JP2022078711A - Heat conductive material and manufacturing method thereof - Google Patents

Abstract

To provide a heat conductive material capable of achieving a dramatic increase of heat conductivity.SOLUTION: A heat conductive material includes a composite particle having a crystalline particle of boron nitride and an amorphous particle of boron nitride smaller than the crystalline particle and adhered on a surface of the crystalline particle. A composite material can be obtained by using a filler having the composite particle as a skeleton. The amorphous particle preferably carries an auxiliary particle (such as a carbon nano-tube) having larger heat conductivity than the crystalline particle. The composite particle can be obtained, for instance, through a calcination process that heats a mixture of a crystalline particle of boron nitride (such as h-BN) and melamine borate. A use of a hydrophobing filler can further help improve the heat conductivity and specific resistance of the composite material.SELECTED DRAWING: Figure 10

Description

The present invention relates to a heat conductive material or the like using boron nitride.

Due to structural changes associated with high density and high performance of electronic devices (semiconductor modules, etc.), electrically insulating materials with high heat dissipation that are different from conventional materials have come to be used. For example, instead of a substrate made of ceramics (AlN or the like) having high thermal conductivity, a composite material in which an insulating material (filler) having high thermal conductivity is dispersed in a resin is used for an insulating sheet or the like. Such (organic / inorganic) composite materials are excellent in moldability, processability, adhesiveness to dissimilar materials, etc., and are relatively inexpensive.

By the way, various ceramic particles (including fibers) are used as fillers for composite materials. For example, particles such as silica (SiO ₂ ), alumina (Al ₂ O ₃ ), and aluminum nitride (Al N). However, silica and alumina have low thermal conductivity, and aluminum nitride reacts with water ( _H2O ) to generate ammonia ( _NH3 ), so that the moisture resistance is low and the long-term reliability is inferior. Therefore, boron nitride (BN), which has high thermal conductivity and high electrical insulation and is chemically stable, is often used as a filler for composite materials.

Boron nitride generally includes a hexagonal normal pressure phase (also referred to as “h-BN” as appropriate) and a cubic high pressure phase (also referred to as “c-BN” as appropriate). Hexagonal boron nitride (h-BN) is usually used as a filler.

h-BN is composed of scaly layers in which hexagonal mesh layers similar to graphite are laminated, and has thermal conductivity anisotropy in which the thermal conductivity differs greatly in the plane direction and the thickness direction. Further, the h-BN particles are likely to be oriented in the plane direction in the resin to be filled. Therefore, for example, even if h-BN particles are highly filled, the heat dissipation (thermal conductivity) in the thickness direction (in the case of a sheet-shaped composite material, the direction from the heat source side to the cooling source side) tends to be insufficient. there were.

Therefore, various proposals have been made to enhance the thermal conductivity (heat dissipation) of the composite material containing h-BN particles, and for example, there is a description related to the following patent document.

JP-A-59-64355 JP-A-2010-260225 JP 2011-184507 JP 2012-171842 Japanese Patent Application Laid-Open No. 2012-255055 JP-A-2018-159062

However, all the patent documents only use a filler made of crystalline h-BN.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a heat conductive material or the like using new boron nitride particles having a structure different from that of the conventional one.

As a result of diligent research to solve this problem, the present inventor has obtained the heat of the composite material by using the composite particles in which the amorphous particles of boron nitride are attached to the crystalline particles of boron nitride as the skeleton of the filler. Succeeded in increasing the conductivity. By developing this result, the present invention described below was completed.

《Heat conductive material》
The present invention
It is a heat conductive material containing composite particles having crystalline particles of boron nitride and amorphous particles of boron nitride that are smaller than the crystalline particles and adhere to the surface of the crystalline particles.

By using the composite particles according to the present invention, it is possible to obtain a heat conductive material (including a composite material) having a high heat conductivity. The reason for this is not clear, but it can be thought of as follows. Amorphous particles in composite particles can carry more substances (auxiliary particles) having high thermal conductivity than crystalline particles. Therefore, it is considered that the heat conduction path is formed between the adjacent crystalline particles via the amorphous particles and the auxiliary particles, and the heat conductivity of the composite material is remarkably improved.

<< Manufacturing method of heat conductive material >>
The present invention is also understood as a method for manufacturing a heat conductive material. For example, the present invention comprises a firing step of heating a mixture of boron nitride crystalline particles and melamine borate, and adheres amorphous boron nitride particles smaller than the crystalline particles to the surface of the crystalline particles. It may be a method of manufacturing a heat conductive material which obtains the composite particles which have been made.

<< Composite / heat conductive member >>
The present invention is also grasped as a composite material or a heat conductive member which is a form of a heat conductive material. For example, the heat conductive material of the present invention may be a composite material in which a filler containing composite particles is dispersed in a matrix. Further, the heat conductive material (composite material) of the present invention may be a heat conductive member such as a heat radiating member, a substrate, or a case.

"others"
(1) The "-material" as used herein means a "material" or a "member". For example, the heat conductive material may be a material (raw material) composed of composite particles or powder thereof, composite particles containing auxiliary particles or powder thereof. Further, the heat conductive material may be a tangible composite material (including a material) having these particles or powder and a base material (matrix) or a binder (binder), or a composite member having a desired shape.

(2) Unless otherwise specified, "x to y" in the present specification include a lower limit value x and an upper limit value y. A range such as "a to b" may be newly established with any numerical value included in the various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value. As used herein, "x to yμm" means xμm to yμm unless otherwise specified. The same applies to other unit systems (W / mK, Ωm, etc.).

It is a model figure which shows typically each filler particle used in an Example. It is an SEM image which observed the filler particle of a sample 1 and a sample 2. It is an SEM image which observed the filler particle of a sample 3 and a sample C. It is an SEM image which observed the composite material of the sample M1 and the sample M3. It is an X-ray diffraction pattern which concerns on the filler particle of a sample 1 and a sample 2. It is a bar graph which shows the thermal conductivity of the composite material of each sample. It is a scatter diagram which shows the relationship between specific resistance and thermal conductivity about the composite material of each sample. It is a graph which shows the influence which the filling rate of a filler has on the thermal conductivity based on the composite material of a sample M31. 3 is a graph showing the effect of the volume ratio of amorphous particles and auxiliary particles on thermal conductivity based on the composite material of sample M3. It is a model figure which shows schematically the composite material which concerns on this invention.

One or more components arbitrarily selected from the present specification may be added to the components of the present invention. The contents described in the present specification appropriately correspond not only to the heat conductive material (including the filler, the composite material, and the member) but also to the manufacturing method thereof and the like. Even a methodical component can be a component related to an object. Which embodiment is the best depends on the target, required performance, and the like.

《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 adhesion of the amorphous particles may be any of chemical bonds (including van der Waals bonds), sintering, adhesion and the like. It should be noted that all of the particles adhering to the surface of the crystalline particles do not have to be amorphous particles. That is, some of the particles attached to the crystalline particles may be crystallized.

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

The volume ratio (% by volume) of each particle is specified from the volume ratio (calculated from the blending amount and density) of the raw material at the time of preparing the composite particle. The volume ratio of auxiliary particles, the volume ratio of each particle to the entire composite material, and the like can also be specified by the same method. When the amorphous particles are obtained from melamine borate, the volume of the amorphous particles becomes 1/5 of the volume of melamine borate by the firing step. Therefore, it is advisable to increase the blending volume of melamine borate to 5 times the desired volume of the amorphous particles. The larger the volume ratio of the amorphous particles, the broader the peak of the X-ray diffraction pattern can be (see FIG. 5).

《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 crystalline particles.

The size and morphology of crystalline particles do not matter. The size of the crystalline particles composed of h-BN has, for example, a maximum length of 1 to 100 μm, 10 to 60 μm, and further 20 to 40 μm. The maximum length can be obtained, for example, from an observation image of a microscope (for example, an SEM image) of crystalline particles. If you dare to say it, the size of the crystalline particles may be the average value obtained by arithmetically calculating the maximum lengths of the crystalline particles existing in one field of view (1500 μm × 1000 μm). Such specification of particle size also corresponds to amorphous particles, composite particles, auxiliary particles and the like as used 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 long spherical, substantially spherical, etc.).

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

《Amorphous particles》
The amorphous particles may be of any size and morphology that can be attached to the surface of the crystalline particles. Amorphous particles are usually smaller than crystalline particles. In this specification, the size of the particles is determined based on the maximum length that can be specified from the above-mentioned observation image.

The maximum length of each amorphous particle adhering to the crystalline particles is, for example, 1/1000 to 1/5, 1/500 to 1/10, and further 1/200 to 1/1 of the maximum length of the crystalline particles. It is about 20. Suffice it to say, the maximum length of the amorphous particles may be, for example, 0.1 to 30 μm, 0.5 to 15 μm, or even 1 to 5 μm. Amorphous particles can have various shapes, for example, amorphous particles are scaly. Of course, the amorphous particles may have other shapes (substantially fibrous, substantially long spherical, substantially spherical, etc.).

Incidentally, the degree of crystallinity of boron nitride (distinguishing between crystalline particles and amorphous particles) is determined from the profile of X-ray diffraction (XRD). There are peaks in the profile of crystalline particles, no clear peaks can be seen in the profile of amorphous particles, and the whole is broad (halo pattern).

《Auxiliary particles》
The heat conductive material may contain auxiliary particles that are supported on the amorphous particles and contribute to the formation of a heat conduction path between the crystalline particles. As auxiliary particles, various particles having different sizes, thermal conductivity, etc. can be used depending on the specifications of the heat conductive material and the like. Auxiliary particles should be at least smaller than crystalline particles. Further, the auxiliary particles may be smaller than the amorphous particles. It is preferable that the auxiliary particles have a higher thermal conductivity than the crystalline particles and / or the amorphous particles.

An example of auxiliary particles is carbon particles. Examples of carbon particles include graphite particles (including carbon black), diamond particles, and nanocarbon particles. Examples of the nanocarbon particles include carbon nanotubes (CNT), carbon nanohorns (CNH), fullerenes, graphene and the like. CNTs are superior to other carbon particles in that they have high thermal conductivity, an aspect ratio of 10 or more, and easily form a thermal conduction path even in a small amount.

The carbon particles have, for example, a maximum length of 0.001 to 5 μm and even 0.01 to 2 μm. The magnitude comparison with other particles may be performed based on the above-mentioned arithmetic mean value of the maximum length in the field of view.

The auxiliary particles may be contained, for example, in an amount of 5 to 20% by volume, further 7 to 15% by volume, based on the total amount of crystalline particles, amorphous particles and auxiliary particles. Looking at the total amount of amorphous particles and auxiliary particles, auxiliary particles are 20 to 60% by volume, further 30 to 55% by volume (40 to 80% by volume, even 45 to 70% by volume for amorphous particles). It should be included. If the number of auxiliary particles is too small, the effect will be poor. If there are too many auxiliary particles, the number of crystalline particles and amorphous particles will be relatively small, and the thermal conductivity may decrease. Further, if the amount of auxiliary particles (carbon particles or the like) having high conductivity becomes excessive, the specific resistance (electric resistivity) of the heat conductive material may decrease.

《Composite material》
The heat conductive material may be a composite material (material or member) composed of a filler composed of composite particles (including auxiliary particles) and a matrix (including a binder; hereinafter simply referred to as “matrix”) for fixing the filler. ..

(1) The filling rate of the filler in the filler matrix is, for example, 50 to 90% by volume and further 60 to 80% by volume with respect to the entire composite material. The thermal conductivity of the composite material changes depending on the filling rate, but even if the filling rate is excessively increased, the thermal conductivity does not improve so much. The filling rate of the filler is specified from the blending amount of the raw materials at the time of production, and the filling rate of the filler in the composite material is calculated and specified from the observation image of the composite material (cross section) as described above.

All or part of the filler (composite particles, auxiliary particles) may be surface-treated to enhance the affinity with the matrix. As a result, the dispersibility, filling property, adhesion and the like of the filler in the matrix can be improved, and the thermal conductivity and / or the specific resistance of the composite material can be improved.

The surface treatment is, for example, a hydrophobizing treatment or a coupling treatment. If the matrix is an organic material (resin, rubber, elastomer, etc.), surface treatment such as silane coupling treatment or fluorine plasma treatment may be performed on the filler. The silane coupling treatment can be carried out 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, there is, for example, hexamethyldisilazane (HMDS: C ₆ H ₁₉ NSi ₂ ). The silane coupling agent usually has a reactive group (silyl group, etc.) corresponding to a functional group (hydroxyki group, methoxy group, ethoxy group, etc.) on the filler (composite particle, auxiliary particle, etc.) side which is an inorganic material. ).

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 1 to 100 parts by mass with respect to 100 parts by mass of the untreated filler. 2 parts by mass. 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.

In addition to subjecting the filler before mixing (including kneading) to a surface treatment in advance, a surface treatment agent (coupling agent or the like) may be added or added when the matrix and the filler are mixed.

(2) Matrix The matrix (including the binder) is made of, for example, an organic material having an insulating property. Specifically, a resin, rubber, elastomer, or the like is usually used as a matrix. The resin may be a thermosetting resin or a thermoplastic resin. The thermosetting resin is, for example, an epoxy resin, a phenol resin, a silicone resin, or the like. The thermoplastic resin is, for example, polystyrene, polymethylmethacrylate, polycarbonate, polyphenylene sulfide and the like. The rubber is, for example, ethylene-propylene-diene rubber (EPDM), butyl rubber, or the like. In the present specification, unless otherwise specified, it is simply referred to as "resin" including rubber and elastomer.

"Production method"
(1) Composite particles Various methods for producing composite particles composed of crystalline particles and amorphous particles can be considered. For example, composite particles are obtained through a firing step of heating a mixture of boron nitride crystalline particles and melamine borate (complex or salt). More specifically, for example, it is preferable that a part or all of the following steps are performed.

The mixture is obtained by mixing, for example, boron nitride powder (h-BN powder or the like) and melamine borate powder. Such mixing is performed using a ball mill, a vibration mill, a V-type mixer, or the like (mixing step). At this time, it is preferable that the laminated h-BN particles can also be pulverized.

When the boron nitride powder is added directly to the prepared solution of melamine borate, the solvent (usually water) may be removed before or after the mixing (and even after filtration). The solvent can be removed by, for example, vacuum drying (evaporation) (drying step). The solvent may be removed, for example, in a normal temperature range (for example, 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 product is obtained, for example, by molding the mixture, CIP (Cold Isostatic Pressing) molding, RIP (Rubber Isostatic Pressing) molding, or the like (molding step). ). It is sufficient that the molded body has a shape that can be crushed after firing. The molding pressure is also sufficient to obtain a molded body that can be handled, and may be, for example, about 50 to 500 MPa and further about 100 to 300 MPa.

When the mixture (including its molded product) is heated, for example, in vacuum or in an inert gas, a calcined product (sintered product) is obtained. The heating temperature may be, for example, 1600 ° C to 2000 ° C, 1700 to 1900 ° C, and further 1750 to 1850 ° C. The heating time may be, for example, 0.3 to 3 hours or even 0.7 to 2 hours. The above-mentioned molding and firing may be performed at the same time by HIP (Hot Isostatic Pressing).

By pulverizing the fired body in an atmospheric atmosphere, for example, a powder composed of composite particles (referred to as “composite powder”) can be obtained (powdering step). The fired body can be crushed using a small crusher, a crusher, or the like.

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

(2) Composite material The composite material in which the filler is dispersed (filled) in the matrix is formed by, for example, compression molding, injection molding, transfer molding, or the like. When the matrix is made of a thermosetting resin, it is preferable that a thermosetting treatment (cure treatment) is performed after molding. The composite material may be in the shape of a final product or a shape close to it, or may be a material to be post-processed or an intermediate material.

By the way, melamine borate is produced by various known methods. For example, when a heated aqueous solution of boric acid and melamine is cooled (cooled), a white powder is precipitated. When the white powder thus obtained is dehydrated and dried, anhydrous borate melamine (C _{3H 6} N _{6.2H 3} BO ₃ ) is obtained.

《Use》
A composite material in which a filler containing composite particles is dispersed in a matrix can exhibit desired thermal conductivity and insulating properties depending on the material of the matrix and the filling rate of the filler. Therefore, for example, a substrate or a case of an electronic device or the like. , Heat dissipation member, etc., or a part of them. The thermal conductivity of the composite material can be, for example, 13 to 60 W / mK, 15 to 40 W / mK, and further 18 to 35 W / mK. The specific resistance of the composite material can be, for example, 10 to 10 ⁴ Ωm and further 10 ² to 10 ³ Ωm.

Various fillers using crystalline particles made of h-BN were prepared, and a composite material in which each filler was filled in a resin as a matrix was produced. The structure (structure) of each filler was observed, and the thermal conductivity and resistivity of each composite material were evaluated. Hereinafter, the present invention will be described with reference to such specific examples.

《Manufacturing of filler》
Five types of fillers (Samples 1 to 3, Sample 31 and Sample C) were prepared. FIG. 1 schematically shows each filler (model). The filler of Sample 1 consists only of crystalline particles of h-BN. The filler of sample 2 is composed of composite particles in which amorphous particles of BN are attached to the crystalline particles. The filler of sample 3 is composed of particles obtained by further modifying the composite particles with carbon nanotubes (CNT). Sample C is composed of particles obtained by modifying crystalline particles with CNT. The filler of the sample 31 is composed of particles obtained by subjecting the particles of the sample 3 to a hydrophobic treatment.

(1) Sample 1 (crystalline particles)
Commercially available h-BN powder (Denkaboron nitride powder SGP manufactured by Denka Co., Ltd.) 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 the crystalline particle source.

(2) Sample 2 (composite particles)
Using melamine borate powder and h-BN powder as raw materials, a filler composed of composite particles was produced.

First, the borate melamine powder was prepared as follows. Boric acid (commercially available reagent: 24 g) was added to pure water (800 ml) heated to 95 ° C., and the mixture was sufficiently stirred to completely dissolve it. Melamine (commercially available reagent 16 g) was added to this boric acid aqueous solution and completely dissolved in the same manner. The mixed aqueous solution was cooled to about 25 ° C. and then dehydrated with a vacuum suction filter. The obtained residue was vacuum dried (40 ° C. × 12 hours) in a vacuum drying oven. When the white powder thus obtained was analyzed by XRD, it was identified as anhydrous borate melamine (C _{3H 6} N _{6.2H 3} BO ₃ ).

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

Further, the filtered and dried mixed powder was CIP molded. CIP molding was carried out by placing the mixed powder in a double vinyl chloride bag. In this way, a molded ingot (about 50 mm × 10 mm) was obtained. The molding pressure at this time was 3 t / cm ² (294 MPa).

The molded mass was placed in a heating furnace to reduce the pressure, and then heated (1800 ° C. × 1 hour) under a nitrogen gas flow (firing step). The obtained fired body was crushed with a tabletop crusher for 0.2 hours (crushing step). The pulverized material was sieved and classified to have a particle size of 1 to 53 μm. In this way, a filler composed of composite particles was obtained. This filler (100% by volume) corresponds to crystalline particles: 80% by volume and amorphous particles: 20% by volume when converted from the respective densities and blending masses.

(3) Sample 3 (composite particles + auxiliary particles)
Using melamine borate powder, h-BN powder and CNT powder as raw materials, a filler composed of composite particles modified with CNT (particles) was produced. NC7000 (average diameter: 9.5 nm, average length: 1.5 μm) manufactured by Nanocyl was used as the CNT powder. Hereinafter, the CNT powder was used as the auxiliary particle source.

It was produced in the same manner as the filler of Sample 3 except that h-BN powder (16 g), melamine borate powder (10 g) and CNT powder (2 g) were weighed and blended. In this way, a filler composed of composite particles modified with CNTs (auxiliary particles) was obtained. This filler (100% by volume) corresponded to crystalline particles: 80% by volume, amorphous particles: 10% by volume, and CNTs: 10% by volume.

(4) Sample 31 (composite particles + auxiliary particles + hydrophobizing treatment)
A filler in which the filler of sample 3 was hydrophobized (silane coupling treatment) was also produced using hexamethyldisilazane (HMDS / SZ-31 manufactured by Shin-Etsu Chemical Co., Ltd.). HMDS was added in an amount of 5 parts by mass with respect to 100 parts by mass of the filler. Specifically, in the hydrophobizing treatment, toluene and HMDS were stirred and mixed (60 ° C. × 5 hr), and then dried in a room temperature vacuum drying oven for 12 hr. In this way, a filler obtained by hydrophobizing the composite particles modified with CNT was obtained.

(5) Sample C (crystalline particles + auxiliary particles)
Using h-BN powder and CNT powder as raw materials, a filler composed of crystalline particles modified with CNT was produced. It was produced in the same manner as the filler of sample 3 except that h-BN powder (18 g) and CNT powder (2 g) were weighed and blended. In this way, a filler composed of crystalline particles modified with CNTs (auxiliary particles) was obtained. This filler (100% by volume) corresponded to crystalline particles: 90% by volume and CNT: 10% by volume.

<< Production of composite materials >>
A composite material in which each filler was dispersed in a matrix was produced. Unless otherwise specified, the filling rate of each filler was set to 70% by volume with respect to the entire composite material (100% by volume). For the matrix (binder), a one-component heat-curable epoxy resin (EP160 / hereafter manufactured by Cemedine Co., Ltd., simply referred to as "resin") was used. Specifically, the composite material was produced as follows.

Each 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 filler particles were coated with a resin. This compound was filled in a mold and compression molded in the uniaxial direction. At this time, the molding pressure was 15 MPa and the mold temperature was 120 ° C. In this way, a columnar composite molded body (φ14 mm × 20 mm) was obtained. The composite molded product was heated in an atmospheric atmosphere (120 ° C. × 30 minutes) to cure the resin. In this way, a composite material in which the filler was dispersed in the resin was obtained. In this embodiment, the composite material using each filler of Sample 1, Sample 2, Sample 3, Sample 31, and Sample C is referred to as Sample M1, Sample M2, Sample M3, Sample M31, and Sample MC in this order. As a reference sample, a composite material containing only CNT powder as a filler was also produced in the same manner (Sample MT).

"observation"
(1) SEM
Each filler particle of Samples 1 to 3 and Sample C was observed with a scanning electron microscope (SEM). The observed images are shown in FIGS. 2 and 3. Further, FIG. 4 shows an observation image of the cross section of each composite material of the sample M1 and the sample M3 observed by SEM.

(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.

"measurement"
(1) Thermal conductivity The thermal conductivity (λ) of the composite material was determined by the nanoflash method. Specifically, from the thermal diffusivity (α) measured by the nanoflash method, the specific heat (Cp) determined by the differential scanning calorimeter (DSC), and the density (ρ) determined by the Archimedes method, λ = The thermal conductivity was calculated as α, Cp, and ρ. For the measurement of the thermal diffusivity, a thin plate-shaped sample cut out from the columnar composite material in the direction perpendicular to the axial direction (pressurization direction) was used. In this way, the thermal conductivity when the pressurizing direction of the composite material (usually the direction substantially orthogonal to the orientation direction) is assumed to be the heat transfer direction is obtained.

(2) Specific resistance The specific resistance of each composite material was measured by the DC four-terminal method in the room temperature range using the above-mentioned disc-shaped sample.

The thermal conductivity and resistivity of each of the obtained samples are summarized in FIGS. 6 and 7. Further, FIG. 8 shows the relationship between the filling rate of the filler and the thermal conductivity based on the sample M31. Further, FIG. 9 shows the relationship between the volume ratio of the amorphous particles and the auxiliary particles (CNT) and the thermal conductivity based on the sample M3. In the case of FIG. 9, the filling rate of the filler with respect to the entire composite material: 70% by volume and the ratio of the crystalline particles to the entire filler: 80% by volume were unchanged. That is, the volume ratio of the amorphous particles and the CNTs was changed while keeping the total ratio of the amorphous particles and the CNTs at 20% by volume with respect to the entire filler.

"evaluation"
(1) Structure (form)
As is clear from FIG. 2, the particles of sample 2 are composite particles in which smaller particles (maximum length of about 1 to 10 μm) are attached to the surface of h-BN particles (particles of sample 1). It could be confirmed. As is clear from FIG. 5, it was also found that the particles of sample 1 are crystalline particles and the small particles observed in sample 2 are amorphous particles.

As is clear from FIG. 3, on the surface of the composite particle, a large amount of CNT (auxiliary particle) is supported on the surface and the side surface of the crystalline particle (h-BN particle) via the amorphous particle of BN. I found it (Sample 3). On the other hand, it was also found that CNT adhered only slightly to the side surface of the crystalline particles of BN (Sample C).

As is clear from FIG. 4, in the composite material (sample M3) in which the composite particles carrying CNTs are used as fillers, the amorphous particles and CNTs are interposed in the gaps between the adjacent crystalline particles (h-BN particles). The particles were in close contact with each other. That is, it was confirmed that the adjacent spaces of the crystalline particles were connected to the amorphous particles by CNTs to form a heat conduction path. Further, in the composite material of the sample M3, the orientation of the crystalline particles was not constant, and the orientation thereof was low.

On the other hand, in the composite material (Sample M1) containing only crystalline particles as a filler, each crystalline particle is oriented in a certain direction (direction substantially orthogonal to the pressurizing direction), and a resin is formed between the crystalline particles. It was in an intervening state. That is, it was confirmed that the contact between the adjacent crystalline particles was blocked by the resin, and it was difficult to form a heat conduction path between them.

(2) Characteristics As is clear from FIG. 6, the composite material (samples M3 and M31) using the filler in which the CNT is supported on the composite particles is the composite material (sample M1) using the filler consisting only of crystalline particles. The thermal conductivity was significantly improved by about 21 to 43% as compared with the above. Further, although the composite material of the sample MC contained CNT having a high thermal conductivity, the thermal conductivity was lower than that of the sample M1. From these facts, it was found that the presence of amorphous particles in the filler particles causes many auxiliary particles (CNTs) to be supported on the surface of the composite particles, thereby significantly improving the thermal conductivity.

As is clear from FIG. 7, it was also found that the composite material of the samples M3 and M31 showed a specific resistance close to that of the composite material of the samples M1 and M2 containing no CNT, although it contained the highly conductive CNT. .. It was also found that the composite material of the samples M3 and M31 had a higher specific resistance than the composite material of the sample MC containing no amorphous particles (the amount of CNT was the same).

Further, as can be seen from the comparison between the sample M3 and the sample M31 shown in FIGS. 6 and 7, it was also found that the thermal conductivity and the specific resistance (electric resistivity) are further improved by using the hydrophobized filler. rice field.

(3) Filling rate As is clear from FIG. 8, the thermal conductivity of the composite material increases according to the filling rate of the filler, but when the filling rate exceeds around 70% by volume, the increasing tendency gradually increases. I also found that it would be.

(4) Ratio of amorphous particles and auxiliary particles As is clear from FIG. 9, the amount of amorphous particles is 40 to 80% by volume and further 45 to 75% by volume with respect to the total amount of auxiliary particles (CNT). It was found that a composite material having high thermal conductivity can be obtained by using the filler particles.

<< Consideration >>
As can be seen from the above results, it was found that the thermal conductivity of the composite material can be significantly improved by using a filler having a boron nitride composite particle as a skeleton. As shown in FIG. 10, such a dramatic improvement in thermal conductivity is a result of the heat conduction path being formed by connecting the crystalline particles with high heat conductivity with the amorphous particles and the auxiliary particles. It is probable that the percoration of.

Claims (12)

A heat conductive material containing composite particles having boron nitride crystalline particles and amorphous boron nitride particles smaller than the crystalline particles and adhering to the surface of the crystalline particles. The heat 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 crystalline particles and the amorphous particles. The heat conductive material according to claim 1 or 2, further comprising auxiliary particles supported on the amorphous particles, which have a higher thermal conductivity than the crystalline particles. The heat conductive material according to claim 3, wherein the auxiliary particles include carbon particles. The heat conductive material according to claim 4, wherein the carbon particles include carbon nanotubes. The heat conductive material according to any one of claims 3 to 5, wherein the auxiliary particles are contained in an amount of 5 to 20% by volume based on the total amount of the crystalline particles, the amorphous particles and the auxiliary particles. The heat conductive material according to any one of claims 3 to 6, 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 auxiliary particles. The heat conductive material according to any one of claims 1 to 7, which is a composite material in which a filler containing the composite particles is dispersed in a matrix. The heat conductive material according to claim 8, wherein the filling rate of the filler is 50 to 90% by volume with respect to the entire composite material. The heat conductive material according to claim 8 or 9, wherein the filler is surface-treated to enhance the affinity with the matrix. It comprises a firing step of heating a mixture of boron nitride crystalline particles and melamine borate.
A method for producing a heat conductive material for obtaining composite particles in which amorphous particles of boron nitride smaller than the crystalline particles are adhered to the surface of the crystalline particles. The method for producing a heat conductive material according to claim 11, wherein the mixture further contains auxiliary particles having a higher thermal conductivity than the crystalline particles and being carried on the amorphous particles.

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