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

Abstract

[Problem] To provide: a graphite flake laminate that exhibits a high heat conductivity and can be used as a sheet-like heat conduction member; and a production method with which it is possible to produce the graphite flake laminate by using a safe and simple production facility. [Solution] Provided is a graphite flake laminate in which a large number of graphite flakes are laminated, wherein the porosity in a cross section in the lamination direction is 20.0% or less, and the median value of the aspect ratios of gaps present between the graphite flakes is 6.5 or more. This material is used as a heat conduction member for a heat dissipation structure or the like. The laminate can be produced by a method in which a specific acid is added to a liquid dispersion in which a carbon material comprising aggregates of graphene, and a carbonate salt or a hydrogencarbonate salt are dispersed in a dispersion medium.

Description

Graphite flake laminate, its manufacturing method, and heat conduction member and heat dissipation structure using the same

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

[0002] Semiconductor elements mounted in various electronic devices such as personal computers, light-emitting diode (LED) light sources, and thin displays, as well as other devices, generate heat during operation, and if the generated heat accumulates, it can adversely affect the operation of the semiconductor elements and peripheral devices. Furthermore, with the advancement of IoT, the development of high-speed communication networks, and the increasing intelligence of various devices, there is an ever-increasing demand for efficient heat dissipation from electronic devices. Similarly, with regard to secondary batteries used as power sources for driving automotive motors, there is also a growing demand for efficient heat dissipation during operation, from the perspectives of increasing capacity and reducing space required when mounted in the vehicle.

[0003] Various cooling means are used to dissipate heat generated by the operation of the various electronic devices and motor drive power supplies described above. For example, methods for cooling electronic components such as semiconductor elements include attaching a fan to the device to cool the air inside the device housing, and attaching a heat sink such as a heat dissipation fin or heat sink to the semiconductor element to be cooled. Furthermore, motor drive power supplies are cooled using cooling media such as air or water, as well as the above-mentioned fans and heat sinks.

[0004] 2. Description of the Related Art Conventionally, when a heat sink is attached to a semiconductor element for cooling, a technique has been proposed in which a thermally conductive sheet is provided between the semiconductor element and the heat sink in order to efficiently dissipate heat from 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 for producing graphene paper by injecting sulfonic acid chloride (HSO ) between the graphene layers of a carbon material that is an aggregate of graphene. ３ After intercalating sulfur dioxide (SO Cl), hydrogen peroxide was added to intercalate the graphene layers. ２ ), hydrogen chloride (HCl), oxygen (O ２ (2003) discloses a technique for producing gases such as fluorine, methyl methacrylate, etc., by which a raw material is flaked to obtain a graphite flake laminate. According to Non-Patent Document 1, this method makes it possible to efficiently and inexpensively mass-produce graphite flake laminates that exhibit a high thermal conductivity of 950 W / m K.

[0006] The technology described in Non-Patent Document 1 achieves high thermal conductivity by introducing a highly oriented organic substance (tannic acid) in a post-process and sintering at a high temperature of about 600°C. However, there is a problem that the thermal conductivity is insufficient if these post-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 large-scale gas removal and neutralization equipment is also required when scaling up.

[0007] Therefore, an object of the present invention is to provide a graphite flake laminate that exhibits high thermal conductivity and can be used as a sheet-like heat conduction member, and a manufacturing method that can manufacture such a graphite flake laminate using safe and simple manufacturing equipment.

[0008] The present inventors conducted extensive research in light of the above-mentioned problems. As a result, they found that the above-mentioned problems can be solved by setting the porosity in a cross section in the stacking direction of a graphite flake laminate formed by stacking a large number of graphite flakes to 20.0% or less and setting the median aspect ratio of voids present between the graphite flakes to 6.5 or more. They also found that such a graphite flake laminate can be produced by exfoliating a carbon material, which is an aggregate of graphene, using carbon dioxide gas generated by a reaction between a (hydrogen)carbonate and an acid. Based on these findings, the present inventors have completed the present invention.

[0009] That is, one embodiment of the present invention is a graphite flake laminate comprising a large number of stacked graphite flakes, having a porosity of 20.0% or less in a cross section in the stacking direction, and having a median aspect ratio of 6.5 or more of voids present between the graphite flakes.

[0010] Another aspect of the present invention is a method for producing a graphite flake laminate, comprising: adding an acid capable of generating carbon dioxide gas from the carbonate or bicarbonate to a dispersion in which a carbon material, which is an aggregate of graphene, and a carbonate or bicarbonate are dispersed in a dispersion medium, thereby generating the carbon dioxide gas from the carbonate or bicarbonate, thereby exfoliating the carbon material to obtain exfoliated graphene; and stacking the exfoliated graphene to obtain a graphite flake laminate.

[0011] FIG. 1 is a cross-sectional schematic diagram illustrating a cross-section of a graphite flake laminate 10 according to one embodiment of the present invention. FIG. 2 is an enlarged view of a portion 10a of the graphite flake laminate 10 shown in FIG. 1. FIG. 3 is an explanatory diagram illustrating the structures of (a) single-layer graphene (graphene sheet) and (b) multilayer graphene. FIG. 4 is a cross-sectional schematic diagram illustrating a cross-section of a heat dissipation structure according to a conventional technique. FIG. 5 is a cross-sectional schematic diagram illustrating a cross-section of a heat dissipation structure according to one embodiment of the present invention. FIG. 6 is a microscopic image of expanded graphite before and after exfoliation treatment in Example 1. FIG. 7 is a microscopic image of a cross-section in the stacking direction of the graphite flake laminates obtained in Comparative Example 1 and Example 1, and a binarized image of the same. FIG. 8 is a cross-sectional image of a cross-section in the stacking direction of the graphite flake laminate obtained in Comparative Example 3. FIG. 9 is an observation image of a cross section in the stacking direction of the graphite flake laminate obtained in Example 3.

[0012] <Graphite Flake Laminate> One embodiment of the present invention is a graphite flake laminate formed by stacking a large number of graphite flakes, having a porosity of 20.0% or less in a cross section in the stacking direction, and having a median aspect ratio of 6.5 or more of voids present between the graphite flakes.

[0013] The present invention provides a graphite flake laminate that exhibits high thermal conductivity and can be used as a sheet-like heat conduction member, and a manufacturing method that can produce such a graphite flake laminate using safe and simple manufacturing equipment.

[0014] Hereinafter, embodiments of the graphite flake laminate according to the present invention will be described with reference to the drawings. However, 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 proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0015] FIG. 1 is a schematic cross-sectional view of a graphite flake laminate 10 according to one embodiment of the present invention. FIG. 2 is an enlarged view of a portion 10a of the graphite flake laminate 10 shown in FIG. 1. The graphite flake laminate 10 shown in FIGS. 1 and 2 is a structure in which a large number of graphite flakes 100 are stacked. Voids 110 exist between the large number of graphite flakes 100. In the graphite flake laminate 10 according to this embodiment, the proportion of voids 110 in the cross section of the laminate 10 in the stacking direction (void ratio) is 20.0% or less (e.g., 7.2%). The median aspect ratio of the voids is 6.5 or more (e.g., 12.8).

[0016] The structure of the graphite flake laminate according to this embodiment will be described below.

[0017] [Graphite Flakes] The graphite flakes constituting the graphite flake laminate are flake-shaped carbon materials having a graphene structure. From the viewpoint of achieving particularly high thermal conductivity in the in-plane direction, the graphite flakes preferably contain single-layer graphene, multilayer graphene, or graphite, and particularly preferably contain multilayer graphene. Here, multilayer graphene has a structure (FIG. 3(b)) in which multiple single-layer graphene (graphene sheets) shown in FIG. 3(a) are stacked in parallel. Graphene sheets have a structure in which carbon atoms having sp2 hybrid orbitals (sp2 carbons) are connected in a two-dimensional honeycomb structure, and are therefore characterized by extremely high thermal conductivity in the in-plane direction. Consequently, the graphite flake laminate also has extremely high thermal conductivity in the in-plane direction, since adjacent graphite flakes are arranged in close contact with each other and with the long axes of the graphite flakes oriented in the in-plane direction. In other words, the graphite flake laminate has a selectively enhanced thermal conductivity in the plane direction relative to the thermal conductivity in the thickness direction (direction perpendicular to the plane direction (stacking direction)).

[0018] As described above, graphite flakes have a structure in which a plurality of graphene layers are stacked, but there is no particular limit to the number of stacked graphene layers in the graphite flakes. In this specification, graphite flakes having up to 10 stacked graphene layers are referred to as "graphene." Graphene having one stacked graphene layer is referred to as "single-layer graphene," and graphene having two to ten stacked graphene layers is referred to as "multilayer graphene." Graphite flakes having 11 or more stacked graphene layers are referred to as "graphite."

[0019] Here, as an example, the number of stacked graphene layers in the graphite flakes 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 plane direction, and further, multilayer graphene is inexpensively available, so it is preferably used in the graphite flake laminate according to this embodiment. Note that as the graphite flakes, only one type may be used alone, or two or more types may be used in combination.

[0020] Graphite flakes are anisotropic in size. In this specification, when the plane with the largest area among the faces constituting the surface of a graphite flake is defined as the XY plane, the longest line segment connecting any two points on the contour of the graphite flake in the XY plane 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 means the arithmetic mean diameter of the major axes of several tens of graphite flakes contained in the graphite flake laminate.

[0021] Furthermore, in this specification, when the plane with the largest area among the faces constituting the surface of a graphite flake is defined as the XY plane, the maximum dimension of the side constituting the XZ plane or the YZ plane is defined as the minor axis of the graphite flake. There are no particular restrictions on the value of the average minor axis of the graphite flakes, 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 minor axis of the graphite flakes refers to the arithmetic average value of the minor axes of several tens of 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 more than 1 and not more than 2,000,000, more preferably 5 to 900,000, and even more preferably 30 to 200,000. The average aspect ratio of the graphite flakes refers to the arithmetic average value of the aspect ratios of several tens of graphite flakes contained in the graphite flake laminate.

[0023] The content of graphite flakes in the graphite flake laminate is not particularly limited, but is preferably 50 to 100% by mass, more preferably 70 to 100% by mass, even more preferably 80 to 100% by mass, particularly preferably 90 to 100% by mass, and most preferably 100% by mass, based on 100% by mass of the total amount of the constituent components of the graphite flake laminate.

[0024] [Other Compounding Components] The graphite flake laminate according to the present embodiment essentially contains graphite flakes, but may further contain other compounding components. However, since the incorporation of other compounding components may decrease the thermal conductivity in the planar direction, it is also preferable not to incorporate other compounding components.

[0025] Such other blending components include, for example, carbon materials other than graphite flakes, thermally conductive fillers other than carbon materials, binders, and the like.

[0026] As the carbon material other than graphite flakes, any conventionally known carbon material can be used, including carbon black, fullerene, carbon nanotube, carbon nanofiber (vapor grown carbon fiber (VGCF, etc.)), carbon nanohorn, carbon microcoil, and carbon nanocoil, from the viewpoint of further improving the thermal conductivity in the planar direction.

[0027] Examples of thermally conductive fillers other than carbon 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 hydrates (e.g., boehmite)), magnesium oxide, titanium oxide, cerium oxide, and zirconium oxide. Examples of oxides include transition metal oxides such as barium titanate, as well as metal ion-doped oxides such as indium tin oxide and antimony tin oxide. Examples of hydroxides include aluminum hydroxide, calcium hydroxide, and magnesium hydroxide. Examples of metals include copper, gold, nickel, tin, iron, and alloys thereof.

[0028] Binders are used for the purpose of improving the binding properties of various blended components, protecting them, etc. 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, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PC Fluorine resins such as vinylidene fluoride-hexafluoropropylene fluoroelastomers (VDF-HFP fluoroelastomers), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluoroelastomers (VDF-HFP-TFE fluoroelastomers), vinylidene fluoride-pentafluoropropylene fluoroelastomers (VDF-PFP ... Examples of suitable fluororubbers include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers), and epoxy resins. Of these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are preferably used.

[0029] [Porosity] Air has an extremely low thermal conductivity. Therefore, it is preferable that the graphite flake laminate according to this embodiment does not contain much air. Quantitatively speaking, in the graphite flake laminate according to this embodiment, the porosity in the cross section in the stacking direction is essentially 20.0% or less, more preferably 15.0% or less, even more preferably 10.0% or less, and particularly preferably 7.2% or less. While there is no particular lower limit for the void ratio, it is usually 1.0% or more, preferably 3.1% or more, and particularly preferably 6.1% or more. That is, in a preferred embodiment, the porosity is 1.0 to 7.2%, more preferably 3.1 to 7.2%. This porosity value can be achieved by employing the manufacturing method described below or adjusting the pressing pressure when pressing the obtained laminate. Furthermore, in this specification, the porosity value described above is the value measured by the method described in the Examples section below. As described in the Examples section, the porosity calculation is performed over a 60 μm × 40 μm field of view in the cross section of the graphite flake laminate in the stacking direction. This range is narrow enough to allow measurement of the porosity by binarization processing, but is wide enough to eliminate arbitrariness in the porosity calculation. This also applies to the calculation of the median aspect ratio of voids, which will be described below.

[0030] [Median Aspect Ratio of Voids Present Between Graphite Flakes] In the graphite flake laminate according to this embodiment, the graphite flakes are neatly stacked. That is, exfoliated graphene particles become two-dimensional graphite flakes, and when these are stacked in a highly oriented manner, the shape of the voids generated between the layers also becomes two-dimensional (planar), and in a cross-sectional image, it appears one-dimensional (linear). Reflecting this state in which the graphite flakes are stacked in a highly oriented and dense manner, the median aspect ratio of the voids present between the graphite flakes is relatively large. Specifically, this median is essentially 6.5 or more, preferably 7.0 or more, and more preferably 7.5 or more. Meanwhile, there is no particular upper limit to the median void size, but it is usually 30 or less. In one embodiment, the median void size is 6.5 to 7.5. This median value can be achieved by employing the manufacturing method described below or by adjusting the pressing pressure when pressing the obtained laminate. In this specification, the median value is a value measured by the method described in the Examples section below. As described in the Examples section, the voids considered when calculating the median value are those with an area of ​​0.04 μm2 present in a cross section of the graphite flake laminate in the stacking direction. ２ 80 μm or more ２ This is a void with a too small area (0.04 μm ２ When the aspect ratio is calculated taking into account the voids (less than 80 μm), the value is always close to 1, making it impossible to define meaningfully. ２ If the aspect ratio is calculated taking into account the voids (micrometers), the large-scale delaminations that exist between the graphene layers will also be included, resulting in a large median value despite the poor thermal conductivity.

[0031] There are no particular limitations on the film thickness of the graphite flake laminate according to this embodiment, and it can be appropriately determined depending on 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 thermally conductive member, etc. As an 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.

[0032] <<Method for Producing Graphite Flake Laminate>> There are no particular limitations on the method for producing the graphite flake laminate according to this embodiment, and any production method that can produce a graphite flake laminate having the structure described above can be appropriately adopted.

[0033] As an example, the graphite flake laminate according to this embodiment can be produced by adding a predetermined acid to a dispersion in which a carbon material, which is an aggregate of graphene, and a carbonate or bicarbonate are dispersed in a dispersion medium. That is, another embodiment of the present invention also provides a method for producing a graphite flake laminate. The production method includes adding an acid capable of generating carbon dioxide gas from the carbonate or bicarbonate to a dispersion in which a carbon material, which is an aggregate of graphene, and a carbonate or bicarbonate are dispersed in a dispersion medium, thereby generating the carbon dioxide gas from the carbonate or bicarbonate and exfoliating the carbon material to obtain exfoliated graphene, and stacking the exfoliated graphene to obtain a graphite flake laminate. This production method allows for the safe production of graphite flake laminates with excellent thermal conductivity in the planar direction using simple production equipment. Furthermore, this production method generates a relatively small amount of gas compared to the method described in Non-Patent Document 1, enabling exfoliation under mild conditions. Furthermore, since the solubility of the salt used is not high (up to 10% in water at room temperature), the pressure generated between the graphene layers during intercalation and gas generation is relatively small, which has the advantage of minimizing wrinkles in the exfoliated graphene.

[0034] There is no particular limitation on the method for obtaining exfoliated graphene and then stacking them to obtain a graphite flake laminate, but an example of such a method includes preparing a coating slurry containing exfoliated graphene and, if necessary, a surfactant, forming the coating slurry into a film, drying and / or firing the film, and then subjecting the film to a pressing process such as hot pressing or roll pressing. This production method will be described below.

[0035] In this manufacturing method, first, a carbon material that is a graphene aggregate is prepared as a raw material. There are no particular limitations on this carbon material, and any material having the structure of a graphene aggregate can be used as appropriate. In particular, from the viewpoint of ease of intercalation with a carbonate or hydrogencarbonate, which will be described later, it is preferable that the carbon material (graphene aggregate) used as the 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 of reducing interfacial thermal resistance by increasing the particle size of the graphite flakes that make up the graphite flake laminate and improving the thermal conductivity of the laminate in the plane direction, it is preferable that the carbon material (graphene aggregate) used as the raw material contains expanded graphite.

[0036] Next, the carbon material prepared above is dispersed in a dispersion medium together with a carbonate or hydrogencarbonate to prepare a dispersion liquid. Water is typically used as the dispersion medium. The amount of the dispersion medium may be determined appropriately so as to allow the solid content to be sufficiently dispersed.

[0037] There is no particular limitation on the type of carbonate or hydrogen carbonate, and it is possible to obtain a carbonate or hydrogen carbonate which is a weak acid by contacting with an acid in a later step. ２Any material capable of generating a gas (carbon dioxide) of alkali metal ions or alkaline earth metal ions may be used. One example is a carbonate or hydrogen carbonate salt containing alkali metal ions or alkaline earth metal ions. Alkali metal ions or alkaline earth metal ions have the advantage of being highly intercalatable because they easily bond to the defects or functional groups (-OH or -COOH) on the edges of graphene. Furthermore, when these cations intercalate, they form carbonate ions (CO ３ ２－ ) and bicarbonate ions (HCO ３ － ) are also considered to be attracted between the graphene layers by Coulomb force. Here, examples of alkali metals include one or more selected from the group consisting of lithium, sodium, and potassium. Examples of alkaline earth metals include one or more selected from the group consisting of calcium, strontium, barium, radium, beryllium, and magnesium. These metals have particularly excellent intercalation properties due to their small atomic radii. Examples of carbonates include lithium carbonate, sodium carbonate, potassium carbonate, calcium carbonate, strontium carbonate, barium carbonate, radium carbonate, beryllium carbonate, and magnesium carbonate. Examples of bicarbonates include lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, calcium bicarbonate, strontium bicarbonate, barium bicarbonate, radium bicarbonate, beryllium bicarbonate, and magnesium bicarbonate. In addition to alkali metal or alkaline earth metal salts, ammonium carbonate, ammonium bicarbonate, and the like may also be used. These carbonates or bicarbonates intercalate between the graphene layers upon contact with the raw carbon material (graphene aggregates). Therefore, the amount (concentration) of these salts in the dispersion medium may be appropriately determined so that a sufficient amount of salt can be intercalated to exfoliate the carbon material (aggregates of graphene).

[0038] Next, an acid is added to the dispersion liquid prepared above. At this time, the acid to be added is an acid that can generate carbon dioxide gas from the carbonate or hydrogencarbonate contained in the dispersion liquid. The acid is typically an acid that has a pKa smaller than the acid dissociation constant (pKa) of carbonic acid in an aqueous solution at 25°C. Here, carbonic acid undergoes two-stage dissociation in an aqueous solution. The acid dissociation constant of carbonic acid at 25°C is pKa for the first stage. １ Although the second stage is small at pKa = 3.60, ２ = 10.25, and due to the existence of an equilibrium with carbon dioxide, the apparent acid dissociation constant pKa is large at 6.35. The aforementioned "acid having a pKa smaller than the acid dissociation constant (pKa) of carbonic acid in an aqueous solution at 25°C" refers to an acid having a pKa smaller than the apparent pKa of carbonic acid (6.35). Note that, if such an acid has multiple acid dissociation constants, the smallest of those acid dissociation constants should be smaller than 6.35. Examples of such acids include acetic acid (4.76), hydrofluoric acid (3.45), citric acid (3.09), chloric acid (1), nitric acid (-2), sulfuric acid (-2), hydrochloric acid (-7), hydrobromic acid (-9), perchloric acid (-10), and iodic acid (-11). These acids may also be used in the form of hydrates, such as citric acid monohydrate. Furthermore, if the amount of acid added is too small, carbon dioxide gas may not be generated sufficiently, and the amount of acid added should be determined appropriately taking such a possibility into consideration.

[0039] When such an acid is added to the dispersion, the anions (carbonate ions or bicarbonate ions) of the carbonate or bicarbonate ionized in the dispersion are converted into carbonic acid (H ２ CO ３ ) is generated, most of which is carbon dioxide (CO ２ ) gas (carbon dioxide gas) is released from the dispersion to the outside. At this time, when carbon dioxide gas is generated and released to the outside, the graphene layers constituting the carbon material (graphene aggregates) are exfoliated. As a result, the carbon material is exfoliated to obtain exfoliated graphene. When adding an acid to the dispersion, a solid or liquid acid may be added as is, or may be added in the form of a solution such as an aqueous solution.

[0040] The exfoliated graphene obtained in this manner may be re-dispersed in a dispersion medium such as water and subjected to a treatment such as ultrasonic stirring, etc. Thereafter, a conventionally known surfactant is added as necessary, and the mixture is further stirred using a stirrer (for example, a rotation / revolution propellerless mixer) or the like to obtain a coating slurry.

[0041] After forming a film from this coating slurry, the film may be dried and / or fired as necessary, and then subjected to a pressing treatment such as hot pressing or roll pressing to obtain a graphite flake laminate.

[0042] There are no particular limitations on the method for forming a film from the coating slurry, and examples thereof include a suction filtration film-forming method, a method using a die coater or an applicator, an electrostatic coating method, etc. Among these, from the viewpoint of promoting the orientation of exfoliated graphene and obtaining a graphite flake laminate having particularly excellent thermal conductivity in the plane direction, film formation by the suction filtration film-forming method is preferred.

[0043] The laminate in which exfoliated graphene is stacked can be subjected to a drying treatment for several minutes to several hours at a temperature of about 40 to 90° C. as needed, and can further be subjected to a firing treatment for several minutes to several tens of minutes at a temperature of about 100 to 150° C. Here, in the production method according to the present embodiment, from the viewpoint of improving the thermal conductivity in the plane direction by bringing the exfoliated graphene particles into close contact with each other in the stacking direction, it is preferable to fire the laminate in which exfoliated graphene is stacked, and it is more preferable to press the laminate in the stacking direction after firing.

[0044] As the pressing treatment, conventionally known pressing treatments such as hot pressing and roll pressing can be appropriately employed. Hot pressing and roll pressing may be used in combination, or either one may be used alone. Among these, roll pressing is preferably employed from the viewpoint that the application of shear force during pressing can improve the orientation of the graphite flakes in the plane direction. Here, there are no particular limitations on the pressing conditions in the hot pressing, and examples of such conditions include a pressing pressure of about 3 to 30 MPa, a temperature of about 100 to 200°C, and a period of several minutes to several tens of minutes. There are also no particular limitations on the pressing conditions in the roll pressing, and examples of such conditions include a pressing pressure of about 200 to 1000 MPa and a rotation speed of about 50 to 200 rpm.

[0045] <<Uses of Graphite Flake Laminate (Heat Conduction Member and Heat Dissipation Structure)>> The graphite flake laminate according to one aspect of the present invention described above has excellent thermal conductivity in the planar direction. Therefore, this excellent thermal conductivity can be utilized as a heat conduction member. Furthermore, this heat conduction member can be disposed near a heat source to form a heat dissipation structure. That is, according to yet another aspect of the present invention, a heat conduction member is provided that includes the graphite flake laminate according to one aspect of the present invention described above. Furthermore, a heat dissipation structure is also provided that includes a heat source and the heat conduction member according to the above aspect that is disposed so as to be in contact with the heat source. Hereinafter, the heat dissipation structure according to this aspect will be described with reference to the drawings, taking as an example a heat dissipation structure for dissipating heat generated from a superbright light-emitting diode (LED) as a heat source.

[0046] In recent years, high-intensity light-emitting diode (LED) lamps have been adopted for the purpose of extending the lifespan and saving power in automotive headlights, and heat sinks are used to cool these high-intensity LEDs. These heat sinks are usually made of metal materials with high thermal conductivity, such as pure aluminum or aluminum alloys, and have a shape such as die-casting, with multiple fins arranged in a row on a flat heat-receiving surface.

[0047] FIG. 4 is a schematic cross-sectional view showing a cross section of such a heat dissipation structure according to the prior art.

[0048] As shown in Figure 4, a conventional heat dissipation structure 1 is used to cool a high-brightness LED module 4 with an output of, for example, 1 W or more. Its basic configuration combines a heat transfer plate 3 made of a good thermal conductor metal or carbon material with a heat sink body 2 made of a thermally conductive resin. The LED module 4 has a structure in which a light emitter 8, which incorporates multiple LED elements and is integrally formed with a lens, is held in the center of a substrate 7, and the substrate 7 is bonded to the heat receiving surface 5 arranged on the heat sink body 2. The heat dissipation structure 1 shown in Figure 4 transfers heat generated from a small heat source when the LED module 4 is driven to the entire heat receiving surface 5 through the heat transfer plate 3 made of a good thermal conductor metal or carbon material and disposed along the heat receiving surface 5 to which the LED module 4 is bonded. The heat is then dissipated into the air by the fins 6 of the heat sink body 2, which are made of a thermally conductive resin with low thermal capacity and high emissivity, thereby suppressing the temperature rise of the LED module 4 (i.e., cooling the LED module 4). The heat dissipation structure 1 having such a configuration is capable of dissipating heat generated by the LED module 4 to a certain extent. However, there is a problem of poor space efficiency because the heat sink body 2 occupies a large volume in the heat dissipation structure 1. Furthermore, because the heat sink body 2 is made of a metal material such as pure aluminum or an aluminum alloy, the heat dissipation structure 1 can also be a factor in increasing the weight of the automobile when applied to, for example, an automobile headlight.

[0049] On the other hand, FIG. 5 is a schematic cross-sectional view showing a cross section of a heat dissipation structure according to one embodiment of the present invention.

[0050] As shown in Fig. 5 , the heat dissipation structure 1 according to one embodiment of the present invention does not include the heat sink body 2, as compared with the heat dissipation structure shown in Fig. 4 . Meanwhile, a graphite flake laminate 10 (see Fig. 1 ) according to one embodiment of the present invention is disposed as a thermally conductive member, covering the entire surface of the heat transfer plate 3 on the side where the LED module 4 is disposed. With this configuration, the heat dissipation structure 1 according to the embodiment shown in Fig. 5 can rapidly transfer heat generated from a small heat source by driving the LED module 4 in the planar direction of the graphite flake laminate 10 and efficiently dissipate the heat to the outside. Furthermore, because the heat dissipation structure 1 does not include the heat sink body provided in the heat dissipation structure according to the embodiment shown in Fig. 4 , it has the significant advantage of being able to significantly reduce the volume and weight of the heat dissipation structure.

[0051] Although not shown in FIG. 5 , it is preferable that the heat dissipation structure 1 further includes a heat sink arranged in contact with the graphite flake laminate 10 serving as a thermally conductive member. The provision of such a heat sink enables more efficient heat dissipation. The term "heat sink" as used herein refers to a component capable of more efficiently dissipating heat conducted from a heat source (LED module 4 in FIG. 5 ) via the graphite flake laminate 10 to the outside. Examples of such heat sinks include heat sinks, heat pumps, and metal housings for electronic devices. When a heat sink is arranged on the graphite flake laminate 10, it is preferable that the heat sink not face the heat source via the graphite flake laminate (thermal conductive member) arranged in contact with the heat source (i.e., the heat source and the heat sink are spaced apart in the planar direction of the graphite flake laminate (thermal conductive member)), in order to fully utilize the excellent thermal conductivity of the graphite flake laminate 10 in the planar direction. In this case, the distance between the heat source and the heat sink in the plane direction 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.

[0052] The above describes a heat dissipation structure according to one embodiment of the present invention, 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 can be used for the purpose of dissipating heat generated by a wide variety of heat sources, without being limited to this.

[0053] Examples of such heat sources include the above-mentioned LED modules, as well as various lasers (sensors) (conventional heat dissipation means are aluminum plates or combinations thereof with cooling fins, Peltier elements, or chiller water cooling); high-performance infrared cameras (IR) (conventional heat dissipation means are aluminum plates or combinations thereof with cooling fins, Peltier elements, or chiller water cooling); head-up displays (HUDs) (conventional heat dissipation means are combinations of heat sinks and spreaders); smartphone elements and batteries (conventional heat dissipation means are air cooling, heat dissipation sheets, or combinations of heat sinks and spreaders); digital camera elements and batteries (conventional heat dissipation means are combinations of heat sinks, spreaders, and housings); and personal computers (PCs). elements and batteries (wherein the conventional heat dissipation means is a combination of a fan, heat sink, spreader, and housing); elements of electronic control units (ECUs) for vehicles, etc. (wherein the conventional heat dissipation means is a combination of a heat sink, spreader, and housing); elements of insulated gate bipolar transistors (IGBTs), which are the main conversion elements of high-power inverters (wherein the conventional heat dissipation means is a combination of a heat sink, spreader, and housing); rotating parts of motors (wherein the conventional heat dissipation means is a combination of a fan and heat sink, or a combination of either of these with water cooling); light source lamps for thin displays (wherein the conventional heat dissipation means is a heat sink); and large-capacity in-vehicle batteries (wherein the conventional heat dissipation means is a heat dissipation sheet, heat dissipation material, or a combination of air cooling or water cooling with a fan and heat sink).

[0054] By applying the heat dissipation structure according to this embodiment to these heat sources, the heat dissipation means conventionally applied to each heat source can be replaced with the graphite flake laminate (thermal conductive member) according to one embodiment of the present invention. As a result, if a heat sink as a conventional heat dissipation means can be eliminated, for example, as described with reference to FIG. 5 , 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, or water cooling with the graphite flake laminate (thermal conductive member) according to one embodiment of the present invention, it is expected that the weight and volume of the heat dissipation structure can also be significantly reduced compared to heat dissipation structures equipped with conventional heat dissipation means.

[0055] The following embodiments are also included within the scope of the present invention: the graphite flake laminate according to claim 1 having the features of claim 2; the graphite flake laminate according to claim 1 or 2 having the features of claim 3; a heat conduction member including the graphite flake laminate according to any one of claims 1 to 3; a heat dissipation structure using the heat conduction member according to claim 4; a heat dissipation structure according to claim 5 having the features of claim 6; a heat dissipation structure according to claim 6 having the features of claim 7; a manufacturing method according to claim 8 having the features of claim 9; a manufacturing method according to claim 9 having the features of claim 10; a manufacturing method according to any one of claims 8 to 10 having the features of claim 11; and a manufacturing method according to any one of claims 8 to 11 having the features of claim 12.

[0056] The present invention will be described in more detail below with reference to examples, although the technical scope of the present invention is not limited to the following examples.

[0057] <<Examples of Preparation of Graphite Flake Laminate>> [Comparative Example 1] A predetermined amount of commercially available graphene nanoplatelets (GNP) (average particle size: 25 μm) was weighed and mixed with chlorosulfonic acid (HSO ３ The graphene was then placed in a solution of 100% hydrogen peroxide (HCl) and allowed to soak. Hydrogen peroxide was then added to the solution in multiple batches, and the mixture was allowed to stand until gas generation ceased. The solution was then centrifuged to recover the wet powder, which was then allowed to stand at room temperature and dried to obtain exfoliated graphene.

[0058] A predetermined amount of the exfoliated graphene obtained above was weighed and added to pure water. Next, this solution was subjected to ultrasonic stirring at 28 kHz and 110 W for 10 minutes. Thereafter, a predetermined amount of surfactant (Daicel CMC1190, manufactured by Daicel Miraize Co., Ltd.) was added to this solution, and the solution was stirred at 2000 rpm for 1 minute using a stirrer (rotation / revolution propellerless mixer) to obtain a coating slurry. The composition ratio (mass ratio) of the coating slurry thus obtained was graphene:pure water:CMC = 1:400:0.05.

[0059] A graphene film was produced on filter paper by a suction filtration film-forming method using the coating slurry obtained above. The obtained graphene film was dried by being held on a hot plate at 60°C for 30 minutes. Next, it was fired by being held in an electric furnace at 130°C for 30 minutes. The fired product was then hot-pressed under conditions of 30 MPa and 200°C for 20 minutes to obtain a graphite flake laminate (film thickness 50 μm) of this comparative example.

[0060] A predetermined amount of commercially available expanded graphite (EG) (average particle size: 200 μm) was weighed and added to pure water. Then, ultrasonic stirring and subsequent operations were performed using this solution. The coating slurry had a composition ratio (mass ratio) of graphene:pure water:CMC=1:100:0.05. The hot-press pressure and temperature were changed to 10 MPa and 130° C., respectively. The graphite flake laminate (film thickness: 60 μm) of this comparative example was obtained in the same manner as in Comparative Example 1.

[0061] [Example 1] A predetermined amount of sodium bicarbonate, which is an alkali metal bicarbonate, was weighed and added to pure water to prepare an aqueous sodium bicarbonate solution. Next, a predetermined amount of commercially available expanded graphite (EG) (average particle size: 200 μm) was weighed and added to this aqueous sodium bicarbonate solution, and ultrasonic stirring was performed three times for 10 minutes at 28 kHz and 110 W.

[0062] Thereafter, a predetermined amount of citric acid monohydrate, which is an acid, is added to this solution in several divided portions, and carbon dioxide (CO ２The solution was then filtered by suction to recover a wet powder, which was then dried on a hot plate at 80°C or lower to obtain exfoliated graphene.

[0063] A predetermined amount of the exfoliated graphene obtained above was weighed and added to pure water. Next, a graphite flake laminate (film thickness 60 μm) of this example was obtained in the same manner as in Comparative Example 2, except that the ultrasonic agitation and subsequent operations were carried out using this solution.

[0064] In this example, a microscopic image of the expanded graphite before exfoliation treatment and a microscopic image of the graphite flakes after exfoliation treatment are shown in Fig. 6. Also, Fig. 7 shows observed images of cross sections in the stacking direction of the graphite flake laminates obtained in Comparative Example 1 and Example 1, and images obtained by binarizing these images (details will be described later). As shown in Fig. 7, in the graphite flake laminate of Example 1, the graphite flakes were highly oriented and stacked at high density, whereas in the graphite flake laminate of Comparative Example 1, although the orientation of the graphite flakes was confirmed, there were large voids between the graphite flake layers due to large-scale peeling.

[0065] A predetermined amount of commercially available expanded graphite (EG) (average particle size: 44 μm) was weighed and added to pure water. Then, the procedure from ultrasonic stirring onward was carried out using this solution, and a graphite flake laminate (film thickness: 50 μm) of this comparative example was obtained in the same manner as in Comparative Example 1, except that the composition ratio (mass ratio) of the coating slurry was graphene:pure water:CMC=1:100:0.1.

[0066] Fig. 8 shows an observation image of a cross section in the stacking direction of the graphite flake laminate obtained in Comparative Example 3. As shown in Fig. 8, in the graphite flake laminate of Comparative Example 1, the orientation of the graphite flakes was random, and large voids due to large-scale peeling existed between the graphite flake layers.

[0067] Example 2 A graphite flake laminate (film thickness 80 μm) of this example was obtained in the same manner as in Example 1, except that commercially available expanded graphite (EG) (average particle size: 44 μm) was used as the expanded graphite and the pressing pressure and pressing temperature during hot pressing were changed to 30 MPa and 200° C.

[0068] Example 3 A graphite flake laminate (film thickness 80 μm) of this example was obtained in the same manner as in Example 1 described above, except that commercially available expanded graphite (EG) (average particle size: 44 μm) was used as the expanded graphite, calcium carbonate, which is an alkaline earth metal bicarbonate, was used instead of sodium bicarbonate, and pressing was performed using an electric roll press (pressing pressure 262 MPa, 100 rpm) at 25° C. instead of the hot press.

[0069] Fig. 9 shows an observation image of a cross section in the stacking direction of the graphite flake laminate obtained in Example 3. As shown in Fig. 9, it was observed that in the graphite flake laminate of Example 3, the graphite flakes were highly oriented and stacked with each other at a high density.

[0070] Furthermore, microscopic Raman spectroscopy was performed on the graphite flake laminates obtained in Comparative Example 1, Example 1, and Example 3 to obtain Raman spectra. Here, a HORIBA HR was used as the Raman analyzer. The measurement conditions were a 100x objective lens, a laser with a wavelength of 532 nm was used as the incident light, and a slit width of 0.1 mm. The measurement range was 0 to 2000 cm. －１The measurement time was 10 seconds, and the number of integrations was 24. As a result, the peak intensities of the D band and G band were significantly reduced, and the peak intensity of the D' band was reduced to below the detection limit. This shows that the graphite flake particles were thinned by the exfoliation treatment in each Example. Furthermore, the ratio of the peak intensity of the D band to the peak intensity of the G band (D / G ratio) was 0.15 in Comparative Example 1, but was reduced to 0.03 in Examples 1 and 3. This shows that the crystallinity of the graphite flake particles was increased by the exfoliation treatment in each Example. This is thought to be because agglomerations were broken down when unexfoliated graphene was exfoliated by the exfoliation treatment, reducing the number of edges and defects of graphene particles present in the plane direction of the graphite flake laminate.

[0071] A predetermined amount of commercially available expanded graphite (EG) (average particle size: 44 μm) was weighed and added to pure water. The graphite flake laminate of this comparative example (film thickness: 80 μm) was obtained in the same manner as in Comparative Example 1, except that the ultrasonic agitation and subsequent steps were carried out using this solution, the composition ratio (mass ratio) of the coating slurry was graphene:pure water:CMC = 1:100:0.05, and the resulting fired product was pressed using an electric roll press (pressing pressure: 262 MPa, 100 rpm) at 25°C and then hot pressed.

[0072] Evaluation Example of Graphite Flake Laminates Measurement of Porosity The porosity of the graphite flake laminates produced in the above-described comparative examples and examples was measured by the following method.

[0073] Specifically, first, ion milling was performed on the cross section of the obtained graphite flake laminate in the stacking direction. The cross section was then observed using a scanning electron microscope (SEM) (observation magnification: 2000x). As a result, it was confirmed that in all comparative examples and examples, graphite flakes were stacked to form a laminate. Next, a binarization process was performed on the observed image with a field of view of 60 μm × 40 μm to separate the carbon material (graphene) portion and the void portion, and the porosity [%] was calculated from the proportion of the void portion in the entire image. The threshold value used in the binarization process was an intermediate value between the brightness of the carbon material (graphene) portion and the brightness of the void portion. The results are shown in Table 1 below.

[0074] [Measurement of Median Aspect Ratio of Voids Present Between Graphite Flakes] For the graphite flake laminates produced in the above-described Comparative Examples and Examples, the median aspect ratio of the voids present between the graphite flakes was measured by the following method.

[0075] Specifically, the area of ​​0.04 μm present in the observation image obtained above (field of view of 60 μm × 40 μm) ２ 80 μm or more ２ The following voids were identified, and the aspect ratio values ​​and their medians were calculated for each void. The aspect ratio values ​​of the voids were calculated by using the long side length L [μm] and the short side length S [μm], where L is the distance between two parallel tangent lines circumscribing the periphery of the selected void, selected so that the distance between the tangent lines is the largest, and S is the distance between two parallel tangent lines circumscribing the periphery of the selected void, selected so that the distance between the tangent lines is the largest, and the long side length L is the distance between two parallel tangent lines orthogonal to the two parallel tangent lines that give the length L. The results are shown in Table 1 below.

[0076] [Evaluation of Thermal Conductivity] The graphite flake laminates produced in the above-described comparative examples and examples were evaluated for thermal conductivity by the following method to assess their performance when used as thermally conductive members.

[0077] First, each sample was cut to a size of 20 mm wide x 130-150 mm long, and the cross-sectional area of ​​the sample was calculated from the thickness of each sample. Next, the sample was placed in a thermostatic chamber at 25°C, and a heat source was installed at one end (0 mm position). Thermocouples were also installed at positions 25 mm, 50 mm, 75 mm, and 100 mm from the end where the heat source was installed. Heat of 2.0 W was generated from the heat source, and the temperature of each thermocouple was measured when it stabilized. The thermal conductivity was calculated from the temperature of the thermocouple located 50 mm from the heat source according to the following formula. The results are shown in Table 1 below.

[0078] (Thermal conductivity calculation formula) Thermal conductivity [W / m K] = Heat transfer amount [W] x distance [m] x (1 / cross-sectional area [m ２ ])×(1 / temperature difference [K])

[0079]

[0080] The results shown in Table 1 indicate that the graphite flake laminates of Examples 1 to 3 according to one embodiment of the present invention have very high thermal conductivity in the in-plane direction because the porosity and the median aspect ratio of voids of a given size are within a given range. This indicates that the graphite flake laminates according to the present invention exhibit excellent performance as thermal conduction members. In contrast, the graphite flake laminates of each comparative example do not achieve sufficient thermal conductivity because the porosity is too high or the median aspect ratio is too small. This is thought to be due to the structural differences, as shown in FIG. 7, which manifest as large differences in thermal conductivity.

[0081] REFERENCE SIGNS LIST 1 heat dissipation structure, 2 heat sink body, 3 heat conduction plate, 4 LED module, 5 heat receiving surface, 6 fin, 7 substrate, 8 light emitter, 10 graphite flake laminate, 10a part of graphite flake laminate, 100 graphite flake, 110 void

Claims

1. A graphite flake laminate comprising a large number of stacked graphite flakes, the void ratio in a cross section in the stacking direction being 20.0% or less, and the median aspect ratio of the voids present between the graphite flakes being 6.5 or more.

2. The graphite flake laminate of claim 1, wherein the graphite flakes comprise single layer graphene, multilayer graphene, or graphite.

3. The graphite flake laminate of claim 1, wherein the porosity is 1.0 to 7.2%.

4. A heat conductive member comprising the graphite flake laminate according to any one of claims 1 to 3.

5. A heat dissipation structure comprising: a heat source; and the heat conduction member according to claim 4, arranged in contact with the heat source.

6. The heat dissipation structure of claim 5, further comprising a heat sink disposed in contact with said thermally conductive member.

7. The heat dissipation structure according to claim 6, wherein the heat sink is disposed at a position not facing the heat source via the thermally conductive member.

8. A method for producing a graphite flake laminate, comprising: adding an acid capable of generating carbon dioxide gas from the carbonate or hydrogen carbonate to a dispersion in which a carbon material, which is an aggregate of graphene, and a carbonate or hydrogen carbonate are dispersed in a dispersion medium, thereby generating the carbon dioxide gas from the carbonate or hydrogen carbonate, thereby exfoliating the carbon material to obtain exfoliated graphene; and stacking the exfoliated graphene to obtain a laminate.

9. The method of producing a graphite flake laminate according to claim 8, wherein the carbonate or bicarbonate comprises a carbonate or bicarbonate containing an alkali metal ion or an alkaline earth metal ion.

10. The method for producing a graphite flake laminate according to claim 9, wherein the alkali metal comprises one or more selected from the group consisting of lithium, sodium and potassium, and the alkaline earth metal comprises one or more selected from the group consisting of calcium, strontium, barium, radium, beryllium and magnesium.

11. The method for producing a graphite flake laminate according to claim 8, wherein the carbon material has a multi-layer structure.

12. The method for producing a graphite flake laminate according to claim 8 or 11, further comprising, after firing the laminate in which the exfoliated graphene is stacked, performing a press treatment on the laminate in a stacking direction.

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