Thermally conductive material and its manufacturing method

Abstract

To provide a heat conductive material which is substantially isotropic and can achieve high heat conductivity.SOLUTION: A heat conductive material contains composite particles having substantially spherical base particles, and flat deposition particles which are bonded to the surface of the base particles through a resin layer. The composite particles may have crosslinked particles having larger particle sizes than those of the deposition particles on a surface side. The heat conductive material may be a filler containing composite particles, and a composite material containing the filler in a matrix. The composite particles are obtained, for example, by pressurizing a mixture in which substantially spherical first particles, flat second particles, and a resin are mixed, and crushing a compact. The composite particles may be produced by multi-step mixing substantially spherical first particles, flat second particles, third particles having larger particle sizes than those of the second particles, and a resin.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The present invention relates to a thermally conductive material and the like. [Background technology]

[0002] As elements, devices, and equipment become more dense and perform better, they generate more heat, and sufficient heat dissipation is required to ensure their functionality and lifespan. For example, in the case of electronic devices (semiconductor modules, etc.), heat is dissipated through heat-dissipating materials with excellent thermal conductivity (heat sinks, housings, heat-dissipating sheets (thermally conductive sheets), thermally conductive grease, etc.). In addition to simple metals, composite materials with excellent formability are often used as heat-dissipating materials. Composite materials typically consist of a filler with excellent thermal conductivity and a matrix that holds the filler (for example, a resin containing elastomer, rubber, etc.).

[0003] Ceramic particles (including fibers) of silica (SiO2), alumina (Al2O3), aluminum nitride (AlN), etc. have been used as fillers. However, because these particles are roughly spherical, the contact rate and contact area between particles is small, making it difficult to form sufficient heat conduction paths between particles.

[0004] Recently, boron nitride (BN) particles have come to be used as a filler due to their excellent thermal conductivity, electrical insulation, and chemical stability. Boron nitride generally exists in a hexagonal normal pressure phase (referred to as "h-BN" where appropriate) and a cubic high pressure phase (referred to as "c-BN" where appropriate). h-BN particles are usually used as a filler. h-BN particles are flat (scale-like or plate-like) particles composed of stacked hexagonal mesh layers similar to graphite. Generally, the thermal conductivity in the plane direction (a-axis (100) direction) is greater than the thermal conductivity in the thickness direction (c-axis (002) direction) (thermal conductivity anisotropy).

[0005] Furthermore, fillers containing a mixture of multiple types of particles with different particle sizes and thermal conductivity properties, and composite materials filled (dispersed, encapsulated) with such fillers have also been proposed, and for example, there are related descriptions in the following patent documents. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2008-106231 [Patent Document 2] Patent Publication No. 2011-184507 [Patent Document 3] Patent Publication No. 2019-43804 Summary of the Invention [Problem to be solved by the invention]

[0007] Patent Document 1 proposes an adhesive sheet (composite material) for electronic devices in which a powder (filler) made by simply mixing boron nitride powder and spherical alumina powder is mixed in an epoxy resin (matrix) (

[0056] , Example 9 in Table 1). The thermal conductivity of this material is only 3.6 W / mK at most.

[0008] Patent Document 2 proposes a sheet (composite material) in which a powder (filler) made by simply mixing alumina, boron nitride, and aluminum nitride is mixed in a silicone resin (matrix) (

[0027] , Table 4). The thermal conductivity of this sheet is only 7.2 W / mK at most.

[0009] Patent Document 3 proposes composite particles (filler) obtained by hot pressing a mixed powder of boron nitride particles and aluminum nitride particles under a nitrogen atmosphere (40 MPa x 1800°C x 2 hours) to obtain a compressed and sintered body, which is then pre-pulverized using a hammer and mortar and wet-impact pulverized using a high-pressure nozzle. A thermally conductive composite material in which these composite particles are mixed in an epoxy resin (matrix) has a thermal conductivity of at most 4 W / mK in the compression direction (z direction) during molding, and a thermal conductivity of at most 5.9 W / mK in the perpendicular direction (x direction).

[0010] The present invention has been made in view of the above circumstances, and has an object to provide a new thermally conductive material and the like. [Means for solving the problem]

[0011] As a result of intensive research aimed at solving this problem, the inventors have discovered that composite particles in which flat particles are bonded to roughly spherical particles with a resin can exhibit high, nearly isotropic thermal conductivity. By expanding on this finding, the present invention, which will be described below, has been completed.

[0012] <Thermal Conductive Material> (1) The present invention is a thermally conductive material containing composite particles having substantially spherical base particles and flat coating particles adhered to the surfaces of the base particles via a resin.

[0013] According to the present invention, the thermal conductivity of a filler (one form of thermally conductive material) containing at least composite particles or a composite material (another form of thermally conductive material) containing the filler can be improved almost isotropically. The reason for this is unclear, but it is thought to be due to the morphology (structure) of the composite particles of the present invention. That is, the composite particles of the present invention are not simply a mixture or contact of roughly spherical particles and flat particles, but rather a state in which flat coating particles are adhered to the outer surface of a roughly spherical base particle via a resin (layer). In other words, the composite particles are in a state in which multiple flat coating particles are closely spaced in various orientations around the roughly spherical outer surface of the base particle. Therefore, the composite particles also have more opportunities to come into contact with each other in various directions (even surface contact) through the coated particles, and sufficient heat conduction paths can be formed almost isotropically. As a result, it is believed that the thermally conductive material of the present invention exhibits the excellent thermal conductivity described above.

[0014] (2) The composite particles according to the present invention may further have bridging particles on the (outermost) surface, which are larger in particle size than the coated particles. The bridging particles increase the contact opportunities between the composite particles and promote the formation of the above-mentioned heat conduction paths. This can further improve the thermal conductivity of the thermally conductive material.

[0015] <<Method for manufacturing thermally conductive material>> (1) The present invention can also be understood as a method for producing a thermally conductive material. For example, the present invention may be a method for producing a thermally conductive material, comprising a preparation step of obtaining a mixture of substantially spherical first particles, flat second particles, and a resin, a molding step of pressurizing the mixture to obtain a molded body, and a pulverization step of pulverizing the molded body, thereby obtaining composite particles in which the second particles (coating particles) are bonded to the surfaces of the first particles (base particles) by the resin.

[0016] The molding step is preferably carried out in a state where the resin in the mixture can easily flow or penetrate between the particles (for example, in a softened or molten state). If the resin is in a solidified state or in a state where it is difficult to flow during molding, it may be molded by heating (i.e., warm molding).

[0017] (2) The present invention may also be a method for producing a thermally conductive material, which includes, for example, a first mixing step of obtaining a first mixture by mixing substantially spherical first particles with a resin, and a second mixing step of obtaining a second mixture by mixing the first mixture with flat second particles, thereby obtaining composite particles in which the second particles are adhered to the surfaces of the first particles with the resin.

[0018] Even if the molding process is not performed, composite particles in which the first and second particles are bonded together via the resin can be obtained by simply performing the mixing process. If the resin is a thermosetting resin, the resin that bonds the first and second particles can be hardened by heating the second mixture (thermosetting process). This results in a filler in which the first and second particles are firmly bonded together by the thermoset resin.

[0019] (3) The present invention may also be a method for producing a thermally conductive material, which includes, for example, a first mixing step of obtaining a first mixture by mixing substantially spherical first particles with a resin, a second mixing step of obtaining a second mixture by mixing the first mixture with flat second particles, and a third mixing step of obtaining a third mixture by mixing the second mixture with third particles having a particle size larger than that of the second particles, and which obtains composite particles in which the second particles are bonded to the surfaces of the first particles by the resin and the third particles are attached to the (top) surface side.

[0020] A resin may be further mixed into the third mixture. This allows the third particles to be actively bonded to the second particles, etc., via the resin. When a thermosetting resin is used, a heat curing step may be performed as appropriate. The heat curing step may be performed both after the second mixing step and after the third mixing step, or only after one of them (especially after the third mixing step).

[0021] (4) The first and second mixing steps may be combined (fused). For example, the present invention may be a method for producing a thermally conductive material that includes a mixing step (conveniently referred to as the "fourth mixing step") for obtaining a mixture (conveniently referred to as the "fourth mixture") of substantially spherical first particles, flat second particles, and a resin, thereby obtaining composite particles in which the second particles are bonded to the surfaces of the first particles with the resin. The third mixing step may be performed after the mixing step.

[0022] The first, second, and third mixing steps may also be combined (fused). For example, the present invention may be a method for producing a thermally conductive material that includes a mixing step (conveniently referred to as the "fifth mixing step") for obtaining a mixture (conveniently referred to as the "fifth mixture") of substantially spherical first particles, flat second particles, third particles larger in particle size than the second particles, and a resin, and that obtains composite particles in which the second particles and the third particles are bonded to the surface sides of the first particles by the resin.

[0023] When a thermosetting resin is used, a heat curing step may be carried out as appropriate. The heat curing step may be carried out after each mixing step, or may be carried out only after any mixing step (especially after the final mixing step).

[0024] <Composite materials / thermal conductive materials> The thermally conductive material of the present invention can be understood as a filler containing at least composite particles (one form of a thermally conductive material), as well as a composite material in which at least composite particles are dispersed in a matrix (or binder) (another form of a thermally conductive material), or as a thermally conductive member made of such a composite material (another form of a thermally conductive material).

[0025] The matrix is ​​sufficient as long as it can contain or support (including hold) the composite particles, and is not limited to a solid, but may be a semi-solid (semi-fluid) or even a liquid (fluid). For example, the thermally conductive material (composite) of the present invention may be a solid such as a heat dissipation member, substrate, case, sheet, or film, or a (semi-)liquid (fluid) such as oil or grease. In this specification, such composites are collectively referred to simply as "thermally conductive material," regardless of their state.

[0026] When crosslinked particles are used, the crosslinked particles may be a part (constituent element) of the composite particle, or may be considered as a separate filler independent of the composite particle. For example, a composite filler in which crosslinked particles and composite particles in which coated particles are adhered to base particles are mixed without being attached (bonded), or a composite material or heat conductive member in which the composite filler is dispersed in a matrix may also be understood as the thermal conductive material of the present invention.

[0027] "others" (1) In this specification, the term "material" means "material" or "component." For example, the thermally conductive material may be composite particles themselves, a filler containing composite particles, an aggregate (powder) thereof, or a tangible or intangible composite material (including raw material) made of composite particles or filler (particles, powder, etc.) and a base material (matrix, binder), or a tangible component made by processing a material made of a composite material into a desired shape.

[0028] (2) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. A new range such as "a to b" can be established by using any numerical value included in the various numerical values ​​or numerical ranges described in this specification as a new lower limit or upper limit value. Unless otherwise specified, "x to y μm" in this specification means x μm to y μm. The same applies to other unit systems (W / mK, Ωm, etc.). [Brief explanation of the drawings]

[0029] [Figure 1] 1A to 1C are schematic diagrams illustrating an example of a manufacturing process for composite particles. [Figure 2A] This is an SEM image of the cross section of a molded body obtained in the manufacturing process of the filler (composite particles) of Sample 1. [Figure 2B] 1 is an SEM image of composite particles according to Sample 1. [Figure 2C] The SEM image and EDX analysis image of the composite particle are shown. [Figure 3] 1 is an SEM image of the filler of sample C0. [Figure 4] 1 is a bar graph comparing the thermal conductivity of composites. [Figure 5] 1 is a bar graph showing the relationship between the particle size ratio (h-BN / AlN) of composite particles and the thermal conductivity of the composite material. [Figure 6] FIG. 2 is a schematic diagram showing the effect of the filler form on the formation of heat conduction paths. [Figure 7] 10A to 10C are schematic diagrams showing another example of a process for producing composite particles. [Figure 8A] 1 is an SEM image of composite particles according to Sample 31. [Figure 8B] 1 is an SEM image of composite particles according to Sample 41. [Figure 9A] 1 is a bar graph comparing the thermal conductivity of composite materials (base particles: AlN). [Figure 9B] 1 is a bar graph comparing the thermal conductivity of composite materials (base particles: Al2O3). [Figure 10] 10A to 10C are schematic diagrams showing another example of a process for producing composite particles. [Figure 11] 1 is a graph comparing the thermal conductivity and porosity of composites containing different fillers. [Figure 12] These are SEM images of the cross sections of these composite materials. [Figure 13] Schematic diagrams showing cross sections of these composite materials. DETAILED DESCRIPTION OF THE INVENTION

[0030] One or more components arbitrarily selected from the present specification may be added to the components of the present invention. The contents described in this specification may apply not only to thermally conductive materials (composite particles, fillers, composite materials, components, etc.) but also to their manufacturing methods, etc. Even method-related components may be considered to be product-related components. Which embodiment is best depends on the target, required performance, etc.

[0031] 《Composite particles》 Composite particles include at least base particles (first particles), coated particles (second particles), and a resin. Composite particles may further include bridging particles (third particles). Each particle may be an agglomerated secondary particle. Composite particles or fillers containing composite particles may also include unbonded base particles, coated particles, and bridging particles. Other particles (e.g., particles made of graphite (including carbon black), diamond, nanocarbon (carbon nanotubes (CNT), carbon nanohorns (CNH), fullerenes, graphene, etc.)) may also be included in the filler or composite material.

[0032] In this specification, the raw material particles that will become base particles are referred to as first particles, the raw material particles that will become coated particles are referred to as second particles, and the raw material particles that will become crosslinked particles are referred to as third particles. The particle shape may change before and after preparation into composite particles.

[0033] (1) Particle shape The base particles (first particles) are approximately spherical, the coated particles are flat, and the crosslinked particles have a particle shape such as a plate, flat, or fibrous shape.

[0034] "Nearly spherical" means that the circularity determined from an observation image (e.g., SEM image) of the base particle is 0.6 or more, and further 0.7 or more. The theoretical upper limit of the circularity is 1, but the practical upper limit is 0.98 or less.

[0035] Circularity is calculated from the maximum length (L) of a particle and its area (S), as follows: 4S / πL 2It can be obtained as follows. Specifically, it can be obtained by performing image processing on the observed image using software (such as ImageJ). Usually, the arithmetic mean value of the circularity obtained for a plurality of particles within the visual field (650 μm × 450 μm) may be adopted as the "circularity" referred to in this specification.

[0036] "Flat shape" means, for example, the aspect ratio (L / t), which is the ratio of the maximum length (L) of the particle to the minimum length (t) of the particle, is, for example, 3 to 300 (more preferably 20 to 200). The minimum length (t) and the maximum length (L) of the particle are obtained from the observed image described above. Usually, the arithmetic mean value of the aspect ratio obtained for a plurality of particles within the above-mentioned visual field may be adopted as the "aspect ratio" referred to in this specification. The same applies to "plate shape", "fiber shape", etc.

[0037] (2) Particle size ratio The particle size ratio (L2 / L1), which is the ratio of the particle size (L2) of the deposited particle to the particle size (L1) of the base particle, is, for example, 0.01 to 2, 0.04 to 1, 0.08 to 0.5 or 0.1 to 0.3. If the particle size ratio is too small, the thermal conductivity of the heat conductive material may decrease. If the particle size ratio is too large, it becomes difficult for the deposited particles to adhere to the outer surface of the base particle.

[0038] The particle size (L3) of the crosslinked particle is preferably larger than the particle size (L2) of the deposited particle (L2 < L3). The particle size ratio (L3 / L2), which is the ratio of the particle size (L3) of the crosslinked particle to the particle size (L2) of the deposited particle, is, for example, 1.1 to 20, 1.4 to 15, more preferably 3 to 10. If the particle size ratio is too small or too large, the cooperativity between the composite particles due to the crosslinked particles may decrease. Note that the deposited particles and the crosslinked particles usually have a smaller particle size than the base particle (L2 < L1, L3 < L1).

[0039] The "particle size" referred to in this specification is the maximum length (L) of a particle. The particle size can be determined from the observation image described above. For multiple particles, the arithmetic mean value of the particle sizes of each particle within the field of view (650 μm × 450 μm) can be used as the "particle size" referred to in this specification. Note that in this specification, particle size is simply referred to as "particle size" regardless of the particle shape (approximately spherical, flat, scaly, plate-like, fibrous, spheroidal, etc.).

[0040] The particle size of the particles constituting the composite material or composite particles can be calculated in the same manner as above for particles separated or extracted from the composite material or composite particles.

[0041] The particle size of the raw material powder (first particles, second particles, or third particles) may alternatively be the 50% diameter (D50: median diameter) determined from the particle size distribution obtained by laser diffraction.

[0042] The particle size of each particle is not critical. Specifically, the particle size of the base particle is, for example, 10 to 200 μm, 20 to 150 μm, 35 to 120 μm, or 40 to 95 μm. The particle size of the coated particle is, for example, 2 to 100 μm, 4 to 75 μm, 8 to 50 μm, or 15 to 35 μm. The particle size of the crosslinked particle is, for example, 5 to 150 μm, 10 to 100 μm, or 15 to 50 μm.

[0043] (3) Volume ratio The volume ratio (appropriately referred to as the "first volume ratio") of the base particles to the total amount of the base particles and the coated particles (appropriately referred to as the "first total amount") is, for example, 0.5 to 0.9, 0.6 to 0.85, or 0.7 to 0.82. When crosslinked particles are included, the volume ratio (appropriately referred to as the "second volume ratio") of the base particles to the total amount (appropriately referred to as the "second total amount") of the base particles, the coated particles, and the crosslinked particles is, for example, 0.4 to 0.85, 0.5 to 0.8, or 0.6 to 0.76. The volume ratio (appropriately referred to as the "third volume ratio") of the crosslinked particles to the coated particles is, for example, 0.1 to 2, 0.2 to 1.5, or 0.3 to 1. If either volume ratio is too small or too large, the thermal conductivity or its isotropy may decrease.

[0044] The volume of a particle is calculated, for example, from its mass (content) and its true density. The volume ratio is calculated from the volume calculated for each particle. For particles that make up a composite material or composite particles, the calculation can be similarly performed for the particles after separation and extraction. The volume at the time of preparation can be calculated from the blending amount (mass) of each raw material powder and its true density.

[0045] (4) Coverage For at least a portion of the composite particles, the proportion of the surface area of ​​the base particle covered by the coating particles (referred to as the "coverage") should be 50% or more, 60% or more, or even 70% or more. An insufficient coverage can result in reduced thermal conductivity and isotropy. The coverage is calculated as follows: The surface of the composite particle is observed using SEM / EDX to measure the component amounts of the base particle and the coating particles. The coverage is calculated as (component amount of the coating particle) / {(component amount of the base particle) + (component amount of the coating particle)}. For example, as shown in the SEM / EDX image in Figure 2C, for a composite particle in which AlN particles are coated with h-BN particles, the coverage is calculated by N amount / (Al amount + N amount) based on the Al amount (component amount of the base particle) and N amount (component amount of the coating particle) determined from the surface of a single particle. The arithmetic average of the coverages calculated for several particles may also be used.

[0046] (5)Material The base particles (first particles) and the coated particles (second particles) are preferably made of a material with high thermal conductivity. Examples of the base particles include approximately spherical particles made of one or more of aluminum nitride (e.g., AlN), aluminum oxide (e.g., Al2O3), silicon oxide (e.g., SiO2), and cubic boron nitride (c-BN). The coated particles or bridging particles include flat or plate-like particles made of hexagonal boron nitride (h-BN). For thermally conductive materials used in electronic devices, both the base particles and the coated particles are preferably made of insulating materials.

[0047] (6) Resin The resin used for adhesion (resin layer, adhesive layer) may be a thermoplastic resin or a thermosetting resin. Thermosetting resins may be appropriately subjected to a thermosetting treatment. The thermosetting resin layer brings the base particles (first particles), the coated particles (second particles), or the bridging particles (third particles) into a stronger and more intimate contact state.

[0048] The resin is sufficient as long as it can bond the particles together. The resin may be, for example, 0.1 to 5 mass%, 0.3 to 3.5 mass%, or 0.5 to 2.5 mass% of the entire composite particle. In terms of volume percentage of the entire composite particle, the resin may be, for example, 1 to 45 volume%, 5 to 40 volume%, 10 to 35 volume%, or 15 to 30 volume%. The resin constituting the composite particle may be the same or different from the resin constituting the matrix of the composite material described below.

[0049] 《Composite material》 A composite material is composed of a filler containing at least composite particles and a matrix (including a binder) that supports, holds, or disperses the filler.

[0050] (1) Filler The filler is contained in an amount of, for example, 50 to 95 volume %, 55 to 85 volume %, or 60 to 80 volume % of the entire composite material. If the filler filling rate is too low, the thermal conductivity of the composite material may also decrease. If the filler filling rate is too high, the thermal conductivity of the composite material does not necessarily increase.

[0051] The filler filling rate (volume %) during the production of a composite material is determined from the blending amount and density of the raw materials. The filler filling rate in the composite material is determined from the total amount of the composite material and the amount of filler separated from the composite material. When the filler cannot be separated, the filler filling rate may be determined indirectly or alternatively from an observation image (e.g., SEM image) of the composite material (cross section).

[0052] The filler may be entirely or partially surface-treated to enhance its affinity with the matrix, which can improve the filler's dispersibility, packing ability, and adhesion in the matrix, thereby improving the thermal conductivity of the composite.

[0053] The surface treatment is, for example, a hydrophobic treatment or a coupling treatment. If the matrix is ​​an organic material (resin, rubber, elastomer, etc.), for example, a silane coupling treatment or a fluorine plasma treatment can be performed. The silane coupling treatment can be performed using various silane coupling agents equipped with reactive groups corresponding to the functional groups on the matrix side (amino group, epoxy group, isocyanate group, vinyl group, acrylic group, etc.). A typical silane coupling agent is, for example, hexamethyldisilazane (HMDS: CH 19 Silane coupling agents usually have reactive groups (silyl groups, etc.) that correspond to functional groups (hydroxyl groups, methoxy groups, ethoxy groups, etc.) present on the inorganic filler (composite particles, etc.).

[0054] The content (blended amount / added amount) of the surface treatment agent is, for example, 0.1 to 3 parts by mass, 0.5 to 2.5 parts by mass, or even 1 to 2 parts by mass per 100 parts by mass of the total untreated filler. If the amount of the surface treatment agent is too small, the effect is poor, and if the amount of the surface treatment agent is too large, the effect is not improved much.

[0055] The surface treatment may be performed on the filler before mixing (including kneading), or may be performed by adding a surface treatment agent (such as a coupling agent) when mixing the matrix and filler.

[0056] (2) Matrix The matrix (including the binder) is, for example, a resin or oil (including resin-based oil, semi-solidified grease, etc.). The resin (including rubber, elastomer, etc.) may be a thermosetting resin or a thermoplastic resin. Examples of thermosetting resins include epoxy resin, phenolic resin, and silicone resin. Examples of thermoplastic resins include polystyrene, polymethyl methacrylate, polycarbonate, and polyphenylene sulfide. Examples of rubbers include ethylene-propylene-diene rubber (EPDM), butyl rubber, etc. Examples of oils include silicone oil and silicone grease.

[0057] 《Manufacturing method》 (1) Composite particles There are various methods for producing composite particles in which flat second particles (adhered particles) are bonded to substantially spherical first particles (base particles) with a resin (binder, adhesive). For example, composite particles can be obtained by crushing a mixture (including a kneaded product) of the first particles, the second particles, and a resin, or by pulverizing a molded product of the mixture. More specifically, the methods are as follows.

[0058] The mixture is obtained, for example, by mixing (including kneading) first particles (powder), second particles (powder), and a resin. Such mixing is performed using a ball mill, a vibration mill, a V-type mixer, or the like (preparation step). At this time, the layered second particles may be pulverized. The mixing may be performed by either a dry method or a wet method. The mixture obtained by wet mixing may be vacuum dried, heated, or the like, as appropriate.

[0059] The mixing (preparation step) of the first particles (powder), the second particles (powder), and the resin may be performed in stages. For example, a first mixing step of mixing the first particles and the resin to obtain a first mixture and a second mixing step of adding the second particles to the first mixture and mixing them to obtain a second mixture may be performed sequentially. This allows the second particles to be efficiently bonded to the first particles while reducing the amount of resin.

[0060] The mixing step may be performed by adding a solvent or the like to adjust the viscosity of the resin. In this case, a drying step may be performed after the second mixing step to remove the solvent or the like by volatilization or evaporation. Furthermore, when a thermosetting resin is used, a thermosetting step in which the second mixture is heated to harden the resin between the first and second particles may be performed at the composite particle manufacturing stage (before the composite material manufacturing stage).

[0061] The mixture may be crushed or pulverized as it is, or the mixture may be pressure-molded and then crushed (a crushing step). A powder consisting of composite particles (referred to as "composite powder") can be obtained by crushing or pulverizing. For example, a small crusher, a crusher, a cutter mill, or the like can be used for crushing.

[0062] The molded body can be obtained, for example, by molding the mixture with a mold, CIP (Cold Isostatic Pressing), RIP (Rubber Isostatic Pressing), HIP (Hot Isostatic Pressing), etc. (molding process). The molding pressure is, for example, 10 to 100 MPa, or even 20 to 50 MPa. The molded body may have any shape that allows it to be crushed.

[0063] The molding process may be cold molding performed at room temperature, or warm molding performed by heating the mixture or mold. Warm molding is preferably performed at a temperature at which the resin softens or melts. This makes it easier to interpose the resin between the first particles and the second particles while suppressing the amount of resin. The warm molding temperature (T) is, for example, −30 to 30°C (|T−(Ts, Tm)|≦30°C) or −20 to 20°C (|T−(Ts, Tm)|≦20°C) relative to the softening point (Ts) or melting point (Tm) of the resin. Depending on the type of resin, the warm molding temperature (usually the mold temperature) is, for example, 80 to 200°C or 100 to 160°C. The warm molding time is, for example, 5 to 60 minutes or 15 to 40 minutes. When a thermosetting resin is used, the warm molding step may also serve as a thermosetting step.

[0064] The composite powder may be adjusted to have an average particle size (median diameter: D50) of 5 to 60 μm, or even 15 to 55 μm.

[0065] (2) Composite material Composite materials are obtained by mixing, kneading, molding, etc., filler and matrix. Composite materials in which the filler is held in a resin can be obtained, for example, by compression molding, injection molding, transfer molding, etc., of the mixture. If the matrix is ​​a thermosetting resin, a thermosetting treatment (cure treatment) may be performed after molding. Composite materials may be in the shape of the final product or a shape close to that, or may be raw materials to be processed, intermediate materials, etc.

[0066] 《Application》 Fillers and composite materials are used, for example, in heat dissipation components (heat dissipation sheets, substrates, cases, etc.) and oils and fats (oil, grease, etc.). The thermal conductivity of the composite material can be, for example, 8 to 40 W / mK, or even 10 to 25 W / mK. Depending on the type of particles to be deposited and their degree of orientation, the thermally conductive material of the present invention can have a difference in thermal conductivity between two perpendicular directions of, for example, 6 W / mK or less, 4 mK or less, or even 2 W / mK or less. Note that the composite material used in electronic devices, etc., has a specific resistance of, for example, 10 5 ~10 12 Ωm and even 10 8 ~10 10 It is preferable that the resistance is Ωm. [Example]

[0067] Various composite materials were prepared by dispersing different fillers in the matrix, and their thermal conductivity properties were evaluated. The present invention will be described in more detail with reference to these specific examples.

[0068] [First Example] 《Making Filler》 Several types of fillers were produced as shown below.

[0069] (1) Sample 1 (composite particles) The raw materials used were commercially available AlN powder (FAN-f50 manufactured by Furukawa Electronics Co., Ltd.), h-BN powder (PT110 manufactured by Momentive Corporation), and epoxy resin (EP-160 manufactured by Cemedine Co., Ltd., a one-component heat-curing epoxy adhesive). This epoxy resin was a highly viscous liquid at room temperature.

[0070] The AlN powder consisted of spherical particles (first particles) with a circularity of 0.90 and a particle size (D50) of 45 μm. The h-BN powder was pulverized using a micronizer (Starburst HJP-25008, manufactured by Sugino Machine Co., Ltd.) to produce cleaved powder. This cleaved powder consisted of flat particles (second particles) with an aspect ratio (AR = maximum length L / thickness t) of 40 and a particle size (D50) of 20 μm (particle size ratio: 0.44). These raw materials were used to prepare composite particles through the steps shown in Figure 1. Specifically, the process is as follows:

[0071] In a polypropylene container, 0.25 g of epoxy resin was mixed with 1 to 10 cc of a solvent (dichloromethane) and 3.85 g of AlN powder (Step I / mixing step). Mixing was carried out at room temperature using a mixer (Thinky Corporation ARE-310 "Mixer") at 2000 rpm for 0.5 min.

[0072] 1.15 g of cleaved h-BN powder was added to the mixture and kneaded (Step II / Kneading Step). The kneading was carried out at room temperature using the above-mentioned apparatus at 2000 rpm for 0.5 min.

[0073] The obtained kneaded material was placed in a vacuum chamber and vacuum dried (30 minutes) at room temperature (Step III / drying step). In this way, a mixture (kneaded material) from which the solvent had been evaporated was obtained. The true density of the AlN particles was 3.3 g / cm. 3 , true density of h-BN particles: 2.27 g / cm 3 The volume ratio of AlN particles (first particles):h-BN particles (second particles) was 7:3 (volume ratio of the coated particles to the total of both particles: 0.3). The epoxy resin (true density: 1.16 g / cm 3 ) corresponds to 30% by volume of the total mixture.

[0074] The mixture was placed in a mold cavity consisting of a die and upper and lower punches, and warm compression molded in one direction (Step IV / molding step). At this time, the temperature of the mold (die) heated by a heater was 130°C, and the molding pressure was 35 MPa. The temperature at which the epoxy resin softened or melted after drying was 80°C.

[0075] The compact (12 mm × 12 mm × 15 mm) removed from the mold was crushed using a benchtop crusher (manufactured by AS ONE Corporation) (Step V / Crushing Step). The crushed powder was sieved to adjust the particle size to less than 53 μm. In this way, composite particles in which h-BN particles were bonded (modified) to AlN particles via resin were obtained.

[0076] (2) Samples 21 to 24 The composite particles of Samples 21 to 24 were also produced using the same process as Sample 1, except that the cleaved powder used in producing Sample 1 was changed to the h-BN powder described below. Sample 21: HGP manufactured by Denka Co., Ltd. / D50: 5 μm, AR: 4 Sample 22: Denka Co., Ltd. GP / D50: 10 μm, AR: 5 Sample 23: SGP manufactured by Denka Co., Ltd. / D50: 20 μm, AR: 10 Sample 24: Momentive PT110 / D50: 45 μm, AR: 18

[0077] (3) Sample C0 A filler was also prepared that was produced through the same process as Sample 1, except that the epoxy resin used in producing Sample 1 was not added.

[0078] (4) Sample C1 The following raw material powders were simply mixed in the mixer described above to obtain a powder, which was used as a filler. AlN coarse powder (FAN-f50 manufactured by Furukawa Electronics Co., Ltd.): 2.18 g AlN fine powder (FAN-f05 manufactured by Furukawa Electronics Co., Ltd.): 0.94 g (Circularity: 0.90, D50: 5 μm) 0.92g of the above-mentioned h-BN cleaved powder The volume ratio of the mixed powder was set to 7:3, where the total amount of the AlN coarse powder and the AlN fine powder: the h-BN cleaved powder.

[0079] (5) Sample C2 The filler used was a powder obtained by simply mixing the aforementioned AlN coarse powder and AlN fine powder in a volume ratio of 7:3, in the same manner as in sample C1.

[0080] (6) Sample C3 The above-mentioned AlN coarse powder was used as a filler as it was.

[0081] 《Production of composite materials》 A composite material was produced in which the filler was held in a matrix. Unless otherwise specified, the filler filling rate was 70% by volume relative to the total composite material (100% by volume). The matrix (binder) used was the epoxy resin (EP-160 manufactured by Cemedine Co., Ltd.; simply referred to as "resin") used in producing the composite particles. The specific steps are as follows:

[0082] The filler and resin were kneaded in a plastic container for 10 minutes. The vacuum-dried kneaded product was crushed to obtain a compound in which the resin adhered to the filler. This compound was filled into a mold and compression-molded in one direction. At this time, the mold temperature was 130°C, the molding pressure was 20 MPa, and the pressurized state was maintained for 30 minutes to thermally cure the resin. This resulted in a cylindrical composite (φ14 mm × 20 mm) in which the filler was held by the resin. In this example, unless otherwise specified, the filler and the composite material using that filler will be referred to by the same sample number (the same applies hereinafter).

[0083] "observation" The cross section of the pre-pulverized green body obtained during the production of filler for Sample 1 was observed with a scanning electron microscope (SEM). The SEM image is shown in Figure 2A.

[0084] Figure 2A shows an SEM image of the filler (composite particle) of Sample 1. Figure 2C shows the SEM image of the composite particle and an EDX analysis image obtained by elemental analysis using energy dispersive X-ray spectroscopy (EDX). The coverage of the AlN particles by the h-BN particles calculated from Figure 2C was approximately 84%.

[0085] The SEM image of the filler in sample C0 is shown in FIG.

[0086] "measurement" The thermal conductivity (λ) of the composite was determined by the nanoflash method (measurement device: NETZSCH LFA447). Specifically, the thermal conductivity was calculated as λ = α·Cp·ρ from the thermal diffusivity (α) measured by the nanoflash method, the specific heat (Cp) determined by a differential scanning calorimeter (DSC), and the density (ρ) determined by the Archimedes method.

[0087] To measure thermal diffusivity, thin plate-shaped samples (referred to as "perpendicular samples") cut from the cylindrical composite material in a direction perpendicular to the axial direction (pressure direction) and thin plate-shaped samples (referred to as "parallel samples") cut parallel to the axial direction were used. The thermal conductivities of the composite materials for Sample 1 and Samples C1 to C3 are shown in Figure 4. The thermal conductivities of the composite materials for Samples 21 to 24 are shown in Figure 5. Note that the "perpendicular direction" and "parallel direction" shown in each figure refer to the thermal conductivity of the perpendicular sample and the thermal conductivity of the parallel sample, respectively.

[0088] "evaluation" (1) Filler As is clear from Figures 2A to 2C (collectively referred to as "Figure 2"), the filler of Sample 1 was found to consist of composite particles in which flat h-BN particles (coating particles) were densely adhered to the surfaces of spherical AlN particles (base particles) via a thin resin layer.

[0089] On the other hand, as is clear from FIG. 3, the filler of sample C0, to which no resin was added during production, was in a state in which the spherical AlN particles and the flat h-BN particles (adhered particles) were almost separated.

[0090] (2) Thermal conductivity of the composite material As is clear from Figures 4 and 5, Sample 1 and Samples 21 to 24, which used composite particles as a filler, all had high thermal conductivity in both the perpendicular and parallel directions, unlike the composites of the other samples. In other words, it was found that composites using composite particles as a filler have excellent thermal conductivity and small anisotropy.

[0091] Furthermore, as is clear from Figure 5, composites containing composite particles in which the particle size ratio of h-BN particles to AlN particles is at least 1 or less have thermal conductivities of 10 W / mK or more in both the perpendicular and parallel directions.

[0092] 《Consideration》 Based on the above results, it can be said that the morphology (structure) of the filler had a significant effect on the thermal conductivity of the composite material. The mechanism behind this is inferred as shown in Figure 6. That is, as shown in Figure 6(a), composite particles in which flat coating particles are adhered to the surface of a spherical base particle allow the particles to come into approximate surface contact with each other at various points, allowing sufficient heat conduction paths to be formed approximately isotropically. It is believed that this allows composite materials containing composite particles to exhibit excellent, approximately isotropic thermal conductivity.

[0093] On the other hand, as shown in Figure 6(b), in fillers consisting only of spherical particles, the contact rate between particles (contact ratio) was small to begin with, and the contact between particles was also point contact. Furthermore, as shown in Figure 6(c), in fillers consisting only of flat particles, the contact rate between particles was also small, and each particle was oriented in one direction. For this reason, it is thought that conventional composites have difficulty in forming sufficient heat conduction paths, and therefore have been unable to achieve high thermal conductivity. Even if the filling ratio of flat particles were increased, the direction of heat conduction would be limited, and it is thought that high thermal conductivity could not be achieved isotropically.

[0094] [Second Example] 《Making Filler》 Several types of fillers were produced as shown below.

[0095] (1) Sample 31 Using the aforementioned AlN powder (FAN-f50 manufactured by Furukawa Denshi Co., Ltd.), h-BN powder (HGP manufactured by Denka Co., Ltd., particle size ratio: 0.11), and epoxy resin (EP-160 manufactured by Cemedine Co., Ltd.) as raw materials, composite particles were prepared by carrying out the steps shown in Figure 7. Specifically, the process is as follows.

[0096] In a polypropylene container, 0.35 g of epoxy resin was mixed with 1 to 10 cc of a solvent (dichloromethane) and 3 g of AlN powder at room temperature (Step A / first mixing step), using a mixer (Thinky Corporation ARE-310 "Mixer" model) operated at 2000 rpm for 0.5 min.

[0097] 1.4 g of h-BN powder was added to the mixture (first mixture) in the container and mixed at room temperature (step B / second mixing step). The mixer was also operated at 2000 rpm for 0.5 min. The transition from step A to step B was carried out continuously without stopping the mixer.

[0098] In addition, AlN particles (true density: 3.3 g / cm 3 ) and h-BN particles (true density: 2.27 g / cm 3 The volume fraction of the AlN particles (first particles):h-BN particles (second particles) was 6:4 (volume ratio of the coated particles to the total of both particles: 0.4). The epoxy resin (true density: 1.16 g / cm 3 ) is 16% by volume of the total mixture.

[0099] The obtained kneaded product (second mixture) was placed in a vacuum chamber and vacuum dried at room temperature for 30 minutes (step C / drying step). The dried kneaded product was heated at 120°C to 150°C (curing temperature Tc to Tc+30°C of the epoxy resin) for 20 minutes (step D / thermal curing step).

[0100] The heated mixture was crushed in a benchtop crusher (WB-1 manufactured by Osaka Chemical Co., Ltd.). The crushed powder was sieved to adjust the particle size to less than 53 μm. Composite particles in which h-BN particles were bonded (modified) to AlN particles via a resin were obtained.

[0101] (2) Sample 41 Composite particles were fabricated using the same process as Sample 31, except that the AlN powder was replaced with Al2O3 powder (CB-A50S manufactured by Showa Denko K.K.). The Al2O3 powder consisted of spherical particles (first particles) with a circularity of 0.9 and a particle size (D50) of 50 μm. The particle size ratio (h-BN particles / Al2O3 particles) was 0.10.

[0102] The composition of each raw material was 0.14 g of epoxy resin, 3.6 g of Al2O3 powder, and 1.4 g of h-BN powder. The Al2O3 particles had a true density of 3.95 g / cm 3Therefore, the ratio of Al2O3 particles (first particles):h-BN particles (second particles) is 6:4 (volume ratio of the coated particles to the total of both particles: 0.4). The epoxy resin accounts for 7% by volume of the entire mixture.

[0103] (3) Samples 32 and 42 Composite particles (sample 32) were also prepared using the same raw materials as sample 31 and manufactured in the same manner as in Example 1. Incidentally, sample 32 and the previously described sample 21 were made using the same raw materials and process, but the volume proportions of the AlN particles, h-BN particles, and epoxy resin were different.

[0104] Composite particles were also prepared using the same raw materials as in Sample 41, in the same manner as in Example 1 (Sample 42).

[0105] (4) Samples C3 and C4 Sample C3 (similar to the first example) and sample C4 used the above-mentioned AlN powder and Al2O3 powder as fillers, respectively.

[0106] 《Production of composite materials》 Composite materials in which fillers made of composite particles of each sample were held in a matrix made of epoxy resin were produced in the same manner as in the first example.

[0107] In addition to EP-160 manufactured by Cemedine Co., Ltd., the epoxy resin may also be, for example, EP-106NL manufactured by Cemedine Co., Ltd. The latter has a lower viscosity (about 1 / 5) than the former, making it easier to knead with the filler and improving the dispersibility of the composite particles in the matrix.

[0108] "observation" SEM images of the fillers (composite particles) of Samples 31 and 41 are shown in FIGS. 8A and 8B (collectively referred to as "FIG. 8"), respectively.

[0109] Each composite particle was subjected to elemental analysis by EDX in the same manner as in Example 1. As a result, the coverage rate of Sample 31 was approximately 95%, and the coverage rate of Sample 41 was approximately 90%.

[0110] "measurement" The thermal conductivity of each sample composite was measured in the same manner as in Example 1. The results are shown in Figures 9A and 9B (collectively referred to as "Figure 9").

[0111] "evaluation" (1) Filler As can be seen from Figure 8, the manufacturing method of this example also produced roughly spherical composite particles in which flat h-BN particles (coated particles) were densely adhered to the surfaces of spherical base particles (AlN particles or Al2O3 particles) via a resin layer.

[0112] (2) Composite material As can be seen from Figure 9, the composite materials (samples 31, 32, 41, and 42) using composite particles as a filler had high thermal conductivity in both the perpendicular and parallel directions. Suffice it to say, the composite materials (samples 31 and 41) according to this embodiment tended to have higher thermal conductivity than the composite materials (samples 32 and 42) according to the first embodiment. This tendency was more pronounced when the base particles were Al2O3 particles (see Figure 9B). For all samples, the difference in thermal conductivity between the perpendicular and parallel directions was slight (less than 3 W / mK).

[0113] [Third Example] As shown in Table 1, a large number of fillers and composite materials using each filler were produced, and the properties of the composite materials were evaluated. The present invention will be described in more detail with reference to these examples.

[0114] 《Making Filler》 <Samples 511-532> The fillers of Samples 511 to 532 shown in Table 1 consist of (secondary) composite particles in which third particles acting as bridging particles are further bonded to the (primary) composite particles shown in Example 2. These secondary composite particles were produced as follows.

[0115] (1) Raw materials For the first particles, AlN powder (FAN-f80 manufactured by Furukawa Denshi Co., Ltd. / circularity: 0.90) with a particle size (L1) of 80 μm was used. Unless otherwise specified, the particle size is the median diameter (D50) (the same applies hereinafter).

[0116] For the second particles, one of the following h-BN powders with different particle sizes (L2) was used. Particle size: 5 μm / Denka HGP Particle size: 10 μm / Denka MGP Particle size: 13 μm / Denka HGP + MGP (mixture of the above powders)

[0117] For the third particles, one of the following h-BN powders with different particle sizes (L3) was used. Particle size: 20 μm / Denka SGP Particle size: 40 μm / Momentive PT110

[0118] The resin used was an epoxy resin (EP-160 manufactured by Cemedine Co., Ltd.).

[0119] (2) Process Each raw material was weighed out in the blending amount (volume %) shown in Table 1 and subjected to each process shown in Fig. 10. Specifically, the process is as follows.

[0120] In a polypropylene container, a solvent (dichloromethane: 1-10 cc) and first particles (AlN powder) were added to the epoxy resin and stirred and mixed at room temperature (Step A / first mixing step). As in Example 2, mixing was performed by operating the mixer at 2000 rpm for 0.5 min. The amount of resin (volume %) added to the entire mixture (first particles, second particles, third particles, and resin) in the first mixing step is also shown in Table 1.

[0121] The second particles (h-BN powder) were added to the mixture (first mixture) in the container and mixed at room temperature (step B / second mixing step). The transition from step A to step B was carried out while the mixer was running at 2000 rpm for 0.5 min.

[0122] The obtained kneaded material (second mixture) was placed in a vacuum chamber and vacuum dried at room temperature for 30 minutes (step C / first drying step). The dried kneaded material was heated at 120°C to 150°C (epoxy resin curing temperature Tc to Tc + 30°C) for 20 minutes (step D / thermal curing step). In this way, composite particles (referred to as "primary composite particles") were obtained in which second particles (h-BN particles) were bonded (modified) to first particles (AlN particles) via the resin.

[0123] Furthermore, in the above-mentioned container, the third particles (h-BN powder) were added to the primary composite particles and stirred and mixed at room temperature (Step E / Third Mixing Step). This mixing was carried out in the same manner as the first mixing step, while adding a solvent (dichloromethane: 1 to 10 cc) to the epoxy resin. The amount of resin (volume %) added to the entire mixture (first particles, second particles, third particles, and resin) in the third mixing step is also shown in Table 1.

[0124] The resulting kneaded product (third mixture) was placed in a vacuum chamber and vacuum dried at room temperature for 30 minutes (step F / second drying step). In this way, composite particles (referred to as "secondary composite particles") were obtained in which second particles (fine h-BN particles) and third particles (coarse h-BN particles) were bonded (modified) to first particles (AlN particles) via the resin.

[0125] <Samples 611-631> The fillers of Samples 611 to 631 were prepared without carrying out the third mixing step and subsequent steps described above, using the raw materials (first particles, second particles, and resin) weighed out in the volume ratios shown in Table 1. In other words, the fillers of Samples 611 to 631 were made of primary composite particles obtained by carrying out the previous steps as described above.

[0126] <Samples C51 and C52> The fillers of samples C51 and C52 were prepared using the raw materials (first particles and resin) weighed out in the volume ratios shown in Table 1, without performing the second mixing step and subsequent steps described above. In other words, the fillers of samples C51 and C52 consisted of the mixture obtained after the first mixing step. In this case, the first mixing step and other steps were performed as described above.

[0127] 《Production of composite materials》 Composite materials in which the filler of each sample was held in an epoxy resin (matrix) were produced in the same manner as in Example 1. However, the amount of matrix resin relative to the total composite material (100% by volume) was as follows: Sample 513: 10% by volume, Samples 511, 512, 514 to 532: 20% by volume, Sample C51: 30% by volume, and Sample C52: 20% by volume.

[0128] "measurement" (1) Thermal conductivity The thermal conductivity of the composite materials was measured in the same manner as in Example 1. The thermal conductivity of parallel samples cut out from each composite material is also shown in Table 1.

[0129] (2) Porosity The porosity of the composite material is also shown in Table 1. Dense The coefficient of elasticity (ρ) was calculated from the degree of elasticity (ρ) and theoretical density (ρth) as {(ρth-ρ) / ρth} x 100(%). ρ was calculated from the measured mass and volume (Archimedes' method) of the composite material. ρth was calculated based on the blending ratio and density of each raw material (particles and resin) used to produce the composite material.

[0130] The thermal conductivities and porosities of Samples 515, 613, and C52 are shown in comparison in FIG.

[0131] "observation" SEM images of the cross sections (planes parallel to the pressure direction during molding) of the composite materials for Samples 515, 613, and C52 are shown in Figure 12. The state of the filler in each composite material is shown schematically in Figure 13 based on the SEM images.

[0132] "evaluation" As can be seen from Table 1 and Figure 11, the samples (samples 511-532 and 611-631) containing (primary) composite particles in which flattened second particles (coating particles) are bonded to the surfaces of roughly spherical first particles (base particles) with resin showed sufficiently small porosity and high thermal conductivity. This tendency was particularly evident in samples 511-532, which contained (secondary) composite particles in which third particles (bridging particles) were attached to the outermost surface.

[0133] On the other hand, samples C51 and C52, which used approximately spherical base particles as fillers, had a large porosity (see FIG. 12) and a small thermal conductivity.

[0134] 《Consideration》 The reason why the thermal conductivity was significantly improved by using secondary composite particles having bridging particles (third particles) as filler is presumed to be as follows from FIGS. 12 and 13.

[0135] As shown in Figure 13(f), in a composite material containing only roughly spherical base particles as filler, contact between particles is poor, and even if contact does occur, it is point-like contact, making it difficult to form a heat conduction path that penetrates the composite material.

[0136] As shown in Figure 13(e), in a composite material that uses primary composite particles as a filler, in which flat coated particles are attached to the surface of an approximately spherical base particle, the composite particles are more likely to come into planar contact with each other, increasing the number of heat conduction paths that penetrate the composite material.

[0137] As shown in Figure 13(d), in a composite material with secondary composite particles as fillers, in which bridging particles larger in diameter than the coated particles are on the outermost surface, in addition to the planar contact between the composite particles, the bridging particles also connect the composite particles. This leads to the formation of many isotropic heat conduction paths between the composite particles, which is thought to have significantly increased the thermal conductivity of the composite material.

[0138] From the above, it has become clear that the thermally conductive material (filler or composite material) of the present invention ensures substantially isotropic and high thermal conductivity.

[0139] [Table 1]

Claims

1. Composite particles include substantially spherical base particles and flat coating particles bonded to the surfaces of the base particles via a resin, the base particles are made of one or more of aluminum nitride, aluminum oxide, silicon oxide, and cubic boron nitride; The particles are a thermally conductive material made of hexagonal boron nitride.

2. 2. The thermal conductive material according to claim 1, wherein a particle size ratio (L2 / L1), which is a ratio of a particle size (L2) of the coating particles to a particle size (L1) of the base particles, is 0.01 to 2.

3. 2. The thermal conductive material according to claim 1, wherein the volume ratio of the base particles to the total amount of the base particles and the adhered particles is 0.5 to 0.

9.

4. The thermally conductive material according to any one of claims 1 to 3, wherein the composite particles are dispersed in a matrix.

5. The composite material further includes bridging particles having a particle size larger than that of the coated particles, 5. The thermally conductive material of claim 4, wherein said bridging particles are made of hexagonal boron nitride.

6. the composite particle further has a bridging particle on the surface side, the bridging particle having a particle size larger than that of the coated particle, 2. The thermally conductive material of claim 1, wherein said bridging particles comprise hexagonal boron nitride.

7. 7. The thermal conductive material according to claim 6, wherein the volume ratio of the base particles to the total amount of the base particles, the coated particles, and the bridging particles is 0.4 to 0.

85.

8. 8. The thermally conductive material according to claim 6, wherein the composite particles are made of a composite material dispersed in a matrix.

9. a preparation step of obtaining a mixture of substantially spherical first particles, flat second particles, and a resin; a molding step of pressing the mixture to obtain a molded body; and a crushing step of crushing the molded body, the first particles are made of one or more of aluminum nitride, aluminum oxide, silicon oxide, and cubic boron nitride; the second particles are made of hexagonal boron nitride; The method for producing a thermally conductive material provides composite particles in which the second particles are bonded to the surfaces of the first particles with the resin.

10. The method for manufacturing a thermally conductive material according to claim 9 , wherein the molding step is performed while the resin is in a softened or molten state.

11. a first mixing step of obtaining a first mixture by mixing substantially spherical first particles and a resin; a second mixing step of mixing the first mixture with flat second particles to obtain a second mixture, the first particles are made of one or more of aluminum nitride, aluminum oxide, silicon oxide, and cubic boron nitride; the second particles are made of hexagonal boron nitride; The method for producing a thermally conductive material provides composite particles in which the second particles are bonded to the surfaces of the first particles with the resin.

12. a first mixing step of obtaining a first mixture by mixing substantially spherical first particles and a resin; a second mixing step of mixing the first mixture with flat second particles to obtain a second mixture; a third mixing step of mixing the second mixture with third particles having a particle size larger than that of the second particles to obtain a third mixture, the first particles are made of one or more of aluminum nitride, aluminum oxide, silicon oxide, and cubic boron nitride; the second particles and the third particles are made of hexagonal boron nitride; The method for producing a thermally conductive material provides composite particles in which the second particles are bonded to the surfaces of the first particles with the resin, and the third particles are attached to the surface side.

13. the resin is a thermosetting resin, The method for producing a thermally conductive material according to claim 11 or 12, further comprising a thermal curing step of heating the second mixture to cure the resin present between the first particles and the second particles.

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