JP7342905B2 - Composite manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a composite material using boron nitride.

Due to structural changes associated with higher density and higher performance of electronic devices (semiconductor modules, etc.), electrical insulating materials with high heat dissipation properties that are different from conventional ones have come to be used. For example, instead of a substrate made of highly thermally conductive ceramics (such as AlN), a composite material in which a highly thermally conductive insulating material (filler) is dispersed in a resin (matrix) is used for insulating sheets and the like. Such (organic/inorganic) composite materials have excellent moldability, processability, adhesion to different materials, etc., and are also relatively inexpensive.

By the way, various ceramic particles (including fibers) are used as fillers for composite materials. For example, particles of silica (SiO ₂ ), alumina (Al ₂ O ₃ ), aluminum nitride (AlN), etc. are used. However, silica and alumina have low thermal conductivity. Furthermore, since aluminum nitride reacts with water (H ₂ O) to generate ammonia (NH ₃ ), it has low moisture resistance and poor long-term reliability. Therefore, boron nitride (BN), which has excellent thermal conductivity, heat resistance, electrical insulation, etc., and is chemically stable, has come to be used as a filler for composite materials.

Boron nitride generally has a hexagonal normal pressure phase (sometimes referred to as "h-BN") and a cubic high pressure phase (sometimes referred to as "c-BN"). Hexagonal boron nitride (h-BN) is usually used as a filler.

h-BN is composed of a scale-like structure in which hexagonal mesh layers similar to graphite are laminated, and exhibits thermal conductivity anisotropy with large differences in thermal conductivity in the plane direction (a-axis direction) and thickness direction (c-axis direction). have Therefore, a composite material has been proposed in which h-BN particles are oriented to increase thermal conductivity (heat dissipation, etc.) in a specific direction (for example, from the heat source side to the cooling source side). Descriptions related to this can be found, for example, in the following patent documents.

JP2012-255055 JP2018-159062 JP2020-45456

Patent Documents 1 to 3 all use h-BN particles made fine by pulverization or the like to suppress the filling amount and improve the thermal conductivity of the composite material. However, their thermal conductivity was not necessarily sufficient.

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a new composite material etc. with high thermal conductivity.

As a result of intensive research to solve this problem, the present inventors found that by controlling the morphology of hexagonal boron nitride particles (simply referred to as "BN particles"), the thermal conductivity of composite materials containing BN particles was significantly increased. succeeded in increasing it. By developing this result, we have completed the present invention described below.

《Composite manufacturing method》
The present invention is a method for producing a composite material, which includes a molding step of obtaining a composite material from a mixture of thermally conductive particles and a binder, wherein the thermally conductive particles include BN particles made of hexagonal boron nitride, and the BN particles include hexagonal boron nitride particles. The method is for producing a composite material in which the particles have a particle size of 1 to 100 μm, a thickness of 0.1 to 5 μm, and an aspect ratio, which is the ratio of the particle size to the thickness, of 10 to 300.

According to the manufacturing method of the present invention, a composite material with excellent thermal conductivity can be obtained. The reason for this is not certain, but it is thought to be as follows. Since the BN particles according to the present invention have a specific morphology (particle size and thickness), they can be easily oriented in a direction of high thermal conductivity (in-plane direction, a-axis direction, (100) direction) in a composite material. It also became easier for adjacent particles to come into contact with each other. As a result, many heat conduction paths are formed between the heat conduction particles (BN particles), and it is thought that the composite material according to the present invention exhibits high thermal conductivity.

《Composite material》
The invention is also understood as a composite material. For example, the present invention provides a composite material in which thermally conductive particles are bound with a binder, the thermally conductive particles include BN particles made of hexagonal boron nitride, and the degree of orientation of the BN particles is 50. % or more, and the contact rate of the BN particles may be 70% or more. For example, the composite material of the present invention preferably has a porosity of 15% or less relative to its entirety.

"others"
(1) As used herein, "material" means "material" or "member." The composite material may be a composite material (such as a raw material) with an indefinite shape, or a composite member that has been molded, processed, etc. into a desired shape.

(2) "x to y" as used herein includes a lower limit x and an upper limit y, unless otherwise specified. A new range such as "a to b" can be established by setting any numerical value included in the various numerical values or numerical ranges described herein as a new lower limit or upper limit. "x~yμm" as used herein means xμm~yμm unless otherwise specified. The same applies to other unit systems (MPa, W/mK, Ωm, etc.).

FIG. 3 is a particle size distribution diagram of each BN powder used in the production of samples. It is a SEM image of each BN powder. This is an SEM image of a cross section of a composite material of Sample 3 and Sample C4. It is a scatter diagram showing the relationship between the degree of orientation and thermal conductivity. It is a scatter diagram showing the relationship between porosity and thermal conductivity. It is a scatter diagram which shows the relationship between the content rate of BN particles and thermal conductivity. It is a scatter diagram showing the relationship between the content rate of BN particles and the porosity. It is a schematic diagram showing the X-ray diffraction measurement regarding the test piece of each sample. It is a schematic diagram which shows the calculation method of a contact rate based on a SEM image.

One or more components arbitrarily selected from this specification may be added to the components of the present invention. The content described in this specification applies not only to composite materials (materials, members, etc.), but also to methods for manufacturing the same. Even method-related components can be material-related components. Which embodiment is best depends on the object, required performance, etc.

《Thermal conductive particles》
The thermally conductive particles include at least hexagonal boron nitride (h-BN) particles (BN particles). The BN particles will be explained in detail below. Note that the thermally conductive particles may include particles of cubic boron nitride (c-BN) and other types of particles (ceramic particles, metal particles, etc.).

The BN particles may have the following specific shapes (for example, particle size, thickness, aspect ratio, etc.).

(1) Particle size The particle size of the BN particles is, for example, 1 to 100 μm, 10 to 60 μm, 14 to 40 μm, and further 18 to 30 μm. If the particle size is too small or too large, the degree of orientation of the BN particles in the composite material may decrease.

Unless otherwise specified, the "particle size" of the BN particles in this specification is the 50% diameter (D50: median diameter) determined from the particle size distribution of a powder made of BN particles (simply referred to as "BN powder"). Particle size distribution is determined by laser diffraction method. In addition, "particle size" as used herein is an index value representing the size of the particle, and the shape of the particle (approximately spherical, approximately disc-shaped, approximately scaly, approximately fibrous, approximately spheroidal, approximately It has nothing to do with spherical shape, etc.).

(2) Thickness The thickness of the BN particles is, for example, 0.1 to 5 μm, 0.3 to 3 μm, 0.5 to 2.5 μm, and further 1 to 2 μm. The thinner the layer, the easier it is for the BN particles to align in a certain direction. If the thickness becomes too large, the BN particles will be difficult to deform, which may cause an increase in porosity in the composite material. Note that the BN particles may be a single h-BN layer having a hexagonal lattice structure, a laminate or an aggregate thereof (agglomerate, secondary particle), and do not necessarily have to be scale-like.

The "thickness" of the BN particles in this specification is the arithmetic mean value of the thicknesses measured for a plurality of BN particles arbitrarily extracted from within a predetermined field of view based on the observed image of the BN powder with a microscope.

(3) Aspect Ratio The aspect ratio of the BN particles is, for example, 10 to 300, 20 to 100, or even 30 to 75. BN particles with too small an aspect ratio cause an increase in porosity and a decrease in contact ratio. Note that the aspect ratio (particle size/thickness) referred to in this specification is calculated based on the "thickness" and "particle size" of the BN particles determined by the method described above.

(4) Bulk Density The bulk density of the BN particles is, for example, 0.1 to 0.6 g/cm ³ , 0.15 to 0.5 g/cm ³ , and further 0.2 to 0.4 g/cm ³ . If the bulk density is too large, the composite material may become brittle due to a decrease in the specific surface area (surface area per unit mass) of the BN particles, a decrease in the ability of the binder (resin) to intervene between the BN particles, etc. The bulk density of BN particles as used herein is determined by measuring the mass of BN powder scooped into a 100 ml measuring cylinder with a spoon. The BN powder was charged into the measuring cylinder without performing any operation (such as tapping) that would affect the volume.

《Binder》
The binder (matrix) is appropriately selected according to the specifications of the composite material. For example, if an organic material such as resin, rubber, or elastomer is used as a binder, the insulation, thermal conductivity, moldability, etc. of the composite material can be easily ensured.

The resin may be a thermosetting resin or a thermoplastic resin. Examples of the thermosetting resin include epoxy resin, phenol resin, and silicone resin. Examples of the thermoplastic resin include polystyrene, polymethyl methacrylate, polycarbonate, and polyphenylene sulfide. Examples of the rubber include ethylene-propylene-diene rubber (EPDM) and butyl rubber. In this specification, unless otherwise specified, the term "resin" includes rubber and elastomer. Note that if the insulating properties of the composite material are not required, a metal material (for example, a low melting point metal) may be used as the binder.

《Composite material》
The composite material is made up of thermally conductive particles (particularly BN particles) bound together with a binder. Examples of indicators for specifying composite materials include degree of orientation, contact ratio, porosity, content, and thermal conductivity.

(1) Degree of orientation The degree of orientation indicates the direction of BN particles in the composite material. The higher the degree of orientation, the higher the thermal conductivity in the orientation direction. The degree of orientation is, for example, 50-100%, 55-95%, 60-90%, 70-85%.

The degree of orientation is identified from the peak intensity ratio based on the profile (XRD pattern) obtained from X-ray diffraction measurement of the composite material (see FIG. 5A). Specifically, for h-BN, the degree of orientation is determined from the following formula, where the peak intensity of the (100) plane: I(100) and the peak intensity of the (002) plane: I(002).
Orientation degree (%) = 100 × I (100) / {I (100) + I (002)}

(2) Contact ratio The contact ratio is an index of the contact ratio (density) of thermally conductive particles (mainly BN particles) in the composite material. The higher the contact ratio, the better the thermal conductivity of the composite material. The contact rate is, for example, 70-99%, 75-97%, or even 80-95%.

The contact ratio was quantitatively determined as follows by image processing a cross-sectional image of the cross-section of the composite material observed under a microscope (see FIG. 5B). First, a binary image is obtained by binarizing a cross-sectional image. An analysis line is drawn in a field of view (for example, 130 μm x 70 μm) arbitrarily extracted from the binary image. Along the analysis line, contact portions (for example, contact portions between BN particles) and non-contact portions (for example, contact portions between BN particles and resin portions or void portions) are determined. From each number (the number of pixels counted along the analysis line), the contact rate is determined by the following equation.
Contact rate (%) = 100 x (number of contact parts) / {(number of contact parts) + (number of non-contact parts)}

Such operations are performed on five analysis lines drawn for each field of view for three fields of view arbitrarily extracted from the binary image. That is, the above-mentioned contact rate is calculated for each of the 15 analysis lines in total. These arithmetic mean values are referred to as the "contact rate" in the present invention.

(3) Porosity Porosity is an indicator of the percentage of the area in the composite material that is free of thermally conductive particles and binder. The smaller the porosity, the better the thermal conductivity of the composite material. The porosity is, for example, 0 to 15%, or even 5 to 10%.

The porosity is determined by the following formula from the apparent density (D) of the composite material determined by the Archimedes method and the theoretical density (Dth) calculated from the compounding amount of raw materials (BN powder, binder, etc.) and true density of the composite material. Calculated by
Porosity (%) = 100 x {1-(D/Dth)}

(4) Content rate The content rate is the volume ratio of the BN particles to the entire composite material. The higher the content of BN particles, the higher the thermal conductivity of the composite material. The content is, for example, 50 to 98% by volume, 65 to 97% by volume, 70 to 96% by volume, and further 80 to 94% by volume.

When manufacturing a composite material, the content is determined from the true density and blending amount of raw materials (for example, thermally conductive particles and binder). In the case of a manufactured composite material, it may be calculated and specified from the area ratio of thermally conductive particles determined by image processing the cross-sectional image of the composite material described above.

(5) Thermal conductivity The thermal conductivity of the composite material can be, for example, 20 to 60 W/mK, or even 35 to 50 W/mK. Note that unless otherwise specified, the thermal conductivity is measured along the main orientation direction of the BN particles (a-axis direction/see FIG. 5A).

"Production method"
(1) Molding process By molding a mixture of thermally conductive particles (filler) and a binder (matrix), a composite material in which they are bound (dispersed) can be obtained.

The molding process is performed, for example, by compression molding, extrusion molding, injection molding, transfer molding, or the like. An appropriate method may be selected depending on the content of thermally conductive particles (particularly BN particles) in the entire mixture and the specifications (characteristics, shape, etc.) of the composite material. A composite material with a high content of BN particles may be manufactured, for example, by compression molding.

Compression molding includes uniaxial compression molding, cold isostatic pressing (CIP), and the like. Uniaxial compression molding is performed by applying a compressive force in a uniaxial direction to a mixture filled into a mold cavity. As a result, a composite material in which scale-like (plate-like) BN particles are oriented in a direction substantially perpendicular to the uniaxial direction (that is, oriented so that the c-axis is along the uniaxial direction) can be efficiently obtained. It will be done.

In the compression step of compressing the mixture to form a composite material, the compression force (molding pressure) is, for example, 5 to 100 MPa, 10 to 50 MPa, or even 20 to 35 MPa. If the compression force is too low, a dense (high thermal conductivity) composite material cannot be obtained, and if the compression force is too high, it may lead to decreased productivity and increased costs.

When filling the mixture (kneaded material, compound, etc.) into the cavity of a mold (mold) or during the compression process, the mixture may be vibrated (vibration process). This makes it easier to obtain a dense composite material with a small porosity. Note that the mixture can be vibrated using a vibrator attached to the mold, an ultrasonic vibration device, or the like.

Further, before the compression step (before the main forming step of turning the mixture into a composite material), a pre-pressing step may be performed to apply a pressure smaller than the compression force of the compression step to the mixture. The pre-pressing step makes it easier to obtain a dense composite material with low porosity. The preloading process may be performed only once or multiple times. The compression force in the preloading process is, for example, 0.1 to 3 MPa, and further 0.5 to 1 MPa. If the pre-compression process is performed in the cavity of the mold filled with the mixture, it is easy to proceed to the original compression process and it is efficient. Note that the filling of the mixture into the cavity may be divided into a plurality of times, and the prepressing step may be performed each time.

When both the vibration step and the preload step are performed, each step may be performed sequentially or in parallel (simultaneously). When performing each process sequentially, the order does not matter, but for example, it is preferable that the preload process is performed after the vibration process.

(2) Coupling agent The mixture may include a coupling agent that increases the affinity between the thermally conductive particles and the binder. This improves the contact (wettability) between the binder and the thermally conductive particles, making it easier for the binder to penetrate between the thermally conductive particles, resulting in a dense and highly thermally conductive composite material.

The coupling agent in the mixture may be one introduced by surface treatment (coupling treatment, hydrophobization treatment, etc.) of the thermally conductive particles, or one added at the time of mixing (kneading) the thermally conductive particles and the binder. If the binder is an organic material (resin, rubber, elastomer, etc.), for example, various silane cups equipped with reactive groups corresponding to the functional groups (amino group, epoxy group, isocyanate group, vinyl group, acrylic group, etc.) on the binder side can be used. It is recommended to use a ring agent.

A typical silane coupling agent is, for example, hexamethyldisilazane (HMDS: C ₆ H ₁₉ NSi ₂ ). In addition, the silane coupling agent usually has a reactive group (silyl group, etc.) that also corresponds to a functional group (hydroxyl group, methoxy group, ethoxy group, etc.) on the side of the thermally conductive particles (BN particles, etc.) that are inorganic materials. Equipped with

The content (compounding amount/addition amount) of the coupling agent is, for example, 0.1 to 3 parts by mass, 0.5 to 2.5 parts by mass, or even 1 part by mass, based on 100 parts by mass of the entire untreated thermally conductive particles. ~2 parts by mass. If it is too little, the effect will be poor, and if it is too much, there will be little improvement in the effect.

(3) Supplementary information The composite material may contain auxiliary particles that contribute to improving its properties. The thermally conductive particles may be only BN particles, or may be a mixture of multiple types of particles having different materials, properties, particle sizes, etc. For example, in addition to BN particles, c-BN particles, carbon particles (nanocarbon particles, graphite particles, diamond particles, etc.), magnetic particles, etc. may be included in the composite material.

When the binder is made of a thermosetting resin, it is preferable to perform a thermosetting treatment (cure treatment) after molding. The composite material may have a final product shape or a shape close to it, or may be a post-processed material or an intermediate material.

BN particles in a desired form can be obtained, for example, by classifying (distributing, etc.) commercially available BN powder. BN powder may be prepared by pyrolysis of melamine borate (C ₃ H ₆ N ₆ .2H ₃ BO ₃ ). Incidentally, melamine borate can be obtained, for example, by cooling (standing to cool) a heated aqueous solution of boric acid and melamine.

《Application》
Composite materials with excellent thermal conductivity and insulation properties are preferably used for, for example, substrates, cases, heat dissipation members, etc. of electronic devices, or parts thereof. The thermal conductivity and specific resistance (electrical resistivity) of the composite material can be adjusted by the content of thermally conductive particles and the type of binder.

A plurality of samples (composite materials) were manufactured using BN powder with different specifications, and the characteristics of each sample were evaluated. Hereinafter, the present invention will be specifically explained while showing such examples.

《BN powder》
(1) Specifications Four types (names: A to D) of BN powders shown in Table 1 and FIGS. 1A and 1B (together referred to as "FIG. 1") were prepared. According to the names shown in Table 1, each BN powder is referred to as A powder, B powder, C powder, and D powder.

FIG. 1A shows the particle size distribution of each powder. The 50% diameter (D50) of each powder determined from the particle size distribution is shown in Table 1 as "particle size". The particle size distribution was measured at room temperature (approximately 20° C.) using a laser diffraction particle size distribution analyzer (LMS-2000E, manufactured by Seishin Enterprise Co., Ltd.).

FIG. 1B is an SEM image obtained by observing each powder with a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies Corporation). Based on the SEM image, the thickness of 5 to 10 BN particles within a field of view (for example, 250 μm×200 μm) was measured. The arithmetic mean value is shown in Table 1 as "thickness".

The aspect ratio (particle size/thickness) of each powder shown in Table 1 was calculated from the "particle size" and "thickness" shown in Table 1.

The bulk density shown in Table 1 was determined by measuring the mass of 100 ml of BN powder by the method described above.

(2) Preparation Each powder was prepared as follows.
Powder A was obtained by pulverizing commercially available h-BN powder (PT110, manufactured by Momentive) using a micronizer (Starburst HJP-25008, manufactured by Sugino Machine Co., Ltd.). Boron nitride powder HGP manufactured by Denka Corporation was used as powder B, boron nitride powder SGP manufactured by Denka Corporation was used as powder C, and PT110 manufactured by Momentive Corporation was used as powder D.

(3) Surface treatment In sample 4, Powder A was used which was subjected to hydrophobization treatment (silane coupling treatment) using hexamethyldisilazane (HMDS/SZ-31 manufactured by Shin-Etsu Chemical Co., Ltd.). 5 parts by mass of HMDS was added to 100 parts by mass of BN powder. Specifically, the hydrophobization treatment was performed by stirring and mixing BN powder, toluene, and HMDS (60° C. x 5 hours), and then drying the mixture in a vacuum drying oven at room temperature for 12 hours. A powder made of BN particles thus made hydrophobic was used in the production of Sample 4.

《Sample production》
A plurality of samples (composite materials) shown in Table 1 were manufactured using each BN powder. A one-component heat-curable epoxy resin (EP160 manufactured by Cemedine Co., Ltd./hereinafter simply referred to as "resin") was used as the binder (matrix). The content of the BN particles shown in Table 1 is the volume ratio to the total of the BN powder and the binder. The content was calculated from the blending amounts of the BN powder and binder and the true density.

Specifically, the composite material was manufactured as follows. Each BN powder and resin were kneaded for 10 minutes in a plastic container. The vacuum-dried kneaded material was crushed to obtain a compound (granular mixture) in which resin was attached to the surface of BN particles.

The compound was divided into three equal parts and filled into a mold (molding mold) in three parts (filling step). Each filling was performed by vibrating the mold with a vibrator (CH25A (manufactured by Exen Co., Ltd.) at 150 Hz (vibration process). Also, after each filling, the compound in the cavity was Light pressure was applied (approximately 3 to 5 MPa) to flatten the upper surface (prepressing step).

After such preforming, uniaxial compression molding (molding process/compression process) was performed in which the entire compound was pressurized in a uniaxial direction. The molding pressures are summarized in Table 1. Note that filling and molding of the compound were performed by heating the mold to 130°C.

In this way, a cylindrical molded body (φ14 mm×20 mm) was obtained. The molded body was heated in the air (120° C. x 30 minutes) to thermoset the resin (binder). A square sheet (12 mm x 12 mm x 2 mm) cut out from the obtained cylindrical composite material was used as a test piece. Cutting was performed along the pressing direction (uniaxial direction) using IsoMet1000 manufactured by Buehler. That is, the sheet was cut out so that the direction of the sheet surface (12 mm x 12 mm) was parallel and the direction of the sheet thickness (1 mm) was perpendicular to the pressing direction.

《Observation/Measurement》
(1) XRD/Orientation degree As shown in FIG. 5A, the surface of each sample was subjected to X-ray diffraction analysis (XRD/Cu-Kα/UltraV manufactured by Rigaku Co., Ltd.). In this way, XRD (2θ=20° to 60°) of each sample was obtained.

The degree of orientation of the BN particles in each sample in the a-axis direction was calculated by the method described above based on XRD. The obtained results are also shown in Table 1.

(2) SEM/Contact Ratio The cross section of each sample was observed using a scanning electron microscope (SEM). SEM images of the cross sections of Sample 3 and Sample C4 are illustrated in FIG. The orientation direction in FIG. 2 is a direction (thickness direction of the test piece) orthogonal to the above-mentioned pressing direction.

The contact rate of each sample was calculated by the method described above based on the SEM image (see FIG. 5B). Image processing was performed using free software (Image J). The obtained results are also shown in Table 1.

(3) Density/Porosity The apparent density (D) of each sample was determined by the Archimedes method. The obtained results are also shown in Table 1.

Further, the porosity was determined from the apparent density (D) and theoretical density (Dth) of each sample using the following formula. Theoretical density (Dth) was calculated from the blending amount of raw materials (BN powder, binder, etc.) used for manufacturing each sample and the true density. The calculated porosity is also shown in Table 1.
Porosity (%) = 100 x {1-(D/Dth)}

(4) Thermal conductivity The thermal conductivity (λ) of each sample was determined by the nanoflash method. Specifically, the thermal diffusivity (α) measured by the nanoflash method (measuring device: NETZSCH LFA447), the specific heat (Cp) determined by a differential scanning calorimeter (DSC), and the density determined by the Archimedes method. (D), the thermal conductivity was calculated as λ=α・Cp・D.

Thermal diffusivity was measured in the orientation direction of the BN particles (a-axis direction/see FIGS. 2 and 5A). The thermal conductivity (in the thickness direction of the test piece) of each sample thus obtained is also shown in Table 1.

Based on the characteristics of each sample shown in Table 1, the relationship between the degree of orientation and thermal conductivity is shown in Figure 3A, the relationship between porosity and thermal conductivity is shown in Figure 3B, and the relationship between BN particle content and thermal conductivity is shown in Figure 3A. FIG. 4A shows the relationship between the content of BN particles and the porosity, and FIG. 4B shows the relationship between the content of BN particles and the porosity. Note that FIGS. 3A and 3B are collectively referred to as "FIG. 3," and FIGS. 4A and 4B are collectively referred to as "FIG. 4."

"evaluation"
From Table 1, FIG. 2, FIG. 3, and FIG. 4, the following was found. Samples 1 to 4 using powder A consisting of BN particles with particle size, thickness, or aspect ratio within a predetermined range all have a higher degree of orientation and a higher contact rate than samples C1 to C5, and are extremely It showed high thermal conductivity. In particular, Sample 4 using BN particles surface-treated with a coupling agent exhibited high thermal conductivity.

It was also found that samples 2 to 4 all had porosity of 10% or less and exhibited high thermal conductivity, despite having a high content of BN particles. Furthermore, it has been confirmed that each sample has a specific resistance of approximately 10 ¹⁴ Ω·m. Thus, it was confirmed that the present invention provides a composite material with high thermal conductivity and excellent insulation properties.

Claims (10)

A method for producing a composite material comprising a forming step of pressurizing a mixture of thermally conductive particles and a binder to obtain a composite material,
The thermally conductive particles include BN particles made of hexagonal boron nitride,
The BN particles are scaly and have a 50% diameter (D50) particle size of 1 to 100 μm determined from the particle size distribution determined by laser diffraction method , and the arithmetic value of the thickness of the BN particles measured within the field of view of the microscopic image. The thickness determined as an average value is 0.1 to 5 μm , and the aspect ratio, which is the ratio of the particle size to the thickness , is 10 to 300,
A method for producing a composite material in which the BN particles are contained in an amount of 70 to 98% by volume based on the entire mixture . The method for producing a composite material according to claim 1, wherein the BN particles have a bulk density of 0.1 to 0.6 g/cm ³ . The method for manufacturing a composite material according to claim 1 or 2 , wherein the mixture includes a coupling agent that increases the affinity between the thermally conductive particles and the binder. The method for producing a composite material according to any one of claims 1 to 3, wherein the molding step includes a compression step of compressing the mixture at 5 to 500 MPa. 5. The method for manufacturing a composite material according to claim 4 , wherein the molding step further includes a pre-pressing step of applying a pressure smaller than the compression force of the compression step to the mixture at least once before the compression step. The method for manufacturing a composite material according to any one of claims 1 to 5 , wherein the molding step further comprises a vibrating step of vibrating the mixture. The method for producing a composite material according to any one of claims 1 to 6, wherein a sheet is obtained by cutting the molded body obtained by pressing the mixture along the pressing direction. The method for producing a composite material according to any one of claims 1 to 7, wherein in the composite material, the degree of orientation of the BN particles is 50% or more, and the contact ratio of the BN particles is 70% or more. The method for producing a composite material according to any one of claims 1 to 8 , wherein the composite material has a porosity of 15% or less relative to the entire composite material. The method for producing a composite material according to any one of claims 1 to 9 , wherein the composite material has a thermal conductivity of 20 to 60 W/mK .

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