Method for producing graphite sheet, graphite sheet, and composite for producing graphite sheet

Abstract

The goal is to provide a graphite sheet with high thermal diffusivity without using a special resin film as a raw material. This problem is solved by a method for producing a graphite sheet using a composite material containing cellulose nanofibers and graphite oxide as a raw material and heat-treating the raw material at a temperature of 2400°C or above.

Description

Technical Field

[0001] The present invention relates to a method for producing a graphite sheet, a graphite sheet, and a composite for producing a graphite sheet. Background Art

[0002] Graphite is a material with excellent heat resistance, chemical resistance, high thermal conductivity, and high electrical conductivity. In particular, graphite sheets, made of crystalline graphite, have recently been used as a heat dissipation material for semiconductor devices and other heat-generating components in various electronic and electrical devices, such as computers and smartphones.

[0003] A known method for producing graphite sheets is the expanded graphite method. This method involves first immersing natural graphite in a mixture of concentrated sulfuric acid and concentrated nitric acid, rapidly heating it to produce expanded graphite. The acid is then removed by washing, and the graphite is processed into sheets by high-pressure pressing. However, the graphite sheets produced by this method have weak strength, inadequate physical properties, and are also subject to issues such as the effects of residual acid.

[0004] In order to solve such problems, a method has been developed in which a special resin film is fired at a high temperature to achieve graphitization (for example, see Patent Document 1). As the resin film used in this method, films containing polyoxadiazole, polyimide, polyphenylene vinylene, polybenzimidazole, polybenzoxazole, polythiazole, polyamide, etc. can be cited. Furthermore, in recent years, methods have been developed in which graphene oxide is subjected to solution film formation, chemical reduction, high-temperature reduction, and high-pressure pressurization (for example, see Patent Document 2). These methods are much simpler than the expanded graphitization method, and the resulting graphite sheets have the advantages of being substantially free of impurities such as acid and having excellent thermal conductivity and electrical conductivity properties close to those of single-crystal graphite.

[0005] [Prior art literature]

[0006] [Patent Document]

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2004-123506

[0008] Patent Document 2: Japanese Patent Application No. 2018-524257 Summary of the Invention

[0009] (Problems to be solved by the invention)

[0010] Conventional methods for producing highly crystalline graphite sheets utilize a method of graphitizing a resin (resin film) by heat treatment at high temperatures. However, this method is limited in the raw materials that can be used.

[0011] In view of the above-mentioned situation, one embodiment of the present invention aims to provide a method for producing a graphite sheet using high-temperature heat treatment, which can produce a graphite sheet with high thermal diffusivity without using a special resin film as a raw material.

[0012] (Methods used to solve problems)

[0013] The present inventors conducted intensive research and found that a graphite sheet with high thermal diffusivity can be produced by heat-treating a composite of cellulose nanofibers and graphite oxide at high temperatures. This led to the completion of the present invention.

[0014] That is, one aspect of the present invention includes the following aspects.

[0015] A method for manufacturing a graphite sheet comprises: heat-treating a composite comprising cellulose nanofibers and graphite oxide at a temperature above 2400°C.

[0016] A composite material for producing graphite sheets, comprising cellulose nanofibers and graphite oxide.

[0017] A graphite sheet with a thermal diffusivity of 3.5 cm 2 / s or more, its surface roughness after pressing is 1.0 μm or more, and its thickness maintenance rate after pressing is 70% or more.

[0018] (Effects of the Invention)

[0019] According to one embodiment of the present invention, a method for manufacturing a graphite sheet using high-temperature heat treatment can be provided, which can achieve the production of a graphite sheet with high thermal diffusivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] [ Figure 1 ] Figure 1 This is a SEM observation result of the surface of the composite material containing cellulose nanofibers and graphite oxide used in Example 14.

[0021] [ Figure 2 ] Figure 2 This is a SEM observation result of the surface of the graphite sheet obtained in Example 14.

[0022] [ Figure 3 ] Figure 3 This is a SEM observation result of the surface of the composite material containing cellulose nanofibers and graphite oxide used in Example 17.

[0023] [ Figure 4 ] Figure 4 This is a SEM observation result of the surface of the graphite sheet obtained in Example 17.

[0024] [ Figure 5 ] Figure 5 This is a SEM observation result of the surface of the composite material containing cellulose nanofibers and graphite oxide used in Example 19.

[0025] [ Figure 6 ] Figure 6 This is a SEM observation result of the surface of the graphite sheet obtained in Example 19.

[0026] [ Figure 7 ] Figure 7 This is a SEM observation result of the cross section of the graphite sheet obtained in Example 19.

[0027] [ Figure 8 ] Figure 8 This is a SEM observation result of the surface of the graphite sheet obtained in Reference Example 1.

[0028] [ Figure 9 ] Figure 9 IR spectra of cellulose nanofibers A, B, and C and paper used in the examples.

[0029] [ Figure 10 ] Figure 10 1 is the XRD spectrum of cellulose nanofibers A, B, C and paper used in the examples. DETAILED DESCRIPTION

[0030] The specific embodiments of the present invention are described in detail below. The present invention is not limited to the various schemes described below, and various changes can be made within the scope shown in the specification. In addition, the embodiments or embodiments obtained by combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, by combining the technical means disclosed in each embodiment, new technical features can be formed. In addition, in this specification, unless otherwise specified, "A to B" expressing a numerical range means greater than A (including A and greater than A) and less than B (including B and less than B).

[0031] <Graphite Sheet>

[0032] First, a graphite sheet produced by a method for producing a graphite sheet according to one embodiment of the present invention will be described. In this specification, the "method for producing a graphite sheet according to one embodiment of the present invention" may be referred to as the "present production method."

[0033] The graphite sheet according to one embodiment of the present invention is produced (manufactured) by the present production method, and is produced by heat-treating a composite of cellulose nanofibers and graphite oxide. In this specification, the "graphite sheet according to one embodiment of the present invention" may be referred to as the "present graphite sheet."

[0034] Because of the above-described structure, this graphite sheet can achieve excellent thermal diffusivity without using a special resin film such as a polyimide film. Special resin films such as polyimide films are very expensive. Therefore, there is a need for a technology that can provide graphite sheets using more affordable materials. On the other hand, because the graphite sheet according to one embodiment of the present invention uses a composite of cellulose nanofibers and graphite oxide as a raw material, it can be provided at a lower price than when using a special resin film. From this perspective, the present invention can also be said to be extremely useful.

[0035] (Thermal diffusivity of graphite sheet)

[0036] The thermal diffusivity of this graphite sheet is 3.5cm 2 / s or more, preferably 5.0cm 2 / s or more, 7.0cm 2 / s or more, 8.0cm 2 / s or more. Thermal diffusivity is 3.5cm 2 Graphite sheets with a thermal diffusivity of more than 1000 nm have excellent heat dissipation properties and are therefore suitable for use as heat dissipation components in fields requiring excellent heat dissipation, such as electronic equipment. In addition, the upper limit of the thermal diffusivity of the graphite sheet is not particularly limited, but can be, for example, 12.0 cm 2 It should be noted that the thermal diffusivity of the graphite sheet is a value measured by the method described in the examples below.

[0037] (Surface roughness of graphite sheet after pressurization)

[0038] When graphite sheets are used as heat dissipation materials in electronic devices, they are typically attached with adhesive tape or bonding tape. However, the present inventors have discovered that the use of conventional graphite sheets can lead to the following adhesion and cohesion issues. Specifically, graphite sheets are typically compressed or rolled to adjust their flexibility or thickness. However, compression or rolling smoothes the surface of the graphite sheet, weakening its adhesion to the adhesive tape or bonding tape. As a result, the adhesion and cohesion between the graphite sheet and the adhesive tape or bonding tape are weakened.

[0039] On the other hand, the present graphite sheet has a moderate surface roughness even after compression or rolling, in other words, even after pressurization. Therefore, it has excellent adhesive force and bonding force even after pressurization. From the perspective of adhesive force and bonding force, the surface roughness of the present graphite sheet after pressurization is preferably 1.0 μm or more, more preferably 1.4 μm or more, further preferably 1.8 μm or more, and even more preferably 2.0 μm or more. In addition, the upper limit of the surface roughness of the present graphite sheet is not particularly limited, but can be, for example, 5.0 μm or less. It should be noted that the surface roughness of the graphite sheet after pressurization is a value measured by the method described in the Examples below.

[0040] Furthermore, the present inventors have discovered that, in graphite sheets after compression or rolling, as the graphite layers in the sheet rearrange into layers, the cohesive force in the thickness direction weakens. This results in a decrease in the interlaminar strength of the graphite layers, which can lead to the problem of graphite layers being easily delaminated.

[0041] However, the present graphite sheet has a moderate surface roughness even after pressurization, and the interior of the pressed graphite sheet also has a moderate roughness. Therefore, the arrangement of the graphite layers in the pressed graphite sheet is moderately irregular, resulting in excellent interlaminar strength. In other words, the present graphite sheet with a moderate surface roughness (e.g., 1 μm or greater) also has excellent interlaminar strength.

[0042] (Thickness retention rate of graphite sheet after pressurization)

[0043] When graphite sheets are used as heat dissipation materials for electronic devices, the graphite sheets are sometimes clamped between the heating component and the radiator for pressing (compression bonding) for use. However, the inventors have found that the previous graphite sheets have the following problems regarding size change (thickness change). That is, when the graphite sheet is clamped between the heating component and the radiator for pressing for use, the graphite sheet is subjected to very high pressure to tighten. The problem with the previous graphite sheets is that the size (thickness) of the graphite sheet will change significantly due to the tightening, thereby reducing the fit between the heating component and the radiator, resulting in reduced heat dissipation performance. In addition, during assembly or maintenance, the heating component, graphite sheet, and radiator are sometimes disassembled. In this case, if the graphite sheet with a significantly changed size (thickness) is recycled, the heat dissipation performance will also be reduced.

[0044] On the other hand, even after being sandwiched and pressed between a heat-generating component and a heat sink, in other words, even after being pressurized, the graphite sheet exhibits little change in size (thickness). In other words, it exhibits excellent dimensional stability. Therefore, it exhibits excellent heat dissipation even after being pressurized.

[0045] The dimensional stability of the present graphite sheet can be evaluated by the thickness retention rate after pressurization. To achieve excellent dimensional stability and provide a graphite sheet with excellent heat dissipation properties, the thickness retention rate of the present graphite sheet after pressurization is preferably 70% or greater, more preferably 75% or greater, even more preferably 80% or greater, and even more preferably 85% or greater. The upper limit of the thickness retention rate of the present graphite sheet is not particularly limited, but may be 100%, 99%, 95%, or 90%. The thickness retention rate of the graphite sheet after pressurization is measured using the method described in the Examples below.

[0046] As described above, the present graphite sheet is a graphite sheet that is excellent not only in thermal diffusivity but also in adhesive strength, adhesion rate, and dimensional stability. Therefore, the present graphite sheet can also be expressed as: thermal diffusivity is 3.5 cm 2 / s or more, a graphite sheet having a surface roughness of 1 μm or more after pressing and a thickness retention rate of 70% or more after pressing.

[0047] The thickness of the present graphite sheet is not particularly limited, but is preferably 1 μm to 50 mm. When the thickness of the present graphite sheet is 1 μm or greater, heat transfer is excellent. Furthermore, when the thickness of the present graphite sheet is 50 mm or less, the sheet maintains good thickness after pressurization and exhibits excellent heat transfer in the thickness direction.

[0048] From the viewpoint of transferring a large amount of heat in a small volume, the density of the graphite sheet is preferably 1.5 g / cm 3 More than 1.8 g / cm 3 The upper limit of the density of the graphite sheet is not particularly limited, but may be, for example, 2.26 g / cm 3 the following.

[0049] <Cellulose Nanofibers>

[0050] As the cellulose nanofiber in one embodiment of the present invention, mechanically defibrillated cellulose nanofibers (for example, cellulose nanofibers obtained by a high-pressure homogenization method, cellulose nanofibers obtained by a grinding method, etc.), TEMPO-oxidized cellulose nanofibers, phosphated cellulose nanofibers, phosphite-esterified cellulose nanofibers, crystalline cellulose, cellulose nanocrystals, carboxymethylated cellulose, carboxymethylated cellulose sodium salt, etc. can be listed. Only one of them can be used, or two or more of them can be used in combination. Among these cellulose nanofibers, TEMPO-oxidized cellulose nanofibers, mechanically defibrillated cellulose nanofibers, and crystalline cellulose are preferred, and TEMPO-oxidized cellulose nanofibers are particularly preferred. This is because the composite comprising TEMPO-oxidized cellulose nanofibers easily forms a graphite layer structure through carbonization and graphitization.

[0051] In addition, when a film is made solely of cellulose nanofibers, the linear expansion coefficient of the film in the plane direction relative to the range of 50 to 150°C is preferably 20 ppm / K or less. In addition, the degree of etherification is also preferably 0.5 to 1.5. In addition, from the perspective of having a good thickness retention rate after pressurization of the obtained graphite sheet, when a film is made solely of cellulose nanofibers, the surface roughness of the film (sometimes referred to as the surface roughness after drying) is preferably 0.1 μm to 3.0 μm, more preferably 0.2 μm to 2.5 μm, and even more preferably 0.5 μm to 2.0 μm.

[0052] Furthermore, the cellulose nanofibers are preferably cellulose nanofibers represented by the following structural formula, because this can provide a graphite sheet having a high thermal diffusivity.

[0053]

[0054] In the formula, R1, R2, and R3 are each independently any one of -OH, -COOH, -HPO3, -H2PO4, -Na2PO4, -CH2OCH2COOH, and -CH2OCH2COONa, and n is an integer greater than 1. Furthermore, R1, R2, and R3 can be used to bond the repeating units expressed in the structural formula to each other and may have a branched structure.

[0055] The cellulose nanofibers expressed in the above structural formula refer to: straight-chain cellulose micron short fibers with a width of 3 to 4 nm formed by regular entanglement of 30 to 40 strings of cellulose molecules and having high crystallinity, micron short fiber bundles with a width of 20 to 30 nm formed by bundles of cellulose micron short fibers, and their aggregates.

[0056] The cellulose nanofibers according to one embodiment of the present invention preferably have a crystallinity of 50% or greater, thereby producing graphite sheets with high thermal diffusivity. The crystallinity is more preferably 55% or greater, even more preferably 60% or greater, and even more preferably 65% ​​or greater. A crystallinity of 70% or greater is particularly preferred. Cellulose nanofibers have both amorphous and crystalline portions, so the crystallinity refers to the percentage of the crystalline portion within the cellulose nanofibers. In the present invention, the crystallinity is measured by X-ray diffraction.

[0057] The crystallinity of the cellulose nanofibers of the present invention was determined using the following procedure. The X-ray diffraction spectrum was measured using a horizontal multi-purpose X-ray diffractometer (Ultima III, manufactured by Rigaku Co., Ltd.) at an X-ray power of 40 kV and 40 mA within the range of 5° ≤ 2θ ≤ 35°. The crystallinity was calculated using the following formula.

[0058] Crystallinity (%)=[(I 22.6-I 18.5) / I 22.6]×100

[0059] In the above formula, I 22.6 is the diffraction intensity of X-ray diffraction on the lattice plane (002 plane) (diffraction angle 2θ=22.6°), and I 18.5 is the diffraction intensity on the amorphous part (diffraction angle 2θ=18.5°).

[0060] Cellulose nanofibers also form the cell walls of plants and can be isolated from a variety of plants. Methods for isolating cellulose nanofibers can be broadly categorized into two types: mechanical defibration, which involves mechanical treatment alone, and pretreatment with chemical or enzymatic treatment followed by mechanical treatment. While fiber diameter (distribution) and fiber length (distribution) vary depending on the production method, they generally produce highly hydrophilic fibers with a high aspect ratio, ranging from 4 to several hundred nanometers in width and micrometer-scale length.

[0061] Mechanical defibration methods include high-pressure homogenization, grinding, collision, bead milling, and twin-screw extrusion. A larger specific surface area after mechanical defibration increases the water retention of the aqueous dispersion of cellulose nanofibers and can form a gel depending on the concentration.

[0062] On the other hand, as methods that combine pretreatment and mechanical fibrillation, TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxygen) oxidation, cationization, and enzyme treatment are known. In the TEMPO oxidation method, only the C6 position of the glucose unit of cellulose is selectively oxidized, thereby introducing a negatively charged carboxyl group. Therefore, with only a slight mechanical treatment in water, it is possible to obtain TEMPO-oxidized cellulose nanofibers with a width of 4 nm and high crystallinity through electrostatic repulsion. Cationization methods also utilize electrostatic repulsion in the same way as TEMPO oxidation methods.

[0063] The IR spectrum of the cellulose nanofibers in one embodiment of the present invention is not particularly limited, but is preferably in the range of 3000 to 3500 cm- 1 The peak has a wave number range of 1030-1070 cm, which makes the interaction with graphite oxide good. -1 The maximum peak exists in the wave number range of 1070-1140 cm-1, and the maximum peak exists in the wave number range smaller than that of the maximum peak, for example, 1070-1140 cm-1. -1 or 1140~1200cm -1 The presence of two peaks within the range of , resulting in a good thickness retention rate of the graphite sheet after pressurization. It should be noted that in this specification, the IR spectrum of cellulose nanofibers refers to the IR spectrum of a cellulose nanofiber membrane when the cellulose nanofibers are prepared as a single sheet (cellulose nanofiber membrane), and more specifically, the value measured by the method described in the Examples.

[0064] The XRD spectrum of the cellulose nanofibers in one embodiment of the present invention is not particularly limited, but preferably has a peak in the range of 20-25° 2θ and a peak in the range of 10-20° 2θ. Having peaks in the above ranges means that the cellulose nanofibers are crystalline, and the resulting thickness retention after pressurization is good, which is preferred. It should be noted that in this specification, the XRD spectrum of cellulose nanofibers refers to the XRD spectrum of a cellulose nanofiber film when the cellulose nanofibers are prepared as a membrane sheet (cellulose nanofiber film) alone, and more specifically, the value measured by the method described in the Examples.

[0065] <Graphite Oxide>

[0066] The graphite oxide according to one embodiment of the present invention (hereinafter sometimes referred to as "the present graphite oxide") is graphite obtained by gas-phase oxidation, chemical oxidation, or electrolytic oxidation of graphite, and is graphite obtained by replacing or modifying a portion of the graphite surface with oxygen or even with oxygen-containing functional groups such as hydroxyl groups and carboxyl groups.

[0067] While various graphites can be used, graphite with a well-developed layer structure and high crystallinity is preferred because the yield of graphite oxide is high and graphite oxide with a small number of basic layers is easily obtained. Preferred examples of such graphites include natural graphite (particularly high-quality graphite), rough graphite (particularly graphite produced at high temperatures), and highly oriented pyrolytic graphite. Expanded graphite, which has its interlayers expanded beforehand, is also preferred. These graphites may be in the form of powders, films, sheets, or the like.

[0068] The average particle size of graphite can be appropriately selected based on the average particle size of graphite oxide particles suitable for the intended use. The average particle size of graphite is preferably 0.1 μm to 500 μm, and more preferably 1 μm to 200 μm. An average particle size of 0.1 μm or greater is preferred because the resulting graphite oxide particles have a larger aspect ratio and greater shape anisotropy than those with an average particle size of less than 0.1 μm.

[0069] The shape of the graphite oxide particles is not particularly limited and may be various shapes. For example, the graphite oxide particles may be spherical or flat.

[0070] In the present graphite oxide, the mass ratio of carbon to oxygen (C / O) is preferably 0.75 to 5.0. If the mass ratio is less than 0.75, it may be difficult to maintain the structure of the graphite. In addition, if the mass ratio is 5.0 or more, the oxygen content in the graphite oxide is low, which may make it difficult to produce a graphite sheet with high thermal diffusivity. That is, in the present graphite oxide, when the mass ratio of carbon to oxygen (C / O) is 0.75 to 5.0, a suitable graphite structure can be maintained, thereby achieving a good thickness retention rate after pressurization, and providing a graphite sheet with excellent thermal diffusivity and surface roughness after pressurization, and therefore preferred. The mass ratio of carbon to oxygen is preferably 4.0 or less, more preferably 3.0 or less, and further preferably 2.0 or less. In addition, with respect to the lower limit of the mass ratio, it is preferably 0.6 or more, more preferably 0.85 or more, and further preferably 1.0 or more. The mass ratio of carbon to oxygen (C / O) in graphite oxide can be measured on a dry film of graphite oxide using a CHN elemental analyzer (PE2400II manufactured by PerkinElmer).

[0071] The average particle size of the graphite oxide is not particularly limited, but is preferably 30 nm to 3 mm, more preferably 50 nm to 1 mm, even more preferably 100 nm to 500 μm, particularly preferably 0.3 μm to 250 μm, and most preferably 0.5 μm to 90 μm, to obtain a graphite sheet with superior thermal diffusivity, surface roughness after pressurization, and thickness retention after pressurization. The average particle size of the graphite oxide can be calculated using a laser diffraction / scattering particle size distribution analyzer (LA-920, manufactured by Horiba, Ltd.).

[0072] As the graphite oxide, a commercially available product may be used, or graphite oxide produced by appropriate synthesis may be used.

[0073] The method for synthesizing the graphite oxide is not particularly limited, but for example, a method in which graphite is oxidized with an oxidant and then subjected to interlayer exfoliation, or a method in which graphite is used as a working electrode for electrolysis and then subjected to interlayer exfoliation, etc. can be cited. As the method for oxidizing with an oxidant, the Brodie method (using nitric acid and potassium chlorate), the Staudenmaier method (using nitric acid, sulfuric acid and potassium chlorate), the Hummers-Offeman method (using sulfuric acid, sodium nitrate and potassium permanganate) and the like can be cited. As the method for performing electrolysis, a method in which an aqueous solution of an acidic substance such as sulfuric acid, nitric acid and perchloric acid is used as an electrolyte solution can be cited. In addition, as the method for interlayer exfoliation, a method in which a mechanical external force is applied, a method in which a heat treatment is performed, a method in which an ultrasonic irradiation is performed and the like can be cited.

[0074] <Complex>

[0075] A composite (hereinafter sometimes referred to as "the present composite" or "the composite") can be a raw material for a graphite sheet according to one embodiment of the present invention, and contains cellulose nanofibers and graphite oxide. Regarding the contents of cellulose nanofibers and graphite oxide in the composite, relative to 100% by weight of the composite, the content of cellulose nanofibers is preferably 5 to 95% by weight, and the content of graphite oxide is preferably 95 to 5% by weight. When the contents of cellulose nanofibers and graphite oxide in the composite are within the above ranges, it is easy to obtain a graphite sheet of good quality, especially one with excellent surface roughness and thickness retention after pressurization. More preferably, the content of cellulose nanofibers is 15 to 85% by weight, and the content of graphite oxide is 85 to 15% by weight; further preferably, the content of cellulose nanofibers is 25 to 75% by weight, and the content of graphite oxide is 75 to 25% by weight.

[0076] From the perspective of improving the thickness retention rate of the resulting graphite sheet after pressurization, the surface roughness of the present composite is preferably 0.3 μm or greater, more preferably 0.4 μm or greater, and even more preferably 0.5 μm or greater. The upper limit of the surface roughness of the present composite is not particularly limited, but can be, for example, 2.5 μm or less. It should be noted that the surface roughness of the composite (raw material film) after pressurization is a value measured by the method described in the Examples below.

[0077] The specific method for obtaining the composite is not particularly limited, but an example thereof includes mixing cellulose nanofibers, graphite oxide, and, if necessary, a dispersion medium to obtain a dispersion, applying or casting the dispersion onto a substrate in the form of a film, drying the film, and peeling the film from the substrate. However, if a commercially available graphite oxide dispersion is used as the graphite oxide, the addition of the dispersion medium is unnecessary.

[0078] The dispersion medium is not particularly limited, but examples thereof include water, DMF, DMAc, DMSO, NMP, dichlorobenzene, toluene, xylene, methoxybenzene, methanol, ethanol, propanol, pyridine, and γ-butyrolactone. Of these, water and methanol having a relative dielectric constant of 15 or greater are preferred from the perspective of preventing aggregation of graphite oxide particles. Water is particularly preferred, and ion-exchanged water is particularly preferred among water.

[0079] The substrate may be a base plate, a film, an endless belt, a stainless steel drum, etc. Examples of the coating method include spin coating and bar coating.

[0080] The shape of the present composite is not particularly limited, but is preferably a membrane (membrane-shaped). When the shape of the composite comprising cellulose nanofibers and graphite oxide is a membrane, the thickness of the membrane is not particularly limited, but is, for example, 1 μm to 50 mm, preferably 2 μm to 1 mm, and more preferably 3 μm to 300 μm. If the thickness of the composite is 1 μm or more, a graphite sheet having good surface roughness after pressurization can be obtained. In addition, if the thickness of the composite is 50 mm or less, a graphite sheet having good thickness retention after pressurization and excellent heat transfer in the thickness direction can be obtained.

[0081] The present composite can be heat-treated to provide the present graphite sheet. Therefore, the present composite can also be considered a composite for producing graphite sheets. Specifically, one embodiment of the present invention provides a composite for producing graphite sheets comprising cellulose nanofibers and graphite oxide. The present composite can also be considered a raw material film for graphite sheets.

[0082] A graphite sheet obtained from a composite material for producing a graphite sheet comprising cellulose nanofibers and graphite oxide as a raw material comprises fibrous carbon derived from the cellulose nanofibers and layered carbon derived from graphite oxide, etc. In other words, the graphite sheet according to one embodiment of the present invention comprises both fibrous carbon and layered carbon. This graphite sheet preferably contains fibrous carbon because it can achieve excellent thickness retention after pressurization.

[0083] <Method for Manufacturing Graphite Sheet>

[0084] In a method for producing a graphite sheet according to one embodiment of the present invention, a composite material comprising cellulose nanofibers and graphite oxide is heat-treated to produce the graphite sheet. This heat treatment releases oxygen atoms, hydrogen atoms, and the like from the cellulose nanofibers and graphite oxide, resulting in graphitization. This method enables the production of a graphite sheet exhibiting excellent thermal diffusivity, adhesive strength, adhesion, and dimensional stability.

[0085] The heat treatment process in the present manufacturing method is described in detail. First, a carbonization process is performed by preheating the composite body as a raw material in a non-oxidizing atmosphere such as nitrogen to carbonize it. In this way, a carbonized film can be obtained. The carbonization process can usually be carried out by heating the composite body to a temperature of 80°C or more and 1500°C or less (for example, 1000°C). In the carbonization process, the heating rate of the composite body is not particularly limited, but for example, it is preferably 0.1°C / min to 10°C / min. In the carbonization process, it is preferred to preheat the composite body for a certain time while maintaining it at a certain temperature or above (for example, above 1000°C). For example, when heating to 1000°C at a rate of 10°C / min, it is preferred to maintain it in a temperature range of 1000°C for about 30 minutes. The carbonization process can be carried out under reduced pressure or while flowing an inert gas. In addition, the carbonization process can also be carried out while applying a load to the composite body that does not cause damage to the composite body.

[0086] Next, the resulting carbon (carbonized film) is placed in an ultrahigh-temperature furnace and graphitized to produce a graphite sheet. During the graphitization process, the carbon (carbonized film) undergoes a rearrangement of its graphite layers, forming highly crystalline graphite. The carbonization and graphitization processes can be performed continuously in the same furnace, or the carbonization process can be followed by a carbon cooling process, followed by a separate graphitization process.

[0087] The heating temperature during the graphitization process is preferably 2400°C or higher, more preferably 2700°C or higher, and further preferably 2800°C or higher. The graphitization process is preferably carried out in an inert gas. The inert gas is not particularly limited, but argon is preferred, and argon to which a small amount of helium is added is more preferred. The carbonized film temperature rise rate during the graphitization process is not particularly limited, but is preferably, for example, 0.1°C / min to 10°C / min. The graphitization process can be carried out under reduced pressure or while an inert gas is flowing.

[0088] The carbonization step and / or graphitization step can also be carried out while applying a load to the composite (raw material film) and / or carbon (carbonized film) using a pressing device or the like. If the carbonization step and / or graphitization step is carried out while applying a load to the raw material film and / or carbonized film, a graphite sheet with a higher thermal diffusivity and a better appearance can be produced. The load is preferably 1 kg / cm 2 More than 10 kg / cm 2 More than 50 kg / cm 2In the method for producing a graphite sheet according to one embodiment of the present invention, the carbonization step may be performed while applying a load only during the carbonization step, the graphitization step may be performed while applying a load only during the graphitization step, or both the carbonization step and the graphitization step may be performed while applying a load. Preferably, both the carbonization step and the graphitization step are performed while applying a load, thereby producing a graphite sheet with a higher thermal diffusivity and a better appearance.

[0089] By carrying out the above steps, a composite composed of cellulose nanofibers and graphite oxide is used as a raw material and subjected to heat treatment at a high temperature, thereby producing a graphite sheet with high thermal diffusivity.

[0090] One embodiment of the present invention may also be the following.

[0091] (1) A method for producing a graphite sheet, comprising: heat-treating a composite comprising cellulose nanofibers and graphite oxide at a temperature of 2400° C. or higher.

[0092] (2) The method for producing a graphite sheet according to (1), wherein the thickness of the composite body is 1 μm to 50 mm.

[0093] (3) The method for producing a graphite sheet according to (1) or (2), wherein the surface roughness of a film sheet produced using the cellulose nanofibers alone is 0.1 μm to 3.0 μm.

[0094] (4) The method for producing a graphite sheet according to any one of (1) to (3), wherein the cellulose nanofibers include at least one selected from the group consisting of mechanically defibrillated cellulose nanofibers, TEMPO-oxidized cellulose nanofibers, phosphated cellulose nanofibers, phosphite-esterified cellulose nanofibers, crystalline cellulose, and sodium carboxymethylated cellulose.

[0095] (5) The method for producing a graphite sheet according to any one of (1) to (4), wherein the cellulose nanofibers have a degree of crystallinity of 50% or more.

[0096] (6) The method for producing a graphite sheet according to any one of (1) to (5), wherein the cellulose nanofibers have a structure represented by the following structural formula.

[0097]

[0098] In the formula, R1, R2, and R3 are each independently any one of -OH, -COOH, -HPO3, -H2PO4, -Na2PO4, -CH2OCH2COOH, and -CH2OCH2COONa, and n is an integer greater than 1. Furthermore, R1, R2, and R3 can be used to bond the repeating units expressed in the structural formula to each other and may have a branched structure.

[0099] (7) The method for producing a graphite sheet according to any one of (1) to (6), wherein the graphite oxide has a C / O ratio of 0.75 to 5.0.

[0100] (8) The method for producing a graphite sheet according to any one of (1) to (7), wherein the average particle size of the graphite oxide is 30 nm to 3 mm.

[0101] (9) The method for producing a graphite sheet according to any one of (1) to (8), wherein the proportion of the cellulose nanofibers in the composite is 5 to 95% by weight.

[0102] (10) A composite material for producing a graphite sheet, comprising cellulose nanofibers and graphite oxide.

[0103] (11) The composite body for producing a graphite sheet according to (10), wherein the surface roughness is 0.3 μm or more.

[0104] (12) A graphite sheet having a thermal diffusivity of 3.5 cm 2 / s or more, its surface roughness after pressing is 1.0 μm or more, and its thickness maintenance rate after pressing is 70% or more.

[0105] (13) The graphite sheet according to (12), comprising fibrous carbon and layered carbon.

[0106] (Example)

[0107] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0108] (Measurement of Thickness of Composite and Graphite Sheet)

[0109] The thickness of the composite and graphite sheet was measured at the four corners and one location in the center using a micrometer OMV-25MX (406-250-30) manufactured by Mitsutoyo Co., Ltd. in a thermostatic chamber at 25°C. The average of the five thickness measurements was used as the thickness of the composite and graphite sheet.

[0110] (Method for measuring thermal diffusivity of graphite sheet)

[0111] The thermal diffusivity of the graphite sheet was measured using a thermal diffusivity measuring apparatus (thermal wave analyzer TA-3 manufactured by Bethe Corporation) in an atmosphere at 20° C. on a graphite sheet sample cut into a shape of 40×40 mm.

[0112] (Surface roughness of graphite sheet after pressurization)

[0113] The surface roughness of a graphite sheet after pressurization was measured in accordance with JIS B 0601. Specifically, the surface roughness of both surfaces of a graphite sheet (cut into 25 mm length x 5 mm width) after pressurization was measured using a surface roughness tester SJ-210 (Code No. 178-2560-11; manufactured by Mitsutoyo Co., Ltd.). The reference range (L) was set at 4 mm, and three measurements were performed on each side of the graphite sheet. The average of these measurements was used as the surface roughness of the graphite sheet after pressurization. The surface roughness of the graphite sheet after pressurization, whichever had the greater surface roughness, was selected as the surface roughness of the graphite sheet after pressurization.

[0114] Separately, the pre-pressurized graphite sheet was pressurized by the following method to obtain a pressurized graphite sheet. First, a graphite sheet (2 cm long x 2 cm wide) was sandwiched between two polyimide films (75 μm thick x 10 cm long x 10 cm wide), and then further sandwiched between two SUS plates (5 mm thick x 10 cm long x 10 cm wide) to obtain a five-layer laminate. Next, a compression molding machine was used to pressurize the graphite sheet in the laminate to a pressure of 300 kg / cm 2 The five-layer stack was pressurized for 1 minute to obtain a pressurized graphite sheet.

[0115] Surface roughness of cellulose nanofibers (films) and raw film

[0116] The surface roughness of cellulose nanofibers (films) and raw film was measured in accordance with JIS B 0601. Specifically, the surface roughness of the cellulose nanofibers (films) and raw film was measured using the same method as described in the "(Surface roughness of graphite sheet after pressurization)" section above, except that the measurement object was changed from a pressurized graphite sheet to a cellulose nanofiber (film) or raw film. Here, the term "cellulose nanofiber (film)" refers to a film-like cellulose nanofiber obtained by drying an aqueous solution containing cellulose nanofibers.

[0117] (Thickness retention rate of graphite sheet after pressurization)

[0118] The thickness retention rate of the graphite sheet after pressurization is calculated by the following formula:

[0119] Thickness retention rate of graphite sheet after pressurization = thickness of graphite sheet after pressurization / thickness of graphite sheet before pressurization.

[0120] It should be noted that the method for measuring the thickness of the graphite sheet before and after pressurization is as described in the above-mentioned "(Measurement of the thickness of the composite and the graphite sheet)" item. Regarding the graphite sheet after pressurization, a graphite sheet obtained by pressurizing the graphite sheet before pressurization according to the following method was used. First, a graphite sheet (length 2 cm × width 2 cm) was sandwiched between two polyimide films (thickness 75 μm × length 10 cm × width 10 cm), and further clamped with two SUS plates (thickness 5 mm × length 10 cm × width 10 cm) to obtain a total of 5-layer stacked body. Then, a compression molding machine was used to pressurize the graphite sheet in the stacked body to a pressure of 300 kg / cm 2 The five-layer stack was pressurized for 1 minute to obtain a pressurized graphite sheet.

[0121] (SEM observation of the composite (raw material film) and graphite sheet)

[0122] The composite (raw material film) and graphite sheet were observed using an ultra-high-resolution scanning electron microscope (FE-SEM). Using an ULTRAplus (manufactured by Carl Zeiss) microscope, the surfaces of the composite (raw material film) and graphite sheet were observed at an accelerating voltage of 5.0 kV using a secondary electron detector (SE2).

[0123] IR of cellulose nanofibers (films)

[0124] The IR spectrum of the cellulose nanofibers was obtained using an infrared spectrometer (Spectrum ONE, manufactured by Perkin Elmer) at 4000 to 650 cm -1 The range of was measured for film-like cellulose nanofibers (cellulose nanofiber membrane) obtained by drying an aqueous solution containing cellulose nanofibers.

[0125] (XRD of cellulose nanofibers (film))

[0126] Regarding the XRD spectrum of cellulose nanofibers, an X-ray diffraction device (X'Pert Pro, manufactured by Malvern Panalytical) was used to irradiate CuKα (λ=1.541 Å), and the film-like cellulose nanofibers (cellulose nanofiber membrane) obtained by drying the aqueous solution containing cellulose nanofibers were measured in the range of 2θ from 5° to 90°.

[0127] (Example 1)

[0128] A dispersion of cellulose nanofibers A (TEMPO oxidized cellulose nanofibers; crystallinity 71%, diameter 1nm~5nm, length 100nm~1μm, surface roughness 0.15μm after drying) and graphite oxide (average particle size 15μm, C / O ratio 1.2) (the proportion of cellulose nanofibers is 75% by weight) is applied in such a way that the thickness after drying reaches 35μm, and dried at room temperature to obtain a composite (raw material film) containing cellulose nanofibers and graphite oxide with a thickness of 35μm.

[0129] The obtained composite body was heated to room temperature to 1000°C in a nitrogen atmosphere, and then maintained at 1000°C for 10 minutes for carbonization to obtain a carbonized film. Then, the obtained carbonized film was sandwiched between graphite plates to obtain a laminate consisting of a carbonized film and a graphite plate. The laminate was first heated in a temperature range of room temperature to 2000°C under vacuum, and then heated to above 2900°C under argon in a temperature range exceeding 2000°C, and then maintained at a temperature above 2900°C for 10 minutes to graphitize and obtain a graphite sheet. The thermal diffusivity of the obtained graphite sheet was 3.5 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0130] [Table 1]

[0131]

[0132] (Example 2)

[0133] A graphite sheet was prepared in the same manner as in Example 1 except that the ratio of cellulose nanofibers in the dispersion of graphite oxide and cellulose nanofibers was changed to 50 wt%. The thermal diffusivity of the obtained graphite sheet was 9.1 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0134] (Example 3)

[0135] A graphite sheet was prepared in the same manner as in Example 1 except that the ratio of cellulose nanofibers in the dispersion of graphite oxide and cellulose nanofibers was changed to 45 wt%. The thermal diffusivity of the obtained graphite sheet was 9.2 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0136] (Example 4)

[0137] A graphite sheet was prepared in the same manner as in Example 1 except that the ratio of cellulose nanofibers in the dispersion of graphite oxide and cellulose nanofibers was changed to 40 wt%. The thermal diffusivity of the obtained graphite sheet was 8.5 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0138] (Example 5)

[0139] A graphite sheet was prepared in the same manner as in Example 1 except that the ratio of cellulose nanofibers in the dispersion of graphite oxide and cellulose nanofibers was changed to 5 wt%. The thermal diffusivity of the obtained graphite sheet was 7.8 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0140] (Example 6)

[0141] A graphite sheet was prepared in the same manner as in Example 3 except that the thickness of the composite after drying was changed to 5 μm. The thermal diffusivity of the obtained graphite sheet was 9.3 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0142] (Example 7)

[0143] A graphite sheet was prepared in the same manner as in Example 3 except that the thickness of the composite after drying was changed to 200 μm. The thermal diffusivity of the obtained graphite sheet was 6.2 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0144] (Example 8)

[0145] A graphite sheet was prepared in the same manner as in Example 3 except that graphite oxide having a C / O ratio of 2.5 was used. The thermal diffusivity of the obtained graphite sheet was 8.9 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0146] (Example 9)

[0147] A graphite sheet was prepared in the same manner as in Example 3 except that graphite oxide having an average particle size of 5 μm was used. The thermal diffusivity of the obtained graphite sheet was 8.5 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0148] (Example 10)

[0149] A graphite sheet was prepared in the same manner as in Example 3 except that graphite oxide having an average particle size of 80 μm was used. The thermal diffusivity of the obtained graphite sheet was 9.2 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0150] (Comparative Example 1)

[0151] A graphite sheet was prepared in the same manner as in Example 1 except that a dispersion of polyvinyl alcohol and graphite oxide (average particle size of about 20 μm, C / O ratio of 35) (polyvinyl alcohol ratio of 90 wt%) was applied to a thickness of 35 μm after drying and dried at room temperature. The resulting 35 μm thick film was used in place of the composite. The thermal diffusivity of the obtained graphite sheet was 0.1 cm 2 Table 1 shows the composition and thickness of the composite used, and the thermal diffusivity of the obtained graphite sheet.

[0152] (Example 11)

[0153] A dispersion of cellulose nanofibers A (TEMPO-oxidized cellulose nanofibers; crystallinity 71%, diameter 1 nm to 5 nm, length 100 nm to 1 μm, surface roughness 0.15 μm after drying) and graphite oxide (average particle size 15 μm, C / O ratio 1.2) (cellulose nanofiber ratio 50 wt%) was applied to a 35 μm thick composite (raw film) containing cellulose nanofibers and graphite oxide and dried at room temperature. The resulting composite had a surface roughness of 0.27 μm.

[0154] The obtained composite was heated at 50 kg / cm 2 The carbonized film was then carbonized by heating the sample to room temperature to 1000°C in a nitrogen atmosphere under a load of 50 kg / cm2 and then maintaining the temperature at 1000°C for 10 minutes to obtain a carbonized film. The obtained carbonized film was then sandwiched between graphite plates to obtain a laminate consisting of the carbonized film and the graphite plates. The laminate was then heated to 1000°C in a nitrogen atmosphere under a load of 50 kg / cm2. 2 Under a load-pressurized state, the graphite sheet was first heated in a temperature range of room temperature to 2000°C under vacuum, then heated to 2900°C or higher under argon in a temperature range exceeding 2000°C, and then maintained at a temperature of 2900°C or higher for 10 minutes to graphitize the sheet. The thermal diffusivity of the obtained graphite sheet was 9.2 cm 2 Table 2 shows the composition and thickness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0155] [Table 2]

[0156]

[0157] (Example 12)

[0158] A graphite sheet was prepared in the same manner as in Example 11 except that a composite (raw film) containing cellulose nanofibers and graphite oxide having a thickness of 25 μm was obtained. The thermal diffusivity of the obtained graphite sheet was 9.3 cm 2 Table 2 shows the composition, thickness, and surface roughness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0159] (Example 13)

[0160] A graphite sheet was prepared in the same manner as in Example 11 except that a composite (raw material film) containing cellulose nanofibers and graphite oxide having a thickness of 10 μm was obtained. The thermal diffusivity of the obtained graphite sheet was 9.4 cm 2 Table 2 shows the composition and thickness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0161] (Example 14)

[0162] A graphite sheet was prepared in the same manner as in Example 12 except that graphite oxide with an average particle size of 30 μm was used. The surface roughness of the raw film was 0.30 μm. The thermal diffusivity of the obtained graphite sheet was 9.4 cm 2 Table 2 shows the composition and thickness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0163] The surfaces of the composite (raw material film) and the obtained graphite sheet used in Example 14 were observed by SEM. Figure 1 and Figure 2 shown. Figure 1 : is a SEM observation result of the surface of the composite (raw material film) used in Example 14, Figure 1 The upper figure shows the observation results at a magnification of 1000 times, the middle figure shows the observation results at a magnification of 5000 times, and the lower figure shows the observation results at a magnification of 10,000 times. Figure 2 This is a SEM observation result of the surface of the graphite sheet obtained in Example 14. Figure 2 The upper graph shows the observation results at a magnification of 1000 times, the middle graph shows the observation results at a magnification of 5000 times, and the lower graph shows the observation results at a magnification of 10000 times. Figure 2 As is apparent from the upper figure (the portion surrounded by a circle), the graphite sheet obtained in Example 14 contains fibrous carbon.

[0164] (Example 15)

[0165] A graphite sheet was prepared in the same manner as in Example 14 except that a composite (raw material film) containing cellulose nanofibers and graphite oxide having a thickness of 10 μm was obtained. The thermal diffusivity of the obtained graphite sheet was 9.5 cm 2 Table 2 shows the composition, thickness, and surface roughness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0166] (Example 16)

[0167] A graphite sheet was prepared in the same manner as in Example 14 except that the ratio of cellulose nanofibers in the dispersion of graphite oxide and cellulose nanofibers was changed to 25 wt%. The thermal diffusivity of the obtained graphite sheet was 9.5 cm 2 Table 2 shows the composition, thickness, and surface roughness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0168] (Example 17)

[0169] A graphite sheet was prepared in the same manner as in Example 14, except that cellulose nanofibers B (mechanically defibrated (high-pressure homogenization) type cellulose nanofibers; crystallinity 50% or higher, diameter 10 nm to 50 nm, length 100 nm to 5 μm, surface roughness 1.0 μm after drying) were used instead of cellulose nanofibers A. The surface roughness of the raw film was 0.51 μm. The thermal diffusivity of the graphite sheet obtained from the raw film was 8.8 cm 2 Table 2 shows the composition and thickness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0170] The surfaces of the composite (raw material film) and the obtained graphite sheet used in Example 17 were observed by SEM. Figure 3 and Figure 4 shown. Figure 3 : is a SEM observation result of the surface of the composite (raw material film) used in Example 17, Figure 3 The upper figure shows the observation results at a magnification of 1000 times, the middle figure shows the observation results at a magnification of 5000 times, and the lower figure shows the observation results at a magnification of 10,000 times. Figure 4 This is a SEM observation result of the surface of the graphite sheet obtained in Example 17. Figure 4The upper graph shows the observation results at a magnification of 1000 times, the middle graph shows the observation results at a magnification of 5000 times, and the lower graph shows the observation results at a magnification of 10000 times. Figure 4 As is apparent from the upper figure (the portion surrounded by a circle), the graphite sheet obtained in Example 17 contains fibrous carbon.

[0171] (Example 18)

[0172] A graphite sheet was prepared in the same manner as in Example 17 except that the ratio of cellulose nanofibers in the dispersion of graphite oxide and cellulose nanofibers was changed to 25 wt%. The thermal diffusivity of the obtained graphite sheet was 9.1 cm 2 Table 2 shows the composition, thickness, and surface roughness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0173] (Example 19)

[0174] A graphite sheet was prepared in the same manner as in Example 14, except that cellulose nanofibers C (mechanically defibrated (grinding) cellulose nanofibers made from crystalline cellulose; crystallinity 50% or higher, diameter 10 nm to 50 nm, length 100 nm to 25 μm, and surface roughness 1.3 μm after drying) were used instead of cellulose nanofibers A. The surface roughness of the raw material film was 0.98 μm. The thermal diffusivity of the resulting graphite sheet was 8.5 cm 2 Table 2 shows the composition and thickness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0175] The surfaces of the composite (raw material film) and the obtained graphite sheet used in Example 19 were observed by SEM. Figure 5 and Figure 6 shown. Figure 5 : is a SEM observation result of the surface of the composite (raw material film) used in Example 19, Figure 5 The upper figure shows the observation results at a magnification of 1000 times, the middle figure shows the observation results at a magnification of 5000 times, and the lower figure shows the observation results at a magnification of 10,000 times. Figure 6 This is a SEM observation result of the surface of the graphite sheet obtained in Example 19. Figure 6 The upper graph shows the observation results at a magnification of 1000 times, the middle graph shows the observation results at a magnification of 5000 times, and the lower graph shows the observation results at a magnification of 10000 times. Figure 6As is apparent from the upper figure (the portion surrounded by a circle) in FIG, the graphite sheet obtained in Example 19 contains fibrous carbon. Furthermore, a cross-sectional photograph of the graphite sheet obtained in Example 19 is shown in FIG. Figure 7 .Depend on Figure 7 It is clear (particularly in the portion surrounded by a circle) that a fibrous pattern (fibrous carbon) can be confirmed in the graphite sheet obtained in Example 19.

[0176] (Example 20)

[0177] A graphite sheet was prepared in the same manner as in Example 19 except that the ratio of cellulose nanofibers in the dispersion of graphite oxide and cellulose nanofibers was changed to 25 wt%. The thermal diffusivity of the obtained graphite sheet was 8.8 cm 2 Table 2 shows the composition, thickness, and surface roughness of the composite used, as well as the thermal diffusivity, surface roughness after pressing, and thickness retention rate after pressing of the obtained graphite sheet.

[0178] (Reference Example 1)

[0179] A graphite sheet was prepared in the same manner as in Example 11 except that a polyimide film (Apical 200AV (manufactured by Kaneka Corporation)) was used instead of the composite material containing cellulose nanofibers and graphite oxide. The thermal diffusivity of the obtained graphite sheet was 9.5 cm 2 Table 2 shows the thickness of the polyimide film used, the thermal diffusivity, the surface roughness after pressing, and the thickness retention rate after pressing of the obtained graphite sheet.

[0180] The surface of the graphite sheet obtained in Reference Example 1 was observed by SEM. Figure 8 shown. Figure 8 This is a SEM observation result of the surface of the graphite sheet obtained in Reference Example 1, showing the observation result at a magnification of 1000 times. Figure 8 It is clear that in the graphite sheet obtained in Reference Example 1, distortion of graphite was confirmed, but fibrous carbon was not confirmed.

[0181] (IR and XRD spectra of cellulose nanofibers)

[0182] The IR spectra and XRD spectra of the cellulose nanofibers (cellulose nanofiber A, cellulose nanofiber B, and cellulose nanofiber C) and the high-quality paper (TANOSEE: α environmentally friendly paper NH type, hereinafter referred to as paper) used in the examples were measured. Figure 9 and Figure 10 shown. Figure 9 10 is the IR spectrum of cellulose nanofibers A, B, C and paper used in the examples. In addition, 11 is the XRD spectrum of cellulose nanofibers A, B, C and paper used in the examples. Figure 9 It can be seen that cellulose nanofiber A has a wavelength of 3000-3500 cm -1 In addition, it can be seen that cellulose nanofibers A, B, C and paper have peaks similar to those of paper at wave numbers of 1030-1070 cm -1 The maximum peak is in the range of 1070-1140 cm-1, which is smaller than the wave number range with the maximum peak. -1 and 1140~1200cm -1 There are two peaks in the range of Figure 10 It was found that the cellulose nanofibers A, B, and C have a peak in the range of 2θ from 20 to 25°, and a peak in the range of 2θ from 10 to 20°.

[0183] (Summarize)

[0184] A comparison between Examples 1-20 and Comparative Example 1 shows that the thermal diffusivity of the graphite sheets obtained in Comparative Example 1, in which a composite of graphite oxide and cellulose nanofibers was heat-treated alone, was significantly poor, making it virtually incapable of functioning as a heat dissipation component. On the other hand, the graphite sheets of Examples 1-19, in which a composite containing cellulose nanofibers and graphite oxide was heat-treated, all exhibited excellent thermal diffusivity. This demonstrates that the present production method can produce graphite sheets with high thermal diffusivity. Furthermore, the results of Reference Example 1 show that, compared to Reference Example 1, in which conventional graphite was produced using a special resin film, namely a polyimide film, as a raw material, the graphite sheets of Examples 1-20 produced using the present production method not only exhibited comparable thermal diffusivity to conventional graphite, but also exhibited significantly superior post-pressurization surface roughness and post-pressurization dimensional stability. This indicates that the graphite sheets obtained using the present production method (the present graphite sheets) also exhibited superior adhesive strength, adhesion rate, and dimensional stability compared to graphite sheets obtained using conventional methods.

[0185] (Industrial Applicability)

[0186] The graphite sheet according to one embodiment of the present invention can be preferably used as a heat dissipation member for semiconductor elements or other heat-generating components mounted in various electronic devices or electrical equipment such as computers.

Claims

1. A method for manufacturing a graphite sheet, comprising: The composite material containing cellulose nanofibers and graphite oxide is heat-treated at a temperature above 2400°C. The thermal diffusivity of the graphite sheet is 3.5 cm 2 / s or more, The graphite sheet has a thickness maintenance rate of 70% or more after being pressed.

2. A method for manufacturing a graphite sheet, comprising: The composite material containing cellulose nanofibers and graphite oxide is heat-treated at a temperature above 2400°C. The thermal diffusivity of the graphite sheet is 3.5 cm 2 / s or more, The content of the cellulose nanofibers is 25 to 75 wt % relative to 100 wt % of the composite.

3. A method for manufacturing a graphite sheet, comprising: The composite material containing cellulose nanofibers and graphite oxide is heat-treated at a temperature above 2400°C. The thermal diffusivity of the graphite sheet is 3.5 cm 2 / s or more, The cellulose nanofibers include at least one selected from the group consisting of mechanically defibrated cellulose nanofibers, TEMPO-oxidized cellulose nanofibers, and crystalline cellulose. The C / O ratio of the graphite oxide is 0.75 to 5.

0.

4. The method for producing a graphite sheet according to any one of claims 1 to 3, wherein The thickness of the composite body is 1 μm to 50 mm.

5. The method for producing a graphite sheet according to any one of 1 to 3, wherein When a membrane is made of the cellulose nanofibers alone, the surface roughness of the membrane is 0.1 μm to 3.0 μm.

6. The method for producing a graphite sheet according to any one of claims 1 to 3, wherein The cellulose nanofibers have a crystallinity of more than 50%.

7. The method for producing a graphite sheet according to any one of claims 1 to 3, wherein The cellulose nanofiber has a structure expressed by the following structural formula: In the formula, R1, R2, and R3 are each independently any one of -OH, -COOH, -CH2OCH2COOH, and -CH2OCH2COONa, n is an integer greater than 1, and R1, R2, and R3 are allowed to bond the repeating units expressed by the structural formula to each other and to have a branched structure.

8. The method for producing a graphite sheet according to any one of claims 1 to 3, wherein The average particle size of the graphite oxide is 30 nm to 3 mm.

9. A composite body for producing graphite sheets, having a thermal diffusivity of 3.5 cm 2 The raw materials of graphite sheets with a diameter of more than 1 / s include cellulose nanofibers and graphite oxide. The cellulose nanofibers include at least one selected from the group consisting of mechanically defibrated cellulose nanofibers, TEMPO-oxidized cellulose nanofibers, and crystalline cellulose. The C / O ratio of the graphite oxide is 0.75 to 5.

0. 10 . The composite for producing a graphite sheet according to claim 9 , wherein the surface roughness is 0.3 μm or more.

11. A graphite sheet with a thermal diffusivity of 3.5 cm 2 / s or more, its surface roughness after pressing is 1.0 μm or more, and its thickness maintenance rate after pressing is 70% or more. 12 . The graphite sheet according to claim 11 , comprising fibrous carbon and layered carbon.

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