Copper paste, wick forming method and heat pipe

Abstract

A copper paste for wick formation in heat pipes, the copper paste containing copper particles, pyrolytic resin particles, a dispersion medium that disperses the copper particles and pyrolytic resin particles, and a pyrolytic resin that is soluble in the dispersion medium.

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 considerations such as miniaturization and close contact with the heat source. Such flat heat pipes are becoming thinner due to volume constraints in small information devices, and the wick thickness must also be thin. Furthermore, in flat heat pipes, the wick surface may have a complex shape, such as an uneven surface. However, when the wick is made of porous sintered powder, conventional manufacturing methods have difficulty producing such thin wicks and wicks with complex shapes. The present inventors have investigated forming a wick using a paste-like composition (copper paste) containing copper particles.

[0005] Copper paste can be used to easily form thin film wicks and wicks with complex shapes by printing. However, there is room for improvement in the porosity of the wick (the proportion of voids in the wick).

[0006] Therefore, an object of the present invention is to provide a copper paste that can form a wick with high porosity. [Means for solving the problem]

[0007] After extensive research, the inventors discovered that the porosity of the wick can be improved by adding particulate heat-decomposable resin (heat-decomposable resin particles) to the copper paste, and thus completed the present invention.

[0008] That is, one aspect of the present invention relates to a copper paste for forming a wick shown in [1] to [7] below, a method for forming a wick shown in [8], and a heat pipe shown in [9].

[0009] [1] A copper paste for forming a wick of a heat pipe, the copper paste containing copper particles, heat-decomposable resin particles, a dispersion medium for dispersing the copper particles and the heat-decomposable resin particles, and a heat-decomposable resin that is soluble in the dispersion medium.

[0010] [2] The copper paste according to [1], wherein the content of the thermally decomposable resin particles is 3 to 30 parts by mass per 100 parts by mass of the total amount of the copper particles and the thermally decomposable resin particles.

[0011] [3] The copper paste according to [1] or [2], wherein the thermally decomposable resin particles have a volume average particle size of 5 to 40 μm.

[0012] [4] The copper paste according to any one of [1] to [3], wherein the 95% thermal decomposition temperature of the thermally decomposable resin particles and the thermally decomposable resin is 450° C. or lower.

[0013] [5] The copper paste according to any one of [1] to [4], wherein the content of the thermally decomposable resin is 1 to 25 parts by mass per 100 parts by mass of the copper particles.

[0014] [6] The copper paste according to any one of [1] to [5], wherein the proportion of copper particles having a particle size of 1.5 μm or less is 10% by volume or more based on the total amount of the copper particles.

[0015] [7] The copper paste according to any one of [1] to [6], wherein the viscosity of the copper paste at 25°C is 10 to 120 Pa·s.

[0016] By using the copper paste of the above aspect, a wick having a high porosity can be formed by printing.

[0017] Incidentally, when forming a thin film wick (e.g., a wick having a thickness of 50 μm or less), it is preferable to use copper particles having a volume average particle size smaller than the film thickness of the wick from the viewpoint of sinterability. However, when copper particles having a small volume average particle size are used, the pore size and porosity also become small, which tends to increase flow resistance due to capillary action. Therefore, it is difficult to form a thin film wick with a sufficiently high porosity using the conventional method described above. On the other hand, with the copper paste described above, pores can be formed using thermally decomposable resin particles, so that a wick with a sufficiently large pore size can be formed even when copper particles having a small volume average particle size (e.g., a volume average particle size of 50 μm or less) are used. Therefore, the copper paste described above is suitable for forming a thin film wick (e.g., a wick having a thickness of 50 μm or less).

[0018] [8] A method for forming a wick for a heat pipe, comprising the steps of printing the copper paste according to any one of [1] to [7], and sintering the copper paste.

[0019] [9] A heat pipe comprising a wick containing a sintered body of the copper paste according to any one of [1] to [7]. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a copper paste that can form a wick having a high porosity. [Brief explanation of the drawings]

[0021] [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 cross-sectional SEM images of the sintered bodies (wicks) of the examples and comparative examples. [Figure 3] FIG. 2 is a diagram showing a cross-sectional SEM image of a sintered body (wick) of an example. [Figure 4]FIG. 2 is a diagram showing a cross-sectional SEM image of a sintered body (wick) of an example. [Figure 5] FIG. 2 is a diagram showing a cross-sectional SEM image of a sintered body (wick) of an example. DETAILED DESCRIPTION OF THE INVENTION

[0022] In this specification, a numerical range indicated with "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this specification, the upper or lower limit of a numerical range of a certain stage may be replaced with the upper or lower limit of a numerical range of another stage. Furthermore, unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more. 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. Furthermore, in this specification, "(meth)acrylic" means at least one of acrylic and its corresponding methacrylic. Furthermore, "(crosslinked)" means both the presence and absence of the prefix "crosslinked."

[0023] Preferred embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments.

[0024] <Copper paste> One embodiment of the copper paste is a wick-forming copper paste used to form a wick for a heat pipe. The copper paste contains copper particles, thermally decomposable resin particles, a dispersion medium for dispersing the copper particles and the thermally decomposable resin particles, and a thermally decomposable resin soluble in the dispersion medium. Hereinafter, the thermally decomposable resin constituting the thermally decomposable resin particles may be referred to as "thermally decomposable resin A," and the thermally decomposable resin soluble in the dispersion medium may be referred to as "thermally decomposable resin B." Each component contained in the copper paste will be described below.

[0025] (copper particles) The copper particles are particles whose main component is metallic copper, and are dispersed in a dispersion medium in the copper paste. The content of copper element in the copper particles may be 90 atm% or more, based on the total amount of metal elements contained in the copper particles. A copper content of 90 atm% or more makes it easier to obtain good sinterability and good thermal conductivity. From this perspective, the content of copper element may be 93 atm% or more or 95 atm% or more, based on the total amount of metal elements contained in the copper particles. In other words, the content of metal elements other than copper element in the copper particles may be 10 atm% or less, 7 atm% or less, or 5 atm% or less, based on the total amount of metal elements contained in the copper particles.

[0026] The copper particles may contain copper oxide. The content of copper oxide in the copper particles may be 20% by mass or less, 15% by mass or less, or 10% by mass or less, based on the total mass of the copper particles. When the copper oxide content is within the above range, volumetric shrinkage due to reduction during sintering, which can cause cracks, peeling, etc., is less likely to occur. From this perspective, the copper particles may not contain copper oxide. The copper oxide may be cuprous oxide or cupric oxide. The copper oxide may be contained in the copper particles as copper oxide contained in a natural oxide film formed on the surface of the copper particles.

[0027] The copper particles may be spherical, blocky, needle-like, flake-like, dendritic (branch-like), approximately spherical, or irregular. Among these, the use of flake-like, dendritic, and irregular-shaped copper particles facilitates the improvement of the wick porosity. Two or more types of copper particles having different shapes may be used as the copper particles. Preferred examples of combinations of copper particles having different shapes include spherical and flaky, spherical and dendritic, and spherical and irregular. When spherical and flaky copper particles are used in combination, the sinterability of the copper particles improves, and a wick (sintered body) having excellent strength and adhesion to the adherend tends to be formed. Similar effects tend to be obtained when spherical and dendritic copper particles are used in combination, and when spherical and irregular-shaped copper particles are used in combination.

[0028] The volume average particle size of the copper particles may be smaller than the film thickness of the wick to be formed. For example, when forming a thin wick with a thickness of approximately 50 μm, the volume average particle size of the copper particles may be 45 μm or less. When copper particles having such a volume average particle size are used, the number of copper particles larger than the target wick film thickness is reduced, the printing shape is improved, and the wick film thickness is more likely to meet the target value. From this perspective, the ratio of the volume average particle size of the copper particles to the thickness of the wick to be formed (volume average particle size of copper particles / wick thickness) may be 0.9 or less, 0.8 or less, or 0.7 or less. The ratio of the volume average particle size of the copper particles to the thickness of the wick to be formed (volume average particle size of copper particles / wick thickness) may be 0.1 or more. Here, the volume average particle size of the copper particles refers to the particle size (d50) at the point where the cumulative curve reaches 50% when the particle size distribution of the copper particles is determined on a volume basis using a light scattering particle size distribution analyzer and the cumulative curve is calculated with the total volume set to 100%.

[0029] From the viewpoint of low-temperature sintering, the 10% volume average particle size of the copper particles may be 5.0 μm or less, 4.0 μm or less, or 2.0 μm or less. The 10% volume average particle size of the copper particles may be 0.1 μm or more. That is, the 10% volume average particle size of the copper particles may be 0.1 to 5.0 μm, 0.1 to 4.0 μm, or 0.1 to 2.0 μm. Here, the 10% volume average particle size of the copper particles refers to the particle size at the point where the cumulative curve is 10% when the particle size distribution of the copper particles is determined on a volume basis using a light scattering particle size distribution analyzer and a cumulative curve is calculated with the total volume set to 100%.

[0030] The proportion of copper particles having a particle size larger than the film thickness of the wick to be formed (e.g., coarse particles such as aggregates of primary particles) among the copper particles may be 10% by volume or less, 7% by volume or less, or 5% by volume or less, based on the total volume of the copper particles. When the proportion of copper particles having a particle size larger than the film thickness of the wick is within the above range, better sinterability is likely to be obtained. From this perspective, the copper paste does not need to contain copper particles having a particle size larger than the film thickness of the wick. From the perspective of even better sinterability, the proportion of copper particles having a particle size more than 0.9 times the film thickness of the wick (e.g., a particle size larger than 45 μm when the wick film thickness is 50 μm) may be within the above range. Here, the proportion of copper particles having a predetermined particle size can be determined by a particle sieving test, measurement of the copper paste using a particle gauge, etc. Note that when copper particles are mixed with other components (e.g., thermally decomposable resin particles), the specific gravity of the copper paste may be, for example, 1.3 to 8.0 g / cm. 3 After adding a liquid of about 1000 ml and centrifuging to separate only the copper particles, the proportion of the copper particles can be determined by the above method.

[0031] From the viewpoint of imparting sinterability, the proportion of copper particles having a particle size of 1.5 μm or less may be 10 vol% or more, 12 vol% or more, or 15 vol% or more, based on the total volume of the copper particles. From the viewpoint of obtaining better sinterability, the proportion of copper particles having a particle size of 1.2 μm or less may be within the above range, and the proportion of copper particles having a particle size of 1.0 μm or less may be within the above range. The proportion of copper particles having a particle size of 1.5 μm or less may be 80 vol% or less, 50 vol% or less, or 30 vol% or less, based on the total volume of the copper particles. The proportion of copper particles having a particle size of 1.5 μm or less may be 100 vol% based on the total volume of the copper particles, but from the viewpoint of increasing the pore size and increasing the porosity, the copper particles may contain copper particles having a particle size greater than 1.5 μm.

[0032] From the viewpoint of dispersibility, the proportion of copper particles having a particle size of 0.1 μm or less among the copper particles may be 5 vol% or less, 4 vol% or less, or 3 vol% or less, based on the total volume of the copper particles, or may be 0 vol%.

[0033] A copper paste containing copper particles having the particle size distribution described above can be obtained, for example, by combining two or more types of copper particles (particle groups) having different volume average particle sizes. The combination of two or more types of copper particles may be, for example, a combination of copper particles having a volume average particle size of 5 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). From the viewpoints of combustibility and mixability, the large-diameter copper particles may be flaky, dendritic, or irregular, and the small-diameter copper particles may be spherical or approximately spherical.

[0034] The amount of large-diameter copper particles added 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 amount of large-diameter copper particles added 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 amount of large-diameter copper particles added 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.

[0035] The amount of small-diameter copper particles added 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 amount of small-diameter copper particles added 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 these viewpoints, the amount of small-diameter copper particles added 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.

[0036] The mass ratio of the content of small-diameter copper particles to the content of large-diameter copper particles (amount of small-diameter copper particles added / amount of large-diameter copper particles added) 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 (amount of small-diameter copper particles added / amount of large-diameter copper particles added) 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.

[0037] 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 these 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.

[0038] (pyrolytic resin particles) The thermally decomposable resin particles are resin particles composed of a thermally decomposable resin (thermolybdenum resin A). The content of the thermally decomposable resin A in the thermally decomposable resin particles may be 90% by mass or more, 93% by mass or more, or 95% by mass or more, based on the total mass of the thermally decomposable resin particles. The thermally decomposable resin particles may consist of only the thermally decomposable resin A.

[0039] The thermally decomposable resin particles can be decomposed at a temperature lower than the sintering temperature. Because the thermally decomposable resin particles have such thermal decomposition properties, voids are formed in the areas where the thermally decomposable resin particles were present during sintering of the copper paste, and a wick with high porosity is formed by sintering the copper paste. Furthermore, since voids are formed in the areas where the thermally decomposable resin particles were present, the porosity of the wick can be easily adjusted to a desired range by adjusting the shape and content of the thermally decomposable resin particles, and the pore size can also be adjusted.

[0040] The 95% thermal decomposition temperature of the thermally decomposable resin particles may be 450°C or lower, 400°C or lower, or 350°C or lower. When the 95% thermal decomposition temperature of the thermally decomposable resin particles is within the above range, the wick can be baked at a low temperature and in a short time, and further, residues of the thermally decomposable resin particles are less likely to be generated in the wick. From this perspective, the 95% thermal decomposition temperature of the thermally decomposable resin A may also be within the above range. The 95% thermal decomposition temperature of the thermally decomposable resin particles may be 120°C or higher. The 95% thermal decomposition temperature is the 95% weight loss temperature measured by TG / DTA measurement. 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.

[0041] The amount of residue (ash content) after thermal decomposition of the thermally decomposable resin particles at the sintering temperature may be 5% by mass or less, or 2% by mass or less, relative to the mass of the thermally decomposable resin particles before thermal decomposition. The smaller the amount of residue after thermal decomposition, the better the sinterability. From this perspective, the amount of residue (ash content) after thermal decomposition of the thermally decomposable resin A at the sintering temperature may also be within the above range. The ash content can be determined by TG / DTA measurement in an inert gas (nitrogen or argon) containing 3 to 5% by mass of hydrogen. Specifically, a sample (thermally decomposable resin particles or thermally decomposable resin A) is held in an inert gas (nitrogen or argon) containing 3 to 5% by mass of hydrogen at the sintering temperature for the sintering time, and the weight change of the sample before and after holding is measured. The ash content can be determined from the resulting weight change. Note that TG / DTA measurement in air is not preferred because oxidative decomposition of the sample progresses, resulting in a smaller amount of residue compared to the amount of residue in a reducing atmosphere.

[0042] The thermally decomposable resin particles can remain in particulate form in the dispersion medium and are dispersed in the dispersion medium in the copper paste. The amount of the thermally decomposable resin particles that dissolves in 100 g of the dispersion medium at 25° C. is, for example, 1 g or less. From this perspective, the amount of the thermally decomposable resin A that dissolves in 100 g of the dispersion medium at 25° C. may also be 1 g or less.

[0043] The thermally decomposable resin A constituting the thermally decomposable resin particles may be a copolymer from the viewpoint of reducing the solubility of the resin particles in a solvent (dispersion medium), and may be a crosslinked body (a thermally decomposable resin having a three-dimensional crosslinked structure) from the viewpoint of further reducing the solubility. Examples of the thermally decomposable resin A include (crosslinked) polycarbonate, (crosslinked) poly(meth)acrylic acid, (crosslinked) poly(meth)acrylic acid ester, (crosslinked) polyester, and (crosslinked) polyether. From the viewpoints of solvent resistance, cost, ease of particle synthesis, and thermal decomposition, the thermally decomposable resin A may be a crosslinked poly(meth)acrylic acid ester.

[0044] The thermally decomposable resin particles may be spherical, lumpy, approximately spherical (for example, long grains), short fiber-like, or irregular in shape.

[0045] The volume average particle size of the thermally decomposable resin particles may be smaller than the film thickness of the wick to be formed, from the viewpoint of facilitating film thickness control and preventing the pore size from becoming too large. For example, the ratio of the volume average particle size of the thermally decomposable resin particles to the wick thickness (volume average particle size of thermally decomposable resin particles / wick thickness) may be 0.9 or less, 0.8 or less, or 0.7 or less. Here, the volume average particle size of the thermally decomposable resin particles refers to the particle size (d50) at the point where the cumulative curve is 50% when the particle size distribution of the thermally decomposable resin particles is determined on a volume basis using a scattering particle size distribution analyzer and a cumulative curve is calculated with the total volume set to 100%.

[0046] The volume average particle size of the thermally decomposable resin particles may be 5 μm or more, 7 μm or more, or 9 μm or more, from the viewpoint of easily forming a wick with a large pore size. The volume average particle size of the thermally decomposable resin particles may be 40 μm or less, 35 μm or less, or 30 μm or less, from the viewpoint of preventing the pore size from becoming too large. From these viewpoints, the volume average particle size of the thermally decomposable resin particles may be 5 to 40 μm, 7 to 35 μm, or 9 to 30 μm.

[0047] The proportion of thermally decomposable resin particles (e.g., coarse particles such as aggregates of primary particles) having a particle size larger than the film thickness of the wick to be formed may be 10% by volume or less, 7% by volume or less, or 5% by volume or less, based on the total volume of the thermally decomposable resin particles, from the viewpoint of improving the print shape and making it easier for the wick film thickness to meet the target value. From this viewpoint, the copper paste may not contain thermally decomposable resin particles having a particle size larger than the film thickness of the wick. From the viewpoint of preventing the pore size from becoming too large, the proportion of thermally decomposable resin particles having a particle size more than 0.9 times the film thickness of the wick (e.g., a particle size larger than 45 μm when the wick film thickness is 50 μm) may be within the above range. Here, the proportion of thermally decomposable resin particles having a predetermined particle size can be determined by particle sieving tests, measurement of the copper paste using a particle gauge, etc.

[0048] The content of the thermally decomposable resin particles may be 3 parts by mass or more, 5 parts by mass or more, or 6 parts by mass or more, relative to 100 parts by mass of the total amount of the copper particles and the thermally decomposable resin particles. In this case, a wick (sintered body) with higher porosity is likely to be obtained. The content of the thermally decomposable resin particles may be 30 parts by mass or less, 25 parts by mass or less, 20 parts by mass or less, 15 parts by mass or less, or 10 parts by mass or less, relative to 100 parts by mass of the total amount of the copper particles and the thermally decomposable resin particles. When the content of the thermally decomposable resin particles is 30 parts by mass or less, a wick (sintered body) with sufficient strength and adhesion is likely to be obtained. When the content is 15 parts by mass or less, a wick (sintered body) with better strength and adhesion is likely to be obtained. When the content is 10 parts by mass or less, a wick (sintered body) with even better strength and adhesion is likely to be obtained. From these viewpoints, the content of the thermally decomposable resin particles may be 3 to 30 parts by mass, 5 to 25 parts by mass, 6 to 20 parts by mass, 3 to 15 parts by mass, 5 to 15 parts by mass, 3 to 10 parts by mass, or 5 to 10 parts by mass, relative to 100 parts by mass of the total amount of the copper particles and the thermally decomposable resin particles.

[0049] The content of the thermally decomposable resin particles may be 19 parts by volume or more, 29 parts by volume or more, or 33 parts by volume or more, relative to 100 parts by volume of the total amount of the copper particles and the thermally decomposable resin particles. In this case, a wick (sintered body) with higher porosity is likely to be obtained. The content of the thermally decomposable resin particles may be 77 parts by volume or less, 72 parts by volume or less, 66 parts by volume or less, 55 parts by volume or less, or 45 parts by volume or less, relative to 100 parts by volume of the total amount of the copper particles and the thermally decomposable resin particles. When the content of the thermally decomposable resin particles is 77 parts by volume or less, a wick (sintered body) with sufficient strength and adhesion is likely to be obtained. When the content is 55 parts by mass or less, a wick (sintered body) with better strength and adhesion is likely to be obtained. When the content is 45 parts by mass or less, a wick (sintered body) with even better strength and adhesion is likely to be obtained. From these viewpoints, the content of the thermally decomposable resin particles may be 19 to 77 volume parts, 29 to 72 volume parts, 33 to 66 volume parts, 19 to 55 volume parts, 29 to 55 volume parts, 19 to 45 volume parts, or 29 to 45 volume parts per 100 volume parts of the total amount of the copper particles and the thermally decomposable resin particles.

[0050] (Pyrolytic resin B) The thermally decomposable resin B is soluble in the dispersion medium. The amount of the thermally decomposable resin B that dissolves in 100 g of the dispersion medium at 25° C. is, for example, more than 5 g.

[0051] In the copper paste, the thermally decomposable resin B is partially or entirely dissolved in the dispersion medium. The amount of thermally decomposable resin B dissolved in the dispersion medium at 25°C may be 5 parts by mass or more, 6 parts by mass or more, or 7 parts by mass or more per 100 parts by mass of the dispersion medium. The amount of thermally decomposable resin B that is not dissolved in the dispersion medium may be 10% by mass or less, 5% by mass or less, or 3% by mass or less of the total mass of the thermally decomposable resin B, from the viewpoint of making it easier to control the size of the pores and suppressing adhesion to the printing mask.

[0052] The thermally decomposable resin B is thermally decomposable in addition to being soluble in the dispersion medium, and can be decomposed at a temperature lower than the sintering temperature. Therefore, the thermally decomposable resin B functions as a binder for the copper particles and the thermally decomposable resin particles in the copper paste, and decomposes during sintering to form voids between the particles.

[0053] The 95% thermal decomposition temperature of the thermally decomposable resin B may be 450°C or lower, 400°C or lower, or 350°C or lower. When the 95% thermal decomposition temperature of the thermally decomposable resin B is within the above range, the thermally decomposable resin B is easily removed at low temperatures, making it possible to perform baking at low temperatures of 500°C or lower, and further, it is less likely that residues of the thermally decomposable resin B will be left in the wick. The 95% thermal decomposition temperature of the thermally decomposable resin B may be 160°C or higher, from the viewpoint of removing only the dispersion medium in the drying process. The above 95% thermal decomposition temperature can be measured in the same manner as the 95% thermal decomposition temperature of the thermally decomposable resin particles.

[0054] The amount of residue (ash content) after thermal decomposition of the thermally decomposable resin B at the sintering temperature may be 5% by mass or less, or 2% by mass or less, relative to the mass of the thermally decomposable resin B before thermal decomposition. The smaller the amount of residue after thermal decomposition, the better the sinterability. The ash content can be measured in the same manner as the ash content of the thermally decomposable resin particles.

[0055] Examples of the thermally decomposable resin B include polycarbonate, poly(meth)acrylic acid, poly(meth)acrylic acid ester, polyester, polyether, etc. From the viewpoints of solubility in a dispersion medium (organic solvent), cost, and thermal decomposition, the thermally decomposable resin B may be polymethacrylic acid ester.

[0056] The content of thermally decomposable resin B 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 copper particles, from the viewpoint of excellent shape retention after printing and drying. The content of thermally decomposable resin B may be 25 parts by mass or less, 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 viewpoint of easy viscosity adjustment and excellent sinterability. From these viewpoints, the content of thermally decomposable resin B may be 1 to 25 parts by mass, 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.

[0057] (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, terpineol (α-terpineol, β-terpineol, γ-terpineol, and mixtures thereof), 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, and propylene glycol. Examples of suitable esters include ethers such as 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; 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.

[0058] The content of the dispersion medium may be 5 parts by mass or more, 50 parts by mass or less, or 5 to 50 parts by mass, relative to 100 parts by mass of the 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 inhibited.

[0059] (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 less than 20 mass%, 10 mass% or less, or 5 mass% or less, based on the total mass of the metal particles contained in the copper paste. The copper paste may not contain other metal particles. In addition, when the copper paste contains other metal particles, the content per 100 parts by mass of copper particles in this specification may be interpreted as the content per 100 parts by mass of metal particles.

[0060] 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.

[0061] 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.

[0062] The thixotropy index (hereinafter also referred to as "TI value") of the copper paste may be 2.0 or more and 20 or less, 3.0 or more and 15 or less, or 4.0 or more and 10 or less. When the TI value of the copper paste is within the above range, the viscosity of the copper paste is 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 attached to the adherend, the viscosity recovers when left to stand, preventing excessive wetting and spreading of the printed matter. The TI value is determined by measuring the viscosity of the paste using an E-type viscometer at 25°C and a rotation speed of 0.5 rpm, where μ 0.5 and is the value calculated by the following formula when the viscosity measured at 25°C and a rotation speed of 5 rpm is μ5. TI value = μ 0.5 / μ5

[0063] The copper paste described above can be prepared by mixing copper particles, thermally decomposable resin particles (particles mainly composed of thermally decomposable resin A), thermally decomposable resin B, a dispersion medium, and other components. The copper paste can be prepared, for example, by dissolving thermally decomposable resin B in a dispersion medium, adding thermally decomposable resin particles and copper particles to the resulting solution, and performing a dispersion treatment. Alternatively, the copper paste can be prepared by mixing the solution obtained by dissolving a thermally decomposable resin in a dispersion medium with a dispersion obtained by mixing thermally decomposable resin particles and copper particles in a dispersion medium and performing a dispersion treatment. After mixing the components, a stirring treatment may be performed. The maximum diameter of the dispersion may be adjusted by a classification operation.

[0064] The dispersion treatment can be carried out using a disperser or a stirrer, examples of which 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.

[0065] The stirring treatment can be carried out using a stirrer, for example, an Ishikawa type stirrer, a rotation-revolution type stirrer, a Raikai mixer, a twin-screw kneader, a three-roll mill, or a planetary mixer.

[0066] 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.

[0067] <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.

[0068] The printing method for the copper paste is not particularly limited, and examples thereof include 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, and electrodeposition coating.

[0069] 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.

[0070] 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.

[0071] From the viewpoint of reducing thermal damage to each component and improving yield, the maximum temperature reached during heat treatment (firing temperature) may be 150°C or higher and 700°C or lower, 200°C or higher and 600°C or lower, or 250°C or higher and 550°C or lower. If the maximum temperature reached during heat treatment is 150°C or higher, sintering tends to proceed sufficiently when the holding time at the maximum temperature reached during heat treatment is 60 minutes or shorter.

[0072] The holding time at the maximum temperature reached during the heat treatment may be from 1 minute to 60 minutes, from 1 minute to less than 40 minutes, or from 1 minute to less than 30 minutes, from the viewpoint of volatilizing all of the dispersion medium and improving the yield.

[0073] 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 may be used. The drying conditions (temperature and time) may be adjusted appropriately depending on the type and amount of the dispersion medium used. The drying conditions (temperature and time) may be drying at 50°C to 180°C for 1 minute to 120 minutes.

[0074] According to the wick formation method described above, a wick with high porosity can be formed by printing. Furthermore, since the above method forms the wick by printing a copper paste, the wick can be easily formed even when the wick formation surface has a complex shape (for example, an uneven shape, a curved shape, a shape with a V-shaped recess, etc.). Furthermore, since the above method has a high degree of freedom in the shape of the wick that can be formed, a wick with a complex shape (for example, a shape with a curve) can be easily formed. Furthermore, it is also possible to form a thin film wick by adjusting the particle size of the copper particles, for example.

[0075] <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 heat pipe can be manufactured by the same method as that of a conventionally known heat pipe, except for the wick formation process. The wick containing the sintered copper paste can be formed according to the wick formation method of the above embodiment. An example of a heat pipe will now be described with reference to the drawings.

[0076] 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.

[0077] The shape of the container 2 is not particularly limited and may be tubular, flat, or the like. When the container 2 is flat, for example, a wick may be formed by printing copper paste in the recesses of a first substrate having recesses formed on its surface, and then the first substrate and a second substrate having recesses formed on their surfaces may be bonded together so that the recesses face each other. This results in a heat pipe 1 having a flat container 2.

[0078] The material of the container 2 is preferably 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.

[0079] 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, and has pores. In other words, 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).

[0080] At least some of the pores in the wick 3 are formed in the region where the thermally decomposable resin particles and thermally decomposable resin B were present. Usually, the pores derived from the thermally decomposable resin particles are larger than the pores derived from thermally decomposable resin B. The size of the pores derived from the thermally decomposable resin particles is approximately the same as the particle size of the thermally decomposable resin particles, and the diameter is, for example, 5 to 40 μm, 7 to 35 μm, or 9 to 30 μm.

[0081] 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, 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 80% by volume or less. The porosity can be obtained by analyzing a cross-sectional image 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, it 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 can be determined, for example, from the apparent density M1 (g / cm) from the volume of the wick and the mass of the wick measured with a precision balance. 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)

[0082] 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, 40 μm or less, or 30 μm or less from the viewpoint of achieving a good balance between flow resistance and capillary force and facilitating thinning of the wick. The average pore diameter is determined by measuring the diameter of the pores in an SEM image of the cross section processed after casting.

[0083] The thickness of the wick 3 may be 100 μm or less, 70 μm or less, 50 μm or less, or 40 μm or less. The thickness of the wick 3 may be 10 μm or more.

[0084] 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]

[0085] 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.

[0086] Example 1 [Preparation of copper paste] 1.941 g of dihydroterpineol (manufactured by Nippon Terpene Chemical Co., Ltd.) as a dispersion medium, 16.964 g of KFA-2000 (methacrylic resin, manufactured by GOO Chemical Industry Co., Ltd., 95% thermal decomposition temperature: 330°C, 33.6 mass% dihydroterpineol solution) as a thermally decomposable resin B, and 0.095 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 obtain a solution. To this solution, 15.066 g of copper particles, CH-0200 (manufactured by Mitsui Mining & Smelting Co., Ltd., spherical copper powder, volume average particle size: 0.2 μm) and 60.264 g of 1400-YF (manufactured by Mitsui Mining & Smelting Co., Ltd., flaky copper powder, volume average particle size: 6.8 μm), and 5.670 g of thermally decomposable resin particles, GR-600T (acrylic resin particles, manufactured by Negami Chemical Industrial Co., Ltd., volume average particle size: 9.4 μm, 95% thermal decomposition temperature: 370 ° C.), were added, and the mixture was stirred at 2000 rpm for 1 minute using a planetary vacuum mixer (Thinky Corporation). After that, the whole was stirred once with a spoon to confirm the absence of solids, and then stirred at 2000 rpm for 2 minutes under reduced pressure to obtain a copper paste. The volume average particle size (d50) of the copper particles (a mixture of CH-0200 and 1400-YF particles) in the copper paste was 5.5 μm, and the 10% volume average particle size was 2.9 μm. The content of thermally decomposable resin particles (Cw) per 100 parts by mass of the total of copper particles and thermally decomposable resin particles was 7 parts by mass, and the content of thermally decomposable resin particles (Cv) per 100 parts by volume of the total of copper particles and thermally decomposable resin particles was 36.4 parts by volume. The content of copper particles with a particle size of 1.5 μm or less (small diameter copper particles) was 20% by volume, based on the total amount of copper particles in the copper paste. The viscosity of the copper paste at 25°C was 10 to 120 Pa·s.

[0087] [Wick Preparation (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 copper paste was printed using a metal squeegee.

[0088] The copper plate printed with the copper paste obtained by printing the copper paste was placed on a hot plate heated to 90°C and dried in air for 20 minutes to prepare a fired sample. The sample was 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 sample was fired at a firing temperature of 600°C, with a heating time of 20 minutes and a holding time of 60 minutes. The gas flow was then stopped, and the sample was cooled by forced air cooling while depressurizing for at least 30 minutes. After returning the pressure to normal with argon gas, the fired sample was removed into the air. This resulted in a sintered body (sintered body 1) of the copper paste having the thickness shown in Table 1. The obtained sintered body 1 was used to construct a heat pipe, and it was confirmed that sintered body 1 functioned as a wick.

[0089] [Wick Preparation (2)] Except for using a 40 μm thick SUS mask with two 25 mm × 5 mm openings, copper paste was printed and fired in the same manner as in wick preparation (1), to obtain a sintered body of the copper paste (sintered body 2) with the thickness shown in Table 1. The obtained sintered body 2 was used to construct a heat pipe, and it was confirmed that the sintered body 2 functioned as a wick.

[0090] [Wick Preparation (3)] Copper paste was printed and fired in the same manner as in wick preparation (1), except that a 500 μm thick SUS mask with two 25 mm × 5 mm openings was used as the SUS mask, to obtain a sintered body of the copper paste (sintered body 3) with the thickness shown in Table 1. The obtained sintered body 3 was used to form a heat pipe, and it was confirmed that the sintered body 3 functioned as a wick.

[0091] <Example 2> A copper paste was obtained in the same manner as in Example 1, except that GR-300T (acrylic resin particles, manufactured by Negami Chemical Industrial Co., Ltd., volume average particle size: 22 μm) was used instead of GR-600T as the thermally decomposable resin particles. The viscosity of the copper paste at 25°C was 10 to 120 Pa·s. Next, wick preparations (1) to (3) were carried out in the same manner as in Example 1, except that the obtained copper paste was used, and sintered bodies of the copper paste (sintered bodies 1 to 3) having the thicknesses shown in Table 1 were obtained. The obtained sintered bodies 1 to 3 were used to construct heat pipes, and it was confirmed that sintered bodies 1 to 3 functioned as wicks.

[0092] <Comparative Example 1> A copper paste was obtained in the same manner as in Example 1, except that no thermally decomposable resin particles were used and the amounts of each component were changed to those shown in Table 1. Next, wick preparations (1) to (3) were carried out in the same manner as in Example 1, except that the obtained copper paste was used, and sintered bodies of the copper paste (sintered bodies 1 to 3) having the thicknesses shown in Table 1 were obtained.

[0093] [Table 1]

[0094] <Rating 1-1> A tape peeling test and porosity measurement were carried out on the sintered bodies 2 of Examples 1 and 2 and Comparative Example 1. The specific evaluation methods are shown below, and the evaluation results are shown in Table 2.

[0095] [Tape peeling test] A 16 mm wide piece of Scotch tape (registered trademark) manufactured by Nichiban Co., Ltd. was applied to the sintered compact 2, 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 any residue on the tape was checked. A was assigned to cases where there was no residue, B to cases where there was a small amount of residue in some areas, and C to cases where there was residue over the entire surface.

[0096] [Porosity measurement] The sample (a laminate of copper plate and sintered compact 2) obtained in wick preparation (2) was placed in a plastic cup and poured with casting resin (Epomount, Refine Tech Co., Ltd.). It was then placed in a vacuum desiccator and degassed under reduced pressure. The resin 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-Tech Corporation) at an applied voltage of 15 kV and a magnification of 500x. The 500x SEM observation images were binarized using the image analysis software Image J, and the porosity (unit: volume %) of the sintered body (wick) was calculated from the ratio of the number of dots in the white and black areas. In this evaluation, three images were observed at different locations, the porosity in each image was calculated, and the average of these was taken as the porosity of the sintered body (wick).

[0097] [Table 2]

[0098] As shown in Table 2, the sintered bodies of Examples 1 and 2 and Comparative Example 1 were all rated A in the tape peeling test, confirming that the strength of the sintered bodies and the adhesion to the adherend surface were good. Furthermore, it was confirmed that the sintered body of Comparative Example 1, which did not use thermally decomposable resin particles, had a low porosity, while the sintered bodies of Examples 1 and 2 had a high porosity.

[0099] <Rating 1-2> In the same manner as in the measurement of porosity, cross-sectional SEM images of the sintered body 3 of Examples 1 and 2 and Comparative Example 1 were observed. The observed cross-sectional SEM images are shown in Fig. 2. Fig. 2(a) is a cross-sectional SEM image of the sintered body 3 of Example 1, Fig. 2(b) is a cross-sectional SEM image of the sintered body 3 of Example 2, and Fig. 2(c) is a cross-sectional SEM image of the sintered body 3 of Comparative Example 1.

[0100] As shown in Figure 2(a) and (b), in Examples 1 and 2, which used thermally decomposable resin particles, it was confirmed that in addition to the pores inherent in the sintered body, pores derived from the thermally decomposable resin particles (pores having substantially the same shape as the thermally decomposable resin particles and a size equivalent to that of the thermally decomposable resin particles) were formed. On the other hand, as shown in Figure 2(c), the pores in the sintered body of Comparative Example 1, which did not use thermally decomposable resin particles, were only the pores inherent in the sintered body formed between the copper particles, and the pore size was small, at several micrometers.

[0101] Example 3 [Preparation of copper paste] 17.575 g of terpineol C (manufactured by Nippon Terpene Chemical Co., Ltd.) as a dispersion medium, 1.330 g of M-6003 (methacrylic resin, manufactured by Negami Chemical Industries Co., Ltd., 95% thermal decomposition temperature: 284°C) as a thermally decomposable resin B, and 0.095 g of lauric acid as an additive (dispersibility improver) were placed in a plastic bottle, mixed using a planetary vacuum mixer (Planetry Vacuum Mixer ARV-310, manufactured by Thinky Corporation), and left overnight to obtain a solution. To this solution, 15.39 g of CT-0500 (Mitsui Mining & Smelting Co., Ltd., spherical copper powder, volume average particle size: 1 μm) and 60.75 g of FCC-115 (Fukuda Metal Foil & Powder Co., Ltd., dendritic copper powder, volume average particle size: 30 μm) were added as copper particles, and 4.860 g of GR-300T (acrylic resin particles, Negami Chemical Industrial Co., Ltd., volume average particle size: 22 μm, 95% thermal decomposition temperature: 350 ° C) were added as thermally decomposable resin particles. The mixture was stirred at 2000 rpm for 1 minute using a planetary vacuum mixer (Thinky Corporation, Planetary Vacuum Mixer ARV-310). The mixture was then stirred once with a spoon to confirm the absence of solids, and then stirred at 2000 rpm for 2 minutes under reduced pressure to obtain a copper paste. The content of thermally decomposable resin particles (Cw) was 6 parts by mass per 100 parts by mass of the total of copper particles and thermally decomposable resin particles, and the content of thermally decomposable resin particles (Cv) was 32.6 parts by volume per 100 parts by volume of the total of copper particles and thermally decomposable resin particles. The content of copper particles with a particle size of 1.5 μm or less (small diameter copper particles) was 20% by volume, based on the total amount of copper particles in the copper paste. The viscosity of the copper paste at 25°C was 10 to 120 Pa·s.

[0102] [Wick Creation] Wick preparation (1) and (2) were carried out in the same manner as in Example 1, except that the copper paste obtained above was used and the firing temperature was changed to 450°C, and sintered bodies of the copper paste (sintered bodies 1 and 2) having the thicknesses shown in Table 3 were obtained. The obtained sintered bodies 1 and 2 were used to form heat pipes, and it was confirmed that the sintered bodies 1 and 2 functioned as wicks.

[0103] <Examples 4 and 5> Copper pastes were prepared in the same manner as in Example 3, except that the amounts of each component were changed to those shown in Table 3 so that the content Cw of thermally decomposable resin particles was 12 parts by mass or 20 parts by mass relative to 100 parts by mass of the total amount of copper particles and thermally decomposable resin particles. The viscosity of the copper paste at 25°C was 10 to 120 Pa·s. Next, wick preparations (1) and (2) were carried out in the same manner as in Example 3, except that the obtained copper paste was used, and sintered bodies of the copper paste (sintered bodies 1 and 2) having the thicknesses shown in Table 3 were obtained. The obtained sintered bodies 1 and 2 were used to construct heat pipes, and it was confirmed that sintered bodies 1 and 2 functioned as wicks.

[0104] <Comparative Example 2> Copper paste was prepared in the same manner as in Example 3, except that no thermally decomposable resin particles were used and the blending amounts of each component were changed to the amounts shown in Table 3. Next, wick preparations (1) and (2) were carried out in the same manner as in Example 1, except that the obtained copper paste was used, and sintered bodies of the copper paste (sintered bodies 1 and 2) having the thicknesses shown in Table 1 were obtained.

[0105] [Table 3]

[0106] <Rating 2-1> The tape peeling test and porosity measurement were carried out in the same manner as in Evaluation 1-1 for the sintered bodies 2 of Examples 3 to 5 and Comparative Example 2. The evaluation results are shown in Table 4.

[0107] [Table 4]

[0108] As shown in Table 4, the sintered body of Example 3 was judged to be A in the tape peeling test, confirming that the strength of the sintered body and the adhesion to the adherend surface were good. Furthermore, it was confirmed that the sintered body of Comparative Example 2, which did not use thermally decomposable resin particles, had a low porosity, while the sintered bodies of Examples 3 to 5 had a high porosity.

[0109] <Rating 2-2> In the same manner as in Evaluation 1-2, cross-sectional SEM images of the sintered bodies 1 of Examples 3 to 5 were observed. The observed cross-sectional SEM images are shown in Fig. 3. Fig. 3(a) is a cross-sectional SEM image of the sintered body 1 of Example 3, Fig. 3(b) is a cross-sectional SEM image of the sintered body 1 of Example 4, and Fig. 3(c) is a cross-sectional SEM image of the sintered body 1 of Example 5.

[0110] As shown in Figure 3(a) to (c), in Examples 3 to 5, which used thermally decomposable resin particles, it was confirmed that in addition to the pores inherent in the sintered body, pores derived from the thermally decomposable resin particles (pores having approximately the same shape as the thermally decomposable resin particles and a size equivalent to the size of the thermally decomposable resin particles) were formed.

[0111] <Examples 6 and 7> Copper pastes were prepared in the same manner as in Example 4, except that EBY (Mitsui Mining & Smelting Co., Ltd., dendritic copper powder, volume average particle size: 6.7 μm) or EAX small diameter (Mitsui Mining & Smelting Co., Ltd., dendritic copper powder, volume average particle size: 13.2 μm) was used instead of FCC-115. The content of copper particles with a particle size of 1.5 μm or less (small diameter copper particles) was 20 vol% based on the total amount of copper particles in the copper paste. The viscosity of the copper paste at 25°C was 10 to 120 Pa s.

[0112] Next, wick preparation (1) and (2) were carried out in the same manner as in Example 4, except that the obtained copper paste was used, and sintered bodies of the copper paste (sintered bodies 1 and 2) having the thicknesses shown in Table 5 were obtained. The obtained sintered bodies 1 and 2 were used to construct heat pipes, and it was confirmed that the sintered bodies 1 and 2 functioned as wicks.

[0113] [Table 5]

[0114] <Rating 3-1> The tape peeling test and porosity measurement were carried out in the same manner as in Evaluation 1-1 for the sintered bodies 2 of Examples 6 and 7. Table 6 shows the evaluation results.

[0115] [Table 6]

[0116] As shown in Table 6, the sintered bodies of Examples 6 and 7 were all rated A in the tape peeling test, confirming that the strength of the sintered bodies and the adhesion to the adherend surface were good. In addition, it was confirmed that the sintered bodies of Examples 6 and 7 had high porosity.

[0117] <Rating 3-2> In the same manner as in Evaluation 1-2, cross-sectional SEM images of the sintered bodies 1 of Examples 6 and 7 were observed. The observed cross-sectional SEM images are shown in Fig. 4. Fig. 4(a) is a cross-sectional SEM image of the sintered body 1 of Example 6, and Fig. 4(b) is a cross-sectional SEM image of the sintered body 1 of Example 7.

[0118] As shown in Figures 4(a) and 4(b), in Examples 6 and 7, which used thermally decomposable resin particles, it was confirmed that in addition to the pores inherent in the sintered body, pores derived from the thermally decomposable resin particles (pores having approximately the same shape as the thermally decomposable resin particles and a size equivalent to the size of the thermally decomposable resin particles) were formed.

[0119] Example 8 A copper paste was prepared in the same manner as in Example 4, except that EAX Large Diameter (manufactured by Mitsui Mining & Smelting, dendritic copper powder, volume average particle size: 16.5 μm) was used instead of FCC-115. The content of copper particles with a particle size of 1.5 μm or less (small diameter copper particles) was 20% by volume based on the total amount of copper particles in the copper paste. The viscosity of the copper paste at 25°C was 10 to 120 Pa s.

[0120] Next, wick preparation (1) was carried out in the same manner as in Example 4, except that the obtained copper paste was used, and a sintered body of the copper paste (sintered body 1) having the thickness shown in Table 7 was obtained. The obtained sintered body 1 was used to construct a heat pipe, and it was confirmed that the sintered body 1 functioned as a wick.

[0121] Example 9 The copper paste of Example 9 was prepared in the same manner as in Example 4, except that CH-0200 was used instead of CT-0500, and CuAtW-250 (Fukuda Metal Foil & Powder Co., Ltd., irregularly shaped copper powder, volume average particle size: 30 μm) was used instead of FCC-115. The content of copper particles with a particle size of 1.5 μm or less (small diameter copper particles) was 20% by volume based on the total amount of copper particles in the copper paste. The viscosity of the copper paste at 25°C was 10 to 120 Pa s.

[0122] Next, wick preparation (1) was carried out in the same manner as in Example 4, except that the obtained copper paste was used, and a sintered body of the copper paste (sintered body 1) having the thickness shown in Table 7 was obtained. The obtained sintered body 1 was used to construct a heat pipe, and it was confirmed that the sintered body 1 functioned as a wick.

[0123] Example 10 A copper paste was prepared in the same manner as in Example 9, except that CT-0500 was used instead of CH-0200, and the amounts of each component were changed to those shown in Table 7 so that the content of thermally decomposable resin particles (Cw) was 25 parts by mass per 100 parts by mass of the total amount of copper particles and thermally decomposable resin particles. The content of copper particles with a particle size of 1.5 μm or less (small diameter copper particles) was 20% by volume based on the total amount of copper particles in the copper paste. The viscosity of the copper paste at 25°C was 10 to 120 Pa s.

[0124] Next, wick preparation (1) was carried out in the same manner as in Example 9, except that the obtained copper paste was used, and a sintered body of the copper paste (sintered body 1) having the thickness shown in Table 7 was obtained. The obtained sintered body 1 was used to construct a heat pipe, and it was confirmed that the sintered body 1 functioned as a wick.

[0125] [Table 7]

[0126] <Rating 4-1> The porosity was measured in the same manner as in Evaluation 1-1 for the sintered bodies 1 of Examples 8 to 10. Table 8 shows the evaluation results.

[0127] [Table 8]

[0128] As shown in Table 8, it was confirmed that the sintered bodies of Examples 8 to 10 had high porosity.

[0129] <Rating 4-2> In the same manner as in Evaluation 1-2, a cross-sectional SEM image of the sintered body 1 of Example 8 was observed. The observed cross-sectional SEM image is shown in Figure 5. As shown in Figure 5, in Example 8, which used thermally decomposable resin particles, it was confirmed that in addition to the pores inherent in the sintered body, pores derived from the thermally decomposable resin particles (pores having substantially the same shape as the thermally decomposable resin particles and a size equivalent to the size of the thermally decomposable resin particles) were formed. [Explanation of symbols]

[0130] 1...heat pipe, 2...container, 3...wick.

Claims

1. A copper paste for forming a wick of a heat pipe, The composition comprises copper particles, thermally decomposable resin particles, a dispersion medium for dispersing the copper particles and the thermally decomposable resin particles, and a thermally decomposable resin that is soluble in the dispersion medium; A copper paste in which the proportion of copper particles having a particle size of 1.5 μm or less is 10% by volume or more based on the total amount of the copper particles.

2. The copper paste according to claim 1, wherein the content of the thermally decomposable resin particles is 3 to 30 parts by mass per 100 parts by mass of the total amount of the copper particles and the thermally decomposable resin particles.

3. The copper paste according to claim 1 or 2, wherein the volume average particle size of the thermally decomposable resin particles is 5 to 40 μm.

4. The copper paste according to any one of claims 1 to 3, wherein the 95% thermal decomposition temperature of the thermally decomposable resin particles and the thermally decomposable resin is 450°C or less.

5. The copper paste according to any one of claims 1 to 4, wherein the content of the thermally decomposable resin is 1 to 25 parts by mass per 100 parts by mass of the copper particles.

6. A copper paste described in any one of claims 1 to 5, comprising large-diameter copper particles having a volume average particle size of 5 to 50 μm and small-diameter copper particles having a volume average particle size of 0.1 to 2.0 μm.

7. The copper paste according to any one of claims 1 to 6, wherein the viscosity of the copper paste at 25 ° C. is 10 to 120 Pa s.

8. 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 7; and sintering the copper paste.

9. A heat pipe comprising a wick containing a sintered body of the copper paste according to any one of claims 1 to 7.

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