Method for producing graphite sheet

Abstract

To provide a method for producing a graphite sheet capable of producing a graphite sheet having high thermal conductivity.SOLUTION: A method for producing a graphite sheet according to the present invention includes the steps of: preparing a slurry containing a graphite powder; forming the slurry into a sheet by orienting graphite particles in the slurry; and firing the graphite particles oriented in the sheet. The slurry contains the graphite powder A having a mean particle size of 40 to 50 μ m, and further contains the graphite powder B having a mean particle size of 1.6 to 2.5 μm and / or the graphite powder C having a mean particle size of 160 to 250 μ m, and the content ratio of the graphite powder having the largest mean particle size is 20 to 85wt%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a graphite sheet. [Background technology]

[0002] Graphite sheets formed by stacking multiple layers of graphene have high in-plane thermal conductivity and are used as components for dissipating heat from electronic control components in automobiles and heat-generating components in general-purpose electronic devices.

[0003] Patent Document 1 discloses that a graphite sheet, which is obtained by uniformly adsorbing an ionic surfactant onto the surface of graphite powder and pressing the resulting graphite deposit with controlled charging properties, has thermoelectric conversion function and high thermal conductivity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-48280 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technology described in Patent Document 1 uses one type of graphite powder, and the difference in particle size of the graphite particles is due only to the particle size distribution. Since the difference in particle size is small, voids tend to form between the graphite particles, making it difficult to increase the packing rate of the graphite particles, and the effect of improving thermal conductivity is not sufficiently achieved.

[0006] The present invention has been made in view of the problems associated with the prior art, and an object of the present invention is to provide a method for producing a graphite sheet that can produce a graphite sheet having high thermal conductivity. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above object, the present inventors have discovered that the above object can be achieved by mixing multiple types of graphite powders with different average particle sizes, and have thus completed the present invention.

[0008] That is, the method for producing a graphite sheet of the present invention includes the steps of preparing a slurry containing graphite powder, orienting the graphite particles in the slurry to form a sheet, and firing the graphite particles oriented in the sheet form. The slurry contains graphite powder A having an average particle size of 40 to 50 μm, and further contains graphite powder B having an average particle size of 1.6 to 2.5 μm and / or graphite powder C having an average particle size of 160 to 250 μm, The graphite powder with the largest average particle size is contained in an amount of 20 to 85 wt %. [Effects of the Invention]

[0009] According to the present invention, by mixing multiple types of graphite powders having different average particle sizes, it is possible to provide a method for manufacturing a graphite sheet that has a high graphite particle filling rate and high thermal conductivity. DETAILED DESCRIPTION OF THE INVENTION

[0010] The method for producing a graphite sheet of the present invention will now be described in detail. The graphite sheet manufacturing method of the present invention is a method for producing a graphite sheet by aggregating graphite particles, and includes the steps of preparing a slurry containing graphite powder, orienting the graphite particles in the slurry to form the slurry into a sheet, and firing the graphite particles formed into the sheet.

[0011] The graphite particles are scale-like particles formed by stacking multiple layers of graphene, which is made up of carbon atoms and forms a honeycomb lattice on the same plane, and have high thermal conductivity in the in-plane direction.

[0012] (Slurry preparation process) The slurry preparation step is a step of mixing a plurality of graphite powders having different average particle sizes in an aqueous medium.

[0013] In the present invention, graphite powder A having an average particle size of 40 to 50 μm is included, and a process is carried out in which this powder A is mixed with graphite powder B having an average particle size of 1.6 to 2.5 μm and / or graphite powder C having an average particle size of 160 to 250 μm.

[0014] At this time, the content of the graphite powder with the largest number average particle size, that is, graphite powder A or graphite powder C, is 20 to 85 wt %.

[0015] If graphite powder with an average particle size of less than 1.6 μm is used, the gaps between the graphite particles will be small, but when made into a graphite sheet, the number of interfaces between the graphite particles will be large, resulting in a decrease in thermal conductivity.

[0016] Furthermore, when graphite powder having an average particle size exceeding 250 μm is used, the number of interfaces between the graphite particles decreases, while the voids between the graphite particles become larger, making it difficult to fill the voids and resulting in a decrease in thermal conductivity.

[0017] The slurry of the present invention essentially contains graphite powder A having an average particle size of 40 to 50 μm. In other words, the voids between particles of graphite powder A are filled with graphite powder B, and the voids between particles of graphite powder C are filled with graphite powder A.

[0018] This makes it possible to suppress an increase in the number of interfaces between graphite particles while filling the gaps that occur between the graphite particles, thereby increasing the packing rate of the graphite particles, and thus producing a graphite sheet with high thermal conductivity.

[0019] In the present invention, the average particle size of graphite particles refers to the average of the major axes of the basal planes of primary particles, and the major axis of the basal planes can be measured as the primary particle size obtained by particle size distribution measurement using light.

[0020] The slurry is preferably prepared by adding graphite powder to an aqueous solution containing an anionic surfactant and stirring the mixture to mix the plurality of graphite powders in the aqueous solution.

[0021] In the present invention, a plurality of graphite powders having different average particle sizes are not simultaneously added to an aqueous solution containing a surfactant, but rather the graphite powders are sequentially added in descending order of average particle size to the aqueous solution, stirred and dispersed, and then the graphite powder with the largest average particle size is added and dispersed.

[0022] Anionic surfactants are adsorbed to defects in the graphite lattice, i.e., the in-plane ends (hereinafter, sometimes simply referred to as "ends") of planar, scaly graphite particles. Therefore, the surfactant connects the ends of the graphite particles, and in areas where voids are generated, some of the adsorption sites of the surfactant are adsorbed to the graphite particles, while other adsorption sites of the surfactant remain untouched and do not reach other graphite particles.

[0023] By adding graphite powder having a particle size smaller than the graphite particles, other adsorption sites of the surfactant are adsorbed to the ends of the graphite particles having a smaller particle size.

[0024] Therefore, small graphite particles enter and fill the gaps created between large graphite particles, and the ends of the scale-like graphite particles are connected by the surfactant to form large, planar graphite particle clusters, improving the thermal conductivity of the graphite sheet.

[0025] The anionic surfactant is preferably water-soluble carboxymethyl cellulose (hereinafter sometimes simply referred to as "CMC"). The above-mentioned CMC has a chain-like structure in which glucose is polymerized in a linear chain, and has many adsorption sites within the molecular chain. As a result, it adsorbs to the edges of flake-shaped graphite particles, surrounding them, attracting smaller graphite particles and narrowing the gaps that form between the graphite particles, thereby improving the thermal conductivity of the graphite sheet.

[0026] Furthermore, in the present invention, as described above, small graphite particles are allowed to fill the gaps generated between large graphite particles, thereby increasing the packing rate of the graphite particles.

[0027] Therefore, it is important that the content of CMC in the aqueous solution is such that the CMC is adsorbed to the ends of the large graphite particles in just the right amount to connect the large graphite particles together, and that in the voids, some of the adsorption sites of the CMC are adsorbed to the ends of the large graphite particles, leaving other adsorption sites free.

[0028] In other words, if the amount of CMC is excessive and there is a large amount of completely free CMC that is not adsorbed to large graphite particles when small graphite powder is added, the small graphite particles will be connected to each other, and the small graphite particles will not be attracted to the gaps between the large graphite particles, making it difficult for the small graphite particles to enter the gaps.

[0029] The content of CMC in the aqueous solution is preferably 0.08 to 0.12 wt %, although it depends on the particle size of the graphite particles, and the total amount of graphite particles added to the aqueous solution is preferably 0.5 to 1.0 wt %.

[0030] Furthermore, the CMC preferably has a viscosity of 1000 to 2000 (mPa·s) in a 1 wt % aqueous solution at 25°C. The viscosity of the above aqueous solution corresponds to the molecular weight of CMC, and CMC with low viscosity has short molecular chains, while CMC with high viscosity has long molecular chains.

[0031] If the viscosity of a 1 wt % aqueous solution at 25°C is less than 1000 mPa·s, the viscosity of the solution is too low, reducing the dispersion stability of the slurry and making it more likely that the orientation of the graphite particle agglomerates will be reduced during the molding process described below. Also, if the viscosity is low (the molecular weight is low), the bonds between the graphite particles due to the CMC will be weaker, making it more likely that gaps will form, leading to a decrease in thermal conductivity.

[0032] Furthermore, if the viscosity exceeds 2000 (mPa·s), the CMC molecular chains become long and the gaps between the graphite particles connected by the CMC become wider, making it difficult to remove the CMC that has entered these gaps during desolvation, which may reduce the graphite sheet's effect in improving thermal conductivity.

[0033] The graphite particles preferably have an ID / IG ratio obtained from a Raman spectrum of 0.15 or less, more preferably 0.1 or less.

[0034] The ID / IG ratio obtained from Raman spectroscopy is an index for evaluating the crystallinity of graphite, where ID is the intensity of the D band derived from defects, and IG is the intensity of the G band specific to graphite. The smaller the ID / IG ratio, the higher the crystallinity and the lower the defect density in the crystal.

[0035] Graphite particles with few defects in the crystals naturally have excellent thermal conductivity. In addition, graphite particles with an ID / IG ratio of 0.1 or less have few defects on their planar crystal surfaces, which prevents CMC from adsorbing to the crystal surfaces of the graphite particles, i.e., in the out-of-plane direction of the planar, scaly graphite particles. Instead, CMC is adsorbed to the edges of the graphite particles, forming clumps of planar graphite particles.

[0036] Therefore, in the production method of the present invention, by setting the ID / IG ratio of the graphite particles to 0.1 or less, the orientation of the graphite particles is improved and the graphite particles are arranged on the same plane without any gaps, and therefore the thermal conductivity of the graphite sheet can be improved beyond that expected from the thermal conductivity of the graphite particles themselves.

[0037] Examples of graphite powder having an ID / IG ratio of 0.1 or less include expanded graphite and graphene nanoplates that have not undergone an oxidation treatment process.

[0038] (molding process) The forming step is a step in which a slurry containing graphite particles is dehydrated while being oriented by applying a force in one direction to form the particles into a sheet.

[0039] The graphite particles in the slurry are not dispersed individually, but are connected by CMC to form graphite particle clusters arranged on the same plane. Therefore, when a force is applied in one direction, the graphite particle clusters interfere with each other and are deposited in one direction with a uniform orientation.

[0040] An example of a method for applying a unidirectional force is a vacuum filtration membrane formation method. Specifically, the entire amount of the slurry is placed on a filtration membrane, and after the liquid level of the slurry has settled, the pressure is reduced while the graphite particles are suspended, before they are deposited, to dehydrate the slurry, and the graphite particle agglomerates are oriented and deposited to form a sheet.

[0041] When the pressure is reduced while the slurry is flowing on the filtration membrane, the force of the slurry flow and the force of the reduced pressure combine on the filtration membrane, changing the direction of the force acting on the graphite particle agglomerates, resulting in a decrease in orientation.

[0042] Furthermore, after the graphite particle agglomerates are deposited, friction makes it difficult for the graphite particle agglomerates to change direction, resulting in a decrease in orientation.

[0043] As described above, by applying a unidirectional force to the graphite particle agglomerates in a floating state before they are deposited, it is possible to produce a graphite sheet in which the graphite particles are highly oriented.

[0044] (Firing process) The firing step is a step of baking the graphite sheet and burning off impurities remaining between the graphite particles. In the present invention, the graphite particles are highly oriented and stacked in the stacking direction with no gaps between them, so that firing at a low temperature is possible. The firing temperature is preferably 100 to 200°C, depending on the thickness of the graphite sheet. The reason for this temperature range is that the impurities remaining between the graphite particles are mainly water, and the temperature is preferably high enough to sufficiently burn off the water. Furthermore, if heat of 350°C or higher is applied in an oxygen atmosphere, the edges and defects of the graphite will decompose into CO2, so firing in an oxygen-free atmosphere is preferred. [Example]

[0045] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0046] [Example] (Preparation of slurry) Carboxymethyl cellulose (1190: Daicel Corporation) was added to pure water to prepare a 1 wt% CMC aqueous solution. A predetermined amount of this CMC solution was weighed and added to pure water. The solution was stirred (2000 rpm) with a Rentaro mixer (rotating / revolving propellerless mixer) for 1 minute and then ultrasonically stirred (28 kHz) for 10 minutes to prepare a 0.1 wt% CMC aqueous solution.

[0047] The graphite powder with the largest particle size was added to the CMC aqueous solution in a predetermined amount shown in Table 1 below, and the mixture was stirred with a Rentaro (2000 rpm) for 1 minute and then ultrasonically stirred (28 kHz) for 10 minutes to disperse the graphite powder with the largest particle size.

[0048] A predetermined amount of graphite particles having small particle diameters shown in Table 1 below was added to the dispersion liquid containing the graphite powder having the largest particle diameter, and the mixture was stirred by a Rentaro (2000 rpm) for 1 minute and then ultrasonically stirred (28 kHz) for 10 minutes to obtain a slurry containing graphite powders having large particle diameters and graphite powders having small particle diameters. The composition ratio (wt %) of this slurry was graphite:CMC:pure water=1:0.1:60-100.

[0049] (molding process) The entire amount of the slurry was poured onto a flat filter. After confirming that the liquid level had settled, pressure reduction was immediately initiated, and graphite particles were formed into a sheet on the filter by a vacuum filtration membrane-forming method. The filter and the resulting sheet were then placed on a hot plate and kept at 60°C for 30 minutes to dry.

[0050] (Firing process) The dried graphite sheet was baked in an electric furnace at 130°C for 30 minutes and pressed with an electric roll press to obtain a graphite sheet. The thickness of the graphite sheet itself was 75 to 120 μm.

[0051] [Comparative Example] A graphite sheet was obtained in the same manner as in the Examples, except that a slurry containing only one type of graphite powder shown in Table 1 was used instead of using multiple types of graphite powder.

[0052] <Evaluation> The thermal conductivity H (unit: W / (m·K)) of the graphite sheet at 25°C was obtained by the following steady-state method. A heat source was placed on the edge of several 20mm x 150mm rectangular reference samples with different thermal resistivities, and thermocouples were placed on the sample surface at the heat source and on the surface 50mm from the edge. After applying a constant power to the heat source, the surface temperature difference between the heat source and the 50mm point was recorded at least 10 minutes after the temperature reached a constant value. The relationship between the inverse of the thermal resistance and this temperature difference was then fitted with an exponential function. For a graphite sheet cut into a size of 20 mm x 100-150 mm, the difference in surface temperature between the heat source position and a point 50 mm away was recorded in the same manner as above, and the thermal conductivity was calculated using the fitting formula mentioned above. The evaluation results are shown in Table 1.

[0053] [Table 1]

[0054] The results in Table 1 show that the graphite sheet manufacturing method of the present invention can produce a graphite sheet having a thermal conductivity at 25°C of 270 to 400 (W / mk).

[0055] Furthermore, a comparison between Example 1 and Example 2 shows that the thermal conductivity improves when the proportion of graphite powder with a large average particle size is 50 wt% or more, a comparison between Example 2 and Example 6 shows that the thermal conductivity improves further when the proportion of graphite powder with a large average particle size is 58 wt% or more, and a comparison between Example 6 and Examples 3 to 5 shows that the thermal conductivity improves most when the proportion of graphite powder with a large average particle size is 60 wt% or more.

[0056] Furthermore, the results of Comparative Examples 1 and 2 show that graphite powder C has large voids and therefore has low thermal conductivity, while the results of Comparative Examples 1 and 4 show that graphite powder B has many interfaces and therefore has low thermal conductivity. This shows that the inclusion of graphite powder A as an essential component can improve thermal conductivity.

Claims

1. preparing a slurry containing graphite powder; orienting the graphite particles in the slurry to form a sheet; and a step of firing the sheet-shaped graphite particles, the slurry contains graphite powder A having an average particle size of 40 to 50 μm, Further, the graphite powder B has an average particle size of 1.6 to 2.5 μm and / or the graphite powder C has an average particle size of 160 to 250 μm, A method for producing a graphite sheet, characterized in that the content of graphite powder with the largest average particle size is 20 to 85 wt %.

2. the slurry preparation step includes a process of adding graphite powder to an aqueous solution containing an anionic surfactant and stirring the resulting mixture; 2. The method for producing a graphite sheet according to claim 1, wherein the process of adding and stirring graphite powders is carried out in the order of graphite powders having larger average particle diameters to graphite powders having smaller average particle diameters, and the plurality of graphite powders are mixed in the solution.

3. the anionic surfactant is water-soluble carboxymethyl cellulose, 3. The method for producing a graphite sheet according to claim 2, wherein the viscosity of a 1 wt % aqueous solution of the water-soluble carboxymethyl cellulose at 25° C. is 1000 to 2000 mPa·s.

4. 4. The method for producing a graphite sheet according to claim 3, wherein the graphite particles have an ID / IG ratio obtained from a Raman spectrum of 0.1 or less.

5. the step of orienting the graphite particles and forming them into a sheet, It is a vacuum filtration membrane formation method, 5. The method for producing a graphite sheet according to claim 4, further comprising the step of starting to reduce the pressure after placing the entire amount of the slurry on the filtration membrane and before the graphite particles are deposited.

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