Thermal diffusion sheet and its manufacturing method

Abstract

To provide a thermal diffusion sheet with improved thermal conductivity by increasing the content of boron nitride in a wet papermaking thermal diffusion sheet and a manufacturing method for the same.SOLUTION: The thermal diffusion sheet is composed of a papermaking sheet including 80-95% by weight of scale-like boron nitride as an inorganic filler having thermal conductivity and 5 to 20% by weight of organic fibers as binder fibers.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a thermal diffusion sheet and a manufacturing method thereof. [Background technology]

[0002] Thermal diffusion sheets and heat dissipation sheets for promoting heat dissipation from heat generating bodies are used in various applications. For example, in vehicle-mounted or stationary power supply devices in which multiple secondary battery cells are stacked to achieve high output and high capacity, it is known that the secondary battery cells generate heat when they are charged and discharged. In order to improve the heat dissipation of such secondary battery cells, a thermal diffusion sheet is attached to the exterior can of the battery cell. In order to improve the heat dissipation with a thermal diffusion sheet, it is required to increase the thermal conductivity. In addition to secondary battery cells, electronic components such as power transistors, CPUs, and SoCs, and light-emitting elements such as LEDs and LDs are also required to have heat dissipation properties because they are heat generating bodies.

[0003] As such a thermal diffusion sheet, one using boron nitride as an insulating heat dissipation filler has been proposed (for example, Patent Documents 1 to 5). In order to improve the heat dissipation performance of such a thermal diffusion sheet, it is considered to increase the boron nitride content.

[0004] However, because boron nitride has low affinity for both water and solvents, it is not easy to disperse boron nitride uniformly in water when attempting to produce a thermal diffusion sheet by wet papermaking. In addition, at high boron nitride content ranges of over 80% by weight, improvement in thermal conductivity becomes sluggish as the content of expensive boron nitride increases. In addition, a large amount of powder falls off the sheet, making it difficult to achieve high content.

[0005] Also, a method for producing a sheet with a high boron nitride content has been proposed, which includes a step of mixing insulating particles, a binder resin, and a solvent to obtain a slurry (Patent Document 5). However, in this case, an organic solvent is used to dissolve the binder resin, which makes the recovery of the solvent and the disposal of waste liquid complicated, and increases the environmental load. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Republished WO2019 / 093305 [Patent Document 2] Special Publication No. 2019-535094 [Patent Document 3] Special Publication No. 2014-501859 [Patent Document 4] Japanese Patent Application Publication No. 10-130496 [Patent Document 5] Republished Publication No. 2020-194972 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a thermal diffusion sheet for wet-laid papermaking in which the blending ratio of boron nitride is increased to improve thermal conductivity, and a method for manufacturing the same.

[0008] The thermal diffusion sheet according to the first embodiment of the present invention is composed of a paper sheet containing 80 to 95 weight % of flaky boron nitride as a thermally conductive inorganic filler and 5 to 20 weight % of organic fibers as binder fibers.

[0009] In addition to the above-mentioned constitution, the thermal diffusion sheet according to the second embodiment of the present invention further contains 1 to 2 weight % of fine fibrous cellulose. With the above-mentioned constitution, by using fine fibrous cellulose with a large specific surface area as an aggregate, it is possible to blend a larger amount of flaky boron nitride.

[0010] Furthermore, in addition to any one of the configurations described above, a thermal diffusion sheet according to a third embodiment of the present invention is characterized in that the organic fiber is an aramid fiber.

[0011] Furthermore, in addition to any one of the configurations above, a thermal diffusion sheet according to a fourth embodiment of the present invention has an average thermal conductivity of 30 W / m·K or more in a plane direction of the thermal diffusion sheet.

[0012] Furthermore, in addition to any one of the configurations described above, a thermal diffusion sheet according to a fifth embodiment of the present invention has an average thickness of 50 to 200 μm.

[0013] Furthermore, in addition to any one of the above configurations, a thermal diffusion sheet according to a sixth embodiment of the present invention has a density of 1.5 to 2.1 g / cm. 3 It is.

[0014] Furthermore, in addition to any one of the above configurations, a thermal diffusion sheet according to a seventh embodiment of the present invention is such that the papermaking sheet is a wetlaid papermaking sheet.

[0015] Furthermore, in addition to any one of the above configurations, the thermal diffusion sheet according to an eighth embodiment of the present invention is such that the paper sheet is subjected to a heat and pressure treatment.

[0016] Furthermore, a method for producing a thermal diffusion sheet according to a ninth embodiment of the present invention includes the steps of forming a sheet by wet papermaking using 80 to 95 weight % of flaky boron nitride as a thermally conductive inorganic filler, 5 to 20 weight % of organic fiber, and 1 to 2 weight % of fine fibrous cellulose, and subjecting the sheet to a heat and pressure treatment. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is an exploded perspective view showing a power supply device according to a first embodiment of the present invention. [Diagram 2] 1 is a schematic cross-sectional view of a thermal diffusion sheet according to a first embodiment. [Diagram 3] FIG. 6 is a cross-sectional view showing a thermal diffusion sheet according to a second embodiment. [Figure 4] 11A to 11C are cross-sectional views showing a process for obtaining a thermal diffusion sheet according to a third embodiment. [Diagram 5]1 is a schematic diagram of a manufacturing apparatus for manufacturing a thermal radiation sheet according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the embodiments of the present invention will be described with reference to the drawings. However, the embodiments shown below are merely examples for embodying the technical idea of ​​the present invention, and the present invention is not limited to the following. In addition, this specification does not specify the members shown in the claims as the members of the embodiments. In particular, the dimensions, materials, shapes, and relative positions of the components described in the embodiments are merely explanatory examples, and are not intended to limit the scope of the present invention, unless otherwise specified. Note that the size and positional relationship of the members shown in each drawing may be exaggerated to clarify the explanation. Furthermore, in the following explanation, the same name and symbol indicate the same or similar members, and detailed explanation will be omitted as appropriate. Furthermore, each element constituting the present invention may be configured as a form in which multiple elements are composed of the same member, and one member serves multiple elements, or conversely, the function of one member can be shared by multiple members. [Embodiment 1]

[0019] The thermal diffusion sheet according to the embodiment of the present invention realizes high thermal conductivity by using a lightweight and non-conductive paper sheet and by incorporating a high content of flaky boron nitride. Such a thermal diffusion sheet is thermally coupled to heat-generating components such as electronic components and can be suitably used as a heat dissipation member. Conventionally, metal heat dissipation members used in heat dissipation fins have a problem that they are heavy and have high conductivity, making them difficult to use in situations where light weight and insulation are required. For example, in-vehicle components are strongly required to be lightweight to improve fuel efficiency, and non-conductivity is required from the viewpoint of safety, etc. In addition, graphite sheets used as heat dissipation members in the past have a problem that they are difficult to use for dissipating heat near antennas or communication modules because they have the property of blocking radio waves. In contrast, the thermal diffusion sheet according to the present embodiment has a low electromagnetic wave shielding property, so it does not block radio waves and has the advantage of being usable as a heat dissipation member for communication components.

[0020] The thermal diffusion sheet according to the present embodiment can be used as a buffer sheet interposed between a heat generating body that requires insulation and a heat sink or other heat dissipation sheet that is directly attached to the heat generating body to dissipate heat. As an example, the exploded perspective view of FIG. 1 shows an example of use in a power supply device in which a plurality of secondary battery cells are stacked. The power supply device 100 shown in this figure is used as an on-board power supply for driving a motor that turns the wheels in an electric vehicle or hybrid vehicle. The power supply device is not limited to automobiles, but can also be used as a power supply for driving electric vehicles such as electric buses, trains, and electric carts, as a backup power supply for factories and base stations, and even as a home storage battery. Furthermore, the thermal diffusion sheet according to the embodiment of the present invention is not limited to power supplies, but can be used for various purposes such as dissipating heat from heat generating bodies. For example, it can be used in various electronic components and electronic devices, such as computer central processing units (CPUs), image processing processors (GPUs), smartphone SoCs, DSPs and microcomputers in embedded devices, semiconductor elements such as transistors, light-emitting diodes (LEDs), semiconductor lasers (LDs), electroluminescence (EL), liquid crystals, and other light emitters, as well as vehicle headlights and lighting, etc. It can be particularly well suited for applications that require not only thermal conductivity but also insulation.

[0021] The power supply device 100 shown in Fig. 1 includes a plurality of secondary battery cells 20 and a thermal diffusion sheet 10 interposed between the secondary battery cells 20. The secondary battery cells 20 have exterior cans 21 in a rectangular cylindrical shape with a bottom, and a plurality of the secondary battery cells 20 are stacked with their main surfaces facing each other. The stacking is performed, for example, by covering both end faces of a battery stack 25 in which the secondary battery cells 20 are stacked with end plates 30, and fastening the end plates 30 together with fastening members. The battery stack 25 is fixed onto a base plate 40 as necessary. The base plate 40 can function as a cooling plate, for example, by circulating a refrigerant inside.

[0022] Each secondary battery cell 20 has an electrode body housed inside an outer can 21, and the open end is sealed with a sealing plate 22. A pair of electrodes 23 and an explosion-proof valve 24 are provided on the sealing plate 22 located on the top surface of the outer can 21 in FIG. 1. The electrodes 23 of the multiple secondary battery cells 20 are connected to each other electrically in series and / or in parallel by connecting the electrodes 23 with a bus bar. The explosion-proof valve 24 is a member that opens when it detects an increase in the internal pressure of the outer can 21, and discharges high-pressure gas from inside the outer can 21. Each explosion-proof valve 24 is connected to a gas duct for guiding the high-pressure gas to the outside as necessary. (Thermal diffusion sheet 10)

[0023] A thermal diffusion sheet 10 is interposed between adjacent secondary battery cells 20. The thermal diffusion sheet 10 is also called a spacer or separator, and provides insulation between adjacent secondary battery cells 20 to prevent the exterior cans 21 from shorting out.

[0024] 1, a configuration is described in which a battery stack 25, in which multiple secondary battery cells 20 are stacked, dissipates heat from the bottom surface using a thermal diffusion sheet 10. However, the present invention is not limited to this configuration, and for example, a thermal radiation sheet may be disposed on the side surface or end surface of the battery stack. Furthermore, the number of secondary battery cells that make up the battery stack is not limited to three, and may be four or more, or two or less.

[0025] A cross-sectional view of the thermal diffusion sheet 10 is shown in FIG. 2. The thermal diffusion sheet 10 shown in this figure is composed of a paper sheet containing 80 to 95% by weight of boron nitride as an inorganic filler and binder fibers. As the boron nitride, hexagonal boron nitride having high thermal conductivity can be suitably used. Depending on its form, it is also known to be divided into flake-like, spherical, etc., and to exist as aggregates of these. In this embodiment, flake-like boron nitride is used.

[0026] The binder fiber contains 5 to 20% by weight of organic fiber. The specific surface area of ​​the organic fiber is 50 to 200 m. 2It is preferable that the specific surface area contains at least one type of the specific surface area of ​​50 to 200 m / g. Here, the specific surface area can be measured by the BET method of JIS Z 8830:2013. 2 As the organic fiber of 1 / g, a known fibrous binder can be used, and specific examples thereof include pulp fibers or fibrid fibers of polyolefin, polyvinyl alcohol, polyacrylonitrile, polyphenylene sulfide (PPS), polyethylene terephthalate (PET), aramid, etc. Among them, aramid pulp is preferable from the viewpoints of heat resistance, electrical insulation, etc.

[0027] Further, the thermal diffusion sheet preferably contains fine fibrous cellulose. Specific examples of fine fibrous cellulose include cellulose fibers of wood pulp, rayon, etc., which have been highly beaten with a homogenizer, a disc refiner, etc., defibrated by an underwater collision method, chemically treated by a TEMPO oxidation method, etc., and combinations thereof, and generally have a fiber diameter of 1 μm or less. In the present invention, the specific surface area is 10 to 100 m. 2 / g. In this way, by using fine fibrous cellulose with a large specific surface area as the aggregate, it is possible to blend a larger amount of scaly boron nitride. If the specific surface area is less than the above range, the boron nitride will not be dispersed uniformly, and the thermal conductivity of the sheet will be insufficient. If it exceeds the above range, the fibers will be too fine, resulting in a problem of reduced yield in the manufacturing process. It is preferable that the fine fibrous cellulose is contained in an amount of 1 to 2 wt %. This provides the effect of uniformly dispersing the inorganic filler in water and improving the yield.

[0028] However, the papermaking sheet does not contain graphite, which allows it to exhibit insulating properties.

[0029] The paper sheet is preferably a wet paper sheet. The paper sheet is preferably subjected to a heat and pressure treatment. The heat and pressure treatment increases the density of the sheet, improving thermal conductivity and suppressing powder fall.

[0030] Furthermore, the average thickness of the sheet is preferably 50 to 200 μm.

[0031] The thermal diffusion sheet preferably has an average thermal conductivity of 30 W / m·K or more in the planar direction.

[0032] The density of the thermal diffusion sheet is 1.5 to 2.1 g / cm 3 It is preferable that: [Embodiments 2 and 3]

[0033] In the example of FIG. 2, the thermal diffusion sheet is formed of a single layer of sheet, but the present invention is not limited to this configuration, and the thermal diffusion sheet may be formed of a multi-layer sheet. For example, the thermal diffusion sheet 20 may be formed of a laminated sheet in which a plurality of sheets are laminated, as in the thermal diffusion sheet 20 according to the second embodiment shown in FIG. 3. Alternatively, the thermal diffusion sheet 30 may be obtained by slicing a laminate SS in which a plurality of sheets are laminated, in a direction intersecting the lamination direction, as in the thermal diffusion sheet 30 according to the third embodiment shown in FIG. 4. In the example of FIG. 4, the thermal diffusion sheet 30 is obtained by cutting in a direction perpendicular to the lamination direction of the sheets as shown by the dashed line, but it may be cut at an angle from the perpendicular direction. (Method of manufacturing thermal diffusion sheet)

[0034] The thermal diffusion sheet is manufactured by forming a sheet containing 80 to 95% by weight of flaky boron nitride, 5 to 20% by weight of organic fiber, and 1 to 2% by weight of fine fibrous cellulose by wet papermaking, and then subjecting the sheet to a heat-pressing treatment. Here, the manufacturing method of the thermal diffusion sheet will be described with reference to FIG. 5. FIG. 5 shows a schematic configuration diagram of a manufacturing apparatus 1000 for manufacturing a thermal diffusion sheet 10. The manufacturing apparatus 1000 shown in FIG. 5 includes a papermaking machine 1 for forming a papermaking slurry into a sheet shape, a dryer 3 for drying the papermaking sheet 9 formed by the papermaking machine 1, and a press machine 2 for hot-pressing the dried papermaking sheet 9 to a predetermined thickness. As shown in this figure, the thermal diffusion sheet is manufactured by a papermaking process in which a papermaking slurry in which boron nitride powder and fiber are suspended in a dispersion liquid is wet-papered to form a sheet-shaped papermaking sheet 9, and a press process in which the papermaking sheet 9 obtained in the papermaking process is hot-pressed to a predetermined thickness. (Paper making process)

[0035] The papermaking process is a process of preparing a papermaking slurry containing boron nitride, and a process of supplying a slurry to the mesh conveyor 4, where the slurry is dispersed on the papermaking surface 4A of the mesh conveyor 4. In the process of supplying a raw material, a papermaking slurry with an adjusted boron nitride content is prepared. The papermaking slurry is dispersed on the mesh conveyor 4 in the papermaking tank 5, and paper is made. In the process of supplying a slurry, the papermaking slurry is dispersed on the moving mesh conveyor 4 to form a layer, and the papermaking sheet 9 is made. In the above papermaking process, a known wet papermaking machine can be used to manufacture the papermaking sheet 9 by the wet method, but a mesh conveyor arranged in an inclined position is preferably used because of its good fiber dispersion and easy adjustment of orientation. (drying process)

[0036] The paper sheet 9 produced in the papermaking process is moved to the dryer 3 as shown in Fig. 5 and dried. The dryer 3 shown in the figure is a drum-type dryer, which dries the paper sheet 9 transported along the outer periphery of the drum 7 with hot air. The dryer 3 shown in the figure is equipped with multiple drums 7, and is structured to supply hot air of 120 to 130°C discharged from the outer periphery of each drum 7 to the paper sheet 9, thereby drying the paper sheet 9 passing through the dryer 3. However, the dryer is not limited to a drum-type dryer, and other dryers capable of drying paper sheets can also be used. (Pressing process)

[0037] The paper sheet 9 dried in the drying process is transferred to the press machine 2 and heat-pressed. The press machine 2 shown in FIG. 5 is equipped with a pair of rollers 8 arranged vertically in a horizontal position, and the paper sheet 9 is heat-pressed by passing between the pair of rollers 8. The paper sheet 9 passing between the pair of rollers 8 is heated to a predetermined temperature and pressurized to a predetermined thickness. The press machine 2 shown in FIG. 5 has a pair of rollers 8 with a diameter of 30 to 100 cm arranged in a parallel position facing each other, and is structured to press the paper sheet 9 passing between the rollers 8 with a pressure of 100 to 700 kg / cm while heating it to a temperature of 100 to 250°C. In this pressing process, the paper sheet 9 is pressed to a thickness of 50 μm to 500 μm, preferably 50 μm to 300 μm, and more preferably 70 μm to 230 μm, to become a thermal diffusion sheet 10. The thermal diffusion sheet 10 manufactured in a continuous long sheet shape is wound into a roll and shipped.

[0038] The manufacturing apparatus shown in Fig. 5 has a papermaking machine 1, a dryer 3, and a press 2 arranged in a straight line, and is configured to manufacture thermal diffusion sheets 10 on a single line. However, the manufacturing apparatus can also heat-press the paper sheet dried in the dryer by using a press machine on a separate line after winding it into a roll. This method allows the movement speed of the manufacturing line for the paper sheet manufactured in the papermaking and drying processes to be different from the movement speed of the manufacturing line for the thermal radiation sheet manufactured by heat pressing using a press machine on a separate line.

[0039] As for the pressing device, in addition to a roller press as shown in Fig. 5, known devices such as a double belt press and a flat plate press can be used. These pressing steps may be carried out alone, or may be carried out multiple times in combination with the same or different types of devices.

[0040] To make a continuous sheet of flake boron nitride, (1) Melting the resin binder and mixing it with the filler (2) A method in which a resin binder is dissolved in a solvent and mixed with a filler Of these, (1) tends to become highly viscous and does not allow for a high filler content, resulting in low thermal conductivity. Also, (2) generally uses organic solvents, which means that recovery of the solvent and disposal of the waste liquid are complicated. In contrast, the papermaking method disperses all the materials in water and can be made into sheets simply by dehydrating and drying, which places an extremely low burden on the environment and is characterized by the fact that it allows for a high filler content. EXAMPLES

[0041] The thermal diffusion sheet 10 according to the first embodiment of the present invention was manufactured by the steps shown below. [Paper making process] (Raw material adjustment process)

[0042] Scaly boron nitride powder with an average particle size of 40 μm and a specific surface area of ​​153 m 2 / g and a pulp-like para-aramid fiber with a specific surface area of ​​0.3 m 2 / g, and a PET fiber (fiber diameter 2 μm, fiber length 3 mm) with a specific surface area of ​​45 m 2 The fine fibrous cellulose having a molecular weight of 1000 to 15000g was prepared in the following mixing ratio: Water was supplied to a mixing tank, and the raw materials were then added and stirred with a propeller to prepare a papermaking slurry of a specified concentration. Papermaking slurry mix ratio: Scaly boron nitride powder: 93% by mass Aramid fiber: 1.5% by mass PET fiber: 4% by mass Fine fibrous cellulose: 1.5% by mass (Slurry supply process, drying process)

[0043] The papermaking slurry was dispersed on a mesh conveyor 4 and formed into a sheet, which was then dried in a drum dryer to form a papermaking sheet 9 having an overall thickness of 400 μm. [Pressing process]

[0044] The obtained paper sheet 9 was subjected to a heat press treatment by passing it between a pair of metal heat press rolls set at a diameter of 50 cm and a temperature of 250°C at a linear pressure of 400 kgf / cm and a speed of 20 m / min, thereby obtaining a thermal diffusion sheet according to Example 1. EXAMPLES

[0045] A thermal diffusion sheet according to Example 2 was produced in the same manner as in Example 1, except that the composition of the papermaking slurry was 90% by weight of boron nitride powder, 2% by weight of pulp-like para-aramid fiber, 6% by weight of PET fiber, and 2% by weight of fine fibrous cellulose. [Comparative Example 1]

[0046] On the other hand, as Comparative Example 1, a thermal diffusion sheet was prepared in the same manner as in Example 1, except that the composition of the papermaking slurry was 45% by weight of scaly boron nitride powder, 45% by weight of boron nitride agglomerated powder, 4% by weight of aramid pulp, 3% by weight of PPS fiber, and 3% by weight of PET fiber. [Comparative Example 2]

[0047] As Comparative Example 2, a thermal diffusion sheet was prepared in the same manner as in Example 1, except that the composition of the papermaking slurry was 40% by weight of flaky boron nitride powder, 40% by weight of boron nitride agglomerated powder, 8% by weight of aramid pulp, 8% by weight of PPS fiber, and 4% by weight of PET fiber. (Thermal Conductivity)

[0048] Heat dissipation was measured for each of the samples of Examples 1 and 2 and Comparative Examples 1 and 2. Two samples were prepared for each sample, and the thickness, density, thermal conductivity in the vertical direction (sheet thickness direction), and thermal conductivity in the in-plane direction were measured. The measurement method was a laser flash method according to JIS R 1611:2010. The measurement device used was a Xe flash analyzer LFA-447 Nanoflash. (Dust resistance)

[0049] Tests were carried out on the samples of Examples 1 and 2 and Comparative Examples 1 and 2 in accordance with the JIS H 8504:1999 tape test method. The adhesive surface of the tape peeled off from the sample was observed, and the results were recorded as × if there was any attached powder, fiber, or the like that had fallen off the sample, and ◯ if there was no attached powder, fiber, or the like. The results are shown in Table 1.

[0050] [Table 1] (heat dissipation)

[0051] As shown in Table 1, Examples 1 and 2 all achieved a thermal conductivity of 45 W / m K or more in the in-plane direction. On the other hand, Comparative Examples 1 and 2 only achieved a thermal conductivity of 25 W / m K or less. The Comparative Examples did not contain fine fibrous cellulose, and so had no choice but to add agglomerated powder with a large particle size in order to increase the yield of boron nitride. Because of its shape, the boron nitride agglomerated powder does not orient in the in-plane direction, so it is not possible to increase the thermal conductivity in the in-plane direction. [Industrial Applicability]

[0052] The thermal diffusion sheet and the method for producing the same of the present invention can be used as a thermal diffusion sheet that is sandwiched between objects that repeatedly expand and contract. For example, the sheet can be suitably used as a heat insulating spacer interposed between secondary battery cells or secondary battery cell modules, a buffer sheet interposed between an explosion-proof valve and a gas duct, or a heat insulating material that protects a drive circuit of an ECU or the like. [Explanation of symbols]

[0053] 1000…Heat radiation sheet manufacturing equipment 100...Power supply device 1…Paper machine 2. Press machine 3...Dryer 4…Mesh conveyor 4A…Paper side 5…Paper making tank 7. Drums 8…Roller 9…Paper sheet 10…Heat diffusion sheet 12...Surface sheet 20...Secondary battery cell 21...Outer can 22...Sealing plate 23...Electrode 24...Explosion-proof valve 25...Battery stack 30...End plate 40...Foundation plate SS…Laminated body

Claims

1. 80 to 95% by weight of flake boron nitride as a thermally conductive inorganic filler; 5 to 20% by weight of organic fiber as binder fiber; A thermal diffusion sheet comprising a paper sheet comprising:

2. The thermal diffusion sheet according to claim 1, further comprising: A thermal diffusion sheet comprising 1 to 2% by weight of fine fibrous cellulose.

3. The thermal diffusion sheet according to claim 1 or 2, The thermal diffusion sheet, wherein the organic fiber is an aramid fiber.

4. The thermal diffusion sheet according to any one of claims 1 to 3, The thermal diffusion sheet has an average thermal conductivity of 30 W / m·K or more in a plane direction.

5. The thermal diffusion sheet according to any one of claims 1 to 4, The thermal diffusion sheet has an average thickness of 50 to 200 μm.

6. The thermal diffusion sheet according to any one of claims 1 to 5, The density of the thermal diffusion sheet is 1.5 to 2.1 g / cm 3 A thermal diffusion sheet.

7. The thermal diffusion sheet according to any one of claims 1 to 6, The thermal diffusion sheet, wherein the papermaking sheet is a wet-laid papermaking sheet.

8. The thermal diffusion sheet according to any one of claims 1 to 7, A thermal diffusion sheet obtained by subjecting the paper sheet to a heat and pressure treatment.

9. A method for manufacturing a thermal diffusion sheet, comprising the steps of: forming a sheet by wet papermaking using 80 to 95% by weight of flaky boron nitride as a thermally conductive inorganic filler, 5 to 20% by weight of organic fiber, and 1 to 2% by weight of fine fibrous cellulose; A step of subjecting the sheet to a heat and pressure treatment; A method for producing a thermal diffusion sheet comprising the steps of:

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