Graphite flake laminate and method for producing same, and heat conduction member and heat dissipation structure each using same

Abstract

Provided are: a graphite flake laminate that exhibits high thermal conductivity and can be used as a sheet-form heat conduction member; and a production method with which it is possible to produce this graphite flake laminate by using safe and simple production equipment.　The present disclosure provides: a graphite flake laminate containing a large number of laminated graphite flakes and a metal material that fills voids between the graphite flakes, the volume ratio of the metal material to 100 vol% of the graphite flakes being 0.21-14.0 vol%; and a method for producing the graphite flake laminate.

Description

Graphite Flake Laminate, Method for Producing the Same, Thermal Conductive Member Using the Same, and Heat Dissipation Structure

[0001] The present invention relates to a graphite flake laminate, a method for producing the same, a thermal conductive member using the same, and a heat dissipation structure.

[0002] In semiconductor elements mounted in various electronic devices such as personal computers, light-emitting diode (LED) light sources, thin displays, and other devices, heat is generated during driving, and if the generated heat accumulates, it has an adverse effect on the driving of the semiconductor element and peripheral devices. Furthermore, with the progress of IoT, the development of high-speed communication networks, and the intelligence of various devices, the demand for efficient heat dissipation from electronic devices is increasing. Similarly, for secondary batteries used as power sources for motor drives in vehicles, from the viewpoints of increasing their capacity and saving space during vehicle mounting, the demand for efficiently dissipating the heat generated during their driving is also strengthening.

[0003] Here, various cooling means are used for the purpose of dissipating the heat generated by driving various electronic devices and power sources for motor drives as described above. For example, as a method for cooling electronic components such as semiconductor elements, a method of attaching a fan to the device to cool the air inside the device housing, or a method of attaching a heat sink such as heat dissipation fins or a heat dissipation plate to the semiconductor element to be cooled is used. In addition, for cooling the power source for motor drives, in addition to cooling using a cooling medium such as air or water, cooling using a fan or a heat sink as described above is also performed.

[0004] Conventionally, when cooling a semiconductor element by attaching a heat sink, a technique of providing a thermal conductive sheet between the semiconductor element and the heat sink has been proposed in order to efficiently release the heat of the semiconductor element.

[0005] As a method for producing graphene paper that can be used as such a thermal conductive sheet, for example, Non-Patent Document 1 (Tongshun Wu, et al., Efficient and inexpensive preparation of graphene laminated film with ultrahigh thermal conductivity, Carbon, Volume 171, 2021, Pages 639-645) describes a method of placing sulfonic acid chloride (HSO4) between the graphene layers of a carbon material which is a graphene aggregate. ３ After intercalating with Cl, hydrogen peroxide is added, and sulfur dioxide (SO) is added between the graphene layers. ２ ), hydrogen chloride (HCl), oxygen (O ２ A technology has been disclosed in which gases such as ) are generated, and the raw material is flaked off to obtain a graphite flake laminate. According to Non-Patent Literature 1, this method makes it possible to efficiently produce a large quantity of graphite flake laminates exhibiting a high thermal conductivity of 950 W / m·K at low energy cost and low cost.

[0006] In the technology described in Non-Patent Document 1, high thermal conductivity is achieved by introducing highly oriented organic matter (tannic acid) in a later process or by sintering at a high temperature of around 600°C. However, there is a problem that the thermal conductivity is insufficient when these later processes are not performed. Furthermore, the technology described in Non-Patent Document 1 also has the problem that additional equipment is required due to the generation of harmful gases, and the introduction of large-scale gas removal and neutralization equipment is also required when scaling up.

[0007] Therefore, the present invention aims to provide a graphite flake laminate exhibiting high thermal conductivity that can be used as a sheet-shaped heat-conducting member, and a manufacturing method that enables the safe and simple production of such a graphite flake laminate using manufacturing equipment.

[0008] The inventors of the present invention conducted intensive studies in view of the above problems. As a result, they found that the above problems can be solved by filling the voids between graphite flakes with a metal material in a graphite flake laminate containing many stacked graphite flakes, and by controlling the volume ratio of the metal material in the laminate within a predetermined range. They also found that such a graphite flake laminate can be manufactured by obtaining a cake layer by removing the dispersion medium from a dispersion liquid in a specific direction from a dispersion liquid in which a carbon material which is an aggregate of graphene, a metal material, and a surfactant are dispersed in a dispersion medium, and then subjecting the cake layer to a drying treatment and / or calcination treatment. Based on these findings, the inventors of the present invention have completed the present invention.

[0009] In other words, one embodiment of the present invention is a graphite flake laminate comprising a large number of stacked graphite flakes and a metal material filling the voids between the graphite flakes, wherein the volume ratio of the metal material to 100 volume percent of the graphite flakes is 0.21 to 14.0 volume percent.

[0010] Another embodiment of the present invention is a method for producing a graphite flake laminate, comprising obtaining a cake layer by removing the dispersion medium from a dispersion in a specific direction from a dispersion in which a carbon material which is an aggregate of graphene, a metal material, and a surfactant are dispersed in a dispersion medium, and then subjecting the cake layer to a drying treatment and / or calcination treatment.

[0011] Figure 1 is a schematic cross-sectional view of a graphite flake laminate 10 according to one embodiment of the present invention. Figure 2 is an enlarged view of a part 10a of the graphite flake laminate 10 shown in Figure 1. Figure 3 is an explanatory diagram for illustrating the structure of (a) single-layer graphene (graphene sheet) and (b) multi-layer graphene. Figure 4 is a schematic cross-sectional view of a heat dissipation structure according to the conventional technology. Figure 5 is a schematic cross-sectional view of a heat dissipation structure according to one embodiment of the present invention. Figure 6 is an observation image of the laminated cross-section of the graphite sheet laminate obtained in Example 5, obtained by observing the laminated cross-section using a scanning electron microscope (SEM). The magnifications of (a) to (c) in Figure 6 are (a) 250x, (b) 2000x, and (c) 10000x, respectively.

[0012] 《Graphite Flake Laminate》 One embodiment of the present invention is a graphite flake laminate comprising a large number of laminated graphite flakes and a metal material filling the voids between the graphite flakes, wherein the volume ratio of the metal material to 100 volume percent of the graphite flakes is 0.21 to 14.0 volume percent.

[0013] The present invention provides a graphite flake laminate exhibiting high thermal conductivity that can be used as a sheet-shaped heat-conducting member, and a manufacturing method that enables the safe and simple production of such a graphite flake laminate using manufacturing equipment.

[0014] The embodiments of the graphite flake laminate according to the above embodiment will be described below with reference to the drawings, but the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from the actual ratios.

[0015] Figure 1 is a schematic cross-sectional view of a graphite flake laminate 10 according to one embodiment of the present invention. Figure 2 is an enlarged view of a part 10a of the graphite flake laminate 10 shown in Figure 1. The graphite flake laminate 10 shown in Figures 1 and 2 is a structure in which a large number of graphite flakes 100 are stacked. There are voids between the large number of graphite flakes 100. In the graphite flake laminate 10 according to this embodiment, the voids are filled with a metal material 110. In the graphite flake laminate 10 according to this embodiment, the volume ratio of the metal material to the volume % of the graphite flakes 100 is 0.21 to 14.0 volume% (for example, 6.30 volume%).

[0016] The following describes the structure of the graphite flake laminate relating to this embodiment.

[0017] [Graphite Flakes] The graphite flakes that make up the graphite flake laminate are flake-shaped carbon materials having a graphene structure. From the viewpoint of having particularly high thermal conductivity in the planar direction, the graphite flakes preferably contain single-layer graphene, multi-layer graphene, or graphite, and among these, it is preferable that they contain multi-layer graphene. Here, multi-layer graphene has a structure in which multiple single-layer graphene (graphene sheets) shown in Figure 3(a) are stacked in parallel (Figure 3(b)). Since graphene sheets have a structure in which carbon atoms with sp2 hybrid orbitals (sp2 carbon) are linked in a honeycomb structure in two dimensions, they have the characteristic of having extremely high thermal conductivity in the planar direction. As a result, the graphite flake laminate also has very high thermal conductivity in the planar direction because the adjacent graphite flakes are arranged so that they are in close contact with each other and the long axes of the graphite flakes are oriented in the planar direction. In other words, graphite flake laminates have a selectively enhanced thermal conductivity in the planar direction compared to the thermal conductivity in the thickness direction (the direction perpendicular to the planar direction (the lamination direction)).

[0018] As described above, graphite flakes have a structure in which multiple graphene layers are stacked, but there are no particular restrictions on the number of stacked graphene layers in graphite flakes. In this specification, graphite flakes with up to 10 stacked graphene layers are referred to as "graphene." Among graphene, those with one stacked graphene layer are referred to as "single-layer graphene," and those with two to ten stacked graphene layers are referred to as "multilayer graphene." Furthermore, graphite flakes with 11 or more stacked graphene layers are referred to as "graphite."

[0019] Here, as an example, the number of graphene layers in the graphite flake is preferably 2 to 100 layers, more preferably 2 to 50 layers, even more preferably 2 to 20 layers, and particularly preferably 2 to 10 layers. As described above, multilayer graphene or graphite has the characteristic of being particularly excellent in thermal conductivity in the planar direction, and furthermore, multilayer graphene is readily available at low cost, so it is suitably used in the graphite flake laminate according to this embodiment. Note that only one type of graphite flake may be used alone, or two or more types may be used in combination.

[0020] Graphite flakes exhibit anisotropy with respect to their size. In this specification, the longest line segment connecting any two points on the contour of the graphite flake in the XY plane, where the plane with the largest area among the surfaces constituting the surface of the graphite flake is defined as the major axis of the graphite flake. There are no particular restrictions on the value of the average major axis of the graphite flakes, but it is preferably 0.1 to 1000 μm, more preferably 0.1 to 500 μm, and even more preferably 0.1 to 100 μm. The value of the average major axis of the graphite flakes refers to the arithmetic mean of the major axes of several dozen graphite flakes contained in the graphite flake laminate.

[0021] Furthermore, in this specification, the short axis of a graphite flake is defined as the maximum dimension of the edge constituting the XZ plane or YZ plane, when the surface with the largest area among the faces constituting the surface of the graphite flake is defined as the XY plane. There are no particular restrictions on the value of the average short axis of the graphite flake, but it is preferably 0.6 to 30 nm, more preferably 0.6 to 15 nm, and even more preferably 0.6 to 3 nm. The value of the average short axis of the graphite flake is intended to mean the arithmetic mean of the short axes of several dozen graphite flakes contained in the graphite flake laminate.

[0022] The ratio of the major axis to the minor axis of the graphite flakes obtained as described above is defined as the aspect ratio. There are no particular restrictions on the average aspect ratio of the graphite flakes, but it is preferably greater than 1 and less than or equal to 2,000,000, more preferably between 5 and 900,000, and even more preferably between 30 and 200,000. The average aspect ratio of the graphite flakes is defined as the arithmetic mean of the aspect ratios of several dozen graphite flakes contained in the graphite flake laminate.

[0023] There are no particular restrictions on the graphite flake content in the graphite flake laminate, but it is preferably 35 to 99% by mass, more preferably 40 to 95% by mass, even more preferably 45 to 90% by mass, particularly preferably 48 to 85% by mass, and most preferably 50 to 70% by mass, based on 100% by mass of the total amount of constituent components of the graphite flake laminate.

[0024] [Metallic Material] The graphite flake laminate according to this embodiment is characterized by further containing a metallic material in addition to graphite flakes. In this specification, "metallic material" means a single metallic element or an alloy (including solid solution). As described above, in a graphite flake laminate, a large number of graphite flakes are stacked on top of each other, but if the laminate is made using only graphite flakes, a certain amount of voids will inevitably exist between the graphite flakes. The graphite flake laminate according to this embodiment further contains a metallic material that fills these voids.

[0025] There are no particular restrictions on the type of metallic material used to fill the voids between graphite flakes. Suitable metallic materials include solder metal, which can melt and fill the voids during the manufacturing of the graphite flake laminate. In particular, the melting point of the metallic material is preferably 350°C or lower, more preferably 300°C or lower, even more preferably 250°C or lower, even more preferably 200°C or lower, particularly preferably 170°C or lower, and most preferably 140°C or lower. If the melting point of the metallic material is 350°C or lower, heat treatments such as drying and / or firing can be performed at relatively low temperatures to melt the metallic material during the manufacturing of the graphite flake laminate. At temperatures exceeding 350°C, there is a risk that defects in graphene may decompose into carbon dioxide under an oxygen atmosphere. In contrast, by setting the melting point of the metallic material to 350°C or lower and performing various treatments at relatively low temperatures, such thermal decomposition and the resulting decrease in thermal conductivity can be prevented. Furthermore, this also contributes to energy conservation. Examples of metallic materials with a melting point of 350°C or lower include one or more selected from the group consisting of lead, cadmium, tin, indium, bismuth, gallium, selenium, polonium, potassium, cesium, sodium, radon, lithium, and rubidium, as well as alloys containing these metals. In particular, from the viewpoint that water can be used as a solvent during film formation, it is preferable that the metallic material contains one or more selected from the group consisting of lead, cadmium, tin, indium, bismuth, gallium, selenium, and polonium, as well as alloys containing these metals. From the viewpoint of minimizing harm to the human body, it is more preferable that the material contains one or more selected from the group consisting of tin, indium, bismuth, gallium, selenium, and polonium, as well as alloys containing these metals, and even more preferable that the material contains one or more selected from the group consisting of indium, bismuth, gallium, selenium, and polonium, as well as alloys containing these metals. Specific examples of these metallic materials include U-Alloy 138G (manufactured by Osaka Asahi Metal Factory Co., Ltd.; a Bi-Sn alloy with a melting point of 138°C).

[0026] There are no particular restrictions on the content of metal material in the graphite flake laminate, but it is preferably 1 to 65% by mass, more preferably 5 to 60% by mass, even more preferably 10 to 55% by mass, particularly preferably 15 to 52% by mass, and most preferably 30 to 50% by mass, based on 100% by mass of the total amount of constituent components of the graphite flake laminate. Furthermore, in the graphite flake laminate, the volume ratio of metal material to 100% by volume of graphite flakes is essentially 0.21 to 14.0% by volume, preferably 0.21 to 10.5% by volume, more preferably 1.05 to 10.5% by volume, even more preferably 3.15 to 10.5% by volume, and particularly preferably 3.15 to 6.30% by volume. Thus, it is remarkable that the thermal conductivity in the plane direction of the graphite flake laminate can be greatly improved by adding only a very small amount of metal material to the graphite flakes. Here, if the volume percentage of the metallic material is less than 0.21 vol%, the metallic material may not be distributed throughout the entire graphite flake laminate, and the effect of improving thermal conductivity may not be sufficiently obtained. On the other hand, if the volume percentage of the metallic material exceeds 14.0 vol%, it may inhibit the orientation of the graphite flakes during the film formation and pressing stages, impairing the high thermal conductivity derived from the graphite flakes. The above volume percentage values ​​shall be calculated by crushing the graphite flake laminate into a powder, measuring the mass of the carbon material and metallic material respectively using methods such as EDX (energy-dispersive X-ray spectroscopy), ICP, specific gravity separation, and GC / MS (EDX method shall be used if the measured values ​​fluctuate), and then calculating the values ​​from the specific gravity of the carbon material and metallic material according to Formula 1 described in the Examples section below.

[0027] In the graphite flake laminate according to this embodiment, the metal material fills the voids between the graphite flakes. The proportion of these voids (porosity in the cross-section in the stacking direction of the graphite flake laminate) is preferably 20.0% or less, more preferably 15.0% or less, even more preferably 10.0% or less, and particularly preferably 7.2% or less. On the other hand, there is no particular limit to the lower limit of this proportion of voids, but it is usually 1.0% or more, preferably 3.1% or more, and particularly preferably 6.1% or more. That is, in one preferred embodiment, the porosity is 1.0 to 7.2%, and more preferably 3.1 to 7.2%. To achieve this porosity value, the manufacturing method described later can be employed, or the press pressure when pressing the obtained laminate can be adjusted. Furthermore, in this specification, the above-mentioned porosity value shall be calculated by observing the layered cross-section using a scanning electron microscope (SEM) (observation magnification 2000x), and by performing a binarization process on a 60 μm × 40 μm field of view of the observed image, dividing the carbon material (graphene) portion and the void portion filled with metal material, and expressing it as the percentage of the void portion to the entire image.

[0028] [Other Components] The graphite flake laminate according to this embodiment essentially contains graphite flakes and metal materials, but may further contain other components. However, since the inclusion of other components may reduce the thermal conductivity in the planar direction, it is also preferable not to include other components.

[0029] Other such components include, for example, carbon materials other than graphite flakes, thermally conductive fillers other than carbon and metallic materials, and binders.

[0030] In addition to graphite flakes, conventionally known carbon materials can be used. From the viewpoint of further improving thermal conductivity in the planar direction, examples include carbon black, fullerenes, carbon nanotubes, carbon nanofibers (vapor-grown carbon fibers (VGCF, etc.)), carbon nanohorns, carbon microcoils, and carbon nanocoils.

[0031] Examples of thermally conductive fillers other than carbon and metallic materials include carbides, nitrides, oxides, hydroxides, and metals. Examples of carbides include silicon carbide, boron carbide, aluminum carbide, titanium carbide, and tungsten carbide. Examples of nitrides include silicon nitride, boron nitride, aluminum nitride, gallium nitride, chromium nitride, tungsten nitride, magnesium nitride, molybdenum nitride, and lithium nitride. Examples of oxides include iron oxide, silicon oxide (silica), aluminum oxide (alumina) (including aluminum oxide hydrate (boehmite, etc.)), magnesium oxide, titanium oxide, cerium oxide, and zirconium oxide. Furthermore, examples of oxides include transition metal oxides such as barium titanate, and even oxides doped with metal ions, such as indium tin oxide and antimony tin oxide. Examples of hydroxides include aluminum hydroxide, calcium hydroxide, and magnesium hydroxide.

[0032] Binders are used to improve the binding properties and protect various components of a compound. Examples of binders include thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene block copolymer and its hydrogenated products, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), and polychlorotrifluoroethylene (PC). Fluororesins such as TFE, ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), Examples include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-per-fluoromethyl vinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers), as well as epoxy resins. Among these, polyimide, styrene-butadiene rubber, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are preferably used.

[0033] There are no particular restrictions on the film thickness of the graphite flake laminate according to this embodiment, and it can be appropriately determined according to the desired thermal conductivity, the size and physical properties of the installation surface of the substrate or heat source on which the laminate is installed as a thermal conductive member, etc. For example, the film thickness of the graphite flake laminate is preferably 10 to 500 μm, more preferably 20 to 400 μm, even more preferably 30 to 300 μm, particularly preferably 40 to 200 μm, and most preferably 50 to 100 μm.

[0034] 《Method for Manufacturing Graphite Flake Laminates》 There are no particular restrictions on the method for manufacturing graphite flake laminates according to this embodiment, and any manufacturing method capable of realizing a graphite flake laminate having the above-described structure can be appropriately adopted.

[0035] As an example, the graphite flake laminate according to this embodiment can be manufactured by a manufacturing method provided in another embodiment of the present invention, which includes obtaining a cake layer by removing the dispersion medium from a dispersion liquid in a specific direction from a dispersion liquid in which a carbon material which is an aggregate of graphene, a metal material, and a surfactant are dispersed in a dispersion medium, and then subjecting the cake layer to a drying treatment and / or calcination treatment. According to such a manufacturing method, a graphite flake laminate with excellent thermal conductivity in the planar direction can be manufactured with safe and simple manufacturing equipment. The manufacturing method will be described below.

[0036] In this manufacturing method, first, a carbon material, which is an aggregate of graphene, is prepared as a raw material. There are no particular restrictions on this carbon material; any material having the structure of a graphene aggregate can be used as appropriate. In particular, it is preferable that the carbon material (graphene aggregate) used as a raw material has a multilayer structure. Examples of carbon materials (graphene aggregates) having such a multilayer structure include expanded graphite, graphene flakes, and graphene nanoplatelets. In particular, from the viewpoint that the particle size of the graphite flakes constituting the graphite flake laminate can be increased to reduce the interfacial thermal resistance and improve the thermal conductivity in the planar direction of the laminate, it is preferable that the carbon material (graphene aggregate) used as a raw material contains graphite such as expanded graphite.

[0037] Next, the carbon material prepared above is dispersed in a dispersion medium together with the metal material and surfactant to form a dispersion. Water is usually used as the dispersion medium. The amounts of the dispersion medium and surfactant should be appropriately determined so as to be sufficient to adequately disperse the carbon material and metal material in the dispersion medium. The details of the metal material are as described above, and the amount used is preferably the volume ratio shown in the section on graphite flake laminate according to one embodiment of the present invention. Furthermore, any surfactant that can disperse the carbon material and metal material in the dispersion medium can be used as appropriate. Examples of surfactants include carboxymethylcellulose (CMC), sodium carboxymethylcellulose (NaCMC), hydroxypropylcellulose, hydroxyethylcellulose, polyvinylpyrrolidone (PVP), any polymer prepared using N-vinylpyrrolidone monomer, polyacrylic acid esters and analogs of polyacrylic acid esters, polyamino acids (e.g., polyalanine, polyleucine, polyglycine), polyamide hydroxyurethane, polylactone, polyacrylamide, xanthan gum, chitosan, polyethylene oxide, polyvinyl alcohol (PVA), polyvinyl acetate, polyacrylic acid, polyethyleneimine (PEI), sugar alcohols (e.g., sorbitol and xylitol), esters of anhydrosorbitol, secondary alcohol ethoxylates, and combinations thereof. In particular, the surfactant preferably contains carboxymethylcellulose (CMC), sodium carboxymethylcellulose (NaCMC), hydroxypropylcellulose, or hydroxyethylcellulose, more preferably contains carboxymethylcellulose (CMC) or sodium carboxymethylcellulose (NaCMC), and especially preferably contains carboxymethylcellulose (CMC).

[0038] In this manufacturing method, the dispersion prepared above is subjected to an operation to remove the dispersion medium in a specific direction, for example, while the dispersion is coated onto the surface of a substrate. This yields a cake layer (for example, a cake layer formed on the surface of the substrate). There are no particular restrictions on the specific means used to perform this operation to remove the dispersion medium, but examples include vacuum filtration film formation, centrifugal separation, and pressing. Among these, it is preferable to perform the solvent removal operation by vacuum filtration film formation, as this is simpler and more reliable, and promotes the orientation of graphite flakes, thereby obtaining a graphite flake laminate with particularly excellent thermal conductivity in the planar direction.

[0039] In the cake layer obtained in this way, the exfoliated graphene exists in a laminated state together with the metal material. In this manufacturing method, the cake layer is subjected to a drying treatment and / or a firing treatment. In a preferred embodiment, a metal material with a melting point of 350°C or lower is used, and these drying and / or firing treatments are carried out under temperature conditions between the melting point of the metal material and 350°C. This configuration has the advantage of sufficiently reducing the risk of thermal decomposition of graphene and the resulting decrease in thermal conductivity, and also contributing to energy saving. For example, the drying treatment can be carried out at a temperature of about 40 to 90°C for several minutes to several hours, and the firing treatment can be carried out at a temperature of about 140 to 200°C for several minutes to tens of minutes, for example, in place of or in addition to the drying treatment.

[0040] Through these processes, the metallic material melts and, unlike the binder, freely deform due to surface tension to fill the gaps between the graphite flakes. Additionally, after these processes, a pressing process such as hot pressing or roll pressing may be further applied. As a result, the graphite flake laminate according to one embodiment of the present invention described above can be obtained. In the manufacturing method according to this embodiment, from the perspective of improving the thermal conductivity in the plane direction by closely adhering the graphite flakes in the stacking direction to each other, it is preferable to perform a firing process on the cake layer in which the graphite flakes are stacked. It is more preferable to further include performing a pressing process in the stacking direction of the dried and / or fired cake layer after the drying process and / or the firing process.

[0041] As the pressing process, conventionally known pressing processes such as hot pressing and roll pressing can be appropriately adopted. The hot pressing process and the roll pressing process may be used in combination, or only one of them may be used. Among these, from the perspective that the orientation in the plane direction of the graphite flakes can be improved by applying a shearing force during pressing, the roll pressing process is preferably adopted. Here, there are no particular restrictions on the pressing conditions during hot pressing. For example, conditions such as a pressing pressure of about 3 to 30 [MPa], a temperature of about 100 to 200 °C, and a duration of several minutes to several tens of minutes are exemplified. Also, there are no particular restrictions on the pressing conditions during roll pressing. For example, conditions such as a pressing pressure of about 200 to 1000 [MPa] and a rotation speed of about 50 to 200 [rpm] are exemplified.

[0042] "Use of Graphite Flake Laminate (Thermal Conductive Member and Heat Dissipation Structure)" The graphite flake laminate according to one embodiment of the present invention described above has excellent in-plane thermal conductivity. Therefore, it can be used as a thermal conductive member by utilizing this excellent in-plane thermal conductivity. The value of the thermal conductivity of the graphite flake laminate according to this embodiment is not particularly limited, but is preferably 320 [W / m·K] or more, more preferably 328 [W / m·K] or more, still more preferably 339 [W / m·K] or more, even more preferably 344 [W / m·K] or more, particularly preferably 358 [W / m·K] or more, and most preferably 362 [W / m·K] or more. The value of the in-plane thermal conductivity shall be the value measured using the method described in the column of Examples described later.

[0043] Further, this thermal conductive member can be arranged near the heat source to constitute a heat dissipation structure. That is, according to still another aspect of the present invention, a thermal conductive member having the graphite flake laminate according to one embodiment of the present invention described above is provided. Furthermore, a heat dissipation structure including a heat source and the thermal conductive member according to the above aspect arranged so as to be in contact with the heat source is also provided. In this heat dissipation structure, it is preferable that a heat radiator is further arranged so as to be in contact with the thermal conductive member. Also, it is more preferable that the heat radiator is arranged at a position not facing the heat source via the thermal conductive member. Hereinafter, the heat dissipation structure according to this embodiment will be described with reference to the drawings by taking as an example a heat dissipation structure for dissipating heat generated from a high-brightness light-emitting diode (LED) as a heat source.

[0044] In recent years, high-brightness light-emitting diode (LED) lamps have been adopted for the purpose of extending the lifespan and saving power of automotive headlights, and a heat sink is used to cool this high-brightness LED. This heat sink is usually composed of a metal material with high thermal conductivity such as pure aluminum or an aluminum alloy, and has a shape in which a plurality of fins are arranged in a row on a flat heat-receiving surface by die-casting or the like.

[0045] Figure 4 is a cross-sectional schematic view schematically showing a cross-section of such a conventional heat dissipation structure.

[0046] As shown in Figure 4, a conventional heat dissipation structure 1 is used, for example, to cool a high-brightness LED module 4 with an output of 1W or more, and has a basic configuration that combines a heat transfer plate 3 made of at least a good thermal conductor metal or carbon material and a heat sink body 2 made of a thermally conductive resin. The LED module 4 has a structure in which a light-emitting body 8, which has multiple LED elements built into it and an integrally formed lens, is held in the center of a substrate 7, and the substrate 7 is joined to a heat receiving surface 5 placed on the heat sink body 2 described above. In the heat dissipation structure 1 shown in Figure 4, the heat transfer plate 3 made of a good thermal conductor metal or carbon material provided along the heat receiving surface 5 to which the LED module 4 is joined, transfers the heat generated from the narrow heat source by the operation of the LED module 4 to the entire heat receiving surface 5, and the heat is dissipated into the air by the fins 6 of the heat sink body 2 molded from a thermally conductive resin having a low heat capacity and high emissivity, thereby suppressing the temperature rise of the LED module 4 (i.e., cooling the LED module 4). With a heat dissipation structure 1 having such a configuration, it is possible to dissipate the heat generated from the LED module 4 to a certain extent. However, the heat sink body 2 occupies a large volume within the heat dissipation structure 1, resulting in poor space efficiency. Furthermore, since the heat sink body 2 is made of a metallic material such as pure aluminum or an aluminum alloy, the heat dissipation structure 1, when applied to, for example, an automobile headlight, can increase the weight of the vehicle.

[0047] On the other hand, Figure 5 is a schematic cross-sectional view showing a cross-section of a heat dissipation structure according to one embodiment of the present invention.

[0048] As shown in Figure 5, the heat dissipation structure 1 according to one embodiment of the present invention has the heat sink body 2 removed compared to the heat dissipation structure shown in Figure 4. On the other hand, a graphite flake laminate 10 (see Figure 1) according to one embodiment of the present invention is arranged as a heat conductive member so as to cover the entire surface of the heat transfer plate 3 on the side where the LED module 4 is located. With this configuration, the heat dissipation structure 1 according to the embodiment shown in Figure 5 can quickly transfer the heat generated from the narrow heat source by the operation of the LED module 4 in the planar direction of the graphite flake laminate 10 and efficiently dissipate the heat to the outside. Furthermore, since it does not have the heat sink body that was present in the heat dissipation structure according to the embodiment shown in Figure 4, it has the extremely excellent advantage of being able to significantly reduce the volume and weight of the heat dissipation structure.

[0049] Although not shown in Figure 5, it is preferable that a heat sink is further arranged in the heat dissipation structure 1 so as to be in contact with the graphite flake laminate 10, which acts as a heat conductive member. By arranging such a heat sink, more efficient heat dissipation can be achieved. Here, "heat sink" refers to a member that can more efficiently release the heat conducted from the heat source (LED module 4 in Figure 5) by the graphite flake laminate 10 to the outside. Examples of such heat sinks include heat sinks, heat pumps, and metal casings of electronic equipment. Furthermore, when a heat sink is further arranged on the graphite flake laminate 10, it is preferable that the heat sink is positioned so as not to face the heat source via the graphite flake laminate (heat conductive member) that is arranged to be in contact with the heat source (i.e., the heat source and the heat sink are separated in the planar direction of the graphite flake laminate (heat conductive member)). In this case, the distance in the planar direction between the heat source and the heat sink of the graphite flake laminate is preferably 1 cm or more, more preferably 5 cm or more, even more preferably 10 cm or more, particularly preferably 15 cm or more, and most preferably 20 cm or more.

[0050] The heat dissipation structure according to one embodiment of the present invention has been described above using a heat dissipation structure having a high-brightness light-emitting diode (LED) as a heat source as an example. However, the heat dissipation structure according to this embodiment is not limited to this and can be used for the purpose of dissipating heat generated by a wide variety of heat sources.

[0051] Examples of such heat sources include, in addition to the LED modules mentioned above, various lasers (sensors) (conventional heat dissipation methods include aluminum plates, a combination of aluminum plates and cooling fins, Peltier elements, or chiller water cooling); high-performance infrared cameras (IR) (conventional heat dissipation methods include aluminum plates, a combination of aluminum plates and cooling fins, Peltier elements, or chiller water cooling); head-up displays (HUDs) (conventional heat dissipation methods include a combination of a heat sink and a spreader); smartphone components and batteries (conventional heat dissipation methods include air cooling, heat dissipation sheets, a combination of a heat sink and a spreader); digital camera components and batteries (conventional heat dissipation methods include a combination of a heat sink, a spreader, and a casing); and personal computers (PCs). Examples include: components and batteries (conventional heat dissipation methods include a combination of a fan, heat sink, spreader, and housing); components of electronic control units (ECUs) for automotive use, etc. (conventional heat dissipation methods include a combination of a heat sink, spreader, and housing); insulated gate bipolar transistors (IGBTs), which are the main conversion elements of high-power inverters (conventional heat dissipation methods include a combination of a heat sink, spreader, and housing); rotating parts of motors (conventional heat dissipation methods include a combination of a fan and heat sink, or a combination of these with water cooling); light source lamps for thin displays (conventional heat dissipation methods include a heat sink); and large-capacity batteries for automotive use (conventional heat dissipation methods include a combination of a heat dissipation sheet or heat dissipation material, air cooling or water cooling, and a fan and heat sink).

[0052] By applying the heat dissipation structure according to this embodiment to these heat sources, it is possible to replace the heat dissipation means conventionally applied to each heat source with the graphite flake laminate (thermal conductive member) according to one embodiment of the present invention. As a result, if the heat sink used as a conventional heat dissipation means can be removed, as explained using Figure 5, for example, the weight and volume of the heat dissipation structure can be significantly reduced. Furthermore, by replacing conventional heat dissipation means such as fans, air cooling, and water cooling with the graphite flake laminate (thermal conductive member) according to one embodiment of the present invention, it is also expected that the weight and volume of the heat dissipation structure can be significantly reduced compared to a heat dissipation structure equipped with conventional heat dissipation means.

[0053] The following embodiments are also included in the scope of the present invention: a graphite flake laminate according to claim 1 having the features of claim 2; a graphite flake laminate according to claim 1 or 2 having the features of claim 3; a graphite flake laminate according to any one of claims 1 to 3 having the features of claim 4; a graphite flake laminate according to any one of claims 1 to 4 having the features of claim 5; a graphite flake laminate according to any one of claims 1 to 5 having the features of claim 6; a graphite flake laminate according to any one of claims 1 to 6 having the features of claim 7; a graphite flake laminate according to claim 7 having the features of claim 8; a heat conductive member including a graphite flake laminate according to any one of claims 1 to 8; a heat dissipation structure using a heat conductive member according to claim 9; a heat dissipation structure according to claim 10 having the features of claim 11; a manufacturing method according to claim 12 having the features of claim 13; a manufacturing method according to claim 12 or 13 having the features of claim 14; a manufacturing method according to claim 12 or 13 having the features of claim 15.

[0054] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples.

[0055] 《Examples of Graphite Flake Laminate Fabrication》 [Example 1] First, the metal material U-alloy 138G (manufactured by Osaka Asahi Metal Factory Co., Ltd.; Bi-Sn alloy with a melting point of 138°C; specific gravity 8.4 g / cm³)３ A predetermined amount of ) was weighed and dispersed in pure water to prepare a metal dispersion. On the other hand, a predetermined amount of carboxymethylcellulose sodium (CMC Daicel 1190, manufactured by Daicel Mirise Co., Ltd., hereinafter also simply referred to as "CMC") was weighed and dissolved in pure water to prepare a 1% by mass CMC solution. Next, a predetermined amount of this 1% by mass CMC solution was weighed and added to pure water, and mixed for 1 minute at 2000 rpm using a planetary stirring type mixing and kneading device "Awatori Rentaro" (ARE-310, manufactured by Thinky Co., Ltd.), followed by ultrasonic stirring treatment (28 kHz) for 10 minutes to prepare a 0.05% by mass CMC solution.

[0056] The 0.05% by mass CMC solution and metal dispersion prepared above were mixed in a predetermined ratio to obtain a mixture. This mixture was mixed at 2000 rpm for 1 minute using the mixing and kneading apparatus described above, and then subjected to ultrasonic stirring (28 kHz) for 10 minutes.

[0057] Next, graphite, a carbon material (specific gravity 1.8 g / cm³) ３ A predetermined amount of the powder was weighed and added to the solution obtained above. Then, the mixture was mixed for 1 minute at 2000 rpm using the above mixing and kneading apparatus, and then ultrasonic stirring (28 kHz) was performed for 10 minutes to obtain a coating slurry. The mass ratio of the components of the obtained coating slurry was graphite:CMC:metal material:pure water = 100:5:1:10000.

[0058] A coating film made of graphite sheets was prepared on filter paper by a vacuum filtration film-forming method using the coating slurry obtained above. The obtained coating film was dried on a hot plate by holding it at 60°C for 30 minutes, and then at 80°C for 30 minutes. Next, the coating film was fired by holding it in an electric furnace at 130°C for 60 minutes. Then, the fired product was subjected to roll pressing using an electric roll press machine. Finally, the product was fired in a muffle furnace at 250°C for 120 minutes to obtain the graphite flake laminate (film thickness 80 μm) of this example.

[0059] Furthermore, the volume ratio of the metallic material to 100% by volume of graphite flakes in the graphite flake laminate obtained above was calculated according to Formula 1 below. As a result, the volume ratio of the metallic material was 0.21% by volume. Also, in Formula 1, the specific gravity of the graphite in sheet form was 1.8 g / cm³. ３ This was adopted. The density of the graphite powder is 2.2 g / cm³. ３ However, in its sheet form, the specific gravity of graphite decreases to the value mentioned above.

[0060]

[0061] [Example 2] The graphite flake laminate of this example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 5% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 1.05% by volume.

[0062] [Example 3] The graphite flake laminate of this example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 10% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 2.10% by volume.

[0063] [Example 4] The graphite flake laminate of this example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 15% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 3.15% by volume.

[0064] [Example 5] A graphite flake laminate of this example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 20% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 4.20% by volume. Here, the graphite sheet laminate obtained in Example 5 is shown in Figures 6(a) to (c) as observed images of the laminated cross section using a scanning electron microscope (SEM). The magnifications for (a) to (c) are 250x for (a), 2000x for (b), and 10000x for (c). As shown in Figure 6, it can be seen that in the graphite sheet laminate of this example, the voids between the graphite flakes are filled with the metal material alloy (U138 alloy G).

[0065] [Example 6] The graphite flake laminate of this example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 30% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 6.30% by volume.

[0066] [Example 7] The graphite flake laminate of this example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 50% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 10.50% by volume.

[0067] [Comparative Example 1] First, a predetermined amount of sodium carboxymethylcellulose (CMC Daicel 1190, manufactured by Daicel Mirise Co., Ltd.) was weighed and dissolved in pure water to prepare a 1% by mass CMC solution. Next, a predetermined amount of this 1% by mass CMC solution was weighed and added to pure water, and mixed for 1 minute at 2000 rpm using a planetary stirring type mixing and kneading device "Awatori Rentaro" (ARE-310, manufactured by Thinky Co., Ltd.). After that, ultrasonic stirring treatment (28 kHz) was applied for 10 minutes to prepare a 0.05% by mass CMC solution.

[0068] Next, graphite, a carbon material (specific gravity 1.8 g / cm³) ３ A predetermined amount of the powder was weighed and added to the 0.05% by mass CMC solution prepared above. Then, the mixture was mixed at 2000 rpm for 1 minute using the above mixing and kneading apparatus, and ultrasonic stirring (28 kHz) was performed for 10 minutes to obtain a coating slurry. The mass ratio of the components of the obtained coating slurry was graphite:CMC:pure water = 100:5:10000.

[0069] A coating film consisting of graphite flakes was prepared on filter paper by a vacuum filtration film-forming method using the coating slurry obtained above. The obtained coating film was dried by holding it on a hot plate at 60°C for 30 minutes. Next, the coating film was fired by holding it in an electric furnace at 130°C for 30 minutes. Then, the fired product was subjected to a roll press treatment using an electric roll press machine to obtain the graphite flake laminate (film thickness 80 μm) of this comparative example.

[0070] [Comparative Example 2] A graphite flake laminate of this comparative example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 70% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 14.70% by volume.

[0071] [Comparative Example 3] A graphite flake laminate of this comparative example was obtained using the same method as in Example 1 described above, except that the content of the metal material (U138 alloy G) in the coating slurry was changed to 100% by mass relative to 100% by mass of graphite. The volume ratio of the metal material relative to 100% by volume of graphite flakes in the obtained graphite flake laminate was calculated to be 21.00% by volume.

[0072] [Evaluation of Thermal Conductivity] The thermal conductivity in the planar direction was evaluated as the performance of the graphite flake laminates prepared in the above-described examples and comparative examples when used as thermal conductive members, using the following method (steady-state method). The results are shown in Table 1 below.

[0073] First, multiple reference samples measuring 20 mm x 150 mm and with different thermal resistivity were prepared. A heat source was placed at one end of each sample, and thermocouples were installed on the sample surface at the heat source location and on the surface 50 mm away from the end. After applying a constant power to the heat source, the surface temperature difference between the heat source location and the 50 mm point was recorded after 10 minutes or more had elapsed since the temperature reached a certain level. Subsequently, a fitting formula was created by fitting the relationship between the reciprocal of the thermal resistance value and the surface temperature difference using an exponential function.

[0074] Next, for graphite sheets cut to 20 mm x 100-150 mm, the surface temperature difference at the heat source location and at the 50 mm point was recorded in the same manner as above, and the thermal conductivity was calculated from the fitting formula prepared above.

[0075] In this measurement, a current of 0.85 A was applied to a heat source with a resistance of 4 Ω to generate 2.9 W of power, thereby heating the heat source. The temperature of each thermocouple was measured 50 minutes after the start of heating.

[0076]

[0077] As can be seen from the results shown in Table 1, the graphite flake laminates of Examples 1 to 7 according to one embodiment of the present invention have a very high thermal conductivity in the planar direction because the voids between the graphite flakes are filled with a metal material and the content of the metal material is controlled to a predetermined range as a volume percent. From this, it can be said that the graphite flake laminate according to the present invention exhibits excellent performance as a thermal conductive member. In contrast, the graphite flake laminates of each comparative example either do not have the voids filled with a metal material (Comparative Example 1) or use too much metal material (Comparative Examples 2 and 3), resulting in insufficient thermal conductivity.

[0078] 1 Heat dissipation structure, 2 Heat sink body, 3 Thermal conductive plate, 4 LED module, 5 Heat receiving surface, 6 Fins, 7 Substrate, 8 Light-emitting element, 10 Graphite flake laminate, 10a Part of graphite flake laminate, 100 Graphite flakes, 110 Metal material.

Claims

1. A graphite flake laminate comprising a large number of stacked graphite flakes and a metal material filling the voids between the graphite flakes, wherein the volume ratio of the metal material to 100 volume percent of the graphite flakes is 0.21 to 14.0 volume percent.

2. The graphite flake laminate according to claim 1, wherein the volume percentage is 0.21 to 10.5 volume%.

3. The graphite flake laminate according to claim 1, wherein the volume percentage is 1.05 to 10.5 volume%.

4. The graphite flake laminate according to claim 1, wherein the volume percentage is 3.15 to 10.5 volume%.

5. The graphite flake laminate according to claim 1, wherein the volume percentage is 3.15 to 6.30 volume%.

6. The graphite flake laminate according to claim 1, wherein the graphite flakes comprise single-layer graphene, multi-layer graphene, or graphite.

7. The graphite flake laminate according to claim 1, wherein the melting point of the metal material is 350°C or lower.

8. The graphite flake laminate according to claim 7, wherein the metallic material contains one or more selected from the group consisting of lead, cadmium, tin, indium, bismuth, gallium, selenium, and polonium, and alloys containing these metals.

9. A heat-conducting member comprising a graphite flake laminate according to any one of claims 1 to 8.

10. A heat dissipation structure comprising: a heat source; and a heat conductive member according to claim 9, disposed in contact with the heat source.

11. The heat dissipation structure according to claim 10, wherein the heat dissipator is further arranged to be in contact with the heat conductive member.

12. A method for producing a graphite flake laminate, comprising: obtaining a cake layer by removing the dispersion medium from a dispersion in a specific direction from a dispersion in which a carbon material which is an aggregate of graphene, a metal material, and a surfactant are dispersed in a dispersion medium; and subjecting the cake layer to a drying treatment and / or calcination treatment.

13. The method for manufacturing a graphite flake laminate according to claim 12, wherein the volume ratio of the metal material to 100 volume percent of graphite flakes in the graphite flake laminate is 0.21 to 14.0 volume percent.

14. The method for manufacturing a graphite flake laminate according to claim 12 or 13, wherein the melting point of the metal material is 350°C or lower, and the drying treatment and / or the firing treatment are carried out under temperature conditions between the melting point and 350°C.

15. A method for producing a graphite flake laminate according to claim 12 or 13, further comprising applying a press treatment in the stacking direction of the dried and / or fired cake layer after the drying and / or firing treatment.

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