JP7452478B2 - Thermal conductive material and its manufacturing method - Google Patents

Description

The present invention relates to a thermally conductive material using boron nitride.

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

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

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

h-BN has a scale-like structure in which hexagonal mesh layers similar to graphite are laminated, and has thermal conductivity anisotropy in which the thermal conductivity is greatly different in the plane direction and the thickness direction. In addition, h-BN particles tend to be oriented in the plane direction in the resin filled. For this reason, for example, even if highly filled with h-BN particles, heat dissipation (thermal conductivity) in the thickness direction (in the case of a sheet-like composite material, from the heat source side to the cooling source side) tends to be insufficient. there were.

Therefore, various proposals have been made to improve the thermal conductivity (heat dissipation) of composite materials containing h-BN particles, and for example, there are related descriptions in Patent Documents 1 to 6 below.

Japanese Patent Publication No. 59-64355 JP2010-260225 JP2011-184507 JP2012-171842 JP2012-255055 JP2018-159062 JP 2-143433 JP2011-51166 JP2013-86433 JP2014-237805 JP2019-513203

However, all of Patent Documents 1 to 6 only use crystalline h-BN as a filler. Note that Patent Documents 7 to 11 describe the use of magnetic powder such as iron powder as a filler for composite materials, but there is no specific description regarding boron nitride.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a new thermally conductive material using boron nitride particles.

As a result of intensive research to solve this problem, the inventors of the present invention have created a composite particle skeleton in which amorphous particles of boron nitride are attached to crystalline particles of boron nitride, and have attached magnetic particles to the amorphous particles. By using a supported filler, we succeeded in increasing the thermal conductivity of the composite material. By developing this result, we have completed the present invention described below.

《Thermal conductive material》
The present invention provides composite particles having crystalline particles of boron nitride, amorphous particles of boron nitride smaller than the crystalline particles and attached to the surface of the crystalline particles, and magnetic particles supported on the amorphous particles. It is a thermally conductive material containing

By using the composite particles according to the present invention, a thermally conductive material (including a composite material) having excellent thermal conductivity can be obtained. Although the reason for this is not certain, it is thought to be as follows. The amorphous particles in the composite particles increase the contact ratio and contact area between the composite particles (particularly between the crystalline particles), and can support auxiliary particles (for example, magnetic particles).

Magnetic particles, which are an example of auxiliary particles, are also usually made of a substance with high thermal conductivity (for example, a metal such as an iron element). A heat conduction path can then be formed between adjacent crystalline particles via the amorphous particles and the magnetic particles. In this way, it is considered that the thermally conductive material (for example, composite material) containing the composite particles according to the present invention has significantly improved thermal conductivity.

Note that since the composite particles according to the present invention include magnetic particles, their orientation can be controlled by a magnetic field. For this reason, in the composite material, composite particles (especially crystalline particles) are oriented in a specific direction (for example, the a-axis direction (100)) that has high thermal conductivity, thereby further increasing the thermal conductivity of the composite material in a predetermined direction. It also becomes possible.

《Method for manufacturing thermally conductive material》
The present invention can also be understood as a method for manufacturing a thermally conductive material. For example, the present invention may be a method for manufacturing a thermally conductive material that includes a firing step of heating a mixture of crystalline particles of boron nitride, melamine borate, and magnetic particles.

《Composite materials/thermal conductive materials》
The thermally conductive material of the present invention is not only understood as a filler made of composite particles, but also as a composite material containing composite particles (filler), and furthermore, a member made of a composite material (thermal conductive member). A composite material, which is a type of thermally conductive material, is made up of a filler containing composite particles dispersed in a matrix. Thermal conductive members, which are one form of thermally conductive materials, include, for example, heat dissipating members, substrates, cases, and the like.

"others"
(1) As used herein, "material" means "material" or "member." For example, the thermally conductive material may be a composite particle or a powder thereof (raw material), or a composite material (material, raw material, member, etc.) having the composite particle (powder) and a base material (matrix) or a binding material (binder).

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

FIG. 3 is a model diagram schematically showing each filler particle used in Examples. 3 is a SEM image of filler particles of Sample 1 and Sample 2. These are an SEM image and an EDX image of filler particles of Sample 3. 3 is an XRD profile of filler particles of Sample 1 and Sample 2. It is a bar graph showing the thermal conductivity of the composite material of each sample. It is a scatter diagram showing the relationship between specific resistance and thermal conductivity for each sample composite material. It is a graph showing the relationship between filler filling rate and thermal conductivity based on the composite material of sample M3. 1 is a model diagram schematically showing a composite material according to the present invention.

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

《Composite particles》
The composite particles include crystalline particles made of boron nitride (BN) and amorphous particles made of boron nitride. Amorphous particles are attached to the surface of crystalline particles. The amorphous particles may be attached by chemical bonding (including van der Waals bonding), sintering, adhesion, or the like. Note that the amorphous particles may be part of all particles attached to the surface of the crystalline particles. Moreover, some of the particles attached to the crystalline particles may be crystallized BN.

The amorphous particles may be contained in an amount of, for example, 5 to 20% by volume, and more preferably 7 to 15% by volume, based on the entire composite particles (or the total amount of crystalline particles and amorphous particles). If there are too few amorphous particles, the effect will be poor. If there are too many amorphous particles, there will be a relatively small number of crystalline particles, and the thermal conductivity may decrease.

The volume ratio (volume %) of each particle is specified from the volume ratio (calculated from the blended amount and density) of the raw material at the time of preparing the composite particles. The volume ratio of the magnetic particles, the volume ratio of each particle to the entire composite material, etc. can also be specified using a similar method. In addition, when obtaining amorphous particles from melamine borate, the volume of the amorphous particles becomes 1/5 of the volume of melamine borate due to the firing process. Therefore, it is preferable to increase the volume of melamine borate by 5 times the desired volume of the amorphous particles. Note that the larger the volume ratio of amorphous particles, the broader the peak of the X-ray diffraction pattern may become (see FIG. 4).

《Crystalline particles》
The crystalline particles are, for example, hexagonal boron nitride (h-BN) or cubic boron nitride (c-BN). Usually, h-BN particles are used as the crystalline particles.

The size and form of the crystalline particles do not matter. The size of the crystalline particles made of h-BN is, for example, a maximum length of about 1 to 100 μm, 10 to 60 μm, and further 20 to 40 μm. The maximum length is determined, for example, from a microscopic observation image (for example, a SEM image) of the crystalline particles. If I were to say so, the size of the crystalline particles may be determined by calculating the average value of the maximum lengths of the crystalline particles present in one field of view (1500 μm×1000 μm). Such particle size specification also applies to amorphous particles, composite particles, magnetic particles, etc. in the present invention. Further, the particle size is also simply referred to as "particle size", regardless of the shape of the particles (substantially scaly, substantially fibrous, substantially spheroidal, substantially spherical, etc.).

The h-BN itself (single layer) has a network-like hexagonal lattice structure, but the crystalline particles themselves may be a single layer of h-BN, or a laminate or aggregate thereof (agglomerate, secondary particle). Therefore, the crystalline particles referred to herein do not necessarily have to be scaly.

《Amorphous particles》
The amorphous particles may have any size or shape as long as they can adhere to the surface of the crystalline particles. Amorphous particles are usually smaller than crystalline particles. In this specification, the size of particles is determined based on the maximum length that can be determined from the observed image described above.

The maximum length of each amorphous particle attached to a crystalline particle is, for example, 1/1000 to 1/5, 1/500 to 1/10, or 1/200 to 1/2 of the maximum length of the crystalline particle. It is about 20. To put it bluntly, the maximum length of the amorphous particles may be, for example, about 0.1 to 30 μm, 0.5 to 15 μm, or even about 1 to 5 μm. Although the amorphous particles can have various shapes, for example, the amorphous particles are scaly. Of course, the amorphous particles may have other shapes (substantially fibrous, substantially spheroidal, substantially spherical, etc.).

Incidentally, the degree of crystallinity of boron nitride (distinguishing between crystalline particles and amorphous particles) is determined from the X-ray diffraction (XRD) profile. The profile of crystalline particles has a peak, while the profile of amorphous particles does not have a clear peak, and the entire profile has a broad (halo pattern) shape.

《Magnetic particles》
The magnetic particles are supported on the amorphous particles and contribute to forming heat conduction paths between the crystalline particles or controlling the orientation of the composite particles. The existing form of the magnetic particles (arrangement within the composite particle) does not matter. The magnetic particles may be in contact with (further attached to, bonded with, etc.) the crystalline particles, as long as part of the magnetic particles is in contact with the amorphous particles. Furthermore, the magnetic particles may be partially exposed or may be entirely surrounded by amorphous particles and crystalline particles.

As the magnetic particles, various particles having different materials, characteristics, sizes, etc. can be used depending on the specifications of the thermally conductive material. The magnetic particles may be soft magnetic particles or hard magnetic particles, and are, for example, simple substances (such as pure Fe), alloys (such as FeNi alloys), or compounds (such as FeNi alloys) of ferromagnetic elements (such as iron group elements: Fe, Co, and Ni). ₃ O ₄ ). In particular, magnetic particles (eg, Fe particles) made of pure metal or alloy are preferred because they have high thermal conductivity.

The magnetic particles preferably have a size (particle diameter) that allows them to be easily supported by amorphous particles that are smaller than crystalline particles. For example, the maximum length of the magnetic particles is preferably 1 to 1000 nm, 10 to 500 nm, or even 50 to 250 nm. Note that the size of the magnetic particles and the size comparison with other particles are performed based on the arithmetic mean value of the maximum length within the field of view described above. Particles having a particle size of 1 μm or less determined in this manner are referred to as "nanoparticles" in this specification.

The magnetic particles may be contained in an amount of, for example, 5 to 20% by volume, or more preferably 7 to 15% by volume, based on the entire composite particles (total amount of crystalline particles, amorphous particles, and magnetic particles). Considering the total amount of amorphous particles and magnetic particles, magnetic particles account for 20 to 60% by volume, and even 30 to 55% by volume (40 to 80% by volume, and even 45 to 70% by volume for amorphous particles). Good to include. If there are too few magnetic particles, the effect will be poor. If there are too many magnetic particles, there will be relatively too few crystalline particles or amorphous particles, and the thermal conductivity may decrease. Furthermore, if there are too many highly conductive magnetic particles (such as iron particles), the insulation properties (specific resistance) of the thermally conductive material may decrease.

The composite particles may include a mixture of multiple types of magnetic particles having different materials, properties, particle sizes, and the like. Different types of particles other than magnetic particles may be supported on the amorphous particles. Examples of foreign particles include carbon particles. Examples of carbon particles include nanocarbon particles (CNT, etc.), graphite particles (including carbon black), diamond particles, and the like. Such particles can serve as auxiliary particles that can improve the thermal conductivity, heat resistance, etc. of the composite particles.

《Composite material》
The thermally conductive material may be a composite material (material, raw material, member, etc.) consisting of a filler made of composite particles and a matrix (including a binder) that fixes the filler.

(1) Filler The filling rate of the filler in the matrix is, for example, 60 to 87% by volume, and more preferably 65 to 85% by volume, based on the entire composite material. The thermal conductivity of a composite material can vary depending on its filling rate, but even if the filling rate is increased too much, the thermal conductivity does not improve much. The filling rate of the filler is specified at the time of manufacturing based on the true density and blending amount of the raw materials. The filling rate of the filler in the composite material may be calculated and specified from the observed image of the composite material (cross section) as described above.

All or part of the filler (composite particles, magnetic particles) may be subjected to surface treatment to increase affinity with the matrix. This improves the dispersibility, filling properties, adhesion, etc. of the filler in the matrix, and can improve the thermal conductivity and/or specific resistance of the composite material.

The surface treatment is, for example, a hydrophobic treatment or a coupling treatment. If the matrix is an organic material (resin, rubber, elastomer, etc.), the filler may be subjected to surface treatment such as silane coupling treatment or fluorine plasma treatment. The silane coupling treatment can be performed using various silane coupling agents having reactive groups corresponding to the functional groups (amino group, epoxy group, isocyanate group, vinyl group, acrylic group, etc.) on the matrix side. A typical silane coupling agent is, for example, hexamethyldisilazane (HMDS: C ₆ H ₁₉ NSi ₂ ). Note that silane coupling agents usually have reactive groups (such as silyl groups, etc.) that also correspond to functional groups (hydroxy groups, methoxy groups, ethoxy groups, etc.) on the side of fillers (composite particles, magnetic particles, etc.) that are inorganic materials. ).

The content (compounding 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, based on 100 parts by mass of the entire untreated filler. Part by mass. If too little of the surface treatment agent is used, the effect will be poor, and if too much of the surface treatment agent is used, the effect will not improve much.

In addition to surface-treating the filler before mixing (including kneading), a surface-treating agent (such as a coupling agent) may be blended or added at the time of mixing the matrix and filler.

(2) Matrix The matrix (including the binder) is made of, for example, an insulating organic material. Specifically, the matrix is usually a resin, rubber, elastomer, or the like. The resin may be a thermosetting resin or a thermoplastic resin. Examples of the thermosetting resin include epoxy resin, phenol resin, and silicone resin. Examples of the thermoplastic resin include polystyrene, polymethyl methacrylate, polycarbonate, and polyphenylene sulfide. Examples of the rubber include ethylene-propylene-diene rubber (EPDM) and butyl rubber. In this specification, unless otherwise specified, the term "resin" includes rubber and elastomer.

"Production method"
(1) Composite particles Various methods for producing composite particles can be considered. Composite particles are obtained, for example, through a firing process in which a mixture of crystalline particles of boron nitride, melamine borate (complex or salt), and magnetic particles is heated. Specifically, for example, some or all of the following steps may be performed.

The mixture is obtained, for example, by mixing boron nitride powder (h-BN powder, etc.), boric acid melamine powder, and magnetic powder (eg, nano iron powder, carbonyl iron powder, etc.). Such mixing is performed using a ball mill, a vibration mill, a V-type mixer, etc. (mixing step). At this time, it is preferable that h-BN particles in a laminated (or agglomerated) state can also be pulverized.

When adding boron nitride powder (and also magnetic powder) directly to the prepared solution of melamine borate, the solvent (usually water) may be removed (drying) before or after mixing (and after filtration). The solvent can be removed, for example, by vacuum drying (evaporation) (drying step). Note that the solvent may be removed, for example, at room temperature (eg, 10 to 40°C).

The dried mixture may be fired as it is, or a molded product obtained by pressure molding the mixture may be fired. The molded body is obtained, for example, by molding the mixture with a mold, CIP (cold isostatic pressing), RIP (rubber isostatic pressing), etc. (molding process) ). Note that it is sufficient that the molded body has a shape that allows it to be crushed after firing. It is sufficient that the molding pressure is such that a molded product that can be handled is obtained, for example, about 5 to 500 MPa, or more preferably about 30 to 100 MPa.

When the mixture (including its molded body) is heated, for example, in a vacuum or in an inert gas, a fired body (sintered body) is obtained. The heating temperature may be, for example, 1600°C to 2000°C, 1700°C to 1900°C, or further 1750°C to 1850°C. The heating time may be, for example, 0.2 to 3 hours, or more preferably 0.5 to 2 hours. Note that the above-described molding and firing may be performed simultaneously by HIP (Hot Isostatic Pressing).

A powder made of composite particles (referred to as "composite powder") is obtained by pulverizing the fired body, for example, in the air (powdering step). Note that the sintered body can be crushed using a small crusher, a crusher, or the like.

The particle size of the composite powder may be classified, for example, by sieving into 1 to 100 μm, or even 1 to 53 μm. The average particle size (median diameter: D50) of the powder may be adjusted to, for example, 5 to 45 μm, or further 16 to 22 μm.

(2) Composite material A composite material in which a filler is dispersed (filled) in a matrix can be obtained by, for example, compression molding, injection molding, transfer molding, or the like. Composite materials formed in a magnetic field can exhibit excellent thermal conductivity by controlling the orientation of composite particles. In addition, if the crystalline particles are h-BN, the direction in which the orienting magnetic field is applied (orientation direction) is, for example, the direction in which h-BN is oriented along its crystal plane (002) (usually the a-axis direction <100>). It is good to say.

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

Incidentally, melamine borate is produced by various known methods. For example, when a heated aqueous solution of boric acid and melamine is cooled (left to cool), a white powder is precipitated. When the white powder thus obtained is dehydrated and dried, anhydrous melamine borate (C ₃ H ₆ N ₆ .2H ₃ BO ₃ ) is obtained.

《Application》
A composite material in which a filler containing composite particles is dispersed in a matrix can exhibit desired thermal conductivity and insulation depending on the material of the matrix and the filling rate of the filler. The thermal conductivity of the composite material can be, for example, 13 to 60 W/mK, 15 to 40 W/mK, or even 18 to 35 W/mK. The specific resistance of the composite material can be, for example, 10 to 10 ⁸ Ωm, 10 ² to 10 ⁷ Ωm, or even 10 ³ to 10 ⁶ Ωm. The composite material may be used for, for example, a substrate, a case, a heat dissipation member, etc. of electronic equipment, or a part thereof.

Various fillers using crystalline particles made of h-BN were prepared, and composite materials were manufactured by filling each filler into a resin matrix. The structure (tissue) of each filler was observed, and the thermal conductivity and specific resistance of each composite material were evaluated. The present invention will be described below with reference to such specific examples.

《Production of filler》
Three types of fillers (Samples 1 to 3) were prepared. FIG. 1 schematically shows each filler (model). The filler of Sample 1 consists only of h-BN crystalline particles. The filler of Sample 2 was composed of composite particles in which amorphous particles of BN were attached to crystalline particles. The filler of sample 3 consists of particles obtained by modifying the composite particles of sample 2 with nano-sized (maximum length 1 μm or less) iron particles (referred to as "nano iron particles"). These nano-iron particles correspond to the magnetic particles in the present invention.

(1) Sample 1 (crystalline particles)
Commercially available h-BN powder (Denka boron nitride powder SGP manufactured by Denka Corporation) was used as the filler (crystalline particles) of Sample 1 (20 g). This powder had a BN purity of 99% or more, a particle size of 18 μm (D50), and a crystallinity (GI value) of 0.9. Hereinafter, the h-BN powder was used as a crystalline particle source.

(2) Sample 2 (crystalline particles + amorphous particles)
A filler consisting of composite particles was produced using h-BN powder and melamine borate powder as raw materials.

First, melamine borate powder was prepared as follows. Boric acid (commercially available reagent: 24 g) was poured into pure water (800 ml) heated to 95° C., and thoroughly stirred to completely dissolve it. Melamine (16 g of a commercially available reagent) was added to this boric acid aqueous solution and completely dissolved in the same manner. This mixed aqueous solution was water-cooled to about 25° C., and then further dehydrated using a vacuum suction filter. The obtained residue was vacuum dried (40°C x 12 hours) in a vacuum drying oven. When the white powder thus obtained was analyzed by XRD, it was identified as anhydrous melamine borate (C ₃ H ₆ N ₆ .2H ₃ BO ₃ ).

Next, h-BN powder (16 g) and melamine borate powder (20 g) were mixed together with acetone (160 g) in a ball mill (12 hours) (mixing step). After filtering the obtained mixed powder, the residue was dried under vacuum at room temperature (evaporation) (drying step). Filtration and drying were performed to volatilize organic solvents such as acetone.

Furthermore, the filtered and dried mixed powder was subjected to CIP molding. CIP molding was performed by placing the mixed powder in a double-layered vinyl chloride bag. In this way, a molded block (approximately 50 mm x 10 mm) was obtained. The molding pressure at this time was 3 t/cm ² (294 MPa).

The molded lump was placed in a heating furnace and the pressure was reduced, and then heated (1800° C. x 1 hour) under a nitrogen gas flow (firing step). The obtained fired body was crushed for 0.2 hours using a tabletop crusher (pulverization step). The pulverized material was sieved and classified to have a particle size of 1 to 53 μm. In this way, a filler consisting of composite particles was obtained. This filler (100 volume %) corresponded to crystalline particles: 80 volume % and amorphous particles: 20 volume % when converted from their respective densities and blended masses.

(3) Sample 3 (crystalline particles + amorphous particles + nano iron particles)
A filler consisting of composite particles modified with nano-iron particles was produced using h-BN powder, melamine borate powder, and nano-iron particle powder (referred to as "nano-iron powder") as raw materials. Nano Iron (average particle size: 25 nm) manufactured by QSI was used as the nano iron powder (magnetic particle source). It was confirmed using a laser diffraction particle size distribution analyzer (LMS-2000E, manufactured by Seishin Enterprise Co., Ltd.) that each particle of the nano iron powder had a maximum length of 40 nm or less (less than 1 μm).

The filler was manufactured in the same manner as Sample 2 except that h-BN powder (16 g), melamine borate powder (10 g), and nano iron powder (7 g) were weighed and blended. In this way, a filler consisting of composite particles modified with nano-iron particles was obtained. This filler (100% by volume) corresponded to crystalline particles: 80% by volume, amorphous particles: 10% by volume, and nano-iron particles: 10% by volume.

《Production of composite materials》
Composite materials were manufactured in which each filler was dispersed in a matrix. The filling rate of each filler was 70% by volume with respect to the entire composite material (100% by volume) unless otherwise specified. A one-component heat-curable epoxy resin (EP160 manufactured by Cemedine Co., Ltd./hereinafter simply referred to as "resin") was used as the matrix (binder). Specifically, the composite material was manufactured as follows.

Each filler and resin were kneaded in a plastic container for 10 minutes. The vacuum-dried kneaded material was crushed to obtain a compound in which filler particles were coated with resin. This compound was filled into a mold and compression molded in a uniaxial direction. At this time, the molding pressure was 15 MPa and the mold temperature was 120°C. The thus obtained cylindrical composite molded body (φ14 mm×20 mm) was heated in the air (120° C.×30 minutes) to obtain a composite material in which the filler was dispersed in the cured resin. In this example, composite materials using fillers of sample 1, sample 2, and sample 3 are referred to as sample M1, sample M2, and sample M3 in this order.

"observation"
(1) SEM/EDX
Each filler particle of Samples 1 to 3 was observed using a scanning electron microscope (SEM). Sample 3 was also subjected to elemental analysis by energy dispersive X-ray analysis (EDX). The observed images are shown in FIGS. 2 and 3.

(2) XRD
The filler particles of Sample 1 and Sample 2 were subjected to X-ray diffraction analysis (XRD/Cu-Kα). The obtained diffraction patterns are summarized in FIG. 4.

"measurement"
(1) Thermal conductivity The thermal conductivity (λ) of the composite material was determined by the nanoflash method. Specifically, λ= Thermal conductivity was calculated as α・Cp・ρ. For the measurement of thermal diffusivity, a thin plate-shaped sample cut out from a cylindrical composite material in a direction perpendicular to the axial direction (pressing direction) was used. In this way, the thermal conductivity was determined assuming that the pressing direction of the composite material (usually a direction substantially perpendicular to the orientation direction) was the heat transfer direction.

(2) Specific resistance The specific resistance of each composite material was measured by the DC four-probe method at room temperature using the above disk-shaped sample.

The thermal conductivity and specific resistance of each sample obtained are shown together in FIGS. 5 and 6. Further, FIG. 7 shows the relationship between filler filling rate and thermal conductivity based on sample M3.

"evaluation"
(1) Structure (form)
As is clear from Figure 2, it can be confirmed that the particles of sample 2 have smaller particles (maximum length of about 1 to 10 μm) attached to the surface of the h-BN particles (particles of sample 1). Ta. As is clear from FIG. 4, it was also found that the particles of Sample 1 were crystalline particles, and the small particles observed in Sample 2 were amorphous particles.

As is clear from Figure 3, in the particles of sample 3 (composite particles), nano iron particles (magnetic particles) are attached to the surface of crystalline particles (h-BN particles) via amorphous BN particles. (Sample 3).

As is clear from FIG. 4, in the composite material (sample M3) in which the filler is composite particles carrying nano-iron particles, amorphous particles and nano-iron particles are present in the gaps between adjacent crystalline particles (h-BN particles). The particles were interposed and the particles were in close contact with each other. In other words, it was confirmed that adjacent crystalline particles were connected by amorphous particles and nano-iron particles, forming a heat conduction path. Note that by applying a magnetic field during molding of the composite material of sample M3, it is also possible to orient the crystalline particles in a desired direction.

In the composite material containing only crystalline particles as filler (sample M1), each crystalline particle is oriented in a certain direction (substantially perpendicular to the pressurizing direction), but the resin blocks the gaps between each crystalline particle ( It was in a state of being divided. In other words, it was found that contact between adjacent crystalline particles was inhibited by the resin, making it difficult to form a heat conduction path between them.

(2) Characteristics As is clear from Figure 5, the composite material using a filler in which composite particles support nano-iron particles (sample M3) is different from the composite material using a filler consisting only of crystalline particles (sample M1). Thermal conductivity was significantly improved compared to

As is clear from FIG. 6, it was also found that the composite material of sample M3 exhibited a large specific resistance of 10 ⁴ Ωm despite containing highly conductive nano-iron particles. This is considered to be because the nano-iron particles are surrounded by crystalline particles and amorphous particles, which are electrical insulators, within the composite particles.

In any case, by using composite particles as a filler in which amorphous particles and nano iron particles (magnetic particles) are supported on the surface of crystalline particles (h-BN), thermal conductivity and specific resistance can be improved to a high level. It was confirmed that a composite material compatible with both can be obtained.

(3) Filling rate As is clear from Figure 7, the thermal conductivity of the composite material increases depending on the filling rate of the filler, but when the filling rate approaches 90% by volume, the thermal conductivity tends to decrease. I also found out that it happens.

《Consideration》
As can be seen from the above results, the thermal conductivity and specific resistance of the composite material can be significantly improved when composite particles made of boron nitride crystalline particles and amorphous particles with magnetic particles supported are used as a filler. I understand. Furthermore, as shown in Figure 8, the dramatic improvement in thermal conductivity is due to the formation of a thermal conduction path by connecting highly thermally conductive crystalline particles with amorphous particles and magnetic particles. It is thought that percolation occurred.

Claims (9)

Composite particles having crystalline particles of boron nitride, amorphous particles of boron nitride smaller than the crystalline particles and attached to the surface of the crystalline particles, and magnetic particles supported on the amorphous particles ,
The magnetic particles are a thermally conductive material containing iron particles . The thermally conductive material according to claim 1, wherein the amorphous particles are contained in an amount of 5 to 20% by volume based on the total amount of the composite particles. The thermally conductive material according to claim 1 or 2, wherein the magnetic particles are contained in an amount of 5 to 20% by volume based on the total amount of the composite particles. The thermally conductive material according to any one of claims 1 to 3, wherein the amorphous particles are contained in an amount of 40 to 80% by volume based on the total amount of the amorphous particles and the magnetic particles. The thermally conductive material according to any one of claims 1 to 4, wherein the iron particles are nanoparticles having a particle size of 1 μm or less. The thermally conductive material according to any one of claims 1 to 5 , which is a composite material in which the filler containing the composite particles is dispersed in a matrix. The thermally conductive material according to claim 6 , wherein a filling rate of the filler is 60 to 87% by volume based on the entire composite material. 8. The thermally conductive material according to claim 6 , wherein the filler is surface-treated to increase its affinity with the matrix. comprising a firing step of heating a mixture of crystalline particles of boron nitride, melamine borate, and magnetic particles;
A manufacturing method for obtaining the thermally conductive material according to any one of claims 1 to 8 .

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