Graphite sheet and its manufacturing method

Abstract

The present disclosure addresses the problem of providing a graphite sheet capable of reducing thermal contact resistance by reducing thermal contact resistance on the surface of the graphite sheet. A graphite sheet (11) is formed by a plurality of graphite layers laminated in the thickness direction. At least a part of a graphite layer that forms a layer other than the outermost layer of the graphite sheet (11) is exposed on the surface of a graphite layer that forms the outermost layer. The graphite sheet (11) has a deemed shear strength of 0.1-0.5 MPa, as measured in a depth range of 0.5-19 μm by a SAICAS method.

Description

[Technical Field]

[0001] The present disclosure relates to a graphite sheet and a method for manufacturing the graphite sheet, and more particularly to a graphite sheet disposed between a heat-generating component and a coolant and a method for manufacturing the graphite sheet. [Background technology]

[0002] In recent years, the number of electric vehicles and hybrid vehicles that use electric motors as the main or auxiliary drive sources for running has been increasing. The inverters that control these vehicles use insulated gate bipolar transistors (IGBTs), and the IGBTs are attached to the coolant with screws or other means to dissipate the heat they generate.

[0003] Regarding such a technique, Patent Document 1 discloses that in a power module with a heat dissipation component, the power module includes a base plate, a ceramic insulating substrate bonded onto the base plate, a semiconductor element bonded onto the ceramic insulating substrate, and a heat dissipation component attached to the base plate side of the power module via a heat dissipation sheet, the flatness of the surface of the base plate opposite to the ceramic insulating substrate is set to 20 μm or less.

[0004] In addition to this method, grease or other materials are used between the IGBT and the coolant to smoothly transfer heat. However, when grease is used, the thermal conductivity is insufficient, and when the IGBT repeatedly heats and cools, its expansion gradually pushes the grease outward, which can deteriorate the thermal conductivity. Another method involves transferring heat by sandwiching a solid thermally conductive sheet such as a graphite sheet, but when such a graphite sheet is fastened with screws, the contact thermal resistance between the graphite sheet surface and the IGBT and the coolant increases, making it difficult to transfer heat effectively.

[0005] The graphite sheets used for such applications are obtained by, for example, pyrolyzing polymer films to graphite. During graphitization, the film is baked at high temperatures, for example, around 2600°C, which causes further crystallization near the surface, making the surface hard. As a result, the parts that come into contact with the IGBT or coolant are too hard to make sufficient contact, resulting in high contact thermal resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-067801 Summary of the Invention

[0007] An object of the present disclosure is to provide a graphite sheet that can reduce contact thermal resistance on the surface and exhibit sufficient heat dissipation effect, and a method for manufacturing the graphite sheet.

[0008] A graphite sheet according to one embodiment of the present disclosure is a graphite sheet having a plurality of graphite layers stacked in the thickness direction, in which at least a portion of each of the graphite layers constituting an outermost layer of the graphite sheet is exposed on the surface of the graphite layer constituting the outermost layer.

[0009] A graphite sheet according to another embodiment of the present disclosure has a deemed shear strength of 0.1 MPa or more and 0.5 MPa or less when measured to a depth of 0.5 μm to 19 μm by the SAICAS method.

[0010] A method for producing a graphite sheet according to one embodiment of the present disclosure includes a first step of obtaining a graphite base material, and a second step of removing a surface layer portion of the graphite base material obtained in the first step to obtain a graphite sheet. [Brief explanation of the drawings]

[0011] [Figure 1]1 is a scanning electron microscope photograph of the surface of a graphite sheet according to an embodiment of the present invention. [Figure 2] 1 is a scanning electron microscope photograph of the surface of a graphite sheet before removing the surface layer. [Figure 3] FIG. 2 is a schematic diagram illustrating a method for measuring the deemed shear strength of a graphite sheet in the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] (1) Overview A graphite sheet and a method for manufacturing the graphite sheet according to an embodiment of the present disclosure will be described. Note that the following embodiment is merely one of various embodiments of the present disclosure. The following embodiment can be modified in various ways depending on the design as long as the object of the present disclosure can be achieved.

[0013] A graphite sheet is sandwiched between an IGBT and a coolant, for example, and fastened with screws, etc., to compress the graphite sheet and bring it into close contact with the IGBT and the coolant, smoothly transferring the heat generated by the IGBT to the coolant. Graphite sheets are usually made by sintering at high temperatures, which causes more crystallization near the surface, making the surface harder. As a result, the parts that come into contact with the IGBT or the coolant are too hard to make sufficient contact, resulting in high contact thermal resistance.

[0014] In response to this problem, the inventors discovered that by giving the surface of a graphite base material obtained by firing at high temperatures a specific configuration, for example by removing the surface layer, the resulting graphite sheet can reduce the contact thermal resistance on the surface and exhibit a sufficient heat dissipation effect, thereby completing the present disclosure.

[0015] That is, the graphite sheet of this embodiment has the following configuration (1) or (2). (1) A graphite sheet in which a plurality of graphite layers are laminated in the thickness direction, and at least a portion of each of the graphite layers other than the outermost layer is exposed on the surface of the graphite layer that constitutes the outermost layer of the graphite sheet. (2) The assumed shear strength measured by the SAICAS method from a depth of 0.5 μm to 19 μm is 0.1 MPa or more and 0.5 MPa or less.

[0016] The graphite sheet (1) can be made to have a sufficiently soft surface by removing the surface layer of the graphite substrate until such a structure appears, and the graphite sheet (2) can be made to have a sufficiently soft surface by removing the surface layer of the graphite substrate until the surface has such a deemed shear strength, thereby reducing the contact thermal resistance at the surface and, as a result, achieving a sufficient heat dissipation effect.

[0017] The method for producing a graphite sheet according to this embodiment includes a first step of obtaining a graphite base material, and a second step of removing a surface layer of the graphite base material obtained in the first step to obtain a graphite sheet.

[0018] According to the method for producing a graphite sheet of the present embodiment, a graphite sheet can be obtained by a simple method.

[0019] (2) Details <Graphite sheet> Graphite sheet 11 according to this embodiment is formed by laminating multiple graphite layers in the thickness direction. A "graphite layer" is a layer made of graphite that forms a single cleavage plane, and includes one or more, preferably one to three, graphene layers. A "graphene layer" is a layer in which carbon atoms are arranged in a hexagonal honeycomb lattice pattern, and is usually a single layer.

[0020] FIG. 1 is a scanning electron microscope (SEM) photograph of the surface of graphite sheet 11 of this embodiment. As can be seen, in graphite sheet 11, at least a portion of the graphite layers constituting the outermost layer of graphite sheet 11 are exposed on the surface of the graphite layer constituting the outermost layer of graphite sheet 11. As can be seen from the SEM photograph of FIG. 1, there are voids in part of the outermost graphite layer of graphite sheet 11, and the surface of the graphite layer below the outermost layer is visible through this void, as well as the edge portion of the graphite layer below the outermost layer. FIG. 2 is an SEM photograph of the surface of a graphite sheet different from this embodiment, before the surface layer portion is removed. The graphite sheet of FIG. 2 does not show a structure in which graphite layers other than the outermost layer are exposed from the outermost graphite layer.

[0021] As shown in the SEM photograph of FIG. 1 , the exposed portions of graphite sheet 11 form multiple recesses. Having multiple recesses on its surface improves adhesion to IGBTs, coolants, and the like. This allows graphite sheet 11 to further reduce the contact thermal resistance on its surface, further improving its heat dissipation effect. Furthermore, the multiple recesses on its surface enable composite formation with a resin material or the like by impregnation or the like. The average depth of the multiple recesses is preferably 3 μm or more and 30 μm or less. When the average depth of the recesses is 3 μm or more, the conformability of the surface layer improves, thereby reducing thermal resistance. Furthermore, composite formation with a resin material or the like by impregnation or the like becomes easier. When the average depth of the recesses is 30 μm or less, the voids are not too large, preventing increased thermal resistance and ensuring the strength of graphite sheet 11. Therefore, when the average depth of the recesses is within the above range, graphite sheet 11 can further reduce the contact thermal resistance on its surface and further improve its heat dissipation effect. The average depth of the recesses is more preferably 5 μm or more and 25 μm or less, and even more preferably 7 μm or more and 20 μm or less. The "average depth of the recesses" is the arithmetic mean value of the depths of each recess determined from a scanning electron microscope photograph or the like, for example, at any 10 points.

[0022] The average equivalent circular diameter of the recesses is preferably 30 μm or more and 100 μm or less. When the average equivalent circular diameter of the recesses is within this range, graphite sheet 11 can further improve the surface adhesion, thereby further reducing the contact thermal resistance on the surface and further improving the heat dissipation effect. The average equivalent circular diameter of the recesses is more preferably 35 μm or more and 80 μm or less, and even more preferably 40 μm or more and 60 μm or less. The "average equivalent circular diameter of the recesses" refers to the arithmetic mean value of the diameter of a circle when the shape of the opening of the recess in a plan view is assumed to be a circle having the same projected area as the projected area of ​​the opening, for example, at any 10 points.

[0023] The average number of recesses per unit area is 5 / mm 2 More than 30 pieces / mm 2 It is preferable that the average number of recesses is 5 / mm or less. When the average number of recesses is within the above range, graphite sheet 11 can further improve the adhesion of the surface, thereby further reducing the contact thermal resistance on the surface and further improving the heat dissipation effect. The average number of recesses is 5 / mm 2 More than 25 pieces / mm 2 More preferably, it is 10 pieces / mm or less. 2 More than 20 pieces / mm 2 It is more preferable that the average number of recesses is equal to or less than 100. The "average number of recesses" can be determined, for example, from an SEM photograph of a certain area of ​​the surface of graphite sheet 11.

[0024] Furthermore, graphite sheet 11 according to this embodiment has a deemed shear strength of 0.1 MPa or more and 0.5 MPa or less when measured by the SAICAS method to a depth of 0.5 μm to 19 μm. Because graphite sheet 11 according to this embodiment has a deemed shear strength within the above range when measured to a depth of 0.5 μm to 19 μm, the surface can be made sufficiently soft. This allows the graphite sheet to easily deform under pressure at the portion in contact with the IGBT or coolant, thereby allowing for close contact with the IGBT or coolant and reducing the contact thermal resistance at the surface. As a result, sufficient heat dissipation can be achieved. If the deemed shear strength is greater than 0.5 MPa, the contact thermal resistance cannot be sufficiently reduced. Conversely, if the deemed shear strength is less than 0.1 MPa, the surface is easily scratched and difficult to handle. The deemed shear strength is preferably 0.2 MPa or more and 0.5 MPa or less, and more preferably 0.3 MPa or more and 0.5 MPa or less.

[0025] The SAICAS method, also known as the Surface and Interfacial Cutting Analysis System method, is an evaluation method in which a material is cut from its surface layer at a low speed with a sharp cutting blade. FIG. 3 is a schematic diagram illustrating a method for measuring the deemed shear strength of a graphite sheet in this embodiment. The arrow D in FIG. 3 indicates oblique cutting, and the arrow x indicates displacement. Using the SAICAS method, the horizontal force (Fh) and vertical force (Fw) applied to the cutting blade when cutting the surface of graphite sheet 11 can be measured, and the deemed shear strength of the surface layer can be calculated from the horizontal force (Fh) applied to cutting blade 12, the cutting angle of cutting blade 12, and the cross-sectional area. Specifically, a graphite sheet 11 was fixed to a SAICAS DN-20 (manufactured by Daipla Wintes), and the cutting edge 12 was made of boron nitride and had a width of 2 mm, a rake angle of 20°, and a clearance angle of 10°. The cutting speed was constant speed mode, with 0.5 μm / s horizontally and 0.05 μm / s vertically. The point at which the horizontal load reached 0.002 N or more was defined as the point at which the cutting edge 12 contacted the graphite sheet 11, and measurements were taken from there up to 19 μm vertically. The deemed shear strength from 0.5 μm to a depth of 19 μm was calculated using the following formula:

[0026] t=Fh×(2A×Cot(φ)) (t: assumed shear strength, Fh: horizontal force, A: cross-sectional area of ​​cutting edge, φ: shear angle) Similarly, when measuring in deeper regions, the measurement is continued and the assumed shear strength is measured based on the slope between each depth.

[0027] Furthermore, if F1 is the deemed shear strength measured from 0.5 μm to a depth of 19 μm, and F2 is the deemed shear strength measured from 20 μm to a depth when the thickness of the graphite sheet is 2a, then it is desirable that 0.5≦F1 / F2<1. That is, it is desirable that F1 is smaller than F2. In this case, graphite sheet 11 has a surface layer with sufficient softness, improving its ability to conform to irregularities and further improving its connection thermal resistance. If F1 is greater than F2, the surface layer becomes too hard, resulting in poor conformability and high connection thermal resistance. It is also desirable that F1 / F2≧0.5. In this case, the surface layer is less susceptible to damage and easier to handle. If F1 / F2 is less than 0.5, the surface layer is more susceptible to damage and handling becomes difficult.

[0028] An example of graphite sheet 11 in this embodiment has a thickness of about 200 μm and a compressibility of about 70% when a pressure of 600 kPa is applied. When graphite sheet 11 is measured by the SAICAS method to a depth of 0.5 μm to 19 μm, the deemed shear strength is, for example, 0.4 MPa, and when measured to a depth of 20 μm to 100 μm, the deemed shear strength is, for example, 0.5 MPa.

[0029] The average thickness of graphite sheet 11 is, for example, 50 μm or more and 2000 μm or less, preferably 100 μm or more and 1000 μm or less, and more preferably 200 μm or more and 800 μm or less.

[0030] Furthermore, it is desirable that the compressibility when a pressure of 600 kPa is applied be 60% or more. This will enable sufficient reduction of the thermal contact resistance. Note that the compressibility here is expressed as a percentage of (T0-T1) / T0, where T0 is the initial thickness and T1 is the thickness when a pressure of 600 kPa is applied and then released.

[0031] The area near the surface of graphite sheet 11 affects the contact thermal resistance, with the area further inside having almost no effect. A small assumed shear strength indicates a soft state, which is due to a low density and a low thermal conductivity. Therefore, it is desirable to make the assumed shear strength higher in the interior than in the surface.

[0032] In this embodiment, when the thickness of graphite sheet 11 is 2a, the deemed shear strength measured from 20 μm to a depth of a is set to, for example, 0.5 MPa, which is larger than the deemed shear strength measured from 0.5 μm to a depth of 19 μm. This reduces the contact thermal resistance and improves the thermal conductivity when the sheet is sandwiched between the IGBT and the coolant and fastened with screws or the like.

[0033] Graphite sheet 11 typically has two surfaces, a front surface and a back surface. Both surfaces preferably have the specific structure described above. Having both surfaces with the specific structure makes both surfaces sufficiently soft, which can further improve the adhesion of the surfaces of graphite sheet 11 and further reduce the thermal contact resistance at the surfaces, thereby further improving the heat dissipation effect.

[0034] <Graphite sheet manufacturing method> Next, a method for producing the graphite sheet in this embodiment will be described.

[0035] The method for producing a graphite sheet according to this embodiment includes, for example, a first step of obtaining a graphite base material, and a second step of removing a surface layer of the graphite base material obtained in the first step to obtain a graphite sheet.

[0036] (1st step) In this process, a graphite substrate is obtained. First, a polyimide film, for example, approximately 100 μm thick, is pyrolyzed to obtain a carbonized film, which is then baked at approximately 2600°C to graphitize it. During pyrolysis and graphitization, gas is generated inside the graphite substrate, causing it to expand in the thickness direction. Therefore, the graphite substrate has a thickness of, for example, approximately 500 μm, and is easily compressed overall when pressure is applied. However, the surface layer has undergone crystallization, making only the surface layer hard. Therefore, even when this graphite substrate is sandwiched between the IGBT and the coolant and fastened with screws, sufficient contact is not achieved, resulting in high contact thermal resistance.

[0037] (2nd process) In this step, the surface layer of the graphite substrate obtained in the first step is removed to obtain a graphite sheet.

[0038] To remove the surface layer, a slicer such as the NP1240C manufactured by Nippi Kikai can be used. Using a slicer to remove the surface layer of the graphite substrate makes it easier to produce graphite sheet 11. First, the graphite substrate obtained by firing is fixed to the slicer. Next, it is sliced ​​at a position approximately 50 μm from the surface. Then, graphite sheet 11 can be obtained by slicing it at a position to obtain a predetermined thickness. If the graphite substrate is thick enough, multiple graphite sheets 11 can be obtained, improving mass productivity.

[0039] According to the manufacturing method of this embodiment, by removing the surface layer in the second step, as described above, (1) at least a portion of the graphite layers constituting the outermost layer of the graphite sheet other than the outermost layer is exposed on the surface of the graphite layer constituting the outermost layer, or (2) the deemed shear strength measured to a depth of 0.5 μm to 19 μm by the SAICAS method can be set within a specific range, allowing for close contact with the IGBT and the coolant and reducing the contact thermal resistance.

[0040] (summary) As is clear from the above-described embodiment, the graphite sheet (11) of the first aspect is a graphite sheet (11) in which a plurality of graphite layers are laminated in the thickness direction, and at least a portion of the graphite layers constituting the outermost layer of the graphite sheet (11) are exposed on the surface of the graphite layer constituting the outermost layer.

[0041] According to the first aspect, the graphite sheet (11) can have a sufficiently soft surface, which can reduce the contact thermal resistance on the surface, and as a result, can exhibit a sufficient heat dissipation effect.

[0042] In the graphite sheet (11) of the second embodiment, the exposed portion of the first embodiment forms a plurality of recesses, and the average depth of the recesses is 3 μm or more and 30 μm or less.

[0043] According to the second aspect, the graphite sheet (11) has good conformability at the surface, resulting in low thermal resistance, and the voids are not too large, preventing high thermal resistance. This reduces the contact thermal resistance at the surface, further improving the heat dissipation effect. Furthermore, the multiple recesses on the surface with this average depth make it easier to combine the graphite sheet (11) with a resin material or the like by impregnation or the like, and also ensures the strength of the graphite sheet (11).

[0044] In the graphite sheet (11) of the third embodiment, in the second embodiment, the average equivalent circle diameter of the recesses is 30 μm or more and 100 μm or less.

[0045] According to the third aspect, the graphite sheet (11) can further improve the adhesion of the surface, thereby further reducing the contact thermal resistance on the surface and further improving the heat dissipation effect.

[0046] In the graphite sheet (11) of the fourth embodiment, the average number of recesses per unit area is 5 / mm in the second or third embodiment. 2More than 30 pieces / mm 2 The following is the result.

[0047] According to the fourth aspect, the graphite sheet (11) can further improve the adhesion of the surface, thereby further reducing the contact thermal resistance on the surface and further improving the heat dissipation effect.

[0048] In the graphite sheet (11) of the fifth embodiment, the assumed shear strength measured by the SAICAS method to a depth of 0.5 μm to 19 μm is 0.1 MPa or more and 0.5 MPa or less.

[0049] According to the fifth aspect, the graphite sheet (11) can have a sufficiently soft surface, which reduces the contact thermal resistance on the surface, and as a result, can exhibit a sufficient heat dissipation effect.

[0050] In the graphite sheet (11) of the sixth embodiment, when the deemed shear strength measured by the SAICAS method in the fifth embodiment from a depth of 0.5 μm to 19 μm is defined as F1, and when the thickness of the graphite sheet (11) is 2a, the deemed shear strength measured from 20 μm to a depth is defined as F2, F1 is smaller than F2.

[0051] According to the sixth aspect, the graphite sheet (11) has a surface layer with sufficient softness, which improves the ability to conform to irregularities and further improves the thermal contact resistance.

[0052] In the graphite sheet (11) of the seventh embodiment, in the sixth embodiment, F1 and F2 satisfy F1 / F2≧0.5.

[0053] According to the seventh aspect, the graphite sheet (11) has a surface that is resistant to scratches and is easy to handle.

[0054] In the graphite sheet (11) of the eighth aspect, in any one of the first to seventh aspects, the compressibility when a pressure of 600 kPa is applied is 60% or more.

[0055] According to the eighth aspect, the contact thermal resistance can be sufficiently reduced.

[0056] A ninth aspect of the method for producing a graphite sheet (11) includes a first step of obtaining a graphite base material, and a second step of removing a surface layer portion of the graphite base material obtained in the first step to obtain a graphite sheet (11).

[0057] According to the ninth embodiment, the graphite sheet (11) can be obtained by a simple method.

[0058] In a tenth aspect of the method for producing a graphite sheet (11), the graphite sheet (11) obtained in the ninth aspect is produced by the second step in such a manner that at least a portion of the graphite layers other than the outermost layer are exposed on the surface of the graphite layer that constitutes the outermost layer of the graphite sheet (11).

[0059] According to the tenth aspect, a graphite sheet (11) having a sufficiently soft surface, which can reduce the contact thermal resistance on the surface and can exhibit a sufficient heat dissipation effect, can be obtained by a simple method.

[0060] In an eleventh aspect of the method for producing a graphite sheet (11), the graphite sheet (11) obtained in the second step in the ninth or tenth aspect has a deemed shear strength of 0.1 MPa or more and 0.5 MPa or less when measured by the SAICAS method to a depth of 0.5 μm to 19 μm.

[0061] According to the eleventh aspect, a graphite sheet (11) having a sufficiently soft surface, which can reduce the contact thermal resistance on the surface and can exhibit a sufficient heat dissipation effect, can be produced by a simple method.

[0062] In a twelfth aspect of the method for producing a graphite sheet (11), in any one of the ninth to eleventh aspects, a slicer is used for the removal in the second step.

[0063] According to the twelfth aspect, by using a slicer to remove the surface layer portion of the graphite substrate, the graphite sheet (11) can be produced more simply. [Industrial Applicability]

[0064] The graphite sheet according to the present disclosure can reduce the contact thermal resistance on the surface and exert a sufficient heat dissipation effect, making it industrially useful. [Explanation of symbols]

[0065] 11 Graphite Sheet 12 cutting edge

Claims

1. A graphite sheet in which a plurality of graphite layers are laminated in the thickness direction, at least a portion of a graphite layer other than the outermost layer is exposed on a surface of the graphite layer constituting the outermost layer of the graphite sheet, the exposed portion forms a plurality of recesses, the recesses having an average depth of 3 μm or more and 30 μm or less, and an average equivalent circle diameter of 30 μm or more and 100 μm or less.

2. The average number of recesses per unit area is 5 / mm ２ 30 pieces / mm or more ２ 2. The graphite sheet of claim 1, wherein:

3. When the deemed shear strength measured by the SAICAS method from a depth of 0.5 μm to 19 μm is defined as F1, and when the thickness of the graphite sheet is 2a, the deemed shear strength measured from a depth of 20 μm to a is defined as F2, The F1 is 0.1 MPa or more and 0.5 MPa or less, The graphite sheet, wherein F1 and F2 satisfy 1 > F1 / F2 ≥ 0.

5.

4. 4. The graphite sheet according to claim 3, which has a compressibility of 60% or more when a pressure of 600 kPa is applied thereto.

5. A method for producing a graphite sheet, comprising: a first step of obtaining a graphite base material; and a second step of removing a surface layer portion of the graphite base material obtained in the first step to obtain a graphite sheet.

6. 6. The method for producing a graphite sheet according to claim 5, wherein at least a portion of the graphite layers other than the outermost layer is exposed on the surface of the graphite layer that constitutes the outermost layer of the graphite sheet obtained in the second step.

7. 7. The method for producing a graphite sheet according to claim 5 or 6, wherein the graphite sheet obtained in the second step has a deemed shear strength of 0.1 MPa or more and 0.5 MPa or less when measured to a depth of 0.5 μm to 19 μm by the SAICAS method.

8. 8. The method for producing a graphite sheet according to claim 5, wherein a slicer is used for the removal in the second step.

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