Thermal Conductive Filler, Thermal Conductive Composite Material, Wire Harness, and Method for Manufacturing Thermal Conductive Filler

Abstract

To provide: a heat-conductive filler that is capable of exhibiting high heat conductivity and that has reduced specific gravity; a heat-conductive composite material and wire harness containing the heat-conductive filler; and a heat-conductive filler production method that enables production of such a heat-conductive filler.SOLUTION: A heat-conductive filler 10 comprises: base particles 11; and a covering layer 12 that covers the base particles 11. The covering layer 12 contains a gel substance 12a that is bonded to the surfaces of the base particles 11 through a chemical bond and that covers the surfaces of the base particles 11, and a heat-conductive substance 12b that is dispersed in layers of the gel substance 12a, and that has higher thermal conductivity and greater specific gravity than the base particles 11 and the gel substance 12a. In addition, in a heat-conductive composite material 1, the heat-conductive filler 10 is dispersed in a matrix material 2. Furthermore, the wire harness comprises the heat-conductive composite material 1.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a thermally conductive filler, a thermally conductive composite material, a wire harness, and a method for manufacturing a thermally conductive filler.

Background Art

[0002] In an insulating member constituting an electric and electronic component, a thermally conductive filler may be added to an organic polymer material for the purpose of enhancing heat dissipation and suppressing the influence of heat generation due to energization or the like. The thermally conductive filler is often composed of an inorganic compound having high thermal conductivity such as alumina, aluminum nitride, boron nitride, or the like.

[0003] In recent years, in various electric and electronic components including automotive electronics, large current and integration have been progressing, and the amount of heat generation during energization tends to increase. As a means of suppressing the influence of such heat generation, for example, in the case of an automotive wire harness, the wire is flattened to increase the surface area of the wire, or the wire is efficiently brought into contact with a highly thermally conductive exterior material, and thus improvement in heat dissipation by improving the shape and structure of the member has been promoted. On the other hand, increasing the thermal conductivity of the material itself that constitutes the insulating member of electric and electronic components such as wire coatings and wire exterior materials is also important for improving heat dissipation.

[0004] If a large amount of filler is mixed into an organic polymer material or the like, the thermal conductivity of the material can be increased. However, when a large amount of filler made of an inorganic compound is mixed into an organic polymer material, the specific gravity of the material increases, making it difficult to reduce the weight of the electric and electronic component. From the perspective of reducing the weight of the entire product such as an automobile, weight reduction is important in the electric and electronic components mounted on the product. Therefore, weight reduction is also desired in materials containing a thermally conductive filler. As a method therefor, attempts have been made to keep the addition amount of the filler small.

[0005] For the purpose of maintaining high thermal conductivity while suppressing the addition amount of the filler, improvements have been made regarding the shape and particle arrangement of the filler. For example, Patent Document 1 discloses a filler having voids inside and a porosity within a predetermined range. Patent Document 2 discloses an inorganic-organic composite composition in which boron nitride particles are dispersed in a resin as a matrix in a state of exfoliated flat particles generated through an exfoliation process of delaminating secondary particles, which are laminates of primary particles. Patent Document 3 discloses a highly thermally conductive composite in which highly thermally conductive fillers having anisotropy in shape are in direct contact with each other to form a network structure in a matrix resin.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Patent Document 9

Patent Document 10

Non-Patent Documents

[0007]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] Inorganic compounds typified by alumina, aluminum nitride, and boron nitride exhibit high thermal conductivity. On the other hand, they have a large specific gravity. When added to an organic polymer material or the like as a filler to form a composite material, it is difficult to achieve high thermal conductivity while keeping the specific gravity of the entire composite material small. In particular, fillers made of oxides such as alumina tend to have a large specific gravity. As described in Patent Documents 1 to 3, by devising the shape and particle arrangement of the filler, the amount of inorganic compound added can be reduced to some extent, but there is a limit. By examining the constituent materials of the filler, if the specific gravity of the filler itself can be reduced, there is a possibility of more highly achieving both weight reduction and high thermal conductivity in the composite material containing the filler.

[0009] For example, if a material with a small specific gravity such as hollow particles and a material with high thermal conductivity can be combined to form a filler, it may be possible to achieve both light weight and high thermal conductivity for the entire filler. Materials in which hollow particles such as glass are combined with other materials are disclosed in Patent Documents 8 to 10 and Non-Patent Document 1 and the like. However, it is not easy to firmly fix a layer of a substance responsible for heat conduction on the surface of particles of an inorganic compound such as glass and provide it with a thickness sufficient to exhibit thermal conductivity. If a heat-conductive substance can be bonded to the surface of hollow particles made of glass or the like via a chemical bond, it is expected that the layer of the heat-conductive substance can be firmly fixed to the hollow particles. However, the density of chemical bonds that can be directly formed on the surface of particles of an inorganic compound such as glass is limited, and the range affected by the chemical bond is limited to a very thin region at the interface between the particle and the heat-conductive substance.

[0010] Therefore, an object of the present invention is to provide a thermally conductive filler that can exhibit high thermal conductivity while keeping the specific gravity low, a thermally conductive composite material and a wire harness including such a thermally conductive filler, and a method for manufacturing a thermally conductive filler that can manufacture such a thermally conductive filler.

Means for Solving the Problems

[0011] The thermally conductive filler of the present disclosure has a base material particle and a coating layer that coats the base material particle. The coating layer is bonded to the surface of the base material particle via a chemical bond and coats the surface of the base material particle. The coating layer includes a gel-like substance and a thermally conductive substance that is dispersed in the layer of the gel-like substance and has a higher thermal conductivity and a larger specific gravity than the base material particle and the gel-like substance.

[0012] The thermally conductive composite material of the present disclosure includes the thermally conductive filler and a matrix material, and the thermally conductive filler is dispersed in the matrix material.

[0013] The wire harness of the present disclosure includes the thermally conductive composite material.

[0014] The method for manufacturing a thermally conductive filler of the present disclosure includes a gel preparation step of preparing the gel-like substance in a state where the thermally conductive substance is dispersed therein, and a coating step of bonding the gel-like substance in which the thermally conductive substance is dispersed, which is prepared in the gel preparation step, to the surface of the base material particle via a chemical bond, thereby manufacturing the thermally conductive filler.

Advantages of the Invention

[0015] The thermally conductive filler according to the present disclosure becomes a thermally conductive filler that can exhibit high thermal conductivity while keeping the specific gravity low. Further, the thermally conductive composite material and the wire harness according to the present disclosure include such a thermally conductive filler. According to the method for manufacturing a thermally conductive filler according to the present disclosure, such a thermally conductive filler can be manufactured.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0017] [Description of Embodiments of the Present Disclosure] First, the embodiments of the present disclosure will be listed and described.

[0018] The thermally conductive filler according to the present disclosure has a base material particle and a coating layer covering the base material particle, and the coating layer is bonded to the surface of the base material particle via a chemical bond and covers the surface of the base material particle. The coating layer includes a gel-like substance and a thermally conductive substance dispersed in the layer of the gel-like substance. The thermally conductive substance has a higher thermal conductivity and a larger specific gravity than the base material particle and the gel-like substance.

[0019] The above-mentioned heat-conductive filler has a coating layer containing a heat-conductive substance and a gel-like substance on the surface of the base material particles. By using a substance with a specific gravity smaller than that of the heat-conductive substance as the base material particles, the specific gravity of the entire filler can be made smaller compared to the case where the entire filler is composed of the heat-conductive substance. On the other hand, the heat-conductive substance contained in the coating layer, which has a higher thermal conductivity than the base material particles, contributes to the improvement of the thermal conductivity of the filler. The gel-like substance constituting the coating layer is a highly viscous substance having a cross-linked structure, and the heat-conductive substance can be stably held by being dispersed in the viscous body. The heat-conductive substance held by the gel-like substance forms a heat conduction path within the coating layer of a single filler particle, between adjacent filler particles, and between the filler particles and other materials surrounding the filler particles, contributing to heat conduction. In this way, even when using base material particles or gel-like substances with low thermal conductivity, due to the contribution of the heat-conductive substance held by the gel-like substance, high thermal conductivity can be exhibited by the entire heat-conductive filler particles. Therefore, in the heat-conductive filler, it is possible to ensure high thermal conductivity while keeping the specific gravity small.

[0020] Furthermore, since the gel-like substance constituting the coating layer is bonded to the surface of the base material particles via chemical bonds, the coating layer is firmly fixed to the surface of the base material particles. Since the gel-like substance forms a network by a cross-linked structure, combined with the effect of chemical bonds with the base material particles, the whole is firmly fixed to the base material particles along the thickness direction of the coating layer, and that state is stably maintained. Therefore, a layer of the gel-like substance having a thickness sufficient to hold the amount of heat-conductive substance necessary to obtain high thermal conductivity is firmly bonded to the surface of the base material particles, and a heat-conductive filler having a small specific gravity and high thermal conductivity can be stably obtained.

[0021] Here, the base material particles are preferably hollow. Then, due to the presence of the hollow portion, the specific gravity of the base material particles can be effectively reduced, and it is excellent in the effect of reducing the specific gravity of the entire heat conductive filler. Although the thermal conductivity of the base material particles becomes low due to the presence of the hollow portion, by providing a coating layer containing a heat conductive substance on the surface, high thermal conductivity can be ensured for the entire heat conductive filler.

[0022] The base material particles are configured as a hollow body of glass and preferably have, on the surface, a functional group capable of forming a chemical bond with the functional group of the gel substance. Hollow body particles of glass with controlled particle size and shape can be obtained relatively easily and inexpensively. In addition, since various functional groups can be easily introduced at high density on the surface of glass particles using a silane coupling agent, it is easy to be a base material particle capable of stably bonding the gel substance constituting the coating layer via a chemical bond.

[0023] The gel substance has a carbonyl group, the base material particles have a basic group on the surface, and the gel substance is preferably bonded to the surface of the base material particles via a chemical bond between the carbonyl group and the basic group. Many substances having a carbonyl group, such as polyacrylic acid, polyvinylpyrrolidone, and their derivatives, are known as those constituting a gel and can be suitably used as the gel substance constituting the coating layer. Since the carbonyl group can form a chemical bond with a basic group, by providing a basic group on the surface of the base material particles, the coating layer can be bonded to the base material particles via a chemical bond.

[0024] In this case, the gel substance preferably contains an organic polymer having a carboxyl group as the carbonyl group. As organic polymers having a carboxyl group, including polyacrylic acid, many substances that form stable gels are known and have high versatility, so they can be suitably used as the gel substance constituting the coating layer of the heat conductive filler.

[0025] The base material particles preferably have a primary amino group as the basic group on the surface. The primary amino group can form an ionic bond with an acidic carbonyl group such as a carboxyl group to form a salt. In addition, the primary amino group can form a covalent bond with a neutral carbonyl group such as a ketone group to form an amide bond or an imide bond. Therefore, by forming a primary amino group on the surface of the base material particles, a gel-like substance having a carbonyl group can be firmly bonded to the base material particles.

[0026] The heat conductive material is preferably configured as particles having anisotropy in shape. Since the heat conductive material has anisotropy in shape, it often exhibits high heat conductivity. However, if it is used as a heat conductive filler as it is, anisotropy occurs in heat conduction, and it becomes difficult to effectively contribute to the improvement of heat conductivity in an arbitrary direction. However, even such a highly anisotropic heat conductive material can be dispersed and held in a gel-like substance, making it difficult for anisotropy to occur in the orientation of the heat conductive material, and enabling it to contribute highly uniformly to heat conduction in each direction. As a result, it can be made into an excellent heat conductive filler that exhibits high heat conductivity in any direction.

[0027] In this case, the heat conductive material is preferably carbon fiber. Carbon fiber has high heat conductivity while having an elongated shape with high anisotropy. However, by forming a coating layer while being held in a gel-like substance, it can provide a heat conductive filler that exhibits high heat conductivity in any direction.

[0028] The heat conductive filler preferably has a specific gravity of 1.8 or less. Then, the low specific gravity of the heat conductive filler can be sufficiently ensured.

[0029] The heat conductive composite material according to the present disclosure includes the heat conductive filler and a matrix material, and the heat conductive filler is dispersed in the matrix material.

[0030] The above-mentioned thermally conductive composite material contains the thermally conductive filler according to the embodiment of the present disclosure described above. Since the thermally conductive filler has a coating layer containing a gel-like substance and a thermally conductive substance formed on the surface of the base material particles, while suppressing the specific gravity of the entire thermally conductive composite material to be small, the high thermal conductivity of the thermally conductive filler can be utilized to enhance the heat dissipation performance.

[0031] Here, the matrix material preferably includes an organic polymer. Many organic polymers have low thermal conductivity, but by mixing the above-mentioned thermally conductive filler having a coating layer containing a thermally conductive substance, high heat dissipation performance can be ensured for the entire thermally conductive composite material. On the other hand, many organic polymers have a relatively small specific gravity, but since the thermally conductive filler to be mixed has a suppressed specific gravity, the specific gravity of the thermally conductive composite material can be suppressed to be small even in the state where the thermally conductive filler is added.

[0032] The thermally conductive composite material preferably has a specific gravity of 1.4 or less. In this case, the specific gravity of the entire thermally conductive composite material can be sufficiently suppressed to be small.

[0033] The thermally conductive composite material preferably has a thermal conductivity at room temperature of 0.9 W / (m·K) or more. In this case, sufficiently high thermal conductivity can be ensured for the entire thermally conductive composite material.

[0034] The wire harness according to the present disclosure includes the above-mentioned thermally conductive composite material.

[0035] Since the above-mentioned wire harness includes the thermally conductive composite material described above, it is possible to utilize high thermal conductivity while suppressing the specific gravity of the constituent members to be small. Therefore, high heat dissipation performance can be obtained while suppressing the mass of the entire wire harness to be small. Therefore, even when heat generation occurs due to energization of the electric wires constituting the wire harness while maintaining the light weight of the wire harness, the influence of the heat generation can be suppressed to be small.

[0036] The method for manufacturing a thermally conductive filler according to the present disclosure includes a gel preparation step of preparing the gel-like substance with the thermally conductive substance dispersed therein, and a coating step of bonding the gel-like substance in which the thermally conductive substance is dispersed, prepared in the gel preparation step, to the surface of the base material particles via chemical bonds, to manufacture the thermally conductive filler.

[0037] According to the above manufacturing method, a coating layer in which a thermally conductive substance is dispersed in a gel-like substance can be formed on the surface of the base material particles, and a thermally conductive filler having a small specific gravity and high thermal conductivity can be easily manufactured.

[0038] [Details of Embodiments of the Present Disclosure] Hereinafter, a thermally conductive filler, a thermally conductive composite material, a wire harness, and a method for manufacturing a thermally conductive filler according to embodiments of the present disclosure will be described in detail with reference to the drawings. A thermally conductive composite material according to an embodiment of the present disclosure is configured to include the thermally conductive filler according to an embodiment of the present disclosure. Further, a wire harness according to an embodiment of the present disclosure is configured to include the thermally conductive composite material according to an embodiment of the present disclosure. Furthermore, the thermally conductive filler according to an embodiment of the present disclosure can be manufactured by the manufacturing method according to an embodiment of the present disclosure.

[0039] In this specification, unless otherwise specified, various physical property values are measured at room temperature in the air. Further, in this specification, for a certain component to be the main component of a certain material means a state in which the component occupies 50% by mass or more of the total mass of all components constituting the material. Furthermore, in this specification, "organic polymer" includes those with a low degree of polymerization such as oligomers.

[0040] [Thermally Conductive Filler] First, a thermally conductive filler (hereinafter, may be simply referred to as "filler") according to an embodiment of the present disclosure will be described.

[0041] (Overall Configuration) As shown in FIG. 1A, the thermally conductive filler 10 according to an embodiment of the present disclosure has a base material particle 11 and a coating layer 12, and is in a particulate form. The coating layer 12 covers the surface of the base material particle 11.

[0042] As shown in FIG. 1B, the coating layer 12 includes a gel-like substance 12a and a thermally conductive substance 12b, and the thermally conductive substance 12b is dispersed in the layer of the gel-like substance 12a. The gel-like substance 12a is bonded to the surface of the base material particle 11 via a chemical bond, whereby the coating layer 12 is fixed to the surface of the base material particle 11. In the gel-like substance 12a, the constituent molecules are crosslinked by a binding force such as a hydrogen bond to form a network-like network, resulting in a highly viscous state. Particles of the thermally conductive substance 12b are held in the structure of the gel-like substance 12a.

[0043] The thermally conductive substance 12b has a higher thermal conductivity than the base material particle 11 and the gel-like substance 12a. Also, the thermally conductive substance 12b has a larger specific gravity than the base material particle 11 and the gel-like substance 12a. As will be described later, the base material particle 11 is preferably configured as a hollow body or a porous body. However, when there are regions in the particle that are not occupied by solid substances, such as the hollow portion of the hollow body and the pores of the porous body, the specific gravity and thermal conductivity of the base material particle 11 are defined for the entire particle including those regions not occupied by solid substances.

[0044] Since the filler 10 has a structure in which a coating layer 12 containing the thermally conductive substance 12b is formed on the surface of the base material particle 11, most of the volume of the filler 10 will be occupied by the base material particle 11. Since the base material particle 11 has a smaller specific gravity than the thermally conductive substance 12b, the specific gravity of the entire filler becomes smaller than when the entire filler is composed of the thermally conductive substance 12b.

[0045] On one hand, the coating layer 12 that coats the surface of the base material particles 11 contains a thermally conductive substance 12b having a high thermal conductivity, thereby enhancing the thermal conductivity of the entire filler 10. As shown in FIG. 1B, the particles of the thermally conductive substance 12b dispersed within the coating layer 12 come into contact with each other within the layer, ensuring thermal conductivity within the coating layer 12. Further, as shown in FIG. 1A, the coating layer 12 on the surface of the filler particles 10 contacts the matrix material 2 surrounding the filler particles 10 and the coating layers 12 on the surfaces of other filler particles 10, so that the thermally conductive substance 12b contained in the coating layer 12 contributes to the heat conduction between the filler particles 10 and the matrix material 2 and between the filler particles 10. Since the coating layer 12 is provided only on the surface of the base material particles 11, the volume of the entire filler 10 can exhibit thermal conductivity with a small volume of the thermally conductive substance 12b while ensuring the volume by the base material particles 11. The adjacent filler particles 10 form a heat conduction path by contacting each other through the surface coating layer 12.

[0046] From the perspective of avoiding an increase in the mass of the filler 10, the specific gravity of the entire filler 10 is preferably 1.8 or less, more preferably 1.5 or less, and still more preferably 1.2 or less. On the other hand, from the perspective of avoiding the situation where the amount of the thermally conductive substance 12b necessary to ensure sufficient thermal conductivity cannot be contained in the coating layer 12 due to excessive reduction of the specific gravity, the specific gravity of the entire filler 10 is preferably 0.5 or more, more preferably 0.8 or more. The specific gravity of the filler 10 can be measured, for example, as the true density of the powdery filler 10 using a pycnometer.

[0047] (Base material particles) As described above, the base material particles 11 have a specific gravity smaller than that of the heat conductive material 12b contained in the coating layer 12. As long as it has such a specific gravity, the specific structure and constituent material of the base material particles 11 are not particularly limited. Examples of the structure of the base material particles 11 include a solid body in which a solid substance occupies the entire area, a hollow body having a hollow portion that is not occupied by the solid substance inside, and a porous body having a large number of fine pores that are not occupied by the solid substance. The form shown in FIG. 1A shows a case where the base material particles 11 are hollow particles having a hollow portion 11a.

[0048] When the base material particles 11 are composed of a solid body, the solid substance itself constituting the base material particles 11 needs to have a specific gravity smaller than that of the heat conductive material 12b. Suitable examples of such solid substances include organic polymers such as various resins, elastomers, and rubbers. Since the base material particles 11 need to bond the gel-like substance 12a to the surface via chemical bonds, when the base material particles 11 are made of an organic polymer, it is preferable that the organic polymer has a functional group capable of forming a chemical bond with the functional group of the gel-like substance 12a. The organic polymer may have such a functional group in the main chain or may have a functional group introduced into the side chain by modification or the like.

[0049] When the base material particles 11 have regions not occupied by solid substances such as hollow bodies or porous bodies, the presence of these regions reduces the specific gravity of the entire base material particles 11. Therefore, even if the solid substance itself as a constituent material has a high specific gravity (density), the specific gravity of the entire base material particles 11 can be maintained lower than that of the heat conductive material 12b compared to the case of a solid body. Thus, as the base material particles 11, those made of various constituent materials can be adopted. In particular, as shown in Fig. 1A, if the base material particles 11 are made of a hollow body, the hollow part (cavity) 11a not occupied by the solid substance is surrounded by the shell 11b formed by the solid material and maintained as a space blocked from the external environment of the base material particles 11. Therefore, it is easy to form the coating layer 12 having a clear layered structure on the surface of the base material particles 11 and maintain the effect of reducing the specific gravity by ensuring a space (hollow part 11a) not occupied by other substances. From these viewpoints, the form in which the base material particles 11 are configured as a hollow body is particularly suitable.

[0050] When the base material particles 11 are configured as a hollow body or a porous body, as the constituent material, in addition to the organic polymers mentioned above for the case of a solid body, various inorganic materials can be preferably adopted. Examples of such inorganic materials include metals or inorganic compounds such as glass and ceramics. Various functional groups can be introduced onto the surface of the inorganic material by surface treatment. The gel-like substance 12a can be bonded to the surface of the base material particles 11 through a chemical bond between these functional groups and the functional groups possessed by the gel-like substance 12a.

[0051] Preferred examples of the inorganic material constituting the base material particles 11 (the shell 11b thereof) include glass. As a material itself, glass has a relatively low specific gravity among various inorganic compounds and has high thermal conductivity compared to organic polymers and the like. Therefore, by using it as the material of the base material particles 11 constituting the thermal conductivity filler 10, it shows a high effect on reducing the specific gravity and increasing the thermal conductivity of the thermal conductivity filler 10. In addition, a technique for producing hollow particles using glass and further controlling the particle size and shape has already been established, and hollow particles of glass can be obtained at a lower cost compared to other types of hollow particles. The type of glass constituting the base material particles 11 is not particularly limited, and various glasses such as soda-lime glass, silica glass, borate glass, borosilicate glass, soda-lime borosilicate glass, lead glass, and phosphate glass can be used. Among these glass types, as will be described later, it is preferable to use those containing silicon atoms capable of forming a siloxane bond with a silane coupling agent, such as soda-lime glass, silica glass, borosilicate glass, and soda-lime borosilicate glass, so that a functional group can be introduced using a silane coupling agent. The material constituting the base material particles 11 may be only one type, or two or more types may be used in combination by mixing or laminating.

[0052] The base material particles 11 have a functional group on the surface that can form a chemical bond with the functional group of the gel-like substance 12a. The type of the functional group of the base material particles 11 is not particularly limited, and depending on the type of the functional group of the gel-like substance 12a, the base material particles 11 may have a functional group on the surface that can react with the functional group of the gel-like substance 12a. The chemical bond may be an electrostatic bond (ionic bond, hydrogen bond) or a covalent bond. When forming an electrostatic bond, a polar group having a polarity opposite to that of the polar group provided in the gel-like substance 12a may be present on the surface of the base material particles 11. The polar group of the base material particles 11 may be ionic or non-ionic. In many cases, since the gel-like substance 12a and its raw material substances are in a negatively charged state, it is preferable that a positively charged polar group is present on the surface of the base material particles 11.

[0053] As will be described later, many compounds having a carbonyl group are known as the compounds constituting the gel. If a functional group capable of forming a chemical bond with the carbonyl group is provided on the surface of the base material particles 11, various gel-like substances 12a can be chemically bonded to the surface of the base material particles 11. Examples of the functional group capable of forming a chemical bond with the carbonyl group include basic groups. Examples of the basic group include an amino group, an amide group, an imide group, etc. Among them, it is preferable that an amino group, particularly a primary amino group, is present on the surface of the base material particles 11. The formation of the chemical bond between these functional groups and the gel-like substance 12a will be described later together with the examples of the gel-like substance 12a.

[0054] As long as the base material particles 11 have at least on the surface a functional group capable of forming a bond with the gel-like substance 12a, the distribution and the introduction method of the functional group are not particularly limited. That is, even if the entire base material particles 11 contain a compound having a polar group as a constituent material, with respect to the base material particles 11 composed of a material substantially free of or containing only a very small amount of the functional group, the functional group may be introduced only on the surface (and its vicinity) by surface treatment or the like. From the viewpoint of simplicity in introducing the functional group to the surface of the base material particles 11, when the base material particles 11 are composed of an organic substance, it is preferable that the organic substance itself constituting the base material particles 11 has a functional group and the functional group is exposed on the surface of the base material particles 11. On the other hand, when the base material particles 11 are composed of an inorganic substance such as glass, it is preferable to introduce the functional group to the surface portion by surface treatment. When the base material particles 11 are composed of glass containing silicon or have a hydroxyl group on the surface, various functional groups can be easily introduced to the surface by using a silane coupling agent.

[0055] The specific shape and particle size of the base material particles 11 are not particularly limited. However, it is preferable to have a highly isotropic shape such as one that can approximate a sphere, etc., in terms of facilitating the formation of the coating layer 12 on the surface and enhancing the affinity with the matrix material 2. From the perspective of suppressing the specific gravity of the entire filler 10 to a small value, etc., the particle size (median diameter D50; the same applies hereinafter) of the base material particles 11 is preferably 1 μm or more, and more preferably 5 μm or more. On the other hand, from the perspective of minimizing the influence on the properties of the matrix material 2 to which the filler 10 is added and increasing the specific surface area, etc., the particle size of the base material particles 11 is preferably 100 μm or less, and more preferably 60 μm or less.

[0056] From the perspective of suppressing the specific gravity of the entire filler 10 to a small value, it is preferable that the specific gravity of the base material particles 11 alone is also small. The specific specific gravity of the base material particles 11 is not particularly limited as long as the specific gravity of the entire base material particles 11 is smaller than that of the heat-conductive substance 12b. However, taking the specific gravity (true density) of the entire base material particles 11 including the hollow part 11a, etc., for example, it is preferably 1.0 or less, and more preferably 0.5 or less. There is no particular lower limit set for the specific gravity of the base material particles 11, but the specific gravity of the base material particles 11 composed of a hollow body of an inorganic material such as glass or a solid body of an organic polymer is generally 0.1 or more.

[0057] (Coating layer) In the coating layer 12, the gel-like substance 12a is bonded to the surface of the base material particles 11 through chemical bonds in a state of covering the surface of the base material particles 11. The heat-conductive substance 12b is dispersed and held within the layer of the gel-like substance 12a.

[0058] The gel-like substance 12a forms bonds between molecules to form a network-like crosslinked structure. The bonds forming the crosslinked structure may be physical bonds such as van der Waals forces, but are preferably stronger chemical bonds. The chemical bonds may be electrostatic bonds such as ionic bonds or hydrogen bonds, or covalent bonds. From the viewpoint of the simplicity of forming the coating layer 12 and the like, electrostatic bonds are preferred, and from the viewpoint of the stability of the crosslinked structure, hydrogen bonds are more preferred.

[0059] The gel-like substance 12a has a functional group capable of forming a chemical bond with the functional group on the surface of the base material particles 11. The coating layer 12 is constituted in a state where the gel-like substance 12a is bonded to the surface of the base material particles 11 by the chemical bond formed between the functional groups of both. In the gel-like substance 12a, the functional group that forms a bond with the surface of the base material particles 11 may be the same as the functional group that forms a crosslinked structure between molecules, or may be contained in the molecule of the gel-like substance 12a as a functional group different from the functional group involved in crosslinking. Preferably, the common functional group is involved in both intermolecular crosslinking and bonding with the base material particles 11. In the gel-like substance 12a having a carbonyl group described later, the carbonyl group forms a bond in the crosslinked structure and, when a basic group such as an amino group is present on the surface of the base material particles 11, can bond the gel-like substance 12a to the surface of the base material particles 11 through bonding with these basic groups.

[0060] When the gel-like substance 12a is bonded to the surface of the base material particles 11 via a chemical bond, the coating layer 12 is firmly fixed to the base material particles 11. The direct influence of this chemical bond only reaches the interface between the coating layer 12 and the base material particles 11 and the very vicinity thereof. However, the tissue within the gel-like substance 12a is continuous due to the crosslinked structure. Therefore, the state in which the entire layer of the gel-like substance 12a is fixed to the base material particles 11 is stably maintained by the chemical bond at the interface and the crosslinked structure within the layer.

[0061] Since the gel-like substance 12a forms a network structure and has high viscosity, as shown in FIG. 1B, when particles of the heat-conductive substance 12b are dispersed in the gel-like substance 12a, a state in which the heat-conductive substance 12b is stably held in the layer of the gel-like substance 12a is maintained. The gel-like substance 12a often constitutes a gel in a state containing water or other polar solvents, but even when those polar solvents are removed, the state in which the heat-conductive substance 12b is dispersed and held in the structure of the gel-like substance 12a is maintained. When the particles of the heat-conductive substance 12b dispersed in the gel-like substance 12a come into contact with each other and a heat conduction path is formed between the particles, the coating layer 12 exhibits heat conductivity as a whole.

[0062] For the retention of the heat-conductive substance 12b inside the gel-like substance 12a, no chemical interaction is required. Therefore, in order to disperse and hold the heat-conductive substance 12b in the layer of the gel-like substance 12a, it is not necessary for the heat-conductive substance 12b to show compatibility with the gel-like substance 12a. Also, since the heat-conductive substance 12b is not fixed to the surface of the base material particles 11 by direct interaction with the surface of the base material particles 11, the type of the heat-conductive substance 12b is not limited by the type of the base material particles 11. That is, various heat-conductive substances 12b can be arranged on the surface of the base material particles 11 in a state of being dispersed in the layer of the gel-like substance 12a without being restricted by the constituent material or shape of the heat-conductive substance 12b, and that state can be stably maintained. Further, from the viewpoint of improving heat conductivity or the like, even when the coating layer 12 is formed thick, the entire layer can be maintained in a state of being stably fixed to the surface of the base material particles 11. Similarly, from the viewpoint of improving heat conductivity or the like, increasing the content density of the heat-conductive substance 12b in the coating layer 12 can be relatively easily performed as long as the gel-like substance 12a can hold the heat-conductive substance 12b in the structure, without being restricted by the formation density of chemical bonds or the like.

[0063] As the thickness of the coating layer 12, from the viewpoint of obtaining a sufficiently high thermal conductivity for the entire filler 10, it is preferable that the coating layer 12 is formed with a thickness having a ratio of [dry volume of the mixed gel containing the gel-like substance 12a and the thermally conductive substance 12b]:[dry volume of the base material particles 11] of 10:90 or more, further 20:80 or more, and 30:70 or more. On the other hand, from the viewpoint of avoiding peeling or damage of the coating layer 12 due to the coating layer 12 being too thick and the difficulty of mixing the filler 10 into the matrix material 2 associated with these phenomena, it is better to suppress the thickness of the coating layer 12 to such an extent that the above ratio is 80:20 or less, further 70:30 or less. Here, the dry volume refers to the volume in a state where liquid components such as water contained in the gel-like substance 12a and other solvents are removed by drying or the like. As the value of the thickness of the coating layer 12, it is generally preferably 1 μm or more, further 5 μm or more, and 30 μm or less.

[0064] Note that even if the thermally conductive substance 12b and the base material particles 11 are added to the matrix material 2 in an independent state without using the gel-like substance 12a, a certain degree of effect of improving thermal conductivity can be obtained. However, in this case, the thermally conductive substance 12b will be dispersed in the matrix resin in a state where it covers a very small area region on the surface of the base material particles 11 or in a state independent of the base material particles 11. By using the gel-like substance 12a as in the filler 10 according to the present embodiment, the thermally conductive substance 12b can be stably held on the surface of the base material particles 11, and thus the thermally conductive substance 12b can contribute efficiently to the formation of a thermal conduction path.

[0065] (gel-like substance) The gel-like substance 12a constituting the coating layer 12 is not particularly limited as long as it can form a gel state and has a functional group capable of forming a chemical bond with a functional group on the surface of the base material particles 11. In the filler 10, the gel-like substance 12a constituting the coating layer 12 may be in a state of swelling by taking in water or other solvents, or may be in a state where those solvents have been removed by drying or extraction. From the viewpoints of the stability and ease of handling of the filler 10, etc., it is preferable that the gel-like substance 12a is in a state where the solvent has been removed.

[0066] As the gel-like substance 12a, many substances composed of polymers having a carbonyl group are known, and in this embodiment, those substances can be preferably adopted. Due to the high polarity of the carbonyl group and the like, molecules having a carbonyl group are likely to form a stable gel. Here, the carbonyl group includes various functional groups having a C=O bond, and in addition to the ketone group, it can be a carboxyl group, an amide group, an ester group, etc. For example, polymers having a carboxyl group such as polyacrylic acid, polymers having an amide group such as polyvinylpyrrolidone, or derivatives of these compounds swell in water or a highly polar solvent and form a stable gel state. Polymers having a ketone group also gel by swelling with polyethylene glycol (PEG). Specific examples of the polymer constituting the gel-like substance 12a include polyacrylic acid, polyacrylate, polyethylene acrylic acid copolymer, polyethylene acrylic acid copolymer salt, polyvinylpyrrolidone, carboxymethyl cellulose, carboxymethyl cellulose salt, alginic acid, alginate, and the like.

[0067] When the molecule constituting the gel-like substance 12a has a carbonyl group, if a basic group such as an amino group is present on the surface of the base material particles 11, the gel-like substance 12a can be stably bonded to the surface of the base material particles 11 by forming a bond between the carbonyl group and the basic group. For example, when the gel-like substance 12a has an acidic carbonyl group such as a carboxyl group, an ionic bond can be formed with the basic group of the base material particles 11 to form a salt. On the other hand, when the gel-like substance 12a has a neutral carbonyl group such as a ketone group, a covalent bond can be formed with the basic group on the surface of the base material particles 11. For example, when a primary amino group is present on the surface of the base material particles 11, an amide bond or an imide bond is formed by a condensation reaction.

[0068] The specific gravity and thermal conductivity that the gel-like substance 12a should have are not particularly specified. However, the gel-like substance 12a is often composed of organic substances and has a lower specific gravity than the heat-conductive substance 12b composed of inorganic substances. The specific gravity of the gel-like substance 12a may be 1.5 or less, preferably 1.2 or less, in the state of the raw material substance (gel-forming polymer) before gelation. Although there is no particular lower limit for its specific gravity, the specific gravity of the organic polymer capable of forming a gel is generally 0.5 or more. Since the gel-like substance 12a has a low specific gravity, combined with the effect of the low specific gravity of the base material particles 11, the specific gravity of the entire filler 10 can be kept low. Also, the gel-like substance 12a is often composed of organic substances, and its thermal conductivity is lower than that of the heat-conductive substance 12b. However, as described above, the heat-conductive substance 12b is dispersed and held in the layer of the gel-like substance 12a to form a heat conduction path, so that high thermal conductivity can be obtained for the entire coating layer 12.

[0069] (Heat-conductive substance) The heat-conductive substance 12b contained in the coating layer 12 is not particularly limited in terms of its type and shape as long as it has a higher thermal conductivity and a larger specific gravity than the base material particles 11 and the gel-like substance 12a and can be dispersed in the gel-like substance 12a. However, from the viewpoints of high thermal conductivity and dispersibility in the gel-like substance 12a, the heat-conductive substance 12b is preferably composed of particles of inorganic substances such as metals, metal compounds, and carbon materials.

[0070] Specific examples of the material constituting the heat conductive material 12b include, as metals, non-magnetic metals such as copper and aluminum, and alloys mainly composed of these elements. Further, as metal compounds, oxides, nitrides, carbides, oxynitrides, carbonitrides, carbon oxides, hydroxides, borides, etc. containing metal elements (including the case of semi-metals such as B and Si), and silicates, aluminates, titanates, etc. of metals can be exemplified. As metal oxides, aluminum oxide, silicon oxide, magnesium oxide, iron oxide, beryllium oxide, titanium oxide, zirconium oxide, etc., as metal nitrides, aluminum nitride, silicon nitride, boron nitride, etc., as metal hydroxides, aluminum hydroxide, magnesium hydroxide, etc., and as metal carbides, silicon carbide, etc. can be cited as suitable examples. Further, as carbon materials that can constitute the heat conductive material 12b, graphite, carbon black, carbon fiber, carbon nanotube, graphite, graphene, artificial diamond, etc. can be mentioned. The heat conductive material 12b contained in the coating layer 12 may be one kind or a plurality of kinds. Also, when using a plurality of kinds of heat conductive materials 12b, they may be mixed or form a composite. The heat conductive material 12b may be surface-treated with an organic substance or the like, but since no chemical interaction is required for the gel-like substance 12a to hold the heat conductive material 12b, from the viewpoint of enhancing the holding property by the gel-like substance 12a, it is not necessary to perform surface treatment on the heat conductive material 12b.

[0071] Among those listed above, in terms of high thermal conductivity and the like, metal oxides such as aluminum oxide and magnesium oxide, and carbon materials such as carbon black and carbon fiber are particularly preferable as the thermal conductive substance 12b. Further, the thermal conductive substance 12b is preferably configured as particles having anisotropy in shape, such as plate-like, needle-like, rod-like, fibrous, etc. Particles of a metal compound or carbon material having anisotropy in shape often have high thermal conductivity in a specific direction due to the anisotropy. The fact that these particles have high thermal conductivity is convenient for use as a thermal conductive material. However, if the particles are oriented and distributed in a specific direction due to the anisotropy of the particle shape, the thermal conductivity in a specific direction is excellent, but the thermal conductivity in other directions becomes low, making it difficult to use as a thermal conductive material. However, in the filler 10 according to the present embodiment, the particles of the thermal conductive substance 12b are dispersed and held in the gel-like substance 12a. Therefore, within the coating layer 12 and as the filler 10 as a whole, the particles of the thermal conductive substance 12b having an anisotropic shape are arranged in a random orientation without taking a specific orientation. Therefore, by utilizing the high thermal conductivity of the particles of the thermal conductive substance 12b having an anisotropic shape, a thermal conductive filler 10 excellent in thermal conductivity in any direction rather than a specific direction can be obtained. As the thermal conductive substance 12b having an anisotropic shape and high thermal conductivity, carbon fiber can be mentioned. Carbon fiber has an elongated and highly anisotropic shape and has a large anisotropy in thermal conductivity. However, by dispersing it in the gel-like substance 12a, the large anisotropy can be eliminated, and by forming a thermal conduction path between adjacent carbon fibers, high thermal conductivity can be exhibited in any direction.

[0072] The particle size of the heat conductive substance 12b is not particularly limited, but from the viewpoint of exhibiting sufficiently high heat conductivity, it is preferably 0.1 μm or more, more preferably 0.3 μm or more on average. On the other hand, in the coating layer 12, from the viewpoint of facilitating dispersion and retention within the structure of the gel-like substance 12a, the particle size of the heat conductive substance 12b is preferably 30 μm or less, more preferably 20 μm or less on average. Further, the particle size of the heat conductive substance 12b is preferably smaller than the particle size of the base material particles 11, and more preferably half or less of the particle size of the base material particles 11. Here, the particle size of the heat conductive substance 12b refers to the approximate diameter when the particle can be approximated as a sphere, and refers to the major axis (the length of the long axis) when the shape has a high degree of anisotropy such as acicular or fibrous shape.

[0073] The content of the heat conductive substance 12b in the coating layer 12 is not particularly limited, but from the viewpoint of making the heat conductivity within the layer of the coating layer 12 sufficiently high and as a result, enabling the filler 10 as a whole to exhibit sufficiently high heat conductivity, the heat conductive substance 12b is preferably contained in an amount such that the ratio of [dry volume of the gel-like substance 12a]:[dry volume of the heat conductive substance 12b] is 90:10 or more, more preferably 50:50 or more, and still more preferably 30:70 or more. On the other hand, from the viewpoint of stably retaining the heat conductive substance 12b inside the structure of the gel-like substance 12a, it is advisable to suppress the content of the heat conductive substance 12b to 1:99 or less, more preferably 10:90 or less, in the above ratio.

[0074] <Method for manufacturing heat conductive filler> Next, a method for manufacturing a heat conductive filler according to an embodiment of the present disclosure, which can manufacture the above heat conductive filler 10, will be described. The heat conductive filler 10 can be manufactured by performing a gel preparation step and a coating step.

[0075] Prior to manufacturing the heat conductive filler 10, the base material particles 11 are prepared. Many solid particles of organic polymers, hollow particles of inorganic compounds such as glass, and porous particles are commercially available, and these particles may be appropriately used.

[0076] When the base material particles 11 do not have a functional group capable of forming a chemical bond with the functional group of the gel-like substance 12a on the surface, it is necessary to perform a surface treatment on the base material particles 11 to introduce a functional group on the surface. As the surface treatment, a compound having a desired functional group may be bonded to the surface of the base material particles 11 by a chemical reaction. At this time, another compound may be interposed between the compound having the functional group and the surface of the base material particles 11. When the base material particles 11 are composed of a substance containing silicon atoms such as glass or a substance having a hydroxyl group on the surface, it is preferable to use a silane coupling agent to introduce a functional group on the surface of the base material particles 11.

[0077] In the gel preparation step, a mixed gel that becomes the coating layer 12 is prepared. That is, the gel-like substance 12a is prepared in a state where the heat conductive substance 12b is dispersed therein. The specific method is not particularly limited, but crosslinking formation and swelling are performed on the raw material substances constituting the gel, such as a gelling polymer, to form the gel-like substance 12a, or after forming the gel-like substance 12a, the particles of the heat conductive substance 12b may be dispersed in the gel-like substance 12a. For example, when using a gelling polymer that swells with a polar solvent such as water and becomes the gel-like substance 12a as a raw material substance, the gelling polymer and the particles of the heat conductive substance 12b may be stirred in a solvent containing a polar solvent such as water. At this time, a small amount of an aqueous metal salt solution may be added so that the gel-like substance 12a is obtained in a salt state. In the gel preparation step, the addition amounts of the raw material substance that becomes the gel-like substance 12a and the heat conductive substance 12b may be selected so that a desired value is obtained as the ratio of [dry volume of the gel-like substance 12a (gelling polymer)]:[dry volume of the heat conductive substance 12b].

[0078] In the coating process, the surface of the base material particles 11 is coated with the mixed gel prepared in the gel preparation process, and further, the gel-like substance 12a is bonded to the surface of the base material particles 11 via chemical bonds. At this time, the base material particles 11 prepared in a state having a predetermined functional group on the surface may be mixed and kneaded with the mixed gel prepared above. Alternatively, a solvent that does not affect the decrease in the gel property of the gel-like substance 12a or the modification of the functional group on the surface of the base material particles 11 may be used, and the base material particles 11 and the mixed gel may be mixed in the solvent. Through these operations of mixing and kneading or mixing in a solvent, the mixed gel surrounds the surface of the base material particles 11, and further, a chemical bond is formed between the functional group of the gel-like substance 12a constituting the mixed gel and the functional group present on the surface of the base material particles 11. For the purpose of promoting the formation of chemical bonds, operations such as heating and addition of a reactant may be appropriately performed. The heating temperature is preferably a temperature below the boiling point of the solvent used.

[0079] The particles obtained through the coating process may be used as the thermal conductivity filler 10 as they are, or may be used as the thermal conductivity filler 10 after appropriately removing the liquid component. The removal of the liquid component can be performed by heating under normal pressure or vacuum, air drying, etc. When the liquid component exists on the surface of the filler particles 10 in a state incorporated in the gel-like substance 12a, etc., during the use of the filler 10, volatilization or flow of the liquid component may occur, which may reduce the handleability of the filler 10. In such a case, it is preferable to remove the liquid component.

[0080] <Thermal Conductivity Composite Material> Next, a thermal conductivity composite material (hereinafter, may be simply referred to as a composite material) according to an embodiment of the present disclosure will be described. As shown in FIG. 1, the thermal conductivity composite material 1 according to this embodiment includes the thermal conductivity filler 10 according to the embodiment of the present disclosure described above and the matrix material 2. The filler 10 is dispersed in the matrix material 2.

[0081] Since the composite material 1 according to this embodiment contains the thermally conductive filler 10 having the coating layer 12 containing the thermally conductive substance 12b on the surface of the base material particles 11 described above, due to the high thermal conductivity of the thermally conductive substance 12b, the composite material 1 as a whole exhibits high thermal conductivity and is excellent in heat dissipation. At the same time, due to the effect of reducing the specific gravity of the thermally conductive filler 10 by the base material particles 11, the composite material 1 as a whole has a small specific gravity.

[0082] The type of the matrix material 2 is not particularly limited, but the matrix material 2 preferably contains an organic polymer, and more preferably, as long as it is mainly composed of an organic polymer. Specific examples of the organic polymer constituting the matrix material 2 include various resins, thermoplastic elastomers, rubbers, and the like. When a resin material is used as the matrix material 2, depending on the desired application, it may be a curable resin, a thermoplastic resin, or a plastic soluble in a solvent. Examples of the type of resin constituting the matrix material 2 include olefin resins such as polyethylene and polypropylene, halogen resins such as polyvinyl chloride, polylactic acid, polystyrene resins, polyvinyl acetate, ABS resins, AS resins, acrylic resins, methacrylic resins, polyamide resins, urethane resins, silicone resins, fluororesins, polyvinyl alcohol, polyimide, polyacetal, polycarbonate, modified polyphenylene ether (PPE), polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, and epoxy resins, or copolymers or polymer alloys of these resins. The matrix material 2 may contain only one kind of organic polymer or may contain a plurality of kinds. Note that the matrix material 2 is not in a gel state and appropriately takes a solid state at room temperature after undergoing curing or the like. The matrix material 2 may appropriately contain additives such as a flame retardant, a filler, and a colorant in addition to the organic polymer.

[0083] The specific gravity of the matrix material 2 itself is not particularly limited, but from the viewpoint of keeping the specific gravity of the entire composite material 1 added with the filler 10 small, it is preferably suppressed to 1.5 or less. There is no particular lower limit for the specific gravity of the matrix material 2, but when an organic polymer is used as the matrix material 2, its specific gravity is generally 0.8 or more. Also, the thermal conductivity of the matrix material 2 itself is not particularly limited, but from the viewpoint of ensuring a high thermal conductivity for the entire composite material 1 added with the filler 10, it is preferably 0.1 W / (m·K) or more. There is no particular upper limit for the thermal conductivity of the matrix material 2, but when an organic polymer is used as the matrix material 2, its thermal conductivity is generally 0.6 W / (m·K) or less. Incidentally, the specific gravity of the matrix material 2 and the composite material 1 can be measured by the water displacement method or the like. Also, the thermal conductivity of these materials can be measured by the laser flash method, the heat ray method, or the like.

[0084] In the composite material 1 according to the present embodiment, the content of the filler 10 may be appropriately determined so that a desired specific gravity and thermal conductivity can be obtained for the entire composite material 1. The higher the content of the filler 10, the higher the thermal conductivity of the composite material 1. Therefore, the content of the filler 10 may be determined with the content at which the desired thermal conductivity is obtained as the lower limit. For example, the content of the filler 10 may be determined so that the thermal conductivity of the composite material 1 is 5 times or more, further 7 times or more, 10 times or more, 15 times or more the thermal conductivity of the matrix material 2 without adding the filler 10. Alternatively, the content of the filler 10 may be determined so that the thermal conductivity of the composite material 1 is 0.9 W / (m·K) or more, further 1.5 W / (m·K) or more, 2.0 W / (m·K) or more, 3.0 W / (m·K) or more. Although the higher the thermal conductivity of the composite material 1 is, the more preferable it is, from the viewpoint of avoiding an increase in specific gravity due to excessive addition of the filler 10, it is advisable to keep it at 50 times or less, further 30 times or less, also 8.0 W / (m·K) or less, further 5.0 W / (m·K) or less the thermal conductivity of the matrix material 2.

[0085] The upper limit of the content of the filler 10 in the composite material 1 is not particularly defined, but the content of the filler 10 may be determined so that the specific gravity of the composite material 1 is suppressed to 1.3 times or less, and further 1.2 times or less, of the specific gravity of the matrix material 2 without adding the filler 10. More preferably, the specific gravity of the composite material 1 may be equal to or less than the specific gravity of the matrix material 2 without adding the filler 10. Alternatively, the content of the filler 10 may be determined so that the value of the specific gravity of the composite material 1 is suppressed to 1.8 or less, further 1.5 or less, and 1.3 or less. Note that the smaller the specific gravity of the composite material 1, the more preferable it is, and the lower limit is not particularly defined.

[0086] When the content of the filler 10 is defined as the ratio of the filler 10 in the entire composite material 1, the content of the filler 10 may generally be 10% by volume or more, further 20% by volume or more, and 30% by volume or more, from the viewpoint of sufficiently improving the thermal conductivity of the composite material 1. Alternatively, the content of the heat conductive substance 12b itself may be 2% by volume or more, further 4% by volume or more, and 6% by volume or more. On the other hand, from the viewpoint of suppressing an increase in the specific gravity of the composite material 1 and avoiding saturation of the effect of improving the thermal conductivity due to the addition of a large amount of the filler 10, it may be 60% by volume or less, or 50% by volume or less. Alternatively, the content of the heat conductive substance 12b itself may be 15% by volume or less, further 12% by volume or less.

[0087] As described above, the composite material 1 according to the present embodiment achieves both high thermal conductivity and low specific gravity. Therefore, the present composite material 1 can be suitably used as a material for constituting a member that requires both light weight and heat dissipation. The specific use of the composite material 1 is not particularly limited. Next, the case of using it as a constituent material of a wire harness will be exemplified in detail. The composite material 1 according to the present embodiment can be manufactured by mixing the powdery filler 10 manufactured by the manufacturing method described above with the matrix material 2 at a predetermined mixing ratio.

[0088] <Wire harness> Finally, a wire harness according to an embodiment of the present disclosure will be described. The wire harness according to this embodiment includes the thermally conductive composite material 1 according to the embodiment of the present disclosure described above. As shown in FIG. 2, the wire harness 5 is provided with a connector 52 including a connection terminal (not shown) at the terminal portion of an insulated wire 51 having an insulating coating provided on the outer periphery of an electric wire conductor. In the wire harness 5, a plurality of insulated wires 51 may be bundled, and in this case, a tape 53 can be used as an exterior material for bundling the insulated wires 51.

[0089] In the wire harness 5 according to this embodiment, the composite material 1 according to the embodiment of the present disclosure described above can constitute various members that require heat dissipation properties. Mainly, it is preferable to use, as an insulating member, a composite material 1 in which a filler 10 is added to an organic polymer as a matrix material 2. Examples of such insulating members include the insulating coating that constitutes the insulated wire 51, exterior materials such as the tape 53 and protective tubes arranged outside the insulated wire 51, adhesives used for adhesion and water stoppage between constituent members, and connector housings that constitute the connector 52. Further, the composite material 1 may be disposed between a protective tube such as a corrugated tube and the insulated wire 51.

[0090] In recent years, in the automotive field, especially in electric vehicles and hybrid vehicles, the current flowing through the electric wires has been increasing, and accordingly, the amount of heat generated from the electric wires has tended to increase. Also, a large number of electric wires and electrical connection members have come to be arranged in close proximity. In these cases, it is important for the various members constituting the wire harness 5 to have high heat dissipation properties from the viewpoint of minimizing the influence of heat dissipation from the electric wires and electrical connection members. In the wire harness 5, by configuring the members that may be affected by heat dissipation in this way using the above-described composite material 1 having high thermal conductivity, it becomes possible to efficiently dissipate heat. Also, in the automotive field, weight reduction of constituent members is an important issue, and by using the above-described composite material 1 with a small specific gravity, it is possible to contribute to weight reduction of the wire harness 5.

Example

[0091] Examples are shown below. The present invention is not limited by these examples. Here, a thermally conductive filler having a coating layer on the surface of base material particles was produced, and the state of the filler particles, as well as the specific gravity and thermal conductivity of the composite material containing the filler, were evaluated. Hereinafter, unless otherwise specified, the production and evaluation of the samples were carried out at room temperature in the atmosphere.

[0092] [Test Method] (1) Production of Filler First, a plurality of fillers having a coating layer on the surface of base material particles were prepared. When producing the filler, the raw material particles were surface-treated, and in the gel preparation step, after preparing a mixed gel, in the coating step, a coating layer was formed on the surface of the raw material particles.

[0093] (1-1) Preparation of Base Material Particles As the base material particles, glass hollow particles having an amino group introduced on the surface were prepared. Specifically, hollow particles made of soda lime borosilicate glass (manufactured by 3M Co., Ltd., "Glass Bubbles iM16K"; median diameter 20 μm; specific gravity 0.46) were prepared as untreated glass hollow particles (GB). Then, 5 g of GB and 100 mL of acetone were placed in an eggplant flask, gently stirred at room temperature, and suspended. Further, while continuing the stirring, 0.5 g of 3-aminopropyltriethoxysilane (AP) ((C2H5O)3Si-C3H6-NH2) was added to the suspension. After stirring at room temperature for 2 hours as it was, a condenser was attached, 200 mL of pure water was added, and the mixture was stirred at 50 °C for 24 hours. Then, it was filtered and air-dried, and further heated in an oven at 140 °C for 24 hours. Through the above steps, base material particles (AP-GB) surface-treated with 3-aminopropyltriethoxysilane were obtained. 3-aminopropyltriethoxysilane is bonded to the surface of the glass hollow particles via a siloxane bond, and amino groups are bonded to the surface of the particles.

[0094] (1-2) Gel Preparation Step As gelation polymer materials serving as raw materials for forming a gel-like substance, the following were prepared. · PAA: Polyacrylic acid (average molecular weight of about 1,000,000; manufactured by Fujifilm Wako Pure Chemical Corporation) · EAA: Ethylene-acrylic acid copolymer (acrylic acid content 15% by mass; manufactured by Sigma-Aldrich)

[0095] Also, as heat conductive substances, the following were prepared. · CF: Carbon fiber (10 μm in length, 150 nm in diameter; vapor-grown carbon fiber VGCF manufactured by Showa Denko KK) · AlO: α-alumina (0.5 μm in diameter; manufactured by Fujifilm Wako Pure Chemical Corporation) · MgO: Magnesium oxide (7 - 15 μm in diameter; manufactured by Ube Materials Co., Ltd.)

[0096] Each gelation polymer material in the input amount described in Table 1, 17 ml of tetrahydrofuran (THF), and 33 ml of pure water were put into an eggplant flask. After attaching a reflux tube, they were stirred at 60°C for 12 hours to dissolve and disperse the gelation polymer. While continuing the stirring, each heat conductive substance was added in the input amount described in Table 1, the reflux tube was attached again, and they were stirred at 60°C for 2 hours to disperse the heat conductive substance. In this way, a mixed gel was prepared.

[0097] (1 - 3) Coating process While continuing the stirring of the mixed gel prepared in the above gel preparation process, base material particles (uncoated GB or AP-GB) in the input amount described in Table 1 were put into the mixed gel. The reflux tube was attached again, and they were stirred at 60°C for 2 hours to disperse the base material particles. Then, the dispersion was cooled to room temperature. Then, while stirring 1 L of isopropanol in another beaker, the above dispersion was added little by little to make a suspension. The obtained suspension was suction filtered, and the obtained filtrate was dried at 80°C for 24 hours. In this way, a heat conductive filler was obtained.

[0098] Table 1 below summarizes the gelling polymer materials, thermally conductive substances, types and specific gravities of base particles, and amounts added used in the preparation of each thermally conductive filler. Also shown in the table are, as values calculated from those amounts and specific gravities, the dry volume ratio of [gelling polymer]:[thermally conductive substance], the dry volume ratio of [mixed gel]:[base particles], and the specific gravity of the filler particles. The dry volume ratio of [gelling polymer]:[thermally conductive substance] is calculated as the ratio of the amounts added of the gelling polymer material and the thermally conductive substance converted to volume, and the dry volume ratio of [mixed gel]:[base particles] is calculated as the ratio of the total value of the volumes of the calculated gelling polymer material and thermally conductive substance and the volume of the base particles, and each is shown with one significant digit. The specific gravity of the filler particles is calculated from the total of the amounts added of each of the gelling polymer, thermally conductive substance, and base particles and the total of the calculated volumes. Note that, as will be described later based on the micrographs of FIGS. 3A and 3B, it has been confirmed that almost all of the gelling polymer material and thermally conductive substance used as raw materials form a mixed gel and cover the surface of the base particles, and as described above, the validity of calculating the dry volume ratio and specific gravity of each component based on the amounts added of the respective raw materials is shown.

[0099]

Table 1

[0100] (2) Preparation of Composite Material Each filler prepared above was dispersed in a matrix material to prepare composite materials for Samples A1 to A14 and Samples B1 to B7. Here, the matrix material constituting the composite material was a cured product of the following two-component epoxy resin. · Epoxy resin: Glycidyl ether of bisphenol A (“jER828” manufactured by Mitsubishi Chemical Corporation; epoxy equivalent: 190 g / eq.) · Epoxy curing agent: Amine type (“ST12” manufactured by Mitsubishi Chemical Corporation; amine value: 345 to 385 KOHmg / g)

[0101] With the mass ratios shown in Table 2 below, various fillers, an epoxy resin main agent, and an epoxy curing agent were mixed in an agate mortar at room temperature and degassed under vacuum at room temperature for 1 minute. Then, the mixture was heated at 100 °C for 10 minutes using a hot press molding machine to cure it. From the cured body, a portion where no bubbles were visually confirmed was cut out to prepare a resin cured body test piece (10 mm × 10 mm × 1 mm). For sample B1, a resin cured body test piece was prepared from only the epoxy resin without adding a filler.

[0102] (3) Evaluation of the state of the filler and the properties of the composite material Each of the fillers prepared above was dispersed in liquid paraffin and observed with an optical microscope to evaluate the state of the filler particles.

[0103] In addition, the specific gravity and thermal conductivity were measured for each of the resin cured body test pieces prepared as composite materials above. The specific gravity was measured by the water displacement method. The thermal conductivity was measured by the laser flash method using a thermal conductivity device ("LFA447" manufactured by NETZSCH). The measurement direction of the thermal conductivity was perpendicular to the surface of the resin cured body test piece.

[0104] [Test results] As representatives of the fillers prepared, optical microscope images of the fillers "30-PC80" and "30-PA80" are shown in Figures 3A and 3B, respectively. In each of the images, the left image is focused on the center of the particles, and the right image is focused on the surface of the particles. In both of the two types of fillers, as seen in the image mainly focused on the center of the particles, the base material particles maintain a hollow state. And, as seen in the image mainly focused on the surface of the particles, a region observed in a somewhat hazy layer-like manner on the surface of the hollow base material particles corresponds to the coating layer formed from the mixed gel. In the image of "30-PC80", in the coating layer, an elongated structure observed like a number of thorns corresponds to the carbon fibers added as a thermally conductive substance. Also, in the image of "30-PA80", in the coating layer, a number of dot-like structures observed like a dark shadow correspond to the alumina particles added as a thermally conductive substance.

[0105] In this way, it is confirmed that from the microscopic image, in the produced filler, a structure is obtained in which a coating layer composed of a mixed gel in which a heat conductive substance is dispersed is formed on the surface of the hollow particles as the base material particles. The mixed gel coats the surface of the hollow particles in a layered manner to form a coating layer, and the presence of the gel-like substance is not confirmed at locations other than the surface of the particles. That is, almost the entire amount of the mixed gel used as a raw material is in a state of coating the surface of the hollow base material particles. From this, as described above, it is confirmed that a coating layer containing a heat conductive substance can be stably formed on the surface of the base material particles by the manufacturing method of mixing the surface-treated base material particles with the mixed gel.

[0106] In the image of "30-PC80" in FIG. 3A, as described above, in the coating layer, a large number of elongated substances protrude from the surface, and this substance is associated with the carbon fiber added as the heat conductive substance. On the surface of the base material particles, the protruding directions of the carbon fibers are random, and the carbon fibers are dispersed in the gel-like substance and thus do not have a specific orientation.

[0107] Table 2 summarizes the measurement results of the composition and properties for the composite materials related to Samples A1 to A14 and Samples B1 to B7. The upper row shows the mixing ratios (unit: mass %) of the filler and the matrix material, the middle row shows the mixing amount of the filler (unit: volume %) and the content of the heat conductive substance (unit: volume %). Here, the content of the heat conductive substance is calculated from the volume ratio of the heat conductive substance in the filler and the mixing amount of the filler in the composite material. The lower row summarizes the measurement results of the specific gravity and the thermal conductivity.

[0108]

Table 2

[0109] Samples A1 to A14 are formed by providing a coating layer in which a heat-conductive substance is dispersed in a gel-like substance on the surface of base particles having an amino group introduced by surface treatment to form a filler, and adding the filler to a matrix material. In these samples A1 to A14, although the filler is added up to a maximum of 50% by volume, the specific gravity of the composite material is equal to or less than the specific gravity of sample B1 without the filler added, or is suppressed to an increase of 10% or less with respect to the specific gravity of sample B1.

[0110] And in samples A1 to A14, in all cases, the thermal conductivity is 1.5 W / (m·K) or more. These values are equivalent to 7 times or more compared to the thermal conductivity of sample B1 without the filler added. From this result, in samples A1 to A14 to which the filler with a coating layer in which a heat-conductive substance is dispersed in a gel-like substance is provided on the surface of the base particles, it can be seen that by including the filler with low-specific gravity hollow base particles, a high thermal conductivity can be obtained while keeping the specific gravity of the entire composite material with the filler added small. It can be interpreted that the coating layer stably holds the heat-conductive substance in the structure of the gel-like substance, and the gel-like substance forms a chemical bond with the base particles, so that the state in which the base particles are coated by the coating layer showing high thermal conductivity with the heat-conductive substance dispersed is stably formed. It is considered that continuous heat conduction paths are formed on the surface of each filler particle by the particles of the heat-conductive substance held in a dispersed state in the gel-like substance coming into contact with each other within the layer of the coating layer. In addition, due to the effect of the volume occupied by the base particles, the coating layers on the surfaces of adjacent fillers come into contact with each other, and it is interpreted that heat conduction paths are also formed between the filler particles, showing a high effect on the improvement of the thermal conductivity.

[0111] Here, Samples B2 to B5 are examined. In Sample B2, the surface-treated glass hollow particles themselves (AP-GB) are added to the matrix material, and the specific gravity is lower compared to Sample B1 without any filler added. However, since the filler does not contain a heat-conductive substance, the thermal conductivity has not improved compared to Sample B1. The thermal conductivity of glass itself is about 1.0 W / (m·K), which is higher than that of the matrix material. However, since it is in the form of hollow particles and contains air inside, phonon scattering occurs inside the particles, making it difficult for heat conduction through the particles. Thus, it is interpreted that the glass hollow particles themselves cannot be used as a heat-conductive filler.

[0112] As the heat-conductive substances, carbon fibers themselves are added at 7.2% by volume in Sample B3, and alumina particles themselves are added at 7.2% by volume in Sample B4, respectively, in their single form. This addition amount is the same as that of Samples A1 to A5 as the content of the heat-conductive substance. However, in these Samples B3 and B4, the thermal conductivity is 0.6 W / (m·K) or less. Samples B3 and Samples A1 and A4, and Samples B4 and Samples A2 and A5 use the same heat-conductive substances, but the thermal conductivities obtained in Samples B3 and B4 are 1 / 4 or less lower compared to Samples A1, A4, and Samples A2, A5. This is considered to be because in Samples B3 and B4, the volume occupied by the filler in the composite material is small, so the contact area between the filler particles is small, and the formation of the heat conduction path between the filler particles is not effectively achieved. In particular, the carbon fibers used in Sample B3 have high anisotropy in shape. In the resin cured product test piece formed by press molding, the fiber axis direction is oriented in the in-plane direction of the test piece, which is perpendicular to the measurement direction of the thermal conductivity. Therefore, it is considered that the heat conduction path along the fiber axis direction, which is effective in improving the measured thermal conductivity, is not effectively formed in the thickness direction of the test piece.

[0113] In Samples B5 and B6, as heat-conductive substances, carbon fiber itself and alumina itself are each added alone, similar to Samples B3 and B4, but the blending amount is increased to 30% by volume. This blending amount is the same as that of Samples A1 to A5 in terms of the filler blending amount (the volume ratio occupied by the filler). In these Samples B5 and B6, compared with Sample B1 without the addition of a filler, the thermal conductivity has been significantly improved. This result indicates that, compared with Samples B3 and B4, the increase in the addition amount of the heat-conductive substance has increased the contact area between the filler particles and formed an effective heat conduction path. However, due to the increase in the volume occupied by the heat-conductive substance itself, the specific gravity of the composite material is nearly 1.4 times that of Sample B1 or even larger. Sample B5 uses the same heat-conductive substance as Samples A1 and A4, and Sample B6 uses the same heat-conductive substance as Samples A2 and A5, and these pairs are compared with each other. Then, in Samples B5 and B6, although the content of the heat-conductive substance is more than four times higher, the obtained thermal conductivity has not improved compared with Samples A1, A4, and Samples A2, A5. In Sample B5 using carbon fiber, the thermal conductivity is rather lower. Thus, from the results of Samples B3 to B6, it can be said that it is difficult to achieve both low specific gravity and high thermal conductivity when using fillers composed of heat-conductive substances alone, such as carbon fiber and alumina. In particular, when using a heat-conductive substance with high anisotropy in shape, such as carbon fiber, alone, it can be said that it is difficult to control the anisotropic orientation of the heat-conductive substance and achieve high thermal conductivity in a desired direction.

[0114] In Sample B7, in addition to the thermally conductive material, the filler is composed of base material particles and a gel-like material. However, the base material particles used are glass hollow particles that have not been surface-treated. In this Sample B7, as the thermally conductive material, it contains the same amount of carbon fiber as Samples A1, A4, and B3. However, the thermal conductivity of Sample B7 is significantly lower compared to the values of Samples A1 and A4 that use the gel-like material, and is close to the value of Sample B3 where carbon fiber is added alone as the thermally conductive material. In this Sample B7, since the base material particles do not have functional groups on their surfaces that can form chemical bonds with the gel-like material, it is considered that the coating layer is not firmly bonded to the surface of the base material particles via chemical bonds. Correspondingly, in Sample B7, the thermally conductive material is not stably maintained in a state of covering the surface of the base material particles while being dispersed in the gel-like material, but rather it is considered to cover only a very small area region on the surface of the base material particles or to be dispersed in the matrix resin independently of the base material particles.

[0115] Finally, Samples A1 to A14 are compared with each other. First, in the group of Samples A1 to A3 and the group of Samples A4 and A5, the types of thermally conductive materials used are different from each other. When these samples are compared with each other, in Samples A1 and A4 that use carbon fiber as the thermally conductive material, the thermal conductivity is significantly higher compared to other samples that use alumina particles or magnesium oxide particles. This corresponds to the fact that carbon fiber has higher thermal conductivity compared to alumina or magnesium oxide. Carbon fiber has high anisotropy in shape. As described above for Sample B3, although it shows high thermal conductivity in the direction along the oriented fiber axis direction, in other directions, the effect of improving thermal conductivity is limited. However, as shown in the microscope image of Figure 3A, by dispersing carbon fiber in the gel-like material, the carbon fiber can be randomly dispersed in the coating layer and blended into the matrix resin, thereby reducing the influence due to the anisotropy of the shape and enabling excellent thermal conductivity to be exhibited in any direction.

[0116] In the groups of Samples A1 and A4, and also in the groups of Samples A2 and A5, the types of gelling polymer materials used are different. However, when comparing the thermal conductivities in those groups, the thermal conductivity does not change significantly regardless of which gelling polymer material is used. From this, it can be said that if a thermally conductive substance can be dispersed and retained, the type of gel-like substance does not have a significant impact on the thermal conductivity of the resulting filler.

[0117] In Samples A1 and A6 - A9, in the mixed gel used as the raw material for the coating layer, the addition amounts of carbon fibers as thermally conductive substances to the gel-like substance ([dry volume ratio of gelling polymer]:[thermally conductive substance]) are different from each other, and correspondingly, the contents of the thermally conductive substances are different from each other. The content of the thermally conductive substance is in the order of Samples A6, A7, A1, A8, A9 being more, and in that order, the thermal conductivity of the composite material is higher. That is, the higher the content of the thermally conductive substance in the filler, the higher the effect of improving thermal conductivity. In Sample A8, the [dry volume ratio of gelling polymer]:[thermally conductive substance] is 30:70, but in the region where the content of the thermally conductive substance is higher than that, a high thermal conductivity of 2.0 W / (m·K) or more is obtained.

[0118] In Samples A1 and A10 - A12, due to the different amounts of the mixed gel used for the base material particles ([dry volume ratio of mixed gel]:[base material particles]), the contents of the thermally conductive substances are different from each other. The content is in the order of Samples A12, A11, A1, A10 being more, and in that order, the specific gravity of the composite material increases and the thermal conductivity is higher. That is, in the filler, the larger the proportion occupied by the coating layer and the thicker the coating layer formed, the higher the effect of improving thermal conductivity. In Sample A1, the [dry volume ratio of mixed gel]:[base material particles] is 30:70, but in the region where the proportion occupied by the coating layer is larger than that, a high thermal conductivity of 2.0 W / (m·K) or more is obtained.

[0119] In Samples A1, A13, and A14, the filler addition amounts in the composite materials are different from each other. In the order of Samples A14, A1, and A13, the filler addition amount is large, and in that order, the specific gravity of the composite material increases, and the thermal conductivity also increases. That is, the more the filler is added, the higher the effect of improving the thermal conductivity. In Samples A1 and A14 where the filler addition amount exceeds 20% by volume, a high thermal conductivity of 2.0 W / (m·K) or more is obtained. However, like Sample A14, even if the filler blending amount is increased to 50% by volume or more, the effect of improving the thermal conductivity becomes limited.

[0120] As described above, the embodiments of the present disclosure have been described in detail. However, the present invention is not limited to the above embodiments at all, and various modifications can be made without departing from the gist of the present invention.

Explanation of Reference Numerals

[0121] 1 (Thermally conductive) composite material 10 (Thermally conductive) filler 11 Base material particles (hollow particles) 11a Hollow part 11b Shell 12 Coating layer 12a Gel-like substance 12b Thermally conductive substance 2 Matrix material 5 Wire harness 51 Insulated wire 52 Connector 53 Tape

Claims

1. Base material particles, A coating layer that coats the base material particles, and has, The coating layer is, A gel-like substance that is bonded to the surface of the base material particles via a chemical bond and coats the surface of the base material particles, A heat conductive material that is dispersed in the layer of the gel-like substance and has a higher thermal conductivity and a larger specific gravity than the base material particles and the gel-like substance, and includes, The base material particles are configured as a hollow body of glass, and have a functional group on the surface that can form a chemical bond with the functional group of the gel-like substance, a heat conductive filler.

2. The heat conductive filler according to claim 1, having a specific gravity of 1.8 or less.

3. A heat conductive composite material including the heat conductive filler according to claim 1 or claim 2 and a matrix material, The heat conductive filler is dispersed in the matrix material.

4. The matrix material includes an organic polymer, and the heat conductive composite material according to claim 3.

5. The heat conductive composite material according to claim 3 or claim 4, having a specific gravity of 1.4 or less.

6. The heat conductive composite material according to any one of claims 3 to 5, having a thermal conductivity at room temperature of 0.9 W / (m·K) or more.

7. A wire harness including the heat conductive composite material according to any one of claims 3 to 6.

8. A gel preparation step of preparing the gel-like substance with the heat conductive material dispersed therein, A coating step of bonding the gel-like substance in which the heat conductive material is dispersed, prepared in the gel preparation step, to the surface of the base material particles via a chemical bond, and includes, A method for manufacturing a heat conductive filler for manufacturing the heat conductive filler according to claim 1 or claim 2.

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