Copper paste, wick forming method and heat pipe

Abstract

A copper paste for forming wick of a heat pipe, the copper paste containing copper particles, a thermally decomposable resin and a dispersion medium, the copper particles including a copper particle of a larger diameter having a volume-average particle diameter of 10-50 μm and a copper particle of a smaller diameter having a volume-average particle diameter of 0.1-2.0 μm.

Description

[Technical Field]

[0001] The present invention relates to a copper paste, a method for forming a wick, and a heat pipe. [Background technology]

[0002] A heat pipe is a passive heat transfer element that utilizes the evaporation and condensation of a working liquid. It contains a working liquid and a component called a "wick" that creates a capillary pump action within a sealed space. Because heat pipes can transport large amounts of heat with a small temperature difference, they have attracted attention as heat dissipation devices for small information devices such as smartphones. For example, Patent Document 1 discloses a heat pipe with a wick made of porous sintered powder. When the wick is made of porous sintered powder, a common method is to deposit sinterable metal powder (e.g., copper powder) in a predetermined location, compress it under pressure, and then sinter it by firing. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-222481 Summary of the Invention [Problem to be solved by the invention]

[0004] While pipe-shaped heat pipes have traditionally been widely used, flat heat pipes called vapor chambers have also come into use due to miniaturization and improved adhesion to the heat source. Such flat heat pipes are becoming thinner due to volume constraints in small information devices, and the wick must also be formed thinner. Furthermore, in flat heat pipes, the surface on which the wick is formed (the wick formation surface) may have a complex shape, such as an uneven surface. However, when the wick is made of porous sintered powder, conventional pressure compression methods have difficulty in producing such thin wicks and wicks with complex shapes.

[0005] Therefore, one object of the present invention is to provide a new method for forming a wick that enables easy formation of a wick even when the thickness of the desired wick is thin or when the surface on which the desired wick is formed has a complex shape. [Means for solving the problem]

[0006] As a result of the inventors' investigations, they discovered that it is possible to form a wick by printing using a copper paste consisting of a combination of two types of copper particles with different particle sizes and a thermally decomposable resin, and thus completed the present invention.

[0007] That is, one aspect of the present invention relates to the following copper paste for forming a wick of a heat pipe.

[0008] [1] A copper paste for forming a wick of a heat pipe, comprising copper particles, a thermally decomposable resin, and a dispersion medium, wherein the copper particles include large-diameter copper particles having a volume average particle size of 10 to 50 μm and small-diameter copper particles having a volume average particle size of 0.1 to 2.0 μm.

[0009] [2] The copper paste according to [1], wherein the 95% thermal decomposition temperature of the thermally decomposable resin is 350°C or lower.

[0010] [3] The copper paste according to [1] or [2], wherein the content of the large-diameter copper particles is 40 to 90 mass% based on the total mass of the copper particles, and the content of the small-diameter copper particles is 10 to 60 mass% based on the total mass of the copper particles.

[0011] [4] The copper paste according to any one of [1] to [3], wherein the content of the thermally decomposable resin is 1 to 20 parts by mass per 100 parts by mass of the copper particles.

[0012] [5] The large-diameter copper particles have a tap density of 1.0 to 4.5 g / cm 3 The copper paste according to any one of [1] to [4], wherein

[0013] [6] The copper paste according to any one of [1] to [5], wherein the viscosity of the copper paste is 10 to 120 Pa·s.

[0014] The above copper paste allows the wick to be formed by printing, so that the wick can be easily formed even when the wick is thin (for example, 70 μm or less) or when the wick formation surface has a complex shape.

[0015] Another aspect of the present invention relates to a method for forming a wick of a heat pipe, the method comprising the steps of printing the copper paste according to any one of [1] to [6] above, and sintering the copper paste.

[0016] Another aspect of the present invention relates to a heat pipe having a wick containing a sintered body of the copper paste according to any one of the above [1] to [6]. [Effects of the Invention]

[0017] According to the present invention, a new method for forming a wick can be provided that enables a wick to be easily formed even when the desired wick is thin or when the surface on which the desired wick is formed has a complex shape. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic cross-sectional view showing a heat pipe according to an embodiment of the present invention; [Figure 2] FIG. 2 is a diagram showing a cross-sectional SEM image of a sintered body (wick) of an example. DETAILED DESCRIPTION OF THE INVENTION

[0019] In this specification, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in this specification, the upper or lower limit of a numerical range in a certain stage may be replaced with the upper or lower limit of a numerical range in another stage. Furthermore, unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more types. In this specification, when multiple substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the multiple substances present in the composition, unless otherwise specified.

[0020] Preferred embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments.

[0021] <Copper paste> The copper paste of one embodiment is a wick-forming copper paste used to form a wick of a heat pipe. The copper paste contains copper particles, a thermally decomposable resin, and a dispersion medium. Each component of the copper paste is described below.

[0022] (copper particles) The copper particles include copper particles having a volume average particle size of 10 to 50 μm (large diameter copper particles) and copper particles having a volume average particle size of 0.1 to 2.0 μm (small diameter copper particles). The volume average particle size can be determined using a light scattering particle size distribution analyzer.

[0023] The volume average particle size of the large diameter copper particles may be 10 μm or more, 15 μm or more, or 20 μm or more, from the viewpoint of easily obtaining pores of a preferred size (for example, pores of 10 μm or more).The volume average particle size of the large diameter copper particles may be 50 μm or less, 45 μm or less, or 40 μm or less, from the viewpoint of easily obtaining a thinner wick after firing (for example, a wick having a thickness of 70 μm or less).From the above viewpoint, the volume average particle size of the large diameter copper particles may be 10 to 40 μm, 20 to 50 μm, or 20 to 40 μm.

[0024] The maximum diameter of the large-diameter copper particles (the diameter of the copper particles having the largest particle diameter among the large-diameter copper particles) may be 70 μm or less, 60 μm or less, or 50 μm or less, from the viewpoint of easily obtaining a thinner wick (for example, a wick having a thickness of 70 μm or less) after firing. The maximum diameter of the large-diameter copper particles is a value measured by a sieving method.

[0025] The maximum diameter of large-diameter copper particles can also be determined by the percentage of large-diameter copper particles obtained by sieving through a sieve with 63 μm openings in accordance with JIS Z 8815: 1994. The percentage of large-diameter copper particles obtained by this method may be 5.0 mass% or less, from the viewpoint of facilitating the production of a thinner wick (e.g., a wick with a thickness of 70 μm or less) after firing.

[0026] The minimum diameter of the large-diameter copper particles (the diameter of the copper particles having the smallest particle diameter among the large-diameter copper particles) may be 0.04 μm or more, 0.06 μm or more, or 0.1 μm or more, from the viewpoint of facilitating the formation of pores of a preferred size (for example, pores of 10 μm or more). The minimum diameter of the large-diameter copper particles is measured in the same manner as the maximum diameter.

[0027] The large-diameter copper particles may be, for example, spherical, blocky, needle-like, flake-like, dendritic, or approximately spherical, or may be irregular. When irregular large-diameter copper particles are used, a wick having a high porosity and moderately large pores is easily obtained. Therefore, when irregular large-diameter copper particles are used, the capillary force of the wick is easily improved. From this perspective, the large-diameter copper particles have a tap density of 0.5 g / cm or less. 3 More than 0.8g / cm 3 or more than 1.0g / cm 3 may be equal to or greater than 4.5 g / cm 3 Below, 4.3g / cm 3 or less, or 4.0 g / cm 3 or less, 1.0 to 4.5 g / cm 3 or 1.0 to 4.0 g / cm 3 Such copper particles tend to have an irregular shape. The tap density of the large-diameter copper particles is a value measured in accordance with JIS Z 2512:2012.

[0028] The content of the large-diameter copper particles may be 40% by mass or more, 60% by mass or more, 70% by mass or more, 75% by mass or more, or 80% by mass or more, based on the total mass of the copper particles, from the viewpoint of ensuring a more preferable porosity and pore size. The content of the large-diameter copper particles may be 90% by mass or less, 87% by mass or less, 85% by mass or less, or 80% by mass or less, based on the total mass of the copper particles, from the viewpoint of achieving a good balance with the amount of small-diameter copper particles added. From the above viewpoints, the content of the large-diameter copper particles may be 40 to 90% by mass, 60 to 87% by mass, 70 to 85% by mass, 75 to 80% by mass, or 80 to 85% by mass, based on the total mass of the copper particles.

[0029] The volume average particle size of the small diameter copper particles is 0.1 μm or more, 0.15 μm or more, or even 0.2 μm or more from the viewpoints of dispersibility and cost. The volume average particle size of the small diameter copper particles is 2.0 μm or less, 1.5 μm or less, or even 1.2 μm or less from the viewpoint of sufficient sinterability. From the above viewpoints, the volume average particle size of the small diameter copper particles may be 0.1 to 1.2 μm, 0.2 to 2.0 μm, or 0.2 to 1.2 μm.

[0030] The maximum diameter of the small-diameter copper particles (the diameter of the copper particles having the largest particle diameter among the small-diameter copper particles) may be 0.1 μm or more, 0.15 μm or more, or 0.2 μm or more from the viewpoints of dispersibility and cost. The maximum diameter of the small-diameter copper particles may be 2.0 μm or less, 1.5 μm or less, or 1.2 μm or less from the viewpoint of sufficient sinterability. The maximum diameter of the small-diameter copper particles is a value measured in the same manner as the maximum diameter of the large-diameter copper particles.

[0031] The minimum diameter of the small diameter copper particles (the diameter of the copper particles having the smallest particle diameter among the small diameter copper particles) may be 0.04 μm or more, 0.06 μm or more, or 0.1 μm or more from the viewpoints of dispersibility and cost. The minimum diameter of the small diameter copper particles is measured in the same manner as the maximum diameter.

[0032] The shape of the small-diameter copper particles may be, for example, spherical, blocky, needle-like, flake-like, dendritic, or approximately spherical. The small-diameter copper particles may also be an aggregate of copper particles having these shapes. From the viewpoint of dispersibility and packing ability, the shape of the small-diameter copper particles may be spherical, approximately spherical, or flake-like. From the viewpoint of combustibility and improving mixability with the large-diameter copper particles when the large-diameter copper particles have the above-mentioned irregular shape, the shape of the small-diameter copper particles may be spherical or approximately spherical.

[0033] The content of the small-diameter copper particles may be 10% by mass or more, 15% by mass or more, or 20% by mass or more, based on the total mass of the copper particles, from the viewpoint of excellent adhesive strength and shape retention of the sintered body. The content of the small-diameter copper particles may be 60% by mass or less, 30% by mass or less, 27% by mass or less, or 25% by mass or less, based on the total mass of the copper particles, from the viewpoint of improving porosity and controlling pore size. From the above viewpoints, the content of the small-diameter copper particles may be 10 to 60% by mass, 15 to 30% by mass, 20 to 27% by mass, or 20 to 25% by mass, based on the total mass of the copper particles.

[0034] The mass ratio of the content of small-diameter copper particles to the content of large-diameter copper particles (content of small-diameter copper particles / content of large-diameter copper particles) may be 0.1 or more, 0.18 or more, or 0.25 or more, from the viewpoint of excellent adhesive strength and shape retention of the sintered body. The mass ratio (content of small-diameter copper particles / content of large-diameter copper particles) may be 1.0 or less, 0.6 or less, or 0.45 or less, from the viewpoint of improving porosity and controlling pore size. From the above viewpoints, the mass ratio may be 0.1 to 1.0, 0.18 to 0.6, or 0.25 to 0.45.

[0035] The content of the copper particles may be 70% by mass or more, 75% by mass or more, or 80% by mass or more, based on the total mass of the copper paste, from the viewpoint of easy viscosity adjustment and excellent printability. The content of the copper particles may be 90% by mass or less, 88% by mass or less, or 85% by mass or less, based on the total mass of the copper paste, from the viewpoint of easy viscosity adjustment and excellent printability. From the above viewpoints, the content of the copper particles may be 70 to 90% by mass, 75 to 88% by mass, or 80 to 85% by mass, based on the total mass of the copper paste.

[0036] (pyrolytic resin) The thermally decomposable resin may be a resin that can decompose at the sintering temperature without leaving any residue. The 95% thermal decomposition temperature of the thermally decomposable resin may be 350°C or lower, 300°C or lower, or 250°C or lower. The 95% thermal decomposition temperature is the 95% weight loss temperature measured by TG / DTA. This temperature is measured not in an oxidizing atmosphere such as air, but in a reducing atmosphere containing hydrogen, formic acid, etc., or in an inert gas atmosphere from which oxygen has been removed.

[0037] The smaller the residue after pyrolysis of the pyrolyzable resin, the better the sinterability of the copper particles. The amount of residue (ash content) at the sintering temperature is usually 5% by mass or less, and may be 3% by mass or less, relative to the mass of the resin before pyrolysis. From the viewpoint of obtaining higher sinterability, it may be 2% by mass or less. The amount of residue after pyrolysis of the pyrolyzable resin can be measured by TG / DTA of the pyrolyzable resin in an inert gas (nitrogen or argon) containing 3 to 5% by mass of hydrogen, as the weight change after holding the resin at the sintering temperature for the sintering time. Note that TG / DTA measurement in air is not preferred because oxidative decomposition proceeds and the amount of residue is smaller than the amount of residue in a reducing atmosphere.

[0038] The thermally decomposable resin may be soluble in the dispersion medium described below. Examples of thermally decomposable resins that are soluble in the dispersion medium include polycarbonate, poly(meth)acrylic acid, poly(meth)acrylic acid ester, polyester, etc. Among these, polymethacrylic acid ester may be selected from the viewpoints of solubility in organic solvents, cost, and thermal decomposition. In this specification, "(meth)acrylic" means at least one of acrylic and its corresponding methacrylic.

[0039] The content of the thermally decomposable resin may be 1 part by mass or more, 2 parts by mass or more, or 3 parts by mass or more relative to 100 parts by mass of the copper particles, from the viewpoint of excellent shape retention after printing and drying. The content of the thermally decomposable resin may be 20 parts by mass or less, 15 parts by mass or less, or 12 parts by mass or less relative to 100 parts by mass of the copper particles, from the viewpoint of easy viscosity adjustment and excellent sinterability. From the above viewpoints, the content of the thermally decomposable resin may be 1 to 20 parts by mass, 2 to 15 parts by mass, or 3 to 12 parts by mass relative to 100 parts by mass of the copper particles.

[0040] (dispersion medium) The dispersion medium is not particularly limited, and may be, for example, a volatile one. Examples of volatile dispersion media include monohydric and polyhydric alcohols such as pentanol, hexanol, heptanol, octanol, decanol, ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, α-terpineol, dihydroterpineol, and isobornylcyclohexanol (MTPH); ethylene glycol butyl ether, ethylene glycol phenyl ether, diethylene glycol methyl ether, diethylene glycol ethyl ether, diethylene glycol butyl ether, diethylene glycol isobutyl ether, diethylene glycol hexyl ether, triethylene glycol methyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, diethylene glycol butyl methyl ether, diethylene glycol isopropyl methyl ether, triethylene glycol dimethyl ether, triethylene glycol butyl methyl ether, propylene glycol propyl ether, and dipropylene glycol. Examples of the esters include ethers such as glycol methyl ether, dipropylene glycol ethyl ether, dipropylene glycol propyl ether, dipropylene glycol butyl ether, dipropylene glycol dimethyl ether, tripropylene glycol methyl ether, and tripropylene glycol dimethyl ether; esters such as ethylene glycol ethyl ether acetate, ethylene glycol butyl ether acetate, diethylene glycol ethyl ether acetate, diethylene glycol butyl ether acetate, dipropylene glycol methyl ether acetate (DPMA), ethyl lactate, butyl lactate, γ-butyrolactone, and propylene carbonate; acid amides such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide; aliphatic hydrocarbons such as cyclohexane, octane, nonane, decane, and undecane; aromatic hydrocarbons such as benzene, toluene, and xylene; mercaptans having an alkyl group having 1 to 18 carbon atoms; and mercaptans having a cycloalkyl group having 5 to 7 carbon atoms.Examples of mercaptans having an alkyl group having 1 to 18 carbon atoms include ethyl mercaptan, n-propyl mercaptan, i-propyl mercaptan, n-butyl mercaptan, i-butyl mercaptan, t-butyl mercaptan, pentyl mercaptan, hexyl mercaptan, and dodecyl mercaptan. Examples of mercaptans having a cycloalkyl group having 5 to 7 carbon atoms include cyclopentyl mercaptan, cyclohexyl mercaptan, and cycloheptyl mercaptan.

[0041] The content of the dispersion medium may be, for example, 5 to 50 parts by mass per 100 parts by mass of copper particles. If the content of the dispersion medium is within the above range, the viscosity of the copper paste can be adjusted to a more appropriate level and sintering of the copper particles is less likely to be hindered.

[0042] (others) The copper paste may further contain metal particles other than copper particles. Examples of other metal particles include nickel, silver, gold, palladium, and platinum particles. The content of the other metal particles may be 0% by mass, 0% by mass or more but less than 20% by mass, 0 to 10% by mass, or 0 to 5% by mass, based on the total mass of the metal particles contained in the copper paste. When the copper paste contains other metal particles, the content per 100 parts by mass of the copper particles in this specification may be read as the content per 100 parts by mass of the metal particles, and the content based on the total mass of the copper particles may be read as the content based on the total mass of the metal particles.

[0043] If necessary, the copper paste may contain appropriate additives such as dispersibility improvers such as organic acids (e.g., lauric acid) and organic amines, wetting improvers such as nonionic surfactants and fluorine-based surfactants, antifoaming agents such as silicone oils, and ion trapping agents such as inorganic ion exchangers.

[0044] From the viewpoint of printability, the viscosity of the copper paste may be 10 to 120 Pa·s. The viscosity is measured using an E-type viscometer at 25°C and a rotation speed of 2.5 rpm. As the E-type viscometer, for example, a VISCOMETER-TV33 viscometer manufactured by Toki Sangyo Co., Ltd. can be used. As a measuring jig for a cone rotor, for example, a 3°×R14, SPP can be used.

[0045] The thixotropy index (hereinafter also referred to as TI value) of the copper paste may be 2.0 to 20, 3.0 to 15, or 4.0 to 10. When the TI value of the copper paste is within the above range, the viscosity of the copper paste is easily reduced by shear force, and therefore, printing becomes easier when the paste is stirred manually or with a stirring device (for example, a rotation-revolution type stirring device (Planetary Vacuum Mixer ARV-310, manufactured by Thinky Corporation)) before printing. Furthermore, after the copper paste is applied to the adherend, the viscosity is easily restored by leaving it to stand, which prevents excessive wetting and spreading of the printed matter. The TI value is measured using an E-type viscometer at 25°C and a rotation speed of 0.5 rpm. 0.5 and viscosity μ5 measured at 25°C and a rotation speed of 5 rpm, using the following formula: TI value = μ 0.5 / μ5

[0046] The copper paste described above can be prepared by mixing large-diameter copper particles, small-diameter copper particles, a thermally decomposable resin, a dispersion medium, and other components. The copper paste can be prepared, for example, by dissolving a thermally decomposable resin in a dispersion medium, then adding large-diameter copper particles and small-diameter copper particles and performing a dispersion treatment. Alternatively, the copper paste can be prepared by mixing a solution obtained by dissolving a thermally decomposable resin in a dispersion medium with a dispersion obtained by mixing large-diameter copper particles and small-diameter copper particles in a dispersion medium and dispersing the mixture. After mixing the components, a stirring treatment can be performed. The maximum diameter of the dispersion can be adjusted by a classification operation.

[0047] The dispersion treatment can be carried out using a disperser or a stirrer. Examples of dispersers and stirrers that can be used in the dispersion treatment include an Ishikawa type stirrer, a Silverson stirrer, a cavitation stirrer, a rotation-revolution type stirrer, an ultra-thin film high-speed rotary disperser, an ultrasonic disperser, a Raikai mixer, a twin-screw kneader, a bead mill, a ball mill, a triple-roll mill, a homomixer, a planetary mixer, an ultra-high pressure type disperser, and a thin layer shear disperser.

[0048] The stirring treatment can be carried out using a stirrer, such as an Ishikawa stirrer, a rotation-revolution type stirrer, a Raikai mixer, a twin-screw kneader, a three-roll mill, and a planetary mixer.

[0049] The classification operation can be carried out using, for example, filtration, natural sedimentation, centrifugation, etc. Examples of filters for filtration include water combs, metal meshes, metal filters, and nylon meshes.

[0050] <Wick formation method> In one embodiment, a method for forming a wick includes printing a copper paste and sintering the copper paste. The copper paste of the above embodiment can be used in this method. By sintering the copper paste, a wick containing a sintered body of the copper paste is obtained.

[0051] The printing method of the copper paste is not particularly limited. For example, the copper paste can be printed using screen printing, transfer printing, offset printing, jet printing, a dispenser, a jet dispenser, a needle dispenser, a comma coater, a slit coater, a die coater, a gravure coater, a slit coater, letterpress printing, intaglio printing, gravure printing, stencil printing, soft lithography, a bar coater, an applicator, a particle deposition method, a spray coater, a spin coater, a dip coater, or electrodeposition coating.

[0052] The method for sintering the copper paste is not particularly limited. For example, the copper paste can be sintered by heat-treating (firing) the copper paste using a hot plate, a warm air dryer, a warm air heating furnace, a nitrogen dryer, an infrared dryer, an infrared heating furnace, a far-infrared heating furnace, a microwave heating device, a laser heating device, an electromagnetic heating device, a heater heating device, a steam heating furnace, or the like.

[0053] The gas atmosphere during the heat treatment may be an oxygen-free atmosphere from the viewpoint of suppressing oxidation of the resulting sintered body. The gas atmosphere during the heat treatment may be a reducing atmosphere from the viewpoint of removing surface oxides of copper particles in the copper paste. Examples of the oxygen-free atmosphere include a nitrogen or rare gas atmosphere, a vacuum atmosphere, etc. Examples of the reducing atmosphere include a pure hydrogen gas atmosphere, a mixed gas atmosphere of hydrogen and nitrogen typified by forming gas, a nitrogen atmosphere containing formic acid gas, a mixed gas atmosphere of hydrogen and rare gas, and a rare gas atmosphere containing formic acid gas.

[0054] From the viewpoint of reducing thermal damage to each component and improving yield, the maximum temperature reached during the heat treatment may be 150 to 700°C, 200 to 600°C, or 250 to 550°C. If the maximum temperature reached is 150°C or higher, sintering tends to proceed sufficiently when the maximum temperature is held for 60 minutes or less.

[0055] The time for which the maximum temperature is maintained may be 1 to 60 minutes, or may be 1 minute or more but less than 40 minutes, or may be 1 minute or more but less than 30 minutes, from the viewpoint of volatilizing all of the dispersion medium and improving the yield.

[0056] The wick formation method may further include a step of drying the copper paste before the step of sintering the copper paste. The gas atmosphere during drying may be air, an oxygen-free atmosphere such as nitrogen or a rare gas, or a reducing atmosphere such as hydrogen or formic acid. The drying method may involve drying at room temperature, heating, or vacuum drying. For heating and vacuum drying, for example, a hot plate, a warm air dryer, a warm air heating furnace, a nitrogen dryer, an infrared dryer, an infrared heating furnace, a far-infrared heating furnace, a microwave heating device, a laser heating device, an electromagnetic heating device, a heater heating device, a steam heating furnace, or a hot plate press can be used. The drying conditions (temperature and time) may be appropriately adjusted depending on the type and amount of the dispersion medium used. The drying conditions (temperature and time) may be, for example, drying at 50 to 180°C for 1 to 120 minutes.

[0057] According to the wick formation method described above, the wick is formed by printing using a copper paste, so that the wick can be easily formed even when the wick formation surface has a complex shape (e.g., an uneven shape, a curved shape, a shape with a V-shaped recess, etc.). Furthermore, in the above method, the copper particles contain large-diameter copper particles and small-diameter copper particles, so sufficient sinterability and shape retention can be obtained without applying pressure during sintering. Therefore, the above method can achieve higher productivity than conventional methods that require pressure. Furthermore, the above method has a high degree of freedom in the shape of the wick that can be formed, so that, for example, it is possible to form a thinner wick and also makes it easy to form wicks with more complex shapes (e.g., shapes with curved portions).

[0058] <Heat pipe> A heat pipe according to one embodiment includes a wick containing the sintered copper paste of the above embodiment. The configuration of the heat pipe, excluding the wick, can be the same as that of a conventionally known heat pipe (such as a vapor chamber). The wick containing the sintered copper paste can be formed according to the wick-forming method of the above embodiment. That is, the heat pipe can be manufactured in the same manner as the conventionally known heat pipe, except for the wick-forming step. An example of a heat pipe will now be described with reference to the drawings.

[0059] 1 is a schematic cross-sectional view showing a heat pipe according to one embodiment. The heat pipe 1 includes a container 2 defining an enclosed space S, and a wick 3 and a working liquid contained in the space S of the container 2. A gas phase space A is provided in the space S defined by the container 2 so that the vaporized working liquid vaporized by a heat source can flow. Although not shown, the working liquid is, for example, water or an organic solvent, and is impregnated into the wick 3.

[0060] The shape of the container 2 is not particularly limited and may be tubular, flat, or the like. When the container 2 is flat, the heat pipe may be formed, for example, by the following method. First, a wick is formed by printing copper paste in the recesses of a first substrate having recesses formed on its surface. Next, the first substrate and a second substrate having recesses formed on their surfaces are bonded together so that the recesses face each other. This results in a heat pipe 1 having a flat container 2.

[0061] The material of the container 2 may be metal from the viewpoints of thermal conductivity, pressure resistance, gas shielding properties, workability, etc. Examples of metals that can be used include copper, copper alloys, aluminum, stainless steel, and carbon steel.

[0062] The wick 3 is disposed on the inner wall surface of the container 2. The wick 3 is a porous body formed by sintering the copper paste of the above embodiment. Therefore, the wick 3 includes a sintered body of the copper paste of the above embodiment. The wick 3 may be integrally formed with the container 2, or may be pre-formed (separately disposed).

[0063] From the viewpoint of the ease of flow of the working liquid due to capillary action, the porosity of the wick 3 (porosity of the sintered body) may be 40% by volume or more, 45% by volume or more, or 50% by volume or more, based on the volume of the wick. The porosity of the wick 3 (porosity of the sintered body) is not particularly limited, but may be, for example, 90% by volume or less or 80% by volume or less, based on the volume of the wick. That is, the porosity of the wick 3 (porosity of the sintered body) may be, for example, 40 to 80% by volume, 45 to 80% by volume, or 50 to 80% by volume, based on the volume of the wick. The porosity can be obtained by analyzing cross-sectional images of the wick observed with a scanning electron microscope, a scanning ion microscope, or the like, using image analysis software. Furthermore, if the composition of the metal material constituting the wick is known, the porosity can also be calculated from the difference between the volume of the wick and the volume of the metal in the wick. The volume of the metal is calculated, for example, from the volume of the wick and the weight of the wick measured with a precision balance to obtain the apparent density M1 (g / cm 3 ) and calculate the density of the metal (e.g., copper has a density of 8.96 g / cm 3 ) and calculate the volume ratio from the following formula (A). Metal volume fraction (volume %) = [(M1) / (metal density)] x 100…(A)

[0064] The average pore diameter of the wick 3 may be 10 μm or more, 15 μm or more, or 20 μm or more, from the viewpoint of achieving a good balance between flow resistance and capillary force. The average pore diameter of the wick 3 may be 50 μm or less, 45 μm or less, or 40 μm or less, from the viewpoint of achieving a good balance between flow resistance and capillary force and from the viewpoint of facilitating thinning of the wick. From the above viewpoints, the average pore diameter of the wick 3 may be 10 to 50 μm, 15 to 45 μm, or 20 to 40 μm. The average pore diameter is determined by measuring the length of pores in an SEM image of the cross section processed after casting.

[0065] The wick 3 may have two or more peaks in the pore size distribution determined by measuring the pores in an SEM image of the cross section processed by casting. Specifically, the wick 3 may have, for example, a first peak at 0.5 to 5 μm and a second peak at 10 to 50 μm. When such pore peaks are present, the small pores with the first peak tend to provide a strong capillary force, while the large pores with the second peak tend to enable rapid transport of a large amount of liquid.

[0066] The heat pipe described above is used, for example, with a heat dissipation member provided on the outer wall of a container. The heat pipe is suitably used as a heat dissipation device for small information devices such as smartphones and tablets. [Example]

[0067] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0068] Example 1 [Preparation of copper paste] 11.7 g of dihydroterpineol (manufactured by Nippon Terpene Chemical Co., Ltd.) as a dispersion medium, 3.0 g of KFA-2000 (a dihydroterpineol solution of acrylic resin, solid content: 24% by mass, 95% thermal decomposition temperature = 330 °C, manufactured by GOO Chemical Co., Ltd.) as a thermally decomposable resin, and 0.3 g of lauric acid as an additive (dispersibility improver) were placed in a plastic bottle and mixed using a planetary vacuum mixer (Planetry Vacuum Mixer ARV-310, manufactured by THINKY Corporation). To this dispersion, 17.0 g of small-diameter copper particles CH-0200 (manufactured by Mitsui Mining & Smelting Co., Ltd., volume average particle size: 0.36 μm) and 68 g of large-diameter copper particles CuAtW-250 (manufactured by Fukuda Metal Foil & Powder Co., Ltd., volume average particle size: 27 μm) were added, and the mixture was stirred at 2000 rpm for 1 minute using a planetary vacuum mixer (Planetry Vacuum Mixer ARV-310, manufactured by Thinky Corporation). The mixture was then stirred once with a spoon to confirm the absence of solids, and then stirred at 2000 rpm for 1 minute under reduced pressure to obtain a copper paste. The CuAtW-250 was amorphous and had a tap density of 3.9 g / cm. 3 The viscosity of the copper paste was 32 Pa·s. The viscosity was measured using an E-type viscometer (VISCOMETER TV-33, manufactured by Toki Sangyo) equipped with an SPP rotor at a temperature of 25°C and a rotation speed of 2.5 rpm. The viscosity value was measured 144 seconds after the start of measurement (JIS3284).

[0069] <Examples 2 to 4 and Comparative Example 1> Copper pastes were prepared in the same manner as in Example 1, except that the amounts of large-diameter copper particles and small-diameter copper particles were changed to those shown in Table 1. In this example, the amounts (unit: parts by mass) shown in the table are solid content amounts.

[0070] <Example 5> A copper paste was prepared in the same manner as in Example 1, except that the amounts of the large-diameter copper particles, the small-diameter copper particles, and the lauric acid were changed to those shown in Table 2, that terpineol C (an isomer mixture of α-, β-, and γ-terpineol, manufactured by Nippon Terpene Chemical Co., Ltd., trade name) was used in the amount shown in Table 2 as the dispersion medium instead of dihydroterpineol, and that a solution of M-6003 (molecular weight Mn = 189,300, 95% thermal decomposition temperature = 284 ° C, manufactured by Negami Chemical Industries Co., Ltd., trade name) in the amount shown in Table 2 was used instead of KFA-2000 as the thermally decomposable resin (a solution prepared by dissolving M-6003 in a predetermined amount of terpineol C). The viscosity of the copper paste, measured in the same manner as in Example 1, was 63 Pa s.

[0071] Example 6 The copper paste was prepared in the same manner as in Example 1, except that the amount of the large-diameter copper particles and the amount of lauric acid were changed to the amounts shown in Table 2, that CT-0500 (manufactured by Mitsui Mining & Smelting Co., Ltd., volume average particle size: 1.11 μm) was used as the small-diameter copper particles instead of CH-0200 in the amount shown in Table 2, that Terpineol C (an isomer mixture of α-, β-, and γ-terpineol, manufactured by Nippon Terpene Chemical Co., Ltd., trade name) was used as the dispersion medium instead of dihydroterpineol in the amount shown in Table 2, and that a solution of M-6003 (molecular weight Mn = 189,300, 95% decomposition temperature = 284 ° C, manufactured by Negami Chemical Industrial Co., Ltd., trade name) in the amount shown in Table 2 (a solution prepared by dissolving M-6003 in a predetermined amount of Terpineol C) was used as the thermally decomposable resin instead of KFA-2000.

[0072] Example 7 162.4 g of Terpineol C (a mixture of α-, β-, and γ-terpineol isomers, manufactured by Nippon Terpene Chemical Co., Ltd.) was mixed with 26.6 g of M-6003 (manufactured by Negami Chemical Industries Co., Ltd.) as a dispersion medium, and the mixture was stirred for 3 hours using a mixer rotor with a stirring blade to completely dissolve the resin, yielding a resin solution. 162.0 g of CT-0500 (manufactured by Mitsui Mining & Smelting Co., Ltd., volume average particle size: 1.11 μm) small-diameter copper particles were added to the resin solution, and the mixture was mixed at 50 rpm for 15 minutes using a planetary mixer (Tk. HIVIS MIX fmodel.03, manufactured by PRIMIX). Then, 567 g of CuAtW-250 (manufactured by Fukuda Metal Foil & Powder Co., Ltd., volume average particle size: 27 μm) as large-diameter copper particles and FC-115 (dendritic copper powder, manufactured by Fukuda Metal Foil & Powder Co., Ltd., volume average particle size: 21 μm, tap density: 1.2 g / cm) were added. 3 81 g of ethanol was added and mixed for 15 minutes at 50 rpm using a planetary mixer. The pressure was reduced and the mixture was mixed for 15 minutes at 50 rpm to obtain a copper paste. The viscosity of the copper paste was measured in the same manner as in Example 1 and was found to be 50 Pa s.

[0073] <Comparative Example 2> A copper paste was prepared in the same manner as in Example 1, except that the amounts of large-diameter copper particles, small-diameter copper particles, and lauric acid were changed to the values ​​shown in Table 2, that terpineol C (a mixture of isomers of α-, β-, and γ-terpineol, product name: Nippon Terpene Chemical Co., Ltd.) was used as the dispersion medium instead of dihydroterpineol in the amount shown in Table 2, and that the thermally decomposable resin (KFA-2000) was not used.

[0074] <Comparative Examples 3 and 4> A copper paste was prepared in the same manner as in Example 6, except that MA-C25 (manufactured by Mitsui Mining & Smelting Co., Ltd., volume average particle diameter: 8 μm) or 1400YF (manufactured by Mitsui Mining & Smelting Co., Ltd., volume average particle diameter: 5 μm) was used instead of large-diameter copper particles CuAtW-250 (manufactured by Fukuda Metal Foil & Powder Co., Ltd., volume average particle diameter: 27 μm).

[0075] <Evaluation> [Evaluation of copper paste printability (1)] The Hull Cell copper plate was divided into three equal parts, each measuring 30 mm long, 67 mm wide, and 300 μm thick. A 70 μm thick SUS mask with two 25 mm x 5 mm openings was placed on the copper plate, and the copper paste was printed using a metal squeegee. A uniformly thick coating without rubbing or unevenness was evaluated as printability A. A coating with a few streaky rubbing and / or unevenness was evaluated as printability B. A coating with rubbing and / or unevenness so severe that the substrate was visible across the entire surface was evaluated as printability C. The results are shown in Tables 1 and 2 as printability (1).

[0076] [Evaluation of copper paste printability (2)] The copper paste was placed in a 5 mL plastic syringe manufactured by Musashi Engineering Co., Ltd. The syringe containing the copper paste was set in a pneumatic dispenser (ML-505X, manufactured by Musashi Engineering Co., Ltd.), a wide printing needle was attached to the tip of the syringe, and the syringe was pressurized at 1 kgf / cm. 2 The ink was discharged onto a copper plate (a Hull Cell copper plate divided into three equal parts, 30 mm long x 67 mm wide x 300 μm thick) at a pressure of 98 kPa (=98 kPa). When the ink was continuously and uniformly discharged and printed, it was rated as printability A; when the discharge was intermittent or very slow, it was rated as printability B; and when the ink could not be discharged or the discharge stopped during discharge, it was rated as printability C. The results of printability (2) are shown in Tables 1 and 2.

[0077] [Evaluation of shape retention before sintering] The copper pastes of the Examples and Comparative Examples were printed using a 70 μm thick stencil and dried. The resulting pre-sintered prints were rubbed with a finger to evaluate their shape retention before sintering. A pre-sintered shape retention rating was given for a print that did not lose its shape when rubbed with a finger, while a pre-sintered shape retention rating of C was given for a print that crumbled into powder when rubbed with a finger and could no longer maintain its shape. The results are shown in Tables 1 and 2.

[0078] [Firing copper paste] The copper plates printed with the copper pastes obtained in the copper paste printability evaluations (1) and (2) were placed on a hot plate heated to 90°C and dried in air for 10 minutes to prepare fired samples. The samples were placed on a glass tray in a tubular furnace (manufactured by AVC Corporation) and set in the furnace. After depressurization, 100 sccm of hydrogen and 900 sccm of nitrogen were introduced. Once the pressure returned to normal, the samples were fired at 600°C, increasing the temperature for 20 minutes and maintaining the temperature for 60 minutes. The gas flow was then stopped, and the samples were cooled by forced air cooling while depressurizing for at least 30 minutes. After returning the pressure to normal with argon gas, the fired samples were removed into the air. This resulted in a sintered body of the copper paste. The samples (sintered bodies) obtained using the copper paste obtained in printability evaluation (1) were used in the following evaluations.

[0079] [Adhesion evaluation (tape peel test)] A 16mm wide piece of Nichiban Cellotape (registered trademark) was applied to the sintered body obtained above, and the tape was firmly rubbed with a fingertip for approximately 10 seconds. Then, within 30 seconds or more but within 5 minutes, the edge of the tape was grasped at an angle as close to 60° as possible and peeled off in 0.5 to 1.0 second, and the adhesion to the tape was checked. A was assigned to the absence of adhesion, B to the presence of a small amount of adhesion in some areas, and C to the presence of adhesion over the entire surface. The results are shown in Tables 1 and 2.

[0080] [Porosity measurement] The sample obtained above was placed in a plastic cup and poured with a casting resin (Epomount, Refine Tech Co., Ltd.). The cup was then placed in a vacuum desiccator and degassed under reduced pressure. The sample was then left at room temperature for 10 hours to harden. The cast sample was cut near the cross section of interest using a Refine Saw Excel (Refine Tech Co., Ltd.) equipped with a resinoid grinding wheel. The cross section was then polished using a polishing machine (Refine Polisher Hv, Refine Tech Co., Ltd.) equipped with waterproof abrasive paper (Carbomac Paper, Refine Tech Co., Ltd.) and buffed with an alumina polishing solution. The sample was then observed using a SEM (TM-1000, Hitachi High-Technologies Corporation) at an applied voltage of 15 kV and various magnifications. The observed images were binarized using Image J, and the porosity (unit: volume %) of the sintered body (wick) was calculated from the dot ratio between the white and black areas. The results are shown in Tables 1 and 2.

[0081] [Measurement of average pore diameter] The pores in the 500x SEM image obtained in the above [Porosity Measurement] were measured using Image J, and the average pore diameter of the sintered body (wick) was calculated by averaging the measurements at 20 points. The results are shown in Tables 1 and 2.

[0082] [Measurement of pore size distribution] Three 500x and 10,000x SEM images were obtained in the above [Porosity Measurement]. The pores in the images were measured using Image J, and the pore size distribution of the sintered body (wick) was determined from the size distribution at 120 locations. Figure 2 shows SEM images of Example 7. Figure 2(a) is a 500x SEM image, and Figure 2(b) is a 10,000x SEM image. In Example 1, pore size peaks were confirmed at 1.2 μm and 20 μm. In Example 6, pore size peaks were confirmed at 1.0 μm and 30 μm. In Example 7, pore size peaks were confirmed at 1.1 μm and 30 μm.

[0083] [Table 1]

[0084] [Table 2]

[0085] [Wick creation] The sintered bodies produced using the copper pastes of the Examples were used to form heat pipes, and it was confirmed that the sintered bodies functioned as wicks. On the other hand, as shown in the above evaluation results, the sintered body produced using the copper paste of Comparative Example 1 had insufficient adhesion, and the copper paste of Comparative Example 1 was unsuitable as a copper paste for forming a wick. Furthermore, the copper paste of Comparative Example 2 had poor printability, and a sintered body could not be produced using the copper paste of Comparative Example 2. Furthermore, the sintered bodies produced using the copper pastes of Comparative Examples 3 and 4 did not function as wicks. [Explanation of symbols]

[0086] 1...heat pipe, 2...container, 3...wick.

Claims

1. A copper paste for forming a wick of a heat pipe, Contains copper particles, a thermally decomposable resin, and a dispersion medium, The copper particles include large-diameter copper particles having a volume average particle size of 10 to 50 μm and small-diameter copper particles having a volume average particle size of 0.1 to 2.0 μm.

2. The copper paste according to claim 1, wherein the thermally decomposable resin has a 95% thermal decomposition temperature of 350°C or less.

3. The content of the large-diameter copper particles is 40 to 90 mass% based on the total mass of the copper particles, The copper paste according to claim 1 or 2, wherein the content of the small diameter copper particles is 10 to 60 mass% based on the total mass of the copper particles.

4. The copper paste according to any one of claims 1 to 3, wherein the content of the thermally decomposable resin is 1 to 20 parts by mass per 100 parts by mass of the copper particles.

5. The large diameter copper particles have a tap density of 1.0 to 4.5 g / cm ３ The copper paste according to any one of claims 1 to 4.

6. The copper paste according to any one of claims 1 to 5, wherein the viscosity of the copper paste is 10 to 120 Pa s.

7. 1. A method for forming a wick for a heat pipe, comprising: A step of printing the copper paste according to any one of claims 1 to 6; and sintering the copper paste.

8. A heat pipe comprising a wick containing a sintered body of the copper paste according to any one of claims 1 to 6.

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