TWI901665B - Copper paste, capillary structure forming method and heat pipe - Google Patents

Abstract

A copper paste is used for forming the capillary structure of a heat pipe. It contains copper particles, thermally decomposable resin, and a dispersion medium. The copper particles contain large-diameter copper particles with a volume average particle size of 10 μm to 50 μm and small-diameter copper particles with a volume average particle size of 0.1 μm to 2.0 μm.

Description

Copper paste, capillary structure formation methods and heat pipes

This invention relates to a copper paste, a method for forming capillary structures, and a heat pipe.

A heat pipe is a passive heat transfer element that utilizes the evaporation and condensation of a working fluid. It comprises a working fluid and a component called a "capillary structure" that generates a capillary pump effect within a sealed space. Heat pipes can transfer large amounts of heat with small temperature differences, and are therefore attracting attention as heat dissipation devices for small information devices such as smartphones. For example, Patent 1 discloses a heat pipe comprising a capillary structure made of porous sintered powder. When the capillary structure is composed of porous sintered powder, the general method is as follows: sintering is achieved by depositing sinterable metal powder (e.g., copper powder) at specified locations, followed by compression and calcination. [Prior Art Documents] [Patent Documents]

Patent Document 1: Japanese Patent Application Publication No. 2003-222481

[Problem to be Solved by the Invention] While heat pipes have traditionally been mostly tubular, flat heat pipes with a vapor chamber are increasingly used for miniaturization and better contact with the heat source. Due to volume limitations in small information devices, these flat heat pipes are being made increasingly thinner, requiring a thinner capillary structure. Furthermore, in flat heat pipes, the surfaces with capillary structures (the surfaces where the capillary structure forms) sometimes have complex shapes such as uneven surfaces. However, when the capillary structure is composed of porous sintered powder, previous compression methods are insufficient to handle the thinness and complex shapes of these capillary structures.

Therefore, one of the objectives of this invention is to provide a novel method for forming capillary structures, which can easily form capillary structures even when the target capillary structure is thin or when the forming surface of the target capillary structure has a complex shape.

[Methods for Solving the Problem] The inventors and others have discovered that by using a copper paste made of two copper particles of different sizes and thermally decomposable resin, a capillary structure can be formed by printing, thus completing the present invention.

That is, one aspect of the present invention relates to copper paste for forming the capillary structure of the heat pipe as shown below.

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

[2] The copper paste as described in [1], wherein the thermally decomposable resin has a 95% thermal decomposition temperature of 350°C or less.

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

[4] The copper paste as described in any of [1] to [3], wherein the content of the thermally decomposable resin is 1 to 20 parts by mass relative to 100 parts by mass of the copper particles.

[5] The copper paste as described in any of [1] to [4], wherein the tap density of the large-diameter copper particles is 1.0 g/ ^cm3 to 4.5 g/ ^cm3 .

[6] The copper paste as described in any of [1] to [5], wherein the viscosity of the copper paste is 10 Pa·s to 120 Pa·s.

According to the copper paste, capillary structures can be formed by printing, so even if the thickness of the capillary structure is thin (e.g., less than 70 μm) and the forming surface of the capillary structure has a complex shape, the capillary structure can be easily formed.

Another aspect of the present invention relates to a method for forming a capillary structure, which is a method for forming a capillary structure of a heat pipe, comprising: a step of printing copper paste as described in any one of [1] to [6]; and a step of sintering the copper paste.

Another aspect of the invention relates to a heat pipe comprising a capillary structure of a sintered body containing copper paste as described in any of [1] to [6].

[Effects of the Invention] According to the present invention, a novel method for forming capillary structures can be provided, which can easily form capillary structures even when the thickness of the target capillary structure is thin and when the forming surface of the target capillary structure has a complex shape.

In this specification, the numerical range indicated by "~" represents the range of minimum and maximum values recorded before and after "~". The upper or lower limit of a numerical range recorded in stages in this specification can be replaced with the upper or lower limit of another numerical range. Furthermore, unless otherwise specified, the materials exemplified in this specification can be used alone or in combination with two or more. Regarding the content of each component in the composition, in cases where multiple substances equivalent to each component are present in the composition, unless otherwise specified, the content refers to the total amount of those multiple substances present in the composition.

The preferred embodiments of the present invention will now be described. However, the present invention is not limited to any of the embodiments described below.

<Copper Paste> One embodiment of the copper paste is a capillary structure forming copper paste used to form the capillary structure of a heat pipe. The copper paste contains copper particles, thermally decomposable resin, and a dispersion medium. The components contained in the copper paste are described below.

(Copper particles) Copper particles contain copper particles with a volume average size of 10 μm to 50 μm (large-diameter copper particles) and copper particles with a volume average size of 0.1 μm to 2.0 μm (small-diameter copper particles). The volume average size can be determined using a light scattering particle size distribution measurement device.

From the viewpoint that it is easy to obtain pores of a better size (e.g., pores of 10 μm or more), the volume average particle size of the large-diameter copper particles is 10 μm or more, 15 μm or more, or 20 μm or more. From the viewpoint that it is easy to obtain a thinner capillary structure after calcination (e.g., a capillary structure with a thickness of 70 μm or less), the volume average particle size of the large-diameter copper particles is 50 μm or less, 45 μm or less, or 40 μm or less. From the aforementioned viewpoint, the volume average particle size of the large-diameter copper particles can be 10 μm to 40 μm, 20 μm to 50 μm, or 20 μm to 40 μm.

From the perspective that a thinner capillary structure (e.g., a capillary structure less than 70 μm thick) can be easily obtained after calcination, the maximum diameter of large-diameter copper particles (the diameter of the copper particle with the largest diameter among large-diameter copper particles) can be less than 70 μm, less than 60 μm, or less than 50 μm. The maximum diameter of large-diameter copper particles is a value obtained by sieving.

One method for determining the maximum diameter of large-diameter copper particles is as follows: According to Japanese Industrial Standard (JIS) Z 8815:1994, this method involves determining the percentage of large-diameter copper particles on the sieve obtained by sieving using a sieve with a sieve aperture of 63 μm. Considering that a thinner capillary structure (e.g., a capillary structure with a thickness of 70 μm or less) can be easily obtained after calcination, the percentage of large-diameter copper particles on the sieve obtained using this method can be 5.0% by mass or less.

From the viewpoint that it is easy to obtain pores of a better size (e.g., pores larger than 10 μm), the minimum diameter of large-diameter copper particles (the diameter of the smallest copper particle among large-diameter copper particles) can be 0.04 μm or larger, 0.06 μm or larger, or 0.1 μm or larger. The minimum diameter of large-diameter copper particles is measured in the same way as the maximum diameter.

Large-diameter copper particles can be, for example, spherical, blocky, needle-like, sheet-like, branch-like, or roughly spherical, and can also be amorphous. When using amorphous large-diameter copper particles, it is easy to obtain capillary structures with high porosity and moderately large pores. Therefore, when using amorphous large-diameter copper particles, the capillary force of the capillary structure is easily improved. From the aforementioned perspective, the tap density of large-diameter copper particles can be 0.5 g/ ^cm³ or higher, 0.8 g/cm³ or ^higher , or 1.0 g/cm³ ^or higher; it can be 4.5 g/cm³ or ^lower , 4.3 g/cm³ or ^lower , or 4.0 g/cm³ or ^lower ; or it can be between 1.0 g/ ^cm³ and 4.5 g/ ^cm³ or between 1.0 g/ ^cm³ and 4.0 g/ ^cm³ . These copper particles tend to be amorphous. The tap density of large-diameter copper particles is a value measured according to JIS Z 2512:2012.

From the perspective of ensuring better porosity and pore size, based on the total mass of copper particles, the content of large-diameter copper particles can be 40% or more by mass, 60% or more by mass, 70% or more by mass, 75% or more by mass, or 80% or more by mass. From the perspective of achieving a good balance with the addition of small-diameter copper particles, based on the total mass of copper particles, the content of large-diameter copper particles can be 90% or less by mass, 87% or less by mass, 85% or less by mass, or 80% or less by mass. From the aforementioned perspective, based on the total mass of copper particles, the content of large-diameter copper particles can 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.

From the perspective of dispersibility and cost, the volume average particle size of the small-diameter copper particles is 0.1 μm or more, 0.15 μm or more, or 0.2 μm or more. From the perspective of exhibiting sufficient sinterability, the volume average particle size of the small-diameter copper particles is 2.0 μm or less, 1.5 μm or less, or 1.2 μm or less. From the aforementioned perspective, the volume average particle size of the small-diameter copper particles can be 0.1 μm to 1.2 μm, 0.2 μm to 2.0 μm, or 0.2 μm to 1.2 μm.

From the perspective of dispersibility and cost, the maximum diameter of small-diameter copper particles (the diameter of the copper particle with the largest diameter among small-diameter copper particles) can be 0.1 μm or more, 0.15 μm or more, or 0.2 μm or more. From the perspective of exhibiting sufficient sinterability, the maximum diameter of small-diameter copper particles can be 2.0 μm or less, 1.5 μm or less, or 1.2 μm or less. The maximum diameter of small-diameter copper particles is a value measured in the same manner as the maximum diameter of large-diameter copper particles.

From the perspective of dispersibility and cost, the minimum diameter of small-diameter copper particles (the diameter of the smallest copper particle among small-diameter copper particles) can be 0.04 μm or more, 0.06 μm or more, or 0.1 μm or more. The minimum diameter of small-diameter copper particles is measured in the same way as the maximum diameter.

The shape of small-diameter copper particles can be, for example, spherical, blocky, needle-like, sheet-like, branch-like, or roughly spherical. Small-diameter copper particles can be aggregates of copper particles having these shapes. From the viewpoint of dispersibility and filling properties, the shape of small-diameter copper particles can be spherical, roughly spherical, or sheet-like. From the viewpoint of combustibility, and from the viewpoint that the mixing with large-diameter copper particles becomes better when the large-diameter copper particles are the aforementioned amorphous shapes, the shape of small-diameter copper particles can be spherical or roughly spherical.

From the viewpoint of excellent adhesion and shape retention of the sintered body, the content of small-diameter copper particles can be 10% by mass or more, 15% by mass or more, or 20% by mass or more, based on the total mass of copper particles. From the viewpoint of improving porosity and controlling void size, the content of small-diameter copper particles can 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 copper particles. From the aforementioned viewpoint, the content of small-diameter copper particles can be 10% by mass to 60% by mass, 15% by mass to 30% by mass, or 20% by mass to 27% by mass, or 20% by mass to 25% by mass, based on the total mass of copper particles.

From the viewpoint of excellent adhesion and shape retention of the sintered body, the mass ratio of small-diameter copper particles to large-diameter copper particles (content of small-diameter copper particles/content of large-diameter copper particles) can be 0.1 or more, 0.18 or more, or 0.25 or more. From the viewpoint of improving porosity and controlling void size, the mass ratio (content of small-diameter copper particles/content of large-diameter copper particles) can be 1.0 or less, 0.6 or less, or 0.45 or less. From the aforementioned viewpoint, the mass ratio can be 0.1 to 1.0, 0.18 to 0.6, or 0.25 to 0.45.

From the perspective of easier viscosity adjustment and improved printability, the copper particle content can 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 perspective of easier viscosity adjustment and improved printability, the copper particle content can 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 aforementioned perspective, the copper particle content can 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.

(Thermolytic Resins) As thermolytic resins, resins that decompose at sintering temperatures without residue can be used. The 95% thermal decomposition temperature of thermolytic resins can be below 350°C, below 300°C, or below 250°C. Furthermore, the 95% thermal decomposition temperature refers to the 95% weight reduction temperature measured in thermogravimetric/differential thermal analysis (TG/DTA). This temperature is not measured in an oxidizing environment like air, but in a reducing environment containing hydrogen, formic acid, etc., or in an inert gas environment where oxygen has been removed.

The less residue left after the thermal decomposition of a thermally decomposable resin, the better the sinterability of the copper particles. The amount of residue (ash content) at the sintering temperature relative to the resin mass before thermal decomposition is typically below 5% by mass, and can be below 3% by mass for even better sinterability, or even below 2% by mass. The amount of residue after the thermal decomposition of a thermally decomposable resin can be determined by measuring the weight change of the TG/DTA of the resin in a passivating atmosphere (nitrogen or argon) containing 3% to 5% by mass of hydrogen, after maintaining the sintering temperature for a specified time. Furthermore, the TG/DTA determination in air involves oxidative decomposition, resulting in less residue compared to that in a reducing environment, which is undesirable.

Thermally degradable resins may be soluble in the dispersion medium described later. Examples of thermally degradable resins that are soluble in the dispersion medium include polycarbonate, poly(meth)acrylic acid, poly(meth)acrylate, and polyester. Among these, poly(meth)acrylate is preferred from the perspectives of solubility in organic solvents, cost, and thermal degradability. Furthermore, in this specification, "(meth)acrylic acid" refers to at least one of acrylic acid and its corresponding methacrylic acid.

From the viewpoint of excellent shape retention after printing and drying, the content of thermally degradable resin can 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 copper particles. From the viewpoint of easy viscosity adjustment and excellent sintering properties, the content of thermally degradable resin can 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 copper particles. From the aforementioned viewpoints, the content of thermally degradable resin can 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 copper particles.

(Dispersion Medium) There are no particular limitations on the dispersion medium; for example, it can be a volatile dispersion medium. Examples of volatile dispersion media include: pentanol, hexanol, heptanol, octanol, decanol, ethylene glycol, diethylene glycol, propylene glycol, butanediol, α-terpineol, dihydroterpineol, isoborneol cyclohexanol (MTPH), and other monohydric and polyhydric alcohols; 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, and diethylene glycol. Ethers such as 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, dipropylene 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; ethylene glycol ethyl ether acetate, ethylene glycol butyl ether acetate, diethylene glycol ethyl ether acetate, diethylene glycol butyl ether acetate, and dipropylene glycol methyl ether acetate (Dipropylene glycol methyl ether acetate). Esters such as methyl ethyl ester (DPMA), ethyl lactate, butyl lactate, γ-butyrolactone, and propyl carbonate; acid amines 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; thiols having alkyl groups with 1 to 18 carbon atoms; and thiols having cycloalkyl groups with 5 to 7 carbon atoms. Examples of thiols having alkyl groups with 1 to 18 carbon atoms include: ethyl thiol, n-propyl thiol, isopropyl thiol, n-butyl thiol, isobutyl thiol, tributyl thiol, pentyl thiol, hexyl thiol, and dodecyl thiol. Examples of thiols that are cycloalkyl groups having 5 to 7 carbon atoms include cyclopentyl thiols, cyclohexyl thiols, and cycloheptyl thiols.

For every 100 parts by mass of copper particles, the content of the dispersion medium can be, for example, 5 to 50 parts by mass. If the content of the dispersion medium is within the range described above, the copper paste can be adjusted to a more suitable viscosity, and the sintering of the copper particles will not be easily hindered.

(Other) Copper paste may also contain other metal particles besides copper particles. Examples of other metal particles include nickel, silver, gold, palladium, and platinum. Based on the total mass of the metal particles in the copper paste, the content of other metal particles can be 0% by mass, or more than 0% by mass but less than 20% by mass, or 0% to 10% by mass, or 0% to 5% by mass. Furthermore, when the copper paste contains other metal particles, the content relative to 100 parts by mass of copper particles in this specification can be replaced with the content relative to 100 parts by mass of metal particles, and the content based on the total mass of copper particles can be replaced with the content based on the total mass of metal particles.

As needed, dispersants such as organic acids (e.g., lauric acid), dispersants such as organic amines, wettability enhancers such as nonionic surfactants and fluorinated surfactants, defoamers such as silicone oil, and ion scavengers such as inorganic ion exchangers can also be added to the copper paste.

From a printability perspective, the viscosity of the copper paste can range from 10 Pa·s to 120 Pa·s. Furthermore, this viscosity is measured using a Type E viscometer at 25°C and a rotation speed of 2.5 rpm. For example, a Type E viscometer can be used, such as the VISCOMETER-TV33 model manufactured by Toki Kogyo Co., Ltd. As a fixture for measuring tapered rotors, a 3°×R14, SPP material is suitable.

The thixotropic index (TI value) of the copper paste can be 2.0–20, 3.0–15, or 4.0–10. If the TI value of the copper paste is within the range described above, the copper paste easily becomes less viscous due to shear force. Therefore, it is easily printed by stirring manually or with a stirring device (such as a planetary vacuum mixer ARV-310, manufactured by Thinky Co., Ltd.) before printing. In addition, after the copper paste adheres to the component that is to be bonded, its viscosity easily recovers by resting, thus suppressing excessive wetting and spreading of the printed material. Furthermore, the TI value is the viscosity μ _{_0.5} measured using a type E viscometer at 25°C and 0.5 rpm, and the viscosity μ _₅ measured at 25°C and 5 rpm, calculated using the following formula: TI value = μ_{_0.5} / μ_₅

The copper paste can be prepared by mixing large-diameter copper particles, small-diameter copper particles, thermally degradable resin, a dispersion medium, and other components. For example, the copper paste can be prepared by dissolving the thermally degradable resin in a dispersion medium and then adding large-diameter and small-diameter copper particles for dispersion treatment. Alternatively, it can be prepared by mixing a solution obtained by dissolving the thermally degradable resin in a dispersion medium with a dispersion obtained by mixing large-diameter and small-diameter copper particles in a dispersion medium for dispersion treatment. The mixture can also be stirred after mixing. The maximum diameter of the dispersion can also be adjusted by fractionation.

Dispersion processing can be carried out using a disperser or a mixer. Examples of dispersers and mixers that can be used in dispersion processing include: Ishikawa mixer, Silverson mixer, cavitation mixer, rotary mixer, ultra-thin film high-speed rotary disperser, ultrasonic disperser, pounder, twin-shaft mixer, bead mill, ball mill, three-rod roll mill, homogenizer, planetary mixer, ultra-high pressure disperser, and thin-layer shear disperser.

Mixing can be performed using a mixer. Examples of mixers that can be used in mixing include: Ishikawa mixers, rotary mixers, pounders, twin-shaft mixers, three-roll mills, and planetary mixers.

Classification operations can be carried out using methods such as filtration, natural sedimentation, and centrifugation. Examples of filters used for filtration include: water combs, metal mesh, metal filters, and nylon mesh.

<Method for Forming a Capillary Structure> A method for forming a capillary structure in one embodiment includes steps of printing copper paste and sintering the copper paste. In this method, the copper paste of the embodiment described above can be used. By sintering the copper paste, a capillary structure of a sintered body containing copper paste can be obtained.

There are no particular restrictions on the printing method of copper paste. For example, you can use screen printing, transfer printing, offset printing, jet printing, dispenser, jet dispenser, needle dispenser, comma coater, slit coater, die coater, gravure coater, slit coat, letterpress printing, gravure printing, stencil printing, soft lithography, bar coat, applicator, particle stacking, spray coater, spin coater. Copper paste is printed using methods such as coating machines, dip coaters, and electroplating.

There are no particular limitations on the sintering method for copper paste. For example, the copper paste can be sintered by heating it using a heating plate, a warm air dryer, a warm air furnace, a nitrogen dryer, an infrared dryer, an infrared furnace, a far-infrared furnace, a microwave heating device, a laser heating device, an electromagnetic heating device, a heater heating device, or a steam furnace.

From the perspective of suppressing the oxidation of the resulting sintered body, the gas environment during heating treatment can be an oxygen-free environment. From the perspective of removing surface oxides from copper particles in copper paste, the gas environment during heating treatment can be a reducing environment. Examples of oxygen-free environments include: nitrogen, rare gas environments, and vacuum environments. Examples of reducing environments include: pure hydrogen environments, mixed gas environments of hydrogen and nitrogen (represented by syngas), nitrogen environments containing formic acid gas, mixed gas environments of hydrogen and rare gases, and rare gas environments containing formic acid gas.

From the perspective of reducing heat damage to components and improving yield, the maximum temperature reached during heat treatment can be 150℃～700℃, 200℃～600℃, or 250℃～550℃. If the maximum temperature is above 150℃, there is a tendency for sintering to be fully carried out when the maximum temperature is held for less than 60 minutes.

From the perspective of ensuring complete evaporation of the dispersion medium and improving yield, the holding time at the highest temperature can be from 1 minute to 60 minutes, or more than 1 minute but less than 40 minutes, or more than 1 minute but less than 30 minutes.

The method for forming the capillary structure may further include a step of drying the copper paste before sintering it. The gas environment during drying can be the atmosphere, an oxygen-free environment such as nitrogen or rare gases, or a reducing environment such as hydrogen or formic acid. The drying method can be room temperature drying, heated drying, or depressurized drying. Heated drying and depressurized drying may utilize, for example, hot plates, warm air dryers, warm air furnaces, nitrogen dryers, infrared dryers, infrared furnaces, far-infrared furnaces, microwave heating devices, laser heating devices, electromagnetic heating devices, heater heating devices, steam furnaces, hot plate pressing devices, etc. The drying conditions (temperature and time) can be adjusted according to the type and amount of dispersion medium used. For example, the drying conditions (temperature and time) can be 50°C to 180°C for 1 minute to 120 minutes.

According to the capillary formation method described above, capillary structures are formed using copper paste and by printing. Therefore, even when the formation surface of the capillary structure has complex shapes (e.g., concave-convex shapes, curved shapes, shapes with V-shaped concave portions, etc.), capillary structures can be easily formed. Furthermore, in this method, since the copper particles contain both large-diameter and small-diameter copper particles, sufficient sinterability and shape retention can be obtained during sintering even without pressure. Therefore, this method offers higher productivity compared to previous methods that required pressure. Furthermore, in the method, since the shape of the capillary structure that can be formed has a high degree of freedom, for example, a thinner capillary structure can be formed, and it is also easy to form a more complex capillary structure (e.g., a shape with curved portions).

<Heat Pipe> One embodiment of the heat pipe includes a capillary structure of a sintered body containing copper paste of the embodiment. The structure of the heat pipe, except for the capillary structure, can be the same as that of conventionally known heat pipes (such as steam chambers). The capillary structure of the sintered body containing copper paste can be formed according to the method for forming the capillary structure of the embodiment. That is, the heat pipe can be manufactured using the same method as conventionally known heat pipes, except for the step of forming the capillary structure. Hereinafter, an example of a heat pipe will be described with reference to the drawings.

Figure 1 is a schematic cross-sectional view of a heat pipe embodiment. The heat pipe 1 includes a container 2 defining a sealed space S, a capillary structure 3 contained within the space S of the container 2, and a working fluid. A vapor phase space A is ensured within the space S defined by the container 2 by allowing the vaporized working fluid, which is vaporized by a heat source, to flow through. Although not shown, the working fluid, such as water or an organic solvent, is contained within the capillary structure 3.

The shape of container 2 is not particularly limited and can be tubular, flat, etc. When container 2 is flat, a heat pipe can be formed, for example, using the following method: First, copper paste is printed into the recesses of a first substrate with recesses formed on its surface to form a capillary structure. Then, the first substrate and a second substrate with recesses formed on its surface are bonded together with the recesses facing each other. This yields a heat pipe 1 comprising a flat container 2.

From the perspectives of thermal conductivity, pressure resistance, gas shielding, and processability, the material of container 2 can be metal. For example, copper, copper alloys, aluminum, stainless steel, carbon steel, etc. can be used as metals.

The capillary structure 3 is disposed on the inner wall surface of the container 2. The capillary structure 3 is a porous body formed by sintering the copper paste of the embodiment described above. Therefore, the capillary structure 3 contains a sintered body of the copper paste of the embodiment described above. The capillary structure 3 can be integrally formed with the container 2, or it can be pre-formed (disposed separately).

From the perspective of the ease of flow of the working fluid caused by capillary phenomenon, based on the volume of the capillary structure, the porosity of the capillary structure 3 (porosity of the sintered body) can be 40% or more, 45% or more, or 50% or more. The porosity of the capillary structure 3 (porosity of the sintered body) is not particularly limited; for example, based on the volume of the capillary structure, it can be 90% or less or 80% or less. That is, for example, based on the volume of the capillary structure, the porosity of the capillary structure 3 (porosity of the sintered body) can be 40% to 80% or 45% to 80% or 50% to 80% of the volume. Porosity can be obtained by analyzing cross-sectional images of capillary structures observed using scanning electron microscopes, scanning ion microscopes, etc., using image analysis software. Alternatively, if the composition of the metallic material constituting the capillary structure is known, it can be calculated based on the difference between the volume of the capillary structure and the volume of the metal within it. The volume of the metal can be obtained, for example, by calculating the apparent density _M1 (g/ ^cm³ ) based on the volume of the capillary structure and its weight measured using a precision balance. Using the calculated _M1 and the density of the metal (e.g., the density of copper is 8.96 g/ ^cm³ ), the volume ratio is calculated according to the following formula (A). The volume percentage of a metal (volume %) = [( _M1 ) / (density of the metal)] × 100··· (A)

From the perspective of achieving a good balance between flow resistance and capillary force, the average pore diameter of the capillary structure 3 can be 10 μm or more, 15 μm or more, or 20 μm or more. From the perspective of achieving a good balance between flow resistance and capillary force and facilitating the thinning of the capillary structure, the average pore diameter of the capillary structure 3 can be 50 μm or less, 45 μm or less, or 40 μm or less. From the aforementioned perspective, the average pore diameter of the capillary structure 3 can be 10 μm to 50 μm, 15 μm to 45 μm, or 20 μm to 40 μm. The average pore diameter can be determined by measuring the length of the pore portion in an SEM image of the cross-section after the injection molding process.

The capillary structure 3, whose pore diameter distribution is determined by measuring the length of the pore portion in the SEM image of the cross-section after injection molding, may have two or more peaks. Specifically, for example, a first peak may be present at 0.5 μm to 5 μm, and a second peak may be present at 10 μm to 50 μm. In the case of such pore peaks, there is a tendency for strong capillary force to be obtained through small pores with the first peak, and for large pores with the second peak to be used for rapid transport of large quantities of liquid.

The heat pipes described above are used, for example, when a heat dissipation component is installed on the outer wall of a container. Heat pipes are particularly suitable as heat dissipation devices for small information devices such as smartphones and tablets. [Example]

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

<Example 1> [Preparation of Copper Paste] 11.7 g of dihydroterpene alcohol (manufactured by Japan Terpene Chemical Co., Ltd.) as a dispersion medium, 3.0 g of KFA-2000 (dihydroterpene alcohol solution of acrylic resin, solid content: 24% by mass, 95% thermal decomposition temperature = 330°C, manufactured by Mutual Chemicals) as a thermally decomposable resin, and 0.3 g of lauric acid as an additive (dispersibility improver) were placed into a plastic bottle and mixed using a rotational stirring device (planetary vacuum mixer ARV-310, manufactured by Thinky Co., Ltd.). 17.0 g of CH-0200 (manufactured by Mitsui Metals Industry Co., Ltd., volume average particle size: 0.36 μm) as small-diameter copper particles and 68 g of CuAtW-250 (manufactured by Fukuda Metal Foil Powder Industry Co., Ltd., volume average particle size: 27 μm) as large-diameter copper particles were added to the dispersion. The mixture was stirred at 2000 rpm for 1 minute using a planetary vacuum mixer (ARV-310, manufactured by Thinky Co., Ltd.). Then, the mixture was stirred once with a spatula to confirm the absence of solids. Under reduced pressure, the mixture was stirred at 2000 rpm for 1 minute to obtain a copper paste. Furthermore, CuAtW-250 is amorphous with a tap density of 3.9 g/ ^cm³ . Additionally, the copper paste has a viscosity of 32 Pa·s. Viscosity was measured using an E-type viscometer (VISCOMETER TV-33, manufactured by Toki Kogyo) equipped with an SPP rotor at a temperature of 25°C and a rotation speed of 2.5 rpm. The viscosity value is the value after 144 seconds from the start of measurement (JIS 3284).

<Examples 2 to 4 and Comparative Example 1> The amounts of large-diameter copper particles and small-diameter copper particles were changed to those shown in Table 1. Otherwise, the copper paste was prepared in the same manner as in Example 1. Furthermore, in this embodiment, the amounts (unit: parts by mass) shown in the table are the amounts of solid components.

<Example 5> The amounts of large-diameter copper particles, small-diameter copper particles, and lauric acid were changed to those shown in Table 2. As a dispersion medium, the amount of terpineol C (a mixture of isomers of α-terpineol, β-terpineol, and γ-terpineol, manufactured by Nippon Terpene Chemical Co., Ltd., trade name) shown in Table 2 was used instead of dihydroterpineol. As a thermally decomposable resin, a solution of M-6003 (molecular weight Mn=189300, 95% thermal decomposition temperature=284°C, manufactured by Nejou Kogyo Co., Ltd., trade name) shown in Table 2 was used instead of KFA-2000. Otherwise, the copper paste was prepared in the same manner as in Example 1. The viscosity of the copper paste was measured to be 63 Pa·s, the same as in Example 1.

<Example 6> The amounts of large-diameter copper particles and lauric acid were changed to those shown in Table 2 to create small-diameter copper particles. The amounts of CT-0500 (manufactured by Mitsui Metals Industry Co., Ltd., volume average particle size: 1.11) shown in Table 2 were used. Instead of CH-0200, terpineol C (a mixture of isomers of α-terpineol, β-terpineol, and γ-terpineol, manufactured by Nippon Terpene Chemical Co., Ltd., trade name) was used as the dispersion medium, and instead of dihydroterpineol, the amount of terpineol C shown in Table 2 was used as the thermally decomposable resin, and instead of KFA-2000, a solution of M-6003 (molecular weight Mn=189300, 95% decomposition temperature=284°C, manufactured by Nejou Kogyo Co., Ltd., trade name) was used. Otherwise, the copper paste was prepared in the same manner as in Example 1.

<Example 7> 162.4 g of terpineol C (a mixture of isomers of α-terpineol, β-terpineol, and γ-terpineol, manufactured by Nippon Terpene Chemical Co., Ltd., trade name) as a dispersion medium was mixed with 26.6 g of M-6003 (manufactured by Negami Kogyo) as a thermally decomposable resin. The mixture was stirred for 3 hours using a mixing rotor with stirring blades until completely dissolved, thereby obtaining a resin solution. 162.0 g of CT-0500 (manufactured by Mitsui Metals Mining Co., Ltd., volume average particle size: 1.11 μm) as small-diameter copper particles was 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 Industry, volume average particle size: 27 μm) and 81 g of FC-115 (dendritic copper powder, manufactured by Fukuda Metal Foil Powder Industry, volume average particle size: 21 μm, tap density: 1.2 g/ ^cm³ ) as large-diameter copper particles were added, and the mixture was mixed at 50 rpm for 15 minutes using a planetary mixer. The mixture was then depressurized and mixed at 50 rpm for 15 minutes to obtain a copper paste. The viscosity of the copper paste was measured to be 50 Pa·s, as in Example 1.

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

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

<Evaluation> [Evaluation of the printability of copper paste (1)] A Hull cell copper plate was divided into 3 equal parts, and a copper plate with a length of 30 mm × width of 67 mm × thickness of 300 μm was prepared. A SUS mask with a thickness of 70 μm and two openings of 25 mm × 5 mm was placed on the copper plate, and copper paste was printed using a metal squeegee. At this time, the case in which a coating surface with no fly-white, spots and uniform thickness can be obtained is defined as printability A, the case in which a coating surface with several stripes of fly-white and/or spots can be obtained is defined as printability B, and the case in which fly-white and/or spots to the extent that the substrate can be seen on the entire surface of the coating surface is defined as printability C. As for printability (1), the results are shown in Tables 1 and 2.

[Evaluation of the printability of copper paste (2)] Copper paste was placed into a 5 mL plastic syringe manufactured by Musashi Engineering Co., Ltd. The syringe containing copper paste was placed in an air compressor (ML-505X, manufactured by Musashi Engineering), and a wide printing needle was installed at the front end of the syringe. The paste was sprayed onto a copper plate (a Hall channel copper plate divided into 3 equal parts, 30 mm in length × 67 mm in width × 300 μm in thickness) under a pressure of 1 kgf/cm ² (=98 kPa). At this point, the situation where printing can be carried out continuously and uniformly is defined as printability A, the situation where printing is intermittent or very slow is defined as printability B, and the situation where printing cannot be carried out or stops during the printing process is defined as printability C. As for printability (2), the results are shown in Tables 1 and 2.

[Evaluation of Shape Retention Before Sintering] Copper paste for both the experimental and comparative examples was printed using a 70 μm thick stencil plate and then dried. The shape retention before sintering was evaluated by examining the prints obtained through finger rubbing. Shape retention before sintering was defined as A, where the print shape remained intact even with finger rubbing; and C, where the print broke into powder and could not maintain its shape when rubbed with a finger. The results are shown in Tables 1 and 2.

[Calming of Copper Paste] A copper plate printed with the copper paste obtained in the printability evaluations (1) and (2) was placed on a heating plate heated to 90°C and dried in air for 10 minutes to prepare a calcination sample. The sample was placed on a glass tray of a tubular furnace (manufactured by AVC Corporation) and placed in the tubular furnace. After depressurization, hydrogen was passed through at 100 sccm and nitrogen at 900 sccm. After returning to atmospheric pressure, calcination was carried out at 600°C for 20 minutes and maintained for 60 minutes. Then, the gas was stopped, and forced air cooling was performed while depressurization for more than 30 minutes. After restoring to normal pressure using argon, the calcined sample was taken out into the air. In this way, a sintered body of copper paste was obtained. The sample (sintered body) obtained using the copper paste obtained in the printability evaluation (1) was used in the following evaluation.

[Adhesion Evaluation (Tape Peeling Test)] A 16 mm wide glass tape (Cellotape, registered trademark) manufactured by Nichiban Co., Ltd. was applied to the obtained sintered body. The tape was rubbed vigorously with fingertips for approximately 10 seconds. Then, after 30 seconds to 5 minutes, the end of the tape was grasped at an angle as close to 60° as possible and peeled off within 0.5 to 1.0 seconds. The adhesion on the tape was then examined. No adhesion was classified as A, a small amount of adhesion was classified as B, and adhesion formed on the entire surface was classified as C. The results are shown in Tables 1 and 2.

[Determination of Porosity] The obtained sample was placed in a plastic cup, and injection molding resin (epomount, manufactured by Refine Tec Co., Ltd.) was poured in. The sample was then placed in a vacuum dryer and depressurized to remove bubbles. The resin was then allowed to harden at room temperature for 10 hours. Using a Refine Saw Excel (manufactured by Refine Tec Co., Ltd.) equipped with a resin bonding grinding wheel, the sample was cut near the section to be observed. The sample was polished using a cutting profile cut with a polishing device (Refine Polisher Hv, manufactured by Refine Tec Co., Ltd.) equipped with water-resistant polishing paper (carbon mac paper, manufactured by Refine Tec Co., Ltd.). The sample was then polished using an alumina polishing slurry. The sample was observed using a SEM device (TM-1000, manufactured by Hitachi High-technologies Co., Ltd.) at an applied voltage of 15 kV and various magnifications. The observed images were binarized using ImageJ, and the porosity (unit: volume %) of the sintered body (capillary structure) was calculated based on the ratio of white to black area points. The results are shown in Tables 1 and 2.

[Determination of Average Pore Diameter] The pore lengths of the 500x SEM images obtained in the [Determination of Porosity] section were measured using Image J. The average pore diameter of the sintered body (capillary structure) was determined by averaging 20 measurements. The results are shown in Tables 1 and 2.

[Determination of Pore Diameter Distribution] Using three 500x and three 10000x SEM images obtained in the [Determination of Porosity] section, the length of the pores mapped onto the image was measured using Image J, and the pore diameter distribution of the sintered body (capillary structure) was determined by the size distribution at 120°. Figure 2 shows the SEM image of Example 7. Figure 2(a) is a 500x SEM image, and Figure 2(b) is a 10000x SEM image. In Example 1, pore diameter peaks were confirmed at 1.2 μm and 20 μm. In Example 6, pore diameter peaks were confirmed at 1.0 μm and 30 μm. In Example 7, aperture peaks were confirmed at 1.1 μm and 30 μm.

[Table 1] Implementation Example 1 Implementation Example 2 Implementation Example 3 Implementation Example 4 Comparative example 1 Large-diameter copper particles CuAtW-250 68 76.5 72.25 59.5 85 Small-diameter copper particles CH-0200 17 8.5 12.75 25.5 0 Thermally decomposable resins KFA-2000 0.72 0.72 0.72 0.72 0.72 solvent dihydroterpenol 13.98 13.98 13.98 13.98 13.98 Additives Lauric acid 0.3 0.3 0.3 0.3 0.3 Printability (1) A A A A B Printability (2) A A A A B Shape retention before sintering A A A A A Tightness A B A A C Porosity (volume %) 49 48 44 41 - Average pore diameter (μm) 20 20 20 15 -

[Table 2] Implementation Example 5 Implementation Example 6 Implementation Example 7 Comparative example 2 Comparative example 3 Comparative example 4 Large-diameter copper particles CuAtW-250 64.8 66.4 56.7 64.8 - - FC-115 - - 8.1 - - - Small-diameter copper particles CH-0200 16.2 - - 16.2 - - CT-0500 - 16.6 16.2 - 16.6 16.6 Other copper particles MA-C25 - - - - 66.4 - 1400YF - - - - - 66.4 Thermally decomposable resins KFA-2000 - - - - - - M-6003 3.8 2.38 2.66 - 2.38 2.38 solvent Terpineol C 14.82 14.535 16.245 18.62 14.535 14.535 Additives Lauric acid 0.38 0.085 0.095 0.38 0.085 0.085 Printability (1) A A A C A A Printability (2) A A A C A A Shape retention before sintering A A A C A A Tightness A B A - A A Porosity (%) 48 36 50 - 30 34 Average pore diameter (μm) 25 30 30 - 1 3

[Capillary Structure Fabrication] Regarding the sintered body fabricated using the copper paste of the embodiments, it was confirmed that the resulting sintered body constitutes a heat pipe and functions as a capillary structure. On the other hand, as shown in the evaluation results above, the sintered body fabricated using the copper paste of Comparative Example 1 lacks sufficient adhesion, and the copper paste of Comparative Example 1 is unsuitable as a copper paste for capillary structure formation. In addition, the copper paste of Comparative Example 2 lacks printability, and the copper paste of Comparative Example 2 cannot be used to fabricate a sintered body. Furthermore, regarding the sintered bodies fabricated using the copper pastes of Comparative Examples 3 and 4, the sintered bodies did not function as a capillary structure.

1: Heat pipe 2: Container 3: Capillary structure A: Vapor phase space S: Enclosed space

Figure 1 is a schematic cross-sectional view showing one embodiment of the heat pipe. Figures 2(a) and (b) are scanning electron microscope (SEM) images showing cross-sectional views of the sintered body (capillary structure) of the embodiment.

1: Heat pipe

2: Container

3: Capillary Structure

A: Vapor phase space

S: Enclosed space

Claims (8)

A copper paste for forming the capillary structure of a heat pipe, comprising copper particles, a thermally decomposable resin, and a dispersion medium, wherein the copper particles comprise large-diameter copper particles with a volume average particle size of 10 μm to 50 μm and small-diameter copper particles with a volume average particle size of 0.1 μm to 2.0 μm. The copper paste as described in claim 1, wherein the thermally decomposable resin has a 95% thermal decomposition temperature below 350°C. The copper paste as described in claim 1 or claim 2, wherein, based on the total mass of the copper particles, the content of large-diameter copper particles is 40% to 90% by mass, and the content of small-diameter copper particles is 10% to 60% by mass, based on the total mass of the copper particles. The copper paste as described in claim 1 or claim 2, wherein the content of the thermally decomposable resin is 1 to 20 parts by mass relative to 100 parts by mass of the copper particles. The copper paste as described in claim 1 or claim 2, wherein the tap density of the large-diameter copper particles is 1.0 g/ ^cm³ to 4.5 g/ ^cm³ . The copper paste as described in claim 1 or claim 2, wherein the viscosity of the copper paste is 10 Pa·s to 120 Pa·s. A method for forming a capillary structure, specifically a method for forming the capillary structure of a heat pipe, comprising: a step of printing copper paste as described in any one of claims 1 to 6; and a step of sintering the copper paste. A heat pipe comprising a capillary structure of a sintered body containing copper paste as described in any one of claims 1 to 6.