Thermal Conductive Material and Method for Producing the Same

Abstract

To provide a heat-conductive material having excellent thermal conductivity.SOLUTION: The invention provides a heat-conductive material including a composite particle having a boron nitride particle and a carbon nanotube particle connected to the boron nitride particle. The heat-conductive material may be a filler including the composite particle or a composite material including the filler in its matrix. Preferably, the carbon nanotube particle is included, e.g., at 5-35 vol.% of the total amount of the boron nitride particle and the carbon nanotube particle. Such a composite particle is obtained by, e.g., a firing step of heating a compact including a boron nitride particle and a carbon nanotube particle to obtain a fired body and a pulverization step of pulverizing the fired body. The firing step is, e.g., run by heating at 1,500-2,000°C in a nitrogen atmosphere.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The present invention relates to a heat conductive material containing boron nitride and the like.

Background Art

[0002] In order to maintain the functions and lifespan of high-density and high-performance electronic devices (such as semiconductor modules), heat dissipation is necessary. Heat dissipation of electronic devices is usually performed through a heat dissipation member (such as a heat sink, a housing, etc.) made of metal or the like. At this time, a heat dissipation sheet (such as a heat conductive sheet, a heat conductive insulating sheet, etc.) that absorbs unevenness, undulations, etc. on the contacting surface is often interposed between the electronic device (heat source) and the heat dissipation member.

[0003] For the heat dissipation sheet, for example, a composite material (including a composition) composed of a filler with excellent thermal conductivity and a resin (including elastomers, rubbers, etc.) with excellent flexibility (elasticity) and adhesion is used.

[0004] As the filler, for example, ceramic particles (including fibers) such as silica (SiO 2 ), alumina (Al 2 O 3 ), and aluminum nitride (AlN) have been used. However, silica and alumina have relatively low thermal conductivities. Also, aluminum nitride reacts with water (H 2 O) to generate ammonia (NH 3 ), so it has low moisture resistance and poor long-term reliability. Therefore, boron nitride (BN) particles, which are excellent in thermal conductivity and electrical insulation and are also chemically stable, are increasingly used as fillers for composite materials.

[0005] Boron nitride generally has a hexagonal system normal pressure phase (also appropriately referred to as "h-BN") and a cubic system high pressure phase (also appropriately referred to as "c-BN"). Usually, hexagonal boron nitride (h-BN) is used as the filler. Note that h-BN is composed of flaky shapes in which hexagonal network layers similar to graphite are stacked, and usually has a thermal conductivity anisotropy in which the thermal conductivity in the plane direction (a-axis (100) direction) is larger than the thermal conductivity in the thickness direction (c-axis (002) direction).

[0006] Various proposals have been made regarding such boron nitride particles. For example, there are descriptions related to the following patent documents.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0008] Patent Documents 1 to 4 only propose composite materials using fillers composed only of boron nitride particles (h-BN particles). Patent Document 5 proposes a thermal conductivity filler composed of boron nitride particles, alumina particles, and aluminum nitride particles. However, the thermal conductivity of the composite material (sheet) using such a filler remains at a maximum of 7 W / mK.

[0009] The present invention has been made in view of such circumstances, and an object thereof is to provide a new thermal conductive material having boron nitride and the like.

Means for Solving the Problems

[0010] As a result of intensive studies to solve this problem, the present inventors have succeeded in obtaining composite particles (fillers) in which carbon nanotube particles are adhered to boron nitride particles, and have newly found that a composite material containing such composite particles exhibits high thermal conductivity. By developing this result, the present invention described below has been completed.

[0011] "Thermal Conductive Material" The present invention relates to a thermal conductive material containing composite particles having boron nitride particles and carbon nanotube particles in contact with the boron nitride particles.

[0012] According to the present invention, it is possible to improve the thermal conductivity of a filler (one form of the thermal conductive material) containing composite particles or a composite material (another form of the thermal conductive material) containing the filler. Although the reason for this is not clear, it is considered that this is because the composite particles according to the present invention are in the following state (structure). That is, the composite particles according to the present invention are composed of particles in which boron nitride particles (appropriately referred to as "BN particles") are modified with carbon nanotube particles (appropriately referred to as "CNT particles"), and it is considered that the BN particles and the CNT particles are in at least a contacting state (a state of being closely attached). In other words, the composite particles according to the present invention are considered to be not in a state where only the BN particles and the CNT particles are simply mixed, nor in a state where only the BN particles and the CNT particles are locally or partially simply in contact.

[0013] However, the contacting state referred to in this specification may be sufficient as long as stable heat conduction is possible between the BN particles and the CNT particles and further between adjacent BN particles through the CNT particles. Therefore, the contacting BN particles and CNT particles 、 may be in a state of being chemically or physically bonded or joined. There is That's fine.

[0014] "Method for Manufacturing a Thermal Conductive Material" The present invention can also be understood as a method for manufacturing a thermal conductive material. For example, the present invention may be a method for manufacturing a thermal conductive material that includes a firing step of heating a molded body containing boron nitride particles and carbon nanotube particles to obtain a fired body, and a pulverizing step of pulverizing the fired body, and obtaining composite particles in which the carbon nanotube particles are in contact with the boron nitride particles.

[0015] "Composite Material / Thermal Conductive Member" The present invention can also be understood as a composite material or a heat conductive member, which is a form of heat conductive material. For example, the heat conductive material of the present invention may be a composite material having composite particles and a matrix (or binder) that holds the composite particles. Further, the heat conductive material (composite material) of the present invention may be a heat conductive member such as a heat dissipation member, a substrate, or a case.

[0016] 《Others》 (1) As used in this specification, the term "~ material" means "material" or "member". For example, the heat conductive material may be the composite particles themselves, a filler containing the composite particles, an aggregate (powder) thereof, etc., or a tangible composite material (including the material) having the composite particles and / or the filler (particles, powder, etc.) and a base material (matrix) or a binder. It may also be a composite member (such as a sheet) having the composite material in a desired shape.

[0017] (2) As used in this specification, "x~y" includes the lower limit value x and the upper limit value y unless otherwise specified. For any numerical value included in the various numerical values or numerical ranges described in this specification, a new range such as "a~b" can be newly established with the new numerical value as the new lower limit value or upper limit value. As used in this specification, "x~y μm" means x μm~y μm unless otherwise specified. The same applies to other unit systems (W / mK, Ωm, etc.).

Brief Description of the Drawings

[0018]

Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0019] One or more components arbitrarily selected from this specification can be added to the components of the present invention. The content described in this specification is applicable not only to heat conductive materials (composite particles, fillers, composites, members, etc.), but also to their manufacturing methods and the like. Even a methodological component can be a component related to an object. Whether any embodiment is the best depends on the object, required performance, etc.

[0020] 《Composite Particles》 The composite particles contain at least boron nitride particles (BN particles) and carbon nanotubes (CNT particles), and the BN particles are modified with CNT particles. More specifically, the BN particles and the CNT particles are in contact with each other, and stable heat conduction can be ensured at least between both particles. This contact is, for example, adhesion, bonding (further connection), joining, etc. The bonding is, for example, sintering, chemical bonding (including van der Waals bonding), etc. The joining may be a case where the BN particles and the CNT particles are directly joined, or a case where an intermediate material (adhesive, etc.) is interposed.

[0021] In the composite particles, a plurality of BN particles may be in contact with each other (bonded, etc.), and a plurality of CNT particles may be in contact with each other (bonded, etc.). Further, the CNT particles may be in contact with each other (bonded, etc.) so as to crosslink a plurality of BN particles. Note that the BN particles and the CNT particles may be in an aggregated state (secondary particle state).

[0022] The in-contact BN particles and CNT particles may substantially maintain their original particle shape (for example, the particle shape at the time of raw material powder), or may be deformed.

[0023] In addition, the composite particles and fillers referred to in this specification may include non-attached BN particles and CNT particles, or may also include other particles (for example, graphite particles (including carbon black), diamond particles, other nanocarbon particles (such as carbon nanohorns (CNH), fullerenes, graphene, etc.)).

[0024] 《Boron Nitride Particles》 The BN particles are composed of, for example, hexagonal boron nitride (h-BN) or cubic boron nitride (c-BN). The BN particles may include both h-BN particles and c-BN particles, but are usually h-BN particles.

[0025] The size and shape of the BN particles are not limited. For h-BN particles, for example, the maximum length is about 1 to 100 μm, 10 to 60 μm, or even 20 to 40 μm. The maximum length can be obtained from, for example, a microscopic photograph of the particles (such as an SEM image). To be specific, it is advisable to take the arithmetic mean of the maximum lengths of the particles present per field of view (1500 μm × 1000 μm) as the size of the particles. The sizes of other particles (composite particles, CNT particles, etc.) referred to in the present invention are also determined in the same way. Note that regardless of the shape of the particles (such as substantially flaky, substantially fibrous, substantially long spherical, substantially spherical, etc.), the particle size is also simply referred to as the "particle diameter".

[0026] Although h-BN itself (single layer) has a network-like hexagonal lattice structure, the BN particles themselves may be h-BN single layers, or their laminates or aggregates (aggregates, secondary particles). Therefore, the BN particles do not necessarily have to be flaky.

[0027] 《Carbon Nanotube Particles》 CNT particles are usually tiny (for example, with a maximum length of 0.001 - 5 μm, and even more preferably 0.01 - 2 μm), and have a thermal conductivity sufficiently higher than that of BN particles. Even a small amount of such CNT particles can form excellent thermal conduction paths between BN particles. Therefore, CNT particles may be contained, for example, in an amount of 5 - 35% by volume, 10 - 30% by volume, and even more preferably 15 - 25% by volume based on the total amount of boron nitride particles and carbon nanotube particles. Whether the amount of CNT particles is too small or too large, the thermal conductivity of the composite particles and thus the composite material may decrease. In particular, when the amount of CNT particles is excessive, the insulating property (electrical resistivity) of the composite material may decrease.

[0028] Note that the volume ratio (vol.%) of the particles can be calculated using the true density of the particles. For the volume ratio during preparation, it can be calculated from the mass and true density of the blended particles (raw material powder). For the volume ratio of the constituent particles in the composite particles, it can be calculated from the mass and volume of the composite particles and the true density of each constituent particle. For the volume ratio of the constituent particles contained in the composite material, it may be calculated in the same way by separating the composite particles from the composite material. Note that the volume ratio of each particle with respect to the entire composite material can also be obtained in the same manner.

[0029] 《Composite Material》 The thermal conductive material may be a composite material (material or member) in which at least a filler containing composite particles is held by a matrix (including a binder).

[0030] (1) Filler The filler may be contained, for example, in an amount of 55 - 95% by volume, and even more preferably 60 - 80% by volume based on the entire composite material. If the filling rate of the filler is too small, the thermal conductivity of the composite material may also decrease. Even if the filling rate of the filler is excessive, the thermal conductivity of the composite material does not necessarily increase.

[0031] Note that the filling rate (volume%) of the filler during the production of the composite material is specified from the blending amount and density of the raw materials. The filling rate of the filler in the composite material is specified from the total amount of the composite material and the amount of the filler separated from the composite material. When the filler cannot be separated, it may be specified indirectly from the observation image of the composite material (cross-section).

[0032] It is preferable that all or part of the filler is subjected to a surface treatment to enhance the affinity with the matrix. By the surface treatment, the dispersibility, filling property, adhesion property, etc. of the filler in the matrix can be improved, and the thermal conductivity of the composite material can be improved.

[0033] The surface treatment is, for example, a hydrophobization 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 may be performed. The silane coupling treatment can be performed using various silane coupling agents having a reactive group corresponding to a functional group (amino group, epoxy group, isocyanate group, vinyl group, acrylic group, etc.) on the matrix side. As a typical silane coupling agent, for example, hexamethyldisilazane (HMDS: C 6 H 19 NSi 2 ) is available. Note that the silane coupling agent usually has a reactive group (silyl group, etc.) corresponding to a functional group (hydroxy group, methoxy group, ethoxy group, etc.) on the filler (composite particles, etc.) side, which is usually an inorganic material.

[0034] The content (blending amount / addition amount) of the surface treatment agent is, for example, 0.1 to 3 parts by mass, 0.5 to 2.5 parts by mass, and further 1 to 2 parts by mass with respect to 100 parts by mass of the entire filler before treatment. With too little surface treatment agent, the effect is poor, and even if the surface treatment agent is excessive, the improvement in effect is small.

[0035] Note that the surface treatment may be performed on the filler before mixing (including kneading), or may be performed by adding a surface treatment agent (coupling agent, etc.) during the mixing of the matrix and the filler.

[0036] (2) Matrix The matrix (including the binder) is made of, for example, an insulating organic material. Specifically, usually, resins, rubbers, elastomers, etc. serve as the matrix. The resin may be a thermosetting resin or a thermoplastic resin. Examples of the thermosetting resin include epoxy resin, phenolic resin, silicone resin, etc. Examples of the thermoplastic resin include polystyrene, polymethyl methacrylate, polycarbonate, polyphenylene sulfide, etc. Examples of the rubber include ethylene - propylene - diene rubber (EPDM), butyl rubber, etc. In this specification, unless otherwise specified, including rubber - elastomers, it is simply referred to as "resin".

[0037] 《Manufacturing Method》 (1) Composite Particles Various methods for manufacturing composite particles in which BN particles and CNT particles are in contact can be considered. For example, the composite particles can be obtained by firing a mixture (or even a molded body) of BN particles and CNT particles. More specifically, it is as follows.

[0038] The mixture can be obtained, for example, by mixing boron nitride powder (such as h - BN powder) and carbon nanotube powder. Such mixing is carried out using a ball mill, vibration mill, V - type mixer, etc. (mixing step). At this time, it is preferable that the laminated h - BN particles can also be pulverized. Note that the mixing may be dry, but wet mixing is easier to suppress the scattering and floating of minute CNT particles.

[0039] The mixture may be fired as it is, or a molded body obtained by pressure - molding the mixture may be fired (firing step). The molded body can be obtained, for example, by injection - molding, CIP (Cold Isostatic Pressing), RIP (Rubber Isostatic Pressing), etc. of the mixture (molding step). Note that the molded body only needs to be in a shape that can be pulverized after firing. The molding pressure is, for example, about 50 - 500 MPa, and further preferably about 200 - 400 MPa.

[0040] By heating the mixture (including its molded body), a fired body (and further a sintered body) can be obtained. The heating temperature is, for example, 1600°C to 2000°C, 1700 to 1900°C, and further 1750 to 1850°C. The heating time is, for example, 0.3 to 3 hours, and further 0.7 to 2 hours. Note that by HIP (Hot Isostatic Pressing / Hot Isostatic Pressing Method), the above-mentioned molding and firing may be performed simultaneously.

[0041] The heating atmosphere is preferably carried out, for example, in a non-oxidizing atmosphere. The non-oxidizing atmosphere is, for example, an inert gas atmosphere or a vacuum atmosphere. The inert gas atmosphere may be a noble gas atmosphere or a nitrogen gas atmosphere.

[0042] By crushing or pulverizing the fired body, a powder composed of composite particles (referred to as "composite powder") can be obtained (powdering process). Note that the pulverization process of the fired body can be carried out using a small pulverizer, a crusher, etc.

[0043] The composite powder may be classified (particle size adjustment) to 1 to 100 μm, and further 1 to 53 μm, for example, by sieving. In terms of the average particle size (median diameter: D50), it may be adjusted to 5 to 45 μm, and further 16 to 22 μm, for example.

[0044] (2) Composite material The composite material in which the filler is held by the matrix is formed, for example, by compression molding, injection molding, transfer molding, etc. When the matrix is made of a thermosetting resin, a thermosetting treatment (curing treatment) may be carried out after molding. The composite material may be in the shape of the final product or a shape close to it, or it may be a material or intermediate material to be post-processed.

[0045] 《Applications》 The composite material is used, for example, in heat dissipation members such as heat dissipation sheets, substrates of electronic devices, cases, etc., and parts thereof. The thermal conductivity of the composite material (especially the thermal conductivity in the plane direction (a-axis direction) of BN particles) can be, for example, 15 to 40 W / mK, and further 20 to 30 W / mK. The specific resistance of the composite material is, for example, 10 to 10 5Ωm, and further 10 2 ~10 4 Ωm can be obtained.

Examples

[0046] A plurality of composite materials in which a filler containing h-BN particles is filled in a resin (matrix) were produced, and their properties (thermal conductivity, specific resistance, etc.) were evaluated. The present invention will be described in more detail while showing such specific examples.

[0047] 《Production of Filler》 Four types of fillers (Sample 1, Sample C1, Sample C2, and Sample C3) were prepared (see Figure 4). Each filler was produced as follows.

[0048] (1) Sample 1 (Composite Particles) As raw materials, commercially available h-BN powder (Denka Boron Nitride Powder SGP / BN purity: 99% or more, particle size: 18 μm (D50)) manufactured by Denka Co., Ltd. and CNT powder (NC7000 manufactured by Nanocyl / average diameter: 9.5 nm, average length: 1.5 μm) were prepared.

[0049] 18 g of h-BN powder and 4 g of CNT powder were wet-mixed in a ball mill (mixture). The true density of h-BN particles: 2.27 g / cm 3 3 , the true density of CNT particles: 2.20 g / cm 3 3 Therefore, their volume fractions correspond to h-BN particles: CNT particles = 8:2 (volume ratio of CNT particles to the total of both particles: 20 vol%).

[0050] Note that the wet mixing was carried out for 12 hours using zirconia balls (ZrO 2 2 / particle size 5 mm): 500 g and acetone: 160 g.

[0051] After distilling acetone from the filtrate excluding zirconia balls using a rotary evaporator, further vacuum drying (at room temperature) was performed. The mixture of h-BN particles and CNT particles thus obtained was put into a double vinyl chloride bag and cold isostatic pressing (CIP) was carried out (forming step). The forming pressure at this time was 3 t / cm 2 (294 MPa).

[0052] The formed body (30 mm × 21 mm × 21 mm) was placed in a sintering furnace and heated at 1800 °C for 1 hour under a nitrogen gas flow (sintering process). Thus, a sintered body of h-BN particles and CNT particles was obtained. 2 The sintered body was pulverized in a cutter mill installed in a draft chamber for several minutes. The pulverized powder was sized to less than 53 μm by sieving. Thus, a filler (composite particles) in which h-BN particles were modified with CNT particles was obtained.

[0053]

[0054] (2) Sample C1 The mixture before forming described above was crushed, and the powder sized to less than 53 μm by sieving was used as the filler. That is, the filler of Sample C1 was not subjected to the forming process and sintering process described above. The other processes were carried out in the same manner as in Sample 1.

[0055] (3) Sample C2 The above-described h-BN powder was used as the filler as it was.

[0056] (4) Sample C3 The above-described CNT powder was used as the filler as it was.

[0057] 《Fabrication of Composite Material》 A composite material in which the filler was held by the matrix was fabricated. The filling rate of the filler was 50 to 90% by volume with respect to the entire composite material (100% by volume) unless otherwise specified. As the matrix (binder), a one-component heat-curing epoxy resin (EP160 manufactured by Cemedine Co., Ltd., simply referred to as "resin") was used. The specific process is as follows.

[0058] ​The filler and the resin were kneaded in a plastic container for 10 minutes. The kneaded material that had been vacuum dried 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 a uniaxial direction. At this time, the mold temperature was set to 120 °C and the molding pressure was set to 20 MPa, and the pressurized state was maintained for 30 minutes to thermoset the resin. Thereby, the composite was heated in an air atmosphere (120 °C × 30 minutes) to obtain a columnar composite (φ14 mm × 20 mm) in which the filler was held by the resin.

[0059] In this example, the composites using the fillers of Sample 1, Sample C1, Sample C2, or Sample C3 are referred to as Sample M1, Sample MC1, Sample MC2, and Sample MC3, respectively, in order.

[0060] 《Measurement》 (1) Thermal conductivity The thermal conductivity (λ) of the composite material was determined by the nano-flash method (measurement device: LFA447 manufactured by NETZSCH). Specifically, the thermal diffusivity (α) measured by the nano-flash method, the specific heat (Cp) determined by a differential scanning calorimeter (DSC), and the density (ρ) determined by the Archimedes method were used to calculate the thermal conductivity as λ = α·Cp·ρ.

[0061] For the measurement of the thermal diffusivity, a thin plate-like sample (appropriately referred to as a "vertical sample") cut out from the columnar composite material in a direction perpendicular to the axial direction (pressurization direction) and a thin plate-like sample (appropriately referred to as a "parallel sample") cut out in a direction parallel to the axial direction thereof were used. The thermal conductivity of the vertical sample is referred to as the thermal conductivity in the "vertical direction", and the thermal conductivity of the parallel sample is referred to as the thermal conductivity in the "parallel direction". The relationship between the filler filling rate and the thermal conductivity is shown in FIGS. 1A and 1B. In this specification, the thermal conductivity in the vertical direction is simply referred to as the "thermal conductivity" as appropriate.

[0062] (2) Orientation degree As shown in Fig. 3, the surface of a test piece (12 mm × 12 mm × 2 mm) cut from the composite material was subjected to X-ray diffraction analysis (XRD / Cu-Kα / UltraV manufactured by Rigaku Corporation). The test piece was cut along the compression direction (uniaxial direction) using an IsoMet1000 manufactured by Buehler

[0063] Using the thus obtained XRD profile (2θ = 20° to 60°), the degree of orientation of the a-axis direction of the BN particles was calculated from the peak intensity ratio shown in the following equation Degree of orientation (%) = 100 × I(100) / {I(100) + I(002)} Here, I(100) is the peak intensity of the (100) plane, and I(002) is the peak intensity of the (002) plane. The relationship between the filling rate and the degree of orientation for each sample is shown in Fig. 1C

[0064] (3) Porosity The apparent density (D) of the composite material was determined by the Archimedes method. From the apparent density (D) and the theoretical density (Dth), the porosity of the composite material was determined by the following equation Porosity (%) = 100 × {1 - (D / Dth)} The theoretical density (Dth) was calculated from the blending amounts and true densities of the raw materials (h-BN particles, CNT particles, binder) used in the production of the composite material. The relationship between the filling rate and the porosity for each sample is shown in Fig. 1D

[0065] (4) Specific resistance The specific resistance of the composite material was measured by the DC four-terminal method at room temperature using a disk-shaped vertical sample. The relationship between the thermal conductivity and the specific resistance for each sample is summarized in Fig. 2

[0066] 《Evaluation》 (1) Thermal conductivity As is clear from Fig. 1A and Fig. 1B, the composite material (sample M1) using the composite particles obtained by firing h-BN particles and CNT particles as a filler (sample 1) exhibited a higher thermal conductivity not only in the vertical direction but also in the parallel direction than other composite materials. This tendency was particularly remarkable when the filling rate was 55% by volume or more, and further 65% by volume or more

[0067] (2) Orientation degree As is clear from Fig. 1C, in the composite material (sample M1) using composite particles as the filler (sample 1), the orientation degree was higher than that of other composite materials in the range where the filling rate was 65% by volume or more. In particular, the orientation degree increased significantly in the vicinity where the filling rate was 70% by volume (for example, 60 - 80% by volume, and more specifically 65 - 75% by volume). However, the orientation degree only reached about 50% - 55% at most.

[0068] (3) Porosity As is clear from Fig. 1D, in the composite material (sample M1) using composite particles as the filler (sample 1), the porosity was generally smaller throughout the whole area than that of other composite materials. In particular, the porosity decreased significantly when the filling rate was 75% by volume or less (for example, 50 - 75% by volume, and more specifically 65 - 73% by volume). From the comparison of Fig. 1A, Fig. 1B and Fig. 1D, it is considered that there is a correlation between the decrease in porosity and the improvement in thermal conductivity.

[0069] (4) Specific resistance and thermal conductivity As is clear from Fig. 2, it was found that the composite material (sample M1) using composite particles as the filler (sample 1) can achieve both high specific resistance (electrical resistivity) and high thermal conductivity in a high - dimensional manner, compared with other composite materials.

[0070] 《Discussion》 Based on the above - mentioned results, it is inferred that the thermal conductivity of the composite material changes depending on the form (structure) of the filler. Specifically, it is as follows.

[0071] As shown in Fig. 4, it is considered that the composite particles (sample 1) have CNT particles attached to h - BN particles and cross - link (connect) the adjacent spaces between h - BN particles. As a result, it is considered that a large number of thermal conduction paths are formed in the composite material (sample M1) using composite particles as the filler, and high thermal conductivity is exhibited.

[0072] It is considered that a large number of voids are formed around the CNT particles freed from the h - BN particles in the filler (sample C1) composed of a simple mixture of CNT particles and h - BN particles. Therefore, it is considered that the thermal conductivity of the composite material (sample MC1) did not improve even when the filling rate of the filler increased.

[0073] For the composite material (sample MC2) using only h-BN particles as the filler (sample C2), the vertical thermal conductivity increased somewhat with the increase in the filling rate, but its basic tendency is considered to be the same as that of the composite material (sample MC1).

[0074] From the above, it was confirmed that the thermal conductive material (filler or composite material) of the present invention is excellent in thermal conductivity.

Claims

1. A heat conductive material comprising composite particles in which carbon nanotube particles are chemically or physically bonded or joined to boron nitride particles, wherein the carbon nanotube particles are contained in an amount of 5 to 35% by volume based on the total amount of the boron nitride particles and the carbon nanotube particles, and the composite particles are obtained by pulverizing a fired body obtained by heating a molded body obtained by pressurizing a mixture containing the boron nitride particles and the carbon nanotube particles at 50 to 500 MPa in a nitrogen atmosphere to 1500 to 2000 °C.

2. The heat conductive material according to Claim 1, wherein the molded body is obtained by cold isostatic pressing (CIP).

3. The heat conductive material according to Claim 1 or 2, comprising a composite material having the composite particles and a matrix holding the composite particles.

4. The heat conductive material according to any one of Claims 1 to 3, wherein the composite particles are contained in an amount of 55 to 95% by volume based on the entire composite material.

5. A firing step of obtaining a fired body by heating a molded body obtained by pressurizing a mixture containing boron nitride particles and carbon nanotube particles at 50 to 500 MPa in a nitrogen atmosphere to 1500 to 2000 °C, and a pulverizing step of pulverizing the fired body, whereby a heat conductive material is obtained in which carbon nanotube particles are chemically or physically bonded or joined to the boron nitride particles.

6. The method for producing a heat conductive material according to Claim 5, wherein the molded body is obtained by CIP.

7. The method for producing a heat conductive material according to Claim 5 or 6, wherein the mixture is obtained by wet mixing.

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