Thermal conduction film and heat dissipation structure using the same

Abstract

[Problem] To provide a means for being able to selectively improve thermal conductivity in a surface direction of a heat conduction sheet. [Solution] A heat conduction film is configured by disposing a scale-like carbon material formed by a plurality of graphene layers such that adjacent scale-like carbon materials contact each other, and such that the major axis of the scale-like carbon material is oriented in the surface direction of the film.

Description

Technical Field

[0001] The present invention relates to a heat conduction film and a heat dissipation structure using the same.

Background Art

[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 by 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 intelligentization 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 increasing.

[0003] Here, for the purpose of dissipating the heat generated by driving various electronic devices and motor drive power sources as described above, various cooling means are used. For example, as a cooling method for 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 the cooling of motor drive power sources, 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 attaching a heat sink to a semiconductor device for cooling, in order to efficiently release the heat of the semiconductor device, a technique of providing a thermal conductive sheet between the semiconductor device and the heat sink has been proposed. As such a thermal conductive sheet, for example, in International Publication No. 2018 / 030430 pamphlet, it has a structure in which a plurality of resin layers including a thermal conductive resin layer containing a thermal conductive plate-like filler such as boron nitride or exfoliated graphite are laminated, and when the vertical plane with respect to the laminated surface is defined as the sheet surface, a thermal conductive sheet is disclosed in which the major axis of the thermal conductive plate-like filler is oriented at an angle of 60° or more with respect to the sheet surface. According to the disclosure of International Publication No. 2018 / 030430 pamphlet, by adopting the above-described configuration, it is said that a thermal conductive sheet can be realized in which the amount of use of the thermal conductive plate-like filler is suppressed while improving the thermal conductivity of the thermal conductive sheet.

Summary of the Invention

Problems to be Solved by the Invention

[0005] Here, as can also be seen from the fact that the thermal conductive sheet disclosed in International Publication No. 2018 / 030430 pamphlet is designed such that the thermal conductivity in the thickness direction of the thermal conductive sheet is 3 W / m·K or more, it is intended to efficiently realize heat conduction in the thickness direction of the thermal conductive sheet. Such a thermal conductive sheet is effective as a thermal conductive sheet provided between the semiconductor device and the heat sink as described above. However, in this thermal conductive sheet, since the thermal conductive plate-like filler is configured such that its major axis is oriented at an angle of 60 to 90° with respect to the sheet surface, the thermal conductivity in the plane direction of the thermal conductive sheet is extremely low. Therefore, this thermal conductive sheet cannot be used in a heat dissipation structure that requires heat dissipation in the plane direction of the thermal conductive sheet.

[0006] Therefore, an object of the present invention is to provide a means capable of selectively improving the thermal conductivity in the plane direction of a thermal conductive sheet.

Means for Solving the Problems

[0007] In view of the above problems, the present inventors have conducted intensive studies. As a result, it has been found that the above problems can be solved by constructing a heat conduction film by arranging flaky carbon materials formed of a plurality of graphene layers such that adjacent flaky carbon materials are in contact with each other and the major axis of the flaky carbon materials is oriented in the plane direction of the film, and the present invention has been completed.

[0008] According to one embodiment of the present invention, there is provided a heat conduction film including flaky carbon materials formed of a plurality of graphene layers, wherein adjacent flaky carbon materials are in contact with each other, and the flaky carbon materials are arranged such that the major axis of the flaky carbon materials is oriented in the plane direction of the film.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0010] 《Heat Conduction Film》 One embodiment of the present invention is a heat conduction film including flaky carbon materials formed of a plurality of graphene layers, wherein adjacent flaky carbon materials are in contact with each other, and the flaky carbon materials are arranged such that the major axis of the flaky carbon materials is oriented in the plane direction of the film. According to the heat conduction film according to this embodiment, a heat conduction film excellent in selective thermal conductivity in the plane direction is provided.

[0011] Hereinafter, embodiments of the heat conduction film according to the above-described present embodiment will be described with reference to the drawings. However, the technical scope of the present invention should be determined based on the description in the claims and is not limited only to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios.

[0012] FIG. 1 is a schematic cross-sectional view schematically showing a cross-section of a heat conduction film according to an embodiment of the present invention.

[0013] As shown in FIG. 1, a heat conduction film 10 according to an embodiment of the present invention contains a large number of multilayer graphenes 100, which are one type of flaky carbon material. Here, adjacent multilayer graphenes 100 are in contact with each other. Also, the multilayer graphenes 100 are arranged such that the major axis of the multilayer graphenes 100 is oriented in the plane direction of the film.

[0014] The multilayer graphenes 100 included in the heat conduction film 10 having the configuration shown in FIG. 1 have a structure (FIG. 2(b)) in which a plurality of single-layer graphenes (graphene sheets) shown in FIG. 2(a) are laminated in parallel. Since the graphene sheet has a structure in which carbon atoms having sp2 hybrid orbitals (sp2 carbon) are connected in a honeycomb structure in the two-dimensional direction, it has the characteristic that the thermal conductivity in its plane direction is extremely high. As a result, the heat conduction film 10 shown in FIG. 1 is also arranged such that adjacent multilayer graphenes 100 are in contact with each other and the major axis of the multilayer graphenes 100 is oriented in the plane direction of the film, so that the thermal conductivity in the plane direction of the film is very high. In other words, it can also be said that the heat conduction film 10 shown in FIG. 1 has the in-plane thermal conductivity selectively enhanced with respect to the thermal conductivity in the thickness direction (the direction perpendicular to the plane direction) of the film.

[0015] Hereinafter, the constituent materials of the heat conduction film according to the present embodiment will be described.

[0016] [Flaky Carbon Material] The heat conduction film according to this embodiment contains a flaky carbon material formed of a plurality of graphene layers. In this specification, the "flaky carbon material" means a carbon material having a flaky shape. This flaky carbon material is formed of a plurality of graphene layers, but there is no particular limitation on the number of stacked graphene layers, and it can be appropriately set within the range having a flaky shape. In this specification, a flaky carbon material having 10 or fewer stacked graphene layers is referred to as "graphene". Among graphene, those having 1 stacked graphene layer are referred to as "single-layer graphene", and those having 2 to 10 stacked graphene layers are referred to as "multi-layer graphene". And a flaky carbon material having 11 or more stacked graphene layers shall be referred to as "graphite".

[0017] Here, as an example, the number of stacked graphene layers in the flaky carbon material is preferably 2 to 100 layers, more preferably 2 to 50 layers, still more preferably 2 to 20 layers, and particularly preferably 2 to 10 layers. Therefore, in the heat conduction film according to this embodiment, the flaky carbon material is preferably multi-layer graphene or graphite, and more preferably multi-layer graphene. As described above, multi-layer graphene or graphite has the feature of being particularly excellent in the in-plane thermal conductivity, and furthermore, multi-layer graphene is available at low cost, so it is preferably used in the heat conduction film according to this embodiment. Note that only one kind of the flaky carbon material may be used alone, or two or more kinds may be used in combination.

[0018] As described above, since the flaky carbon material has a flaky shape, there is anisotropy in its size. In this specification, among the surfaces constituting the surface of the flaky carbon material, when the surface with the largest area is defined as the XY plane, the longest line segment connecting any two points on the contour of the flaky carbon material in the XY plane is defined as the major axis of the flaky carbon material. There is no particular limitation on the value of the average major axis of the flaky carbon material, 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. Note that the value of the average major axis of the flaky carbon material means the arithmetic average diameter of the major axes of dozens of flaky carbon materials included in the heat conduction film.

[0019] Also, in this specification, among the surfaces constituting the surface of the flaky carbon material, when the surface with the largest area 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 flaky carbon material. There is no particular limitation on the value of the average minor axis of the flaky carbon material, 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. Note that the value of the average minor axis of the flaky carbon material means the arithmetic average value of the minor axes of dozens of flaky carbon materials included in the heat conduction film.

[0020] And the value of the ratio of the major axis to the minor axis of the flaky carbon material obtained as described above is defined as the aspect ratio. There is no particular limitation on the value of the average aspect ratio of the flaky carbon material, but it is preferably greater than 1 and 2,000,000 or less, more preferably 5 to 900,000, and even more preferably 30 to 200,000. Note that the value of the average aspect ratio of the flaky carbon material means the arithmetic average value of the aspect ratios of dozens of flaky carbon materials included in the heat conduction film.

[0021] As the flaky carbon material, a commercially available product or a processed product obtained by processing a commercially available product may be used, or a material prepared by oneself may be used. Since the manufacturing methods of the multi-layer graphene and graphite described above are widely known, detailed descriptions thereof are omitted here. In recent years, technologies for mass-producing flaky carbon materials such as multi-layer graphene at low cost have been developed, and the procurement cost of flaky carbon materials has also been reduced. Therefore, according to this embodiment, it can be said that it is highly advantageous in that a heat conduction film excellent in the in-plane thermal conductivity can be manufactured at a low cost. Conventionally, since there has been no technology capable of manufacturing flaky carbon materials at such a low cost, it is considered that there has been no motivation in the conventional technology to arrange the flaky carbon materials so as to be oriented in the plane direction of the film to produce a heat conduction film excellent in in-plane thermal conductivity.

[0022] There is no particular limitation on the content of the flaky carbon material in the heat conduction film, but it is preferably 5 to 90% by mass, more preferably 10 to 90% by mass, and still more preferably 15 to 90% by mass with respect to 100% by mass of the total amount of the constituent components of the heat conduction film.

[0023] [Other compounding components] The heat conduction film according to this embodiment essentially contains a flaky carbon material, but may further contain other compounding components. Examples of such other compounding components include heat conductive fillers other than the flaky carbon material, binders, thickeners, and the like. In particular, since the heat conduction film contains a binder, the flaky carbon materials contained in the heat conduction film are firmly bound to each other. For this reason, the mechanical strength of the heat conduction film is improved, and the heat conductive network formed by the flaky carbon materials is less likely to be cut. As a result, even when used for a long period of time, a decrease in the in-plane thermal conductivity is minimized, and a heat conduction film excellent in durability is provided.

[0024] Examples of the heat conductive filler other than the flaky carbon material include carbon materials, carbides, nitrides, oxides, hydroxides, metals, etc. Examples of the carbon materials include carbon black, diamond, fullerene, carbon nanotube, carbon nanofiber (vapor grown carbon fiber (such as VGCF)), carbon nanohorn, carbon microcoil, carbon nanocoil, etc. Examples of the carbides include silicon carbide, boron carbide, aluminum carbide, titanium carbide, tungsten carbide, etc. Examples of the nitrides include silicon nitride, boron nitride, aluminum nitride, gallium nitride, chromium nitride, tungsten nitride, magnesium nitride, molybdenum nitride, lithium nitride, etc. Examples of the oxides include iron oxide, silicon oxide (silica), aluminum oxide (alumina) (including hydrated products of aluminum oxide (such as boehmite)), magnesium oxide, titanium oxide, cerium oxide, zirconium oxide, etc. Further, examples of the oxides include transition metal oxides such as barium titanate, and furthermore, those doped with metal ions, such as indium tin oxide, antimony tin oxide, etc. Examples of the hydroxides include aluminum hydroxide, calcium hydroxide, magnesium hydroxide, etc. Examples of the metals include copper, gold, nickel, tin, iron, or their alloys, etc.

[0025] There is no particular limitation on the content of the heat conductive filler other than the flaky carbon material in the heat conductive film, but it is preferably 1 to 50% by mass, more preferably 5 to 45% by mass, and still more preferably 10 to 40% by mass with respect to 100% by mass of the total amount of the components constituting the heat conductive film. Further, the total content of the flaky carbon material and the heat conductive filler other than the flaky carbon material in the heat conductive film is preferably 5 to 95% by mass, more preferably 15 to 85% by mass, and still more preferably 25 to 75% by mass.

[0026] Binders are used for purposes such as improving the formability of coating films during the production of heat conductive films, improving the binding properties of various compounding components, and protection. Therefore, in a preferred embodiment of this form, the heat conductive film further contains a binder. Examples of binders include, for example, 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 its hydrogenated product, styrene-isoprene-styrene block copolymer and its hydrogenated product, etc. thermoplastic polymers, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), fluororesins such as polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), etc. vinylidene fluoride-based fluororubbers, epoxy resins, and the like.Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are preferably used.

[0027] There is no particular limitation on the content of the binder in the heat conduction film. However, as described above, a resin binder is usually used as the binder. However, the heat conduction characteristics of the resin binder are generally low. Therefore, when forming the heat conduction film according to this embodiment, the blending of the binder becomes a factor that increases the thermal resistance and inhibits heat conduction. From this perspective, the content of the binder is preferably 50% by mass or less, more preferably 1 to 40% by mass, still more preferably 5 to 35% by mass, and particularly preferably 5 to 30% by mass with respect to the total amount of 100% by mass of the constituent components of the heat conduction film.

[0028] Examples of the thickener include cellulose nanofibers (CNF) (such as carboxymethyl (CM)-modified CNF), polyvinylpyrrolidone (PVP), sodium alginate, polyacrylic acid, polyacrylamide, carboxymethyl cellulose (CMC), hydroxypropylmethyl cellulose (HPMC), starch, xanthan gum, pectin, and the like.

[0029] There is no particular limitation on the content of the thickener in the heat conduction film. However, with respect to the total amount of 100% by mass of the constituent components of the heat conduction film, it is preferably 1 to 60% by mass, more preferably 5 to 50% by mass, and still more preferably 10 to 40% by mass.

[0030] [Structure of the heat conduction film] In the heat conduction film according to this embodiment, as shown in FIG. 1, it is necessary to have a structure in which adjacent flaky carbon materials are in contact with each other. This is because, by having adjacent flaky carbon materials in contact with each other in this way, a high thermal conductivity in the plane direction of the heat conduction film can be achieved. Ideally, it is preferable that all the flaky carbon materials contained in the heat conduction film are in contact with adjacent flaky carbon materials, but it is not necessarily required that all the flaky carbon materials are in contact with adjacent flaky carbon materials. In a preferred embodiment of this embodiment, when observing a cross section perpendicular to the plane direction of the heat conduction film, it is preferable that the flaky carbon materials are continuous with adjacent flaky carbon materials, so that a thermally conductive network is formed from one end to the other end in the plane direction of the heat conduction film. Quantitatively expressing this, among the particles of the flaky carbon materials contained in the heat conduction film, preferably 10% or more, more preferably 20% or more, still more preferably 30% or more, particularly preferably 40% or more, and most preferably 50% or more are in contact with adjacent flaky carbon materials.

[0031] Further, in the heat conduction film according to the present embodiment, as shown in FIG. 1, it is also characterized in that the flaky carbon materials are arranged such that the major axes of the flaky carbon materials are oriented in the plane direction of the film. Here, the "major axis of the flaky carbon material" means an axis oriented in any direction on the XY plane described above, and corresponds to the axis in the XY plane direction in a cross section perpendicular to the plane direction of the heat conduction film. In this specification, whether the flaky carbon materials are arranged such that the major axes of the flaky carbon materials are oriented in the plane direction of the film shall be determined from the value of the "orientation ratio" measured in the column of the examples described later. Specifically, if the value of the orientation ratio is less than 45°, it shall satisfy the requirement that "the flaky carbon materials are arranged such that the major axes of the flaky carbon materials are oriented in the plane direction of the film". Regarding the value of this orientation ratio, if it is less than 45°, there is no particular limitation, but preferably it is 44.5° or less, more preferably 44° or less, still more preferably 43.5° or less, even more preferably 43° or less, particularly preferably 42.5° or less, and most preferably 42° or less. If the value of the orientation ratio is within these ranges, a sufficiently excellent in-plane thermal conductivity can be achieved. On the other hand, there is no particular limitation on the lower limit value of the orientation ratio either. Ideally, it is 0°, but substantially, due to the overlap of the flaky carbon materials, the orientation ratio takes a value greater than 0°. From the viewpoint of easy manufacturing, the lower limit value of the orientation ratio is preferably 3° or more.

[0032] Here, the thermal conductivity of air is very low. Therefore, it is preferable that the heat conduction film according to the present embodiment contains little air. Expressing this quantitatively, in the heat conduction film according to the present embodiment, the ratio of voids occupying a cross section perpendicular to the plane direction of the film, as a value per unit area, is preferably 20% or less, more preferably 19% or less, still more preferably 18% or less, particularly preferably 17% or less, and most preferably 16% or less. On the other hand, there is no particular limitation on the lower limit value of this void ratio, but usually it is 5% or more. In this specification, as the value of the void ratio described above, the value of the "porosity" measured in the column of the examples described later shall be adopted.

[0033] There is no particular limitation on the film thickness of the heat conduction film according to this embodiment, and it can be appropriately determined according to the desired thermal conductivity of the heat conduction film, the size, physical properties, etc. of the formation surface of the support or heat source on which the heat conduction film is formed. For example, the film thickness of the heat conduction film according to this embodiment is preferably 10 to 200 μm, more preferably 15 to 100 μm, and still more preferably 20 to 50 μm.

[0034] Also, there is no particular limitation on the in-plane thermal conductivity of the heat conduction film according to this embodiment, but it is preferably 57 W / m·K or more, more preferably 60 W / m·K or more, still more preferably 63 W / m·K or more, and particularly preferably 65 W / m·K or more. On the other hand, there is no particular limitation on the upper limit value of the in-plane thermal conductivity, and usually the higher the better. As an example, it is 1500 W / m·K or less. In this specification, the value of the in-plane thermal conductivity of the heat conduction film shall be the value measured by the method described in the column of the examples described later.

[0035] 《Method for Manufacturing Heat Conduction Film》 There is no particular limitation on the method for manufacturing the heat conduction film according to this embodiment, and a manufacturing method capable of realizing a heat conduction film having the above-described structure, specifically, a manufacturing method capable of arranging the flaky carbon materials in contact with each other and oriented in the in-plane direction of the film can be appropriately adopted.

[0036] When manufacturing the heat conduction film according to this embodiment, usually, a coating slurry is prepared by dispersing the flaky carbon material and other compounding components (thermal conductive fillers other than the flaky carbon material, binders, thickeners, etc.) added as necessary in an appropriate solvent. Then, this coating slurry is coated on an appropriate support to form a coating film, and this coating film is dried and fired to manufacture the heat conduction film according to this embodiment on the above support. Here, there is no particular limitation on the support, and usually a foil-shaped or plate-shaped metal can be used. Also, if it can withstand the heating temperature during drying and firing of the coating film, the heat conduction film according to this embodiment may be directly manufactured on the surface of a heat source or the like where heat dissipation is to be realized using the heat conduction film according to this embodiment.

[0037] When manufacturing the slurry for coating, water is usually used as the solvent. There are no particular restrictions on the composition of the slurry for coating. For the solid content, the composition in the slurry for coating may be determined in consideration of the values described above as the content in the heat conduction film. Also, the amount of the solvent may be appropriately determined as an amount that can sufficiently disperse the solid content and can sufficiently ensure the coatability during coating.

[0038] Examples of coating means capable of realizing the structure peculiar to the heat conduction film according to this embodiment as described above include a die coater, an applicator, electrostatic coating, and the like. Here, for example, when coating the slurry for coating using a die coater, when the slurry for coating is supplied from the die manifold at a constant flow rate and passes through the slit located at the tip of the die, the flaky carbon material contained in the slurry for coating is oriented in a certain direction. Next, when the slurry for coating discharged from the tip of the slit adheres to the coating film formation surface such as a support, it is pressurized in a direction perpendicular to the coating film formation surface by the tip of the slit. As a result of these behaviors, in the coating film composed of the slurry for coating coated using a die coater, the flaky carbon material is oriented in the plane direction of the coating film. Also, when the slurry for coating passes through the above gap, the voids in the coating film are crushed and the amount of voids decreases. Further, when the slurry for coating contains a binder, the aggregation of the binder is also suppressed when the slurry for coating passes through the above gap. And since this three-dimensional structure is maintained even after subsequent drying and firing, the finally manufactured heat conduction film has the characteristics described above.

[0039] Also, when applying the coating slurry using an applicator, first, a predetermined amount of the coating slurry is placed on the coating film forming surface such as a support. Next, by moving the applicator so that the coating slurry passes through the gap between the applicator and the coating film forming surface (this gap width defines the film thickness of the coating film), a coating film having a uniform film thickness can be formed on the coating film forming surface. At this time, the flaky carbon material contained in the coating slurry becomes oriented in the plane direction of the coating film when the slurry passes through the gap between the applicator and the coating film forming surface. Further, when the coating slurry passes through the above gap, the voids in the coating film are crushed and the amount of voids decreases. Furthermore, when the coating slurry contains a binder, the aggregation of the binder is also suppressed when the coating slurry passes through the above gap. Since the three-dimensional structure in these coating films is retained even after subsequent drying and firing, the finally manufactured heat conduction film has the above-described characteristics.

[0040] Furthermore, when applying the coating slurry using electrostatic coating, for example, a large number of droplets in which the fine particles of the slurry are negatively charged are sprayed onto the positively charged coating film forming surface. Here, when the droplets collide with the coating film forming surface, the flaky carbon material becomes oriented in the plane direction of the coating film forming surface by the electrostatic force. Also, when the droplets collide with the coating film forming surface, the voids in the coating film are crushed by the pressure applied to the droplets, and the amount of voids decreases. Furthermore, when the coating slurry contains a binder, the aggregation of the binder is also suppressed by the pressure applied to the droplets when the droplets collide with the coating film forming surface. Since the three-dimensional structure in these coating films is retained even after subsequent drying and firing, the finally manufactured heat conduction film has the above-described characteristics.

[0041] 《Uses of the Heat Conduction Film (Heat Dissipation Structure)》 The heat conduction film according to one embodiment of the present invention described above has excellent in-plane thermal conductivity. Therefore, a heat dissipation structure can be configured using this excellent in-plane thermal conductivity. That is, according to another embodiment of the present invention, there is provided a heat dissipation structure including a heat source and the heat conduction film according to one embodiment of the present invention described above arranged to be in contact with the heat source. 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.

[0042] In recent years, high-brightness light-emitting diode (LED) lamps have been adopted for the purpose of extending the lifespan and reducing power consumption of automotive headlights, and heat sinks are used to cool these high-brightness LEDs. This heat sink is usually made 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.

[0043] FIG. 3 is a schematic cross-sectional view schematically showing a cross-section of a heat dissipation structure according to such a conventional technique.

[0044] As shown in FIG. 3, the heat dissipation structure 1 according to the prior art is used, for example, for cooling a high-brightness LED module 4 with an output of 1 W or more, and has a basic configuration in which a heat transfer plate 3 made of at least a good heat-conducting metal or carbon material and a heat sink body 2 made of a thermally conductive resin are combined. Further, the LED module 4 has a structure in which a light emitter 8 incorporating a plurality of LED elements and integrally forming a lens is held at the center of a substrate 7, and the substrate 7 is joined to a heat receiving surface 5 disposed on the above-described heat sink body 2. The heat dissipation structure 1 shown in FIG. 3 transfers the heat generated from a narrow heat source due to the driving of the LED module 4 to the entire heat receiving surface 5 by the heat transfer plate 3 made of a good heat-conducting metal or carbon material provided along the heat receiving surface 5 for joining the LED module 4, and dissipates the heat into the air by the heat sink body 2 formed of a thermally conductive resin having a low heat capacity and a high emissivity, thereby suppressing the temperature rise of the LED module 4 (that is, cooling the LED module 4). According to the heat dissipation structure 1 having such a configuration, it is possible to dissipate the heat generated from the LED module 4 to some extent. However, since the heat sink body 2 occupies a large volume in the heat dissipation structure 1, there is a problem that the space efficiency is poor. Further, since the heat sink body 2 is made of a metal material such as pure aluminum or an aluminum alloy, the heat dissipation structure 1 also becomes a factor in increasing the weight of the vehicle when applied to, for example, an automotive headlight.

[0045] On the other hand, FIG. 4 is a cross-sectional schematic view schematically showing a cross section of a heat dissipation structure according to an embodiment of the present invention.

[0046] As shown in FIG. 4, compared with the heat dissipation structure shown in FIG. 3, the heat sink main body 2 is removed from the heat dissipation structure 1 according to an embodiment of the present invention. On the other hand, a heat conduction film 10 (see FIG. 1) according to an embodiment of the present invention is disposed so as to cover the entire surface of the heat transfer plate 3 on the side where the LED module 4 is disposed. By having such a configuration, the heat dissipation structure 1 according to the embodiment shown in FIG. 4 can quickly transfer the heat generated from a narrow heat source due to the driving of the LED module 4 in the plane direction of the heat conduction film 10 and efficiently dissipate the heat to the outside. Further, since the heat sink main body provided in the heat dissipation structure according to the embodiment shown in FIG. 3 is not provided, there is an extremely excellent advantage that the volume and weight of the heat dissipation structure can be significantly reduced.

[0047] Although not shown in FIG. 4, in the heat dissipation structure 1, it is preferable that a heat radiator is further disposed so as to be in contact with the heat conduction film 10. By disposing such a heat radiator, more efficient heat dissipation can be realized. Here, the "heat radiator" means a member that can more efficiently release the heat conducted from the heat source (LED module 4 in FIG. 4) by the heat conduction film 10 to the outside. Examples of such a heat radiator include a heat sink, a heat pump, and a metal housing of an electronic device. Further, when a heat radiator is further disposed on the heat conduction film 10, from the viewpoint of fully utilizing the excellent in-plane heat conductivity of the heat conduction film 10, the heat radiator is disposed at a position not facing the heat source (that is, the heat source and the heat radiator are separated from each other in the plane direction of the heat conduction film) via the heat conduction film disposed so as to be in contact with the heat source. At this time, the distance in the plane direction of the heat conduction film between the heat source and the heat radiator is preferably 1 cm or more, more preferably 5 cm or more, still more preferably 10 cm or more, particularly preferably 15 cm or more, and most preferably 20 cm or more.

[0048] The above has described a heat dissipation structure having a high-brightness light-emitting diode (LED) as a heat source as an example of the heat dissipation structure according to an aspect of the present invention. However, the heat dissipation structure according to this aspect is not limited to this and can be used for the purpose of dissipating heat generated in various heat sources.

[0049] Examples of such heat sources include, for example, in addition to the above-described LED module, various lasers (sensors) (conventional heat dissipation means are aluminum plates and combinations thereof with cooling fins, Peltier elements, or chiller water cooling); high-performance infrared cameras (IR) (conventional heat dissipation means are aluminum plates and combinations thereof with cooling fins, Peltier elements, or chiller water cooling); head-up displays (HUD) (conventional heat dissipation means are combinations of heat sinks and spreaders); elements and batteries of smartphones (conventional heat dissipation means are air cooling, heat dissipation sheets, combinations of heat sinks and spreaders); elements and batteries of digital cameras (conventional heat dissipation means are combinations of heat sinks, spreaders, and housings); elements and batteries of personal computers (PCs) (conventional heat dissipation means are combinations of fans, heat sinks, spreaders, and housings); elements of in-vehicle electronic control units (ECUs) such as for vehicle use (conventional heat dissipation means are combinations of heat sinks, spreaders, and housings); elements of insulated gate bipolar transistors (IGBTs) which are main conversion elements of high-power inverters (conventional heat dissipation means are combinations of heat sinks, spreaders, and housings); rotating parts of motors (conventional heat dissipation means are combinations of fans and heat sinks, or combinations thereof with water cooling); light source lamps of thin displays (conventional heat dissipation means are heat sinks); in-vehicle large-capacity batteries (conventional heat dissipation means are heat dissipation sheets, heat dissipation materials, air cooling, or combinations of water cooling, fans, and heat sinks), and the like.

[0050] 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 heat conduction film according to one embodiment of the present invention. As a result, for example, if the heat sink as the conventional heat dissipation means can be removed as described with reference to FIG. 4, the weight and volume of the heat dissipation structure can be significantly reduced. Further, by replacing the conventional heat dissipation means such as a fan, air cooling, and water cooling with the heat conduction film according to one embodiment of the present invention, it is also expected that the weight and volume of the heat dissipation structure can be greatly reduced as compared with the heat dissipation structure provided with the conventional heat dissipation means.

Example

[0051] Hereinafter, the present invention will be described in more detail with reference to examples. However, the technical scope of the present invention is not limited only to the following examples.

[0052] 《Production Example of Heat Conduction Film》 [Example 1] A predetermined amount of commercially available graphene (manufactured by Kanto Chemical Co., Inc., including multi-layer graphene) and vapor-grown carbon fiber (VGCF, manufactured by Showa Denko K.K.) were weighed, and stirred at a rotational speed of 2000 rpm for 1 minute using a stirrer (a rotation / revolution propellerless mixer) to obtain a carbon material mixture.

[0053] Next, a predetermined amount of pure water was weighed, added to the carbon material mixture obtained above, and then stirred at a rotational speed of 2000 rpm for 1 minute using the above stirrer.

[0054] On the other hand, a predetermined amount of styrene-butadiene rubber (SBR, manufactured by JSR Corporation) was weighed as a binder, added to the carbon material mixture mixed with pure water above, and then stirred at a rotational speed of 2000 rpm for 1 minute using the above stirrer.

[0055] Next, a predetermined amount of carboxymethyl cellulose nanofiber (CMized CNF, manufactured by Nippon Paper Industries Co., Ltd.) was weighed, added to the carbon material mixture mixed with the binder above, and then stirred at a rotational speed of 2000 rpm for 1 minute using the above stirrer.

[0056] Then, a predetermined amount of pure water was weighed, added to the carbon material mixture in which the CNF was CM-treated above, and then stirred for 1 minute at a rotational speed of 2000 rpm using the above stirrer to obtain a coating slurry. The composition ratio (mass %) of the coating slurry thus obtained was graphene: VGCF: binder: CM-treated CNF: pure water = 5: 7.5: 3.75: 8.75: 75 (that is, the composition ratio (mass %) of the solid content was graphene: VGCF: binder: CM-treated CNF = 20: 30: 15: 35).

[0057] As a support for forming the heat conduction film, aluminum foil (film thickness 20 μm) was prepared. Next, the coating slurry obtained above was applied to one surface of the support using a die coater (self-propelled coater) to form a coating film (film thickness 30 μm) composed of the coating slurry, and a laminate composed of the support and the coating film was obtained.

[0058] Thereafter, the laminate obtained above was left standing on a hot plate at 40 ° C for 30 minutes to dry the coating film. Next, this laminate was further left standing in an electric furnace at 130 ° C for 30 minutes to bake the coating film, and the heat conduction film (film thickness 30 μm) of this example was produced on the surface of the support.

[0059] [Comparative Example 1] On one surface of the same support (aluminum foil) as in Example 1, the coating slurry obtained in Example 1 described above was applied using a spray gun (manufactured by Anest Iwata Corporation) to form a coating film (film thickness 30 μm) composed of the coating slurry, and a laminate composed of the support and the coating film was obtained.

[0060] Thereafter, the laminate obtained above was left standing on a hot plate at 40 ° C for 30 minutes to dry the coating film. Next, this laminate was further left standing in an electric furnace at 130 ° C for 30 minutes to bake the coating film, and the heat conduction film (film thickness 30 μm) of this comparative example was produced on the surface of the support.

[0061] [Comparative Example 2] The coating slurry obtained in Example 1 described above was transferred to another container, and a support (aluminum foil) similar to that in Example 1 was immersed in this coating slurry. Then, after pulling up the immersed support from the coating slurry and holding it for 1 minute, the excess coating slurry was removed to form a coating film made of the coating slurry on the surface of the support. And the sample thus obtained was left standing in an electric furnace at 60°C for 5 minutes to dry the coating film.

[0062] Thereafter, the sample obtained above was immersed in the coating slurry from the opposite direction to the above. Then, after pulling up the immersed support from the coating slurry and holding it for 1 minute, the excess coating slurry was removed to form a coating film made of the coating slurry on the surface of the support again. And the sample thus obtained was left standing in an electric furnace at 60°C for 5 minutes to dry the coating film.

[0063] Thereafter, the sample obtained above was left standing in an electric furnace at 130°C for an additional 30 minutes to bake the coating film, and a heat conduction film (film thickness: 30 μm) of this comparative example was produced on the surface of the support.

[0064] 《Evaluation Example of Heat Conduction Film》 [Measurement of the Orientation Ratio of Flaky Carbon Materials in the Heat Conduction Film] Regarding the heat conduction films produced in the above-described examples and comparative examples, the orientation ratio of flaky carbon materials (graphene) was evaluated by the following method. The results are shown in Table 1 below.

[0065] Specifically, using a micro X-ray diffraction measurement apparatus, pole figure measurement of the graphite (002) plane inside the heat conduction film was performed. At this time, the longitudinal direction (MD) and the width direction (TD) of the heat conduction film in the obtained pole figure were defined, and the full width at half maximum of the diffraction intensity peak was calculated from the cross-sectional profile of the pole figure in the width direction (TD), and this was used as the orientation degree. By performing the measurement of the full width at half maximum of the diffraction intensity peak using such a pole figure while changing the incident angle θ during X-ray diffraction measurement in the range of 0 to 90°, the orientation degree can be calculated by the following mechanism. That is, if the orientation of the flaky carbon material (graphene) is random, since the flaky carbon material (graphene) corresponding to each incident angle θ is uniformly present, the diffraction intensity peak becomes broad (flat). On the other hand, if the orientation of the flaky carbon material (graphene) is aligned in a specific direction, since the diffraction intensity is detected biased toward the incident angle θ in a specific region, the diffraction intensity peak becomes sharp. In this measurement method, the smaller the value of the orientation degree, the higher the degree of orientation of the long axis of the flaky carbon material (graphene) in the plane direction of the heat conduction film. Also, in Table 1 below, the fact that the orientation degree is described as "random" means that the value of the orientation degree could not be calculated by the above method due to the random orientation of the flaky carbon material.

[0066] [Measurement of the porosity of the heat conduction film (the ratio of voids in the cross-section perpendicular to the plane direction)] Regarding the heat conduction films produced in the above-described examples and comparative examples, the porosity (the ratio of voids in the cross-section perpendicular to the plane direction) was measured by the following method. The results are shown in Table 1 below.

[0067] Specifically, a cross-section perpendicular to the plane direction of the heat conduction film was observed using a scanning electron microscope (SEM), and the area of the voids present in a 20 μm × 20 μm observation field in the obtained SEM observation image was measured. Then, the porosity was calculated as the percentage of the area of the voids measured in this way with respect to the area of the observation field (area of voids / area of observation field × 100 [%]). As the cross-section preparation process used for SEM observation, ion milling was performed.

[0068] Here, according to the SEM observation of the heat conduction film produced in Example 1, it was also confirmed that adjacent graphene (scaly carbon material) was in contact with each other.

[0069] [Evaluation of Thermal Conductivity] For the heat conduction films produced in the above-described Examples and Comparative Examples, the thermal conductivity was evaluated by the following method. The results are shown in Table 1 below.

[0070] First, each heat conduction film (with a support) was cut into strips of 150 mm × 20 mm to prepare evaluation samples for thermal conductivity.

[0071] A heat source (output 2 W) was placed at one end of the evaluation sample prepared above. At this time, thermocouples were placed at three locations on the surface of the evaluation sample. The locations of the thermocouples were such that the central part of the heat source (between the heat source and the heat conduction film) was the first location, and the positions 25 mm and 50 mm in the longitudinal direction of the evaluation sample from there were the second and third locations, respectively.

[0072] The evaluation sample with the thermocouples placed above was left standing in a thermostat at 25°C. Then, the switch of the heat source was turned on to apply heat from the heat source to the heat conduction film, and the temperature profile of the thermocouples was obtained. At this time, heating was continued until the temperature of the thermocouples became stable, and the temperature at each measurement point when the temperature of the thermocouples became stable was obtained. And from the temperatures measured in this way, the thermal conductivity of the heat conduction film was calculated according to the following calculation formula for thermal conductivity. Note that as the distance [m], the distance between the first thermocouple and the third thermocouple (50 mm (=0.05 [m])) was used. Also, as the cross-sectional area [m 2 , the cross-sectional area of the heat conduction film (20 mm × 30 μm (=6 × 10 -7 [m])) was used. And as the temperature difference [K], the temperature difference between the first thermocouple and the third thermocouple was used.

[0073] (Calculation Formula for Thermal Conductivity) Thermal Conductivity [W / m·K] = Heat Transfer Amount [W] × Distance [m] × (1 / Cross-sectional Area [m 2) × (1 / temperature difference [K])

[0074]

Table 1

[0075] From the results shown in Table 1, it can be seen that the heat conduction film of Example 1 according to an embodiment of the present invention has a structure in which the flaky carbon materials (graphene) are arranged such that the adjacent flaky carbon materials are in contact with each other and the long axes of the flaky carbon materials are oriented in the plane direction of the film, whereby the heat conductivity in the plane direction is very high. On the other hand, it can also be seen that the heat conduction film of the comparative example has a structure in which the long axes of the flaky carbon materials (graphene) are not oriented in the plane direction of the film, resulting in inferior heat conductivity.

Explanation of reference numerals

[0076] 10 Heat conduction film, 100 Multilayer graphene (flaky carbon material).

Claims

Claim 1. A heat conductive film comprising: (A) a scaly carbon material made of graphene or graphite; (B) a carbon nanotube or a carbon nanofiber; and (C) a resin binder selected from the group consisting of styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated product, styrene-isoprene-styrene block copolymer and its hydrogenated product, wherein the contents of (A) to (C) are, respectively, (A) 15 to 90% by mass, (B) 5 to 45% by mass, and (C) 5 to 30% by mass based on 100% by mass of the total amount of the components constituting the heat conductive film; adjacent (A) scaly carbon materials are in contact with each other, and the (A) scaly carbon materials are arranged such that the major axis of the (A) scaly carbon materials is oriented in the plane direction of the film, and the proportion of voids in a cross section perpendicular to the plane direction of the heat conductive film is 5% or more and 20% or less per unit area. Claim 2. The heat conductive film according to claim 1, wherein the orientation ratio of the (A) scaly carbon material is 42° or less. Claim 3. The heat conductive film according to claim 1 or 2, wherein when observing a cross section perpendicular to the plane direction of the heat conductive film, a thermally conductive network is formed from one end to the other end in the plane direction of the heat conductive film because the (A) scaly carbon materials are continuous with adjacent (A) scaly carbon materials. Claim 4. The heat conductive film according to any one of claims 1 to 3, wherein 50% or more of the particles of the (A) scaly carbon material contained in the heat conductive film are in contact with adjacent (A) scaly carbon materials. Claim 5. A heat generating source, a heat conductive film according to any one of claims 1 to 4, arranged to be in contact with the heat generating source, and a heat dissipation structure comprising the same. Claim 6. The heat dissipation structure according to claim 5, further comprising a heat radiator arranged to be in contact with the heat conductive film. Claim 7. The heat dissipation structure according to claim 6, wherein the heat radiator is arranged at a position not facing the heat generating source through the heat conductive film.

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