Heat dissipation sheet and production method for heat dissipation sheet

Abstract

A heat dissipation sheet (100) for dissipating heat generated by a heat dissipation target, said heat dissipation sheet (100) containing a liquid metal layer (10) that is deformable in a normal temperature environment and a sealing sheet (20) that seals the liquid metal layer (10).

Description

Heat dissipation sheet and method for manufacturing the heat dissipation sheet

[0001] The present invention relates to a heat-dissipating sheet and a method for manufacturing the heat-dissipating sheet.

[0002] Patent Document 1 discloses a heat dissipation sheet made of graphite. Metals such as graphite are hard and difficult to deform, which limits the range of application of such a sheet. Patent Document 2 also discloses a heat dissipation sheet made by mixing a deformable liquid metal with a polymer. However, since the polymer is dominant, the thermal conductivity is low and sufficient heat dissipation is not achieved.

[0003] JP 2002-050725 Michael D. Bartlett, Navid Kazem, Matthew J. Powell-Palm, Xiaonan Huang, Wenhuan Sun, Jonathan A. Malen, and Carmel Majidi “High thermal conductivity in soft elastomers with elongated liquid metal inclusions” Proceedings of the National Academy of Sciences (PNAS)<URL: https: / / www.pnas.org / doi / 10.1073 / pnas.1616377114>

[0004] An object of the present invention is to provide a heat-dissipating sheet that has the ability to conform to the shape of a heat-dissipating object and has high heat-dissipating properties, and a method for manufacturing the heat-dissipating sheet.

[0005] The invention disclosed in this application to solve the above problems has various aspects, and representative aspects thereof are outlined below.

[0006] (1) A heat dissipation sheet for dissipating heat generated from a heat dissipation target, comprising: a liquid metal layer that is deformable in a room temperature environment; and a sealing sheet that seals the liquid metal layer.

[0007] (2) In (1), the liquid metal layer and the sealing sheet have stretchability and flexibility.

[0008] (3) In the heat dissipation sheet according to (1) or (2), the liquid metal layer contains a gallium-based metal.

[0009] (4) In any one of (1) to (3), the liquid metal layer contains a metal powder that has low reactivity with the liquid metal contained in the liquid metal layer.

[0010] (5) In (4), the metal powder has a higher thermal conductivity than the liquid metal.

[0011] (6) In the heat dissipation sheet according to (4) or (5), the metal powder is copper powder.

[0012] (7) In the heat dissipation sheet according to (4) or (5), the metal powder includes at least one of iron powder, nickel powder, tungsten powder, silver powder, platinum powder, and gold powder.

[0013] (8) In (6), the ratio of the weight of the copper powder to the total weight of the liquid metal layer is less than 16 wt %.

[0014] (9) In (6), the ratio of the weight of the copper powder to the total weight of the liquid metal layer is 14 wt % or less.

[0015] (10) In (6), the ratio of the weight of the copper powder to the total weight of the liquid metal layer is 12 wt % or less.

[0016] (11) In (6), the ratio of the weight of the copper powder to the total weight of the liquid metal layer is 10 wt % or less.

[0017] (12) In (6), the ratio of the weight of the copper powder to the total weight of the liquid metal layer is 7 wt % or less.

[0018] (13) In (6), the ratio of the weight of the copper powder to the total weight of the liquid metal layer is 5 wt % or less.

[0019] (14) In any one of (1) to (13), the encapsulating sheet is a polymer nanosheet having adhesiveness to the heat dissipation target.

[0020] (15) The heat dissipation sheet according to any one of (1) to (14), wherein the material of the sealing sheet is a styrene-butadiene-styrene block copolymer.

[0021] (16) In any one of (1) to (15), the sealing sheet includes a first sealing layer covering the upper surface of the liquid metal layer and a second sealing layer covering the lower surface of the liquid metal layer, and the first sealing layer and the second sealing layer are in close contact with each other at the peripheral portion of the liquid metal layer.

[0022] (17) In the heat dissipation sheet according to (16), the film thickness of the first sealing layer and the second sealing layer is 100 nm or more and 500 nm or less.

[0023] (18) In (1), the liquid metal layer includes a eutectic metal (GaInSn) of gallium (Ga), indium (In), and tin (Sn), and copper powder whose weight ratio to the total weight of the liquid metal layer is 5% or less, the material of the sealing sheet is a styrene-butadiene-styrene block copolymer, the sealing sheet includes a first sealing layer covering the upper surface of the liquid metal layer and a second sealing layer covering the lower surface of the liquid metal layer, the first sealing layer and the second sealing layer are in close contact with each other at the periphery of the liquid metal layer, and the film thickness of the first sealing layer and the second sealing layer is 100 nm or more and 500 nm or less.

[0024] (19) A method for manufacturing a heat dissipation sheet for dissipating heat generated from a heat dissipation target, comprising the steps of: preparing a first sealing layer; forming a frame portion on the first sealing layer so as to surround an area where liquid metal is to be applied; applying the liquid metal to the area; spreading the liquid metal applied to the area into a sheet-like shape corresponding to the film thickness of the frame portion; providing a second sealing layer so as to sandwich the liquid metal together with the first sealing layer; cutting the first sealing layer and the second sealing layer between the liquid metal and the frame portion to separate the portion where the frame portion is to be formed; and adhering the first sealing layer and the second sealing layer to each other at the peripheral portion of the liquid metal.

[0025] (20) A method for manufacturing a heat dissipation sheet according to (19), comprising the step of mixing metal powder into the liquid metal before the step of applying the liquid metal.

[0026] According to the above aspects (1) to (20) of the present invention, it is possible to provide a heat-dissipating sheet that has the ability to follow the shape of a heat-dissipating object and has high heat dissipation properties, and a method for manufacturing the heat-dissipating sheet.

[0027] 1 is a cross-sectional view schematically showing a heat dissipation sheet according to the present embodiment; FIG. 2 is an exploded perspective view showing a heat dissipation sheet according to the present embodiment; FIG. 3 is an experimental result showing the relationship between the number of days elapsed since copper powder was mixed into GaInSn and the state of GaInSn mixed with copper powder, for each copper powder content; FIG. 4 is a diagram showing the thermal conductivity of GaInSn itself, the heat dissipation sheet according to the present embodiment, and a heat dissipation sheet of a comparative example; FIG. 5 is a graph showing an example of experimental results for each copper powder content; FIG. 6 is a graph showing the relationship between the film thickness of a sealing sheet and the thermal conductivity of a heat dissipation sheet; FIG. 7 is a diagram showing each step in a method for manufacturing a heat dissipation sheet; FIG. 8 is a diagram showing each step in a method for manufacturing a heat dissipation sheet;

[0028] Hereinafter, an embodiment of the present invention (hereinafter referred to as the present embodiment) will be described in detail with reference to the drawings.

[0029] [Overview of Heat Dissipation Sheet 100] Fig. 1A is a cross-sectional view schematically showing a heat dissipation sheet according to this embodiment. Fig. 1B is an exploded perspective view showing a heat dissipation sheet according to this embodiment. As shown in Figs. 1A and 1B, the heat dissipation sheet 100 includes a liquid metal layer 10 and a sealing sheet 20 that seals the liquid metal layer 10. Note that in Figs. 1A and 1B, and Figs. 5A and 5B described below, the width, thickness, shape, etc. of each part are shown schematically for ease of understanding, but the present invention is not limited to these disclosures.

[0030] As disclosed in the above-mentioned Patent Document 1, heat dissipation sheets using metals such as graphite are known. However, metals such as graphite are hard and difficult to deform, which limits the range of application of such sheets as heat dissipation sheets. Therefore, in this embodiment, a deformable liquid metal is used as the heat dissipation metal.

[0031] However, considering the corrosiveness to other metals, short circuits due to leakage, and effects on the human body, it is difficult to use liquid metal as it is. Therefore, it is possible to use liquid metal by covering it with a sheet. However, because liquid metal has a relatively low thermal conductivity, a thick sheet covering the liquid metal cannot achieve sufficient heat dissipation. Furthermore, in the heat dissipation sheet disclosed in Patent Document 2, which is made by mixing liquid metal and polymer, covering the liquid metal with a polymer can solve problems such as corrosiveness, but experimental results by the inventors of the present application showed that the thermal conductivity is less than one-third of that of the liquid metal itself.

[0032] Therefore, in this embodiment, a thin polymer nanosheet is used as the encapsulating sheet 20, and a configuration is adopted in which the encapsulating sheet 20 seals the liquid metal. This suppresses corrosiveness to other metals, short circuits due to leakage, and effects on the human body, while providing shape conformability and high heat dissipation. Here, shape conformability refers to the ability to deform to conform to the shape of the heat dissipation target, and includes flexibility and stretchability. Note that the encapsulating sheet 20 is not limited to one that completely seals the liquid metal, and it may have gaps to the extent that problems such as corrosiveness to other metals, short circuits due to leakage, and effects on the human body are unlikely to occur.

[0033] The thermal dissipation sheet 100 according to this embodiment may be attached to the surface of a heat dissipation target (heat generating body) such as a circuit board or battery of an electronic device. Furthermore, because the thermal dissipation sheet 100 has shape-conforming properties, it can be used to dissipate heat generated by stretchable and flexible electronic devices such as wearable devices worn on the human body.

[0034] Specific examples of the liquid metal layer 10 and the sealing sheet 20 that constitute the heat dissipation sheet 100 will be described below.

[0035] [Liquid Metal Layer 10] In this embodiment, a eutectic metal of gallium (Ga), indium (In), and tin (Sn) (hereinafter referred to as GaInSn) is used as the liquid metal that is the material of the liquid metal layer 10. However, the liquid metal is not limited to this, and may be a metal that is in a semi-solid state (paste state) under room temperature and has deformability such as flexibility and stretchability. In other words, the liquid metal may be a metal whose melting point is below room temperature (approximately 15 to 30°C). Examples of metals that are the main components of the liquid metal include Ga (gallium, melting point 29.76°C), In (indium, melting point 156.60°C), Sn (tin, melting point 231.93°C), Pb (lead, melting point 327.46°C), and Bi (bismuth, melting point 271.40°C). The liquid metal may be an alloy of these metals, such as GaInSn, GaIn, GaSn, or BiInSn.

[0036] The liquid metal layer 10 may be in a sheet shape, as shown in Figures 1A and 1B. This shape allows heat to be transferred in the in-plane direction and heat to be released from the upper or lower surface, thereby enhancing the heat dissipation effect. In this embodiment, the thickness of the liquid metal layer 10 is set to 200 μm. However, the thickness is not limited to this, and may be 10 μm or more and 200 μm or less.

[0037] Here, GaInSn, which is a liquid metal, has high fluidity and poor wettability with respect to the encapsulating sheet 20, making it difficult to apply it in a sheet form on the encapsulating sheet 20. Therefore, in this embodiment, copper powder is mixed with GaInSn as a filler to reduce the fluidity and improve the wettability. Specifically, 5 wt % or 10 wt % of copper powder is mixed with GaInSn. Here, wt % (weight percent) refers to the ratio of the weight of copper powder to the total weight of the liquid metal layer 10 (also simply referred to as the content in this specification).

[0038] FIG. 2 shows the experimental results showing the relationship between the number of days elapsed since copper powder was mixed into GaInSn and the state of the GaInSn mixed with copper powder, for each content of copper powder.

[0039] 2, when the copper powder content is 5 wt%, GaInSn is in a liquid state with high fluidity and poor wettability on day 1. When the copper powder content is 5 wt%, GaInSn becomes a paste state on day 3 and after, and remains in that state even on day 30.

[0040] When the copper powder content was 10 wt %, the GaInSn became a paste state from the first day and became solid on the 30th day.

[0041] When the copper powder content was 15 wt %, the GaInSn became a paste state from the first day and became solid on the third day.

[0042] As can be seen from the above experimental results, when the copper powder content is low (or when no copper powder is present), the liquid state becomes poorly wettable. Therefore, it is preferable to mix copper powder with GaInSn rather than forming the liquid metal layer 10 using only GaInSn. On the other hand, when the copper powder content is high, the liquid metal layer solidifies in a short period of time. In the solidified state, it becomes difficult to apply the solidified layer to the sealing sheet 20 in a sheet-like form. The photograph in Figure 2 shows that GaInSn mixed with 10 wt% and 15 wt% copper powder solidified without spreading into a sheet-like form after 30 days.

[0043] From the above, the copper powder content should be 10 wt% or less, which allows the paste state to be maintained for several days, and is preferably 5 wt% or less. The copper powder content should be greater than 0 wt%, so it is preferable that it is, for example, 1 wt% or more and 10 wt% or less. The copper powder content may be around 5 wt% as long as it is greater than 0 wt%, and is preferably, for example, 1 wt% or more and 7 wt% or less.

[0044] The reason why the wettability improves when copper powder is mixed in is thought to be that GaInSn reacts with copper powder to form an oxidized portion. The formation of the oxidized portion makes the mixture paste-like, which makes it possible to apply the mixture in a sheet form on the sealing sheet 20. However, there is a concern that the presence of the oxidized portion may decrease the thermal conductivity.

[0045] 3A is a diagram showing the in-plane thermal conductivity of GaInSn itself, the heat dissipation sheet according to the present embodiment, and a comparative heat dissipation sheet. As shown in FIG. 3A , according to the findings and experimental results of the inventors of the present application, the in-plane thermal conductivity of GaInSn itself is 23.2 [W / m₄K], while the in-plane thermal conductivity of the heat dissipation sheet 100 containing 5 wt% copper powder is 19.8 [W / m₄K], and the in-plane thermal conductivity of the heat dissipation sheet 100 containing 10 wt% copper powder is 27.1 [W / m₄K]. Thus, the in-plane thermal conductivity of the heat dissipation sheet 100 containing 5 wt% copper powder is lower than that of GaInSn. This is thought to be due to the formation of an oxidized portion by adding copper powder and the covering with the sealing sheet 20.

[0046] However, the thermal conductivity of the thermal dissipation sheet 100 is more than 10 times that of the composite thermal dissipation sheet shown in Figure 3A, and it can be said that it has sufficient heat dissipation properties. As shown in Figure 3A, the thermal conductivity of the composite thermal dissipation sheet was 1.25 [W / mK]. The composite thermal dissipation sheet is a thermal dissipation sheet prototyped by the inventors of the present application based on the disclosure of Patent Document 2, and is made by mixing liquid metal (GaInSn) and polymer (Ecoflex (registered trademark) 00-30) in a 50:50 ratio.

[0047] On the other hand, the thermal conductivity of the heat dissipation sheet 100 containing 10 wt% copper powder was higher than that of GaInSn. This is thought to be because the oxidized liquid metal formed a layer surrounding the copper powder (hereinafter referred to as the oxide layer), and these oxide layers aggregated together, forming a thermal path through the copper powder. In other words, thermal conductivity decreases when the oxide layer is formed discretely. However, by increasing the copper powder content, the amount of aggregation of the oxide layer increases, thereby increasing the thermal path and increasing the thermal conductivity. Note that, as shown in Figure 3A, the thermal conductivity of GaInSn is 23.2 [W / mK], while the thermal conductivity of copper powder is approximately 400 [W / mK]. Thus, it is preferable for the metal powder mixed with the liquid metal to have a higher thermal conductivity than the liquid metal.

[0048] The metal powder to be mixed with the liquid metal is not limited to copper, and any metal powder with low corrosivity or reactivity to the liquid metal is preferred. This is because a metal powder with high reactivity to the liquid metal may cause a chemical reaction, such as corrosion of the liquid metal. Specifically, the metal powder may be copper (Cu), iron (Fe), nickel (Ni), tungsten (W), silver (Ag), platinum (Pt), gold (Au), or a mixture thereof. Furthermore, the powder to be mixed with the liquid metal is not limited to metal, and may be carbon (C). Furthermore, it is preferable that the metal powder has a small diameter to facilitate mixing with the liquid metal. Specifically, the diameter of the metal powder is preferably approximately 10 to 25 μm. However, this is not limited to this, and the diameter of the metal powder may be nanoscale.

[0049] Figure 3B is a graph showing an example of experimental results for each copper powder content. Specifically, the upper left of Figure 3B shows the relationship between copper powder content and thermal diffusivity. The upper right of Figure 3B shows the relationship between copper powder content and in-plane thermal conductivity. The lower left of Figure 3B shows the elongation rate of heat dissipation sheet 100 when the copper powder content is 12 wt%. The lower right of Figure 3B shows the elongation rate of heat dissipation sheet 100 when the copper powder content is 16 wt%.

[0050] From the upper graph of Fig. 3B, it can be said that the copper powder content should be 16 wt% or less, because sufficient thermal diffusivity and thermal conductivity in the in-plane direction can be obtained at copper powder contents ranging from 1 wt% to 16 wt%.

[0051] Furthermore, the lower graph in Figure 3B indicates that a copper powder content of less than 16 wt% is preferable. When the copper powder content is 12 wt%, the maximum elongation of the heat dissipation sheet 100 is approximately 270%, which means that it can be stretched up to approximately 3.7 times its original size, and therefore can be said to have sufficient extensibility. On the other hand, when the copper powder content is 16 wt%, the extensibility of the heat dissipation sheet 100 cannot be said to be sufficient. Thus, increasing the copper powder content improves thermal conductivity, but reduces extensibility.

[0052] According to the experimental results shown in the graphs of FIG. 3B, the copper powder content is preferably 1 wt% or more and less than 16 wt%. Considering that a copper powder content of 15 wt% as shown in FIG. 2 tends to solidify, the copper powder content is preferably 1 wt% or more and 14 wt% or less. The copper powder content is preferably 1 wt% or more and 12 wt% or less. The copper powder content is preferably 1 wt% or more and 10 wt% or less. The copper powder content is preferably 1 wt% or more and 7 wt% or less. The copper powder content is preferably 1 wt% or more and 5 wt% or less. The copper powder content is preferably 1 wt% or more and 3 wt% or less.

[0053] 1B , the sealing sheet 20 includes a sealing layer 21 that covers the upper surface side of the liquid metal layer 10 and a sealing layer 22 that covers the lower surface side of the liquid metal layer 10. The sealing layer 21 and the sealing layer 22 are in close contact with each other at the peripheral portion of the liquid metal layer 10.

[0054] The encapsulating sheet 20 is preferably made of a thin, stretchable, and flexible material. In this embodiment, a polymer nanosheet made of SBS (styrene-butadiene-styrene block copolymer) is used as the encapsulating sheet 20. A polymer nanosheet made of SBS has high adhesive properties. Therefore, the heat dissipation sheet 100 has adhesive properties on its upper and lower surfaces, and can be attached to a heat dissipation target without using adhesives or adhesive tape.

[0055] FIG. 4 is a graph showing the relationship between the film thickness of the encapsulating sheet and the thermal conductivity of the heat dissipation sheet. The horizontal axis of FIG. 4 shows the film thickness of the encapsulating layer 21 and the encapsulating layer 22 of the encapsulating sheet 20 made of SBS. The vertical axis of FIG. 4 shows the ratio of the thermal conductivity of the heat dissipation sheet 100 when the thermal conductivity of GaInSn is set to 1. FIG. 4 also shows the thermal conductivity in the in-plane direction (in-plane thermal conductivity) and the thermal conductivity in the vertical direction (direction perpendicular to the in-plane direction). The relationship between the film thickness of the encapsulating sheet and the thermal conductivity of the heat dissipation sheet shown in FIG. 4 was calculated using known theoretical calculations.

[0056] 4, if the thickness of the sealing layer 21 and the sealing layer 22 is greater than 1000 nm, the thermal conductivity in the in-plane direction decreases. Therefore, it is preferable that the thickness of the sealing layer 21 and the sealing layer 22 be 1000 nm or less.

[0057] Furthermore, if the thickness of the sealing layer 21 and the sealing layer 22 is greater than 500 nm, the thermal conductivity in the vertical direction drops sharply. Therefore, it is preferable that the thickness of the sealing layer 21 and the sealing layer 22 be 500 nm or less.

[0058] Furthermore, there are technical limitations on the thickness of the sealing layers 21 and 22, and they can be fabricated to a thickness of 100 nm or more. Furthermore, the sealing layers 21 and 22, which are made of SBS, have the property that the thinner they are, the better their adhesion becomes. Therefore, it is preferable that the thickness of the sealing layers 21 and 22 be 100 nm or more and 500 nm or less (the range of the strip-shaped region in FIG. 4 ). By fabricating the sealing layers 21 and 22 within this range, the thermal conductivity in the vertical direction can be maintained at approximately 80% of that of GaInSn, and the thermal conductivity in the in-plane direction can be made equivalent to that of GaInSn, while also ensuring adhesion.

[0059] The material of the encapsulating sheet 20 is not limited to SBS. The material of the encapsulating sheet 20 may be, for example, (a) a styrene-based elastomer, (b) a silicone-based elastomer, (c) an olefin-based elastomer, (d) a urethane-based elastomer, (e) a polyester-based elastomer, (f) a polyamide-based elastomer, (g) an acrylic elastomer, or (h) a rubber-modified epoxy resin. These may be used alone or in combination of two or more.

[0060] Examples of (a) styrene-based elastomers include, in addition to the above-mentioned styrene-butadiene-styrene block copolymer (SBS), styrene-ethylene-butylene-styrene block copolymer (SEBS), styrene-isoprene-styrene block copolymer (SIS), styrene-ethylene-propylene-styrene block copolymer (SEPS, a hydrogenated product of SIS), styrene-ethylene-propylene block copolymer (SEP, a hydrogenated product of styrene-isoprene block copolymer), styrene-isobutylene-styrene block copolymer (SIBS), polystyrene copolymer (PS), and the like.

[0061] (b) Silicone-based elastomers include those primarily composed of organopolysiloxane, such as polydimethylsiloxane (PDMS), polymethylphenylsiloxane, and polydiphenylsiloxane. These may be partially modified with vinyl groups, alkoxy groups, or the like. A thin film of organopolysiloxane can be obtained, for example, by treating a base material containing a siloxane compound with a curing agent to polymerize and / or crosslink the material. Depending on the type of primary reactive group in the base material, a compound having an alkenyl group can be used as the curing agent if the base material has a hydrosilyl group as the primary reactive group, and a compound having a hydrosilyl group can be used if the base material has an alkenyl group as the primary reactive group.

[0062] (c) Examples of olefin-based elastomers include copolymers of α-olefins having 2 to 20 carbon atoms, such as ethylene, propylene, 1-butene, 1-hexene, and 4-methylpentene. Specific examples include ethylene-propylene copolymer (EPR), ethylene-propylene-diene copolymer (EPDM), copolymers of α-olefins with non-conjugated dienes having 2 to 20 carbon atoms, such as dicyclopentadiene, 1,4-hexadiene, cyclooctadiene, methylenenorbornene, ethylidenenorbornene, butadiene, and isoprene, and carboxy-modified NBR obtained by copolymerizing methacrylic acid with a butadiene-acrylonitrile copolymer.

[0063] (d) Examples of urethane-based elastomers include those having structural units of a hard segment made of a low-molecular-weight glycol and a diisocyanate, and a soft segment made of a high-molecular-weight (long-chain) diol and a diisocyanate.

[0064] (e) Examples of polyester-based elastomers include those obtained by polycondensation of dicarboxylic acid or its derivatives and diol compounds or its derivatives, particularly those obtained by copolymerizing a polyester structure with a polyether structure.

[0065] (f) Examples of polyamide elastomers include polyether block amide types and polyether ester block amide types, which use polyamide for the hard segment and polyether or polyester for the soft segment.

[0066] (g) Acrylic elastomers are primarily composed of acrylic esters, and examples thereof include ethyl acrylate, butyl acrylate, methoxyethyl acrylate, and ethoxyethyl acrylate. Furthermore, cross-linking monomers such as glycidyl methacrylate and allyl glycidyl ether can be used. Furthermore, acrylonitrile and ethylene can also be copolymerized. Specific examples include acrylonitrile-butyl acrylate copolymers, acrylonitrile-butyl acrylate-ethyl acrylate copolymers, and acrylonitrile-butyl acrylate-glycidyl methacrylate copolymers.

[0067] (h) Examples of rubber-modified epoxy resins include those obtained by modifying some or all of the epoxy groups of bisphenol F epoxy resin, bisphenol A epoxy resin, salicylaldehyde epoxy resin, phenol novolac epoxy resin, or cresol novolac epoxy resin with both-end carboxylic acid-modified butadiene-acrylonitrile rubber, terminal amino-modified silicone rubber, etc. Among these, styrene-based elastomers and silicone-based elastomers are preferably used.

[0068] The encapsulating sheet 20 may contain other components such as known additives within a range that does not impair the effects of the present invention. Examples of known additives include antioxidants, weather stabilizers, heat stabilizers, lubricants, crystal nucleating agents, ultraviolet absorbers, colorants, surfactants, fillers, etc. These may be used alone or in combination of two or more.

[0069] [Method of Manufacturing Heat-Dissipating Sheet 100] An outline of the method of manufacturing heat-dissipating sheet 100 will be described with reference to Figures 5A and 5B. Figures 5A and 5B are diagrams showing the steps in the method of manufacturing the heat-dissipating sheet.

[0070] First, a 5 wt % PVA (polyvinyl alcohol) aqueous solution is applied to a 50 μm-thick PET film 31 using a gravure coater, and the PVA layer 32 is formed by drying at 80° C. Furthermore, an SBS solution, in which SBS is dissolved in a solvent, THF (tetrahydrofuran), at a concentration of 5 wt %, is applied to the PVA layer 32 to form the sealing layer 21. This produces a laminate shown in step a1. Alternatively, a laminate shown in step b1 may be produced by a similar procedure.

[0071] Next, the sealing layer 21 and the PVA layer 32 are peeled off from the PET film 31 using adhesive tape 33 (step a2). Then, liquid metal (GaInSn) 110 is applied onto the sealing layer 21 (step a3), and the liquid metal 110 is formed into a sheet shape by screen printing (step a4). The tape 33 may be, for example, a commercially available masking tape.

[0072] The tape (frame) 33 is preferably formed so as to surround the area where the liquid metal 110 is to be applied. This allows the tape 33 to function as a mask, and the liquid metal 110 can be applied to the area surrounded by the tape 33. Furthermore, by spreading the liquid metal 110 into a sheet using a screen printing technique, the film thickness of the liquid metal 110 becomes a thickness corresponding to the film thickness of 33. As described above, it is preferable that 5 wt % or 10 wt % of copper powder is mixed into the liquid metal 110 in advance. This improves the wettability of the liquid metal 110 with respect to the sealing layer 21, making it easier to form the liquid metal 110 into a sheet.

[0073] On the other hand, adhesive tape 43 is used to peel off the sealing layer 22 and the PVA layer 42 from the PET film 41 (step b2). The PVA layer 42 is then dissolved by immersing it in pure water for 5 minutes (step b3), and the mesh layer 44 is attached to the sealing layer 22 (step b4). The mesh layer 44 is used to prevent wrinkles in the sealing layer 22, and attaching the mesh layer 44 also makes it easier to handle the laminate shown in step b4.

[0074] Furthermore, the laminate produced in steps a1 to a4 is stacked on top of the laminate produced in steps b1 to b4, sandwiching the liquid metal 110 between the sealing layers 21 and 22 (step c1). The sealing layers 21 and 22 are then cut between the liquid metal 110 and the tapes 33 and 34, separating the portions where the tapes 33 and 34 will be formed (step c2). The PVA layer 32 is then dissolved, and the sealing layers 21 and 22 are tightly attached to each other around the periphery of the liquid metal 110, completing the heat dissipation sheet 100 (step c3). The mesh layer 44 shown in step c3 may be peeled off before using the heat dissipation sheet 100.

[0075] The method for manufacturing the heat dissipation sheet 100 is not limited to that described here, and the steps may be interchanged, some of the steps may be omitted, or other steps may be added.

[0076] [Evaluation of Heat Dissipation Properties of Heat Dissipation Sheet 100] Figure 6 is a graph showing an example of experimental results relating to the evaluation of the heat dissipation properties of a heat dissipation sheet (copper powder content of 5 wt%). Plots connected by a solid line in Figure 6 represent the temperature on the flexible circuit board. Plots not connected by a solid line represent the temperature when heat dissipation sheet 100 is attached to the flexible circuit board.

[0077] Furthermore, the graph on the left side of Figure 6 shows the temperature when the heat dissipation sheet and flexible circuit board are not bent, and the graph on the right side of Figure 6 shows the temperature when the heat dissipation sheet and flexible circuit board are bent with a curvature of 8.5 mm.

[0078] In this experiment, a heater was fabricated in which metal patterns were formed near the -7 mm, 0 mm, and 7 mm positions on the flexible circuit board. In addition, in this experiment, 1.0 A / 1.0 V was applied to the heater, and the state was measured and photographed with an infrared thermo camera.

[0079] As shown in Figure 6, when no heat dissipation sheet was used (w / o (without) LM (Liquid Metal) Sheet), the temperature rose significantly in the area of ​​the flexible circuit board where the metal pattern, which is the heat generating element, was formed.

[0080] Furthermore, as shown in Figure 6, when the heat dissipation sheet 100 was used (with LM Sheet), it was possible to suppress temperature rise in both the unbent and bent states. As shown in Figure 6, by using the heat dissipation sheet 100, the temperature was almost uniform at around 40°C, and it can be seen that heat was diffused in the surface direction. In this way, the heat dissipation sheet 100 can be effectively used for flexible heat-generating objects.

[0081] 7A and 7B show an example of experimental results for evaluating the heat dissipation properties of a heat dissipation sheet (containing 5 wt % copper powder). Fig. 7A shows a schematic diagram of the experiment, and Fig. 7B shows an example of the experimental results for evaluating the heat dissipation properties of the heat dissipation sheet.

[0082] In this experiment, as shown in Figure 7(a), a heat dissipation sheet 100 was attached to a stretchable substrate, and an LED, which is a heat-generating element, was placed on the heat dissipation sheet 100. Furthermore, as shown in Figure 7(b), the LED was left on for approximately 500 seconds, and then the temperature change of the LED when the LED was turned off was confirmed. Furthermore, the temperature change of the LED was also confirmed when the stretchable substrate was stretched to twice its original size.

[0083] The graph on the left side of Fig. 7(b) shows the temperature change of the LED when the stretchable substrate is not stretched and when the heat dissipation sheet 100 is used and when it is not used. The graph on the right side of Fig. 7(b) shows the temperature change of the LED when the stretchable substrate is stretched to twice its original size and when the heat dissipation sheet 100 is used and when it is not used.

[0084] As shown in Figure 7(b), when the stretchable substrate was not stretched, the maximum temperature was approximately 62°C without the heat dissipation sheet 100, while the maximum temperature was approximately 45°C with the heat dissipation sheet 100, resulting in a reduction of the LED temperature by approximately 17°C. When the stretchable substrate was stretched twice its original size, the maximum temperature was approximately 63°C without the heat dissipation sheet 100, while the maximum temperature was approximately 43°C with the heat dissipation sheet 100, resulting in a reduction of the LED temperature by approximately 20°C. As such, the LED temperature could be reduced regardless of the stretch state of the stretchable substrate (heat dissipation sheet 100). The reason the temperature could be reduced more when the stretchable substrate was stretched is thought to be because the heat dissipation sheet 100 also stretched to fit the shape of the stretchable substrate, increasing the planar area of ​​the liquid metal layer 10 and enabling more effective heat transfer in the in-plane direction. Thus, the heat dissipation sheet 100 can be effectively used for stretchable heat-generating objects.

[0085] [Summary] As described above, the heat dissipation sheet 100 according to this embodiment is composed of a liquid metal layer 10 that is deformable under room temperature conditions and a nanoscale thin film encapsulating sheet 20. This allows the sheet to conform to the shape of a heat dissipation target and achieve high heat dissipation performance. Therefore, it can be used to dissipate heat generated from a stretchable or flexible heat dissipation target. Furthermore, since the liquid metal layer 10 in the heat dissipation sheet 100 is formed in a sheet shape, it has high in-plane thermal conductivity, enabling the temperature of the heat dissipation target to be lowered uniformly throughout. Furthermore, since the heat dissipation sheet 100 has adhesive properties on its top and bottom surfaces, it can be attached to a heat dissipation target without the use of additional adhesives or the like. Furthermore, since the liquid metal layer 10 in the heat dissipation sheet 100 is sealed within the encapsulating sheet 20, it is possible to suppress corrosion to other metals, short circuits due to leakage, and adverse effects on the human body.

[0086] Although the embodiments of the present invention have been described above, the specific configurations shown in these embodiments are merely examples and are not intended to limit the technical scope of the present invention. Those skilled in the art may modify these disclosed embodiments as appropriate, and it should be understood that the technical scope of the invention disclosed in this specification also includes such modifications.

Claims

1. A heat dissipation sheet for dissipating heat generated from a heat dissipation target, comprising: a liquid metal layer that is deformable in a room temperature environment; and a sealing sheet that seals the liquid metal layer.

2. The heat dissipation sheet according to claim 1, wherein the liquid metal layer and the sealing sheet are stretchable and flexible.

3. The heat dissipation sheet according to claim 1, wherein the liquid metal layer contains a gallium-based metal.

4. The heat dissipation sheet according to claim 1, wherein the liquid metal layer contains a metal powder that has low reactivity with the liquid metal contained in the liquid metal layer.

5. The heat dissipation sheet according to claim 4, wherein the metal powder has a higher thermal conductivity than the liquid metal.

6. The heat dissipation sheet according to claim 4, wherein the metal powder is copper powder.

7. The heat dissipation sheet according to claim 4, wherein the metal powder includes at least one of iron powder, nickel powder, tungsten powder, silver powder, platinum powder, and gold powder.

8. The heat dissipation sheet according to claim 6, wherein the weight ratio of the copper powder to the total weight of the liquid metal layer is less than 16 wt %.

9. The heat dissipation sheet according to claim 6, wherein the weight ratio of the copper powder to the total weight of the liquid metal layer is 14 wt % or less.

10. The heat dissipation sheet according to claim 6, wherein the weight ratio of the copper powder to the total weight of the liquid metal layer is 12 wt % or less.

11. The heat dissipation sheet according to claim 6, wherein the weight ratio of the copper powder to the total weight of the liquid metal layer is 10 wt % or less.

12. The heat dissipation sheet according to claim 6, wherein the weight ratio of the copper powder to the total weight of the liquid metal layer is 7 wt % or less.

13. The heat dissipation sheet according to claim 6, wherein the weight ratio of the copper powder to the total weight of the liquid metal layer is 5 wt % or less.

14. The heat dissipation sheet according to claim 1, wherein the sealing sheet is a polymer nanosheet that has adhesiveness to the heat dissipation target.

15. The heat dissipation sheet according to claim 1, wherein the material of the sealing sheet is a styrene-butadiene-styrene block copolymer.

16. The heat dissipation sheet described in claim 1, wherein the sealing sheet includes a first sealing layer covering the upper surface of the liquid metal layer and a second sealing layer covering the lower surface of the liquid metal layer, and the first sealing layer and the second sealing layer are in close contact with each other at the peripheral edge of the liquid metal layer.

17. The heat dissipation sheet according to claim 12, wherein the first sealing layer and the second sealing layer have a thickness of 100 nm or more and 500 nm or less.

18. The heat dissipation sheet according to claim 1, wherein the liquid metal layer comprises a eutectic metal (GaInSn) of gallium (Ga), indium (In), and tin (Sn), and copper powder whose weight ratio to the total weight of the liquid metal layer is 5% or less; the material of the sealing sheet is a styrene-butadiene-styrene block copolymer; the sealing sheet comprises a first sealing layer covering an upper surface of the liquid metal layer and a second sealing layer covering a lower surface of the liquid metal layer; the first sealing layer and the second sealing layer are in close contact with each other at the periphery of the liquid metal layer; and the film thickness of the first sealing layer and the second sealing layer is 100 nm or more and 500 nm or less.

19. A method for manufacturing a heat dissipation sheet for dissipating heat generated from a heat dissipation target, comprising the steps of: preparing a first sealing layer; forming a frame on the first sealing layer so as to surround an area where liquid metal is to be applied; applying the liquid metal to the area; spreading the liquid metal applied to the area into a sheet shape corresponding to the film thickness of the frame; providing a second sealing layer so as to sandwich the liquid metal together with the first sealing layer; cutting the first sealing layer and the second sealing layer between the liquid metal and the frame to separate the portion where the frame will be formed; and adhering the first sealing layer and the second sealing layer to each other at the periphery of the liquid metal.

20. The method for manufacturing a heat dissipation sheet according to claim 19, further comprising the step of mixing metal powder into the liquid metal before the step of applying the liquid metal.

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