Patent Application
20170015560
The technical field relates to a graphite plate used as a heat-conductive material, and a production method thereof.
As one example of a conventional method for obtaining a graphite plate with excellent heat conductivity, a polymer-graphitization method in which a polymer film is subjected to a heat treatment under an inert gas atmosphere such as nitrogen, argon or helium has been known (for example, see Japanese Patent Nos. 2057739 and 2975098, Publications).
Furthermore, as an example of a method for obtaining a graphite material that is used for gaskets, sliding members, crucibles, heating elements, etc., a method in which a kneaded mixture including a carbon material powder such as coke and a binding material such as tar and pitch is baked, and then, the mixture is heated so as to be converted into a graphite material has been known (for example, see Japanese Patent No. 5033325, Publication), although the produced graphite material has inferior heat conductivity.
Conventional graphite plates are flat and smooth materials that have glossy surfaces or frosted-glass-like non-glossy surfaces. Accordingly, when heat is conveyed from the graphite plates to other components, the contact areas therebetween becomes small, and the heat loss due to the contact thermal resistance is inevitable. Therefore, for example, heat-conductive pastes are coated onto surfaces of graphite plates, thereby improving their adhesiveness, and such materials are practically used.
However, since heat conductivities of the heat-conductive pastes are lower than that of graphite in the above-mentioned conventional arts, there is a problem that the heat conductivities of the resulting products are inferior.
Furthermore, when heat-conductive pastes are additionally used, there are also problems of increased numbers of required components, increased numbers of the production steps/increased amount of time required for the production, scattering of the pastes during the repair, and quality deterioration over time/due to the environment.
Hence, the purpose of the present application is to provide a graphite plate that does not require use of any heat-conductive pastes and that exhibits small contact thermal resistance, and a production method thereof.
In order to achieve the above-described purpose, provided is a graphite plate, having a surface roughness Ra from 10 μm to less than 40 μm, and a surface-unevenness variation of 0.01% to 0.135% in any span 80 mm long within the surface of the graphite plate. Furthermore, provided is a method for producing a graphite plate, including: subjecting a polymer film to a heat treatment in an inert gas, wherein the heat treatment is Conducted at 2400° C. to 3200° C., and a pressure of 10 kg/cm2 to 100 kg/cm2 is applied to the polymer film at 2000° C. or higher.
The graphite plate is capable of reducing the heat loss due to the contact thermal resistance -without using any heat-conductive pastes.
Hereinafter, embodiments will be described with reference to drawings.
The graphite plate according to an embodiment is a graphite crystalline body that has a certain level of surface roughness (satin-like roughness) on its surface. Furthermore, in the production method thereof, one or more pieces of polymer films are graphitized in a state in which they are layered.
A method for producing a graphite plate using a polymer film as a material will specifically be described. A polymer film 3 described below is used as a material, and is kept inside a heat-resistant vessel 1 (as shown in
The polymer films 3, which serves as a material, is placed on the bottom of the heat-resistant vessel 1. While heating the heat-resistant vessel 1, the polymer film 3 is pressed by a block 2. As for the heating method, resistance heating, induction heating or the like can be used. An inert gas (argon, helium, nitrogen, etc. ) is used for the atmosphere. As a result, the polymer film 3 is turned into a graphite plate 5.
The graphite plate 5 according to an embodiment has a structure in which hexagonal-mesh-like two-dimensional graphite crystals are layered. Accordingly, the graphite plate 5 has heat conductivities different between the surface direction and the thickness direction,
Since the heat is vibrationally conveyed therethrough, the graphite plate does not have a heat conductivity of 700 W/mK or higher if all of covalent bonds in the hexagonal-mesh-like structure are broken. Furthermore, when all of covalent bonds in the hexagonal-mesh-like structure remain intact, the graphite plate has a heat conductivity of 1500 W/mK. Consequently, the graphite plate 5 according to embodiments, which has a structure in which hexagonal-mesh-like two-dimensional graphite crystals are layered, may have a heat conductivity of 700 W/mK to 1500 W/mk in the surface direction.
Graphite crystals are connected to one another through the van der Waals' forces, and therefore, their connections are not rigid in the same manner as covalent bonds. Accordingly, the heat conductivity in the thickness direction is small. If all the connections through the van der Waals' forces are broken, the graphite plate does not have a heat conductivity of 2 W/mK or higher in the thickness direction. On the other hand, when all the connections remain intact, the graphite plate has a heat conductivity of 20 W/mK. Consequently, the graphite plate 5 according to embodiments, which has a structure in which hexagonal-mesh-like two-dimensional graphite crystals are layered, may have a heat conductivity of 2 to 20 W/mk in the thickness direction.
As a result of the above-mentioned features, a graphite plate 5 according to an embodiment may have a heat conductivity of 700 W/mK to 1500 W/mK in the surface direction, and a heat conductivity of 2 W/mK to 20 W/mK in the thickness direction.
The density of the graphite plate 5 is also determined based upon a state of breakage of the crystal structure. The apparent density is 2.2 g/cm3 in a state where the covalent bonds and the connections through the van der Waals' forces remain intact. In a state where, while the covalent bonds and the connections through the van der Waals' forces are broken, the structure in which the crystals are layered is maintained, the density is 1.0 g/cm3. If the layer structure is broken, the density becomes smaller than 1.0 g/cm3 Accordingly, the graphite plate 5 according to an embodiment, which is a graphite crystal body, has a density of 1.0 g/cm3 to 2.2 g/cm3
A graphite plate 5 according to an embodiment may have a thickness of 2.5 μm to 2 mm. If the thickness of the graphite plate 5 is smaller than 25 μm, pressure cannot be evenly applied to the graphite plate 5, and the surface cannot be controlled. Consequently, in that case, the surface of the graphite plate 5 cannot be arranged in a satin-like fashion.
If the thickness of the graphite plate 5 is larger than 2 mm, it is difficult to remove a gas from a central portion thereof, and the surface cannot be arranged in a satin-like fashion while the overall crystallinity is maintained at a high level.
A graphite plate 5 according to an embodiment has a satin-like surface. This makes it possible to reduce the heat loss due to the contact thermal resistance against other components. Accordingly, it is not required to use any heat-conductive pastes, etc. serving as materials for reducing the contact thermal resistance, and the graphite plate 5 can be used even in the field of high-temperature industrial apparatuses in which heat-conductive pastes are deteriorated.
The polymer film, used as a material for the graphite plate 5 may be made of a polymer with a benzene ring, and examples thereof include polyimide, polyamide, polyoxadiazole, polybeneothiazole, polybenzobisthiazole, polybenzoxazole, polybenzobisoxazole, polyparaphenyienevinyiene, polyphenylene benzimidazole, polyphenylene benzbisimidazole, and polythiazole. The polymer film is preferably made of at least one polymer selected from these polymers. This is because, in that case, the resulting graphite plate 5 has enhanced heat conductivity.
The thickness of the polymer film used herein may be 2 μm to 150 μm, preferably 12 μm to 125 μm. If the thickness is smaller than 12 μm, wrinkles may easily be generated on the resulting graphite plate 5 due to static electricity. If the thickness is smaller than 2 μm, the surface of the resulting graphite plate 5 may be distorted by wrinkles. If the thickness is larger than 125 μm, the range of conditions for controlling elimination of a gas therefrom, may be narrow, and the control of the elimination of a gas may be difficult. If the thickness is larger than 150 μm, it may be impossible to eliminate a gas therefrom, and the surface of the resulting graphite plate 5 may be uneven.
In particular, a polymer film with a thickness of 25 μm to 75 μm hardly has wrinkles thereon, and it is easy to control elimination of a gas therefrom. Accordingly, in that case, it is easy to produce a homogenous graphite plate 5.
The development of hexagonal-mesh-like two-dimensional crystals and the layer crystal structure of graphite is determined by a given temperature for the heat treatment. If the temperature is lower than 2400° C., the hexagonal mesh-like two-dimensional graphite crystals and the structure in which the two-dimensional crystals are layered cannot be produced, and therefore, such a temperature is unfavorable. In such a case, the transfer of atoms does not occur.
When the temperature for the heat treatment is 2600° C. or higher, the structure in which hexagonal-mesh-like two-dimensional graphite crystals are layered is produced as the whole, and therefore, such a temperature is preferable.
If the temperature for the heat treatment is higher than 3200° C., the graphite plate 5 starts to sublime, and therefore such a temperature is unfavorable.
As a result, it is required that the temperature for the heat treatment is within a range from 2400° C. to 3200° C. Furthermore, the temperature for the heat treatment is preferably 2600° C. or higher.
The two-dimensional crystallization (crystallization in the surface direction) of the graphite plate 5 is a phenomenon that is caused on the scale of several hundred nanometers, and is determined mainly by the temperature for the heat treatment as described above.
Meanwhile, by applying pressure to the polymer film 3 during the heat treatment, the surface of the graphite plate 5 is folded on the micrometer scale. By controlling the surface of the graphite plate 5 so as to have such surface properties on the micrometer scale, the heat loss due to the contact thermal resistance is reduced.
The thermal resistance of the surface of the graphite plate 5 is reduced by controlling the surface properties of the graphite plate 5 to a certain range. The surface properties are described below, and are ascertained by referring to the section of Examples.
The surface roughness is 10 μm or higher and less than 40 μm, and the flatness is within a range from 0.01% to 0.135%.
When conventional graphite plates are produced, no pressure is applied thereto, or various types of pressure-applying methods are used. However, in the present embodiment, a pressure of 10 kg/cm2 to 100 kg/cm2 is applied to the surface of the graphite plate 5 in the vertical direction at. 2400° C. to 3200° C.
If the applied, pressure is lower than 10 kg/cm2, the surface roughness becomes 40 μm or higher, and the flatness becomes larger than 0.135%.
If the applied pressure is higher than 100 kg/cm2, the surface roughness becomes less than 10 μm, and the flatness becomes less than 0.01%.
By applying a pressure of 10 kg/cm2 to 100 kg/cm2 to the polymer film at 2400° C. to 3200° C. in the heat treatment, the surface roughness can foe adjusted to 10 μm or higher and less than 40 μm, and the flatness can be adjusted to 0.01% to 0.135%, thereby reducing the contact thermal resistance against other components.
In cases where the temperature for the heat treatment is lower than 2400° C., the surface of the polymer film 3 cannot foe controlled even if pressure is applied, to the polymer films 3.
In the process of the heat treatment, oxygen, nitrogen and hydrogen are eliminated from the polymer film 3, which is a compound of carbon, oxygen, nitrogen and hydrogen. Thus, only carbon is caused to remain therein, and carbon is caused to be recrystallized, and, consequently, a graphite plate 5 with high crystallinity can be obtained. If the graphite plate 5 does not have high crystallinity, it may not have a heat conductivity of 700 W/m to 1500 W/mK.
Since the elimination of oxygen, nitrogen and hydrogen is carried out in forms of gases, the surface of the material is distorted. By control ling the elimination, the surface can be formed in a satin-like fashion. If the applied pressure is lower, distortion of the surface of the material becomes larger. If the applied pressure is higher, the surface of the material becomes flat and smooth. In addition, if the pressure is applied, thereto at less than 2000° C., a homogenous graphite plate 5 may not be produced.
Heat energies affect, the material from the surface, so that there is a time lag between the recrystallizations in the surface and in the internal portion. Consequently, the surface can be formed in a satin-like shape while the overall crystallinity can be maintained at a high level.
With regard to the heat treatment of the polymer film 3, the temperature may be decreased once to room temperature between the step in which oxygen, nitrogen, and hydrogen are eliminated, therefrom and the step in which carbon is recrystallized, and thus, the heat treatment may be carried out twice in order to be increased.
The polymer film(s) 3 is placed inside the heat-resistant vessel 1. In that case, in order to increase the number of the polymer films to be processed in each treatment, carbon plates 4 each having a thickness of about 5 mm may be placed between the polymer films 3 inside the heat-resistant, vessel 1 as shown in
It is required that the heat-resistant vessel 1 and the block 2 are both resistant to a temperature of 3200° C. or higher and that they are materials that don't cause impurities. In addition to these requirements, it is required that they each have a structure that makes it possible to apply pressure to the polymer film(s) 3. Therefore, it is required that they bear a pressure of 100 kg/cm2.
Shapes of the heat-resistant vessel 1 and the block 2 are not necessarily limited to a rectangular or round shape. It is required that there is no temperature variation inside the heat-resistant vessel 1, and the heat-resistant vessel 1 and the block 2 are preferably made of carbon not containing any impurities.
For the heat treatment, an inert gas is used in order to prevent, oxidation of the heat-treated material. For the inert gas, helium, nitrogen, and argon are preferable, and argon is particularly preferable. In order to prevent the air from penetrating into the furnace, the gas pressure is not limited as long as it is higher than the ordinary pressure. However, if the gas pressure inside the furnace, is higher than 0.2 MPa, the gases of oxygen, nitrogen and hydrogen, to be eliminated may be difficult to discharge therefrom. In addition, if the gas pressure inside the furnace is low, release of the gases to be eliminated rapidly occurs, resulting in breakage of the surface.
Moreover, even if the surface is not broken, generation of the gases to be eliminated becomes non-uniform, and the homogeneity may be impaired. Furthermore, the larger the thickness of the polymer film 3 used as a material, the more difficult it becomes for the gases to be eliminated to pass therethrough. Consequently, in that case, the resulting graphite plate is likely to have such a tendency of impaired homogeneity.
As described below, samples were produced, and were evaluated. Conditions and results are shown in Table 1. However, those for Example 4 are not listed in Table 1 since the method for layering materials was only changed.
A polyimide film (Kapton 100H manufactured by DU PONT-TORAY CO., LTD,; thickness: 25 μm) was cut into pieces 100 mm square, and thirty of the pieces were stacked. The stacked pieces were put into the heat-resistant vessel 1 in FIG. 1, and were heated to 1000° C., at a rate of 1° C./minute within a range of 450° C. to 650° C., in an atmosphere of nitrogen gas, using an electric furnace. Subsequently, oxygen, nitrogen and hydrogen were eliminated, and then, while the materials were heated to 3000° C. at 10° C./minute in an atmosphere of argon gas, a pressure of 50 kg/cm2 was applied thereto, thereby producing a graphite plate 5. The graphite plate 5 produced by this method had a surface roughness of 16.3 μm and a flatness of 0.040%. The heat conductivity thereof was 1160 W/mK. However, a thermocouple 6 indicated 124° C., and the sample was evaluated as “excellent.”
A polyimide film (Kapton 200H manufactured by DU PONT-TORAY CO., LTD.; thickness: 50 μm) was cut into pieces 100 mm square, and thirty of the pieces were stacked. The stacked pieces were put into the heat-resistant vessel 1 in
A polyimide film (Kapton 300H manufactured by DU PONT-TORAY CO,, LTD.; thickness: 75 μm.) was cut into pieces 100 mm square, and thirty of the pieces were stacked. The stacked pieces were put into the heat-resistant vessel 1 in
A polyimide film (Kapton 100H manufactured by DU PONT-TORAY CO., LTD.; thickness: 25 μm) was cut into pieces 100 mm square, and 3 sets of stacks each having thirty of the pieces were prepared. Four carbon plates each having a thickness of 5 mm were placed between them. The stack was put into the heat-resistant vessel 1, and was pressed with a block 2, as shown in
In accordance with the production method In Japanese Patent No. 2057739, a polyimide film (Kapton 100H manufactured by DU PONT-TORAY CO., LTD.; thickness: 25 μm) was cut into pieces 100 mm square, and thirty of the pieces were stacked. The stacked, pieces were put into the heat-resistant vessel 1 in
It is considered that, since the surface was not formed in a satin-like shape, and contact points against the other component were few, the heat loss due to the contact thermal resistance was increased
In accordance with the production method in Japanese Patent No. 2975098, a polyimide film (Kapton 200H manufactured by DU PONT-TORAY CO., LID,; thickness: 75 μm) was cut into pieces 100 mm square, and thirty of the pieces were stacked. The stacked pieces were put into the heat-resistant vessel 1 in
It is considered that the hexagonal-mesh like two dimensional crystal structure was broken in the resulting graphite plate. This is implied by the decreased heat conductivity. Furthermore, it is considered that, due to the breakage of the two-dimensional crystal structure, the graphite plate could not be in contact with the counterpart member while having sufficient elasticity.
In accordance with the production method in Japanese Patent No. 2975098 a polyamide film (Kapton 300H manufactured by DU PONT-TORAY CO., LTD.; thickness: 75 μm) was cut into pieces 100 mm square, and thirty of the pieces were stacked. The stacked pieces were put into the heat-resistant vessel 1 in
It is considered that the hexagonal-mesh-like two-dimensional crystal structure was broken in the resulting graphite plate. This is implied by the decreased heat conductivity. Furthermore, it is considered that, due to the breakage of the two-dimensional crystal structure, the graphite plate could not be in contact with the counterpart member while having sufficient elasticity.
In accordance with the production method In Japanese Patent No. 2975098, a polyimide film (Kapton 300H manufactured by DU PONT-TORAY CO., LTD.; thickness: 75 μm) was cut into pieces 100 mm square, and thirty of the pieces were stacked. The stacked pieces were put into the heat-resistant vessel 1 in
It is considered that the hexagonal-mesh-like two-dimensional crystal structure was broken in the resulting graphite plate. This is implied by the decreased heat conductivity. Furthermore, it is considered that, due to the breakage of the two-dimensional crystal structure, the graphite plate could not be in contact with the counterpart member while having sufficient elasticity.
A photo of the surface of a graphite plate 5, laser-based profile(r)(c)try are shown in
Surface roughness was evaluated based on Ra (arithmetic mean roughness) in the JIS standard.
The cross-sectional structure shown in
Results of Examples 1 to 3 and Comparative Examples 1 to 4 are summarized in the graph of
However, in the materials used herein, only within a certain, range, there is a proportional relationship between the surface roughness and the flatness. Furthermore, properties of the contact thermal resistance within that range is preferred.
Based on the graph, a region where the surface roughness is from. 10 μm to less than 40 μm and the flatness is from 0.010% to 0.135% is a discontinuous area (specific or critical area) to the other area. Furthermore, a region of examples where the surface roughness is from 16.3 μm to 24.7 μm and the flatness is from 0.04% to 0.09% is preferable,
Furthermore, a relationship between the contact thermal resistance and the heat conductivity is shown in
In addition, this is not associated with the thickness of the graphite plate 5.
The graphite plate according to the embodiments can be a heat-conductive material used inside high-performance and downsized electronic devices, and can particularly be used in notebook computers, tablets, smartphones, portable phones, wearable devices, digital cameras, and digital movie cameras. Furthermore, the graphite plate can be used In industrial apparatuses that exceed the limit of heat resistance of heat-conductive pastes, which are used in order to reduce the heat loss due to the contact thermal resistance, outdoor-use devices that ultraviolet rays penetrate into, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-141730 | Jul 2015 | JP | national |
| 2016-081746 | Apr 2016 | JP | national |
