Thermally conductive sheet and manufacturing method thereof

Abstract

To provide a thermally conductive sheet having high thermal conductivity not only in the thickness direction of a sheet but also in one direction along the surface direction of the sheet.SOLUTION: A thermally conductive sheet 10 includes a scale-like filler 12 in a polymer matrix 11, the long axis direction of the scale-like filler 12 is oriented along either a first direction which is the thickness direction of the thermally conductive sheet or a second direction perpendicular to the first direction, and the horizontal axis direction perpendicular to the long axis direction on the scale-like face is oriented along the other of the first direction and the second direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a thermally conductive sheet and a method for producing the same. [Background technology]

[0002] In electronic devices such as computers, automobile parts, and mobile phones, heat sinks and other heat dissipators are commonly used to dissipate heat generated from heat-generating elements such as semiconductor elements and mechanical parts. It is known that a thermally conductive sheet is placed between the heat-generating element and the heat dissipator to improve the efficiency of heat transfer to the heat dissipator. A thermally conductive sheet generally contains a polymer matrix and a thermally conductive filler dispersed in the polymer matrix. In some thermally conductive sheets, the anisotropic filler, which has an anisotropic shape, is oriented in one direction to enhance thermal conductivity in a specific direction.

[0003] A thermally conductive sheet in which anisotropic fillers are oriented in one direction can be manufactured by, for example, creating multiple primary sheets in which anisotropic fillers such as fibrous fillers are oriented along the sheet surface direction by stretching or other methods, stacking multiple primary sheets together, and vertically slicing the resulting sheet. This manufacturing method (hereinafter also referred to as the "flow orientation method") produces a thermally conductive sheet composed of multiple stacked unit layers of minute thickness. Furthermore, the anisotropic fillers can be oriented in the thickness direction of the sheet, improving thermal conductivity in the thickness direction (see, for example, Patent Document 1). The high thermal conductivity of a thermally conductive sheet in the thickness direction enables it to efficiently dissipate heat generated by heat-generating elements inside electronic devices. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-27144 Summary of the Invention [Problem to be solved by the invention]

[0005] Meanwhile, heat spots, where temperatures rise locally, can occur inside electronic devices. To eliminate heat spots, thermal diffusion sheets with excellent thermal conductivity in the plane direction can be used. Furthermore, since the heat resistance of electronic elements generally varies depending on the type of element, for example, if elements with low heat resistance are present on a substrate, it is necessary to prevent heat transfer in that direction. In this case, it is necessary to increase the thermal conductivity in a specific direction within the plane while decreasing the thermal conductivity in other directions. However, conventional thermal diffusion sheets have poor thermal conductivity in the thickness direction, making them less efficient at transferring heat generated by the heat-generating element to the heat sink. Furthermore, because they diffuse heat isotropically, it is difficult to suppress heat transfer in a specific direction. On the other hand, thermally conductive sheets in which anisotropic fillers are oriented in the thickness direction of the sheet, such as those obtained by conventional flow orientation methods, are highly efficient at transferring heat generated by a heat generating element to a heat sink, but it is difficult to increase thermal conductivity in the direction along the surface of the sheet.

[0006] Therefore, an object of the present invention is to provide a thermally conductive sheet that has high thermal conductivity not only in the thickness direction of the sheet but also in one direction along the surface direction of the sheet. [Means for solving the problem]

[0007] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by the following constitution, and have completed the present invention. That is, the present invention provides the following [1] to

[12] . [1] A thermally conductive sheet containing a scale-like filler in a polymer matrix, A thermally conductive sheet in which the long axis direction of the scale-like filler on the scale surface is aligned along either a first direction which is the thickness direction of the thermally conductive sheet or a second direction which is perpendicular to the first direction, and the horizontal axis direction which is perpendicular to the long axis direction on the scale surface is aligned along the other of the first direction and the second direction. [2] The thermally conductive sheet according to [1] above, wherein the scale-like filler is oriented so that the long axis direction is along the first direction and the horizontal axis direction is along the second direction. [3] The thermally conductive sheet described in [1] above, wherein the scale-like filler is oriented so that the horizontal axis direction is along the first direction and the long axis direction is along the second direction. [4] A thermally conductive sheet according to any one of [1] to [3] above, wherein the first aspect ratio, expressed as the ratio of the length in the long axis direction to the length in the horizontal axis direction of the scaly filler (length in the long axis direction / length in the horizontal axis direction), is 1.5 or more. [5] The thermally conductive sheet according to any one of the above [1] to [4], wherein the average particle size of the scaly filler is 20 μm or more. [6] The thermally conductive sheet according to any one of the above [1] to [5], wherein the flaky filler contains flaky graphite powder. [7] The thermally conductive sheet according to any one of the above [1] to [6], wherein the scaly filler contains scaly boron nitride powder. [8] The thermally conductive sheet according to any one of the above [1] to [7], further comprising a fibrous filler in the polymer matrix. [9] The thermally conductive sheet according to [8] above, wherein the fibrous filler is carbon fiber.

[10] A laminated sheet has a plurality of unit layers, and at least one of the plurality of unit layers contains the scaly filler; The thermally conductive sheet according to any one of the above [1] to [9], wherein a plurality of unit layers are laminated along a third direction perpendicular to the first and second directions.

[11] The thermally conductive sheet according to any one of the above [1] to

[10] , further comprising a non-anisotropic filler in the polymer matrix.

[12] A method for producing a thermally conductive sheet according to any one of the above [1] to

[11] , preparing a mixture containing a resin that is a precursor of the polymer matrix and the scaly filler; a step of subjecting the mixture to a flow orientation treatment to obtain a primary sheet while orienting the scaly filler; a step of stacking the primary sheets to obtain a laminated block; cutting the laminated block along the lamination direction; A method for manufacturing a thermally conductive sheet comprising: [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a thermally conductive sheet that has high thermal conductivity not only in the thickness direction of the sheet but also in one direction along the surface direction of the sheet. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic perspective view showing a first embodiment of a thermally conductive sheet. [Figure 2] FIG. 2 is a schematic perspective view showing a scaly filler. [Figure 3] FIG. 2 is a schematic perspective view showing an example of a method for manufacturing a thermally conductive sheet. [Figure 4] FIG. 2 is a schematic perspective view showing a second embodiment of a thermally conductive sheet. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the thermally conductive sheet according to the embodiment of the present invention will be described in detail. [First embodiment] FIG. 1 is a schematic diagram of a thermally conductive sheet 10 according to a first embodiment, and FIG. 2 is a schematic diagram illustrating the details of a scaly filler 12. The thermally conductive sheet 10 according to the first embodiment includes a polymer matrix 11 and a scaly filler 12 dispersed in the polymer matrix 11. As shown in FIG. 2, the length direction of the scaly filler 12 on the scale surface is defined as a major axis Y, the direction perpendicular to the major axis direction on the scale surface is defined as a horizontal axis X, and the thickness direction of the scaly filler 12, which is perpendicular to the major axis Y and horizontal axis X, is defined as a thickness direction Z. The scaly filler 12 is a thermally conductive filler that enhances the thermal conductivity of the thermally conductive sheet 10.

[0011] In the thermally conductive sheet 10, the scaly filler 12 is oriented such that its major axis direction Y is aligned with a first direction, which is the thickness direction of the thermally conductive sheet 10, and its horizontal axis direction X is aligned with a second direction perpendicular to the first direction. Here, the second direction is a direction in the plane of the sheet. Therefore, the thermally conductive sheet 10 has good thermal conductivity not only in the thickness direction but also in one direction in the plane of the thermally conductive sheet 10. In this specification, a direction perpendicular to both the first and second directions is referred to as a third direction. The third direction is a direction in the plane of the thermally conductive sheet 10. The thermally conductive sheet 10 has good thermal conductivity not only in the thickness direction but also in one direction in the plane direction, which enhances the heat dissipation effect while also dissipating heat in the plane direction, making it less likely that heat spots will occur. Furthermore, because the thermal conductivity is not very high in directions other than the one direction in the plane direction, it is possible to prevent heat from being transferred in that direction, for example, if there is an element with low heat resistance on the substrate.

[0012] The thermally conductive sheet 10 may contain, in addition to the scaly filler 12, an anisotropic filler other than the scaly filler 12 as the thermally conductive filler dispersed in the polymer matrix 11, and specifically, as shown in Fig. 1, it preferably contains a fibrous filler 13. By containing the fibrous filler 13 in addition to the scaly filler 12, for example, the presence of the fibrous filler 13 between the scaly fillers 12 forms a good thermal conduction path, resulting in high thermal conductivity.

[0013] The fibrous filler 13 is oriented so that its fiber axis direction is along the first direction, which is the thickness direction of the sheet. By orienting the fibrous filler 13 along the first direction, the thermal conductive sheet 10 can further increase the thermal conductivity in the thickness direction of the sheet (first direction), making it easier to make the thermal conductivity along the first direction sufficiently higher than the thermal conductivity along the second direction.

[0014] The thermally conductive sheet 10 also preferably contains a non-anisotropic filler (not shown) as the thermally conductive filler dispersed in the polymer matrix 11. By containing the non-anisotropic filler, the thermally conductive sheet 10 has a thermally conductive filler appropriately interposed between the anisotropic fillers such as the scaly filler 12, thereby further improving the thermal conductivity. In this specification, an anisotropic filler is a filler that has anisotropy in shape and is capable of orientation. An anisotropic filler usually has one of its aspect ratios greater than 2. A non-anisotropic filler is a filler that has substantially no anisotropy in shape and does not orient in a predetermined direction even under an environment in which an anisotropic filler orients in a predetermined direction, such as under the action of a shear force described below. As described below, a non-anisotropic filler is, for example, one whose aspect ratio is 2 or less.

[0015] In the present invention, the thermally conductive filler contained in the polymer matrix 11 may be a scaly filler 12 alone, or both the scaly filler 12 and the fibrous filler 13, or a combination of the scaly filler 12 and a non-anisotropic filler. Furthermore, a combination of the scaly filler 12, the fibrous filler 13, and a non-anisotropic filler may be used.

[0016] Hereinafter, the materials constituting the thermally conductive sheet according to this embodiment will be described in more detail. (polymer matrix) The polymer matrix 11 is a member that holds a thermally conductive filler such as the scale-like filler 12, and is preferably made of a flexible rubber-like elastic material. The polymer matrix is ​​formed from a resin that is its precursor. Note that the term "precursor" as used herein refers not only to a substance that becomes the polymer matrix 11 by reacting as described below, but also to a substance that does not react and is the same as the polymer matrix 11. In order to incorporate an anisotropic filler such as the scaly filler 12 into the polymer matrix 11 in an oriented state, the resin must be fluid during the orientation process. For example, if the resin precursor of the polymer matrix 11 is a thermoplastic resin, the anisotropic filler can be oriented by heating and plasticizing it. Alternatively, if a reactive liquid resin is used, the anisotropic filler can be oriented before curing and then cured while maintaining that state to obtain a cured product with an oriented anisotropic filler. Thermoplastic resins have a relatively high viscosity, and plasticizing them to a low viscosity can cause thermal degradation of the resin, so reactive liquid resins are preferred.

[0017] The reactive liquid resin is preferably a rubber or gel that is liquid before the reaction and hardens under predetermined conditions to form a crosslinked structure. A crosslinked structure refers to a structure in which at least a portion of the polymer is crosslinked three-dimensionally to form a hardened product that does not melt when heated. Furthermore, since a mixed composition is prepared by adding an anisotropic filler to the liquid resin and orienting the filler in the fluid liquid resin, the resin preferably has low viscosity and is capable of hardening under predetermined conditions after orientation.

[0018] Examples of the curing method for such reactive liquid resins include thermosetting and photosetting, but since they contain a large amount of filler such as a scaly filler that blocks light, it is preferable to use thermosetting rubber or gel. More specifically, examples include silicone resin, urethane rubber that utilizes the reaction of polyol and isocyanate, and acrylic rubber that utilizes the radical reaction or cationic reaction of acrylate, but it is preferable to use silicone resin.

[0019] The silicone resin is not particularly limited as long as it is an organopolysiloxane, but it is preferable to use a curable silicone resin. When the silicone resin is a curable type, it is obtained by curing a curable silicone composition. The silicone resin may be an addition reaction type or other types. In the case of an addition reaction type, the curable silicone composition preferably comprises a silicone compound as a main component and a curing agent that cures the main component.

[0020] The silicone compound used as the main agent is preferably an alkenyl group-containing organopolysiloxane, and specific examples include vinyl group-containing organopolysiloxanes such as vinyl group-containing polydimethylsiloxane, vinyl group-containing polyphenylmethylsiloxane, vinyl group-containing dimethylsiloxane-diphenylsiloxane copolymer, vinyl group-containing dimethylsiloxane-phenylmethylsiloxane copolymer, and vinyl group-containing dimethylsiloxane-diethylsiloxane copolymer.

[0021] The curing agent is not particularly limited as long as it can cure the silicone compound that is the main component, but organohydrogenpolysiloxane, which is an organopolysiloxane having two or more hydrosilyl groups (SiH), is preferred. The hardness of the primary sheet (described later) can be adjusted by adjusting the number of hydrosilyl groups, molecular weight, and compounding ratio of the curing agent to the base resin. Specifically, the hardness of the primary sheet can be reduced by using a curing agent with fewer hydrosilyl groups per molecule or a large molecular weight, or by reducing the compounding ratio of the curing agent to the base resin.

[0022] The content of the polymer matrix in the thermally conductive sheet, expressed in volume % (filling rate), is preferably 15 to 50 volume %, more preferably 20 to 45 volume %, relative to the total volume of the thermally conductive sheet.

[0023] (scaly filler) It is preferable that the scaly filler 12 has a first aspect ratio, which is expressed as the ratio of the length in the long axis direction Y to the length in the horizontal axis direction X (length in the long axis direction Y / length in the horizontal axis direction X), of 1.5 or more. By setting the first aspect ratio to 1.5 or more, the thermal conductivity in the first direction (thickness direction) can be made significantly higher than the thermal conductivity in the second direction (one of the surface directions). This prevents excessive heat transfer in the surface direction while increasing the thermal conductivity in the thickness direction, making it easier to improve the heat dissipation effect. Furthermore, from the viewpoint of making the thermal conductivity in the first direction (thickness direction) sufficiently higher than the thermal conductivity along the surface direction, the first aspect ratio is more preferably 1.7 or more. However, the first aspect ratio need only be 1 or greater. If the first aspect ratio is, for example, less than 1.5, it becomes difficult to achieve a significant difference in thermal conductivity between the first direction and the second direction, but it is suitable for applications requiring high thermal conductivity in both the thickness direction and the surface direction. The first aspect ratio is, for example, 5 or less, preferably 3 or less, and more preferably 2.5 or less, in order to impart a certain level of thermal conductivity in the second direction as well.

[0024] From the viewpoint of facilitating orientation in the first direction (thickness direction) and enhancing thermal conductivity, the scaly filler 12 preferably has a second aspect ratio, expressed as the ratio of the length in the major axis direction Y to the length in the thickness direction Z (length in the major axis direction Y / length in the thickness direction Z), of 3 or more, and more preferably 6 to 300. Furthermore, in order to reduce the viscosity of the mixture containing the various materials, the second aspect ratio is more preferably 8 to 15. On the other hand, from the viewpoints of preventing the scaly filler 12 from falling off from the cured product and enhancing thermal conductivity, the second aspect ratio is more preferably 15 to 300. The second aspect ratio is typically greater than the first aspect ratio.

[0025] Furthermore, the average particle size of the scaly filler 12 is preferably 20 μm or more. The average particle size is the average length of the major axis direction Y. When the average particle size is 20 μm or more, the scaly filler 12 is more easily oriented along the first direction (thickness direction), and the fillers are more easily brought into contact with each other, ensuring a heat transfer path and making it easier to increase thermal conductivity, particularly thermal conductivity in the first direction. From the viewpoint of improving thermal conductivity, the average particle size of the scaly filler 12 is more preferably 30 μm or more, even more preferably 40 μm or more, and even more preferably 60 μm or more. Furthermore, in order to reduce the bulk of the scaly filler 12 and facilitate high loading in the polymer matrix 11, the average particle size of the scaly filler 12 is preferably 400 μm or less, more preferably 300 μm or less, even more preferably 200 μm or less, and even more preferably 150 μm or less. One type of scaly filler 12 may be used alone, or two or more types may be used in combination. For example, the scaly filler 12 may have at least two different average particle sizes.

[0026] The aspect ratio (first and second aspect ratios) and average particle size of the scaly filler 12 can be determined by observing the scaly filler 12 with a microscope and measuring each length. For example, the matrix component of the thermally conductive sheet 10 is dissolved to separate the scaly filler 12. The longitudinal lengths of 50 randomly selected scaly fillers 12 are measured using an electron microscope or optical microscope, and the average (arithmetic mean) of the measured lengths is used as the average particle size. In this case, care must be taken to avoid applying a large shear force to the scaly filler 12 so as not to crush it. If it is difficult to separate the scaly filler 12 from the thermally conductive sheet 10, the longitudinal lengths of the scaly filler 12 in the longitudinal direction Y can be measured using an X-ray CT scanner, and the average (arithmetic mean) of the measured lengths can be used as the average particle size. Similarly, the length in the longitudinal direction Y, the length in the transverse direction X, and the length in the thickness direction Z (i.e., the thickness) of any 50 scale-like fillers 12 can be measured, and the first and second aspect ratios can be calculated from the ratio of the average values ​​(arithmetic mean values). In this specification, "arbitrary" means something selected at random.

[0027] Examples of the scaly filler 12 include scaly carbon powder, scaly silicon carbide powder, scaly aluminum nitride powder, scaly boron nitride powder, and scaly aluminum oxide powder. Among these, from the viewpoint of thermal conductivity, at least one selected from scaly graphite powder and scaly boron nitride powder is preferred. Furthermore, from the viewpoint of improving thermal conductivity, particularly thermal conductivity in the first direction, scaly graphite powder is more preferred as the scaly filler 12. In flake graphite powder, the crystal planes of graphite are aligned in the in-plane direction of the flake surfaces, and the flake surfaces have high thermal conductivity in the in-plane direction. Therefore, by aligning the flake surfaces in a specific direction, the thermal conductivity in a specific direction can be increased. Preferably, the flake graphite powder has a high degree of graphitization.

[0028] The content of the scaly filler 12 in the thermally conductive sheet 10 is preferably 8 to 400 parts by mass per 100 parts by mass of the polymer matrix. By setting the content of the scaly filler 12 to 8 parts by mass or more, the thermal conductivity in the first and second directions is easily increased, while by setting the content to 400 parts by mass or less, the viscosity of the liquid composition described below is easily adjusted, resulting in good orientation of the scaly filler 12. From these perspectives, the content of the scaly filler 12 in the thermally conductive sheet 10 is more preferably 40 to 300 parts by mass, and even more preferably 70 to 200 parts by mass. Furthermore, the content of the scaly filler 12, expressed as a volume-based filling rate (volume filling rate), is preferably 5 to 50 volume %, more preferably 8 to 40 volume %, and even more preferably 13 to 30 volume %, relative to the total amount of the thermally conductive sheet. As mentioned above, the thermally conductive sheet 10 may be used in combination with other anisotropic fillers such as the fibrous filler 13, but when the scale-like filler 12 is used in combination with the fibrous filler 13, the preferred total amount of the scale-like filler 12 and the fibrous filler 13 is as described below.

[0029] As described above, the scaly filler 12 is oriented such that the major axis direction Y is aligned with the first direction of the thermally conductive sheet 10 and the horizontal axis direction X is aligned with the second direction of the thermally conductive sheet 10. Here, the long axis direction Y being along the first direction means that the proportion of the number of scale-like fillers 12 whose angle (orientation angle) between the long axis direction Y and the first direction of the thermal conductive sheet 10 is less than 30° exceeds 50% of the total amount of scale-like fillers, and this proportion preferably exceeds 80%. Furthermore, the horizontal axis direction X being along the second direction means that the proportion of the number of scale-like fillers 12 whose angle with the horizontal axis direction X relative to the second direction of the thermally conductive sheet 10 is less than 30° exceeds 50% of the total amount of scale-like fillers, and this proportion preferably exceeds 80%. In addition, from the viewpoint of increasing the thermal conductivity in the first direction, it is preferable that the angle (orientation angle) formed by the long axis direction Y of the scaly filler 12 with respect to the first direction be greater than or equal to 0° and less than 30°, and this angle is the average value of the orientation angles of a certain number of scaly fillers 12 (for example, 50 arbitrary scaly fillers 12). Furthermore, from the viewpoint of increasing the thermal conductivity in the second direction, it is preferable that the angle formed by the horizontal axis direction X of the scaly filler 12 with respect to the second direction be greater than or equal to 0° and less than 30°, and this angle is the average value of the angles formed by a certain number of scaly fillers 12 (for example, 50 arbitrary scaly fillers 12).

[0030] (fibrous filler) As described above, the thermally conductive sheet 10 preferably contains a fibrous filler 13 dispersed in a polymer matrix 11. From the viewpoint of enhancing thermal conductivity by facilitating the orientation of the fiber axis direction of the fibrous filler 13 in the first direction, the aspect ratio of the fibrous filler 13 is preferably 4 or more, more preferably 7 to 100, and even more preferably 15 to 50. The aspect ratio means the length in the fiber axis direction of the fibrous filler 13 (fiber length) / fiber diameter.

[0031] In this embodiment, the first aspect ratio of the scaly filler 12 and the aspect ratio of the fibrous filler 13 can also be said to be the ratio of the length of the anisotropic filler in the first direction to the length of the anisotropic filler in the second direction. Therefore, the weighted average value of the first aspect ratio of the scaly filler 12 and the aspect ratio of the fibrous filler 13 (also referred to as the "aspect ratio in the first direction / second direction") can be said to be a ratio indicating the degree to which the anisotropic filler is oriented in the first direction relative to the second direction. The weighted average value of the aspect ratio is the average value obtained by weighting the aspect ratio of each anisotropic filler (the first aspect ratio for the scaly filler 12, and the aspect ratio for the fibrous filler 13) by the amount of blending (volume ratio). Specifically, the aspect ratio of the first direction / second direction may be 1 or more, preferably 1.5 or more, more preferably 1.7 or more, and even more preferably 3 or more. In this embodiment, an aspect ratio of 1.5 or more increases the thermal conductivity in the thickness direction, improving the heat dissipation effect when used in electronic devices and the like. Furthermore, the aspect ratio of the first direction / second direction is preferably 8 or less, more preferably 7 or less, and even more preferably 5 or less. In this embodiment, an aspect ratio of 8 or less increases the thermal conductivity in the surface direction, making it easier to prevent heat spots and the like.

[0032] The average fiber length of the fibrous filler 13 is preferably 20 to 500 μm, and more preferably 80 to 400 μm. When the average fiber length is 20 μm or more, the fillers in the thermally conductive sheet are in proper contact with each other, ensuring a heat transfer path and improving the thermal conductivity of the thermally conductive sheet 10. On the other hand, when the average fiber length is 500 μm or less, the bulk of the fibrous filler 13 is reduced, allowing for high packing. Furthermore, even if a conductive fibrous filler 13 is used, the conductivity of the thermally conductive sheet 10 is prevented from becoming higher than necessary. The average fiber length can be calculated by observing the fibrous filler 13 under a microscope. For example, the matrix component of the thermally conductive sheet 10 is dissolved and the fiber lengths of 50 randomly selected fibrous fillers 13 are measured using an electron microscope or optical microscope, and the average (arithmetic mean) of the measured fiber lengths can be used as the average fiber length. In this case, care must be taken to avoid applying a large shear force so as not to crush the fibers. Furthermore, if it is difficult to separate the fibrous filler 13 from the thermally conductive sheet 10, the fiber lengths of the fibrous filler 13 can be measured using an X-ray CT scanner, and the average fiber length can be calculated. Similarly, the diameter of the fibrous filler 13 can be measured using an electron microscope, an optical microscope, or an X-ray CT device.

[0033] Examples of the fibrous filler 13 include carbon fiber, metal fiber, ceramic fiber, polyparaphenylene benzoxazole fiber, etc. Among these, carbon fiber is preferred. As the carbon fiber, graphitized carbon fiber is preferred. Graphitized carbon fiber has graphite crystal planes aligned in the fiber axis direction, providing high thermal conductivity in the fiber axis direction. Therefore, by aligning the fiber axis direction in a specific direction, thermal conductivity in a specific direction can be increased. Graphitized carbon fiber with a high degree of graphitization is preferred.

[0034] As the graphitized carbon material such as the graphitized carbon fiber described above, the following raw materials can be graphitized. Examples include condensed polycyclic hydrocarbon compounds such as naphthalene, condensed heterocyclic compounds such as PAN (polyacrylonitrile), and pitch. However, it is particularly preferable to use graphitized mesophase pitch, polyimide, or polybenzazole, which have a high degree of graphitization. For example, by using mesophase pitch, the pitch is anisotropically oriented in the fiber axis direction in the spinning process described below, thereby obtaining graphitized carbon fibers with excellent thermal conductivity in the fiber axis direction. The use of mesophase pitch in graphitized carbon fiber is not particularly limited as long as it is spinnable, and mesophase pitch may be used alone or in combination with other raw materials. However, using mesophase pitch alone, that is, graphitized carbon fiber containing 100% mesophase pitch, is most preferred from the viewpoints of high thermal conductivity, spinnability, and stable quality.

[0035] The graphitized carbon fiber can be one that has been subjected to spinning, infusibilization, and carbonization in sequence, and then pulverized or cut to a predetermined particle size before graphitization, or one that has been carbonized, pulverized, or cut and then graphitized. When pulverization or cutting is performed before graphitization, condensation polymerization reactions and cyclization reactions are more likely to proceed on the newly exposed surface during the graphitization treatment, thereby increasing the degree of graphitization and enabling the production of graphitized carbon fibers with even improved thermal conductivity. On the other hand, when spun carbon fiber is pulverized after graphitization, the graphitized carbon fiber is stiff and therefore easy to pulverize, and carbon fiber powder with a relatively narrow fiber length distribution can be obtained by pulverization for a short period of time. One type of fibrous filler 13 may be used alone, or two or more types may be used in combination. For example, as the fibrous filler 13, at least two fillers having different average fiber lengths may be used.

[0036] As described above, the fiber axis direction of the fibrous filler 13 is oriented along the first direction. Here, the fiber axis direction along the first direction means that the proportion of the number of fibrous fillers 13 whose major axes form an angle of less than 30° with respect to the first direction exceeds 50% of the total amount of fibrous filler, and this proportion preferably exceeds 80%. In order to increase thermal conductivity, it is preferable that the orientation direction of the fibrous filler 13 be such that the angle (orientation angle) between the fiber axis direction of the fibrous filler 13 and the first direction is greater than or equal to 0° and less than 5°, and this angle is the average value of the orientation angles of a certain number of fibrous fillers 13 (for example, 50 pieces of arbitrary fibrous fillers 13).

[0037] When the scaly filler 12 and fibrous filler 13 are contained, the mass ratio of the scaly filler 12 to the fibrous filler 13 (scaly filler / fibrous filler) is preferably 20 / 80 to 95 / 5, more preferably 30 / 70 to 90 / 10, and even more preferably 55 / 45 to 80 / 20. By setting the mass ratio to 20 / 80 or more, the amount of the scaly filler 12 can be maintained at a certain level or more, which makes it easier to improve thermal conductivity not only in the first direction but also in the second direction. Furthermore, by setting the mass ratio to 95 / 5 or less, the effect of the fibrous filler 13 can be more easily achieved, making it easier to improve thermal conductivity in the first direction, for example.

[0038] The total content of the scaly filler 12 and the fibrous filler 13 in the thermally conductive sheet 10 is preferably 10 to 500 parts by mass relative to 100 parts by mass of the polymer matrix. By setting the total content to 10 parts by mass or more, it becomes easier to increase the thermal conductivity, and by setting it to 500 parts by mass or less, it becomes easier to achieve an appropriate viscosity for the liquid composition described below, resulting in good orientation of the fillers. From these viewpoints, the total content of the scaly filler 12 and the fibrous filler 13 in the thermally conductive sheet 10 is more preferably 50 to 350 parts by mass, and even more preferably 80 to 250 parts by mass. Furthermore, the total content, expressed as a volume-based filling rate (volume filling rate), is preferably 2 to 50 volume %, more preferably 8 to 40 volume %, and even more preferably 15 to 30 volume % relative to the total amount of the thermally conductive sheet.

[0039] The thermal conductivity of the scaly filler 12 and the fibrous filler 13 along the anisotropic direction (i.e., the long axis direction, the fiber axis direction) is generally, but not limited to, 30 W / (m·K) or more, and preferably 100 W / (m·K) or more. The upper limit of the thermal conductivity is not particularly limited, but is, for example, 2000 W / (m·K) or less. The thermal conductivity is measured by the laser flash method. Furthermore, the scaly filler 12 and the fibrous filler 13 may be conductive or insulating. If the scaly filler 12 and the fibrous filler 13 are insulating, the insulation in the thickness direction of the thermally conductive sheet 10 can be improved in this embodiment, making it suitable for use in electrical devices. In the present invention, being conductive means, for example, a filler having a volume resistivity of 1×10 9 Ω·cm or less. Also, having insulating properties means that the volume resistivity is, for example, 1×10 9 This refers to a case where the resistance exceeds Ω·cm.

[0040] (non-anisotropic filler) As described above, the thermally conductive sheet 10 preferably contains a non-anisotropic filler (not shown) in the polymer matrix 11. The non-anisotropic filler is a material that imparts thermal conductivity to the thermally conductive sheet 10 together with anisotropic fillers such as the scaly fillers 12. By including the non-anisotropic filler, the filler is interposed between the oriented anisotropic fillers such as the scaly fillers 12, resulting in a thermally conductive sheet with higher thermal conductivity. A non-anisotropic filler is a filler that has substantially no anisotropy in shape, and is a filler that does not orient in a predetermined direction even in an environment in which an anisotropic filler such as the scaly filler 12 orients in a predetermined direction, such as under the action of shear force described below.

[0041] The aspect ratio of the non-anisotropic filler is preferably less than 2, and more preferably 1.5 or less. By setting the aspect ratio to less than 2, an increase in the viscosity of the liquid composition described below can be prevented, enabling high filling.

[0042] The non-anisotropic filler may be conductive but is preferably insulating, and in the thermally conductive sheet 10, the fillers (i.e., the scaly filler 12, or the scaly filler 12 and the fibrous filler 13, and the non-anisotropic filler) to be blended are preferably insulating. If these are insulating, in this embodiment, the insulation of the thermally conductive sheet 10 in the thickness direction is more easily improved.

[0043] Examples of non-anisotropic fillers include metals, metal oxides, metal nitrides, metal hydroxides, carbon materials, oxides, nitrides, carbides, etc. The shape of the non-anisotropic filler may be spherical or irregular powder. In the non-anisotropic filler, examples of metals include aluminum, copper, nickel, etc.; examples of metal oxides include aluminum oxide, such as alumina, magnesium oxide, and zinc oxide; and examples of metal nitrides include aluminum nitride. Examples of metal hydroxides include aluminum hydroxide. Furthermore, examples of carbon materials include spherical graphite. Examples of oxides, nitrides, and carbides other than metals include quartz, boron nitride, and silicon carbide. Among the above, examples of non-anisotropic fillers having insulating properties include metal oxides, metal nitrides, metal hydroxides, and metal carbides. Among the non-anisotropic fillers listed above, aluminum oxide and aluminum are preferred because they have high thermal conductivity and are readily available in spherical form, while aluminum hydroxide is preferred because it is readily available and can enhance the flame retardancy of the thermally conductive sheet. Of these, aluminum oxide is more preferred.

[0044] The average particle size of the non-anisotropic filler is preferably 0.1 to 50 μm, more preferably 0.5 to 35 μm, and particularly preferably 1 to 20 μm. By setting the average particle size to 50 μm or less, problems such as disturbance of the orientation of an anisotropic filler such as a scaly filler are less likely to occur. Furthermore, by setting the average particle size to 0.1 μm or more, the specific surface area of ​​the non-anisotropic filler does not become larger than necessary, and even if a large amount is added, the viscosity of the liquid composition is less likely to increase, making it easier to load a large amount of the non-anisotropic filler. The average particle size of the non-anisotropic filler can be measured by observation using an electron microscope, etc. More specifically, similar to the measurements of the scaly filler 12 and the fibrous filler 13, the particle sizes of 50 random non-anisotropic fillers can be measured using an electron microscope, optical microscope, or X-ray CT device, and the average value (arithmetic mean value) can be used as the average particle size. The non-anisotropic filler may be used alone or in combination of two or more. When two or more types of each filler are used, the average particle size of each filler is calculated without distinguishing between them.

[0045] The content of the non-anisotropic filler in the thermally conductive sheet 10 is preferably in the range of 50 to 1500 parts by weight, more preferably 200 to 800 parts by weight, and even more preferably 250 to 550 parts by weight, per 100 parts by weight of the polymer matrix. By setting the content at 50 parts by weight or more, the amount of non-anisotropic filler present in the gaps between the anisotropic fillers, such as the scaly filler 12, is at a certain level or greater, resulting in good thermal conductivity. On the other hand, by setting the content at 1500 parts by weight or less, the effect of improving thermal conductivity according to the content can be obtained, and the non-anisotropic filler does not inhibit the thermal conduction of the anisotropic fillers, such as the scaly filler 12. Furthermore, by setting the content within the range of 200 to 800 parts by weight, the thermal conductivity of the thermally conductive sheet 10 is excellent, and the viscosity of the liquid composition is also favorable. The content of the non-anisotropic filler, expressed in volume %, is preferably 10 to 75% by volume, more preferably 30 to 60% by volume, and even more preferably 35 to 50% by volume, based on the total volume of the thermally conductive sheet.

[0046] (Additional ingredients) In the thermally conductive sheet 10, various additives may be further blended into the polymer matrix 11 to the extent that the functionality of the thermally conductive sheet 10 is not impaired. Examples of additives include at least one selected from dispersants, coupling agents, adhesives, flame retardants, antioxidants, colorants, and anti-settling agents. Furthermore, when curing the curable silicone composition as described above, a curing catalyst that accelerates curing may be blended as an additive. Examples of curing catalysts include platinum-based catalysts.

[0047] [Unit Layer] The thermally conductive sheet 10 is manufactured by, but is not limited to, the manufacturing method described below, and is composed of a plurality of unit layers 14. Each unit layer 14 in the thermally conductive sheet 10 contains a scale-like filler 12. As shown in FIG. 1 , the plurality of unit layers 14 are stacked along a third direction, and adjacent unit layers 14 are bonded to each other.

[0048] Each unit layer 14 may contain, as the thermally conductive filler, only the scaly filler 12, or may contain both the scaly filler 12 and the fibrous filler 13, or may contain the scaly filler 12 and a non-anisotropic filler (not shown in FIG. 1).Furthermore, it may contain the scaly filler 12, the fibrous filler 13, and the non-anisotropic filler. Furthermore, each unit layer 14 has substantially the same composition. Therefore, the contents of the scaly filler 12, fibrous filler 13, non-anisotropic filler, and polymer matrix in each unit layer 14 are the same as those in the thermally conductive sheet, and the contents and filling rates of the scaly filler 12, fibrous filler 13, non-anisotropic filler, and polymer matrix 11 in each unit layer 14 are also as described above.

[0049] In each unit layer 14, the scale-like filler 12 is oriented so that the major axis direction Y is along the first direction and the horizontal axis direction X is along the second direction, as described above. Furthermore, when the thermally conductive sheet 10 contains a fibrous filler 13, the fibrous filler 13 is oriented so that the fiber axis direction is along the first direction in each unit layer 14. Furthermore, in each unit layer 14, the polymer matrix 11 is a component that holds the thermally conductive filler described above, and in each unit layer 14, the polymer matrix 11 is blended so that each of the thermally conductive fillers described above is dispersed therein.

[0050] (thermal conductivity) The thermal conductivity of the thermally conductive sheet 10 in the first direction is, for example, 5 W / (m·K) or more, preferably 8 W / (m·K) or more, and more preferably 11 W / (m·K) or more. By setting the thermal conductivity at or above these lower limits, the thermal conductivity of the thermally conductive sheet 10 in the thickness direction can be made excellent. There is no particular upper limit, but the thermal conductivity of the thermally conductive sheet 10 in the thickness direction is, for example, 50 W / (m·K) or less. The thermal conductivity is measured according to a method in accordance with ASTM D5470-06.

[0051] The scale-like filler 12 is oriented such that its horizontal axis direction X is aligned with the second direction. Therefore, high thermal conductivity is also exhibited in the second direction. The thermal conductivity of the thermally conductive sheet 10 in the second direction is preferably 2.5 W / (m·K) or more, more preferably 3 W / (m·K) or more, and even more preferably 4.5 W / (m·K) or more. There is no upper limit to the thermal conductivity of the thermally conductive sheet 10 in the second direction, but it is, for example, 50 W / (m·K) or less.

[0052] Furthermore, by orienting the scaly filler 12 as described above, the thermal conductivity of the thermally conductive sheet 10 in the third direction (the direction perpendicular to the second direction along the surface direction) is lower than the thermal conductivity in the first and second directions. The thermal conductivity of the thermally conductive sheet 10 in the third direction is preferably less than 4.5 W / (m·K), more preferably less than 3 W / (m·K), and even more preferably less than 2.5 W / (m·K). The lower limit of the thermal conductivity of the thermally conductive sheet 10 in the third direction is not particularly limited, but is, for example, 0.2 W / (m·K) or more.

[0053] Furthermore, the thermal characteristic level in the second direction, calculated by the following formula, is preferably 10% or more. When the thermal characteristic level is 10% or more, the thermally conductive sheet 10 has anisotropic thermal conductivity in the planar direction, allowing heat to be transferred in one planar direction while preventing heat transfer in other directions. From this perspective, the thermal characteristic level in the second direction is more preferably 20% or more, and even more preferably 50% or more. Furthermore, the thermal characteristic level in the second direction may be 100% or less, but from the viewpoint of improving the thermal conductivity in the thickness direction more than the thermal conductivity in the surface direction to provide excellent heat dissipation, it is preferably 90% or less, and more preferably 80% or less. Thermal property level in the second direction (%) = (λ2-λ3) / (λ1-λ3) × 100 λ1: Thermal conductivity in the first direction λ2: Thermal conductivity in the second direction λ3: Thermal conductivity in the third direction

[0054] The thermally conductive sheet 10 has a Type E hardness of, for example, 70 or less. When the thermally conductive sheet 10 has a Type E hardness of 70 or less, flexibility is ensured, and, for example, conformability to heat generating bodies and heat dissipating bodies is improved, which tends to result in good heat dissipation properties. Furthermore, when used on an adherend with large irregularities, extreme flexibility is preferable, and the Type OO hardness of the thermally conductive sheet 10 is preferably 62 or less. With a Type OO hardness of 62 or less, the thermally conductive sheet 10 becomes an extremely flexible thermally conductive sheet, and exhibits excellent conformability to heat-generating bodies and heat-dissipating bodies. From the perspective of improving flexibility and achieving excellent conformability, the Type OO hardness of the thermally conductive sheet 10 is preferably 50 or less, and more preferably 45 or less. On the other hand, the Type OO hardness of the thermally conductive sheet 10 is not particularly limited, but is, for example, 15 or more, preferably 18 or more, and more preferably 25 or more. Furthermore, when priority is given to the handleability of the thermally conductive sheet 10, the Type E hardness of the thermally conductive sheet 10 is preferably 15 or more, and particularly preferably 35 or more. The softer the hardness of the thermally conductive sheet 10, the less stress it can exert on the heating element, the heat sink, or the substrate on which they are mounted when compressed, which is preferable. However, by setting the hardness to 15 or more on the Type OO hardness, the thermally conductive sheet 10 can be made to have good handleability and be easily attached to the adherend. In particular, a Type E hardness of 35 or more can achieve an excellent balance between handleability and softness. The above-mentioned type E hardness and type OO hardness are values ​​measured using a predetermined durometer in accordance with the method specified in ASTM D2240-05.

[0055] The type OO hardness of the thermally conductive sheet 10 and the primary sheet described below was measured in accordance with ASTM D2240-05. The type OO hardness was measured by measuring the hardness of both sides of a 10 mm test piece and calculating the average value. However, if the thickness is less than 10 mm, multiple sheets are stacked to adjust the thickness of the test piece to 10 mm, or to a thickness greater than 10 mm but closest to 10 mm.

[0056] In this embodiment, an anisotropic filler, such as the scaly filler 12, or the scaly filler 12 and the fibrous filler 13, may be exposed on either of the two surfaces 10A, 10B of the thermally conductive sheet 10. The exposed scaly filler 12, or the scaly filler 12 and the fibrous filler 13, may protrude from each of the two surfaces 10A, 10B. By exposing the anisotropic filler on each of the surfaces 10A, 10B of the thermally conductive sheet 10, each of the surfaces 10A, 10B becomes a non-adhesive surface. The two surfaces 10A, 10B of the thermally conductive sheet 10 can be cut, for example, by cutting with a blade (described later), thereby exposing the scaly filler 12, or the scaly filler 12 and the fibrous filler 13, on each of the surfaces 10A, 10B. However, one or both of the surfaces 10A and 10B may be an adhesive surface without exposing the anisotropic filler.

[0057] The thickness of the thermally conductive sheet 10 is changed as appropriate depending on the shape and application of the electronic device in which the thermally conductive sheet 10 is mounted. The thickness of the thermally conductive sheet 10 is not particularly limited, but is preferably in the range of 0.1 to 5 mm, for example.

[0058] Furthermore, the thickness of each unit layer 14 is not particularly limited, but is preferably 0.1 to 10.0 mm. By setting the thickness of each unit layer 14 within the above range, it becomes possible to orient the major axis direction Y and the horizontal axis direction X of the scale-like filler 12 along the first and second directions, respectively, by flow orientation, which will be described later. Furthermore, when a fibrous filler 13 is used, it becomes easier to orient the major axis direction of the fibrous filler 13 along the first direction. From these viewpoints, the thickness of each unit layer 14 is more preferably 0.3 to 5.0 mm, and even more preferably 0.5 to 3 mm. The thickness of the unit layer 14 is the length 14L along the third direction.

[0059] The thermally conductive sheet 10 has a compression ratio of, for example, 10 to 65%, and preferably 20 to 65%, when compressed at 0.276 MPa (=40 psi). If the compression ratio is equal to or greater than these lower limits, flexibility is increased, making it easier to compress and use the sheet inside an electronic device, etc. Furthermore, if the compression ratio is 65% or less, the unit layers 14 do not expand due to the pressure applied when stacking the unit layers 14 during production of the thermally conductive sheet 10, making it easier to properly manufacture the thermally conductive sheet 10. From the viewpoint of further improving flexibility, a compression ratio of 25% or more is more preferable. From the viewpoint of preventing expansion of the unit layers 14 during production and improving production efficiency, a compression ratio of 60% or less is preferable, and 55% or less is more preferable.

[0060] As described below, the thermally conductive sheet 10 can prevent the compression ratio from becoming too high and adjust it to within the above-mentioned predetermined range by bonding the primary sheets together using VUV irradiation. The compression ratio in the present invention is measured when the thermally conductive sheet 10 is compressed in a direction perpendicular to the bonding surface where the unit layers 14 are bonded together. Specifically, the compression ratio is measured in a first direction (thickness direction) of the thermally conductive sheet 10. The compression ratio is a parameter that indicates the ratio of the amount of compression to the initial thickness, expressed as "(T1-T2) / T1," where T1 is the initial thickness before compression and T2 is the thickness when compressed with a predetermined pressure. The compression ratio may be measured by, for example, cutting the thermally conductive sheet into a size of 10 mm x 10 mm and sandwiching the test piece between a base with a flat surface and a presser that presses in parallel.

[0061] The thermally conductive sheet 10 is used inside electronic devices, etc. Specifically, the thermally conductive sheet 10 is interposed between a heat generating element and a heat dissipating element, and transfers heat generated by the heat generating element to the heat dissipating element by thermal conduction, where the heat is dissipated from the heat dissipating element. Here, examples of the heat generating element include various electronic components used inside the electronic device, such as a CPU, a power amplifier, and a power supply. Examples of the heat dissipating element include a heat sink, a heat pump, and a metal housing of the electronic device. When the thermally conductive sheet 10 is used, for example, both surfaces 10A and 10B are in close contact with and compressed against the heat generating element and the heat dissipating element, respectively. As described above, the thermally conductive sheet 10 of this embodiment has high thermal conductivity in the first direction (thickness direction), and therefore has excellent heat dissipation properties, and also has a certain thermal conductivity in the planar direction, which makes it easy to prevent the occurrence of heat spots, etc. Furthermore, since the thermal conductivity cannot be increased in any direction other than the planar direction, it is possible to prevent heat transfer in the direction in which, for example, an electronic device contains an element with low heat resistance.

[0062] <Method for manufacturing thermally conductive sheets> Next, an example of a method for manufacturing the thermally conductive sheet 10 will be described. This manufacturing method includes a mixture preparation step of preparing a mixture containing a resin that is a precursor of a polymer matrix and at least scaly filler 12 as a thermally conductive filler, a primary sheet preparation step of subjecting the mixture to flow orientation treatment to obtain a primary sheet while orienting anisotropic fillers such as scaly filler 12, a lamination step of stacking the primary sheets to obtain a laminated block, and a cutting step of cutting the laminated block along the lamination direction. Each step is described in detail below.

[0063] (Mixture preparation process) In the mixture preparation step, a mixture (liquid composition) is prepared containing a resin that is a precursor of the polymer matrix (for example, a curable silicone composition in the case of a silicone resin) and a scaly filler 12. The mixture may further contain a fibrous filler 13, a non-anisotropic filler, or additional components. The liquid composition is usually in the form of a slurry. The components that make up the liquid composition may be mixed using, for example, a known kneader, kneading roll, mixer, or the like.

[0064] Here, the viscosity of the liquid composition is preferably 100 to 10,000 Pa·s. When the viscosity is 100 Pa·s or higher, by applying shear force to the filler in the orientation process to flow and form it into a sheet, the major axis direction Y of the scaly filler 12 is easily oriented in the flow direction (one direction in the sheet plane) and the transverse axis direction X is oriented along the sheet plane and perpendicular to the flow direction (another direction in the sheet plane). Furthermore, a viscosity of 10,000 Pa·s or lower improves coatability. From these perspectives, the viscosity of the liquid composition is more preferably 300 to 3,000 Pa·s, and even more preferably 400 to 2,000 Pa·s. The viscosity is measured using a rotational viscometer (Brookfield viscometer DV-E, spindle SC4-14) at a rotational speed of 1 rpm, and the measurement temperature is the temperature at which the liquid composition is applied.

[0065] The viscosity of the liquid composition can be adjusted by the type and amount of the thermally conductive filler described above. It can also be adjusted appropriately by adjusting the components constituting the resin. For example, when the liquid composition is a curable silicone composition, the above viscosity can be achieved by appropriately adjusting the molecular weight of the components constituting the curable silicone composition (e.g., alkenyl group-containing organopolysiloxane, organohydrogenpolysiloxane, etc.). Furthermore, an organic solvent may be blended into the liquid composition as needed to adjust the viscosity to the above range, but it is preferable not to blend an organic solvent.

[0066] (Primary sheet preparation process) In the primary sheet preparation step, the liquid composition is formed into a sheet while applying shear force to obtain a primary sheet. The liquid composition may be applied to a substrate film, for example, using a coating applicator such as a bar coater or doctor blade, or by extrusion molding or nozzle discharge. This method applies shear force along the coating direction (flow direction) of the liquid composition. By forming the liquid composition into a sheet while applying shear force, the scale-like filler 12 is oriented so that its longitudinal axis direction Y is aligned with the flow direction (one direction in the sheet plane) and its transverse axis direction X is aligned with a direction perpendicular to the flow direction (another direction in the sheet plane). Furthermore, when a fibrous filler is blended into the liquid composition, the fibrous filler 13 is oriented so that its fiber axis is aligned with the flow direction.

[0067] Next, the liquid composition formed into a sheet is cured, dried, or the like as necessary to obtain a primary sheet. In the primary sheet, as described above, the major axis direction Y of the scaly filler 12 is oriented in one plane direction, and the horizontal axis direction X is oriented in the other plane direction. Furthermore, when the liquid composition contains, for example, a curable silicone composition, the curing of the liquid composition is carried out by curing the curable silicone composition. The liquid composition may be cured by heating, for example, at a temperature of about 50 to 150°C. The heating time is, for example, about 10 minutes to 3 hours. When a solvent is blended into the curable liquid composition, the solvent may be evaporated by heating during curing.

[0068] The thickness of the primary sheet obtained by curing is preferably in the range of 0.1 to 10 mm. By setting the thickness of the primary sheet within this range, as described above, the anisotropic filler, particularly the scaly filler 12, can be properly oriented in the plane direction by shear force. Furthermore, by setting the thickness of the primary sheet to 0.1 mm or more, it can be easily peeled from the base film. Furthermore, by setting the thickness of the primary sheet to 10 mm or less, deformation of the primary sheet due to its own weight can be prevented. From these viewpoints, the thickness of the primary sheet is more preferably 0.3 to 5.0 mm, and even more preferably 0.5 to 3.0 mm.

[0069] The type OO hardness of the primary sheets is preferably 6 or more. By setting the hardness to 6 or more, the primary sheets do not expand significantly even when pressure is applied when stacking the primary sheets, making it possible to produce a laminated block with sufficient thickness. From this perspective, the type OO hardness of the primary sheets is more preferably 10 or more, and even more preferably 15 or more. Furthermore, from the viewpoint of ensuring flexibility of the resulting thermally conductive sheet 10, the type OO hardness of the primary sheet is preferably 55 or less, more preferably 50 or less, and even more preferably 40 or less. Furthermore, from the viewpoint of improving the handleability of the resulting thermally conductive sheet 10, the Type E hardness of the primary sheet is preferably 70 or less, and more preferably 40 or less. Moreover, the Type E hardness of the primary sheet is preferably 10 or more, and more preferably 30 or more.

[0070] (Lamination process) Next, the multiple primary sheets 17 obtained in the primary sheet preparation step are stacked so that the orientation directions of the anisotropic fillers are the same (see FIGS. 3(a) and 3(b)). That is, the multiple primary sheets 17 are stacked so that one direction along the longitudinal axis direction Y of the above-mentioned scale-like fillers 12 and the other direction along the transverse axis direction X are aligned with each other. The stacked multiple primary sheets 17 are then bonded together to obtain a laminated block 18. For example, when the resin of the multiple stacked primary sheets 17 is a thermoplastic resin, the laminated block 18 may be formed by press molding to melt and bond the polymer matrix 11 in the primary sheets 17. Alternatively, the primary sheets 17 may be bonded to each other by applying a known adhesive or the like between the primary sheets 17. Furthermore, if the precursor of the polymer matrix is ​​curable, multiple semi-cured primary sheets 17 may be stacked, and after stacking, each primary sheet 17 may be fully cured, and the fully cured primary sheets 17 may be bonded to each other and integrated to form a laminated block 18.

[0071] Furthermore, when the polymer matrix is ​​a silicone resin or the like, at least one surface of the obtained primary sheet 17 may be irradiated with VUV to activate at least one surface, and the primary sheets 17, 17 may be bonded together by that surface. VUV stands for vacuum ultraviolet light, and refers to ultraviolet light with a wavelength of 10 to 200 nm. Examples of VUV light sources include an excimer Xe lamp and an excimer ArF lamp.

[0072] When the primary sheet 17 contains, for example, a silicone resin (organopolysiloxane) as described above, VUV irradiation activates the irradiated surface. As described below, the primary sheet 17 is overlapped with another primary sheet 17 so that one activated surface becomes the overlapping surface, resulting in strong adhesion between the primary sheets 17. Although the mechanism is unclear, it is presumed that when the silicone resin is irradiated with VUV, the C-Si bond of the organopolysiloxane changes to an Si-O bond such as Si-OH, and the Si-O bond firmly bonds the primary sheets 17. That is, the primary sheet 17 and the primary sheet (unit layer 14, 14) are bonded together by the formation of bonds between the organopolysiloxane molecules. Furthermore, bonding the primary sheets 17 together by VUV irradiation does not significantly impair their flexibility in the direction perpendicular to the stacking direction. Therefore, the compression ratio can be easily adjusted to fall within a predetermined range. The VUV irradiation conditions are not particularly limited as long as they can activate the surface of the primary sheet 17. For example, the cumulative light amount is 5 to 100 mJ / cm 2 2 Preferably, the cumulative light amount is 10 to 50 mJ / cm 2 It is recommended to irradiate VUV so that

[0073] Here, it is sufficient that either one of the overlapping surfaces that contact each other of the primary sheets 17 has been irradiated with VUV in advance. By irradiating one surface with VUV, adjacent primary sheets 17, 17 are bonded together by that activated surface. Furthermore, from the viewpoint of further improving adhesiveness, it is preferable that both overlapping surfaces have been irradiated with VUV. That is, as shown in Figure 3(a), it is preferable that the primary sheet 17 is overlapped so that one side 17A that has been irradiated with VUV comes into contact with another primary sheet 17, and in this case, it is preferable that the other side 17B of the other primary sheet 17 that comes into contact with the one side 17A is also irradiated with VUV.

[0074] Although VUV irradiation can bond the primary sheets 17 simply by overlapping them as described above, pressure may be applied in the lamination direction of the primary sheets 17 to achieve stronger bonding. The pressure should be such that the primary sheets 17 do not deform significantly, and can be applied using, for example, a roller or press. For example, when using a roller, the pressure is preferably 0.3 to 3 kgf / 50 mm. The laminated primary sheets 17 may be heated as appropriate, for example, when applying pressure, but because primary sheets 17 activated by VUV irradiation can be bonded without heating, it is preferable not to heat the laminated primary sheets 17. Therefore, the temperature during pressing is, for example, 0 to 50°C, and preferably about 10 to 40°C.

[0075] (cutting process) Next, as shown in FIG. 3(c), the laminated block 18 is cut with a blade 19 along the lamination direction (third direction) of the primary sheets 17 to obtain the thermally conductive sheet 10. In this case, the laminated block 18 is preferably cut in a direction perpendicular to one direction (first direction) along which the longitudinal axis of the scaly filler 12 runs. The blade 19 may be, for example, a double-edged or single-edged blade such as a razor blade or a utility knife, a round blade, a wire blade, or a saw blade. The laminated block 18 is cut with the blade 19 by, for example, a push-cutting, shearing, rotating, sliding, or other method.

[0076] [Second embodiment] Next, a first embodiment of the present invention will be described with reference to FIG. In the first embodiment, the direction along the long axis direction Y of the scaly filler 12 was the thickness direction of the sheet (first direction), but as shown in Figure 4, in the thermally conductive sheet 20 of this embodiment, the direction along the long axis direction Y (see Figure 2) of the scaly filler 12 is a direction (second direction) perpendicular to the thickness direction of the sheet, and the direction along the horizontal axis direction X is the thickness direction of the sheet (first direction). With this configuration, in this embodiment, as in the first embodiment, the thermal conductivity is good not only in the thickness direction but also in one direction along the surface direction perpendicular to the thickness direction. However, since the direction along the major axis direction Y of the scaly filler 12 is the second direction, the thermal conductivity in one direction along the surface direction (second direction) is higher than the thermal conductivity in the thickness direction (first direction). Therefore, the thermal conductive sheet 20 of this embodiment can be suitably used in applications requiring high thermal conductivity along the surface direction.

[0077] Regarding thermal conductivity, the thermal conductivity in the first direction is preferably 2.5 W / (m·K) or more, more preferably 3 W / (m·K) or more, even more preferably 4.5 W / (m·K) or more, and is, for example, 50 W / (m·K) or less. The thermal conductivity in the second direction is higher than the thermal conductivity in the first direction, for example, 5 W / (m·K) or more, preferably 8 W / (m·K) or more, more preferably 11 W / (m·K) or more, and for example, 50 W / (m·K) or less. The thermal conductivity in the third direction is lower than the thermal conductivity in the first and second directions, and is preferably less than 4.5 W / (m K), more preferably less than 3 W / (m K), and even more preferably less than 2.5 W / (m K), and is, for example, 0.2 W / (m K) or greater. As mentioned above, the thermal property level in the second direction is preferably 10% or greater, but typically exceeds 100%. Furthermore, in this embodiment, the weighted average of the first aspect ratio of the scaly filler 12 and the aspect ratio of the fibrous filler 13 can be said to be the aspect ratio of the second direction / first direction. Specifically, the aspect ratio of the second direction / first direction in this embodiment may be 1 or more, preferably 1.5 or more, more preferably 1.7 or more, even more preferably 3 or more, preferably 8 or less, more preferably 7 or less, and even more preferably 5 or less.

[0078] The thermally conductive sheet 20 of this embodiment may contain other fillers such as a fibrous filler 13 and a non-anisotropic filler, as in the first embodiment. When the fibrous filler 13 is blended, the fiber axis direction of the fibrous filler 13 is preferably also oriented along the second direction. Furthermore, the physical properties and dimensions, such as the sheet thickness, the unit layer thickness 14L, the Type E hardness of the thermally conductive sheet, and the compression rate when compressed in the thickness direction at 0.276 MPa, are as described in the first embodiment above. The other configurations in the second embodiment are also the same as those in the first embodiment above, and detailed descriptions thereof will be omitted. The method for manufacturing the thermally conductive sheet 20 in this embodiment may be the same as that in the first embodiment, except that in the cutting step, the scaly filler 12 is cut in a direction perpendicular to the one direction along which the horizontal axis of the scaly filler 12 runs.

[0079] In the above description of each embodiment, the unit layers 14 in the thermally conductive sheet 20 have been described as having substantially the same composition, but the compositions of the unit layers 14 may be different from each other. For example, the contents of the scaly filler 12, or the scaly filler 12 and the fibrous filler 13, do not need to be the same in each unit layer 14, and the contents of the scaly filler 12 or the fibrous filler 13 in some unit layers 14 may be different from the contents of the scaly filler 12 or the fibrous filler 13 in other unit layers 14. Similarly, the contents of the non-anisotropic filler in some unit layers 14 may be different from the contents of the non-anisotropic filler in other unit layers 14. Furthermore, the type of at least one of the scaly filler 12, the fibrous filler 13, and the non-anisotropic filler in some unit layers 14 may be different from the types of these in the other unit layers 14. Furthermore, it is not necessary for all of the multiple unit layers 14 to contain the scaly filler 12, and some of the unit layers 14 may contain the scaly filler 12; for example, at least one of the multiple unit layers 14 may contain the scaly filler 12. That is, in each of the above embodiments, it is not necessary for the entire region of the thermally conductive sheet 20 to contain the scaly filler 12 along one of the first and second directions, and it is sufficient for a portion of the thermally conductive sheet 20 to contain the scaly filler 12 along either the first or second direction. Similarly, some of the multiple unit layers 14 may contain fibrous filler 13, while the others may not contain fibrous filler 13. Furthermore, some of the multiple unit layers 14 may contain non-anisotropic filler, while the others may not contain non-anisotropic filler.

[0080] As described above, by appropriately adjusting the content and type of the scale-like filler 12 and fibrous filler 13 in each unit layer 14, the thermal conductivity of some unit layers 14 may be made higher than the thermal conductivity of other unit layers 14. In such a case, unit layers 14 with high thermal conductivity and unit layers 14 with low thermal conductivity may be arranged alternately, but do not have to be arranged alternately.

[0081] Similarly, the conductivity of some unit layers 14 may be lower than that of other unit layers 14. In such a case, unit layers 14 with high conductivity and unit layers 14 with low conductivity may be arranged alternately, but this is not necessary. By making the conductivity of some unit layers 14 lower than that of the other unit layers 14, the some unit layers 14 with low conductivity hinder conduction along the third direction (see FIG. 1). Therefore, the conductivity of the entire thermally conductive sheet 20 in the third direction is low, making it easier to ensure insulation. Note that, in order to make it easier to ensure insulation, it is preferable that the unit layers 14 with low conductivity do not contain a conductive thermally conductive filler but contain an insulating thermally conductive filler.

[0082] Alternatively, some of the multiple unit layers 14 may be unit layers 14 with relatively high thermal conductivity, and the remaining unit layers 14 may be unit layers 14 with optical transparency. The unit layers 14 with thermal conductivity are layers containing a thermally conductive filler such as the thermally conductive filler or the scaly filler 12, as described above. On the other hand, the unit layers 14 with optical transparency may be layers that do not contain a thermally conductive filler. With this configuration, the entire thermally conductive sheet 20 has consistent thermal conductivity and optical transparency along the thickness direction. The unit layers 14 with thermal conductivity and the unit layers 14 with optical transparency may be arranged alternately, but this is not required.

[0083] Furthermore, the orientation direction of the long axis direction Y of the scaly filler 12 in each unit layer 14 does not need to be the same direction (i.e., one of the first and second directions). That is, in the present invention, it is sufficient that the orientation direction of the long axis direction Y in at least some unit layers 14 is one of the first direction or the second direction, and the orientation direction of the transverse axis direction X is the other of the first direction or the second direction. For example, the first directions in each unit layer 14 may be stacked sequentially so as to be 90° apart from each other, or may be changed by any angle.

[0084] Of course, the components other than the thermally conductive filler may be changed for each unit layer 14. For example, the type of polymer matrix 11 in some unit layers 14 may be different from the type of polymer matrix 11 in other unit layers 14. Furthermore, the presence or absence of an added component in some unit layers 14, the type and amount of the added component, etc. may be different from those of the other unit layers 14. For example, the hardness (Type E hardness or Type OO hardness) of some unit layers 14 may be made different from the hardness of other unit layers 14 by making the type or amount of silicone resin or at least part of the type or amount of thermally conductive filler of some unit layers 14 different from that of other unit layers 14. [Example]

[0085] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0086] The evaluation methods in the present examples are as follows. [Measurement of viscosity of liquid composition (mixture)] The viscosity of each liquid composition was measured using a viscometer (BROOKFIELD rotational viscometer DV-E) with a rotor of spindle SC4-14 at a rotation speed of 1 rpm and a measurement temperature of 25° C. The results are shown in Table 1.

[0087] [Thermal Conductivity] The thermal conductivity of the prepared thermally conductive sheet in the thickness direction (first direction) was measured according to ASTM D5470-06. The thermal conductivity in the second and third directions was also measured according to ASTM D5470-06. The results are shown in Table 1. The thermal conductivity in the second direction is the thermal conductivity measured on a test piece (2 mm thick) cut from the laminated block of each example described below so that the second direction is the thickness direction, and the thermal conductivity in the third direction is the thermal conductivity measured on the primary sheet (2 mm thick) of each example. The level of thermal properties in the second direction is expressed as a percentage. Specifically, it was calculated using the following formula, with the same as the first direction being "100%" and the same as the third direction being "0%." Thermal property level in the second direction (%) = (λ2-λ3) / (λ1-λ3) × 100 λ1: Thermal conductivity in the first direction λ2: Thermal conductivity in the second direction λ3: Thermal conductivity in the third direction

[0088] [Type E hardness] The Type E hardness was measured according to the standard of ASTM D2240-05 using a 10 mm test piece made by stacking five thermally conductive sheets and primary sheets obtained in each Example and Comparative Example. The results are shown in Table 1. [Compression Ratio] The thermally conductive sheets obtained in each Example and Comparative Example were prepared into samples with an outer dimension of 10 mm x 10 mm, and the compression ratio was measured when compressed at 0.276 MPa (=40 psi) as described in the specification. The results are shown in Table 1.

[0089] [Example 1] A curable silicone composition was prepared by mixing an alkenyl-containing organopolysiloxane (base resin) and a hydrogen organopolysiloxane (curing agent) (total 100 parts by weight, volumetric packing ratio 38% by volume), 180 parts by weight (volumetric packing ratio 30% by volume) of boron nitride powder (average major axis length 40 μm, first aspect ratio = 1, second aspect ratio = 4-8, thermal conductivity 100 W / (m·K)) as a scaly filler, and 340 parts by weight (volumetric packing ratio 32% by volume) of aluminum oxide (spherical, average particle size 3 μm, aspect ratio 1.0) as a non-anisotropic filler, to obtain a slurry-like liquid composition (mixture). The viscosity of the liquid composition at 25°C was 480 Pa·s.

[0090] The liquid composition was applied unidirectionally to a polyethylene terephthalate (PET) substrate film at 25°C using a bar coater as an applicator. The scale-like filler was oriented such that the long axis direction X was aligned with the application direction and the horizontal axis direction X was aligned with the sheet surface direction and perpendicular to the application direction. Next, the applied liquid composition was cured by heating at 120°C for 0.5 hours to obtain a primary sheet with a thickness of 2 mm.

[0091] Both sides of each of the obtained primary sheets were irradiated with a VUV irradiation device (trade name Excimer MINI, manufactured by Hamamatsu Photonics) at room temperature (25°C) in the atmosphere with an integrated light dose of 20 mJ / cm on the surface of the primary sheet. 2 VUV was irradiated under the conditions of Next, 100 VUV-irradiated primary sheets were stacked and pressed with a roller at a pressure of 1.6 kgf / 50 mm in an environment of 25°C to obtain a laminated block. The obtained laminated block was sliced ​​with a cutter blade parallel to the stacking direction and perpendicular to the direction along the long axis of the scale-like filler, to obtain a thermally conductive sheet with a thickness of each unit layer of 2 mm and a sheet thickness of 2 mm. In the thermal conductive sheet, the scaly filler was oriented such that the major axis was aligned along the thickness direction (first direction) and the horizontal axis was aligned along the direction perpendicular to the first direction in the sheet surface direction (second direction). This was also the case in the following examples.

[0092] Example 2 The liquid composition was prepared in the same manner as in Example 1, except that flake graphite powder (average major axis length 130 μm, first aspect ratio = 2, second aspect ratio = 6 to 13, thermal conductivity 400 W / (m·K)) was used as the flake filler, and the blending amounts of each filler were changed as shown in Table 1. The volume filling rate of the silicone resin was 38% by volume, the volume filling rate of the scaly filler was 23% by volume, the volume filling rate of the non-anisotropic filler was 39% by volume, and the viscosity of the liquid composition at 25°C was 600 Pa·s.

[0093] Example 3 The liquid composition was prepared in the same manner as in Example 1, except that flake graphite powder (average major axis length 80 μm, first aspect ratio = 1.85, second aspect ratio = 4 to 8, thermal conductivity 400 W / (m·K)) was used as the flake filler, and the blending amount of each filler was changed as shown in Table 1. The volume filling ratio of the silicone resin and each filler was the same as in Example 2, and the viscosity of the liquid composition at 25°C was 750 Pa·s.

[0094] Example 4 The liquid composition was prepared in the same manner as in Example 1, except that flake graphite powder (average major axis length 40 μm, first aspect ratio = 1.7, second aspect ratio = 3 to 6, thermal conductivity 400 W / (m·K)) was used as the flake filler, and the blending amount of each filler was changed as shown in Table 1. The volume filling ratio of the silicone resin and each filler was the same as in Example 2, and the viscosity of the liquid composition at 25°C was 940 Pa·s.

[0095] Example 5 The liquid composition was prepared in the same manner as in Example 4, except that graphitized carbon fiber (average fiber length 100 μm, aspect ratio 10, thermal conductivity 500 W / (m·K)) was further blended as a fibrous filler, and the blending amount of each filler was changed as shown in Table 1. The fibrous filler was oriented so that the fiber axis direction was along the thickness direction (first direction), and this was also the case in the following examples and comparative examples. The volume filling rate of the silicone resin was 38% by volume, the volume filling rate of the scaly filler was 9% by volume, the volume filling rate of the fibrous filler was 14% by volume, and the volume filling rate of the non-anisotropic filler was 39% by volume. The viscosity of the liquid composition at 25°C was 750 Pa·s.

[0096] Example 6 The liquid composition was prepared in the same manner as in Example 5, except that the blending amount of each filler was changed as shown in Table 1. The volume filling rate of the silicone resin was 38% by volume, the volume filling rate of the scaly filler was 14% by volume, the volume filling rate of the fibrous filler was 9% by volume, and the volume filling rate of the non-anisotropic filler was 39% by volume, and the viscosity of the liquid composition at 25°C was 540 Pa s.

[0097] Example 7 The liquid composition was prepared in the same manner as in Example 4, except that the blending amounts of each filler were changed as shown in Table 1. The volume filling rate of the silicone resin was 38% by volume, the volume filling rate of the scaly filler was 22% by volume, and the volume filling rate of the non-anisotropic filler was 40% by volume, and the viscosity of the liquid composition at 25°C was 960 Pa s.

[0098] (Comparative Example 1) The liquid composition was prepared in the same manner as in Example 1, except that no scaly filler was used and the blending amounts of each filler were changed as shown in Table 1. The volume filling rate of the silicone resin was 37% by volume, the volume filling rate of the fibrous filler was 20% by volume, and the volume filling rate of the non-anisotropic filler was 43% by volume, and the viscosity of the liquid composition at 25°C was 360 Pa s.

[0099] (Comparative Example 2) The liquid composition was prepared in the same manner as in Example 1, except that no scaly filler was used and the blending amounts of each filler were changed as shown in Table 1. The volume filling rate of the silicone resin was 38% by volume, the volume filling rate of the fibrous filler was 22% by volume, and the volume filling rate of the non-anisotropic filler was 40% by volume, and the viscosity of the liquid composition at 25°C was 450 Pa s.

[0100] [Table 1]

[0101] In the thermally conductive sheets of the above examples, the thermal conductivity was improved not only in the thickness direction (first direction) but also in one direction along the surface direction (second direction) by containing a scaly filler and orienting the scaly filler so that the major axis direction Y is the first direction and the transverse axis direction X is the second direction. As a result, the thermal conductivity in the thickness direction and one direction along the surface direction was improved, and the thermal resistance in these directions was reduced. In contrast, the thermal conductive sheets of each comparative example did not contain a scaly filler material with the longitudinal axis direction Y oriented along the first direction and the transverse axis direction X oriented along the second direction, so the thermal conductivity in both the thickness direction and one direction along the surface direction was not improved, and therefore the thermal resistance values ​​in both the thickness direction and one direction along the surface direction could not be reduced. [Explanation of symbols]

[0102] 10,20 Thermally conductive sheet 10A One side 10B The Other Side 11 Polymer matrix 12 Scaly filler 13 Fibrous fillers 14 Unit Layer 17 First Seat 18 Stacked Blocks 19 Cutlery

Claims

1. A thermally conductive sheet containing a scale-like filler in a polymer matrix, the flaky filler is flaky graphite powder, the scale-like filler is oriented such that the major axis direction of the scale surface is along either a first direction which is the thickness direction of the thermal conductive sheet or a second direction which is perpendicular to the first direction, and the horizontal axis direction which is perpendicular to the major axis direction on the scale surface is oriented along the other of the first direction and the second direction; A thermally conductive sheet, wherein the first aspect ratio of the scaly filler, expressed as the ratio of the length of the long axis direction to the length of the horizontal axis direction of the scaly filler (length of the long axis direction / length of the horizontal axis direction), is 1.5 or more and 5 or less.

2. The thermally conductive sheet according to claim 1 , wherein the scale-like filler is oriented such that the major axis direction is along the first direction and the horizontal axis direction is along the second direction.

3. The thermally conductive sheet according to claim 1 , wherein the scaly filler is oriented such that the horizontal axis direction is aligned with the first direction and the major axis direction is aligned with the second direction.

4. The thermally conductive sheet according to any one of claims 1 to 3, wherein the average particle size of the scaly filler is 20 µm or more.

5. The thermally conductive sheet according to any one of claims 1 to 4, further comprising a fibrous filler in the polymer matrix.

6. The thermally conductive sheet according to claim 5 , wherein the fibrous filler is carbon fiber.

7. The laminated sheet has a plurality of unit layers, and at least one of the plurality of unit layers includes the scaly filler material; 7. The thermally conductive sheet according to claim 1, wherein a plurality of unit layers are laminated along a third direction perpendicular to the first and second directions.

8. 8. The thermally conductive sheet according to claim 1, further comprising a non-anisotropic filler in the polymer matrix.

9. A method for producing a thermally conductive sheet according to any one of claims 1 to 8, preparing a mixture containing a resin that is a precursor of the polymer matrix and the scaly filler; a step of subjecting the mixture to a flow orientation treatment to obtain a primary sheet while orienting the scaly filler; stacking the primary sheets to obtain a laminated block; cutting the laminated block along the lamination direction; A method for manufacturing a thermally conductive sheet comprising:

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