JP2024060280A - Thermally conductive organic-inorganic composite material with high inorganic filler loading - Google Patents

Abstract

[Problem] The object of the present invention is to provide a method for achieving both high density and high thermal conductivity by increasing the frequency of contact between particles to form a heat transfer path without simply increasing the content of insulating inorganic filler, without surface treatment or special treatment of filler particles, or without pressure molding. [Solution] The organic inorganic heat dissipation material of the present invention is an organic inorganic heat dissipation material composed of two types of insulating granular inorganic fillers with different particle sizes, which are not particles such as plate-like, scaly, fibrous, or rod-like, and a matrix resin, in which the volume ratio of the blended amounts is insulating granular thermally conductive inorganic filler:matrix resin=50:50-60:40, the particle size of the larger insulating granular thermally conductive inorganic filler is 200-500 μm, the particle size of the smaller insulating granular thermally conductive inorganic filler is 2-30 μm, and the volume ratio of the two types of insulating granular thermally conductive inorganic fillers mixed is large particles:small particles=5:1-1:1. [Selected Figure] Figure 2

Description

The present invention relates to an organic-inorganic composite material that has a high degree of contact between insulating inorganic fillers and is suitable as a heat dissipation material that also has thermal isotropy.

Organic-inorganic composites for heat dissipation applications are composed of resin and inorganic substances with high thermal conductivity, and much research has been conducted on increasing the thermal conductivity of the resin itself, as well as the shape, dispersibility, surface coating, orientation, and percolation of the inorganic fillers.
Furthermore, with the electrification of automobiles, there is a demand for heat countermeasures for batteries, motors, communication devices, electronic devices, home appliances, etc. to be made thinner and lighter in order to improve convenience, and there is an increasing need for high-performance heat dissipation materials incorporated into such devices.

In light of the above situation, various organic-inorganic composite heat dissipation materials that contain resin and inorganic substances with high thermal conductivity have been developed (Patent Documents 1 and 2, etc.).

Whether it is a ceramic sintered body or an organic-inorganic composite, it is important to form a heat transfer path to improve thermal conductivity. When the amount of inorganic filler contained in the organic-inorganic composite is increased to 30, 40, and 50 vol%, the thermal conductivity increases, but this is largely due to the effect of increased contact between inorganic fillers and the formation of more heat transfer paths, rather than simply the effect of an increase in the amount of inorganic filler.

However, because hexagonal boron nitride is a plate-shaped crystal with structural anisotropy, it is difficult to achieve high packing. For example, even if the filler particles are ideal spheres, they can only be packed at a maximum of about 60-70%. With plate-shaped crystals, facets are formed, which reduces the packing degree. If these gaps are filled with resin, the density of the composite itself can be prevented from decreasing, but the degree of contact between the thermally conductive inorganic fillers decreases, making it impossible to ensure a heat transfer path.

Since actual fillers are polyhedral, the particles contact each other at points or surfaces, but when considering ideal spheres, there is only one point of contact between two spheres, regardless of their size (radius). When a thermally conductive inorganic filler with a small particle size is used, the number of interparticle junctions increases and it is possible to reduce the gaps between the particles, but this is not suitable for increasing the thermal conductivity of the composite because the thermal conductivity of the particles themselves is low. Conversely, in the case of a thermally conductive inorganic filler with a large particle size, the thermal conductivity of the particles themselves is higher than that of small particle fillers, but the number of interparticle junctions is low and the gaps between the particles are large, making it impossible to form many heat transfer paths.

In order to compensate for the above-mentioned drawbacks and to increase the filler loading and ensure a heat transfer path, it has been proposed to fill the material with a mixed filler containing two or more types of fillers with different shapes (Patent Documents 3 to 5).
Patent Document 3 discloses a thermally conductive molding composition using two types of thermally conductive fillers with different aspect ratios.
Patent Document 4 discloses a thermally conductive resin composition containing a thermally conductive inorganic filler having a shape or size different from that of a fibrous thermally conductive inorganic filler.

JP 2018-159062 A JP 2016-79353 A JP 2002-535469 A JP 2022-059896 A

The object of the present invention is to provide a method for achieving both high density and high thermal conductivity by increasing the frequency of contact between particles to form a heat transfer path, rather than simply increasing the content of insulating inorganic filler, without surface treatment, special treatment of the filler particles, or pressure molding.

To achieve the above objective, a heat transfer path is secured by filling two or more types of insulating granular inorganic filler with different particle sizes. In other words, by filling the area near the contact points of larger particles with smaller particles, the frequency of contact between the particles is increased (Figure 1). It was found that this achieves a higher density and thermal conductivity than mixing one type of inorganic filler at the same volume fraction, and this led to the completion of the present invention. In other words, the present invention is characterized by the following configuration and solves the above problems.

[1] An organic inorganic heat dissipation material composed of two types of insulating granular inorganic fillers having different particle sizes and a matrix resin, the volume ratio of the blended amounts being insulating granular thermally conductive inorganic filler:matrix resin=50:50 to 60:40, the particle size of the large insulating granular thermally conductive inorganic filler being 200 to 500 μm, the particle size of the small insulating granular thermally conductive inorganic filler being 2 to 30 μm, the large insulating granular thermally conductive inorganic filler being cubic boron nitride, the small insulating granular thermally conductive inorganic filler being any one of hexagonal boron nitride, silicon nitride, aluminum nitride, synthetic diamond, nanodiamond, aluminum oxide, magnesium oxide, and talc, and the mixing volume ratio of the two types of insulating granular thermally conductive inorganic fillers being large particles:small particles=5:1 to 1:1.
[2] The organic/inorganic heat dissipation material according to [1], wherein the matrix resin is any one of a phenolic resin, a polyimide, an epoxy resin, a urea resin, a melamine resin, a silicone resin, and a cellulose nanofiber.
[3] A method for producing an organic/inorganic heat dissipating material composed of two types of insulating granular inorganic fillers with different particle sizes and a resin, comprising the steps of: blending a large-particle insulating granular inorganic filler, cubic boron nitride, with a small-particle insulating granular thermally conductive inorganic filler, hexagonal boron nitride, silicon nitride, aluminum nitride, synthetic diamond, nanodiamond, aluminum oxide, magnesium oxide, or talc, in a volume ratio of 5:1 to 1:1 (step 1); mixing and kneading the insulating granular inorganic filler blended in step 1 with a matrix resin in a volume ratio of insulating granular thermally conductive inorganic filler:matrix resin = 50:50 to 60:40 (step 2); and casting the kneaded product obtained in step 2 into a molding container and heating and curing it (step 3).

According to the present invention, it is possible to provide an organic-inorganic composite that has high density and high thermal conductivity by filling the spaces between larger inorganic filler particles with smaller inorganic filler particles without pressure molding or filler surface treatment.

FIG. 1 is a schematic diagram illustrating the concept of a thermally conductive organic-inorganic composite according to an embodiment of the present invention. 1 is a secondary electron image of a sample surface of a lightweight organic/inorganic heat dissipation material produced in an embodiment of the present invention. 1 is a graph showing the compounding ratio, measured density, and thermal conductivity of all organic/inorganic heat dissipation materials produced in the examples and comparative examples of the present invention. 1 is a graph showing the relationship between the filler volume fraction and thermal conductivity obtained from the Bruggeman model formula. (The thermal conductivity of the resin was fixed at 0.2 W/m/K, and the thermal conductivity of the filler was calculated at 20, 80, and 200 W/m/K. Since the particle size of the filler is not reflected in this model formula, the filler thermal conductivity was used to distinguish the results.)

The thermally conductive organic-inorganic composite of the present invention is an organic-inorganic heat dissipation material composed of two types of insulating granular inorganic fillers with different particle sizes and a matrix resin, in which the volume ratio of the blended amounts is insulating granular thermally conductive inorganic filler:matrix resin = 50:50 to 60:40, the larger insulating granular thermally conductive inorganic filler is cubic boron nitride, the smaller insulating granular thermally conductive inorganic filler is any one of hexagonal boron nitride, silicon nitride, aluminum nitride, synthetic diamond, nanodiamond, aluminum oxide, magnesium oxide, and talc, and the mixed volume ratio of the two types of insulating granular thermally conductive inorganic fillers is large particles:small particles = 5:1 to 1:1.

The large and small insulating granular thermally conductive inorganic fillers used in the present invention are all granular particles. The granular particles of the present invention are particles that are approximately spherical, approximately regular polyhedral, blocky, etc., with a ratio of the long axis to the short axis of the particle being 2 or less, and do not include plate-like, scale-like, fibrous, rod-like, etc. particles with a ratio of the long axis to the short axis of 2 or more.

The thermally conductive organic-inorganic composite of the present invention is a thermally conductive organic-inorganic composite having high density and high thermal conductivity, which is composed of either a thermosetting resin or a thermoplastic resin and an insulating granular thermally conductive inorganic filler, has a heat transfer path with improved contact frequency between the insulating granular thermally conductive inorganic fillers, and has a thermal conductivity of 5 W/m/K or more, preferably 6 W/m/K or more, and more preferably 10 W/m/K or more.

The particle size of the large insulating granular thermally conductive inorganic filler is 200 to 500 μm, preferably 300 to 450 μm. The particle size of the small insulating granular thermally conductive inorganic filler is 2 to 30 μm, preferably 3 to 20 μm.

The insulating granular thermally conductive inorganic filler is more difficult to mix or knead with the matrix resin as the amount of the filler is increased, and it is necessary to add an organic solvent before mixing or kneading. In addition, the filler must be packed in by applying pressure, such as by pressure molding, extrusion molding, injection molding, roll press molding, blade molding, or heat press molding. Therefore, if the filler is not sufficiently degassed, air bubbles may remain, and the filler particles may be defective or damaged by being pressed in, impairing their thermal properties, and the thermal conductivity of the thermally conductive organic-inorganic composite may not increase as much as expected. In the present invention, by adding filler particles with significantly different particle sizes, the small particles fill the gaps between the large particles, and the insulating granular thermally conductive inorganic fillers have the advantage of being able to form a heat transfer path by increasing the frequency of contact between them.

The amount of insulating granular thermally conductive inorganic filler in the organic/inorganic heat dissipation material of the present invention is preferably a volume ratio of insulating granular thermally conductive inorganic filler:matrix resin = 50:50 to 60:40.

The Bruggeman model formula can be used as a model formula for predicting the thermal conductivity of a composite system. This formula assumes that the filler has a spherical shape, and can calculate the thermal conductivity relative to the volume fraction of the filler in the composite. The Bruggeman model formula is as follows:
1-V=(lambda _c -lambda _f )/(lambda _m -lambda _f )*(lambda _m /lambda _c ) ^1/3
Here, V is the volume of the filler, λ _c is the thermal conductivity of the composite, λ _f is the thermal conductivity of the filler, and λ _m is the thermal conductivity of the matrix resin.

According to the Bruggeman model formula mentioned above, it is difficult to achieve high thermal conductivity when the amount of thermally conductive inorganic filler added is 50 vol% or less, and the effect of the thermal conductivity of the thermally conductive inorganic filler is also small. When a large amount of thermally conductive inorganic filler is added, such as 70 vol% or 80 vol%, the thermal properties of the thermally conductive inorganic filler become dominant (Figure 4), but there is an overwhelming shortage of resin required for dispersion and kneading, so solvent must be added, and the process becomes complicated by the addition of a desolvation process in the subsequent process, and there is a concern that the strength of the composite will decrease due to the small amount of resin, so it can be said that the effective limit for the amount of thermally conductive inorganic filler added is around 60 vol%.

The size of the insulating granular thermally conductive inorganic filler used in the present invention is such that the particle size of the large insulating granular thermally conductive inorganic filler is 200 to 500 μm, preferably 300 to 450 μm, and the particle size of the small insulating granular thermally conductive inorganic filler is 2 to 30 μm, preferably 3 to 20 μm. The mixing volume ratio of the two types of insulating granular thermally conductive inorganic fillers is preferably large particles:small particles=5:1 to 1:1.
By mixing two types of particles according to the above-mentioned compounding ratio, rather than adding the insulating granular thermally conductive inorganic filler having the particle size described above alone, the interparticle contacts are increased, and an organic/inorganic heat dissipation material having high thermal conductivity can be obtained.

Examples of the resin include thermosetting resins such as phenol resin, polyimide resin, epoxy resin, urea resin, melamine resin, and silicone resin, and thermoplastic resins such as polyvinyl alcohol, polyvinyl butyral, and acrylic resin.

The thermally conductive organic-inorganic composite material of the present invention can be molded into various shapes of heat dissipation materials, for example, from thin plates to bulk bodies, by changing the shape of the mold used during casting. The thermally conductive organic-inorganic composite material of the present invention has a thickness of 1 mm or more, preferably 2 mm or more. The upper limit of the thickness can be manufactured without restriction according to the thickness of the bulk material used, but is usually several tens of centimeters or less.

Figure 1 is a schematic diagram explaining the concept of a thermally conductive organic-inorganic composite according to an embodiment of the present invention. In the diagram, one large particle (○) corresponds to 100 small particles (●). The diagram shows that the number of contacts obtained when one large particle (○) is replaced with 100 small particles (●) is greater than the number of contacts obtained when four single fillers are mixed with resin. The insulating granular thermally conductive inorganic filler forms a network of heat transfer paths by filling the gaps between the large particles with small particles according to the concept shown in Figure 1, thereby achieving high thermal conductivity.

Figure 2 shows secondary electron images of a bulk organic/inorganic heat dissipation material observed from the normal direction. The top row in the figure is an SEI image, and the bottom row is an LEI image; the LEI image shows the unevenness of the sample surface even more clearly. As per the concept in Figure 1, small particles fill the gaps between larger particles, making the sample surface containing the small particles smooth, and it can be confirmed that a heat transfer path is formed.

Next, a method for producing the thermally conductive organic-inorganic composite material of the present invention will be described.
First, two types of insulating granular inorganic filler having different particle sizes are uniformly mixed and blended in a volume ratio of larger particle size insulating granular inorganic filler to smaller particle size insulating granular thermally conductive inorganic filler of 5:1 to 1:1 (step 1).
Next, the insulating granular inorganic filler and the matrix resin blended in step 1 are mixed and kneaded in a volume ratio of insulating granular thermally conductive inorganic filler:matrix resin=50:50 to 60:40 (step 2).
The kneaded product obtained in step 2 is poured into a molding vessel and heated and hardened (step 3).

The resins used are phenolic resin, polyimide, epoxy resin, urea resin, melamine resin, and silicone resin for thermosetting resins, and polyvinyl alcohol, polyvinyl butyral, acrylic resin, etc. for thermoplastic resins.

When mixing filler and resin, the following methods can be used depending on the type of resin. Dissolve the resin in a solvent and add the filler before mixing (resole-type phenolic resin, polyvinyl alcohol, polyvinyl butyral, acrylic resin, etc.). Add the filler to liquid resin and mix (urea resin, melamine resin, etc.). Add the filler to raw monomer material with added hardener and mix (polyimide, epoxy resin, silicone resin, etc.). It is preferable to mix the resin and filler by kneading in order to thoroughly blend the filler and resin.

The resulting mixture (kneaded product) is poured into a mold for molding, and heated to harden the resin, allowing heat dissipating materials of various shapes to be produced by cast molding.
The mold used for the slip casting may be made of any of Teflon (registered trademark), silicone, and metal, so long as it can withstand heat treatment at 150° C. or higher and is releasable.

By using the above-described manufacturing method, the gaps between the large-particle insulating granular thermally conductive inorganic filler particles are filled with the small-particle insulating granular thermally conductive inorganic filler particles, thereby forming a heat transfer path, and thus an organic/inorganic heat dissipation material exhibiting high thermal conductivity can be produced.
In other words, the manufacturing method of the organic/inorganic heat dissipating material of the present invention utilizes insulating granular thermally conductive inorganic fillers of different sizes, so that the particles are filled without any gaps as shown in the conceptual diagram of Figure 1, thereby forming an internal structure as shown in Figure 2, and it is presumed that this makes it possible to produce an organic/inorganic heat dissipating material with higher thermal conductivity than an organic/inorganic heat dissipating material composed of only one type of insulating granular thermally conductive inorganic filler.

(Measurement of bulk density of heat dissipation material)
The bulk density of the heat dissipating materials prepared in the examples and comparative examples was calculated by Archimedes' method.
(Thermal conductivity of heat dissipation material)
The thermal conductivity of the heat dissipating materials prepared in the examples and comparative examples was calculated by evaluating the thermal diffusivity using LFA467 manufactured by Netzsch Japan and by the following formula (1).
(Thermal conductivity) = (Thermal diffusivity) × (Specific heat) × (Density) (1)
The specific heat in formula (1) is a physical property value inherent to a substance, and was therefore calculated using the alligation rule.
The density is an actually measured density.

Example 1
3.48g of cubic boron nitride (Global Diamond Co., Ltd.: FBN-300), 0.45g of hexagonal boron nitride, and 0.92g of epoxy resin were weighed out so that the volume ratio of cubic boron nitride (Denka Co., Ltd.: SGPS grade) to epoxy resin was 50:10:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and hexagonal boron nitride was added thereto and allowed to blend well. Next, the mixture of the cubic boron nitride and hexagonal boron nitride mixed powder and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. The mixture was then cured in a dryer at 155° C. for 1 hour to obtain a pellet-shaped heat dissipation material, which had a measured density of 1.90 g/cm ³ and a thermal conductivity of 10.57 W/m/K.

Example 2
2.78 g of cubic boron nitride (Global Diamond Co., Ltd.: FBN-300), 0.90 g of hexagonal boron nitride, and 0.92 g of epoxy resin were weighed out so that the volume ratio of cubic boron nitride (Global Diamond Co., Ltd.: SGPS grade) to hexagonal boron nitride (Denka Co., Ltd.: SGPS grade) and epoxy resin was 40:20:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and hexagonal boron nitride was added thereto and allowed to blend well. Next, the mixture of the cubic boron nitride and hexagonal boron nitride mixed powder and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. The mixture was then cured in a dryer at 155° C. for 1 hour to obtain a pellet-shaped heat dissipation material, which had a measured density of 1.81 g/cm ³ and a thermal conductivity of 8.95 W/m/K.

Example 3
2.09g of cubic boron nitride (Global Diamond Co., Ltd.: FBN-300), 1.36g of hexagonal boron nitride, and 0.92g of epoxy resin were weighed out so that the volume ratio of cubic boron nitride (Global Diamond Co., Ltd.: SGPS grade), epoxy resin was 30:30:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and hexagonal boron nitride was added thereto and allowed to blend well. Next, the mixture of the cubic boron nitride and hexagonal boron nitride mixed powder and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. The mixture was then cured in a dryer at 155° C. for 1 hour to obtain a pellet-shaped heat dissipating material, which had a measured density of 1.83 g/cm ³ and a thermal conductivity of 6.56 W/m/K.

Example 4
3.48g of cubic boron nitride (Global Diamond Co., Ltd.: FBN-300), 0.65g of aluminum nitride, and 0.92g of epoxy resin were weighed out so that the volume ratio of cubic boron nitride (Furukawa Electronics Co., Ltd.: AlN-f05-A01 grade) and epoxy resin was 50:10:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and aluminum nitride was added thereto and allowed to blend well. Next, the mixture of cubic boron nitride, aluminum nitride mixed powder, and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. It was then cured in a dryer at 155°C for 1 hour to obtain a pellet-shaped heat dissipation material. The measured density was 2.61 g/cm ³ , and the thermal conductivity was 11.65 W/m/K.

Example 5
2.78 g of cubic boron nitride (Global Diamond Co., Ltd.: FBN-300), 1.30 g of aluminum nitride, and 0.92 g of epoxy resin were weighed out so that the volume ratio of cubic boron nitride (Furukawa Electronics Co., Ltd.: AlN-f05-A01 grade) and epoxy resin was 40:20:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and aluminum nitride was added thereto and allowed to blend well. Next, the mixture of cubic boron nitride, aluminum nitride mixed powder, and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. It was then cured in a dryer at 155°C for 1 hour to obtain a pellet-shaped heat dissipation material. The measured density was 2.68 g/cm ³ , and the thermal conductivity was 10.13 W/m/K.

Example 6
2.09g of cubic boron nitride (Global Diamond Co., Ltd.: FBN-300), 1.96g of aluminum nitride, and 0.92g of epoxy resin were weighed out so that the volume ratio of cubic boron nitride (Furukawa Electronics Co., Ltd.: AlN-f05-A01 grade) and epoxy resin was 30:30:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and aluminum nitride was added thereto and allowed to blend well. Next, the mixture of cubic boron nitride, aluminum nitride mixed powder, and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. It was then cured in a dryer at 155°C for 1 hour to obtain a pellet-shaped heat dissipation material. The measured density was 2.63 g/cm ³ , and the thermal conductivity was 7.67 W/m/K.

<Comparative Example 1 and Comparative Example 3>
Cubic boron nitride (Global Diamond Co., Ltd.: FBN-300) was weighed out at 4.18 g, and epoxy resin was weighed out at 0.92 g, so that the volume ratio of cubic boron nitride to epoxy resin was 60:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Rika Co., Ltd.: Rikacid MH-700G) mixed at a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and cubic boron nitride powder was added thereto and allowed to blend well. Next, the cubic boron nitride powder and epoxy resin mixture were filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. Then, the mixture was cured in a dryer at 155°C for 1 hour to obtain a pellet-shaped heat dissipation material.
The measured density of the obtained heat dissipation material was 1.97 g/cm ³ , and the thermal conductivity was 4.17 W/m/K.

<Comparative Examples 2 and 5>
4.18 g of cubic boron nitride and 0.92 g of epoxy resin were weighed out so that the volume ratio of hexagonal boron nitride (Denka Co., Ltd.: SGPS grade) and epoxy resin was 60:40. The epoxy resin was a total weight of a mixture of a base agent (Nippon Kayaku Co., Ltd.: GAN) and a hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) in a weight ratio of 10:8. First, the epoxy resin base agent and the hardener were mixed, and a mixed powder of cubic boron nitride and hexagonal boron nitride was added thereto and allowed to blend well. Next, the hexagonal boron nitride powder and epoxy resin mixture were filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. Then, the mixture was cured in a dryer at 155°C for 1 hour to obtain a pellet-shaped heat dissipation material. The measured density was 1.30 g/cm ³ and the thermal conductivity was 2.90 W/m/K.

<Comparative Example 4>
3.91g of aluminum nitride and 0.92g of epoxy resin were weighed out so that the volume ratio of aluminum nitride (Furukawa Electronics Co., Ltd.: AlN-f05-A01 grade) and epoxy resin was 60:40. The epoxy resin was a total weight of a mixture of a base agent (Nippon Kayaku Co., Ltd.: GAN) and a hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) in a weight ratio of 10:8. First, the epoxy resin base agent and hardener were mixed, and aluminum nitride powder was added thereto to allow them to blend well. Next, the mixture of aluminum nitride powder and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. It was then cured in a dryer at 155°C for 1 hour to obtain a pellet-shaped heat dissipation material. The measured density was 2.43g/cm ³ and the thermal conductivity was 3.47W/m/K.

<Comparative Example 6>
2.26 g of hexagonal boron nitride (Denka Co., Ltd.: SGPS grade), 0.45 g of hexagonal boron nitride (Showa Denko Co., Ltd.: UHP-2S grade), and 0.92 g of epoxy resin were weighed out so that the volume ratio of hexagonal boron nitride (Denka Co., Ltd.: SGPS grade), hexagonal boron nitride (Showa Denko Co., Ltd.: UHP-2S grade), and epoxy resin was 50:10:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and hexagonal boron nitride was added to it and allowed to blend well. Next, the mixture of hexagonal boron nitride, nanodiamond mixed powder, and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. The mixture was then cured in a dryer at 155° C. for 1 hour to obtain a pellet-shaped heat dissipation material, which had a measured density of 1.44 g/cm ³ and a thermal conductivity of 4.15 W/m/K.

<Comparative Example 7>
The volume ratio of hexagonal boron nitride (Denka Co., Ltd.: SGPS grade), hexagonal boron nitride (Showa Denko Co., Ltd.: UHP-2S grade), and epoxy resin was 40:20:40, so that 1.81 g of the former hexagonal boron nitride, 0.90 g of the latter hexagonal boron nitride, and 0.92 g of the latter epoxy resin were weighed. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and hexagonal boron nitride was added thereto and allowed to blend well. Next, the mixture of hexagonal boron nitride, nanodiamond mixed powder, and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. The mixture was then cured in a dryer at 155° C. for 1 hour to obtain a pellet-shaped heat dissipating material, which had a measured density of 1.40 g/cm ³ and a thermal conductivity of 3.73 W/m/K.

<Comparative Example 8>
The volume ratio of hexagonal boron nitride (Denka Co., Ltd.: SGPS grade), hexagonal boron nitride (Showa Denko Co., Ltd.: UHP-2S grade), and epoxy resin was 30:30:40, so that 1.36 g of the former hexagonal boron nitride, 1.36 g of the latter hexagonal boron nitride, and 0.92 g of epoxy resin were weighed. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed in a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and hexagonal boron nitride was added to it and allowed to blend well. Next, the mixture of hexagonal boron nitride, nanodiamond mixed powder, and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. The mixture was then cured in a dryer at 155° C. for 1 hour to obtain a pellet-shaped heat dissipation material. The measured density was 1.58 g/cm ³ and the thermal conductivity was 4.31 W/m/K.

<Comparative Example 9>
2.71 g of hexagonal boron nitride and 0.92 g of epoxy resin were weighed out so that the volume ratio of hexagonal boron nitride (Showa Denko K.K.: UHP-2S grade) to epoxy resin was 60:40. The epoxy resin was the total weight of the main agent (Nippon Kayaku Co., Ltd.: GAN) and the hardener (Shin Nippon Kayaku Co., Ltd.: Rikacid MH-700G) mixed at a weight ratio of 10:8. First, the epoxy resin main agent and the hardener were mixed, and the mixed powder of cubic boron nitride and hexagonal boron nitride was added to it and mixed well. Next, the mixture of hexagonal boron nitride, nanodiamond mixed powder, and epoxy resin was filled into a silicone rubber tablet molding mold with a diameter of 10 mmφ and a depth of 2 mm. It was then cured in a dryer at 155°C for 1 hour to obtain a pellet-shaped heat dissipation material. The measured density was 1.25 g/cm ³ and the thermal conductivity was 1.66 W/m/K.

The results of the examples and comparative examples are summarized in Table 1 in terms of the filler combinations, the volume ratio of the filler in the organic/inorganic heat dissipation material, the theoretical density, measured density, relative density, and thermal conductivity of the heat dissipation materials produced.

Figure 2 shows surface observation images (secondary electron images) of the organic-inorganic heat dissipation materials produced in Example 2 of the present invention and Comparative Examples 1 and 2. It can be seen that the surface becomes smooth when the large particles are replaced with small particles, and that the gaps between the large particles are filled.

Figure 3 is a graph plotting thermal conductivity versus filler composition ratio for all organic/inorganic heat dissipation materials prepared in the examples and comparative examples of the present invention. The thermal conductivity of the organic/inorganic heat dissipation materials (examples) in which large and small particles coexist exceeds that of organic/inorganic heat dissipation materials (comparative examples) composed of only large particles or only small particles, suggesting that the coexistence of large and small particles improves the filling degree, reduces voids, and forms more heat transfer paths than a single particle alone.

The organic inorganic heat dissipation material of the present invention is suitable as a heat dissipation material having isotropic high thermal conductivity for electronic devices, electric appliances, secondary batteries, and sliding members. In addition, by further evolving the conventional idea and method of preparing an organic inorganic heat dissipation material by mixing fillers of different sizes, and finding the optimal compounding ratio, a heat transfer path is secured by highly filling inorganic particles expected to have high thermal conductivity, and a thermal conductivity that is more than twice as high as that of an organic inorganic heat dissipation material prepared from a single type of filler can be achieved.

Claims (3)

An organic inorganic heat dissipation material composed of two types of insulating granular inorganic fillers with different particle sizes and a matrix resin, in which the volume ratio of the blended amounts is insulating granular thermally conductive inorganic filler:matrix resin = 50:50 to 60:40, the particle size of the large insulating granular thermally conductive inorganic filler is 200 to 500 μm, the particle size of the small insulating granular thermally conductive inorganic filler is 2 to 30 μm, the large insulating granular thermally conductive inorganic filler is cubic boron nitride, the small insulating granular thermally conductive inorganic filler is any one of hexagonal boron nitride, silicon nitride, aluminum nitride, synthetic diamond, nanodiamond, aluminum oxide, magnesium oxide, and talc, and the mixed volume ratio of the two types of insulating granular thermally conductive inorganic fillers is large particles:small particles = 5:1 to 1:1. The organic/inorganic heat dissipation material according to claim 1, wherein the matrix resin is any one of a phenolic resin, a polyimide, an epoxy resin, a urea resin, a melamine resin, a silicone resin, and a cellulose nanofiber. A method for producing an organic/inorganic heat dissipating material composed of two types of insulating granular inorganic filler with different particle sizes and a resin, the method comprising the steps of: blending a large-particle insulating granular inorganic filler, i.e., cubic boron nitride, with a small-particle insulating granular thermally conductive inorganic filler, i.e., hexagonal boron nitride, silicon nitride, aluminum nitride, synthetic diamond, nanodiamond, aluminum oxide, magnesium oxide, or talc, in a volume ratio of 5:1 to 1:1 (step 1); mixing and kneading the insulating granular inorganic filler blended in step 1 with a matrix resin in a volume ratio of insulating granular thermally conductive inorganic filler:matrix resin=50:50 to 60:40 (step 2); and casting the kneaded product obtained in step 2 into a molding container and heating and curing it (step 3).