can be a known carbon material that acts as a reducing agent for boron oxide. Examples of carbon sources include: amorphous carbon such as carbon black, activated carbon, and carbon fiber; crystalline carbon such as diamond, graphite, and nanocarbon; and pyrolytic carbon obtained by thermal decomposition of monomers or polymers. Among these, amorphous carbon is preferred as the carbon source from the viewpoint of high reactivity, and carbon black is even more preferred from the viewpoint of industrially controlled quality. Examples of carbon black include acetylene black, furnace black, and thermal black.

[0052] When the carbon source is granular, its average particle size is preferably 0.01 μm or larger, and more preferably 0.05 μm or larger. Within this range, the larger the average particle size, the easier the carbon source is to handle. Alternatively, the average particle size of the carbon source is preferably 5 μm or smaller, more preferably 4 μm or smaller, and even more preferably 3 μm or smaller. Within this range, the smaller the average particle size, the higher the reactivity of the carbon source.

[0053] (Oxygen-containing calcium) Oxygen-containing calcium compounds are any compounds containing calcium and oxygen. During the heating process, oxygen-containing calcium compounds prevent the volatilization of boron oxide by forming a high-melting-point complex oxide with boron oxide, and also function as a catalyst in the reaction of directly nitriding boron carbide. Oxygen-containing calcium compounds are preferably salts of an organic or inorganic acid with calcium. Examples of oxygen-containing calcium compounds include calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate. Among these, calcium oxide and calcium carbonate are preferred as oxygen-containing calcium compounds. Oxygen-containing calcium compounds may be one type or a combination of two or more types.

[0054] When the oxygen-containing calcium compound is in granular form, its average particle size is preferably 0.01 μm or more, more preferably 0.05 μm or more, and even more preferably 0.1 μm or more. Furthermore, the average particle size of the oxygen-containing calcium compound is preferably 200 μm or less, more preferably 120 μm or less, and even more preferably 80 μm or less.

[0055] (Boron Carbide) Boron carbide may be any known material. Boron carbide promotes the formation of aggregated particles during the heating process. When boron carbide is granular, its average particle size is preferably 30 μm or more, more preferably 50 μm or more, and even more preferably 70 μm or more. Within this range, the larger the average particle size, the more likely it is that aggregated particles with a longer length La, which contributes greatly to improving thermal conductivity, will be formed. Furthermore, the average particle size of boron carbide is preferably 250 μm or less, more preferably 180 μm or less, and even more preferably 150 μm or less. Within this range, the smaller the average particle size, the less likely it is that coarse aggregated particles will be formed.

[0056] (Ratio of components) In the raw material mixture, the ratio of the mass of boron oxide converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) is 0.68 or higher, preferably 0.78 or higher. The higher the ratio within this range, the less carbon remains after the reductive nitriding reaction, thus increasing the effect of improving the dielectric strength of the resin composition by the hexagonal boron nitride aggregate particles produced. Alternatively, the ratio is 0.95 or lower, preferably 0.81 or lower. The lower the ratio within this range, the less excessive growth of the primary particles constituting the aggregate particles due to the reductive nitriding reaction can be avoided, thus reducing the number of hexagonal boron nitride aggregate particles produced and reducing the softening of the aggregate particles.

[0057] In the raw material mixture, the B standard for boron oxide is B ２ O ３The content of the oxygenated calcium compound, converted to CaO based on Ca, relative to 100 parts by mass of the total mass converted to C of the carbon source, is 8 parts by mass or more, preferably 12 parts by mass or more. Within this range, the higher the content of the oxygenated calcium compound, the more boron oxide forms a complex oxide with the oxygenated calcium compound, making it less likely for the boron oxide to volatilize, thus allowing for favorable control of the particle size of the hexagonal boron nitride powder produced. Furthermore, the content is 25 parts by mass or less, preferably 21 parts by mass or less. Within this range, the lower the content of the oxygenated calcium compound, the lower the melting point of the complex oxide of boron oxide and the oxygenated calcium compound, thus facilitating the reductive nitridation reaction during the heating process.

[0058] In the raw material mixture, the B standard for boron oxide is B ２ O ３ The content of boron carbide per 100 parts by mass of the total of the mass converted to C in the carbon source and the mass converted to CaO on a Ca basis of the oxygen-containing calcium compound is 10 parts by mass or more, preferably 15 parts by mass or more. Within this range, the higher the boron carbide content, the more the generation of a sufficient amount of aggregated particles is promoted. Also, the content is 35 parts by mass or less, preferably 26 parts by mass or less. Within this range, the lower the boron carbide content, the less boron carbide, which is less susceptible to reductive nitridation compared to boron oxide, is produced, and the amount of carbon remaining after the reductive nitridation reaction is reduced, thus increasing the effect of improving the dielectric strength of the resin composition by the hexagonal boron nitride aggregated particles produced.

[0059] (Heating Process) The heating process involves heating the raw material mixture under a nitrogen atmosphere. During the heating process, the reduction nitridation reaction of the boron element contained in the raw material mixture proceeds, yielding hexagonal boron nitride aggregate particles and hexagonal boron nitride powder containing them. (Heating Conditions) In the heating process, the raw material mixture is heated under a nitrogen atmosphere. Nitrogen is supplied to the reaction system by known methods. The nitrogen atmosphere mainly consists of nitrogen gas, for example, containing 90% or more by volume of nitrogen gas. The nitrogen atmosphere may also contain a non-oxidizing gas such as argon gas or helium gas as the remainder.

[0060] In the heating process, the heating temperature is 1700°C or higher, preferably 1800°C or higher. The higher the heating temperature within this range, the easier the reaction proceeds, and the more efficiently boron nitride can be obtained. Alternatively, the heating temperature can be 1900°C or lower, preferably 1850°C or lower. The lower the heating temperature within this range, the more the grain growth of the primary particles constituting the aggregated particles is suppressed, resulting in denser aggregated particles, and thus making it easier to increase the strength of the aggregated particles.

[0061] In the heating process, the heating time can be adjusted according to the composition of the raw material mixture and the heating temperature, but as an example, it may be between 5 hours and 30 hours. A heating time of 5 hours or more is advantageous from the viewpoint of allowing the reduction-nitridation reaction to proceed sufficiently. A heating time of 30 hours or less is advantageous from the viewpoint of carrying out the heating process at a low cost.

[0062] The heating process can be carried out using a known reactor capable of controlling the reaction atmosphere. Examples of reactors include atmosphere-controlled high-temperature furnaces that perform heating by high-frequency induction heating or heater heating. The heating method may be batch, pusher, or continuous.

[0063] (Grinding Process) The grinding process is a process of grinding the boron nitride powder obtained in the heating process. By performing the grinding process, the core-shell particles of the boron nitride layer-boron oxide nucleus contained in the powder are decomposed, and hexagonal boron nitride particles from which the boron nitride layer has been decomposed can be suitably obtained. The grinding process can be carried out using known grinding equipment, and examples of grinding equipment include stone mill grinders, ball mills, hammer mills, roll crusher pin mills, jet mills, mortars, etc.

[0064] (Acid Washing Process) The acid washing process is a process of washing the boron nitride powder obtained by the heating process, or by the grinding process if performed, with acid. The boron nitride powder obtained by the reductive nitridation reaction in the heating process may contain impurities such as complex oxides consisting of boron oxide and calcium oxide. Therefore, the boron nitride powder may be washed with acid to remove impurities. Examples of acids include hydrochloric acid, nitric acid, sulfuric acid, and acetic acid. The method of acid washing is not particularly limited, and known methods may be used. As an example, the boron nitride powder is placed in a container and the powder is brought into contact with 5 to 10 times the amount of dilute hydrochloric acid (5 to 20% by mass HCl) for 6 hours or more. In the acid washing process, stirring may be performed with a stirring blade or the like in order to wash efficiently.

[0065] After acid washing, the powder may be washed with water to remove any remaining acid. One method of washing with water is to filter the used acid, then disperse the powder in an equal amount of pure water, and filter again.

[0066] The drying conditions for the powder obtained by acid washing or water washing are preferably a drying temperature of 50°C to 250°C, in air or under reduced pressure. The drying time is not particularly limited, but it is preferable to dry it for a time such that the moisture content approaches 0% as closely as possible under the aforementioned drying conditions.

[0067] (Classification Process) The classification process is a process for classifying hexagonal boron nitride powder. The classification process allows for the removal of unwanted particles of a certain particle size contained in the powder. Classification can be carried out by known methods. Examples of classification include the removal of coarse particles using a sieve and the removal of ultrafine particles using airflow classification.

[0068] <Additional Notes> The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0069] <Summary> As can be understood from the above description, the present invention encompasses the following aspects.

[0070] Embodiment 1: Hexagonal boron nitride aggregated particles formed by the aggregation of hexagonal boron nitride primary particles, wherein a rectangular parallelepiped having at least one face parallel to the face having the largest projected area among the faces of the aggregated particles, circumscribing the aggregated particles and having the smallest volume, has lengths La, Lb, and Lc of three mutually orthogonal sides (where La ≥ Lb ≥ Lc), where La is 70 μm or more and 250 μm or less, the ratio La / Lc is 2.8 or more, and the ratio Lb / Lc is 2.8 or more, the hexagonal boron nitride aggregated particles.

[0071] Embodiment 2: Hexagonal boron nitride aggregated particles of Embodiment 1, wherein the ratio La / Lc is 9 or less and the ratio Lb / Lc is 9 or less.

[0072] Embodiment 3: Hexagonal boron nitride powder comprising hexagonal boron nitride aggregated particles according to Embodiment 1 or 2.

[0073] Embodiment 4: The hexagonal boron nitride powder according to Embodiment 3, wherein M is the mass of particles remaining on a sieve with a mesh size of 70 μm out of 100 parts by mass of the hexagonal boron nitride powder, and R is the number ratio of hexagonal boron nitride aggregate particles among the particles remaining on the sieve, and the product M・R is 10 or more and 40 or less.

[0074] Embodiment 5: Hexagonal boron nitride powder according to Embodiment 3 or 4, wherein the carbon content is 300 ppm or less.

[0075] Embodiment 6: A resin composition comprising hexagonal boron nitride powder according to any one embodiment of Embodiments 3 to 5 and a resin.

[0076] Embodiment 7: A method for producing aggregated hexagonal boron nitride particles, comprising a mixing step of mixing boron oxide, a carbon source, an oxygen-containing calcium compound, and boron carbide to obtain a raw material mixture, wherein the boron oxide contains 20% by volume or more of particles with a particle size of 75 μm or less, and 30% by volume or more of particles with a particle size of 425 μm or more and 725 μm or less, and the ratio of the mass of the boron oxide converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) of the raw material mixture is 0.68 or more and 0.95 or less, and the B of the boron oxide is used as the basis for B ２ O ３For every 100 parts by mass of the total of the mass converted to and the mass converted to C of the carbon source, the oxygenated calcium compound is contained in an amount of 8 to 25 parts by mass, converted to CaO on a Ca basis, and B on a B basis of the boron oxide. ２ O ３ A manufacturing method comprising: a mixing step of adding 10 to 35 parts by mass of boron carbide to 100 parts by mass of the total of the mass converted to C of the carbon source and the mass of the oxygenated calcium compound converted to CaO on a Ca basis; and a heating step of heating the raw material mixture at a temperature of 1700°C to 1900°C under a nitrogen atmosphere.

[0077] Embodiment 8: The method for producing boron oxide according to Embodiment 7, wherein the boron oxide has a volume-average particle size of 150 μm or more and 300 μm or less.

[0078] Embodiment 9: The manufacturing method of Embodiment 7 or 8, further comprising a grinding step of grinding the obtained boron nitride powder after the heating step.

[0079] An embodiment of the present invention is described below. In each of the examples and comparative examples, hexagonal boron nitride aggregate particles and powders containing them were produced under various conditions, the properties of the obtained powders were measured, and the properties of resin compositions containing the powders as fillers were evaluated.

[0080] [Example 1] Boron oxide powder A with a volume average particle size of 35 μm and boron oxide powder B with a volume average particle size of 520 μm were used as raw materials. Boron oxide powder A contained many large particles (particle size 425 μm to 725 μm), and boron oxide powder B contained many small particles (particle size 75 μm or less). Boron oxide powders A and B were mixed to prepare a boron oxide mixed powder having a volume average particle size of 220 μm, a particle ratio of 35 volume% of particles with a particle size of 75 μm or less, and a particle ratio of 40 volume% of particles with a particle size of 425 μm to 725 μm.

[0081] 720 g of boron oxide mixed powder, 275 g of carbon black, 230 g of calcium carbonate, and 180 g of boron carbide were mixed using a ball mill to obtain a raw material mixture. The ratio B / C of the mass of boron oxide converted to B to the mass of carbon black converted to C in the raw material mixture was 0.81.２ O ３ The amount of calcium carbonate converted to CaO on a Ca basis was 13 parts by mass per 100 parts by mass, which is the sum of the mass converted to C in carbon black and the mass converted to C in carbon black. ２ O ３ The boron carbide content was 16 parts by mass per 100 parts by mass, which is the sum of the mass converted to carbon, the mass converted to carbon (C) of carbon black, and the mass converted to CaO based on the Ca of calcium carbonate. 1 kg of the raw material mixture was subjected to a reduction nitridation reaction by heating it at 1800°C for 6 hours in a graphite Tamman furnace under a nitrogen gas atmosphere.

[0082] The obtained crude hexagonal boron nitride powder was pulverized and placed in a container. For acid washing, five times the volume of dilute hydrochloric acid (7% by mass HCl) was added to the powder, and the mixture was stirred at 700 rpm for 24 hours. After acid washing, the dilute hydrochloric acid was filtered, and the filtered boron nitride powder was dispersed in the same volume of pure water as the used dilute hydrochloric acid. The pure water was then filtered. This operation was repeated five times, and the powder was then vacuum-dried at 200°C for 6 hours.

[0083] The powder obtained after drying was sieved through a 120 μm mesh to obtain the hexagonal boron nitride powder of Example 1. Microscopic observation, specific surface area measurement, and carbon content measurement of the obtained hexagonal boron nitride powder were performed using the methods described later.

[0084] [Example 2] The same procedure as in Example 1 was followed to obtain the hexagonal boron nitride powder of Example 2, except that the boron oxide mixed powder was changed to a boron oxide mixed powder having a volume average particle size of 205 μm, a particle size of 75 μm or less (23% by volume), and a particle size of 425 μm to 725 μm (33% by volume).

[0085] [Comparative Example 1] The same procedure as in Example 1 was followed to obtain the hexagonal boron nitride powder of Comparative Example 1, except that the boron oxide mixed powder was changed to a boron oxide mixed powder having a volume average particle size of 260 μm, a particle size of 75 μm or less of 8 volume%, and a particle size of 425 μm to 725 μm of 31 volume%.

[0086] [Comparative Example 2] The same procedure as in Example 1 was performed, except for the following changes, to obtain the hexagonal boron nitride powder of Comparative Example 2. Note that Comparative Example 2 is the technology described in Patent Document 1 above. - Instead of the boron oxide mixed powder, boron oxide powder was used that had a volume average particle size of 161 μm, a particle size of 75 μm or less (25% by volume), and did not contain particles with a particle size of 425 μm to 725 μm. - Carbon black and calcium carbonate were used as raw materials other than boron oxide. In the raw material mixture: The ratio B / C of the mass of boron oxide converted to B and the mass of carbon black converted to C was set to 0.63; B based on the B of boron oxide ２ O ３ The amount of calcium carbonate converted to CaO on a Ca basis shall be 8 parts by mass per 100 parts by mass of the total mass converted to C of carbon black and the total mass converted to C of carbon black; and the amount of boron oxide converted to B on a B basis. ２ O ３ The boron carbide content was set to 0 parts by mass relative to 100 parts by mass of the total of the mass converted to carbon, the mass converted to carbon (C) of carbon black, and the mass converted to CaO based on the Ca of calcium carbonate. Boron carbide was not used. The temperature of the heating step for the reduction nitridation reaction was changed to 1940°C.

[0087] [Microscopic observation of powder] First, 100 parts by mass of hexagonal boron nitride powder from each of the examples and comparative examples were passed through a sieve with a mesh size of 70 μm, and the mass M of the particles remaining on the sieve was measured.

[0088] Next, the particles remaining on the sieve were observed using a SEM equipped with a rotatable sample stage. For each observed particle, 100 randomly selected particles were used to construct a hypothetical rectangular parallelepiped having at least one face parallel to the face of the particle with the largest projected area, circumscribing the particle, and having the smallest volume. The lengths La, Lb, and Lc of the three sides of the rectangular parallelepiped (where La ≥ Lb ≥ Lc) were then measured.

[0089] The average number of particles with length La, and the average number of particles with aspect ratios La / Lc and La / Lb were calculated for 100 selected particles. The proportion R of particles among the 100 selected particles with length La between 70 μm and 250 μm, and aspect ratios La / Lc and Lb / Lc of 2.8 or higher was calculated. The product M・R was also calculated.

[0090] [Measurement of Specific Surface Area] The specific surface area of ​​the hexagonal boron nitride powder in each of the examples and comparative examples was measured using Micromeritics' FlowSorb III 2310 (product name).

[0091] [Measurement of Carbon Content] The carbon content of the hexagonal boron nitride powder in each of the examples and comparative examples was measured using EMIA-110 (product name) manufactured by Horiba, Ltd.

[0092] [Preparation of Resin Sheets, Evaluation of Thermal Conductivity and Dielectric Strength] Hexagonal boron nitride powder was packed into epoxy resin to prepare resin sheets for both the examples and comparative examples, and their thermal conductivity and dielectric strength were evaluated. Specifically, a base resin mixture was prepared with 100 parts by mass of epoxy resin (JER828, manufactured by Mitsubishi Chemical Corporation), 5 parts by mass of a curing agent (imidazole-based curing agent, Curesol 2EDS4MZ, manufactured by Shikoku Chemicals Co., Ltd.), and 210 parts by mass of a solvent, methyl ethyl ketone. Next, the base resin mixture and hexagonal boron nitride powder were mixed so that the ratio of epoxy resin to hexagonal boron nitride powder was 32% by volume of epoxy resin and 68% by volume of hexagonal boron nitride powder, and the mixture was stirred in a rotation / revolution mixer (MAZERUSTAR, manufactured by Kurabo Industries Ltd.) to obtain varnish. This varnish was applied to a PET film to a thickness of approximately 250-300 μm using a PI-1210 automatic coating machine manufactured by Tester Industries Co., Ltd., and after drying, it was cured under reduced pressure at a temperature of 200°C, a pressure of 5 MPa, and a holding time of 30 minutes to produce a resin sheet with a thickness of 200 μm.

[0093] The thermal conductivity of the resin sheet was calculated by analyzing it using a thermal wave analysis device. The calculated thermal conductivity was evaluated according to the following criteria. A thermal conductivity evaluation of "4" or "3" indicates that the resin sheet has excellent thermal conductivity. 4: 16 W / m·K < thermal conductivity 3: 15 W / m·K < thermal conductivity ≤ 16 W / m·K 2: 14 W / m·K < thermal conductivity ≤ 15 W / m·K 1: thermal conductivity ≤ 14 W / m·K

[0094] Furthermore, the dielectric strength of the resin sheet was measured using a dielectric strength tester (manufactured by Tama Densoku Co., Ltd.). The measured dielectric strength was evaluated according to the following criteria. A dielectric strength evaluation of "4" or "3" indicates that the dielectric strength of the resin sheet is excellent. 4: Dielectric strength < 60kV / mm 3: 55kV / mm < Dielectric strength ≤ 60kV / mm 2: 50kV / mm < Dielectric strength ≤ 55kV / mm 1: Dielectric strength ≤ 50kV / mm

[0095] [Results] Table 1 shows the manufacturing conditions, powder properties measurement results, and resin sheet evaluation results for the examples and comparative examples.

[0096]

[0097] In Table 1, *1: The B / C ratio refers to the ratio of the mass of boron oxide converted to B in the raw material mixture to the mass of carbon black converted to C (mass converted to B / mass converted to C). *2: The amount of CaO refers to the amount of boron oxide in the raw material mixture, based on B. ２ O ３ This refers to the content of calcium oxide converted to CaO on a Ca basis, per 100 parts by mass, which is the sum of the mass converted to C and the mass converted to C of carbon black. *3: The amount of boron carbide refers to the amount of boron oxide in the raw material mixture, converted to B on a B basis. ２ O ３ This refers to the boron carbide content per 100 parts by mass, which is the sum of the mass converted to C in carbon black and the mass converted to CaO based on the Ca of calcium oxide.

[0098] In Comparative Examples 1 and 2, boron oxide with a particle size below a specified percentage and boron oxide with a particle size below a specified percentage were used. However, the aspect ratios La / Lc and Lb / Lc of the resulting aggregated particles were low, and the aggregated particles were spherical.

[0099] As can be seen from the comparison between Examples 1 and 2 and Comparative Examples 1 and 2, in Examples 1 and 2, by using large-particle boron oxide and small-particle boron oxide in specific proportions or more, the aspect ratios La / Lc and Lb / Lc of the aggregated particles became larger. Also, in Example 1, as can be seen from the product M・R value, it was possible to produce a larger number of aggregated particles with a large aspect ratio. Furthermore, in Examples 1 and 2, the length of the long side La did not decrease significantly compared to Comparative Examples 1 and 2, but remained at a similar level. In addition, in Examples 1 and 2, the carbon content was lower than in Comparative Example 1 and at a similar level to Comparative Example 2. Due to these effects, in Example 2, the resin sheet showed excellent thermal conductivity and dielectric strength, and in Example 1, the resin sheet showed particularly excellent thermal conductivity and dielectric strength.

[0100] Figure 3 shows SEM images of the hexagonal boron nitride powder of Example 1. Figure 3(a) is an image observed at a magnification of 200x, and Figure 3(b) is an image observed at a magnification of 500x. As can be seen from Figure 3(a), the hexagonal boron nitride powder of Example 1 contained many plate-like aggregated particles. Furthermore, as can be seen from Figure 3(b), the aggregated particles had an aggregated structure throughout, composed of multiple randomly oriented primary particles.

[0101] One embodiment of the present invention, hexagonal boron nitride powder, can be used, for example, as a filler for resins used in electronic components.

[0102] 10 Hexagonal boron nitride aggregate particles 10a, 10b Hexagonal boron nitride product layer 100a, 100b Boron oxide 101 Carbon source 102 Oxygen-containing calcium compound 103 Boron carbide Sxy plane Ex, Ey, Ez side La, Lb, Lc side length

Claims

1. Hexagonal boron nitride aggregated particles, comprising hexagonal boron nitride primary particles aggregated, wherein a rectangular parallelepiped having at least one face parallel to the face of the aggregated particle having the largest projected area, circumscribing the aggregated particle and having the smallest volume, has lengths La, Lb, and Lc of three mutually orthogonal sides, respectively (where La ≥ Lb ≥ Lc), and La is 70 μm or more and 250 μm or less, the ratio La / Lc is 2.8 or more, and the ratio Lb / Lc is 2.8 or more.

2. The hexagonal boron nitride aggregated particles according to claim 1, wherein the ratio La / Lc is 9 or less and the ratio Lb / Lc is 9 or less.

3. Hexagonal boron nitride powder comprising hexagonal boron nitride aggregated particles as described in claim 1.

4. The hexagonal boron nitride powder according to claim 3, wherein M is the mass of particles remaining on a sieve with a mesh size of 70 μm out of 100 parts by mass of the hexagonal boron nitride powder, and R is the number ratio of hexagonal boron nitride aggregated particles among the particles remaining on the sieve, and the product M・R is 10 or more and 40 or less.

5. The hexagonal boron nitride powder according to claim 3, wherein the carbon content is 300 ppm or less.

6. A resin composition comprising hexagonal boron nitride powder according to any one of claims 3 to 5 and a resin.

7. A method for producing aggregated hexagonal boron nitride particles, comprising a mixing step of mixing boron oxide, a carbon source, an oxygen-containing calcium compound, and boron carbide to obtain a raw material mixture, wherein the boron oxide contains 20% by volume or more of particles with a particle size of 75 μm or less, and 30% by volume or more of particles with a particle size of 425 μm or more and 725 μm or less, and the raw material mixture has a ratio (mass converted to B / mass converted to C) of 0.68 or more and 0.95 or less, and the boron oxide has a B value. ２ O ３ For every 100 parts by mass of the total of the mass converted to and the mass converted to C of the carbon source, the oxygenated calcium compound is contained in an amount of 8 to 25 parts by mass, converted to CaO on a Ca basis, and B on a B basis of the boron oxide. ２ O ３ A manufacturing method comprising: a mixing step of adding 10 to 35 parts by mass of boron carbide to 100 parts by mass of the total of the mass converted to C of the carbon source and the mass of the oxygenated calcium compound converted to CaO on a Ca basis; and a heating step of heating the raw material mixture at a temperature of 1700°C to 1900°C under a nitrogen atmosphere.

8. The manufacturing method according to claim 7, wherein the boron oxide has a volume-average particle size of 150 μm or more and 300 μm or less.

9. The manufacturing method according to claim 7 or 8, further comprising a grinding step of grinding the obtained boron nitride powder after the heating step.

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