KR102720208B1 - Fabrication method of a capillary-driven heat pipe using electroless plating - Google Patents

Abstract

The present invention relates to the manufacture of a capillary-driven heat pipe. After forming a polymer template into a desired final shape by 3D printing or other methods, an outer shell is formed by electroless homogeneous plating, and a core is formed by electroless porous plating and blackening. Since electroless plating has the characteristic of laminating a metal material of uniform thickness on all surfaces regardless of the position or direction in three-dimensional space, using this method, a very light copper outer shell and core can be formed regardless of the shape of the polymer template. Therefore, the outer shell and core manufactured by electroless plating proposed in the present invention can be manufactured to have a much lighter weight since the thickness can be precisely adjusted as needed, and can be manufactured in an ultra-small size, which is advantageous in the manufacture of a heat pipe that requires cooling of a device with severe internal space restrictions such as a mobile device, and since it is manufactured based on a 3D printed polymer template, it is possible to manufacture small quantities with a desired specific size and shape to fit the installation part without secondary processing.

Description

{FABRICATION METHOD OF A CAPILLARY-DRIVEN HEAT PIPE USING ELECTROLESS PLATING}

The present invention relates to the manufacture of a heat pipe in which a heat exchange or heat transfer process is performed by the phase change and movement of a working fluid accommodated in a vacuum chamber, and more particularly, to the manufacture of a so-called capillary-driven heat pipe in which the movement of a liquid working fluid is performed by the capillary phenomenon in a wick as a medium.

Heat pipes are designed for the purpose of efficient heat conduction and diffusion, and are widely used for connecting to external cooling devices in electrical devices, especially computers, where the space near the heat source is narrow and it is difficult to attach a cooling device. The most widely used type is as shown in Fig. 1. In the case of the heat pipe of Fig. 1, the space within the pipe-shaped impermeable outer shell (evelope (sealed outer wall)) is maintained at a pressure lower than the atmospheric pressure, and an evaporator and a condenser are arranged at both ends, and a passageway for a working fluid is provided between them. A wick, which serves as a storage and movement medium for the liquid working fluid, is formed mainly on the wall surface of the vacuum chamber over the entire area of the evaporator, the condenser, and the passageway between them, and the center between the wall surfaces forms an empty space. In this case, the empty space in the center of the passageway forms a passage through which the working fluid moves from the vaporizer to the condenser. (Reference: [1] Faghri, A, 2016, Heat Pipe Science and Technology, Second edition, Global Digital Press.)

The operation of the heat pipe according to the above drawing 1 will be explained as follows. When a high-temperature external heat source comes into contact with the outer surface of the vaporizing section, the liquid working fluid stored in the wick vaporizes, and the gaseous working fluid is pushed along the empty space in the center of the passageway section to the condensing section on the opposite side as the pressure increases. The temperature of the condensing section is usually lowered by a heat sink (cooling fins and fan) installed on the outside of the condensing section. Therefore, the gaseous working fluid condenses and changes into a liquid. The liquid working fluid is returned to the original vaporizing section along the wick provided on the wall of the passageway section by capillary action. As this vaporization and condensation of the working fluid is repeated, a thermodynamic cycle is completed that moves between both ends of the heat pipe, and heat is transferred much more efficiently than by simple conduction. Therefore, the temperature difference in the longitudinal direction of the heat pipe becomes smaller. A heat pipe that operates on this principle is called a capillary-driven heat pipe because the liquid working fluid returns to the origin along the wick by capillary action, or a vacuum chamber type heat pipe because the internal pressure is lowered to facilitate the phase transformation of vaporization/condensation even at low temperatures.

In such capillary-driven heat pipes, wick materials that act as storage and movement media for liquid working fluids through capillary phenomena are traditionally made of sintered powder metal, metal fabric (textile or mesh), and grooves processed on the wall surface, as shown in Fig. 2. These wicks have pores that are small enough to allow the capillary phenomenon to occur well, but the porosity, which is the ratio of the internal empty space, is about 10 to 30%.

As an example of a traditional capillary-driven heat pipe, a seamless tube is first manufactured (mainly) with copper, and then a mandrel with a diameter slightly smaller than the inner diameter of the tube is placed inside. Copper powder is placed in the space between the tube wall and the mandrel, and the tube is heated at a temperature slightly lower than the melting point of copper for a long time so that the copper powder sinteres and adheres to the inner wall, thereby creating a sintered metal wick. In addition, a metal thread can be separately woven into a cylindrical shape and inserted so as to adhere closely to the inner wall of the tube, or a method can be used to create a groove in the inner wall in the final stage of the seamless tube manufacturing process.

As another example, a flat heat pipe has been developed to transfer heat over a wider area. That is, a wick is arranged on the inner surface of a flat outer shell, and columns or walls are arranged in the inner space to reinforce the structural rigidity in order to prevent the collapse of the outer shell in a vacuum. Figures 3(a) and (b) use open-cell foam metal and grooves as wicks, respectively. (References [2] X. Ji, J. Xu, A.M. Abanda, Copper foam based vapor chamber for high heat flux dissipation, Exp. Therm. Fluid Sci. 40 (2012) 93-102.), [3] M. Wang, W. Cui, Y. Hou, Thermal spreading resistance of grooved vapor chamber heat spreader, Appl. Therm. Eng. 153 (2019) 361-368.)

In recent years, there has been an increasing need for much thinner flat heat pipes than before for cooling mobile devices. Against this background, Ji et al. proposed a form in which a simple flat plate was covered on top of a plane in which cones were regularly arranged using a powder composed of copper particles with a diameter of 1 μm (Fig. 4(a)). The total thickness was 3.4 mm. (Reference [4] Ji, X. B.; Xu, J. L.; Li, H. C.; Huang, G. H. Switchable Heat Transfer Mechanisms of Nucleation and Convection by Wettability Match of Evaporator and Condenser for Heat Pipes: Nanostructured Surface Effect. Nano Energy 2017, 38, 313325.) In addition, Yang et al. proposed a form in which parallel grooves were dug on a silicon wafer using MEMS technology, a number of pillars (pin-fin) were arranged in a regular pattern (array), and then a simple flat plate was covered on top of it (Fig. 4(b)). The total thickness was 1.5 mm. (Reference [5] Yang, K. S.; Lin, C. C.; Shyu, J. C.; Tseng, C.Y.; Wang, C. C. Performance and Two-Phase Flow Pattern for Micro Flat Heat Pipes. Int. J. Heat Mass Transfer 2014, 77, 11151123.)

In addition, Lewis et al. proposed a structure in which a copper fabric is placed on a flat plate and a separate thin plate with copper pillars arranged in a regular pattern is covered using MEMS technology and electroplating (Fig. 5(a)). The total thickness is 0.5 mm. (Reference [6] Lewis, R.; Xu, S. S.; Liew, L. A.; Coolidge, C.; Yang, R. G.; Lee, Y. C. Thin Flexible Thermal Ground Planes: Fabrication and Scaling Characterization. J. Microelectromech. Syst. 2015, 24 (6), 20402048.) Tang et al. proposed a structure in which a copper fabric woven into a tube shape is pressed and placed in the center of a similarly pressed copper tube so that the copper fabric acts as both a core and a support for the flat plate, and the space on both sides of the copper fabric acts as a passage for the vapor to move (Fig. 5(b)). The core thickness is 0.24 mm. In this case, the total thickness is estimated to be about 0.5 mm, although not specified. (Reference [7] Tang, Y.; Tang, H.; Li, J.; Zhang, S. W.; Zhuang, B. S.; Sun, Y. L. Experimental Investigation of Capillary Force in a Novel Sintered Copper Mesh Wick for Ultra-Thin Heat Pipes. Appl. Therm. Eng. 2017, 115, 10201030.)

Most recently, in 2021, Luo et al. formed a 100 μm thick dendrite layer on one surface of two copper foils through copper electroplating, then arranged them so that they faced each other and placed a support plate with multiple parallel strip-shaped perforations between them, so that the dendrite plating layer served as a wick and the strip-shaped perforations of the support plate served as a vapor passage. The total thickness was 0.5 mm. (Reference [8] J. L. Luo, D. C. Mo, Y. Q. Wang, and S. S. Lyu, Biomimetic Copper Forest Wick Enables High Thermal Conductivity Ultrathin Heat Pipe. ACS Nano, 2021, 15, 66146621.) Since this method uses electroplating, there is a problem that it cannot be applied when the shape of the wick in the heat pipe is not flat, since the formation of the dendritic tissue is sensitive to the distance and arrangement between the electrodes.

However, the conventional capillary-driven heat pipes mentioned above, as well as the heat pipes that are being newly attempted recently, have complicated manufacturing methods and are particularly limited in their application to recent mobile devices. Specifically, conventional heat pipes are manufactured by forming a wick inside a copper tube manufactured through traditional mechanical processing consisting of extrusion and drawing processes as an outer shell and then manufacturing them through a step of injecting a working fluid and a vacuum and sealing process. In this case, it is technically difficult to manufacture the copper tube below a certain thickness through mechanical processing, that is, to make it small and lightweight. In addition, since the wick is manufactured by arranging or forming sintered metal, foam, fabric, etc., or by forming it through groove or MEMS precision processing, or by electroplating, etc., the process difficulty is high and the porosity is relatively low at 50% or less, so the amount of working fluid injected and the heat capacity that can be transferred are small. In addition, due to the limitations of the outer shell formation method itself or the influence of the wick formation method, it is difficult to freely form the outer shell other than a simple rod or flat shape, so it is difficult to avoid restrictions related to the installation space.

Meanwhile, various types of ultra-low-density materials have been introduced recently. For example, in 2011, an innovative ultra-low-density material called Microlattice was introduced. The ultra-low-density material consists of a multiscale hierarchical structure in which tubes with a diameter of several hundred μm and a length of several mm, made of extremely thin metal films with a thickness of 100 nm, form a periodic truss, so that the density can be reduced to about 1/1000 of that of water, while still having relatively high strength and stiffness and enabling elastic recovery even after large deformation. (Fig. 7, Reference [9] T.A. Schaedler, A.J. Jacobsen, A. Torrents, A.E. Sorensen, J. Lian, J.R. Greer, L. Valdevit, W.B. Carter, Science 334 (2011) 962-965.) Subsequently, ultra-low-density materials with similar structures such as Nanolattice and Mechanical metamaterials appeared. (Fig. 8 (a), (b), Reference [10] L.R. Meza, S. Das, J.R. Greer, Science 345 (2014) 1322-1326. [11] X. Zheng, H. Lee, T.H. Weisgraber, M. Shusteff, J. DeOtte, E.B. Duoss, J.D. Kuntz, M.M. Biener, Q. Ge, J.A. Jackson, S.O. Kucheyev, N.X. Fang, C.M. Spadaccini, Science 344 (2014) 1373-1377.) In addition, the inventors of the present invention have introduced Shellular, a thin film structure in the form of a triply periodic minimal surface (TPMS), as an ultra-low-density material. (Fig. 9, [12] S.C. Han, J.W. Lee, K. Kang, Adv. Mater. 27 (2015) 5506-5511. [13] S.C. Han, K. Kang, Materials Today 31 (2019) 31-38.) All of these materials are made by manufacturing a truss-shaped template with a polymer, then depositing metal or ceramic on the surface through electroless plating, chemical vapor deposition (CVD), or atomic layer deposition (ALD), and then etching the polymer template inside. All of these deposition methods have the characteristic of depositing solid materials with a uniform thickness on all surfaces regardless of the location or orientation in three-dimensional space.

The above-mentioned ultra-low-density materials have been continuously discussed regarding their applications, but to date, there has been no confirmed case of applying the materials to a heat pipe, which is the subject of the present invention. Against this backdrop, the inventors of the present invention have anticipated that if the above-mentioned ultra-low-density materials can be implemented as a lightweight thin-film structure having a uniform wall thickness regardless of the shape of the template, it will be possible to implement an innovative heat pipe having a significantly lighter and freer appearance by resolving the limitations of conventional capillary-driven heat pipes if this can be utilized as the outer shell of the heat pipe, and have sought specific measures for this.

Patent No. 10-2289462 Publication Patent No. 10-2019-0118037

[1] Faghri, A, 2016, Heat Pipe Science and Technology, Second edition, Global Digital Press. [2] X. Ji, J. Xu, A.M. Abanda, Copper foam based vapor chamber for high heat flux dissipation, Exp. Therm. Fluid Sci. 40 (2012) 93-102.) [3] M. Wang, W. Cui, Y. Hou, Thermal spreading resistance of grooved vapor chamber heat spreader, Appl. Therm. Eng. 153 (2019) 361-368. [4] Ji, X. B.; Xu, J.L.; Li, H. C.; Huang, G. H. Switchable Heat Transfer Mechanisms of Nucleation and Convection by Wettability Match of Evaporator and Condenser for Heat Pipes: Nanostructured Surface Effect. Nano Energy 2017, 38, 313325. [5] Yang, K.S.; Lin, C. C.; Shyu, J. C.; Tseng, C.Y.; Wang, C. C. Performance and Two-Phase Flow Pattern for Micro Flat Heat Pipes. Int. J. Heat Mass Transfer 2014, 77, 11151123. [6] Lewis, R.; Xu, S. S.; Liew, L.A.; Coolidge, C.; Yang, R. G.; Lee, Y. C. Thin Flexible Thermal Ground Planes: Fabrication and Scaling Characterization. J. Microelectromech. Syst. 2015, 24 (6), 20402048. [7] Tang, Y.; Tang, H.; Li, J.; Zhang, S. W.; Zhuang, B. S.; Sun, Y. L. Experimental Investigation of Capillary Force in a Novel Sintered Copper Mesh Wick for Ultra-Thin Heat Pipes. Appl. Therm. Eng. 2017, 115, 10201030. [8] J. L. Luo, D. C. Mo, Y. Q. Wang, and S. S. Lyu, Biomimetic Copper Forest Wick Enables High Thermal Conductivity Ultrathin Heat Pipe. ACS Nano, 2021, 15, 66146621. [9] T.A. Schaedler, A.J. Jacobsen, A. Torrents, A.E. Sorensen, J. Lian, J.R. Greer, L. Valdevit, W.B. Carter, Science 334 (2011) 962-965. [10] L.R. Meza, S. Das, J.R. Greer, Science 345 (2014) 1322-1326. [11] X. Zheng, H. Lee, T.H. Weisgraber, M. Shusteff, J. DeOtte, E.B. Duoss, J.D. Kuntz, M.M. Biener, Q. Ge, J.A. Jackson, S.O. Kucheyev, N.X. Fang, C.M. Spadaccini, Science 344 (2014) 1373-1377. [12] S.C. Han, J.W. Lee, K. Kang, Adv. Mater. 27 (2015) 5506-5511. [13] S.C. Han, K. Kang, Materials Today 31 (2019) 31-38.) [14] C. A. Deckert, “Electroless Copper Plating A Review: Part I,” PLATING & SURFACE FINISHING, pp. 48-55 (1995). [15] J. Sudagar, J. Lian, W. Sha, "Electroless nickel, alloy, composite and nano coatings - A critical review.," Journal of Alloys and Compounds, 571 (2013) 183-204. DOI: 10.1016/j.jallcom.2013.03.107, [16] Tomoyuki FUJINAMI, Kentei KOU, Hideo HONMA, "Formation of the Porous Film by Electroless Copper Plating," Japan Electronics Vol.1 No.1 (1998) 66-69. [17] Heon-Cheol Shin and Meilin Liu, “Copper Foam Structures with Highly Porous Nanostructured Walls,” Chem. Mater. 16 (2004) 5460-5464. [18] Min, Jingchun; Wu, Xiaomin; and Gao, Frank, “Hydrophilic Treatments of Copper Finned Tube Evaporators” (2008). [19] International Refrigeration and Air Conditioning Conference. Paper 930. [20] M. Maldovan and E. L. Thomas, “Periodic Materials and Interference Lithography, 2009 WILEY-VCH Verlag GmbH & Co. KGaA, ISBN: 978-3-527-31999-2 [21] Y. Jung, S. Torquato, Fluid permeabilities of triply periodic minimal surfaces, Phys. Rev. E 72 (2005) 056319. [22] Cheng Han Wu, Failure Study of Shellulars under Internal Pressure. Master's thesis, Chonnam National University Graduate School, 2019.

The present invention has been made to solve the problems of the above-mentioned prior art, and aims to provide a capillary-driven heat pipe that is advantageous in being ultra-small and lightweight, can be implemented with a free external shape, and is easy to manufacture.

The inventors of the present invention, in the process of researching and developing a capillary-driven heat pipe related to the above-mentioned problem, (i) manufactured a template having a free shape and size by 3D printing or other methods, and formed a first electroless plating layer having a uniform thickness using the template to form an ultra-low-density thin film structure as the outer shell of the heat pipe, and (ii) formed an impermeable or porous second electroless plating layer on the surface of the ultra-low-density thin film structure, that is, the surface of the first electroless plating layer, and subjected the porous electroless plating layer to a hydrophilic treatment such as blackening to form the core of the heat pipe, thereby confirming that the heat pipe manufactured in this way can have sufficient capillary force to be applied to the capillary-driven heat pipe, and not only is the manufacturing method simple and easy, but also uses a very thin electroless plating layer with a uniform thickness as the material for the outer shell and the core regardless of the position or direction in three-dimensional space, so that it can be made small and lightweight and can be implemented with a free external shape, so that it can be used for mobile devices. It was noted that the present invention can be advantageously applied to the device. The recognition of the above-mentioned problem and the gist of the present invention based on it are as follows.

(1) A capillary-driven heat pipe including an outer shell having an internal space shielded from the outside and composed of an impermeable wall; and a wick provided on an inner wall surface of the outer shell to store a liquid working fluid and serve as a moving medium of the liquid working fluid; wherein each of the outer shell and the wick is composed of a first electroless plating layer and a second electroless plating layer, and the first electroless plating layer is impermeable and at least the second electroless plating layer is hydrophilically treated.

(2) The capillary-driven heat pipe of (1), characterized in that the second electroless plating layer is non-permeable.

(3) The capillary-driven heat pipe of (1), characterized in that the second electroless plating layer is porous.

(4) A capillary-driven heat pipe according to any one of (1) to (3), characterized in that the outer shell has a TPMS structure shape.

(5) A capillary-driven heat pipe including an outer shell having an internal space shielded from the outside and composed of an impermeable wall; and a wick provided on an inner wall surface of the outer shell to store a liquid working fluid and serve as a moving medium of the liquid working fluid; wherein the outer shell and the wick are composed of a single impermeable electroless plating layer, and at least a surface of the impermeable electroless plating layer on the side provided as the wick is hydrophilically treated.

(6) A method for manufacturing a capillary-driven heat pipe, comprising: (a) forming an outer shell and a wick; (b) injecting a working fluid into an inner space of the outer shell; and (c) forming a vacuum for the inner space of the outer shell and then sealing it; wherein step (a) comprises: (a-1) forming a template; (a-2) and forming an impermeable first electroless plating layer forming the outer shell and a second electroless plating layer forming the wick using the template; and (a-3) performing a hydrophilic treatment on at least the second electroless plating layer; wherein step (a-2) involves removing the template before or after formation of the porous electroless plating layer.

(7) The method for manufacturing a capillary-driven heat pipe of (6), characterized in that the step (a-2) is performed by: (a-2-1) forming the first electroless plating layer on the exposed surface of the above-mentioned template; (a-2-2) removing the template; and (a-2-3) forming the second electroless plating layer on the exposed surface of the first electroless plating layer.

(8) A method for manufacturing a capillary-driven heat pipe according to (7), characterized in that it further comprises a step of performing pretreatment to prevent plating on one of the exposed surfaces of the first electroless plating layer between steps (a-2-2) and (a-2-3).

(9) The method for manufacturing a capillary-driven heat pipe of (6), characterized in that the step (a-2) is performed by: (a-2-1) forming a first electroless plating layer on the exposed surface of the template; (a-2-2) forming a second electroless plating layer on the exposed surface of the first electroless plating layer; and (a-2-3) removing the template.

(10) A method for manufacturing a capillary-driven heat pipe according to (9), characterized in that it further comprises a step of performing pretreatment to prevent plating on one of the exposed surfaces of the template prior to the step (a-2-1).

(11) A method for manufacturing a capillary-driven heat pipe according to any one of (6) to (10), characterized in that the second electroless plating layer is non-permeable.

(12) A method for manufacturing a capillary-driven heat pipe according to any one of (6) to (10), characterized in that the second electroless plating layer is porous.

(13) A method for manufacturing a capillary-driven heat pipe according to any one of (6) to (10), characterized in that the template has a TPMS structure shape.

(14) A method for manufacturing a capillary-driven heat pipe, comprising: (a) a step of forming an outer shell and a wick; (b) a step of injecting a working fluid into an inner space of the outer shell; and (c) a step of forming a vacuum for the inner space of the outer shell and then sealing it; wherein the step (a) comprises: (a-1) a step of forming a template; (a-2) and a step of forming an impermeable electroless plating layer constituting the outer shell and the wick using the template as a medium; and (a-3) a step of hydrophilizing a surface of at least a side of the impermeable electroless plating layer that serves as a wick.

According to the present invention, a template having a free shape and size is manufactured by 3D printing or other methods, and a first electroless plating layer having a non-permeable thickness is formed using the template to form an ultra-low-density thin film structure as the outer shell of a heat pipe, thereby making it advantageous in terms of miniaturization and weight reduction and freely implementing a shape compared to an outer shell manufactured by conventional mechanical processing such as extrusion or drawing.

In addition, by forming a second electroless plating layer on the surface of the ultra-low-density thin film structure, that is, the surface of the non-permeable first electroless plating layer, and performing a hydrophilic treatment such as a blackening treatment on the second electroless plating layer to form a wick of a heat pipe, the thickness of the wick can be precisely controlled uniformly as necessary regardless of the shape of the outer shell, which is not only advantageous for weight reduction, but also secures excellent capillary force, thereby implementing excellent transferable heat capacity despite the thin thickness. In particular, when the second electroless plating layer is formed to be porous, it is advantageous because the retention amount of the working fluid can be increased.

Since such heat pipes have excellent transferable heat capacity and can be manufactured in an ultra-small, lightweight, and diverse shape, they can be advantageously applied to cooling devices with severe restrictions on the internal space in which they are installed, such as mobile devices.

In addition, the heat pipe according to the present invention can be easily manufactured through a series of processes including a step of manufacturing a polymer template, a step of forming a two-stage electroless plating layer, a step of etching the polymer template under the plating layer, a step of injecting a working fluid, and a step of forming and sealing a vacuum; and all of the unit processes can be performed with small equipment that does not generate noise, vibration, heat, etc., so that it can be advantageous for small-scale production of a variety of products.

Figure 1 is a structural diagram of a conventional capillary-driven heat pipe.
Figures 2 to 6 are examples of various wick structures of a conventional capillary-driven heat pipe.
Figures 7 to 9 are structural diagrams of conventional ultra-low-density materials.
Figure 10 is a manufacturing process diagram of a heat pipe according to an embodiment of the present invention.
FIG. 11 is an exemplary diagram of various shapes of heat pipes according to an embodiment of the present invention.
FIG. 12 is an example of a TPMS structure that can be implemented with a heat pipe of the present invention.
Figure 13 is a process diagram for manufacturing a heat pipe prototype according to an embodiment of the present invention.
FIG. 14 is an infrared photograph taken in an operating state of a heat pipe according to an embodiment of the present invention.
Figure 15 is an explanatory drawing of heat pipe specifications compared based on similar performance between a prototype of the present invention and a known commercial product.

Hereinafter, the present invention will be described in detail through examples. Prior to this, the terms or words used in this specification and claims should not be interpreted as limited to their conventional or dictionary meanings, and should be interpreted as meanings and concepts that conform to the technical idea of the present invention based on the principle that the inventor can appropriately define the concept of the term in order to explain his or her own invention in the best way. Therefore, the configuration of the embodiments described in this specification is only one of the most preferred embodiments of the present invention and does not represent all of the technical ideas of the present invention, so it should be understood that there may be various equivalents and modified examples that can replace them at the time of filing the present invention. Meanwhile, in the drawings, the same or similar reference numbers are given to the same or equivalent parts, and also, throughout the specification, when a part is said to "include" a certain component, this does not exclude other components, but means that other components can be further included, unless specifically stated otherwise.

The present invention comprises a heat pipe using two electroless plating layers, a first electroless plating layer and a second electroless plating layer, wherein the first plating layer provided as an outer shell is manufactured to be impermeable to prevent a working fluid from escaping from the inside to the outside of the heat pipe, and the second plating layer provided as a wick may be manufactured to be impermeable or porous, but is characterized by being hydrophilically treated. For reference, in the following description, 'homogeneous metal (layer)' means 'impermeable electroless plating metal (layer)' or 'impermeable electroless plating layer', and 'porous metal (layer)' means 'porous electroless plating metal (layer)' or 'porous electroless plating layer'.

Fabrication of capillary driven heat pipes

Figure 10 shows a manufacturing process of a capillary-driven heat pipe according to an embodiment of the present invention.

First, a template is made of, for example, a polymer material that is easy to mold and remove. For this purpose, any known polymer structure manufacturing process, including 3D printing and injection molding, can be used. Figs. 10(a) and (b) are schematic diagrams for manufacturing the outer shell and wick of a heat pipe using polymer templates manufactured in positive and negative template forms, respectively. Fig. 10 assumes that the final shape of the heat pipe is a cylindrical rod, and illustrates the change in its cross-section according to the process. Meanwhile, in the description of Fig. 10 below, it is assumed that the electroless plating layer constituting the wick is manufactured to be porous, but even if the wick is manufactured to be non-permeable, the overall process sequence is the same.

Specifically, FIG. 10(a) shows the steps of: producing a positive polymer template; forming a homogeneous metal shell on its surface by electroless plating; removing a portion of the shell from both ends to expose the polymer inside, and then etching it by a physical, chemical, or thermal method to leave only the homogeneous metal shell; forming a porous metal layer on the inner/outer surface of the shell by electroless plating; and performing a hydrophilic treatment on the porous metal layer (not shown in the drawing). In fact, the porous metal layer on the outer surface of the shell cannot be used as a wick, so it can be considered a waste of plating solution. In order to prevent plating of this part, a pretreatment for preventing plating may be performed by coating a non-plating material such as paraffin on one of the exposed surfaces of the shell made of the homogeneous metal in the intermediate step between the third template removal and the fourth porous metal layer formation (not shown in the drawing). Accordingly, the porous metal layer may not be formed on the surface of the homogeneous metal on which plating prevention has been performed.

FIG. 10(b) illustrates the steps of manufacturing an intaglio polymer template; forming a homogeneous metal shell on its inner surface by electroless plating; forming a porous metal layer on the inner surface of the shell by electroless plating; removing the outside of the template or part of both ends of the outside to expose the inner polymer and then etching the same by a physical, chemical or thermal method to leave only the homogeneous metal shell; and performing a hydrophilic treatment on the porous metal layer (not shown in the drawing). Although not shown in FIG. 10(b), in the case of the intaglio template, since the exposed surface may be both the inner and outer surfaces, by performing a plating prevention pretreatment of coating the non-plating material described above on one of the exposed surfaces of the template before the second homogeneous metal formation, the surface on which the subsequent homogeneous metal and porous metal layers are formed can be restricted from the beginning, thereby preventing waste of the plating solution.

Meanwhile, Fig. 10(b) shows a case where a negative template is used, in which a two-stage electroless plating layer is first formed only on the inner surface of the template and then the template is removed. However, as in the case where a positive template is used, the template may be removed in the middle of forming the two-stage electroless plating layer. In this case, as described above, it goes without saying that pretreatment for plating prevention may be performed by coating a non-plating material on one of the exposed surfaces of the outer shell made of a homogeneous metal in the middle of the template removal and the porous metal layer formation.

The above polymer template can be manufactured using a typical polymer product manufacturing process such as injection molding, or by assembling or combining already manufactured polymer products such as 3D printing or Lego toys. If there are scratches or unevenness on the template surface, a mechanical method such as sandpaper polishing, a method of momentarily applying heat to the surface to locally melt it, or a method of chemically dissolving only the surface can be applied. As a chemical surface treatment method, the Han's treatment using two solvents suggested by the inventors of the present invention in Patent No. 10-2289462 can also be used.

The electroless plating method and plating solution required for forming the homogeneous metal shell and the porous metal layer core can be referred to known literature data. (References: [14] C. A. Deckert, "Electroless Copper Plating A Review: Part I," PLATING & SURFACE FINISHING, pp. 48-55 (1995). / [15] J. Sudagar, J. Lian, W. Sha, "Electroless nickel, alloy, composite and nano coatings - A critical review.," Journal of Alloys and Compounds, 571 (2013) 183-204. DOI: 10.1016/j.jallcom.2013.03.107, / [16] Tomoyuki FUJINAMI, Kentei KOU, Hideo HONMA, "Formation of the Porous Film by Electroless Copper Plating," Japan Electronics Vol. 1 No. 1 (1998) 66-69. / [17] Heon-Cheol Shin and Meilin Liu, "Copper Foam Structures with Highly Porous Nanostructured Walls," Chem. Mater. 16 (2004) 5460-5464.) For example, if hydrogen bubbles generated during the electroless plating process are allowed to escape from the plating surface, homogeneous plating can be achieved, otherwise porous plating can be achieved. Factors controlling this include various factors such as acidity and temperature in addition to surfactants.

After forming a homogeneous metal shell and a porous metal layer wick through the above two electroless plating processes, it is preferable to increase hydrophilicity by chemically treating, for example, with NaOH and K ₂ S ₂ O _8, the porous metal layer used as the wick to promote capillary phenomenon. (References: [18] Min, Jingchun; Wu, Xiaomin; and Gao, Frank, "Hydrophilic Treatments of Copper Finned Tube Evaporators" (2008). / [19] International Refrigeration and Air Conditioning Conference. Paper 930.) By increasing the capillary force of the wick through this hydrophilic treatment, even if the volume fraction of the wick is small, the transferable heat capacity of the heat pipe can be increased, which can be advantageous for product miniaturization. The material for the hydrophilic treatment can vary depending on the material of the processable metal layer.

The heat pipe according to the present invention can be manufactured in various shapes because it forms a plating layer on the outer shell and the wick using electroless plating that can laminate a uniform surface layer regardless of position and shape using a polymer template as a medium. Fig. 11 shows an example. Fig. 11(a) shows a typical rod shape, Figs. 11(b) to 11(d) are for two-dimensional heat transfer, and Figs. 11(e) and 11(f) are for three-dimensional heat transfer. Looking at Figs. 11(a) to 11(e), it can be seen that the cross-section of the cylindrical elements is made circular in order to withstand the internal vacuum, that is, the external pressure, and the connecting portions of the elements have a smooth, curved fillet.

In addition, the heat pipe for three-dimensional heat transfer here is designed to have a large surface area so that heat can be dissipated by the surrounding air flow through natural convection or forced convection, thereby omitting a separate heavy cooling fin. In particular, Fig. 11(f) is a P-surface among the triple periodic minimal surface (TPMS) shapes. The TPMS is a surface that has a constant mean curvature at all points on the surface, and the mean curvature here means the average value of the maximum and minimum curvatures in two perpendicular directions at one point on the three-dimensional surface. The TPMS divides the space into two intertwined sub-volumes. That is, the TPMS can be recognized as an interface between the two sub-volumes.

As an example, Figures 12(a) and (b) are conceptual diagrams showing that the P-surface and G-surface, which are types of TPMS, divide two subspaces, respectively. In addition, the TPMS having the zero mean curvature mentioned above divides the space into two continuous subvolumes, and although the volume ratio of these two subvolumes is the same as 1:1, even if the volume ratios are different, a surface with a minimum surface area with a constant mean curvature dividing the two subvolumes can be defined, and this surface can also be called a TPMS (Reference: [20] M. Maldovan and E. L. Thomas, "Periodic Materials and Interference Lithography, 2009 WILEY-VCH Verlag GmbH & Co. KGaA, ISBN: 978-3-527-31999-2). Since such TPMS has a uniform mean curvature anywhere on the surface, it is reported that the thin film structure in the form of TPMS has various advantages. Specifically, since it is composed of one continuous and non-intersecting surface, a polymer template is created in the form of one subvolume and electroless plated to form the outer and It is possible to form a wick. In addition, since each subspace surrounded by a smooth curved surface has a large surface area and excellent permeability when a fluid flows inside (Reference: [21] Y. Jung, S. Torquato, Fluid permeabilities of triply periodic minimal surfaces, Phys. Rev. E 72 (2005) 056319.), it is expected that the cooling efficiency by natural convection and forced convection will also be excellent.

Meanwhile, Korean Patent Publication No. 10-2019-0118037 discloses a thin film structure having an interface shape between two subspaces, and in particular, a TPMS-type thin film structure has a uniform average curvature and can thus withstand high internal pressure. Accordingly, it was proposed to apply such a shell structure as a pressure vessel, and it was reported that the ratio of the internal subspace (space where high-pressure gas is stored) to the total space of such a TPMS pressure vessel can be freely adjusted for production, and that the internal pressure strength of the pressure vessel increases as the ratio of the internal space decreases (Reference: [22] Cheng Han Wu, Failure Study of Shellulars under Internal Pressure. Master's thesis, Chonnam National University Graduate School, 2019.). Therefore, it is expected to be desirable as an outer structure of a hip pipe that must withstand a vacuum.

Prototype fabrication and performance testing of heat pipes

Fig. 13 shows a process for manufacturing a hip pipe prototype according to the present invention. An acrylic rod having a diameter of 5 mm was used as a polymer template. After uniformly roughening the entire surface with sandpaper, homogeneous plating was performed at a low temperature (40 ^o C). The plating solution and plating process were in accordance with ELC-250 of Youngin Plachem Co., Ltd. and the process specified by the company. Next, both end surfaces were polished to remove the plating film, and then the acrylic template inside was etched by soaking in 40 ^o C acetone for 8 hours. Since the homogeneous plating film must have a sufficient thickness (at least 40 um for a diameter of 5 mm) to support a vacuum as the outer shell of the heat pipe, if it is determined that the thickness after etching is insufficient, additional homogeneous plating can be performed. In this case, additional plating is performed on both sides of the initial plating film, so the plating speed can be increased. Then, it is immersed in a porous plating solution (ELC-250-A 140 ml, ELC-250-B 60 ml, sodium hypophosphite 80 g, Surfynol® 104 6 g, HCHO 8 ml) and plated until the thickness of the porous layer becomes 100 μm. In this case, Surfynol® 104 6 g acts as a surfactant. Then, it is chemically treated with NaOH and K ₂ S ₂ O ₈ to increase the hydrophilicity. [References: [18] Min, Jingchun; Wu, Xiaomin; and Gao, Frank, "Hydrophilic Treatments of Copper Finned Tube Evaporators" (2008). / [19] International Refrigeration and Air Conditioning Conference. Paper 930.]. Then, a small copper plate is placed on one end of the pipe and sealed by brazing, and a perforated copper plate and a separate copper pipe are placed on the other end and brazed to create a connection for vacuum and working fluid injection. Then, a small amount (0.7 g) of working fluid, water, is injected, and the air inside the pipe is removed using a vacuum pump and sealed to complete the heat pipe prototype.

As shown in Fig. 14(a), the completed heat pipe prototype was fitted into a heating block, heated by applying electricity, and an infrared image of the specimen and its surroundings was taken using an infrared camera. Fig. 14(b) is an infrared image observed when the heat pipe is operating normally. From the almost no color difference between the lower evaporator and the upper condenser, it can be seen that the temperature difference is actually almost 3 ^o C. Fig. 14(c) is an infrared image observed when the heat pipe is not operating normally due to a small hole formed on the outer shell during the plating process. It can be seen that the colors between the evaporator and the upper condenser are significantly different, and that the temperature difference is actually quite severe, almost 30 ^o C.

The performance of the heat pipe prototype was compared with a commercial product (Oriental Electronics, HPC6100) of similar size. Fig. 15(a) is a comparative photograph of a commercial product and a prototype manufactured according to the present invention. Fig. 15(b) is a table comparing the diameter, length, and weight. Since the prototype manufactured according to the present invention was manufactured by attaching a thick copper plate and a connecting pipe to both ends for convenience of temporary manufacturing, the weight of these was excluded, and only the weight of the outer shell formed by electroless plating, the wick, and the injected working fluid were indicated. The weight of the prototype manufactured according to the present invention is only about 10% of that of the commercial product. Fig. 15(c) is a comparison of the infrared image measured by applying the same heat input to the commercial product in the device of Fig. 14(a) with Fig. 14(b) measured on the prototype of the present invention. It can be seen that the temperature distribution is very similar and uniform. The above results indicate that the prototype according to the present invention has almost the same heat transfer capability while the weight is only 10% of that of the commercial product.

The above description relates to specific embodiments of the present invention. As described above, the above embodiments according to the present invention are disclosed for the purpose of explanation and are not to be understood as limiting the scope of the present invention. It should be understood that various changes and modifications are possible without departing from the essence of the present invention by those skilled in the art.

Meanwhile, in the above-described embodiments, when the first electroless plating layer and the second electroless plating layer are formed by temporally separate processes or when there is a difference in properties between homogeneous and porous, the distinction can be clear, but if the properties of the two are, for example, homogeneous, the first electroless plating layer and the second electroless plating layer can be conceptually distinguished from the perspective of the purpose of being assigned to the outer shell or the core, but can be physically recognized as a single electroless plating layer having a sufficient thickness to suit the purpose. For example, in the above-described embodiment, when the second electroless plating layer is configured to be impermeable like the first electroless plating layer, the two can be integrated into a single electroless plating layer through a single process, and in this case, it is also possible to manufacture it in a form in which the inner sidewall of the one electroless plating layer (i.e., the inner surface forming the inner space of the heat pipe) is hydrophilically treated. A sufficient thickness for such a single electroless plating layer may be secured, for example, by only a primary opaque electroless plating process on a template, or by performing a primary opaque electroless plating process of a predetermined thickness based on a template and then performing a secondary opaque electroless plating layer on one or both sides of the primary opaque electroless plating layer while removing the template. In this case, rapid growth of the secondary opaque electroless plating layer can be promoted by performing the electroless plating process on both sides of the primary opaque electroless plating layer.

Furthermore, in the heat pipe of the present invention, which is a two-stage structure of the first and second electroless plating layers according to the above-described embodiment or a single electroless plating layer according to a changeable embodiment, the surface to be hydrophilically treated is basically intended to be the inner surface of the heat pipe, but it is not to be interpreted restrictively. That is, since the presence or absence of the hydrophilic treatment on the outer surface does not matter in performing the operation of the heat pipe, it should be understood to mean that at least the hydrophilic treatment on the inner surface of the heat pipe is sufficient for the heat pipe to operate. In terms of the process as well, if there is no problem in use even if the hydrophilic treatment is performed on both sides, it may be more complicated to perform a separate preventive treatment to prevent the hydrophilic treatment on the outer surface.

Accordingly, all such modifications and changes may be understood to fall within the scope of the invention disclosed in the claims or their equivalents.

Claims (14)

A capillary-driven heat pipe comprising: an outer shell having an inner space shielded from the outside and formed of an impermeable wall; and a wick provided on an inner wall surface of the outer shell to store a liquid working fluid and serve as a moving medium of the liquid working fluid;
The above outer shell is formed with a first electroless plating layer having a uniform thickness on its surface using an external template,
The above wick is formed by a second electroless plating layer having a uniform thickness on at least one surface of the first electroless plating layer,
A capillary-driven heat pipe, characterized in that the first electroless plating layer is non-permeable and at least the second electroless plating layer is hydrophilically treated. A capillary-driven heat pipe, characterized in that in the first paragraph, the second electroless plating layer is non-permeable. A capillary-driven heat pipe, characterized in that in the first paragraph, the second electroless plating layer is porous.
In any one of paragraphs 1 to 3,
A capillary-driven heat pipe characterized in that the outer shell has a triply periodic minimal surface (TPMS) structure shape. delete A method for manufacturing a capillary-driven heat pipe, comprising: (a) a step of forming an outer shell and a wick; (b) a step of injecting a working fluid into an inner space of the outer shell; and (c) a step of forming a vacuum in the inner space of the outer shell and then sealing it;
The step (a) above comprises: (a-1) a step of forming a template; (a-2) a step of forming a first non-permeable electroless plating layer forming the outer shell and a second electroless plating layer forming the wick using the template as a medium; and (a-3) a step of performing a hydrophilic treatment on at least the second electroless plating layer.
A method for manufacturing a capillary-driven heat pipe, characterized in that the step (a-2) involves a step of removing the template before or after formation of the second electroless plating layer. In the 6th paragraph, the step (a-2)
(a-2-1) A step of forming the first electroless plating layer on the exposed surface of the Sangik template;
(a-2-2) a step of removing the above template; and
(a-2-3) A method for manufacturing a capillary-driven heat pipe, characterized in that it is performed by forming the second electroless plating layer on the exposed surface of the first electroless plating layer. A method for manufacturing a capillary-driven heat pipe, characterized in that in claim 7, the method further comprises a step of performing pretreatment to prevent plating on one of the exposed surfaces of the first electroless plating layer between steps (a-2-2) and (a-2-3). In the 6th paragraph, the step (a-2)
(a-2-1) a step of forming a first electroless plating layer on the exposed surface of the template;
(a-2-2) a step of forming the second electroless plating layer on the exposed surface of the first electroless plating layer; and
(a-2-3) A method for manufacturing a capillary-driven heat pipe, characterized in that it is performed by a step of removing the template. A method for manufacturing a capillary-driven heat pipe, characterized in that in claim 9, prior to step (a-2-1), the method further comprises a step of performing pretreatment to prevent plating on one of the exposed surfaces of the template. A method for manufacturing a capillary-driven heat pipe, characterized in that the second electroless plating layer is non-permeable in any one of claims 6 to 10. A method for manufacturing a capillary-driven heat pipe, characterized in that the second electroless plating layer is porous in any one of claims 6 to 10. A method for manufacturing a capillary-driven heat pipe, characterized in that the template has a TPMS structure shape in any one of claims 6 to 10. A method for manufacturing a capillary-driven heat pipe, comprising: (a) forming an outer shell and a wick; (b) injecting a working fluid into an inner space of the outer shell; and (c) forming a vacuum in the inner space of the outer shell and then sealing it;
A method for manufacturing a capillary-driven heat pipe, characterized in that the step (a) comprises: (a-1) a step of forming a template; (a-2) and a step of forming an impermeable electroless plating layer constituting the outer shell and the core using the template; and (a-3) a step of hydrophilizing the surface of at least a side of the impermeable electroless plating layer that serves as a core.